The ScanDeNi® process could turn an existing under-performing activated sludge plant into an asset

Abstract

With tightening up of effluent discharge standards from wastewater treatment facilities, many plants are facing costly augmentations and in many cases completely new plants will have to be constructed. The ScanDeNi process was developed in Sweden for increased nitrogen removal at the Västerås Sewage Treatment Plant (STP), 125,000 p.e. near Stockholm, and can be described as a modified contact stabilisation process with pre-denitrification and a selector stage for nitrification. The STP was upgraded at a cost of some 25 Mill. SEK (2.5 Mill. USD). It has been successfully in operation since 1998, exceeding all expectations. The process is showing the following major advantages. 25-35% less volume for the same Sludge Retention Time (SRT) and secondary sedimentation sludge load, compared to conventional pre-denitrification; or a 25-35% higher load can be applied within the same volume with the same removal efficiencies. The selector mechanism appears to be not limited to the nitrifying bacteria alone. Other microorganisms appear to be responsible for the reduction of surface active matter from the return activated sludge (RAS), as well as in the reject stream from sludge dewatering, resulting in an increase in alpha-values of approximately 50%. Due to the high alpha-values less aeration is required, resulting in significant operating cost savings. 'Automatic' creation of anaerobic conditions, enabling biological phosphorus removal. Whilst rarely a concern in warmer climates, BNR plants in cold climates in winter often lose their capacity to nitrify. The Västerås STP has consistently maintained excellent effluent quality even with effluent temperatures as low as 7 degrees C, and at an SRT of some 7-9 days, proving the effectiveness of the nitrifier selector. The ScanDeNi process could offer excellent effluent discharge standards (T-N < 10 mg/L, T-P < 0.5 mg/L) in smaller tank volumes and at a significantly lower operating cost, compared to conventional pre-denitrification systems.