Aerobic biological treatment of a colloidal wastewater

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Virginia Polytechnic Institute and State University


The purpose of this study was to examine a modification of the activated sludge process known as contact stabilization. Both batch and continuous flow experiments were conducted under highly controlled laboratory conditions using a wastewater containing colloidal organic material to delineate process fundamentals.

The colloidal wastewater used was prepared from yogurt, beef extract and various mineral salts. Batch type experiments were conducted at 0, 23, 43, and 54 percent colloidal percentages based on a chemical oxygen demand, COD basis. Four different food to micro-organism ratios were used at each colloidal percentage. Measurements of filtered and settled CID, oxygen uptake and mixed liquor suspended solids, MSS, were used to monitor waste stabilization.

Continuous flow type experiments were conducted on two model treatment units. One unit was similar to a completely mixed activated sludge process, and the other a model of the contact stabilization process. Both of these units were operated at steady state conditions on a sludge age basis to compare treatment between each unit. Appropriate measurements were made to determine percent waste stabilization and waste sludge production at each steady state condition studied.

From the results of the batch study, it could be concluded that not all colloidal wastes are subject to a period of adsorption onto biological floc and then released back into solution as was believed previously. Furthermore, using batch experiments for toxicity determinations as suggested previously was found to lead to erroneous results.

Results from the continuous flow portion of the study indicated that the completely mixed and contact stabilization processes both have similar COD removal efficiencies when compared on a total sludge age basis. It was also found that sludge production was lower in the contact stabilization system when compared to the completely mixed system. A mathematical model was developed based on results of the continuous flow experiments that can be used by design engineers when designing a process and can also be used by a treatment plant operator to optimize process operation.