UV has made tremendous gains in the last 25 years, and is now recognized as a primary method of disinfecting wastewater. All of the typical wastewater organisms are rendered nonviable by a correct dose of UV light; ETS maintains comprehensive databases of organisms and the amount of UV required to achieve 1, 2, 3 or higher log removals to meet and exceed a discharge permit.
Municipal wastewater was originally disinfected using chlorine. As awareness grew regarding the potential for carcinogenicity, UV light gradually replaced the use of chemicals. The earliest UV systems utilized racks of lamps immersed into an open channel. These open, gravity flow channels suffer from many disadvantages; poor hydraulic mixing, very large footprint, expensive to build, and are vulnerable to fluctuations in flowrate.
All of the ETS products are designed for insertion into a pipe, and ETS engineers will be pleased to demonstrate how chlorine contact tanks can be retrofitted with a closed vessel UV system.
System sizing is a function of flowrate, transmittance of the waste stream, total suspended solids (TSS), microbial challenge, discharge requirements, and degree of standby (if required). Occasionally a wastewater facility will operate the UV system seasonally and will sometimes isolate and mothball a UV facility when disinfection is not required to meet a discharge permit.
Low pressure UV lamps can become vulnerable to phenomenon called photo-repair. Two separate mechanisms exist; light repair occurs following disinfection by low pressure UV lamps and exposure to sunlight. This leads many open channel plants to cover the open channel. Dark repair is a far slower mechanism and less of an issue in municipal installations.
Wastewater applications include backwash water, screen washes, watershed management, stream discharge, storm water overflows and increasingly the reuse of wastewater. Many wastewater facilities are being renamed Water Reclamation Facilities (WRFs); and now watershed management professionals are starting to recognize the value of this precious commodity.