Wastewater
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.
Municipal wastewater does not need to be disinfected using UV light in an open channel. Open gravity flow channels suffer from many disadvantages; poor hydraulic mixing, very large footprint, expensive to build, high headloss, and vulnerable to fluctuations in flow rate.
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 used to house a new UV installation.
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 a phenomena called photorepair. 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 a municipal installations.
Wastewater applications include backwash water, screen washes, watershed management, stream discharge, stormwater overflows and increasingly the reuse of wastewater. Many wastewater facilities are being renamed Water Reclamation Facilities (WRFs); and finally watershed management professionals are starting to recognize the value of this precious commodity.
UV is used both as a primary disinfectant, and also to dechlorinate reuse water at the factory gate. The regulations governing the disinfection of reclaimed (reuse) water are necessarily onerous, and are defined by the National Water Research Institute (NWRI) guidelines. The wastewater is highly polished, often beyond regular tertiary standards. Membranes are increasingly used to prepare the waste stream with regard to turbidity, and these streams have high UV transmittance levels.
Applications for reuse include: irrigation, both for agriculture, landscape and golf, and in coastal regions for direct injection into the aquifer. Aquifer Storage and Recovery (ASR) schemes are now being implemented in Florida and other coastal, water stressed areas to prevent saline incursion into the aquifer.
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