September 2000Analytical Instrumentation & Environmental Monitoring
Traditionally it has been acceptable to filter raw water to a reasonably good clarity (0,5 to 1,5 NTU) and then disinfect the water with chlorine, UV radiation or ozone to kill all the bugs.
In the past any of the reasonably good turbidity instruments were adequate for the analysis of drinking water. Softdrink bottling companies, however, further filtered this commercial drinking water prior to using it to produce soft drinks. This generated a requirement for more sensitive, accurate, repeatable and measurement stable analysers.
A new threat to the safety of water supplies, namely protozoal pathogens, has emerged in the last 20 years. The protozoan parasites Giardia and Cryptosporidium, also found in South African waters, have become a recognised cause of diarrhoea in humans and are life threatening to the immuno-suppressed, the very young and old people.
Certain unique characteristics of Protozoan cysts and oocysts contribute to the fact that they are recognised as the main causes of waterborne parasitic diseases.
* The unique structure and composition of these cysts and oocysts renders then resistant to disinfectants such as chlorine. The only sure way to destroy them is to boil the water for more than 2 minutes at +65°C or freeze them to -10°C.
* Their size (less than 10 mm) makes it difficult to detect or to physically remove (an exceptionally good filtration system can filter them out).
* Bacterial indicators traditionally used are inadequate to detect their possible presence (sophisticated fluorescence microscopes are typically used).
Since 1983 there have been at least 69 major outbreaks of waterborne/foodborne cryptosporidium parvum.
In 1993 there was an outbreak of cryptosporidium in Milwaukee (hometown of GLI) which infected 403 000 people. The suspected cause was 'treatment deficiencies of lake water'. GLI was able to demonstrate to the local water authorities that by using the GLI low range turbidimeter system (LRTS), they could have an advance warning of the possible presence of protozoan parasites in their filtered water. This has led to sales in excess of 10 000 LRTS units to water treatment authorities worldwide.
Continuous turbidity monitoring is one of the best ways to detect changes in water quality. The GLI LRTS has its zero electronically checked once every minute and is sensitive to particle sizes smaller than 10 mm. Analysers using a different technology would regard this size of particles as a dissolved solid and not be able to sense their presence.
LRTS technology
This system uses a patented four-beam method of two light sources and two photodetectors spaced at 90° intervals around the sample chamber. Two measurement phases provide four independent measurements from two light sources. During phase 1, photodetector #2 provides a 90° scattered light active signal, while photodetector #1 provides a forward scattered light reference signal. During phase 2 the process is reversed.
The microprocessor uses a ratiometric algorithm to calculate the turbidity value from the four readings.
This method mathematically cancels the error effects from aging or fouling of the components, and compensates for colour effects. The turbidity system therefore only requires calibration to conform to regulatory requirement, and for this purpose a patented glass calibration cube, certified to a known US EPA approved formazin standard, is available. This extremely stable standard ensures reproducibility of calibration and measurement accuracy.
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