Based on the economic results of the PNS optimization and previous SPI evaluation of different intercrops, a footprint for the PNS results was calculated. The evaluation includes every substrate, transport, net electricity and infrastructure for fermenters and CHP units. SPIonExcel already provides a huge database of LCIA datasets which can be used for modeling the scenarios. In case of intercrops substrate the SPI value for conservation tillage + self-loading trailer from Table 4 was used.

The overall footprint points out the environmental impact for one year of production. In case of the optimum solution it would need 93.08 km2 of area which has to be reserved to embed the production sustainably into nature. The overall footprint is shared between both products according the amount of output and the price per MWh (electricity: 205 €/MWh; heat: 22.5 €/MWh). Price allocation of the footprint leads to a higher footprint for the higher valued product.

Scenario 1 has a benefit from the ecological point of view and almost equal revenue according to Table 9. For scenario 2 there is only a slightly difference to the optimum solution because of two small CHP units instead one.

Main impact categories are in every case ‘fossil carbon’, ’emissions to water’ and ‘air’. This mainly derives from the utilization of net electricity which contributes around 45 % to the whole footprint. Main contribution to this categories stemming from net electricity and
machinery input in agriculture which are still mainly fossil based. This is also the main optimization potential for a further decrease of the footprint.

5PI for emissions to soil

■ SPI foremissions to water

■ SPI for emission to air

■ SPI for renewable

■ SPI for fossil C

SPI for non renewable SPI for area

Scenario 2

Compared to other electricity provision system the optimum solution from the PNS has an ecological benefit in footprint ranging from 61 to 96 % which is pointed out in Figure 7. Although the footprint of the optimum solution could be optimized by using the produced electricity for itself and not selling to the grid (which has economic reasons because of high feed-in tariffs) the ecological benefit compared to other sources is obvious. Every contribution to a greener net infects simultaneously all net participants.

5. Conclusion

The three pillar principle of sustainability serves as conceptual framework to conclude this study. Not only economic and ecological factors are important to implement innovative structures. Often we forget about the social component, the third pillar of sustainability. Not to do so farmers’ opinion about intercrops where taken into account. It turned out that intercrops production also abuts on farmers’ psychological barriers and the need of intensive cooperation among farmers in the surrounding of a biogas plant. In conjunction with economic risk and high investments, determining farm management for at least 15 years it becomes obvious, that well-considered decisions are to be made. Therefore, it is not astonishing that farmers hesitate, if economic benefits do not clearly compensate social an managerial risks of biogas production from intercrops. Furthermore, the situation that biogas production from corn is favorable regarding practicability in comparison to biogas production from intercrops, reduces farmers motivation to decide for the latter. But even the growing and harvesting of intercrops requires additional work and the strict time frame to cultivate fields, the risk of soil compaction through harvest and potential lower yields of main crops after winter intercrops are counter­arguments to cooperate with farmers already running biogas plants. Higher feed-in tariffs for biogas from intercrops seem to be inevitable and sensitization of decision makers and farmers is needed to emphasize that the planting of intercrops holds many advantages and that intercrops reduce the ecological footprint decisively. Although a higher energy input for agricultural machines is required because of the additional workload for intercrops. In summary the energy balance per hectare including biogas production points out a benefit. In times of green taxes a reduction of CO2 emissions can diminish production costs. More biogas output per hectare raises the income beside minimized mineral fertilizer demand reduces costs and lowers the ecological footprint. Furthermore, biogas production from intercrops contributes to a reduction of nitrate leaching and nitrous oxide emissions from agriculture. With the transport optimization in-between the network the ecological footprint decreases caused by intelligent fermenter set-up going along with less transport kilometers and fuel demand. A farmer association running an optimal network described before lowers the investment risk and ensures continuous operation and stable substrate availability. On the other hand an association has the potential to strengthen the community and the social cohesion of regions. Some of the advantages mentioned before effect the regional value added positively. On closer examination it could be shown that intercrops can play an important role in sustainable agriculture for the future by running a social and ecological acceptable network and still being lucrative for the operators and the region. Finally biogas production from intercrops does not affect the security of food supply. On the contrary it may even increase productivity in the case of stockless organic farming.

6. Acknowledgment

The research presented here was carried out under the project "Syn-Energy" funded by the Austrian Climate and Energy Fund and carried out within the program "NEUE ENERGIEN 2020" (grant Number 819034).

In 1996, biodegradability of plant fibre paper was studied, which lower mechanical properties under certain environmental conditions, and eventually fragmented or completely degraded. Weight reduction and observation methods were employed by Gao Yujie (1996), Zhang Wenqun (1994), Wang Weigang (2003), Li Zhiming (2004) and Wang

Wei to study the biodegradability (2009), observation method only can describe the film degradation process and the weight reduction method can quantitatively explain the degradation process of biodegradable film.

In this chapter, the physical properties and chemical composition of the biogas residue produced by anaerobic fermentation using ruminant feces were determined and analyzed; manufacturing technology and performance of biogas residue film was studied by the methods of central composite quadratic rotatable orthogonal experiment; biogas residue fibre mulching for planting eggplant was studied with the method of random plot experiment.

Sustainable recycling of organic wastes demands clear regulations of recycled wastes, the used recycling methods and the controlling of products. These regulation processes for the digestate are different in certain countries, respected the elaboration and the used limits.

In Hungary, the digestate is regarded as other non-hazardous waste if the ingestate does not contain sewage or sewage sludge, while in the presence of these materials the conditions of the digestate utilisation depend on the quality of the given material.

In Scotland the BSI PAS110:2010 digestate quality assurance scheme is applied. If a digestate complies with the standards for the quality, the usage criteria and the certification system stated in the worked scheme, the Scottish Environment Protection Agency (SEPA) does not apply the waste regulatory control for it.

In Swiss the digestate which suits the limits, can be used as soil conditioner and fertilizer in "bio"-agriculture.

In Germany the origin of the input materials determines the quality label of digestate product by biowaste and renewable energy crops. Digestates have to fulfil the minimum quality criteria for liquid and solid types which determine the minimum of nutrients and the maximum of pollutions in the digestate. Pollutions mean toxic elements, physical contaminants and pathogen organisms. The quality of digestate products is regularly controlled by "Bundesgutegemeinschaft Kompost e. V." (BGK) (Siebert et al., 2008).

Chandrasekar (2006) demonstrated the gas processing unit (GPU). In stage one biogas from the digester will be cleaned of moisture droplets, particulates and hydrogen sulfide. The cleaned gas mixture, which consists primarily of methane (CH4) and carbon dioxide (CO2), will be then converted in stage 2 to ultra-high purity hydrogen in a steam reformer. As a first step to realize this vision, a GPU was installed (Fig. 19) which has been successfully removing over 99% of hydrogen sulfide (H2S) along with most of the water droplets and particulates. A steam reformer has been also installed.

In the GPU biogas from the digester is pressurized to over 3 inches water column by a blower. It then passes through a coalescing filter to remove most of the particulates and water droplets. Water collected in the coalescing filter gets automatically drained out once it reaches a certain level. The biogas is then heated to about 85 oF in a heater before it passes through two successive activated carbon beds where H2S is converted into elemental sulfur. The process has been optimized so that bed replacement is needed only once every six months. The configuration of dual beds allows for continuous operation even when one bed is being replaced. The bed manufacturer should be contracted to replace the used beds, thereby obviating the need for the farmer to handle the sulfur. The design requires minimum operation and maintenance and has been set up to be controlled through a computer that will also monitor the incoming gas pressure, control and monitor the blower as well as monitor the exit H2S concentration and shut the blower/GPU if the exit concentration is greater than the set point. If the GPU shuts down, biogas will automatically feed the engine generator like before to produce electricity. A simple schematic of the GPU is shown in Figure 20.

As pointed out by Barber et al., (2010) perennial grasses benefit the environment in numerous ways. They help to reduce climate change, increase energy efficiency and will constitute a sustainable energy resource for the world. Switchgrass, the most widely used perennial grass for biofuels, is also in such a manner, beneficial to both farmers as well as energy consumers in general. Perennial grasses are crucial to the ecosystem to create a sustainable energy resource for the world and also to limit the use of fossil fuels. These grasses are important because they can produce ethanol, an energy source that emits much less carbon dioxide than other fossil fuels. Reducing carbon dioxide emissions is important because carbon dioxide emissions in the atmosphere constitute one of the leading causes of climate change. Barry (2008) pointed out that 1 bale of switchgrass can yield up to about 50 gallons of ethanol. As reported by Rinehart (2006), researchers are using switchgrass as a biofuel so that they can successfully reduce carbon dioxide emissions. Switchgrass has a high energy in and out ratio because of lignin, the byproduct of the cellulose conversion that stores internal energy for its energy transformation process. Ethanol reduces carbon dioxide emissions by approximately ninety percent when compared to gasoline and consequently, carbon dioxide in the ozone layer of our atmosphere will slowly begin to deplete itself as biofuels created from switchgrass, other grasses and other ethanol sources are utilized. As a rule, all species of the grass family (poaceae) contain starch and should be able to yield ethanol.

When considering the rheology for biogas reactors their viscosity is estimated to correspond to a given TS of the reactor fluid. This is mainly based on historically rheological data from sewage sludge with known TS values. However, problems may arise when using these TS relationships for other types of substrates which may impose other rheological characteristics of the reactor fluids. Furthermore, often low consideration is given to possible viscosity changes due to variation in feedstock composition etc.

Shift in the viscosity and elasticity properties of the reactor material related to substrate composition changes can alter the prerequisites for the process regarding mixing (dimension of stirrers, pumps etc. or reactor liquid circulation) and likely also foaming problems (Nordberg & Edstrom, 2005; Menendez et al., 2006). It may also call for changes in the post treatment requirements and end use quality of the organic residue e. g. dewatering ability, pumping and spreading on arable land (Baudez & Coussot, 2001). The additions of enzymes can be used to reduce the viscosity of the substrate mixture in the digester significantly and avoid the formation of floating layers (Weiland, 2010; Morgavi et al., 2001). All these factors affect the total economy for a biogas plant.

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.