Effect of organics on sulfur-utilizing autotrophic denitrification under mixotrophic conditions
Introduction
Biological denitrification has been used successfully to remove nitrogen from wastewater. Usually, either autotrophic or heterotrophic denitrification systems are used for this purpose. Heterotrophic denitrification is very efficient in terms of nitrate removal provided adequate amounts of organic carbon are available (Flere and Zhang, 1999, Zhang and Lampe, 1999). However, when organic carbon in the wastewater is insufficient compared to the nitrogen content, expensive chemicals, like methanol or similar organic compounds, must be added. For this reason, sulfur-based autotrophic denitrification has been receiving more attention recently due to two advantages: (1) there is no need for an external organic carbon source, like methanol or ethanol, which lowers the cost and risks of the process; and (2) there is less sludge production, thereby minimizing sludge handling (Batchelor and Lawrence, 1978, Claus and Kutzner, 1985, Koenig and Liu, 1996, Zhang and Lampe, 1999). However, autotrophic denitrification increases the sulfate concentration in the wastewater and consumes alkalinity. When wastewater with very high nitrate concentrations is treated, significant sulfate production might become a problem due to sulfide production if anaerobic conditions develop. Alkalinity consumption in high-nitrate, low-alkalinity wastewater would increase the overall treatment costs, because supplementary alkalinity must be added to maintain a neutral reactor pH. In an effort to reduce alkalinity consumption, sulfur and limestone autotrophic denitrification (SLAD) systems have been increasingly studied (Flere and Zhang, 1999, van der Hoek et al., 1998, Zhang and Lampe, 1999). Although the use of limestone seems to be an economical and effective method of compensating for alkalinity consumption in the autotrophic denitrification process, it also has the disadvantage of increasing the hardness and the total dissolved solids (TDS) level of the effluent (Driscoll and Bisogni, 1978). When wastewater containing high nitrate is treated, it is difficult to provide alkalinity with limestone, due to the limited rate of CaCO3 dissolution and it is difficult to reduce the high concentration of sulfate.
With regard to microbiology, bacteria capable of oxidizing reduced sulfur compounds like sulfide, sulfur, or thiosulfate can be classified physiologically into four types: obligate chemolithoautotrophs, facultative chemolithoautotrophs, chemolithoheterotrophs, and heterotrophs. Obligate chemolithoautotrophs capable of denitrification, like Thiobacillus denitrificans and Thiomicrospira denitrificans, are virtually restricted to an autotrophic mode of growth, since they cannot obtain energy from the oxidation of organic compounds, being able to utilize organic compounds only to a limited extent (Kuenen, 1979). In contrast, facultative chemolithoautotrophic denitrifiers, such as Thiobacillus versutus, Thiobacillus thyasiris, Thiosphaera pantotropha, and Paracoccus denitrificans are not only able to grow autotrophically, by using reduced sulfur compounds as an energy source, but are also capable of heterotrophic growth. Hence, these bacteria can apparently adapt to different environments (i.e. autotrophic, heterotrophic, or mixotrophic conditions) (Matin, 1978). Because in nature, these bacteria are likely to encounter autotrophic and heterotrophic conditions, it is of considerable interest that their nitrate removal characteristics under mixotrophic conditions be determined. However, no detailed information is available on the effect of organic compounds on sulfur-based autotrophic denitrification.
The objectives of the study described by this paper were; (1) to determine the effects of organic compounds, such as methanol and landfill leachate, on denitrification in an upflow packed-bed column, while providing a comparison among obligately autotrophic, mixotrophic, and heterotrophic denitrification, and (2) to evaluate the feasibility of a sulfur-utilizing autotrophic denitrification process to treat nitrate-containing wastewater containing organic substances, or to treat nitrate-containing wastewater by adding smaller amounts of organic compounds than indicated by the heterotrophic stoichiometric requirements, thereby promoting simultaneous autotrophic and heterotrophic denitrification.
Section snippets
Column reactors
Six laboratory columns were constructed of Plexiglass tubes having an inner diameter of 30 mm and a volume of 250 ml (Fig. 1). Each column was packed with different types of media, as illustrated in Table 1. Sulfur particles were irregular in shape with diameter ranging from 4 to 4.75 mm and porosity from 48 to 50%. The ceramic beads and granular activated carbon (GAC) particles used in columns 4 and 6, respectively, were similar in size to the sulfur particles. To prevent the effect of
pH variation and alkalinity
The influent pH for all tests described below was maintained at 7±0.2. Denitrification, in common with other biological processes, is a function of pH. The optimum pH for most strains of heterotrophic denitrifying bacteria has been reported to be between 7 and 8 (Knowles, 1982). Autotrophic colorless sulfur denitrifiers have an optimum pH range of 6–9 (Holt et al., 1994). Eq. (1) suggests that 1.28 moles of hydrogen ions are produced when 1 mole of nitrate-nitrogen is reduced (equivalent to
Acknowledgements
This work was supported in part by the G7 Environmental Engineering Technology Development project and in part by the Brain Korea 21 program and by the Advanced Environmental Monitoring Research Center (ADEMRC) of KOSEF.
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