The compost filter is a fairly new method for pre-treating wastewater. There are two different methods: two chamber compost filters or compost filter bags. Greywater or domestic wastewater flows directly into this filter. The solids stay in the filter and are decomposed and transformed into humus by aerobic digestion; the liquids are drained at the bottom and forwarded to the constructed wetland. As it is an aerobic process, there are neither biogas emissions nor bad odours. From time to time, the operator has to add bulking material like straw or wood chips, to enforce the dehydration and to avoid clogging of the filter. (compiled by Beat Stauffer (seecon international gmbh))