What pump selection considerations are needed to handle wastewater?
The type of material in the fluid should be considered when selecting a proper pump for wastewater treatment. Fluid streams within a wastewater treatment plant are characterized by their properties as follows: large solids, grit, sludge, scum and flocculated materials.
The wastewater fluid stream may contain one or more of these characteristics. These properties will create different considerations that need to be addressed when selecting a pump for the intended service. Therefore, the user should identify and communicate to the pump manufacturer the nature of the fluid for each specific application.
Generally wastewater pumps are required to handle solids. Rotodynamic pumps that are specifically designed to deal with solids and minimize clogging are referred to as solids-handling pumps. Solids may include household and commercial solids, large solids, stringy material (such as rags and hair), sanitary waste, plastic scraps, food waste, sticks, leaves, abrasive materials (such as sand, grit, stones and pieces of metal), and other inorganic and organic solids.
Solid materials that have recently become more prevalent in wastewater flows are: personal wipes, towels, cleaning cloths and household cleaning materials that are marketed to be discarded through the sewer system. These materials, in addition to the stringy materials and rags, can bind together to create a large mass that can lead to clogging issues in the pump and associated piping.
Grit pump applications require that the pump materials and pump speed to be selected to resist abrasion from grit. To limit the effect of grit on downstream equipment, grit-removal equipment is normally installed at the head of the plant.
Sludge pumping applications can contain stringy material, grit and grease and may be viscous in consistency. Some sludge can also contain a significant volume of entrained gases, such as hydrogen sulfide, due to organic bacterial action. The pumps should address these characteristics and minimize the potential for clogging and gas binding.
Scum is commonly present in stormwater pumping and similar applications. Scum consists of all of the floatable materials that are skimmed off the surface of primary and secondary clarifiers. This can consist of any materials that are light enough to float on the water surface. Scum normally consists of oils, grease, fats, wax, soaps, food wastes, hair and light plastic materials. The scum is removed from the clarifiers and sent to the digester or dewatering system.
Finally, pumps need to be able to handle flocculated materials. Pumps that transfer flocculated materials (smaller particles that have agglomerated into larger particles) need to keep the material in the flocculated state and not shear it. Flocculated materials are normally associated with clarifiers where the lighter solids have been flocculated to aid in their removal from the process.
For more information regarding wastewater treatment and pumping, refer to HI’s Wastewater Treatment Plant Pumps: Guidelines for Selection, Application, and Operation available at www.pumps.org.
