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.
Differences in the nature of soluble organic matter were measured for various full-scale wastewater treatment processes. Conventional activated sludge, pure oxygen activated sludge, biofiltration, granular activated carbon, and tertiary sand filtration were evaluated. Effluent soluble organic matter was analyzed by ultrafiltration for the apparent molecular weight distribution of soluble organic carbon and UV-absorbing material. The effects of annual season, secondary treatment process, and tertiary treatment process upon the properties of the effluent soluble organic matter were statistically significant at the 99% level. Effluent properties from the various treatments were sufficiently different to support the concept of the selection of appropriate treatments to minimize the effluent concentration of specific fractions of the soluble organic material as required for specific water reuse applications.
An aerated wastewater stabilization lagoon, or any subsection of the overall lagoon, should function as an oxidation reactor, a clarifier, and a sludge digester. A relatively simple set of calculations can be used to evaluate these three functions for zones within a lagoon system. Only SBOD, TSS, and NH4-N or PO4-P measurements are needed to support the analysis. The evaluation should be limited to bacterially dominated zones of a lagoon system, i.e. lack of significant algal influence.
A relatively simple set of calculations was presented in 1994 to evaluate the effectiveness of each ASB cell as reactor, clarifier, and digester. The steady-state model, which incorporated estimates of solids settling and benthal feedback of BOD5 and nutrients, has been a reasonable diagnostic tool for municipal and industrial applications. Results have aided in understanding normal system function, the nature of chronic inefficiencies of individual systems, and appropriate modifications to meet changes in discharge requirements. For applications in the pulp and paper industry, several changes have been incorporated recently. Nitrogen limitation is not needed in modeling pulp and paper ASB reactions. Slowly biodegradable material is modeled as a contributor to soluble BOD5, and this contribution becomes a significant factor in the latter segments of an ASB. Phosphorus availability is modeled as a stoichiometric control of soluble BOD5 uptake. Anoxic microorganisms are assumed to be responsible for a portion of the soluble BOD5 consumption in the first ASB aeration zone. Finally, the long-term nutrient capture in ASBs is modeled as 3% for nitrogen and 28% for phosphorus.
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