The anaerobic continuous stirred tank reactor (CSTR) is the most basic bioreactor configuration. The major advantage of the CSTR is its simplicity in construction and operation. However, large bioreactor volumes are required to provide the high retention time necessary to sustain the slow growing anaerobic microbial mass inside the bioreactor, which raises the cost of the process. Therefore, for an efficient anaerobic system with relatively small bioreactor volume, the design of anaerobic digesters should aim at providing an optimum environment for the growth of the anaerobic microorganisms given the complexity of their physiology and the syntrophic and/or antagonistic interactions among them. Lettinga (1995) specified certain criteria:

• High retention of the active biomass (microorganisms) inside the bioreactor.

• Sufficient contact between the biomass and the substrate.

• High reaction rates and elimination of the limiting transport phenomena.

• Suitable environment for the adaptation of the biomass to various types of feedstocks.

• Suitable environment for all organisms under the operating conditions.

Depending on the solid content of the feedstocks, different bioreactor configurations can be used:

• Low solid content feedstocks (e. g. secondary wastewater treatment, wastewater from food industry, hydraulic flush manure systems; swine)

— Anaerobic lagoons — fixed, floating, or submerged covers

— Completely mixed reactors

— Anaerobic filter reactors

— Fluidised bed reactors

— Upflow anaerobic sludge blanket reactors (U ASBR)

— Anaerobic baffled reactors (ABRs).

• Medium solid content feedstocks (e. g. dairy manure, ‘scraped’ swine manure, municipal or food industries sludge)

— Plug flow reactors

— Completely mixed reactors

— Contact reactors.

• High solid content feedstocks (e. g. organic fraction of municipal solid wastes, agricultural residues, food processing waste; food residuals; pulp — paper sludge)

— Plug flow

— Completely mixed

— Leach-bed.

A brief description of the main bioreactor types follows:

• Fixed-bed anaerobic reactor (anaerobic filter): The wastewater is introduced from the bottom or the top of a column which is filled with inert material (rocks, cinder, plastic or gravel). The filling material provides the surface upon which microorganisms are attached forming a biofilm. The microorganisms can also be retained through entrapment in the micro-porous structure of the filling material. Clogging is a typical problem with this type of digester. The organic load of the wastewater must be low to medium. Recirculation must be applied so that the organic load in the entrance is maintained between 8 and 12 g/L. Wastewaters containing significant amounts of suspended solids or constituents that cause precipitation of organic and inorganic compounds are not suitable for this bioreactor type. The filling material must provide large void space to avoid clogging (95%) and have large specific surface (100-200 m2/m3).

• Expanded and fluidised bed anaerobic digester: This type of configuration allows a more effective mass transfer from the liquid phase to the membrane, because fine filling material is used (0.2-0.5 mm). The upflow velocity must be high enough (through recirculation) to maintain the expansion of the bed between 15% and 30%, while if the expansion raises up to 300%, the bed is characterised as fluidised (Hall, 1992). Energy consumption required to provide recirculation is the main disadvantage of this bioreactor. The wastewater must contain low suspended solids as in the case of the fixed bed bioreactors.

• UASBR: The UASBR was designed as an alternative to wastewater treatment without the operating problems of bioreactors with filling materials but incorporating the concept of biomass immobilisation (Lettinga et al., 1980). In this bioreactor type, the microorganisms are agglomerated to form a dense structure (granule) with excellent settling properties and strength under adverse conditions. The granular sludge blanket remains in the bottom of the bioreactor. The feed is introduced from the bottom and the motion of the flow is upwards. The upflow velocity is very important since it influences the formation of the granules. Typical upflow velocities range between 0.5 and 3 m/h (Annachhatre, 1996). The biogas produced is often entrapped in the granules making them lighter and buoyant with their potential wash out. An effective three phase separator on the top of the bioreactor results in the retention of the granule and their return to the sludge blanket. UASBR is a reliably tested technology for the treatment of a wide range of wastewaters (from municipal wastewater to high strength agro industrial wastewater) with low solid content. It has low installation, operation and maintenance costs. More than 900 full-scale units are currently being operated all over the world (Garcia et al., 2008). Hybrid systems have been developed to combine the characteristics of a UASBR and an anaerobic filter, expanded or fluidised bed reactor. Hybrid UASBR have been used to treat a variety of industrial wastewaters over the years (Banu and Kaliappan, 2008; Sunil Kumar et al., 2007; Ramakrishnan and Gupta, 2006; Sandhya and Swaminathan, 2006).

• ABR: It is a rectangular tank with baffles. The wastewater flows above and below a series of baffles successively coming into contact with the biomass which is accumulated in the bottom of the bioreactor (McCarty and Bachmann, 1992). This bioreactor type is simple in structure, with no moving parts or mixers. The biomass is not necessary to have good settling properties as in the UASB, in order to be retained in the bioreactor. It is an efficient system at low retention times and its operation is stable under sudden changes in the organic loading rate (Barber and Stuckey, 1999). A modification of this bioreactor type led to the periodic anaerobic baffled reactor (PABR) which is based on the periodic feeding mode to all compartments. In PABR, the switching frequency of the feed allows flexibility in operation; the PABR can be operated as a simple ABR, if the switching frequency is set to zero, and, in the extreme case of very high switching frequency, as a single-compartment upflow bioreactor (Skiadas and Lyberatos, 1998; Stamatelatou et al., 2009).

• Plug flow: It is a long narrow insulated and heated tank. The digested material flows from one end of the tank to the other as fresh feedstock enters the bioreactor. The bioreactor can be placed horizontally or vertically. It is used in the case of solid feedstocks. In order to provide mixing, various practices are applied (de Baere, 2008). In the Dranco process (vertical, downflow plug flow digester), the fresh feedstock is mixed with a portion of the digested material and is introduced from the top of the bioreactor. The same concept can be applied while the plug flow reactor is placed horizontally (Kompogas process). In this case slowly rotating impellers inside the reactor can aid the horizontal movement of the mixture, also serving for mixing, degassing and suspension of the heavier particles. In another plug-flow type configuration (Valorga process), the horizontal flow is circular and biogas injection at intervals under pressure through a network of nozzles provides mixing.

• Leach bed: The feedstock is loaded in a vertical bioreactor to form a bed through which a liquid stream percolates as a leachate and is recirculated to the top of the same reactor where it is produced (Biocel process). This process is implemented in Lelystad, The Netherlands (ten Brummeler, 1999).

• Complete mixed anaerobic digester — anaerobic contact process. It usually consists of a round insulated tank, above or below ground. Heating is provided through coils with hot water inside the tank or an external heat exchanger. Mixing is achieved through a motor driven mixer, recirculation of the mixed liquor or biogas. The cover can be floating or fixed. In the case of low solid content feedstocks and in order to enhance the biomass concentration in the bioreactor, a modification of the complete mixed anaerobic digester led to the anaerobic contact process. In this configuration, the bioreactor is followed by a settling tank (or inclined parallel plates, membranes, etc.; Defour et al., 1994) to separate the sludge from the supernatant. The sludge is recycled to the bioreactor increasing the biomass concentration.

• Covered anaerobic lagoon: It is a large earthen impoundment, lined with appropriate geomembranes and covered with a flexible or floating gas tight cover. They are used mostly for manure treatment. No heat and mixing are provided, therefore the ambient temperature is prevailed making this type of digester unsuitable in cold climatic conditions.

There are more parameters according to which an anaerobic system can be

characterised:

1 Temperature of operation: All digesters usually operate within two temperature ranges, either at 35-40°C (mesophilic) or 50-60°C (thermophilic). Mesophilic anaerobic digestion is applied for digesting rumens of animals and feedstock from industrial and farm activities, while thermophilic anaerobic digestion is more suitable for sanitation of pathogen-bearing feedstocks. Another advantage of thermophilic anaerobic digestion is the fast conversion rates of the feedstock (induced by the fast metabolism of the microorganisms due to the high temperature) and, consequently, the lower retention time (and reactor volume) required. However, the psychrophilic range of temperatures (<20°C) have also been studied, especially in lagoons and swamps. Thorough studies on reactor design and in-depth parametric analysis for psychrophilic consortia are lacking. It has been acknowledged, however, that, in psychrophilic conditions, systems favouring biomass accumulation are required to secure high efficiency (Kotsyurbenko et al, 1993; Lettinga et al., 2001). Another possibility has been to apply genetic engineering in the attempt to introduce stable enzymes, active in cold temperatures to give improved catalysts for the biomethanation process (Kashyap et al., 2003).

2 Solid content of digesting mixture: When the solid content of the digesting mixture is less than 3-4% (little or no suspended solids), then the digesters are usually a single phase liquid system. Digesters treating solids are characterised as wet or dry depending on whether the solid content is up to 12-15% or more. Wet anaerobic systems are in a slurry form and can still be mixed through agitation, while for the dry anaerobic systems, the plug-flow type digesters are most suitable.

3 Number of bioreactors: The anaerobic systems may consist of a single bioreactor or a combination of bioreactors of different or the same design. Especially, in the case of anaerobic digestion of solid or slurry feedstocks, the use of more than one bioreactors is a common practice. Typically, two stages are used, with the first one being the hydrolytic-acidogenic step and the second one being the methanogenic step. In a two-stage process, it is possible to optimise the operational conditions of both steps since they take place in different bioreactors. The application of this concept has resulted in a great variety of two-stage configurations. The main advantage of the two-stage systems is the process stability in the case of feedstocks that would cause an unstable performance in single stage systems.

Two or three bioreactors of leach-bed type may be used in series, as in the sequential batch anaerobic composting (SEBAC) process (Chynoweth et al., 1992, 2006). Leachate is transferred from a ‘mature’ bioreactor to a bioreactor filled with the fresh feedstock and recycled to the top of the ‘mature’ bioreactor until methanogenic conditions in the first stage prevail. Then, the bioreactor is switched to internal leachate recirculation until methanogenesis is completed. The volatile fatty acids from the first stage are transferred through the leachate into the ‘mature’ bioreactor with active methanogenic populations, while microbes from the second ‘mature’ bioreactor are recycled to the first one, enhancing its microbial activity.

In a similar concept, another configuration also uses batch loading to stimulate rapid volatile fatty acid production in a two-stage system. It combines one or more high solid bioreactors of leach-bed type in the first stage with a high rate and low solids bioreactor (such as an anaerobic filter or a UASBR) in the second stage (Zhang and Zhang, 1999; Lehtomaki et al., 2008). The high-solids reactors are loaded and the leachate from the batch reactors is continuously circulated through a single low-solids digester. The effluent of the second bioreactor, with reduced organic load and high alkalinity, is pumped back to the first stage bioreactor(s).

Temperature phased systems is another case of multistage configuration with each stage operating at a different temperature. This process has been implemented most often using thermophilic digestion (with a temperature range of 45°C to 65°C) as the first phase, followed by mesophilic digestion (with a temperature range of 25°C to 42°C). This is designed to produce a final product with a minimal odour level, better dewatering properties and low content in pathogens.

4 Continuous or batch mode of operation: There are systems that operate in a continuous mode, while others are loaded in batches and, upon completion of the waste degradation up to a degree, are emptied and left with a 10-15% of the digested content as a seed for the next cycle of batch (sequencing batch reactors).

5 Small or large scale systems: Anaerobic digestion has been extensively applied in agriculture at small scale in the form of on-farm digesters, and the produced biogas is utilised for heat as well as electricity production. The solid and liquid residues from the anaerobic digesters can be recycled in the farm. The digesters are constructed as simple as possible in order to be economic. They are heated containers, shaped like silos, troughs, basins or ponds and may be placed underground or on the surface. They may be batch type (much simpler to construct and maintain) or continuous type. On-farm digesters usually operate at a mesophilic range of temperatures at a typical retention time of 10-30 d.

However, operation of large scale systems have the advantage of being more economically profitable; integrated farm waste management takes all factors into account (feedstock, products) with the aim of maximising the economies of scale and of eliminating the impacts to the environment. Moreover, very large scale anaerobic digestion plants, the so-called ‘centralised anaerobic digestion plants’, have been developed to use feedstock from a variety of sources. The primary source of feedstock is farm wastes, but also other non-toxic types of wastes, such as those coming from food processing industries or the organic fraction of the source-sorted municipal solid wastes, can be introduced in a centralised anaerobic digestion (CAD) facility. The anaerobic digesters may be either mesophilic or thermophilic and operate at typical retention times of 12-20 d. Process control schemes are usually applied in this scale since it is affordable to employ trained staff. Farmers may have an additional income through tipping fees by providing the feedstock to a CAD, but they may also benefit more through applying the digestate on their farms as a fertiliser. The location of CAD plants is also crucial and they usually serve either a single large farm or several farms within a radius of about 10 km.