The influence of the high and low molecular weight fractions of a bleach kraft mill effluent on the microbial activity of activated sludge

The high molecular weight (HMW) fraction (> 1000 Da) of bleached kraft mill effluent (BKME) is the principal source of adsorbable organic halide (AOX), colour and chemical oxygen demand in treated effluent. The influence of HMW material on the microbial activity of activated sludge microorganisms was studied in a series of batch growth experiments using fractionated effluent. The HMW material in the untreated effluent comprised 80% of the AOX but only 20% of the soluble COD. Multivalent metal ions had a greater affinity for this HMW fraction. The addition of HMW material to the LMW fraction had a stimulatory effect on microbial activity as measured by increases in biomass and substrate utilization. Chemical characterization and subsequent growth experiments revealed that nitrogen, which was deficient in the LMW effluent, was primarily associated with the HMW material. The association of nitrogen with the HMW fraction may have an important role on the behaviour of nitrogen within a biological treatment system and in the receiving environment.