Substrates

The feedstock supply to anaerobic digestion is called a substrate (food) for the microorganism’s growth. Their properties have a major influence on process efficiency and stability. The composition of the substrate is necessary for both the quality and quantity of biogas formed.

10.5.1 Selection of Substrates

Various types of organic feedstocks can be potentially utilized for the production of biogas, probably a lot more than used nowadays. The main raw source of materials used for the production of biogas today in Sweden is the municipal sewage sludge from treatment plants. Some other commonly used substrates for the production of biogas are waste from slaughter household, food waste and industries feed, and animal manure. Some examples of other materials used include fryer fat, grease trap waste, waste generated from paramedical industries and dairy farms, distillation waste, and grass silage. Soon, waste from the agricultural sector and different crop wastes will also become an important possible substrate for the production of biogas. Some other less general feedstocks that are being currently evaluated for the production of biogas include microalgae, feathers, grass, and woody biomass. The net amount of biogas production today concerning output energy is around 1.3 TWh/ year, though theoretical energy potential in the domestic waste that excludes forest residue is approximately 15 TWh/year (Nordberg,).

10.5.2 Pre-Treatments

Lignocellulose wastes, for example, forestry by-products and waste from meat, poultry, fish industries, and sewage sludge, have higher biogas potential. However, a very complex flow structure of the microbial cell from the sewage sludge and lignocellulose makes the hydrolysis process a rate-controlling step in the digestion system. The pre-treatment process is a well-established method to enhance the degradation of the above-mentioned wastes.

General pre-treatment approaches include physical (e.g., ultrasonic, irradiation, heat, and pressure), chemical (e.g., oxidation, ozonation, acids, and alkali treatments), and biological (e.g., the addition of bacteria/fungi/enzymes) treatments. The pre-treatment will enhance the availability of the feedstock to microbial degradation by breaking down the physical structure, reducing the particle size, changing the porosity of biomass, and increasing the surface area. Various studies that explored the impact on the yield of methane due to pre-treatment of different feedstocks and different methods showed improved efficiency of the process. Even though some studies have investigated the effect of pre-treatment on the microorganism communities and its relationship to enhanced methane yields, most of the investigations conducted to date are on anaerobic digestion of waste sludge with various results (Westerholm and Schniirer 2019).

10.5.3 Sanitation

It is essential to sanitize (kill) some pathogens present in the substrate to prevent contamination from reaching the digestate and substrate materials. The most general method to disinfect the substrate in the biogas production process is to heat the substrate at 70°C for about 1 hour. This is the remedy that is essential for low-problem animal manure in the EU Regulation EEC 2002. Another alternative method to Appert's method is pasteurization that gives a consistent level of sanitation, which has been in operation since 2007. Preconditioning is one of the methods which reduces the concentration of bacteria by 1,000,000 times and its heat resistant capacity by a thousand times. The presence of infective microorganisms from the substrate can impact the usefulness and quality of anaerobic residue (Schniirer and Jarvis 2010).

10.5.4 Thickening

The dry solid contents of the substrate can be increased by permitting the feedstock to pass via a screw or press. This is necessary in order to reduce the digester’s volumetric loading. The free volume might be used to expand the organic loading, which would increase the quantity of gas yield. The problem in removing a quantity of water is that it involves some risk to the primary nutrients; for example, salts and some kind of organic feedstock can be dissolved into the water, causing it to lose its substrate for biogas generation. The dehydration process also causes increased wear on the wheel, the mixer, and the grinders (Schniirer and Jarvis 2010).

10.5.5 Reduction of Particle Size/Increased Solubility

There are a variety of pre-treatment methods used for the substrate in the biogas production process that can help in enhancing the decomposition behavior. The most general is mechanical pre-treatment, which is used for screws, blenders, androtary cutter mill knives. Degradation can be achieved by biological, chemical, and thermal treatments, and/or by adding acids/bases, hydrolytic enzymes, electroporation, and ultrasound methods. The total surface area of feedstock can be increased by also using grinding. The organisms is widely used in most of the methods for degrading material because organisms are active in the hydrolytic process. The total surface area depends on particle size: the shorter the particle is, the higher the overall surface area present in it (Vazquez-Padm et al. 2009).

Toxicity

Various substances prevent the biogas production process, and methane producers are typically very sensitive when disturbed. The unwanted substance will get into the anaerobic digestion system by poorly arranged or polluted feedstock or it can be generated with the decay of a precursor unwanted substance. The impact of the toxic material may change, and the process might differ depending on some parameter, for example, the concentration of a substance, temperature, pH, or type of organisms available. If anaerobic digestate substrate residue is used as a fertilizer in the landfill area, there will be a different toxic substance produced that may adversely impact the soil microorganisms.

10.6.1 Inhibition Levels

The difference in the concentrations of different substances can lead to the stoppage of the process. These different results in the level of inhibition are affected by various factors such as antagonism, synergism, complex formation, and adaptation. The microorganisms present in the system can sometimes recover before a disturbance, although the inhibition process is often irreversible. This means that organisms are not able to restore the repressive effects, and even repressive producing material will disappear. This process will be started again and fresh microorganisms will be produced. In another way, the microorganisms might be undamaged but inhibited later during an biogas generation period that can be regenerated. At this time, it is imperative to discuss the lag period, i.e., the period when microorganisms stop developing or in which their growth rate is slowed down because of inhibitory effects (Zhou, Brown, and Wen 2019; Chen, Cheng, and Creamer 2008).

10.6.2 Inhibiting Substances

Various substances will be inhibitory to the generation of the biogas process. A material is determined to be inhibitory to the biogas generation process when it results in opposing shifts in the microorganism’s population or prevents bacterial development. The level of inhibition is generally shown by the reduction of stable levels of accumulation of organic acids and methane production. Some common inhibitors are ammonia, sulfide, and light metal ions (i.e., aluminum, calcium, magnesium, potassium, sodium) (Chen, Cheng, and Creamer 2008). Based on the source, the waste contains inhibitory or even toxic materials. Because of variation in the microorganisms, the composition of the waste, and operating parameters, the level of the inhibitory substance may vary every consecutive batch in the digestor. Before going to the anaerobic digestion process, it is essential to choose the correct material for doing anaerobic process and also to incorporate new methods to remove toxins before the anaerobic digestion process will help in making the waste treatment method effective.

Monitoring and Evaluation of the Biogas Production Process

It is necessary to cautiously monitor the generation of the biogas process, thereby spotting problems earlier and correcting them before they deteriorate the process. A few organisms like methane production organisms are very sensitive and may stop developing or wash away from the process. One example is that the temperature of the process must be monitored closely, as microorganisms are sensitive to temperature fluctuation. pH, fatty acid concentration, and ammonia and carbon dioxide content of biogas are all essential parameters that need to be monitored through the biogas process.

Monitoring Involved in the Biogas Process

The biogas production process is continuous; hence, it requires regular maintenance and supervision. Apart from this, mixer, pumps, gas collecting facilities, etc., must be working properly, and hence a frequent status check is essential. In addition to this, it is necessary to monitor the digestate tank and substrate tank because some microbial processes occur there too. The substrate tank must be kept at atmospheric temperature to avoid the decomposition process from making an impact on foaming and low pH, or the anaerobic digestor tank covered to avoid unintentional methane emission and nitrous oxide. It is necessary to monitor the daily basis of the biogas production process, and the inspection staff should be trained well. It is quite beneficial to make monitoring schedules and reports on a weekly or monthly basis to ensure the stability of operating parameters; for example, pH, temperature, alkalinity, ammonium, gas flow, and fatty acids are to be monitored.

Some parameters will be measured for longer periods between the biogas process. There are some laboratory procedures and existing sampling techniques to be conducted frequently for sampling, and some new methods that will facilitate continuous monitoring of the process. The biological process will be examined at various locations and approximate sampling places within the biogas generation process, including the inlet and outlet of the anaerobic digester and the sanitation or substrate tank. The best way to take a sample is at the period of mixing and pumping the slurry. Samples should be tested at shorter intervals if a disparity or problem is suspected. The sampling method at the location is significant. The biological process will be evaluated at various locations. Samples should be taken in the same way at all times.

10.7.1.1 Loading and Retention Time

The loading rate is important to obtain a constant decomposition rate. The time available for biological degradation is decided based on the retention time. Based on the microbiological view, these two process parameters are important for process efficiency. The organisms grow in a uniform substrate in a particular period and they need sufficient time to break down the substrate at a larger rate. The retention time and load are governed at a relatively high level; the uniform loading of the anaerobic digestor will ensure the analyzing of the substrate composition from the tank. Now, dry solids and volatile solids are mixed completely in the tank. It is tough to predict the exact rate of organic loading required; in this case, trial and error will be used, especially when a start-up new substrate is provided. The retention period is governed by several factors, such as characteristics of substrate and process temperature. Usually, slowly degraded materials like cellulose material require longer retention times as compared to easily degraded materials, for example, food waste.

The thermophilic process often runs at slightly lower retention time as compared to mesophilic processes from bacterial activity increases by an increment in temperature. Generally, the mesophilic process needs a retention period of approximately 1 day and a thermophilic process requires 12 days (Schniirer and Jarvis 2010). Most of the co-anaerobic digestion facilities in Sweden work at quite a higher retention time; about 20 to 30 days is usual. Here two shorter and longer times are used for degradation of the materials with higher water concentration, which include digestate sludge and industrial wastewater; those must be re-intro- duced into the anaerobic digestion process to retain the organisms or otherwise they will be washed away. In this situation, the hydraulic retention period will be decreased and the solid retention time will increase. The retention time of the process is only a couple of days (Kroeker et al. 1979).

10.7.1.2 Substrate Composition

The anaerobic digestion rate is strongly affected by the composition, complexity, and availability of the substrate present in the system (Ghaniyari-Benis et al. 2009; Zhao, Wang, and Ren 2010). Various types of carbon sources will support diverse microbial groups. Before doing the anaerobic digestion process, we need to characterize the substrate for lipid, carbohydrate, fiber, and protein content (Lesteur et al. 2010). Apart from this, the substrate must also be characterized by the quality of methane that can be generated by using anaerobic circumstances.

The carbohydrates are an essential organic component obtained from MSW for the generation of biogas (Dong et al. 2009). However, starch can act as a low- cost feedstock for the production of biogas, in contrast to glucose and sucrose. Experimentally obtained results show that the concentration of initial and substrate total solid content in the anaerobic reactor can disturb the efficiency of the whole biogas production process and the methane production amount during the process (Fernandez, Perez, and Romero 2008).

10.7.1.3 Gas Quantity

The quantity of biogas is very important to measure the condition of the process. Gas production rate reduction does not relate to the loading of the fresh substrate if the system is working optimally. The interrelation between the amount of biogas generated and the quantity of natural content provided also helps to evaluate the performance of the whole process. The usual natural gas production process produces biogas at a magnitude of 1 to 4 m3 per m3 of digester tank volume per day. The biogas plant should be able to accumulate this quantity of biogas all day. It is useful to interact with some devices to measure the amount of biogas produced in the collection unit. There are different varieties of flow meters used nowadays for this resolution. The quantity of biogas is frequently represented in cubic meters (m3); for example, the quantity of biogas at 0°C and at an outside environmental pressure of 1.01325 bar (Schniirer and Jarvis 2010).

10.7.1.4 Gas Composition

The composition in biogas is another essential parameter to determine the process condition. A small quantity of methane and an increased fraction of C02 suggests that methane CH4 generation was reserved. This is the indication that a few problems occurred in this process. All the compounds of gases consisting of biogas are generated at the time of decomposition by a different substance with the assistance of microorganisms. Natural biogas contains mainly carbon dioxide and methane, with other gases in very small quantities (i.e., hydrogen, nitrogen, ammonia, and sulfite). Commonly, biogas is saturated along with water vapors (Murphy 1949).

The composition of biogas can be analyzed by passing the produced biogas continuously over an analyzer. Another method is to get the sample and segregate gas from the gaseous phase for consecutive investigations. This procedure is regularly used when this process is analyzed from the laboratory. Various analysis methods may be applied. Using an anaerobic fermentation tube called an Einhorn saccharom- eter is a very fast method to find out the concentration of C02. This has a strong (7M) solution of lye, and within it, an identified quantity of biogas sample was injected. Hence, C02 will dissolve in the lye, and at the same time, methane will generate a gas bubble inside a tube. The CO, concentration can be evaluated by measuring the entire quantity of gas and then comparing this to the initial injected volume. Based on this procedure, it is essential to know the truth that immediate variation in the pH will cause the discharge of salt of bicarbonate, which dissolves all the organic substances in the anaerobic digester tank, in the form of carbon dioxide. The measured CO, concentration becomes higher than normal (Schniirer and Jarvis 2010).

Process Efficiency

The anaerobic digestion process relies upon the effective conversion of a biological substance into value-added products collectively known as biogas, which contains methane as a main ignitable constituent. The anaerobically generated biogas could be used as a combustible source for home lighting, cooking, and heating and also some additional applications. The anaerobic digestion processes largely depend upon mutual interaction of microorganisms in decomposing complex and organic matters to solvable monomers, for example, fatty acids, amino acids, glycerol, and simple sugars. For anaerobic digestion, process efficiency is a vital component to understand the actual biological process and possible chemical reactions. Even though there are many benefits of anaerobic digestion, poor operation stability will delay the technology from being widely implemented (Dupla et al. 2004).

Many factors will affect the stability and performance of the anaerobic system. Some chemical processes which are related to hydrogen partial pressure, interspecies hydrogen transfer, and microbial electrochemical systems are implemented to enhance the process efficiency of the anaerobic system through enhancing the microbial interactions. The above factors accelerate the methanogenic process and enhance the performance of the process.

 
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