Optimised percolate recirculation strategy for batch dry anaerobic digestion

Dry-batch anaerobic digestion (AD) is an increasingly used treatment for the organic fraction of municipal solid waste (OFMSW), avoiding disposal to landfill of millions of tonnes every year while allowing for renewable energy production from waste. Despite the well-known benefits, dry ADs are associated with operational problems resulting from operation at solids contents 20%, including localised accumulation of inhibitors (e.g. volatile fatty acids (VFA), free ammonia), poor mixing, limited treatment capacity, slow reaction kinetics and biogas yields much lower than those attainable in laboratory scale. Addition of inoculum to the feedstock is one of the most common strategies attempting to solve inhibition problems, but it reduces volume available for waste treatment. Recent studies have proposed recirculation of percolate formed during dry AD as an alternative or in combination to inoculum addition. The resulting benefits are capped by the lack of scientific understanding of the mechanisms by which process is improved. Therefore, this study aims to investigate an optimised percolate recirculation strategy that can maximise biogas production and treatment capacity in dry digestion units operated in batch mode, and to do so for the several feedstocks most commonly used in dry AD.
The specific objectives to fulfil the aim are:
1) Elucidate the mechanisms by which percolate recirculation improves dry AD performance
2) Determine suitability of percolate recirculation to mitigate localised inhibition due to ammonia or VFA accumulation
3) Propose a percolate recirculation strategy (volume and frequency) that allows optimal biogas generation and treatment capacity
4) Understand required variations on percolate recirculation strategies depending on feedstock type and use or not of bulking agents
5) Quantify benefits of a dynamic percolate recirculation strategy that varies during batch fermentation according to evolution of substrate degradation
This project is a collaboration between Cranfield University (UK) and UTC (France) and will require the student to spend approximately half of the PhD time on each institution.