Physical properties of chemically conditioned sludges

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Virginia Polytechnic Institute and State University

The problems of sludge handling and disposal have significantly increased in the past decade because of more stringent discharge limits which create more disposable sludges and because regulations have been enacted which control the ultimate disposal of sludge residues. These problems have resulted in a need for producing sludges with both good dewatering characteristics and low water content so as to minimize the ultimate volume of disposal solids. Because of the need for more information concerning sludge dewatering this project was undertaken. The objectives of this study were to quantify the physical properties of sludges following chemical conditioning by a variety of conditioning chemicals.

Water and wastewater sludge samples from full-scale Blacksburg water and wastewater treatment plants were used. The iron sludge was made in the laboratory. Each one of these samples was subjected to dewatering studies using laboratory-scale vacuum filtration equipment and sand drying beds.

This study indicated that the best conditioner for activated sludge is ferric chloride. Addition of ten percent ferric chloride (on dry solids basis) enhanced the dewatering process and slightly increased the shear strength. Polymer was also an excellent conditioner for activated sludge. A polymer dose of 0.6 percent on a dry solids basis increased the shear strength slightly and increased the dewatering rate of activated sludge. The increase in solids concentration of activated sludge conditioned with polymer was smaller than that by conditioning with ferric chloride. Since the volume of sludge to be disposed has an impact on economics, polymer conditioning might be preferred over ferric chloride conditioning.

Lime conditioning of activated sludge appeared to be useless. Conditioning with a mixture of 15 percent lime and 5 percent ferric chloride on dry solids basis did not improve the dewatering rate of activated sludge but conditioning with 20 and 25 percent lime and 5 percent ferric chloride for each mixture appeared to modestly improve the dewatering rate and greatly increase the shear strength of activated sludge. However, conditioning with mixture of lime and ferric chloride appears to be unreasonable when ten percent ferric chloride alone can produce a better-conditioned sludge. Polymer appeared to be an excellent conditioner for alum and iron sludges.