Different dilutions of the biogas stream produced in the AD at an initial concentration of 3000 ppmv of H2S were prepared by mixing the biogas with humidified air. Two empty-bed residence times (EBRT), 31 and 85 s, were chosen for the performance of the reactor during the H2S and VFA biodegradation tests. Increasing mass loading rates from 99 g/m3h to 400 g/m3h (corresponding to 850 and 3000 ppmv H2S) were used for evaluating H2S removal at both EBRT. Gas samples of the inlet and outlet ports of the biofilter were periodically collected and diluted in 10 L Tedlar bags before taking measurements to determine the H2S and VFA consumption in the biofiltration system. Details of the analysis conditions were previously reported in Ramirez-Saenz et al., 2009.

The level of pH has an effect on the enzymatic activity in the micro-organisms, since each enzyme is in activity only in one specific range of pH, and it has its maximum activity with its optimal pH (Ahring, 1994). A stable pH indicates system equilibrium and digester stability. A falling pH decrease can point toward acid accumulation and digester instability. Gas production is the only parameter that shows digester instability faster than pH. The range of acceptable pH for the bacteria participating in digestion is from 5.5 to 8.5, though the closer to neutral, the greater the chance that the methanogenic bacteria will function (Golueke, 2002). Most methanogens function in a pH range between 6.7 and 7.4, and optimally between 7.0 and 7.2. The greatest potential for a digester failure is a result of acid accumulation. This would occur if the amount of volatile solids loaded into the digester as fresh waste increased sharply. Maintaining pH is especially delicate in the start-up because fresh waste must undergo acid forming stages before any methane forming can begin, which will lower the pH. To raise the pH during the early stages, operators must add a buffer to the system, such as calcium carbonate or lime.

In all HRTs, significant higher ethanol productions in the ultrasound-assisted fermentation process than in the control fermentation process were recorded (p<0.05). When the HRT was 12 h, the ethanol concentration without ultrasonic treatment was 9.87 g L-1 and it was significant lower by 2.85 g L-1 than the production in the process stimulated with low intensity ultrasounds (p<0.05) (Fig. 9). Lactose consumption was only 62.1%, but application of ultrasound increased it to 69.7% (p<0.05) (Fig. 10). The best results were obtained with the longest HRT of 36 h. Ethanol concentration increased to the value of 26.30 g L-1 when the culture has been sonicated, while in the fermentation process without ultrasound irradiation was only 23.60 g L-1 (p<0.05), (Fig. 9). Lactose consumption was as high as 98.9% in ultrasound-assisted fermentation unit and was significant higher by 6.5% than the consumption in the reactor without ultrasonic irradiation (p<0.05) (Fig. 10). High ethanol production and lactose consumption were observed with shortening HRT to 24 h. S. cerevisiae stimulated with low intensity ultrasound produced 24.85 g ethanol L-1, while the lactose consumption was 95.6% (Fig. 9-10). In the control fermentation unit there was 21.79 g L-1 and 89.5%, respectively. The differences were statistically significant (p<0.05). Under the HRT of 36 h, in the fermentation process with ultrasound irradiation the maximum ethanol yield of 0.532 g g-1 lactose was observed, whereas using biocatalyst S. cerevisiae without ultrasound exposure gave the result as 0.511 g g-1 (Fig. 11) (p<0.05). Shortening the HRT to 24 h allowed remaining high ethanol yield of 0.520 g g-1 with sonicated S. cerevisiae, but in the control fermentation process it was as low as 0.487 g g-1 (p<0.05). When the HRT was 12 h the ethanol yield was only 0.365 and 0.318 g g-1, respectively (p<0.05).

There were only few experiments investigating the enhancing ethanol production by ultrasonic stimulation of S. cerevisiae. Schlafer et al. (2000) improved biological activity of S. cerevisiae by low energy ultrasound assisted bioreactors operated at a frequency of 25 kHz and a power input of 0.3 W L-1. The ethanol production without ultrasonic treatment varied between 3-12 g L-1, while ultrasonic stimulation increased it to 30 g L-1. The highest ethanol concentrations were obtained with a cycle regime of ultrasound exposure and a pause, because during continuous ultrasound irradiation no stimulation in the ethanol fermentation process was recorded.

Lanchun et al. (2003) investigated the influence of low intensity ultrasound on physiological characteristic of S. cerevisiae. The results of their study showed, that ultrasounds in the frequency of 24 kHz and the power efficiency of 2 W with 1 s irradiation time every 15 s and 30 min duration cycle, stimulated the material transport and improved the cell’s metabolism by changing the osmotic pressure of membrane. Consequently, transfer of substance was speeded up, enzyme synthesis was driven up and enzyme activity was enhanced.

The positive results of the ultrasound treatment on the ethanol production by co­immobilized S. cerevisiae seemed to be a combination of different processes, including activating the yeast by improving the mass transfer rate of nutrients in the liquid, enhancing the uptake of foreign substances and the release of intracellular products in cells, improving the cell growth and degassing of CO2 (Lanchun et al. 2003; Liu et al., 2007; Liu et al. 2003b). Stimulating enzyme activity is done by increasing in the mass transfer rate of the reagents to the active site (Liu et al., 2007). Ultrasounds irradiation can cause thermal and mechanical stress to biological materials (Liu et al., 2003b). High energy ultrasonic waves break the cells

and denaturate enzymes (Liu et a!., 2007; Pitt & Ross, 2003). Low energy ultrasounds can produce a variety of effects on biological materials, including the inhibition or stimulation cellular metabolisms, enzyme activity, alteration of cell membranes and other cellular structures (Liu et al., 2007; Liu et al. 2003a). According to Xie et al. (2008), cavitation is the primary basis of biological effects of low intensity ultrasound. Cavitation bubbles produced by low intensity ultrasound can cause acoustic microstreaming (Xie et al., 2008). The microstreaming surrounding the cells can cause shear stress and enhance the mass transfer, which may stimulate metabolic activities inside the cells (Liu et al., 2003b; Pitt & Ross, 2003; Xie et al., 2008). When ultrasonic intensity is sufficiently low, a stable cavitation occurs and leads to the enhancement of mass transfer and fluid mixing, which produces positive effects on the rate of biological reactions in the exposure systems (Liu et al., 2007).

The growth activity of yeast cells is hardly changed within the early period of sonication regardless of either damage to cell wall, or complete inactivation of the yeast located in the cavitation zone (Tsukamoto et al., 2004). Short sonication time up to 5 min of irradiation indicated bactericidal effects, but the cells were able to repair the damages. According to Guerrero et al. (2005) yeasts, inclusive with S. cerevisiae, are highly resistant to ultrasound damage. Moreover, at relatively low intensity of ultrasounds, microorganisms can adapt to the irradiation exposure and their biological activity increases (Liu et al., 2007). With relatively short irradiation period, cell damage and membrane permeability induced by ultrasounds appear to be temporary and reversible. Lanchun et al. (2003) also stated that sonication cannot influence on fermentation strength of S. cerevisiae descendants.

Fig.3-9 showed the effect of rosin and wet strength agent on wet tensile strength when other factors were held at 0 level. When rosin was lover than 0 level, wet tensile strength increased with the increase of wet strength agent; When rosin was higher than 0 level, the increase of wet tensile strength became flat with wet strength agent increased, the maximum occurred when wet strength agent was held at 3%, and rosin was held at 0.4%. The reason was that the increase of rosin, affected the adsorption of the fibre to wet strength agent, to a certain extent, reduced the effect of wet strength agent, wet tensile strength would not increase or decrease.

3.4.3 Effect of bauxite and wet strength agent on wet tensile strength

Fig.3-10 showed the effect of bauxite and wet strength agent on wet tensile strength when other factors were held at 0 level. When bauxite was at any level, wet tensile strength gradually increased with the increase of wet strength agent ;When bauxite was lower than 0 level, wet tensile strength increased, When bauxite was higher than 0 level, wet tensile strength decreased, the maximum occurred when wet strength agent was held at 3%, and bauxite was held at 4.5%.The reason was that the increased amount of added bauxite in the slurry system, leaded to adsorption of anionic trash in fibre system, affected the adsorption to the wet strength agent, resulted in decrease of wet tensile strength.

Tanzania’s energy demand is characterised by a low per capita consumption of commercial energy (petroleum and electricity) and a high dependence on non-commercial energies, including biomass fuels in the form of firewood, charcoal and bio-waste. Renewable energy technologies currently in use in the country include improved wood-fuel stoves and charcoal production practices, biogas, windmills, and solar thermal and photovoltaics (PV). The applications of these technologies are at various stages of development in terms of demonstration and commercialization.

Tanzania has no renewable energy policy at the moment but only the general energy policy framework for all kinds of energy.

The National Energy Policy (2003) objectives are to ensure availability of reliable and affordable energy supplies and their use in a rational and sustainable manner in order to support national development goals. The National Energy Policy, therefore, aims to establish an efficient energy production, procurement, transportation, distribution and end — use systems in an environmentally sound and sustainable manner (URT, 2003). It also supports research and development of renewable energy and promotes the use of efficient biomass and end-use technologies. The main elements of the policy are: [21]

policy, which is the main guidance for change, backed by legislation and regulation. The ministry also facilitates mobilisation of resources into areas where market forces fail to ensure adequate energy services. The policy put guidance for licensing operators, monitoring markets and performance; and applying any other necessary regulatory measures.

Within the Ministry of Energy and Minerals there is a Rural Energy Agency (REA) for rural electrification. The policy acknowledges that around 80% of the population has very low purchasing power and depends mainly on wood-fuel for cooking and kerosene for lighting, which have negative consequences to the environment and the quality of life, especially to the rural poor. Rural electrification is a case of long-term national interest and a prerequisite for a balanced socio-economic growth for all in Tanzania through enabling rural poor accessing sustainable clean energies.

However, energy policy has attracted criticism in different ways. Stakeholders feel that consideration of improving clean energy by rural poor needs to be on the application of appropriate technologies that are affordable, environmentally sound and well adapted to local needs as explained in the Policy. Also, while gender issues have received attention at micro level in terms of technological interventions such as cookstoves, biogas, solar cookers, and wood plantations, they have yet to be addressed in macro level policies. Women’s needs for energy vary depending on whether they are in urban or rural areas, their stage of economic development and whether they are economically active. Parikh (1995) makes a plea to include gender issues in macro level energy policies such as energy investment, imports and pricing. Also there is inadequate information and data on how the ongoing and planned power sector reform can be modified to address the existing challenges, particularly with regard to electrification of the poor (Karekezi and Kimani, 2002). A study by Barnes and Floor (1996) highlights constraints towards improving clean energy in rural development and these include the widespread inefficient production and use of traditional energy sources fuelwood and charcoal which pose economic, environmental, and health threats. Also the highly uneven distribution and use of modern energy sources such as electricity, petroleum products and liquefied or compressed natural gas, pose important issues of economics, equity, and quality of life. The policy does not provide adequate strategies on overcoming these. Many developing countries including Tanzania has general energy policies pertaining to the development of electricity, oil and renewable energy sub­sectors for the benefit of the public and the economy. However, the absence of sharply focused, pro-rural energy policy and/or their policy instruments has been the major challenges towards the observed stagnation of some initiatives like the biogas (Habtetsion and Tsighe, 2002). The Energy Policy formulation in Tanzania takes place in the context of great uncertainty, due to mainly pressures exerted by conflicting interests (Mwandosya and Luhanga 1993).

Within the Energy Policy, biogas has received a low profile or recognition. There is no specific policy statement to explain and strategies for the promotion of biogas technology in rural Tanzania; rather everything is dumped in the category of renewable energy. Omer and Fadalla (2003) recommends that biogas technology must be encouraged, promoted, invested, implemented, and demonstrated, but especially for remote rural areas.

The main challenges facing biogas technology is inappropriate institutional structure and/or gaps in the structure, in addition to lack of corporate culture; poor incentives; and, poor linkages among the various stakeholders concerned in energy for rural development (Habtetsion and Tsighe, 2002). Progressive government intervention is needed to shift reform process towards a more responsible development path of renewable energy (Wamukonya, 2004). Generally speaking, the database for the context of renewable energy in Tanzania is not well documented and the renewable energy technology including biogas is still at an infant stages. So many efforts have been done by individuals of which, most of them have not been documented. The financial capital coupled with poor technology (Mwerangi, 2008) and lack of sustainable institutional framework for renewable energy developments hinders the development of biogas. This trend tallies with Uddin (1999) comment that lack of policy mechanisms, institutional development and financing exist as major barriers for Thailand

Another policy issues is lack of credits. A high proportion of the respondents in this study area indicated high cost and that there were no credit facilities in the area of study. There is also an issue of awareness and culture. A study by Mwakaje (2005) show that a large number of people who have not accessed biogas technology especially from the Muslim community have a perception that biogas is a dirty thing. However, being close to Lomwe Secondary School in Kilimanjaro Region, Tanzania and observing physically the functioning of biolatrine, many neighbour households including the Muslims were motivated to adopt the technology. The challenge was the amount of waste to feed the biodigestor and of course the cost to incur. Improving credit accessibility may have significant impact on biogas adoption in Rungwe district and Tanzania at large. Factors influencing socio-political and community acceptance are increasingly recognized as being important for understanding the apparent contradictions between general public support for renewable energy innovation and the difficult realization of specific projects (Wustenhagen et al., 2007).

Section 19 of the Energy Industry Act stipulates that the operators of gas distribution networks are obliged, taking into account conditions set out in section 17 for the network connection of LNG facilities, de-centralised generation and storage facilities, other transmission or gas distribution systems and direct pipelines, to determine minimum technical requirements for design and operation, and to publish them on the Internet.

The minimum technical requirements set out according to these sections shall ensure the interoperability of the grids and shall be justified objectively and not be discriminatory. Interoperability includes in particular the technical connection requirements and conditions for grid-compatible gas properties, including gas generated from biomass or other types of gas, as far as they are technically able to be injected into the gas supply grid or transported through this grid without compromising security (Energy Industry Act, 2005). For ensuring technical security section 49 applies: energy facilities shall be constructed and operated so that technical security is guaranteed. Thereby, subject to other legislation, generally accepted technical rules are to be observed. Compliance with the generally recognized technical rules shall be presumed if, in the case of plants for the generation, transmission and distribution of gas, the technical rules of the German Association of the Gas and Water Industry have been complied with.

Conditions for gas grid access are described in the Gas Network Access Ordinance (GasNZV, 2005) in part 11a "special arrangements for injecting biogas into the natural gas grid". The Gas Network Access Ordinance regulates the conditions under which the gas network operators are obliged to grant transportation customers access to the gas networks.

There are three major types of digesters that have been in use in developing countries: Chinese fixed dome digester, the Indian floating drum digester and the more recent tube digesters. These reactors are small in size (5-10 m3) and mostly used at household level to deliver the energy demand for household cooking and lighting. The advantages of these reactors are that they are inexpensive compared to sophisticated systems, can be built with locally available material, are easy to handle and do not have moving parts which are prone to failure. The working principle of these reactors is the same although there are substantial differences between them. The substrate enters through the inlet pipe into the digester tank where the substrate has an average retention time of 10-30 days. The biogas is collected above the slurry and leaves the tank through a gas pipe into the top cover. In the fixed dome digester, the top is made of concrete or bricks as the rest of the digester below ground. The floating cover type has steel cover floating on the slurry, which is above ground, whereas the rest of the digester is also below the ground. The digested slurry leaves the digester through an outlet pipe and is collected in outlet pit. However, these digesters have several limitations. Each of the digester type does not have facilities for mixing the slurry or for maintaining a certain temperature in the digester and controlling it. There are also no facilities to remove sand, stones and other non-digestible materials, which will over the years, accumulate and decrease the volume of the digester and hence will reduce its efficiency. The accumulation of inert and non-degradable material makes it necessary to stop the process from time to time and remove the materials, thereby increasing labour and maintenance cost of the technology.

There is also lack of adequate coordinating framework as one of the most important weakness of energy institutions in Africa. Lack of coordination among institutions and conflicting interests are obstacles to good penetration of biogas technology into the African market. Rationalising functions and building institutions around them will improve the situation (Davidson, 1992). Constant persuasion and active campaigns can help reduce institutional inertia and resistance to adoption of biogas technology. Most renewable energy technologies require long development periods and dedicated stakeholders are important for building up experiences and competencies. New technologies often need to be nurtured for over decades, before sufficient socio-technical momentum emerges. Alignment between the technical, economic, regulatory and social context can provide the basis for building up momentum, until the biogas technology is able to survive on its own. Many African countries have a National programme having a three-pronged focus: sanitation, rural energy, and organic fertilizer usage, aimed at promoting domestic and agricultural based plants and this will help in promoting and implementing biogas plants. There are also now many biogas service providers in many African countries that specialize in the construction of biogas plants. The major focus of the biogas service providers is on sanitation. The service providers have used the hygiene-promoting aspect of biogas plants to market the technology.

There is need for continuous improvement of the biogas technology because its implementation is intrinsically the exploitation of the technical advantages. In some instances biogas plants have not worked effectively because of lack of support, lack of repairs and poor design. Lack of knowledge about biogas technology is often cited as a reason for non-adoption of biogas in some countries in Africa. Where people have installed biogas reactors, problems arising from the bad quality of the installed units and the poor operations and maintenance capacity of users have led to poor performance and even abandonment of biogas digesters. In some instance, the demonstration effect has been one of failure and has served to deter rather than enhance biogas adoption. A survey in Kenya of about 21 existing plants in 1986 found only 8 out of 21 functional and 13 out of 21 not functional or never finished (Day et al., 1990). According to the authors, the major problems associated with existing biogas plants in Kenya include inadequate design and construction, poor maintenance, and poor social acceptance. The effect of individual economic status is also important to consider in the assessments of biogas technology. Ni and Nyns (1996) reported that most surveys have revealed that biogas is more accepted by upper and middle-income farmers. The obvious effect of the income of individuals is the ability of investment to install a digester system and above all to maintain it operational. The regular operation of a biogas plant is more difficult to achieve than its initial installation. The routine operation and maintenance of the digester system need much physical work that is usually laborious and messy, making the biogas benefits less attractive.

The following parameters were determined: total and soluble COD, pH, total solids, mineral solids, volatile solids, total suspended solids, mineral suspended solids, volatile suspended solids, total volatile fatty acids (TVFA), alkalinity and total phenolic compounds. All analyses were carried out according to the recommendations of the Standard Methods of American Public Health Association (APHA, 1989).

In each steady-state experiment, samples were collected and the above parameters analysed. The pH and gas volume were determined daily, whilst the remaining parameters were measured at least five times per week on five different samples taken on different days to ensure that representative data were obtained.

2. Results and discussion

2.1 Influence of substrate concentration and OLR on the COD removal efficiency and operational parameters

The anaerobic degradability studies were carried out using two different two-phase OMSWs with COD concentrations of 35 g COD/L (OMSW 1) and 150 g COD/L (OMSW 2). The experiments were performed using progressive influent substrate concentrations, those corresponding to the OMSW 1 being the first ones and those corresponding to the OMSW 2 carried out at the end of the study.

Tables 2 and 3 summarize the steady-state operating results including HRT, OLR, methane production rates (rcm), total and soluble CODs, VS, TVFA, alkalinity and TVFA/alkalinity ratio for the OMSW 1 and OMSW 2, respectively (Borja et al., 2002).

OLR (g COD/(L d))

Fig. 1. Variation of the percentage of COD removed with the OLR for the two OMSWs used (■: OMSW 1; •: OMSW 2).

Values are the averages of 5 determinations taken over 5 days after the steady-state conditions had been reached. The differences between the observed values were less than 3 % in all cases. (* rCH4: methane production rates)

Table 2. Steady-state results under different experimental conditions for the OMSW 1 with a COD of 35 g/L.

Table 3. Steady-state results under different experimental conditions for the OMSW 2 with a COD of 150 g/L.

As can be seen in Figure 1 the percentage of COD removed decreased with increased OLR for the two influent substrate concentrations studied. The percentage of COD removal decreased from 93.3% to 83.2% when OLR increased from 0.86 to 4.14 g COD/(L d) for the most diluted substrate (OMSW 1). For the most concentrated influent (OMSW 2) OLRs were varied from 3.00 to 15.03 g COD/ (L d) and COD removal efficiencies higher than 88% were obtained at an OLR of 12.02 g COD/ (L d). Even under a higher OLR of 15.03 g COD/(L d), corresponding to an HRT of 10 days, COD removal was 82.9%.

The total effluent CODs of the anaerobic reactor increased with increased OLR for the two influent substrate concentrations studied, as summarized in Tables 2 and 3. Such an increase in the effluent COD was paralleled by a similar increase in the effluent total volatile fatty acids (TVFA). This seems to indicate that, at higher OLR, the effluent total COD and mainly soluble COD is largely composed of the unused volatile acids produced in the reactor.

Given that the buffering capacity of the experimental system was found to be at favourable levels with excessive total alkalinity present at virtually all loadings, the efficiency of the process and the rate of methanogenesis was not very affected. The experimental data obtained in this work indicate that a total alkalinity of about 1.7 g/L as CaCO3 is sufficient to prevent the pH from dropping to below 7.0 at an OLR of 9.05 g COD/(L d) for the most concentrated substrate used (OMSW 2).

The pH in the reactor was always higher that 7.0 for all the HRTs and OLRs studied corresponding to the most diluted OMSW studied. In addition, pH values equal or higher than 6.9 were observed for OLRs lower than 12.02 g COD/(L d) and HRTs higher than 12.5 d when the most concentrated influent was processed, with pH of 7.2 as a maximum value achieved. This high stability can be attributed to carbonate/bicarbonate buffering. This is produced by the generation of CO2 in the digestion process which is not completely removed from the reactor as gas. Buffering in anaerobic digestion is normally due to bicarbonate, as carbonate is, generally, negligible if compared to the bicarbonate (carbonate/bicarbonate ratio is equal to 0.01 for pH 8.2) (Speece, 1983). The buffering guards against possible acidification of the reactor giving a pH of the same order as the optimal for methanogenic bacteria (Wheatley, 1990).

The TVFA/ Alkalinity ratio can be used as a measure of process stability (Wheatley, 1990): when this ratio is less than 0.3-0.4 the process is considered to be operating favourably without acidification risk. As was observed in Tables 2 and 3 the ratio values were lower than the suggested limit value for OLRs lower than 9.05 g COD/ (L d) in the experiments corresponding to the highest influent substrate concentrations studied (OMSW 2). For this substrate, between HRTs of 50.0 and 16.6 days, the TVFA/Alkalinity ratio was always lower than the above-mentioned failure limit and the TVFA values were always lower than 1,08 g/L (as acetic acid). However, at a HRT of 10.0 days, a considerable increase of the TVFA/ Alkalinity ratio until a value of 0.95 was observed in the reactor, which was mainly due to a considerable increase in the TVFA concentration (1.57 g/L as acetic acid) with simultaneous decrease in alkalinity (1.32 g/L, as CaCO3).

Gas mixing occurs when streams of different gas qualities are united to a single stream. In pipeline systems this means that different gas sources meet in a T-type or Y-type of pipeline. A simple example of two streams of volume (V) with two calorific values (H) is given below (see figure 3). The resultant value depends on the product of volumes or flow (Q) and the amount of each calorific value according to:

V1*H1 + V2 * H2

Hi,2= v:+v

H Zi(V, * Hi) Zi(Qi * Hi)

Hs ZiVi ZiQi

H1,2 , Hs = resulting calorific value V1 , V2 = volume of stream #1, #2 H1 , H2 = calorific value of stream #1, #2 Qi = flow of stream i ( = dV/dt)

The mixing of the flows and thus the resulting value is not perfect in the vicinity of the mixing point. This is due to the pipe dimensions which may be large or different and the flow characteristic: laminar or turbulent. The mixing process is better and faster if the flow is turbulent. If the flow is laminar mixing may take a long way and time as even a layering effect may occur. In order to speed up mixing in such cases static mixer pipes, which have small obstacles inside to provoke little turbulence, will be built in the line.

Process Network Synthesis (PNS) was used as a tool for economic decisions to get an optimal technology solution for biogas production with particular consideration of feedstock which is not in competition with food or feed production. Ecological evaluation of the resulting optimal PNS solution through footprint calculation was based on the Sustainable Process Index (SPI). These calculations are based on the data, which was gathered in three field tests, and the practical experiences, that were gained in the growing and harvesting of intercrops on more than 50 hectares of arable land. Besides the determination of dry matter yields of different kinds of intercrops and intercrop mixtures the effects on ground water, soil and nutrient management were investigated in the field experiments with time-domain-reflectometry, soil water and mineral nitrogen content measurement. Additionally, the potential biogas production was measured by means of biogas fermenter lab scale experiments.