Evaluation of the biomethane yield from anaerobic co-digestion of nitrogenous substrates

Abstract

This paper examines three substrates for anaerobic co-digestion: abattoir waste; cheese waste and food waste. These substrates were assessed in detail for suitability for biomethane production. Biomethane potential (BMP) assays were carried out in mono and co-digestion for the three substrates and two mixes: T1 (40% abattoir waste; 50% cheese waste and 10% food waste on a wet weight basis) and T2 (30% abattoir waste; 40% cheese waste and 30% food waste). The C:N ratio of both mixes was below optimum. Low levels suggest that the production of free ammonia (NH₃) in semi-continuous digestion was of primary concern. Both mixes were digested in a semi-continuous process for 25 weeks. The recommended operating condition for T1 was a loading rate of 3 kg VS mn⁻³ day⁻¹ at a retention time of 23 days. The biomethane yield was 305 L CH₄ kg⁻¹ volatile solids (VS) which was 87% of the BMP value and equivalent to 61% biodegradability. For T2 (with the higher C:N ratio) a higher loading rate of 4 kg VS mn⁻³ day⁻¹ at a lower retention time of 15 days was recommended. The biomethane yield was 439 L CH₄ kg⁻¹ VS (99% of the BMP value and 84% biodegradibility). At these conditions, levels of total ammonical nitrogen (TAN) were 4109 and 4831 mg L⁻¹ for T1 and T2, respectively. These values are on the large side according to the literature. The temperature was reduced to 35°C to minimize toxicity associated with TAN. Ratios of volatile acids to bicarbonate were typically in the range of 0.2–0.3 suggesting stable operation.

Keywords:

• biomethane
• biogas
• abattoir waste
• food waste

Acknowledgements

Researchers were funded by Science Foundation Ireland (SFI), the Irish Research Council for Science, Engineering and Technology (IRCSET) and Bord Gais Energy (BGE). Laboratory equipment was funded by Bord Gais Networks (BGN).