7.1 Small pump stations
7.1.1 General considerations
ACTEW has a preference for gravity sewerage systems because of the ease of operation and the minimisation of hydrogen sulphide corrosion of the sewers. For systems that will be owned and operated by ACTEW, sewage pumping should only be adopted where shown on ACTEW's Sewerage Strategy Plans, or after discussion and agreement with ACTEW regarding the economics of pumping versus other alternatives.
The design of pump stations is heavily influenced by the locality. There is no single set of standards that can apply to all locations. Standard Drawing No. WSS 055 has been prepared to provide basic guidelines in the design of each station. It is recommended that the agent consult ACTEW at the earliest possible time to quantify site specific issues relating to proposed sewer pump stations.
Sewage pumping may be appropriate:
- where sewerage facilities are required to serve developments that cannot drain to existing or proposed gravity systems;
- to prevent excessive depth of sewer lines in flat terrain (not generally the case in Canberra);
- to provide surcharge relief on overload sewerage systems by transfer to a different catchment;
- to lift sewage into a sewage treatment plant.
The following general requirements are for small neighbourhood pump stations intended to be owned and operated by ACTEW, and which handle PWWF's up to about 50 litres per second. The more detailed aspects of station design should be discussed with ACTEW at an appropriate stage during design.
For stations larger than 50 litres per second, design requirements will normally be specified in an ACTEW brief tailored to the individual project.
Where a proposed pump station is associated with a single specific lease, and the station and rising main are to remain the responsibility of the lessee, ACTEW's main concern will be to ensure that the design and operation of the station and rising main are such as to prevent hazardous conditions, corrosion, or odour problems within the receiving ACTEW system.
7.1.2 Station catchment and siting
The station location and duty shall be based on the service areas shown on ACTEW's Sewerage Strategy Plans using flow estimating procedures as set out in Clause 3.1.1.
The siting and depth shall be chosen to command the service areas by gravity while allowing adequate incoming grades. In determining station depth, it is important that the constraints governing wet well depth be clearly identified, and excessive depth be avoided. Measures such as concrete encasement of gravity sewers under roadways and floodways to reduce depth of cover, and in appropriate cases relaxation of "slime control" criteria for critical sewers (refer Clause 3.1.3 (i)), should be used.
Station siting shall provide optimum integration of the station with adjacent land uses, landscaping, all weather access, hardstand areas for maintenance vehicles, and satisfactory overflow arrangements. Minimum turning radii around the station and access tracks shall be suitable for an 8.8 metre service vehicle as per Austroads Standards Australia — AS HB72. The vehicle must be able to be reversed or driven to enable parking (with the rear or side of the vehicle next to the well). Access track grades steeper than 2% shall be cement stabilised, and tracks steeper than 5% must be sealed. In areas where the station is at the end of a dead-end access, the hardstand must allow for the truck to be turned around within the pavement area.
Security fencing shall enclose the key components. Appropriate warning signs shall be visible.
Designers should be aware that the noise of mechanical equipment may cause irritation to adjacent residents. Keep stations well away from adjacent residential development and/or provide appropriate soundproofing. Covers and vents that are prone to flooding or are required for station access during floods shall be above the 100 year ARI flood level of any adjacent waterway or pond. Siting should attempt to minimise pump station, gravity and rising main costs, and ensure economic and timely availability of power supply, telephone and town water connections necessary for commissioning and operation.
7.1.3 Design objectives
The overall objectives for sewage pump station design are as follows:
- safe working conditions for operations and maintenance personnel;
- ease of accessibility and operation;
- long term reliability;
- minimum capital, and operating and maintenance costs of the station, gravity mains and rising mains;
- unobtrusive location;
- energy efficiency.
With the exception of temporary facilities intended to be superseded within a few years, station components shall be designed for the following design lives:
- Civil works — 100 years;
- Electrical and mechanical equipment — 15 to 25 years.
Works shall make appropriate provision for initial and foreseeable future design loadings. The economics of staging, temporary cut-off walls or curtailments, initial use of reduced impeller sizes, or planned upgrading of the pump capacity should be considered.
For ease of maintenance, and in minimising stores inventories, equipment should be of standard types. The equipment should be interchangeable with, or preferably identical to equipment incorporated into existing ACTEW stations. Designs incorporating new or unusual equipment require the prior approval of ACTEW.
7.1.4 Station arrangement and services
For stations up to 50 L/s capacity, unless there are good reasons to the contrary and prior written agreement from ACTEW is obtained, a single wet well configuration with submersible centrifugal pumps should be adopted. Valves and meters shall be housed in isolated chambers at shallower levels.
These stations should be oriented such that the incoming line is perpendicular to the axis of the pumps. The valve chamber shall be integral with the wet well, and located on the side opposite the incoming sewer for cast in-situ construction. For pre-cast wet wells, the valve pit is located adjacent to the wet well. Switch and control gear shall be located such that access to the wet well or valve pit does not interfere with the operation of the station.
Water supply shall be provided for washdown purposes and shall terminate in a 25mm quick coupler connection. To prevent cross-contamination of the town supply, an approved backflow preventer shall be fitted in accordance with the Water and Sewerage Regulations 2001 (Reference 10.1) and AS/NZS 3500. A telephone connection is required for telemetry of remote alarms.
Power supply, switching, control, and telemetry equipment should be located in a vandal-proof, above ground, painted metal kiosk.
Permanently installed lifting equipment is not required for individual loads less than 500kg. For heavier loads, permanent lifting facilities (such as gantries or booms) shall be provided.
Adequately drained hardstand areas shall be provided, and shall be of a size sufficient for two service vehicles. Access covers to wet well and valve pit(s) shall be arranged either to prevent vehicle access, or designed for vehicle loadings.
Excessive flows, blockages, and operational failure of the pump may occur, dictating the need to provide an overflow for incoming sewage. The overflow pipe can be located in the collection maintenance hole, with the invert level located above the obvert level of the incoming sewer pipe. Suitable means of disposal of the flow from the overflow pipe is generally site specific, and shall be discussed with ACTEW and the Environment ACT at an early stage in the design. In general, overflows shall not be permitted to enter leases, and should be collected by the stormwater drainage network as soon as practicable. The overflow pipe outlet shall be arranged to prevent stormwater from entering the sewer system up to the 100 Year ARI flood level of any adjacent watercourse or waterbody.
In general all components of the station which require manual handling shall be designed to conform with the ACT Standard and Code of Practice on Material Handling (Reference 10.14).
7.1.5 Station capacity and pumping units
Pump configurations should be:
- two fixed speed pumps each sized for PWWF; or
- three pumps each sized for 50% of PWWF; or
- two variable speed pumps each capable of PWWF.
Pumps and motors shall be sized to handle the full range of flows and delivery heads expected to be encountered (including wet well levels varying between minimum operating and "flood" levels). Flood levels shall be calculated as the maximum level in the wet well to which incoming sewage can surcharge (including due to flooding of any adjacent watercourse). Units shall be selected for optimum efficiency under the most commonly expected average flow conditions. Motors shall be non-overloading for free discharge. Details on the preferred pumps should be obtained from ACTEW prior to the start of the final design.
Impellers shall be of the "non-clog" type capable of passing DN75.
Each pump shall be provided with a re-circulation flush valve, which mixes the sludge that accumulates at the base of the pumps. Details on the preferred flush valves should be obtained from ACTEW prior to the start of the final design.
7.1.6 Station inlet
Pump stations shall be provided with a single inlet pipe. Sewage from additional pipes may enter the station via the collection maintenance hole located adjacent to the station.
Provision for isolating the station from incoming gravity flows is required to permit maintenance or other works to be undertaken. This is usually achieved via a penstock (or knife-gate valve) located on the end of the inlet pipe within the station. The penstock shall be operable from the surface via an extension spindle manufactured from a non-corrosive material (preferably stainless steel 316L).
7.1.7 Wet well
All structures to be designed to accommodate appropriate structural loads as outlined in AS 3735 and AS 1170. The station will be provided with appropriate brackets to enable erection of safety equipment over the well. Details are to be obtained from ACTEW prior to design taking place.
The volume of the wet well is a function of the incoming sewage flow rates and the pump capacity. Where feasible, the wet well volume between switching levels shall be sufficient to limit pump starts, under any conditions, to 12 per hour.
The active volume of the wet well may be determined from the following formula:
WV = wet well volume (L)
Qp = pump capacity (L/s)
S = number of starts per hour
Switching levels shall be as follows:
- Bottom water level (BWL)
The BWL is to be set as low as possible (to minimise dead storage), while still maintaining sufficient submergence to provide adequate suction head at the pump inlet, and to prevent any vortex actions. For small pumps the BWL may be set approximately one quarter of the way up the motor housing. For larger pumps the manufacturer will need to be consulted.
- Top water level (TWL)
The TWL is defined by the volume between this level and the BWL (required to limit pump starts to the maximum permitted number per hour). The duty pumps are switched on at this level.
- Maximum top water level (MTWL)
The MTWL is set 150mm below the invert level of the incoming sewer pipe. The standby pumps are switched on at this level.
The shape of the wet well shall incorporate the following features:
- benching of the base of the wet well (to direct flow to the pump suction);
- adequate (but not excessive) clearance between the pump inlet and the base of the wet well (to enable removal of solids and to provide efficient pump operation);
- reduction in the plan area of the wet well below the minimum water level needed for pump operation (to optimise pump efficiency);
- sufficient area to install machinery to the tolerances required by the pump manufacturer (to enable the operation of level regulators and permit access via ladders);
- access to the wet wells of pump stations is required for cleaning, removal of obstructions and maintenance or replacement of pumps and level regulators. Consideration shall be given to maintenance access to the bottom of the wet well. Means of access shall include movable ladder extensions with suitable brackets for extensions as required (refer to Standard Drawing No. WSS 023), and ladders permanently fixed within the wet well. Where ladders are used, these shall be manufactured from a non-corrosive material, preferably stainless steel 316L. The maximum length of any ladder is to be 6.0 metres, with landings required at 6.0 metre intervals for deeper wells. A vertical clearance of 2.1 metres shall be provided beneath each landing.
Openings shall be provided in the concrete roofs of pump stations (designed to facilitate the installation and removal of pumps). The covers fitted to a pump station shall be sealed (gastight) metal covers, Class B (Class D if trafficable). Elsewhere covers shall be made from aluminium.
Plate covers shall be hinged, and a post (or similar) provided to rest the opened covers. Covers shall be light enough to be lifted by one person. The top of the structure shall have no projections that constitute a trip hazard. All locks and hinges shall be recessed.
The wet well will be provided with an automatic well washing system. The wash cycle is to parallel each pump cycle. Details on the preferred well washing systems should be obtained from ACTEW prior to design taking place.
The well shall be painted with grease repelling epoxy.
Pipework shall be designed to suit the pumping units and to comply with the following:
- reflux valves shall be provided on the discharge side of each pump;
- isolating valves shall be provided on suction lines (on the discharge side of each pump downstream of the reflux valve), on any cross connections, to permit operation of any one pump independently of other(s) removed for maintenance, and on the rising main(s) immediately downstream of the junctions of discharge lines from the individual pumps;
- dismantling joints shall be provided to permit piecework within the station to be removed consistently with normal maintenance requirements;
- pipework shall be sized to give a minimum velocity of 1.0 metre per second. Maximum velocities shall be arranged to prevent undue abrasion of pipe linings or excessive headloss;
- a drainage line shall be provided allowing sewage within the rising main to be emptied into the pump station (to facilitate maintenance on the rising main).
7.1.9 Mechanical equipment
All parts shall be accessible for maintenance and replacement. Allowance shall be made in the design of equipment for wear and tear, including that due to pumping of abrasive solids. Provision shall be made for any necessary adjustments.
7.1.10 Electrical equipment
All pump stations are to be equipped with the following:
- electricity supply meter: records the energy consumption of the total pump station;
- power supply unit: this distributes power to all the control and starter units, with power generally being 3 phase 415 volts. The features of the unit are the main switch, voltmeter, phase failure relay LED, and circuit breakers;
- common auto control unit: this unit houses all the control equipment for the automatic operation of the station. Components within this unit include the duty selector switch, top water level override button, maximum top water level override button, and the flood level indicating light;
- motor starter units: these units control each pump and house all of the motor protection equipment and alarm lights. The components include the control selector switch, thermal overload indicating light, thermistor LED, seal failure light, seal failure reset button, ammeter, hours run meter, and the number of starts meter;
- an hours run meter and ammeter for each pump;
- telemetering unit;
- programmable logic controller (PLC) to ACTEW specification;
- auxiliary board: includes a twin 10 ampere general purpose outlet and a light switch to turn on the internal fluorescent tube.
The programming of the PLC's will be carried out by ACTEW at the developers expense prior to the handover, the developer needing to make the necessary arrangements with ACTEW for this to occur.
7.1.11 Flow metering
Flow metering requirements shall be obtained from ACTEW prior to commencing design.
7.1.12 Protection and alarms
Protection and alarm requirements shall be obtained and verified by ACTEW prior to commencing design. At most locations ACTEW will request that a security system be installed.
In environmentally sensitive locations the Environment ACT may require that emergency storage and or backup generators be provided adjacent to the pump station.
If storage or generators are considered necessary, the degree of protection required will be determined by the locality of the station and the nature of the receiving water body. Some guidelines on integration of storage tanks to pump stations is provided on Standard Drawing No. WSS 055. Storage structures introduce maintenance constraints, ACTEW should be consulted during the planning and design to assure that the technology implemented complies with ACTEW's operating and maintenance capability.
For small wet well pump stations with submersible pumps, natural ventilation of the wet well using ash-down vents is generally adequate, however, such ventilation may not be appropriate for sites where public access is likely. In such localities tall mechanised vents will need to be constructed to ensure gasses are adequately dispersed. In very sensitive areas such as public parks, and sites in close proximity to residential leases, odour scrubbing may need to be considered.
Stations or tanks deeper than 6.0 metres, or those that exceed a total internal volume of 50m3, will be fitted with a DN375 PVC pipe attached to the wall of the station. This pipe will extend from beneath the cover slab surface of the station to 200mm above the MTWL. A DN400 metal cover will be constructed directly above the DN375 conduit, a second DN600 metal cover being provided on the opposite side of the station. This system will be used in conjunction with a portable fan to force out gasses that may have accumulated at the base of the structure (prior to entry).
7.1.14 Public safety and security
The pump station and surrounds are to be designed to avoid hazard to the public and to resist vandalism. This would include consideration of:
- gently sloping banks around the station;
- concealed electrical cables;
- high standard of locking doors etc;
- fencing of the station site, if appropriate;
- provision of intruder alarms on cabinets or buildings;
- use of anti-graffiti paint for above ground components.
7.1.15 Operation and maintenance manuals
Draft operation and maintenance manuals for the pump station are required to be produced and presented by the developer to ACTEW at least three weeks prior to the handover date. ACTEW will examine these draft manuals and comment on their suitability within two weeks. The draft manuals are to include (but not be limited to) final wiring diagrams, a final ladder diagram, and final PLC software (on floppy disk). Final manuals are to be presented at handover.
7.2 Rising mains
Rising mains are to be designed to minimise septicity of sewage while in traverse, and to ensure ease of operation and maintenance of the main.
To minimise septicity, sizing shall be determined by minimum velocity requirements that will limit both slime growth and the detention time of the sewage in the pipe. In general, rising main velocities between l and 3 meters per second are acceptable.
To minimise potential operations and maintenance problems the smallest possible diameter pipe shall be used, with a minimum size of DN80 for carrying raw sewage and DN50 for carrying septic tank effluent. Rising mains shall be graded so as to be continuously rising. In the terminal (discharge) region the vertical alignment of the pipe should be designed to minimise the length of a partly full pipe between pumping cycles (refer to Clause 5.1.19). If such grading is not practical, automatic air valves and manually operated scour valves may be required. In such cases, proposals should be discussed with ACTEW at an early stage in the design.
Corrosion and rupture resistant pipe materials shall be used for the rising main. Rising mains shall be located outside leased land to protect lessees from the effects of possible pipe rupture.
The rising main, up to the discharge maintenance hole, is considered an integral part of the pump station. It is therefore required to be handed over at the same time as the pump station.
Issues relating to high septicity, due to long retention periods during early stages of development, shall be resolved using hydrogen sulphide control principles as set out in the document entitled Hydrogen Sulphide Control Manual — Septicity, Corrosion and Odour Control in Sewerage Systems (Reference 10.13).
Refer to Standard Drawing No. WSS 055 for a typical rising main connection to gravity sewer.