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Top Tips for... turbidity monitoring

The impact that high levels of turbidity and suspended solids can have on the aquatic environment makes it a particularly important parameter to measure. Here, Dr John Gaffney, Continuous Water & Gas Analyser Product Manager at ABB, explains why and outlines some top tips for achieving maximum performance from turbidity measurement equipment.

ABB’s ATS430 turbidity sensorABB’s ATS430 turbidity sensor

Protecting the UK’s waterways against pollution is a full-time job, making continuous, accurate measurement of potential contaminants an essential part of any water treatment process. Turbidity is a particularly important parameter to measure because of the impact suspended solids can have on the aquatic environment.

When measuring turbidity, one of the first challenges is to define a unit of turbidity that can be used to compare measurements from different devices. Turbidity units have no physical value, making it difficult to compare values, especially where they are measured using different devices and different methods of measurement.

Two standards aimed at helping the production of turbidimeters that are comparable to each other are the EPA 180.11 method and the ISO7027 standard. Both standards use a scattering angle of 90°, which is called nephelometric arrangement, but each of them uses a different wavelength and different prescriptions on the geometry of the illumination and detection.

The EPA standard was designed to harmonize the design of turbidimeters that used incandescent light sources, with illumination wavelengths peaking in the green region of the spectrum. These devices work well at low turbidity values, owing to the shorter wavelength.

EPA measurements are expressed in Nephelometric Turbidity Units, or NTU.

The ISO standard, on the other hand, stipulates the use of light in the near infrared region. Using a longer wavelength reduces the effect of absorption by organic contaminants on the measurement significantly. ISO measurements are expressed in Formazin Nephelometric Units, or FNU.

Whichever method is used, certain key steps can be taken to ensure that sensors used for measuring turbidity and TSS offer maximum performance and accuracy.

1. Orientation
Proper orientation of sensors is vital to maintaining quality data when monitoring turbidity and suspended solids. In both NTU and FNU, a 90-degree angle is designated relative to the incident light beam. The sensor is placed in this position as it is most sensitive to the scattered light coming from the suspended particles.
However, in some applications space can be an issue, making it difficult to accommodate a sensor at a 90-degree angle. In these instances, a 45-degree angle is preferable.

In fact, if space is severely limited, any angle is more desirable than installing a sensor vertically, which can impact data quality due to particulates and air bubbles damaging the sensor face.

It is also important to place the sensor downstream to reduce turbulence and stop particles from crashing into the sensor.

2. A clean sensor is an accurate sensor
Measuring turbidity is a murky job. While in long term operation, the sensor can get extremely dirty due to the accumulation of bio-film, manganese and iron oxide.

Correct installation and positioning can help overcome this but having some form of cleaning mechanism to automatically remove contaminants before they accumulate is preferable. The automatic cleaning method is a vital component which eliminates the issue of optical contamination and prolongs the sensor lifespan without the need for any manual cleaning.

3. Maintain calibration
To safeguard against inaccurate data, maintaining the turbidity sensor to a high working order is critical. With the complex nature of replicating turbidity and suspended solids characteristics, two main methods have become commonplace.

The first method of calibration involves formazine, a synthetic insoluble solution and closest method to real samples as it accurately replicates real-life characteristics when light is scattered through the solution.

However, formazine can be dangerous to operators as it is a known carcinogen, so robust health and safety measures are needed. Additional care is needed due to formazine’s instability at low concentrations which negatively impacts its shelf life. Monitoring the shelf life of formazine is important as replacement costs can quickly rise and using unstable formazine can compromise data accuracy.

The second method uses a dry standard, which simplifies calibration whilst improving operator safety by removing the need to use carcinogenic formazine. Each dry standard calibration is tested against a primary formazine standard to boost accuracy before a hockey puck like disc is used. The discs are then sealed with a polymer gel that accurately maintains its NTU to ensure precise calibration, helping to avoid data corruption.

4. Use a flow cell
There are a few extraction methods for measuring turbidity and suspended solids but using a flow cell is one technique that can establish a standardised and constant flow rate. A stable flow rate at a suitable velocity offers the best chance in maintaining an accurate distribution of particulates which supports accurate data collection. Without the correct flow rate velocity, particulate deposition can occur which affects the monitoring system.

To reduce anomalous readings, a flow cell is mounted onto a sensor to ensure a uniform flow and minimise back scatter and refraction from the sensor’s infrared beam.

However, a flow cell does need a pump in the absence of pressure head or gravity. In this setup, costs rise as pump maintenance needs to be carried out. Sample extraction can also be marginally affected as the water is removed from the process, increasing the risk of sample contamination.

5. Sample standardisation
Any sample that is to be measured must remain stable to mimic application conditions. Conditions need to be carefully monitored to ensure the sample remains as close to its original state as possible. Preventative methods are necessary, with any changes needed to be recognised and adequately reversed to return the sample to its original state to ensure accuracy is not affected.

In the case of monitoring turbidity and suspended solids to safeguard against inaccurate data, turbulence should be minimised to guard against potential errors. In highly aerated applications, the control of air bubbles is also vital when extracting a sample; too many and data anomalies will occur.

SUMMARY

The important role played by continuous water quality analysers in safeguarding the quality of both potable and treated waste water makes it critical to ensure they are kept in top working order. Following the steps outlined in this article will provide a useful starting point to help design an effective maintenance strategy.

For more information on ABB’s products and services in this field, call 01480 488080, or email:
enquiries.mp.uk@gb.abb.com.

Topic: Water Quality
Tags: treatment , Water Quality

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