Trends in Micropollutant Biotransformation along a Solids Retention Time Gradient

Abstract

For many polar organic micropollutants, biotransformation by activated sludge microorganisms is a major removal process during wastewater treatment. However, our current understanding of how wastewater treatment operations influence microbial communities and their micropollutant biotransformation potential is limited, leaving major parts of observed variability in biotransformation rates across treatment facilities unexplained. Here, we present biotransformation rate constants for 42 micropollutants belonging to different chemical classes along a gradient of solids retention time (SRT). The geometric mean of biomass-normalized first-order rate constants shows a clear increase between 3 and 15 d SRT by 160% and 87%, respectively, in two experiments. However, individual micropollutants show a variety of trends. Rate constants of oxidative biotransformation reactions mostly increased with SRT. Yet, nitrifying activity could be excluded as primary driver. For substances undergoing other than oxidative reactions, i.e., mostly substitution-type reactions, more diverse dependencies on SRT were observed. Most remarkably, characteristic trends were observed for groups of substances undergoing similar types of initial transformation reaction, suggesting that shared enzymes or enzyme systems that are conjointly regulated catalyze biotransformation reactions within such groups. These findings open up opportunities for correlating rate constants with measures of enzyme abundance such as genes or gene products, which in turn should help to identify enzymes associated with the respective biotransformation reactions.

Impacts of solids retention time and antibiotic loading in activated sludge systems on secondary effluent water quality and microbial community structure

“…This is supported by Achermann et al. (), which showed that biodegradation rate constants standardized to biomass concentration increased rapidly for SRT <7 days but remained relatively constant for SRT ≥7 days. In other words, changes in microbial community structure are important for systems with shorter SRTs (e.g., 2 days vs. 7 days) but not necessarily systems with longer SRTs (e.g., 7 days vs. 20 days), which might explain why nitrification has historically been linked to improved TOrC removal (Clara et al., ; Suarez et al., ).…”

“…Treatment criteria associated with biochemical oxygen demand (BOD), nitrification/denitrification, and phosphorus removal can generally be achieved by incorporation of selective oxic zones and/or changes in solids retention time (SRT). More recently, a number of studies have documented the positive correlation between SRT and attenuation of highly sorbing and/or biodegradable trace organic compounds (TOrCs) (Achermann et al., ; Clara, Kreuzinger, Strenn, Gans, & Kroiss, ; Gerrity et al., ; Leu, Chan, & Stenstrom, ; Melcer & Klecka, ; Oppenheimer, Stephenson, Burbano, & Liu, ; Salveson et al., ; Suarez, Lema, & Omil, ). A related issue that is becoming increasingly important for the wastewater industry is the prevalence of antibiotics (Michael et al., ) and antibiotic resistance (Rizzo et al., ).…”

Clustering micropollutants based on initial biotransformations for improved prediction of micropollutant removal during conventional activated sludge treatment

“…72 However, it has also been reported that the effect of an increase in SRT is not always clear and may vary significantly depending on the tested compounds. 21,66,68 We observed a weak association between SRT and R% for micropollutants in the SMX and DCL clusters, and even a strong negative association between SRT and R% for micropollutants in the NAP cluster. Previous studies have also reported weak or negative associations between SRT and removal of sulfamethoxazole, diclofenac, and naproxen 10,21,58,73 which supports our findings.…”

“…We therefore hypothesize that accurate and robust predictions of micropollutant removal during wastewater treatment can be achieved only for groups of micropollutants that undergo the same types of enzyme-catalyzed biotransformations, as has also been suggested by some recent experimental work. 21,22 The lack of fundamental insights on the fate of micropollutants in WWTPs presents one of the biggest challenges in optimizing their removal during wastewater treatment. To address this challenge and to test our hypotheses, we designed a study that aimed to identify the drivers of micropollutant removal in WWTPs.…”

Microbial residence time is a controlling parameter of the taxonomic composition and functional profile of microbial communities

“…Briefly, six automated sequencing batch reactors (6 × 12 L) treating local municipal wastewater after primary clarification were operated in parallel at MRTs of 1, 3, 5, 7, 10, or 15 days (d) as detailed previously29 and summarized in Supplemental Table 1. Forty-eight days (time-point 1; TP1) and 187 days (time-point 2, TP2) after start-up, activated sludge samples were collected for DNA (at the start of the previously described biotransformation experiment29) and RNA (5 hours after the start of the experiment) extraction.…”