Improving bioenergy production in anaerobic digestion systems utilising chicken manure via pyrolysed biochar additives: A review

There is growing interest in the use of anaerobic digestion (AD) to treat organic waste resulting in volume reduction and stabilisation of the waste together with the production of renewable energy in the form of methane. Currently, livestock manure represents the largest organic waste stream in most countries, causing various negative environmental impacts. However, livestock manure can be used as a readily available feedstock for AD, reducing its environmental impact while simultaneously generating bioenergy. However, the AD of certain livestock manures such as chicken manure is not without constraints due to the high protein nature of the waste. These constraints are typically high digester pH, unfavourably low C:N ratio as well as the build-up of ammonia (NH3). Ammonia production will inevitably lead to NH3 inhibition, which can irreversibly terminate AD and methane production. To mitigate the NH3 stress that accompanies the use of chicken manure as a feedstock, various methods have been previously employed such as in situ NH3 stripping, bentonite addition, Se supplementation, Fe2+ and Ni2+ supplementation, co-digestion, water extraction and biochar addition. Pyrolysed biochar addition represents a new, innovative, and promising method due to its current role in the circular economy. This review provides an in-depth analysis into the current uses and knowledge of biochar for the purpose of NH3 stress mitigation. In addition, the review describes and compares factors that can potentially improve the efficacy of biochar in averting NH3-build up and suggests possible future studies.