Proceedings

Improving anaerobic digestion by carbon dioxide enrichment: increasing biogas production and carbon dioxide uptake

Bajón Fernández, Y., McAdam, E., Soares, A., Vale, P., Cartmell, E., Cranfield Water Science Institute, Cranfield University

(free)

Carbon dioxide (CO2) enrichment may be applied in anaerobic digestion (AD) to improve methane generation and to obtain a ‘closed loop’ AD process through carbon sequestration by using the CO2 stream produced during biogas upgrading. Methane yield increases of 30% and 40% CO2 uptake during sewage sludge digestion have previously been reported. In this study nitrogen and CO2 (g) were diffused initially into water to quantify the time required to reach CO2 saturation conditions with the diffusers in place (2 mins for injections of 1Lpm and 0.5 CO2 molar fraction). The CO2 was then diffused into batch anaerobic digesters (1 L volume) at different molar fractions of CO2 (0.3, 0.6 and 0.9) containing food waste material to determine the benefits for biogas production, CO2 uptake and solids reduction during digestion. Enhancements in the renewable energy production were obtained, reaching 13% increased methane yields when enriching the digestion process with CO2 at 0.9 mole fraction. Associated reductions regarding the overall CO2 emissions of around 11% were observed, thus proving the potential of AD to act as a carbon sink when treating food waste material. Any initial acidification due to CO2 enrichment was buffered during the digestion process. Therefore, the CO2 could be injected periodically, which could further enhance the observed benefits in terms of methane production and carbon uptake.

Keywords: anaerobic digestion, carbon capture, CO2, CH4, food waste, optimisation.

Anaerobic digestion (AD) is the most widespread management solution for sewage sludge with increasing application for food waste. The majority of previous studies which have aimed to improve AD have focused on increasing the biodegradability of the input material rather than on the optimisation of the digester operation itself. These studies have led to the development of sludge pre-treatments which can have high parasitic energy demands and varying levels of success in terms of soluble chemical oxygen demand (sCOD) and volatile fatty acid (VFA) release.

Few studies have assessed the potential of optimising anaerobic processes by enriching the reactor or the input material to be treated with carbon dioxide (CO2). As early as 1994, (Sato & Ochi 1994) stated associated benefits of up to 30% increased specific methane yields when enriching with CO2 AD treating sewage sludge. Salomoni et al. (2011) and Alimahmoodi & Mulligan (2008) further confirmed the potential of CO2 biological conversion in anaerobic processes: two-phased anaerobic digesters (TPAD) and Upflow Anaerobic Sludge Blanket (UASB) reactors respectively.

A transformation of CO2 into CH4 in anaerobic processes considers the carbon as a raw material rather than as a waste, and the digesters themselves as carbon sinks in which the CO2 is utilised rather than stored as in the conventional CO2 capture and storage (CCS) techniques. Further benefits are envisaged if the increasing implementation of biogas upgrading technologies is considered, which leads to a waste stream concentrated in CO could lead to a ‘closed CO2 loop’ for A wide implementation of the CO conventional AD optimisation technologies since it could be potentially implemented without high operational or capital costs. For example, relatively low capital investments could be required if t CO2 was introduced in the recirculation or feed stream rather than in the digester itself or if current gas mixing systems could be adapted for the CO

This paper examines the potential of CO during food waste digestion.

 

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