Department Process Engineering

Im Rahmen des Projekts ReTREAT wurden verschiedene Verfahren auf deren Eignung als biologische Nachbehandlung nach einer Ozonung getestet. Der Fokus lag dabei – neben allgemeinen Reinigungseffekten – insbesondere auf dem Abbau von labilen Reaktionsprodukten sowie deren ökotoxikologischen Wirkungen.

Ozonation is a water treatment process for disinfection and/or micropollutant abatement. However, ozonation of bromide-containing water leads to bromate (BrO₃^–) formation, a potential human carcinogen. A solution for mitigating BrO₃^– formation during abatement of micropollutants is to minimize the ozone (O₃) concentration. This can be achieved by dosing ozone in numerous small portions throughout a reactor in the presence of H₂O₂. Under these conditions, O₃ is rapidly consumed to form hydroxyl radical (^·OH), which will oxidize micropollutants. To achieve this goal, a novel process (“MEMBRO₃X”) was developed in which ozone is transferred to the water through the pores of polytetrafluoroethylene (PTFE) hollow fiber membranes. When compared to the conventional peroxone process (O₃/H₂O₂), the MEMBRO₃X process shows better performance in terms of micropollutant abatement and bromate minimization for groundwater and surface water treatment. For a groundwater containing 180 μg/L bromide, a 95% abatement of the ozone-resistant probe compound p-chlorobenzoic acid yielded <0.5 μg/L BrO₃^–, whereas in the conventional peroxone process, 8 μg/L BrO₃^– was formed. In addition, the efficacy of the MEMBRO₃X process was demonstrated with river water and lake water.

Combined partial nitritation–anammox (PN/A) systems are increasingly being employed for sustainable removal of nitrogen from wastewater, but process instabilities present ongoing challenges for practitioners. The goal of this study was to elucidate differences in process stability between PN/A process variations employing two distinct aggregate types: biofilm [in moving bed biofilm reactors (MBBRs)] and suspended growth biomass. Triplicate reactors for each process variation were studied under baseline conditions and in response to a series of transient perturbations. MBBRs displayed elevated NH₄⁺ removal rates relative to those of suspended growth counterparts over six months of unperturbed baseline operation but also exhibited significantly greater variability in performance. Transient perturbations led to strikingly divergent yet reproducible behavior in biofilm versus suspended growth systems. A temperature perturbation prompted a sharp reduction in NH₄⁺ removal rates with no accumulation of NO₂^– and rapid recovery in MBBRs, compared to a similar reduction in NH₄⁺ removal rates but a high level of accumulation of NO₂^– in suspended growth reactors. Pulse additions of a nitrification inhibitor (allylthiourea) prompted only moderate declines in performance in suspended growth reactors compared to sharp decreases in NH₄⁺ removal rates in MBBRs. Quantitative fluorescence in situ hybridization demonstrated a significant enrichment of anammox in MBBRs compared to suspended growth reactors, and conversely a proportionally higher AOB abundance in suspended growth reactors. Overall, MBBRs displayed significantly increased susceptibility to transient perturbations employed in this study compared to that of suspended growth counterparts (stability parameter), including significantly longer recovery times (resilience). No significant difference in the maximal impact of perturbations (resistance) was apparent. Taken together, our results suggest that aggregate architecture (biofilm vs suspended growth) in PN/A processes exerts an unexpectedly strong influence on process stability.