Upgrading the Chemical Precipitation Process Using a Fixed Film Biological Reactor: A Case Study of the Taskila Plant of Oulu, Finland

Abstract

This paper is a case study about a design process resulting for the first time in Finland in the application of a fixed film reactor (biofilter) for upgrading an existing wastewater treatment plant. The need for the upgrade arose from the need to increase the efficiency of the existing plant to bring it into compliance with the revised discharge requirement. More particularly, the requirement was to increase reduction of the carbonaceous biochemical oxygen demand (BOD). The plant, which is presently the largest, and also one of the last, chemical precipitation plants in Finland, had, throughout its more than twenty years of existence, removed phosphorus with efficiency, which would also be helpful in meeting the new requirements.

The evaluation of the existing plant in terms of past performance, condition of the structures and hydraulic capacity was prerequisite before the final process concept was selected. The assessment that the plant was reliable and hence also that the chemical process was predictable contributed to the choice of the applied process configuration. The efficient chemical precipitation was to remain as the first stage of the process, and this was to be supplemented by a biological stage designed specifically for the removal of carbonaceous BOD. Further development for nitrogen treatment shall be based on subsequent stages designed particularly for nitrification and denitrification resulting in a C + N + DN configuration.

Two alternative biological applications were considered; conventional suspended growth activated sludge process and high rate fixed film biofiltration. Technical and economic comparison of these options as well as recorded experiences of the users of similar plants elsewhere in Europe convinced us that, as long as the performance of the chemical pre-treatment is steady and predictable, the high rate filtration process is the more economically and technically competitive solution for upgrading the chemical treatment plant. The investment costs are substantially lower than those of the equivalent conventional activated sludge plant, there is not much difference in the operating expenses, modular structure offers flexible future expansion of the process and the process provides high quality effluent.