Water Journal : Water Journal July 2012
UV disinfection refereed paper technical features 64 JULY 2012 water Disinfectant dose The UV system's reduction equivalent dose (RED) can be established based on validation testing or as required by a local regulator. For example, one facility must operate at an MS2 RED of 40 mJ/cm2 or greater at all times. Other systems can operate based on a T1- or MS2-phage RED and a validation factor based on the UVDGM and calculated by the UV manufacturer to achieve target disinfection. This approach typically results in a lower RED to achieve the same level of disinfection (e.g. MS2 RED of 25 mJ/cm2 for 3-log Cryptosporidium inactivation) and can help to reduce capital and operating costs of the UV system as long as the UV reactor has sufficient turndown capability to take advantage of the lower RED. Table 1 shows the required UV Dose to achieve log inactivation of a target pathogen per the UVDGM. The RED divided by the validation factor must be greater than or equal to the required UV dose to ensure compliance of the desired log inactivation is achieved. Off-specification water Off-specification events must be monitored continuously and recorded at specified intervals, typically every five minutes, but may be more frequent if required by the regulator. Total off-specification volume is typically recorded monthly, compared with total water produced per month, and makes up less than 5% of the total water volume produced in a month. However, a local regulatory agency may require more stringent requirements. For example, most regulators require a treatment goal of no off-specification water, but allow 1--5% off-specification water if unusual or emergency conditions occur. Some regulators require recording off- specification water by time, not volume. On the other hand, some utilities don't track monthly off-specification volume, because local regulators don't require it. The production of off-specification water cannot be purposeful, but usually minor off-specification events such as the closing of isolation valves during a UV reactor emergency shutdown are allowed, as long as they are quantified by the utility. Depending on the system, it may be required to limit the flow through the UV reactor to avoid producing off- specification water if the UV transmittance (UVT) drops significantly below the design value. Table 2 shows an example from one utility that reduced the flow through the UV reactor from 100 MLD down to 10 MLD as UVT dropped from 85% to 70% during seasonal water quality changes. At another utility, zero off-specification was the ultimate regulatory target. This is very difficult to achieve. Therefore, a recycle line was installed downstream of the UV reactors (see Figure 1). In the event of an off-specification alarm, the recycle line would open and return the water to the filter clearwell. This was made possible since the UV reactors were located downstream of low lift pumps and the UV reactors were conveniently located above the clearwell. This would not be an option in many gravity systems. Tracking of UV System Components Replaceable parts The UVDGM recommends tracking the age of replaceable parts, such as UV lamps. Although all of the participating utilities track lamp age, few regularly track the age of other parts, such as ballasts or sleeves. Some record the date when a component is installed. In one case, a local regulator requires medium-pressure lamps to be replaced every 5,000 hours of operation. In most other cases, the utility determines lamp-replacement frequency based on operating experience. For most systems, medium-pressure lamps usually operate for 9,000 hours or more. Although none of the participating utilities have low-pressure, high-output (LPHO) lamps, the expected life for LPHO lamps is 12,000 hours or longer. It is typically up to the utility to determine the appropriate replacement frequency of lamps if this activity is not regulated. Participating utilities usually replace ballasts and sleeves only when problems occur, and this is generally rare. Power outages and quality Power outages or poor power quality can shut down UV reactors and create off-specification water. Most utilities have a backup power generator for long-duration power outages. Concerns regarding short-term power outages (less than five minutes) or power quality are usually addressed with an isolated power supply cabinet, power conditioning equipment, and an uninterruptable power supply for the UV reactor control system's programmable logical controller (PLC) to avoid loss of UV dose logic. Uninterruptable power supplies for the full system connected power load were installed only at utilities receiving power of very poor quality from their electric utilities. System Maintenance Activities The UVDGM also recommends maintenance activities. Most of the utilities check their control systems daily to ensure normal operation, Table 1. Required UV dose (mJ/cm2) to achieve desired log inactivation of target pathogen. Required UV Dose (mJ/cm2) Log Inactivation 0.5 1 1.5 2 2.5 3 3.5 4 Cryptosporidium 1.6 2.5 3.9 5.8 8.5 12 15 22 Giardia 1.5 2.1 3 5.2 7.7 11 15 22 Virus (Adeno) 39 58 79 100 121 143 163 186 Note: Required UV Dose < RED / VF Table 2. Maximum UV reactor flow versus UVT. Influent UVT (%/cm) Max. UV Reactor Flow (MLD) >85% 100 80% 50 70% 10 Figure 1. A UV recycle line.
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