Effect of redox conditions in the degradation of (RS)-MCPP during biological treatment of synthetic wastewater in an anaerobic membrane bioreactor (AMBr)

Abstract

The aim of this research is to investigate the effect of mecoprop (RS-MCPP) degradation in an anaerobic membrane bioreactor (AMBr) under a range of redox conditions (methanogenic, nitrate reducing and sulphate reducing). A method was developed for the analysis of (RS)-MCPP in aqueous form using High Performance Liquid Chromatography (HPLC). The study was carried out in five phases. Initially, Phase I investigated the effect of elevated (RS)-MCPP on AMBr under methanogenic conditions. In this phase, increasing concentrations of (RS)-MCPP (5 - 200 mg. L-1) promoted a progressive increases in the (RS)-MCPP substrate utilisation rate (SUR) (0.15 to 3.19 [ig-mgVSS' 1. d"'). However, the COD removal efficiency decreased gradually from 98 % to 94 % as the (RS)-MCPP concentration increased in the feed from 50 - 200 mg. L-1, respectively. Phase II investigated the effect of denitrifying conditions on (RS)-MCPP degradation. Results showed that (RS)-MCPP removal efficiency and (RS)-MCPP SUR improved from 2- 47 % and 0.5 to 60.7 Rg. mgVSS-l. d-1, respectively as the COD/N-NO3-1 ratio and OLR were reduced from 250 to 0.2 and 1.51 to 0.07 kgCOD M, 3 A-1, respectively. Phase III investigated the effect of sulphate-reducing conditions on (RS)-MCPP degradation. Results showed that sulphidogenic conditions were less effective than nitrate reducing conditions with an average (RS)-MCPP removal and (RS)-MCPP SUR at 20(±10.7) % and 16.4 ýtgm. gVSS-lffl, respectively. Phase IV investigated the effect of HRT on the kinetics of (RS)-MCPP degradation. The robustness and stability of AMBr was also investigated in response to hydraulic shock loads on the biomass imposed. Results showed that the AMBr is stable to a large transient hydraulic shock loads, and it recovered rapidly to baseline performance after the hydraulic shock load had ended. High (RS)-MCPP degradation (up to 75 %) and utilisation rates (43 gg. mgVSS'1. d") were observed at long HRT (16.9 days). Phase V was a confirmatory experiment study to investigate the possible effects of trace quantities of oxygen that had been present in the reactor headspace and the effluent tank during Phase IV. This confirmed that the system performance towards (RS)-MCPP biodegradation had not been affected by trace oxygen levels when comparison are made. The molecular-based method, fluorescent in situ hybridisation (FISH) was used for the direct identification and enumeration of microbial communities in the AMBr sludge. In general, FISH results showed populations were dominated by methanogenic archaea in methanogenic phase (Phase I and IV) comprising between 55-66 % of the DAPI count. However, as anoxic condition were introduced in Phase II and III (nitrate reducing and sulphate reducing), the results identified a definite population shift, with the predominance of eubacteria (70-86 % of the DAPI count). The two methanogenic genera detected were Methanosaeta and Methanosamina. Of these two genera, the former was dominant in the reactor accounting for 57-100 % of the total methanogenic archaea. Furthermore, a comparison of (RS)-MCPP utilisation rates in each phase of the research revealed wide ranges (RS)-MCPP degradation efficiency and (RS)-MCPP SUR, however, although changes were observed in the composition of the bacterial population, no direct linkage could be made with any of the individual groups identified by FISH.