Inactivating with UV
Breakthrough research conducted by Calgon Carbon Corp. in 1997 proved that ultraviolet (UV) disinfection is very effective at inactivating Cryptosporidium and Giardia at low doses. Many utilities are now using or considering UV disinfection in their plants as an additional barrier for protozoa disinfection, to get disinfection credits for Cryptosporidium and Giardia and to lower chlorine doses.
Statistical analysis of laboratory data indicated that three-log inactivation of Cryptosporidium and Giardia can be achieved at UV doses of 12 and 11 mJ/sq cm, respectively. Doses for other log-inactivation credits are given in the Long Term 2 Enhanced Surface Water Treatment Rule (LT2) and the UV Disinfection Guidance Manual (UVDGM) as shown in Figure 1.
The UVDGM requires UV systems to demonstrate that the UV reactor can deliver the required dose through validation testing to receive a credit for Cryptosporidium, Giardia or virus inactivation. Validation testing must determine a set of operating conditions that can be monitored by the control system to ensure that the UV dose required for a given pathogen-inactivation credit is consistently delivered during the operation of the UV system. The validation must be done at full scale and take into account the following operating conditions: flow rate; UV intensity, measured by a UV-intensity sensor; lamp status; lamp aging; lamp-sleeve fouling; UV transmittance (UVT) of the water; inlet and outlet piping; dose distributions arising from the velocity profiles through the reactor; failure of UV lamps or other critical components; inactivation of the surrogate microorganisms used during validation testing; and relative inactivation of the target organism.
Validation involves the use of surrogate organisms, such as MS2 coliphage, that are added to the water stream ahead of the UV reactor, along with agents that decrease the UVT of the water and thereby simulate waters of different quality. Effluent samples are collected at different operating conditions of UVT, flow, lamp power and number of lamps. The reduction in viable organisms is used to obtain a UV dose that can be correlated with the various operating parameters to generate operating curves for the reactor at a water treatment plant (WTP). The use of independently traceable process measurements ensures that the reactor is providing adequate removal of the target organism, thus protecting public health.
Typical Validation Procedure
Validation normally takes place at an offsite facility and covers all possible operating conditions of the full-scale UV reactor. This includes UVT from the minimum expected (70/80%T) up to the maximum (98%T), flows from the minimum to maximum and lamp output from the minimum to maximum.
A third-party validation report is provided that details the results of validation, including UV reactor details, the validation piping arrangement, test methods and results and equations that can be used to predict the performance of the UV reactor. This model is used in the operating system to calculate the dose as a function of flow, UV sensor reading and UVT. It includes a full statistical analysis to prove conformity with the UVDGM criteria and provide dose requirements for achieving different levels of Cryptosporidium and Giardia inactivation.
Operation at a WTP can be ensured by using the equations that correlate the data to measured operating conditions during validation. The plant operator, engineer or regulator can easily check the performance using the online flow and certified reference UV sensors.
Dose Monitoring & Control
To operate the system solely on flow and sensor irradiance, it is important to design the reactor with a sensor position such that dose is more or less proportional to irradiance regardless of water transmittance or lamp power. Placed too close to the lamp, lamp output dominates; placed too far away from the lamp, water UVT has the predominant impact. With the correct placement, both have an equal effect, proportional to dose in a Sentinel UV System. This can permit the operation of the reactor without the need for a UVT analyzer.
The German DVGW W294 standard provides for the certification of UV sensors used in a UV reactor. The Sentinel reactors use a DVGW-certified sensor. This means that the duty and reference sensors are DVGW standard-certified, providing assurance to the operator and regulator. In addition, the duty sensors can be automatically calibrated online, ensuring all duty sensors are reading close to each other and to the reference sensor.
West View Water Authority (WVWA) in Pittsburgh became the largest surface water plant in North America to install UV disinfection when it installed a Sentinel system to disinfect a plant flow of 40 million gal per day in January 2001. A 48-in. Sentinel reactor was utilized that employs six 20-kW lamps.
WVWA installed the unit as an added treatment barrier with no change to its existing treatment process. The unit is installed following the clearwell in a common 48-in. line. Installation was extremely simple and accomplished in a 12-hour period during a scheduled plant shutdown.
This unit has been provided to WVWA with a complete service package in which all operating parameters are monitored from the Calgon Carbon office in Pittsburgh. Service technicians are automatically prompted for routine maintenance items such as lamp replacement based on the hours of operation. In the event of an alarm condition, the service technician is automatically notified. WVWA expects to gain disinfection credits for Cryptosporidium when the LT2 becomes law. For now, the UV system provides an added barrier of protection against pathogens entering the distribution system.