Post digestion methane emissions: quantification, mitigation, and appropriate measures: PAPER & PRESENTATION

Bungay, S.1, Mann, J.2, Goodwin, T.2., 1Mott MacDonald, UK 2Marches Biogas, UK



Anaerobic digestion is widely used as a method for stabilising sewage sludge. Under anaerobic conditions, the organic matter in the sewage sludge is broken down and converted to biogas. The amount and rate of biogas produced is a function of both the biodegradability of the sewage sludge, which typically comprises of readily biodegradable, slowly biodegradable, and unbiodegradable fractions; and the conditions within the anaerobic reactor (organic loading rate, temperature, and solids residence time).

The biogas potential and residual biogas potential are typically measured using biomethane potential (BMP) and residual biogas potential (RBP) tests, which use standard conditions for temperature, mixing, and duration. The BMP is a measure of the biogas potential, whereas the RBP is a measure of the residual biogas following AD.

This paper demonstrates that the breakdown of the organic fraction of the sewage sludge and subsequent biogas production follows a first order kinetic degradation reaction, which allows kinetic modelling to be used to predict the BMP over different timescales, including the ultimate biogas yield (YBIOGAS, MAX). The paper also investigates the actual biogas yield (YBIOGAS, ACTUAL) from digested sludge (liquid and caked) under ambient conditions and evaluates the effect of dehydrating and rewetting digested sludge on biogas production. Using a combination of the modelling and biogas potential testing, the theoretical process emissions from secondary digesters and post-digestion storage tanks are calculated, and appropriate measures for mitigating these losses are proposed

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