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SRT and MCRT Calculator

Solids retention time is the average number of days solids stay in the system. It controls nitrification, floc quality and sludge production.

SRT and MCRT

Formula

SRT, days = (Aeration volume, MG x MLSS, mg/L x 8.34)
        / [(WAS flow, MGD x WAS conc, mg/L x 8.34) + (Effluent flow, MGD x Effluent TSS, mg/L x 8.34)]

MCRT adds the clarifier solids inventory to the numerator.
Inputs used
  • Aeration basin volume (in service) (MG)
  • MLSS (mg/L)
  • WAS flow (gpd)
  • WAS (RAS) concentration (mg/L)
  • Effluent flow (MGD)
  • Effluent TSS (mg/L)
  • Clarifier volume (optional, for MCRT) (MG)
  • Average clarifier solids (optional, for MCRT) (mg/L)

Worked example (White River WRRF, fictional)

  1. Inventory = 3.6 MG x 3,000 mg/L x 8.34 = 90,072 lb
  2. Wasted = 0.096 MGD x 9,000 mg/L x 8.34 = 7,206 lb/d
  3. Effluent loss = 8.0 MGD x 8 mg/L x 8.34 = 534 lb/d
  4. SRT = 90,072 / (7,206 + 534) = 11.6 days

Tips

  • Use 7 day averages for MLSS, WAS and effluent TSS.
  • Use the aeration volume in service. For aerobic SRT, leave out anoxic and anaerobic zones.
  • Always include effluent solids; during storms they can be a large share of the total.

Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.

This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.