BACKGROUND
Provisions for water storage is provided to:
a) Have sufficient water for fire demands
b) Have an available quantity to meet the demand
c) Allow the water treatment process to be set at a given flow rate
d) Reduce the cost associated with meeting short-term peak flow rates
e) Reduce the need for additional pumping stations
f) Meet water demands when the treatment facilities are unable to provide water due to a power failure, maintenance activities, etc.
g) Blend water sources

Water supply systems must be managed to protect the water against deterioration between the time it leaves the plant and the time it reaches the consumer.

The storage of a treated water supply depends upon the proper design, construction, water quality maintenance, and operation and maintenance of water storage tanks to provide protection of the treated water in the storage system.

Design and Construction
The water storage system should be designed and constructed to prevent the introduction of substances that impact the water quality such as
a) making them inaccessible to the public;
b) providing air vents that are screened;
c) providing covered storage tanks to lessen the chance of air borne, bird, animal or such debris from entering;
d) protection from rain and surface drainage;
e) lined or specially coated tanks where the materials are corrosion resistant or should minimize corrosion. whenever the investment is justified.

Types of Water Storage Tanks and Reservoirs
There are many ways to store treated water. Typically, local geological considerations, weather, fire demands, the availability of high-capacity wells, etc. dictate the type of storage vessel. The picture to the left is the inside of a 5 MG reservoir under construction. Some of these types are:

a) Steel tanks which are most commonly ground level or elevated; must be coated to reduce corrosion impacts
b) Concrete cast-in-place tanks are ground level, cast in place over a web of reinforcing bar in large forms.
c) Concrete reservoirs are concrete lined earthen depressions

Reservoir and Tank Location
Pressure loss due to friction losses in the main, generally influence the location. 35 to 100 psi pressure in the water distribution system. (Remember that 1 psi = 2.31 ft of water height)

Operationally, and for maintenance reasons, several small tanks (as shown by the image of the two smaller tanks side by side) are preferable to one or two large tanks:
a) lessens the pressure differentials in the system
b) allows greater flexibility in maintenance and operation.

There are two broad types of storage applications:
a) Tanks are usually placed in an elevated portion of the distribution system, whenever possible, as illustrated by the picture of the Government Hill Water Tank, Anchorgae Alaska, which is still standing today.... survived a 9.2 earthquake.... (the nearby elementary school split into two parts when the gound split beneath it!)
b) Ground level storage applications are not as desirable, as it requires the constant pumping from the storage vessel, and emergency power for the pumping requirements.

In either case, there are the economies of scale to be taken into account: the relationship of the quantity of water to be stored, site constraints, capacity (function of height) vs/surface area of the floor, etc

Communities are recognizing the need for aesthetic values, and often hillside and elevated tank locations that are quite visible, are not desirable. Efforts to minimize the visual impact include: landscaping and constructing earthen berms to hide the vessel’s outline. Multiple uses, such as parks, open spaces, etc. are being considered. Roofs of ground level storage tanks are serving other uses such as parking lots, parks, tennis courts, etc.

Tank and Reservoir Components
The following are components in a tank or reservoir installation:
Valve vault

Inlet pipe: in elevated reservoir, pipe is both inlet and outlet
Outlet pipe: w/silt stop; ground level have separate inlet and outlet.
May have one pipe for both inlet and outlet
Drain Pipe
Overflow pipe: in case of failure of water level controls - air gap w/ sewer or storm drain- have splash plate to reduce erosion.
Vents: ½" screen, smaller plugs
Instrumentation
Water level and level alarms, and may have chlorine residual, etc. depending on the system
May control pumps for supply
Ladders and access hatches: safety cages, rest platform ½ way up; ladders also inside w/ platform.
Lighting: depends on height of structure and location w/ FAA regulations. Potential good neighbor problems w/ lighting.

Operation and Maintenance
Cold weather - turn over the storage water contents to reduce ice buildup, and reduce water age, and potential THM increase.
Warm weather - turn over to enhance water quality - chlorine residual or water age; reduce water temperature if a small metal reservoir.

Operation is usually controlled by manual adjustment of treated water flow rates, or by automatic control of pumping facilities based on water demand.
Manual control: calculation of expected demand, and adjustment of water treatment production based on experience.
Automatic control may be accomplished by:
a) rising/falling tank levels initiate well pumping sequences
b) rising/falling tank levels initiate clearwell pumping sequences and water plant production

Maintenance activities may include:
a) Disinfection of all newly-constructed, repaired or recently opened tanks, as per the AWWA standard C652.
1) 6% of tank w/50mg/L chlorine for 6 hrs, then fill hold for 24 hours
2) 10mg/L for 6 hours if previously chlorinated; 24 hr detention time if not chlorinated prior.
3) Brush on or spray on a 200 mg/L chlorine solution (air-pac application method)

Bacteriological test: must pass first or two consecutive passes before placing in service.

b) Maintenance of a measurable chlorine residual in all parts of the system at all times to protect the water distribution system, and to minimize the growth of biological slimes in the system. This includes the water storage components.

c) Regularly scheduled cleaning of all reservoirs and storage tanks, depending on the type and rate of accumulation of deposited solids. May be accomplished with divers while the storage unit is in service or may be removed from service and hosed out.

d) Regularly scheduled inspection of the coatings, and structural components all reservoirs and tanks. (As shown on the picture to the left, some rust started showing after a few years. This was properly recoated.)

e) Avoidance of reservoir draw-down to a point of limiting water demand response. If a reservoir is "over-drawn" you must disinfect it, according to the regulations covering reservoir disinfection, prior to placing it back into service!

f) Cathodic protection for water storage tanks that cannot be protected against corrosion by either water treatment practices or by corrosion-resistant coatings. High silicon, cast-iron anodes in rings (w/117volt input, 12 amp, 12 volt output cathodic protection system)

g) The creation and maintenance of a water storage record-keeping system that records all operational, maintenance, and related items to the system.

Monitoring Water Quality
Assuring water quality throughout the entire water distribution system is assured by:
a) establishment and execution of a sampling plan, which includes tanks and reservoirs
b) sampling for chlorine residuals, THMs, etc. as dictated by the water system requirements

Where chlorine residuals are difficult to maintain in the distribution system, the following may be discussed for implementation:
a) Higher chlorine residuals at the treatment facility usually result in higher THM formation in the distribution system.
b) Lower chlorine residuals at the treatment facility, with a corresponding re-chlorination at storage facilities, may help maintain lower THMs and yet maintain a better chlorine residual in the entire distribution system.
c) Water quality impacts by water storage facilities are lessened by water "turn-over rates" or the detention time that the treated water is in the storage facility.

We will discuss the distribution system piping by itself later. It is important to recognize that the distribution piping works in concert with the storage facilities, and must be operated as the total system that it is.

SAFETY
a) Elevated tank training - falls from elevated heights - harness/lines
b) are most always confined spaces
c) portable power tools - power
d) Personal protective gear: SCBA, gloves, etc as required

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