by Jim Becker, Pump Solutions Group
May 23, 2012

These pumps enhance cost efficiencies and reduce breakdowns.


No matter where a community is located, one thing remains constant—it will produce both solid and liquid waste. Wastewater includes household wastes (from toilets, showers and sinks), storm water runoff and industrial waste from commercial facilities.

Most modern communities dispose of wastewater through a sewer system, where it is collected and transported through a network of pipes and pump stations until it reaches a wastewater treatment facility. These treatment facilities remove physical, chemical and biological contaminants from the wastewater and produce a liquid waste stream that is suitable for reuse or for discharge back into the environment.

Once it reaches a treatment facility, wastewater undergoes an extensive treatment process that ensures that it is sanitary enough for reuse or release back into the environment. This process can be extremely harsh and involves specialized, industrial pumping equipment. Pumps are used during virtually every step in the treatment process and serve many purposes—including inlet raw wastewater pumping, grit removal, chemical feed and sludge transfer.


The Challenge

To say that the wastewater treatment process is unforgiving and abrasive on pumping equipment would be an understatement. Pump breakdowns due to erosion and clogging are constant. These breakdowns only increase when the incorrect pumping technology is used. Selecting the correct pump for each application is essential.

“One of the biggest problems we have seen when it comes to pumping systems, is that if you don’t select the correct pump for the right application—and it might be an excellent pump—it still isn’t going to function properly,” says Scott Maring, assistant superintendent of wastewater treatment for the Metropolitan Sewer District of Cincinnati, Ohio. “If we buy a pump that is able to handle a certain type of application but just beats itself up in the process, it’s not going to be very reliable and is going to have lots of problems that are symptomatic of other things. Many factors go into why we purchase a pump. It has to be able to perform reliably in its service and be maintainable. It is critical that the pump is perfect for the application. If not, we would be replacing a lot of pumps.”


Sludge Transfer

One application in which extra caution is needed prior to selecting a pump is sludge transfer. As mentioned above, once wastewater reaches a treatment facility, it must go through different treatment processes. Two of these processes include primary and secondary treatment, and both produce large amounts of sludge.

During primary treatment, wastewater flows into a primary clarifier or sedimentation tank. Once collected, the wastewater is held until the larger, heavier solid objects settle to the bottom, while the oil, grease and lighter solids float to the surface. What settles is called primary sludge, which is then removed from the tank by mechanical scrapers and pumps. Occasionally, primary sludge is pumped out of the facility to a different location where it is treated and reused in different ways.


Blackmer plant
Wastewater treatment facilities use specialized pumping equipment to help provide sanitary water. 


Only about 60 percent of the suspended solids are removed during primary treatment. Therefore, the wastewater must undergo a secondary treatment process. This secondary treatment involves removing biological matter and soluble materials that require oxygen for decay from the wastewater, as well as the removal of any additional suspended solids. Secondary treatment can be accomplished through many methods. One of the most common methods is the activated sludge process.


Activated Sludge Process

During the activated sludge process, wastewater and biological sludge are pumped into an aeration tank and combined. Air is then injected into the mix to help break down the biological matter. This mixture then flows to a second clarifier, or settling tank, where the activated sludge settles and is pumped out. Typically, about 35 percent of the sludge is then returned to the aeration tank to help maintain a high population of microbes to break down the organic material. This sludge is commonly referred to as return activated sludge. Because more activated sludge is produced than is needed for return sludge, any surplus activated sludge, commonly referred to as waste activated sludge, is pumped out of the tank for disposal.

Due to the difficulty inherent in pumping sludge, gravity is used to transfer sludge whenever possible. However, whenever gravity is not a viable transfer method, heavy-duty pumping equipment is required to do the job. Since sludge is a viscous, thick material that often contains large amounts of grit and grease, not every type of pumping technology is an ideal choice. When a pump has not been specifically designed for sludge transfer applications, the end-user often experiences a variety of problems.

“If grit gets into the sludge, it can be destructive to the equipment,” explains Maring. “So if we don’t get the grit out early, it just wears on all the pump’s rotating pieces. Grit, along with grease, can thicken the sludge and make it very difficult to pump.”


Primary Sludge and Waste-Activated Sludge