Nitrate (NO3−) can reach both surface water and groundwater as a consequence of agricultural activity (including excess application of inorganic nitrogenous fertilizers and manures), from domestic and industrial wastewater disposal and from oxidation of nitrogenous waste products in human and animal excreta, including septic tanks. Some groundwaters may also have nitrate contamination as a consequence of leaching from natural vegetation. Nitrate levels in excess of 50 ppm in drinking water have been linked to health problems such as methaemoglobinaemia, in particular in infants, as well as gastric cancers.
Several processes are known for removing nitrates from water, such as, ion exchange, reverse osmosis and biological denitrification. In ion exchange processes water is passed through a column containing an anion exchange resin. When this resin is fully loaded with nitrate ions, the resin is regenerated by exchanging with a solution containing a different ion and the resulting nitrate solution needs to be discharged as a waste. In reverse osmosis, the water is passed through a membrane that retains nitrate ions. The resultant concentrated solution of retained ions must then be discarded. Biological denitrification involves the use of bacteria, which are problematic since their capability of removing the nitrogen-containing components is temperature-dependent.
Thus, the drawbacks of these processes is that produce relatively concentrated nitrate solutions which need to be discharged, that they are temperature dependent and that they lead to significant costs.
Electrochemical processes for removing nitrates (i.e. electrodenitrification) from water have also been described in the prior art, such as in patent application ES 2 400 506 A2. In these processes a nitrate ion reducing reaction to ammonia and nitrogen gas occurs at a cathode and, at an anode, chloride ions present in the water are oxidized to hypochlorite. In these processes a nitrate ion reducing reaction to ammonia and nitrogen gas occurs at a cathode, chloride ions present in the water are oxidized to hypochlorite at an anode, chlorine dissociates in water into hypochlorite and chloride, and said hypochlorite reacts with ammonia to produce nitrogen gas, according to the reactions shown below.NO3−+6H2O+8e−→NH3+90H−NO3−+3H2O+5e−→½N2+60H−2Cl−→Cl2+2e−Cl2+H2O→HClO+HCl2NH3+3HClO→N2+3HCl+3H2O
Levels of ammonium and chlorine derivatives present in drinking water may not exceed certain values, 0.5 ppm and 2 ppm, respectively. In order to simultaneously obtain acceptable nitrate, ammonium and chlorine levels using an electrodenitrification process, the time needed for the reduction of nitrate concentration to the desired level must be the same as that for complete oxidation of ammonium to nitrogen gas. Otherwise, ammonium will remain in the water or chloramine compounds will appear by reaction of free chlorine in excess with remaining ammonium. A similar situation occurs for water discarded into public wastewater systems, wherein levels of ammonium may not exceed certain values, generally of 50 ppm. In this case, in order to simultaneously obtain acceptable nitrate and ammonium levels using an electrodenitrification process, the time needed for the reduction of nitrate concentration to the desired level must be the same as that for oxidation of ammonium to nitrogen gas until the desired level of ammonium is reached.
Patent application ES 2400506 A2, discloses an electrochemical cell for electrodenitrificating water wherein acceptable nitrate, ammonium and chlorine levels are obtained. However, said patent application only provides 2 examples in which the nitrate content in the water to be treated is 125 ppm and 123 ppm at about 200 ppm of chloride concentration. However, this patent application does not provide any information regarding the fractal surface of the cathode and the inert surface of the anode. The inventors have observed, that these parameters influence the overall design of electrochemical cells suitable for achieving acceptable ammonium, combined chlorine and nitrate levels, as shown in the in the comparative examples of the present application.
Therefore, there is still a need to provide a process for treating water whereby acceptable nitrate, ammonium and chlorine levels are simultaneously obtained.