Denitrification of groundwater: pilot-plant testing of cotton-packed bioreactor and post-microfiltration

The use of raw cotton as carbon source in the denitrification of drinking water was tested in a field pilot-plant. The reactor treated water from a well in which the concentration of nitrate varied from 22 mg N l(-1) in summer to a minimum of 9 mg l(-1) in winter. The experimental reactor had a capacity of approximately 9 m(3) and could be packed with up to 1500 kg of unprocessed cotton. The highest rate of denitrification observed was 0.36 kg N m(-3) d(-1), at a feed Fate of 6 m3 h(-1). However, this performance could be sustained only temporarily as the relatively high water pressure caused serious compression of the bed. The long-term (six months) performance of the system was studied at feed rates of 0.8 and 1.5 m(3) h(-1). The process was stable and 80-100% of the influent nitrogen was removed. The increase in DOC at the outlet was usually less than 7 mg l(-1), and the number of bacteria was in the order of 10(5)-10(6) CFU ml(-1). Crossflow microfiltration was an effective post-treatment for the removal of bacteria and elimination of turbidity.