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DC Water Operator: Wastewater Treatment: Process Control, Nitrogen Removal, and Solids

DC Water Operator · Work through your exam, one topic at a time.

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Wastewater Treatment: Process Control, Nitrogen Removal, and Solids

What this domain covers

The Wastewater Treatment domain covers preliminary, primary, secondary, and advanced treatment, solids handling, and the process-control math that keeps a plant in permit compliance. DC Water's Blue Plains Advanced Wastewater Treatment Plant is the reference point: roughly 370 MGD capacity, enhanced nitrogen removal for Chesapeake Bay limits, UV disinfection, Cambi thermal hydrolysis ahead of anaerobic digestion, and Class A biosolids.

Process train and process control

  • Preliminary treatment screens and grit-removes; primary clarifiers remove settleable solids and scum.
  • Secondary treatment is usually activated sludge: influent BOD and TSS are metabolized in the aeration basin, and mixed liquor settles in the clarifier. Return activated sludge (RAS) keeps the biomass; waste activated sludge (WAS) controls solids retention time (SRT).
  • Key controls: F/M, MLSS/MLVSS, SRT, dissolved oxygen, and sludge volume index (SVI). Higher SRT means lower F/M, less excess sludge, and the longer sludge age nitrifiers need.
  • Nitrogen removal is two-step: nitrification (ammonia to nitrate, consuming oxygen and about 7.1 lb alkalinity per lb ammonia-N) then denitrification (nitrate to nitrogen gas, consuming carbon and returning alkalinity).
  • UV disinfection depends on dose and UV transmittance, so effluent TSS matters.

Worked example: SRT

Aeration tank volume 2.0 MG, MLSS 2,500 mg/L, WAS 0.15 MGD at 8,000 mg/L. Solids in aeration = 2.0 × 2,500 × 8.34 = 41,700 lb. Solids wasted = 0.15 × 8,000 × 8.34 = 10,008 lb/day. SRT = 41,700 ÷ 10,008 ≈ 4.2 days. If the target were 10 days for nitrification, you would cut WAS to about 0.06 MGD - the reason cold-weather nitrification upsets are managed through solids, not just air.

Traps that cost marks

  • Confusing SRT (solids inventory ÷ wasting rate) with hydraulic detention time.
  • Treating BOD and CBOD as interchangeable, or using TSS where VSS is required.
  • Forgetting alkalinity and oxygen demand when nitrification is required.
  • Reporting removal efficiency as influent/effluent instead of (in − out) ÷ in.
  • Ignoring that UV performance falls when transmittance falls, even at constant flow.