Extended anaerobic conditions in the biological wastewater treatment: Higher reduction of toxicity compared to target organic micropollutants.

Extended anaerobic conditions during biological wastewater treatment may enhance the biodegradation of micropollutants. To explore this, we combined iron-reducing or substrate-limited anaerobic conditions and aerobic pilot-scale reactors directly at a wastewater treatment plant. To investigate the detoxification by these processes, we applied two in vitro bioassays for baseline toxicity (Microtox) and reactive toxicity (AREc32) as well as in vivo bioassays with aquatic model species in two laboratory experiments (Desmodesmus subspicatus, Daphnia magna) and two on-site, flow-through experiments (Potamopyrgus antipodarum, Lumbriculus variegatus). Moreover, we analyzed 31 commonly occurring micropollutants and 10 metabolites. The baseline toxicity of raw wastewater was effectively removed in full-scale and reactor scale activated sludge treatment (>85%), while the oxidative stress response was only partially removed (>61%). A combination of an anaerobic pre-treatment under iron reducing conditions and an aerobic nitrification significantly further reduced the residual in vitro toxicities by 46-60% and outperformed the second combination consisting of an aerobic pre-treatment and an anaerobic post-treatment under substrate-limiting conditions (27-43%). Exposure to effluents of the activated sludge treatment did not induce adverse in vivo effects in aquatic invertebrates. Accordingly, no further improvement in water quality could be observed. Compared to that, the removal of persistent micropollutants was increased. However, this observation was restricted to a limited number of compounds and the removal of the sum concentration of all target micropollutants was relative low (14-17%). In conclusion, combinations of strictly anaerobic and aerobic processes significantly enhanced the removal of specific and non-specific in vitro toxicities. Thus, an optimization of biological wastewater treatment can lead to a substantially improved detoxification. These otherwise hidden capacities of a treatment technology can only be uncovered by a complementary biological analysis.