Although the wastewater treatment process is designed to remove any large solids before primary and secondary treatment occurs, removal of these objects is not flawless. Often, primary sludge and return or waste activated sludge will still contain large, stringy solids. These solids frequently clog the pumping equipment. This happens when solids come in contact with the pump’s impellers, especially if the pump’s discharge port is too small to pass these objects. Additionally, even pumps designed to pass large solids can experience clogs when stringy material gets weaved into a mass and gets caught in the pump. Regardless of the specific cause, service technicians must disassemble and clean the pump whenever a clog occurs. This shuts down the treatment process and can result in unnecessary operating costs.
“Grit and grease are two of the biggest killers in this type of pumping application, as are the large objects that clog the pump,” says Maring. “Because of these issues, it wears valves, damages instrumentation, and clogs a pump’s piping. Once the pump’s pipe is restricted, it’s essentially changing the dynamics of the pumping system. If the pipe is choked off so that it’s smaller, the pump has to work harder to squeeze the sludge through a smaller hole. This is not very efficient.”
In addition, pumping equipment accounts for a large amount of the energy consumed at a wastewater treatment facility. Because of this, a pump that is able to continually perform at its best efficiency point (BEP) while pumping sludge is the ideal option. Pumps represent a large part of the facility’s total investment in equipment. Pump maintenance and replacement can be costly, which increases the cost of investment, so pumps that minimize maintenance and downtime are the preferred choice.
“Fixing and replacing pumps is a significant part of our job,” says Maring.
|Pumps represent a large part of the facility's total investment.|
Therefore, the pumping equipment selected for sludge transfer applications must not only be designed to handle large solids but also be cost-effective and require low maintenance. Fortunately for those involved in the transfer of sludge, a pumping technology is available that offers a smooth, cost-effective operation with trouble-free maintenance.
The preferred pump for demanding wastewater applications is the centrifugal vortex pump. This type pump has been designed with a semi-open impeller recessed into the volute section of the pump. A recessed impeller design allows for the clear passage of solids through the ports. Because of this, vortex pumps are ideal at handling the large, stringy and abrasive solids often associated with primary and activated sludge.
“When it comes to the pumps we use during our primary sludge applications, we typically use centrifugal pumps that feature end-suction, open-face type impellers that can pass at least three-inch solids,” says Maring.
Unlike typical centrifugal pumps, the impeller of a vortex pump is recessed into the back of the pump’s casing and creates a liquid vortex in the open casing which directs solids, slurry, sludge, grit and stringy or fibrous material through the pump.
A vortex design offers some significant advantages during sludge transfer applications. Since the impeller is recessed and not in the primary flow path, it minimizes contact with the process fluid and the majority of the solids that pass through the pump, allowing for clear passage of solids with minimal clogging. Basically, whatever goes into the pump will be pumped out as long as the solids are nominally smaller than the ports. This is ideal when handling liquids that contain long stringy material and larger diameter solids. Lowering the amount of clogs results in fewer breakdowns.
In addition to allowing for the clear passage of solids, the vortex pumping action also offers other advantages over traditional pumping technology. Because only 15 to 20 percent of solids come into contact with the pump’s impeller, erosion is greatly reduced. The design also allows for the handling of entrained air far beyond the capabilities of a standard centrifugal pump. Additionally, because of their concentric casing design, recessed impeller pumps offer more reliable operation at low flows and can even shutoff for extended periods of time.
Because pumping equipment can represent a large part of the facility’s investment in equipment, the total cost of ownership should be a major consideration when selecting a pump. Not only will centrifugal vortex pumps help reduce maintenance costs, but typically will be much less expensive to purchase compared to other pumping technologies. For example, on a price-per-given-flow basis, centrifugal vortex pumps are typically much less expensive to purchase than positive displacement (PD) pumps.