Total nitrogen removal using a pure-oxygen predenitification configuration

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Takach, Tibor
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A study was conducted utilising a pure-oxygen activated sludge system in a predenitrification configuration. The objectives of the study were to: (1) assess the feasibility of a predenitrification configuration at short anoxic and aerobic hydraulic retention times; (2) to determine how predenitrification affects process stability; (3) to determine the potential for total nitrogen removal using short anoxic hydraulic retention times; and (4) to optimise the system by varying the recycle ratio. Two parallel systems were operated at 20$\sp\circ$C and with a fixed aerobic hydraulic retention time of 2.5 hours. The hydraulic retention time of the Train B anoxic reactor was decreased from 2.5 hours to 1.5 hours and the recycle ratio was varied from 1 to 2.5 Q$\rm \sb{RAW}.$ The study found that 1.5 hour anoxic hydraulic retention time and a 2.5 hour aerobic hydraulic retention time was sufficient for total nitrogen removals. Total nitrogen removals were dependent on the recycle ratio and the influent carbon loading. Removals approaching the theoretical maximum of 65% at a recycle ratio of 1.5 Q$\rm\sb{RAW}$ were observed during the study. Returned dissolved oxygen to the anoxic zone and a lack of a readily available carbon source limited the maximum recycle ratio to 1.8 Q$\rm\sb{RAW}.$ Reliable process stability was observed throughout the study and can be attributed to an increase in the buffering capacity of the mixed liquor through alkalinity recovery in the anoxic zone. The effluent pH was observed to be 6.4 and 6.5 for Train A and Train B respectively and the effluent alkalinity remained constant at 195 mg/L and 200 mg/L. Specific nitrification rates were observed to be 2.6 and 2.7 mgN/gVSS-h while the specific denitrification rates were 3.85 and 4.3 mgN/gVSS-h for Train A and Train B respectively.