The fate of particulate and dissolved nickel (Ni) and copper (Cu) was determined at various stages in a 7.3 mJ/s (167 mgd) tertiary treatment plant consisting of primary sedimentation, off-line primary effluent equalization, first-stage (secondary) activated sludge, second-stage (nitrification) activated sludge, prefilter chloramination, filtration, disinfection and dechlorination. At micrograms/liter levels, particulate Cu and Ni removals were higher than dissolved Cu and Ni removals. Only secondary activated sludge removed the dissolved Cu. No other process removed either dissolved Cu or Ni. The dissolved Cu concentration in the second stage nitrification activated sludge plant effluent was higher than that in the first stage secondary activated sludge plant effluent. Prefilter chloramination solubilized particulate Cu and decreased the overall Cu removal by filtration. The dissolved Cu removed by the activated sludge systems varied seasonally; the higher removals seemed to be associated with increased activated sludge growth. Water Environ. Res., 68, 1]72 (]997).

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An effect of sludge age (SRT) on the operation cost of a 10 MGD activated sludge plant was evaluated using an activated sludge computer model. For a completely-mixed system an increase in SRT generally reduced operating cost, while for a system with an anaerobic selector the reverse was generally observed. Nitrification significantly increased operating cost and, if not required, can be avoided by lowering SRT. The effect of SRT on the operating costs of BNR systems cannot be predicted theoretically; field studies were required to determine this effect. This paper presents the results of such a field study. Deviation of the SRT from its optimum value may carry substantial financial penalties in addition to operating problems. Precise maintenance of SRT using an automatic control system is recommended.