NITROGEN REMOVAL TECHNOLOGY IS ADVANCED BY THE USE OF CONTINUOUS BACKWASH SAND FILTERS

This paper presents a case history of the novel application of a continuous backwash sand filtration design at three distinct wastewater treatment plants inside the Roosevelt Roads US Naval base in Puerto Rico. The three plants (1.4, 2.2, and 2.9 MGD) have a 3 mg/l nitrogen, 10 mg/l TSS, and 10 mg/l BOD discharge limitations and have been meeting these limitations with discharges as low as 0.5 mg/l nitrogen, 5 mg/l TSS, and 5 mg/l BOD. The stringent nitrogen discharge was required to help reduce the eutrophication potential of coral reefs in the receiving waters. The biological denitrification removal system is a tertiary treatment system, which follows clarified trickling filter effluent at two of the wastewater treatment plants. The remaining denitrification system is also a tertiary treatment system that follows secondary clarification of an extended aeration activated sludge process. These facilities' innovative biological denitrification systems consist of methanol storage, injection equipment, and continuous backwash low headloss sand filters followed by chlorination, dechlorination and cascade aeration. The dynamic moving nature of the tertiary filter's sand bed allows for the controlled growth of the denitrifying bacteria and the continuous release of the nitrogen gas. This innovation simplifies plant operations since it eliminates the need to “bump” the sand bed for gas release and shutdown of the system to backwash the solids and bacteria accumulation. Both of these procedures are required for conventional static bed systems. The paper will present the key design aspect of the denitrification system along with current operating parameters and performance data.