Effect of cycle duration on phosphorus and nitrogen transformations in biofilters

A system consisting of two identical biofilters aerated in cycles was used to achieve nitrification and phosphorus removal. The influent contained 12 mg/L NO3-N, 10 mg/L NH4-N, 50 mg/L COD and 8.5 mg/L P. The specific surface area of the packing media was approximately 140 m2/m3 and the empty bed residence time (EBRT) in each biofilter was maintained at 34 min. The two biofilters were exposed to alternating unaerated/aerated conditions in cycles. The flow direction was reversed at the end of each cycle and the influent was always introduced into the unaerated biofilter. The aeration cycle duration (CD) was varied in the range of 3–12 h. Varying the CD had significant impact on the performance of the system and resulted in environmental conditions which inhibited or enhanced nitrogen and phosphorus transformations. Carbon removal, nitrification, denitrification, and phosphorus release and uptake were dependent on the CD. The results demonstrate the role of CD in creating environmental conditions suitable for optimising phosphorus removal and modifying nitrogen transformations in biofilters.