DESIGN AND OPERATION OF NUTRIENT REMOVAL PLANTS FOR VERY LOW EFFLUENT CONCENTRATIONS

Out of a total of six plants with nitrogen removal in Norway, four plants use the MBBR (Moving Bed Biofilm Reactor) process. In all four cases the MBBR process was chosen because it offered a very compact treatment solution and because it had the lowest investment costs as well as the lowest total annual costs. All the MBBR plants use the combined denitrification process (pre-denitrification + post-denitrification) for nitrogen removal, followed by chemical precipitation for phosphorus removal. This type of design offers a lot of operation flexibility and enables the plants to produce very low effluent concentrations. External carbon sources used for post-denitrification at these plants are ethanol, methanol and monopropylene glycol. The plants have maximum design flows from 1,125 to 7,200 m3/h (7.1 to 45.6 mgd) and at least two parallel trains. All plants have primary clarifiers. Two of the plants use sedimentation for final solids separation, while the other two use dissolved air flotation. The paper presents typical design data for nitrification and denitrification in MBBRs, plus results from guarantee tests where a single train is operated at design load, as well as results from normal operation of the plants. With influent concentrations above 50 mg total N/L, the Gardermoen WWTP has demonstrated effluent concentrations