Phosphorus recovery from dairy manure via biocrystallization in a UASB reactor: A techno-economic and environmental assessment

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Recovering phosphorus from manure allows to cycle a valuable nutrient back to agriculture while offering farmers a way to comply with the increasingly restrictive legislation concerning livestock manure application on agricultural land. To this end, a novel biocrystallization process in an Upflow Anaerobic Sludge Bed (UASB) reactor has been developed to recover phosphorus by retaining it in the sludge bed of the reactor, while producing biogas as a valuable by-product. Previous research has proven that this process is effective at recovering phosphorus at a lab level, but the economic feasibility and environmental performance of the process are still unknown. The present study analyses how this technology would perform from an environmental and economic perspective, to identify bottlenecks that could be addressed from the early stages of development.

The process was assessed using a conceptual design which consisted of an industrial facility treating the excreted manure of 11 farms in the region of Friesland, the Netherlands. The manure which was mechanically separated on-farm was later digested in the UASB reactor. The P-rich sludge obtained was dried to obtain a 2% P wt CaP fertilizer, while the effluent was stripped to later recover the ammonia as diammonium sulfate with a 7% wt N content.
The environmental assessment suggests that the P biocrystallization process could retain up to 41% of the initial P, while reducing the Global Warming Potential (GWP) associated with a conventional manure treatment by 36%. On the other hand, the techno-economic assessment, suggests that the P biocrystallization process could lead to a treatment cost of 26 € per ton of manure, which is nearly twice the value expected for a conventional manure treatment under the studied conditions. The elevated costs can be mainly linked to the high capital costs caused by the elevated energy requirement triggered by the drying unit needed to reduce the water content of the CaP fertilizer and by the high temperatures needed for the thermophilic digestion.

The analysis concluded that the P biocrystallization process is likely to present relevant environmental benefits when compared to conventional manure management systems, however, the economic constraints might impose a drawback for its implementation. Further research is suggested at a lab level to find methods to reduce the water content in the sludge and to study the effects of reducing the temperature of the anaerobic digestion (e.g., working at mesophilic conditions).