Effects of Mixing Ratio, Contact Time and Dissolved Oxygen on Efficiency of Biosorption for Primary Carbon Diversion

Wastewater treatment plants are being transformed from energy sinks to energy-neutral or even energy-positive utilities. Conventional primary wastewater treatment diverts about half of the influent particulate organics to potential energy-generating anaerobic digestion but allows the soluble fraction to pass on to energy-consuming secondary oxidation. Enhanced removal of particulates can be accomplished via physical processes, but chemicals or biological processes are necessary for diversion of a portion of the soluble organics. The performance of biosorption (with waste activated sludge) followed by either flotation or micro screening separation has been evaluated in terms of some controllable operating parameters. Results show that the mixing ratio (or dose) of biosorbent, dissolved oxygen between 0.5 and 1.0 mg l-1 and contact times between 15 and 60 minutes have only minimal impact on soluble COD removal performance which can be from 30 to 35%. The use of polymer has a large impact on either DAF or micro screen separation in terms of TSS removal performance with increases from 30 to 50% without polymer to over 60% with polymer. Biosorption was found to follow zero-order kinetics for the truly soluble organics (ffCOD) fraction at a rate of 3.0 mg sorbed per minute per gram TSS sorbent which is mass transfer limited and does not reach equilibrium and thus cannot be fitted with an isotherm model. Sorption of the colloidal organic fraction (cCOD) is enmeshed in the floc matrix and is practically instantaneous.