Kepp, U., Patrick Nolan Consulting Ltd(free)
The majority of anaerobic digestion plants built today is to the design of typical agricultural plants from central Europe. These plants have been optimised for energy recovery from energy crops and manures. The treatment of food waste in these plants is possible when addressing the different characteristics of food waste and energy crops in the design and through operation. This paper highlights typical parameters for potential feedstock and how operation can be organised for maximum biogas yields.
Keywords: Anaerobic digestion, agricultural design, hydrolysis kinetics
Anaerobic digestion (AD) has proven to be a viable option for treatment of organic waste, high in water and with low energy content when combusted. AD has shown to be superior to composting (Edelmann, 2000) due to the ability to recover energy contained in organic solids as a fuel with a wide spectre of applications. In addition energy use during treatment is lower for AD than for composting processes.
With an increasing number of AD plants in operation for more than 15-20 years, operational plant life is likely to exceed political drivers like:
• Carbon support schemes.
• The EU agricultural politics to take land out of food production.
In addition delivery contracts agreed during construction of the plant will be renewed and are likely to be changed.
It is therefore important that an AD installation is flexible to treat different types of feedstock in order to respond to changing economic conditions. Flexibility may demand the installation of new pre-treatment equipment or the construction of additional balance tanks for improved management of liquid and solids feedstock.
The first plants for treatment of organic waste fractions were designed similar to sewage sludge digestion plants. Large tank volumes and high installation costs led to the development of both dry processes including percolation processes and the typical horizontal tanks with a gas cone on top for agricultural installations treating energy crops and manure.