Hybrid activated sludge/biofilm process for the treatment of municipal wastewater in a cold climate region: a case study

A hybrid activated sludge/biofilm process was investigated for wastewater treatment in a coldclimate region. This process, which contains both suspended biomass and biofilm, usually referredas IFAS process, is created by introducing plastic elements as biofilm carrier media into aconventional activated sludge reactor. In the present study, a hybrid process, composed of anactivated sludge and a moving bed biofilm reactor was used. The aim of this paper has been toinvestigate the performances of a hybrid process, and in particular to gain insight the nitrificationprocess, when operated at relatively low MLSS SRT and low temperatures. The results of a pilotscalestudy carried out at the Department of Hydraulic and Environmental Engineering at theNorwegian University of Science and Technology in Trondheim are presented. The experimentalcampaign was divided into two periods. The pilot plant was first operated with a constant HRT of4.5 hours, while in the second period the influent flow was increased so that HRT was 3.5 hours.The average temperature was near 11.51C in the overall experimental campaign. The averagemixed liquor SRT was 5.7 days. Batch tests on both carriers and suspended biomass wereperformed in order to evaluate the nitrification rate of the two different biomasses. The resultsdemonstrated that this kind of reactor can efficiently be used for the upgrading of conventionalactivated sludge plant for achieving year-round nitrification, also in presence of low temperatures,and without the need of additional volumes.