Return Activated Sludge (RAS) Rate
The recycle rate that keeps enough biomass in the aeration basin.
RAS rate is typically expressed as a percentage of influent flow, commonly 25–75% for conventional activated sludge systems.
Increasing RAS rate raises MLSS in the aeration basin (up to a point) by returning more settled solids from the clarifier rather than wasting them.
Too little RAS starves the aeration basin of biomass and lowers MCRT; too much RAS can overload the clarifier hydraulically and reduce sludge blanket depth control.
Adjust RAS rate in small, deliberate increments and give the system 1–2 sludge ages to stabilize before evaluating the effect.
RAS concentration can be estimated from the settled sludge volume in a 30-minute test: RAS % solids ≈ MLSS ÷ (settled volume fraction), a useful field check between full lab TSS results.
A sludge blanket depth that's rising even with RAS rate unchanged usually means clarifier underflow solids have become more concentrated (often from an SRT or F/M shift) rather than an actual RAS pumping problem — check MLSS and wasting trends before assuming a mechanical RAS pump issue.
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