Biological Wastewater Treatment Upset From Shock Load
An activated sludge plant treats 5000 m³/d of petrochemical wastewater with a design influent COD of 1200 mg/L and a 6-hour hydraulic retention time in the aeration basin (1250 m³). Mixed liquor suspended solids are maintained at 3500 mg/L. A production upset sends a 12-hour shock load of 4000 mg/L COD with a high fraction of phenolic compounds. Within 24 hours the effluent COD rises from 80 to 600 mg/L, dissolved oxygen in the basin climbs from 2.0 to 5.5 mg/L, and the sludge volume index rises from 120 to 260 mL/g.
1. Interpret the DO and SVI signals to explain what is happening biologically.
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Rising DO with falling COD removal is the key signal: the biomass is no longer consuming oxygen at the normal rate, meaning either the biomass is inhibited or the substrate is not bioavailable. Phenolic shock loads are known to inhibit nitrifiers and some heterotrophs; the high DO indicates the oxygen demand collapsed, not that aeration increased. The SVI jump from 120 to 260 mL/g indicates filamentous bulking or deflocculation - the biomass is stressed and the floc structure is breaking down, which also worsens settling and can cause solids carryover. The combination (high DO, high effluent COD, high SVI) points to toxicity/inhibition rather than organic overload, because an organic overload would drive DO down, not up. The 12-hour duration is long enough to shock the population but short enough that recovery is possible if the toxicant is stopped.
2. What immediate and recovery actions would you take, and how would you monitor recovery?
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Immediate: stop or divert the phenolic stream, reduce or stop wasting to retain biomass, and consider adding a readily biodegradable carbon source (e.g., acetate or methanol) to sustain the heterotrophs while the toxicant passes. Increase aeration only if DO falls - currently DO is high, so cut aeration to save energy and avoid stripping volatiles. Recovery: monitor effluent COD, DO, SVI, and specific oxygen uptake rate (SOUR). A recovering sludge shows SOUR rising back toward baseline, DO falling as demand returns, and SVI trending down. If the sludge is badly deflocculated, consider seeding with fresh sludge or adding a coagulant/polymer to the clarifier. Long-term: install a equalization tank or a toxicity warning (online UV or respirometry) to catch the next shock before it reaches the basin, and evaluate whether the phenolic stream needs pretreatment (e.g., Fenton or activated carbon) before discharge to the biological system.