Denitrification by Fluidized Biofilm Reactor

The Rancho California Wastewater Reclamation Plant, which has a design capacity of 23.6 Ml/d, is located in Southern California approximately 64 km northeast of the City of San Diego. The effluent from this plant is used for irrigation and groundwater recharge. In order to protect the local groundwater from nitrate contamination, the plant must nitrify and subsequently denitrify to a nitrate-nitrogen concentration of 2.5 mg/l. The influent nitrate-nitrogen concentration, to the denitrification facility, is normally in the range of 15 to 20 mg/l. Denitrification is achieved by three upflow, fluidized bed biofilm reactors.

The reactor is designed for a loading of 3.0 kg of N0₃−N per day per m³ of expanded bed volume. The upflow velocity is 0.4 cm/s and the reactor has an empty bed contact time of 10 minutes at design flow. Each reactor has a design capacity of 7.5 Ml/d. The biofilm is grown on a media of sand with an effective size of 0.6mm and a uniformity coefficient of 1.2. The reactor has a concrete perforated underdrain, 0.6 meters of support gravel and 1.2 meters of sand. Following a two month start-up period, the facility has consistently removed 95 to 100 percent of the nitrate nitrogen. The objective of this paper is to present the design criteria and evaluate one year of operating performance.