Design and Operation Experience of Advanced Effluent Treatment with Ozonation and Subsequent Biofiltration at the Paper Mill SCA-Graphic Laakirchen AG - Austria

摘要

The paper mill “SCA GRAPHIC LAAKIRCHEN AG” produces 485.000 to of graphic paper per year. To clean the effluent of the paper production a wastewater treatment plant is located on site. An amount of 6,500,000 m3 of wastewater is treated per year, mechanically first, and in a 3-step biological plant. The COD-load into the wastewater treatment plant is equivalent to the effluent of a city with approx. 220,000 inhabitants. he trend towards brighter paper grades requires more intense bleaching of mechanical pulp and increased usage of chelating agents and optical brighteners. These chemicals are hard to be degraded with biological waste water treatment. Higher bleaching dissolves more lignin which is hard to be degraded as well. As a result the COD removal efficiency in the biological stage of the waste water treatment plant drops and additional measures for enhanced COD reduction have to be implemented in order to keep given discharge limits. Ozonation and subsequent bio-filtration of mechanical and biological pre-treated wastewater from the paper production provide: 1. Enhanced COD-reduction, which cannot be obtained by using onventional mechanical and biological treatment alone 2. Controllable COD-reduction in a very wide range and therefore elimination of COD-peaks which go along with special production situations 3. Reduction of the treatment costs by concentration to COD-peaks and the combination of ozonation and subsequent bio-filtration 4. Decrease of the color of the ozonated wastewater and therefore better possibilities for reuse of the treated effluent in the paper mill. In 1996/97 extensive pilot trials were performed at SCA Graphic Laakirchen AG to investigate this system for enhanced COD degradation. The promising results led to a full scale installation in 2004 at the same site. The plant at SCA Graphic Laakirchen AG has been designed to treat 26,000 m3/d of biological pre-treated wastewater. With a capacity of 75 kg/h of ozone it is possible to eliminate 1,830 kg/d of biologically not degradable hard COD. The installed electrical equipment (ozone generator, pumps) has a maximum power consumption of 1,600 kW. Especially the possibility for excellent control of the COD-removal rate allows for economical usage and therefore acceptable operational costs in relation to the paper production. The average values for 2005 of the full scale plant show costs of 1.33 EUR per kg COD eliminated or 0.53 EUR per ton of paper produced (when 30 % of the production is of brighter grade). Investment costs for the ozonation step amounted to 3.508 Mio. EUR.