Black & Veatch's Frank Rogalla looks at UV disinfection of potable water
The use of ultraviolet (UV) light to accomplish disinfection has long been established in wastewater treatment, especially in North America. UV disinfection is now gaining popularity in the treatment of drinking water, after large scale successful operation in Helsinki, Stockholm and Wahnbach, Germany.
The UK’s first UV plant for potable water disinfection is now under design in the London area. While wastewater treatment utilises feedback measurement of effluent water quality to determine regulatory compliance, water treatment regulators rely on indirect measurement of disinfection using a concept similar to the calculation of CT values (concentration multiplied by exposure time) for chemical inactivation. With regulations pertaining to Cryptosporidium inactivation looming on the horizon, the US Environmental Protection Agency will soon issue a guidance manual to provide states with direction on the proper installation and operation of UV disinfection systems.
use of UV in water
Various communities in the US and Europe pioneered the
application of UV disinfection for municipal water and wastewater treatment in the early 1900s.
Early experiments with both UV and ozone appear to have met with difficulties, while chlorine found favour for reasons related to economics and ease of use.
Interest in UV has resurged due to its efficacy as a disinfectant, advances in equipment reliability and Crypto. inactivation requirements, as well as restrictions on chlorinated and brominated disinfection byproducts in drinking water. The technology’s surge in popularity for potable water treatment is directly attributable to the discovery that radiation in the UV-C range is effective for inactivation of pathogens such as Crypto. and Giardia.
A Tool in the Toolbox
The Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) is a product of the US regulatory negotiations process and offers UV disinfection as one of many “tools in a toolbox”. In fact, the cost-effective use of UV disinfection to inactivate Crypto. is a fundamental premise of the agreement.
Whereas the initial rules established a two-log removal criterion for Crypto. the second issue will establish further removal/inactivation requirements. Raw water quality will determine which systems must document additional removal efficiencies or provide inactivation. Most systems with inactivation requirements will need to use alternate disinfection techniques because free chlorine and chloramines are largely ineffective against Crypto.
While the new agreements addressed UV for disinfection of surface water, it was clear the USEPA needed to provide basic design criteria by which to determine adequate disinfection before UV could be widely implemented.
The agency therefore drafted a guidance manual for UV disinfection in conjunction with the proposed regulations. The stakeholder draft of the UV Disinfection Guidance Manual was issued for limited review and comment in October 2001. Final guidance on this issue will be provided in the next draft of the manual, scheduled for distribution this summer. Of the draft manual
content, dose values, validation and monitoring requirements,
and guidance for a short-term situation in which a UV system does not provide the required dose (eg during a power interruption) are expected to draw the most interest.
UV Dosage explained
The UV dose required to achieve the desired inactivation of the target pathogen is critical for utilities considering use of UV disinfection under the new rules. During the negotiations phase, the dose most often taken under consideration was 40 mJ/cm2.
The dose values presented in the design table are based on bench-scale tests where precise measurement can be used to calculate the exposure time and the UV rate to calculate the dose. In large-scale, flow-through reactors, UV dosage can vary greatly with the flow characteristics of the reactor, the placement of the lamps, and the lamps’ germicidal output. To account for the variation between the ideal conditions of bench-tests and non-ideal conditions found in full-scale applications, a safety factor will be proposed to
modify the dose values. These values indicate that a dose of 40 mJ/cm2 would provide three-log inactivation of Giardia and Crypto. Much higher doses are required to achieve inactivation of Adenovirus.
With UV, no simple chemical residual measurement can be used to determine if microbes have been inactivated to a sufficient degree. Instead, UV reactors are ‘validated’ to perform under certain conditions of flow, water quality and UV lamp output.
Validation tests would entail seeding the challenge microbe into the UV disinfection unit and measuring the inactivation achieved by the reactor. Each reactor will have a UV dose equivalent assigned to them by comparing the inactivation achieved to a UV dose-response curve for the challenge microbe using a collimated beam. Each validated system must monitor operations to demonstrate the system is being operated within its validated parameters. Monitored parameters are expected to include flow rate, lamp outage and irradiance, as well as water quality parameters.
The sensors used to determine irradiance would be used as a check on the reactor’s performance. Each system would then report UV reactor operation outside of its validated conditions.
One of the biggest questions for UV systems is how to handle short-term incidents where the lamp output dims or is reduced below the validated condition. For example, a momentary power ‘blip’ may cause a UV system to shut off power to a lamp.
This would require a cooling-down period, a restrike of the arc inside the lamp and a power-output recovery period.
Because water treatment plants do not typically shut down for such short-duration incidents, the time during which water passes through the reactor and the lamp is being brought back up to power could be considered as not meeting the requirement to provide continuous disinfection. These conditions have been described as an ‘off-specification’ period.
One solution is to provide an uninterruptible power supply (UPS) system to carry the UV reactor through these short periods.
With power requirements for UV systems on recent bids ranging from 0.25-1.3kW/Mld, the costs for a UPS system capable of maintaining operation under all conditions could be significant.
The need for a backup system must be balanced with the quality of power supply available at the system site and the utility’s ability to meet the requirements of the final guidance manual.
addressing Design Issues
Retrofitting UV disinfection into an existing treatment plant requires consideration of:
- available head – with pressure-drop varying from 152.4mm-914.4mm, fitting UV reactors into a facility’s hydraulic profile is a major issue,
- power cost – operating a UV system in an energy-efficient manner is important. This includes being able to reduce UV power usage when water quality is high or flow is low. Transmittance and intensity sensors can help utilities identify such opportunities,
- space – with most UV disinfection expected to be installed following filtration, available space in existing pipe galleries can be quickly overrun to provide adequate straight runs of pipe for valves, flowmeters and the UV reactor,
- validation requirements – systems that validate on site must incorporate flow disposal during testing. Diversion to backwash water storage ponds, NPDES permitted discharge, or sewer connections may be required,
- redundancy – existing guidance from the US National Water Resources Institute (NWRI) on what type of redundancy is required for an online reactor is subject to interpretation. One interpretation requires redundant reactors in series, which significantly impacts system cost,
- maintenance requirements – although cleaning maintenance is an issue for some high hard or iron and manganese waters, UV system maintenance should not be a major concern.
what the future holds
While use of UV for treatment of surface waters is promising, application for Crypto. inactivation currently requires a fee.
Calgon Carbon Corporation (CCC) holds a US patent for the use of UV to disinfect potable water containing Crypto. and has indicated in letters to USEPA, American Water Works Association and others that users must obtain a licence before water is treated with UV for Crypto. inactivation. In the US, the licence fee with continuous-wave UV technology has been fixed at 0.3p/m3.
This fee must be paid regardless of the type of UV equipment or UV vendor that is selected if the system will be used to inactivate Crypto. within the dosage range described in the patent.
While the patent is being challenged in several court cases, would-be users of UV disinfection should be aware of the potential operating costs if the patent is judged to be valid.
While UV disinfection is of great interest in water treatment, its widespread use in the US hinges on the final version of the new surface water treatment rules, results of individual system monitoring for Crypto. concentrations in raw water and the final content of the USEPA guidance manual.
Although many utilities await final recommendations
before adopting UV disinfection, a good number – including
West Valley Water District in Rialto, California – have
determined the benefits outweigh the remaining questions and
are proceeding with UV installation.