Anaerobic Filter

Anaerobic filters are used as secondary treatment step in household greywater or blackwater treatment systems. They are also used, together with other treatment units in a so-called decentralised wastewater treatment system (DEWATS) (e.g. for biodegradable industrial wastewater).

An anaerobic filter is an attached biofilm system (fixed bed or fixed film reactor, see also fixed film activated sludge) that aims at removing non-settleable and dissolved solids (MOREL & DIENER 2006). As septic tanks or anaerobic baffled reactors, anaerobic filters are based on the combination of a physical treatment (settling) and a biological treatment (see also anaerobic digestion). It comprises a watertight tank containing several layers of submerged media, which provide surface area for bacteria to settle. As the wastewater flows through the filter usually from bottom to top (up-flow), it comes into contact with the biomass on the filter and is subjected to anaerobic degradation (MOREL & DIENER 2006).

Anaerobic filters are used for wastewater with a low percentage of suspended solids and narrow COD/BOD ratio. It is suitable for domestic wastewaters and all industrial wastewater, which has a lower content of suspended solids. Pre-treatment in settlers or septic tanks may be necessary to eliminate solids of larger size before they are allowed to enter the filter (SASSE 1998). Recovering biogas may be considered in case of a BOD concentration >1,000 mg/L (SASSE 1998).

With this technology, suspended solids and BOD removal can be as high as 90%, but is typically between 50% and 80%. Nitrogen removal is limited and normally does not exceed 15% in terms of total nitrogen (TN). Total Coliform reduction is 1 to 2 log units (MOREL & DIENER 2006). The hydraulic retention time (HRT) compared to the tank volume should be in the range of 1.5 and 2 days for pre-settled blackwater (SASSE 1998) and 0.7 to 1.5 days for greywater (MOREL & DIENER 2006). For domestic wastewater, the constructed anaerobic filter volume (voids plus filter mass) may be estimated at 0.5 m3 /capita, for smaller units it may come to 1 m3 /capita (SASSE 1998).

The construction of anaerobic filters is similar to septic tanks but generally there are several chambers and additional baffels in order to ensure upflow. Consequently, non-settleable and dissolved solids are also treated as they are brought into close contact with the active bacterial mass fixed on the filter material. The bacteria tend to fix themselves on reactor walls and filter material (e.g. gravel, rocks, cinder or specially formed plastic pieces) and anaerobically digests the dispersed or dissolved organic matter within short retention times. The larger the surface for bacterial growth, the quicker the digestion. A good filter material provides 90 to 300 m² surface area per m³ of occupied reactor volume (SASSE 1998; MOREL 2006). A rough surface provides a larger area, at least in the starting phase. Later on, the biofilm that grows on the filter mass quickly closes the smaller groves and holes (SASSE 1998). The digestion process itself is similar to the one taking place in ABRs, but in ABRs the contact of the solids and the bacteria is provided by discharging wastewater to the bottom directly into the biomass, which is settled in the sludge.

Pre- and primary treatment is essential to remove solids and garbage that may clog the filter. The majority of settleable solids are removed in a sedimentation chamber in front of the anaerobic filter. Small-scale stand-alone units typically have an integrated settling compartment, but primary sedimentation can also take place in a separate Settler or another preceding technology (e.g., existing Septic Tanks). Designs without a settling compartment are of particular interest for (Semi-) Centralized Treatment plants that combine the anaerobic filter with other technologies, such as the Anaerobic Baffled Reactor (ABR).

Anaerobic filters are usually operated in upflow mode because there is less risk that the fixed biomass will be washed out (SASSE 1998). In down flow systems flushing of the filter for the purpose of cleaning is easier. A combination of up-flow and down-flow chambers is also possible (SASSE. 1998). Anaerobic upflow filters with rock-filled beds, with a similar appearance as aerobic trickling filters are today promoted and commercialised as decentralised, on-site wastewater collection and treatment systems; simple to construct and to operate (ROSE 1997) . The water level should cover the filter media by at least 0.3 m to guarantee an even flow regime (MOREL & DIENER 2006) . The hydraulic retention time (HRT) is the most important design parameter influencing filter performance. An HRT of 12 to 36 hours is recommended.

The ideal filter should have a large surface area for bacteria to grow, with pores large enough to prevent clogging. The surface area ensures increased contact between the organic matter and the attached biomass that effectively degrades it. Ideally, the material should provide between 90 to 300 m² of surface area per m³ of occupied reactor volume (SASSE 1998; MOREL 2006) . Typical filter material sizes range from 12 to 55 mm in diameter and decreases from bottom to top . Materials commonly used include gravel, crushed rocks or bricks, cinder, pumice, or specially formed plastic pieces, depending on local availability. Filters with two to three filter layers and a minimum depth of 0.8 to 1.2 m are recommended (SPERLING & CHERNICHARO 2005). Generally, simple anaerobic filters start with a first layer of large-sized cinder or rocks (5 to 15 cm), which are bedded on a perforated concrete slab about half a meter above ground parallel to the flow direction (SASSE 1998).

The connection between the chambers can be designed either with vertical pipes or baffles. Accessibility to all chambers (through access ports) is necessary for maintenance. The tank should be vented to allow for controlled release of odorous and potentially harmful gases.

The quality of treatment in well-operated anaerobic filters has been reported to be in the range of 50 to 90% BOD removal depending on the influent (SASSE 1998; MOREL & DIENER 2006). Total Suspended Solids (TSS) removal can be 50 to 80%; total nitrogen can be removed up to 15% and total Coliform reduction is 1 to 2 log units (MOREL & DIENER 2006). The hydraulic retention time (HRT) compared to the tank volume should be in the range of 1.5 and 2 days for pre-settled blackwater (SASSE 1998) and 0.7 to 1.5 days for greywater (MOREL & DIENER 2006). For domestic wastewater, the constructed anaerobic filter volume (voids plus filter mass) may be estimated at 0.5 m³ /capita, for smaller units it may come to 1 m³ /capita (SASSE 1998).

Depending on land availability and the hydraulic gradient of the sewer, the anaerobic filter can be built above or below ground (TILLEY et al. 2008). However, most often they are below the ground surface to save space, to reduce health risks and to provide insulation and protection against cold climates. However, access to inlet and outlet should be provided to allow regular checking of the state of the biofilm and the volume of accumulated sludge.

To prevent pollution of the groundwater, anaerobic filters should be watertight but they should still not be constructed in areas with high groundwater tables or prone to flooding. To prevent the release of potentially harmful gases, the anaerobic filters should be vented (TILLEY et al. 2008).

Under normal operating conditions, users do not come in contact with the influent or effluent. Effluent, scum and sludge must be handled with care as they contain high levels of pathogenic organisms. The effluent contains odorous compounds that may have to be removed in a further polishing step. The effluent still contains pathogens and should be treated further if reused in agriculture (e.g. in a waste stabilisation pond system or a surface, horizontal or vertical flow constructed wetland), directly used for fertigation or discharged properly (e.g. in a leach field or soak pit). Care should be taken to design and locate the facility such that odours do not bother community members.

The reactor vessel of anaerobic filters can be integrated in or is very similar to a septic tank and can be constructed at relatively low cost with locally available material. Filter material can be simple gravel, rocks, cinder or specially formed plastic pieces depending on the availability and the financial means. Pre-fabricated plastic or fibreglass tanks may also be available and may be less costly, depending on the context. Desludging is required regularly, preferably with a vacuum tanker. However, anaerobic filters still remain a rather low cost option (not really more expensive than septic tanks) for an acceptable treatment efficiency of dissolved compounds and some pathogens.

An anaerobic filter requires a start-up period of 6 to 9 months to reach full treatment capacity since the slow growing anaerobic biomass first needs to be established on the filter media. To reduce start-up time, the filter can be inoculated with anaerobic bacteria, e.g., by spraying septic tank sludge onto the filter material (SASSE 1998) . The flow should be gradually increased over time. It can be helpful to start first with lower hydraulic loads (e.g. 1/4 of the future daily flow, SASSE 1998) and then to increase the flow rate slowly over the following three months. Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the anaerobic filter.

Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Over time, solids will clog the pores of the filter. As well, the growing bacterial mass will become too thick, break off and eventually clog pores. When the efficiency decreases, the filter must be cleaned. This is done by running the system in reverse mode (backwashing) or by removing and cleaning the filter material (SASSE 1998; MOREL & DIENER 2006) . As with septic tanks, desludging of the primary settling chamber should be done at regular intervals. Both, desludging and cleaning of the filter material can be a health-hazard and appropriate safety precautions should be taken (TILLEY et al. 2008).

Anaerobic filters produce biogas. If it is not recovered, which is recommendable, there should be venting to evacuate the gases, but precaution should be taken for foul odours in densely populated areas.

Anaerobic filter tanks should be checked from time to time to ensure that they are watertight.