Anaerobic Digestion (Large-Scale)

Large-scale biogas reactors are designed for the conversion of the organic fraction of large volumes of slurries and sludge into biogas. Typical substrates are excess sludge from large-scale wastewater treatment plants, agricultural and food industry wastes (e.g. manure, from stock framing, sugar refining, starch production, coffee processing, alcohol generation, slaughterhouses etc.) or industrial wastes (e.g. from paper manufacturing, biotechnological industries, etc.) (WERNER et al. 1989).

Biogas production form agriculture and food industry waste slurry. Products are electricity, clean fuel, carbon credits and liquid and solid fertiliser. Source: BIOPACT (n.y.)

Energy crops are sometimes also added in order to increase biogas yields. The produced biogas can be recovered and generally transformed into heat in a gas turbine or into combined heat and power (CHP) in cogeneration plants and fed into the public grid (MES et al. 2003; JENSSEN et al. 2004; WRAPAI 2009) (see also conversion of biogas to electricity at large scale). It can also be upgraded to natural gas quality, compressed and used to power motor vehicles. The sludge that remains after digestion is rich in nutrients and can be used as a soil amendment in agriculture, generally after anaerobic composting as final treatment step.

The generation of combined heat and power as well as natural gas from agricultural products. Overall scheme. Source: UNKNOWN (n.y.)

To date, such large-scale biogas digesters have been mainly applied and developed in industrialised countries resulting in plants with sophisticated equipment and operational techniques (e.g. wet and dry digestion processes, heated and unheated reactors, batch and continuous reactors etc.). They vary in design and complexity but have all in common to require expert planning, design and staff for operation and maintenance. Given increasing electricity and fuel prices, the dissemination of large-scale biogas plants also gains in interest in less industrialised countries (BRUYN 2006).

A typical egg-shaped biogas reactor in Germany (left) and an agricultural fixed-dome biogas reactor. Sources: MIKLED (n.y.) and KLIMA SUCHT SCHUTZ (n.y.)

Vertical continuously stirred tank reactor (CSTR, left) and horizontal plug-flow reactor (PFR, right) — two examples for large-scale agricultural biogas digesters in Europe. Source: BRUYN (2006)

As small-scale digesters (see also biogas digester small scale), large-scale anaerobic digesters treating slurries are generally designed according to the wet digestion process with 10 to 20 % of total solids (TS) (see also anaerobic treatment of waste and wastewater). The volumes of the reactors are ranging from several hundred to several thousand m3. Due to the size of plant, the respective objectives and special requirements concerning operation and substrates, the anaerobic treatment of waste materials and wastewater at large-scale requires a different set of planning mechanisms, plant types and implementation factors (WERNER et al. 1989). The designer must know that he cannot ‘simply’ enlarge the plants for a small-scale plant to any degree (SASSE 1988). When a low-tech solution is required, it is possible to construct several low-tech small and decentralised biogas plants instead of one single larger digester in order to facilitate operation and maintenance (ICRC 2009).

The most common forms of large-scale digesters are batch reactors and continuous-flow, plug-flow and continuously stirred tank reactor (PFR and CSTR). Fed-batch reactors (accumulation systems) are sometimes also applied (MES et al. 2003). Completely mixed and batch systems are generally built vertically, plug-flow reactors are generally horizontal reactors. Horizontal reactors are often constructed similar to floating or expandable plastic dome plants (see biogas digester small scale) but much larger. Fixed dome are also used, but require large volumes of retention tanks for sludge expansion.

Sustainable sanitation concept (energy recovery and nutrient recycling) of the city of Braunschweig, Germany: The wastewater treatment plants covers its energy needs with the production of biogas from excess sludge combined with biogas recovered from landfills and green waste digestion. Agricultural plants, digesting energy crops (corn) produce biogas, which is transformed to combined heat and power which is fed into the grid of Braunschweig. Source: VEOLIA WATER (2010)

The anaerobic digestion is either carried out in the mesophilic (20 to 35 °C) or the thermophilic range (50 to 60 °C). Thermophilic processes produce more biogas in shorter time. However, mesophilic processes are often preferred as high temperatures require higher input energy to obtain operation temperatures and the production of ammonia, which is toxic for the anaerobic microorganism producing the biogas, is higher at higher temperatures (ISAT & GTZ 1999)  (see also anaerobic treatment of waste and wastewater). 

One way to optimise large-scale anaerobic digesters is to use multi-stage digestion, which allows to more accurately control pH and temperature, the main process factors influencing the performance of the digestion bacteria. In such systems, the four stages of anaerobic digestion (hydrolysis and acidogenesis from acetogenesis and methanogenesis) are separated in different consecutive compartments (MES et al. 2003). Consequently, the optimum conditions for each type of bacteria must be maintained in a smaller volume (e.g. high temperatures for methanogenesis), resulting in a simplified maintenance and in energy savings.

Sewage or municipal wastewater can be treated by anaerobic digestion, but due to the liquid nature of such wastes, the process requires high-rate anaerobic digestion reactors (e.g. upflow anaerobic sludge blanket reactors).

However, when sewage is treated in a conventional large-scale wastewater treatment plant (e.g. in an activated sludge system, see factsheet activated sludge), the anaerobic treatment of the excess sludge is often integrated into the overall process (TBW 2001). In activated sludge systems, sludge means the total solid material that results from sedimentation and bacterial activity and growth during the aerobic wastewater treatment. During the process, much of the sludge is recirculated (see picture below), while the excess sludge is treated together with the sludge from primary sedimentation in an anaerobic digester.

Overall scheme of a high-tech activated sludge wastewater treatment plant combined with sludge digestion and biogas production. Source: WIKIPEDIA (2010)

Typical excess sludge digesters work often in mesophilic, sometimes thermophilic conditions. The sludge retention time is about one month (TBW 2001). To generate appropriate reactor temperatures, a heating system is required. The biogas plant’s energy requirements for heating and other uses can partly, sometimes fully, be covered by utilising the produced gas (transformation in a CHP). Excess heat and power is fed in the local heat and power grid. The excess gas can also be compressed and sold as natural gas (e.g. for vehicles).

The remaining sludge is post-treated most often by aerobic composting. In Central Europe, where the digestion of excess sludge from wastewater treatment plant is common (TBW 2001), the use of the remaining sludge in agriculture is often restricted due to the risk of over-fertilisation (eutrophication) and the high content of heavy metals and other pollutants which can be introduced into the soil and water by applying the sludge on the fields. Therefore, the remaining slurry is generally dewatered and disposed of in landfills or incinerators. To recover the nutrients contained in the remaining sludge (mainly P and N but also some other trace minerals) high-tech solutions (e.g. ASH DEC) are being developed and more and more frequently used (see also advanced nutrient recovery). Though this process serves the purpose of recycling, it is highly energy and cost intensive and requires high-tech solutions.