Water-cleavage takes place as a result of the cathodic reduction. OH–- ions are formed, while hydrogen (H2) is released. Oxidation takes place on the anode. Magnesium ions are released into the solution and react with the P and N in the water to form struvite. Here, magnesium is the limiting reactant. As a result of the electrolytic water-splitting at the cathode, the pH value of the wastewater increases and remains constant at pH 9. This has the advantage that no base has to be added for the struvite precipitation process.
Feasibility study in a pilot plant
In the course of a feasibility study the process was tested using a pilot plant with a flow rate of up to 1 m3/h at a sewage treatment plant with biological phosphorus elimination. The average phosphorus 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 37 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 electrochemical 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 obtained which can then be used directly as a fertilizer. The total quantity of chemicals that would have to be used at the treatment plant would decrease by 40 tons or 20 percent per year.
Industrial-scale implementation with flat-panel reactors
With the results from the first pilot tests, in which tubular electrolytic cells were used, the process was further developed with flat-panel reactors for industrial use.