Secondary and tertiary effluent from water treatment plants, although relatively low in BOD and suspended solids in efficiently operated installations, contains a number of pathogenic micro-organisms that can cause serious diseases in people and animals.

Disease can be transmitted orally by ingestion of infected water or by consumption of contaminated shellfish, or merely as a result of contact with the skin when swimming in contaminated water.

The risk of disease is increased by the discharge of infected water close to swimming areas, drinking water intakes, and shellfish-farming locations or when used for agricultural purposes in the interests of water conservation.

The incorporation of a medium-pressure, broad-spectrum germicidal irradiation system after final filtration of treated waste water can provide an economical means of disinfection to meet the microbial standards required for safe discharge or re-use without the use of additional chemicals, which could add to the contamination load.

Design of the UV system must consider the minimum UV transmission value of the effluent and the peak flow.

Industrial wastewater treatment plants may incorporate a medium-pressure UV system in conjunction with ozone. This advanced oxidation process offers a powerful water treatment combination both for disinfection and for reduction of difficult organic compounds.

Consider the following additional advantages of medium-pressure systems:

• Compared with conventional low-pressure mercury lamp systems, the use of high intensity, medium-pressure models results in a dramatic reduction of number of lamps, space requirement, maintenance time, and installation and lamp replacement costs.
• UV radiation of broad germicidal action applied at the correct dose rate destroys the ability of pathogens to metabolize and reproduce and is successful against viruses and bacteria.
• Medium-pressure UV systems with high energy output per unit size are particularly applicable in this respect, being specifically designed for this purpose. Some units incorporate an automatic wiper with interval control to suit effluent quality.
• UV irradiation treatment eliminates the need for chemical handling and storage on site, or systems for dosing chemicals into the treatment stream.
• There are cost savings for smaller waste water treatment installations when compared with open channel technology.