4.1.1 Presedimentation
4.1.2 Rapid mix
4.1.3 Flocculation
4.1.4 Sedimentation
4.1.5 Solids contact unit
4.1.6 Tube or plate settlers
4.2.1 Rapid rate gravity filters
4.2.2 Rapid rate pressure filters
4.2.3 Diatomaceous earth filtration
4.2.4 Slow sand filters
4.2.5 Direct filtration
4.2.6 Deep bed rapid rate gravity filters
4.2.7 Biologically active filters
4.3.1 Chlorination equipment
4.3.2 Contact time and point of application
4.3.3 Residual chlorine
4.3.4 Testing equipment


The design of treatment processes and devices shall depend on evaluation of the nature and quality of the particular water to be treated, the desired quality of the finished water and the mode of operation planned.


Plants designed for processing surface water shall

a. provide a minimum of two units each for rapid mix, flocculation and sedimentation,

b. permit operation of the units either in series or parallel where softening is performed and should permit series or parallel operation where plain clarification is performed,

c. be constructed to permit units to be taken out of service without disrupting operation, and with drains or pumps sized to allow dewatering in a reasonable period of time,

d. provide multiple-stage treatment facilities when required by the reviewing authority,

e. be started manually following shutdown,

f. minimize hydraulic head losses between units to allow future changes in processes without the need for repumping.

4.1.1 Presedimentation

Waters containing high turbidity may require pretreatment, usually sedimentation either with or without the addition of coagulation chemicals.

a. Basin design - Presedimentation basins should have hopper bottoms or be equipped with continuous mechanical sludge removal apparatus, and provide arrangements for dewatering.

b. Inlet - Incoming water shall be dispersed across the full width of the line of travel as quickly as possible; short-circuiting must be prevented.

c. Bypass - Provisions for bypassing presedimentation basins shall be included.

d. Detention time - Three hours detention is the minimum period recommended; greater detention may be required.

4.1.2 Rapid mix

Rapid mix shall mean the rapid dispersion of chemicals throughout the water to be treated, usually by violent agitation. The engineer shall submit the design basis for the velocity gradient (G value) selected, considering the chemicals to be added and water temperature, color and other related water quality parameters.

a. Equipment - Basins should be equipped with mechanical mixing devices.

b. Mixing - The detention period should be not more than thirty seconds.

c. Location - The rapid mix and flocculation basin shall be as close together as possible.

4.1.3 Flocculation

Flocculation shall mean the agitation of water at low velocities for long periods of time.

a. Basin Design.- Inlet and outlet design shall prevent short-circuiting and destruction of floc. A drain and/or pumps shall be provided to handle dewatering and sludge removal.

b. Detention - The flow-through velocity shall be not less than 0.5 nor greater than 1.5 feet per minute with a detention time for floc formation of at least 30 minutes.

c. Equipment - Agitators shall be driven by variable speed drives with the peripheral speed of paddles ranging from 0.5 to 3.0 feet per second.

d. Piping - Flocculation and sedimentation basins shall be as close together as possible. The velocity of flocculated water through pipes or conduits to settling basins shall be not less than 0.5 nor greater than 1.5 feet per second. Allowances must be made to minimize turbulence at bends and changes in direction.

e. Other designs - Baffling may be used to provide for flocculation in small plants only after consultation with the reviewing authority. The design should be such that the velocities and flows noted above will be maintained.

f. Superstructure - A superstructure over the flocculation basins may be required.

4.1.4 Sedimentation

Sedimentation shall follow flocculation. The detention time for effective clarification is dependent upon a number of factors related to basin design and the nature of the raw water. The following criteria apply to conventional sedimentation units:

a. Detention time - Shall provide a minimum of four hours of settling time. This may be reduced to two hours for lime-soda softening facilities treating only groundwater. Reduced sedimentation time may also be approved when equivalent effective settling is demonstrated or when overflow rate is not more than 0.5 gpm per square foot (1.2 m/hr).

b. Inlet devices - Inlets shall be designed to distribute the water equally and at uniform velocities. Open ports, submerged ports, and similar entrance arrangements are required. A baffle should be constructed across the basin close to the inlet end and should project several feet below the water surface to dissipate inlet velocities and provide uniform flows across the basin.

c. Outlet devices - Outlet devices shall be designed to maintain velocities suitable for settling in the basin and to minimize short-circuiting. The use of submerged orifices is recommended in order to provide a volume above the orifices for storage when there are fluctuations in flow. Outlet weirs and submerged orifices shall be desined as follows:

1. The rate of flow over the outlet weirs or through the submerged orifices shall not exceed 20,000 gallons per day per foot (250 m3/day/m) of the outlet launder.

2. Submerged orifices should not be located lower than three (3) feet below the flow line.

3. The entrance velocity through the submerged orifices shall not exceed 0.5 feet per second.

d. Velocity - The velocity through settling basins shall not exceed 0.5 feet per minute. The basins must be designed to minimize short-circuiting. Fixed or adjustable baffles must be provided as necessary to achieve the maximum potential for clarification.

e. Overflow - An overflow weir (or pipe) should be installed which will establish the maximum water level desired on top of the filters. It shall discharge by gravity with a free fall at a location where the discharge will be noted.

f. Superstructure - A superstructure over the sedimentation basins may be required. If there is no mechanical equipment in the basins and if provisions are included for adequate monitoring under all expected weather conditions, a cover may be provided in lieu of a superstructure.

g. Sludge collection - Mechanical sludge collection equipment should be provided.

h. Drainage - Basins must be provided with a means for dewatering. Basin bottoms should slope toward the drain not less than one foot in twelve feet where mechanical sludge collection equipment is not required.

i. Flushing lines - Flushing lines or hydrants shall be provided and must be equipped with backflow prevention devices acceptable to the reviewing authority.

j. Safety - Permanent ladders or handholds should be provided on the inside walls of basins above the water level. Guard rails should be included. Compliance with other applicable safety requirements, such as OSHA, shall be required.

k. Sludge removal - Sludge removal design shall provide that

1. sludge pipes shall be not less than three inches in diameter and so arranged as to facilitate cleaning,

2. entrance to sludge withdrawal piping shall prevent clogging,

3. valves shall be located outside the tank for accessibility,

4. the operator may observe and sample sludge being withdrawn from the unit.

l. Sludge disposal - Facilities are required by the reviewing authority for disposal of sludge. (see Section 4.11).

4.1.5 Solids contact unit

Units are generally acceptable for combined softening and clarification where water characteristics, especially temperature, do not fluctuate rapidly, flow rates are uniform and operation is continuous. Before such units are considered as clarifiers without softening, specific approval of the reviewing authority shall be obtained. Clarifiers should be designed for the maximum uniform rate and should be adjustable to changes in flow which are less than the design rate and for changes in water characteristics. A minimum of two units are required for surface water treatment. Installation of equipment

Supervision by a representative of the manufacturer shall be provided with regard to all mechanical equipment at the time of

a. installation, and

b. initial operation. Operating equipment

The following shall be provided for plant operation:

a. a complete outfit of tools and accessories,

b. necessary laboratory equipment,

c. adequate piping with suitable sampling taps so located as to permit the collection of samples of water from critical portions of the units. Chemical feed

Chemicals shall be applied at such points and by such means as to insure satisfactory mixing of the chemicals with the water. Mixing

A rapid mix device or chamber ahead of solids contact units may be required by the reviewing authority to assure proper mixing of the chemicals applied. Mixing devices employed shall be so constructed as to

a. provide good mixing of the raw water with previously formed sludge particles, and

b. prevent deposition of solids in the mixing zone. Flocculation

Flocculation equipment

a. shall be adjustable (speed and/or pitch),

b. must provide for coagulation in a separate chamber or baffled zone within the unit,

c. should provide the flocculation and mixing period to be not less than 30 minutes. Sludge concentrators

a. The equipment should provide either internal or external concentrators in order to obtain a concentrated sludge with a minimum of waste water.

b. Large basins should have at least two sumps for collecting sludge with one sump located in the central flocculation zone. Sludge removal

Sludge removal design shall provide that

a. sludge pipes shall be not less than three inches in diameter and so arranged as to facilitate cleaning,

b. entrance to sludge withdrawal piping shall prevent clogging,

c. valves shall be located outside the tank for accessibility, and

d. the operator may observe and sample sludge being withdrawn from the unit. Cross-connections

a. Blow-off outlets and drains must terminate and discharge at places satisfactory to the reviewing authority.

b. Cross-connection control must be included for the potable water lines used to backflush sludge lines. Detention period

The detention time shall be established on the basis of the raw water characteristics and other local conditions that affect the operation of the unit. Based on design flow rates, the detention time should be

a. two to four hours for suspended solids contact clarifiers and softeners treating surface water, and

b. one to two hours for the suspended solids contact softeners treating only groundwater.

The reviewing authority may alter detention time requirements. Suspended slurry concentrate

Softening units should be designed so that continuous slurry concentrates of one per cent or more, by weight, can be satisfactorily maintained. Water losses

a. Units shall be provided with suitable controls for sludge withdrawal.

b. Total water losses should not exceed

1. five per cent for clarifiers,

2. three per cent for softening units.

c. Solids concentration of sludge bled to waste should be

1. three per cent by weight for clarifiers,

2. five per cent by weight for softeners. Weirs or orifices

The units should be equipped with either overflow weirs or orifices constructed so that water at the surface of the unit does not travel over 10 feet horizontally to the collection trough.

a. Weirs shall be adjustable, and at least equivalent in length to the perimeter of the tank.

b. Weir loading shall not exceed

1. 10 gallons per minute per foot of weir length for units used for clarifiers,

2. 20 gallons per minute per foot of weir length for units used for softeners.

c. Where orifices are used the loading rates per foot of launder rates should be equivalent to weir loadings. Either shall produce uniform rising rates over the entire area of the tank. Upflow rates

Unless supporting data is submitted to the reviewing authority to justify rates exceeding the following, rates shall not exceed

a. 1.0 gallon per minute per square foot of area at the sludge separation line for units used for clarifiers,

b. 1.75 gallons per minute per square foot of area at the slurry separation line, for units used for softeners.

4.1.6 Tube or plate settlers

Proposals for settler unit clarification must include pilot plant and/or full scale demonstration satisfactory to the reviewing authority prior to the preparation of final plans and specifications for approval. Settler units consisting of variously shaped tubes or plates which are installed in multiple layers and at an angle to the flow may be used for sedimentation, following flocculation. General Criteria

a. Inlet and outlet considerations -- Design to maintain velocities suitable for settling in the basin and to minimize short-circuiting.

b. Drainage -- Drain piping from the settler units must be sized to facilitate a quick flush of the settler units and to prevent flooding other portions of the plant.

c. Protection from freezing -- Although most units will be located within a plant, outdoor installations must provide sufficient freeboard above the top of settlers to prevent freezing in the units. A cover or enclosure is strongly recommended.

d. Application rate for tubes-- A maximum rate of 2 gpm per square foot of cross-sectional area (4.8 m/hr) for tube settlers, unless higher rates are successfully shown through pilot plant or in-plant demonstration studies.

e. Application rate for plates -- A maximum plate loading rate of 0.5 gpm per square foot (1.2 m/hr) based on 80 percent of the projected horizontal area.

f. Flushing lines -- Flushing lines shall be provided to facilitate maintenance and must be properly protected against backflow or back siphonage.


Acceptable filters shall include, upon the discretion of the reviewing authority, the following types:

a. rapid rate gravity filters (4.2.1),

b. rapid rate pressure filters (4.2.2),

c. diatomaceous earth filtration (4.2.3),

d. slow sand filtration (4.2.4),

e. direct filtration (4.2.5),

f. deep bed rapid rate gravity filters (4.2.6),

g. biologically active filters (4.2.7),

h. membrane filtation (see policy statements on Reverse Osmosis, and Membrane Filtration for Treating Surface Sources), and

i. bag and cartridge filters (see policy statement on Bag and Cartridge Filters for Public Water Systems).

The application of any one type must be supported by water quality data representing a reasonable period of time to characterize the variations in water quality. Experimental treatment studies may be required to demonstrate the applicability of the method of filtration proposed.

4.2.1 Rapid rate gravity filters Pretreatment

The use of rapid rate gravity filters shall require pretreatment. Rate of filtration

The rate of filtration shall be determined through consideration of such factors as raw water quality, degree of pretreatment provided, filter media, water quality control parameters, competency of operating personnel, and other factors as required by the reviewing authority. In any case, the filter rate must be proposed and justified by the designing engineer to the satisfaction of the reviewing authority prior to the preparation of final plans and specifications. Number

At least two units shall be provided. Where only two units are provided, each shall be capable of meeting the plant design capacity (normally the projected maximum daily demand) at the approved filtration rate. Where more than two filter units are provided, the filters shall be capable of meeting the plant design capacity at the approved filtration rate with one filter removed from service. Where declining rate filtration is provided, the variable aspect of filtration rates, and the number of filters must be considered when determining the design capacity for the filters. Structural details and hydraulics

The filter structure shall be designed to provide for

a. vertical walls within the filter,

b. no protrusion of the filter walls into the filter media,

c. cover by superstructure,

d. head room to permit normal inspection and operation,

e. minimum depth of filter box of 8 1/2 feet,

f. minimum water depth over the surface of the filter media of three feet,

g. trapped effluent to prevent backflow of air to the bottom of the filters,

h. prevention of floor drainage to the filter with a minimum 4-inch curb around the filters,

i. prevention of flooding by providing overflow,

j. maximum velocity of treated water in pipe and conduits to filters of two feet per second,

k. cleanouts and straight alignment for influent pipes or conduits where solids loading is heavy, or following lime-soda softening,

l. washwater drain capacity to carry maximum flow,

m. walkways around filters, to be not less than 24 inches wide,

n. safety handrails or walls around filter walkways,

o. construction to prevent cross connections and common walls between potable and non-potable water. Washwater troughs

Washwater troughs should be constructed to have

a. the bottom elevation above the maximum level of expanded media during washing,

b. a two-inch freeboard at the maximum rate of wash,

c. the top edge level and all at the same elevation,

d. spacing so that each trough serves the same number of square feet of filter area,

e. maximum horizontal travel of suspended particles to reach the trough not to exceed three feet. Filter material

The media shall be clean silica sand or other natural or synthetic media approved by the reviewing authority, having the following characteristics:

a. a total depth of not less than 24 inches and generally not more than 30 inches,

b. an effective size range of the smallest material no greater than 0.45 mm to 0.55 mm,

c. a uniformity coefficient of the smallest material not greater than 1.65,

d. a minimum of 12 inches of media with an effective size range no greater than 0.45 mm to 0.55 mm, and a specific gravity greater than other filtering materials within the filter.

e. Types of filter media:

1. Anthracite - Clean crushed anthracite, or a combination of anthracite and other media may be considered on the basis of experimental data specific to the project, and shall have

a. effective size of 0.45 mm - 0.55 mm with uniformity coefficient not greater than 1.65 when used alone,

b. effective size of 0.8 mm - 1.2 mm with a uniformity coefficient not greater than 1.85 when used as a cap,

c. effective size for anthracite used as a single media on potable groundwater for iron and manganese removal only shall be a maximum of 0.8 mm (effective sizes greater than 0.8 mm may be approved based upon onsite pilot plant studies).

2. Sand - sand shall have

a. effective size of 0.45 mm to 0.55 mm,

b. uniformity coefficient of not greater than 1.65.

3. Granular activated carbon (GAC) - Granular activated carbon media may be considered only after pilot or full scale testing and with prior approval of the reviewing authority. The design shall include the following:

a. The media must meet the basic specifications for filter media as given in Section through d except that larger size media may be allowed by the reviewing authority where full scale tests have demonstrated that treatment goals can be met under all conditions.

b. There must be provisions for a free chlorine residual and adequate contact time in the water following the filters and prior to distribution (See 4.3.2.d and 4.3.3).

c. There must be means for periodic treatment of filter material for control of bacterial and other growth.

d. Provisions must be made for frequent replacement or regeneration.

4. Other media - Other media will be considered based on experimental data and operating experience.

5. Torpedo sand - A three-inch layer of torpedo sand should be used as a supporting media for filter sand, and should have

a. effective size of 0.8 mm to 2.0 mm, and

b. uniformity coefficient not greater than 1.7.

6. Gravel - Gravel, when used as the supporting media shall consist of cleaned and washed, hard, durable, rounded silica particles and shall not include flat or elongated particles. The coarsest gravel shall be 2 1/2 inches in size when the gravel rests directly on the strainer system, and must extend above the top of the perforated laterals. Not less than four layers of gravel shall be provided in accordance with the following size and depth distribution:
2 1/2 to 1 1/2 inches 5 to 8 inches
1 1/2 to 3/4 inches 3 to 5 inches
3/4 to 1/2 inches 3 to 5 inches
1/2 to 3/16 inches 2 to 3 inches
3/16 to 3/32 inches 2 to 3 inches

Reduction of gravel depths and other size gradations may be considered upon justification to the reviewing authority when slow sand filtration or when proprietary filter bottoms are specified. Filter bottoms and strainer systems

Departures from these standards may be acceptable for high rate filters and for proprietary bottoms. Porous plate bottoms shall not be used where iron or manganese may clog them or with waters softened by lime. The design of manifold-type collection systems shall:

a. minimize loss of head in the manifold and laterals,

b. assure even distribution of washwater and even rate of filtration over the entire area of the filter,

c. provide the ratio of the area of the final openings of the strainer systems to the area of the filter at about 0.003,

d. provide the total cross-sectional area of the laterals at about twice the total area of the final openings,

e. provide the cross-sectional area-of the manifold at 1 1/2 to 2 times the total area of the laterals.

f. lateral perforations without strainers shall be directed downward. Surface wash or subsurface wash

Surface or subsurface wash facilities are required except for filters used exclusively for iron or manganese removal, and may be accomplished by a system of fixed nozzles or a revolving-type apparatus. All devices shall be designed with

a. provision for water pressures of at least 45 psi (310 kPa),

b. a properly installed vacuum breaker or other approved device to prevent back siphonage if connected to the treated water system,

c. rate of flow of 2.0 gallons per minute per square foot of filter area (4.9 m/hr) with fixed nozzles or 0.5 gallons per minute per square foot (1.2 m/hr) with revolving arms,

d. air wash can be considered based on experimental data and operating experiences. Air scouring

Air scouring can be considered in place of surface wash

a. air flow for air scouring the filter must be 3-5 standard cubic feet per minute square foot of filter area (0.9 - 1.5 m3/min/m2) [m/min] when the air is introduced in the underdrain; a lower air rate must be used when the air scour distribution system is placed above the underdrains,

b. a method for avoiding excessive loss of the filter media during backwashing must be provided,

c. air scouring must be followed by a fluidization wash sufficient to restratify the media,

d. air must be free from contamination,

e. air scour distribution systems should be placed below the media and supporting bed interface; if placed at the interface the air scour nozzles shall be desiged to prevent media from clogging the nozzles or entering the air distributin system.

f. piping for the air distrubtion system shall not be flexible hose which will collapse when not under air pressure and shall not be a relatively soft material which may erode at the orifice opening with the passage of air at high velocity.

g. air delivery piping shall not pass down through the filter media nor shall there be any arrangement in the filter design which would allow short circuiting between the applied unfiltered water and the filtered water,

h. consideration should be given to maintenance and replacement of air deliver piping,

i. the backwash delivery system must be capable of 15 gallons per minute per square foot of filter surface area (37 m/hr); however, when air scour is provided the backwash rate must be variable and should not exceed 8 gallons per minute per square foot (20 m/hr) unless operating experience shows that a higher rate is necessary to remove scoured particles from filter surfaces.

j. the filter underdrains shall be designed to accommodate air @scour piping when the piping is installed in the underdrain, and

k. the provisions of Sectin shall be followed. Appurtenances

a. The following shall be provided for every filter:

1. influent and effluent sampling taps,

2. an indicating loss of head gauge,

3. an indicating rate-of flow meter. A modified rate controller which limits the rate of filtration to a maximum rate may be used. However, equipment that simply maintains a constant water level on the filters is not acceptable, unless the rate of flow onto the filter is properly controlled. A pump or a flow meter in each filter effluent line may be used as the limiting device for the rate of filtration only after consultation with the reviewing authority.

4. where used for surface water, provisions for filtering to waste with appropriate measures for backflow prevention.

b. It is recommended the following be provided for every filter:

1. a continuous or rotating cycle turbidity recording device for surface water treatment plants,

2. wall sleeves providing access to the filter interior at several locations for sampling or pressure sensing,

3. a 1 to 1 1/2 inch pressure hose and storage rack at the operating floor for washing filter walls,

4. particle monitoring equipment as a measns to enhance overall treatment operations where used for surface water. Backwash

Provisions shall be made for washing filters as follows:

a. a minimum rate of 15 gallons per minute per square foot (37 m/hr), consistent with water temperatures and specific gravity of the filter media. A rate of 20 gallons per minute per square foot (50 m/hr) or a rate necessary to provide for a 50 percent expansion of the filter bed is recommended. A reduced rate of 10 gallons per minute per square foot (24 m/hr) may be acceptable for full depth anthracite or granular activated carbon filters,

b. filtered water provided at the required rate by washwater tanks, a washwater pump, from the high service main, or a combination of these,

c. washwater pumps in duplicate unless an alternate means of obtaining washwater is available,

d. not less than 15 minutes wash of one filter at the design rate of wash,

e. a washwater regulator or valve on the main washwater line to obtain the desired rate of filter wash with the washwater valves on the individual filters open wide,

f. a rate-of-flow indicator, preferably with a totalizer, on the main washwater line, located so that it can be easily read by the operator during the washing process,

g. design to prevent rapid changes in backwash water flow.

h. backwash shall be operator initiated. Automated systems shall be operator adjustable. Miscellaneous

Roof drains shall not discharge into the filters or basins and conduits preceding the filters.

4.2.2 Rapid rate pressure filters

The normal use of these filters is for iron and manganese removal. Pressure filters shall not be used in the filtration of surface or other polluted waters or following lime-soda softening. General

Minimum criteria relative to rate of filtration, structural details and hydraulics, filter media, etc., provided for rapid rate gravity filters also apply to pressure filters where appropriate. Rate of filtration

The rate shall not exceed three gallons per minute per square foot (7.2 m/hr) of filter area except where in plant testing as approved by the reviewing authority has demonstrated satisfactory results at higher rates. Details of design

The filters shall be designed to provide for

a. loss of head gauges on the inlet and outlet pipes of each filter,

b. an easily readable meter or flow indicator on each battery of filters. A flow indicator is recommended for each filtering unit,

c. filtration and backwashing of each filter individually with an arrangement of piping as simple as possible to accomplish these purposes,

d. minimum side wall shell height of five feet. A corresponding reduction in side wall height is acceptable where proprietary bottoms permit reduction of the gravel depth,

e. the top of the washwater collectors to be at least 18 inches above the surface of the media,

f. the underdrain system to efficiently collect the filtered water and to uniformly distribute the backwash water at a rate not less than 15 gallons per minute per square foot of filter area (37 m/hr),

g. backwash flow indicators and controls that are easily readable while operating the control valves,

h. an air release valve on the highest point of each filter,

i. an accessible manhole to facilitate inspection and repairs for filters 36 inches or more in diameter. Sufficient handholes shall be provided for filters less than 36 inches in diameter. Manholes should be at least 24 inches in diameter where feasible,

j. means to observe the wastewater during backwashing,

k. construction to prevent cross-connection.

4.2.3 Diatomaceous earth filtration

The use of these filters may be considered for application to surface waters with low turbidity and low bacterial contamination, and may be used for iron removal for groundwaters providing the removal is effective and the water is of satisfactory sanitary quality before treatment. Conditions of use

Diatomaceous earth filters are expressly excluded from consideration for the following conditions:

a. bacteria removal,

b. color removal,

c. turbidity removal where either the gross quantity of turbidity is high or the turbidity exhibits poor filterability characteristics,

d. filtration of waters with high algae counts. Pilot plant study

Installation of a diatomaceous earth filtration system shall be preceded by a pilot plant study on the water to be treated.

a. Conditions of the study such as duration, filter rates, head loss accumulation, slurry feed rates, turbidity removal, bacteria removal, etc., must be approved by the reviewing authority prior to the study.

b. Satisfactory pilot plant results must be obtained prior to preparation of final construction plans and specifications.

c. The pilot plant study must demonstrate the ability of the system to meet applicable drinking water standards at all times. Types of filters

Pressure or vacuum diatomaceous earth filtration units will be considered for approval. However, the vacuum type is preferred for its ability to accommodate a design which permits observation of the filter surfaces to determine proper cleaning, damage to a filter element, and adequate coating over the entire filter area. Treated water storage

Treated water storage capacity in excess of normal requirements shall be provided to:

a. allow operation of the filters at a uniform rate during all conditions of system demand at or below the approved filtration rate, and,

b. guarantee continuity of service during adverse raw water conditions without by-passing the system. Number of units.

See Section Precoat

a. Application - A uniform precoat shall be applied hydraulically to each septum by introducing a slurry to the tank influent line and employing a filter-to-waste or recirculation system.

b. Quantity - Diatomaceous earth in the amount of 0.1 pounds per square foot of filter area (0.49 - 0.98 kg/m2) or an amount sufficient to apply a 1/16 inch coating should be used with recirculation. When precoating is accomplished with a filter-to-waste system, 0.15 - 0.2 pounds per square foot of filter area is recommended. Body feed

A body feed system to apply additional amounts of diatomaceous earth slurry during the filter run is required to avoid short filter runs or excessive head losses.

a. Quantity - Rate of body feed is dependent on raw water quality and characteristics and must be determined in the pilot plant study.

b. Operation and maintenance can be simplified by providing accessibility to the feed system and slurry lines.

c. Continuous mixing of the body feed slurry is required. Filtration

a. Rate of filtration - The recommended nominal rate is 1.0 gallon per minute per square foot of filter area (2.4 m/hr) with a recommended maximum of 1.5 gallons per minute per square foot (3.7 m/hr). The filtration rate shall be controlled by a positive means.

b. Head loss - The head loss shall not exceed 30 psi (210 kPa) for pressure diatomaceous earth filters, or a vacuum of 15 inches of mercury (-51 kPa) for a vacuum system.

c. Recirculation - A recirculation or holding pump shall be employed to maintain differential pressure across the filter when the unit is not in operation in order to prevent the filter cake from dropping off the filter elements. A minimum recirculation rate of 0.1 gallon per minute per square foot of filter area (0.24 m/hr) shall be provided.

d. Septum or filter element - The filter elements shall be structurally capable of withstanding maximum pressure and velocity variations during filtration and backwash cycles, and shall be spaced such that no less than one inch is provided between elements or between any element and a wall.

e. Inlet design - The filter influent shall be designed to prevent scour of the diatomaceous earth from the filter element. Backwash

A satisfactory method to thoroughly remove and dispose of spent filter cake shall be provided. Appurtenances

The following shall be provided for every filter:

a. sampling taps for raw and filtered water,

b. loss of head or differential pressure gauge,

c. rate-of-flow indicator, preferably with totalizer,

d. a throttling valve used to reduce rates below normal during adverse raw water conditions,

e. evaluation of the need for body feed, recirculation, and any other pumps, in accordance with Section 6.3.

f. provisions for filtering to waste with appropriate measures for backflow prevention (see Section 4.11). Monitoring

a. A continuous monitoring turbidimeter with recorder is required on the filter effluent for plants treating surface water.

b. Particle monitoring equipment should be provided as a means to enhance overall treatment operations for plants treating surface water.

4.2.4 Slow sand filters

The use of these filters shall require prior engineering studies to demonstrate the adequacy and suitability of this method of filtration for the specific raw water supply, Quality of raw water

Slow rate gravity filtration shall be limited to waters having maximum turbidities of 10 units and maximum color of 15 units; such turbidity must not be attributable to colloidal clay. Raw water quality data must include examinations for algae. Number

At least two units shall be provided. Where only two units are provided, each shall be capable of meeting the plant design capacity (normally the projected maximum daily demand) at the approved filtration rate. Where more than two filter units are provided, the filters shall be capable of meeting the plant design capacity at the approved filtration rate with one filter removed from service. Structural details and hydraulics

Slow rate gravity filters shall be so designed as to provide:

a. a cover,

b. headroom to permit normal movement by operating personnel for scraping and sand removal operations,

c. adequate access hatches and access ports for handling of sand and for ventilation,

d. filtration to waste,

e. an overflow at the maximum filter water level, and

f. protection from freezing. Rates of filtration

The permissible rates of filtration shall be determined by the quality of the raw water and shall be on the basis of experimental data derived from the water to be treated. The nominal rate may be 45 to 150 gallons per day per square foot of sand area (100 to 360 m/day), with somewhat higher rates acceptable when demonstrated to the satisfaction of the approving authority. Underdrains

Each filter unit shall be equipped with a main drain and an adequate number of lateral underdrains to collect the filtered water. The underdrains shall be so spaced that the maximum velocity of the water flow in the underdrain will not exceed 0.75 feet per second. The maximum spacing of laterals shall not exceed 3 feet if pipe laterals are used. Filtering material

a. Filter sand shall be placed on graded gravel layers for a minimum depth of 30 inches.

b. The effective size shall be between 0.15 mm and 0.30 mm. Larger sizes may be considered by the reviewing authority; a pilot study may be required.

c. The uniformity coefficient shall not exceed 2.5.

d. The sand shall be cleaned and washed free from foreign matter.

e. The sand shall be rebedded when scraping has reduced the bed depth to no less than 19 inches. Where sand is to be reused in order to provide biological seeding and shortening of the ripening process, rebedding shall utilize a "throw over" technique whereby new sand is placed on the support gravel and existing sand is replaced on top of the new sand. Filter gravel

The supporting gravel shall conform to the size and depth distribution provided for rapid rate gravity filters. See,6. Depth of water on filter beds

Design shall provide a depth of at least three to six feet of water over the sand. Influent water shall not scour the sand surface. Control appurtenances

Each filter shall be equipped with:

a. loss of head gauge,

b. an orifice, Venturi meter, or other suitable metering device installed on each filter to control the rate of filtration,

c. an effluent pipe designed to maintain the water level above the top of the filter sand. Ripening

Slow sand filters shall be operated to waste after scraping or rebedding during a ripening period until the filter effluent turbidity falls to consistently below 1 NTU.

4.2.5 Direct filtration

Direct filtration, as used herein, refers to the filtration of a surface water without prior settling. The nature of the treatment process will depend upon the raw water quality. A full scale direct filtration plant shall not be constructed without prior pilot studies which are acceptable to the reviewing authority. In-plant demonstration studies may be appropriate where conventional treatment plants are converted to direct filtration. Where direct filtration is proposed, an engineering report shall be submitted prior to conducting pilot plant or in-plant demonstration studies. Engineering report

In addition to the items considered in Section 1.1, "Engineering Report", the report should include a historical summary of meteorological conditions and of raw water quality with special reference to fluctuations in quality, and possible sources of contamination. The following raw water parameters should be evaluated in the report:

a. color,

b. turbidity,

c. bacterial concentration,

d. microscopic biological organisms,

e. temperature,

f. total solids,

g. general inorganic chemical characteristics,

h. additional parameters as required by the reviewing authority.

The report should also include a description of methods and work to be done during a pilot plant study or, where appropriate, an in-plant demonstration study. Pilot plant studies

After approval of the engineering report, a pilot study or in-plant demonstration study shall be conducted. The study must be conducted over a sufficient time to treat all expected raw water conditions throughout the year. The study shall emphasize but not be limited to, the following items:

a. chemical mixing conditions including shear gradients and detention periods,

b. chemical feed rates,

c. use of various coagulants and coagulant aids,

e. flocculation conditions,

f. filtration rates,

g. filter gradation, types of media and depth of media,

h. filter breakthrough conditions, and

i. adverse impact of recycling backwash water due to solids, algae, trihalomethane formation and similar problems.

Prior to the initiation of design plans and specifications, a final report including the engineer's design recommendations shall be submitted to the reviewing authority.

The pilot plant filter must be of a similar type and operated in the same manner as proposed for full scale operation.

The pilot study must demonstrate the minimum contact time necessary for optimum filtration for each coagulant proposed. Pretreatment - Rapid mix and flocculation

The final rapid mix and flocculation basin design should be based on the pilot plant or in-plant demonstration studies augmented with applicable portions of Section 4.1.2, "Rapid Mix" and Section 4.1.3, "Flocculation." Filtration

a. Filters should be rapid rate gravity filters with dual or mixed media. The final filter design should be based on the pilot plant or in-plant demonstration studies augmented by applicable portions of Section 4.2.1, "Rapid Rate Gravity Filters." Pressure filters or single media sand filters shall not be used.

[b. Surface wash, subsurface wash or air scour shall be provided for the filters in accordance with and

c. Provisions for filtration to waste with appropriate measures for backflow prevention may be required by the reviewing authority. Control and operationnote 1

a. A continuous recording turbidimeter should be installed on each filter effluent line and on the composite filter effluent line.

b. Additional continuous monitoring equipment to assist in control of coagulant dose may be required by the reviewing authority. Siting requirements

The plant design and land ownership surrounding the plant shall allow for the installation of conventional sedimentation basins should it be found that such are necessary.

4.2.6 Deep bed rapid rate gravity filters

Deep bed rapid rate gravity filters, as used herein, generally refer to rapid rate gravity filters with filter material depths greater than 48 inches. Filter media sizes are typically larger than those listed in

Deep bed rapid rate gravity filters may be considered based on pilot studies pre approved by the reviewing authority.

The final filter design shall be based on the pilot plant studies and shall comply with all applicalble portions of Section 4.2.1. Careful attention shall be paid to the design of the backwash system which usually includes simultaneous air scour and water backwash at subfluidization velocities.

4.2.7 Biologically active filters

Biologically active filtration, as used herein, refers to the filtration of a surface water (or groundwater with iron, manganese or significant natural organic material) which includes the establishment and maintenance of biological activity within the filtration media.

Objectives of biologically active filtration may include control of disinfection byproducts, increased disinfectant stability, reduction of substrates for microbial regrowth, breakdown of small quantities of synthetic organic chemicals, reduction of ammonia-nitrogen, and oxidation of iron and manganese. Biological activity can have an adverse impact on turbidity, particle and microbial pathogen removal, disinfection practices; head loss development; filter run times and distribution system corrosion. Design and operation should ensure that aerobic conditions are maintained at all times. Biologically active filtration often includes the use of ozone as a pre-oxidant/disinfectant which breaks down natural organic materials into biodegradeable organic matter and granular activated carbon filter media which may promote denser biofilms.

Biologically active filters may be considered based on pilot studies pre approved by the reviewing authority. The study objective must be clearly defined and must ensure the microbial quality of the filtered water under all anticipated conditions of operation. The pilot study shall be of sufficient duration to ensure establishment of full biological activity; ofthen greater than three months is required.

The final design shall be based on the pilot plant studies and shall comply with all applicable portions of Section 4.2.1.


Chlorine is historically the preferred disinfecting agent. Disinfection may be accomplished with gas and liquid chlorine, calcium or sodium hypochlorites, chlorine dioxide, or ozonation. Ozonation is fast becoming a reliable means of primary disinfection for a surface water treatment plant to meet the requirements of the Federal Safe Drinking Water Act, Surface Water Treatment Rule (SWTR). See Interim Standard for Ozone for more information on the design and operation. Other disinfecting agents will be considered, providing reliable application equipment is available and testing procedures for a residual are recognized in "Standard Methods for the Examination of Water and Wastewater," latest edition. Disinfection is required at all surface water supplies and at any groundwater supply of questionable sanitary quality or where other treatment is provided. Disinfection with chloramines is not recommended for primary disinfection to meet the CT requirements in a surface water treatment plant or a plant treating groundwater under the influence of a surface water. "CT" is the product of disinfectant residual and disinfectant contact time. The required amount of CT needed is contained in the EPA Guidance Manual to the SWTR. Continuous disinfection is recommended for all water supplies.

4.3.1 Chlorination equipment Type

Solution-feed gas chlorinators or hypochlorite feeders of the positive displacement type must be provided. (see Part 5). Capacity

The chlorinator capacity shall be such that a free chlorine residual of at least 2 milligrams per liter can be maintained in the water after contact time of at least 30 minutes when maximum flow rate coincides with anticipated maximum chlorine demand. The equipment shall be of such design that it will operate accurately over the desired feeding range. Standby equipment

Where chlorination is required for protection of the supply, standby equipment of sufficient capacity shall be available to replace the largest unit. Spare parts shall be made available to replace parts subject to wear and breakage. If there is a large difference in feed rates between routine and emergency dosages, a gas metering tube should be provided for each dose range to ensure accurate control of the chlorine feed. Automatic switch-over

Automatic switch-over of chlorine cylinders should be provided, where necessary, to assure continuous disinfection. Automatic proportioning

Automatic proportioning chlorinators will be required where the rate of flow or chlorine demand is not reasonably constant. Eductor

Each eductor must be selected for the point of application with particular attention given to the quantity of chlorine to be added, the maximum injector waterflow, the total discharge back pressure, the injector operating pressure, and the size of the chlorine solution line. Gauges for measuring water pressure and vacuum at the inlet and outlet of each eductor should be provided. Injector/diffuser

The chlorine solution injector/diffuser must be compatible with the point of application to provide a rapid and thorough mix with all the water being treated. The center of a pipeline is the preferred application point.

4.3.2 Contact time and point of application

a. Due consideration shall be given to the contact time of the chlorine in water with relation to pH, ammonia, taste-producing substances, temperature, bacterial quality, trihalomethane formation potential and other pertinent factors. The disinfectant should be applied at a point which will provide adequate contact time. All basins used for disinfection must be designed to minimize short circuiting. Additional baffling can be added to new or existing basisns to minimize short circuiting and increase contact time.

b. At plants treating surface water, provisions should be made for applying chlorine to the raw water, settled water, filtered water, and water entering the distribution system.

c. As a minimum, at plants treating groundwater, provisions should be made for applying the disinfectant to the detention basin inlet and water entering the distribution system.

d. The contact time provided will depend on the type of disinfectant used along with the parameters mentioned in 4.3.2.a. As a minimum, the system must be designed to meet the CT standards set by the reviewing authority in accordance with the SWTR. If primary disinfection is accomplished using ozone, chlorine dioxide, or some other chemical that does not provide a residual disinfectant, then chlorine must be added to provide a residual disinfectant as mentioned in 4.3.3.

4.3.3 Residual chlorine

a. Minimum free chlorine residual at distant points in a water distribution system should be 0.2 to 0.5 milligrams per liter. Minimum combined chlorine residuals, if appropriate, should be 1.0 to 2.0 milligrams per liter at distant points in the distribution system.

b. Higher residuals may be required depending on pH, temperature and other characteristics of the water.

4.3.4 Testing equipment

a. Chlorine residual test equipment recognized in the latest edition of Standard Methods for the Examination of Water and Wastewater shall be provided and should be capable of measuring residuals to the nearest 0.1 milligrams per liter. It is recommended that larger systems, as a minimum, use the DPD method that utilizes the digital readout with a self contained light source.

b. Automatic chlorine residual recorders should be provided where the chlorine demand varies appreciably over a short period of time.

c. All treatment plants having a capacity of 0.5 million gallons per day or greater should be equipped with recording chlorine analyzers monitoring water entering the distribution system. (see Section 2.9).

d. All surface water plants that serve a population greater than 3300 must have equipment to measure chlorine residuals continuously entering the distribution system.

4.3.5 Chlorinator piping Cross-connection protection

The chlorinator water supply piping shall be designed to prevent contamination of the treated water supply by sources of questionable quality. At all facilities treating surface water, pre- and post-chlorination systems must be independent to prevent possible siphoning of partially treated water into the clear well. The water supply to each eductor shall have a separate shut-off valve. No master shut-off valve will be allowed. Pipe material

The pipes carrying elemental liquid or dry gaseous chlorine under pressure must be Schedule 80 seamless steel tubing or other materials recommended by the Chlorine Institute (never use PVC). Rubber, PVC, polyethylene, or other materials recommended by the Chlorine Institute must be used for chlorine solution piping and fittings. Nylon products are not acceptable for any part of the chlorine solution piping system.

4.3.6 Housing

Adequate housing must be provided for the chlorination equipment and for storing the chlorine. (see Part 5).

4.3.7 Other disinfecting agents

Although disinfecting agents other than chlorine are available, each has usually demonstrated shortcomings when applied to a public water supply. Proposals for use of disinfecting agents other than chlorine must be approved by the reviewing authority prior to preparation of final plans and specifications.


The softening process selected must be based upon the mineral qualities of the raw water and the desired finished water quality in conjunction with requirements for disposal of sludge or brine waste, cost of plant, cost of chemicals and plant location. Applicability of the process chosen shall be demonstrated.

4.4.1 Lime or lime-soda process

Design standards for rapid mix, flocculation and sedimentation are in Section 4.1. Additional consideration must be given to the following process elements. Hydraulics

When split treatment is used, the bypass line should be sized to carry total plant flow, and an accurate means of measuring and splitting the flow must be provided. Aeration

Determinations should be made for the carbon dioxide content of the raw water. When concentrations exceed 10 milligrams per liter, the economics of removal by aeration as opposed to removal with lime should be considered if it has been determined that dissolved oxygen in the finished water will not cause corrosion problems in the distribution system. (see Section 4.5). Chemical feed point

Lime and recycled sludge should be fed directly into the rapid mix basin. Rapid mix

Rapid mix basins must provide not more than 30 seconds detention time with adequate velocity gradients to keep the lime particles dispersed. Stabilization

Equipment for stabilization of water softened by the lime or lime-soda process is required. (see Section 4.8). Sludge collection

a. Mechanical sludge removal equipment shall be provided in the sedimentation basin.

b. Sludge recycling to the rapid mix should be provided. Sludge disposal

Provisions must be included for proper disposal of softening sludges. (see Section 4.11). Disinfection

The use of excess lime shall not be considered an acceptable substitute for disinfection. (see Section 4.3). Plant start-up

The plant processes must be manually started following shut-down.

4.4.2 Cation exchange process

Alternative methods of hardness reduction should be investigated when the sodium content and dissolved solids concentration is of concern. Pre-treatment requirements

Iron, manganese, or a combination of the two, should not exceed 0.3 milligrams per liter in the water as applied to the ion exchange resin. Pre-treatment is required when the content of iron, manganese, or a combination of the two, is one milligram per liter or more. (see Section 4.6). Waters having 5 units or more tubidity should not be applied directly to the cation exchange softener. Design

The units may be of pressure or gravity type, of either an upflow or downflow design. Automatic regeneration based on volume of water softened should be used unless manual regeneration is justified and is approved by the reviewing authority. A manual override shall be provided on all automatic controls. Exchange capacity

The design capacity for hardness removal should not exceed 20,000 grains per cubic foot (46 kg/m3) when resin is regenerated with 0.3 pounds (0.14 kg) of salt per kilogram of hardness removed. Depth of resin

The depth of the exchange resin should not be less than three feet. Flow rates

The rate of softening should not exceed seven gallons per minute per square foot of bed area (17 m/hr) and the backwash rate should be six to eight gallons per minute per square foot of bed area (14 - 20 m/hr). Rate-of-flow controllers or the equivalent must be installed for the above purposes. Freeboard

The freeboard will depend upon the specific gravity of the resin and the direction of water flow. Generally, the washwater collector should be 24 inches above the top of the resin on downflow units. Underdrains and supporting gravel

The bottoms, strainer systems and support for the exchange resin shall conform to criteria provided for rapid rate gravity filters. (see Sections and Brine distribution

Facilities should be included for even distribution of the brine over the entire surface of both upflow and downflow units. Cross-connection control

Backwash, rinse and air relief discharge pipes should be installed in such a manner as to prevent any possibility of back-siphonage. Bypass piping and equipment

A bypass must be provided around softening units to produce a blended water of desirable hardness. Totalizing meters must be installed on the bypass line and on each softener unit. The bypass line must have a shutoff valve and should have an automatic proportioning or regulating device. In some installations, it may be necessary to treat the bypassed water to obtain acceptable levels of iron and/or manganese in the finished water. Additional limitations

Silica gel resins should not be used for waters having a pH above 8.4 or containing less than six milligrams per liter silica and should not be used when iron is present. When the applied water contains a chlorine residual, the cation exchange resin shall be a type that is not damaged by residual chlorine. Phenolic resin should not be used. Sampling taps

Smooth-nose sampling taps must be provided for the collection of representative samples. The taps shall be located to provide for sampling of the softener influent, effluent and blended water. The sampling taps for the blended water shall be at least 20 feet downstream from the point of blending. Petcocks are not acceptable as sampling taps. Sampling taps should be provided on the brine tank discharge piping. Brine and salt storage tanks

a. Salt dissolving or brine tanks and wet salt storage tanks must be covered and must be corrosion-resistant.

b. The make-up water inlet must be protected from back-siphonage. Water for filling the tank should be distributed over the entire surface by pipes above the maximum brine level in the tank. The tanks should be provided with an automatic declining level control system on the make-up water line.

c. Wet salt storage basins must be equipped with manholes or hatchways for access and for direct dumping of salt from truck or railcar. Openings must be provided with raised curbs and watertight covers having overlapping edges similar to those required for finished water reservoirs.

d. Overflows, where provided, must be protected with corrosion resistant screens and must terminate with either a turned downed bend having a proper free fall discharge or a self-closing flap valve.

e. Two wet salt storage tanks or compartments designed to operate independently should be provided.

f. The salt shall be supported on graduated layers of gravel placed over a brine collection system.

g. Alternative designs which are conducive to frequent cleaning of the wet salt storage tank may be considered. Salt and brine storage capacity

Total salt storage should have sufficient capacity to store in excess of 1 1/2 carloads or truckloads of salt, and provide for at least 30 days of operation. Brine pump or eductor

An eductor may be used to transfer brine from the brine tank to the softeners. If a pump is used, a brine measuring tank or means of metering should be provided to obtain proper dilution. Stabilization

Refer to Section 4.8 Waste disposal

Suitable disposal must be provided for brine waste (See Section 4.11). Where the volume of spent brine must be reduced, consideration may be given to using a part of the spent brine for a subsequent regeneration. Construction materials

Pipes and contact materials must be resistant to the aggressiveness of salt. Plastic and red brass are acceptable piping materials. Steel and concrete must be coated with a non-leaching protective coating which is compatible with salt and brine. Housing

Bagged salt and dry bulk salt storage shall be enclosed and separated from other operating areas in order to prevent damage to equipment.

4.4.3 Water quality test equipment

Test equipment for alkalinity, toal hardness, carbon dioxide content, and pH should be provided to determine treatment effectiveness.


Aeration may be used to help remove offensive tastes and odors due to dissolved gases from decomposing organic matter, or to reduce or remove objectionable amounts of carbon dioxide, hydrogen sulfide, etc., and to introduce oxygen to assist in iron and/or manganese removal. The packed tower aeration process is an aeration process applicable to removal of volatile organic contaminants.

4.5.1 Natural draft aeration

Design shall provide

a. perforations in the distribution pan 3/16 to 1/2 inches in diameter, spaced 1 to 3 inches on centers to maintain a six inch water depth,

b. for distribution of water uniformly over the top tray,

c. discharge through a series of three or more trays with separation of trays not less than 12 inches,

d. loading at a rate of 1 to 5 gallons per minute for each square foot of total tray area (2.5 -12.5 m/hr),

e. trays with slotted, heavy wire (1/2 inch openings) mesh or perforated bottoms,

f. construction of durable material resistant to aggressiveness of the water and dissolved gases,

g. protection from loss of spray water by wind carriage by enclosure with louvers sloped to the inside at a angle of approximately 45 degrees,

h. protection from insects by 24-mesh screen.

4.5.2 Forced or induced draft aeration

Devices shall be designed to

a. include a blower with a weatherproof motor in a tight housing and screened enclosure,

b. insure adequate counter current of air through the enclosed aerator column,

c. exhaust air directly to the outside atmosphere,

d. include a down-turned and 24-mesh screened air outlet and inlet,

e. be such that air introduced in the column shall be as free from obnoxious fumes, dust, and dirt as possible,

f. be such that sections of the aerator can be easily reached or removed for maintenance of the interior or installed in a separate aerator room,

g. provide loading at a rate of 1 to 5 gallons per minute for each square foot of total tray area (2.5 - 12.5 m/hr),

h. insure that the water outlet is adequately sealed to prevent unwarranted loss of air,

i. discharge through a series of five or more trays with separation of trays not less than six inches,

j. provide distribution of water uniformly over the top tray,

k. be of durable material resistant to the aggressiveness of the water and dissolved gases.

4.5.3 Spray aeration

Design shall provide

a. a hydraulic head of between 5 - 25 feet,

b. nozzles, with the size, number, and spacing of the nozzles being dependent on the flowrate, space, and the amount of head available,

c. nozzle diameters in the range of 1 to 1.5 inches to minimize clogging,

d. an enclosed basin to contain the spray. Any openings for ventilation, etc. must be protected with a 24-mesh screen.

4.5.4 Pressure aeration

Pressure aeration may be used for oxidation purposes only if pilot plant study indicates the method is applicable; it is not acceptable for removal of dissolved gases. Filters following pressure aeration must have adequate exhaust devices for release of air. Pressure aeration devices shall be designed to

a. give thorough mixing of compressed air with water being treated,

b. provide screened and filtered air, free of obnoxious fumes, dust, dirt and other contaminants.

4.5.5 Packed tower aeration

Packed tower aeration (PTA) which is also known as air stripping involves passing water down through a column of packing material while pumping air counter-currently up through the packing. PTA is used for the removal of volatile organic chemicals, trihalomethanes, carbon dioxide, and radon. Generally, PTA is feasible for compounds with a Henry's Constant greater than 100 (expressed in atm mol/mol) at 12 °C) but not normally feasible for removing compounds with a Henry's Constant less than 10. For values between 10 and 100, PTA may be feasible but should be extensively evaluated using pilot studies. Values for Henry's Constant should be discussed with the reviewing agency prior to final design. Process Design

a. Process design methods for PTA involve the determination of Henry's Constant for the contaminant, the mass transfer coefficient, air pressure drop and stripping factor. The applicant shall provide justification for the design parameters selected (i.e. height and diameter of unit, air to water ratio, packing depth, surface loading rate, etc.). Pilot plant testing shall be provided. The pilot test shall evaluate a variety of loading rates and air to water ratios at the peak contaminant concentration. Special consideration should be given to removal efficiencies when multiple contaminations occur. Where there is considerable past performance data on the contaminant to be treated and there is a concentration level similar to previous projects, the reviewing authority may approve the process design based on use of appropriate calculations without pilot testing. Proposals of this type must be discussed with the reviewing authority prior to submission of any permit applications.

b. The tower shall be designed to reduce contaminants to below the maximum contaminant level (MCL) and to the lowest practical level.

c. The ratio of the column diameter to packing diameter should be at least 7:1 for the pilot unit and at least 10:1 for the full scale tower. The type and size of the packing used in the full scale unit shall be the same as that used in the pilot work.

d. The minimum volumetric air to water ratio at peak water flow should be 25:1. The maximum air to water ratio for which credit will be given is 80:1.

e. The design should consider potential fouling problems from calcium carbonate and iron precipitation and from bacterial growth. It may be necessary to provide pretreatment. Disinfection capability shall be provided prior to and after PTA.

f. The effects of temperature should be considered since a drop in water temperature can result in a drop in contaminant removal efficiency. Materials of Construction

a. The tower can be constructed of stainless steel, concrete, aluminum, fiberglass or plastic. Uncoated carbon steel is not recommended because of corrosion. Towers constructed of light-weight materials should be provided with adequate support to prevent damage from wind.

b. Packing materials shall be resistant to the aggressiveness of the water, dissolved gases and cleaning materials and shall be suitable for contact with potable water. Water Flow System

a. Water should be distributed uniformly at the top of the tower using spray nozzles or orifice-type distributor trays that prevent short circuiting. For multi-point injection, one injection point for every 30 in2 (190 cm2) of tower cross-sectional area is recommended.

b. A mist eliminator shall be provided above the water distributor system.

c. A side wiper redistribution ring should be provided at least every 10 feet in order to prevent water channeling along the tower wall and short circuiting.

d. Sample taps shall be provided in the influent and effluent piping.

e. The effluent sump, if provided, shall have easy access for cleaning purposes and be equipped with a drain valve. The drain shall not be connected directly to any storm or sanitary sewer.

f. A blow-off line should be provided in the effluent piping to allow for discharge of water/chemicals used to clean the tower.

g. The design shall prevent freezing of the influent riser and effluent piping when the unit is not operating. If piping is buried, it shall be maintained under positive pressure.

h. The water flow to each tower shall be metered.

i. An overflow line shall be provided which discharges 12 to 14 inches above a splash pad or drainage inlet. Proper drainage shall be provided to prevent flooding of the area.

j. Butterfly valves may be used in the water effluent line for better flow control, as well as to minimize air entrainment.

k. Means shall be provided to prevent flooding of the air blower.

l. The water influent pipe should be supported separately from the tower's main structural support. Air Flow System

a. The air inlet to the blower and tower discharge vent shall be protected with a noncorrodible 24-mesh downturned screen to prevent contamination from extraneous matter. It is recommended that a 4-mesh screen also be installed prior to the 24-mesh screen on the air inlet system.

b. The air inlet shall be in a protected location.

c. An air flow meter shall be provided on the influent air line or an alternative method to determine the air flow shall be provided.

d. A positive air flow sensing device and a pressure gauge must be installed on the air influent line. The positive air flow sensing device must be a part of an automatic control system which will turn off the influent water if positive air flow is not detected. The pressure gauge will serve as an indicator of fouling buildup.

e. A backup motor for the air blower must be readily available. Other Features that shall be Provided

a. A sufficient number of access ports with a minimum diameter of 24 inches to facilitate inspection, media replacement, media cleaning and maintenance of the interior.

b. A method of cleaning the packing material when iron, manganese, or calcium carbonate fouling may occur.

c. Tower effluent collection and pumping wells constructed to clearwell standards.

d. Provisions for extending the tower height without major reconstruction.

e. An acceptable alternative supply must be available during periods of maintenance and operation interruptions. No bypass shall be provided unless specifically approved by the reviewing agency.

f. Disinfection application points both ahead of and after the tower to control biological growth.

g. Disinfection and adequate contact time after the water has passed through the tower and prior to the distribution system.

h. Adequate packing support to allow free flow of water and to prevent deformation with deep packing heights.

i. Operation of the blower and disinfectant feeder equipment during power failures.

j. Adequate foundation to support the tower and lateral support to prevent overturning due to wind loading.

k. Fencing and locking gate to prevent vandalism.

l. An access ladder with safety cage for inspection of the aerator including the exhaust port and de-mister.

m. Electrical interconnection between blower, disinfectant feeder and well pump. Environmental Factors

a. The applicant must contact the appropriate air quality office to determine if permits are required under the Clean Air Act.

b. Noise control facilities should be provided on PTA systems located in residential areas.

4.5.6 Other methods of aeration

Other methods of aeration may be used if applicable to the treatment needs. Such methods include but are not restricted to spraying, diffused air, cascades and mechanical aeration. The treatment processes must be designed to meet the particular needs of the water to be treated and are subject to the approval of the reviewing authority.

4.5.7 Protection of aerators

All aerators except those discharging to lime softening or clarification plants shall be protected from contamination by birds, insects, wind borne debris, rainfall and water draining off the exterior of the aerator.

4.5.8 Disinfection

Groundwater supplies exposed to the atmosphere by aeration must receive chlorination as the minimum additional treatment.

4.5.9 Bypass

A bypass should be provided for all aeration units except those installed to comply with maximum contaminant levels.

4.5.10 Corrosion control

The aggressiveness of the water after aeration should be determined and corrected by additional treatment, if necessary. (see Section 4.8).

4.5.11 Quality control

Equipment should be provided to test for DO, pH, and temperature to determine proper functioning of the aeration device. Equipment to test for iron, manganese, and carbon dioxide should also be considered.


Iron and manganese control, as used herein, refers solely to treatment processes designed specifically for this purpose. The treatment process used will depend upon the character of the raw water. The selection of one or more treatment processes must meet specific local conditions as determined by engineering investigations, including chemical analyses of representative samples of water to be treated, and receive the approval of the reviewing authority. It may be necessary to operate a pilot plant in order to gather all information pertinent to the design. Consideration should be given to adjusting pH of the raw water to optimize the chemical reaction. Testing equipment and sampling taps shall be provided as outlined in Sections 2.8 and 2.10.

4.6.1 Removal by oxidation, detention and filtration Oxidation

Oxidation may be by aeration, as indicated in Section 4.5, or by chemical oxidation with chlorine, potassium permanganate, ozone or chlorine dioxide. Detention

a. Reaction - A minimum detention time of 30 minutes shall be provided following aeration to insure that the oxidation reactions are as complete as possible. This minimum detention may be omitted only where a pilot plant study indicates no need for detention. The detention basin should be designed as a holding tank with no provisions for sludge collection but with sufficient baffling to prevent short circuiting.

b. Sedimentation - Sedimentation basins shall be provided when treating water with high iron and/or manganese content, or where chemical coagulation is used to reduce the load on the filters. Provisions for sludge removal shall be made. Filtration

Filters shall be provided and shall conform to Section 4.2.

4.6.2 Removal by the lime-soda softening process

See Section 4.4.1.

4.6.3 Removal by manganese greensand filtration

This process, consists of a continuous feed of potassium permanganate to the influent of a manganese coated media filter.

a. Provisions should be made to apply the permanganate as far ahead of the filter as practical and to a point immediately before the filter.

b. Other oxidizing agents or processes such as chlorination or aeration may be used prior to the permanganate feed to reduce the cost of the chemical.

c. An anthracite media cap of at least six inches shall be provided over manganese coated media.

d. Normal filtration rate is three gallons per minute per square foot (7.2 m/hr).

e. Normal wash rate is 8 to 10 gallons per minute per square foot (20 - 24 m/hr) with manganese greensand and 15 to 20 gallons per minute (37 - 49 m/hr) with manganese coated media.

f. Air washing should be provided.

g. Sample taps shall be provided

1. prior to application of permanganate,

2. immediately ahead of filtration,

3. at the filter effluent, and

4. should be provided at points between the anthracite media and the manganese coated media.

4.6.4 Removal by ion exchange

This process of iron and manganese removal should not be used for water containing more than 0.3 milligrams per liter of iron, manganese or combination thereof. This process is not acceptable where either the raw water or wash water contains dissolved oxygen or other oxidants.

4.6.5 Sequestration by polyphosphates

This process shall not be used when iron, manganese or combination thereof exceeds 1.0 milligrams per liter. The total phosphate applied shall not exceed 10 milligrams per liter as PO4. Where phosphate treatment is used, satisfactory chlorine residuals shall be maintained in the distribution system. Possible adverse effects on corrosion must be addressed when phosphate addition is proposed for iron sequestering.

a. Feeding equipment shall conform to the requirements of Part 5.

b. Stock phosphate solution must be kept covered and disinfected by carrying approximately 10 milligrams per liter free chlorine residual. Phosphate solutions having a pH of 2.0 or less may be exempted from this requirement by thereviewing authority.

c. Polyphosphates shall not be applied ahead of iron and manganese removal treatment. The point of application shall be prior to any aeration, oxidation or disinfection if no iron or manganese removal treatment is provided.

4.6.6 Sequestration by sodium silicates

Sodium silicate sequestration of iron and manganese is appropriate only for groundwater supplies prior to air contact. On-site pilot tests are required to determine the suitability of sodium silicate for the particular water and the minimum feed needed. Rapid oxidation of the metal ions such as by chlorine or chlorine dioxide must accompany or closely precede the sodium silicate addition. Injection of sodium silicate more than 15 seconds after oxidation may cause detectable loss of chemical efficiency. Dilution of feed solutions much below five per cent silica as SiO2 should also be avoided for the same reason.

a. Sodium silicate addition is applicable to waters containing up to 2 mg/l of iron, manganese or combination thereof.

b. Chlorine residuals shall be maintained throughout the distribution system to prevent biological breakdown of the sequestered iron.

c. The amount of silicate added shall be limited to 20 mg/l as SiO2, but the amount of added and naturally occurring silicate shall not exceed 60 mg/l as SiO2.

d. Feeding equipment shall conform to the requirements of Part 5.

e. Sodium silicate shall not be applied ahead of iron or manganese removal treatment.

4.6.7 Sampling taps

Smooth-nosed sampling taps shall be provided for control purposes. Taps shall be located on each raw water source, each treatment unit influent and each treatment unit effluent.

4.6.8 Testing equipment shall be provided for all plants.

a. The equipment should have the capacity to accurately measure the iron content to a minimum of 0.1 milligrams per liter and the manganese content to a minimum of 0.05 milligrams per liter.

b. Where polyphosphate sequestration is practiced, appropriate phosphate testing equipment shall be provided.


Sodium fluoride, sodium silicofluoride and hydrofluosilicic acid shall conform to the applicable AWWA standards. Other fluoride compounds which may be available must be approved by the reviewing authority.

4.7.1 Fluoride compound storage

Fluoride chemicals should be isolated from other chemicals to prevent contamination. Compounds shall be stored in covered or unopened shipping containers and should be stored inside a building. Unsealed storage units for hydrofluosilicic acid should be vented to the atmosphere at a point outside any building. Bags, fiber drums and steel drums should be stored on pallets.

4.7.2 Chemical feed equipment and methods

In addition to the requirements in Part 5, fluoride feed equipment shall meet the following requirements:

a. scales, loss-of-weight recorders or liquid level indicators, as appropriate, accurate to within five percent of the average daily change in reading shall be provided for chemical feeds,

b. feeders shall be accurate to within five percent of any desired feed rate,

c. fluoride compound shall not be added before lime-soda softening or ion exchange softening,

d. the point of application of fluorosilicic acid, if into a horizontal pipe, shall be in the lower half of the pipe,

e. a fluoride solution shall be applied by a positive displacement pump having a stroke rate not less than 20 strokes per minute,

f. a spring opposed diaphragm type anti-siphon device shall be provided for all fluoride feed lines and dilution water lines,

g. a device to measure the flow of water to be treated is required,

h. the dilution water pipe shall terminate at least two pipe diameters above the solution tank,

i. water used for sodium fluoride dissolution shall be softened if hardness exceeds 75 mg/l as calcium carbonate,

j. fluoride solutions shall be injected at a point of continuous positive pressure or a suitable air gap provided,

k. the electrical outlet used for the fluoride feed pump should have a nonstandard receptacle and shall be interconnected with the well or service pump,

l. saturators should be of the up-flow type and be provided with a meter and backflow-protection on the makeup water line.

4.7.3 Secondary controls

Secondary control systems for fluoride chemical feed devices shall be required by the reviewing authority as a means of reducing the possibility for overfeed; these may include flow or pressure switches or other devices.

4.7.4 Protective equipment

Protective equipment, as outlined in Section 5.3.4, shall be provided for operators handling fluoride compounds. Deluge showers and eye wash devices shall be provided at all fluosilicic acid installations.

4.7.5 Dust control

a. Provision must be made for the transfer of dry fluoride compounds from shipping containers to storage bins or hoppers in such a way as to minimize the quantity of fluoride dust which may enter the room in which the equipment is installed. The enclosure shall be provided with an exhaust fan and dust filter which place the hopper under a negative pressure. Air exhausted from fluoride handling equipment shall discharge through a dust filter to the outside atmosphere of the building.

b. Provision shall be made for disposing of empty bags, drums or barrels in a manner which will minimize exposure to fluoride dusts. A floor drain should be provided to facilitate the hosing of floors.

4.7.6 Testing equipment

Equipment shall be provided for measuring the quantity of fluoride in the water. Such equipment shall be subject to the approval of the reviewing authority.


Water that is unstable due either to natural causes or to subsequent treatment should be stabilized. The expected treated water quality shall be evaluated to determine what, if any, treatment is necessary.

4.8.1 Carbon dioxide addition

a. Recarbonation basin design should provide

1. a total detention time of twenty minutes,

2. two compartments, with a depth that will provide a diffuser submergence of not less than 7.5 feet nor greater submergence than recommended by the manufacturer as follows:

a. a mixing compartment having a detention time of at least three minutes,

b. a reaction compartment.

b. Plants generating carbon dioxide from combustion shall have open top recarbonation tanks in order to dissipate carbon monoxide gas.

c. Where liquid carbon dioxide is used, adequate precautions must be taken to prevent carbon dioxide from entering the plant from the recarbonation process.

d. Provisions shall be made for draining the recarbonation basin and removing sludge.

4.8.2 Acid addition

a. Feed equipment shall conform to Part 5.

b. Adequate precautions shall be taken for operator safety, such as not adding water to the concentrated acid. (See Sections 5.3 and 5.4).

4.8.3 Phosphates

The feeding of phosphates may be applicable for sequestering calcium in lime-softened water, for corrosion control, and in conjunction with alkali feed following ion exchange softening.

a. Feed equipment shall conform to Part 5.

b. Stock phosphate solution must be kept covered and disinfected by carrying approximately 10 milligrams per liter free chlorine residual. Phosphate solutions having a pH of 2.0 or less may be exempt from this requirement by the reviewing authority.

c. Satisfactory chlorine residuals shall be maintained in the distribution system when phosphates are used.

4.8.4 "Split treatment"

Under some conditions, a lime-softening water treatment plant can be designed using "split treatment" in which raw water is blended with lime-softened water to partially stabilize the water prior to secondary clarification and filtration. Treatment plants designed to utilize "split treatment" should also contain facilities for further stabilization by other methods.

4.8.5 Alkali feed

Water with low alkalinity or pH should be treated with an alkali chemical.

4.8.6 Carbon dioxide reduction by aeration

The carbon dioxide content of an aggressive water may be reduced by aeration. Aeration devices shall conform to Section 4.5.

4.8.7 Other treatment

Other treatment for controlling corrosive waters by the use of calcium hydroxide, sodium silicate and sodium bicarbonate may be used where necessary. Any proprietary compound must receive the specific approval of the reviewing authority before use. Chemical feeders shall be as required in Part 5.

4.8.8 Water unstable due to biochemical action in distribution system

Unstable water resulting from the bacterial decomposition of organic matter in water (especially in dead end mains), the biochemical action within tubercles, and the reduction of sulfates to sulfides should be prevented by the maintenance of a free chlorine residual throughout the distribution system.

4.8.9 Control

Laboratory equipment shall be provided for determining the effectiveness of stabilization treatment.


Provision shall be made for the control of taste and odor at all surface water treatment plants. Chemicals shall be added sufficiently ahead of other treatment processes to assure adequate contact time for an effective and economical use of the chemicals. Where severe taste and odor problems are encountered, in-plant and/or pilot plant studies are required.

4.9.1 Flexibility

Plants treating water that is known to have taste and odor problems should be provided with equipment that makes several of the control processes available so that the operator will have flexibility in operation.

4.9.2 Chlorination

Chlorination can be used for the removal of some objectionable odors. Adequate contact time must be provided to complete the chemical reactions involved. Excessive potential trihalomethane production through this process should be avoided by adequate bench-scale testing prior to design.

4.9.3 Chlorine dioxide

Chlorine dioxide has been generally recognized as a treatment for tastes caused by industrial wastes, such as phenols. However, chlorine dioxide can be used in the treatment of any taste and odor that is treatable by an oxidizing compound. Provisions shall be made for proper storing and handling of the sodium chlorite, so as to eliminate any danger of explosion. (See Section 5.4.3.)

4.9.4 Powdered activated carbon

a. Powdered activated carbon should be added as early as possible in the treatment process to provide maximum contact time. Flexibility to allow the addition of carbon at several points is preferred. Activated carbon should not be applied near the point of chlorine or other oxidant application.

b. The carbon can be added as a pre-mixed slurry or by a dry-feed machine as long as the carbon is properly wetted.

c. Continuous agitation or resuspension equipment is necessary to keep the carbon from depositing in the slurry storage tank.

d. Provision shall be made for adequate dust control.

e. The required rate of feed of carbon in a water treatment plant depends upon the tastes and/or odors involved, but provision should be made for adding from 0.1 milligrams per liter to at least 40 milligrams per liter.

f. Powdered activated carbon shall be handled as a potentially combustible material. It should be stored in a building or compartment as nearly fireproof as possible. Other chemicals should not be stored in the same compartment. A separate room should be provided for carbon feed installations. Carbon feeder rooms should be equipped with explosion-proof electrical outlets, lights and motors.

4.9.5 Granular activated carbon

See Section for application within filters.

4.9.6 Copper sulfate and other copper compounds

Continuous or periodic treatment of water with copper compounds to kill algae or other growths shall be controlled to prevent copper in excess of 1.0 milligrams per liter as copper in the plant effluent or distribution system. Care shall be taken to assure an even distribution.

4.9.7 Aeration

See Section 4.5.

4.9.8 Potassium permanganate

Application of potassium permanganate may be considered, providing the treatment shall be designed so that the products of the reaction are not visible in the finished water.

4.9.9 Ozone

Ozonation can be used as a means of taste and odor control. Adequate contact time must be provided to complete the chemical reactions involved. Ozone is generally more desirable for treating water with high threshold odors. (See Ozone Policy Statement.)

4.9.10 Other methods

The decision to use any other methods of taste and odor control should be made only after careful laboratory and/or pilot plant tests and on consultation with the reviewing authority.


A microscreen is a mechanical supplement of treatment capable of removing suspended matter from the water by straining. It may be used to reduce nuisance organisms and organic loadings. It shall not be used in place of

a. filtration, when filtration is necessary to provide a satisfactory water, or

b. coagulation, in the preparation of water for filtration.

4.10.1 Design

a. shall give due consideration to

1. nature of the suspended matter to be removed,

2. corrosiveness of the water,

3. effect of chlorination, when required as pre-treatment,

4. duplication of units for continuous operation during equipment maintenance;

b. shall provide

1. a durable, corrosion-resistant screen,

2. by-pass arrangements,

3. protection against back-siphonage when potable water is used for washing,

4. proper disposal of wash waters. (See Section 4.11).


Provisions must be made for proper disposal of water treatment plant waste such as sanitary, laboratory, clarification sludge, softening sludge, iron sludge, filter backwash water, and brines. All waste discharges shall be governed by regulatory agency requirements. The requirements outlined herein must, therefore, be considered minimum requirements as state water pollution control authorities may have more stringent requirements. In locating waste disposal facilities, due consideration shall be given to preventing potential contamination of the water supply.

Alternative methods of water treatment and chemical use should be considered as a means of reducing waste volumes and the associated handling and disposal problems.

4.11.1 Sanitary waste

The sanitary waste from water treatment plants, pumping stations, and other waterworks installations must receive treatment. Waste from these facilities must be discharged directly to a sanitary sewer system, when available and feasible, or to an adequate on-site waste treatment facility approved by the appropriate reviewing authority.

4.11.2 Brine waste

Waste from ion exchange plants, demineralization plants, or other plants which produce a brine, may be disposed of by controlled discharge to a stream if adequate dilution is available. Surface water quality requirements of the regulatory agency will control the rate of discharge. Except when discharging to large waterways, a holding tank of sufficient size should be provided to allow the brine to be discharged over a twenty-four hour period. Where discharging to a sanitary sewer, a holding tank may be required to prevent the overloading of the sewer and/or interference with the waste treatment processes. The effect of brine discharge to sewage lagoons may depend on the rate of evaporation from the lagoons. Whenever applicable, reference should be made to the U.S. EPA Suggested Guidelines for Disposal of Drinking Water Treatment Wastes Containing Radioactivity.

4.11.3 Lime softening sludge

Sludge from plants using lime to soften water varies in quantity and in chemical characteristics depending on the softening process and the chemical characteristics of the water being softened. Recent studies show that the quantity of sludge produced is much larger than indicated by stoichiometric calculations. Methods of treatment and disposal are as follows:

a. Lagoons

1. Temporary lagoons which must be cleaned periodically should be designed on the basis of 0.7 acres per million gallons per day per 100 milligrams per liter of hardness removed based on a usable lagoon depth of five feet. This should provide about 2 1/2 years storage. At least two but preferably more lagoons must be provided in order to give flexibility in operation. An acceptable means of final sludge disposal must be provided. Provisions must be made for convenient cleaning.

2. Permanent lagoons should have a volume of at least four times that for temporary lagoons.

3. The design of both temporary lagoons and permanent lagoons should provide for

a. location free from flooding,

b. when necessary, dikes, deflecting gutters or other means of diverting surface water so that it does not flow into the lagoons,

c. a minimum usable depth of five feet,

d. adequate freeboard of at least two feet,

e. adjustable decanting device,

f. effluent sampling point,

g. adequate safety provisions, and

h. parallel operation.

b. The application of liquid lime sludge to farm land should be considered as a method of ultimate disposal. Approval from the appropriate reviewing authority must be obtained. When this method is selected, the following provisions shall be made:

1. Transport of sludge by vehicle or pipeline shall incorporate a plan or design which prevents spillage or leakage during transport.

2. Interim storage areas at the application site shall be kept to a minimum and facilities shall be provided to prevent washoff of sludge or flooding.

3. Sludge shall not be applied at times when washoff of sludge from the land could be expected.

4. Sludge shall not be applied to sloping land where washoff could be expected unless provisions are made, for suitable land, to immediately incorporate the sludge into the soil.

5. Trace metals loading shall be limited to prevent significant increases in trace metals in the food chain, phytotoxicity or water pollution.

6. Each area of land to receive lime sludge shall be considered individually and a determination made as to the amount of sludge needed to raise soil pH to the optimum for the crop to be grown.

c. Discharge of lime sludge to sanitary sewers should be avoided since it may cause both liquid volume and sludge volume problems at the sewage treatment plant. This method should be used only when the sewerage system has the capability to adequately handle the lime sludge.

d. Mixing of lime sludge with activated sludge waste may be considered as a means of co-disposal.

e. Disposal at a landfill can be done as either a solid or liquid if the landfill can accept such waste, depending on individual state requirements.

f. Mechanical dewatering of sludge may be considered. Pilot studies on a particular plant waste are required.

g. Calcination of sludge may be considered. Pilot studies on a particular plant waste are required.

h. Lime sludge drying beds are not recommended.

4.11.4 Alum sludge

Lagooning may be used as a method of handling alum sludge. Lagoon size can be calculated using total chemicals used plus a factor for turbidity. Mechanical concentration may be considered. A pilot plant study is required before the design of a mechanical dewatering installation. Freezing changes the nature of alum sludge so that it can be used for fill. Acid treatment of sludge for alum recovery may be a possible alternative. Alum sludge can be discharged to a sanitary sewer. However, initiation of this practice will depend on obtaining approval from the owner of the sewerage system as well as from the regulatory agency before final designs are made.

Lagoons should be designed to produce an effluent satisfactory to the regulatory agency and should provide for

a. location free from flooding,

b. where necessary, dikes, deflecting gutters or other means of diverting surface water so that it does not flow into the lagoon,

c. a minimum usable depth of five feet,

d. adequate freeboard of at least two feet,

e. adjustable decanting device,

f. effluent sampling point,

g. adequate safety provisions, and

h. a minimum of two cells, each with appropriate inlte/outlet structures to facilitate independent filling/dewatering operations.

4.11.5 "Red water" waste

Waste filter wash water from iron and manganese removal plants can be disposed of as follows: Sand filters

Sand filters should have the following features:

a. Total filter area shall be sufficient to adequately dewater applied solids. Unless the filter is small enough to be cleaned and returned to service in one day, two or more cells are required.

b. The "red water" filter shall have sufficient capacity to contain, above the level of the sand, the entire volume of wash water produced by washing all of the production filters in the plant, unless the production filters are washed on a rotating schedule and the flow through the production filters is regulated by true rate of flow controllers. Then sufficient volume must be provided to properly dispose of the wash water involved.

c. Sufficient filter surface area should be provided so that, during any one filtration cycle, no more than two feet of backwash water will accumulate over the sand surface.

d. The filter shall not be subject to flooding by surface runoff or flood waters. Finished grade elevation shall be established to facilitate maintenance, cleaning and removal of surface sand as required. Flash boards or other non-watertight devices shall not be used in the construction of filter side walls.

e. The filter media should consist of a minimum of twelve inches of sand, three to four inches of supporting small gravel or torpedo sand, and nine inches of gravel in graded layers. All sand and gravel should be washed to remove fines.

f. Filter sand should have an effective size of 0.3 to 0.5 mm and a uniformity coefficient not to exceed 3.5. The use of larger sized sands shall be justified by the designing engineer to the satisfaction of the reviewing authority.

g. The filter should be provided with an adequate under-drainage collection system to permit satisfactory discharge of filtrate.

h. Provision shall be made for the sampling of the filter effluent.

i. Overflow devices from "red water" filters shall not be permitted.

j. Where freezing is a problem, provisions should be made for covering the filters during the winter months.

k. "Red water" filters shall comply with the common wall provisions contained in Sections 7.1.3 and 8.8.1, which pertain to the possibility of contaminating treated water with an unsafe water.

The reviewing authority must be contacted for approval of any arrangement where a separate structure is not provided. Lagoons

Lagoons shall have the following features:

a. be designed with volume 10 times the total quantity of wash water discharged during any 24-hour period,

b. a minimum usable depth of three feet,

c. length four times width, and the width at least three times the depth, as measured at the operating water level,

d. outlet to be at the end opposite the inlet,

e. a weir overflow device at the outlet end with weir length equal to or greater than depth,

f. velocity to be dissipated at the inlet end. Discharge to community sanitary sewer

Red water can be discharged to a community sewer. However, approval of this method will depend on obtaining approval from the owner of the sewerage system as well as from the regulatory agency before final designs are made. A holding tank is recommended to prevent overloading the sewers. Recycling "Red Water" Wastes

Recycling of supernatant or filtrate from "red water" waste treatment facilities to the head end of an iron removal plant shall not be allowed except as approved by the reviewing authority.

4.11.6 Waste filter wash water

Waste filter wash water from surface water treatment or lime softening plants should have suspended solids reduced to a level acceptable to the regulatory agency before being discharged. Many plants have constructed holding tanks and returned this water to the inlet end of the plant. The holding tank should be of such a size that it will contain the anticipated volume of waste wash water produced by the plant when operating at design capacity. A plant that has two filters should have a holding tank that will contain the total waste wash water from both filters calculated by using a 15 minute wash at 20 gallons per minute per square foot. In plants with more filters, the size of the holding tank will depend on the anticipated hours of operation. It is recommended that waste filter wash water be returned at a rate of less than 10 percent of the raw water entering the plant. Filter backwash water should not be recycled when the raw water contains excessive algae, when finished water taste and odor problems are encountered, or when trihalomethane levels in the distribution system may exceed allowable levels. Particular attention must be given to the presence of protozoans such as Giardia and Cryptosporidium concentrating in the waste water stream. Water utilities may need to treat filter waste water prior to recycling or avoid reclaiming filter wash water given the increased risk to treated water quality.


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