Slowly or non-biodegradable organic compounds, such as lignin, have a great impact on the effuent color and chemical oxygen demand (COD) discharges at many mill-sites refning wood chemically or semi-chemically. For example, in a modern kraft pulp mill, the quantity of the high molecular weight (HMW) compounds in bleaching fltrates varies typically between 15-20 kgCOD/ADt, depending on wood raw material, brownstock washing effciency and bleaching conditions. The removal of HMW COD at a wastewater treatment plant (WWTP), on the other hand, is strongly affected by the surface charges of colloids. In a conventional pulp WWTP including secondary biological treatment, the acting adsorbent media is mainly biosludge. However, the negative surface charge of lignin released during the alkaline pulping process usually hinders the adsorption to biosludge and therefore lignin based compounds causes rather high COD concentrations in the fnal effuent of WWTP. It is known that polyvalent metals such as Fe3+, Al3+ or Ca2+ can be used to precipitate lignin from black liquor and pulp mill effuents. In this study, the utilization of different solid waste or side streams containing calcium from integrated pulp and paper mill were studied in laboratory scale to remove dissolved COD from pulp mill wastewaters in highly alkaline conditions. The results from precipitation studies with chemically activated fy ash from a solid fuel boiler, waste lime from causticizing and coating sludge from a paper mill were very promising. The highest reductions of COD concentrations were measured from alkaline bleaching fltrates. It was also observed that the adsorbable organic halogen (AOX) and the color were signifcantly lowered during the alkaline COD reduction process (ACRP) as introduced in this study. Based on the laboratory scale studies and the COD mass balance calculations, the effects of ACRP as a pretreatment unit prior to activated sludge process was estimated. In this specifc case, the total COD reduction in WWTP was found to increase from 65% to 80%, when ACRP was addressed to alkaline bleaching fltrates. ACRP would also reduce the need for wastewater neutralization caused by the acidity of current wastewaters.
|Tila||Julkaistu - 2017|
|OKM-julkaisutyyppi||A1 Julkaistu artikkeli, soviteltu|