Dewatering municipal sludges by osmosis

Abstract

Treatment, handling, and disposal of municipal sludges constitute a difficult and expensive aspect of wastewater treatment. One of the steps in the procedure is dewatering—the reduction of water content to something less than 85%. Conventionally, the dewatering is done by vacuum filtration or by centrifugation, though some other means, such as filter presses, are sometimes used. With most of these processes, some sort of sludge pretreatment, such as the addition of lime or ferric chloride, is normally used. Such additions are not only expensive but render the resulting dewatered sludge less amenable to incineration, because of potentially corrosive products of combustion, or to the spreading of the sludge on agricultural land, because of the unwanted lime, in some areas, and iron. Vacuum filtration of biological sludges rarely produces a sludge of less than 82% moisture, with some passthrough of fines to the filtrate. Centrifugation can produce a drier sludge but normally at the cost of an even higher passthrough of fines. The research for this paper shows that sludges can be dewatered to 70% or less less water content, by osmosis, without the addition of lime or ferric chloride or other pretreatment, though some sort of sludge thickening prior to the osmotic process would lessen the time in the process. Although the work was all done at bench scale, a proposed commercial design is presented.