Optimised acid phase digester operation
Grand, A., MMI Engineering Ltd, UK
(free)
Abstract The digestion of sewage sludge to generate biogas for renewable energy is one method of offsetting carbon dioxide emissions. Recent technological advances have resulted in the development of Acid Phase Digestion (APD) which gives improved biogas generation over single stage anaerobic digesters.
MMI Engineering was asked by MHW to assist in the development of two Acid Phase Digesters of similar design for two sites operated by Severn Trent Water. The digesters are formed of an inner circular tank (Tank 1) with a concentric outer tank (Tank 2). The tanks are hydraulically connected by two openings in the inner tank wall and are positioned on structural safety grounds. The main purpose of the work was to determine, using Computational Fluid Dynamics (CFD), the amount of fluid that passes between the tanks during the feed of new sludge.
This paper describes the process and merits of Acid Phase Digestion, the purpose of the work, the methods implemented in the CFD model and the results of the study.
Keywords Digestion, Computational Fluid Dynamics, Acid Phase, Gas mixing
Introduction Digestion encompasses a number of separate processes and reactions. These are hydrolysis, acidification, acetate formation and methane formation. Hydrolysis is the process of the breaking down and dissolving the complex organic compounds (carbohydrates, fats and proteins) of the suspended solids in the digester water. The sugars, amino acids and fatty acids contained in the dissolved solids react with fermenting bacteria to created volatile fatty acids (VFAs). Bacteria act on these fatty acids to convert them to acetic acid (acetate), carbon dioxide and hydrogen in the acetate formation stage. The final process is the bacterial breakdown of the acetates into methane. At all stages after hydrolysis other by-products are created that lead to biogas normally consisting of 30 to 40% carbon dioxide, some hydrogen, hydrogen sulphide and other gases dependent on the makeup of the feedstock. The single-phase anaerobic digester incorporates all these stages of digestion in one vessel. Therefore this digester must provide conditions favourable to all the bacteria and enzymes present in these processes and some of the stages may have their efficiency compromised (US EPA 2006).
