Techno-economic feasibility and environmental life cycle impacts of struvite production from pig waste with ferrochrome slag as a magnesium source

Ferrochrome slag (FCS), a by-product of the iron and steel industry, has been evaluated as a potential alternative magnesium source in the recovery of struvite. The actual benefits of FCS-derived magnesium in recovering struvite as a low-cost fertilizer are poorly understood. This study evaluated the techno-economic viability and environmental sustainability of recovering struvite from pig waste using FCS as a magnesium source. Five comparable scenarios were modeled in Aspen Plus® to quantify input and output flows, which were subsequently used to estimate the economic and environmental performances of struvite production. All five Scenarios were designed to produce 364.56 kg of struvite via processing of 8 m of digestate and 0.4 tonnes of slag per hour. Scenario 1 produced struvite only, Scenario 2 produced struvite and 62.02 kg/h of an iron/aluminium catalyst, Scenario 3 produced struvite and 486.09 m 3 /h of biogas, Scenario 4 produced struvite, catalyst (62.02 kg/h), and biogas (486.09 m 3 /h), while Scenario 5 considered recovery of struvite, catalyst (62.02 kg/h (343.48 m 3 /h), biogas, and digestate (865.35 kg/h) as products. All 5 scenarios are not economically viable but NPVs increase with the recovery of more products from -$38.77 million (Scenario 1) to -$8.00 million (Scenario 5). There is a general decrease in the environmental impacts including global warming potential (10.74–6.53 kg CO 2 eq. per kg of slag), and fossil resource scarcity (3.12–1.75 kg oil eq./kg of slag) from Scenario 1 to Scenario 5 attributed to onsite co-recovery of struvite with biogas (Scenarios 3–5) compared to offsite recovery where the digestate has to be transported from a pig farm (Scenarios 1 and 2). Also, when the economic allocation is used, the environmental impacts associated with struvite decrease as more products are recovered. For instance, the global warming potential for struvite decreases from 10.74 kg CO2 eq. in Scenario 1–1.81 10.74 kg CO2 eq. per kg of struvite in Scenario 5. This study demonstrates that the recovery of struvite using FCS-derived magnesium is not attractive, but its recovery together with iron/aluminium catalysts, biogas, and digestate significantly enhances both the economic viability and environmental sustainability of nutrient recovery processes. • Economic and environmental impacts of struvite recovery were evaluated. • Onsite recovery of struvite improves the NPV compared to off-site recovery. • Struvite co-recovery with other products increases NPV. • HCl used in Mg recovery from FCS increases costs and environmental impacts. • There is need to evaluate more sustainable sources or methods of Mg recovery.