iv.The pH

The pH is an important parameter in the operation of the digester, and the changes in pH value is is different at different states of the digestion. Biogas yield is

#### *Biogas Production and Process Control Improvements DOI: http://dx.doi.org/10.5772/intechopen.113061*

optimized at pH range of between 6.5 and 7.2 [101, 102]. At a pH of about 5, acidforming bacteria are at their most productive, while methane-forming bacteria thrive at a pH value of 6.2 or higher [33]. The bacteria population used in the bio methane process does best between 6.8 and 7.2, but may survive in a higher pH range [103]. When the pH of the digester falls outside of that range, the methane-producing bacteria deteriorates, providing an unideal environment for the microorganisms responsible for methane synthesis to survive. With a continuous decrease in pH below 6.2, the biogas productivity is reduced due to the accumulation of VFA, as they are toxic to the methane-producing bacteria. For the digester to generate the required biogas, the microorganism activeness is required as well as regular attention to the pH regulation [99, 104].

#### v.C/N ratio

The C/N ratio determines the suitability of biomass for anaerobic digestion. Where the C/N ratio is high indicates low nitrogen content needed for microbial growth which leads to low uptake of nitrogen by methanogens needed for protein production. The result is wastage of carbon and low biogas production [101, 105]. The overall carbon to nitrogen ratio of the substrate. Although very high C/N ratios foster population expansion of methanogens, which has little to no influence on the carbon residue in the substrate and results in poor methane output, the optimal ratio for anaerobic digestion is between 20 and 30. Overabundant ammonia reduces methane generation and poses a threat to methanogenic bacteria when the carbon to nitrogen ratio is too low [99].

#### vi.Toxicants/Inhibitors

Inhibitors (i.e., which are toxic to the anaerobic digestive process) of methanogenesis, such as antibiotics and other residues, result in less methane synthesis and a rise in the concentration of volatile acids. Avoiding a high nitrogen to carbon ratio is important since it increases the likelihood of creating hazardous circumstances for the bacteria [33].

The most common inhibitory substances in substrates for the anaerobic digestion process are (CO2), (NH3), long chain fatty acids (LCFA), H2S. Methanogens are more sensitive to toxic material than other groups of bacteria. Short-chain fatty acids being the main inhibitors for methanogens. The concentration of ammonia is important but needs control with less than 200 mg/l being suitable for biogas generation. Nitrogen is a nutrient, hence C:N ratio of 30:1 improves biogas productivity by supplying nutrients to micro-organisms for the microbial activity. The methanogenic phase is also inhibited by the effect of ammonia which is rich in swine waste, poultry waste, swine waste, and high proteinaceous sludge. It is the free NH3 and not ammonium ion (NH4 + ) that is responsible for inhibitory effect. Studies show that ammonia with a concentration of 1.77–14 g/l of total ammonia nitrogen can reduce the biogas yield by about 50%. The inhibition by ammonia is influenced by the temperature and the pH. Changes in the temperature and pH can lead to process dynamics disturbance leading to as much as 30% loss in biogas yield. Thermophiles are more sensitive to LCFA inhabitation compared to mesophiles. LCFA are found slaughterhouse waste, agricultural residues, olive oil, food waste, wastewater, etc. which act as inhibitory substances in the process of anaerobic digestion [101].

#### vii.Hydraulic retention time (HRT)

*The hydraulic retention time (HRT) refers to the* time taken by the biodegradable material in the bioreactor. Factors influencing the *HRT include the* temperature inside the digester, technologies applied, and the type of feedstock used. The recommended HRT for mesophilic digester is 10–40 days while the thermophilic digester is about 14 days. A very short retention time may have the bacteria washed out of the digester before getting multiplied which leaves the digester in a state of standstill, while longer retention time increases the volume of the reactor to handle the same feedstock. To reduce the retention time and reactor volume, an optimum loading rate should be maintained for optimizing methane production, with 2–3 weeks being regarded as generally optimum for lignocellulosic material to degrade and produce biogas [101].

#### viii.Co-digestion

The performance of a digester can significantly be enhanced by co-digestion which helps supply the necessary missing nutrients to micro-organisms for higher efficiency. Common materials for co-digestion includes leftover foods, cow manure, vegetable wastes, and fruit. Co-digestion with properly selected feedstock materials can significantly increase biogas yield [101].

#### ix.Redox conditions

The digestor's Redox conditions dictate the degree and velocity of contaminant biodegradation. Aerobic biodegradation rates are typically substantially higher than anaerobic biodegradation rates for most pollutants. While oil that has settled into anaerobic sediments may be quite tenacious, petroleum-based hydrocarbons that penetrate the aerobic zone of waters in lakes and rivers are susceptible to microbial destruction. Highly reduced hydrocarbons, like the alkanes, have low molecular weights and require oxygen to degrade. Methane and other alkanes cannot undergo anaerobic degradation. Hexadecane (C16H34), a high-molecular-weight alkane, may be present in some conditions. These chemicals rarely degrade and are often found in petroleum-contaminated areas. However, heavily chlorinated molecules like perchloroethene (PCE) resist oxygen decomposition. Anaerobic environments can bio transform these substances [106].

For a biodigester, the methanogenic bacteria require redox condition between −300 and − 330 mV for optimal performance [107]. Redox condition is important for fermentative H2 production process in which anaerobic bacteria are crucial and rate limiting. The efficiency with which substrates are metabolized, proteins and other storage resources are synthesized, and metabolic waste products are excreted, are all impacted by the Redox state. Pyruvate undergoes an oxidation-reduction reaction (AB) that produces VFAs and H2 under acidic circumstances. Methane and CO2 are produced by mesophilic bacteria during neutral processes, while solventogenesis occurs during basic operations. The repression of mesophilic bacteria indirectly promotes H2 producers within the system. The anaerobic bacteria operate below pH 6, but MB optimum range is between 6.0 and 7.5. The pH range of 5.5–6.0 is ideal to avoid both methanogenesis and solventogenesis. Good H2 is realized at a pH of about 6 compared to near-neutral. However, highly acidic pH (<4.5) inhib-its H2 production as it inactivates AB. Hydrogenase enzyme activity gets inhibited by maintaining low or high pH beyond optimum range [108].
