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Phosphorus Recovery from Sewage Treatment Works Executive Summary of MSc Thesis by Mathis Rogner Centre for Environmental Policy Imperial College London Academic Year: 2008 – 2009 Supervisors: Nick Voulvoulis, Guy Ohandja Objectives: Five main objectives were identified: • To identify the drivers for phosphorus recovery. • To identify technologies for recovery from wastewater and sludge for further investigation. • To compare recovered phosphorus products. • To compare phosphorus recovery technologies given holistic set of comparison criteria. • To evaluate and confirm best performing technologies by case study a case study. Introduction Phosphorus is one of the most essential elements limiting plant growth and therefore, crop yields. Human activities have disrupted the natural cycle of phosphorus to a point where phosphorus rock resources are diminishing, while at the same time, phosphorus is accumulating in the natural environment, often with negative consequences. Thus, there is a growing consensus among sanitary engineers that wastewater is not necessarily a “waste”; rather a potential supply of valuable resources, one of which is phosphorus. There exist a number of emerging technologies to recover phosphorus from sewage treatment works. The fundamental aim of the project is to identify, evaluate and compare various technologies for phosphorus recovery from wastewater or wastewater sludge. It aims to deliver a clear overview of the strengths and weaknesses of the technologies investigated, to assist policy and decision makers in deciding which recovery process to concentrate investment in further research and development. Methodology The first phase of this work involved the identification of drivers for phosphorus recovery by a review of relevant literature. By reviewing scientific literature and existing and emerging E.U. and U.K. legislation, the drivers for recovery were identified. A review of existing literature was also used to identify and select the recovery technologies for subsequent comparison. The technologies identified for comparison were pre-screened, in that their development needed to have progressed beyond the lab-scale, and at least a pilot scale plant was required to have been constructed. The literature review was the main method to collect information on the various possible recovered phosphorus products and the performance characteristics of the technologies themselves. These included reports released by the technology manufacturers themselves, as well as independent reviews of the technologies. Both the recovered phosphorus products and the recovery technologies were compared using a method based on Multi-Criteria Analysis. A holistic set of comparison criteria were determined, and the products and technologies were compared using qualitative and quantitative rankings. The products were compared based on their prospective use in agriculture, in the efficiency and ease of recovery, and in their associated environmental benefits. Recovery technologies were compared according to product quality (purity), cost, operational efficiency, and associated environmental benefits. For each product and technology, the comparisons were optimised for each of the above parameters, to show the how each option performs relative to the single parameter. It is important to note that the wastewater and sludge technologies were compared separately as the nature of the processes involved make a direct comparison impractical. In order to evaluate and confirm the best performing technologies after comparison, a case study using real data from the Whitlingham Trowse STW was performed. This involved using input flow data and nutrient concentrations to model the amount of chemicals required to be added and the final output of recovered products. Results Three main drivers were identified for phosphorus recovery. These were: sustainability and environmental concerns, existing and emerging legislation, and economic drivers. In addition to the well documented risk to the environment due to eutrophication, it was determined that the current practice of extraction and use of phosphorus fertilisers is not sustainable. The current methods produce hazardous and toxic by-products. Diminishing high quality reserves mean that the processing will require more energy and will produce more by-products. Meanwhile, the agricultural demand for phosphorus fertilisers is only expected to increase in the future. Legislation regulating the discharge of phosphorus into surface and ground waters, and also regulating the application of wastewater sludge to agriculture are typically in place to protect the environment for discharges associated from STWs. It was determined that phosphorus discharge consents will only become more stringent in the future, requiring more phosphorus to be removed to sludge. However, stricter regulation on the applicability of sludge to land will also lessen the route for agricultural sludge disposal. This provides an impetus to recover phosphorus separately thus removing it from both wastewater and sludge. Recovery technologies will only be viable if they can compete with the conventional processing of phosphate rock. As discussed earlier, reserves are diminishing, thus increasing the price of phosphate rock and also fertilisers. This allows for recovered phosphorus products to be competitive in the near future. The potential recovered phosphorus products were identified to be ferric phosphate, calcium phosphate and struvite. In comparison, it was determined that struvite is the best product for use in the U.K. As ferric phosphate cannot be utilised as a fertiliser or a raw feed in phosphate processing, and while calcium phosphate can be a substitute raw material in phosphate rock processing, no industry of this type currently exists in the U.K. Of the wastewater technologies studied, it was determined that fluidised bed reactors and the ViroFilter technology were the best performing processes over all parameters. Sludge technologies compared resulted in KREPRO and Aqua-Reci, a sludge treatment technology based on supercritical water oxidation, showed the most promise. A summary of these comparisons and their scores are shown in Figures 1 and 2. The case study modelled the use of fluidised bed reactors, ViroFilter, KREPRO and Aqua-Reci technologies at Whitlingham Trowse STW. It was able to highlight the exorbitant chemical requirement for sludge technologies. It did, however, show that the use of fluidised bed reactors requires reasonable chemical addition, and can produce up to 232 g struvite per cubic meter wastewater treated. Optimisation Parameter FBR Unitika REM-NUT ViroFilter No Weightings 14 18 18 13 Product Quality 18 26 22 29 Cost 23 32 37 17 Operational Efficiency/Use 23 23 33 15 Environmental Benefits 28 36 26 22 Figure 1: Summary of Wastewater Technologies Comparison, Optimisation of Each Parameter Optimisation Parameter KREPRO KEMICOND AQUA-RECI None 14 11 13 Cost 28 14 23 Product Quality 20 27 21 Operation Efficiency/Ease 22 11 26 Environmental Benefits 21 19 23 Figure 2: Summary of Sludge Technologies Comparison, Optimisation of Each Parameter Discussion, Conclusions and Policy Implications The decision to implement phosphorus recovery in a sewage treatment works will, inevitably, fall upon the shoulders of the economic viability of the recovery process. However, increasingly stringent regulations on phosphorus concentrations in both wastewater and sludge can provide further impetus to develop phosphorus recovery technologies today. The comparison of wastewater and sludge technologies yielded that when evaluated in within the context of the U.K., wastewater technologies would prove to be a better investment. Sludge treatment technologies are more expensive and require a lot of added hazardous chemicals. These technologies were developed in countries where the disposal of sludge to land is severely limited or prohibited. The U.K. still allows application of treated sewage sludge to agriculture, and as such, the main limitations preventing application is the concentration of heavy metals and nutrient concentrations. Thus recovering phosphorus from the wastewater stream, either directly after secondary treatment, or after anaerobic digestion, allows for more sludge to be continually disposed of to land. Of the wastewater treatment technologies, the fluidised bed reactors show the most promise. They are well established in both Japan and in Canada and are proven to be able to produce a marketable struvite product. They are also easily retrofitted to existing assets in STWs and can treat a variety of different influents and flows. The ViroFilter technology is also very interesting in that it is a completely passive operation, in that it does not require operator attention, and is ideal for smaller unmanned STWs. However, further research is required to test the effectiveness of the expended filter material as a fertiliser. A major limitation of this study was the lack of reliable data. As many reports were issued by the manufacturer or on their request, the information released was selective in that only positive aspects of technologies were printed. It was very difficult to find reliable cost data as the technologies were developed at different times in different countries. Manufacturers are also reluctant to release detailed cost data to non-buyers. As the comparisons are relative this is not a big issue, however it limits the value of the case-study as many gross assumptions were made. Looking forward, the aim of this report is to give an overview of recovery technologies in a single concise document. A technology should not be evaluated in isolation; rather, it should be compared to the alternative options available. However, before a decision to construct new technologies in STWs can be made, further research into the individual selected technology should occur. This would be best performed by a life-cycle analysis. As the viability of a recovery technology relies on the marketability of the recovered product, a market analysis is recommended to be undertaken to evaluate whether or not the recovered product can be sold for profit within the U.K., or more importantly, in the near vicinity of the STW itself.
"Recovery of Phosphorus from Sewage Treatment Works - Phosphorus "