The soil moisture content of each treatment and analysis of variance of the soil moisture content were respectively shown in table 4-5 and table 4-6.

Table 4-6 showed that there were significant differences among treatments. Multiple comparisons of the soil moisture content among treatments were shown in table 4-7.

Mean difference (%)

Notes: The significance of symbols was as same as table 4-4.

Table 4-7. Multiple comparisons of the soil moisture content among treatments

The results showed that there were significant differences between three kinds of biogas residue fibre film, plastic film and control; there were significant differences between black film and control; there were significant differences between plastic film and black film. There were significant differences between A, C of biogas residue fibre film and black film;

30 25 20 15 10 5 0
СЯ
0 5 10 15 Soil depth/ cm
50 45 40 35 30 25 20 15 10 5 0
30 25 20 15 10 5 0
0 5 10 15 Soil depth/cm (d) Florescence am 8:00
Soil depth/cm (f) Florescence pm 19:00

there were not significant differences between B of biogas residue fibre film and black film; there were not significant differences between three kinds of biogas residue fibre film and plastic film. It can be seen, soil moisture of biogas residue fibre film was significantly higher than control and black film, and there were not significant difference as compared with plastic film.

Kaseamchochoung et al. (2006) also studied the effect of molecular weight of chitosan on flocculation. They controlled the deacetylation of chitosan samples at 83 ± 2% and studied two levels of molecular weight (3.5x10s and 1.4×106 dalton; Da). They found that the low molecular weight chitosan had a higher flocculation efficiency than the high molecular weight chitosan. Following Gregory (1993), they suggested that a possible explanation is that the longer polymers make more surface contacts per molecule and possibly saturate the cell surfaces, leaving no space for other polymers from different cell particles to initiate bridging.

2.1 Effect of environmental pH and ionic strength

Kaseamchochoung et al. (2006) found that the progression of anaerobic digestion in a UASB may cause pH to drop to 6 or even lower. At pH 6 and 7, approximately 90% flocculation was obtained by adding 2 mg chitosan/ g o. d. sludge of chitosan M70 and M85. However, at pH 5, approximately 95% flocculation was obtained at the same chitosan concentration (Fig. 3).

Similar results were obtained by Roussy et al. (2004). They studied chitosan efficiency at three different pH values (pH 5, 6.3, and 9). They found that a lower chitosan dosage (87% DD) was required at pH 5, while a significantly higher dosage of chitosan was required at pH 9 to obtain a residual turbidity below a fixed limit of 5 formalin turbidity units. Their explanation was that two possible mechanisms were possible at pH 5 —(a) coagulation by charge neutralization and (b) flocculation by entrapment in the polymer network. However, at pH 9 only the latter mechanism is possible, but its effect can only be significant at a high chitosan concentration.

Kaseamchochoung et al. (2006) found that both chitosan M70 and M85 were able to flocculate anaerobic sludge even when the system pH dropped to 5. A small degree of restabilization was observed after the charge neutralization point (CPN). That is, the percentage of flocculation dropped only slightly after the CPN, whereas zeta potential values became positive. A possible explanation given in Kaseamchochoung et al. (2006) is that the charge density of chitosan is greatly influenced by pH (Strand et al., 2001). Because the intrinsic pKa of chitosan is close to 6.5, most amine groups are protonated at pH 5, but become significantly less protonated when the pH increases. The polymer is therefore more highly positively charged at pH 5 than at pH 7. At pH 7, chitosan with 70%DD contains a lower charge density than chitosan with 85%DD, and the performance of chitosan (70%DD) would be noticeably lower at a low chitosan dosage (Fig. 2). Kaseamchochoung et al. (2006) suggested that charge density may play an important role in the flocculation mechanism and that this is not surprising because electrostatic forces are typically the main cause of polyelectrolyte adsorption on an oppositely charged surface. They concluded that chitosan has the potential to be used as an effective cationic bioflocculant, which is able to function either in acidic or neutral conditions, and that only relatively small amounts of chitosan (less than 4 mg/g dried sludge) are required.

chitosan concentration (mg/g o. d, studge}

Fig. 4. Percent flocculation as a function of chitosan M70 concentration in sludge suspension at different pH values and ionic strengths (from Kasemchochoung et al., 2006. Reprinted with permission from Water Environment Research. Volume 78, No. 11, pp. 2211 to 2214, Copyright © 2006 Water Environment Federation, Alexandria, Virginia.)

In addition to pH, ionic strength of a medium is also a major factor affecting flocculation. Kaseamchochoung et al. (2006) investigated the effect of ionic strength on flocculation by chitosan of high (0.1 M) and low (0.01 M) ionic strength. At pH 7, ionic strength did not signficantly influence the pattern of flocculation by chitosan M70 and the flocculation remained at approximately 95%. In contrast, at pH 5, chitosan M70 performed significantly better in the high-ionic-strength medium. Under the low ionic strength condition, the flocculation dropped from approximately 95% to 45% (Fig. 4). A possible explanation for the effect of salt was obtained from classical theories of colloidal stability (Strand et al., 2001). The extension of the double layer, which causes electrostatic repulsion between charged colloids and the range of repulsion forces, decreases with increasing ionic strength in the surrounding medium. Therefore, bacterial cells should be able to come closer and thus flocculate better in a high ionic strength medium.

For the production of compliant H gas with technical combustion characteristics matching North Sea I specifications, conditioning by admixing LPG is examined below.

The figures 2 to 5 show the potential composition of biogas mixtures, based upon methane levels in the processed bio-gas of 94, 96, 98 and 99,5 vol..-%, to which propane/butane (in a ratio of 95 / 5) is added. The necessary LPG admixtures for the desired calorific range for a methane content of 94, 96 and 98 vol -% in the treated biogas, lie above the limits as defined in G 486-B2 and DIN 51624, at 9.4 to 12.6 Vol -%, from 8.1 to 11.3 Vol -% and 6.8 to 9.9 vol -%. For processing to a methane content of 99.5 Vol -%, the limit according to G 486-B2 for pressures <100 bar is numerically satisfied up to a propane / butane admixture of 6.5 vol -%. The applicability criteria as described in section 2 apply. On the basis of this restriction, only an admixture of 5.8 to 6.5Vol.-% of LPG for an initial methane content of 99.5Vol.-% is possible. This would then cover a calorific range of 11.971 to 12.080 kWh/m3.

— Wobbe-Index	— relative Dichte
—■— LPG-Zugabe
Methan
16,5
0,74
16,0 H-Gas-Grenze
0,72
0,70
15,5
0,68
15,0
0,66
0,64
14,5
0,62
14,0
0,60
0,58
13,5
0,56
13,0
H in kWh/m8
Fig. 2. Possible H gas mixtures by admixing LPG to an initial concentration of 94 Vol. -% methane
0,74
0,72
0,70
0,68 ф 0,66 I Q Ф > 0,64 Ф
0,62
0,60
0,58
0,56

■— relative Dichte
LPG Zugabe	— Wobbe-Index
Methan
16,5
0,74
16,0
0,72
0,70
15,5
0,68
15,0 s” ф 14,5
0,62
14,0
0,60
0,58
13,5
0,56
13,0
Fig. 4. Possible H gas mixtures by admixing LPG to an initial concentration of 98 Vol. -% methane
—Methan —LPG Zugabe —Wobbe — Index —relative Dichte

Table 8 shows a summary of LPG additions necessary to achieve an average target calorific value of approx.12.2 kWh/m3.

4.1 Target properties: Weser ems L gas

For the production of compliant, low calorific L gas, conditioning by the addition of air is described in the following sections.

Figures 6 to 9 inclusive show possible fuel gas mixtures with a calorific value range of 9.653 to 10.047 kWh / m3, which can be achieved by the addition of air.

Figure 6 shows the possible mixture compositions, when air is added to an initial methane content of 94 vol -% . When interpreting this, please note that the data points for the Wobbe index, the methane content and the air belong together along the line of constant calorific value. A +/- 2% range has been set for the calorific value limits, based on the calorific value defined in DVGW worksheet G 260. All values with a Wobbe index of less than 13 kWh / m3 and an O2 content of less than 3 vol -% meet requirements. All other boundary conditions are shown in the table above.

Figure 7 show the possible mixture compositions, when air is added to initial methane content of 99,5 Vol.-%.

The two cases presented with initial methane contents of 94 and 99.5 Vol -% in the biogas, clearly show that increases in methane content also make necessary increased amounts of air, in order to achieve the desired calorific value and Wobbe index.

12,0

s*

11,5

Table 9. Air admixture to Weser Ems L-Gas and corresponding Wobbe-Index

The higher the initial content of methane in the biogas is, the greater is the approximation to the maximum compliant O2 content of 3 vol -% from conditioning.

Thus the O2 levels upon reaching the lower calorific value band are:

1,776% vol. O2 2,177% vol. O2 2,562% vol. O2 2,836% vol. O2

Reaching of the required calorific value band (9.653 to 10.047 kWh / m3) is possible from all four initial methane contents.

Figure 8 shows a summary of the air admixture ranges of the four initial methane levels. The red line indicates the maximum permissible volume fraction of 3% of O2 in the mixture.

Households in Africa, particularly in the rural areas are increasingly facing energy supply problems. According to United Nations (2010) there are approximately 60% of the total African population living in the rural areas. Biomass in form of wood, cow dung, and crop residues biomass constitutes 30% of the energy used in Africa and over 80% used in many sub-Saharan countries such as Burundi (91%), Rwanda and Central Africa Republic (90%), Mozambique (89 %), Burkina Faso (87%), Benin (86%), Madagascar and Niger (85%) (cited in United Nations Economic and Social Council, 2007). The availableness of these traditional fuels (wood, dried dung and agricultural waste) is declining (Deutsche Gesellschaft fur Internationale Zusammenarbeit (GIZ) GmbH and Integrated Science and Technology ISAT, undated), while the commercialised fuels (e. g. charcoal) are very expensive and their availability unreliable. Domestic biogas provides an opportunity to overcome these challenges in the rural areas. This is because biogas production makes use of domestic resources such as agricultural crop wastes and animal wastes such as pigs, cattle, and poultry as well as human excreta. Biogas production using the existing domestic resources therefore, has a potential to provide a number of benefits to the rural communities in Africa. Biogas plants that are well functioning can provide a wide range of direct benefits to the users particularly in the rural areas. Many of these benefits are directly linked to the Millennium Development Goals of reducing income poverty, promoting gender quality, promoting health and environmental sustainability.

One of the key factors in the success of microbial-mediated processes is an adequate understanding of process microbial, more specifically the study of microscopic organisms involved in wastewater degradation and byproduct formation. The low growth rate, the specific nutrient and trace mineral requirements of methanogens, coupled with their susceptibility to changes in environmental conditions demand meticulous process control for stable operation (Khanal, 2008). The biochemistry mainly involves enzyme-mediated chemical changes (the chemical activities of microorganism), type of substrate (kind wastewater) microorganism can destroy or transform to new compounds, and the step-by­step pathway of degradation (Sachdeva et al., 2000).

Consumer data, especially small "home" consumers, are read once a year, only. When executing computations with small consumers their hourly values are deduced from yearly readings using standardized methods (standard load profiles, SLP). In all computations — when SLP’s are involved — it must be kept in mind that this method influences the accuracy of the computing results when a short time period is considered. Opposite to the former small consumers the big consumers are measured and registered continuously; so computations can be made very accurate even in short periods.

2. Operational aspects

2.1 Smooth operation goal

Network operators favor smooth operating conditions for a number of reasons:

• Avoid sudden pressure changes (could generate shock waves, higher gas velocity)

• Avoid bigger and many flow changes (leads to pressure changes, regulator instability and wear out)

• Deliver/provide constant gas quality (i. e. colorific value), (operate within allowed limits).

In reality more or less big changes are likely to occur each hour (minute) due to changes of the consumption, scheduled feed-in according to delivery contracts, natural variation of gas quality of gas sources/production fields.

A case study, as part of the so called Syn-Energy[19] project, was carried out in a spa town in Upper Austria wherein the set-up of the supply chain was seen as one of the key parameters. Beside detailed analyses of intercrops (e. g. biogas content, yields) a main focus was to find a network in respect of a higher degree of decentralization for biogas production. This can be achieved e. g. with several separated decentralized fermenters that are linked by biogas pipelines to a single combined heat and power plant. The specific data for intercrops were used to carry out the evaluations. Of note was to show how intercrops can affect networks from an ecological and economical point of view.

The results of kinetic considerations based on modified Gompertz equation (Eq. 4) are shown in table 6. Independently from the kind of food waste (in the active of concentration) it was observed that the increase of the volume of generated hydrogen, small drops in reaction rate and prolongation of the lag phase.

2. Conclusions

The presented results shows that the waste studied in this paper represent a vary good substrate in photophermentation by Rhodobacter sphaeroides. Light intensity of 9 klx and inoculum concentration of 0.36 g dry wt/l (30% v/ v) were used as the most effective (high light conversion efficiency and short duration of the process). The studied wastes has to be treated with high temperature (20 min in 120oC). This pretreatment significantly increases H2 production. The optimum concentrations of wastes were estimated: 40% v/ v for dairy waste and 10% v/v for brewery waste with high COD. These wastes represent the effective (comparable with L-malic acid) nutrient for hydrogen production. Higher wastes concentrations inhibit the process as it initiate fermentation which starts to compete with hydrogen production and additionally increases NHp concentration, which also negatively affect the process. Brewery waste with low COD shows low efficiencies and needs to be concentrated to supply sufficient concentration of organic compounds. An application of untreated dairy wastewater containing suspensions in efficient hydrogen generation process can be performed only at controlled acidity (pH = 7.0). Kinetic measurements proved that the rate of hydrogen generation drops with concentration of the waste and prolongs the lag phase.

3. Acknowledgements

These studies were supported by Polish Ministry of Science and Higher Education (grant no: N N204 185440).

One of the main problem of digestate (and other N fertilizer) application is the N leaching. However, Renger & Wessolek (1992) and Knudsen et al. (2006) found that the N leaching was dependent on the use of cover crops. Similar results were reported by Moller & Stinner (2009) who did not find differences in the soil mineral N content among different manuring systems in the case of winter wheat, rye and spelt in autumn, before use of cover crops. That means that the use of cover crops is an appropriate method to avoid N leaching and to compensate for higher N application. From the same experiment, Moller et al. (2008) reported average soil mineral N content in spring. In this case they found significant higher soil mineral N content of the digested slurry treated samples (Table 4).

Digestate contains high proportion of NH4-N therefore it would be expected to increase NH4-N content of treated soil. However, digestate applied in the fall could easily be nitrified by early spring (Rochette et al., 2004; Loria et al., 2007). This predisposed N loss with occurrence of wet conditions.

Generally, the digestate application does not cause any significant changes in the total-N and available P content, while the available K content was increased by the application of biogas residue (Olsen et al., 2008). Similar results have found Vago et al. (2009), who reported the significant increase of 0.01 M dm-3 CaCl2 extractable P content even after 5 L m-2 digestate treatment, while the K content of soil was significantly increased by 10 L m-2 digestate dose only.

Reinforced concrete is obtained by adequately mixing specific proportions of aggregates (gravels and sand), cement, and water (Bartali, 1999). The water:cement ratio is 0.53 L kg-1 and the cement: sand: gravel mass ratio is 1:2.2:3.7 for floors, driveways, structural beams, and columns (Lindley & Whitaker, 1996). Cylindrical cast-in-place concrete tanks are commonly used in biogas plants for storing liquid manure during long periods. A serviceable tank should be watertight to prevent groundwater pollution and corrosion of the reinforcing rods. Therefore, these tanks should be designed to withstand different design loads, i. e. the loads of the soil outside the digester which is buried underground level and loads of the liquid stored inside the digester. Liquid manure is often stored in large cylindrical concrete tanks, which are partially underground. The dimensions of these tanks vary from 18 to 33 m in diameter with heights from 2.4 to 4.9 m and a uniform wall thickness varying from 150 to 200 mm (Ghafoori & Flynn, 2007; Godbout et al., 2003).

The internal volume of the tank can be calculated by multiplying the volume of substrates that should be stored in the tank by 1.10 in order to consider 10% as headspace. The cement mass (kg), gravels volume (m3), and sand volume (m3) required to build the tank can be calculated by multiplying the concrete volume of the tank by the constants C, G, and S, respectively, where C represents the mass of cement required to make 1 m3 of concrete (325 kg m-3), G is the volume of gravel required for 1 m3 concrete (0.8 m3 of gravel per m-3 of concrete), and S is the volume of sand required for 1 m3 concrete (0.4 m3 of sand per m-3 of concrete). The type of iron rods should be selected. The different types (N0D m-1, where N is the number of iron rods per meter length, and D is the diameter of the iron rod) are 606 m-1 (0.666 kg m-1) and 608 m-1 (0.888 kg m-1). In the case of constructing a tank without a concrete top, both types can be used. On the other side, in the case of building a tank with a concrete top, the type 608 m-1 must be used with two iron grids (Samer, 2008, 2010, 2011; Samer et al., 2008). The thickness of digester wall should be 35 cm and is built using reinforced concrete to bear the loads of the materials stored in the digester. Tables 1 through 3 show the typical digester specifications for a commercial biogas plant, the required quantities of construction materials to build the digester, and the quantities of the substrates.

1Consider a duration of 3 days for mixing and pumping, daily manure deposition of 6 m3 day-1, 1.8 m3 cow-1 month-1, and 100 cows

2Consider a storage duration of 7 days and liquid organic matter deposition of 3 m3 day-1 3Consider 40 days of storage duration and liquid substrate deposition of 2 m3 day-1 4Consider digester load of 4 kg m3 day-1 and density of 1.2 kg m-3

5Total quantity of substrates (10750 m3) that should be stored in a digester having a capacity of 11820 m3

Table 3. Quantities of the substrates