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Removing BDOC by ozonation.

The goal of disinfection in water treatment practice is to maximize protection from bacterial contamination while minimizing the formation of disinfection byproducts (DBPs). New regulations from EPA have imposed stricter disinfection requirements while limiting the allowable concentration of DBPs, a dilemma that is most evident in ozone treatment. The potential for bacterial regrowth increases in many situations after ozonation.

Ozonation alone has not consistently reduced DBP concentrations upon subsequent chlorination. Accordingly, if treatment goals are to be met, ozonation may need to be followed by an additional treatment, such as biological treatment to remove BDOC. As expected, the removal of TOC through ozonation alone generally increased with the ozone dosage. The amount of TOC removed by biodegradation alone also increased with ozone dosage, but the rate of increase reached a plateau above an ozone dose of 1 mg of ozone per mg of TOC. The total amount of TOC removed after ozonation plus compete biodegradation increased with ozone dosage. The additional removal at higher ozone dosages, however, is caused by direct oxidation, rather than an increase in the formation of biodegradable material. Biodegradation of samples by recirculating the water through columns containing an acclimated population of microorganisms attached to a fixed media resulted in reproducible measurements of BDOC. As others have found, this BDOC method is a promising technique and could be standardized by selecting a given column size, media size and type, recirculation flow rate, and seeding period. Long-term chlorine demand decreased with increased ozone dosage and was lower still in ozonated samples that were biodegraded. The decline in long-term (seven-day) chlorine residual could be modeled as a first-order reaction. Water samples that were both ozonated and biodegraded were able to maintain substantially higher and longer-lasting chlorine residuals than untreated water. Bacterial regrowth potential can be measured in batch tests using sand media in an attached acclimated biofilm as a seed. The attached bacteria surviving chlorination will regrow in the water sample, simulating regrowth in a distribution system.

Because of the longer-lasting chlorine residuals in ozonated and biodegraded water, the appearance of bacteria was effectively delayed. This delay increased with higher chlorine and ozone dosages and lasted even longer when the samples were biodegraded. Removal of organic carbon in ozonated water through biodegradation is an effective treatment method for removing some DBP precursors, increasing chlorine residual, and reducing biological regrowth. Biological treatment clearly improves overall water quality and reduces one of the main disadvantages of ozonation-biological regrowth. The beneficial effects shown by this research will almost certainly not be as expensive in plant operation, because biodegradation of the water samples was carried to its maximum level. Also, the economics of such treatment have not been considered, but the results of this research indicate that biological treatment is a promising technology for improving water quality.

Ozonation And BDOC Removal: Effect On Water Quality." By Lori A. Cipparone, engineer, Texas Water Development Board, Austin, Texas; Alicia C. Diehl, graduate research assistant, and Gerald E. Speitel, Jr., associate professor, Civil Engineering Department, University of Texas, Austin, Texas. Journal AWWA, February 1997.
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Title Annotation:bacterial contamination
Author:Billings, Clayton H.
Publication:Public Works
Date:May 1, 1997
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