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It is widely assumed that non-aerated selectors are very efficient in nutrient removal, and especially anaerobic basins may largely contribute to good sludge settleability as well. However, based on results measured in full-scale, this paper draws attention to the fact that with decreasing availability of readily biodegradable carbon source (rbCOD) being experienced worldwide, oxygen penetration into non-aerated basins through the uncovered surface may no longer be considered negligible. When the oxygen mass transfer is significant compared to the available influent rbCOD, non-aerated selectors should be regarded as basins with low dissolved oxygen (low DO) concentrations that may underperform with respect to nutrient removal and favor the growth of filaments, especially during low-loaded conditions. In order to fully exclude oxygen penetration, floating seals have been developed and applied at the North-Budapest Wastewater Treatment Plant in Hungary. Comparative full-scale studies showed prevention of significant amounts of influent rbCOD loss (up to 60 mg/L) through the application of this new technology. This amount of saved, non-oxidised but fermented carbon source could be accordingly used for enhancing biological P-removal. Due to the elimination of microaerophilic conditions, the undesirable growth of filamentous bacteria could also be avoided, leading to significantly better activated sludge settling.

Part-time aeration is frequently applied in activated sludge systems in order to decrease treatment costs by achieving nitrification and denitrification in the same basin. However, measurements and mathematical simulations carried out at the North-Budapest Wastewater Treatment Plant (Budapest, Hungary), clearly show that especially in the increasingly characteristic shortage of readily biodegradable carbon-source, this technique may lead to high effluent nitrate and/or ammonia concentrations at decreased temperatures. This situation may be worsened when co-digestion of external wastes with high N-content increase the ammonia concentration of sludge processing return flows. In these cases, denitrification should rather be enhanced in pre-anoxic zones. Pronounced pre-denitrification leads to better usage of the influent carbon-source and to considerably less methanol demand when dosing external carbon-source proves to be necessary.

In order to reduce the pollution load of the Danube, the North-Budapest Wastewater Treatment Plant has been upgraded to enhanced nitrogen removal by establishing a new activated sludge treatment line and modifying the existing unit for nitrification and denitrification. As both the influent flow rate and the influent chemical oxygen demand (COD), biological oxygen demand (BOD5) and total suspended solids (TSS) concentration levels remained far below the design values, setting one fourth of the reactor volume out of operation in the Old Line, and operating the nitrification reactor of the New Line with part-time aeration proved to be possible. Analytical data as well as simulation studies supported the advantage of the intermittent-aeration process in efficient N-removal. However, the lengths of the aerated periods have to be increased with decreasing temperature, and thereby effluent total nitrogen (TN) concentration can increase due to decreasing denitrification efficiency. Potential occurrence of low-dissolved oxygen (DO) bulking should be hindered through applying an efficient anoxic selector system.