Phosphorus consumption. From linear to circular flow

Abstract

Wastewater and waste sludge are generated in all parts of the world during domestic and industrial activities. Conventional wastewater treatment methods generate a highly concentrated municipal sludge, which needs to be disposed of effectively without leading to secondary pollution. Animal manure and post generated manure wastewater are another environmental concern. Both of the above wastewater and sludge/manure are rich in organic/inorganic forms of carbon, nitrogen (N) and phosphorus (P). Anthropogenic and industrial activities in the global biogeochemical cycles have resulted in a drastic one-way mobilization of these resources into the atmosphere and the environment. The current intensive agriculture requires huge quantities of nitrogen (N) and Phosphorus (P) containing fertilizers. The industrial production of ammonia and nitrates is quite energy demanding; however, nitrogen is abundantly present in the nature and therefore it is a non-restricted resource for nitrogen derivate production. Unlike nitrogen, P can be obtained primarily from mineral deposits available only in few geographic locations. The phosphate rock reserves are finite and the current intensive fertilizer production based on economically mined rocks could last only another 50-100 years. Since phosphates are available only in limited geographic locations and the recognition that geologic phosphates are a non-renewable resource, Phosphorus recovery becomes a crucial for sustainable food production as EU depends for 90% on import of phosphate rocks (European Commission 2017). Within the EU only Finland has some phosphate rocks. The list of supplying countries is quite short; more than 70% of the present known global reserves of phosphate rock are located in Morocco as of all mined and processed phosphate rock (2009).  Phosphorus, being a finite resource with deficits starting approximately from the year 2070 due to increased demand might also result in high prices and reliance on single point sources, giving them monopoly over the market. The EU phosphorus flows show that the main losses of phosphorus in the food sector are through sewage sludge, other waste water and food waste. In general, phosphorus can be recycled, mainly from wastewater (e.g. sewage water), manure and organic waste (e.g. wasted food). Thus, wastewater can be considered as a renewable source of N and P. Instead of releasing the N and P rich wastewater into coastal and inland waters increasing eutrophication risk their utilization can be beneficial resulting in multiple benefits like nutrient recovery, water reclamation for reuse and maintenance of ecological balance in aquatic systems. Presently, the recovery and reuse of P is still far from being a main stream practice. Yet, the techniques already accepted and applied differ by the origin of the used matter (wastewater, sludge, ash) are mainly focused on the process of precipitation. One of these techniques is struvite precipitation, which can be implemented in wastewater treatment plants that use enhanced biological or semi biological/chemical phosphorus removal. Struvite (magnesium ammonium phosphate or MAP (MgNH4PO4·6H2O)) is formed by a basic precipitation reaction in different stages of the wastewater treatment process where magnesium (Mg 2+ ), ammonium (NH4 + ) and orthophosphate (PO4 −3 ). The article discusses the progress in extracting P from sewage sludge and animal manure, the conditions to create optimal conditions for struvite precipitation in such media and the way to overcome the problems associated with choosing the right Mg source, pH adjustment and the non-acceptable level of organic matter in the initial suspension.