ADVANCED TREATMENT OF PAPER-MILL EFFLUENTS BY A 2-STAGE ACTIVATED-SLUDGE PROCESS

The work presented in this paper concerns the application of a two-sta ge aerobic activated sludge process for treatment of effluents from pa per mills in Denmark. The paper describes both pilot-scale test result s and full-scale experience with the process. The treatment process is characterised by a high-load first stage (2-4 kg COD/kg MLSSxd) follo wed by a low-load second stage to secure full nitrification and denitr ification of remaining nitrogen compounds. The results of continuous p ilot-scale tests show that it is possible to obtain a reduction of mor e than 85% of the incoming CODsol/l and a 99% reduction of the incomin g BOD5, resulting in an effluent quality of 230 mg CODsol/l and less t han 10 mg BOD5/l. As indicated, practically all the biodegradable orga nic substances are removed by the process. The remaining fraction of s oluble organics measured as COD is considered to be non-biodegradable by conventional biological treatment systems, The results produced in the pilot-scale tests are confirmed by the effluent qualities obtained in a full-scale treatment plant at another paper mill, involving an i dentical process concept. During the pilot-scale tests, special attent ion has been paid to the removal of organic compounds, organic nitroge n as well as nutrients and nitrification. In addition, the sludge char acteristics and the oxygen requirements have been considered.