From full-scale biofilters to bioreactors: Engineering biological metaldehyde removal

Abstract

Polar, low molecular weight pesticides such as metaldehyde are challenging and costly to remove from drinking water using conventional treatment methods. Although biological treatments can be effective at treating micropollutants, through biodegradation and sorption processes, only some operational biofilters have shown the ability to remove metaldehyde. As sorption plays a minor role for such polar organic micropollutants, biodegradation is therefore likely to be the main removal pathway. Here, the biodegradation of metaldehyde was monitored, and assessed, in an operational slow sand filter. Long-term data showed that metaldehyde degradation improved when inlet concentrations increased. A comparison of inactive and active sand batch reactors showed that metaldehyde removal happened mainly through biodegradation and that the removal rates were greater after the biofilm was acclimated through exposure to high metaldehyde concentrations. This suggested that metaldehyde removal was reliant on enrichment and that the process could be engineered to decrease treatment times (from days to hours). Through-flow experiments using fluidised bed reactors, showed the same behaviour following metaldehyde acclimation. A 40% increase in metaldehyde removal was observed in acclimated compared with non-acclimated columns. This increase was sustained for more than 40 days, achieving an average of 80% removal and compliance (< 0.1 µ L-1) for more than 20 days. An initial microbial analysis of the acclimated and non-acclimated biofilm from the same filter materials, showed that the microbial community in acclimated sand was significantly different. This work presents a novel conceptual template for a faster, chemical free, low cost, biological treatment of metaldehyde and other polar pollutants in drinking water. In addition, this is the first study to report kinetics of metaldehyde degradation in an active microbial biofilm at a WTW.

Achieving drinking water compliance levels for metaldehyde with an acclimated sand bioreactor

“…The observed degradation rates achieved after acclimatisation are the same as those reported for batch kinetic trials with non-acclimatised sand and around half the rate observed with acclimated sand at 50 g L - (Rolph et al, 2019). However, the rates are significantly lower than those reported with isolated culture trials treating high concentrations (Thomas et al, 2017).…”

“…Thereafter, the rate of degradation observed in the upflow trials was more than twice that of the down flow direction at 0.045 g L -1 h -1 compared to 0.092 g L -1 h -1 . Comparative beaker batch trials with nonacclimated sand reached a maximum degradation rate of 0.17 g L -1 h -1 , substantially higher than observed in the current trials (Rolph et al, 2019). Likewise, very high degradation rates of 668.8 g L -1 h -1 have been reported during batch trials with isolated metaldehyde degraders exposed to high concentrations of metaldehyde (Thomas et al, 2017).…”

“…All equipment was either autoclaved or rinsed with nitric acid and then rinsed with acetone to prevent metaldehyde contamination. Both glassware and plastic containers were previously tested in batch and no adsorption detected (Rolph et al, 2019).…”

“…Currently, metaldehyde is the pesticide of greatest concern in the UK accounting for 1131 of the 1571 pesticide compliance failures reported between 2000 and 2016 (Cosgrove et al, 2019). Elevated levels are most commonly observed from late summer to winter, especially when a period of heavy rainfall follows a prolonged dry period (Cosgrove et al, 2019, Rolph et al, 2019. Reported concentrations are typically up to 10 g L -1 in the field and up to 1-2 g L -1 at the treatment works and last for between 2 weeks and 2 months (Ramos et al 2019;Cosgrove et al, 2019).…”

“…However, recent findings suggest that metaldehyde may be a suitable primary carbon source even at low concertation (Thomas et al, 2017) offering the potential for selective enrichment of metaldehyde degrading organisms which then remove metaldehyde in periods of elevated concentration (Costerton et al, 1995;Wingender and Jaeger, 2002). Support for this has been provided in batch tests treating spiked real waters where the degradation rates over a 72 hour period increased in line with spike concentration (Rolph et al, 2019).…”