COMPARATIVE VIRTUES OF ASB AND AS SYSTEMS

The purpose of this paper is to undertake a comparative analysis of the activated sludge (AS) and aerated stabilization basin (ASB) treatment processes with a focus on application to the pulp and paper industry. AS systems can achieve higher BOD percent removals, but for the systems studied, the minimum effluent BOD concentrations of the two systems were equivalent. A key advantage of the AS system is a very small footprint which not only leads to advantages in overall size, but, potentially, in off-gas collection and leakage control. ASB systems have typically lower capital, energy, and O&M costs than comparable AS systems. ASB systems are more forgiving in responding to influent quality and flow variations and are simpler to operate. For the systems studied the nutrient discharges were equivalent, but supplemental nutrient addition rates were much higher for AS systems. The waste solids process is a continuous, separate operation in an AS system. Settled secondary solids in an ASB system reduce in volume through hydrolysis to typically ten percent concentrations. At periods up to twenty years the accumulated solids must be dredged, dewatered, and disposed. ASBs, because of their longer hydraulic retention times, tend to have their effluent wastewater approach ambient air conditions. This is desirable during summer conditions, but can be a treatment problem during the wintertime – particularly in northern Canadian sites. Loss of an insulating foam cover is a greater problem for ASB systems. AS systems typically require supplemental cooling equipment to resolve high influent temperature conditions.For the authors each system has advantages that make the selection of one system over the other a site-specific decision. Clearly, when the footprint is a major factor the AS system is favored. For reliability, ease, and cost of operation it appears that the ASB is favored. For pulp and paper application, the reliability of a wastewater treatment system to eliminate the need for production curtailments is a critically important factor in comparing AS and ASB systems. In this area, the authors have found the ASB system to be superior.