Technical Report

Potential of Co-digestion

2003

What is Co-digestion?

Co–digestion is the simultaneous digestion of a homogenous mixture of two or more substrates. The most common situation is when a major amount of a main basic substrate (e.g. manure or sewage sludge) is mixed and digested together with minor amounts of a single, or a variety of additional substrates. The expression co–digestion is applied independently to the ratio of the respective substrates used simultaneously. Until quite recently anaerobic digestion (AD) was a single substrate, single purpose treatment. For example manure was digested to produce energy; sewage sludge was anaerobically stabilized and industrial waste water was pretreated before final treatment in a wastewater treatment plant. Today, the limits and the possibilities of AD are better known and co-digestion has therefore become a standard technology. This report aims to highlight the advantages of co-digestion and to describe a number of successful case stories.

Fields of Application

Generally co-digestion is applied in wet single-step processes such as continuously stirred tank reactors (CSTR). The substrate is normally diluted with dry solid contents of around 8 to 15%. Wet systems are particularly useful when the digestate can be directly applied on fields and green lands without solid’s separation. More detailed information on co-digestion process technology can be found in the IEA Task 37 study “Potential of Co-digestion – Limits and Merits” (Source: http://www.novaenergie.ch/iea-bioenergy-task37/index.htm) Since agriculture requires high quality fertilisers without pollutants, co-digestion lends itself towards blends of well controlled industrial wastes, grass clippings from parks, food industry wastes, dairy wastes etc. together with dilute substrates such as animal manures or sewage sludge. There are two major drivers which helped to promote co-digestion:

1. 1. Digesters in waste water treatment plants are usually oversized. Addition of co-substrates helps to produce more gas and consequently more electricity at only marginal additional cost. The extra electricity produced allows to cover the energy needs of waste water treatment at a reasonable cost.
   2. Agricultural biogas production from manure alone (which has a relatively low gas yield) is economically not viable at current oil prices. Addition of co-substrates with a high methane potential not only increases gas yields but above all increases the income through tipping fees.