Biogas Settler

Sedimentation is also used for the removal of grit (see pretreatement technologies), for secondary clarification in activated sludge treatment (see activated sludge), after chemical coagulation/precipitation, or for sludge thickening. This technology information sheet discusses the use of settlers as primary clarifiers, which are typically installed after a pretreatement technology , and specifically focuses on the use of one type of settlers, the anaerobic biogas settlers (see also anaerobic digestion).

Settlers can achieve a significant initial reduction in suspended solids (50-70% removal) and organic material (20-40% BOD removal) and ensure that these constituents do not impair subsequent treatment processes.

Settlers may take a variety of forms, sometimes fulfilling additional functions. They can be independent tanks or integrated into combined treatment units. Several other technologies in this Compendium have a primary sedimentation function or include a compartment for primary settling:

• the septic tank, where the low sludge removal frequency leads to anaerobic degradation of the sludge.
• the anaerobic baffled reactor and the anaerobic filter both usually include a settler as the first compartment. However, the settler may also be built separately, e.g., in municipal treatment plants or in the case of prefabricated, modular units.
• the biogas reactor, which can be considered as a settler designed for anaerobic digestion and biogas production.
• the Imhoff tank and the upflow anaerobic sludge blanket reactor, designed for the digestion of the settled sludge, prevent gases or sludge particles in the lower section from entering/returning to the upper section.
• the waste stabilisation ponds, of which the first anaerobic pond is for settling
• the sedimentation/thickening ponds, which are designed for the solid-liquid separation of faecal sludge
• the solids-free sewer, which includes interceptor tanks at the building level.

Biogas settlers function much like septic tanks with the difference that biogas is recovered. Biogas, a mixture of methane (CH4) and carbon dioxide (CO2) can be used either directly for cooking and lighting or it can be transformed into heat in a gas heater system or into combined heat and power (CHP) in a cogeneration unit (MES et al. 2003; JENSSEN et al. 2004; WRAPAI 2009). Find more information in the small-scale conversion of biogas to electricity or the direct use of biogas factsheet. The nutrient-rich remaining sludge can be used as fertilizing soil amendment in agriculture (see also reuse of urine and faeces in agriculture and reuse of blackwater and greywater). The liquor, optionally after further treatment, can be used for fertigation.

The main purpose of a settler is to facilitate sedimentation by reducing the velocity and turbulence of the wastewater stream. Settlers are circular or rectangular tanks that are typically designed for a hydraulic retention time of 1.5-2.5 h. Less time is needed if the BOD level should not be too low for the following biological step. The tank should be designed to ensure satisfactory performance at peak flow. In order to prevent eddy currents and short-circuiting, as well as to retain scum inside the basin, a good inlet and outlet construction with an efficient distribution and collection system (baffles, weirs or T-shaped pipes) is important.

Depending on the design, desludging can be done using a hand pump, airlift, vacuum pump, or by gravity using a bottom outlet. Large primary clarifiers are often equipped with mechanical collectors that continually scrape the settled solids towards a sludge hopper in the base of the tank, from where it is pumped to sludge treatment facilities. A sufficiently sloped tank bottom facilitates sludge removal. Scum removal can also be done either manually or by a collection mechanism.

The efficiency of the primary settler depends on factors like wastewater characteristics, retention time and sludge withdrawal rate. It may be reduced by wind-induced circulation, thermal convection and density currents due to temperature differentials, and, in hot climates, thermal stratification. These phenomena can lead to short-circuiting.

Several possibilities exist to enhance the performance of Settlers. Examples include the installation of inclined plates (lamellae) and tubes, which increases the settling area, or the use of chemical coagulants.

Design principles of a biogas settler

Biogas settlers are similar in construction and design as fixed-dome or floating drum biogas plants (see also small-scale biogas digesters). However, in opposition to biogas reactors, biogas settlers are designed for the retention of biomass and are thus typical high-rate biogas reactors. Other high-rate biogas plants are anaerobic baffled reactors (ABRs); anaerobic filters; and up-flow anaerobic sludge blanket reactors. High-rate biogas reactors are characterised by a mixed flow regime: the liquid (e.g. flushing, anal cleansing or greywater) flows through (continuous flow), while the sludge (e.g. faeces, paper etc.) is retained (batch) and treated over a long time until it is removed and used as fertiliser. Thus, biogas settlers are characterised by relatively short hydraulic retention times (HRT) for the liquor and high sludge retention times (SRT) for the solid fraction (organics and inorganics). The settled sludge is transformed into biogas by anaerobic digestion (see also anaerobic treatment of waste and wastewaters). Gas bubbles to the top of the reactor are collected for use.

Biogas settlers can achieve up to 80 to 85 % of removal of biological oxygen demand (BOD) and chemical oxygen demand (COD) (WSP 2007), but the efficiency strongly depends on the settling properties of the organic matter and the HRT and SRT. As a large fraction of the organic matter is volatilised into biogas, sludge production is very low and the reactors need to be emptied only once every several years (i.e. = SRT).

Biomass retention in the biogas settlers may be achieved by gravity settling. Thus, as in any settling unit there needs to be an outlet for the supernatant liquid in the upper part of the reactor on the opposite site from the inlet. Baffles of any kind further enhance the retention of the biomass by holding back unsuspended solid compounds of the inflowing wastewater.

Application of biogas settlers

Anaerobic biogas settlers are most often used as a primary settling treatment in decentralised wastewater treatment systems (e.g. DEWATS). In a typical DEWATS, biogas settlers are followed by a secondary anaerobic treatment using fixed-bed reactors (e.g. ABRs or AFs) and a tertiary aerobic treatment for polishing (surface, horizontal or vertical flow constructed wetlands or maturation ponds) (SCHMIDT 2008; MUELLER 2009). The final effluent can be used for fertigation. The requirement of treatment of the liquor flowing out of the settler depends on the HRT (WAFLER 2008). In the case of small plants, with relatively high HRTs, fixed-bed reactors may not be required if the liquor is reused for fertigation.

Biogas settlers as a primary treatment are adapted best to receive black- and brownwater and industrial high-strength wastewater as long as the wastes are biodegradable (e.g. slaughterhouse wastewater). Brownwater from urine diverting toilets has the advantage to contain less nitrogen than blackwater. This nitrogen is in the form of ammonia, which will be transformed into carbon dioxide, ammoniac and urea, both toxic to bacteria (MANG 2005).

Greywater can be added to the system after the biogas settler to be treated along with the supernatant coming from the settling unit. Besides these wastes which are brought to the DEWATS system by sewers, they can also be used for the off-site treatment of faecal sludge or night-soil from on-site sanitation systems (optionally other organic wastes can be added for co-digestion), which have been transported to the decentralised system by cartage (EAWAG/SANDEC 2008; MULLER 2009). The remaining sludge is generally more or less hygienised, depending on the SRT and the mixing of the old sludge with the new one. As anaerobic digestion mostly removes organics, large amounts of the nutrients remain in the sludge, making it to a valuable solid fertiliser. Almost 100 % of the phosphorus and about 50 to 70 % of the nitrogen as ammonium remains in the digested sludge (JOENSSEN 2004). To enhance the stabilisation of the sludge before reuse, it can be transformed into humus by small or large-scale composting (e.g. together with organic wastes) or further proceeded in drying beds (planted or unplanted) or thickening ponds and settling tanks. After these steps, the final product will be hygienically safe.

To prevent the release of odorous gases, frequent sludge removal is necessary. Sludge and scum must be handled with care as they contain high levels of pathogenic organisms; they require further treatment and adequate disposal. Appropriate protective clothing is necessary for workers who may come in contact with the effluent, scum or sludge.

In strictly anaerobic biogas settlers, most of the pathogens are destroyed (FAO 1996). However, the state of hygienisation of the effluent slurry of biogas digesters strongly depends on the influent concentration in pathogenic microorganisms, the retention time, the temperatures and the mixing of the old sludge with the fresh one. High temperatures and long retention times are more hygienic (SASSE 1988). If more than 55°C were achieved for one to a few days inactivation can be considered as efficient (SCHOENNING & STENSTROEM 2004). To enhance the stabilisation of the remaining sludge before reuse aerobic composting is an adapted post-treatment.

The costs of settlers mainly depend on their size and type. Investment cost of anaerobic settlers is moderate and the potential of self-help for construction is relatively high. However, planning and design requires skilled labour and experienced experts. Both biogas and fertilising sludge make biogas sanitation technology highly interesting from an economic point of view.

In settlers that are not designed for anaerobic processes, regular sludge removal is necessary to prevent septic conditions and the build-up and release of gas which can hamper the sedimentation process by re-suspending part of the settled solids. Sludge transported to the surface by gas bubbles is difficult to remove and may pass to the next treatment stage.

Frequent scum removal and adequate treatment/disposal, either with the sludge or separately, is also important.

Anaerobic settlers do have to be emptied less frequent as the organic material is partly  transformed into gas. Accumulated slurry in the bottom of the reactor needs to be de-sludged every two to five years, depending on the type of reactor (UNEP 2002, MANG 2005). For the operation and maintenance of anaerobic biogas settlers, operational requirements are very low and no professional operator is required as long as the plant is well maintained by skilled users. Starting with the seeding of some sludge form a septic tank or another anaerobic digester speeds up the digestion and prevents the digester from running acid (SASSE 1998).