It is a well-established fact in the water treatment industry that polar, low molecular weight pesticides are very difficult to remove from water using conventional methods. One of the main examples of this is metaldehyde, used frequently in slug pellets. It often seeps into water supplies after rain, so contamination is hard to avoid. As a result, metaldehyde is responsible for most of the pesticide-related drinking water failures in the UK. Whilst biological treatment is often effective against micropollutants, only some operational biofilters have be able to remove metaldehyde. Due to the minor role of the other methods used, biodegradation is likely to be the main removal pathway for metaldehyde.
STREAM researcher Catherine Rolph monitored and assessed the biodegradation in an operational slow sand filter. The long-term data showed that the degradation of metaldehyde did improve when the inlet concentrations increased. Active and inactive sand batch reactors were compared, and that showed that metaldehyde removal happened mainly through biodegradation.
The removal rates were greater if the biofilm was acclimated through high metaldehyde concentrations – there was a 40% increase in metaldehyde removal in acclimated columns compared to non-acclimated. This suggested that metaldehyde removal was reliant on enrichment, which means the process could be engineered to decrease the treatment times from days to hours. It is also important to note that this increase was sustained for more than 40 days, with an average of 80% removal, reducing metaldehyde concentration to be compliant with the approved levels. The study also looked into the microbial makeup of the acclimated and non-acclimated biofilms, and the microbial communities were found to be slightly different. This implies that as biofilms become acclimated, the bacteria present changes to be more appropriate for breaking down metaldehyde.
This study provides a new basis for faster, chemical free, low cost, biological treatment of metaldehyde, as well as other polar pollutants, in drinking water. Whilst this is a first study into this method, the results imply that this is a promising option after further investigation, especially since it was done at full scale and for more than 40 days.
Full article: From full-scale biofilters to bioreactors: Engineering biological metaldehyde removal, C Rolph, R Villa, B Jefferson, A Brookes, A Choya, G Iceton, F Hassard, Science of the Total Environment
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