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ePaper created 2016-03-17, 15:06:18 | version 1.45.01
112 1 2 Recovery and recycling of phosphorus The expansion of the bioeconomy coupled at the same time with the increasing demand for food worldwide is resulting in a growing demand for fertilizers, especially those containing phosphorus. However, the supply of phosphorus fertilizers is determined by the declining purity of the storage sites combined with a decrease in the phosphorus concentra- tions, so that the costs for both extraction and processing are rising. Likewise, the costs for the production of synthetic nitrogen fertilizers are increasing due to the high primary energy requirements. A way out of this situation in terms of sustainability is the recycling of the essential nutrient elements phosphorus (P), nitrogen (N) and potassium (K). These have to be recovered from material cycles in industrial production, the reutilization of foodstuffs, municipal wastewater and from the bioenergetic processing of waste. Fraunhofer IGB is developing and implementing sustainable, cost-efficient technologies and strategies for the integrated management of resources. One of the key areas is the devel- opment of new technologies for recovering nutrients from wastewater and organic waste. Electrochemical phosphate precipitation from wastewater Fraunhofer IGB has developed the ePhos® process to recover ammonium (NH4 + ) and phosphate (PO4 3– from the filtrate water resulting from municipal wastewater treatment. The phosphate is precipitated electrochemically without any use of chemicals. During this process, phosphate precipitates as magnesium-ammonium-phosphate (MgNH4PO4*6 H2O, MAP or struvite) (Fig. 1). The electrochemical phosphorus precipitation takes place in an electrolytic cell consisting of an inert cathode and a sacrificial anode of magnesium ig Water molecules are split by the cathodic reduction, forming OH– ions that raise the pH-value. As a result, it is not neces- sary in the ePhos® process to adjust the pH value by dosing chemicals. Oxidation occurs at the anode: magnesium ions are released into the solution and react with the phosphorus and nitrogen in the water to form struvite. Feasibility study in a pilot plant In the course of a feasibility study the process was tested using a pilot plant ig with a flow rate of up to m3 /h at a sewage treatment plant with biological phosphorus elimina- tion in North Germany. We were able to demonstrate to the client that the phosphorus precipitation and recovery from the centrate water by means of the electrochemical ePhos® process can be carried out at the client’s treatment plant, so that a full-scale plant would solve major operational problems caused by the fluctuating orthophosphate concentrations ll the trials were carried out successfully. The average phospho- rus elimination rate from the centrate water of the digested sludge dewatering and the phosphorus conversion to struvite was more than 80 percent. The phosphorus concentration in the centrate water was reduced by an average of 180 mg/L to 20.8 mg/L. The phosphorus load that no longer has to be treated when the filtrate water is recirculated, decreases by percent; this amounts to 9284 kilograms annually and results in a reduction of sludge production by 7 percent. The design of the process for the client’s plant shows that the electro- chemical phosphate precipitation would require approx. 10 tons of magnesium in the form of sacrificial electrodes per year. From this, approx. 73 tons of struvite per year would be ePhos® – ELECTROCHEMICAL RECOVERY OF PHOSPHORUS Iosif Mariakakis, Uwe Claußnitzer, Jennifer Bilbao, Siegfried Egner ENVIRONMENT anode (oxidation) cathode (reduction) 2 H2O H2 2 OH– e– + – Mg2+ Mg 12