This article outlines the process of production and benefits of utilization of biogas from manure for small scale animal farmers, especially those in developing countries. Apart from the sanitary benefit of proper management of manure, this article highlights other benefits that can be derived from animal manure as a source of energy. Thus, encouraging these farmers to store manure from their animal farms for use, thereby changing waste to valuable resources.

Manure is the waste produced in large quantities and varies with the size of the farm, species of animals and the nutrients in the feed fed to the animals. This waste product, if not properly managed, poses a serious threat in the environment. This is because animal waste contains nitrogen and phosphorus in quantities that are harmful to human health and to aquatic animals if these nutrients meet with water bodies. To avoid environmental risk, government agencies in the developed world regulate the storage, treatment, processing, management, and land application of the manure. In the developing world, as at date, such regulations are rarely in place. Thus, animal wastes are littered all over the communities where the animals are reared, with the continuous causation of environmental hazard and societal nuisance.

The Need for Biogas

Due to other constraints, manure cannot be applied in the field immediately after it is produced. Traditionally, the manure is stored until there is an opportunity for its land application. In the interval between its production and field application, manure might be used for energy generation in digesters. The type of digester used on farms is typically determined by the manure management practices in place and the type of animal manure that is fed into the digester. Slurry and liquid manure from cattle and pigs can be used as feedstock for biogas production in digesters. Biogas recovery from animal waste may hold the key to unlocking the financial and environmental benefits of managing manure generated from livestock operations and organic wastes from food processing sectors. It also helps to reduce the greenhouse gas emission from methane (Agstar, 2011). Biogas is produced by the activities of bacteria that breaks down the biodegradable components of the manure in the absence of oxygen in an airtight chamber. The process is called anaerobic digestion.

Anaerobic digestion is the microbial fermentation of substrate in the absence of oxygen. This results in a mixture of gases comprising methane, carbon dioxide and other gases such as nitrogen and sulphur as impurities. The raw materials fed into the digester are organic materials with high moisture content. Examples of such organic materials are livestock waste (manure), food waste, residues from garden or orchard cleaning. The gas mixture produced in the biodigester is called biogas and can be used as fuel. Biogas contains 55 to 65-% methane, 30 to 35-% carbon dioxide and other gases. The proportions of the gases depend on the raw materials and other process parameters like the Hydraulic Residence Time (HRT) and temperature. Energy content of biogas is about 60% (depending on the methane content) compared to natural gas (Noorolahi et al, 2014) and can be easily adapted for use for replacement of natural gas.

Formation of the Biogas in Digester

The process of the formation of the biogas from the fed-in raw materials can be broken into four steps. The three last steps involve different species of bacteria which break down the output of the step before it into substances necessary for the final formation of biogas from the organic materials fed in the digester. The steps are as follows:

1.  Hydrolysis: this is the addition of water to the organic raw materials. It causes breakdown of carbohydrate to sugar and glucose and conversion of proteins into amino acids. This conversion is usually the slowest of the four steps (Vogeli et al., 2014).  
2.  The second step is the action of bacteria which act on the acids from the first step to make them ready for the next stage of the conversion process.
3. This step involves another species of bacteria which act on the substances formed in the previous stage to convert them into the forms ready for the final stage of the anaerobic process
4. In this last step, methane forming bacteria convert the products of the earlier steps into biogas containing mainly methane and carbon dioxide. This final product needs to be collected and may be stored for use as energy.

Factors which affect the biogas produced are the environmental temperature, pH, the feedstock, bacteria involved and the HRT. The digester should be airtight to avoid the leaking away of the biogas. Digesters must also be anaerobic or oxygen free. Temperatures at which the bacteria act on the substrates for its conversion into biogas have been divided into psychrophilic (41-59 ⁰F), mesophilic (95-100 ⁰F) and thermophilic (118-140 ⁰F) temperatures. Research has been carried out across the three temperature ranges to determine the best temperature in different instances and with diverse feedstocks. Essentially, temperature determines the bacteria species that would be involved in the biogas production. Different bacteria species thrive at different temperature ranges. Higher temperatures more rapidly convert manures to biogas. The optimum pH for a stable anaerobic digestion and high biogas yield lies in the range of 6.5 to 7.5 (Vogeli et al., 2014).

Types of Digesters

There are many types of digesters depending on the construction materials and the technology which was adopted. Red bricks, synthetic membranes and metal sheets are some of the materials which can be used to build digesters. Initial and maintenance costs of the digesters vary with materials used, the size and technology adopted. Bricks and mortals are satisfactory but expensive, while steels corrode quickly in presence of gases containing hydrogen sulfide. Some of the popular digester types are classified into fixed dome and floating gas holder biogas plants. The fixed dome biogas plant has no moving parts (Figure 1). It has a mixing tank and an inlet channel through which the feedstock is passed into the plant. Conversion of the waste to the biogas takes place in the main chamber of the digester. The gas mixture is collected via the gas piping placed at the top of the digestion chamber. The residual of the process is let out through the outlet and the overflow tank, while the gas is collected from the hose at the top of the digester.

The floating dome type of biogas plant (Figure 2) has a floating “balloon” at the top of the digester which can be used to store biogas for periods of higher demand and monitor the volume of gas produced. The moving component of the floating gas holder type makes it more expensive to build and maintain as compared with the fixed dome. However, it has the advantage of operators being able to assess the amount of biogas already accumulated at any time.

Benefits of Biogas Plants

Biogas plants have numerous advantages and benefits which translate into better economics for the farm and ensure good stewardship of resources and the environment. Biogas produced on the farm can supplement the current energy needed on the farm. If a large quantity of biogas is generated on the farm, some can be sold to neighbors for additional income for the farm. Moreover, the generation of biogas reduces the dependency on fossil fuels which are nonrenewable and mitigates attendant global warming continuously being caused by the combustion of fossil fuels. In other words, biogas produces a clean fuel that helps in the control of air pollution resulting from burning of fossil fuels.

Another advantage of biogas production from manure is that it reduces water pollution by the initial decomposition of the manure in the digester before the manure is applied to the field. Anaerobic digestion for animal waste results in substantial environmental benefits. These include reductions in biological oxygen demand, human pathogens, hydrogen sulfide and odor-related components of manure (Ciborowski, 2001). As a result of the biological process which occurred in the digester, its residue has less objectionable odor but conserves critical nutrients in the manure. This residue can be applied to land similar to other animal manure. When applying it to land, observe all precautions and regulations to protect the environment.

References:

Agstar (2011). Recovering value from waste. Anaerobic Digester system Basics. United State Environment protection Agency

Noorolllahi Y, Kheirrroutz M, Asl H.f, Youdefi H and Hajinezhad A. (2014). Biogas production potential from livestock manure in Iran. Renewable and sustainable Energy reviews.

Vigeli Y, Lohri C.R, Gallardo A, Diener S and Zurbrugg C. (2014). Anaerobic Digestion of Biowatse in Developing Countries. Swiss Institute of Aquatic Science and technology. 

Ciborowski P. (2001). Anaerobic digestion of livestock manure for pollution control and Energy production. Feasibility Assessment.