Treatment Technologies for Organochlorine-Containing Sludges and Concentrates from External Treatment of Pulp and Paper Wastewaters

Abstract

Literature data and results from current Swedish research have been used to compile characteristics of and disposal methods for sludges and concentrates from external treatment of wastewaters from the pulp and paper industry. Very little sludge from aerobic lagoon treatment in Sweden is handled, and most of the solids are deposited in the receiving waters. In North America, Finland and Japan, where activated sludge treatment is used to a great extent, sludge handling is an important part of kraft mill effluent treatment. Typically, 5-15 kg of dry solids has to be handled per ton of pulp with a process discharge of 15-20 kg of BOD5/t90- For chemical coagulation, the amounts of sludges produced are higher, typically 40-80 kg dry solids/t90. Especially in Japan, chemical coagulation is used to a great extent. Laboratory and pilot- plant experiments with Ultrafiltration of alkaline stage and total bleach plant effluents suggest that concentrates with a total solids content of approximately 20 and 50 kg dry solids/t90 are produced. The concentrations of organochlorine compounds in sludges and concentrates have not hitherto been well documented, and nor have environmental effects of different disposal methods. Chlorinated phenols, catechols and guaiacols have been found in variable concentrations; chlorinated dibenzodioxins and furans have also been detected. Vacuum filters and especially belt filter presses have been used to dewater sludges. Typical dry solids concentrations achieved vary from 20-35 % for primary sludges to 15-20 % for secondary sludges. Anaerobic digestion may be applied before dewatering of sludges, and methane yields of 200-400 1 CH4/kg VSrem have been reported. Sludge has been disposed of in landfills or incinerated. Leachates from landfilling of pulp and paper industry sludges may contain high levels of soluble organic material, but available information suggests that organochlorine compounds do not leach to a substantial extent. Incineration of pulp mill sludges and concentrates seems to be an increasingly attractive method.