Treatment of waste seawater by electrolysis using charcoal electrodes

Dissolved inorganic nitrogen is one of the major sources for eutrophication of lakes and coastal seas. Electrolysis is one of the expected tools for treatment of waste water. If metallic electrodes are used, the metal ions diffuse into water in the prolonged use. It costs too much to use platinum or gold for electrodes, which are difficult to diffuse into water. In the present study, carbon was proposed for the material of electrodes to decompose dissolved inorganic nitrogen in seawater. Among several kinds of carbons, active carbon was selected since diamond and graphite are expensive. Time variability of dissolved inorganic nitrogen was monitored in waste seawater to investigate the effects of electrolysis on decomposition of dissolved inorganic nitrogen. The seawater with the salinity level of about 3% was produced artificially. Ammonium was added to the water until its concentration was a few mg/L. The distance between the electrodes was 15 mm. Voltage was set to be 4V. As a result, ammonium, nitrite, and nitrate were decomposed successfully in the case of direct electric current power. The rates of decomposition of ammonium, nitrite, and nitrate were 2 mg/day, 2 mg/day, and 6 mg/day, respectively. However, the pH value increased since the hypochlorite was generated between the anode and cathode. The rate of decomposition of ammonium may be overestimated since the ammonium is converted into ammonia if the pH value increases. In the case of the alternating electric current, the pH value was able to be stabilized, however the rate of decomposition of dissolved inorganic nitrogen decreased. As future studies, it is indispensable to carry out the experiments that change the pulse width in a wider range. In addition, the concentrations of substances in the seawater should be monitored more precisely. Then the electrolysis system will be applied to marine aquaculture site for feasibility test, in comparison with the existing methods such as aeration.