To transition towards sustainability, circularity principles need to be implemented throughout all economic sectors, including agricultural. Innovative manure treatment technologies have proven to increase local nutrient circularity and decrease local environmental problems. However, there is a lack of knowledge about potential side-effects beyond a farm’s gate. Only if all effects are included into the assessment and a technology still results in environmentally favourable practices, can the technology be deemed circular and sustainable. To provide a holistic picture of the new treatment technologies, life cycle assessments (LCA) were conducted to identify benefits and drawbacks when including life cycle stages and impact categories not directly linked to the farm and its particular challenges. The environmental performance of treatment technologies in areas of intense agricultural production was evaluated. Furthermore, their impact profiles were compared with that of a baseline scenario without the implementation of such treatment. The assessed technologies were (i) pig slurry acidification, (ii) duckweed as liquid manure treatment, including its use as compound feed ingredient, and (iii) insect larvae for the treatment of agro-residue and their use as compound feed ingredient. Respective baseline scenarios were (i) no acidification, (ii) land application without further treatment, and (iii) land application of crop-residues and industrial composting of solid fraction pig manure.

The treatment scenarios showed different environmental profiles. Slurry acidification reduced impacts in categories mostly related to direct agricultural emissions, such as methane and ammonia. For impact categories associated with material and energy provision, acidification increased the impacts in some cases. The efficiency of slurry acidification depended on local environmental and regulatory conditions, and beneficial impacts were more likely to occur in Denmark than in the Netherlands or Spain.

Duckweed ponds performed worse than direct field application of the treated liquid fraction mostly because the latter results in (greater) savings in potassium fertiliser production. If direct land application is not possible or recommended, the focus should be on long residence times in duckweed ponds to allow for maximum nutrient uptake. Insect composting of Brussels sprouts stems and endive roots performed worse than their field application because non-residue insect feed and energy use resulted in environmental impacts too great to be compensated for by decreased production of soybean meal or fishmeal.

Depending on energy source and impact category, industrial composting performed better or worse than composting through insects.

Overall, the studies suggest that technologies fostering circularity do not necessarily foster sustainability.

In addition to the LCAs, a comparative assessment between qualitative expert judgements and the quantitative LCA results of the aforementioned technologies was conducted to identify the strengths and weaknesses of such assessment compared to LCAs. It was found that expert judgments might serve for initial screening but cannot provide information on overall environmental performances of a technology.

The PhD project sheds light on the presumably counter-intuitive discordance of circularity and sustainability. However, this disagreement was found to be conditional, and it is pointed at options that may enable their alignment and harmonisation. To achieve such alignment, the contribution of various stakeholders, ranging from technology providers, farmers, policymakers, the public and LCA-specialist, is recommended. Thus, a collaborative effort in which challenges are identified, addressed, and evaluated can assist policymakers in implementing solutions aimed at moving the agricultural industry toward a greener future.