Compost Filter

A compost filter, called ‘Rottebehälter’ in German, is a novel alternative pre-treatment method (instead of a septic tank, biogas settler, anaerobic baffled reactor, imhoff tank or UASB reactors) typically applied prior to constructed wetlands (see also vertical flow, horizontal flow and free-surface constructed wetlands) or waste stabilisation ponds. Composting filters are particularly useful where biogas emissions have to be avoided. As it is an aerobic process, no bad odours occur if it is correctly operated. Compost filters used in combination with the above mentioned constructed wetlands or ponds are mainly used for semi-centralised or on-site wastewater treatment facilities for small communities after a simplified sewer system (see also simplified and condominal sewers and solids-free sewers) or single households; effluents of the system may be used for irrigation (see also drip irrigation or fertigation). There are two types of compost filters (chamber or bags).

Two chamber compost filter

This system is also used in cold climates like Switzerland. Experience has shown that it works well up to 40 p.e. The wastewater flows directly into the composting filter. It consists of two chambers; each chamber has a capacity of one year. As soon as the first chamber is full, the influent pipe can be switched to the second chamber for the following year. In the meantime, the faecal sludge in the first chamber is dewatered, and the rotting process (aerobic digestion) successively decomposes the material (BALLY and BITTNER 2009).

Two chamber composting filters consist of two concrete chambers, fibrous web (geomembrane), drainage slabs, gravel, and wood chips. The compost filter is usually covered to prevent the entry of precipitation water and to protect people from falling into the chambers. A proper construction of the drainage and filter layer is important. The filter layer should not be too coarse, otherwise solids and sewage sludge could end up in the collection chamber. This could damage the pump or the following treatment facility (e.g. vertical flow constructed wetland). However, if the pores in the composting filter are too small, it may lead to clogging and anaerobic conditions, or overflowing of the filter (LACK 2006).

The filter passage in a compost filter is short and oxygen depletion hardly occurs. This is also why a properly maintained compost filter does not smell bad. A slight smell can however occur as is known from sewer systems. Due to the low oxygen consumption, the percentage of O₂ in the water is still high enough to support microbiological processes in the subsequent constructed wetland (see vertical flow, horizontal flow and free-surface constructed wetlands) (BALLY and BITTNER 2009).

Compost filter bags

So far this type has only been used for single households or up to 70 p.e. for example in ecological sanitation (ecosan) projects, where concentrated blackwater requires pre-treatment (HOFFMANN et al. 2010).

The raw blackwater passes through a filter bag (made of plastic material) into a chamber with a ventilation pipe. The liquid effluent from the compost filter is collected below the filter bags and normally needs to be pumped to the constructed wetland, as the hydraulic head loss in compost filters is about 1.5 m. The solid components of the blackwater (i.e. faeces and toilet paper) are retained in the straw bed, which is contained in the filter bag (HOFFMANN et al. 2010).

The final product

The final product (after it has been fully aerated and left without addition of new material for 6 to 12 months) is black, compact material, which looks and smells like black soil or humus. Nevertheless, the material still needs secondary composting (see small and large scale composting) as it still contains pathogens such as helminth eggs (HOFFMANN et al. 2010).

Compared to other water based systems, construction costs are relatively low. However, it is still more expensive than a dry toilet or composting toilet system. Nevertheless it is a valuable option for primary treatment in decentralised sanitation systems in combination with constructed wetlands or waste stabilisation ponds. A compost filter needs expert design and constant input of straw or wood chips.

The compost filter bag needs regular maintenance. Once a week dry straw has to be added. Generally, 2 to 4 filter bags are used in alternating modes in two separate chambers (the dimensions of the chambers depend on the number of users); the retained solids are composted during the resting phase of 6 months, during which the second bag is used. Volume reduction during resting phase can be up to 75% (HOFFMANN et al. 2010).

An operator must maintain the active chamber of the two-chamber filter regularly: dry material such as straw or wood chips must be added weekly to monthly. This avoids clogging of the filter and advances the dehydration process. It is recommended, that the added material be arranged all over the compost filter surface. It should be slightly accumulated directly below the influent. If the filter is correctly maintained and operated, no unpleasant odour can develop.

The chambers need to be covered in order to prevent people (especially children) from falling in. The active chamber contains fresh excreta. The material of the inactive chamber is less hazardous, but could still contain pathogens. Therefore, gloves are recommended for any maintenance or repair work of the filter. The decomposed material should be composted again, as a further hygienisation (see small and large scale composting). It is also important to apply this material correctly if it is used for agriculture.