The implementation of the biogas technology on large scale may be prevented or slowed down by a number of constraints. They may be grouped as follows: political, social-cultural, financial, informational, institutional, technical and training (Omer and Fadalla, 2003, Ni and Nyns, 1996). Some of the difficulties encountered in the development of anaerobic treatment for biogas production in developing countries are in Table 3.

There is lack of coherent biogas technology strategy in many sub-Saharan African countries despite the increase in the price of conventional fuel on a daily basis, and their rising demand mainly to technical and non-technical factors. The main contentious problems of biogas commercialisation in sub-Saharan African countries relate to economics and political will and many site-specific issues. Some of these issues are informed by local dynamics of perceptions; influenced by personal, social and institutional factors and beliefs, as well as internal conflicts, due to perceived environmental, social and ecological risks, that were aggravated by miscommunication and the lack of understanding.

• Inexperienced contractors and consultants, resulting in poor-quality plants, and poor choice of materials.

• Lack of reliable information on the potential benefits of the technology by financial institutions.

• Complete absence of academic, bureaucratic, legislation and commercial infrastructure in the region/country.

• Lack of knowledge on the system in practice, sometimes even in research institutes and universities.

• Community acceptance issues and poor ownership responsibility by users.

• Complete absence of pilot studies, and no full-scale experience.

• No properly educated operators, lack of credibility, lack of technical knowledge on maintenance and repair.

• Uninformed or poorly informed authorities and policy makers.

• Failure by government to support biogas technology through focussed energy policy.

• Research at universities is frequently considered to be too academic in nature, even

____ when it is quite applied._______________________________________________________

Table 3. Some of the difficulties thwarting development of biogas technology in Africa (Mwakaje, 2007; Murphy, 2001, Lettinga, 2001; Lettinga, 1995, Switzenbum, 1995; Tafdrup, 1995; Iza et al., 1991)

1.1 Equipment

An anaerobic reactor with a working volume of 1 litre equipped with magnetic stirring and placed in a thermostatic chamber at 35 °C was used. The reactor had an upper settling zone designed to minimize loss of the biomass responsible for the process. The reactor was fed daily by means of an external feeder and liquid effluent removed daily through a hydraulic seal, comprising 25 cm liquid column, designed to prevent air from entering the reactor and biogas from leaving. This reactor has been described in detail elsewhere (Martin et al., 1991).

The methane volume produced in the process was measured using a 5 litre Mariotte reservoir fitted to the reactor. A tightly closed bubbler containing a NaOH solution (3 M) to collect the CO2 produced in the process was intercalated between the two elements. The methane produced displaced a given volume of water from the reservoir, allowing ready determination of the gas (Martin et al., 1991).

1.2 Inoculum

The reactor was inoculated with methanogenically active biomass from a laboratory-scale anaerobic reactor processing olive mill wastewater. The composition and features of the biomass used were: pH, 7.2; total solids (TS), 60.3 g/L; mineral solids (MS), 19.3 g/L; volatile solids (VS), 41.0 g/L; total suspended solids (TSS), 59.9 g/L; mineral suspended solids (MSS), 18.8 g/L; volatile suspended solids (VSS), 41.1 g/L.

1.1 Biogas to the grid

When the biogas finally had been produced, treated und conditioned it will either be fed into a nearby gas pipeline system or grid or it will be burned and transformed into electric power which is fed into the electric grid. In the following chapters we review and discuss the aspects of the gas grid, only.

Normally, the gas grid used by biogas plants will be a low or medium pressure operated distribution network. Certain conditions may require that the biogas is compressed to a higher level and being fed into a high pressure transportation network.

Typical locations of biogas plants and the major cities the corresponding gas transportation and distribution network are shown in figure 1.

1.2 The location paradox

Typical biogas plants have preferred locations in rural areas, where the renewable material grows and the transportation is short and easy. In opposition to the rural places the areas of higher gas demand or actual consumption are located in or near urban areas. So, in most cases the biogas must be transported in pipelines over a longer distance spreading also over a wider network area before the biogas is consumed totally. Depending on the network structure and the total consumption figures there exist mixing areas of biogas and natural gas or pure biogas.

Due to the importance of anaerobic digestion as a treatment process, different dynamic models exist, such as the AM2 which was developed jointly by researchers of the INRA of Narbonne and the INRIA of Sophia-Antipolis in 2001 (Olivier et al., 2001). It is based on experimental results obtained on the fixed bed reactor of the INRA of Narbonne. This model is made of two steps: acidogenesis and methanogenesis corresponding to acido-acetogens and methanogens bacteria populations, respectively. As a more recent and elaborate model, the ADM1, was developed by an IWA group (Batstone et al., 2002). Its main feature is the consideration of the principal steps of anaerobic digestion process that are, respectively, substrate disintegration (non biological step), hydrolysis, acidogenesis, acetogenesis and finally the methanogenesis with seven different bacteria groups.

Since its development in 2002 and up to now, the ADM1 has been tested and used on different substrates where a great number of research works are reported in the literature. As examples, one can cite (Blumensaat and Keller, 2005) who modified the initial version of ADM1 for the simulation of a dynamic behaviour of a pilot scale digester using sludge, in order to ensure a faultless model implementation. They obtained accurate results for the cases of low to medium loading rates. However the accuracy showed a decline with the increase of the loading rate.

Wayne and Parker, 2005) considered the application of the ADM1 to a variety of anaerobic digestion configurations where the results showed, in most of the considered cases, that the model was able to reproduce the trends of the experimental results.

(Feng et al., 2006) found that the ADM1 is not sensitive to the distribution ratio of carbohydrates, proteins and lipids, whereas the fraction of short chain fatty acids (SCFA) in the influent is rather more important.

Consequently, the great capabilities of ADM1 in modelling different types of substrates and calculations have been the motivating factor to use it in the present work to evaluate the performances of a co-digestion process for the treatment of organic municipal solid waste and waste activated sludge in the above mentioned 2000 m3 reactor working at a temperature of 37°C.

As mentioned above the ADM1 (Anaerobic Digestion Model No. 1) was developed by the IWA group (Batstone et al., 2002) with the objective to build a full mathematical model based intimately on the phenomenological model in order to simulate, at best, anaerobic reactors. It includes, as a first step, the disintegration of solid complexes (non biological step) into carbohydrates, lipids, proteins and inert material (soluble and particulate inert). The second step is the hydrolysis process of the disintegration products under an enzymatic action to produce sugars, amino acids and long chain fatty acids (LCFA), successively. Then, amino acids and sugars are fermented to produce VFA, hydrogen and carbon dioxide (acidogenesis). Then LCFA, proprionic acid, butyric acid and valeric acid are anaerobically oxided into acetate, carbon dioxide and hydrogen (acetogenesis). Finally, methane can be produced through two paths: the first one is based on acetate whereas the second one is through the reduction of carbon dioxide by molecular hydrogen. The organic species and molecular hydrogen, in this model, are expressed as COD (Chemical Oxygen Demand), whereas inorganic nitrogen and inorganic carbon species are expressed through their molecular concentrations.

Extensions and modifications were brought to ADM1 to enlarge its prediction capabilities by, taking into account other factors such as, for instance, the sulfato-reductors or the degradation of certain substrates (Wolfsberger and Halubar, 2006) and (Batstone and Keller, 2003). Moreover, Usama Zaher (Usama, 2005) considered the toxic effects of cyanide as an inhibition process for acetate.

All biological processes require presence of specific enzymes. Processes of reduction of protons as well as oxidation of hydrogen (see reaction below)require at least presence of three enzymes: iron hydrogenase, nickel-iron hydrogenase and nitrogenase.

H2 ^ 2H+ + 2e — (1)

Hydrogenase exists in a number (~ 40) of prokariota, both aerobic and anaerobic, as well as in certain eukariota, e. g. in photosynthetic algae (Nickolet, 2000). Hydrogenases show different but significant sensitivity towards oxygen and light. Among more than 100 discovered hydrogenases essentially only those containing Fe and Ni atoms in active center are considered as the most attractive:

— [Fe] hydrogenase — containing only Fe atoms is the most sensitive towards oxygen inhibition but almost 100 times more active than [Ni-Fe] hydrogenases

— [Ni-Fe] or [Ni-Fe-Se] — these two types of hydrogenases indicate much higher affinity towards hydrogen than [Fe] hydrogenase (Darensbourg, 2000).

Active centers of both hydrogenases are composed from iron-sulfur clusters coordinated by carbonyl (CO) or cyanide (CN-) ligands.

Iron hydrogenases are the two directions enzymes because they catalyze both reduction of protons to the molecular hydrogen and the reverse reaction. There are three forms of these enzymes: monomeric — build only from the subunit controlling catalysis, dimeric, trimeric and tetrameric. Active centers located in these enzymes are not uniform either, however, all of them contain H-cluster (see Fig. 4) (Nicolet, 2000). Applying FTIR, EPR and XRD spectroscopy for analysis of monomeric hydrogenase, isolated from Clostridium pasterianum, it was found that H-cluster is composed from two basic units: [4Fe-4S] single group, responsible for electron transport, and the unique arrangement of [2Fe] capable to perform the reverse oxidation reaction of hydrogen. The regular cluster [4Fe-4S] is linked with four cysteine and sulfur atom of one of these forms the bridge bond between [4Fe-4S] and [2Fe]. In this dimeric system, the octahedral iron atoms are linked through two sulfur atoms (see Fig. 5) (Darensbourg, 2000). Moreover, it was found that these atoms are coordinated with five non-protein ligands (CO and CN-1) and water molecule. The bridge sulfur atoms forms additionally the 1,3- propanodithiol structure. The presence of covalent bond between sulfur atoms influence the charge of H-cluster and electric properties (Nicolet, 2000).

Fig. 4. Scheme of iron hydrogenase in Desulfovibrio desulfuricans (Dd) and Clostridium pasterianum (Cp). F — double cluster of [4Fe-4S], L-large subunit of H-cluster, S — small subunit of H cluster, Fd — [2Fe-2S] cluster related to ferredoxin. Pink color represents the unique structure of [4Fe-4S]. In Dd hydrogenase large and small subunits are connected via cysteine, whereas in Cp hydrogenase these units are linked with protein chain.

In active center of hydrogenase it is possible to identify such aminoacids as methionine and histidyne (Das, 2006). These two amino acids become attached to active center during formation of channels (for H2 and H+) connecting enzyme surface with reaction slit. The comparison of H-clusters in two strains of bacteria Clostridium pasteurianum (Cp) and Desulfovibrio desulfuricans (Dd) shows that in both cases the [2Fe] group is involved in hydrogen bond formation with lysine. However, when the second iron atom in Cp is engaged with serine, in the case of Dd, alanine is involved instead. In the case of fermentative bacteria of the Clostridium family in the large unit of monomeric iron hydrogenase it was confirmed a presence of three excessive systems: the [2Fe-2S] structure, rarely existing [4Fe-4S] structure with slit and space constructed from two [4Fe-4S] systems (Vignais, 2006).

Nickel-iron hydrogenase isolated from Desulfovibrio gigas and Desulfofibrio vulgaris is composed from large subunit a (60 kDa) containing Ni-Fe active center and small subunits в (30 kDa) equipped with three iron-sulfur clusters. These clusters are involved in electron transfer between active centers, donors and acceptors. All these clusters are located in the strait lines in which [3Fe-4S] appears between two [4Fe-4S] structures (Vignais, 2006).

The active center f [Ni-Fe] hydrogenase exhibits the unique location of ligands (see Fig. 6)

Here, four molecules of cysteine coordinate one three valent nickel atom. Two of them coordinate simultaneously iron, also located in active center. This kind of arrangement induce formation of sulfur bridges between nickel and iron atoms. Moreover, non-protein ligands such as SO, CO, CN and CO, CN are located in active centers of D. vulgaris and D. gigas, respectively. Nickel and iron atoms are bonded with monoatomic sulfur (D. vulgaris) or oxygen (D. gigas) bridges. Generated space is an ideal place for hydrogen reduction with electrons transported by iron-sulfur clusters from the surface of enzyme. The change of nickel valance form III to 0 and the return to basic state together with reconstruction of sulfur (or oxygen) bridge is observed in this catalytic cycle.

Nitrogenase is considered as the essential part of nitrogen circulation system in the living world. Nitrogen present in the air, needs to be transformed into compounds acceptable by living organisms. The diazotrophic microorganisms, including the PNS bacteria, are able to transform atmospheric nitrogen into NH3. There three types of nitrogenases built of two separate protein units: dinitrogenase (either Mo-Fe protein, or V-Fe protein, or Fe-Fe protein) and reductase (Fe protein). The main task of reductase is the delivery of electrons of high reductive potential to nitrogenase which uses them to different reduce N2 to NH3. Six electrons are involved in this process to reduce the oxidation degree of nitrogen from 0 to 3. The enzyme also transfers two other extra electrons to protons with final formation of one molecule of H2. Reduction of nitrogen to ammonia is highly energy consuming process because of the necessity of breaking the stable triple bond in nitrogen molecule and needs 16 ATP molecules per one molecule of nitrogen:

N2 + 8H+ + 8e + 16ATP ^ 2NH3 + H2 + 16ADP +16Pi (2)

Both components, nitrogenase and reductase are iron-sulphur proteins, in which iron is bonded with sulphur both in cysteine and the inorganic sulphide.

Reductase (Figure 7) is a dimer with mass of 30 kDa composed of four iron atoms and four inorganic sulphides (4Fe-4S). The site for ATP/ ADP bounding is located on the surface of this subunit. Reductase transfers electrons from the reduced ferredoxin towards dinitrogenase. This process occurs during hydrolysis of ATP with simultaneous dissociation of the complex.

Fig. 7. Reductase structure — nitrogenase component (Berg, 2002).

Dinitrogenase is a tetramer of the structure a2p2 and molecular weight of 240kDa (Figure 8). At the interface between the a and в subunits there is the P unit through which electrons are able to penetrate. Two cubo-octahedrons of 4Fe-4S are linked via sulphur atoms from cysteine residues. The flow electrons is realized from P unit to coenzyme Fe-Mo. This coenzyme is built of two units of M-3Fe-3S linked via sulphur atoms. In one unit M stands for Mo, while in the other one for Fe. Atmospheric nitrogen is transformed in the central part of coenzyme Fe-Mo. Multiple interactions of Fe-N type weaken the triple bond in molecular nitrogen which lowers the activation limit for nitrogen reduction (Berg, 2002). The synthesis of nitrogenase strongly depends on the light access to the medium and its intensity. Catalytic stability of nitrogenase is ensured by alternating light and dark 12-hour periods (day and night sequence) (Meyer, 1978). In the absence of molecular nitrogen and with large quantities of energy provided by ATP (Koku, 2002) nitrogenase catalyses hydrogen generation (see eq.3). Nitrogenase acts as a safety valve regulating cell reduction potential (Kars, 2010).

2H+ + 2e + 4ATP ^ H2 + 4ADP +4Pi (3)

There are two main inhibitors of nitrogenase during hydrogen photobiogeneration: molecular oxygen and nitrogen. In the presence of molecular nitrogen occurs competitive nitrogen fixation reaction and this stops almost completely hydrogen evolution. Ammonium ions at concentrations higher than 20 gmol are successful but reversible inhibitors of hydrogen generation (Waligorska, 2009) as well. The nitrogen necessary for the cell functioning is usually provided by ethanolamine and glutamate.

Fig. 8. Dinitrogenase construction (Berg, 2002).

However, glutamate can be the source of nitrogen inhibiting hydrogen evolution similarly as non-ammonium compounds. It can when glutamate becomes the source of carbon after the other sources are exhausted (Koku, 2002). In order to avoid such situation a medium with a relatively high ratio of organic carbon to nitrogen should be applied (e. g. malate to glutamate =15/2 (Eroglu, 1999).

Analysis of variance of the yield was shown in table 4-11.

Table 4-11 showed that there were significant differences between the yields of each treatment. Multiple comparisons of the eggplant yield among treatments were shown in table 4-12.

Notes: The significance of symbols were as same as table4-4.

Table 4-12. Multiple comparisons of the eggplant yield among treatments

The results showed that there were significant differences between B treatment of biogas residue fibre film and control, no significant differences between A, C treatment and control, significant differences between black film and control. There were extremely significant differences between the plastic film, A, C treatment and control, there were significant differences between plastic film and B treatment, no significant differences between plastic film and the black film. It can be seen, the yield of B treatment increased 47% as compared with control.

5. Conclusions

1. Biogas residue are mainly composed of the fiber, non-metallic minerals, minerals, their mass percentage are 64%, 35% and 1% respectively; cellulose, hemicellulose, lignin and ash are 44.8%,21.9%,15.6% and 17.7% respectively in the biogas residue fiber. The mass percent of biogas residue cellulose is over 5% more than that of rice straw, wheat straw and corn stalk; the mass percent of the hemicellulose is 5% less than that of rice straw, wheat straw and corn stalk.

2. An optimum factors combination is rosin 0.4%, bauxite 4% and wet tensile strength 1.8%, beating degree 40SR°, grammage 80 g/ m2, in this case, for the biogas residue fibre film, dry tensile strength can attain more than 30N, wet tensile strength can attain more than12N, the degradation period can attain 35 days to 60 days.

3. The rank of importance of the five factors on the dry tensile strength: grammage, beating degree, wet strength agent, bauxite and rosin; on the wet tensile strength: wet strength agent, bauxite, rosin, grammage and beating degree; on the degradation: grammage, wet strength agent, bauxite, rosin and beating degree.

4. Biogas residue fibre film has significant effect against weeds as compared with plastic film and bare field.

5. There was no significant difference of conserving moisture between biogas residue fiber mulching and the plastic film.

6. In the period of eggplant growth, the rank of the total accumulated temperature: plastic film, black film, biogas residue fibre film and bare field.

7. The eggplant yield of biogas residue fiber mulching whose grammage is 80g/ m2 is 47% more than bare field, and slightly lower than that of the plastic film.

8. It is feasible to make biodegradable mulching from the biogas residue, in accordance with the agronomic requirement

Palm oil mill effluent (POME) contains high COD and biochemical oxygen demand (BOD). POME consists of a wide range of biological substances from complex biopolymers such as proteins, starches and hemicelluloses to simple sugars and amino acids. POME may also contain dissolved oil and fatty acids, glycerin, crude oil solids and short fibers as well as soluble materials that are harmful to the environment. Since POME is discharged at high temperatures (80-90oC), both mesophilic and thermophilic temperatures have been widely applied for POME treatment by anaerobic digestion.

2.2 Effect of chitosan on UASB treating POME

It has been reported that thermophilic operation of anaerobic reactors provides some advantages over mesophilic operation in areas such as higher rates of substrate degradation and biogas production. However, mesophilic reactors can be preferable because of greater process stability (Mustapha et al., 2003; Poh & Chong, 2009). Operating temperature is a major factor that greatly influences digester performance (Choorit & Wisarnwan, 2007; Poh & Chong, 2009; Yu et al., 2002).

The effects of chitosan as a sludge granulation accelerator during the transition from mesophilic (37oC) to thermophilic condition (57oC) has been investigated by Khemkhao et al. (2011). They used two UASB reactors, with a working volume 5.3 L, both of which they inoculated with mesophilic anaerobic sludge. The sludge was then acclimatized to a thermophilic condition with a stepwise temperature increase of 5oC from 37 to 57oC. The OLR ranged from approximately 2 to 9.5 g COD/L-d. One of the reactors was then injected with a chitosan dosage of 2 mg chitosan g/VSS on the first day of operation and the second reactor was used as a control.

At all times during the operation of the two reactors, the UASB with chitosan addition was found to have 5% higher COD removal efficiency and 16 L/d higher biogas production rate (7.82 L/g VSS removed-d) than that of the control. The methane contents of both reactors were found to be similar, with approximately 78% methane content for UASB with chitosan addition and 76% for the control. The effluent VSS in both reactors was found to increase with increase of OLR. The UASB with chitosan addition was found to have 6 to 23% lower effluent VSS than that of the control. Khemkhao et al. (2011) concluded that the UASB with chitosan addition had consistently better performance than the control.

2.3 Effect of chitosan on microbial diversity in UASB treating POME

The mechanism of anaerobic digestion in methane production consists of a series of complex metabolic interactions between various types of microorganisms in the absence of oxygen. Anaerobic digestion is mediated through processes of hydrolysis, acidogenesis, acetogenesis and methanogenesis. Khemkhao et al. (2011) used 16S rRNA targeted denaturing gradient gel electrophoresis (DGGE) fingerprints to study the microbial communities during anaerobic digestion. They found that bacteria and methanogens could both be detected in the UASB reactors operating both with and without chitosan addition.

The table 11 shows the determined rates of admixture for LPG to attain the appropriate target calorific value range. With the LPG quantities shown, the respective initial concentrations of methane, the entire calorific value range of the respective base gas quality is covered, with some restrictions.

For the practical implementation of the listed quantities of the LPG admixtures, limits as given in Table 12 are to be observed, in accordance with the requirements presented on the need for the use and applicability of SGERG88 and AGA8 procedures and the resulting maximum admixture quantities according to Table 12. Due to these limits, defined in DVGW G 486-B2, it will not be possible in every case to reach the upper calorific value range at higher pressures. In addition, the availability of appropriate measuring technology for higher liquid gas fractions is limited. At the very high degrees of methane processing, there are limitations on attaining the lower calorific value range, since when processing to 99.5 Vol -% methane, an H gas with a calorific value of 11.009 kWh / m3 results.

Table 13 shows the maximum possible, compliant LPG admixture, a propane / butane mixture of 95 / 5 Mass.-%. As a result, it is clear that processing to a maximum methane content of 99.5% vol. with the maximum permissible LPG admixtures, a calorific value of maximum 11,361 kWh/ m3 (NTP) is possible at pressures above 100 bar, and a maximum of 12,075 kWh/m3 (NTP) at pressures below 100 bar.

Figure 14 shows the admixture required to achieve the corresponding H gas properties. The limits on the maximum concentration of propane are also shown according to DVGW regulations G 486 supplementary sheet 2 a ppendix B. Admixtures to achieve the properties of North Sea I / North Sea II H gas are, based upon all levels of methane processing, above the limits. A compliant mixture is not possible in this case, or needs to be tested on an individual basis.

96 98

Methankonzentration [Vol.-%]

Fig. 14. LPG quantities necessary to achieve the target calorific value

In order to achieve higher calorific values, alternative conditioning measures can be employed. For example, ready-made mixtures with customized propane / butane ratios can be used for conditioning. This can increase the calorific value, but technical and physical effects, such as condensation behaviour, methane number and k-number deviations need to be considered.

As indicated in Table 3, there are some digesters have been installed in a number of sub­Saharan Africa. These have mainly been pilot or demonstration projects aimed at testing the technical viability of small-scale biogas technology at a limited scale (Hivos, 2009a). These pilot projects have mostly been funded by non-governmental organizations and built for health clinics, schools, and small-scale farmers. While the small-scale biogas plants are located throughout Africa, only a few of them are operational (Parawira, 2009). There is also limited documentation on whether the existing biogas digesters have been successful in achieving the benefits highlighted in section 3.1. Some country specific examples is Tanzania, Ivory Cost and Burundi, which have produced biogas from animal and human waste using the Chinese fixed-dome digester and the Indian floating-cover digester (Omer and Fadalla, 2003). These have not been reliable and in many cases, poor performance has been reported (Omer and Fadalla, 2003). Thus, the plants have only operated for a short period due to poor technical quality (Mshandate and Parawira, 2009).

Currently, a number of different organizations are establishing biogas initiatives in Africa, particularly in rural areas, in order to supply cleaner burning energy solutions. These initiatives are at different stages of development such as: prefeasibility, feasibility, design and implementation to a limited extent. For instance, Burkard (2009) reports on five biogas case studies in Kenya which were to utilize agricultural leaves, residues from floriculture, and residues from vegetable production and canning. In 2010, it was reported that the Dutch government was to spend 200 million Kenyan Shilling to set up 8000 biogas digesters throughout the country. The initiative was targeting farmers practising zero grazing (Daily Nation, 2010). Similar projects are being implemented in Ethiopia, Uganda, Senegal, Burkina Faso, and Tanzania. There are also some other initiatives such as biogas for better life, which is at various stages of biogas development in Ethiopia, Kenya, Uganda, Sudan, Zambia, Malawi, South Africa, Lesotho, Swaziland, Mali, Senegal, and Ghana1. The Netherland Development Organization (SNV) has been supporting the development of National Biogas programmes in East Africa (Ethiopia, Kenya, Uganda, Tanzania, Rwanda) and West Africa (Senegal and Burkina Faso)[3] [4]. While there are few documented successful small-scale biogas plants in the rural areas of Africa, this section will present some selected country specific biogas projects.

1.1.2 Rwanda

Rwanda has a population of 10.2 million people of which 81% of this population reside in the rural areas in 2010 (United Nations, 2007). One of the famous biogas programmes is the Kigali Institute of Science and Technology (KIST) large-scale biogas plants developed and installed in prisons. The aim of these plants was to treat toilet wastes and generate biogas for cooking. The first plant prison which was operational in 2001, and by 2011, KIST has managed to build and operationalize biogas plants in 10 prisons. Each prison is supplied with a linked series of underground biogas digesters, in which the waste decomposes to produce biogas. After this treatment, the bio-effluent is safe to be used as fertiliser for production of crops and fuel wood. The project was funded by Red Cross and the plant consists of five interlocking chambers. KIST’s project saves 50% of wood for cooking and it won Ashden Award in 2006. The projects construction is managed by KIST, who also provides training to both civilians and prisoners.

Another biogas programme is the National Biogas Programme which is promoted by the Rwanda Ministry of Infrastructure, through the support by the Netherlands development organization. The programme aims at reducing firewood use by the households. The Ministry of Infrastructure estimates that 441 units have been installed to date, and approximately 15 000 households will be using biogas by end of 2011 for cooking and lighting[5]. The Ministry of Infrastructure of Rwanda is also collaborating with other ministries (e. g. Ministry of Education) in order to develop biogas plants in schools, clinics and community institutions.

(Fannin, 1987). In the natural environments, the optimum temperature for the growth of methane forming archaea is 5-25 °C for psychrophilic, 30-35 °C, for mesophilic, 50-60 °C, for thermophilic and >65 °C for hyprethermophilic (Tchobanoglous and Burton, 1996).

It is generally understood that higher temperature could produce higher rate of reaction and thus promoting higher application of organic loading rate (OLR) without affecting the organic removal efficiency (Chae et al., 2007; Choorit and Wisarnwan, 2007; Poh and Chong, 2009). Using palm oil mill effluent as the substrate, Choorit and Wisarnwan (2007) demonstrated that when the digester was operated at thermophilic temperature (55 °C), showed higher OLR application than the that of mesophilic (17.01 against 12.25 g COD/ m3-d) and the methane productivity was also higher (4.66 against 3.73 L/L/d) (Choorit and Wisarnwan, 2007). A similarly study by Chae et al (2007), indicated that the higher temperature of 35 °C led to the highest methane yield as compared to 30 °C and 25 °C although the methane contents only changed slightly.

Using cheese whey, poultry waste and cattle dung as substrates, Desai et al. (1994) showed that when the temperature was increased from 20, 40 and 60 °C, the biogas production and methane percentage increased as well. The digestion rate temperature dependence can be expressed using Arrhenius expression:

rt = r30(1.11)(£_30) (7)

where t is temperature in °C, and rt, r30 are digestion rates at temperature t and 30°C, respectively. Based in Eq. 7, the decrease in digestion rate for each 1 °C decreased in temperature below the optimum range is 11%. Similarly, the calculated rate at 25 °C y 5 °C are 59 and 7% respectively, relative to the rate at 30 °C (Dasai et al., 1994).

Although the thermophilic anaerobic process could increase the rate of reaction, the yield of methane that could be achieved over the specified organic amount is the same regardless of the mesophilic or thermophilic conditions. That value is 0.25 kg CH4/kg COD removed or 0.35 m3 CH4/kg COD removed (0 °C, 1 atm) which is derived by balancing the following equation (Eq. 8), taking into account the different operating conditions worked, can be explained that the values obtained for methane production is different in many scientific reports:

CH4+2O2 ^ CO2+2H2O (8)

Although thermophilic condition could result in higher application of organic loading rates and better destruction of pathogens, at the same time it is more sensitive to toxicants and temperature control is more difficult (Gerardi, 2003; Choorit and Wisarnwan, 2007). Furthermore, biomass washout that could lead to volatile fatty acids accumulation and methanogenesis inhibition could also occur if the thermophilic temperature could not be controlled (Poh and Chong, 2009). As a result, in tropical regions mesophilic temperatures are the preferred choice for anaerobic treatment (Yacob et al., 2005, Sulaiman et al., 2009).