Use of biofilter for the treatment of pesticide contaminated water Despite many measures to reduce the risk of pesticides spreading to sensitive environments, their residues are still often found in surface and groundwater. Point sources are a significant cause of such contamination, mainly associated with localized situations, e.g., filling of the tank and washing of the sprayer. According to Sweden's official statistics, 54% of farmers choose some form of concrete surface connected to a collecting tank as a safe pesticide handling place. The collected liquid is later spread on biologically active soil, such as fallow or on stump. However, farmers’ experience indicates that there can be large amounts of water to handle annually, and chemical residues in the spread liquid can have a negative effect on crops. There is a need to improve the existing and future pesticide handling sites to manage large volumes of water safely, easily, and economically. An appropriate and simple solution is the use of a biofilter to treat the collected liquid. The biofilter has been developed based on the Swedish biobed and is used in several countries. It consists of several 1 m3 – plastic containers filled with biomix, and the different units are stacked in a vertical pile and connected with plastic valves and pipes. Contaminated water is collected and circulated through the biofilter and pesticide residues are retained in the biomix where they are degraded by microbial activity. The aim of the project was to adapt the use of the biofilter to Swedish conditions and gain knowledge about how it should be operated. Since water dynamics are an important factor in the biofilter function, the specific goals of the project were to study the effect of the inflow rate on water balance, microbial activity, and pesticide retention in a typical Swedish biomix. For the project a pilot biofilter was built at RISE workshop in Uppsala. A list of the materials needed, the approximate cost of building a biofilter and a preliminary instruction manual for the construction were produced within the frame of the project. The cost of building a 3-unit biofilter was estimated to be approx. 15,000 Swedish crowns. The study was divided into two trials under controlled conditions. First, the effect of inflows on water content and microbial activity in the biomix was investigated. A constant flow rate of 25 L/d was tested in one of the units and an increasing flow of 7, 22 and 40 L/d in another unit. The results showed that the outflow increased with flow rate and was between 50 and 96% of the inflow. Water content in the biomix was lower and fluctuated more on the surface compared to the bottom of the biomix and water retention capacity decreased over time. A tendency to reduced carbon content and microbial activity (measured as respiration rate) over time was observed with the flows > 20 L/d. The levels of glyphosate and diflufenican in the effluent were very low, 0,1 % of the levels in incoming water, regardless the flow. The higher flow reduced the retention capacity of bentazone, i.e., a higher inflow led to higher levels of bentazone in the effluent. According to our results, a typical Swedish biomix, under the conditions tested in this study, can treat a flow lower than 20 L/d without having a major impact on microbial activity and on the pesticide retention capacity. Assuming that the biofilter can be used for 210 days/year (not in winter), approximately 4000 L can be treated in one year with a flow of 20 L/d. Biofilters are a good option for farms with indoor or outdoor pesticide handling areas under roof. For farms with an outdoor concrete area without a roof, where precipitation also ends up in the collection tank, the volumes to treat become too high for a biofilter.