Evaluation of UV disinfection systems for large‐scale secondary effluent

Because of the lack of readily available information about the influence of temperature on microorganism reactivation processes subsequent to inactivation with UV radiation, a series of batch reactivation studies were performed at 5, 10, 15, 20, 25, and 30°C. A special effort was made to model the reactivation process to enable the effect of the temperature variable to be quantified. Because an earlier-proposed kinetic model (K. Kashi UV light is one of the most practical methods used for disinfection in wastewater treatment systems because it can inactivate bacteria, viruses, bacterial spores, and oocysts of protozoa (6, 9). Unlike those for other technologies, such as chlorination and ozonation, one of the technological advantages often put forward for UV disinfection is its lack of sensitivity to temperature variations. This independence from temperature would be expected if UV were a simple photochemical reaction (11). Severin et al. (12) performed a series of batch inactivation studies with bacteria, yeast, and viruses and demonstrated that the effect of temperature on UV inactivation was very small. Abu-ghararah (1) found that the effect of water temperature on the kinetics of the UV disinfection process in the normal operating range of most treatment plants (20 to 40°C) is not statistically significant.However, the physiological effect of inactivation is not well understood and is complicated by the ability of many organisms to repair UV damage inflicted to their nucleic acids. While the infliction of damage is a purely photochemical reaction that occurs in the few seconds while water is exposed to UV radiation in the disinfection channels, the interplay of reactions which eventually lead to inactivation includes some biochemical reactions which can be affected by temperature. The latter phenomena can take place during the irradiation period, but it is mostly during the postirradiation time that reactivation processes occur if adequate conditions exist.The principal inactivating effect of UV irradiation is the formation of photoproducts in DNA. Of these photoproducts, the most important is the pyrimidine dimer formed between adjacent pyrimidine molecules on the same strand of DNA, which can interrupt both the transcription and the replication of the DNA. The formation of a dimer can be reversed by two repair mechanisms: photoreactivation and dark repair. Systematic quantitative study of photoreactivation, the more important of the two mechanisms, has suggested a two-step reaction scheme (4).Step 1 is the formation of a complex between a photoreactivation enzyme (PRE) and the dimer to be repaired. This step does not require light, but it is dependent on temperature, pH, and ionic strength (8).Step 2 is the release of PRE and repaired DNA. The restoration of the dimer to its original monomerized form is absolutely dependent upon light energy intensity. The reaction occurs in less than a millisecond; consequently, the limiting step of the whole reactivation process is the formation of the PREdimer complex. A...

Section: Methodsmentioning

confidence: 99%

Photoreactivation and Dark Repair in UV-Treated Microorganisms: Effect of Temperature

Salcedo

Andrade²,

Quiroga

et al. 2007

“…The UV system is considered one of the more effective disinfection techniques for bacteria and viruses in drinking water and wastewater (26). Although there have been many studies on UV inactivation of microorganisms such as Escherichia coli (14), relatively few studies on UV inactivation of C. parvum have been conducted. This is because, based on assessment by in vitro excystation and vital-dye methods, UV irradiation was considered ineffective in inactivating C. parvum (2,23,27).…”

mentioning

confidence: 99%

Efficacy of UV Irradiation in Inactivating Cryptosporidium parvum Oocysts

Morita

Namikoshi

Hirata

et al. 2002

115

To evaluate the effectiveness of UV irradiation in inactivating Cryptosporidium parvum oocysts, the animal infectivities and excystation abilities of oocysts that had been exposed to various UV doses were determined. Infectivity decreased exponentially as the UV dose increased, and the required dose for a 2-log 10 reduction in infectivity (99% inactivation) was approximately 1.0 mWs/cm 2 at 20°C. However, C. parvum oocysts exhibited high resistance to UV irradiation, requiring an extremely high dose of 230 mWs/cm 2 for a 2-log 10 reduction in excystation, which was used to assess viability. Moreover, the excystation ability exhibited only slight decreases at UV doses below 100 mWs/cm 2 . Thus, UV treatment resulted in oocysts that were able to excyst but not infect. The effects of temperature and UV intensity on the UV dose requirement were also studied. The results showed that for every 10°C reduction in water temperature, the increase in the UV irradiation dose required for a 2-log 10 reduction in infectivity was only 7%, and for every 10-fold increase in intensity, the dose increase was only 8%. In addition, the potential of oocysts to recover infectivity and to repair UV-induced injury (pyrimidine dimers) in DNA by photoreactivation and dark repair was investigated. There was no recovery in infectivity following treatment by fluorescent-light irradiation or storage in darkness. In contrast, UV-induced pyrimidine dimers in the DNA were apparently repaired by both photoreactivation and dark repair, as determined by endonuclease-sensitive site assay. However, the recovery rate was different in each process. Given these results, the effects of UV irradiation on C. parvum oocysts as determined by animal infectivity can conclusively be considered irreversible.

“…Moreover, UV irradiation produces no hazardous by-products like many of the conventional chlorination processes (22). Therefore, water treatment plants that utilize UV disinfection processes have increased in number in recent decades (14).…”

mentioning

confidence: 99%

Determination of Pyrimidine Dimers in Escherichia coli and Cryptosporidium parvum during UV Light Inactivation, Photoreactivation, and Dark Repair

Oguma¹,

Katayama²,

Mikami

et al. 2001

156

102

UV inactivation, photoreactivation, and dark repair of Escherichia coli and Cryptosporidium parvum were investigated with the endonuclease sensitive site (ESS) assay, which can determine UV-induced pyrimidine dimers in the genomic DNA of microorganisms. In a 99.9% inactivation of E. coli, high correlation was observed between the dose of UV irradiation and the number of pyrimidine dimers induced in the DNA of E. coli. The colony-forming ability of E. coli also correlated highly with the number of pyrimidine dimers in the DNA, indicating that the ESS assay is comparable to the method conventionally used to measure colony-forming ability. When E. coli were exposed to fluorescent light after a 99.9% inactivation by UV irradiation, UV-induced pyrimidine dimers in the DNA were continuously repaired and the colony-forming ability recovered gradually. When kept in darkness after the UV inactivation, however, E. coli showed neither repair of pyrimidine dimers nor recovery of colony-forming ability. When C. parvum were exposed to fluorescent light after UV inactivation, UV-induced pyrimidine dimers in the DNA were continuously repaired, while no recovery of animal infectivity was observed. When kept in darkness after UV inactivation, C. parvum also showed no recovery of infectivity in spite of the repair of pyrimidine dimers. It was suggested, therefore, that the infectivity of C. parvum would not recover either by photoreactivation or by dark repair even after the repair of pyrimidine dimers in the genomic DNA.