Author: Oliver Lawal, Director of Technology, Berson UV-techniek
Introduction
UV disinfection technology is one of the fastest growing water treatment technologies today. The development of new UV technologies over this period has been a perfect example of an industry investing to meet market demand – in this case demand for an effective, low cost, and environmentally friendly disinfection technology. An increasing awareness of harmful disinfection byproducts from traditional chemical disinfection solutions, combined with technological advances in component design and manufacture, process control, hydraulics and microbiology have resulted in UV systems that are as reliable and cost efficient as their chemical equivalents. With the implementation of standard methods for the testing, design and operation of UV systems have now given the technology the transparency needed for wide scale adoption globally.
With a mandate to ensure public water systems provide safe drinking water to their users, many governmental regulatory bodies around the world have adopted UV disinfection standards. The vast majority of these drinking water standards reference either the German UV Devices for the Disinfection for Drinking water Supply standard, commonly known as DVGW (Deutsche Vereinigung des Gas und Wassserfaches), or the US Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule standard, known as the UVDGM (Ultraviolet Disinfection Guidance Manual).
While the goal of both standards is to ensure safe drinking water, their approach and outcomes, in terms of operating equipment, is very different. An understanding of these two approaches, their similarities and differences, is therefore helpful to designers, regulators and users alike.
Overview of the DVGW Standard
The German Gas and Water Association (DGVW) first published UV guidelines in 1994, following up with more formal regulations in 1997. The most recent update, known as Work Sheet 94, issued in 2006, has been implemented as part of the German Drinking Water regulations, thus making compliance a legal requirement in Germany. Along with similar standards established in Austria (ÖNorm 2001 and ÖNorm 2003), these standards are recognized throughout the world and form the basis of many other national standards.
The core ethos of the DVGW (and ÖNorm) standard is that a UV system should be proven to continuously deliver a minimum germicidal fluence of 40mJ/cm2 under all operational conditions. The measured UV Intensity must therefore remain above a specified value for all ranges of flow and UV- transmittance that will occur during operation. The justification for selecting 40mJ/cm2 as an appropriate UV fluence level is based upon the knowledge that many harmful pathogens can be inactivated up to a level of 4-log by exposure to a UV fluence of 40mJ/cm2 (see figure 1).
Figure 1: UV Fluence requirements to ensure 4-log inactivation of multiple pathogens
So as to ensure that a given UV system is able to provide the 40mJ/cm2 disinfection level, the DVGW standard defines a detailed microbiological examination method, or bioassay. Tests are performed by simulating operating conditions at full scale using B. subtillis spores as a pathogenic surrogate. Subsequent operating UV systems must be constructed and operated under identical conditions, ensuring that at least one fixed UV sensor continuously monitors the germicidal radiation, ensuring it remains above the specified minimum.
The structure of the DVGW standard, shown in Table 1, allows different stakeholders to easily access the information they require. Operators and engineers can look to Part 1 to assist with the planning of both technical and commercial factors of UV systems. The information in Parts 2 and 3, on the other hand, provides manufacturers, testing agencies and regulators valuable details regarding design and validation.
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Part 1
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Requirements on quality, function and
operation |
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Part 2
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Testing of quality, function and
operation |
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Part 3
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Measurement port and sensors for the
radiometric monitoring of UV disinfection systems, requirements, testing and
calibration |
Table 1: Structure of the DVGW standard
One key benefit of the latest DVGW standard is that is harmonizes the allowed UV sensor types with the Austrian standard (although specific sensor calibration processes are yet to be fully implemented).
Some people have pointed to the lack of targeting of specific pathogens as a limiting factor to the implementation of this standard. In addition, as some pathogens, such a Cryptosporidium and Giardia, are inactivated at significantly lower UV fluence levels than 40 mJ/cm2, the capital and operational costs of DVGW compliant UV systems can be high for very large flows. On the plus side, while still keeping the 40mJ/cm2 disinfection level, the 2006 DVGW standard does allow for the generation of performance curves, allowing the operator to limit energy wasting overdosing situations.
The DVGW standard has formed the backbone of drinking water regulations worldwide for almost 15 years and despite of its limitations, it will continue to provide valuable information to operators, engineers, manufacturers and regulators with regards to the design, testing and operation of UV systems for the protection of public drinking water supplies.
Overview of USEPA UVDGM Guideline
While the German DVGW standard states its validity for all water disinfection facilities using UV treatment and covers a broad range of target pathogens, the USEPA’s Ultraviolet Disinfection Guidance Manual (UVDGM) is more limited in its scope. It is specifically designed to cover all public water systems that use UV disinfection for the treatment of surface water (or groundwater under the direct influence of surface water). More specifically it assists with the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), requiring additional treatment based on source water Cryptosporidium concentrations and current treatment practices, where UV disinfection is one treatment option.
In summary, the UVDGM outlines how compliance to the very specific inactivation targets for specific pathogens can be achieved, in accordance with the water source and existing treatment employed. Table 2, (Table 1.4 within the UVDGM) shows the specific UV dose requirements for the specifically targeted pathogens.
Table 2: Summary of Microbial and Disinfection Byproduct Rules
One of the core premises of the UVDGM is that UV drinking water systems should be designed, tested and operated in accordance with the targeting of specific pathogens. As such it approaches equipment validation, sizing and operation in a different way than the DVGW standard. The choice of a surrogate test microorganism is not specified. Instead, uncertainty factors are used to account for differences in dose response characteristics. Additional uncertainty factors are used to account for further experimental variations as well as for UV sensors.
As with the DVGW standard, the UVDGM guideline structures information in order to allow stakeholders ease of access (see Table 3). The threefold stated objectives are summarized as follows:
- Provide operators and designers technical information on selecting, designing and operating compliant UV installations
- Provide tools and guidance to regulators in assessing UV installations throughout design, start-up and operation
- Provide manufacturers and testing agencies standards for design and validation.
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Chapter 1 & 2
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Introduction and overview of disinfection
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Chapter 3 & 4
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Planning analysis and design considerations
for UV facilities |
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Chapter 5
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Validation of UV reactors
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Chapter 6
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Start-up and operations of UV facilities
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Table 3: Structure of the UVDGM
The UVDGM does recognize both the German DVGW and Austrian ÖNorm standards, granting compliant UV systems a 3-log Cryptosporidium and Giardia inactivation credit. In practice, however, such systems are over-sized when compared with those that have undergone specific UVDGM validation testing.
The equipment validation and operational verification methods outlined in the UVDGM provide a robust, transparent basis for public drinking water UV systems targeting specific pathogens. Together with the draft 2003 guidelines, they have driven the expansion of the use of UV as a safe disinfection technology both in the US and, increasingly, globally.
Summary
Expanding their influence beyond their national borders, both the German DVGW standard and the USEPA UVDGM have played important parts in helping UV disinfection technology become one of the fastest growing water treatment technologies globally.
Although both standards seek to improve the safety of public water supplies, the latest revisions of the two methods take different approaches, with significant differences in the subsequent capital and operating costs, even when comparing identical UV system designs.
It is hoped that by explaining the similarities and differences, both technical and commercial, this article provides stakeholders with much useful information for the planning, designing, validating and operating UV systems for drinking water applications.
References:
1. DVGW (2006). UV Devices for the Disinfection for Drinking water Supply – Parts 1, 2 and Deutsche Vereinigung des Gas und Wassserfaches, Bonn, Germany.
2. USEPA (2006). Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule. EPA 815-R-06-007. U.S. Environmental Protection Agency, Office of Water, Washington, DC, USA.
Kolch, Andreas (2007). UV Disinfection of Drinking Water – the new DVGW Work Sheet 94 Parts 1-3. IUVA News, Volume 9/No.2, June
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