- 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 rate of 6 m 3 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.