Sewage pumps are essential in circumstances where wastewater must be lifted to a higher level before it can be transferred further on by gravity or being delivered to a wastewater treatment facility. Unscreened solids and fibrous materials suspended in the wastewater stream can form large rope-like tangles, which can potentially block pipes and valves as well as jam up pumps in a lift station. Often this results in costly and labor-intensive manual cleaning in order to correct the clogging situation and restore the function of the pump station.
Flushing wipes and larger volumes of the fibrous material is a growing problem that is overwhelming lift stations around the world. Higher environmental standards and fresh water crisis push engineers even further to design toilets and appliances with less consumption of the fresh water, which means increasing the density of the solids in sewage flow. As a result, pump clogging has become a common problem in almost all sewage pumping stations in our modern world and it occurs relatively frequently. However, in most cases, clogging is not the pump's fault! Solids handling sewage pumps are designed to handle "normal sewage" and not the big rag balls.
To address the clogging problem in a pump station and reduce the risk of clogging, a system approach is needed, and a variety of parameters from pump design characteristics and suitable impeller selection to the station geometry and control system philosophy must be considered and studied. Some of the important sewage lift station design considerations are listed below.
Process design
- Coarse screening and grit removal before passing raw sewage to the lift station.
- Considering emergency tanks and overflow line.
- Redundancy and spares.
- Maintain cleansing velocity especially when grinder pumps are in service.
- Reviewing all sewage piping design requirements and parameters (backflow prevention, air and vacuum control, cleaning ports, no-flow and dead zones, water hammer and transient pressure surge, minimum and maximum velocity in system, turbulence and friction losses, anchoring and stress analysis, etc.).
- Providing proper maintenance facilities and safe access to the station.
Lift station design
- Well sizing (active capacity, reserve capacity, flood protection, retention time, pumps duty cycle and starts frequency, pumps hydraulic requirements and suction design, etc.).
- Hydraulic considerations (benching, internal liner, inlet design and turbulence, foam and grease control, forcemain and risers design, surge pressure and supporting, vortex flow and air-entraining, etc.).
A self-cleaning tank insert (KSB Amaclean) is a good sample of a tank internal liner. It enhances wastewater flow to the pumps, prevents fiber deposits and no-flow zone formation, and increases pump efficiency and solids handling capabilities.
Pumps with increased solid free passage
- Employing impellers with a large / adequate solid free passage reduces the risk of pump clogging.
- In general, increasing the free passage size has a negative effect on the pump's hydraulic efficiency.
Most municipalities and design standards recommend a minimum of 3" free passage. A new generation of pump impellers, such as the KSB K-Max closed multi-vane impeller and F-Max vortex impeller, can offer 3" and above of solid free passage along with high efficiency rates.
Grinder pumps
- Using cutter or grinder pumps can reduce the risk of clogging (e.g., KSB submersible pumps equipped with a cutter device, S-impeller).
- Grinders are an alternative to coarse screening. However, to avoid the problem of shredded material recombining in downstream treatment units, and the disadvantages that cutting solid materials into small pieces may create for wastewater treatment processes, using grinder pumps is limited to low flow applications and small residential stations. For pumps under 4" with less than 3" of solid free passage, using grinder pumps is an option to limit pump clogging frequency. However, the physical removal of the coarse material from the sewage effluent, or passing solids to the next process stage by maintaining adequate free passage, is recommended rather than the use of shredding or cutting devices that reintroduce the material to the treated sewage.
- Grit removal should be placed upstream of the cutter pumps to avoid damage to the cutters caused by grit particles.
Limit solids contact with pumps
Pump stations with a solids separation system, such as AmaDS made by KSB, are ideally suited for use in challenging environments:
- Areas with a low density of building development
- Pumping across distances with long discharge pipes
- Topographies where gravity discharge lines cannot be used
- Regions with limited power supply
Benefits:
- Avoiding or limiting direct contact of solids with the pump to minimize blockage and pump maintenance cost.
- Economic efficiency by using pumps with smaller free passages.
- Operating reliability by preventing clogging and reducing erosion and wearing problems.
- Ease of maintenance, as the dry installation provides safe and easy access to all important components.
- With a redundant design of AmaDS, the system can be serviced or repaired while it is in operation.
Automation and monitoring
A system approach, close monitoring and optimum operation can minimize maintenance costs and increase overall performance.
- Using a programmable mixing and flushing system instead of non-monitored common mechanical flush valves.
- Tank self-cleaning program.
- Monitoring and controlling pump duty cycles.
- Implementing monitoring instrumentation and devices for close and effective control.
- System failure monitoring and reporting.
- Running pumps near their best efficiency points.
- Careful and smart control of VFD systems.
- Flood control program.
- Central / online alarming system.
- Control system redundancy and reliability.
- Continuous monitoring of the system curve to detect abnormal pump operation.