Abstract:Phosphorus is an essential nutrient for every organism on the Earth, yet it is also a potential environmental pollutant, which may cause eutrophication of water bodies. Wastewater treatment plants worldwide are struggling to eliminate phosphorus from effluents, at great cost, yet current research suggests that the world may deplete the more available phosphorus reserves by around 2300. This, in addition to environmental concerns, evokes the need for new phosphorus recovery techniques to be developed, to meet future generations needs for renewable phosphorus supply. Many studies have been, and are, carried out on phosphorus recovery from wastewater and its sludge, due to their high phosphorus content. Chemical precipitation is the main process for achieving a phosphorus-containing mineral suitable for reuse as a fertilizer, such as struvite. This paper reviews the current status and future trends of phosphorus production and consumption, and summarizes current recovery technologies, discussing their possible integration into wastewater treatment processes, according to a more sustainable water-energy-nutrient nexus.

Combined sewer overflows (CSOs) are of major environmental concern for impacted surface waterbodies. In the last decades, major storm events have become increasingly regular in some areas, and meteorological scenarios predict a further rise in their frequency. Consequently, control and treatment of CSOs with respect to best practice examples, innovative treatment solutions and management of sewer systems is an inevitable necessity. As a result, the number of publications concerning quality, quantity and type of treatments has recently increased. This review therefore aims to provide a critical overview on the effects, control and treatment of CSOs in terms of impact on the environment and public health, strict measures addressed by regulations, and the various treatment alternatives including natural and compact treatments.Drawing together the previous studies, an innovative treatment and control guideline is also proposed for the better management practices.

Phosphorus is an essential nutrient for plants, which use it as a basic fertilizer component and is expected to increase significantly in the coming years due to higher food crops demand. Unfortunately, the available phosphorus natural reserves are not renewable, and estimates indicate their rapid decline in the future. Urban wastewater, due to its abundance and relatively high phosphorus content, is an excellent candidate for phosphorus recovery, while the element's removal from urban effluents was introduced in the 1970 s to limit the undesired effects of eutrophication. In this study, the process of struvite (and related phosphorous compounds) crystallization was investigated for three different sludge types, and the results were compared. While most studies focus strictly on anaerobically digested sludge for high struvite precipitation efficiency, this study investigated the possibility of inducing precipitation on both aerobic (extended aeration) and anaerobic sludges produced by two wastewater treatment plants in northern Italy. Analysis of precipitates from sludge samples was compared, where the focus was on the aerobic sludge, and its potential for struvite recovery. The effect of different reaction parameters was studied under different operating conditions, and the use and effects of Ca(OH) 2 addition as an inexpensive potential pH adjustment reagent was investigated.

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Combined phosphorus (P) removal and recovery from wastewater is a sensible and sustainable choice in view of potential future P-resource scarcity, due to dwindling primary global reserves. P-recovery from wastewater, notwithstanding the relatively small fraction of total global amounts involved (less than 1/5 of total global use ends up in wastewater) could extend the lifespan of available reserves and improve wastewater cycle sustainability. The recovery of the resource, rather than its mere removal as ferric or aluminum salt, will still allow to achieve protection of receiving waters quality, while saving on P-sludge disposal costs. To demonstrate the possibility of such a recovery, a strategy combining enhanced biological phosphorus removal and mineral P-precipitation was studied, by considering possible process modifications of a large treatment facility. Process simulation, a pilot study, and precipitation tests were conducted. The results demonstrated that it would be possible to convert this facility from chemical -precipitation to its biological removal followed by mineral precipitation, with minimal structural intervention. Considerable P-recovery could be obtained, either in form of struvite or, more sustainably, as calcium phosphate, a mineral that also has possible fertilizing applications. The latter would present a cost about one order of magnitude lower than the former.

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Phosphorus is a potential environmental pollutant, which could lead to the eutrophication of water bodies. For this reason, wastewater treatment plants worldwide are often designed and operated to eliminate phosphorous from effluents, at substantial cost. At the same time, phosphorus is an essential nutrient for agriculture and, consequently, human life. Data seem to suggest that the world will run out of phosphorus by around 2300, in the best case scenario, although even shorter estimates exist. This situation evokes the need for more efficient phosphorus recovery technologies, in order to meet current water quality requirements and—at the same time—critical future phosphorous needs. Chemical precipitation is the main process for achieving a phosphorus-containing mineral suitable for reuse as a fertilizer, where Struvite is an example of such a product. In this study chemical equilibrium of struvite precipitation was simulated using US Geological Survey (USGS)’ PHREEQC model, and results were compared to laboratory precipitation tests to evaluate struvite recovery efficiency under various conditions. pH had the most significant effect on the results and P recovery of >90% was achieved at pH = 9.5. Simulations indicated that struvite precipitation is affected by the presence of Amorphous Calcium Phosphate (ACP) and calcite in the final product of the process. The model showed great potential for predicting equilibrium conditions, and could be very helpful for future optimization of the process.

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