Tertiary denitrification of the secondary effluent in biofilters packed with composite carriers under different carbon to nitrogen ratios

A new type of biofilter packed with composite carriers was designed for tertiary denitrification of the secondary effluent with removal of both oxidized nitrogen and suspended solids (SS). At the empty bed residence time of 15 min and organic carbon to nitrate nitrogen (C/NO3-N) ratios of 2, 1.5 and 1 g/g, the removal percentage of NO3-N was 67%, 58% and 36% in the ethanol biofilter, and was 61%, 43% and 26% in the acetate biofilter, respectively. The biofilters packed with composite carriers removed SS effectively, with the effluent turbidity in both biofilters of less than 3 NTU. During the operating cycle between the biofilter backwashings, the NO3-N removal percentage decreased initially after backwashing, and then gradually increased. Under C/NO3-N ratios of 2, 1.5 and 1 g/g, the NO3-N reduction rate was 1.75, 1.04 and 0.68 g/m/d in the ethanol biofilter, and was 1.56, 1.07 and 0.76 g/m/d in the acetate biofilter, respectively. In addition, during denitrification, the ratio of the consumed chemical oxygen demand to the removed NO3-N was 5.06-8.23 g/g in the ethanol biofilter, and was 4.26-8.6 g/g in the acetate biofilter.