There exist several mechanisms for water disinfection, including chlorination and chloramination, ozonation, and UV irradiation. Ordinarily, only one disinfection technique is used in a treatment train. However, there are potential benefits to combining water treatment methods in sequence, including energy and cost savings and a safer product water.
The secret behind saving money through a combined disinfection process lies in the different mechanisms utilized in oxidation versus irradiation. Oxidation relies on physical interactions between oxidants, such as free chlorine or ozone radicals, and target molecules in order to decompose the molecules. Thus, oxidation works best when there is a high concentration of molecules to degrade so that oxidants maximize their number of interactions.
On the other hand, UV disinfection depends on the ability of UV light to penetrate the water and reach target molecules. Consequently, UV disinfection is actually more effective at lower concentrations, particularly when the molecules present are able to absorb or scatter UV light. A combined process uses the most effective means of each water treatment method by first treating with oxidants and then applying UV irradiation to a water with higher UV transmittance.
Both chlorination and chloramination lead to the formation of disinfection by-products (DBPs), which form when the oxidants react with organic matter. Chlorination leads to the formation of products such as trihalomethanes (THMs) and haloacetic acids (HAAs). Chloramination leads to the formation of N-nitrosodimethylamine (NDMA), which is potentially more hazardous than THMs and HAAs.
A reduction in the amount of chlorine needed, or the ability to avoid chlorine compounds altogether, is the best method to avoid DBPs. UV disinfection is a good alternative, but can be costly in energy expenditure. A combination of UV with chlorination/chloramination would reduce the DBP formation potential. Other oxidizing agents do not form DBPs, such as ozone, hydrogen peroxide, and peracetic acid. However, these processes can be cost prohibitive. The cost for achieving a water treatment goal with these oxidants may be reduced through application in combination with UV disinfection.
A combination process is best suited to applications where transmittance can be improved through chemical oxidation. Wastewater disinfection is such an application—a typical transmittance for secondary treated wastewater is 65%. The transmittance may be even lower if ferric chloride is used as a coagulant, since ferrous compounds can interfere with UV transmittance.
Other applications that can benefit from a combined process are those with a confined footprint or material supply chain. Some examples include remote and isolated needs such as water treatment aboard ships or at mining locations. The efficiency gained by maximizing the outputs of both disinfection processes reduces the size and energy needs to deliver the same level of treatment.
Lastly, achieving a high degree of disinfection may be more affordable with a combined process than with an individual process. Thus, combined disinfection is suitable for applications where exceptional levels of disinfection are required, such as the food and beverage industry and the aquaculture industry, where disinfected water allows for denser farming.
UV disinfection efficiency depends on two factors: the UV Transmittance of incoming water and the intensity of UV light applied during treatment. A high UVT value indicates that incoming water has low levels of dissolved solids and organics. Therefore, UV light transmission will be highly efficient, leading to high levels of photo-oxidation and biological inactivation. Conversely, if UVT levels are low, then efficiency drops.
In a combined or multi-step treatment process, the required UV dose will depend on the efficacy of the previous process. If the previous process removes a significant amount of solids and organics, a lower UV dose will be required. Inline measurement of UV transmittance using a UVT probe will provide the information needed for process optimization and cost savings through accurate UV dosing.
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