Characterisation and fate of bleached kraft mill effluents from a New Zealand pulp and paper mill

Abstract

The wastewater discharges from the Kinleith mill, a New Zealand integrated bleached kraft pulp and paper mill, were characterised and their treatability assessed. The chlorination stage bleaching effluents from the mill’s second bleach plant [(C30D70)EoDED sequence] contained a group of novel chlorinated compounds. Fourteen compounds were isolated from the effluents by a combination of liquid-liquid extraction, column chromatography, and preparative gas chromatography. Mass spectral and ¹H and ¹³C NMR data showed these compounds to be hydroxylated and/or chlorinated derivatives of Pinus radiata monoterpenes. The major compounds were a dichlorobornane and four dichloro-p-menthane-1,8-diols. The chlorinated monoterpenes were detected in total concentrations of 1400-12 300 μg L⁻¹ [70-600 g air-dried tonne⁻¹ (ADT) bleached pulp] and they were the major class of low molecular weight extractable organic compounds present in the chlorination stage effluent. The principal factor determining their formation appeared to be the high concentration of monoterpenes remaining in the Pinus radiata brown stock produced in the mill’s continuous digester. The biological activity of the chlorinated monoterpenes was assessed. The chlorinated monoterpene alcohols were base labile with a 94% decrease in concentration being observed within 4 hours at pH 12. The chlorinated monoterpene hydrocarbons exhibited a lesser degree of alkaline lability. Acute toxicity tests on the monoterpene alcohols gave EC₅₀ concentrations of 60-200 mg L⁻¹, indicating that these compounds display relatively little toxicity. The monoterpene alcohols were also tested for mutagenicity and genotoxicity. Some of these compounds produced mutagenic and genotoxic responses. An assessment of the bioaccumulation potential of the chlorinated monoterpene alcohols showed them to have log Kₒw values of 1.37-2.1. Therefore, these compounds are unlikely to exhibit a significant bioaccumulation propensity. Treatment of the chlorination stage effluents in an aerated lagoon treatment system removed 80% of the chlorinated monoterpene alcohols but only a small fraction of the monoterpene hydrocarbons. It was concluded that these compounds were unlikely to produce significant environmental effects in the recipient. The mill has two secondary treatment systems. The relative effectiveness of each of these was assessed. The two systems operate in different configurations. Treatment system A, which receives general mill wastewaters and chlorination stage bleaching discharges utilises deep, aerated lagoons and has a 4.5 day retention time. Treatment system B, which receives alkali extraction bleaching wastewaters and foul condensates, uses a lagoon system with a retention time of 51 days. Detailed chemical analyses of the untreated and treated wastewaters were made. Mass balances were calculated for a range of physical parameters and for specific chlorinated and non-chlorinated organic constituents. Significant differences in the treatability of various constituents were found. In particular, while system A was able to reduce levels of adsorbable organic halide (AOX) by 65%, no significant reduction in AOX occurred in system B. In contrast, system B reduced levels of chloroacetic acids by 84% while system A did not achieve any statistically significant removal of these compounds. The treatability of chlorophenolic compounds also differed. System A was unable to remove chlorophenols and chloroguaiacols while system B did not reduce levels of chlorocatechols. The removal of AOX from effluents treated in system A was high compared to published data and an assessment was made of possible mechanisms for the observed AOX removal. Much of this removal took place in a short section (3.3 hr residence time) of the system’s main lagoon. The initial AOX decrease in the aqueous phase could be achieved in part by settling of AOX-containing suspended solids from the influent wastewaters. In addition, lime and bacterial solids present in the treatment system were able to adsorb AOX from the influent wastewaters. Only a small proportion of the organic chlorine removed was found in sludges. A mass balance of aqueous and solid phases indicated that over 99% of the removed AOX was mineralised.