By Milan Rubcic, Sydney Water
A typical wastewater system may comprise of a network of sewers and sewage pumping stations (SPS) that collect and transport sewage from where it is generated to a wastewater treatment plant for treatment.
Wherever possible, the wastewater system should be designed to maximise the use of gravity to convey the sewage. Gravity wastewater systems do not use energy nor require maintenance at anywhere near the same rate as pumping stations. Also, they are immune to many pumping problems, such as power and controls failures causing overflows, excessive detention times in wet wells and pressure mains with associated odour and corrosion issues, sudden variations in flow loads on downstream facilities causing surcharges or poor treatment performances etc.
However, large catchments, steep or relatively flat terrain and other variations in the local landscape may preclude or limit the viability of a gravity system. When this happens, one option is to pump sewage by SPS(s) through pressure main(s) into the most suitable nearby sewer, where it can drain by gravity to another pumping station or a wastewater treatment plant. A gravity wastewater system draining to one or more SPSs is generally the most common option. However, alternative systems such as vacuum or pressure sewerage systems, which also rely upon pumps, may be considered. A multiple criteria analysis is often used in determining the most suitable system.
The number of SPSs in Australia continues to grow with over 4600 reported in WSAA facts 2003-4, including over 680 within Sydney Metropolitan Area alone. Pumping stations should be designed and constructed to minimise the risk of adverse environmental impacts and support a totally integrated wastewater system that can be operated and maintained at the lowest life-cycle cost. SPSs and pressure mains are made up of a multitude of civil, electrical and mechanical items including pumps, structures, power and control equipment, telemetry systems, pipes, fittings and valves.
Both, positive displacement and centrifugal pumps are nowadays used in SPSs to pump sewage.
Positive displacement pumps are, generally, employed where pumping heads and flows are outside the capability of centrifugal pumps. Progressive cavity pumps with macerators are the most commonly used type of positive displacement pumps in raw sewage and sludge applications. The pumps are typically installed in dry wells / chambers or superstructures if suction lift is not too excessive (they are usually self-priming). SPSs with positive displacement pumps are generally more complex, have a larger footprint and are more expensive to build and run than SPSs fitted with centrifugal pumps.
Centrifugal pumps are the most common type of pumps used in SPSs. These can be employed in dry (dry well) or submersible (wet well) installations. Centrifugal pumps are not self-priming and are usually installed below minimum water level or otherwise need to be fitted with priming devices.
Conventional (dry well) pumping stations with vertical sewage pumps driven by air cooled electric motors were normal design practice until the development of the submersible motor driven pumping units. A typical conventional SPS consists of side-by–side inground wet and dry wells (often an integral concrete substructure divided by a wall) and a superstructure providing personnel access into the dry well and housing electrical and control equipment. The pumps and pipework are installed in the dry well, which also accommodates ancillary equipment such as access steelwork, ventilation and lighting required for safe entry and operation / maintenance. Sometimes the electrical equipment is also installed on a mezzanine level in the dry well.
There are many conventional dry well type pumping stations still in operation. Some authorities, however, prefer to convert dry wells into wet wells when refurbishing existing conventional SPSs or, as a minimum, replace the dry well pumps with submersible pumping units and relocate the electrical equipment to above ground level so that they are not damaged if the dry well floods. Dry well installed submersible pumps are also used in vacuum pumping stations to transfer sewage collected in vacuum pots. Some authorities prefer dry well installations over wet wells for large, very deep and/or critical pumping stations due to easier access to the pumps for condition monitoring, inspection and in-situ maintenance.
Since the development of submersible electric motors, most water authorities across Australia have adopted submersible, wet well sewage pumping stations as a norm. They were found to be more cost effective (only one well is required), safer to operate (mainly from ground level with no need to enter confined spaces), have a lesser profile / visual impact, require a smaller footprint, have lower noise levels, are less expensive to run and maintain, have similar layouts and features which can lead to easier standardisation etc.
Due to their similarity and popularity, Australian water authorities, lead by Water Service Association of Australia (WSAA), produced a Sewage Pumping Station Code of Australia WSA 04 which sets out the requirements for minimum acceptable technical criteria for design and construction of submersible sewage pumping stations, associated structures and pressure mains. The first version of the Code was published in 1999 and its current version 2.1 was issued in 2005. Some water agencies, such as Sydney Water have subsequently issued their own editions of the Code to include their specific requirements.
The Code covers small to medium size pumping stations and pressure mains up to 200 L/s and DN 375 with two pumps (1 duty + 1 standby). However, its main principles are usually adopted for design of larger pumping stations as well.
With exception of a few positive displacement, vacuum pumping stations and pressure sewerage systems due to site specific reasons, all new sewage pumping stations constructed within Sydney Water’s area of operation (Sydney metropolitan, Blue Mountains and Illawarra) over the last 25 years were of submersible wet well type. They range in flow rate from less than 5L/s to more than 2000L/s. The largest one (SP1174), comprising of six (4 duty + 2 standby) 310kW variable speed submersible pumping units has been constructed within SWC’s Quakers Hill Wastewater Treatment Plant and is currently being commissioned. The station has a capacity of 2200L/s with a provision for future upgrade up to 3300L/s.
A typical submersible sewage pumping station consists of a wet well, valve chamber, inlet maintenance hole, electrical kiosk or switchroom and supporting systems.
The wet well is a circular or sometimes rectangular inground concrete structure that accommodates pumps, their discharge riser pipes, incoming sewer line with isolating valve and level sensing equipment. The wet well floor is benched to provide adequate hydraulic flow to the pumps and achieve self-cleansing. Its control volume between pump cut-in and cut-out levels should be sufficient to limit the number of pump starts per hour to no more than that recommended by the manufacturer. Wet well roof provides suitably sized openings for pumps, level sensors and personnel access. All the openings are fitted with water and gas tight covers.
A typical submersible pumping unit consists of a single stage centrifugal pump driven by an electric motor via a common rotor/impeller shaft, forming a compact and completely watertight vertical pumping unit. The motor is cooled by the pumped liquid so that no external cooling is required.
The pumping units are supplied with a discharge connection (also known as duck-foot bend, discharge pedestal or discharge bend), lifting chains, guide rails, electrical power and control cables. The lifting chain and guide rails are of a suitable length to reach the access opening at the top of the wet well. The guide rail arrangement permits easy lowering and lifting of the submersible pumps in the vertical plane. When lowered down the guide rails the discharge end of the pump automatically connects to the discharge bend installed at the wet well floor. The weight of the pump facilitates a watertight seal between the two.
The pumps can be fitted with an automatic flush valve for flushing of the wet well. The valve opens at each pump start for approximately thirty seconds to stir up the sludge settled in the wet well. Preferably, the valve opening / closing operation shall be induced hydraulically by the pump flow and pressure, thus eliminating the need for electrical components and cables.
Submersible sewage pumps are today available up to and above 600kW. There are a number of suppliers in Australia, but for the purpose of standardisation of spare parts and maintenance procedures, most water authorities have a single or dual supplier policy.
SPS discharge valves are usually installed within an inground valve chamber. These include pump non-return and stop valves, pressure main stop and scour valves and bypass connection stop and non-return valves. In Sydney Water submersible SPSs, the valve chamber has open grid type cover with hinged access hatches and access through the cover to all stop valve spindles to reduce the need to access the valve chamber. Permanent ladders are provided for personnel access into the chamber when required. Non-return valves suitable for unscreened raw sewage applications include long body swing check and ball check type valves. Either resilient seated or metal seated gate valves or eccentric plug valves can be used for flow isolation.
Inlet maintenance hole collects all the sewage from the upstream catchment. It is usually co-sited and positioned close to the wet well and connected to it via low and high level drain pipes. It is sized to enable personnel access and accommodate level sensing equipment, isolating flange to the high level drain pipe, and an emergency bypass pump to enable pumping into the pressure main when the wet well is isolated for maintenance or the main pumps fail. It is usually connected to an emergency relief structure (gas check maintenance hole) to provide controlled sewage overflow in case of excessive inflow or SPS failure. The inlet maintenance hole can also be designed to collect grit.
All the electrical and control equipment is usually installed within an outdoor electrical kiosk positioned next to the wet well. Motors larger than 5.5 kW usually use assisted start i.e. soft starters or in special cases variable speed drives (VSD), while up to 5.5 kW direct-on-line (DOL) starters are adequate.
Motors and starters shall be matched to obtain the required torque to run the pump under all operating conditions and provide adequate margin over the maximum power required at the pump shaft (usually 15%). They should ensure correct starting and running sequences and provide adequate protection for the motor under its starting and operating conditions. The types of starters selected for motors shall be determined by the rating of the motor and its starting currents. Starting characteristics of the motors, including maximum starting currents, shall comply with power supply authority’s requirements
The use of variable speed drives in sewage pumping stations is limited to situations where hydraulic control is required (e.g. pumping directly to sewage treatment plants), where their application significantly improves the cost of pumping, where they can assist with water hammer attenuation during normal operation, or where the pumping station needs to cope with a wide range of flows and heads (e.g. staged development).
VSDs above 22kW and large motor starters usually require additional cooling or are physically too big for an outdoor kiosk and are installed in air conditioned switchrooms.
SPS supporting systems may include ventilation, water supply, telecommunication, power supply, site security and access, odour control, chemical dosing, emergency storage structure etc.
Pumping stations wet well and inlet maintenance hole are usually fitted with natural ventilation system, comprising of low level induct and high level educt vent shafts. Where odour is found to be a problem, a forced ventilation or even an activated carbon odour control unit may be considered
Water supply is provided for wet well, emergency storage structure and inlet maintenance hole wash down and general cleaning around the site. Hydrant and a vandal proof tap are usually considered adequate. A reduced pressure zone device must be provided in the water service to prevent contamination of the water supply system.
Telecommunication system facilitates remote monitoring and control (SCADA or IICATS) of the asset.
Where sewage septicity is a problem, chemical dosing may need to be provided to reduce corrosion and odour downstream of the SPS pressure main discharge point. Chemical dosing is often required in new catchments where due to low inflows during early development stages sewage detention times in the wet well and pressure main are excessively long.
Wet well, valve chamber, inlet maintenance hole hatches and electrical kiosk doors are locked to prevent unauthorised entry and vandalism. Additional site security measures, such as perimeter fencing, motion / entry sensors etc. may also be provided. The type of fencing (eg. chain link, palisade, cattle) depends on the asset location, visual appearance, community consultation and vandalism experienced in the area. SPS located within public parks are usually adequately landscaped and screened instead of fencing for aesthetic reasons.
Emergency storage structures are usually provided to prevent dry weather overflows in case of pumping station failure. The storage structure is sized such that in conjunction with the upstream reticulation system, wet well and the inlet maintenance hole it can contain the maximum sewage flow into the station in dry weather for the total time it takes from the high level alarm to when the contingency plan is implemented. For most Sydney Water SPSs this time is 4 hours, although more than 6 hours may be required for remote or difficult to access SPSs. This emergency storage is usually considered in wet weather as a balance storage to ‘shave off’ the peak inflows and thus reduce the SPS pumping capacity.