Why is grit removed early in the treatment process?
- To lower the BOD load on the aeration basin
- To stop odors forming in the primary clarifier
- To prevent pump wear and deposits in tanks
- To reduce the chlorine demand of the effluent
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C. To prevent pump wear and deposits in tanks
Grit is sand, gravel and other heavy inorganic material. It abrades pumps and piping and settles in channels and digesters, taking up volume. Because it is inorganic, removing it does little for BOD or odor.
Reference: Preliminary treatment: grit removal
What is the purpose of return activated sludge (RAS)?
- To return settled organisms to the aeration tank
- To remove excess microorganisms from the process
- To carry clarified effluent on to disinfection
- To feed raw primary sludge into the digester
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A. To return settled organisms to the aeration tank
RAS is sludge settled in the secondary clarifier and pumped back to the head of the aeration tank to maintain the population of microorganisms (MLSS). Removing excess solids from the system is the job of waste activated sludge.
Reference: Activated sludge terminology: RAS
Wasting activated sludge (WAS) is the operator's main tool for controlling:
- Influent flow
- Effluent pH
- Sludge age
- Aeration tank depth
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C. Sludge age
Microorganisms multiply as they consume the waste. Removing a controlled amount each day sets how long solids stay in the system (sludge age or MCRT) and so controls the MLSS and the F/M ratio.
Reference: Activated sludge process control: wasting and MCRT