Figure 3: 254nm spectrum of a UV amalgam lamp and the effective spectrum for destroying bacteria (e-Coli in accordance with DIN 1031 Part 10) (Photo: Heraeus Noblelight GmbH, Hanau) |
Water disinfection using UV radiation offers a number of benefits: micro-organisms are unable to build up resistance; there are no disinfection by-products; the process is entirely free of chemicals so that there is no effect on water composition, smell or taste.
As well as reducing the danger of illness, the above points are also very important for the zoo inhabitants, as they affect the way that the animals accept their living conditions and considerations such as stress, eating habits, growth, life expectancy and birth rate. Furthermore, at around 0.01Euro/m³, UV light is the most cost-effective, non-chemical water disinfection technique.
The killer blow
The "killer" UV dose, the so-called lethal dose is known for nearly all micro-organisms. Principally, it depends on the cell structure of the pathogen. Consequently, bacteria are more receptive to UV than fungi, which because of their stable cell walls with pigments require a higher lethal dose.
For example, for a disinfection rate of 99.9%, a lethal dose of 9.0 mWs/cm² is required for Coli bacteria, while for the mould fungus Aspergillus Niger it is around 396 mWs/cm². In addition, the required UV dose is also affected by diverse physical parameters in the water such as transmission, suspended solids and iron content.
Water is mainly disinfected using the flow through technique in UV water reactors. The systems offer flow rates of 30 to 700 m³/h and several reactors can be connected in series or parallel.
According to the type and capacity of the system, in the reactor cylinder there can be up to twenty two 220 W UV low pressure amalgam lamps or up to fourteen 440 W UV low pressure high power amalgam lamps from the specialist light source manufacturer Heraeus Noblelight (Fig. 4).