 Disinfection By Products
With respect to the research results on chlorine-byproduct- formation in drinking water, disinfectionby- products are of particular interest even for UV disinfection.
On a theoretical base every disinfectant should induce by-products. The point which has to be identified is the CT-Value. An ideal disinfectant can be used with CT-values which below the limit of by-product formation. Intensive research showed that UV doses which have to be applied for drinking water are in the range of 400 J/m2 whereas by-product- formation (Photooxidation) occurs at UV doses in the kilo Joule range only.
A special focus should be given on emissions below 200 nm which transfers nitrate into nitrite very easily.
This particular medium pressure problem can be solved due to the combination of that lamp type with quartz materials which absorb wave length in that range.
Microorganisms
Since examination of drinking water was established nearly 100 years ago the standards are still the same in our days. Although these standards are appropriate to monitor the hygienic safety of drinking water from a basic standpoint, the past decades showed that there are still organisms to be considered which are not included in standard drinking water examinations.
These organisms are e.g.:
- Legionella (Bacterium)
- Mycobacteria (Bacterium)
- Aeromonas (Bacterium)
- Cryptosporidium (Protozoa)
- Giardia lamblia (Protozoa)
Especially protozoa are the new focus of water authorities throughout the world since Cryptosporidium caused the Milwaukee outbreak (400.000 infected people). Since that time a couple of other outbreaks demonstrated the urgent need for new solutions in monitoring and treatment of drinking water (UK, Australia). Whereas traditional disinfection with chlorine is not sufficient against these parasites a couple of other strategies were successful (Multi barrier concept and monitoring, filtration, ozonation). However non of these are really the sort of safety barrier which are equivalent to a real disinfection concept. UV seemed to be not the right solution as well, but recent research showed evidence for UV to be the most effective method. In experiments based on infectivity instead of viability of Cryptosporidium was eliminated with UV doses which were typical for drinking water applications. Nevertheless the research has to continue to verify and reproduct these results.
UV Reactor Design
In contrast to waste water open channel systems all drinking water installations are subject to water pressure which results in the use of closed vessel systems with different irradiation geometries:
Thin Film Geometry
The flow direction is parallel in orientation to the lamps. Due to certain water quality requirements the system can be adapted to very poor water conditions (60% T 254 in 10 mm). Better water qualities are allowing to use round reactors. The geometry would be the same. These types of reactors are capable of flows between 1 m2/h (single lamps
system) up to 400 m2/h.
Positive Geometry
These kind of reactors are designed for industrial and drinking water applications. The water is
flowing through the quartz sleeve ,whereas the lamps are outside of the water. Reflectors are focusing the UV light into the quartz sleeve. This kind of reactors are capable of flows up to 95 m2/h.
Modular Geometry
The water flow direction is perpendicular in orientation to the lamp. This kind allows to use a
modular design concept. Several rows of lamps after each other are theoretically not flow limited. But for practical reasons this type is limited to maximum flows of 20 MGD per unit.
The reactor design is strongly related to the lamp type. All given examples are based on low pressure high intensity lamp types.
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Drinking Water
