Chapter Eight: Drinking Water Storage Tanks

1 Chapter Eight: Drinking Water Storage Tanks

The main function of Service Reservoirs (SR) or Storage Tanks is to cater for daily demands and especially peak demands of drinking water. Operator checks the amount of drinking water in the storage reservoir and the corresponding water levels at particular times of the day. Procedures for operating the Service Reservoir will depend upon the design of its storage capacity and on the water demand balance.

1.1 Procedures for Operation of Service Reservoirs

Service Reservoirs have to be operated as per the design requirements. Generally, the service reservoirs are constructed at elevated places to supply water during periods of high water demand and hence the SRs are filled in low water demand period. At times pumps may be used only for filling the SR before the next supply timing or can be used also during supply hours to maintain the levels in the SR. Normally, small changes in the distribution system such as pipeline extensions or the addition of few more connections will not require additional storage requirement. Major system changes such as addition of larger size of main pipelines and increase in large number of connections may require additional storage.

1.1.1 Operation of Service Reservoirs during Abnormal Conditions

Abnormal operating conditions arise:

• Whenever demand for water goes up suddenly due to fire demand, or due to excessive demand on one command area/zone of a system,

• Due to failure or breakdown of water supply of another zone of the distribution system,

• Breakdown or out of service pumps or pipelines or power breakdowns or out of service SRs.

The operator must have a thorough knowledge of the distribution system emanating from the SRs. Closure or adjustment of valves at strategic points in the distribution system can focus or divert the flow of water towards the affected areas. Emergency plans must be developed in advance to cope with such situations.

1.1.2 Storage Level and Capacity

Most of the distribution systems establish a pattern of levels for assuring the required supplies at the required pressures. The maximum water levels to be maintained in the SR at each morning should be known to ensure that the system demands are met for the day. It is also desirable to have an indication of levels of SR in the pump house or any appropriate place easily accessible by the operator through use of SCADA systems.

Usually water levels are read at the same time each day and the readings recorded. Checks of water levels at other times of the day will enable one to determine if any unusual consumption conditions have occurred. If any significant increase in consumption is anticipated the operations should ensure a corresponding increase in supply into the SR.

In case of intermittent supply, timings for supply of water in the areas are fixed in advance in large command areas. The water can be supplied to sub-zones during particular fixed hours by operation of the necessary valves.

Routine valve operations are normally done at the SRs. Problems in operation of valves in SRs can also be caused by valve seat getting jammed, and hence it cannot be opened, or non-seating of valves, and hence cannot be closed properly. Sometimes two valves are fixed in series on the outlet and the downstream valve only one is usually operated. Whenever the valve under operation is jammed, the upstream valve is closed and the jammed valve is repaired. Such an arrangement enables repair of valves without emptying the SR.

In some SRs a bypass line is provided direct from the inlet line to the outlet line for drawing water without feeding the service reservoirs (SR). Identification of the valves as to their intended purpose such as inlet, outlet, scour/washout, by-pass etc. and their direction of opening are to be prominently marked. The operator/manager shall ensure that all valves in a SR are in good working condition and are operated as per the schedule for such operation.

1.1.3 Water Quality at Service Reservoirs

Water from all SRs should be periodically sampled to determine the quality of water that enters and leaves the SR. Sampling data can help in setting up periodic cleaning of SR. Common cause of physical water quality problems includes collection of sediment, rust and chemical precipitates. Water quality in the SR may also deteriorate due to excessively long periods of stagnant conditions. Whenever seasonal demand rises, residual chlorine to be maintained properly.

1.2 Plans for Operation and Maintenance of Service Reservoirs

The plan for O&M of the service reservoirs shall contain operational procedures, maintenance procedures and the manufacturer’s information in respect of the instruments/devices.

1.2.1 Procedures for Operations

The operational procedures will inter-alia contain:

(a) Information of design details for the reservoir such as: capacity in litres, size and depth of storage; size of piping/locations of control valves of inlet, outlet, scour and overflow; source of feeding the reservoir; hours of pumping or gravity feeding into the reservoir; rate of flow into the reservoir; hours of supply from the reservoir and quantity to be supplied from the reservoir; areas to be served/ supplied; highest and lowest elevations to be commanded from the SR and the water levels to be maintained in the SR for command of the entire area,

(b) Key plan showing the alignment of pipe connections, by pass lines, interconnections and location of valves, flow meters, pressure gauges and alignment of out-fall drain to lead off the scour and overflow water from the reservoir,

(c) Schedule of suppliers’ names, addresses and telephone numbers of the equipment installed in the SR such as valves, flow meters, level indicators, etc.,

(d) Step by step operating instructions indicating how to operate and control various valves located on the inlets and outlets, so as to ensure the required quantity of water is supplied to the command areas at the desired pressures during the period required to be displayed,

(e) A record sheet for each valve showing direction for turning, number of turns, inspections, repairs and whether opens or closed. The direction of operation of valves shall be clearly marked as “open” or “close”,

(f) The name of the valve and piping such as washout, inlet, outlet, by pass, overflow etc. shall be painted clearly and repainted regularly. In the case of mechanized operation of valves, the steps to include starting, running and stopping the operations,

(g) Different inlet pipes in the service reservoir from different source should be marked with different colour paint.

1.2.2 Maintenance of Service Reservoirs

• Service Reservoirs (SRs) have to be inspected regularly and the line department can prescribe frequency of inspections. Inspection can be done once every two weeks and once a month during the rainy and dry seasons respectively.

• Leakage from structure of SR and through the pipes and valves has to be attended to on priority. It is advisable to resort to pressure grouting to arrest leaks from structures, and

• Sometimes an additional coating of cement mortar plastering is also done using water proof compound to arrest leaks from the structure.

Maintenance is concerned with mainly protection against corrosion both externally and internally. Corrosion of roof slab of reinforced cement concrete (RCC) reservoirs due to the effect of chlorine is also common. Internal corrosion is prevented by cleaning and painting at regular intervals. Toxic paints should not be used for painting interior surface of SRs. Food grade epoxy painted shall only be used for internal surface of SRs. Anticorrosive painting (epoxy) is also done to the interiors when corrosion due to chlorine is expected. Painting of steel tanks once in a year and external painting with waterproof cement paint for exteriors of reinforced concrete tanks once in 5 years is usually done. The inside of painted SR shall be disinfected before putting into use for a period sufficient to give chlorine residuals of at least 0.2 mg/l. Manhole covers and vent pipes shall always be properly placed and maintained

The maintenance procedures shall include step by step procedure for every piece of equipment in SRs such as pipes inside the tank (In-let, out-let, wash-out, over-flow) valves, specials and flow meters following the procedures as per the manufacturers’ catalogues.

(a) Pipes (Inlet, outlet, washout, overflow) and specials

(i) All the pipe fittings should be leak proof, any leakage nearby reservoir may affect the safety of reservoir,

(ii) Overflow pipe should be connected with the distribution system after the sluice valve installed on delivery pipeline,

(iii) Concrete platform as protection works shall be provided around the service reservoir, if not provided, so as to safeguard the reservoir foundation from any leakages/overflow of water.

(b) Valves

All valves should be inspected regularly in specified frequency of inspection and following activities shall be undertaken:

(i) Lubrication is required to be done regularly,

(ii) Spindles that develop leaks should be repacked,

(iii) Rust and sediment in the valve is removed by shutting the disc hard in the seat, then opening about a quarter way and closing tightly several times; the increased velocity usually flushes the obstructions away,

(iv) Valve chambers of the SR also require maintenance to ensure that the interiors of chambers are not silted up and also ensure that the covers are in good condition and are in position,

(v) Sluice valve chamber shall not be water logged.

(c) Cleaning of Reservoirs

Routine inspection is the best way to determine when a tank requires maintenance and cleaning. A visual inspection can be made from the roof manhole with water level lowered to about half full or less. Alternatively a detailed inspection can be made after draining the tank and then cleaning or washing. Best time of the year to take up cleaning of SRs is during the period of lowest water consumption. The following activities are normally involved in cleaning of a tank/SR:

(i) Make alternate arrangement for water supply to consumers served by the SR,

(ii) Close the inlet line before commencing cleaning of SR,

(iii) Do not empty SR. and always keep minimum water level at 200-300 mm in the SR,

(iv) Close the outlet valve so that no water will be used while the tank is being cleaned,

(v) Drain and dispose of the remaining water and silt,

(vi) Wash the interior of tank walls and floor with water hose and brushes,

(vii) Inspect the interior of walls and ceiling of tank for signs of peeling off or deterioration,

(viii) Apply disinfectant (Supernatant of Bleaching powder) to the walls and floor before start of filling the tank/SR,

(ix) The higher frequency of cleaning of SR depends on the extent of silting, development of bio films and results from water quality monitoring. Generally cleaning of Service Reservoir may be periodically done,

(x) Date of last cleaning and the next due date of cleaning may be displayed on the outer surface of the SRs.

1.2.3 Records and Reports

1.2.3.1 Record System

A record system has to be developed which should be realistic and apply to the operating problems involved at the particular SR site. The most efficient way to keep records is to plan what data is essential and then prepare the formats followed by the persons to fill the data, frequency and to whom the record is to be sent for review and report. Sample records to be maintained at a SR site are given below for guidance. The following details shall be recorded:

1.2.3.2 Records of Maintenance

The records on each of the following maintenance/repair works along with the cost of materials and labour shall be maintained along with date:

(a) Water levels in the SR,

(b) Time and relevant operation of control valves with time of opening and closure or throttling position of the valves,

(c) Daily flow meter readings both on the inlets and outlets,

(d) At least one a day Residual chlorine readings of inflow water and outflow water.

(e) Gland ropes of the valves/Spares at the SR were changed,

(f) Manhole covers were changed/replaced,

(g) Water level indicator was repaired or replaced,

(h) Reservoir was cleaned,

(i) Out-fall drain for scour and overflow was last cleaned,

(j) Ladder was changed, when the structure of the reservoir was last repaired to attend to structural defects or arrest leakage,

(k) Reservoir/Pipes was last painted,

(l) Total cost of repairs and replacements at the SR in previous year along with breakup of material cost and labour cost with amount spent on outside agencies for repairs and replacements.

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