EFFECTS OF PH AND ALKALINITY ON SULFUR-DENITRIFICATION IN A BIOLOGICAL GRANULAR FILTER

Two laboratory-scale biological Filters were operated to investigate t he effects of alkalinity and pH on removal of nitrate and nitrite in s ulfur denitrification filter processes. The concentration of sodium bi carbonate in the feed media was changed from 120 to 240 mg/l during ab out 3 months in a filter (Run A). The other filter was initially fed w ith 300 mg/l and then with 240 mg/l (Run B). The performance of the fi lter was monitored by measuring pH, nitrate, nitrite, sulfate, alkalin ity, and thiosulfate, Nitrate concentration in effluent rapidly decrea sed to lower levels within several days for both filters after inocula tion of enrichment culture of sulfur denitrifiers. However there was a large difference in removal of nitrite. When rapid removal of nitrate took place, nitrite accumulation was observed and remained while the bicarbonate concentration was 120 and 150 mg/l. On the other hand the nitrite accumulation disappeared when more bicarbonate (240 and 300 mg /l) was supplied. The experimental results indicated that the nitrite accumulation was closely related to pH condition and alkalinity level in the filter. The stable data of effluent water quality for 5 cases w ere collected and the relationship discussed between nitrite concentra tion and pH in effluents. The relationship indicated a strong pH depen dency on nitrite accumulation below pH of 7.4. The pH condition around 7 is not so inhibitory to biological activity. Therefore, the pH with in the biofilm would be low enough to suppress the nitrite reduction b y sulfur denitrifiers, while the pH in effluent was not in the inhibit ory range. it was recommended to keep the pH higher than 7.4 to preven t nitrite accumulation in the sulfur denitrification filter. Copyright (C) 1996 IAWQ.