Articles & Case Studies

Advanced UV Water Treatment

Posted: Thursday 9th August 2007

The benefits of disinfection by ultraviolet (UV) light are well documented. In fact, Malcolm Snowball of Chelmsford based, GB Environmental published the first definitive paper on germicidal kill mechanisms as far back as 1988 and was one of the three original scientists to discover that UV light of certain wavelengths kills Cryptosporidium.

There are a number of suppliers in the UV market and Malcolm Snowball believes that a high level of healthy competition is one of the factors that have driven his company to develop advanced patent protected techniques which maximise the efficiency and efficacy of UV disinfection.

Snowball believes that there are three important features of an effective UV system:

  1. Continuously clean UV lamps
  2. Complete UV exposure of suspended particles
  3. Effective irradiation levels maintained at all times

Continuously clean lamps

Inevitably, as water passes through any UV disinfection chamber; proteins, oils, fats, general debris, minerals and other calcination products in the water leave deposits on the quartz tube forming a homogenous film. It is paramount that a cleaning mech­anism is in place to remove this film because it will absorb most of the UVC radiation and seriously affect disinfection.

Systems with inadequate cleaning mechanisms are severely compromised since UV only kills micro-organisms by disrupting their DNA upon direct illumination.

Manufacturers of UV disinfection equipment tend to use a simple sleeve that automatically slides along each quartz tube at timed intervals. Analysis by GB Environmental found that such cleaning mechanisms tend to work on loosely held particulate, but are ineffective on the softer constituents of the film, namely the oils, fats and proteins, which they spread across a larger surface area of the quartz tube. This has a shielding effect, reducing the ability of UV rays to pass through the quartz and into the water. The analysis found that proteins act as the "glue" which gathers and binds other particles and thereby exacerbates the problem.

In order to resolve this issue, GB Environmental has developed a unique photocatalytic-oxidation process which uses cleaning heads that revolve around the lamps and react with fouling material to keep the tubes spotlessly clean; bond-breaking without the need for additional chemicals.

The Tioxispring™ cleaning mechanism contains four fluorocarbon segments, which are spring loaded around each quartz sleeve, providing a scraping and raking action across the surface of the sleeve, whilst being driven both radially and axially along it.

As each part of the quartz tube passes through the cleaning mechanism, the two Tioixsprings™ produce a unique catalytic process that cleanses the tube of any remaining fouling. Since the springs are catalytic oxidizers, which function as an aggressive chemical free cleaner, any debris attaching itself to the surface of the quartz is broken down by the oxidation process.

Figure 1. shows a GB Environmental sleeve still in excellent condition after 12 months of continuous service.

The cleaning process should be fully auto­matic, taking place either at pre-determined intervals or following initiation by a radiation detector when radiation falls below a preset limit. If care is taken in sizing and applying the UV disinfector to the application correctly, it will provide a highly consistent biological kill performance with minimum maintenance and service.
The detector windows on GB Environmental systems are cleaned every time the lamp sleeves are cleaned.

Complete UV exposure of suspended particles

The radiation intensity from a UV lamp falls off proportionally to the square of the distance from the lamp, therefore it is important that all micro-organisms are exposed as close to the lamps as possible in order to maximise disinfection. To achieve this, it is necessary for the water flow through the reaction chamber to be highly turbulent without a significant drop in pressure.

The geometric profile of the cleaning mechanism developed by Malcolm Snowball acts as a static mixer producing highly turbulent flow with a low pressure drop. Consequently, micro-organisms are vigorously rotated and brought into close proximity to the lamps, which provides the UVC wavelengths with a multitude of attack angles with which to destroy micro-organism DNA.

Effective irradiation levels maintained at all times

The correct size of UV chamber is clearly a prerequisite for the installation of any UV disinfection technology. However, there has been much debate regarding the type of UV lamps used.

A wide variety of lamps are used in UV disinfection, although broadly speaking the most commonly used lamps tend to fall into three categories; low pressure mercury discharge lamps, amalgam lamps, and medium pressure mercury discharge lamps. All three effectively act to kill pathogenic waterborne micro-organisms, although some are better suited to different levels of water flow.

Low pressure mercury discharge lamps are rarely used in UV disinfection systems these days because of their relatively low germicidal output together with a susceptibility to output variations caused by temperature changes.

The majority of GB Environmental's second-generation UV lamps are amalgam due to their high efficiency, low running costs, low surface temperature, and longer working lives. The amalgam lamps provide an economical method of water disinfection with high levels of reliability. Cool operation also ensures that they tend not to be prone to rapid fouling, which often results when the quartz sleeve encasing the lamp rises in temperature.

Just as no one sized chamber fits all, so the same is true for the type of UV lamps required to adequately disinfect water. If, for example, the choice is between Amalgam lamps and Medium Pressure lamps for a body of water to be treated in a large drinking water treatment plant (in excess of 180ML/day), medium pressure mercury discharge lamps would be the most suitable, since their high UV germicidal output over the entire germicidal range 220nm - 280nm provides the smallest footprint per cubic meter of treated water, albeit at a substantially higher running cost.

Amalgam lamps are very efficient at converting electrical energy into 254nm UV light. However, in order to maintain effective disinfection it is important that UV lamps are provided with the correct power. It is not sensible to design excessively powerful (and therefore inefficient) lamps or to provide the lamps with an inappropriate level of power. For these reasons Snowball has designed a control system that matches lamp power exactly to its needs under any conditions. Systems are sized so that even in the event of one lamp failing, the remaining lamps will still provide the minimum dose of UV for the required disinfection.

Snowball recommends that UV disinfection equipment should be sized according to the worst case scenario. Each UV disinfector should be designed to deliver the correct dose using the Averaging Point Source Summation Technique. This is a complex mathematical calculation, undertaken at the worst point in the reaction chamber during maximum water flow conditions with lamps at the end of their life. In order to impart an additional safety margin, Snowball discounts one lamp when making the calculation.
There are three key components within the calculation. Firstly, lamp power at the germicidal wavelengths {220nm to 280nm) is dependent upon the lamp type and how the lamp drive (ballast) operates the lamp. Secondly, it is important that transmissivity (a measure of the water's ability to transmit UV light at the germicidal wavelengths) is measured appropriately, since certain contam­inants in water can prevent UV light from passing effectively through the water. Thorough transmissivity tests should be conducted in the worse possible conditions. Thirdly, the chamber must be sized according to peak liquid flow in the chamber, since this governs the retention time component of the dose formula. This is calculated as follows:

Dose D = I x t = mj/cm2

"I" is the intensity at the worst position in the chamber and "t" is the retention time of the liquid passing through the chamber.

The technologies described in this article were acknowledged by Frost and Sullivan when GB Environmental was awarded the 2006 European Excellence in Technology Award.

In summary, the early excitement with the benefits of UV treatment, created in the late ‘80s and early ‘90s, has been tempered by the need to resolve the key issues affecting both the efficiency and efficacy of UV disinfection systems. However, Malcolm Snowball believes that the technology has advanced considerably in recent years and now offers a reliable, consistent, chemical-free kill of all the major water pathogens including cryptosporidium.
Furthermore, since UV disinfection does not involve chemicals, it produces no harmful end products. The technology is easy to use and control, it is not temperature sensitive, and it does not suffer from over-dosage issues.

To contact GB Environmental visit www.gb-environmental.com or email info@gb-environmental.com




Read the magazine online

December 2018

About the magazine »
Magazine archive »


Advertisements

Information for advertisers »

Sykes Pumps
Verder Prominent Fluid Controls buttonwood marketing Water Aid Harvey Communications British Water Huber Pulsar Button June 13 wateractive
NO DIG 2018