Europe produces approximately 1.4 billion tonnes of manure annually [101]. Of total greenhouse gas (GHG) emissions in Europe, manure management alone contributes around 17% [41]. In this study, liquid swine manure from intensive production systems in Spain is used as the basis for evaluating treatment strategies.
A decentralized, farm-scale system treating 8,500 tonnes of manure per year is evaluated. Two treat- ment pathways are considered: (i) a biogas production route using anaerobic digestion (AD) with on- site combined heat and power (CHP), followed by solid–liquid separation, composting, and microalgae cultivation for nutrient recovery from the liquid fraction; and (ii) a bioplastic production route via ammo- nia stripping, followed by acidogenesis for volatile fatty acid (VFA) production, which are subsequently used for microbial polyhydroxyalkanoate (PHA) accumulation, with chloroform–hypochlorite extraction, and downstream composting and algae cultivation of residual streams.
The major nutrient flows, carbon (C), nitrogen (N), phosphorus (P), and potassium (K), are tracked throughout the system to identify which treatment performs best in terms of nutrient recovery, envi- ronmental impact reduction, and economic feasibility. A stoichiometric macroscopic mass-balance model was constructed based on an elemental substrate composition derived from Spanish field data (C1H2.6O0.75N0.054, d.o.r =4.9 e−/Cmol). Environmental impacts were assessed using life cycle as- sessment (LCA) in SimaPro with the ReCiPe 2016 Midpoint (Hierarchist) method, applying a system expansion approach within a cradle-to-gate system boundary. This allows for a direct comparison of the environmental performance of both processes by substituting for all products. The economic fea- sibility of both scenarios was evaluated through a techno-economic analysis (TEA), including capital expenditures (CAPEX) and operating expenditures (OPEX). With a discount rate of 3.24%, the net present value (NPV) is calculated. Under base-case yields (30% biodegradability in AD and 54% aci- dogenesis efficiency), the biogas route produces 2.2 kg of biogas, 25 kg of compost, and 7.2 kg of algae per tonne of manure. The bioplastic route yields 2.9 kg of PHA, 8.9 kg of ammonium sulfate, 36 kg of compost, and 1.2 kg of algae. Nutrient retention favors the bioplastic route for nitrogen (38% vs. 85%), while phosphorus is largely conserved in both systems (≥93%). Based on environmental impact, the biogas route performs better across multiple impact categories, including global warming potential (GWP) (23 vs. 120 kg CO2-eq per tonne manure), fossil resource scarcity (FPMF), and freshwater eutrophication. However, marine eutrophication is 23% lower in the bioplastic scenario and 22% for terrestrial acidification. Neither scenario is economically viable at this scale, as break-even requires production volumes 18–31 times higher at maximal yields. Only the biogas route achieves a positive gross profit when improvements in yield, product prices, and EU subsidies are considered. In conlclu- sion, valorization of swine manure is technically (close to) feasible and environmentally beneficial for biogas, and technically feasible but environmentally and economically premature for bioplastics. ... Read more