Denitrification in tertiary filtration: application of an up-flow filter.

The present paper shows the results obtained through an experimental work performed at the wastewater treatment plant of Rome, aimed at studying the performances of a tertiary filter regarding combined removal of suspended solids, COD, and nitrates. The up-flow sand filter was fed by the effluent coming from the secondary settling tank of the plant. The filter bed height was of 80 cm of silica sand. After a start up period, a study of particulate and soluble COD removal process was made, to establish the need of methanol in the denitrification process. Total COD removal efficiency was 60% on average, 55% due by soluble COD removal and 5% by particulate one. In the last phase of the experimental activity methanol was fed as carbon source, sodium sulfite was supplied to produce anoxic environment within the filter and the denitrification efficiency was studied. Nitrates removal rates after an acclimation period of 10 days increased up to 60%, with an effluent NO3-N of 8 mg/L. Denitrification rate was 2.4 kg/m3 d for water temperatures of 25 degrees C. Regarding methanol demand and biologic kinetics, the biomass yield coefficient was 0.3 kg(COD-X)/kg(me). Consequently 2.7 kg of methanol was required per kilogram of denitrified nitrogen.