4.1 Facility layout

The anaerobic digestion can be accomplished in one digester, and thus the facility is called ‘single-stage biogas facility’. In other facility layouts, the anaerobic digestion can be carried out in two stages, i. e. in two different tanks in order to optimize the operating conditions, and thus the facility is called ‘two-stage biogas facility’. The single-stage facility is a simple design with a longer track record, and has lower capital costs and technical problems. The two-stage facility has shorter retention time as each stage design is optimized. There is a potentially higher biogas production from two-stage facilities, but they require higher capital costs.

Subsequent to the site investigations such as the soil specifications ground water level, the facility layout should be planned. The commercial biogas plants consists of a fermenter and a secondary fermenter or so called "follow-up fermenter", where both have identical dimensions, usually as follows: height of 6 m, internal diameter of 23 m, and external diameter of 23.70 m. This implies that the thickness of the fermenter wall is usually 35 cm. A residue storage tank is annexed to the fermenters, where the tank has an internal diameter of 25 m, external diameter of 25.70 m and a height of 6 m (Fig. 4). The solids feeder is located adjacent to the fermenter, and the tanks are surrounded by green belt from all sides except one side where the horizontal silos are located.

First Stage Second Stage

(b) General process scheme of the two-stage anaerobic digestion process (Blumensaat and Keller, 2005)

(c) BIOGAS NORD GmbH

4.2 Dimensions marking

The marking of dimensions (Fig. 5) for biodigester unit should be performed prior to start of excavation work. The marking is considered as preparation for excavation and construction works. An operational area of 3 m width around the digester should be considered, where the workers will use this area to achieve the construction works around the tank base in order to prepare the structure of the concrete base, i. e. the bottom of the digester.

4.3 Excavation works

The depth of digging depends on the specifications of the soil. The inclination of the sides should be 30 cm for each meter depth for the cohesive soil, 60 cm to one meter for the light soil, and 90 cm for the sandy soil.

The bottom of the pit should be concave, where the center of the digester should be the most concave. Generally, the pit shape depends on the design of the digester, where in the case of round-shaped household units (Figs. 6a and b) a guide wood post is installed in the center of pit bottom. A string is linked to the post and used to set the round-shape of the pit. On the other hand, in the case of commercial biogas plants (Fig. 7), total station, teodolit or laser leveling is used for surveying. For large digesters, i. e. for commercial biogas plants, bulldozers are used to achieve the excavation (Fig. 7).

4.4 Preparation of the digester’s bottom

The pit’s bottom should be cleaned, and the gridiron is built (Fig. 8) using a pre-selected type of iron rods as either 606 m-1 or 608 m-1. Subsequently, the concrete mixture is poured (Fig. 9 and 10). The water:cement ratio is 0.53 L kg-1 and the cement:sand:gravel mass ratio is 1:2.2:3.7. The thickness of the concrete base ranges between 10-25 cm depending on the soil’s specifications and the ground water level.

In case of commercial biogas plants, the digester is huge as its diameter may reach 25 m; therefore, the concrete structure should be reinforced (Fig. 11). Hence, the iron rods are used to build 2 iron grids to reinforce the digester wall starting from the digester bottom plate. The standard length of iron rods is 12 m. The standard iron rods are cut to shorter iron rods, and they are then used to build up the tank. Subsequently, either wood panels or pre­constructed metal sheets are used to enclose the iron grids and to form a container for the fluid concrete. When the digester wall is built, about one third of the internal wall of the tank is covered by a protection layer in order to protect the internal face of the wall against corrosion.

In case of household units, burnt-clay bricks are used to build the digester (Fig. 12) and they should be able to tolerate a pressure up to 100 kg cm-2 owing to the fact that the walls of the digester are exposed to the pressures of the soil and the moving equipment near to the digester. A mortar of cement and sand mixture by 1:4 is used. The construction works are followed up till the appropriate height, and the entry or exit holes of the pipes are blocked by a filling material.

Building the digester is associated with integrating the heating tubes. Building the wall starts with structuring the iron grids which will be encased by wood panels or pre­constructed metal sheets, and before pouring the concrete, the heating tubes should be integrated (Figs. 11 and 13). The heating tubes are made of polyvinyl chloride (PVC), inside these tubes hot water flows to heat the digester. The water temperature is either 35 oC or 55 oC depending on the used bacteria as either mesophilic of thermophilic bacteria, respectively. On the other hand, in other designs the heating tubes are installed on the internal surface area of the digester wall.

The static yield stress (ts) is the yield stress measured in an undisturbed fluid while dynamic yield stress is the shear stress a fluid must be exposed to in order to become liquid and start flowing. The fact that both dynamic yield stress and static yield stress sometimes may appear is explained by the existence of two different structures of a fluid. One structure is not receptive to the shear stress and tolerates the dynamic yield stress, while a second structure (a weak gel structure) is built up after the fluid has been resting a certain period of time (Yang et al., 2009). When these two structures merge, a greater resistance to flow is generated translated to the static yield stress.

The formation of the weak gel structure may be a result from chemical interactions among polysaccharides or between proteins and polysaccharides (Yang et al., 2009). The weak gel structure is quite vulnerable and, thus easily interrupted by increasing shear rates.

1.2 Non-Newtonian fluids

Non-Newtonian fluids do not show a linear relationship between shear stress and shear rate. This is due to the complex structure and deformation effects exhibited by the materials involved in such fluids. The non-Newtonian fluids are however diverse and can be characterised as e. g. pseudoplastic, viscoplastic, dilatant and thixotropic fluids (Schramm, 2000).

Methanogenic anaerobic digestion is a classical anaerobic bioconversion process that has been practiced for over a century and used in full-scale facilities worldwide. This is a complicated process that involves a mixture of population of microorganisms and several gasses and liquid products, thus strict process control and product purification are required. Biogas production have been demonstrated in numerous studies with great success like can see in the Table 6 (Gavrilescu, 2005).

3. Biogas production from agro-industrial wastewaters

3.1 Case vinasses of tequila

Tequila is a Mexican regional alcoholic beverage obtained from the fermentation of sugars from the cooked stems of blue agave (Agave tequilana Weber var. azul). Its production and

commercialization is verified and certified by the Mexican Tequila Regulatory Council (CRT) (NOM-006-SCFI-2005, 2006). In 2008 the CRT registered 139 producers and 1,018 brands of Tequila (bottled in Mexico and in foreign countries, CRT 2008). Based on the number of employees, only 7% are large factories and the rest are small and medium factories, with a grand total of around 30,500 direct employees (National Tequila Industry Chamber, CNIT 2009). Therefore, this industry represents an important economic activity for the 180 Mexican municipalities within the appellation d’origine controlee granted in 1995 for Tequila.

Tequila production has had an important increase from 2004 to 2008, as it is shown in Fig. 3. In 2010 about 187.3 million liters of Tequila (55% Alc. Vol.) has been produced with a projection for annual growth of at least 10% (CNIT 2010); there is also a decrease in production of Tequila between 2000 and 2003, due to the agave crisis (Dalton 2005). Although exhaustive reviews regarding the treatment of different distillery wastewaters are published elsewhere (Satyawali and Balakrishnan 2008; Mohana et al. 2009), it is considered that special attention should be paid to distillery effluents from the Tequila industry due to their complex composition. This section present the potential generation of energy from wastewater treatments to generate biogas from the Tequila industry.

rSb d djb <& А Л A8 rh flo A fib rib ilb

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Year

Fig. 3. Dynamics of Tequila production (55% Alc. Vol.). (calculated from CNIT 2010)

The production of Tequila generates large quantities of bagasse and vinasses. Bagasse is a residual solid; it is generated in the elaboration of Tequila and is produced during the extraction of juice from the cooked heads of agave. Vinasses are the liquid residues that are generated and remain in the bottom of the still after the distillation of the must of fermented agave.

liter of Tequila produced, 1.4 kg of bagasse and 10-12 L of vinasses are generated. Under this basis of calculation, it is estimated that the production of Tequila in 2010 generated 262.2 million kilograms of bagasse and 1,873.0 million liters of vinasses.

In the majority of the Tequila factories, bagasse is converted into compost, which is also done in the agave plantations. However, approximately 80% of the vinasses are discharged directly into water bodies (rivers, streams, lakes, reservoirs) and municipal sewer systems or directly onto the soil without receiving adequate treatment for discharge. This common practice causes a deterioration of different degrees to the water bodies receiving the discharges due to low pH, high temperature and elevated concentrations of both BOD and COD of these effluents. On the contrary, if the vinasses receive appropriate treatment and management, they can be used as a source of nutrients and organic matter in agricultural activities; they can also be a potential source of renewable energy. A summary of the physicochemical characteristics of the vinasses generated from the process of producing traditional Tequila (100% agave) is shown in Table 7 (Lopez-Lopez, 2010).

The anaerobic biological process has been utilized for treating Tequila vinasses on laboratory, pilot and industrial scales due to technical and economical advantages over aerobic processes (Linerio and Guzman 2004; Mendez, et al. 2009). On a laboratory scale, Lopez-Lopez and coworkers (2011), Mendez and coworkers (2009) showed an anaerobic digester capable of removing 90-95% of organic material as COD; generating significant amounts of biogas rich in methane. The most common system found at an industrial level in treating Tequila vinasses is of anaerobic type. Fig.4 shows the amount of energy that can be generated if the entire volume of vinasses is treated.

The acidogenic step consists of a degradation of produced components from the hydrolysis step, by the action of acidogenic and fermentative bacteria. It leads to the formation of a mixture of: organic acids, volatile fatty acid (VFA), alcohols, hydrogen, carbon dioxide, ammonium, etc.

Examples of the various products of the fermentation of glucose are shown in the following Table 1:

The dominant route depends on several factors such as the concentration in substrate, pH and hydrogen concentration (Balk et al., 2002). Under a very high organic load, the lactic acid production becomes significant (Mattiasson, 2004). With low pH (lower than 5) the production of ethanol is high, whereas with lower pH (lower than 4) there is a strong production of the volatile fatty acids (VFA) (Ren & al., 1997).

However, the partial hydrogen pressure has a great influence on the fermentation route where a low value encourages the fermentation to acetate and hydrogen is favoured (Thauer, 1977).

A solution of dried permeate from UF whey permeate from the Dairy Plant in Wolsztyn, Poland, was used as a substrate in this study. The solution was prepared by dissolving dried permeate in distilled warm water to obtain 50 g L-1 lactose concentration in wastewater, while the initial COD was 56 g L-1.

Fermentation process was carried out in three UASB reactors with an active volume of 5 L. There were the gas-liquid-solid (G-L-S) separators on the top of each reactor. Whey permeate solution was pumped continuously to the bottom part of the reaction tank by means of the peristaltic pump. The necessary mixing was achieved through the upward wastewater flow and a stirrer operated at 40 rpm. The reactors were water-jacketed and operated at a constant temperature of 25°C ± 1°C. The pH of mixed liquid in the reactors was controlled automatically at pH 4.76 — 4.86 with 2 M NaOH.

For start-up of continuous culture, 1 L of the beads culture medium were grown at 25°C for 24 h in a 2 L Erlenmeyer flask filled with 0.5 L of UF whey permeate after heat sterilization (120°C, 20 min). The concentration of lactose in whey permeate was 50 g L-1. Mixing was achieved by stirring with a magnetic stirrer at 200 rpm. The cell suspension was then aseptically transferred to each UASB reactor which was kept in batch operation for 24 h before switching on the continuous feeding. The reactors were operated at the HRTs of 12, 24 and 48 h. At each HRT the reactors were operated till they had reached the steady-state (the steady-state conditions were evidenced when the standard deviations of the ethanol concentrations and lactose concentrations in the effluent distillate were within 3%).

1.1.2.2 Analytical methods

Lactose concentrations and ethanol concentrations in the effluent distillate were determined according to Standard Methods (PN-67/A-86430; PN-A-79528-3:2007). The biomass concentration of yeast (dry matter) was calculated according to Standard Methods (P-78/C — 04541). The samples were analyzed in triplicate and results were reproducible within 3% standard deviation.

1.2.4 Effect of beating degree and bauxite on dry tensile strength

Fig.3-3 showed the effect of beating degree and bauxite on dry tensile strength when other factors were held at 0 level. With the increase of beating degree and bauxite, dry tensile strength firstly increased and then slowly decreased, the maximum occurred when the two factors were held at 0 level. The reason was that with the beating degree increased, the fibre sub-wire broom degree was high, the exposure of hydrogen bonding of the fibre surface increased, the bonding forces between the fibres enhanced, so that the film strength increased; when beating degree was more than a certain value, the single fibre strength was destroyed, the bonding force decreased, led to the decrease of strength, adding bauxite excessively to strength had side effect, thus resulting in strength decreased.

The source of energy varied from one category to another across the three wealth ranks. Nevertheless, fuelwood dominated energy sources in all the three categories, where over 77% of the respondents were using fuelwood for cooking (Table 4), followed by biogas, very few of the respondents were using charcoal. No-one was using electricity for cooking.

The respondents were asked whether they would like to have a biogas facility in their homes or not, and almost all (96%) said yes, they are willing to install biogas facilities.

Source: Survey data 2006

Table 4. Wealth Categories and Sources of Energy for Cooking (%)

Frank Burmeister, Janina Senner and Eren Tali

Gaswarme — Institut e. V. Essen, Germany

1. Introduction

In the following sections, recommendations supported by schematics are given for the injection of compliant processed biogas into natural gas grids. Based on the characteristics of the natural gases distributed in Germany and taking into account the applicable

• Laws, technical rules and regulations

• Billing procedures

• and the physical and technical conditions to be taken into account solutions for each individual supply case are given.

In order to feed biogas into a natural gas grid, unwanted components need to be removed from the gas and the burning properties of the gas need to be adjusted to those of the rest of the gas in the grid. In this way, the correct operation of the gas-burning appliances and the accuracy of the billing of retail customers is assured.

The purified biogas is conditioned depending upon the properties of the base gas (Fig.1). In the case of L gas, the calorific value or Wobbe index is realised by adding air, or air and LPG. In the case of H gas characteristics, the addition of LPG is required to adjust the calorific value to that of the usually higher calorific H base gas.

Schematic recommendations include the answers to the key questions listed below and allow a simple "read out" of the target properties, taking into account the current regulatory requirements. Gas utilities (GU) and operators can already in the planning phase determine the feed options and requirements with the help of the graphs. The key questions are:

1. What qualities and technical characteristics of combustion must processed biogas have at the very least so that it can be fed into grids in which the natural gases typical in Germany are present as base gases, without having to make changes to the grid?

2. What additional aspects need to be considered when feeding processed biogas into the existing natural gas grid, taking into account fairness in the billing process, properties (functionality of end-user equipment) and cost effectiveness?

The following conditions are to be observed in addition to point 1 : Engine applications, natural gas filling stations (methane, K number, condensation of higher hydrocarbons) and industrial customers.

2. Basic concepts and regulations

2.1 Characteristics of the base gas

The term base gas refers to the natural gas provided by the gas utilities (GUs) in the respective coverage areas without the addition of biogenic gas. The classification of the various natural gases distributed in Germany into H and L gases is made according to worksheet DVGW-G 260 (German Association of Gas and Water [DVGW], 2008). The Wobbe Index, which is a measure of the thermal energy released on the burner of a gas appliance or the energy transported through a pipe has a special significance here.

The Wobbe Index is an important variable to assess the interchangeability of fuel gases. When replacing one fuel gas with another, the output of the burner changes in proportion to the ratio of the Wobbe index. Its definition according G 260 is given in equation 1:

WSn = —SdL 1 d = ^Gas’n With the relative density and the calorific value HS n (1)

Phuft, n

The upper value of the Wobbe index total range should not be exceeded. A shortfall in the lower value is acceptable under certain conditions and subject to a time limit. Both limits are specified in the German regulations. The nominal value listed in G 260 is used for setting the gas appliances used. Technically, the local variation range could be omitted, since the gas appliances are set to a nominal value. Currently however there are still many appliances set to differing values. The major boundary conditions are dictated by the Wobbe Index total range, the calorific range and the relative density, since conditioning with air and / or liquefied gas influences precisely these variables.

For the calorific value of a gas mixture, equation 2 states:

—,n = Zr—ni or Pn = ZriPn, i [

1 Indexing "S" (superior) is the formation with calorific value and "n" standard conditions

2 ri denotes the volume fraction of component i

When considering equation 2 and table 1, it is clear that even small volumes of higher hydrocarbons affect the parameters of combustion of the gas mixture, due to the greater density and calorific values. The same applies to air and carbon dioxide.

In addition to the basic requirements for the gas properties, limits for accompanying substances are specified in worksheet G 260, which may not be exceeded.

Important boundary conditions are determined by the dew points, the oxygen content and the sulphur content. Information on the dew points is formulated so that condensation can be excluded. As far as the oxygen content is concerned, the grids in Germany can be regarded as dry and therefore the limit of 3 vol -% is to be applied. It should be noted at this point that at the long-range transport level significantly lower O2contents, usually in the low ppm range are to be observed (EASEE gas, CPB European Association for the Streamlining of Energy Exchange-gas Common Business Practice, 2005) for cross-border distribution (H gas).

The raw gas must be cleaned, processed (according to G 260) and compressed to the pressure of the grid operator. Under no circumstances should health risks arise from processed gas. For injection into the distribution network of a local GU, the gas must be

odorized according to G 280-1(DVGW, 2004). In addition, the presence of certain gas accompanying substances such as H2S must be monitored regularly. Furthermore, for a time and heat equivalent transfer, the calorific value for billing purposes must also be known.

After processing the raw gases for the public gas supply, these can be used according to G 260 as an exchange gas (G 260 Section 4.4.2) or as additional gas (G 260 4.2, 4.3), (gas for conditioning) and be made available to the grid operator at the transfer interface. (Note: it should be noted that additional gas feeds are only possible in a single pipeline under certain circumstances.)

Put simply, it can be said that the conditions given in worksheets G 260 and G 262 (DVGW, 2007) ensure that the customers’ appliances will work correctly. Sensitive industrial processes sometimes require much tighter limits on the gas properties (e. g. glass and ceramics production). The DVGW worksheet G 260 is very often a component of supply contracts and is an expression of the flexibility of the gas sector, which is necessary in the procurement of natural gas, in order to deliver natural gas from various gas fields into the transport and distribution system of the German gas industry and on to the customer. Due to its geographical location, historical and political development, in Germany natural gases from the most diverse of foreign origins as well as natural gases from its own sources are thus forwarded to the customers, with the guarantee of security of supply, functionality of the natural gas applications and fair billing.

In summary, in order to inject biogas into the natural gas grid, the above requirements must be met. In addition to excluding the gas accompanying substances by cleaning and processing the biogas, further conditioning to adjust the Wobbe Index and the calorific value to the target grid is required, depending upon the case in point.

The processed, conditioned biogas is considered to be an exchange gas, if it meets the requirements set out in G 260, G 262 and G 685. Furthermore, during conditioning with liquid gas, limits according to G 486 (DVGW, 1992) need also to be considered.

Biogas technology is viewed as one of the renewable technologies in Africa that can help eases its energy and environmental problems. To date, some digesters have been installed in several sub-Saharan countries, utilising a variety of waste such as from slaughterhouses, municipal wastes, industrial waste, animal dung and human excreta. Small-scale biogas plants are located all over the continent but very few of them are operational. In most African countries, for example, Burundi, Ivory Coast, and Tanzania, biogas is produced through anaerobic digestion of human and animal excreta using the Chinese fixed-dome digester and the Indian floating-cover biogas digester, which are not reliable and have poor performance in most cases (Omer and Fadalla, 2003). These plants were built for schools, health clinics and mission hospitals and small-scale farmers, in most cases by non­governmental organisations. In Africa the interest in biogas technology has been further stimulated by the promotional efforts of various international organisations and foreign aid agencies through their publications, meetings and visits. Most of the plants have only operated for a short period due to poor technical quality. Table 3 gives a list of the African countries with biogas production units as at 2007. There is thus a need to introduce more efficient reactors to improve both the biogas yields and the reputation of the technology. The development of large-scale anaerobic digestion technology in Africa is still embryonic, but with a lot of potentials.

Some of the first biogas digesters were set up in Africa in the 1950s in South Africa and Kenya. In other countries such as in Tanzania, biogas digesters were first introduced in 1975 and in others even more recently (South Sudan in 2001). To date, biogas digesters have been installed in several sub-Saharan countries including Burundi, Botswana, Burkina Faso, Cote d’Ivoire, Ethiopia, Ghana, Guinea, Lesotho, Namibia, Nigeria, Rwanda, Zimbabwe, South Africa and Uganda (Winrock International, 2007). Biogas digesters have utilized a variety of inputs such as waste from slaughterhouses, waste in urban landfill sites, industrial waste (such as bagasse from sugar factories), water hyacinth plants, animal dung and human excreta. Biogas digesters have been installed in various places including commercial farms (such as in chicken and dairy farms in Burundi), a public latrine block (in Kibera, Kenya), prisons in Rwanda, and health clinics and mission hospitals (in Tanzania) (Winrock International, 2007). However, by far the most widely attempted model is the household biogas digester — largely using domestic animal excreta (Table 2). This is due to the fact that this technology is closely linked to poverty alleviation and rural development. The biogas produced from these household-level systems has been used mostly for cooking, with some use for lighting.

Global experience shows that biogas technology is a simple and readily usable technology that does not require overtly sophisticated capacity to construct and manage. It has also been recognized as a simple, adaptable and locally acceptable technology for Africa (Gunnerson and Stuckey, 1986; Taleghani and Kia, 2005). There are some cases of successful biogas intervention in Africa, which demonstrate the effectiveness of the technology and its relevance for the region. The lessons learned from biogas experiences in Africa suggest that having a realistic and modest initial introductory phase for Biogas intervention; taking into account the convenience factors in terms of plant operation and functionality; identifying the optimum plant size and subsidy level; and; having provision for design adaptation are key factors for successful biogas implementation in Africa (Biogas for better life, 2007). Biogas technology has multiple beneficial effects.

Anaerobic digestion (AD) is an attractive treatment for this waste of difficult disposal. AD processes transform the organic matter contained in a certain waste in biogas as main product. This process is carried out for different kind of microorganisms which work in a coordinate and interdependent chain until biogas obtaining.

Anaerobic treatment of moderate and high strength wastes with high biodegradable content presents a number of advantages in comparison to the classical aerobic processes: a) quite a high degree of purification with high-organic load feeds can be achieved; b) low nutrient requirements are necessary; c) small quantities of excess sludge are usually produced and finally, d) a combustible biogas is generated. The production of biogas enables the process to generate or recover energy instead of just energy-saving; this can reduce operational costs as compared with other processes such as physical, physico-chemical or biological aerobic treatments (Borja et al., 2006).

Previous works carried out at pilot-scale have shown that most of agro-industrial residues, such as sugar beet pulp, potato pulp, potato thick stillage and brewer’s grains, can be treated anaerobically with an efficient solids stabilisation and energy recovery, if the applied process-type (one or two stages) is selected according to the C:N ratio of the residues. These works demonstrated that at hydraulic retention times (HRT) of between 10 and 20 days, normally, the 50-60% of the organic matter was degraded. The ultimate anaerobic biodegradability was higher and lied between 76% (brewer’s grain) and 88% (potato pulp), which demonstrated that more than 60% of the available energy potential could be used in the industrial processes. The gas production varied between 300 and 500 m3 biogas per ton of dry matter with a methane content of 60-70%. The undigested solids, which were separated from the effluent of the reactors could be completely stabilised after a short aerobic post-treatment to be used as a soil conditioner (Borja et al., 2006).

A number of kinetic models have been proposed for the process of anaerobic digestion. Early models were based on a single-culture system and used the Monod equation or variations. More recently, several dynamic simulation models have been developed based on a continuous multi-culture system; these correspond to the major bioconversion steps in anaerobic digestion but again make the assumption that culture growth obeys Monod type kinetics. Doubt has been expressed by several investigators on the validity of applying the Monod equation to waste treatment as the specific growth rate is expressed only as a function of the concentration of the limiting substrate in the reactor. The Monod equation contains no term relating to input substrate concentration; this implies that the effluent substrate concentration is independent of the input concentration. Experimental results do not always agree with this implication; for example the anaerobic digestion of dairy manure, beef cattle manure at mesophilic and thermophilic temperatures, rice straw or poultry litter (Borja et al., 2003).

Deviation from the Monod relationship in many digestion systems may be due to their complexity. This complexity has necessitated the use of generalized measures of feed and effluent strength, namely total Chemical Oxygen Demand (COD) and volatile solids (VS), which may not truly reflect the nature of the growth-limiting substrate. Utilizable carbon in the digester is derived from the hydrolysis of polymeric compounds, constituting the waste, by exo-enzymes in the extracellular medium or on the surface / vicinity of the microorganisms: only these hydrolysed, assimilable compounds can be considered as the growth-limiting substrate in terms of the Monod relationship. Extra-cellular hydrolysis is often considered the rate-limiting step in anaerobic digestion of organic wastes (Borja et al.,

2003) and for a model to be truly valid this must be taken into account.

Multi-culture system kinetics may be desirable in view of the heterogeneous nature of the microbial population performing the various bioconversion steps involved. However, the kinetic models based on this premise necessarily involve a number of kinetic equations and coefficients making them highly complex, as shown by the reported models (Borja et al., 2003). Complexity does not necessarily equate to accuracy and there is still a strong case in favour of a simpler kinetic treatment based on a single culture system. Methanogenesis is particularly suited to this approach as there is a strong holistic characteristic in the process. Various cultures and bioconversion steps in digestion are interdependent and the whole process has certain self-regulatory characteristics within the process limits.

Kincannon and Stover (1982) proposed a widely used mathematical model to determine the kinetic constants for immobilized systems and high-rate reactors. In this model the substrate utilization rate is expressed as a function of the organic loading rate by monomolecular kinetics for biofilm reactors such as rotating biological contactors and biological filters (Kapdan and Erten, 2007). A special feature of the modified Stover-Kincannon model is the utilization of the concept of organic loading rate as the major parameter to describe the kinetics of an anaerobic filter in terms of organic matter removal and methane production (Buyukkamaci and Filibeli, 2002; Kapdan and Erten, 2007).

The modified Stover-Kincannon model allows to calculate the maximum substrate utilization rate by the microorganisms (Rmax) and the saturation constant (Kb) in anaerobic digestion processes (Yu et al., 1998). Therefore, this model allows determining the effluent substrate concentration for a known volume of reactor and an initial concentration of the substrate. The modified Stover-Kincannon model has been used for different substrates and reactor configurations: anaerobic hybrid reactors treating petrochemical waste (Jafarzadeh et al., 2009), anaerobic treatment of synthetic saline wastewater by Halanaerobium lacusrosei (Kapdan and Erten, 2007), anaerobic digestion of soybean wastewaters (Yu et al., 1998) and molasses (Buyukkamaci and Filibeli, 2002) in a filter and in a hybrid reactor, respectively.

The aim of the present study was focused on the AD of two-phase OMSW at two different influent substrate concentrations and on the determination of kinetics constants of the system using the above-mentioned modified Stover-Kincannon model.