Kwangsoon Choi

Characterization of dissolved organic matter in effluents from wastewater treatment plants

Dissolved organic matter (DOM) in effluents from sewage and human-wastes treatment plants (STPEs and HWTPEs) was fractionated using resin adsorbents into six classes: aquatic humic substances (AHS), hydrophobic bases (HoB), hydrophobic neutrals (HoN), hydrophilic acids (HiA), hydrophilic bases (HiB), and hydrophilic neutrals (HiN). DOM-fraction distribution varied substantially depending on the kind of wastewater and the type of treatment process. AHS and HiA dominated in all effluents, collectively accounting for more than 55% of the DOM measured as dissolved organic carbon (DOC). In particular, HiA were the most abundant fraction in the effluents, constituting 32-74% of the DOM. AHS appeared to be the second most dominant fraction and varied considerably, accounting for 3-28% of the DOM. The HoN fraction also varied from 0-21%. AHS, HoN, and HiA were the fractions that changed substantially and differed characteristically among the samples studied. The ratios of ultraviolet absorbance to DOC (UV: DOC ratio) in all effluents exhibited a common relationship: AHS> total DOM > HiA. Nevertheless, the ratios were substantially different between STPEs and HWTPEs and among HWTPEs. For HWTPEs, the effluent from the chemical coagulation process had the highest UV: DOC ratios. On the other hand, the effluent from the ultrafiltration and activated carbon adsorption processes had the lowest ratios. Molecular size distribution of the effluents was determined by size exclusion chromatography and found to exhibit a relatively narrow size range and low weight-averaged molecular weights ranging from 380 to 830 g mol(-1). The weight-averaged molecular weight of DOM increased as the UV: DOC ratio of total DOM increased.

Eutrophication of reservoirs in South Korea

Abstract Natural lakes in South Korea are limited in number and generally quite small. As a result, reservoirs and regulated rivers are the major sources of freshwater for society. About 18 000 reservoirs have been constructed in South Korea, and they are particularly important for domestic water supply. Thirteen of the major reservoirs were surveyed in this general assessment of the trophic state of South Korean reservoirs. Ten reservoirs were from the upper or middle reaches of rivers (including eight of the ten largest reservoirs in Korea), and three were estuarine reservoirs. Reservoirs in the mountainous district of South Korea were usually mesotrophic, whereas the estuarine reservoirs were highly eutrophic. Because total nitrogen to total phosphorus ratios were always between 18 and 163, phosphorus was probably more limiting than nitrogen for algal growth. However, hydraulic residence time and light penetration may be limiting in the nutrient-enriched downstream reservoirs. In winter, algal density was lowest in deep reservoirs, perhaps due to deep mixing. During the same season, algal density was high in shallow reservoirs, perhaps due to a favorable hydraulic residence time. Factors contributing to the observed eutrophication patterns, including nutrient runoff from agriculture, animal farms, fish aquaculture, and urban areas, are discussed. According to the national budget of phosphorus, fertilizer and livestock manure are major source of phosphorus, contributing 133 400 and 73 700 tons of phosphorus per year, respectively, while human excretion discharges 30 000 tons P year−1. Reduction of the application of fertilizer, proper treatment of manure, and conservation of topsoil may be the most practical and effective measures to prevent further lake eutrophication.

Characteristics of Dissolved Organic Carbon in Three Layers of a Deep Reservoir, Lake Soyang, Korea

Vertical and seasonal distributions of dissolved organic carbon (DOC) were investigated in a deep reservoir by considering the biodegradability and optical properties of DOC from three different layers during the stratified season. DOC in the epilimnion was characterized by relatively labile compounds that may have originated from phytoplankton. DOC in the metalimnion was variable in its composition and was possibly affected by turbid water inputs to the reservoir during the summer monsoon season. DOC in the hypolimnion always showed refractory characteristics, with low decomposition rates and high ultraviolet (UV) absorption.

Photoalteration of dissolved organic matter (DOM) released from Microcystis aeruginosa in different growth phases: DOM-fraction distribution and biodegradability

The photoalteration of dissolved organic matter (DOM) produced in different growth phases of the blue-green alga, Microcystis aeruginosa, was investigated by comparing the biodegradability and distribution of fractions of algal DOM after different ultraviolet (UV) treatments. The distribution of DOM-fractions (based on hydrophobic-hydrophilic and acidic-basic breaks) showed that two of the fractions, hydrophilic acids (HiA) and bases (HiB), were more abundant in all growth phases of M. aeruginosa than the other three fractions, hydrophobic acids (HoA), hydrophobic neutrals (HoN), and hydrophilic neutrals (HiN). The proportion of HiB increased, while the HiA fraction decreased with aging of the algae. After UV treatment, all algal DOM became recalcitrant to bacterial degradation without complete photo-degradation. This was more pronounced in DOM from older cultures (stationary phase) as compared to DOM from the exponential growth phase. The DOM distribution was also significantly different after UV exposure, implying photoalteration to the chemical composition of algal DOM. The proportions of the HiB fraction decreased as a percent of the total dissolved organic carbon pool by 1.5-8.1 % after UVA treatment and by 5.3-15.8 % after UVB treatment. In contrast, the HiA fraction increased by similar amounts. Analyses of fluorescent properties and some carboxylic acids confirmed the changes to the HiB and HiA fractions. However, the increased HiA fraction may not be linked to the recalcitrance of algal DOM after UV exposure, since there was no difference in the biodegradability of this fraction before and after UV treatments. The initially labile HiB fraction, however, became less available to bacteria after UV exposure depending on intensity and thus may be linked to the recalcitrance of algal DOM after UV exposure. Our results confirm earlier reports that algal DOM can be changed in its chemical composition as well as biodegradability by UV radiation, and suggest that the HiB fraction may be important in the formation of recalcitrant algal DOM.

Temporal and Vertical Variability in the Relationship Among Organic Matter Indices in a Deep Reservoir Ecosystem

ABSTRACT Temporal and vertical variability in the relationship between biochemical oxygen demand (BOD), chemical oxygen demand (CODMn), and total organic carbon (TOC) were investigated in a deep reservoir (Lake Soyang, Korea) and compared with other major reservoirs in Korea. The average oxidation efficiencies of BOD and COD were low (16% and 36%, respectively), compared with TOC analysis. Weakly positive correlations could be observed between TOC vs. BOD (r = 0.64) and TOC vs. CODMn (r = 0.66) in the upper layer, whereas significant correlations were not observed in the middle and deep layers of the water column. In the upper layer, CODMn was more strongly correlated with particulate organic carbon (POC) than with dissolved organic carbon (DOC), even though DOC is ten times more abundant than POC. This might imply that DOC in the lake is more resistant to permanganate oxidation than POC. The oxidation efficiency of BOD, an indicator of biodegradability, varied among water layers (upper > deep > middle layer). The lower efficiency in the middle layer may be due to an input of recalcitrant allochthonous organic matter during the summer monsoon, when water is known to flow into this layer of the lake. The oxidation efficiency of CODMn also varied significantly by water layer (upper ≅ middle > deep layer). Because of poor and variable oxidation efficiencies, BOD and CODMn measured at any particular time or location should be used with caution as indices of organic matter content in lakes.