By Mehran Masoudi
Solid and fibrous materials in wastewater streams can block or clog pumps, valves and piping systems. Abrasive materials, such as sand, can cause premature wear in pump components.
Large flow variations, especially those that are encountered in pump stations that need to deal with a combination of wastewater and stormwater, can make it impossible to keep pumps running in their most efficient operating range. Low velocity flows in pipes and tank structures can lead to solid wastes settling out into hard-to-move masses, while very high-velocity flows can cause air entrapment and cavitation at pump intakes.
While it is impossible to prevent all such problems, there are ways to prevent some of them.
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Screens are the first line of defence
Clearly, the best way of dealing with solid materials in the waste stream is to intercept and remove them before they can get into the wastewater treatment process. Screens are very effective at this task and care should be taken to keep them clear and in good repair.
Settling basins near the plant intake are also very valuable, since they allow dense, fine-grain solids, such as sand, to settle out before they come into contact with machinery.
Chose the right pump configuration for the job
Some pump/impeller combinations are better at handling solids-laden wastewater streams. Pumps with large free passages can pass relatively large solid objects without becoming blocked. Special impellers, such as the KSB’s D-impeller, have specially contoured leading edges to help avoid becoming clogged with tangles of fibrous materials. Vortex, or free-flow impellers, such as KSB’s newly-designed F-Max model, offer large free passages and are effective when pumping fluids with high levels of dissolved gases.
A key to optimizing pumping efficiency is to match pump characteristics to local operational requirements. Manually clearing blocked pumps can be very time and resource consuming. Pumps intended to handle raw or minimally screened sewage should be selected with good anti-clogging characteristics in mind. Pumps installed at the later stages of the wastewater treatment cycle are less likely to encounter solid materials and can be fitted with impellers that deliver good energy efficiency.
“Chopper”, “grinder” or “cutter” impellers are designed to break up solids into smaller pieces that can pass through the pump without causing blockages. However, when solids are broken into smaller bits, they can more easily pass through screens. This means that they become harder to separate from the waste stream and can cause problems with downstream biological digestion processes.
Keep things moving
In many pumping applications, low flow rates are regarded as a good thing, since they reduce friction losses. For wastewater though, low flows can lead to sludge material settling out in tanks, wells, or in awkward places in the piping system. Careful design of pump stations and related piping systems can keep flow rates high enough that solids are kept in motion. Low flow rates, which implies running on the far left side of the pump’s characteristic curve, can cause cavitation at the discharge nozzle.
Very rapid flows can also cause problems, such as excessive turbulence, the formation of vortices and the entrainment of air in the stream of water entering the pump station. These can create uneven, two-phase flows near the pump suction nozzle and result in rough running and damage due to cavitation.
Avoiding these flow-related problems requires careful design of pump station structures and piping systems.
Stormwater plus wastewater?
Facilities designed to handle a combination of wastewater and stormwater can be difficult to design, considering the extreme variation in flow rates between “normal” and “peak storm” conditions. They should be designed with separate sewage and stormwater sections. The sewage section is relatively small, designed to deal with the lower “normal” wastewater flows and maintain minimum flow velocities. The stormwater section is large enough to handle extreme conditions. Incoming water normally enters the sewage section. It only flows into the stormwater section when volumes exceed the capacity of the sewage section.
A “dissipation chamber” in the stormwater section allows turbulent inflows to settle before they reach the pump intakes. Splitter plates between the pumps keep turbulent flow conditions that might become established near one pump intake from affecting neighbouring pumps.
Multiple pumps make it easier to manage highly variable flow rates. The number of pumps turned on would depend on the volume of water that must be handled. Each running pump would be operated near to its best efficiency point.
Mehran Masoudi is a Project Engineer with KSB Pumps Inc. This article appears in ES&E Magazine’s October 2017 issue.
