Biogas technology utilizing animal waste is not new in Tanzania; it was introduced in the country as early as the 1950s by private stakeholders. In 1975, the government through the Small Industries Development Organisation (SIDO) introduced the Indian design (floating gasholder digester) in primary and secondary schools, rural health centres and a number of other institutions. In 1982, the Parastatal Organization Centre for Agricultural Mechanization and Rural Technology (CAMARTEC) increased the dissemination of this technology in the northern regions. About 1 year later, that is around 1983, technical cooperation between Tanzania and the Federal Republic of Germany led to the introduction of the Biogas Extension Services (BES). CAM ARTEC and the Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) were in-charge of implementing this project and the latter seconded an interdisciplinary team of social scientists, mechanical engineers and agriculturists to Tanzania (Sasse et al., 1991). Between 1984-1985 more strategies were developed to boost biogas adoption. Household plants were offered with a digester volume of 8, 12 and 16m3, and in 1990 the programme comprised standardized plants of sizes 12, 16, 30 and 50m3 for households and institutions (Mwakaje, 2008). The development work towards sustainable reliability and user friendliness resulted in extensive integration of biogas plants into the work routines of farmers. Over the period, CAMARTEC were involved in building capacity by training technicians in biogas plant construction. A »biogas unit» scheme was introduced and this integrated biogas plants, livestock housing with a concrete floor (Mwakaje, 2008). CAMARTEC was also providing advice on the utilization of slurry, gas pipeline systems, burners and lamps; and women were specifically instructed on how to use and manage the plants. The Ministry of Energy and Minerals in collaboration with donors was also promoting biogas use in the Dar es Salaam region. Its main activity was to support the dissemination of biogas technology in the region through facilitating training for private craftsmen, built demonstration plants and undertaking monitoring and evaluation. Up to 1989, only 200 units of biogas had been installed all over the country (Sasse et al., 1991) but in 1992 this had increased to 600 plants national-wide. Nevertheless, as Mwakaje (2008) noted despite all the efforts, the biogas technology did not diffuse much to the rural poor communities in many parts of the country where indoor fed dairy cattle are kept. Reasons for this poor diffusion of the biogas technology included high installation and maintenance costs and inadequate awareness about the technology. The conventional units being built in the country were large and expensive, costing approximately US$ 1400 for one unit (Rutamu, 1991) to USD 2200 depending on the size of digester (IRA, 2005). Furthermore, repair and maintenance required highly skilled labour and most component parts, constructed mainly from concrete and steel, were far out of the financial reach of smallholder farmers (Mwakaje, 2008). This slow pace of biogas technology development by CAMARTEC raised a number of criticisms among stakeholders. For example, the Evangelical Lutheran Church of Tanzania (ELCT) blamed CAMARTEC its commercially oriented and strictly standardized dissemination programme. The ELCT claimed that the programme had not been adapted to Tanzanian conditions as it only served the rich farmers (Sasse et al., 1991). But also most of the CAMATERC activities were concentrated mainly in the two regions of Kilimanjaro and Arusha in a country with more than 20 regions. On the other hand, the Ministry of energy and minerals’ activities were concentrated in the Dar es Salaam region where unfortunately indoor fed dairy cattle are limited to a few households.

Reacting to some of these criticisms, the government of Tanzania changed the biogas technology dissemination strategy in the country. In the years starting 2000 polythene tubular digesters were promoted to reduce production cost through using local materials and simplified installation and operation costs (Mwakaje, 2008). The type of plastic needed for polythene was locally manufactured in Tanzania, maintenance and repair were simple, cheap, and did not require skilled labour and the cost of construction was low. A model promoted by the Sustainable Rural Development (SURUDE) was a low-cost design suitable for poor farmers (CEBITEC, 2003) in rural areas. The material cost was about US$ 100. However, this type of biodigester had one major disadvantage in that it could be easily sabotaged (torn out). This is because the plastic materials of the biodigestor are normally placed on the surface outside the house and therefore could easily be destroyed (Mwakaje 2008).

The cover membrane consists of a weather protection foil, gasholder membrane or gas collector, clamp hose, level indicator for the gas collector, excess and low-pressure safeguard, and a supporting structure (Fig. 24). A desulphurization unit is integrated into the inner membrane (gasholder) to reduce the hydrogen sulfide (H2S). The collector is sealed gas-tight with two cone-shaped foils and a clamp rail.

There are three structure types for supporting the cover, which are: (1) a steel post supports directly the cover, (2) a wood structure (incl. a wood post) supports the cover, and (3) a steel post supports a wood structure which in turn supports the cover.

Fig. 24. Air supported double membrane cover (MT-ENERGIE GmbH & Co. KG)

6.2 Monitoring and controlling

The individual facility components are monitored by computer technology even from afar. At the same time technical procedures, as well as input and output quantities are neatly documented. The computer system consists of an on-site control system installed on a computer and sometimes another remote access system on another computer located far away from the site, e. g. in the company’s headquarter. The remote computer is connected to the site through routers (ISDN or DSL), allowing a remote control of the site. Any of the aforementioned computers is able control the biogas plant through controlling the central plant control system which is connected to the digester running components (agitator, pump, feeding system…etc.), the cogeneration unit, and the biogas analyzer. A control system allows for real-time local and remote operation and monitoring as well as data gathering (Fig. 25).

After the anaerobic digestion of manure to produce biogas, a nutrient-rich substrate which is still very beneficial to plants remains. This observation is supported by the findings of Thomsen (2000). These studies agree that only small differences of between 0.5 and 2.0% are usually measurable in the aggregate nutrient concentrations when digested manure is compared to the undigested form. Adelekan et al., (2010) did a comparative study of the effects of undigested and anaerobically digested poultry manure and conventional inorganic fertilizer on the growth characteristics and yield of maize at Ibadan, Nigeria. The pot experiment consisted of sixty (60) nursery bags, set out in the greenhouse. The treatments, thoroughly mixed with soil, were: control (untreated soil), inorganic fertilizer, (NPK 20:10:10) applied at the 120 kgN/ha; air-dried undigested and anaerobically digested manure applied at 12.5 g/pot, or 25.0 g/ pot or 37.5 g/pot, and or 50.0 g/ pot. Plant height, stem girth, leaf area, number of leaves at 2, 4, 6 and 8 weeks after planting (WAP) and stover mass and grain yield were measured. Analysis of variance (ANOVA) at P < 0.05 was used to further determine the relationships among the factors investigated. Generally, results in respect of plants treated with digested manure, were quite comparable with those treated with undigested manure and inorganic fertilizer, right from 2WAP to 6WAP. Stover yield was increased to as much as 1.58, 1.65 and 2.07 times by inorganic fertilizer, digested and undigested manure, respectively while grain yields were increased by only 200% with inorganic fertilizer, but by up to 812 and 933% by digested and undigested manure, respectively. The paper concluded that digested poultry manure enhanced the growth characteristics of the treated plants for the maize variety used. As observed, the order of grain yield was undigested manure > digested manure > inorganic fertilizer. These results agree with those reported by Agbede et al., (2008) for sorghum (Sorghum vulgare), Akanni (2005) for tomato (Lycopersicon esculentum) and Adenawoola and Adejoro (2005) for jute (Corchorus olitorus L).

Organic manures play a direct role in plant growth as a source of all necessary macro and micronutrients in available forms during mineralization. Thereby, they improve both the physical and physiological properties of soil (El Shakweer et al., 1998; Akanni, 2005), thus enhancing soil water holding capacity and aeration (Kingery et al., 1993; Abou el Magd et al., 2005; Agbede et al., 2008). Organic manures decompose to give organic matter which plays an important role in the chemical behavior of several metals in soil through the fulvic and humic acid contents which have the ability to retain metals in complex and chelate forms (Abou el Magd et al., 2006). They release nutrients rather slowly and steadily over a longer period and also improve soil fertility status by activating soil microbial biomass (Ayuso et al., 1996; Belay et al., 2001). They thus, ensure a longer residual effect (Sherma and Mittra, 1991), support better root development and this leads to higher crop yields (Abou el Magd et al., 2005). Improvement of environmental conditions and public health as well as the need to reduce cost of fertilizing crops are also important reasons for advocating increased use of organic manures (Seifritz, 1982). While the practice of anaerobic digestion of biomass for biogas production is increasing, the use of the digested manure for crop production should concurrently be encouraged, judging by its potential to enhance the growth and yield of crops.

Also, the Herchel-Bulkley model indicated that reactor fluid A performed as a pseudo­Newtonian fluid called Bingham plastic, since the yield stress-value was > 0 (0.24 Pa) and a flow behaviour index of 1.06 (Table 4). Results obtained by the Ostwald and Bingham models confirmed a Bingham plastic behaviour of reactor A. However, since the X0-value was almost 0 and the n-value 1 it was also closely performing as a Newtonian fluid which is consistent with the flow curve appearance (Fig. 2). However, when studying the viscosity curve (Fig. 4) the results showed an initial viscosity decrease and then a constant viscosity indicating a pseudo-Newtonian fluid behaviour.

The Herschel-Bulkley and Ostwald models both indicated a pseudoplastic behaviour of reactor D, since the X0-value was 0 and n < 1 (Table 4). The Bingham model gave a yield stress of 0.33 Pa which did not indicate Newtonian or Bingham plastic behaviour. Thus, the common results for reactor D strongest indicate a pseudoplastic fluid behaviour.

Reactor B was hard to define also when modelling the rheogram data values of figure 4. The regression values were low for all three mathematical models (Table 4). However, the Herschel-Bulkely model had a flow behaviour index n>1 indicating that the fluid acted as a shear thickening (dilatant) fluid, but the Ostwald and Bingham models indicated pseudoplastic and Bingham plastic behaviours, respectively. When the static yield stress appeared in the reactor B rheogram (Figures 2 and 4), the flow behaviour index showed shear thickening fluid behaviour (n=3.4) and a limit viscosity of 8 mPa*s. This also corresponded to a low consistency value (5*1010). At the static yield stress of 24 Pa (Fig. 5), the flow behaviour index showed shear thickening fluid behaviour (n=1.41) and a limit viscosity of 22 mPa*s. This also corresponded to a low consistency value (5*10-4). As soon as the fluid was measured again, n decreased (0.70) showing a pseduoplastic behaviour and K increased (0.11) indicating that the consistency of the reactor material was higher. The limit viscosity was 17 mPa*s. These results showing a time dependency and structure recovery strengthen the arguments for a thixotropic fluid behaviour of reactor B. Once the stirring has ended and the fluid was at rest, the fluid structure starts to rebuild. Therefore, the viscosity become time dependent. This information is important to consider for biogas reactor performance, e. g. when applying semi-continuous mixing.

Also, fluids from reactor C and E were hard to define from modelling of the rheogram data because they gave indications for fluids being between pseudoplastic and Bingham plastic behaviours, i. e. the t0-values were >0 (2.89 and 2.38) and n <1 (Table 4).

2. Conclusion

The biogas reactor fluids investigated were behaving viscoplasticly, since they had yield stress and one of them was also thixotropic, due to its partial structure recovering. However, the reactor treating slaughterhouse waste was very close to act as a Newtonian fluid. Also, there was a difference in dynamic — and limit viscosities depending on the substrates used. The results demonstrated that similar TS values did not necessarily correspond to similar flow and viscosity behaviours. Nor, did biosludge from two different Swedish paper mill industries with similar TS show similar viscosity values.

To encounter problems related to involvement of new substrates and/or co-digestions in existing facilities, investigations for possible viscosity changes are needed. Ongoing research will hopefully provide an important basis for predictions of changes in rheology linked to the composition of the organic materials, which are translated in the process. This is important in order to achieve proper designs in relation to possible variation in substrate mixes in conjunction with new constructions, but also to better control material flows in the existing facilities to avoid disturbances in the reactor performance.

The data obtained for the microbial population by DGGE were further analyzed by Jaccard’s (3) and Sorensen-Dice’s (4) indexes. Similarity indices are frequently used to study the coexistence of species or the similarity of sampling sites. A matrix of similarity coefficients, between either species or locations, may be used to analyze changes in microbial populations over time or at different locations (Real and Vargas, 1996). Jaccard’s index is one of the most useful and widely used indices to determine similarity between binary samples. Jaccard’s index may be expressed as follows:

] =—— ^—— (3)

nA+nB-nAB

where nAB is the number of bands in both samples A and B, nA is the number of bands present in sample A, and nB is the number of bands present in sample B.

Sorensen-Dice’s index, also known as Sorensen’s similarity coefficient, is also used to compare the similarity of two samples. It can also be applied to the presence/absence of data. The index is described by the expression, the terms of the equations are the same as describe above:

г 2пав

Jn —

nA+nB

2. Results

The pH is relatively easy to measure, and is often the only parameter of the liquid phase which is measured on line. The change of the pH can be an indicator, for the stability of anerobic digestion process. Since the micro-organisms can grow at only one specific pH range. The effluent pH can also affect the pH in the digester. The use of the pH as an indicator is normally based on the fact that a decrease of the pH corresponds to the accumulation of VFA. Some anaerobic systems apply the control of the pH where an acid or a base is added to ensure the suitable pH for the microbial growth.

2.4 Alkalinity

Alkalinity is a better alternative than the pH to indicate the accumulation of VFA, because the increase in VFA will directly consume alkalinity before the great change of pH. However, it is proved that the total alkalinity (TA) measured by the titration of the sample with pH 4.3 is not very sensitive because of the combination of result of VFA and bicarbonate to the TA (Hill & Bolte, 1989). Partial Alkalinity (PA) or bicarbonate alkalinity measured by titration of sample in pH 5.75 has an empirical correlation to the VFA accumulation (Wang & al., 2005). However, one does not observe this during the VFA accumulation at the time of the ammonia overload, because this latter increases the alkalinity of the system (Wang & al., 2005).

A 5 L UASB reactor with 4.5 L working volume (R1) was made of stainless steel and cylindrical in shape (Fig. 12). The reactor was constantly stirred at 50 rpm. The pH of mixed liquid in R1 was controlled automatically with 6 M NaOH. The temperature in R1 was maintained at 37°C by inserting the reactor in the thermostatic chamber. For start-up, the reactor was filled with undiluted UF whey permeate and was operated anaerobically at a batch mode. When hydrogen production reached its peak value, the bioreactor feeding mode was turned to a continuous one at a HRT of 24 h (OLR of 10 g COD L-1 d-1) or at a HRT of 12 h (OLR of 20 g COD L-1 d-1). The R1 performance (biogas production and composition in H2 and CH4, COD, Total Volatile Fatty Acids — TVFA concentration, pH) was monitored twice a week throughout the experimental period (84 days).

A 5 L UASB reactor with 4.5 L working volume (R2) was made of stainless steel and cylindrical in shape (Fig. 12). The reactor was constantly stirred at 50 rpm. The pH of mixed liquid in R2 was controlled automatically at pH 7.2 (±0.05) with 6 M NaOH. The temperature in R2 was maintained at 37°C by inserting the reactor in the thermostatic chamber. The R2 was fed with the effluent from R1, which was collected in a 3 L container used as a storage tank which was constantly stirred at 50 rpm (Fig. 12). The temperature in the tank was maintained at 37°C by placing it in the thermostatic chamber. Overflow effluent flowed out in the top part of the storage tank and was collected in a separate container. The R2 was operated at an HRT of 3 d. The R2 performance (biogas production and composition in CH4, COD, TVFA concentration, pH) was monitored twice a week throughout the experimental period, from day 51 to 84. Before R2 was fed with R1 effluent, the diluted UF whey permeate had been used as a feedstock to reach the OLR of 2 g COD L-1 d-1 and HRT of 3 d.

The rule of optimization based on film performance meeting the agronomic requirement to reduce energy consumption and save raw materials as much as possible was applied to determine the optimum combination of the factors. The result was that when rosin was held at 0.8%, bauxite was held at 4%, wet strength agent was held at 1.8%, beating degree was held at 35 SR°, grammage was held at 80 g/m2, the performance that was dry tensile strength was greater than 30N, wet tensile strength was greater than 12N, the degradation period was 35 days to 60 days could be obtained, seeing in Fig 3-13.

3.5 Manufacturing technology

Based on the above research results, manufacturing technology of the biogas residue fibre film was obtained, seeing Fig 3-14.

The flocculation efficiency of chitosan is sensitive to its characteristics. The most important characteristics of chitosan for flocculation efficiency are the degree of deacetylation and molecular weight since these are the main factors that affect particle size, particle formation and aggregation in the flocculation process. However, environmental conditions, i. e. pH and ionic strength, are also important in the dissolution and the charge of chitosan for flocculation process.

The composition, the combustion-related characteristic data and the accompanying substances in the "processed biogas" — summarized under the term "gas properties" — are crucial for injection. In particular, these are the data described in the DVGW work sheet G 260 "gas properties", G 262 "use of regeneratively produced gases" and in the DVGW Worksheet G 685 "gas billing" and G 486 and are defined within their respective limits. Table 7 shows the principal conditioning parameters for injecting into a grid.

The methane content in biogas is not subject to any defined restriction. However methane is, according to the latest technical data, the major combustible component and thus largely determines the calorific value and hence the Wobbe index, as long as no liquid gas admixture is included. According to worksheet DVGW-G 262, the maximum permissible CO2 content is 6 Vol.-%. Since biogas consists mostly of methane and carbon dioxide, processing to at least a minimum methane content of 94 vol -% is assumed. Together with the fractions of oxygen and nitrogen, the biogas has a maximum carbon dioxide content of 5,6 Vol.-%.

According G 260, the maximum permissible O2 volume fraction is 3% in dry networks, otherwise 0.5%. German natural gas grids are considered to be dry.

It is important to note that CO2 and O2 in combination with moisture can lead to corrosion in pipes, fittings and equipment.

In addition to the above conditions, of course the information on sulphur compounds and other impurities and accompanying substances given in G 260 is also to be noted. The data for the dew point temperatures lie below the requirements defined in G 260 (about 4 — 7 ° C (soil temperature at 1 m depth) at line pressure).