Feasibility study of thermophilic aerobic biological treatment of bleached kraft pulp mill effluent

Recently, system closure has become a priority in the pulp and paper industry as a way to reduce water consumption. However, closure has led to the generation of a warm effluents with high concentrations of dissolved, colloidal and suspended pollutants. Presently, cooling is required prior to secondary biological treatment. A potentially attractive alternative approach is to treat effluent at a high temperature. The objective of this study was to investigate the feasibility of aerobic biological treatment of bleach Kraft pulp mill effluent (BKME) at temperatures up to 60 degrees C with particular attention to the effect of temperature on the settling characteristics of the sludge. Four laboratory-scale sequencing batch reactors operating in parallel at different temperatures of 35, 45, 55 and 60 degrees C were used to treat bleached Kraft pulp mill effluent from a hardwood based modem elementary chlorine free (ECF) pulp mill. Mixed liquor and treated effluent from these reactors were used to characterize the treated effluent and settling characteristics of the microbial floc by measuring AOX, COD, lignin and sludge volume index (SVI). The results of this research indicated that the SBRs operating at 35,45,55 and 60 degrees C were capable of achieving the averages of COD and AOX removal from 70+/-10%, 68+/-9%, 54+/-8% and 56+/-10% and 68+/-6%, 65+/-5.0%, 62+/-10% and 60+/-12%, respectively. Chemical compound analysis of feed and effluent treated at different temperatures showed a better removal of long chain fatty acids at thermophilic conditions (60 degrees C) in comparison to the mesophilic (35 degrees C) conditions. Small, weak and dispersed flocs with only few filamentous organisms were observed at 55 and 60 degrees C. These dispersed/pinpoint flocs were responsible for an increase in suspended solids concentrations in the treated effluents from 15 to 70 mg/L at higher temperatures. The trend of zone settling and SVI results varied greatly with temperature with quick settling and a minimum SVI (i.e. best settling) occurring at 45 degrees C. All these results clearly indicate that it is feasible to operate a biological treatment for bleached Kraft pulp mill effluent at the temperature of 45 degrees C.