GB1079087A - Process and apparatus for dehydrating waste solids concentrates - Google Patents

Abstract

1,079,087. Dehydration of sewage sludge and other waste solids concentrates. C. GREENFIELD. Nov. 29, 1965, No. 50579/65. Heading BIB. A waste solids concentrate, such as sewage sludge or black liquor from the paper industry, is dehydrated by mixing it with a relatively non-volatile organic liquid, e.g. tallow oil, fuel oil or glycerine, and subjecting the mixture to a plurality of heating stages at successively higher temperature, wherein water is evaporated, the evolved vapours from each stage (except the first) being employed as heating- medium in the preceding stage, and withdrawing a substantially anhydrous slurry of waste solids in organic liquid from the last stage. Sewage slurry is introduced through line 1 into tank 2 where it is mixed with 50 parts of tallow oil. The mixture is pumped through line 4 into the first stage 5 of a backward flow triple-effect evaporator, where water is boiled off at a temperature of 110‹F. and a pressure of 2" Hg. absolute, by vapour supplied from the second stage through line 7. The vapour evolved in the first stage is conducted from chamber 8 to direct-contact condenser 10. The partially dehydrated slurry is transferred through line 32 to the second stage 13, where it is boiled at 150‹F. by steam conducted through line 14 from the third stage. The slurry is then pumped through line 16 to the third stage 17, where the remainder of the water is boiled off at 250‹F. by steam at 300‹F. supplied through line 19 from boiler 18. The dehydrated slurry, containing 15% solids and only 1% water, is delivered through line 21 to centrifuge 22, from which a clear oil stream is withdrawn through line 23. A stream containing 30% oil and 70% solids is delivered through line 25 to expeller 26, from which dry solids are conveyed to grinder 28 and then blown through line 29 to the furnace of boiler 18, or withdrawn through line 30 for sale as a fertilizer. Level control in each stage of the evaporator is maintained by a sensing element in the sump of each stage which transmits signals to a throttle valve following the pump in the line supplying slurry to that stage. The evaporator stages may be of the single pass, forced recirculation, flash, falling film or rotary wiped film type. The dehydration of an aqueous dispersion of powdered coal, cement or spent lime is referred to.