Settleability-Based RAS Rate (from SVI)
The RAS number straight from the settleometer, no RAS_SS lab result needed.
Example
You enter
- Plant influent flow Q (MGD) 4
- Mixed-liquor solids MLSS (mg/L, target) 2500
- Sludge volume index SVI (mL/g, from the settleometer) 100
You get
- Achievable return solids (settleability ceiling) 10,000 mg/L
- Return ratio 33% of plant flow
- RAS flow rate 1.33 MGD
Checked against published worked examples from: WEF / Sacramento activated-sludge operator practice.
Details, formula, and sources
ScopeThe achievable return concentration is Xr = 1,000,000 / SVI, so the ratio for a target MLSS is R = MLSS / (Xr - MLSS) and Q_RAS = R x Q. Poor settling forces a much higher return - holding 2,500 mg/L on a good SVI of 100 needs a 33% return, but a bulking sludge at SVI 150 wanting 3,000 mg/L needs 82%, and if the pumps cannot deliver it the MLSS target is unreachable. The settleometer path to the same mass balance a RAS flow rate gives from a measured RAS_SS. An operating aid, not a process design.
Formula or tableXr = 1,000,000 / SVI; ras_ratio = MLSS / (Xr - MLSS); Q_RAS = ras_ratio x Q.
Source and editionSettleability-based RAS from the sludge volume index (WEF MOP / Sacramento activated-sludge manuals), by name.
Free accessThe settleability-based RAS relation is public wastewater-operations material; the settleability and clarifier performance govern the actual return.
What governsEstimate. AHJ and licensed professional govern.
API field namesplant_flow_mgd, mlss_mg_l, svi_ml_g, achievable_ras_ss_mg_l, ras_ratio_pct, q_ras_mgd
- Settleability ceiling the achievable return concentration is about Xr = 1,000,000 / SVI mg/LWEF / Sacramento manuals
- Higher SVI, higher RAS poor settling forces a higher return ratio; a bulking sludge can make a target MLSS unreachable within pump capacityWEF / operator practice