MUNICIPAL WASTEWATER TREATMENT IN A HYBRID ACTIVATED SLUDGE BIOFILM REACTOR: A PILOT PLANT EXPERIENCE

A hybrid activated sludge/biofilm process (IFAS) was investigated. This process which contains both suspended and attached biomass, is created by introducing plastic elements as biofilm carriers material into a classical activated sludge system. This process is suitable for upgrading existing activated sludge wastewater treatment plants which are no longer able to comply with the effluent standards, and in particular in those cases where the existing plant does not nitrify or only nitrify during summer. However, the IFAS processes are relatively new and there are still uncertainties with respect to their design. A main task is to find a rational approach to the design of nitrification in hybrid reactors. The aim of this paper is, therefore, to gain insight the nitrification process in a hybrid reactor and, in particular, to establish the influence of mixed liquor SRT. In this paper the first results of a pilot-scale study carried out at the Department of Hydraulic and Environmental Engineering at Norwegian University of Science and Technology (NTNU) are presented. An IFAS-process composed of an activated sludge and a moving bed biofilm reactor (MBBR) process, also called HYBAS, was used. The carriers used in the study were Kaldnes K1 carriers. The pilot plant was constituted by three identical 30 L aerated reactors in series, in the second of which were put the carriers, with a filling ratio of 60%.The pilot plant was operated with a constant flow of 23 L/h and a HRT of 3.75 hours. During the experimental period, batch tests on both carriers and suspended biomass were performed in order to evaluate the nitrification rate of the two different biomasses. The results from the pilot plant study demonstrated that this kind of reactor can efficiently be used for upgrading of conventional activated sludge plant for stable nitrification without the need of additional volume.