How Anaerobic Digestion Works

Anaerobic digestion (AD) is the conversion of organic material such as slurry, energy crops and food waste by micro-organisms in a sealed airtight container. Organic waste is retained in a digester tank for 20 and 40 days, and the digester is fed continuously.

Digesters produce biogas, which is 60% methane and digestate which has low odour and contains valuable plant nutrients.  The biogas is cleaned and used as a fuel in a combined heat and power plant to produce renewable electricity which can be used locally or exported to the grid.

How Anaerobic Digestion works:

• Waste is collected in a reception pit and pumped into a buffer tanks.
• Water is added to achieve between 12 and 15% dry solids.
• The waste is macerated to reduce particle size to 12mm.
• Waste is transferred to the digester with is mixed using paddle stirrers from the top where it is heated and retained for 20 – 40 days.
• The biogas is extracted and treated to remove Hydrogen sulphide moisture and then is used by a combined heat and power gas engine to produce electricity.

Temperature of operation:

Mesophilic digestors operate between 30-38 C, and thermophilic digestors operate from 49-57 C.  Different types of bacteria survive in different temperature ranges.  Digester temperature is related to the time and space required for digestion and the level of sterilization of the digestate.

Digester stages:

Single stage digester design uses one vessel or container for processing organic material; multi stage digesters have more than one in series.  Single stage is more economical and simpler, but produces less biogas and may take longer to digest. Multiple stage digester designs are optimized at each stage to support maximum bacteria activity.

Biogas yield from Anaerobic Digestion

Maximising biogas yield from a digestor depends on getting the moisture content, pH, carbon, nitrogen ratio, blend and retention time in digester right. Typical on-farm feed-stocks are slurry, vegetable waste, grain and meal, silage and dairy waste. Gas yield will depend on the calorific value of feedstock.

Selection of AD technology

There are several AD system configurations, which need to be taken into account when selecting suitable technologies for your project:

Wet verses dry processes

A wet AD process treats waste with solids content < 15% dry solids (ds), for use with, for example, manures and sludge. A dry AD process treats waste with solids from 15 – 40% ds for use with, for example, energy crops. 

Mesophilic verses Thermophilic Digestion

Increasing temperature in a digester increases the biochemical reactions.  Mesophilic digesters operate at temperatures of about 35^OC with feedstock retention between 15 – 30 days, while thermophilic digesters operate at temperature of about 55^OC with feedstock retention between 12-14 days.

Single verses Multi stage Digestion

Multi-stage reactors are generally split into 2 stages, the hydrolysis, as the first and the methanogenesis as the second, so that better control of digestion kinetics can be maintained and thus increase the biogas yield.

Batch verses Continuous

Biogas production can also be maximised if continuous processes are adopted in AD rather than simple and less expensive “batch” processes.