EVALUATION OF MOVING BED BIOFILM REACTOR TECHNOLOGY FOR ENHANCING NITROGEN REMOVAL IN A STABILIZATION POND TREATMENT PLANT

The Sanitation Districts of Los Angeles County (Sanitation Districts) operate the Palmdale Water Reclamation Plant (WPR) to provide wastewater treatment and disposal services for a city of 140,000. The Palmdale WRP is a secondary treatment system that uses oxidation ponds for BOD stabilization. Partial removal of nitrogen occurs within the facility, primarily as a result of assimilation by algae that subsequently settle to the pond bottom. Recent detections of nitrate nitrogen (NO3-N) in local groundwater have been attributed to the longstanding practice of land application of the nitrogen-rich plant effluent. In response to this groundwater impairment, the Sanitation Districts will upgrade the Palmdale WRP by 2009 with a tertiary treatment facility designed for biological nitrogen removal.The Sanitation Districts also established a research program to assess methods to reduce nitrogen discharges from the Palmdale WRP in the interim. The evaluation of moving bed biofilm reactor (MBBR) technology described herein comprised part of that effort. The MBBR study was conducted in two phases. In Phase I, the MBBR was evaluated as an adjunct system to the existing ponds. The MBBR was operated to nitrify primary pond effluent for subsequent denitrification in the secondary pond. In Phase II, the MBBR was evaluated as a complete BOD and nitrogen removal system that could replace the oxidation ponds.The Phase I study provided data to characterize the nitrification performance of the MBBR as a function of temperature, ammonia nitrogen (NH3-N) loading and dissolved oxygen concentration. Nitrification performance in MBBR was found to be more sensitive to dissolved oxygen levels than activated sludge systems. This characteristic was important for maintaining efficient nitrification performance during cold temperature operation.In Phase II, the MBBR achieved an average nitrogen removal rate of 0.15 kg N/day per cubic meter of bioreactor volume (9.4 lb N/1000 ft3-day). This rate was greater than the rate observed at many conventional activated sludge systems that have implemented nitrogen removal. The solids yield from the pilot scale MBBR, 0.26 kg of waste biosolids per kg of COD removed, was lower than that observed at these full-scale plant. The Sanitation Districts concluded that MBBR is an effective technology that merits future consideration. MBBR appears capable of achieving treatment objectives in smaller biological reactors. Savings in clarifier design may also be possible. However, these advantages must be weighed against the costs for purchasing the MBBR media and higher energy costs for aeration.