Technical Papers

The Classifying Selector was introduced to the wastewater industry in 2001, after several successful full-scale applications. The Classifying Selector concept distinguishes itself from the earlier Surface Foaming Wasting scheme in that negative selection pressure is maintained so that nuisance foam causing organisms cannot gain a foothold in sufficient numbers to cause nuisance foams. The propensity of the nuisance causing organism to attach to bubbles and establish a rising velocity is used to enrich them in a surface mixed liquor layer where they are wasted. This paper reintroduces the concept because neither standard texts nor WEF MOP’s properly describe it, and as a result, the benefits of the Classifying Selector have not been broadly obtained in our industry. With prevention of foam trapping and with proper design of the Classifying Selector, activated sludge plants designed for nutrient removal can operate in a manner that prevents foam formation. In certain types of processes that are inherently foam trapping situations, the only solution is Surface Foam Wasting, as foam cannot be eliminated. For these cases, the paper offers guidance as well as highlights where process development activity is required.