Pumping facilities and treated water storage

Chapter 7: Pumping facilities

The requirements described in this chapter apply to raw and treated water pumping stations and booster pumping stations. Pumping facilities should be designed to maintain the quality of pumped water, for example, by minimizing retention time and ensuring adequate flows and velocities in the distribution system. Appropriate design measures to help ensure the security of water pumping facilities should also be incorporated (Section 3.24 Security).

7.1 General

The design of the pumping station is also governed by a number of acts and regulations that do not fall under the jurisdiction of the ministry, including but not limited to Ontario Ministry of Labour regulations, building, electrical, and fire codes.

Groundwater well pumps and backwash pumping are discussed in Chapter 4 Source Development and Chapter 5 Treatment, respectively. Positive displacement pumps used for chemical pumping are discussed in Section 6.2.6 Chemical Feed Equipment and Control.

7.2 Station types

The three types of pumping facilities addressed in this chapter are raw water pumping (commonly called low lift pumping), treated water pumping (commonly called high lift pumping) and booster pumping stations.

Pumping stations commonly use either horizontal centrifugal pumps, vertical turbine pumps or submersible pumps. Typically, horizontal splitcase centrifugal pumps are equipped with side suction and side discharge, while larger units may have bottom suction. Refer to the Hydraulic Institute (HI) ANSI/HI Pump Standards for appropriate uses of different pump types.

7.3 General design considerations

7.3.1 Firm Capacity & Station Capacity

Raw water pumping stations should be provided with firm capacity, which is defined as:

• Capacity of the raw water pumping station able to supply the water treatment plant design capacity with the largest unit out of service.

Consideration should also be given to other circumstances such as distance from the water treatment plant and the availability of raw water storage.

Treated water and booster pumping stations should be provided with firm capacity, which is defined as:

• Capacity of the pumping station with the largest unit out of service if the station supplies a pressure zone with adequate storage available for fire protection and balancing. Sizing of storage facilities is discussed in Section 8.4 Sizing of Storage Facilities; and
• Capacity of the pumping station with the two largest units (including the fire pump[s], if any) out of service if the pumping station serves a pressure zone that does not have adequate floating storage available and is the sole source of supply in the area.

Pumping station structures, major piping and appurtenances should be designed for at least the 20-year estimated flow or if practicable, for the ultimate service area requirements. Alternatively, the initial design should be such as to permit expansion to the ultimate capacity. The initial design should allow for additional pumping units, standby power facilities, transformers, and other mechanical, electrical and treatment equipment as will be required in future. Consideration should also be given to the potential consequences of over-sizing of the initial equipment, especially with respect to metering devices.

7.3.2 Site Considerations & Protection

System hydraulics, protection against interruption of service by fire, flood, freezing or any other hazard should also be considered during site selection.

The station should be elevated to a minimum of 1 m (3 ft) above the 100-year flood elevation, or 1 m (3 ft) above the highest recorded flood elevation, whichever is higher, or protected to such elevations, unless a hydraulic analysis is completed to identify a more appropriate elevation. Consideration should also be given to the requirements of the local Conservation Authority (visit the Conservation Ontario website for more information).

The station should be readily accessible at all times. To allow for servicing, vehicle access to the pumping station should be provided. The grade around the station should lead surface drainage away from the station. The distance from the pumping station exit, as well as the need for additional exits, is governed by the Building Code (O.Reg. 350/06) made under the Building Code Act, 1992. Protection to prevent vandalism and entrance by animals or unauthorized persons should be provided.

Pumping stations should have adequate space for the safe servicing of all equipment. Stations should be of durable construction, fire and weather resistant and have outward-opening doors. Where permitted by code, inward opening doors may be considered. Floor elevation should be at least 150 mm (6 in) above finished grade. The underground structure should be waterproofed. All floors should slope to a suitable drain and drain in such a manner that the pumped water will not be contaminated. Building drains should be constructed in such a way as to prevent any potential break or leakage from entering any wet well structure. Building drains should not be built within water bearing structures. Means for drainage from pump glands without discharging onto the floor should be provided. A sump pump should be supplied to ensure that any miscellaneous water entering the station is removed. Sump pump systems should be alarmed for flood conditions. A vent or a vent system should be provided for all process related structures (i.e., wet well). The vents should be equipped with a 180o bend with insect screen at the outlet. For vents on critical structures, the design should incorporate solid/liquid protection from entry into the vent by vandalism or sabotage.

If diesel or other fuelled engines are to be used for standby power, the requirements of Section 9 of the Environmental Protection Act (EPA) must be satisfied and an application for air approval may be necessary (Section 3.13 Emissions of Contaminants to Air). If the isolation from the engine exhaust to the nearest point of impingement is not sufficient to dissipate the air contaminants to within the regulated levels, an exhaust stack may be required.

Any electrical controls, switch gear, or transformers located outside the pumping station should be properly housed and fenced in accordance with the local hydro requirements.

7.3.3 Pumping Wet Wells

The floor should be sloped to a sump for easy cleaning and draining. The well should be covered and protected from contamination. Well access routes should be adequately sealed against liquid penetration. A 100 mm (4 in) curb should surround any floor openings to prevent floor drainage entering the well and the well should be adequately vented. The wet well should be designed with two pumping compartments or other means to allow the well to be taken out of service for inspection, maintenance or repair, while still maintaining a portion of the station capacity in service.

The design of the wet well will depend substantially on the number, type and size of the pumps required. The design should conform to the recommendations of the Hydraulic Institute standard ANSI/HI 9.8-1998 Pump Intake Design.

7.3.4 Equipment Servicing

Minimum spacing around pumps, piping, and fittings should comply with the requirements of the Building Code (O.Reg. 350/06) made under the Building Code Act, 1992, and the ANSI/HI Pump Standards. Pumping stations should be provided with lifting devices such as crane-ways, hoist beams, eyebolts or other facilities for servicing or removal of pumps, motors, valves, piping or other heavy equipment without the need for heavy manual labour and with minimum disturbance to the system. Cranes and any other lifting devices should be rated to a capacity exceeding that of the heaviest equipment unit anticipated with an appropriate safety margin. Openings in floors, roofs or wherever else needed should be provided for removal of heavy or bulky equipment. Such openings should be designed to bear the loads of equipment traversing them.

A convenient tool board, or other facilities as needed, should be provided for proper maintenance of the equipment.

7.3.5 Stairways & Ladders

Stairways or ladders should be provided between all floors and in pits or compartments which must be entered, or as required by the Building Code (O.Reg. 350/06) made under the Building Code Act, 1992.

Stairways or ladders should have handrails on both sides and treads of non-slip material. Stairs are preferred in areas where there is frequent traffic or where supplies are transported by hand. Stair and tread dimensions should conform to the requirements of the Building Code (O.Reg. 350/06) made under the Building Code Act, 1992.

Stainless steel may be considered for submerged conditions.

7.3.6 Heating, Ventilation & Dehumidification

Provisions should be made for adequate heating for the comfort of the operator and the safe and efficient operation of the equipment. For pumping stations not occupied by personnel, only enough heat need be provided to prevent freezing of equipment or treatment processes.

Ventilation should conform to existing local and/or provincial codes. Adequate ventilation should be provided for all pumping stations for operator comfort and dissipation of excess heat from the equipment. Forced ventilation of at least six changes of air per hour at the high fan speed and three changes of air per hour at the low fan speed (regular operation), and manual-off position should be provided for all confined rooms, compartments, pits and other enclosures below ground floor, as well as any area where unsafe atmosphere may develop or where excessive heat build up may occur.

In areas where excess moisture could cause hazards to safety or damage to equipment, means for dehumidification should be provided.

7.3.7 Lighting

Pumping stations should be adequately lighted throughout. Moisture resistant lighting should be considered. All electrical work is governed by the requirements of the Electrical Safety Code, (O.Reg. 164/99) under the Electricity Act, 1998. More information regarding electrical systems is provided in Section 3.10 Electrical Components. Emergency lighting and illuminated exit signs should be provided at appropriate locations in case of power failure.

Provision of multiple lighting levels may also be considered for various duties, such as minimum lighting for walkthroughs and minor checks and high-level lights for major maintenance.

7.3.8 Sanitary & Other Conveniences

All pumping stations that are staffed for extended periods on a regular basis should be provided with potable water, lavatory and toilet facilities. Plumbing should be so installed as to prevent contamination of a public water supply. Sanitary wastes should be discharged in accordance with Section 3.27 Water Treatment Plant Residuals and Sanitary Waste.

Eyewash and safety showers should be provided where potentially harmful chemicals are present in the pumping facility.

7.3.9 Controls

Pumps, their prime movers and accessories should be controlled to ensure that they will operate in such a manner that will prevent motor overload. Control systems should limit the number of start/stop sequences (according to the pump and electrical equipment manufacturer recommendations) and prevent coincidental simultaneous starting of numerous pumps. Where duplicate pumps are installed, provision should be made for alternation of service cycles. Provision should be made to prevent energizing the motor in the event of a backspin cycle. Electrical controls should be located above grade. Equipment should be provided, or other arrangements made, to prevent hydraulic surge pressure from activating controls which switch on pumps or activate other equipment outside the normal design cycle of operation.

The type of control for pump operation is an important consideration for pump specification and selection, and will depend on whether the pump(s) are part of an open or closed pumping system.

In closed systems (treated water, booster pumping), a control valve (not normally supplied by the pump manufacturer) should be provided to ensure proper operation of the pump. An air release valve should also be provided in the pump discharge to let the air out of the discharge pipe at pump start-up.

Pressure control is commonly used for pump operation in both open systems and closed systems. However, care must be taken for pump selection when this type of control is used as it can have a significant effect on the operation of a pump, specifically, the designer should aim to select a process pump characterized with a steep flow rate versus total dynamic head curve (pump curve) or ensure that the pump does not operate on the flat part of the curve. A combination of flow control and pressure control may be used in smaller systems. Temperature and level sensing controls may also be required. Whatever control system is utilized, operation of the pumps near their maximum efficiency points should be maintained.

An adequately sized pressure relief by-pass may be required to minimize pump cycling and prevent pump damage for pumps operating in the shut-off head condition. More information regarding pump control systems and automation is provided in Section 9.6 Automated/ Unattended Operation.

7.3.10 Standby Power

Dedicated standby power is required to ensure that water may be treated and/or pumped to the distribution system during power outages to meet the average day demand. Alternatives to dedicated standby power may be considered with proper justification.

The need for standby power and the extent of equipment requiring operation by standby power should be individually assessed for each treatment plant (Section 3.12 Standby Power). The following approach should be adopted:

• In cases where no or inadequately sized floating storage is available for fire protection, it is recommended that full standby power be supplied. Typically, the size of the pumping stations in these instances is small and the standby power requirements low. As a result, it is normally the most economical approach to use standby diesel or natural gas generator sets; and
• Where adequate floating storage is available, the need to utilize standby power is less critical. A common approach is to provide sufficient standby power for the pump capacity equal to the average day demand rate. This should supply adequate quantities of water required in the event of a major power outage.

If standby power is required, it should be provided by means of an emergency standby generator set preferably; for small systems, a direct drive engine may be acceptable. The designer is referred to the AWWA Emergency Power Source Planning for Water and Wastewater publication.

The fuel storage and fuel lines should be designed to protect the water supply from contamination (Section 3.12.1 Diesel Fuel Storage) and in accordance with applicable Ontario regulations which govern the design and installation of fuel storage tanks.

Carbon monoxide detectors with audible alarms should be provided when generators are housed within pumping stations. Sound studies may be required to ensure that the operation of standby power equipment does not exceed ambient noise level limits. In some cases, noise attenuation measures may be required.

If the generator and motor are not sized to simultaneously run all equipment in the pumping station, non-essential equipment should be on separate power circuits which are not energized by the standby power unit.

7.3.11 Transformers

Suitable transformers should be supplied to meet all requirements in the pumping station. For larger pumping stations, it is recommended that either dual transformers and switchgear be supplied, or standby power be provided, such that continuous operation of at least half of the pumping station can be maintained.

7.3.12 Automatic & Remote Controlled Stations

All automatic stations should be provided with automatic alarms which will report when abnormal conditions or equipment faults occur. All remote controlled stations should be electrically operated and controlled and should have signaling apparatus of proven performance. Installation of electrical equipment is governed by the Electrical Safety Code (O.Reg. 164/99) under the Electricity Act, 1998.

7.3.13 Safety

Stations should be designed in such a manner as to ensure the safety of the operators and maintenance staff in accordance with the Confined Spaces Regulation (O.Reg. 632/05) made under the Occupational Health and Safety Act (OHSA). Typically, the following points should be considered:

• Any moving equipment should be covered with suitable guards to prevent accidental contact;
• Equipment that starts automatically should be suitably signed to ensure that operators are aware of this situation;
• Local lockouts on all equipment should be supplied so that maintenance personnel can ensure that they are completely out of service;
• Local emergency stops should be provided;
• Provision of fire/smoke detectors, fire extinguishers and sprinkler systems (where appropriate);
• All stairways and walkways should be properly designed with guardrails; and
• Minimizing confined spaces, where applicable.

More detailed information regarding safety should be obtained from the Ontario Ministry of Labour.

7.4 Pumping considerations

Pumps should be specified so that the full range of flows anticipated can be provided with pumps operating in the vicinity of their optimum efficiency points, with due regard to the hydraulic design of the discharge piping. This is often accomplished by selecting pumps which have wide band efficiencies and an appropriate operating curve.

The number of pumps should be consistent with the pattern of flow required and the method of flow control. It is recommended that at least three pumps be provided for operating flexibility; a minimum of two pumps is required, one as a redundant standby, with consideration of firm capacity of the pumping station (Section 7.3.1 Firm Capacity and Station Capacity).

The station design should allow for future additional pump units and where possible, the piping should be large enough for an increase in pump size to be accommodated. Adequate space should be provided for the installation of these additional units and to allow safe servicing of all equipment.

The pumping station design should provide for flooded suctions at all times, otherwise, an adequate priming system should be provided with sufficient capacity to prime pumps within a short period, i.e., 1 to 2 minutes, as recommended by the pump manufacturer. To accomplish this, the designer should carefully review potential operating levels of wet wells or storage reservoirs and pump elevations.

Pumps should be selected which have maximum efficiencies at the average head condition, but which can meet the maximum flow and pressure conditions. Particular attention should be paid to pump suction piping design to ensure net positive suction head (NPSH) available exceeds pump specifications with respect to NPSH required to avoid cavitation.

Adequate control should be provided which is capable of controlling pump operation over the entire range of flows expected. Where this is accomplished by control valves, the valves should be located on the pump discharge to maintain stable control and avoid cavitation. Other more energy efficient types of control, such as variable frequency drive systems, may be preferable provided that they allow for stable pump control. Alternatively, multiple pumps of different sizes or variable capacities may be provided to cover the expected range of flows and to ensure that they operate at their optimal efficiencies.

Large water systems (raw and treated) typically operate using pressure (level) control. For small water supply systems, where substantial seasonal variations in flow exist, it may be necessary to provide duplicate flow and pressure pump control systems one suitable for very low flows (which normally occur in winter) and one suitable for the design maximum flows.

Pumping station headers should be adequately protected from transient pressure surges which may occur if pumps stop on power failure. Protection may be provided either by appropriate valves or hydraulic transient surge tanks. If pump discharge valves are provided, they should be slow acting type and properly controlled to avoid high transient pressures in the system on opening or closing. These valves should be interlocked with the pump operation to provide such protection (Section 7.7.4 Surge Arrestor Systems).

7.4.1 Raw Water Pumping

For raw water pumping stations remote from the treatment plant, the designer should consider the use of slow opening pump discharge valves to minimize hydraulic transients.

The raw water wet well should be provided with an overflow of sufficient size or adequate surge volume to handle intake surges which occur on power failure (when all pumps stop). When designing to handle intake surge, a minimum Hazen-Williams coefficient of one hundred and fifty (C =150) should be applied. All electrical equipment and pump motors, except for immersible and submersible pumps, should be located above the surge water level.

Provisions should be made to prevent downsurge during hydraulic transients after power failure, which may cause problems if pump suctions become exposed and air becomes entrapped in the raw water pump suction piping.

7.4.2 Treated Water Pumping

The minimum number of pumps to be provided is two, in addition to any pumps required to provide fire flows. The minimum capacity should be equal to the maximum day demand and the actual capacity will be dictated by the distribution system and storage design and capacities. In each case, the firm capacity should be equal to or greater than the design capacity required. In case of vertical turbine pumps, the pump intake should be deeper than the lowest reservoir operating level and air release valves should be installed on the pump discharge.

Where backwash pumps draw from the same well as treated water pumps, the variations in well level and consequent variations in suction requirements should be considered when selecting the backwash pumps.

7.4.3 Booster Pumping Stations

The purpose of booster pumping stations is to maintain adequate pressures and flows in water distribution systems as a result of both changes in ground elevation and distance from the source of supply. This section addresses booster pumping stations of the two most common types: without associated storage for the service area (in-line), and with associated storage.

Booster pumping stations should be designed to service specific areas of a water distribution system based on defined limits. These areas are generally isolated from the remainder of the system by control valves.

Booster pumping stations may have incorporated with part of their operation, elevated or ground storage reservoirs which will serve to supply extreme demands, such as peak hour, fire flow and other emergency requirements.

Pumps taking suction from ground level storage tanks should be designed with adequate NPSH.

Booster pumps should be located or controlled so that automatic shutoff or low pressure controller maintains at least 140 kPa (20 psi) in the suction line under all operating conditions. A valve to control the pressure across the pump should be considered if suction and discharge pressures vary. Pumps taking suction from ground level storage facilities should be equipped with automatic shutoffs or low pressure controllers as recommended by the pump manufacturer, and the suction piping should be designed to allow for complete use of the full amount of water in the storage facility. Automatic or remote control devices should have a range between the start and cutoff pressure, or a cut off based on the low water level in the storage facility, which will prevent excessive cycling. A pump bypass should be available and alarms should be installed for such conditions.

Each booster pumping station should contain not less than two pumps with capacities such that firm station capacity can be satisfied with the largest pump out of service. In addition to the other requirements of this section, in-line booster pumps should be accessible for servicing and repairs.

Booster pumps should not be allowed for any service from the public water supply main, unless owned and operated by the water authority or if an air gap or backflow preventer upstream of the booster pump is provided.

Booster pumping stations, either alone or in conjunction with storage, should be capable of meeting the various demand requirements of the area being serviced. Analysis should be undertaken to determine each of the following conditions:

• Peak hourly flows when consumption is highest;
• Night flows when the consumption rate is at its lowest value and reservoirs remote from the source of supply are being filled by the booster station; and
• Fire flows which can occur at any time and which must be added to the maximum day rate.

Discharge pressure from the pumping station should be adequate to ensure that the pressure in the district to be served is within the range of 275 kPa (40 psi) and 700 kPa (100 psi), during peak and minimum demand periods. In the case of fire flows, it may be acceptable to allow the pressure in the system to drop to a level no lower than 140 kPa (20 psi).

If no or inadequate storage is available, the proposed booster pumping station should be designed in a manner that will allow it to supply all of the extreme flow conditions mentioned above.

Booster pumping stations serving pressure zones with adequate storage should be designed for the maximum day rate, as it may be cost prohibitive both in terms of pumping station capacity and watermain design to supply all extreme flow conditions directly from the booster pumping station (Chapter 8 Treated Water Storage).

7.5 Pumping considerations for systems serving fewer than 500 people

7.5.1 Raw Water Pumping

Where low lift pumps are provided on a surface water source, a minimum of two units, each capable of the design flow, should be provided.

Low lift pumps should be submersible or vertical turbine and operation of the pumps should be regulated by high and low water level sensing devices located in the treated water storage reservoir. Control should be provided for the low lift raw water pumps to operate singly or together in automatic or manual modes.

Where filtration with backwash is provided, the low lift pumps should be controlled so that the discharge rate to the treatment units does not exceed the capacity of the treatment unit(s) remaining in operation during backwashing (as applicable). This can be accomplished by either providing sufficient additional storage at the treatment plant to permit complete shut-down of all treatment units during the backwash cycle or by installing a rate-of-flow controller on the low lift pump discharge which throttles/limits flow to the treatment unit to a capacity equal to that of the unit(s) remaining in service. In either case, the controls should be connected to the backwash pump cycle for automatic activation.

Refer to Section 7.4.1 Raw Water Pumping for more information regarding raw water pumping.

7.5.2 Treated Water Pumping

Where high lift pumping is necessary, at least two pumps should be provided with each pump designed to deliver a minimum of the design maximum day flow at the desired head. In many instances, particularly for smaller systems with large flow variations, it may be desirable to provide a third domestic high lift pump with this pump sized to meet a lesser flow than the maximum day requirement of the system. In this case, this pump should be designed to lead during lower flow conditions. During normal periods of domestic demand, the smaller pump would provide an adequate supply of water, while the large pumps would only operate to accommodate higher demands or in the event of failure of the lead pump.

Where fire protection is to be provided via the communal water supply/distribution facility, a third high lift pump (fire pump) should be provided and the capacity of that pump should be at least equal to the minimum required fire flow.

In instances where the distribution system is not provided with an elevated storage tank and a ground storage reservoir located at the site of the treatment facility is the only storage, it may be necessary to provide pump(s) sized for the peak domestic demand, or maximum day demand plus the required level of fire flow. In this case, pump operations should be controlled by pressure switches. Pressure regulating valves (PRVs) with pressure relief to the storage reservoir under low demand conditions should be provided for pressure regulation in the distribution system. In many instances, it may be advisable to provide pressure tanks for pump control in order to minimize the number of start-stop cycles and hence, wear and tear, on the pumping equipment.

Refer to Section 7.4.2 Treated Water Pumping for more information regarding treated water pumping and Chapter 8 Treated Water Storage and Chapter 10 Distribution System Piping and Appurtenances for distribution system pressures requirements.

7.5.3 Pumphouses

For each water supply installation, a pumphouse should be constructed. The designer should refer to all applicable codes and regulations under the Occupational Health and Safety Act (OSHA), the Building Code Act, 1992 and the Fire Protection and Prevention Act, 1997 and local zoning by-laws.

Refer to Section 7.3 General Design Considerations for more information regarding the design of pumping stations.

The design of the pumping station discharge piping should minimize the number of high points. Any high points in the piping system should be equipped with a manually operated air release valve which has been threaded to permit the future installation of an automatic valve should this be found necessary.

7.5.4 Controls

The following controls may be provided between the storage reservoir and the high lift and low lift pumping equipment:

• A low level set-point to shut-off the high lift and fire pumping equipment when the water level in the reservoir drops to a pre-determined low level;
• A high level set-point to shut down the low lift pumps when the water level in the reservoir has reached a pre-determined high level; and
• Level sensors to operate the low lift pumps sequentially.

Pressure switches should be mounted on the discharge line from the high lift pumping station to operate the high lift pumping equipment sequentially. A pressure gauge should also be installed on the discharge of each high lift pump. Elapsed time meters should be provided for all high lift pumps. Output from the high lift pumping station to the distribution system should be metered with a recording type flow meter.

The start-stop operation of the fire pump should be arranged between the municipality/owner and the local fire officials. Indication of the operation status of the pump should be relayed to a central operating point where 24-hour surveillance is provided.

Refer to Section 7.3.9 Controls for more information regarding pumping system controls.

7.5.4.1 Pressure Tanks

Pressure (hydropneumatic) tanks are used in small closed systems to maintain acceptable system pressures without the need for frequent stops and starts of the pumps. They should only be used in very small systems. When considering the use of pressure tanks, the designer should consider the implications of loss of pressure in the distribution system in the event of a power outage or pump failure (i.e., the need to issue a boil water advisory). Pressure tank storage should not be used for chlorine contact or fire protection purposes; fire flow requirements are typically provided by by-passing the pressure tanks. The designer should also consider the impacts of changes in pressure on the operation of the fire pump(s).

Pressure tanks should meet applicable American Society of Mechanical Engineers (ASME) code requirements. The maximum allowable working pressure should be marked on each tank.

Location

Pressure tanks should be located above grade and be completely housed. Enough space should be provided around the tank for inspection and maintenance.

Sizing

The capacity of the wells and pumps in a hydropneumatic system should be equal to the peak instantaneous demand. The active storage volume of the hydropneumatic tanks should be sufficient to limit pump cycling to the manufacturer recommendations. The maximum cycling frequency should be determined for the largest pump when consumer demand is half of the capacity of the largest pump or combination of pumps operated by the same pressure switch.

Hydropneumatic tanks should only be designed for pump control to minimize the number of starts, and should never be used as means for providing disinfectant contact time or storage in any drinking-water system.

Piping

Pressure tanks should have means for isolation to permit operation of the system while a tank is being repaired or maintained.

Appurtenances

An automatic pressure release valve, mechanical means for adding air, including an air filter, and sight glass or other tank level indicator should be provided for maintenance of proper air/water volumes in the pressure tank at all times.

Control equipment consisting of a pressure gauge and pressure operated start-stop controls for the pumps should be provided for the pressure tank system. A shut-off valve should not be installed between the pump and the pressure operated start-stop controls.

The pressure relieving device should prevent the pressure from rising more than 10% above the maximum allowable working pressure. The discharge capacity of the pressure relieving device should be adequately sized. Pressure gauges should have a range of no less than 1.2 times the pressure at which the pressure relieving device is set to function.

An access hatch and drain should also be provided. Where practical, the access hatch should be 600 mm (24 in) in diameter.

7.6 Pumps & Motors

7.6.1 Suction Lift & Priming

Suction lift should be avoided, if possible. If suction lift is necessary, it should be less than 4.5 m (15 ft) and provisions should be made for priming the pumps. To avoid pump cavitation, the NPSH required for a selected pump should be checked against the NPSH available for a given system to ensure that the latter is greater than the former.

Prime water must not be of lesser quality than that of the water being pumped. Means should be provided to prevent either backpressure or backsiphonage backflow.

When an air-operated ejector is used, the screened intake should draw clean air from a point at least 3 m (10 ft) above the ground or other source of possible contamination. Vacuum priming may be used. Compressed air systems should be oil-free and filtered to prevent contamination of treated water.

7.6.2 System Head Curves

The design engineer should determine projected points of operating head and flow for at least the following conditions:

• Average day;
• Maximum day;
• Peak hour; and
• Minimum hour.

Pumps should be selected to ensure that they will operate satisfactorily over the necessary pumping ranges that can be expected at the pumping station (including stages of new development and associated water demand increases in the serviced area). The pumps should be capable of meeting at least the following criteria:

• The rated point which would generally correspond to the maximum day consumption rate;
• The rated point for efficiency evaluation, i.e., the point at which the pump would normally run and at which the pump should be selected for best efficiency;
• The possible operating range which would be the range over which the pump must operate from a minimum total dynamic head (TDH) to a maximum total dynamic head; and
• The minimum submergence level in the case of a vertical turbine unit, or the NPSH required in the case of horizontal centrifugal unit, should also be specified.

All four of these criteria should be evaluated when a pump is being selected. The unit should operate at a TDH considerably less than the ultimate rated point (shut-off head). As a result, the maximum efficiency point should be specified as to be the point at which the pump will normally run.

7.6.3 Constant Speed versus Variable Speed Pumping Units

In certain instances, for pumping stations located in a service area without the provision of adequate storage and where variations in pressure are critical, it may be desirable to use variable speed pumps with pressure control to meet the demand. The provision of variable frequency drives for high lift pumping is particularly advantageous in drinking water distribution systems with little or no system storage. Consideration may be given to operating variable and fixed speed units together to avoid forcing a pump into a shut-off position on its curve.

7.6.4 Water Seals

Water seals should not be supplied with water of a lesser quality than that of the water being pumped. Where pumps are sealed with treated water and are pumping water of lesser quality, either an approved reduced pressure principle backflow preventer or a break tank open to atmospheric pressure should be provided. Where a break tank is provided, an air gap of at least 150 mm (6 in) or two pipe diameters, which ever is greater, should be provided between the feeder line and the flood rim of the tank.

7.6.5 Motors & Starters

Each pump should be operated by a motor capable of operating the pump at any point on the head discharge curve. Pump motors over 15 hp should be certified by the supplier to have undergone standard commercial testing and be rated as "premium" energy efficient. Motors should be located at such a level in the pumping station that they can not be flooded. Alternatively, immersible/submersible motors can be used.

A suitable time delay between pump stop and the subsequent pump start should be provided to allow the shaft to come to a complete stop. Soft start motor starters should be considered when standby power is provided. Staggered pump start is recommended to reduce in-rush load on the generator.

7.7 Appurtenances

7.7.1 Valves/ Check Valves

Pumps should be adequately valved to permit satisfactory operation, maintenance and repair of the equipment. If foot valves are necessary, they should have a net valve area of at least 2.5 times the area of the suction pipe and they should be screened. Each pump should have a positive-acting check valve on the discharge side between the pump and the shut-off valve.

Valves, either of the gate or butterfly type, should be used for pump isolation and control valves (hydraulically activated globe or butterfly) for pump discharge operation. Typically, on larger installations [i.e., 250 mm (10 in) or greater], butterfly valves should be used. Gate or ball valves, especially for suction isolation, may be used for smaller sized piping.

Check valves should be mounted horizontally on pump discharges so that the valve will close slowly and automatically if station or pump flow stops.

7.7.2 Gauges & Meters

Each pump should have a standard pressure gauge on its discharge line and a compound gauge on its suction line. Pressure transducers or pressure indicating transmitters (PITs) for each pump should be provided in larger stations. Means should be provided for measuring the discharge flow of each pump. All stations should have flow rate indicating, totalizing and recording metering of the total water pumped. A magnetic flow meter, an insert Venturi meter or a mechanical meter may be used for this purpose. Flow meters are discussed in greater detail in Section 3.18 Flow Metering. Mercury containing devices should not be used.

Sufficient space should be provided to ensure the accuracy of the metering device (for appropriate length of approach piping, consult the manufacturer recommendations). Valved by-passes should be provided for all meters to facilitate meter maintenance and calibration.

The designer should take into account the piping and orientation requirements for meter installation and application specifications. The designer should also note that air entrainment in process streams can have a negative impact on meter accuracy and functionality, and should be eliminated through air relief designs.

7.7.3 Suction & Discharge Piping

Suction and discharge piping should be designed and arranged in a way that meets the recommendations of the Hydraulic Institute. Piping should be designed so that the friction losses will be minimized. Piping should have watertight joints and the contents must not be subject to contamination or leakage. Piping should be protected against surge or water hammer and provided with suitable restraints where necessary. Piping should be designed such that each pump has an individual suction line or that the lines be manifolded such that they will ensure similar hydraulic and operating conditions.

Suction and discharge piping should be designed and arranged in such a way that it is easily accessible, with sufficient room to service all valves, meters and other appurtenances, and to permit their removal with minimum disturbance to the system. Piping should be arranged to allow ready disassembly from pump to shut off valves, and include a flexible type coupling to permit proper alignment of the piping and pump. Couplings should be adequately protected against thrust. Pump elbows should be supported to remove all bending moments, either steady or shock, from pump nozzles.

The following points should be considered in the design of the various components of the system:

• Suction piping should be designed in such a way to ensure that the NPSH requirements for the pumping unit involved are satisfied (positive suction design is recommended over negative suction);
• Suction piping should be at least one size larger than the pump nozzle, as straight and short as possible, and connected by an eccentric reducer to prevent air accumulation and cavitation;
• Suction piping velocities should be maintained according to Table 7-1:

• An isolating valve should be installed on the suction side of the pump;
• Discharge piping velocities should range according to Table 7-2:

• Piping should be designed to prevent the formation of air pockets; and
• All joints should be restrained in a manner that will not permit joints to pull apart.

7.7.4 Surge Arrestor Systems

A hydraulic transient analysis should be undertaken during the design of pumping stations to ensure that the transients resulting from events such as pumps starting, stopping, and full load rejection during power failure do not adversely affect either the customers on the water system or the piping in the station or the system. Methods of surge protection that can be used to protect stations include:

• Surge anticipator systems that dissipate over-pressure from the discharge lines;
• Slow closing and opening control valves on pump discharges;
• Hydropneumatic surge tanks on discharge headers; or,
• Variable speed pumping units.

Any discharge from such a system may be connected directly back to the water well or storage reservoir or may discharge to a drainage system provided that, in treated water situations, an adequate air gap is included to prevent backflow.

Surge relief valves or slow acting check valves should be provided to minimize hydraulic transients. Slow closing valves should operate during or immediately after power failure. The type and arrangement of check valves and discharge valves are dependent, in some part, on the potential hydraulic transients that might be experienced in the pumping station. In smaller pumping stations, mechanically operated check valves should be adequate. In large stations, consideration should be given to the method of starting and stopping the pumps. An electrically operated butterfly or hydraulically activated globe style valve, coupled with a check valve on the discharge, should be utilized for the starting and stopping sequence of a pump. Other types of valves may incorporate both the isolating valve and check valve characteristics into one common valve; however, suitable isolating valves should be available in the event that maintenance is required on combination type valves.

Chapter 8: Treated water storage

This chapter provides guidance for the design of treated water storage in drinking-water supply systems.

8.1 General

Treated water storage facilities should be designed to allow maintenance of adequate flows and pressures in the distribution system during peak hour water demand, and to meet critical water demands during fire flow and emergency conditions. Storage volumes should be designed based on projected design populations.

One of the most important design objectives for water storage is to minimize the chance of contamination of the treated water. The designer should keep in mind that the purpose of water storage is to ensure continuity of supply and maintain system pressure. The materials and designs used for treated water storage facilities should provide structural stability and durability as well as preserve the quality of the stored water.

In addition to the information in these guidelines, the designer should refer to all applicable AWWA Standards which relate to the design of treated water storage facilities.

8.2 Types of treated water storage facilities

The following are types of treated water storage facilities:

At the water treatment plant:

• Clearwells;
• Reservoirs;
• Pumping wet wells; and
• Pressure tanks.

In the distribution system:

• Elevated tanks;
• Standpipes; and
• Reservoirs.

The type(s) of water storage facility (facilities) selected will depend on many factors such as function, the size of service area, topography, costs, the balance between water treatment capacity and demand, and the amount of storage required at the water plant and in the distribution system.

8.3 Pressure considerations

The minimum required water level and the location for a distribution system storage facility should provide acceptable service pressures throughout the distribution system as described below and as outlined in Section 10.2 Hydraulic Design. System water demands in excess of maximum day requirements are normally met from storage. Storage facilities should be designed to maintain adequate pressure in the distribution system at the average day water demand in the event of a power failure or other emergency. This lessens the potential for groundwater intrusion and contamination of the system.

The maximum variation between high and low levels in elevated distribution system storage tanks should be such that the normal pressures in the distribution system do not go above 700 kPa (100 psi) nor below 275 kPa (40 psi) during normal demand periods. The normal operating pressure in water distribution systems should generally be in the range of 350 kPa to 480 kPa (50 to 70 psi) under maximum daily flow. However, pressures outside of this range may be dictated by distribution system size and/or topography. Pressures as low as 140 kPa (20 psi) may be acceptable when fire demands are experienced in conjunction with maximum day consumption rates. When static pressures exceed 700 kPa (100 psi), pressure reducing devices should be provided on mains or service connections in the distribution system.

8.4 Sizing of storage facilities

Storage facilities should have sufficient capacity, as determined by engineering studies, to meet water demands that exceed the daily water supply capacity of the treatment plant and, where fire protection is provided, fire flow demands. Emergency planning including the provision of stand-by power would also influence the sizing of storage facilities. The water storage requirements (exclusive of storage needed for the operation of the water treatment plant) are described in Section 8.4.2 Sizing Treated Water Storage for Systems Providing Fire Protection and Section 8.4.3 Sizing Treated Water Storage for Systems Not Providing Fire Protection which follow.

Where appropriate for larger distribution systems, hydraulic and water quality models should be used for sizing new storage facilities and for selecting locations for re-chlorination facilities if needed.

8.4.1 Chemical Disinfection Contact & Water Treatment Plant Storage

Any volume required to provide chemical disinfection contact time is not available for storage and should not be included in storage calculations. Refer to Section 5.9 Disinfection for more information on primary disinfection and contact time.

8.4.2 Sizing Treated Water Storage for Systems Providing Fire Protection

The following method for sizing water storage needs may not fulfill the fire protection requirements of the municipality insurance company or the Fire Underwriters Survey. For fire flow requirements, refer to the latest edition of the Fire Underwriters Survey document Water Supply for Public Fire Protection¹². Historically, small municipalities in Ontario have used the following criteria.

Fire protection is a municipal responsibility and the municipality may elect to provide for higher fire flow requirements or entirely forgo fire protection by way of the drinking-water distribution system. The designer should, therefore, ensure that he/she is aware of all applicable requirements.

Total Treated Water Storage Requirement = A + B + C Where: A = Fire Storage; B = Equalization Storage (25% of maximum day demand); and C = Emergency Storage (25% of A + B). The maximum day demand referred to in the above equation should be calculated using existing flow data whenever possible, otherwise the factors in Table 8.2 may be used. Where existing data is available, the required storage should be calculated on the basis of the demand characteristics within the system.

Maximum Day Demand = Average Day Demand × Maximum Day Factor

The above equation is for the calculation of the storage needs for a system where the water supply system is capable of satisfying only the maximum day demand. For situations where the water supply system can supply more, the storage requirements can be reduced accordingly.

The physical configuration of the water containing portion of the storage facility should be such that the equalization volume (B) is located between the top water level (TWL) of the storage facility and that elevation necessary to produce a minimum pressure of 275 kPa (40 psi) in the majority of the system under peak hourly flow. The fire (A) and emergency (C) component volumes (i.e., A + C) should be located between that elevation necessary to produce 275 kPa (40 psi) under peak hourly flow conditions and that elevation necessary to produce a minimum 140 kPa (20 psi) under the maximum day plus fire flow condition.

Should a standpipe with a booster pumping station at the base be proposed, the equalization volume (B) would normally be located between the TWL and that elevation necessary to produce 275 kPa (40 psi) in the majority of the system under peak hourly flow. The fire (A) and emergency (C) components can be below this 275 kPa (40 psi) elevation provided the booster pump is designed/sized to increase system pressures to a minimum 140 kPa (20 psi) under the maximum day plus fire flow condition.

8.4.3 Sizing Treated Water Storage for Systems Not Providing Fire Protection

If the drinking-water system is not being used or will not be used for providing fire protection, the volume of storage should be 25% of the design year maximum day plus 40% of the design year average day.

8.4.4 Treatment Plant Storage

Significant storage may be required at water treatment plants for the proper operation of the plant. This storage is in addition to the storage requirements described in Section 8.4.2 Sizing Treated Water Storage for Systems Providing Fire Protection and Section 8.4.3 Sizing Treated Water Storage for Systems Not Providing Fire Protection. Treatment plant storage can be provided in treated water wet wells, clearwells and/or reservoirs.

The plant storage should be sized (together with distribution system storage capacity) to minimize on/off cycling of the treated water pumps. Plant storage should be sized such that distribution system demands and in-plant water use (e.g., filter washing, chemical systems, and domestic use) can be met while maintaining relatively constant flow through the plant rather than fluctuating filtration rates.

8.4.4.1 Filter Wash Water

Storage for filter wash water should be sized, in conjunction with backwash pump units and treated water storage, to provide adequate backwash water (Section 5.6.2 Rapid Rate Gravity Filters and Section 5.7.2 Membrane Filtration). Consideration should be given to the consequences of backwashing of several filters in rapid succession and/or worst case conditions when peak demand and backwash water requirements coincide.

8.4.5 Sizing Considerations for Systems Serving Fewer Than 500 People

If the system municipality/owner has decided that fire protection is not to be provided and the source is only capable of the maximum day, the minimum effective storage to be provided should be the average daily flow. Appropriate allowances for lawn watering and in-plant process requirements, as needed should be added to the minimum volume.

Where it has been decided that fire protection is to be provided via the communal water supply and distribution system, the minimum volume of the storage facility should be increased by an amount equal to the minimum fire flow for two hours. The allowance for lawn watering is not needed where fire protection is provided via the communal water supply and distribution system. In sizing storage facilities for small systems, the designer should also consider the importance of maintaining water quality, preventing freezing during the winter and excessive warming of the water during the summer.

8.5 Location of storage/site selection considerations

The following factors should be considered in choosing a location for distribution system storage:

• The relationship of distribution system hydraulics (including topography) and water demands in various parts of the system;
• Pumping and transmission costs;
• Safety considerations;
• Aesthetic considerations and public property owner acceptance;
• Future expansion; and
• Site access.

A location in an area of the highest water demand and/or low pressure should be preferred. Storage facilities should ideally be located on the highest point

of ground in the area. For large distribution systems, the placement of one storage tank at central location should be evaluated against smaller units with equivalent total volume in other parts of the system. The designer should be aware that flow reversals may create sediment uptake and dispersal. This may be a more significant issue where the storage tank is located at an extremity of the distribution system.

In addition, the following factors should be considered in choosing a location for ground-level or buried reservoirs:

• Structural costs based on soil conditions and location of the groundwater table (the bottom of the reservoir should ideally be above the groundwater table, or alternatively site drainage should be provided);
• Where groundwater levels have the potential to cause floating, structural measures to prevent floating should be incorporated in the design;
• Excavation costs based on cut and fill considerations; and
• Any opening in the reservoir should not be less than 600 mm (24 in) above the original ground level or the level of the 100 year flood (or the highest flood of record).

Sewers, drains, septic tanks and tile fields, standing water and similar potential sources of contamination should be kept at least 15 m (50 ft) away from the reservoir. Where this separation cannot be obtained, sewers constructed of watermain quality pipe, pressure tested in place at a pressure of 350 kPa (50 psi) without leakage may be used at distances greater than 6 m (20 ft) in accordance with the Ontario Ministry of Transportation Ontario Provincial Standard Specification 701 (OPSS 701) of the Ontario Provincial Standards for Roads and Public Works (OPS).

8.6 Security & Protection

All existing and future water storage structures should be completely covered and watertight to prevent contamination. Any openings should have covers to prevent the entry of birds, animals, insects, runoff and excessive dust. The installation of additional equipment such as antennae should not affect internal coatings or compromise water quality.

Where pipelines are located underneath or close to a reservoir, the use of rigid pipe or pipe with adequate joint flexibility is recommended to minimize potential damage due to differential settling or movement of the reservoir.

Fencing or other security measures, as well as locks on valve and vent housings and access hatches, should be provided at the storage site along with other precautions (e.g., alarms) to guard against illegal entry, vandalism and sabotage. The designer should consult the AWWA Security Guidance for Water Utilities document.

8.7 Controls & Implementation

Adequate instrumentation to control water levels in storage facilities should be provided. Level indicating devices should provide readings at a central location and overflow and low-level alarms, activated by separate and independent devices, should sound at location(s) which will be monitored 24 hours a day. Local level indicators should be provided by a pressure gauge on the tank piping, a level indicating transmitter or other means. For elevated tanks, level control instrumentation should be sufficiently precise to prevent wasting storage or tank overflows.

High lift and/or booster pumps should be designed to operate to maintain storage facility water levels within a range to maintain adequate distribution system pressure. Altitude valves or equivalent controls should be installed on elevated storage when more than one tank is required within a single supply pressure zone or where the storage facility would overflow at allowable high distribution system pressure.

8.8 Design considerations

8.8.1 Construction Materials

All chemicals and water contacting materials used in the construction and operation of drinking-water systems should meet all applicable quality standards as described in Section 3.26 Chemicals and Other Water Contacting Materials.

The use of concrete form release agents that contain diesel oil or other potential water tainting components should be avoided. Designers should be aware that concrete spalling has seriously damaged water saturated concrete during the freeze/thaw cycles that are typical in Ontario.

8.8.2 Maintaining Water Quality

Stagnation and excessive retention time in the distribution system may result in a deterioration of water quality. The deterioration may be indicated by loss of disinfectant residual, formation of disinfection by-products and bacterial re-growth. Therefore, storage facilities should be designed to prevent stagnation and minimize detention times. This may be assisted by providing separate inlet and outlet piping, a two-cell design, baffle walls, diffusers and/or by strategic location of the inlet and outlet piping. Where there is more than one cell, the inlet should be to one cell and the outlet from another. It should be possible to operate with one cell out of service. In standpipes where only the upper portion of the stored water provides useful system pressure, the water should be circulated through the storage facility to maintain quality and minimize ice formation. For smaller systems, circulation may be required to prevent high water temperatures.

If it is not possible to have sufficient turnover of water in a storage facility to maintain water quality, a pumped recirculation system should be provided and a booster disinfection system may be required.

Water quality deterioration in storage may be particularly rapid where sequestering agents are used with hard water or where natural organics react rapidly with a free chlorine residual. In such cases, the use of monochloramine as the secondary disinfectant should be considered by the designer. The designer may also consider the provision of re-chlorination systems at storage facilities in order to maintain acceptable chlorine residuals. Refer to Section 5.9 Disinfection for more information on secondary disinfection.

The designer should refer to a publication such as the AwwaRF report Maintaining Water Quality in Finished Water Storage Facilities (Project #254).

8.8.2.1 Equipment Containing Mercury

Except for UV lamps, equipment containing mercury should not be used within a drinking-water system. UV systems should be designed to protect the water from mercury contamination. Refer to Section 5.9 Disinfection for more information on UV systems.

8.8.3 Isolation & Drainage

The system should be designed to ensure that pressure can be maintained even when the storage facilities are drained for cleaning and maintenance. Consideration should be given to the installation of air release/vacuum relief valves on the distribution side of the isolation valve(s).

The storage facility drain should discharge to the ground surface with no direct connection to a sewer or storm drain. If a connection to a sewer or storm drain is unavoidable, an atmospheric gap of twice the drain diameter should be provided. Floors should be sloped towards the sump to facilitate cleaning.

All buried reservoirs should be designed with two or more cells which can be operated independently, and a separate wet well when applicable. Through valving it should be possible to isolate one of the two cells without affecting the operation of the other cell. Two cells can often be provided as a result of construction phasing requirements.

8.8.4 Vents

Treated water storage facilities should be vented. The overflow pipe should not be considered a vent. Vents should:

• Allow air into the tank at a rate greater than the rate at which water is withdrawn in order to avoid the development of vacuum/pressure within the tank;
• Prevent the entrance of surface water and rainwater;
• Exclude birds and animals;
• Exclude insects and dust, as much as this function can be made compatible with effective venting;
• Be located away from areas which will be subject to severe snow drifting; and
• Eliminate entry of solid/liquid agents as a result of vandalism or sabotage.

On ground-level facilities, the vent should terminate in an inverted "U". The opening should be 600 to 900 mm (24 to 36 in) above the roof or groundcover and covered with twenty-four mesh non-corrodible screen. Where a valve house or pump house is provided, the vents should be located within the structure.

On elevated tanks and standpipes, vents should open downward and be fitted with either twenty-four mesh [0.70 mm (0.03 in) openings] non-corrodible screen or with finer mesh non-corrodible screen in combination with an automatically resetting pressure-vacuum relief mechanism.

8.8.5 Overflow

All water storage facilities should be provided with an overflow which discharges 300 to 600 mm (12 to 24 in) above the ground surface over a drainage outlet structure or a splash pad which drains away from the storage facility. No overflow should be connected directly to a sewer or storm drain. If such a connection is unavoidable, an air gap equal to twice the overflow pipe diameter should be provided. All overflow pipes should be located such that any discharge is visible. Alarms should be installed to alert the operator of an overflow event.

When an internal overflow pipe is used on elevated tanks, it should be located in the access tube. On other types of storage facilities, the overflow pipe should be located on the outside of the structure. Overflows should open downward and be fitted with twenty-four mesh [0.70 mm (0.03 in) openings] non-corrodible screen. The screen should be installed within the overflow pipe at a location least susceptible to damage by vandalism. If a flapper valve is used, the screen should be installed upstream of the valve. The screen should be located such that it can easily be replaced following an overflow event. The overflow pipe should be of sufficient diameter to permit discharge of water in excess of the maximum potential filling rate.

8.8.6 Roof & Sidewall

Watertight roofs should be provided. There should be no opening in the roof or sidewall except properly constructed piping for inflow and outflow, vents, access hatches, overflows, risers, drains, pump and valve mountings and control ports. Particular attention should be given to the sealing of roof structures which are not integral to the tank body.

Any pipes running through the roof or sidewall of a metal storage facility should be welded or properly gasketed. In concrete structures, these pipes should be connected to standard wall castings which were poured in place during the forming of the concrete or pipes should be sealed using rubber link type seals. Wall castings should have water stop flanges or seepage rings embedded in the concrete. Openings in the roof or top of a storage facility designed to accommodate control apparatus or pump columns should be curbed and sleeved with proper additional shielding to prevent contamination from surface water or floor drainage. Where possible, valves and controls should be located outside the storage facility so that the valve stems and similar projections will not pass through the roof or top of the reservoir. As an alternative approach, such valves and controls may pass through the roof or reservoir top provided they are located within a valve or pump house structure on the roof and are curbed.

The roof of the storage facility should be well drained. Downspout pipes should not enter or pass through the reservoir. If the design includes parapets or similar construction which would tend to hold water and snow on the roof, drainage should be provided.

Particular attention should be paid to expansion joint design for large structures to ensure long lasting protection against infiltration from deteriorating joints during expansion and contraction movement. For elevated tanks and standpipes, the use of heat trace cables on the roof may be necessary to prevent the build up of ice.

8.8.7 Grading

The grading around ground-level facilities should direct water away from the tank and prevent standing surface water within 15 m (50 ft). Side slopes should have a grade no steeper than 3:1 to facilitate landscape maintenance.

8.8.8 Safety

Safety of the employees must be considered in the design of the storage facility. As a minimum, the design should conform to applicable laws and regulations of the Province. See Section 7.3.13 Safety. Confined space entry requirements should be considered.

Ladders, ladder guards, railings, handholds and entrance hatches should be provided where applicable. The design should incorporate easily accessible fall arrest systems for use by employees or emergency response workers for access to the exterior and interior of the structure.

8.8.9 Internal Catwalk

Every catwalk over treated water in a storage facility should be located above the high water level and have a solid floor with sealed edges raised to 100 mm (4 in) designed to prevent contamination.

8.8.10 Silt Stop

The discharge pipes from water storage facilities should be located in a manner that will prevent the flow of sediment into the distribution system. Silt stops should be provided.

8.8.11 Disinfection & Backflow Prevention

Treated water storage facilities should be disinfected in accordance with AWWA Standard C652: Disinfection of Water-Storage Facilities before being placed into operation after construction, maintenance or repairs as required by the Procedure for Disinfection of Drinking Water in Ontario (Disinfection Procedure).

The designer should consider the need for dechlorination of highly chlorinated water that will be discharged to the environment in the course of draining for maintenance, overflows or other purposes.

Consideration should be given to backflow prevention during the initial fill of a storage facility or following maintenance.

8.8.12 Provisions for Sampling

Smooth-nosed sampling tap(s) should be provided for collection of water samples for both bacteriological and chemical analyses. The sample tap(s) should be easily accessible and located on the tank side of isolation valves. Sample suction locations should be placed in such a way as to be representative of the desired sample location.

Sample lines should be stainless steel from the sampling point to the sampling tap.

8.8.13 Freezing

Treated water storage facilities, especially the riser pipes, overflows and vents should be designed to prevent freezing which will interfere with proper functioning and cause potential damage to the structure.

Alternatives to be considered to avoid freezing include insulation, variable level operation, internal heating via heat tracing cables, hot water recirculation, separate inlet (high) and outlet (low) piping or a combination of these methods. If a water circulation system is used, the circulation pipe should be located separately from the riser pipe.

Refer to Chapter 12 Challenging Conditions for further information on avoiding freezing.

8.8.14 Access

Treated water storage facilities should be designed with convenient and safe access [i.e., a minimum 900 mm × 1060 mm (36 in × 42 in) opening is recommended] to the interior for sample collection, cleaning and maintenance. Where space permits, at least two (2) access hatches should be provided above the waterline into each water compartment. The number and location of access hatches should comply with the requirements of the Occupational Health and Safety Act (OHSA).

For elevated storage tanks, at least one of the access hatches should be framed at least 100 mm (4 in) above the surface of the roof at the opening. It should be fitted with a solid watertight cover which overlaps the framed opening and extends down around the frame at least 50 mm (2 in), be hinged on one side and have a locking device. All other access ways should be bolted and gasketed.

For ground-level facilities, each access hatch should be elevated at least 450 mm (18 in) above the top of the tank or groundcover and should be fitted with a solid, watertight cover which overlaps a framed opening and extends down around the frame at least 50 mm (2 in). The frame should be at least 100 mm (4 in) high. Alternatively, the cover should have an integral perimeter trough and drain. Each cover should be hinged on one side using non-removable hinges and should have a locking device. All accesses should have a high degree of security to prevent unauthorized access.

8.8.15 Corrosion Protection

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

Coatings formulated without the use of volatile solvents (termed 100% solids) and NSF/ANSI Standard 61: Drinking Water System Components - Health Effects certified should be used on the inside surface of steel structures to avoid the trapping of solvents and the resulting water tainting.

Cathodic protection of steel water structures should be provided and conform to the provisions of AWWA Standard D104: Automatically Controlled, Impressed-Current Cathodic Protection for the Interior of Steel Water Tanks. Considerations should be given to potential ice damage to cathodic protection equipment.

8.9 Treatment plant storage

8.9.1 Clearwells & Reservoirs

A minimum of two compartments should be provided in clearwells and reservoirs. The plant should be able to operate for short periods with the clearwell or reservoir out of service. The design should also include adequate measures for circulation, an overflow and vents (Section 8.8 Design Considerations).

When part of a clearwell or reservoir is to be used for disinfectant contact time (in addition to the storage volume), special attention should be given to baffling. Refer to Section 5.9 - Disinfection for more information about disinfectant contact time.

8.9.2 Adjacent Storage

Except where it is an inherent and necessary feature of the treatment process unit, drinking water should not be stored or conveyed in a compartment adjacent to non-drinking water when the two compartments are separated by a single wall. Pipes carrying non-drinking water should not be installed through storage facilities containing drinking water.

8.9.3 Other Treatment Plant Storage Facilities

Other treatment plant storage facilities such as chlorine contact tanks and wet wells for treated water should be designed as treated water storage facilities.