Anaerobic Baffled Reactor (ABR)

Anaerobic baffled reactors (ABR) are septic tanks  that have been upgraded with a series of baffles along the treatment chamber. The upflow chambers provide enhanced removal and digestion of organic matter. As septic tanks, ABRs are based on a physical treatment (settling) and a biological treatment (anaerobic digestion).

An ABR consists of a tank and alternating hanging and standing baffles that compartmentalise the reactors and force liquid to flow up and down from one compartment to the next, enabling an enhanced contact between the fresh wastewater entering the reactor and the residual sludge, containing the microorganisms responsible for anaerobic digestion of the organic pollutants. The compartmentalised design separates the solids retention time from the hydraulic retention time, making it possible to anaerobically treat wastewater at short retention times of only some hours (EPA 2006). Solids high treatment rates are high, while the overall sludge production is characteristically low (FOXON et al. 2004). They are simple to build and simple to operate, as well as very robust to hydraulic and organic shock loading (SASSE 1998). Yet, both sludge and effluent still need further treatment.

ABRs are suitable for a wide range of wastewater, including high-strength industrial wastewater, but its efficiency increases with higher organic load. Therefore, ABRs are particularly suited for influents with a high percentage of non-settleable suspended solids and a narrow COD/BOD ratio (SASSE 1998). ABRs are typically applied in DEWATS, usually in combination with several other treatment steps. A typical DEWATS could be a five component system of first three anaerobic steps consisting of a biogas settler; an ABR and an anaerobic filter; followed by an aerobic treatment unit such as a constructed wetland (Free-Water Surface CV, Horizontal Subsurface Flow CV or Vertical Flow CV) and a maturation pond (WHO 2009). BOD may be reduced by up to 90%, which is far superior to its removal in a conventional Septic Tank.

ABRs are a combination of the principles of septic tanks, moving bed reactors and up-flow anaerobic sludge blanket reactors. The difference to MBRs and UASBs lies in the fact that it is not necessary for the sludge blanket to float; and that effluent retention is not necessary since a part of the active sludge that is washed out from one chamber is trapped in the next (SASSE 1998). The majority of settleable solids are removed in a sedimentation chamber in front of the actual ABR. 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 ABR with other technologies, or where prefabricated, modular units are used. 

Typical inflows range from 2 to 200 m³ per day. Critical design parameters include a hydraulic retention time (HRT) between 48 to 72 hours, upflow velocity of the wastewater below 0.6 m/h and the number of upflow chambers (3 to 6). 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. Usually, the biogas produced in an ABR through anaerobic digestion is not collected because of its insufficient amount. The tank should be vented to allow for controlled release of odorous and potentially harmful gases.

The reactor always starts with a settling chamber for larger solids and impurities (SASSE 1998) followed by a series of at least 2 (MOREL & DIENER 2006), sometimes up to 5 (SASSE 1998) up-flow chambers. The wastewater enters the chambers at the bottom and needs to pass through the sludge to move up and to the next compartment. Thereby particles settle against the up-stream (SASSE 1998). As the wastewater passes through the sludge, intensive contact between the active biomass in the resident sludge and newly incoming wastewater occurs. To equally distribute the entering liquid in the chambers, they should be designed as relatively short compartments (< 75 cm of length and < 50% to 60% of the height, SASSE 1998). To retain any possible scum formed in the up-flow chamber, the outlets of each tank as well as the final outlet should be placed slightly below the liquid surface (SASSE 1998).

The up-flow velocity is the most crucial parameter for dimensioning, especially with high hydraulic loading. It should not exceed 2.0 m/h (SASSE 1998; MOREL & DIENER 2006). Based on a given HRT, the up-flow velocity increases in direct relation to the reactor height. Therefore, the reactor height cannot serve as a variable parameter to design the reactor for the required HRT. The limited upstream velocity results in large but shallow tanks. It is for this reason that the baffled reactor is not economical for larger plants (SASSE 1998). The organic load should be below 3 kg COD/m³/day. Higher loading-rates are possible with higher temperature and for easily degradable substrates (SASSE 1998).

During the anaerobic digestion, biogas is produced, which can be recovered and reused in the kitchen or for driving pumps and other equipment when necessary. Methane concentration increases steadily from the first compartment to the last (WANG et al. 2004). The methane producing activity of anaerobic sludge in different compartments depends on the substrate, which suggests that the proper anaerobic consortium in each separate compartment develops in accordance to the substrate available and the specific environmental conditions (WANG et al. 2004). The use of the produced biogas in the kitchen might be the most realistic and easiest way to reuse the biogas in decentralised systems. If the gas is not recovered, the tanks need to be vented to prevent the release of the potentially harmful gases (TILLEY et al. 2008).

To increase the treatment efficiency (especially regarding pathogens), the last chamber may be an anaerobic filter (WSP 2008).

Treatment performance

Treatment performance of ABRs is in the range of 65% to 90% COD (Chemical Oxygen Demand) removal, corresponding to about 70% to 95% of BOD (Biological Oxygen Demand) (SASSE 1998; MOREL & DIENER 2006; BORDA 2008). This is far superior to that of a conventional septic tank (30 to 50 %, UNEP 2004). The majority of the settleable solids are removed in the sedimentation chamber at the beginning of the ABR, which typically represents 50 % of the total volume of TSS (TILLEY et al. 2008). The special design also allows for an enhanced treatment of non-settleable solids and a Total Suspended Solids (TSS) removal of up to 90% can be achieved (SINGH 2008). The tanks put in series also help to digest substances that are difficult to degrade, predominantly in the rear part, after easily degradable matters have been digested in the front part already (SASSE 1998). Consequently, recycling of effluent would have a slightly negative effect on treatment quality. ABRs can be designed for a daily inflow in a range of some m3/day up to several hundreds of m3/day (FOXON et al. 2004; TILLEY et al. 2008). The Hydraulic Retention Time (HRT) in ABRs is relatively short and varies from only a few hours up to two or three days (FOXON et al. 2004; MOREL & DIENER 2006; 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. Care should be taken to design and locate the facility such that odours do not bother community members.

In general, the quality of ABR effluents has been shown to consistently meet guidelines for irrigation regarding the removal of organics (e.g. COD or BOD) for reuse in agriculture, but not for discharge to surface water (FOXON et al. 204). The effluents do also contain high amounts of nutrients, ammonia and phosphorus and these nutrients may be regarded as a resource from an agricultural point of view (FOXON et al. 2004). The problem is though, that pathogen removal is generally not satisfactory for the reuse in agriculture and when, only very restricted reuse is recommended.

Septic tank are generally low cost. However, the costs vary depending on the availability of materials and economy of scale (EAWAG/SANDEC 2008). In any case, ABRs have a high potential to be used in DEWATS. As they do not require any electricity and are simple to construct and operate, they are generally cheaper than more mechanical, centralised technology options. ABRs can be constructed with locally available material. However, expert design is required.

An ABR requires a start-up period of several months to reach full treatment capacity since the slow growing anaerobic biomass first needs to be established in the reactor. To reduce start-up time, the ABR can be inoculated with anaerobic bacteria, e.g., by adding fresh cow dung or septic tank sludge. The added stock of active bacteria can then multiply and adapt to the incoming wastewater. In principle, it is advantageous to start with a quarter of the daily flow and then slightly increase loading rates over three months, allowing the bacteria enough time to multiply before suspended solids are washed out (SASSE 1998). As a long start up time is required for the anaerobic digestion of the sludge, the ABR technology should not be used when the need for a treatment system is immediate (TILLEY et al. 2008). Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the ABR.

Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Process operation in general is not required, and maintenance is limited to the removal of accumulated sludge and scum every 1 to 3 years (EAWAG/SANDEC 2008) . This is best done using a Motorized Emptying and Transport technology or a Human-powered Emptying and Transport Technology to avoid that humans get in contact with the sludge and are exposed to health risks (TILLEY et al. 2008) . The desludging frequency depends on the chosen pre-treatment steps, as well as on the design of the ABR. When emptying the tanks, it is vital that some active sludge is left in each of the compartments to maintain a stable treatment process (SASSE 1998).

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