Anaerobic Digestion Technology

Anaerobic digestion is a naturally occurring process through which organic matter (like manure, food-processing wastes or crop residues) are stabilized by microorganisms in the absence of air.

Specially designed and insulated tanks, known as anaerobic digesters, facilitate the anaerobic digestion process under a controlled atmosphere and produce biogas and effluent.

During this process, some organic compounds are converted to methane (CH₄) and carbon dioxide (CO₂) gases. This mixture of gases is known as biogas. The composition of biogas is 50 to 75 per cent methane and 25 to 45 per cent carbon dioxide.  This high methane content makes biogas a fuel in power generators, engines, boilers and burners.

Chemical Oxygen Demand (COD) is a parameter that is widely used to provide a fast and reliable estimate of the quantity of organic material in waste. Every kilogram of COD removed from waste during anaerobic treatment typically produces 0.35 cubic meters of biogas.

Typical COD Range for Wastes and Biogas Production

·          Based on 80% COD removal efficiency

Technology

Anaerobic treatment technologies vary in terms of the length of time required for treating the waste, the size, configuration and complexity of the reaction vessel and the operating temperature. They range from relatively large, simple plug flow covered lagoons to high-rate fluidized bed systems and also include hybrid systems that combine several modern technologies to reduce the size of the facility and to increase the efficiency of the system.

The three main roles of anaerobic digestion systems are:

1.       Waste treatment: where the purpose of the treatment is to reduce the environmental impact of organic wastes prior to their disposal.

2.       Recycling of organic waste: transforms organic wastes into a stable organic soil improver, liquid fertilizer and energy.

3.       Conversion of biomass into a useful fuel: similarly to bioethanol or biodiesel production, crops are harvested and undergo anaerobic processing to provide a fuel product that can be transported and stored.

Biogas Utilization:

Electricity can be produced from biogas using internal combustion engines and power turbines. While producing electricity, heat energy can also be recovered by using co-generators.

Produced electricity can be used to meet the electricity demand of the same facility, while the excess electricity may be sold to the electricity supplier using the grid.

Biogas may also potentially be added to natural gas lines if carbon dioxide is removed.

Effluent Utilization:

The effluent coming out from the digester after the completion of the digestion process is known as digestate. Digestate has a nutrient value and can be applied on land like manure. Digestate also has significantly less odour compared to stored manure.
 

Costs and Benefits:

The capital costs of anaerobic digester plants may range from a few hundred thousand to a few million dollars depending on the size and amount of waste processed. However, most other waste processing technologies require a similar or higher capital investment and do not generate revenue. Additionally, carbon offsets, land application of digestate on cropland offsets fertilizer costs, and tipping fees for the feed materials can also be a part of the revenue stream.

Apart from all financial benefits, anaerobic digesters have environmental benefits that reduce greenhouse gas emissions, odour, ground/surface water contamination, and
pathogens.