Water Quality and Ozone Water Treatment

Aquaculture Water Quality and the Affect of Ozone

(To obtain more information on recirculating aquaculture systems, a complete set of tables, figures and references shown on this and related pages please refer to the book "Recirculating Aquaculture Systems, 2nd Edition", M. B. Timmons, et al, 2002, Cayuga Aqua Ventures, Ithaca, NY. You can obtain a copy of the publication from Cayuga Aqua Ventures by visiting their website at www.c-a-v.net.)

Dissolved Organic Matter

Dissolved organic carbon (DOC) is, by convention, defined as total organic carbon (TOC) after filtration through a 0.45 m membrane filter. In freshwater supplies, humic substances originating from the terrestrial environment are often the most significant contributor to the DOC, conferring a brownish-yellow color on the water. In wastewater, proteins, carbohydrates, lipids, and organic amines will elevate the concentration of DOC. Oxidizing disinfectants like ozone will lose bactericidal strength through reaction with organic matter. The reaction products will generally have weak or no bactericidal activity. Hoigne (1988) has shown that aqueous ozone reactivity can be ascribed to two mechanisms: direct reactions involving molecular ozone and reactions of active hydroxyl-radical intermediates produced by ozone decomposition.

Humic substances of natural waters are relatively resistant to ozonation. Sufficient contact time will produce small amounts of acetic, oxalic, formic and terephthalic acids, phenolic compounds and carbon dioxide. Generally, ozonated organic matter is more biodegradable than the original compounds. The instantaneous ozone demand of surface waters with DOC content of 2.5–3.5 mg/L has been reported to be in the range of 0.50–0.75 mg/L (Roustan et al. 1998). Surface water ozone demands of 0.4–0.5 mg ozone/mg DOC after 5 min of exposure at pH 7.5 was found by Graham (1999). The ozonolysis of carbon-carbon double bonds in organic molecules is an example of an ozone-demanding reaction.

Inorganic Compounds

Oxidizing disinfectants will react with inorganic compounds in accordance with their oxidation potential. Ozone will be involved in several redox reactions due to its high oxidation potential. Metal and heavy metal ions are oxidized to form stable compounds with low solubility. Ferrous and manganeous ions will react to ferric and manganic ions, respectively, which in turn will react with OH- to form an insoluble precipitate. Bromide will be oxidized to bromate through several intermediary steps, while the reaction with chloride is limited by poor kinetics. The conversion of ammonia to nitrite is a slow, pH-dependent, first order reaction, while the nitrite is rapidly oxidized to nitrate. The latter reaction may have a significant effect on ozone disinfection capacity in wastewater treatment systems with incomplete nitrification. Venosa (1983) reports that as much as 2 mg/L ozone was required to oxidize 1 mg/L of nitrite-N.

pH

Extreme pH values may inactivate microorganisms, or limit their growth. The activity of many disinfectants depends on pH. Small changes in the hydrogen ion concentration may influence the disinfection performance. The pH dependence of the biocidal activity of ozone is not clear. Reduced effect at high pH towards poliovirus and rotavirus as well as the cysts of the parasite Naegleria gruberi has been observed (Vaughn et al. 1987; Wickramanayake et al. 1991). However, the opposite relation was evident for Giardia muris cysts, which were more sensitive at pH 9 than at pH 5 and 7 (Wickramanayake et al. 1991). Changes in pH have little to no impact upon UV effectiveness.

Temperature

In general, the microbial inactivation rate by chemical disinfectants will increase with increasing temperature. Farooq et al. (1977) found a higher degree of ozone inactivation of Mycobacterium fortuitum at elevated temperatures. On the other hand, UV inactivation seems relatively insensitive to temperature. Negligible effects were observed in the range 5– 35C when pure cultures of E. coli, Candida parapsilosis and bacterial virus f2 were exposed to UV light in batch reactors (Severine et al. 1983).

Additional information on ozone chemical reactions can found by following the link embedded in this line. The following links on color removal and iron and manganese removal are primarily related to drinking water, but also provide general information on ozone reactions for these materials.