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 foam wasting schemes 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. Neither standard texts nor the Water Environment Federation's Manuals of Practice adequately describe this, and as a result, the benefits of foam elimination obtainable through use of the classifying selector concepts have not been broadly obtained in our industry. In certain types of processes that are inherently foam trapping situations, the only solution is surface foam wasting, as foam cannot be eliminated. Potential efficiency gains possible in these situations are addressed.

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In 2008, the Metropolitan Council Environmental Services (MCES) in Saint Paul, Minnesota must meet new requirements for effluent phosphorus discharges at two large wastewater treatment facilities. To meet these standards at current flows, MCES staff searched out a cost effective solution and retrofitted both plants with air mixed selectors. However, it was unclear how the air mixed selector configurations would perform under increased future loadings. By conducting systematic stress testing of future flow and loading conditions, it was demonstrated that the air mixed selector configuration with minor modifications would provide reliable phosphorus removal eliminating the need for a mechanically mixed selector configuration. Implementing the air mixed selector configuration will save MCES $1.5 Million in capital expenditures and reduce annual operating costs by roughly $60,000 per year.

Computational fluid dynamic (CFD) modeling was used to evaluate primary settling tank (PST) performance and a novel wet weather chemically enhanced high rate treatment (CE9HRT) tank configuration. Total suspended solids removal performance of three PST configurations under dry and wet weather conditions were evaluated using various baffling, launder, and sludge removal improvements. CFD modeling of wet weather treatment tanks with chemically treated raw influent showed CE9HRT can meet project requirements for wet weather flow discharges. The use of a combination of full9scale and bench9scale testing was employed to extend the application of the model to chemically treated influent.

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