In Sweden, the majority of the local Swedish Waterway Authorities have maintained that phosphorous is the limiting factor of biological plant growth. The part played by phosphorous in this regard has been the topic of progressive discussion in recent years, particularly with respect to its influence in the environment of the coastal waters of Sweden. Nitrogen oxidation processes, and also nitrogen reduction processes are at present the subject of much scientific study in Sweden. Table 1 below shows different nitrogen sources found in Sweden and Finland.
TABLE 1 ______________________________________ Nitrogen Load Sweden Finland tons/year % tons/year % ______________________________________ Natural 58 000 40.5 44 300 51.0 Forestry and Agriculture 52 400 36.6 22 200 25.6 Municipal Sewage 24 900 17.4 13 000 15.0 Industry 7 550 5.3 6 900 8.0 Fish Farms 260 0.2 320 0.4 .SIGMA. 143 310 86 720 ______________________________________
Almost half of the nitrogen load on the Swedish environment is derived from "natural" sources, i.e. from meteorological precipitation, erosion, or fixation. 37% is derived from forestry and agriculture, 17% from municipal sewage, and 5% from industrial emission. Fish farming contributes only 0.2% of the total load. Ammonium nitrogen, by which is meant the nitrogen in the ammonium compound, and nitrogen as nitrate are both good plant growth sources. Nitrite is often considered to inhibit biological activities when present in large quantities, since there is then the risk that nitrosoamines will form. Ammonium nitrogen may be poisonous to fish, and may also choke the oxygen in the recipients.
A general review of the various nitrogen processes is given in Table 2 below. Solely municipal sewage, biological oxidation processes, and nitrogen separation processes will be dealt with in the following, since the present invention pertains to the treatment of sewage water. In the absence of pronounced industrial influences, municipal sewage water can be maintained at a normal total nitrogen content of about 40 mg/1.30-50% of this nitrogen is in the form of ammonium nitrogen, while the remaining nitrogen is organically found. The major part of this organically bound nitrogen is converted to ammonium nitrogen in the biological stage.
TABLE 2 __________________________________________________________________________ Methods for the chemical reduction of nitrogen Methods Influence on Suitable for + (pros & cons) Cost __________________________________________________________________________ Biological Nitrification orgN; NH.sub.4.sup.+ Municipal and Sensitive Relatively industrial process expensive sewage Denitrification NO.sub.2.sup.- ; NO.sub.3.sup.- Municipal and Dependent on Relatively industrial organic expensive water carbon Assimilation orgN; NH.sub.4.sup.+ Polishing of Temp. light in algae pond NO.sub.2.sup.- ; NO.sub.3.sup.- water of low requires nitrogen org. carbon content Physical Chemical Ammonia NH.sub.4.sup.+ Increase conc. High pH Extremely stripping where &lt; 80% required, expensive red. is ice formation required in winter Ion exchange NH.sub.4.sup.+ ; NO.sub.3.sup.- Drinking Process Highly NO.sub.2.sup.- water regeneration expensive Break-point NH.sub.4.sup.+ Low residual Org. chlorine High opera- chlorination contents in compounds tional costs clean water Electrolysis NH.sub.4.sup.+ Technical problems Osmosis NH.sub.4.sup.+ ; High contents Membrane Ozone NO.sub.3.sup.- ; clean water problems NO.sub.2.sup.- Precipitation NH.sub.4.sup.+ High contents Requires PO.sub.4.sup.3- NH.sub.4 MgPO.sub.4 __________________________________________________________________________
There is therefore a need for improved elimination of nitrogen from sewage water.