Anaerobic Digestion (General)

Biogas sanitation technologies are based on the collection of waste and wastewater in airtight chambers in which anaerobic digestion transforms the organic matter into biogas and nutrient rich slurry. The produced biogas can be recovered and used 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 cogeneration plants thereby replacing other fuel sources (MES et al. 2003; JENSSEN et al. 2004; WRAPAI 2009) (factsheets on conversion of biogas to electricity on small and large scale, or on direct use of biogas). The biogas can also be upgraded to natural gas quality, compressed and used to power motor vehicles. Methane (CH₄) is the valuable component under the aspect of using biogas fuel. Biogas that contains about 60 to 70 % of CH₄ has a calorific value of about 6 kWh/m³ what corresponds to about half an L of diesel oil (ISAT/GTZ 1999). The remaining sludge is biologically more or less stable and rich in nutrients, which makes it a valuable soil conditioner and fertiliser.

Anaerobic digestion is a well-established treatment technology suited for wastewater or wastes containing high levels of organic matter. Anaerobic waste and wastewater treatments are cheaper and simpler to operate than aerobic processes, as there is no need for energy for the aeration system and the reduction of the sludge volumes is relatively high as a large fraction of the organic matter is volatilised into the biogas. Moreover, the collection of the biogas reduces the emission of greenhouse gases to the atmosphere and gives a source of renewable energy.

Anaerobic digestion is one of the oldest technologies used for waste treatment (MUELLER 2007). The industrialisation of anaerobic digestion began in 1859 with the first plant in Bombay, India. These early biogas plants were based on a simple waste dumping pond, which was covered to collect the gas. This type of plant is still used to day for the treatment of some very diluted wastes (BURKE 2001).

In developing countries, mainly basic agricultural small-scale biogas reactors have been promoted. In more industrialised countries, anaerobic digesters were continuously improved and more sophisticated equipment and operational techniques emerged resulting in the use of large-scale closed tanks and heating and mixing equipment (MUELLER 2007). Today, the interest in biogas as a renewable, green energy is soaring facing the global energy and climate crisis. Electricity and fuel pricing as well as grid access are the key factors for the dissemination of biogas plants (BRUYN 2006). Almost any organic waste can be transformed to energy by anaerobic digestion and various technologies are available, depending on the substrate and the context.

• Small-scale agricultural biogas reactors for the treatment of animal manure. Kitchen or garden wastes can be co-treated or reactors are can be linked to toilets as safe sanitation option (see the factsheet biogas digesters small-scale)
• Large-scale biogas reactors for the treatment of slurries form agriculture and industry and organic solid waste (see the factsheet biogas digesters large-scale and anaerobic treatment of organic solid waste). These types of reactors are also used for the treatment of excess sludge from large municipal wastewater treatment plants.
• Small-scale and decentralised biogas reactors for the treatment of organic solid waste in developing countries (still relatively new, see the factsheet anaerobic treatment of organic solid waste).
• Anaerobic settlers for the treatment of municipal wastewater (domestic and industrial), faecal sludge (sludge and liquid) and sewage: biogas settlers or modified septic tanks, anaerobic baffled reactors (ABRs), anaerobic filters (AFs), up flow anaerobic sludge blanket reactors. These pre-settling units are often used in decentralised wastewater treatment systems (DEWATS).

Anaerobic biogas reactors can be divided into “high-rate” systems, involving biomass retention and “low-rate” systems without biomass retention.

High-rate systems are characterised by a relatively short hydraulic retention time (HRT), but long sludge retention time (SRT). Typical high-rate systems are biogas settlers, ABRs, AFs and UASB reactors, used for the pre-treatment (e.g. biogas settler) and treatment (e.g. ABRs, AFs and UASB) of rather heterogeneous flows such as municipal wastewater. In high-rate systems, the sludge is retained and transformed into biogas, while the liquor flows out of the tanks for further treatment and/or disposal.

Low-rate systems are characterised by a relatively long hydraulic HRT, which is equal to the SRT as the sludge and liquid enter and leave the tank in a more or less homogeneously mixed slurry. Low-rate systems are suited for all kinds of biodegradable slurries (e.g. animal manure, excess sludge from municipal wastewater plants, mixed organic solid wastes). Typical low-rate anaerobic biogas digesters are batch reactors, fed-batch reactors (accumulation systems), plug-flow reactors (PFR) or continuously stirred tank reactors (CSTR) (see the factsheets biogas digesters small and large scale) and biogas reactors for the treatment of organic solid waste).

The process used for all these technologies is basically the same: anaerobic digestion. However, depending on scale and type of waste stream to be treated, complexity of design, construction and operation varies strongly.

Anaerobic digestion is the degradation of organic material by microbial activity in the absence of air transforming it into biomass and biogas, a mixture of methane (CH₄), carbon dioxide (CO₂) and some trace gases:

Typical composition of biogas. Sources: YADAVA & HESSE (1981); FAO (1996); PIPOLI (2005); GTZ (2009

The process is generally carried out in four stages: hydrolysis; fermentation and acidification; acetogenesis, and methanogenesis. To achieve this sequence of four steps, various bacteria (e.g. fermenting, acetogenic and methanogenic bacteria) need to work together. None of these types of bacteria is able to produce biogas products alone (ISAT/GTZ 1999).

Hydrolysis describes the cleavage of a chemical compound through the reaction with water. Thereby, a hydrogen atom (H) is added to one part of the split chain, while the remaining hydroxyl group of the water (OH) is added to the other. Hydrolysis is the first step of anaerobic digestion in which insoluble complex molecules such as carbohydrates and fats are broken down to short sugars, fatty acids and amino acids.
Fermentation is the second step of anaerobic digestion. Fermentative bacteria transform sugars and other monomeric organic products from hydrolysis into organic acids, alcohols, carbon dioxide (CO₂), hydrogen (H) and ammonia (NH₃).

Acetogenesis is the third step of anaerobic digestion. Products from fermentation (organic acids, alcohols) are converted into hydrogen (H₂), carbon dioxide (CO₂) and acetic acid (CH₃COOH). To produce acetic acid, acetogenic bacteria need oxygen and carbon. For this, they use the oxygen solved in the solution or bounded-oxygen. Hereby, the acid-producing bacteria create an anaerobic condition, which are essential for the methane-producing microorganisms responsible for the further step.

Methanogenesis is the fourth and final step of anaerobic digestion. Methanogenic bacteria (methanogens), which are strictly anaerobic, transform the acetic acid, carbon dioxide and hydrogen into a mixture of methane (CH₄, 50–75 %), carbon dioxide (CO₂, 50-75 %) and varying quantities of nitrogen, hydrogen sulphide and other components. This mixture is called biogas (PIPOLI 2005; GTZ 2009).

For the digestion to be effective, it should operate as a finely balanced, living system – carefully controlled and closely monitored – in order to create optimal conditions for the growth of the bacteria responsible for anaerobic digestion. Therefore, several factors should be considered for design and processing of biogas treatment units. The most important of these parameters are described below.

Temperature: mesophilic and thermophilic digestion

Temperature is one of the most important parameters influencing the performance of anaerobic digestion processes. Anaerobic digestion is theoretically possible between approximately 3 to 70 °C (ISAT/GTZ 1999). As in all other microbial processes, the rate of metabolism increases along with the temperature: the higher the temperature, the shorter the retention time (WERNER et al. 1989). Depending on the temperature, the digestion process will be more or less long and therefore, temperature is directly linked to the retention time.
The following types of digestion are distinguished according to the temperature in the digester (SASSE 1988): psychrophilic digestion (10-20 °C); mesophilic digestion (20-35 °C); and thermophilic digestion (50-60 °C, HRT > 8 days). The conventional operational temperature levels for anaerobic digesters are mesophilic and thermophilic. Thermophilic processes produce more biogas in shorter time but require higher input energy to obtain operation temperatures. Further, at higher temperature, not only methane production can be increased but also the generation of free ammonia, which can have an inhibitory effect on the digestion performance (ISAT/GTZ 1999). Therefore, mesophilic systems are generally more interesting. If the temperature of the biomass is below 15°C, gas production will be so low that the biogas plant is no longer economically feasible.

The optimal pH-level

There are two groups of bacteria in terms of pH optima, namely acidogens and methanogens. The best pH range for acidogens is 5.5 – 6.5 and for methanogens is 7.8 – 8.2 (MANG n.y.). The operating pH for combined cultures is 6.5-7.5 (MES et al. 2003). Since methanogenesis is considered as a rate-limiting step, pH close to neutral is optimum. Generally, pH is self-regulating and there is now need for adjustment. However, in the case of dysfunction of a system, an inappropriate pH may be the reason for the disturbed microbial process (MES et al. 2003).

Content of total solids (TS): dry (high-solid) and wet (low-solid) fermentation

Low-rate digesters can be designed to operate either in a high-solids (dry) or low-solids (wet) digestion process. High-solids digesters have TS contents above 20%, whereas below 20%, the digestion process is called wet. The most often, low-rate processes for the treatment of excess sludge, faecal sludge and agricultural or industrial slurries are wet processes. Wet (also low-solids) digesters can transport material through the system using standard pumps that require significantly lower energy input. Dry processes have the advantage to require less space, but they are somehow more complex and relatively new, however they have a large potential for the treatment of municipal organic solid waste. The TS content of high-rate systems after separation of the sludge from the liquid is generally similar to the one of slurries for wet digestion, but the concept of dry and wet mode is not applicable to high-rate anaerobic systems.

Mixing

For a well operating system, mixing has to be provided by mechanical stirring, gas circulation (bubbling) or displacement under gravity (MUELLER 2007). Intensive mixing is important for the process to allow the bacteria the contact with every degradable material. Mixing therefore improves the processing rate of reactor system (MUELLER 2007).

Methane production potential: COD and anaerobic biodegradability

In all kinds of biogas technologies, it is the chemical oxygen demand (COD), which is generally used to quantify the amount of organic matter in waste streams and predict the potential for biogas production. Another very useful parameter to evaluate substrates for anaerobic digestion is the anaerobic biodegradability and hydrolysis constant. The total anaerobic biodegradability is measured by the total amount of methane produced during a retention time of at least 50 days (MES et al. 2003).

Reactor set-up: batch or continuous

The most common forms of low-rate biogas reactors are batch reactors, plug-flow reactors (PFR) and continuously stirred tank reactors (CSTR). High-rate reactors a mixed concept: continuous mode for liquid and batch mode for settled sludge. Wet systems often also apply fed batch reactors (accumulation systems, MES et al. 2003).

Batch systems are the only systems that allow controlling accurately the hydraulic or solid residence time (HRT and SRT). But as the batch needs to be opened and emptied regularly, odour emissions can occur. In continuous reactors (PFR and CSTR) the HRT/SRT are determined by dividing the volume of the reactor by the flow.

One-stage and multi-stage processing

One-stage digestion, where all the process takes place in a single reactor, saves costs and is easier to operate. Small-scale plants in general are designed as one-stage processes. Large-scale plants however, often apply two- or multi-stage processes. The idea of multi-stage processes is that the digestion is mediated by a sequence of bio-chemical reactions, which do not necessarily share the same optimal environmental conditions. The systems involve separation of hydrolysis and acidogenesis from acetogenesis and methanogenesis phases (MES et al. 2003). This allows to more accurately control the main process factors (oxygen, temperature and pH) influencing the performance of the digestion bacteria. This is particularly interesting where heating is applied to improve the methanogenesis process as the volume to be heated is considerably lowered, resulting in an energy savings.