1.1.1 Biogas residue utilization status

The research results showed that biogas residue was one of the residue after organic matter anaerobic fermentation for biogas, which was mainly composed of undecomposed raw materials solid and new generation microbial biomass (Lin Jianfeng, 2003).As cellulose, hemicellulose, lignin and other substance of fermentation materials remained in the biogas residue in the fermentation process, so biogas residue basically retains all the components of anaerobic fermentation production besides the gas. Biogas residue generally contains organic matter 36% to 49.9%, humic acid 10.1% to 24.6%, crude protein 5% to 9%, total nitrogen 0.8% to 1.5%, total phosphorus 0.4% to 0.6%, and total potassium 0.6% to 1.2%. The requirement of N, P,K of anaerobic fermentation process is very low, therefore, the majority of N, P,K and other elements of fecaluria were not being used, and eventually left in the biogas residue and biogas slurry(Bian Yousheng,2005;Xie Tao, 2007). As microbial groups and undecomposed raw materials, so the biogas residue had its unique property (Zhang Quanguo, 2005).

Organic matter of biogas residue is not only a good fertilizer, but also conducive to microbial activity and the formation of granular structure, the organic matter surface can absorb amounts of soluble effective nutrients, under the soil microorganism’s action, it can continuously provide compatible nutrients for growing (Zhang Wudi, 2003). Currently, the utilization of biogas residue is mainly as following (Guo Qiang et al., 2005):

Soil OM decreases in crop soils in Europe and in other continents therefore using amendments for increasing the soil OM content has a particular interest.

Digestate contains high amount of volatile fatty acid (C2-C5) which could be decomposed within few days in the soil (Kirchmann & Lundwall, 1993). The greatest rate of decomposition were observed in the first day after the treatment (Marcato et al., 2009) but the mineralization rate were high during the first 30 days (Plaza et al., 2007). Moreover, the C-mineralization values from the soil incubation assay showed that the results of raw slurry were similar to the effect of compost being in the start of composting process while the digested slurry had similar C-mineralization rate in the soil samples than that of the matured compost (Marcato et al., 2009).

1.3 Effect of digestate on the microbiological activity of soil

Soil microbial community has an important role in the fertility of soil and its alteration after intervention to the soil (e. g. manuring, soil improving, soil pollution) could be indicate more sensitive these changes than changes in the soil physical and chemical properties.

Among the different organic wastes like compost, biogas residue, sewage sludge and different manures with and without mineral N, the biogas residue was more efficient for promoting the soil microbiological activity. The high amount of easy-degradable carbon increased the substrate induced respiration (SIR), which was enhanced by the higher carbon content resulted from the higher litter and root exudates of higher plant growth. In accordance with these results, the largest proportion of active microorganisms was found in the digestate treated samples (Odlare et al., 2008; Kirchmann, 1991). Similarly, the activity of invertase was significantly higher in the digestate treated samples than that in control ones (Makadi et al., 2006).

Besides the macro — and micronutrient content of digestate which are important not for the crops but for soil microorganisms too, it contains growth promoters and hormones, also. Therefore it could be used for stubble remains to facilitate their decomposing. Makadi et al. (2007) compared the effect of digestate and Phylazonit MC bacterial manure on the growth of silage maize (Zea mays L. ‘Coralba’) as a second crop after winter wheat and on the enzyme activities of soil. Digestate was used at the rate of 50% of the total N demand of silage maize while the Phylazonit MC was used at 5 L ha-1 dose. Their results of the changes in enzyme activities are summarized in Table 6.

The maximum of the degradation of disaccharides, indicated by the invertase activity, was found in the 3rd week after Phylazonit MC treatment, while it was found only after the 9th week in the digestate treated soil samples. The Phylazonit MC contains only bacteria and promoting agents of bacterial activity for degrading the soil OM. Contrarily, in the digestate treated samples the degradation of disaccharides takes place at similar rate through 9 weeks because of the OM content of digestate used. Changes in catalase activity indicate the effect of nutrient content of digestate to the increasing microbial metabolism.

A wood or steel structure in form of umbrella is built, and then a mesh network is relayed on the umbrella structure (Figs. 14 and 15). The air supported double membrane cover, which includes the gas holder, is mounted over the structure. The flexible membrane of the gas collector, i. e. holder, moves up and down as a function of the gas pressure. On the other hand, the storage tank covers are numerous: (1) closed cover (concrete, plastic, and tent), (2) straw cover, (3) granulate (perlite), (4) swimming vinyl covering, and (5) open as open lagoons and open storage tanks.

The technology that should be installed includes the filling indicator, tubes, measuring devices and meters, electricity network, fiber cables…etc. Afterwards, the gas collector should be installed as well as the excess and low pressure safeguard and the air support fan. Figure 16 shows an overview of the technology installation.

(a) MT-ENERGIE GmbH & Co. KG (b) BIOGAS NORD GmbH Fig. 16. Technology installation

Pseudoplastic fluids become thinner when the shear rate increases, until the viscosity reaches a plateau of limit viscosity. This behaviour is caused by increasing the shear rate and the elements suspended in the fluid will follow the direction of the current. There will be a deformation of fluid structures involving a breaking of aggregates at a certain shear rate and this will cause a limit in viscosity. For pseudoplastic fluids the viscosity is not affected by the amount of time the shear stress is applied as these fluids are non-memory materials i. e. once the force is applied and the structure is affected, the material will not recover its previous structure (Schramm, 2000). Some examples are corn syrup and ketchup.

1.2.1 Viscoplastic fluids

start flowing. One type of these, the Bingham plastic, requires the shear stress to exceed a minimum yield stress value in order to go from high viscosity to low viscosity. After this change a linear relationship between the shear stress and the shear rate will prevail (Ryan, 2003). Examples of Bingham plastic liquids are blood and some sewage sludge’s.

1.2.2 Dilatant fluids

Dilatant fluids become thicker when agitated, i. e. the viscosity increases proportionally with the increase of the shear rate. Like for the pseudoplastic fluids the stress duration has no influence, i. e. when the material is disturbed or the structure destroyed it will not go back to its previous state. Some examples of shear thickening behaviour are honey, cement and ceramic suspensions.

In general, all types of wastewater can be used as substrates as long as they contain carbohydrates, proteins, fats, cellulose and hemicelluloses as main components. It is important that the following points are taken into consideration when selecting the wastewater industrial.

The content of organic substance should be appropriate for the microorganisms selected in anaerobic process. [12]

in the bioreactor depends on the origin of the liquid. In the Table 8, is shows the methane production rate from wastewater of different types.

In all previous cases, the wastewaters are discharged directly into the body of water,

causing several environmental pollution in addition to the loss of the energetic potential

contained in the effluents.

The acetogenic step allows the transformation of the acids, resulting from acidonenic step to acetate, and carbon dioxide, by the action of the acetogenic bacteria. This operation is carried out by different types of bacteria.

2.2.2 Methanogenesis

The mehanogenic step consists of the transformation of acetate, hydrogen and carbon dioxide into methane. For that, there are two main system routes:

1. Aceticlastic methanogens : acetate + H2 ^ CO2 + CH4

2. Hydrogenotrophic methanogens: CO2 + 4 H2 ^ 2 H2O + CH4

There are other minor routes which have a low importance. In the anaerobic digesters, approximately 60 to 70% of methane are produced by the Aceticlastic methanogens routes (Oles, 1997).

The growth of methanogens bacteria is slow: 3 days in 35°C (Schink, 1997). As they are the most sensitive micro-organisms of the ecosystem, they govern the total kenetics of the process (Ramsay & Pullammanappallil, 2001). Moreover, they are sensitive to the presence of inhibitors such as VFA.

During the methanogenic phase, the products of fermentation such as acetate and H2 / CO2 are converted into CH4 and CO2 by methanogenic bacteria. Methanogenes bacteria can grow directly on H2 / CO2, acetate and all other compounds with only one carbon such as formate, methanol and the methylamine (Punal & al., 2003).

The methanogenic step is influenced by the operating conditions of the digester, such as temperature, hydraulic loading rate, organic loading rate, and the influent substrate composition (McHugh & al., 2003).

1.1.4 Materials and methods

1.1.4.1 Microorganisms

The yeast Saccharomyces cerevisiae B-4 obtained from Institute of Agricultural and Food Biotechnology Warsaw, Poland, was used for assessment ultrasound exposition to ethanol production. The yeast cultures were cultivated on YPG slants (2% glucose, 2% peptone, 1% yeast extract) supplemented with 2% agar, at pH 5.0 and 30 °C for 24 h. The active cultures
for inoculation were prepared by growing the yeast in a 1 L baffled shake-flask containing sterile water and 100 mL YPG medium at 30 °C for 24 h on orbital shaker table at 200 rpm to a concentration of approximately 108 cells mL-1. The cultures in baffled shaken flasks of 100 mL were used to prepare the inocula. After 24 h of incubation at 30 °С, the precultures were centrifuged at 3800 rpm for 10 min and the cells were resuspended in sterile water to obtain 106 cells mL-1. Enzyme |3-D-galactosidase (optimum temperature 30 °С, optimum acidity pH 4.5-5.0, activity 8.7 AU mg-1 d. m. of the preparation), from Aspergillus oryzae manufactured by the SIGMA company (USA), was used for co-immobilization process. The amount of yeast and enzyme was 3% free cell inoculum and 4 cm3 enzyme solution. The yeast culture was co-immobilized in the 2% (w/v) sodium alginate by dropping yeast and enzyme into 150 cm3 0.09 mol L-1 solution of CaCl2 with 10% glucose. The cell beads were washed with sterile water and were stored as a fermentation medium in physiological solution at 8°C.

Fig. 3-4. Response surface and contour plots for the effects of grammage and rosin on dry tensile strength: beating degree was held at 40SR°, bauxite was held at 4%, wet strength agent was held at 1.8%

The sources of fuelwood in the district are communal forests, private forests, farms and timber residues. The distance to the fuelwood sources ranged from 1.1km for the less poor and slightly well-off households and 2.25 km for the ‘poor’ categories. The average distance for all respondents was 1.40km. The short distance for accessing fuelwood by the slightly well off and less poor is partly because a high proportion of them have private forests near their homes. Most people in the district use fuelwood from their own planted trees. Communal land is very limited in the district.

A high proportion of the communal forests have been severely degraded which makes fuelwood not easily available. Women spend 3-4 hours looking for fuelwood. This means that households with biogas facility were saving 3-4 hours wasted in collecting fuelwood. The saved time is used for other economic activities (e. g. farming and marketing) as well as leisure (e. g. resting and listening to the news and other entertainment). On the other hand, if the use of biogas for cooking will increase the demand for fuelwood in the district may decrease which is likely to benefit the poor because most of them do not have dairy cattle for biogas plant installation.

For billing, two parameters need to be determined: The volume flow of the fuel gas under standard conditions (T = 0 ° C, p = 1.01325 bar) and the calorific value for billing purposes. The determination of the standard volume flow from the operational flow is done using the procedure described in G 685, taking into account the temperature and atmospheric pressure. At pressures greater than 1 bar, real gas behaviour should be taken into account (G 486). The calorific value for billing purposes is determined from the calorific values of the feed for each billing interval (such as 1 month (special contract customers) or 1 year (residential customers)) in a coverage area (the total area, which is supplied by the GU, not necessarily contiguous). If the calorific values of the feed change over time, then these are determined arithmetically or by volume-weighted methods over the month.

If gases with different calorific values are distributed, then the following, according to G 685 (DVGW, 2008), applies:

according to the regional location of the customer. The gas delivered to customers may not deviate in calorific value by more than 2% from the calorific value for billing purposes. To check this, the mean values and the quantity-weighted average in the downstream network are to be determined.

Since worksheet G 486 is particularly applicable due to the admixing of propane and butane as considered in this report, essential aspects are explained below.