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Anaerobic Digestion Process: From Waste To Energy

Anaerobic digestion is the process of micro-organisms breaking down organic material in the absence of oxygen.

After mixed waste has been segregated and the inorganic recyclable fractions removed, the organic fractions enter the AD process.

The Anaerobic Digestion Process

Organic waste material (for example food wastes or sewage sludge) is stored in an oxygen free environment (usually a sealed tank or container and commonly referred to as a digester) in the presence of a population of anaerobic micro-organisms.

In this anaerobic environment, biochemical reactions occur that convert organic polymers from the feedstock into methane rich biogas and nutrient rich digestate.

Initially the polymers are broken down into sugars and fatty acids by a process called hydrolysis. In this reduced state, micro-organisms can get to work on the material using three other processes, acidogenesis, acetogenesis and methanogenesis. Each process is facilitated by specific types of micro-organism.

At the end of the anaerobic digestion process, biogas (predominantly methane with carbon dioxide and some trace contaminants) is produced. Any material remaining from the initial feedstock that has not been converted by microbial activity is bound as a solid or semi-solid digestate.

Anaerobic digestion works best at temperatures of 30 – 60°C. Between 30 and 40°C, mesophilic bacteria are the principal micro-organisms present. At 40 – 60°C, thermophilic bacteria are used.

The anaerobic digestion process typically takes anything between 14 and 40 days to complete but this varies upon numerous factors including feedstock type, micro-organism population, temperature and pH. The full process is detailed below in steps via a flow diagram.

Primarily, anaerobic digestion is seen as an opportunity to combat greenhouse gas emissions. As one output from the process is a renewable form of fuel (biogas), there is less of a burden on fossil fuels for energy and also a reduction in greenhouse gases being released from landfill.

Typically, anaerobic digestion plants of suitable size are combined with a means of heat and/or power generation. Gas engines are one common example, these may be subject to regulation under the Environmental Permitting Regulations (2010) if the net thermal input is greater than 0.4MW.

The biogas produced from an anaerobic digestion facility can be combusted in air to create heat and power for the facility. Any excess electricity can also be exported to the grid. Pollutant emissions from a combustion plant such as this do require periodic quantification.

A by-product of the anaerobic digestion process is the solid or semi-solid digestate. Typically digestate is nutrient rich and after pasteurisation at high temperature to remove pathogens may be suitable for use with or without further processing as an organic fertilizer.

Experts In AD and Biogas Emissions

Envirocare offers a fully MCERTS & UKAS accredited monitoring service and consultancy advice to biogas combustion plants and anaerobic digestion facilities.

The monitoring of exhaust gas emissions from biogas combustion plants is a mandatory requirement for anaerobic digestion process operators. Operators who burn biogas in combustion engines to produce electricity are required to have the emissions of CO, NOx, VOC & SO2 assessed annually to demonstrate compliance with limits.

Call us on 01274 738668 or fill out our Envirocare Enquiry Form for any queries regarding our wide range of stack emissions monitoring services.