The types of mechanical mixing (Fig. 21) are: vertical mixing, horizontal mixing, and side mixing. Submersible motor mixing devices are usually used in commercial biogas plants. Each device is provided by a cable and gear protection system (Fig. 22). Light agitation increases the velocity of digestion, differently from heavy agitation which decreases the velocity of reaction. In digesters with capacities higher than 100 m3, it is necessary to install equipment to provide agitation of the contents.

A rotational rheometer RheolabQC coupled with Rheoplus software (Anton Paar) was used for different reactor fluids, which recorded the rheograms’ and allowed subsequent data analysis. The temperature was maintained constant at 37+0.2 °C. The reactor fluid volume used for each measurement was 17 ml. Reactor fluids from mesophilic (37°C) lab-scale reactors (4 L running volume), with a hydraulic retention time (HRT) of 20 days, were sampled.

Five lab-scale reactors (A-E) were sampled before the daily feeding of substrates. All reactors had been running for at least three HRTs prior to sampling. The different substrates treated were slaughter household waste, biosludge from pulp — and paper mill industries, wheat stillage and cereal residues. The TS values ranged between 3.1-3.9 % for four of the reactors while one was at 7.7 % (Table 1).

Rheological measurements were carried out with a three-step protocol where (1) the shear rate increased linearly from 0 to 800 s-1 in 800 sec., (2) maintaining constant shear rate at 800 s-1 in 30 sec, (3) decreasing linearly the shear rate from 800 to 0 s-1 in 800 sec., according to Bjorn et al. (2010). For each sample three measurements were carried out and performed immediately after sampling or stored at +4 °C pending analysis.

The fluid behaviour was interpreted by the flow — and viscosity curves according to Schramm (2000), and the dynamic viscosity, limit viscosity and yield stress were noticed. The three most common mathematical models for non-Newtonian fluids; Herschel Bulkley model; Ostwald model (Power Law) and Bingham model, were applied in order to transform rheogram data values to the rheological behaviour of the fluids. Flow behaviour index (n) and consistency index (K) were studied.

The fruit and vegetable waste samples were analyzed for total solids (TS) and volatile solids (VS) according to the standard methods of the American Public Health Association (APHA, 2005).

Biogas production in the anaerobic digester was periodically measured using a water displacement setup in which the biogas was passed through a 5% NaOH solution (Anaerobic Lab Work, 1992). Biogas samples were taken periodically from the gas collection lines prior to the water displacement setup, and the gas composition was analyzed using a gas chromatograph (GowMac Series 550, Bethlehem, PA) equipped with a thermal conductivity detector. A CTR1-packed column (Alltech Co., Deerfield, IL) was used for the analysis. The analysis conditions were the same as those reported previously (Garcia-Pena et al., 2009). VFA samples were analyzed in a gas chromatograph (Buck Scientific, East Norwalk, CT) as previously reported (Garcia-Pena et al., 2009).

Biofilter samples were analyzed for H2S consumption by measuring H2S concentrations of the inlet and outlet of the biofilter using a gas analyzer (Testo 350XL, Clean Air Engineering, Inc., Pittsburgh, PA).

Several studies proved that the intensity of mixture in an CSTR digester has an effect on the process inhibition and the re-establishment of the organic overload (Hill & Bolte, 1989). Other researchers (Hill, 1990) studied the accumulation of acetate and propionate in a CSTR digester which treats municipal solid waste and the biosolides with an aggressive starting and an organic overload. They noted that while acetate was consumed thereafter, propionate persisted in the whole system and it started to decrease only after the reducing of mixture intensity. They also noted that a digester with a reduced mixture can tolerate a higher organic load than the digester with an intensive mixture.

2.2 Composed toxic/inhibiting

The inhibiting compounds are one or the other present already in the substrate or product during degradation. The majority of the inhibitors are formed during the degradation of the substrate, such as VFA, LCVA, ammonia and sulphide. Some inhibitors are present already in substrate, such as the heavy LCVA, and metals.

The VFA is the main intermediate in anaerobic digestion, and it accumulates under the action of the non balance of the process. With low pH, the VFA becomes more toxic, due to the increase of the non dissociated fraction.

Ammonia comes mainly from the degradation of protein. A study on 18 central biogas stations in Denmark, proved that ammonia was significatif factor affecting the stability of the process (Hawkes & al., 1994). A concentration about 2 gN/l of ammonia will have no inhibiting effect on acetoclastic methanogens (Hill & Holmberg, 1988). However, the activity of methanogens is decreased during the increase in ammonia concentration, and total inhibition is reached for a concentration of 10 gN/l.

The utilization of milk permeate to ethanol in continuous fermentation by co-immobilized S. cerevisiae is possible. The optimal ultrasonic intensity and irradiation period are varied in each biological process enhanced by ultrasound and should be find experimentally. According to this experiment, stimulation of yeasts activity could be achieved in the presence of low intensity ultrasound (1 W L-1, 20 kHz), and 1 min every 6 h irradiation period is favorable to increase ethanol production efficiency. Moreover, the short exposure of yeast to ultrasound could reduce the operation costs comparing with continuous irradiation.

For the continuously operating bioreactors, the maximum rates of sugar utilization were 98.9 and 92.4% for the yeast with ultrasound exposure and without ultrasound exposure (p<0.05), respectively. The maximum ethanol yield was 0.532 g g-1 lactose, while using S. cerevisiae without ultrasound exposure 0.511 g g-1. The study showed that there is no need to extend the HRT over 36 h or more, because most of the lactose was converted into ethanol during 24 h (95.6% in the ultrasound-assisted fermentation).

All results obtained here raises the new perspectives for disposal UF whey permeate.

3.5.1 Effect of grammage and bauxite on degradation period

Despite the over 60 years of biogas promotion in Tanzania the technology has not well developed in Rungwe district to date. This study revealed a number of issues that led to the stagnation of the technology. One, energy policy framework has put low profile of biogas in the rural energy development strategies. The technology has been dumped in the cluster of renewable energy which basically concentrates on major types of energy such as biomass (liquid biofuel and fuelwood). Today, there is a lack of adequate indigenous capacity to design, manufacture, market and distribute as well as install and maintain biogas technologies. Two, the cost of installing biogas facility of USD 550-675 is high for many of the rural poor to afford. Three, there is a tendency of risk averse among the poor to adopt new technologies including biogas. Demonstrating the technology and its related benefits might change the pace of adoption. Four, there is also an issue related to water availability. Where water is far from home creates another burden especially for women who at the end of the day they have to choose between running the biogas facilities or producing food for the family, definitely the latter will prevail. Five, the poor performance of milk marketing is linked with poor government policies, low level of management, inadequate milk markets and difficulties arising from the predominance of direct marketing (Kisusu et al., 2000. Other constraints facing dairy producers include lack of improved technology at farm level and weak institutional support (Somda et al., 2004) small size of farms and their distance from markets, animal health and reproductive problems and lack of good-quality animal feeds in sufficient quantities (Swai and Kurimuribo, 2011). Smallholder dairy producers often face problems of high transaction costs when it comes to the question of marketing their small quantities of milk to distant markets.

The recommendations are that the government should accommodate and institutionalize the planning of biogas technology dissemination energy in rural areas. Sensitisation should be enhanced, and support services should be provided towards optimisation of the biogas production process so that potential benefits are realized (Langeni, 2010). In this regard, addressing technical as well as non-technical factors is essential for the sustainability of biogas development and for decision making processes in the energy sector. The government should facilitate access to credit through providing information and also guarantee farmers to get credits. The government should help the farmers access milk markets through providing marketing information and selling of processed products. Modalities of the arrangements should be to link farmers to markets need to take into account socio-economic and agro-climatic diversities (Chakrabarti and Mukhopadhyay, 2009). There should be educational and awareness campaigns on biogas benefits and successes, the provision of financial and non-financial incentives to households could bolster wider biogas energy acceptance in developing countries (Walekhwa, 2009). Lastly, the government in collaboration with stakeholders should provide water near homes as strategy to facilitate biogas adoption.

6. Acknowledgement

The author would like to thank the Research on Poverty Alleviation (REPOA) for funding this study, without their support this work could have not been accomplished. The University of Dar es Salaam is appreciated for granting permission to undertake this study. Furthermore, thanks should go to the district authorities for facilitating this study. Last and not least, the author is grateful to the farmers for their patience and participating in this research interviews.

After the basic principles and the definitions of terms, there follows three examples of the calorific value adaptation of biogas, before it is injected into the natural gas grid. In these selected cases, one deals with the conditioning for an H gas grid and the other two with L gas grids, where conditioning is based on the addition of air, and on LPG and air. Other cases are described in the DVGW study "Developing a scientific basis for injecting biogas into natural gas grids."

First, the most important combustion-related characteristic data are listed for the selected base gases, in order to summarise the requirements imposed on the biogas, in particular with respect to the Wobbe index and calorific value.

Then the characteristics of the processed biogas with a methane content of 94 — 99,5 Vol.-% will be compiled in order to determine the conditioning necessary to adapt to the base gas. Processed biogases with these methane levels are generally H gases.

To attain the Wobbe index of an L gas, air must be added, which lowers the calorific value. In the case of L gases with higher calorific values, liquid gas needs to be added. When higher demands are placed on the calorific value (H gases), a liquid gas addition is necessary. A processed biogas containing 99.5 Vol -% methane has a calorific value of 11.0 kWh / m3.

For the calculations of the compositions in the following sections, the values from the following table 3 will be used. The composition of air is taken to be 20.95 Vol.-% oxygen and 79.05 Vol.-% nitrogen. All flow rates are standard flow rates.

The site-specific issues that have limited the scope of biogas technology in sub-Saharan Africa include the availability of water and organic materials for effective biodigester operation. Limited water availability poses a constraint for biogas operation in some countries because biogas plants typically require water and substrates such as manure to be mixed in an equal ratio. Small-scale farmers frequently lack sufficient domestic animals to obtain enough manure for the biodigester to produce sufficient gas for lighting and cooking. Even where households keep sufficient numbers of animals, semi nomadic or the free grazing system of many communities in sub-Saharan Africa makes it difficult to collect dung to feed digesters (Abbey, 2005). In countries where houses are clustered together as in Nigeria, a community plant might be more feasible (Akinbami et al., 2001).

In assessing the economic viability of biogas projects one should distinguish four major areas of applications: individual household units, community plants, large-scale commercial plants and industrial plants. In each of these cases, the financial feasibility of the facility depends largely on whether outputs in the form of gas and slurry can substitute for costly feeds which were previously purchased, the efficiencies with which the fuel is used or possible equipment which could lead to higher efficiencies. If ‘externalities’ such as employment, import substitution, energy security, environmental protection, and so on are considered then the economics change usually in favour of the biogas technology (Hall et al., 1992).

All too often, projects intended to introduce new energy technologies are conceived without proper understanding of the needs, problems, capabilities and priorities of the targeted users. Most of the Chinese and Indian biogas plants introduced in Africa are not functional due to many reasons. One of the major reasons of the failure is the separation of national interests and individual family/community interests (Ni and Nyns, 1996). There is need to learn from the past experiences and adapt the biogas technology from Europe and Asia for local African circumstances. There is also the need for bottom-up approach that takes the user interest into account. The Botswana biogas water pumping programme of the mid — 1980s is a good example of how a misunderstanding of the target communities’ needs and problems lead to project failure. The Botswana government’s effort was to introduce biogas as the main pumping fuel in some areas. Water supply is a priority in Botswana due to its arid climate. The problems that arose were not technical but rather socio-economic. The villages targeted to ‘benefit’ from the biogas-pumped water felt disadvantaged in that they had to pay for the water they collected with cattle dung while other villages paid nothing by using the usual government or donor-supplied diesel engines. The benefits of biogas were important to the government as a means of reducing dependence on imported diesel. The perception from the point of view of the intended project beneficiaries was different. Today the biogas plants are disused. The principal reason is that real acceptance of the biogas technology depends on individual interests that do not totally respond to those at the national level. This suggests the necessity of understanding fully the individual interests of a project.

Renewable energy projects conceived without carefully consulting the intended recipients and beneficiaries face serious acceptance problems and fail prematurely due to abandonment. Numerous large-scale demonstration projects such as a sophisticated integrated biogas engine generator system at Kushinga Phikelela near Marondera in Zimbabwe collapsed when weaned from donor support. The reasons of failure had mainly to do with supply of spare parts which had to be procured with scarce foreign currency and lack of local capacity and funds to maintain demonstration installation. The host institute did not need the biogas technology since it has grid electricity and hence neglected it.

Some potential users are reluctant to try the biogas digesters out of concern about sanitation. Use of human wastes from for biogas production and the subsequent digested sludge, for example in schools, as a source of fertiliser faces cultural and health resistance. Even though the anaerobic digestion process naturally reduces the pathogen load, handling biogas feedstock particularly human excreta and using biogas slurry as fertiliser does pose some risk of infection (Brown, 2006). A major difficulty is utilising manure sources properly. There is usually lack of enough supply of manure for efficient and sustainable biogas production. Liquid manure is preferred for most biogas plants, but households may not be accustomed to storing and handling it. People also find it difficult to collect, store and deliver fresh manure to the digester. Liquid manure must be stored in pits or other installations that require investment of time and labour. Therefore promotion of liquid manure digesters requires additional education and training to ensure sustainability. The problems also include that animals must be penned for effective collection of animal dung, farmers must own a sufficient number of livestock to generate continuous flows of biogas, and the initial costs for the required infrastructure may be deterrent (Karekezi, 1994b). The effort of maintenance and control on biogas plants often does not meet the level of literacy skills of rural population.

It is also important to realise that lack of information on improved technologies such as biogas technology at all levels, government, energy institutions, and consumers, poses a very serious problem for technology penetration. Poor infrastructures prevent access to even the vast information available in the public domain about biogas technology and its application. Generating interest among the various stakeholders and setting up information systems using relatively cheap devices now available can assist greatly. Setting up or strengthening existing information systems is very important for the use of renewable energy technologies such as biogas. These systems should be capable of coordinating energy and energy-related information activities with appropriate means for collection, filtering, storage, retrieval and dissemination. In order to promote the implementation and proper use of anaerobic digestion technology, it is important to initiate long-term anaerobic digestion and other renewable energy training and capacity-building programmes, and to perform scientific work in this field (through appropriate research). It is important to

establish contacts between research and university groups and experienced contractors, and to initiate collaboration with polluting industries, i. e., to interest them in the system, either for use as an environmental protection method, or for energy production. In addition, experts should provide reliable and pertinent information about the biogas technology and its potential to local authorities, politicians, and the public in general. It demands a lot of efforts in achieving an efficient transfer of knowledge from research centres and universities to state sanitation companies, consulting engineers firms and government environmental control agencies. There is also need and to obtain grants from the government or international organisations, and industry for pilot-plant and/or demonstration-scale projects (Foresti, 2001; Karekezi, 1994a).

To overcome some of the socio-cultural barriers, intensive educational and campaign programmes may have to be mounted to raise the awareness consciousness of the benefits of this technology. A case in point is that of a full-scale digester installed to treat opaque beer brewery wastewater in Harare which is just being used to treat the wastewater but the biogas from plant is currently vented to the atmosphere (Parawira et al., 2005). Further benefits of the plant could be realised by tapping the energy generated by the anaerobic process in the form of methane.

The volumetric methane production rates as a function of OLR are illustrated in Figure 2. As can be seen the volume of methane produced per day increased linearly with increased OLR up to OLR values of 3.45 and 12.02 g COD/(L d) for the influents OMSW 1 and OMSW 2, respectively. After these values a slight decrease was observed in the cases studied over the different ranges tested. Apparently, the activity of methanogenic bacteria was not impaired up to OLR values of 12.02 g COD/(L d) for the most concentrated influent (OMSW 2) used because of the appropriate stability and adequate buffering capacities provided in the experimental system. Nevertheless, the methane production rate decreased slightly from 2.12 to 2.05 L CH4/ (L d) when the OLR was increased from 12.02 to 15.03 g COD/(L d).

This decrease in the methane production at the highest OLR values might be attributed to an inhibition of the methanogenic bacteria at high OLR values, which caused an increase in effluent TVFA contents and TVFA/ Alkalinity ratio, as can be seen in Table 3. Specifically, TVFA content increased from 1.25 to 1.57 g/L (as acetic acid) when the OLR was increased from 12.02 to 15.03 g COD/(L d).

The experimental data listed in Tables 2 and 3 were used to determine the methane yield coefficient, Yp. As the volume of gas produced per day, Тень is assumed to be proportional to the amount of substrate consumed, then:

Тень = Yp q (So — S) (1)

where S0 and S are the substrate concentrations (expressed as g COD/L) at the digester inlet and effluent, respectively, and q is the feed flow-rate. By plotting Eq (1) in the form тен4 against q (S0 — S) (Figure 3), the following values of the methane yield coefficients with their 95% confidence limits were obtained for the two substrate concentrations used: 0.300 (+ 0.001) and 0.200 (+ 0.006) L methane STP/g COD removed when the OMSW 1 and OMSW 2, respectively, were processed. These values agree with the data reported in the literature for anaerobic treatment of food industry wastewaters (Borja et al., 1995; Maqueda et al., 1998; Martin et al., 1993). Taking into account that, theoretically, 0.35 L of methane is produced per gram of COD removed when the starting compound is glucose (Wheatley, 1990), the effectiveness of the anaerobic process in converting OMSW into methane at mesophilic temperature is demonstrated.

q(So-S) (g COD removed/d)