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

In Denmark drinking water supply is based on groundwater which is treated by aeration followed by filtration in rapid sand filters. Unfortunately pesticide contamination of the groundwater poses a threat to the water supply, since the simple treatment process at the waterworks is not considered to remove pesticides from the water phase and pesticides are detected in 24% of the active Danish waterworks wells. This study aimed at investigating the potential of microbial pesticide removal in rapid sand filters for drinking water treatment. Removal of the pesticides MCPP, bentazone, glyphosate and the degradation compound p-nitrophenol was investigated in the rapid sand filters at Islevbro and Sjælsø waterworks plant I and II. Microcosms were set up with sand from rapid sand filters, water and an initial pesticide concentration of 0.03-0.38 μg/L. In all the investigated waterworks the concentration of pesticides in the water decreased – MCPP decreased to 42-85%, bentazone to 15-35%, glyphosate to 7-14% and p-nitrophenol 1-3% – from the initial concentration over a period of 6-13 days. The largest microbial removal was observed at Sjælsø waterworks Plant II, where the pesticides were partially mineralised – up to 43% of the initial glyphosate was found as CO₂ after 6 days. At Sjælsø waterworks Plant II the contact time in the primary rapid sand filter was 43 minutes. It was found that less than 20 minutes was needed to biologically remove more than 50% of the initial bentazone (concentration 0.1 μg/L). It is therefore certain that there is a potential for microbial removal of pesticides from contaminated groundwater in Danish waterworks.