A free process control resource from Wastewater Solutions, Inc.help@ActivatedSludge.com  |  503-701-2766
ActivatedSludge.comPractical wastewater process control

RAS and WAS

RAS keeps the biomass in the process. WAS removes the extra growth and sets the sludge age. Together they are the operator's main controls.

RAS and WAS relationshipInfluent enters the aeration basin, mixed liquor flows to the clarifier, effluent leaves. Settled sludge returns to the aeration basin as RAS, and a smaller stream is removed as WAS to control sludge age.InfluentAeration basinMLSS 2,500 to 3,500 mg/LSecondaryclarifierEffluentRAS: returns settled organisms (often 40 to 100% of Q)WASto thickeningRAS controls where the solids are. WAS controls how many there are (SRT).
Solids flow around the loop: mixed liquor to the clarifier, RAS back to the basin, WAS out to solids handling.

RAS: return activated sludge

RAS pumps draw settled sludge from the clarifier floor and send it to the head of the aeration basin (or the anoxic or anaerobic zone in BNR plants). Typical RAS rates are 40 to 75% of influent flow; White River designs for 50% (4.0 MGD at 8.0 MGD) with five 2 MGD pumps (four duty, one standby), flow paced to influent.

Four ways to estimate RAS:

  1. Mass balance: RAS = Q x MLSS / (RAS TSS minus MLSS)
  2. Settleometer: RAS ratio = SSV30 / (1,000 minus SSV30)
  3. SVI: estimated RAS TSS = 1,000,000 / SVI, then mass balance
  4. Blanket depth: adjust to hold the target blanket

See How Much RAS Should I Return? for worked examples.

WAS: waste activated sludge

WAS removes the daily growth of biomass and sets SRT. It is usually drawn from the RAS line (thick) or directly from mixed liquor (thin but steady).

WAS (lb/d) = inventory (lb) / target SRT minus effluent solids (lb/d)

WAS (gpd) = WAS (lb/d) / (WAS TSS x 8.34) x 1,000,000

White River design: 90,072 lb / 11.6 d minus 534 lb/d = 7,231 lb/d. At 9,000 mg/L, about 96,000 gpd.

WAS goes to thickening (White River uses gravity belt thickeners), then digestion and dewatering. See Solids Handling.

Mass balance check

At steady state:

Solids into clarifier = solids out in RAS + WAS + effluent

(Q + R) x MLSS = R x RAS TSS + WAS + effluent solids (all in lb/d)

If this does not close within about 10 to 15%, check flow meters and sampling.

Daily routine

  1. Measure blanket depth in each clarifier.
  2. Run settleometer and SVI.
  3. Check RAS TSS and MLSS.
  4. Compare RAS rate with the four methods.
  5. Calculate WAS from the SRT target.
  6. Record changes and reasons.