Biogas from Animal Wastes

The Middle East and North Africa region has a strong animal population. The livestock sector, in particular sheep, goats and camels, plays an important role in the national economy of regional countries. Millions of animals are imported in MENA each year from around the world. In addition, the region has witnessed very rapid growth in the poultry sector.

Animal Waste Management

Animal waste is a valuable source of nutrients and renewable energy. However, most of the waste is collected in lagoons or left to decompose in the open which pose a significant environmental hazard. The air pollutants emitted from manure include methane, nitrous oxide, ammonia, hydrogen sulfide, volatile organic compounds and particulate matter, which can cause serious environmental concerns and health problems.

In the past, livestock waste was recovered and sold as a fertilizer or simply spread onto agricultural land. The introduction of tighter environmental controls on odour and water pollution means that some form of animal waste management is necessary, which provides further incentives for biomass-to-energy conversion. The biogas potential of animal manure can be harnessed both at small- and community-scale.

Anaerobic Digestion of Animal Waste

Anaerobic digestion is a unique treatment solution for animal wastes as it can  deliver  positive  benefits  related  to  multiple  issues,  including  renewable  energy,  water pollution, and air emissions. Anaerobic digestion of animal manure is gaining popularity as a means to protect the environment and to recycle materials efficiently into the farming systems. Waste-to-Energy (WTE) plants, based on anaerobic digestion of cow manure, are highly efficient in harnessing the untapped renewable energy potential of organic waste by converting the biodegradable fraction of the waste into high calorific gases.

The establishment of anaerobic digestion systems for livestock manure stabilization and energy production has accelerated substantially in the past several years. There are thousands of digesters operating at commercial livestock facilities in Europe, United States, Asia and elsewhere. which are generating clean energy and fuel. Many of the projects that generate electricity also capture waste heat for various in-house requirements. Biogas projects based on animal waste play a key role in rural development in developing countries.

Major Considerations

The main factors that influence biogas production from animal manure are pH and temperature of the feedstock. It is well established that a biogas plant works optimally at neutral pH level and mesophilic temperature of around 35^o C. Carbon-nitrogen ratio of the feed material is also an important factor and should be in the range of 20:1 to 30:1.

Animal manure has a carbon – nitrogen ratio of 25:1 and is considered ideal for maximum gas production. Solid concentration in the feed material is also crucial to ensure sufficient gas production, as well as easy mixing and handling. Hydraulic retention time (HRT) is the most important factor in determining the volume of the digester which in turn determines the cost of the plant; the larger the retention period, higher the construction cost.

Process Description

The fresh animal manure is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digesters where stabilization of organic waste takes place.

In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.

Biogas contain significant amount of hydrogen sulfide (H₂S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H₂S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H₂S into elemental sulfur.

Biogas can be used as domestic cooking, industrial heating, combined heat and power (CHP) generation as well as a vehicle fuel. The digested substrate is passed through screw presses for dewatering and then subjected to solar drying and conditioning to give high-quality organic fertilizer.