The response to step overloads of laboratory-scale contact process and UASB reactors and a pilot-scale anaerobic filter is examined. The reactors were operated on ice-cream waste water and changes in operational parameters including VFA levels and COD reduction were monitored. In the laboratory-scale experiments, feed strength was increased from 4500 to 22,000 mg [O] per litre for periods of 2 to 12 hours in contact process and one and two-stage UASB reactors. UASB stability was improved by supplementing with trace metals. Two-stage UASB's gave better COD removals under shock loading than single-stage UASB's or contact process reactors. In an 8 hour step overload experiment on the pilot-scale reactor, bicarbonate alkalinity was shown to be a good indicator of instability.

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