The microbial degradation of pentachlorophenol (PCP) and lindane in the activated sludge process was studied. 14 steady-state experimental runs were performed in single-stage activated sludge reactors under various operating conditions. PCP and lindane mixed with three other model compounds were added to the feed as secondary substrate, with synthetic sewage as primary substrate. Reactor concentrations (dissolved) of the model compounds were in the range of 1-200 µg/l.

For PCP, the biodegradation increased with increasing solids retention time SRT. This indicates that degradation takes place by catabolic growth of a specific fraction of the biomass. At SRT > 8 days the 1st order biodegradation rate constant was about 2.5 × 10−3 1/(mg MLSS day) at a temperature of 15°C.

For lindane, the reverse relation was observed. Increased degradation was observed with increasing degradation of primary substrate. This indicates the presence of other mechanisms such as co-metabolism or probably reductive dechlorination.

In relation to upgrading of wastewater treatment plants to biological nutrient removal, xenobiotic compounds behaving like PCP are expected to be increasingly degraded. For compounds behaving like lindane, co-metabolic activity can be stimulated in plug-flow reactors rather than CSTR, and anaerobic zones in the activated sludge process may play an important role for the degradation.

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