Abstract:
Bangladesh produces an astounding amount of around 52.8 million tonnes of agricultural waste annually. Anaerobic digestion (AD) has been traditionallyused for processing biogenic residues in Bangladesh, however, the inadequate handling of digestate slurry as by-product is a major flaw of this system. Rich in nutrients (nitrogen, phosphorus, calcium etc.) and organics, digestate is difficult to handle in its slurry form and may cause environmental pollution, hindering efficacy of AD.
Hydrothermal carbonization (HTC) is an emergingtechnology for upgrading low-quality and high moisture biomass to carbonaceous material calledhydrochar at moderate temperature and pressure-a suitable means for recoveringnutrients from organic waste. For Bangladesh, integration of AD with HTC has excellent potential for co-producing biomethane and biofertilizer, however no study evaluated the environmental implications of such integration.
In this study, the mentionedgaps were addressed in two ways-employing hydrothermal carbonizations of AD-digestate for the characterization ofhydrochar and conducting life cycle assessment (LCA) comparing a standalone AD system (anaerobic mono-and co-digestion) with proposed AD-HTC integration forevaluating environmental implications. Five impact categories, such as Climate Change, Eutrophication Potential, Acidification Potential, Terrestrial/Human Toxicity, and Malodor emissions were determined. For anaerobic digestion technologies, life cycle impact assessment revealed co-digestion yielded better environmental performance than mono-digestion by reducing climate change (117%), eutrophication potential (54.5%) and terrestrial toxicity (55.7%), however the key hotspot for both was open storage of digestate. AD-HTC system however performed significantly better by reducing all of the impacts compared with standalone AD system with the highest reductions occurring for ecotoxicity and eutrophication potential. Process water from HTC was found to be a key pollutant for AD-HTC system, thus requiring post treatment in the form of nutrient recovery.
