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Extractable organics and colour in a bleached kraft mill effluent land application system and recipient ground waters

This thesis describes a study of the behaviour of extractable organics and colour in a land application system used to treat highly coloured, organic-rich, alkali extraction stage and foul condensate effluents from a traditional bleached kraft mill. The land application system comprised 44 unlined seepage basins, totalling 86 ha in area, into which approximately 10,000 m³ d⁻¹ effluent was loaded for approximately eight months of each year. The study aimed to determine (1) the effectiveness of the system for treating major effluent organics, including monoterpenes, fatty acids, resin acids, chlorophenolics, and colour, and (2) the effects of the system on organic contamination of sediments, aquifer materials, and ground waters. The study was carried out in four parts, investigating: (i) the behaviour of extractable organics and colour in the effluent drains and seepage basins; (ii) the penetration of extractable organics into the sediments beneath the seepage basins; (iii) the organic chemistry of shallow ground waters beneath the seepage system, and of deeper off-site ground waters; and (iv) behaviour of extractable organics and colour in laboratory soil-column simulating the land application process. In addition, a review of land application of pulp mill effluents was carried out, and analytical methods for determining extractable organics in effluents, sediments, and ground waters were developed. Concentrations of extractable organics and chlorophenolics were reduced by >95% over a 40 day period in the seepage basins. Removal rates decreased in the order fatty acids > chlorophenolics ≈ monoterpenes > resin acids. Resin acids underwent reductive and oxidative transformations, producing transient intermediates including 13-abietenoic acid and 13β-hydroxyabietanoic acid. A suite of relatively stable transformed species, dominated by the saturated compound abietanoic acid, remained in the seepage pond after 40 days. Colour removal over the same period was approximately 25%. High concentrations of effluent-derived extractable organics were found in surficial sediments beneath the seepage basins. Major compounds were resin acids, largely saturated species, and diterpene hydrocarbon transformation products. Diterpene hydrocarbons were dominated by retene, fichtelite, and dehydroabietin. Surficial sediments contained concentrations of total resin acids and diterpene hydrocarbons of up to 10,000 mg kg⁻¹. Concentrations dropped with depth, but high levels were found in discrete zones at depths of up to 5 m beneath the surface. This indicated that effluent movement was occurring through permeable conduits rather than via uniform infiltration through the soil, and was consistent with the fractured geology of the site. Ground waters taken from seven well clusters, sampling depths between 2 and 15 m beneath the seepage basins, contained elevated sodium and chloride concentrations (200-400 mg L⁻¹) and spatially variable colour levels (100-2000 CPU). Effluent-derived extractable organics were also found in variable concentrations. Major compounds were methyl-substituted 2-cyclopentenones, resin acids, and diterpene hydrocarbons. Resin acids and diterpene hydrocarbons were the generally dominant compound classes, concentrations totalling 20-2600 μg L⁻¹. Preliminary assessment of chlorophenolics found low levels, totalling 2-4 μg L⁻¹ in the most contaminated ground waters. The results indicated that contamination of shallow ground water was occurring, but the nature and level of this contamination was highly spatially variable. As found for the sediment studies, these findings were consistent with effluent infiltration through heterogeneous, fractured, sub-surface geology. The hydrogeology of the area was determined to be dominated by ground water movement through fractured zones in four major ignimbrite aquifers. Wells were placed at depths between 9 and 110 m, and at distances from immediately adjacent to 2.7 km from the seepage area to sample ground water from these zones. Contamination was found largely within the highly fractured, high flow zones of two of these units, the Marshall A and, to a lesser extent, the Marshall B ignimbrite aquifers. Concentrations of effluent-sourced extractable organics (largely resin acids and diterpene hydrocarbons) and colour were highest immediately beneath the seepage area, maximum concentrations of 110 μg L⁻¹ total resin acids and diterpene hydrocarbons, and 900 CPU colour being found. Concentrations dropped rapidly with distance from the ponds, decreasing by approximately 90% within about 100 m from the seepage area. There was evidence for retardation of organics relative to sodium and chloride in the off-site aquifers. Laboratory simulations of effluent infiltration through seepage site soils were carried out under two application regimes (permanently flooded, and flood-dry cycle) to determine the range of treatment possibilities occurring in the seepage system. Leachate chemistry was monitored over an approximately two year period, and soil extractives measured at the end of the study. Mass balance was used to estimate effluent treatment efficiency. The flood-dry (aerobic) application regime resulted in essentially compete degradative removal of extractable organics. Under the flooded (anaerobic) regime, extractable organic removal was limited for all constituents other than fatty acids, which were >90% removed. Resin acids were strongly retained by the soil, approximately 72% remaining in the soil at the end of the study. Approximately 25% was recovered in the leachate. Monoterpenes were highly mobile, moving rapidly through the soil. A relatively stable suite of transformed compounds was found in the leachates. Monoterpene removal was estimated to be approximately 55%. Under flooded conditions, transformation rather than degradative removal of extractable organics occurred. Colour intensification, rather than reduction, occurred at times in both application regimes. At the end of the study, colour removal in the flood-dry cycle regime was 2%, and in the flooded regime an increase of 51% was measured. In summary, the study found that the land application system resulted in large decreases (>95%) in the concentrations of extractable organics, but little overall treatment of effluent colour. Ground waters affected by effluent infiltration were highly variable in composition, but in general contained elevated colour levels, and moderate-to-low concentrations of extractable organics. Resin acids and diterpene hydrocarbons were the dominant class of extractable organics found in sediments and ground waters beneath the seepage system.
Type of thesis
The University of Waikato
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