Denitrification in a Non-nitrifying Activated Sludge System Employing Recirculation from a Tertiary Nitrification Unit

Abstract: A non-nitrifying, denitrifying activated sludge system has been demonstrated to give high denitrification rates and to be a useful component in a municipal wastewater treatment plant where partial nitrogen removal is required. One useful configuration is where clarified effluent from a non-nitrifying activated sludge system is nitrified in a tertiary nitrification unit and recirculated to the activated sludge system for denitrification.

Such a system was operated continuously in pilot scale, using influent wastewater to the Rya Wastewater Treatment Plant (in Göteborg on the West Coast of Sweden). Influent flow, recirculated flow and sludge withdrawal were controlled as in a full-scale application. Results from one year of operation indicate average effluent inorganic nitrogen concentrations in the order of 10 g N/m3.

Recirculation of clarified effluent is a controlling factor. The variation of wastewater flow and quality influences design, operation and treatment results of such a system. Extrapolations from pilot plant data were made using simple models in order to investigate the effect of different circumstances on effluent nitrogen concentration and mass flow in a system based on recirculation of clarified and nitrified effluent to a denitrifying, non-nitrifying, activated sludge system. Effects of the capacities of the secondary settlers and the nitrification unit as well as denitrification limitations were investigated. Further, the effect of varying influent flow patterns (caused by the fraction of storm-water connected or by varying weather) on effluent ammonium concentration and nitrogen removal was illustrated. At low flows the quality and quantity of the carbon source will be crucial to nitrogen removal. At high flows the hydraulic capacities limit nitrogen removal (although effluent concentrations will still be low due to dilution). Further improvements can be made if the system is controlled and if sludge liquors are returned directly to the nitrification unit instead of to the head of the plant. Apart from the 1-2 g N/m3 less nitrogen released, recirculation flows can be decreased. Little further improvement can be obtained by separate treatment of sludge liquor.

Denitrification rates in a denitrifying, non-nitrifying activated sludge system with a solids retention time (SRT) in the order of 3-5 days can be expected to be higher than in a nitrifying system where the SRT is in the order of 10 to 20 days or higher. This is mainly due to the higher respiration rates recorded in systems with a low SRT (and high growth rate). This was verified in the pilot plant. If the nitrate concentration was not limiting, denitrification rates were in the order of 10-15 g N/(kg VSS-h), althoughr at high effluent nitrate concentrations. At low effluent nitrate concentrations under normal operation the average denitrification rate was 4.7 g N/(kg VSS-h). Denitrification rates were high not only where primary settled wastewater was introduced, but also in the deoxygenation zone where no new wastewater was introduced. Mass balances over the activated sludge system and estimates of nitrification in the activated sludge system due to seeding of nitrifiers from the NTF indicate nitrification, and thus denitrification, rates in the aerated tanks in the order of 0.5 - 2.5 g N/(kg VSS-h).