In recent years, the European Union has taken significant steps to address climate change and support the United Nations’ Sustainable Development Goals through the Paris Agreement and initiatives such as Farm-to-Fork and the EU Green Deal. The general idea is to minimize emissions and promote nutrient recycling in the wasteagriculture nexus as we transition from a linear to a circular economy with the goal of achieving climate neutrality. A potential strategy for achieving this transition is to redefine the role of anaerobic digestion facilities or ‘biorefineries’ in the agriculturewaste nexus. These facilities, once primarily known for energy production, now convert waste biomass into bioenergy and nutrient-rich products. Adding to this, the Russian-Ukrainian conflict has brought to light the energy and geopolitical crisis, which has increased emphasis on the role of waste biorefineries in recovering nutrients and energy from waste side streams. As a result, there is a need for robust life cycle assessments to understand the environmental trade-offs linked to waste biorefinery value chains.

The primary objective of this PhD thesis is to aid future practitioners in navigating the methodological challenges associated with life cycle assessments of value chains linked to waste biorefineries, with Flanders, Belgium as an illustration. The thesis explores the agricultural, technological and LCA methodological aspects of waste biorefineries. LCAs for different technologies of various technology readiness levels to recuperate energy and/ or nutrients have been performed in each of the four main chapters.

The first research chapter examined the effects of incorporating roadside grass clippings as a co-feedstock at decentralised waste biorefineries processing pig manure.The LCA model showcased a consequential LCA perspective and considered the market situation and waste management cascade of the region. The study concluded that despite roadside grass clippings being a constrained resource, utilizing it as a monodigestion or co-digestion feedstock is still more environmentally friendly than conventional management pathways, such as composting, which releases significant amounts of biogenic CO2.

The second research chapter estimated the environmental impacts of implementing emissions abatement techniques at pig stables and its downstream implications on waste biorefineries and the pig manure value chain. Our findings indicated that implementing emissions abatement measures at the source can result in potential climate change benefits. Specifically, modifying stable construction to separate manure in the form of faeces and urine can reduce NH3 emissions at the source while producing a feedstock suitable for anaerobic digestion and a bio-fertilizer in the form of pig urine.Furthermore, our analysis showed that air scrubbers, another emissions abatement technique, outperformed an adapted stable construction in terms of potential acidification and eutrophication impacts, despite their energy demand.

The third research chapter focussed on assessing the environmental impacts and tradeoffs of managing surplus liquid fraction of manure. The LF-management solutions under consideration included nitrification-denitrification with and without ammonia stripping-scrubbing as a pre-treatment step, as well as nutrient up-concentration by membrane filtration and/or vacuum evaporation. A regionalised life cycle assessment was also carried out and inventory data was partially derived from pilot facilities. Nutrient up-concentration from LF via reverse osmosis and vacuum evaporation indicated a better environmental performance when compared to ammonia stripping-scrubbingand nitrification-denitrification. The direct land application seemed to be a better option for managing nitrification-denitrification wastewater than post-treatment and subsequent discharge of effluent in constructed wetlands. This was mostly due to the avoidance of synthetic K fertilizer production from the land application of the K-rich biological effluent. Furthermore, regionalisation indicated that 60-80% of the environmental impacts occurred in the foreground system, primarily in Belgium and France.The final research chapter focussed on addressing the multi-functionality or the ”products” versus ”waste” conundrum whilst setting the functional unit of LCAs linked to waste biorefineries. Our study, which focused on struvite recovery from a municipal wastewater treatment plant, proposed a combined functional unit that considered both the product and waste aspects of the system. We concluded that an absolute comparison of manufacturing a bio-fertilizer (in this case struvite) versus synthetic fertilizer (P fertilizer processed from phosphate rock) is unreasonable since the environmental benefits are inevitably skewed in favour of the latter. This is because, in our study, the system boundaries for struvite recovery included burdens from the wastewater treatment, which was roughly 200 times higher than the processes involved in synthetic P fertilizer manufacture (mining and beneficiation). Therefore, if there is a case to be built for circular economy and nutrient recovery through secondary products, then the LCA should be based on measuring the relative difference in potential impacts between alternatives (in our case, before versus after struvite recovery).This is justified because even if struvite is not recovered, the wastewater would still have to be treated. Furthermore, a prospective LCA was carried out to evaluate potential future developments and policies regarding wastewater sludge management.Results indicated complete mono-incineration of WWS, ash processing to recover P and the subsequent land application appears to be less sustainable in terms of climate change, human toxicity, and terrestrial acidification relative to the status quo, i.e., coincineration with municipal solid waste and valorisation at cement kilns. Impacts on fossil depletion, however, favour mono-incineration over the status quo.

Throughout the research chapters, there was a common trend towards utilizing a consequential approach. Despite the potential difficulties associated with collecting data and comprehending market information, the studies showed that by considering model uncertainty, more dependable outcomes can be obtained that can be used to aid decision-making.