Colour formation from pre and post-coagulation treatment of Pinus radiata sulfite pulp mill wastewater using nutrient limited aerated stabilisation basins

Potential short and long term effects of coagulated Pinus radiata (sulfite process) pulp wastewater mixed with uncoagulated paper mill wastewater on colour formation in aeration stabilisation basins were investigated by simulation studies. Changes in the character of organics in the simulated ASB treatments were assessed from changes in the molecular weight (MW) distributions of compounds that contributed to colour formation. Data from these simulations were also compared with data from the full scale ASB that was used to treat non-coagulated wastewater at the time of this study. The laboratory scale ASB treatment comprised two parallel sets of 3 tanks (Simulations A and B) that were operated over three hydraulic retention times (HRTs) of 84 days in total. The influent used for both simulations was post-coagulated pulp mill wastewater mixed with paper mill wastewater (1:2). Sludge collected from the full scale ASB pond system was correspondingly added to the tanks of both simulations for inoculation with indigenous microorganisms. In addition, settled sludge collected after coagulation of pulp mill wastewaters was added to Simulation B to investigate the effects of alum flocculated material. The wastewaters of Simulations A and B had SUVA256 and 280 values of between 1 and 2 while those of the full-scale system were between 2 and 4. Significant differences were found in colour formation between the two simulations. Peak fitting of HPSEC-UV chromatographs (260 nm and 280 nm) indicated removal of low molecular weight (LMW, <800 Da) UV absorbing compounds in ASB treatment. Colour was attributed to MW compounds of >800 Da. Peaks of HMW compounds, ∼1718 and 1775 measured at 260 nm and 280 nm, respectively were significantly higher in Simulation A at the 1 and 3 HRT compared with Simulation B, which corresponded in higher colour formation in Simulation A. A survey of the full-scale ASB showed a significant decrease in MW of organic compounds throughout the treatment process, with colour formation apparent when compounds had a MW >3000 Da. These compounds were likely to be aromatic and lignin-derived. The addition of alum sludge had significant effects on the extent of colour development.