Smyth, M.1, Brookes, A.2, Riches, S., Bungay, S.3
Anaerobic digestion (AD) involves the sequential breakdown of organics into (principally) methane and carbon dioxide. Nitrogen is largely preserved in AD, however a proportion is transformed from organically bound nitrogen to ammonium nitrogen. The degree of transformation will directly influence ammonium-N levels in the digestate, return liquors post-dewatering, and ultimately the ability of the activated sludge plant to meet its ammonia consent. Quantifying ammonification is therefore important to water companies and will influence the decision making process in new build schemes and plant upgrades. Operationally, where high ammonium levels are challenging to either liquor treatment or digester operation, nitrogen sources can be mapped through the process and changes to plant operation made. Operational changes can include relocation of sludge imports, reduction in digester throughput, reduced dwell time in primary tanks and increasing the sludge age.
This paper details the steps needed to develop a predictive excel-based tool for digester ammonium-N levels. The model takes into account the total nitrogen content of the feed sludge, dry solids content, proportion of readily available nitrogen and the rate of volatile solids destruction. It draws upon data from an Anglian Water study at four sites, which have been used to calibrate and refine the model.
Thanks go to Anglian Water for their co-operation on the project and sharing the data. Secondly to Helix ECL, where the model was first developed and subsequently evolved with Aqua Enviro.
Keywords: ammonia, anaerobic digestion, liquor treatment, total nitrogen, volatile solids destruction