Date of Award

Spring 1989

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)


Civil and Environmental Engineering

First Advisor

Zanoni, A. E.

Second Advisor

Katz, William J.

Third Advisor

Novotny, Vladimir


The High Rate Activated Sludge(HRAS) process is a short term modification of the conventional process with extremely high loading factors. The effluent from the process is relatively high in suspended solids concentration and as a result fails to meet effluent standards even though substrate conversion is at its maximum. This research was undertaken to find a solution to improve the quality of the effluent from the HRAS process. Addition of flocculating agents to enhance the separation of solids was the means considered. The initial phase of the research was directed towards finding a suitable aeration (detention) time. Experiments were conducted at detention times of 3, 6, 9, and 12 hours. During this phase treatment efficiencies with respect to filtered effluent were found to be in the range of 88-99% compared to treatment efficiencies of 80-90% in the case of unfiltered effluent, confirming the fact that dispersed solids in the effluent were responsible for the inferior effluent quality. After reviewing the results from this phase, a detention time of 12 hours was found suitable for experiments in the next phase. The second phase involved testing flocculating agents to enhance separation of the dispersed microorganisms and to improve the quality of the effluent from the process. Percol 700 series polymers (both anionic and cationic) and ferric chloride were considered. Differences in COD removal efficiencies between unfiltered and filtered effluent supernatant samples were used to monitor the effectiveness of the flocculating agents. It was found that both cationic polymers and ferric chloride were effective in flocculating the dispersed solids, while the anionic polymers hindered settling of the solids and led to deterioration in effluent quality. It was possible to reduce the average difference in COD removal efficiency from 20% in the controls to around 3-5% by using Percol 701, a low molecular weight cationic polymer or ferric chloride. It was also noticed that low molecular weight polymers were more effective than high molecular weight polymers.



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