The treatment of nitrocellulose industry wastewaters, characterized by high nitrate (1450-1990 mg/L) concentrations and low pH (1.45-1.57), was investigated by using denitrification filters. Methanol was added as external organic carbon source. The filter was filled with ceramic media and 50 to 100% effluent recirculation was applied. Alkalinity was produced during denitrification and increase in pH was observed. The system achieved 94-99% nitrate removal and effluent pH between 6.4 and 7.8, when pH of the influent wastewater was adjusted to 4. When the filter was filled with limestone media, 74 to 98% nitrate removal was achieved without any alkali addition. The denitrification rate was 229 mg NO3-N/m(3)/d. The two filters operated also in series, limestone filter being the first in the series receiving wastewater. The serial system was capable of reducing nitrate completely and neutralizing the pH into the weak alkali solution (7.08-7.82). The biologically produced alkalinity through denitrification could be beneficially used for the in situ neutralization and treatment of high influent nitrate containing acidic waters. These studies show that up-flow denitrification filters are suitable systems for treating acidic nitrocellulose industry wastewaters.