A simplified method for the design and sizing of anaerobic digestion systems for smaller farms

Anaerobic digestion (AD) as a waste treatment practice has existed for nearly 200 years and has become an accepted option for many farming and small-scale residential operations. Many developing countries now encourage the use of AD in order to meet new environmental regulations and/or to provide small amounts of energy resulting from methane generated during the process. This development has been met with some difficulty due to the lack of resources and knowledge of the systems in many of the rural communities in which these digesters are placed. A properly designed AD system can help prevent soil and water pollution as well as help mitigate methane emissions by capturing them for use as a potential energy source. This paper focuses on providing guidance to the proper design and sizing of an AD system for typical small farms, which account for the majority of dairy farms worldwide. A focus was on the implementation of such systems as they might be applied in Central America, although the aspects studied here can be applied for AD systems handling animal waste streams practically anywhere. We provide a method for sizing of anaerobic digester systems based on design standards from the US National Resource Conservation Service and using field sampled data of holding pen wash water runoff. An overview of the decision process for alternative designs is given, and simple-to-use nomographs are presented for use in sizing of an anaerobic digester system for smaller (non-industrial)-scale farms.