The effect of mean cell residence time on the dewatering characteristics of a biological sludge

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Zentkovich, Terry L.
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Virginia Polytechnic Institute and State University

The effect that mean cell residence time (MCRT) had on the dewaterability of biological sludges was examined in this study. Aeration basin sludge and waste activated sludge from a full scale domestic wastewater treatment facility, in addition to sludges produced from two laboratory scale reactors fed with a synthetic substrate and a primary effluent-dog food mixture, respectively, were used to perform dewatering tests. The sludges were evaluated at various MCRT values for optimal dewatering resistances, optimal conditioning requirements, and optimal compressibility conditions. Specific resistance determinations were made using a 3uchner funnel apparatus to evaluate all of the above mentioned parameters. Also particle size analyses were performed on all sludges to investigate how particle size affected dewatering resistance and conditioner requirements, and also to investigate how MCRT affected particle size. All particle size determinations were made using a HIAC PC-320, twelve channel particle size analyzer.

Results from the study revealed that plants can operate under extended aeration and still maintain good sludge dewatering characteristics. Likewise, by varying MCRT shifts in particle size distribution and corresponding changes in dewatering resistance were noted in the laboratory reactors. However, no optimum MCRT with respect to dewatering could be founded. Particle size proved to be the most important parameter affecting dewatering, and it was affected by conditioning, periods of anaerobiosis, and MCRT in the laboratory reactors.