Understanding WWTP Flow Systems and Progressive Cavity Pumps

Plant Influent

Wastewater or other liquid, raw or partially treated, flowing into a treatment plant.

Raw Wastewater Screens or Comminutors

Raw Wastewater Screens are a pre-treatment process to remove large suspended or floating solids from raw sewage to prevent subsequent plugging of pipes or damage to pumps.
Comminutors are a shredding or grinding device that reduces the size of gross suspended materials in wastewater without removing them from the liquid.

The Main Wastewater Pumps

These pumps are the first to see wastewater, therefore they are designed to meet rugged standards, Since the pumps must deliver a wide range of flows, Centrifugal pumps are usually the accepted choice because of their smooth channels and impellers with large sized openings to prevent clogging. However, ACCA pumps are being used on high head lift stations to wastewater plants. A Progressive Cavity Pump can be used at this point to sample the raw wastewater.

Grit Chamber

There are several types of grit chambers from which to choose. Some are aerated and others depend on proportional weirs or other devices to regulate velocity and grit separation. The grit chambers are designed to remove inorganic materials that could cause excessive equipment wear or interfere with the operation of other plant equipment. Examples of grit would be sand, coffee grounds, and cigarette filter tips. Aerated grit chambers provide a period of wastewater detention for the purpose of trapping grit through the use of an air-induced rotation of the wastewater. Compressed air, usually from the plant’s process air supply, is used. Cyclone Grit Removal is a process whereby a centrifugal force is used to separate grit from organics in a cone-shaped unit. A recessed impeller pump discharges a slurry of grit and organic matter tangentially (high velocities of fluid entering the cyclone) near the upper end of the degritter at the perimeter. This discharge creates a vortex that produces a grit slurry at the lower, narrower opening, and a larger volume containing volatile material at the upper port, The grit flows out the bottom and the degritted flow enters the plant to receive further treatment.

Two other grit chambers used are hand-cleaned chambers and mechanically cleaned chambers. The important thing to remember is if grit is not removed early in the treatment process, inevitably it will cause problems later.

WWTP Progressive Cavity Pumps
Progressive Cavity Pumps in WWTP Flow System

Primary Sedimentation and Grease Skimming Tanks

The purpose of primary sedimentation is to separate the settleable and floatable solids from the wastewater for appro­priate handling. Primary settling tanks remove the larger sized solids from wastewater before it is discharged to receiving water or secondary treatment units. Solids that readily settle are collected from the bottom of tanks as primary sludge. Floating materials such as grease and oils are skimmed from the top of tanks and removed as primary scum. ACCA pumps can be used for three different applications during this process, the primary sludge removal to the digester, the primary scum removal, and the primary wastewater to the aeration tanks.

The advantages of a Progressive Cavity Pump would be:

  • High Solids Handling Ability.
  • Reversibility – easy unclogging of most plugged suction lines, thereby cleaning scum and grease that normally clings to the I.D. of the pipelines.
  • Metered Flows.
  • High Head Capabilities.
  • Open Throat Suction Ports available for hig
  • solids content and thickened grease.
  • Cleanliness and Elimination of Odor.
  • No Valves — ACCA PCP is a valveless pump with no clogging problems.
  • Non-Pulsating Discharge Flow.
  • Quiet Operation — Less than that required by government standards.