Ozonation is a widely applied water treatment process, used for oxidation of contaminants, as well as for the disinfection of water. However, the conventional ozonation process demands a high energy requirement and deep tanks to ensure effective mass transfer and oxidation.Microbubble technologies have emerged which have the potential to improve gas-liquid contacting. Microbubbles have diameters of 1-100 µm, while conventional bubbles used in ozonation are between 2 -6 mm. Microbubbles have many favorable characteristics that make them suitable for ozonation. In this review, the attributes of microbubbles for ozonation have been compared with those of conventional bubbles. The higher interfacial area and lower rise velocity of microbubbles compared with conventional bubbles means that ozone in the gas phase can be more efficiently transferred into the liquid phase. This is due to a higher contact time and increased contact area of the bubble with the bulk liquid. The analysis reveals that the volumetric mass transfer coefficient can be significantly enhanced through the use of microbubbles. In addition, the steady state dissolved ozone concentration was positively impacted by the use of microbubbles. Microbubbles were shown to be able to oxidize a broader range of organic compounds more quickly than for conventional bubbles. However, the review highlighted that comparison of microbubbles with conventional bubbles is not always carried

Results of a feasibility study using an actual coal-gasification wastewater confirm the promising nature of a coupling-dephenolization scheme which we have proposed previously. Kinetic results indicate that the aerobic coupling of phenols, which is the heart of the proposed scheme, occurs at a sufficiently high rate in the actual wastewater to make the scheme practical. A cost anal sis of the scheme indicates that for a commercial-scale coal gasification plant with a daily output of 200 X 10' J of synthetic natural gas and 3 X lo6 gal of wastewater, the treatment cost for a 99% removal of phenols from a wastewater containing 4 g of phenol/L is $8.6/103 gal (1 980 dollar); this compares favorably with the estimated costs of $12.91 1 O3 gal for a biooxidation scheme and $12.81 lo3 gal for a solvent extraction scheme. Additional advantages of the coupling scheme are its simplicity and fast treatment under mild conditions, ability to withstand process interruptions, easy disposal of organic precipitates by incineration, and the ability to remove other pollutants such as sulfides and aromatic amines. I n t r o d u c t i o nA recent kinetic study from our laboratory (Lim et al., 1983) indicates that aqueous phenols, in the presence of oxygen and a catalytic amount of cuprous chloride, undergo a facile oxidative coupling reaction to form insoluble coupling products. The reaction involves primarily the oxidative oligomerization of phenols, although it is also accompanied by a limited amount of hydroxylation reaction. The coupling products share the characteristics of humic acids which are known to be formed in the permanganate oxidation of phenol (Ficek and Boll, 1980); they have been tentatively identified as low molecular weight humic acids.The results have led us to propose a novel dephenolization scheme for treating full-strength coal-conversion wastewaters which are not easily handled by such conventional wastewater treatment schemes as solvent extraction (Beychok, 1974;Cavanaugh et al., 1977;Greminger et al., 1982) and biooxidation (Benefield et al., 1977;Cavanaugh et al., 1977; Holladay et al., 1978; Luthy and Tallon, 1980). Phenol extractants are inherently polar in nature; they are sufficiently soluble in water to warrant an expensive solvent recovery step in an extraction scheme.Biooxidation, on the other hand, is limited to dilute phenolic wastewaters (typically less than 1000 mg/L of phenols); it is slow and highly sensitive to process interruptions and fluctuations in temperature, solution pH, nutrients, and loadings (Holladay et al., 1978; Luthy and Tallon, 1980; Vela and Ralston, 1978).We have tested the feasibility of our proposed coupling-dephenolization scheme using an actual full-strength wastewater that was generated in a fixed-bed coal gasifier (run no. 99) at the Morgantown Energy Technology Center (METC), Morgantown, WV (Fernald, 1983). In this paper we present the results of the feasibility study along with the results of a cost analysis of the proposed scheme. The results show that the coupling...

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