Principles and techniques for conditioning of waste-activated sludge by direct slurry freezing

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Virginia Polytechnic Institute and State University


Direct freezing has been extensively and successfully used for the desalination of saline waters, and for the concentration and recovery of by-products from industrial wastes. In this investigation the direct slurry freeze conditioning of waste-activated sludge was studied, and it was shown that it produces a considerable improvement in dewatering. Dewatering characteristics were adequately defined by the specific resistance and coefficient of compressibility values, by settling, and by filter cake quality when vacuum filtration and gravity drainage on sand beds were utilized.

The sludge samples for the principal part of the investigation were obtained from three different sources: two extended aeration plants treating domestic sewage and a conventional activated sludge plant treating combined industrial domestic wastes. During the latter part of the research, waste activated sludge samples from a plant treating pulp and paper waste and from aerobic digesters were also conditioned by the process.

Gravity settled sludge concentrations after freezing and thawing ranged from 10 to 14 percent. Filter cake moisture contents after vacuum filtration of the conditioned sludge ranged from 40 to 80 percent while cake moisture after gravity drainage on sand beds followed by seven days of air drying ranged from 30 to 90 percent. Very little change in supernatant quality occurred during direct slurry freeze conditioning. Direct microscopic and wet sieve analysis indicated that the principal mechanism of conditioning was the promotion of flocculation.

It was found that the degree of conditioning is a function of the "butane contact" or freezing time with better conditioning occurring with longer detention times. Variables affecting conditioning such as the feed solids A concentration and the butane flow rate were evaluated as were various factors that affect the dewatering of the conditioned sludge.

An economic evaluation indicated that the direct slurry freezing process has an economic advantage when compared to other sludge conditioning processes such as indirect freezing and heat treatment. A cost estimate of $6 to $20 per ton of dry solids processed was projected.