A BOD:N:P ratio of 100:5:1 is often used as a benchmark for nutrient addition in nutrient limited wastewaters. The impact of varying nitrogen levels, whilst maintaining phosphorus constant, was studied in a simulated aerated lagoon (BOD:N of 100:0; 100:1.3; 100:1.8; 100:2.7 and 100:4.9). A synthetic wastewater was prepared using methanol, glucose and acetate as the combined carbon source, ammonium chloride as the nitrogen source and dipotassium hydrogen phosphate as the phosphorus source. Nitrogen levels did not impact organic carbon removal, but did strongly influence floc structure. With no supplemental nitrogen, growth was dispersed. Increasing the nitrogen level increased filamentous growth, with a marked change in filamentous species occurring between a BOD:N ratio of 100:1.8 and 100:2.7. Nitrogen fixation occurred at a BOD:N ratio of 100:0; 100:1.3 and 100:1.8, with nitrogen loss at BOD:N ratios of 100:2.7 and 100:4.9. At a BOD:N ratio of 100:4.9, ammonium discharge was significantly greater (1.8 mg/L) than at the lower nitrogen levels (0.04 – 0.18 mg/L). Phosphorus behaviour was more variable, however significantly more phosphorus was discharged at the lowest nitrogen level than at the highest (p<0.05). Based on readily available nitrogen, the BOD:N ratio at which nitrogen fixation no longer occurred was around 100:1.9.

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