Plumbing Africa February 2018 | Page 31

HEALTH AND SANITATION • • • Disinfectant type and residual – Some reactor components may not be tolerant of certain disinfectants or high doses, so UV equipment manufacturers should be consulted about exposure of UV reactors to chemical disinfectants. UVT – Components in the water can absorb UV light and reduce the dose delivered to the microorganisms from the UV reactor. UVT (which can be calculated from UV absorbance) is a key parameter in making sure that the UV reactor is properly sized for the facility. Iron and manganese – These constituents can foul quartz sleeves, leading to decreased UV output. Iron concentrations >0.1 mg/L may cause operational issues. The operation of a small UV reactor is typically governed by two key parameters: the flow through the reactor and UV sensor reading(s). Over time, UV sensors will drift out of calibration. For this reason, the readings from a UV duty sensor installed in the reactor should be compared against a reference sensor temporarily inserted in the reactor. Public Water Systems (PWSs) typically make these sensor checks on a monthly basis. If the calibration ratio between the duty and reference sensor readings is >1.2, then follow-up actions such as recalibration or replacement of the UV sensor should be taken. For installations that use an online www.plumbingafrica.co.za 29 UVT monitor to control UV output, weekly comparisons between online and benchtop UVT measurements are recommended. INSTALLATION CONSIDERATIONS There are several sources of design guidance for the application of UV disinfection to potable water supplies. These references cover a range of applications, from those producing only a few gallons per day to millions of gallons per day. The following checklist is tailored to institutional settings for legionella control: • Hydraulics should allow for even flow through the reactor. Control valves and reducers should be avoided within five pipe diameters upstream of the UV reactor. Pipe expansions should also be avoided for at least 10 pipe diameters upstream of the reactor to avoid jetting and swirling flow through the UV reactor. • Redundancy or other measures should be built in to allow a UV reactor to be taken out of service for cleaning, lamp replacement, and other maintenance. • Valves to isolate UV reactors may be necessary. In some cases, such as when UV reactors are flooded with cleaning chemicals, special valve arrangements may be beneficial on the outlet and inlet piping. • Power quality analysis includes review of sub-second power interruptions and voltage sags at the location of Continued on page 31 >> February 2018 Volume 23 I Number 12