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(r) Under war conditions, when the demand for fertilizer material was great, unusual conditions existed which, in general, stimulated the use of all air- or heat-dried sludges. The indications are that adjustments will eventually occur, with lowered prices for all heat-dried material. For the sewage treatment plants offering air-dried material gratis, the demand should continue with increased use as the value of the material is demonstrated. Disposal of Digested Sludge 39. (1) In his report dated 15th December, 1948, Mr. Porter estimated that (on the assumption that trade wastes will have to be dealt with by the Drainage Board) there will be about 3,000 tons of wet sludge per week in the first few years and that ultimately the quantity may exceed 7,000 tons per week. After pointing out that the disposal of wet sludge in a sanitary manner is one of the main problems in sewage purification and that if, as is proposed, the digestion process with sludge-gas production is adopted for Auckland there will remain for disposal a considerable quantity of sludge with a water content of more than 91 per cent., he discussed (and opposed as tooexpensive) the carrying of the sludge to the open sea. He then referred to various other methods of disposing of digested sludge, and it will be convenient to quote his statement on these in full: — Other methods of disposal of digested sludge are : (a) Pressing by machine, with sale of sludge cake for its manurial value. This process is somewhat expensive and is not now usually included in new sludge disposal works. At some works, sludge cake is heaped in the open, and ground to produce a fertilizer base. A sample of ground undigested sludge cake from Bradford is available for inspection. (b) Elutriation, vacuum filtration, flash drying, and incineration (or grinding to produce fertilizer). This process is used by West Kent Main Sewerage Board, and by a number of American cities, and will be used at the new Colne Valley Sewage Works. It has the disadvantages of requiring large quantities of heat and chemicals, and investigations at Manchester and Sheffield have proved it to be somewhat expensive. It could only be operated in New Zealand if a regular supply of necessary chemicals could be guaranteed at a low price for many years in advance. A sample of ground digested primary sludge from West Kent is available for inspection. » (c) Air-drying on scoria beds, covered with sand. This process involves use of considerable areas of land, but is relatively cheap to operate. Air-dried sludge has a sale value of possibly 10s. per ton, which would hardly pay for the cost of lifting and transport. (d) Air-drying, followed by rapid grinding at high temperatures. This is the most up-to-date British process and on test has produced a good quality fertilizer, of which a sample (from West Middlesex) is available for inspection. Material of similar quality is sold commercially in Great Britain at a retail price of 255. per cwt. The wholesale value on yearly contract should be £6 to £lO per ton. Middlesex County Council is now installing plant to produce about 4,000 tons per annum, and if satisfactory sales are obtained can produce ultimately about 20,000 tons per annum. This process is recommended for use on Motukorea, and, if satisfactory there, for use at any works which may be established at Mangere or Southdown. The amount which might be produced in 1965 is 6,000 tons per annum approximately. The annual (loan and operating) cost of sludge disposal by this method would be about £40,000. This amount would be offset by value of gas produced, about £lO,OOO per annum, and value of fertilizer produced, say £25,000 net per annum. In a full operational year, therefore, the income from sludge treatment would almost equal the overall cost of sludge disposal. Notes from West Middlesex Works indicate that the final moisture content should be about 10 per cent., and that if relatively full scale oxidation is carried out the material would contain at least 3 per cent, organic nitrogen and about 2 per cent, phosphoric oxide with traces of potash and other valuable materials. The powder is in such condition that it could readily be fortified by the addition of phosphates and potash to any degree required. (e) Composting with town wastes. In 1965, organic wastes available in Auckland would be about 15,000 tons per annum. Composts should initially contain about' 1 ton wet sludge to 1 ton organic wastes. As the output of wet digested sludge will be about 70,000 tons per annum, a composting scheme depending on town and industrial wastes only, is quite impractical as a means of disposing of the Board's output of sludge, as it would leave nearly 55,000 tons per annum to be disposed of by other means. (2) It will be noted that Mr. Porter recommended that the process of air-drying followed by rapid grinding at high temperatures should be adopted for the Brown's Island scheme, and if it is found to be satisfactory there, that the same process should be used in the southern side of the isthmus.

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