FINISHED WATER STORAGE


7.0 GENERAL
7.1 PLANT STORAGE
7.2 HYDROPNEUMATIC TANKS
7.3 DISTRIBUTION STORAGE

7.0 GENERAL

The materials and designs used for finished water storage structures shall provide stability and durability as well as protect the quality of the stored water. Steel structures shall follow the current AWWA standards concerning steel tanks, standpipes, reservoirs, and elevated, tanks wherever they are applicable. Other materials of construction are acceptable when properly designed to meet the requirements of Part 7.

7.0.1 Sizing

Storage facilities should have sufficient capacity, as determined from engineering studies, to meet domestic demands, and where fire protection is provided, fire flow demands.

a. Fire flow requirements established by the appropriate state Insurance Services Office should be satisfied where fire protection is provided.

b. The minimum storage capacity (or equivalent capacity) for systems not providing fire protection shall be equal to the average daily consumption. This requirement may be reduced when the source and treatment facilities have sufficient capacity with standby power to supplement peak demands of the system.


7.0.2 Location of ground-level reservoirs

a. The bottom of reservoirs and standpipes should be placed at the normal ground surface and shall be a above maximum flood level.

b. When the bottom must be below normal ground surface, it shall be placed above the groundwater level. At least 50 per cent of the water depth should be above grade. Sewers, drains, standing water and similar sources of possible contamination must be kept at least fifty feet from the reservoir. Water main pipe, pressure tested in place to 50 psi without leakage, may be used for gravity sewers at distances greater than 20 feet and less than 50 feet.

c. The top of a reservoir shall not be less than two feet above normal ground surface. Clearwells constructed under filters may be excepted from this requirement when the total design gives the same protection.


7.0.3 Protection

All finished water storage structures shall have suitable watertight roofs which exclude birds, animals, insects, and excessive dust.


7.0.4 Protection from trespassers

Fencing, locks on access manholes, and other necessary precautions shall be provided to prevent trespassing, vandalism, and sabotage.


7.0.5 Drains

No drain on a water storage structure may have a direct connection to a sewer or storm drain. The design shall allow draining the storage facility for cleaning or maintenance without causing loss of pressure in the distribution system.


7.0.6 Inlet/outlet and baffle wall

System should be designed to facilitate turn over of water in the reservoir.

7.0.7 Overflow

All water storage structures shall be provided with an overflow which is brought down to an elevation between 12 and 24 inches above the ground surface, and discharges over a drainage inlet structure or a splash plate. No overflow may be connected directly to a sewer or a storm drain. All overflow pipes shall be located so that any discharge is visible.

a. When an internal overflow pipe is used on elevated tanks, it should be located in the access tube. For vertical drops on other types of storage facilities, the overflow pipe be located on the outside of the structure.

b. The overflow of a ground-level structure shall open downward and be screened with twenty-four mesh noncorrodible screen installed within the pipe at a location least susceptible to damage by vandalism.

c. The overflow pipe shall be of sufficient diameter to permit waste of water in excess of the filling rate.


7.0.8 Access

Finished water storage structures shall be designed with reasonably convenient access to the interior for cleaning and maintenance. At least two (2) manholes shall be provided above the waterline at each water compartment where space permtis.

a. shall be framed at least four inches, and preferably six inches, above the surface of the roof at the opening; on ground-level structures, manholes should be elevated 24 to 36 inches above the top or covering sod;

b. shall be fitted with a solid watertight cover which overlaps the framed opening and extends down around the frame at least two inches,

c. should be hinged at one side,

d. shall have a locking device.


7.0.9 Vents

Finished water storage structures shall be vented. Overflows shall not be considered as vents. Open construction between the sidewall and roof is not-permissible. Vents

a. shall prevent the entrance of surface water and rainwater,

b. shall exclude birds and animals,

c. should exclude insects and dust, as much as this function can be made compatible with effective venting. For elevated tanks and standpipes, four-mesh noncorrodible screen may be used;

d. shall, on ground-level structures, terminate in an inverted U construction with the opening 24 to 36 inches above the roof or sod and covered with twenty-four mesh noncorrodible screen installed within the pipe at a location least susceptible to vandalism.


7.0.10 Roof and sidewall

The roof and sidewalls of all structures must be watertight with no openings except properly constructed vents, manholes, overflows, risers, drains, pump mountings, control ports, or piping for inflow and outflow.

a. Any pipes running through the roof or sidewall of a finished water storage structure must be welded, or properly gasketed in metal tanks. In concrete tanks, these pipes shall be connected to standard wall castings which were poured in place during the forming of the concrete. These wall castings should have seepage rings imbedded in the concrete.

b. Openings in a storage structure roof or top, designed to accommodate control apparatus or pump columns, shall be curbed and sleeved with proper additional shielding to prevent the access of surface of floor drainage water into the structure.

c. Valves and controls should be located outside the storage structure so that the valve stems and similar projections will not pass through the roof or top of the reservoir.

d. The roof of concrete reservoirs with earthern cover shall be sloped to facilitate drainage. Consideration should be given to installation of an impermeable membrane roof covering.


7.0.11 Drainage of roof

The roof of the storage structure shall be well drained. Downspout pipes shall not enter or pass through the reservoir. Parapets, or similar construction which would tend to hold water and snow on the roof, will not be approved unless adequate waterproofing and drainage are provided.


7.0.12 Safety

The safety of employees must be considered in the design of the storage structure. As a minimum, such matters shall conform to pertinent laws and regulations of the area where the reservoir is constructed.

a. Ladders, ladder guards, balcony railings, and safely located entrance hatches shall be provided where applicable.

b. Elevated tanks with riser pipes over eight inches in diameter shall have protective bars over the riser openings inside the tank.

c. Railings or handholds shall be provided on elevated tanks where persons must transfer from the access tube to the water compartment.

d. Confined space entry requirements shall be considered.


7.0.13 Freezing

All finished water storage structures and their appurtenances, especially the riser pipes, overflows, and vents, shall be designed to prevent freezing which will interfere with proper functioning.


7.0.14 Internal catwalk

Every catwalk over finished water in a storage structure shall have a solid floor with raised edges so designed that shoe scrapings and dirt will not fall into the water.


7.0.15 Silt stop

The discharge pipes from all reservoirs shall be located in a manner that will prevent the flow of sediment into the distribution system. Removable silt stops should be provided.


7.0.16 Grading

The area surrounding a ground-level structure shall be graded in a manner that will prevent surface water from standing within 50 feet of it.


7.0.17 Painting and/or cathodic protection

Proper protection shall be given to metal surfaces by paints or other protective coatings, by cathodic protective devices, or by both.

a. Paint systems shall meet NSF standard 61 and be acceptable to the reviewing authority. Interior paint must be properly applied and cured. After curing, the coating shall not transfer any substance to the water which will be toxic or cause tastes or odors. Prior to placing in service, an analysis for volatile organic compounds is advisable to establish that the coating is properly cured. Consideration should be given to 100 % solid coatings.

b. Wax coatings for the tank interior should not be used on new tanks. Recoating with a wax system is discouraged; however, the old wax coating must be completely removed to use another tank coating.

c. Cathodic protection should be designed and installed by competent technical personnel; a maintenance contract should be provided.


7.0.18 Disinfection

a. Finished water storage structures shall be disinfected in accordance with current AWWA Standard C652. Two or more successive sets of samples, taken at 24-hour intervals, shall indicate microbiologically satisfactory water before the facility is placed into operation.

b. Disposal of heavily chlorinated water from the tank disinfection process shall be in accordance with the requirements of the state pollution control agency.

c. The disinfection procedure (AWWA C652 chlorination method 3, section 4.3) which allows use of the chlorinated water held in the storage tank for disinfection purposes is not recommended. When that procedure is used, it is recommended that the initial heavily chlorinated water be properly disposed in order to prevent release of water which may contain various chlorinated organic compounds into the distribution system.


7.0.19 Provisions for Sampling

Appropriate sampling tap(s) shall be provided to facilitate collection of water samples for both bacteriologic and chemical analyses.


7.1 PLANT STORAGE

The applicable design standards of Section 7.0 shall be followed for plant storage.

7.1.1 Washwater tanks

Washwater tanks shall be sized, in conjunction with available pump units and finished water storage, to provide the backwash water required by Section 4.2.1.11. Consideration must be given to the backwashing of several filters in rapid succession.


7.1.2 Clearwell

Clearwell storage should be sized, in conjunction with distribution system storage, to relieve the filters from having to follow fluctuations in water use.

a. When finished water storage is used to provide contact time for chlorine (see Section 4.3.2) special attention must be given to size and baffling. (See Section 7.1.2.b below.)

b. To ensure adequate chlorine contact time, sizing of the clearwell should include extra volume to accommodate depletion of storage during the nighttime for intermittently operated filtration plants with automatic high service pumping from the clearwell during non-treatment hours.

c. An overflow and vent shall be provided.

d. A minimum of two clearwell compartments shall be provided.


7.1.3 Adjacent compartments

Finished water must not be stored or conveyed in a compartment adjacent to unsafe water when the two compartments are separated by a single wall.


7.1.4 Basins and wet-wells

Receiving basins and pump wet-wells for finished water shall be designed as finished water storage structures.


7.2 HYDROPNEUMATIC TANKS

Hydropneumatic (pressure) tanks, when provided as the only storage facility, are acceptable only in very small water systems. When serving more than 150 living units, ground or elevated storage designed in accordance with Section 7.0.1 should be provided. Pressure tank storage is not to be considered for fire protection purposes. Pressure tanks shall meet ASME code requirements or an equivalent requirement of state and local laws and regulations for the construction and installation of unfired pressure vessels.

7.2.1 Location

The tank shall be located above normal ground surface and be completely housed.

7.2.2 Sizing

a. The capacity of the wells and pumps in a hydropneumatic system should be at least ten times the average daily consumption rate. The gross volume of the hydropneumatic tank, in gallons, should be at least ten times the capacity of the largest pump, rated in gallons per minute. For example, a 250 gpm pump should have a 2,500 gallon pressure tank.

b. Sizing of hydropneumatic storage tanks must consider the need for chlorine detention time, as applicable, independent of the requirements in 7.2.2.a above.

7.2.3 Piping

The tank shall have bypass piping to permit operation of the system while it is being repaired or painted.

7.2.4 Appurtenances

Each tank shall have an access manhole, a drain, and control equipment consisting of pressure gauge, water sight glass, automatic or manual air blow-off, means for adding air, and pressure operated start-stop controls for the pumps. Where practical the access manhole should be 24 inches in diameter.


7.3 DISTRIBUTION STORAGE

The applicable design standards of Section 7.0 shall be followed for distribution system storage.

7.3.1 Pressures

The maximum variation between high and low levels in storage structures providing pressure to a distribution system should not exceed 30 feet. The minimum working pressure in the distribution system should be 35 psi (240 kPa) and the normal working pressure should be approximately 60 to 80 psi (410 -550 kPa). When static pressures exceed 100 psi (690 kPa), pressure reducing devices should be provided on mains in the distribution system.

7.3.2 Drainage

Storage structures which provide pressure directly to the distribution system shall be designed so they can be isolated from the distribution system and drained for cleaning or maintenance without necessitating loss of pressure in the distribution system. The drain shall discharge to the ground surface with no direct connection to a sewer or storm drain.

7.3.3 Level controls

Adequate controls shall be provided to maintain levels in distribution system storage structures. Level indicating devices should be provided at a central location.

a. Pumps should be controlled from tank levels with the signal transmitted by telemetering equipment when any appreciable head loss occurs in the distribution system between the source and the storage structure.

b. Altitude valves or equivalent controls may be required for a second and subsequent structures on the system.

c. Overflow and low-level warnings or alarms should be located at places in the community where they will be under responsible surveillance 24 hours a day.


| General | | Design | | Source | | Treatment | | Chemical Application | | Pumping Facilities | | Finished Water Storage | | Distribution Systems |
Appendix A Contents
Back to codes
Next section