Microbial degradation of pesticides in rapid sand filters used for drinking water treatment

Abstract

Groundwater is used as drinking water source all over the world. However, large parts are contaminated by pesticides at low concentrations (sub µg/L), due to anthropogenic activities. These pesticides can adversely impact human health, and have legal implications. Thus, it is important to identify sustainable methods to remove pesticides from polluted water sources. Aeration of anaerobic groundwater, followed by biological rapid sand filtration is a widespread technology in drinking water treatment. Even though these systems are not designed for removal of trace contaminants, they have shown potential for microbial degradation of pesticides and their degradation products. If pesticides can be removed in rapid sand filters, it is of large commercial interest due to the importance in maintaining a simple, sustainable water treatment. To take advantage of the microbial pesticide degradation and identify associated risks, it is necessary to understand the extent of pesticide degradation and the governing microbial processes in the water treatment. The objective of this PhD thesis was to investigate the potential for microbial pesticide degradation at waterworks treating groundwater and to investigate, which microbial processes govern the degradation, in order to suggest how pesticide degradation can be stimulated in water treatment systems. In a full-scale waterworks the rapid sand filters removed a phenoxy acid (herbicide) contamination from drinking water and investigations showed a potential for removing several pesticides in filter sand from different waterworks. The largest biological pesticide removal was observed in filter sand from a waterworks characterised by methane-rich groundwater. Thus, it was investigated for a connection between pesticide degradation and methane oxidation. In an enrichment culture, methanotrophs contributed to the degradation of phenoxy acids. However, a phenoxy acid was degraded in filter sand from 10 different waterworks receiving groundwater with varying concentrations and absence of methane. The omnipresent phenoxy acid degradation demonstrated, that degradation in rapid sand filters was not associated with methane oxidation. Based on the present investigations and literature, it was suggested that phenoxy acid degradation in rapid sand filters is due to primary metabolism, and that degradation might be stimulated by enriching naturally occurring specific degraders in the sand filters upon exposure to phenoxy acid contaminated groundwater. A suite of evidence showed that the herbicide bentazone was co-metabolically transformed to hydroxy-bentazone by the methanotrophic enrichment culture. Subsequently, it was investigated whether bentazone degradation was also connected with methane oxidation in drinking water treatment systems. In waterworks wells in Denmark, bentazone was detected significantly less frequently in wells with methane than in wells without methane. Similarly, the biological bentazone removal in filter sand from 14 waterworks correlated significantly with the maximum methane concentration in the raw water and did not correlate with other water quality parameters, such as the ammonium concentration. Furthermore, the connection between bentazone degradation and methane oxidation in filter sand was demonstrated by inhibition experiments, in which acetylene inhibited both the methane oxidation and the bentazone removal. The main biotransformation pathways clearly showed the importance of initial hydroxylation reactions during bentazone degradation in filter sand. However, bentazone was further degraded in filter sand and showed that not only methanotrophs, but also other heterotrophs contributed to the degradation. Methanotrophic biomass from the aeration tanks clearly demonstrated a bentazone degradation, which depended on the presence of methane. Transformation yields describing the bentazone removal versus the methane oxidation were in same order of magnitude for all investigated media: methanotrophic enrichment cultures, filter sand and biomass from aeration tanks, which strongly indicated that the same degradation process governed bentazone removal in the different media. It was suggested that full-scale waterworks operates like a sequential reactor system, where methanotrophs are grown in the aeration tanks and transported to the rapid sand filters where they can perform co-metabolic pesticide biodegradation. It was suggested that bentazone removal can be stimulated at waterworks, by stimulating growth of methanotrophs. Overall, this PhD demonstrated a substantial potential for biological pesticide degradation in drinking water treatment systems. While the omnipresent phenoxy acid degradation potential was probably due to specific degraders, bentazone degradation was connected with the methane oxidation. Based on the present investigations and literature, it was suggested that phenoxy acid degradation can be stimulated by enrichment of naturally occurring degraders in filter sand, and that bentazone degradation can be stimulated by stimulating growth of methanotrophs in the water treatment.