Continuous, accurate measurement of potential contaminants is an essential part of any water treatment process. The impact that high levels of turbidity and suspended solids can have on the aquatic environment makes it a particularly important parameter to measure. This article explains why and outlines some top tips to help you get maximum performance from turbidity measurement equipment.

Maximizing the clarity of water in a stream, river or ocean is a key requirement for a healthy and balanced aquatic ecosystem. The clearer the water, the greater the ability of light to penetrate to aquatic plants which generate the oxygen needed for aquatic life.

Particles larger than two microns are generally considered to be total suspended solids (TSS). As such, suspended solids include silt, sediment, bacteria, clay, algae and non-settleable solids, all of which can affect the transition of light through water. Although some will naturally settle over time, some will stay suspended in the water.

In municipal and wastewater applications, excessive suspended solids levels can trigger a chain of events that can steadily deplete the level of oxygen in the water needed to sustain a healthy aquatic ecosystem. As particles build up, they scatter the daylight passing through the water, reducing its strength. With less direct light available for effective photosynthesis, aquatic plants struggle to produce the oxygen needed for the survival of other aquatic life including fish, amphibians and waterborne insects.

Within the wastewater treatment plant itself, excess turbidity can also be a sign of of a poorly operating treatment process. It‘s important to address this as residual suspended solids breaking through from sewage digestion processes will continue to consume oxygen as they enter the receiving water. Controlling suspended solids is equally important in industrial wastewater applications, where there is the added risk of toxic or metallic compounds escaping into the environment.

As an optical determination of water clarity, turbidity provides an estimation of the total suspended solids in the water. Where turbidity is determined by the amount of light scattered off these particles, it can be used to estimate the total suspended solids level. It is important to note that other dissolved species such as dissolved organic matter may absorb light instead of scattering it, which can affect the accuracy of the determination.

Being able to establish a benchmark level of normal turbidity allows any deviations to be identified. With advances in monitoring technology, particularly those that provide continuous measurement, it is now possible to achieve a real-time picture of both turbidity and total suspended solids levels, allowing accurate detection and pinpointing of any deviations or irregularities. This information is useful both for discharge compliance monitoring and as a means of assessing the operational efficiency of waste water treatment processes. 

The following are some key steps that can help ensure that sensors used for measuring turbidity and total suspended solids offer maximum performance and accuracy.

Tip #1 - should a flow cell be used?

For applications of lower suspended solid concentrations (perhaps below 1000mg/l), a flow cell may be preferred.

Using a flow cell allows a sufficient and consistent flow rate to be established past the sensor optics.  A sufficient flow rate is important for ensuring sampling of the entire particle size distribution, whilst a consistent flow rate is important in terms of minimising the impact of air bubbles on the optical measurement. Furthermore, the use of a flow cell designed specifically for turbidity monitoring limits the influence of both refraction and stray light on the measurement, improving accuracy and repeatability.

In the absence of pressure head or gravity, a pump would be required to move sampled water in to the flow cell. In this setup, costs rise as the pump will need to be maintained throughout the operational life of the instrument. A further by-product of using a flow cell is water loss; following measurement, sampled water is often sent to drain rather than being recovered.

Where a flow cell may not provide the best solution, other sensor mounting methods can be considered.  Alternatives include dip mount systems, chain mount immersion systems, and hot tap retractable assemblies, all of which remove the need for a pump. In all cases, sensor orientation and positioning within the flow path remain a critical consideration for obtaining good quality and consistent data.

Tip #2 - make sure the sensor is correctly installed

Proper orientation of sensors is vital to maintaining quality data when monitoring turbidity and suspended solids

Dip mounting is one of the most common methods used to measure turbidity and suspended solid levels. In an ideal scenario, the measuring end of the sensor should be placed mid-channel of the liquid being monitored. Having the sensor pointing away from the chamber surface also helps to limit light refraction, which can otherwise affect the measurement.

To ensure optimal measurement, the measuring end of the sensor should also be a minimum of 300mm or 12 inches below the surface of the fluid being measured. If the channel is too shallow, an angled elbow will be required to ensure the sensor is fully immersed in the sample. If the sample is heavily aerated, using an angled elbow can also help to reduce the build-up of air bubbles on the sensor face.

To further minimize the impact of air bubbles, the sensor should never be positioned vertically. A vertical orientation can severely limit sensor performance due to increased risk of air bubbles and other particulates coming into contact with the sensor face, as well as increasing the risk of damage that can impact on data quality.

Positioning of the sensor is also important. Placing the sensor downstream and having it facing away from the directional flow reduces turbulence and stops other particles from crashing into and damaging the sensitive face of the sensor.

Tip #3 - consider hot tapping

When space is at a premium and operators cannot achieve the required angle via the dip mount method or chain mount immersion, another method is available.  The hot tap retractable method places a sensor directly into an existing pipeline without shutting it off, eliminating the time, cost and disruption out of installing a sensor.

Tip #4 - keep your sensor clean

Not surprisingly, measuring turbidity and suspended solids can be quite 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.

While correct sensor installation can help to tackle this, it is also ideal to have some form of cleaning mechanism in place to automatically remove contaminants before they can accumulate.

The automatic cleaning method is a vital component which eliminates the issue of optical contamination and prolongs the sensor lifespan without the need for manual cleaning.  By reducing fouling and particulate contact with the sensor face, both potential measurement errors and unwanted downtime can be kept to a minimum.

Tip #5 - 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 calibration methods have become commonplace.

The first method of calibration involves formazine, a synthetic insoluble solution created by combining hydrazine sulphate and hexamethylenetetramine. By creating a wide range of particles shapes and sizes, formazine is the closest method to real samples as it accurately replicates real-life characteristics when light is scattered through the solution.

However, as a known carcinogen, formazine can be dangerous to operators. Robust health and safety measures are needed if this method of calibration is to be considered.  Additional care is also needed due to formazine’s instability at low concentrations which negatively impacts its shelf life.

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.  Once verified against the result, clear labelling is necessary to ensure operators can accurately check the calibration of the sensor. This method also has the added benefit of reducing operator error. The discs that are used are sealed with a polymer gel that accurately maintains its NTU to ensure precise calibration, helping to avoid data corruption.

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 properly installed and maintained in top working order. Following the steps outlined in this article will provide a useful starting point to help ensure that turbidity and total suspended solids levels are measured as accurately as possible.