In order to enhance the performance of the biogas generation process, and to prevent process failure, certain operating parameters such as temperature, pH, nutrient addition, mixing ratio, and retention period, all need to be controlled. Microorganisms are highly sensitive to pH changes. Buffering is necessary for pH control and therefore an essential step in the overall operation (Garba and Sambo, 1992).

Temperature is an important physicochemical factor in the degradation of organic wastes and as such the anaerobic process is dependent on temperature. The temperature has significant effect on biogas production especially when fresh plant material is involved. Two temperature ranges have been reported to affect tile overall process of biogas production. These are; tile mesophilic temperatures and thermophilic temperatures. Tile mesophilic temperature range of 30-40°C has been reported to effectively aid in degradation of organic wastes that are not lignified. Increased biogas production was reported in tile digestion of fresh water weed known as Pistia stratiotes (water lettuce) at mesophilic temperature of 30 °C.). Tile mesophilic temperature range is preferred when fresh plant material is involved (Maishanu, 1992) Also it is easier to maintain the digester at this temperature. Methane-producing bacteria are, very sensitive to sudden thermal changes and therefore any drastic change in temperature should be carefully avoided so that no abrupt decrease in gas production occurs. The digestion process must thus be designed to operate at constant temperature conditions. Temperatures above 65°C cause gas production to stop (Garba and Sambo, 1992).

The anaerobic digestion process can be operated over a pH range of 6.0 — 7.0. As organic acids are produced during the breakdown of cellulose when the pH drops below 7.0, there is a significant inhibition of rnethanogellic bacteria and tile acid conditions of a pH of 4.0 are toxic to these bacteria. At a pH of 4.0, tile production of gas will be very low and later stops (Garba and Sambo, 1992). Several steps such as the introduction of bacteria having cellulolytic capacity, preheating the media material, milling the media material, chemical treatments with NaOH, etc, and drying have been shown to improve biogas yield (Itodoet al., 1998). In the absence of water hyacinth, cow manure can be as an alternative raw material so that the plant will be kept running even after the water hyacinth has been eradicated.

The relationship between the amount of carbon and nitrogen present in organic materials is expressed in terms of the Carbon/Nitrogen (C/N) ratio. A C/N ratio ranging from 20 to 30 is considered optimum for the anaerobic digestion (FAO, 1996). If the C/N ratio is very high the nitrogen will be consumed rapidly by methanogens for meeting their protein requirements and will no longer react on the leftover carbon content of the material. As a result, gas production will be low. On the other hand, if the C/N ratio is very low, nitrogen will be liberated and accumulated in the form of ammonia. Solid concentration in the feed material is also crucial to ensure sufficient gas production as well as easy mixing and handling. The concentration of total solids in the input suspension can be varied within the range of 20 to 100g/liter. In practice, it is recommended to limit the total solids concentration to the range of 20 to 30 g/liter. In Nepal, 6 kg of cow dung per m³ of digester liquid volume is used (FAO, 1996). Cow dung has a solid concentration of about 20% and therefore, it is recommended that dung and water are mixed in a 1:1 ratio to attain the desired level of solids. One kilogram of dung produces about 40 liters of biogas. A family-size biogas plant (two cubic meters) requires 50 kg of dung and an equal amount of water to produce 2000 liters of gas per day. This amount of gas suffices the daily cooking requirement of a family consisting of four to five members.

All waste materials fed to a biogas plant consist of solid substances volatile organic matter and non-volatile matter (fixed solids) and water. During the anaerobic fermentation process, volatile solids undergo digestion and non-volatile solids remain unaffected. According to a finding by The Energy and Resources Institute, fresh cattle waste consists of approximately 20% total solid (TS) and 80% water. TS, in turn, consists of 70% Volatile solids and 30% fixed solids. For optimum gas yield through anaerobic fermentation, normally, 8-10% TS in feed is required. This is achieved by making a slurry of fresh cattle dung in water in the ratio of 1:1. However, if the dung is in dry form, the quantity of water has to be increased accordingly to arrive at the desired consistency of the input (i.e., the ratio could vary from 1:1.25 to even 1:2). If the dung is too diluted, the solid particles will settle down into the digester and if it is too thick, the particles impede the flow of the gas formed at the lower part of the digester. In both cases, gas production will be less than optimum (Anonymous, 1981). It is also necessary to remove inert materials such as stones from the inlet before feeding the slurry into the digester. Otherwise, the effective volume of the digester will decrease.

Bacteria take up the available substrates in dissolved form. Therefore, biogas production and the water content of the initial material are interdependent. Rilling (2005) reported that when the water content is below 20% by weight, hardly any biogas is produced. Optimum moisture content has to be maintained in the digester and the water content should be kept in the range of 60-95 %. Anaerobic digestion of organics will proceed best if the input material consists of roughly 8 % solids. For domestic digesters, TS content should not be too high, otherwise, the substrate would not slide easily through the inlet of the digester and if toxins are present, such as ammonium in high concentrations, high TS is likely to affect bacteria more than when the substrate is diluted. Alternatively, TS content should not be too low, otherwise, the feedstock is very dilute, and a large digester volume is required. Water content is one of the very important parameters affecting AD of solid wastes. There are two main reasons viz.; (a) water makes possible the movement and growth of bacteria by facilitating the dissolution and transport of nutrients and (b) water reduces the limitation of mass transfer of non-homogenous or particulate substrates.

A number of studies have cited the inhibitory effects of free ammonia (NH₃) on the metabolism of methanogens. As ammonia is added to a digester, the pH increases until a chemical equilibrium is reached. However, as ammonia inhibits methanogen metabolism, VFAs accumulate, resulting in a lower pH and a lower concentration of free ammonia. Sterling et al., (2001) concluded that total biogas production was unaffected by small increases in ammonia nitrogen while higher increases reduce biogas production to 50% of the original rate. However, the underlying reason for this effect is still unknown. It was also found that the free ammonia concentration not only affects the acetate-utilizing bacteria, but also the hydrolysis and acidification processes. By controlling the pH value lower it is possible to control the production of ammonia but it lowers the yield of biogas production.

To start up a new anaerobic process, it is critical to use inoculums of microorganisms to commence the fermentation process. The common seeding materials include digested sludge from a running biogas plant or material from sewage. Digested sludge is the best inoculum source for anaerobic thermophilic digestion of the treatment of an organic fraction of municipal solid waste at dry conditions (30% TS). Inoculums caused biogas production rate and efficiency to increase more than two times as compared to a substrate without inoculums. The addition of fresh cow dung to the batch reactor as part of the starter improves biogas production. When poultry litter was used as inoculum for an anaerobic process methane yield was best on a digester with a content of 75% of inoculum (Jagadish et al., 2012).

The production of biogas is also affected by the particle size of the substrate. Too big a particle size is problematic for microbes to digest and it can also result in blockage in the digester, whereas small particle size gives a large surface area for substrate adsorption and thus allows the increased microbial activity followed by an increase in the production of gas. A large amount of biogas was obtained from the grounded water hyacinth than chopped water hyacinth. Degradation of the substrate and biogas production potential of the water hyacinth could be significantly increased by pre-treatment such as a reduction of particle size. These results suggest that the reduction of the particle size of the substrate in conjunction with the optimized microbial growth could improve the methane yields in anaerobic digestion processes (Yasini and Isack, 2016).

The close contact between microorganisms and the substrate material is important for an efficient digestion process. The agitation of the digester contents has a number of benefits, one of the most obvious beings that it helps to mix up material, evening out any localized concentrations, thus also helping to stop the formation of “dead zones” or scum. In addition, it increases the waste‘s availability to the bacteria, helps remove and disperse metabolic products, and also acts to ensure a more uniform temperature within the digester. There have been some suggestions that efficient mixing enhances methane production, but the evidence is inconclusive, so it seems likely that this may only be of noticeable benefit for some systems or operational regimes. Mixing also promotes heat transfer, particle size reduction as digestion progresses, and release of produced gas from the digester contents. There is a significant stirring effect on the anaerobic digestion only when seed sludge from a biogas plant was used as a starter. In this case, the experiments without stirring yielded, without starter, only about 50% of the expected biogas for the investigated substrates.

Total solids mean the number of solid particles in the unit volume of the slurry and they are usually expressed in the percentage form (FAO/CMS, (1996). The percentage of total solid should be between 5% and 12% while other sources reported that the best biogas production occurs when total solid is ranged from 7% to 10% because of avoiding solids settling down or impeding the flow of gas formed at the lower part of the digester. Therefore; dilution of organic substrate or wastes with water to achieve the desirable total solids percentage is required. The total solid concentration of 99 g TS/l produces the maximum biogas yield as per the experiment done with different concentrations of total solid in an anaerobic digestion process (Sajeena, 2013).

It is quite natural that some amount of oxygen can reach anaerobic digesters unintentionally as the reactors are operated within an aerobic open environment, especially through interactions with the surroundings such as by feeding and mixing. Most anaerobic digesters are therefore subjected to a minute and varying aerobic loading conditions. The possible effects of such aeration are neither extensively quantified nor handled in standard AD models. It is commonly perceived that oxygen acts as an inhibitory and toxic agent in AD due to the involvement of a strictly anaerobic microorganism group of acetogens and methanogens (D. Bothejuet al., 2010). Also, the aerobic conversion of soluble organic matter into CO₂ by aerobic respiration is likely. Thus, it was believed that reactor instabilities, slow start-ups, low methane yields, and even total reactor failures might occur due to oxygen entering anaerobic digesters. Due to this negative perception, inoculums used in anaerobic digesters are even de-aerated before commencing reactor operation; sometimes oxygen scavenging chemicals (e.g. sodium sulfide) are also added. Conversely, improved hydrolysis of particulate matter in AD is observed in the presence of oxygen. All non-soluble and long-chain organic matter should go through this initial hydrolysis stage before fermentation or methanogenesis, in which the particulate matter would undergo decomposition and solubilization by the activity of enzymes (such as protease, amylase, etc.) that are extracellularly excreted by fermentative (acedogenic) bacteria. Since hydrolysis is often the rate-limiting reaction stage when the substrate is composed of particulate organic matter enhanced hydrolysis can greatly benefit the overall process efficiency. It is commonly known that hydrolysis rates are significantly higher under aerobic and anoxic conditions compared to anaerobic conditions. Botheju et al., (2010) demonstrated the possibility of the existence of an optimum oxygenation level that would yield a maximum methane generation in AD.

The retention time is the theoretical time that a particle or volume of liquid added to a digester would remain in the digester. It is calculated as the volume of the digester divided by the volume added per day, and it is expressed as days. The solid retention time represents the average time that the solids remain in the system. The solid retention time can be determined by dividing the weight of volatile solids in the system by the weight per unit time of volatile solids leaving the system. The hydraulic retention time (HRT) is equal to the solid retention time in completely mixed non-recycled digester systems. Retention time has an effect on gas production as shown in the results of (1974). There is a minimum retention time which allows the slowest growing bacteria to regenerate. Then there is a minimum. Retention time is required to achieve a satisfactory stabilization of the solids, which Dague shows as 10 days for sewage sludge at 35°C. If the retention time is cut in half, the gas production rate will drop and the process may fail due to a condition called wash out where the bacterial cultures decrease to the point that they are no longer effective. If the retention time is greater than 10 days at 35°C., the gas production levels out, and very little additional gas is produced for the additional time. Therefore, long detention times lead to low efficiency of the process. There is a tendency to refer to gas production rates in terms of volumes of gas produced per volume of digester volume. If maximum gas is desired, the ratio may be four or more volumes/volume per day. However, if the purpose of the digester is to produce and store a soil conditioner and to have some gas, as is the case in China, then the ratio may be very low and has no meaning. Retention time along with the temperature is important from the standpoint of the destruction of pathogens. If improved health is a consideration, certain minimum values should be exceeded.

Gas production is a function of the solid materials and their biodegradability in the digester. The more concentrated the solids, the smaller the digester and the lower the cost of the system. In sewage treatment plants, efforts are made to concentrate the solids to reduce the volume and the costs of the digester. The literature about the Indian digesters as reported by ESCAP (1980) implies that an optimal solids concentration of 7 to 9% of the feed should be used. However, systems have been designed to use as little water as possible. For example, Amon (2007) points out the advantage of dairy manure (10 to 13% solids) are that it can be added to the digester directly without dilution. If the manure has stood for a few days some water may have to be added to slurry the material for introduction into the digester. In Israel the manure is scrapped from the cattle pens with bedding which is either asphalt or concrete. Sterling (2001) reports that he has fed this manure with only small quantities of water at 16 to 18% solids content without difficulty. Batch dry digesters have operated with solids concentrations at 60%. The batch dry digesters with high solids concentrations appear to be an effective means of producing gas cheaply. Currently efforts are underway to manage and optimize the gas production for crop residues and urban solid wastes in landfills having very high solids.

The rate at which biomass is supplied to the digester is referred to as the volumetric organic loading rate and is commonly expressed in terms of grams of volatile solids per liter of digester capacity per day (gm VS/l-day). Different loading rates can be obtained by either changing the concentration of the solids in the influent or varying the flow through the digester. In practice the solids concentrations tend to be kept constant, and thus the flow rate is changed.