Effects of pH and precipitation on autohydrogenotrophic denitrification using the hollow-fiber membrane-biofilm reactor

Experiments carried out in a hollow-fiber, membrane-biofilm reactor (HFMBR) showed that the optimum pH for autotrophic denitrification was in the range 7.7-8.6, with the maximum efficiency at 8.4. Increasing the pH above 8.6 caused a significant decrease in nitrate removal rate and a dramatic increase in nitrite accumulation. The pH rose by 1.2 units when a large buffer was not added, suggesting that some field applications may require pH control. Precipitation of Ca2+ occurred in every experiment. Precipitation was the largest sink for carbonate, and it also offset alkalinity production by denitrification. Although the alkalinity increased in most cases, systems with a high carbonate buffer and high PH accentuated precipitation, and the net change in alkalinity was negative. The long-term success of field applications of the HFMBR may depend upon the interactions among calcium concentration, total carbonate concentration, pH, and alkalinity changes. (C) 2002 Elsevier Science Ltd. All rights reserved.