| Summary: | The objective of the research program was to demonstrate the feasibility of removing nitrogen from highly nitrogenous wastes by a shortened nitrification/denitrification pathway. This process bypasses the nitrite oxidation step to nitrate and the concomitant nitrate reduction to nitrite. The success of the shortcut is dependent on selectively inhibiting the nitrite oxidation step, allowing nitrite to accumulate in the aerobic environment, and be subsequently reduced anaerobically to a gaseous product. Seven runs lasting between 13 and 359 days were undertaken using bench-scale, activated sludge cells operated in series.
Parameters investigated as potential selective inhibitors were: dissolved oxygen, nitrous acid, anaerobiosis, and free ammonia. Of these, only free ammonia (at 5 to 10 mg NH₃-N/L) was found to be effective as a differential inhibitor of unacclimated populations. Nitrite build-up was achieved by intermittent contact of the nitrite oxidizers with the high, free ammonia level in the cell at the front-end of the system.
Comparison of this shortened nitrogen removal pathway with the traditional mechanism revealed: 1) a 40% reduction of COD demand during denitrification; 2) 63% higher rate of denitrifica-tion; 3) Two thirds reduction in biomass yield during anaerobic growth; 4) no apparent nitrite toxicity effects. The degree of nitrite build-up was found to be inversely related to the aerobic residence time. High nitrite concentrations could not be sustained indefinitely, due to acclimation of the nitrite oxidizers to free ammmonia. Measures investigated to overcome the effects of acclimation were: 1) reduction of the sludge age; 2) extension of contact time to high, free ammonia levels; 3) raising free ammonia levels; 4) use of a potentially more inhibitory waste; 5) double substrate inhibition; 6) provision of intermediary recycle, or internal denitrification; 7) temporary reduction of free ammonia levels; 8) temporary stoppage of feed. Of these, the most effective was internal denitrification, due to the continued ability of free ammonia to maintain some degree of inhibition to an acclimated population of nitrite oxidizers. As a result, their nitrifying activity lagged that of the ammonia oxidizers by several hours, allowing nitrite build-up to sustain itself. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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