Smyth, M.1, Minall, R.1, Stead, T.2, Walker, J.3.
Over one third of the UK’s sewage sludges are pre-treated by Cambi’s thermal hydrolysis process, which precedes anaerobic digestion. Of the many installed plants, the investment in thermal hydrolysis will be realized over a 25-year period and therefore taking any opportunity to maximize methane production should be fully investigated to deliver the best possible return on investment.
This paper looks at the role that calorific value analysis and energy balance modelling can play in optimizing digester throughput. It reviews historical methods for evaluating feedstock energy content, including COD: volatile solids.
Samples, analysis (for calorific value) and data were taken from two advanced AD plants (Davyhulme SBAP and Seafield STC). This highlighted the energetic value of primary sludge compared to secondary activated and humus sludges; and that wastewater treatment plant operation at elevated mixed liquor concentrations not only reduces energy availability, but also leads to increased diversion rates of biogas to boiler operation rather than electrical output, post CHP.
In addition, the analysis showed that the energy content of sludge increases through digestion, when expressed on a dry solids basis. This shows that for Seafield and Davyhulme THP-AD shorter chain carbohydrates are preferentially converted to methane over longer chain proteins and fats.
Thanks go to United Utilities and Veolia for providing samples for analysis, sharing data, being co-operative and responsive.
Keywords: Calorific value; energy in sludge; methane yield; Sankey diagram; thermal hydrolysis