Turbidity becomes clearer

Turbidity is important in determining water quality, but how can the water industry ensure accurate measurement? John Kupczak of Siemens Automation and Drives outlines the extent of the problem for water companies, and offers a solution

The effect of pollution on water quality is growing. Reports of impure drinking water and resulting disease have forced countries to respond by setting more stringent guidelines.

Turbidity is created by solid particles in liquids. When a light beam is passed through, the suspended particles scatter it. Measuring the intensity of the scattered light provides a turbidity measurement.

Studies have shown a clear correlation between turbidity and the number of bacteria in a water sample. In particular, turbidity measurements can indicate the presence of cryptosporidium, a water-born parasite which can cause severe diarrhoea and sickness.

As a result, the EU 1998 Drinking Water Directive set a non-mandatory indicator value for turbidity of 1NTU (nephelometric turbidity unit) in the water leaving treatment works. When this value is exceeded, the regulations require the water company to investigate and report the results to Drinking Water Inspectorate (DWI).

Enhanced reduction of turbidity would help to ensure that the basis for bacterial growth is reduced. Ideally, turbidity should remain at the lowest possible levels, well below 0.5NTU, and often down to 0.1NTU.

Increased regulation has brought increased pressure to ensure water quality is kept high, but traditional turbidity meters have a series of problems.

False highs
All turbidity monitors suffer from zero point drift, and need regular calibration to counter this. The calibration regime set by most manufacturers is usually one to three months, by which time is can be extremely difficult to measure the increasingly lower values.

Legislation requires that an individual meter should be used on outlet of each filter. Since all turbidity meters suffer from calibration drift, the cost can be considerable and they require significant human resources (monthly or quarterly calibrations are normal). Instruments will give false high values due to zero point shift, if such calibrations are not performed, thus as an operator you will be reporting higher turbidity values than you need to.

Furthermore, there is a need for high-resolution instruments when measuring ultra-low turbidity values, particularly as turbidity values as low as 15FNU (formazin nephelometric units) are now common in UK water treatment works¡¯ (WTW) filter outlets. Errors of 100% are not uncommon, the principle cause being zero point drift.

A second problem with traditional instruments, and in particular closed flowcell turbidimeters, is the gradual fouling of the optical windows, even in apparently clean water samples. Immersed probes can foul even faster simply because of flow patterns around the sensor often leading to an enhanced build up of deposits.

When measuring turbidity with light, if the water contains absorbent substances, such as coloured constituents, these can reduce the light intensity and falsify the results.
One recent solution to the problems outlined was non-contact falling stream measurement which, while offering automatic calibration and low costs, did not suit all the applications for turbidity monitoring.

It is clear that a new solution is needed. Modern sensors with semi-conductor light sources and microprocessor controls are capable of maintaining reliability and ensuring maintenance-free measurement.

To cover as many turbidity applications as possible, any solution must be able to monitor samples under pressure. They must have an option for reduced flow; use the latest DWI-approved materials; offer manual calibration if required. Any solution should have an option to measure higher turbidity, have a smaller footprint, and offer reduced costs.

Next generation
An answer may lie in a new generation of turbidity meters, such as the AquaScat from Siemens, which utilises a contactless free-falling water stream that eliminates all problems of window fouling and, as a result, minimises service. Furthermore, as stray light is reduced significantly it becomes possible to measure low turbidity levels precisely, even below 0.01FNU. The AquaScat can measure 90¢ª scattered light to ISO 7027, the international standard for water quality.

The key performance criteria of a contact-less, free-falling turbidimeter is factory calibration and a dual-beam measurement method, which ensures the calibration remains stable, reducing the need for servicing.

Water quality is becoming an increasingly important issue, particularly as the threats to drinking water quality are increasing all the time. Reports of impure water are more frequent and traditional turbidity instruments are unable to keep pace. Fortunately, the new generation of instruments is able to respond to the threat.

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