Anaerobic Digestion (Organic Waste)

Biogas reactors are gastight chambers in which organic wastes are degraded and converted into biogas by microorganism in absence of air via a process called anaerobic digestion. The process lasts over a couple of weeks up to several months. Biogas is recovered and used either directly for cooking and lighting (at household level) or it is 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). See also the factsheets about the conversion of biogas to electricity at small or large scale or the direct use of biogas factsheet. It can also be upgraded to natural gas quality, compressed and used to power motor vehicles. Nutrients remain in the sludge, which can be composted and used as soil amendment in agriculture. Liquid effluents are either treated and rejected, or reused in fertigation.

Anaerobic digestion is an established technology for the treatment of animal manure or wastewater (MES et al. 2003). In many industrialised countries, the conversion of municipal organic waste has become increasingly popular in recent years as a sustainable technology producing green energy. Consequently, various high-tech designs of different scales have evolved. In developing countries, many biogas support programmes have focused on rural families using animal manure and human faeces as feedstock in order to reduce the use of firewood by providing people with biogas, improve soil fertility and reduce indoor air pollution (VOEGELI & ZRUBRUEGG 2008). Today, facing the soaring challenge of municipal solid waste management in most urban areas in low and middle-income countries, anaerobic digestion is considered a promising option also for treatment of solid waste management in developing countries as well (www.eawag.ch; VOEGELI & ZURBRUEGG 2008; VOEGELI & LOHRI 2009). This factsheet presents the basic principals of anaerobic digestion as a treatment of solid waste and illustrates some of the many different technologies available today.

All biogas digesters are basically designed following the same process of anaerobic digestion. Anaerobic digestion is a four-stage process consisting of hydrolysis; fermentation (conversion of non-soluble organic biomass to soluble organic compounds); acidification (e conversion of soluble organic compounds to volatile fatty acids and CO₂, followed by the conversion of volatile fatty acids to acetate and H2); and finally methane formation. The final product, biogas, is a mixture of methane (CH₄), carbon dioxide (CO₂) and other trace gases (see also anaerobic digestion, general factsheet).

There are many ways in which anaerobic digestion can occur. The simplest reactors are covered waste dumps, where anaerobic digestion can occur naturally in uncontrolled systems. As mentioned above, today there is a large range of different types and designs of anaerobic digester technologies for the treatment of organic waste available. Even though the process for all these technologies is always the same (i.e. anaerobic digestion), depending on the composition of the substrate and the volume of the waste stream, complexity of design, construction and operation vary strongly.

The main differences regarding the design of these technologies exist regarding the biogas potential of the substrate; dry and wet processes; mesophilic and thermophilic processes; mechanically mixed and no-mixed reactors, batch and continuous reactors and one-stage or multi-stage processes etc. In the following, a short overview on all these parameters is given (refer to anaerobic treatment, general factsheet).

Some reference values of biogas production potential of different substrates are given in the table below. The table also illustrates the potential of petrol equivalents saved. Conventional digesters apply a wet digestion process (total solids (TS) below 20%). Wet digestion has the advantage to provide good fluidity of the slurries and optimal contact of the microorganism and the waste. The new technology, dry digestion (TS > 20 %), allows high gas yields using less space, but the process has only recently been developed and is applied only at high-tech level. Mesophilic digestion is generally more stable even though gas yields are lower. Thermophilic processes produce more biogas in shorter time but require input energy and bear the risk of the production of toxic ammonia(ISAT/GTZ 1999 Vol. I). Batch systems are the lowest-tech of all systems and also the cheapest (VANDEVIVERE et al. 2002). Their major drawbacks are a large footprint and a lower biogas yield due to the impairment of the percolation process via channelling and clogging (VANDEVIVERE et al. 2002). Batch systems have a high potential for application in developing countries. Multi-stage processes separate the different stages of anaerobic digestion in order to achieve optimal growth conditions for the microorganism and to allow the heating of the methanogenic stage only (reducing required energy inputs). However, multi-stage systems are the most complex, and most expensive, of all systems biogas systems for solid waste (VANDEVIVERE et al. 2002).Low-tech small scale wet digesters generally do not provide any mechanical mixing as the gas bubbles rising through the sludge to the top of the reactor provide sufficient mixing. Large-scale, and in particular dry digestion processes require the installation of a mechanical stirring (or transporting device in the case of plug-flow reactors).

Energetic potential of municipal solid waste (MSW). Source: HOLLIGER (2008)

Most anaerobic digesters for the treatment of organic solid waste provide a post treatment of the remaining sludge before it can be reused as soil amendment in agriculture. Aerobic composting, the post-treatment used most often, allows both nitrification and the degradation of lignin, which is not possible under anaerobic conditions (HOLLIGER 2008).

Below, some examples of small-scale and large-scale anaerobic digesters for the treatment of organic solid wastes are given.

ARTI stands for Appropriate Rural Technology Institute, which has developed the ARTI compact biogas plant in 2003 for the treatment of organic waste at the household level (1 to 2 Kg of food waste per day). The plant is sufficiently compact to be used also by urban households. Approximately 2000 such plants are currently in use in Maharashtra, India, in both urban and rural households (WRAPAI 2009). In Africa, only few ARTI biogas plants have been installed so far (Tanzania, VOEGELI & LOHRI 2009; VOEGELI et al. 2009).

ARTI compact biogas plants are based on a simple and low-cost floating-drum design applying a wet digestion process. The plants are made from conventional polythene water tanks (two tanks, with volumes of typically 0.75 m3and 1 m3, MUELLER 2007) and standard plumber piping. The smaller tank is the gasholder and is inverted over the larger one, which holds the mixture of decomposing feedstock and water (LOHRI 2009). An inlet is provided for adding feedstock, and an overflow for removing the digested residue. The overflow liquid can be mixed with the feedstock and recycled back into the plant to maintain optimal moisture condition for a wet digestion process (MUELLER 2007). A pipe takes the biogas to a collection balloon or directly to the kitchen.

BIOTECH is a nodal agency of the Ministry of Non-Conventional Energy Sources in Kerala, South India (MUELLER 2007). BIOTECH has been developing and installing biogas plants that generate biogas from domestic biodegradable waste like cooked food waste, vegetable waste and waste water from kitchen (1 m3 for a 3 to 5 member family meets about 50 % of cooking needs, MUELLER 2007) (see picture above). BIOTECH has also developed reactors for the decentralised anaerobic digestion of organic market waste or municipal solid or slaughterhouse waste. The electricity generated from the decentralized plants for market wastes is used for street lightning and distributed to households.

As the ARTIs, the BIOTECH market level plants are based on a floating-drum design including a recirculation loop to optimise moisture content (HEEB 2009). The feedstock (often market wastes) is put in the inlet tank from where it is flushed into the main digester tank for anaerobic digestion (3 m diameter and 3 m depth, HEEB 2009). To increase the retention time for solids, there is a baffle in the middle of the tank and orthogonal to the flow direction. The baffle holds back unsuspended solid compounds of the waste, whereas liquids can easily flow over (much like septic tanks or biogas digesters, see also factsheet (see also small scale digesters or biogas settlers). This leads to an increased retention time for unsuspended solids and therefore to an improved decomposition of this compounds. The digested liquor (digestate) flows into the effluent tank from where a pump, run with electricity of the produced biogas, transports it up into an overhead storage tank. From the tank, the digestate is then used to flush the feedstock from the inlet tank into the digester tank avoiding the need of fresh water for flushing (HEEB 2009).

KOMPOGAS is a high-solid thermophilic dry digestion process developed in Switzerland (see also anaerobic treatment of waste and wastewaters). It is a compact plant consisting of a horizontal functioning as plug-flow reactor (PFR). The reactors are fed regularly, mostly every day. Most of the feedstock for KOMPOGAS plants comes from municipalities that support source-separated collection of organic wastes or from the food industry. Before the waste is filled into the reactor, ferrous and plastic materials are removed, it is mixed, and it is sent through a grinder (OSTREM 2004).

Slowly rotating intermittent propellers push the waste through the digester, having also the effect of homogenising and degasing the pulp and of keeping heavier particles in suspension (OSTREM 2004). The retention time is of approximately 20 days (MES et al. 2003). The digested material is then dewatered; some of the press water is being recirculated to the reactor to provide an inoculum and to maintain optimal moisture conditions. The excess liquid is sold as liquid fertiliser. The solid fraction is composted (3 to 4 weeks) and sold as soil amendment. The produced biogas is generally transformed in a CHP unit providing 100 % of the facility needs as well as additional electricity for sale. In some cases, the biogas is upgraded to natural gas standards for use in vehicles or input to the natural gas network (OSTREM 2004). Due to the mechanical requirements of the system, the size of the reactors is limited. To enhance capacity an additional reactors need to be installed in parallel (OSTREM 2004).

In Thailand, the development of alternative energy sources is critical, as the government has set 2011 as the target date for 8% of the nation’s total energy reduction (MUELLER 2007). This has given large rise to various large-scale biogas projects. The Rayong Municipality has constructed a wet fed-batch high-solids plant for the treatment of the organic fraction of the municipal solid waste.

The plant is comprised of two systems; a process that converts waste to biogas and fertiliser and a biogas-fired cogeneration process (CHP). Additional to the organic solid waste from the municipality, the Rayong plant uses food, vegetables and fruit wastes and night soil as waste materials. The plant has a capacity to handle 60 tons of waste per day turning out into an output of 5800 tons organic fertiliser and electricity of about 5 million kWh (MUELLER 2007).