Batch experiments were conducted with raw wastewater and preaerated microbial sludge to investigate the removal behavior of the contact stabilization activated sludge process. This process was originally developed based on observed rapid drops in BOD which were attributed to rapid adsorption of colloidal organics. To date no data have been presented verifying that rapid colloidal adsorption occurs or is responsible for rapid BOD decreases.

Food to Microorganism (F:M) ratios were varied from 0.047 to 0.447 mg COD/mg MLSS during batch tests conducted with thirty minute contact times. Mixed liquor samples were collected at predetermined times, poisoned with mercuric chloride to halt microbial activity, and settled. Samples of the supernatant were drawn off and the COD of organics in different size ranges determined using a sequential filtering technique. Colloidal COD was defined to be the difference between the COD of organics passing a 1.5 µm filter and those passing a 0.03 µm filter.

Results indicated that rapid colloidal removal occurred. Colloidal COD concentrations decreased from initial values (maximum 85 mg/l) to near 0 mg/l within five minutes after mixing. Such behavior was only observed for the colloidal fraction and occurred regardless of the initial colloidal COD, total COD, or F:M ratio. It was concluded that physical coagulation was most likely the mechanism responsible for rapid colloidal removal.

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