Ghana has a population of 24.8 million of which 48.5% live in the rural areas (United Nations, 2007). Netherlands Development Organization (2007) estimates that Ghana has a potential to realise 280 000 domestic biogas plants, that is capable of producing 6000 m3 of liquid fertiliser, which would increase yield by 25%. However, low perception of biogas has modern energy has made Ghana not to realise the full potential of biogas utilisation (Bensah and Brew-Hammond, 2008). Bensah and Brew-Hammond (2010) highlights the status of the biogas development in Ghana, in which only about 200 units have been installed, thus lagging behind in comparison with other African countries such as Rwanda, Kenya and Tanzania. Some initiatives such as Biogas Technology West Africa Ltd[6] , funded by UNIDO has implemented a number of biogas digesters in Ghana for schools, hospitals and colleges. These are mainly underground masonry dome systems in the range of 60 m3 to 160 m3 volume. One example of these projects is Keta secondary school plant for 1200 users, and has a capacity of 80 m3. The plant is built in sandy, water logged area and it makes use of human waste. The gas is used for cooking. The future development of biogas in Ghana will however not be left to private investors and initiatives if the benefit to the rural communities is to be realised. Bensah and Brew-Hammond (2010) argues that, for successful future development of biogas in Ghana, there is a need for establishing a government body that solely focuses on promoting biogas.

1.1.3 Mozambique

Mozambique has a population of 22.6 million people, in which 61.6% reside in rural area in 2010 (United Nations, 2007). Similar to Ghana, the Mozambique government does not have an agency solely supporting the development of biogas. Some initiative such as Biogas Technology West Africa Ltd[7] is however, also undertaking a biogas power project in the country. The project is an electric power system powered by a biogas-fired internal combustion engine generator at Mpunsa Village, Chicualacuala District, Gaza Province in Mozambique.

As far as the anaerobic digestion process is concerned, it is more appropriate to discuss alkalinity and pH together because these parameters are related to each other and very promising to ensure a suitable environment for successful methanogenesis process. Alkalinity is produced in the wastewaters as results of the hydroxides and carbonates of calcium, magnesium, sodium, potassium or ammonia and may also include borates, silicates and phosphates (Tchobanoglous and Burton, 1991). The alkalinity plays an important pH controlling role in the anaerobic treatment process by buffering the acidity derived from the acidogenesis process (Gerardi, 2003; Fannin, 1987).

Methane producing methanogens are known to be strongly affected by pH (Poh and Chong, 2009) and could only survive on a very narrow range of pH (Table 2) (Gerardi, 2003).

Table 2. The optimum pH range for selected methanogens (Gerardi, 2003; Steinhaus et al.2007, Tabatabaei et al., 2011)

As such, the methanogenic activity will be severely affected once the optimum pH range is not met. Steinhaus and coworker studied the optimum growth conditions of Methanosaeta concilii using a portable anaerobic microtank (Steinhaus et al., 2007). They reported an optimum pH level of 7.6 revealing that even little variations on both sides of the optimum pH suppressed the growth of the methanogens. Several studies have also reported reactor failure or underperformance simply due to pH reduction caused by accumulation of high volatile fatty acids in the anaerobic treatment system (Fabian and Gordon, 1999; Poh and Chong, 2009; Tabatabaei et al., 2011).

In a study using synthetic wastewater in the thermophilic temperature, was found that at the pH of above 8.0, the methanogenesis was strongly inhibited and the value recorded for acetotrophic methanogenic test was zero (Visser et al., 1993). When investigating the role of pH in anaerobic degradation test; Fabian and Gordon (1999), found out that the acidification led to the low performance of the anaerobic degradation, however the biodegradation was significantly increased once the wastewater when the pH was adjusted to above 6.5.

Intermeshing is a basic concept in pipeline/ network planning: it provides intrinsic redundancy for gas delivery in case of trouble/break at single points of pipeline. It supports continuous operation and pressure of the system; as it is most important to keep the whole pipeline system under pressure all the time (if the pressure would drop to zero, oxygen could enter the pipeline system exposing some areas or the system to the risk of explosion). Exaggerated use of intermeshing lead to higher investment cost in pipes and decrease a cost- efficient network structure in principle (besides, an item of passionate discussions among planners). Intermeshed networks need, because of their complexity, simulation support to get detailed insight into physical state and variables of the pipeline system.

2.5 Online — and offline-simulation

An advanced feature of simulation or operating mode is online-simulation. Here the simulation is coupled with a SCADA system and is executed corresponding to the cycle of the data acquisition (minutes to hours). It is a good tool to watch and control the network by additional detailed information almost in real time. This type of simulation is very demanding as it requires correct and complete data all the time what must be carefully prepared. Offline-simulation is the normal application it can be executed when needed — at arbitrary time.

Table 9 overviews the results of the three optimizations described before.

It turned out that the profitability of a fermenter on location 2 is lower than on the other locations. It was never preferred in any optimum structure. The other locations have one advantage — the shared usage of biogas pipelines whereas low additional costs for location 1 have to be born. There are never heating pipelines from the different locations to the center considered in the optimum technology networks. Just the biogas is transported; heat is produced centrally and distributed within a district heating network, although additional biomass furnaces are required. In scenario 1 the missing corn silage availability was compensated by a higher amount of intercrops, referring to the CH4 content, and it shows the best revenue, because of higher plant utilization and higher revenue for electricity and heat production. Although in the optimal scenario the amount of corn relating to the total feedstock was not even 17 % of the total (dry matter) the compensation for corn with intercrops results in higher revenue. For more corn that intercrops compensate in the input the impact would be even higher. Therefore it is obvious that intercrops can be a profitable feedstock to run a biogas plant. For the case study the availability of intercrops would have to be raised as described before which would lead to the best technology network for the region.

The system has two limiting factors; on the one hand the distances between the fermenter locations and the feedstock providers accompanying different transport costs and on the other hand the limited resource availability. It could be shown that it is not lucrative to run a central CHP with higher capacity (500 kWel) as feed-in tariffs are lower and less revenue can be gained. Nevertheless, from the point of view of sustainability, it would be preferable to substitute two smaller CHPs with a bigger one. An adaptation of reimbursement schemes to the solutions presented is recommended.

1.1.2.1 Research status of biodegradable film materials

Biodegradable materials research began in the 1960s. The initial study was mainly to add natural polymers with biodegradable properties (such as starch, etc.) to generic plastic, then get the so-called biodegradable materials. St. Lawrence starch-company developed a starch — polyethylene or polypropylene blends in Canada (Qiu Weiyang, 2002). With human understanding of biodegradability of macromolecule, the research focus began to turn to biodegradable materials (Qiao Haijun, 2007), which can be classified as microorganism synthetic polymer, chemical synthetic polymer and natural polymer.

biodegradable film, it includes structural degradation film, biodegradable film containing inorganic salts and adding starch. (3) photo — biodegradable film; (4) plant fibre film.

In China, the major research was additive photo-degradation film and synthetic photo­degradation film. The research focused on using light stabilizers to control degradation period. Since 1997, 944-polymeric efficient light stabilizer, BW-6911 new light stabilizer were developed, which replaced the severe irritation and sensitization GW-504/2002 anti-aging system. American Dupont CO. , Ltd produced copolymers of ethylene and CO, American OCC and DOW CO. , Ltd had used this technology to produce film and develop industrial production (Xiong Hanguo, 2004). The disadvantages of photo-degradation film were susceptible to external environment, which was difficult to control the degradation period, and covering field, the part into soil can not be degraded, so its application was limited (Xu Xiangchun, 2006).

The degradation of biodegradable film was caused by microbes in natural environmental condition. It was divided into additive biodegradable film and completely biodegradable film according to degradation mechanism and damage style.

At present, additive biodegradable film was composed of plastic, starch, compatibility agents, self-oxidants, processing additives. Typical varieties were polyethylene starch biodegradable film (Liu Ming et al., 2008). There were institutes of physics and chemistry, Beijing University of Technology, Guangdong biodegradable plastic CO., Ltd more than 20 research institutes. The research focused adding starch or modified starch into PE.

The main varieties of completely biodegradable film were PLA, PCL, and PHB and so on. United States used PCL to produce synthetic polymer biodegradable film (KAM Abd I J — kader, 2002). Warner-Lambert developed a new type of resin, which was made of 70% amylopectin starch and 30% amylose starch (He Aijun, 2002). It had good biodegradability, was considered a significant development in material science.

Photo-degradation film was made of additive photo-sensitizer, auto-oxidants, and anti­oxidants as microbial culture medium in general polymer.

Plant fibre film has good ventilation, wet and dry strength and good biodegradability. Chinese academy of agricultural sciences successfully developed the environmentally — friendly hemp film (Fu Dengqiang, 2008). In addition, paper films composed of different materials were produced. South China Technology University used sugar cane and starch as materials to manufacture a kind of fully degradable film (Tan Chengrong, 2002). Japan manufactured biodegradable film with 1%-10% chitosan cloth softwood mechanical pulp original paper in 1990. The demand of environmental film increased in Washington State University, France, Germany, Italy, Canada, Netherlands and South Korea and other countries, leading to the environmentally-friendly film industry rapid development (Han Yongjun, 2008).

Crop yield is very important economical parameter of plant production but nowadays the quality of foods is becoming more and more important. Digestate treatment seems to be very effective to increase the protein content of plants. Banik and Nandi (2004) investigated biogas residual slurry manures (solid digestate) used as supplement with rice straw for preparation of mushroom beds. The application of biomanure increased the protein content of mushroom 38.3-57.0%, while the carbohydrate concentrations were decreased. Results can be seen in Table 10.

respectively. Changes in amino acid composition of test plants were also very favourable, because almost every essential and non-essential amino acid quantity was increased significantly after digestate treatment. In line with these results the oil content of the treated plants decreased significantly.

Qi et al (2005) examined the effect of fermented waste as organic manure in cucumber and tomato production in North China. Before the vegetables transplantation, the diluted fermented residual dreg was applied 20-30 cm below the soil surface at a rate of 37,500 kg ha-1, while liquid digestate was sprinkled to the soil surface in three vegetables growing stages and on the vegetable leaves once time. They found increasing yield (18.4% and 17.8%) and vitamin C content (16.6% and 21.5%) of treated cucumber and tomato, respectively.

As the results show, the digestate application in solid or liquid form could result significant improvement in the quality of foods without damaging the environment, which is very important for the sustainable environment and healthy life.

6.1 Computer-aided design

A software was developed by Samer (2010) to plan and design biogas plants, specify the dimensions of the different tanks (raw slurry tank, liquid organic matter tank, digester tank, secondary digester tank, and residue storage tank), and compute the amounts of construction materials (iron rods, cement, sand, and gravel) required to build the concrete constructions. Furthermore, the software is able to calculate the capital investment and the fixed costs, the variable costs, and the total costs. Figure 18 shows the user interfaces of the input and output data windows.

(b) User interface for input data of digester tank

The Herschel Bulkley model is applied on fluids with a non linear behaviour and yield stress. It is considered as a precise model since its equation has three adjustable parameters, providing data (Pevere & Guibaud, 2006). The Herschel Bulkley model is expressed in equation 5, where to represents the yield stress.

T = t0 + К * y" (5)

The consistency index parameter (К) gives an idea of the viscosity of the fluid. However, to be able to compare К-values for different fluids they should have similar flow behaviour index (n). When the flow behaviour index is close to 1 the fluid’s behaviour tends to pass from a shear thinning to a shear thickening fluid. When n is above 1, the fluid acts as a shear thickening fluid. According to Seyssiecq and Ferasse (2003) equation 5 gives fluid behaviour information as follows:

To = 0 & n = 1 ^ Newtonian behaviour To > 0 & n = 1 ^ Bingham plastic behaviour T0 = 0 & n < 1 ^ Pseudoplastic behaviour T0 = 0 & n > 1 ^ Dilatant behaviour

1.3.1 Ostwald model

The Ostwald model (Eq. 6), also known as the Power Law model, is applied to shear thinning fluids which do not present a yield stress (Pevere et al., 2006). The n-value in equation 6 gives fluid behaviour information according to:

T = К * y(n_1) (6)

n < 1 ^ Pseudoplastic behaviour n = 1 ^ Newtonian behaviour n > 1 ^ Dilatant behaviour

1.3.2 Bingham model

The Bingham model (Eq. 7) describes the flow curve of a material with a yield stress and a constant viscosity at stresses above the yield stress (i. e. a pseudo-Newtonian fluid behaviour; Seyssiecq & Ferasse, 2003). The yield stress (t0) is the shear stress (t) at shear rate (y) zero and the viscosity (л) is the slope of the curve at stresses above the yield stress.

t = T0 + л * y (7)

T0 = 0 ^ Newtonian behaviour T0 > 1 ^ Bingham plastic behaviour

A lava rock biofilter was used to evaluate the degradation of H2S from the AD gas stream. The experimental setup for the biofilter used in this study was previously described (Ramirez-Saenz et al 2009). The gas stream was humidified and fed in the top of the biofilter using a mass flow controller. Sample ports were located in the output and input of the gas stream. For H2S degradation experiments, the biofiltration system was fed at the top with an air-diluted gas stream originated from the ADR, as previously reported (Ramirez-Saenz et al., 2009). Periodic water additions (once a week) were used to control moisture loss and to avoid SO4-2 accumulation. Recirculation was provided at a flux of 0.5 L/min over 1 h. All experiments were conducted at room temperature (20-25°C).

The factors affecting the production of biogas are mainly based on the operating conditions of the digester, such as pH and temperature which influence directly the micro-organisms. The perturbations in effluent (including the concentration of substrate and its composition in toxic compounds and inhibitors) can also affect the volume and the quality of the produced biogas. Sometimes, the toxic compounds are not present at the beginning in the effluent waste, but they are produced inside the digester starting from degradation of substrate (example: VFA and ammonia).

2.1 Substrate

The type and the composition of the substrate determine directly the quality of the produced biogas. In anaerobic process the substrate is often measured in term of chemical oxygen demand (COD) or of total volatile solids (TVS). It is significant to distinguish between the degradable and the inert fraction, because a considerable fraction of the COD in effluent is inert (Nielsen, 2006). The waste which contains a high percentage of water has a weak methane yield by COD or VS.

Organic waste contains various compounds: mainly saccharides (which are divided into two fractions, easily and slowly degradables), lipids (easily degradable), proteins (easily degradable), VFA (easily degradable), as well as others compounds (Moosbrugger & al., 1993). The production of methane is generally in the range from 100 to 400 L CH4 / kg VS.

On the other hand, the majority of organic waste contain a high fraction of the substrate easily degradable, which gives a high production of methane and VFA. It is thus significant to control the organic and hydraulic loading according to the capacity of the digester when the process functions are at low charge that gives also a low production rate of biogas. Although this can prevent the rupture of the process, it is not very ecomical because the capacity of the process is not completly used. The increase in the charge gives more biogas but also there is the risk of the overload, with, as a consequence, the accumulation of the VFA. The high concentration of VFA decreases the pH and making them more toxic for methanogens bacteria.

with others like sulphur, the phosphorus, the potassium, the calcium, the magnesium and the iron which are required (McMahon & al., 2001). The majority of the nutriments can be inhibiting if they are present at high concentrations.