A two-year survey of adsorbable organic halide (AOX) removals was conducted for eight full-scale wastewater treatment systems operated at pulp mills in North America. The resulting AOX database, one of the most comprehensive currently available, supports several significant findings.
AOX removals averaged 46% for the two activated sludge systems and 34% for the five aerated stabilization basins (ASBs). Both the activated sludge facilities and ASBs averaged removing over half of the influent low molecular weight AOX. The average removal of high molecular weight AOX varied among mill sites from 1% to 47% and was not correlated with the type of treatment process used. The best-performing treatment facility, located at a sulfite mill receiving nearly 100% hardwood furnish and using an activated sludge treatment system, was able to consistently remove 53% of the AOX, 62% of the low molecular weight AOX, and 47% of the high molecular weight AOX in bleaching wastewaters. Factors responsible for this high performance are not currently understood. Effluent AOX was strongly correlated (R2 = 0.90) to the influent loading for ASBs. In-mill improvements such as the use of oxygen and peroxide as well as chlorine dioxide substitution reduce influent AOX loading, apparently with minimal impact on treatment plant removal efficiency.
Floc breakup in biological wastewater treatment occurs in response to hydrodynamic stresses imposed by aeration, recirculation, and mixing. This size reduction is of particular concern because it leads to solids carry-over and adversely affects process controllability. A laboratory study of floc size reduction has shown how the hydrodynamic environment causes breakup and the extent to which it proceeds at particular levels of dissipation. The structure of jet flows was found to be well-suited for the reduction of floc size.
A single-stage aerobic biological treatment system was developed specifically to convert organically bound chlorine to inorganic chloride. In initial laboratory tests, greater than 90% reduction of AOX was achieved in synthetic dichlorophenol, commercial pentachlorophenol, and combined kraft wastewaters, and less than a week was required for startup/acclimation. A six-month field test of the process on a pentachlorophenol wastewater was very successful under highly variable influent conditions. No chlorinated byproducts were detected, and measurements strongly indicated that dehalogenation had occurred.
Recent experiments found Cl/El wastewater to be most easily treatable by the aerobic process, followed by combined lagoon influent, El wastewater, and finally lagoon effluent. However, results have indicated that application of the treatment process to kraft AOX reduction can become feasible only if further process improvements can be defined.
Analytical procedures were developed for determining the apparent molecular weight (AMW) distribution of organic halide in both liquid and solid pulp and paper process components. The techniques included molecular weight separations using ultrafiltration (UF) and determinations using adsorption-pyrolysis-microcoulometry (AC/MC). Correction coefficients for rejection properties of UF membranes were determined using a diafiltration technique.
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