Biophotolysis of water, fermentation and photofermentation of organic substrates are considered to be the best biological methods of hydrogen generation. Reversibility, lack of toxic substances generated in these processes, mild conditions for microbiological reactions, as well as operation at low pressure of these processes are the conditions required for

modern microbiological systems. Moreover, the possibility of application of different waste waters (containing organic carbon) in these processes is an additional benefit.

Fermentation is the process generating basically two gaseous metabolites: hydrogen and carbon dioxide. The volatile fatty acids (VFA) and alcohols represent liquid metabolites of dark fermentation. The low yield of generated hydrogen and high concentration of CO2 (almost 50%) in gaseous products are the main disadvantages of microbiological hydrogen generation. In contrary, high reaction rate and possibility of biodegradation of many organic substances can be assigned to the benefits of this process.

In photofermentation, the photosynthetic heterotrophoic bacteria under anaerobic conditions and in the absence of nitrogen generate hydrogen in presence of organic compounds. Nitrogenase is the enzyme catalyzing hydrogen generation reaction. Presence of molecular nitrogen or nitrogen compounds directs the reaction route towards ammonia formation. The possibility of application of wide spectrum of light (400-950 nm), lack of methabolism generating molecular oxygen, as well as possibility of use of organic substances originating from wastes are the main advantages of photobiological method of hydrogen generation.

Both fermentation and photofermentation require presence of anaerobic microorganisms and the light in case of photofermentation. Photosynthesis, and in consequence also photofermentation is the series of complex reactions transforming energy of light into chemical energy.

The photosynthetic apparatus is localized in invaginations of the cytoplasmic membrane. The photosystem is built of three multimeric (transmembrane) proteins: antennas making the light-harvesting complex (LHC), the reaction centre (RC) and the complex of cytochromes bci (Fig.1) (Vermeglio, 1999). The LHC antennas contain molecules of bacteriochlorophyll and carotenoides. The carotenoides play a double role in LHC systems;

they absorb light from the visible part of the light spectrum in which bacteriochlorophyll is not active and protect the antenna system against damage by singlet oxygen (Isaacs, 1995, Jones, 1997). The majority of the purple bacteria have two different antenna complexes known as LH1 and LH2. The number of LH2 complexes depends on such parameters like light intensity and partial pressure of oxygen, while the number of LH1 complexes is directly correlated with that of the reaction center (RC) to form RC-LH1 center. High ratio of pigment molecules to RC (e. g. 100 molecules of chlorophyll to one RC) increases the area capable of light absorption. Upon absorption of photon by LHC, the reaction centre becomes excited with simultaneous charge separation in a time shorter than 100 picoseconds (ps). The high reaction rate of this process is a consequence of the mutual arrangement of LH1 and RC: one RC is surrounded by a ring of 15-17 LH1 subunits. The closed structure of LH1 complexes in combination with the dense packing of bacteriochlorophyll molecules ensures fast delocalization of the excited state and possibility of energy transfer towards the reaction centre from every point of the ring (Vermeglio, 1999). The reaction centre is an integral part of protein membrane composed of three polypeptides (subunits L, M and H), containing four molecules of bacteriochlorophyll a (PA, PB, BA, BB), two molecules of bacteriofeophityne a (HA, HB), two molecules of ubichinone(QA, QB), one molecule of carotenoid (Crt) and one atom of non-heme iron (Fig.2).

All pigments are linked to the heterodimeric protein skeleton of L and M subunits forming five transmembrane protein helixes (Paschenkoa, 2003). The main source of electrons is the "special pair" of the excited bacteriochlorophylls a located close to side of the cytoplasmic membrane. The excitation is realized by direct absorption of light by the "special pair" of bacteriochlorophylls absorbing at 870 nm and by energy transfer from other pigment molecules located at RC or LHC. The transfer of electrons from the special pair to bacteriopheophytin, located in the middle of the dielectric cytoplasmic membrane occurs in 3-4 ps. This reaction is probably intermediated by a transient product of monomeric bacteriochlorophyll BA. In the next 200 ps the electron is transferred to ubiquinone Qa (connected with RC) and subsequently to ubiquinone QB. The transfer of electron to ubiquinone QB is accompanied by its protonation. The full reduction of ubiquinone QB requires two subsequent cycles in RC after which electrons finally leave RC with electrostatically neutral doubly reduced ubiquinol QH2 (Jones, 1997). The two protons required for protonation originate from cytoplasmic space. In the next step ubiquinol is oxidized by the bc2 cytochrome complex. This complex caused reduction of the [Fe2S2] unit which is a part of cytochrome (part of Rieske unit) and releases two protons to periplasmic space. Then the cycle of electron transfer is closed by recombination of cytochrome c2 by reduction of the special pair of bacteriochlorophylls. The cyclic transfer of electrons is accompanied by transfer of protons from cytoplasm to periplasm leading to the proton gradient between the two sides of cytoplasmic membrane, which is the most important effect of photosynthesis because it stimulates ATP synthesis and reduction of NAD+ (Vermeglio, 1999). Protons accumulated on the periplasmic space of the membrane return to the cytoplasmic space through the ATP synthase channel, which closes the transfer of protons (Paschenkoa, 2003).

4.1.1 The effect of soil depth on soil temperature among treatments in different growth stages

The soil temperatures that varied with soil depth of the six treatments were shown in Fig.4-2.

Figure 4-2 showed that the mulching temperature was higher than the control during the whole growth period, the temperature of plastic film was the highest, the temperature of black film was higher than the biogas residue fibre film, and the temperature of A, B, C of the biogas residue fibre film was slightly higher than the control. It could be seen from the temperature curve of am7:00 and pm 14:00, soil temperature gradually decreased with the soil deepening, but, the temperature of soil surface decreased at pm 19:00, the soil temperature slightly increased with soil deepening in the stage of revival and flowering, and the soil temperature was constant in maturity; mulching had a certain warming effect in the stage of revival and flowering, and had not warming effect in maturity, the reason was that the film had been degraded.

According to Kaseamchochoung et al. (2006), chitosan with 85%DD and MW of 3.5x10s Da yielded the highest flocculation efficiency and versatility to changes in environmental pH and ionic strength.

Lertsittichai et al. (2007) studied the efficiency of chitosan in a UASB reactor treating tropical fruit-processing industry wastewater. The details of their study were as follows. The fruit canning factory wastewater consisted mainly of sugar. The wastewater characteristics were: COD 5,130 to 5,520 mg/L, volatile fatty acid (VFA) 703 to 1,834 mg/L, pH 5 to 6 and ionic strength of 0.028 to 0.036 M.

Two identical UASB reactors with a working volume of 30 L were employed for the comparative study. The startup period was operated at a hydraulic retention time (HRT) of 85 hours, corresponding to an organic loading rate (OLR) of 1.45 g COD/L-d. Chitosan at a concentration of 2 mg/ g suspended solids was added to the reactor on the second day and the same amount was added on the 37th operating day. The HRT of both reactors were reduced in a stepwise fashion, at 85, 65, 45, and 35 hours, when the COD removal was higher than 80% for at least 3 times the HRT.

Throughout the operation of the process, the OLR values ranged from approximately 1 to 4 g COD/L-d. Lertsittichai et al. (2007) found that the UASB with chitosan addition gave 9 to 59% lower COD effluent and had a 4 to 10% higher removal efficiency than the control UASB. The low VFA values corresponded to high biogas production because VFA is an intermediate for methane production. The UASB with chitosan addition gave a lower VFA value and a 35% higher biogas production rate than the control (Fig. 5).

Effluent VSS refers to biomass washout. Lertsittichai et al. (2007) found that the biomass washout increased during the initial operation period of both reactors. After 35 days, the biomass washout decreased due to granule formation. The biomass washout from the UASB with chitosan addition was 16 to 68% lower than that from the control. The UASB with chitosan addition was found to consistently have 24 to 37% higher average particle sizes than the control, corresponding to the lower biomass washout.

In addition, Lertsittichai et al. (2007) found that the UASB with chitosan addition consistently had a 6 to 41% longer solids retention time (SRT) than the control corresponding to a lower effluent VSS and a higher average particle size. The VSS from the bottom sampling ports of the UASB with chitosan addition was higher than that of control, leading to greater overall sludge density. From their observations, Lertsittichai et al. (2007) concluded that chitosan helped sludge pellet development. They gave the possible explanation that the cell surfaces of bacteria carry negative charges, and the electrostatic interactions between them are repulsive. Therefore, a cationic polymer, such as chitosan, assists the flocculation of the bacteria leading to faster sludge formation and a higher density of sludge retained in the reactor.

Overall, Lertsittichai et al. (2007) used only small amounts of chitosan (two injections with 2 mg chitosan/ g suspended solids at each injection). They saw no sign of inhibition to biomass activity. Throughout the course of their experiment at a mesophilic temperature (35oC), the UASB with chitosan addition clearly showed superior performance to the reactor without chitosan, with 9 to 59% lower effluent COD, 4 to 10% higher COD removal, up to 35% higher biogas production rate, and decreased washout of biomass and increased granular size.

For the production of compliant, high calorific L gas, conditioning by the addition of air and LPG is described in the following section.

Please note the following when interpreting the diagrams below: The field of admixtures includes a range of 0 — 20 Vol -% for presentational purposes. In practice, for technical and economic reasons, it is desirable to make the least possible admixtures with a "target" Wobbe Index of 12.4 kWh / m3 for example (setting of the gas appliances). In this context it should be noted that according to G 486 appendix B, the mole fractions of propane are not to exceed 3.5 mol% (6 mol% at p <100 bar) and butane max.1.5 mol% in natural gas, in order make a conversion of standard and operating conditions using the AGA8-DC92 equation of state.

The "field" of the possible mixtures is bounded by the Wobbe Index of 13 kWh / m3, the given calorific value limits, the max. oxygen volume fraction of 3%, and the maximum propane/butane or air admixture. For each value of air addition, there is always a value for the propane / butane addition.

The following figures apply only to the four initial properties of the biogas used.

Figures 9 and 10 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% to a biogas with an initial methane content of 94 vol -% and 96 vol -%.

H in kWh/m3

S, n

H in kWh/m3

S, n

HS in kWh/m3

S, n

Figures 11 and 12 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% with an initial methane content of 98 vol -% and 99,5 vol -%.

The red area represents the required calorific value range from 9.97 to 10.4 kWh / m3. The green dots show the possible, compliant mixtures that lie within all the conditions to be fulfilled.

The bulk of the rural population in Africa have no access to electricity. According to World Economic Outlook (2010), only 14% of sub-Saharan African has access to electricity. It is thus estimated that 582 million rural people in sub-Saharan Africa did not have access to electricity in 2009 (World Economic Outlook, 2010). North Africa is an exception because 98.4% of rural population is electrified and only 2 million did not have access to electricity in 2009 (World Economic Outlook, 2010). Biogas is a potential off-grid, clean energy fuel solution for rural areas of Africa (Amigun and von Blottnitz, 2010), that can provide energy services such as cooking, heating and lighting.

1.1.1 Environmental benefits

commercialized (Hiemstra-van der Horst and Hovorka, 2009). The high dependence on woodfuel in the sub-Saharan Africa has resulted in an alarming rate of tree felling and deforestation (cited in United Nations Economic and Social Council, 2007). According to the United Nations Environmental Programme (2011), nearly half of the forest loss in Africa is due to removal of wood fuel. The estimated deforestation rate in Africa is twice the world rate (AfriNews, 2008). More than 15 million hectares of tropical forests are depleted or burnt every year in order to provide for small-scale agriculture or cattle ranching or for use as fuel wood for heating and cooking (United Nations Convention to Combat Desertification, 2004). Some alarming and worrying deforestation facts in Africa include (AfriNews, 2008): loss of over 90% of West Africa’s original forest — currently, only a small proportion remains; between 1980 and 1995, an area of 1.1 million ha was cleared every year; only one tree is replanted for every 28 trees cut down. In Uganda where 90% of the population lives in rural areas and directly depends on land for cultivation and grazing, forestland has shrunk dramatically. In Nigeria, it is feared that the country will be left without forest due to the present level of deforestation activities.

Forests are required in order to build a resilient natural ecosystem as they moderate climate, act as water reservoirs and are habitat to wildlife. The loss of ground cover due to deforestation thus results in secondary problems such as exposing the soil to erosion during heavy rainfall, flooding, increased evaporation, drought, and increase in the greenhouse gas emissions. Familiar country specific example is the recent frequent droughts and floods experienced in East African countries, particularly Kenya, Somali, Uganda and Ethiopia, that have been associated with deforestation (IRIN, 2006; Mekonnen, 2006). Similarly, the declining rainfall in the West African countries is also attributed to deforestation. The use of alternative energy such as biogas has a potential to reduce the demand for wood and charcoal use, hence reducing greenhouse gas emissions improving water quality, conserving of resources — particularly trees and forests — and producing wider macroeconomic benefits to the nation (Amigun and Blottnitz, 2010) due to reduced deforestation. In addition, the slurry and waste from the biogas plants provides a high quality fertiliser that can be used to improve the soil fertility and increase productivity in agriculture dependent rural communities in Africa.

The anaerobic digestive process is a natural biological process in which an interlaced community of bacteria cooperates to obtain a stable and auto-regulated fermentation through assimilation, transformation and decomposition of the residual organic matter present in waste and wastewater into biogas. This is a complex multistep process in terms of chemistry and microbiology, where the organic material is degraded to basic constituents to obtain methane gas under the absence of an electron acceptor such as oxygen. The common metabolic pathway and process microbiology of anaerobic digestion is shown in Fig. 1 (Khanal, 2008).

Generally, the anaerobic digestion process consists of four stages; the first one is called hydrolysis (or liquefaction), it consists in the transformation of complex organic matter such as proteins, carbohydrates and lipids into simple soluble products like sugars, long-chain fatty acids, amino acids and glycerin, this stage is carried out by the action of extracellular enzymes excreted by the fermentative (group 1) (Khanal, 2008).

In the second step, called the acidogenic stage fermentative bacteria use the hydrolysis products to form intermediate compounds like organic acids, including volatile fatty acids (VFA). Theses VFA along with ethanol are converted to acetic acid, hydrogen and carbon dioxide by other group of bacteria known as hydrogen-producing acetogenic bacteria (group 2) (Khanal, 2008).

Organic acids are oxidized partially by bacteria called acetogenic in the third stage, which produce additional quantities of hydrogen and acetic acid. The acetogenesis is regarded as thermodynamically unfavorable unless the hydrogen partial pressure is kept below 10-3 atm, pathway efficient removal of hydrogen by the hydrogen-consuming organisms such as hydrogenotrophic methanogens and/or homoacetogens (Zinder, 1988).

Finally, in the fourth stage, both acetic acid and hydrogen are the raw material for the growth of methanogenic bacteria, converting acetic acid and hydrogen to biogas composed mainly of methane, carbon dioxide and hydrogen sulfide (Khanal, 2008).

Acetate, H2 and CO2 are the primary substrate for methanogenesis. On chemical oxygen demand (COD) basis about 72% of methane production comes from the decarboxylation of acetate, while the remainder is from CO2 reduction (McCarty, 1964). The groups of microorganisms involved in the generation of methane from acetate are known as acetotrophic or aceticlastic methanogens (group 3). The remaining methane is generated

from H2 and CO2 by the hydrogenotrophic methanogens (group 4). Since methane is largely generated from acetate, acetotrophic methanogenesis is the rate-limiting step in anaerobic wastewater treatment. The synthesis of acetate from H2 and CO2 by homoacetogens (group 5) has not been widely studied. Mackie and Bryant (1981) reported that acetate synthesis through this pathway accounts for only 1-2% of total acetate formation at 40°C and 3-4% total solids at 60°C in a cattle waste digester.

1.1 Process microbiology

a. Fermentative Bacteria (group 1): This group of bacteria is responsible for the first stage of anaerobic processes. The anaerobic species belonging to the family of Streptococcaceae and Enterobacteriaceae and the genera of Bacteroides, Clostridium, Butyrivibrio, Eubacterium, Bifidobacterium and Lactobacillus are most commonly involved in this process (Novaes, 1986).

b. Hydrogen-Producing Acetogenic Bacteria (group 2): This group of bacteria metabolizes higher organic acids (propionate, butyrate, H2, etc.), ethanol and certain aromatic compounds (i. e. benzoate) into acetate, H2 and CO2 (Zinder, 1998). The anaerobic oxidation of these compounds is not favorable thermodynamically by hydrogen-producing bacteria in a pure culture, however in a coculture of hydrogen — producing acetogenic bacteria and hydrogen-consuming methanogenic bacteria, these exists a symbiotic relationship between these two groups of bacteria. It is important to point out that during anaerobic treatment of complex wastewater such as vinasses or slaughterhouse, as many as 30% of the electrons is associated with propionate oxidation. Thus, these chemical appears to be more critical than oxidation of other organic acids and solvents (Deublein and Steinhaunser 2008).

c. Homoacetogens Bacteria (group 3): Homoacetogenesis has attracted much attention in recent years because of its final product acetate, an important precursor to methane generation. The responsible bacteria are either autotrophs or heterotrophs. The autotrophic homoacetogens utilize a mixture of hydrogen and carbon dioxide, with CO2 serving as the carbon source for cell synthesis. The heterotrophics homoacetogens, on the other hand, use organic substrate such as formate and methanol as a carbon source while producing acetate as the end product (Eq. 1 to 4) (Khanal, 2008).

CO2 + H2 ^ CH3COOH + 2H2O (1)

4CO + 2H2O ^ CH3COOH + 2CO2 (2)

4HCOOH ^ CH3COOH + 2CO2 + 2H2O (3)

4CH3OH + 2CO2 ^ 3CH3COOH + 2CO2 (4)

Acetobacterium woodii and Clostridium aceticum are the two mesophilic homoacetogenic bacteria isolated from sewage sludge (Novaes1986). Homoacetogenic bacteria have a high thermodynamic efficiency; as result there is no accumulation of H2 and CO2 during growth on multicarbon compounds (Zeikus 1981).

d. Metanogenic Bacteria (group 4 and 5): Methanogens are obligate anaerobes and considered as a rate-limiting specie in anaerobic treatment of wastewater. Abundant methanogens are found in anaerobic environments rich in organic matter such as swamps, marches, ponds, lake and marine sediments, and rumen of cattle. Most methanogens can grow by H2 as a source of electrons via hydrogenase as shown in the follow reaction (Eq. 5) (Khanal, 2008):

4H2 + CO2 ^ CH4 + 2H2O (5)

The source of H2 is the catabolic product of other bacteria in the system, such as hydrogen — producing fermentative bacteria, especially Clostridia (group 1) and hydrogen-producing acetogenic bacteria (group 2). The hydrogenotrophic pathway contributes up to 28% of the

methane generation in an anaerobic treatment system. It bears mentioning that there are many H2-using methanogens that can use formate as a source of electrons for the reduction de CO2 to methane, as show in reaction (Eq. 6):

4HCOO — + 2H+ ^ CH4 + CO2 + 2HCO3- (6)

In wintertime gas consumption is high and normally biogas is a smaller amount of the total consumption causing normally no transportation problems. But in summertime when the gas consumption is very low some areas face the situation that biogas production is higher than the total consumption in a distribution (sub-) network. In effect, biogas floods the network but could not be consumed, even if the regulating devices would have been changed to different settings to retain natural gas flow. So the exceeding or all biogas must be transported to other areas via additional pipes which have to be provided by the network operator.

2.2 Reverse feeding to high pressure trunk lines

So called "Reverse Feeding" to the transportation network is required to solve the problem of excess biogas production in a local area. An extra pipeline leads the biogas that has been previously compressed to an appropriate point (nearest one) of the transportation network. The level of compression depends on the pressure level of the transportation system (occasionally this can be up to 80 bar).

2.3 Odorization and deodorization

If biogas is fed into a distribution network it must be odorized before. Odorization adds the typical alarming and disgusting smell to the gas that warns human beings in case of leakage. If in the situation that excess biogas exists in a network area odorized biogas must be deodorized before entering the transportation network that has no odorized gas (odorization will be added downstream at the distribution level, only).

Figure 1 shows three potential decentralized locations for biogas production. As there is a spa town located in the considered region it was not possible to contemplate a fourth, central location for a fermenter as it would infringe with the touristy activity there. There is already an existing district heating network in town that should be extended. The heat needed could be either generated by a centrally placed CHP with biogas transported via pipelines or heat produced with decentralized CHPs could be used for fermenter heating and/or transported via long — distance heat pipelines to the town. In the first case, with central CHP, fermenter heating is provided by wood chip furnace.

The fermentation could work with different feedstock types to find out the most lucrative way of using intercrops, manure, grass silage and corn silage. Corn as additional feedstock was taken into consideration for economic reasons, because it is favored under current economic conditions. For the optimization it was assumed that proportional to the availability of manure biomass in an amount of 34 % intercrops, 18 % grass silage and 16 % corn silage (referring to fresh weight) per livestock unit can be supplied. As there are several farmers in and around the considered region eight provider groups (1-8 according to Table 6 and black bordered providers in Figure 1) were defined. The substrate costs were the same for each group.

The providers differed in the amount of available resources as well as in the distance to each possible fermenter location, which directly correlates with transport distances and costs. Transport costs included fix costs for loading and unloading and variable costs depending on the distance (including unloaded runs). For solid substrates fixed costs of 2 €/t fresh weight were taken into account. Similarly, the conversion was made for the variable costs, which were assumed with 0.49 €/km. Fixed transport costs for manure were defined with 20 €/1 dry mass with variable costs of 5 €/t dry mass per kilometer. For grass and corn silage a storage was taken into account. As it is not possible to bring the investment costs down to one number because they are highly depending on the local basic conditions a fix investment of 150,000 € for a silage storage was taken into account. As soon as a location is chosen by the PNS a storage has to be included there. Two locations mean two times investment costs to store the silage that is used for biogas production.

Transportation of heat and biogas could be achieved via pipeline networks. Network energy demands as well as losses caused by transporting were included. Regarding heat it was assumed that the total produced heat amount could be used for district heating. As location 1 and 3 are in one line to the spa town one biogas pipeline could be used for both locations to transport biogas to the central CHP. Therefore no additional costs arise for a biogas pipeline from location 1, if location 3, which is farther away, supplies the center with biogas.

Because of different transport distances the PNS could decide which provision group and amount of substrate should be used to get the most economical optimum solution. The fermentation could run with various substrate feeds. Dependent on them fermenter sizes, costs and exposure times differed. Seven different fermenters were part of the PNS to find the most lucrative way of substrate input. The feeds are shown in Table 7.

In Table 8 the substrate parameters are described. The optimization was based on two different cost situations (maximum and minimum) concerning substrate provision.

Figure 2 shows the so called maximum structure for the PNS optimization, which includes all input and output materials with energy and material flows with economic parameters like investment or operating costs and prices. For the optimization three fermenter sizes (up to a capacity that serves a 250 kWel CHP) were available for biogas production. Four combined heat and power plant capacities (up to 500 kWel) were involved in the maximum structure. The fermenters could be heated by decentralized CHPs or with a wood chip furnace on site in case the biogas is transported to a central CHP.

The biomass furnace that could be a choice to provide fermenter heating was not implemented as separate technology in PNS’ maximum structure, but a price of 5 ct/kWh heat was assumed (Wagner, 2008). Produced electricity could be fed into electricity providers’ grid, thus benefiting from feed-in tariffs according to Austrian’s Eco-Electricity Act (RIS, n. d.).

The PNS optimization shows that the technology network providing the most benefit for the region includes two different locations (1 and 3) for biogas generation. At location 3 biogas is produced with substrate feed 4, a mixture consisting of manure, intercrops, grass and corn silage. The fermenter runs 7.800 full load hours and is able to provide a 250 kWel CHP with biogas. At location 1 the set up includes a fermenter with same capacity but different load. Substrate mixture 7 is used for biogas production which contains manure, intercrops and grass silage. Both fermenters are heated with a biomass furnace on site. All provider groups can supply the fermenters with at least one substrate. The optimal technology network includes two central 250 kWel CHPs supplied via biogas pipelines with biogas from both

locations. For the pipeline coming from location 1 no additional costs have to be incurred because the pipeline would be part of the routing from location 3 to the center. The produced heat covers the central heat demand for a price of 2.25 ct/kWh. The electricity is fed into the grid and feed-in tariffs of 20.5 ct/kWh can be gained. Figure 3 depicts the optimum structure for a situation with maximum substrate costs as listed in Table 8.

location 3

Fig. 3. Optimum structure of a technology network generated with PNS

With this technology network and 15 years payout period a total annual profit of around 196,350 € can be achieved (interest rates are not included). The total material costs including electricity consumed from the grid and costs for fermenter heating add up to approx. 438,000 €/yr with additionally 60,300 € per year for transportation. The total investment costs for this solution would be around 2,895,000 € including district heating and biogas network as well as the costs for fermenters and CHPs.

With minimal substrate costs (see Table 8) there is no change in the optimal structure, but the revenue is higher commensurate to the lower substrate costs (one-third reduction). The revenue for the structure with minimal substrate costs excluding interest accounts for a yearly amount of about 280,400 €.

Chen Haitao, Li Lixia, Wang Hanyang and Liu Lixue

Northeast Agricultural University,

China

1. Introduction

Plastic mulching cultivation technology originated from Japan in 1955. This technology was introduced to China in 1978, and it was comprehensively spread after a series of tests. Plastic film mulching technology was widely used due to increasing temperature, preserving soil moisture, increasing yield, preventing soil erosion etc. To 1986, the using area of plastic film in China had leap to the first in the world. The application of mulching cultivation technology is an effective way to improve crop yield, and has promoted the development of agriculture. The application of plastic film was known as the third revolution following fertilizer, seeds in agriculture, also called "white revolution" (Zhang Ying,2005).

However, plastic is a polymer material, which is non-perishable, difficult degradation, experiments show that the degradation of plastics needs 200 years in the soil (Bian Yousheng, 2005). With the unceasing expanding of mulching area of plastic film and the increasing of using years, the residual plastic film that was not degraded and continued to be accumulated in farmland, large numbers of residual film formed barrier layer gradually, which could hinder the development of crop root and the absorption of moisture and nutrients, hinder machinery tillage, damage crops growth environment, lead to soil compaction, poor permeability, reduction of crop yield and severe environmental pollution, the phenomenon that a large number of plastic film left in farmland make "white revolution " which has brought Gospel to agricultural production be transformed into "white pollution" (Liu Junke, 2000). In order to protect farmland ecological environment, domestic and international measures were actively developed and a variety of environmentally friendly biodegradable plastic film which can produce oxidized photochemical and biochemical effects were promoted, the use of biodegradable plastic film would be an effective way to completely solve the "white pollution" (Yang Huidi, 2000).

In recent years, biodegradable plastic film material and degradation process has become a research focus (Li Xianfa, 2004; Sun Jianping, et al, 2000; Zhang Chunhong et al, 2007). Currently, photolysis film, biodegradable film, photolysis/biodegradable film were researched more, the degradation process of film depended on biodegradation, photo­degradation and chemical degradation, and the effect of efficiency, synergy and coherence between the three main degradation process. Although a variety of biodegradable plastic film were developed in China, due to the high cost, the poor economy and the difficulty of

controlling the degradation of biodegradable film, the promotion and development were hindered. So, the development of degradable film which has good economy and low cost has become the main research direction.

In China, the construction of biogas started in the 1970s, and so far, with over 30 years of history. Biogas technology mainly used manure, straw and other organic substances as raw material to produce biogas by anaerobic fermentation (Zhang Yongmei, 2008). In recent years, the promotion and application of biogas technology was rapid in China, at the end of 2005, 17 million household biogas digesters and 140000 sewage purification digesters were built in rural areas, with an annual output of about 80 billions m3 gas. By the end of 2006, the total number of the rural household biogas has reached 22 millions, biogas construction had entered a new stage of rapid development. Currently, biogas residue were mainly used for planting, breeding, sideline and processing industry (Lu Mei et al, 2007; Guo Qiang, et al, 2005; RK Gupta, 2002), which was difficult to get high value resources utilization, so if its solid waste after anaerobic digestion could achieve high value resource utilization, the economic efficiency of industrial production of biogas could be improved and good ecological and social benefits would be produced (Tian Xin, 2008; Ye Xujun, et al, 2000).

In addition, because ruminant animals mainly fed on coarse fodder, such as rice straw, wheat straw, corn stalk, etc. The conversion rate of straw fibre was not high, the main reason for this phenomenon was that 20% to 70% of the fibre can not be degraded by rumen bacteria of ruminants, the result was that the biogas residue which was from ruminant manure fermentation contained large amounts of wood cellulose (An Juan, 2005).

Therefore, the full biodegradable biogas residue fibre film made of ruminant animal manure or fibre residue of straw fermentation for biogas and a certain percentage of plant fibre, using cleaner production processes, had moisture conservation and weeding property. The film could be completely degraded by microorganisms in the period of crop growth, restore in the soil, improve soil organic matter content, and meanwhile not pollute the environment, all of these would lay the foundation for applications and promotion of biodegradable film, to sustain agricultural sustainable development of our country had important realistic and historical significance.

After the application of the digestate in 5 and 10 L ha-1 dosages, the Cd, Co, Cu, Ni and Sr content of soil solutions did not change. The Zn content decreased significantly, while the amount of manganese (Mn) increased by almost 40% (Vago et al., 2009) (Table 5).

The increasing soluble P content of digestate treated soil decreased the available Zn content in the soil solution by building slightly soluble zinc-phosphate residue (Vago et al., 2009).