Problem: The most wastewater produced in modern countries is treated by centralised wastewater treatment plants which have a low technological flexibility and high financial costs. The phosphate in the wastewater, a nutrient, is burned and diverted into the rivers. Since the reserves of phosphate rock are limited this thesis tries to define a way to recover it by electro-coagulation.
Solution: Novaquatis (EAWAG) propose to separate the urine from wastewater and to treat it separately in a small scale reactor to recover the phosphate.
A method for the phosphate recovery from urine is the struvite precipitation. Struvite is usually obtained by the addition of magnesium mineral. The dosage control and storage of the powder aren’t well adapted to small reactors. The electro-coagulation of phosphate has potential to be a better solution at the household scale.
Process: The electro-coagulation of phosphate from urine is based on the electro-dissolution of a magnesium electrode. The dissolved magnesium reacts with phosphate and ammonium, present in urine, to struvite. The struvite is recovered and sold as fertilizer.
Thesis: The electro-coagulation isn’t yet a well known process. The thesis tried to define and to optimize the whole process. The design of a small scale reactor, a simple controlling and monitoring possibilities are studied. Finally the economical and technical suitability of the process is evaluated.
Method: Two metal plates, used as electrodes, are put into a one litre reactor filled with a synthetic solution simulating urine. The working electrode is made of magnesium and the counter-electrode of stainless steel. A constant voltage was applied, inducing the dissolution of magnesium. Many variables were measured during the phosphate elimination, like pH, current, conductivity, etc. A series of experiments was set up to optimize the process.
Experiment: To understand the process some basic experiments were done. Afterwards the electro-coagulation was performed with different voltages to find out an optimal one. It appeared that in some conditions the magnesium plate was covered with a white layer, a struvite layer, which inhibits the electro-coagulation process. How to prevent the formation of this layer is described.
Results: The first results were promising, the struvite was actually produced and the phosphate almost completely removed. The optimal voltage was defined in the range of +1.0V to +1.25V avoiding the struvite layer formation on the working electrode by lower voltage and the electrolysis of the solution at higher voltage.
A nice way to monitor the process is to follow the changes of the conductivity of the solution and the change of the working electrode potential. The rate of electro-dissolution of magnesium is dependent on the current, so by setting the current, we can control the process well, even stop it.
Suitability: The different experiments showed the process of electro-coagulation is undoubtedly a technologically suitable way to recover phosphate from urine. The process can only be profitable considering the saving costs by the wastewater treatment plant. The cost of the production of the magnesium electrode is difficult to be compensated by the selling of the struvite fertilizer.
Conclusion: The result of this thesis will certainly promote the process of electro-coagulation, since the process is working well and easy to control. The different open points are mentioned in order to confirm the promising results described in this project.