Page 1 © Entec UK Limited August 2009 1. ANAEROBIC DIGESTER MIXING Mixing does not mean simply stirring the substrate. The mixing of the digester has to be sensitively adjusted to give the microbes the best possible environment for the least whole life cost. One of the most important factors of a s uccessful anaerobic digestion plant is the mixing. This applies not only to the digester but also to the inlet tanks, blending tanks, separation systems, digesters themselves and post d igestion storage. The requirements for each of these s tages are different but ef fective mixing is essential. So what is ‘effective mixing’? Design engineers would define it as achieving the optimised contact between the substrates for the minimum whole life cost. Guidance on what this means in reality can be obtained from the manufacturers that have been in the business for a long time. They can provide advice on what system will be most appropriate for your project, but they will, of course, be keen to offer their own products. The following guidance is intended to provide the reader with sufficient understanding to deal knowledgably with the mixing needs of their pro ject. For simplicity, the guidance concentrates on the needs of the anaerobic digestion tank itself as this is the key and most sensitive component in the system. Mixing in the digester is critical as it fulfils the functions of; • Bringing the feedstock and microbes into the intimate contact they need to successfully digest • Avoiding stratification • Avoiding excessive build up of heavy deposits at the bottom of the digester • Minimising the production of crusts and foams on the top of the digesting sludge The successful completion of these functions results in the maximised gas production and quality of digestate. As these are the primary benefits of digestion, it is clear that mixing is a key component of the digestion plant.
