GB825629A - Method of purifying sewage water - Google Patents

Abstract

825,629. Treating sewage ; fertilizers. ELEKTROKEMISK A. S. Feb. 14, 1956 [Feb. 14, 1955; Nov. 15, 1955], No. 4616/56. Class 111. [Also in Group XXXVI] A process for purifying sewage water containing phosphates comprises preferably removing from it the coarser impurities, mixing it with a solution containing magnesium compounds, e.g. sea water, and alkalising before or after mixing, whereby phosphates and hydroxides are precipitated. The sewage water may be first mixed with from 10 to 15% of sea water or the like, and then alkali, e.g. lime milk, is added in quantity sufficient to raise the pH to above 9À5. The precipitated phosphates and hydroxides may be floated to the surface by air injected into the mixture, and skimmed off. Instead of sea water the necessary amount of magnesium may be added in the form of dolomite, preferably burnt. The alkali may be burnt lime or burnt dolomite. Sewage water from pipe-line 12a and sea water from pipe-line 6a pass through rotary meters 12 and 11 respectively and then, mixed, into the annular space between a tubular metal casing 9 forming the cathode of an electrolytic cell and a diaphragm 7 within a perforate tube 8. During electrolysis the liquid in the cathode compartment becomes alkaline so that phosphates and hydroxides are precipated. Hydrogen is evolved at the cathode and brings the precipitated substances to the liquid surface where the resulting foam 18 passes through apertures 16 in the tube 9 into a surrounding vessel 15. The foam is removed at 19 by a scraper or by air injected at 20, is dehydrated by precipitation in settling tanks, and may be dried for use as fertilizer. Sea water only, metered at 6, passes through a valve 5 into the anode compartment formed between the diaphragm 7 and a graphite anode 1. Chlorine is evolved at the anode, and the chlorinated anolyte discharged at 22 is mixed with the purified sewage water, discharged from the vessel 15 at 21 in a separate vessel (not shown) so that the sewage water is disinfected by the chlorine dissolved in the anolyte. In a modification purified alkaline sewage water from the cathode compartment may be added, together with lime, untreated sewage water; the quantities must be adjusted so that the pH of the mixture is above 9À5. In a further modification, sea water and a part of the total alkali necessary may first be added untreated sewage water and the mixture then passed into the cathode compartment and electrolysed until the pH is at least 9À5.