Treatment of the sludge by anaerobic digestion reduces its fermentability and produces a digestate which is suitable to recycle to agriculture. To be beneficial to soils, AD sludge should contain active organic matter and labile organic carbon. However, to be beneficial to the wider environment, it must be sufficiently biodegraded to minimise any fugitive post-treatment emissions of CH4 and CO2, which are potent greenhouse gases. Uncontrolled emissions of CH4 can pose an explosion risk. Therefore, the treatment process should maximise the capture and utilisation of CH4, and once treatment is complete, any further production of CH4 should be halted through inhibition or deactivation of the methanogens in the digestate. 

Separating the solids and liquid fractions of the AD sludge facilitates the treatment of ammonia, and by partitioning the nutrients, it enables targeted application based on crop needs. Treating the separation liquors in a dedicated biological nitrogen removal plant reduces the emissions of ammonia to atmosphere, compared to storing and open-air land spreading whole digestate.

This work investigates post AD biogas production; the deactivation and inhibition of the production of this biogas; and the biological treatment of the separation liquors using nitrification-denitrification and partial nitritation-anammox processes; developing 1) a novel approach to calculating biogas production using a combination of kinetic modelling and biogas potential testing, 2) a new stability standard for AD sludge, and 3)  two novel conditioning treatments to deactivate and inhibit the post AD biogas production using thermal treatment and aeration. The experimental methodology quantified the unmeasured ‘lost’ gas produced, between sampling and analysis.