Suspended Solids Monitor – SoliSense®

Suspended solids are particles suspended in water. They occur naturally and can be seen for example in rivers after a rainfall event when the river appears ‘muddy’.

Water and wastewater treatment is largely about removing particles and organics, and then disinfecting water. Being able to measure suspended solids or turbidity can help in determining the best treatment, for example by measuring the suspended solids in the raw water coming in to a plant. Typically, suspended solids in waste water treatment has specific names such as MLSS (Mixed Liquor Suspended Solids) or RAS (Return Activated Sludge) etc. Measuring the suspended solids using an online suspended solids monitor around a wastewater treatment plant can provide vital information on the efficiency and efficacy of the treatment process.

Typically, suspended solids are measured in mass per volume units such as mg/l or mgl^-1. Sometimes when the solids are quite high they will be measured in g/l or %.

Suspended solids is a ‘unit per volume measurement’ e.g. 1 mg of suspended solids in 1 litre of water. Turbidity is a measurement of ‘cloudiness’. Although each suspended material will have a different relationship (which will change over the range) a useful ‘rule of thumb’ is that 1 mg/L = 2 NTU although this can range from 1 mg/L = 0.5 NTU to 1 mg/L = 2 NTU.

Did you know Pi also manufacture and install a range of online Turbidity Analysers?

This depends entirely on how ‘optically clean’ the medium is. So, for example, if we are measuring sand (silica) in a clean water suspension we can measure up to 50% solids or 500,000 mg/l. If, however, we are measuring black sludge in a wastewater plant it may be as low as 5% or 50,000 mg/l. Typically, in most wastewater works it is 8% or 80,000 mg/l.

This depends entirely on how ‘optically clean’ the medium is. So, for example, if we are measuring sand (silica) in a clean water suspension we can measure up to 50% solids or 500,000 mg/l. If, however, we are measuring black sludge in a wastewater plant it may be as low as 5% or 50,000 mg/l. Typically, in most wastewater works it is 8% or 80,000 mg/l.

Each SoliSense^® has temperature compensation built in so the effect of temperature on the online total suspended solids meter measurement is negligible. It should be noted though that the SoliSense^® is a large mass that can take time to reach thermal equilibrium so any process where the temperature is changing rapidly >2°C per 10 min could see some temperature effect on the reading.

Optical measurements for Total Suspended Solids monitoring have been available for many years and are used in many water, waste water, and industrial settings. However, did you know that…
…the use of multiple path lengths can extend the range of optical sensors?
…the use of longer wavelength emitters can reduce the effect of colour on the readings?
…the use of multiple emitter intensities can mean that no zero is required and can eliminate the effect of background light (patented by Pi)?

Total Suspended Solids Monitoring (TSS)

TSS monitoring is essential for controlling many industrial processes, especially in Waste Water Treatment plants when an understanding of the solids load and solids movement around the plant is essential to efficient plant outcomes.

The standard method of gravimetric sampling (passing a known volume of TSS containing water through a filter, then drying the filter and measuring the weight gain to give a measure of mass/volume – TSS) is slow and cumbersome and subject to errors, e.g. sampling errors. What is needed is a device that can measure the TSS continuously and online.

The most common devices are based on the measurement of the optical properties of the sample, commonly backscatter.

Multiple path lengths for the greatest range

There are some fundamental problems with using backscatter to measure TSS. The first is the range that a sensor can measure over. This is largely determined by the path length of the sensor. The longer the path length, the greater the sensitivity but the smaller the range.

This is because as solids increase, the backscatter (signal) increases but so does the attenuation (blocking) of the light until their effect is equal (the ‘hump’) after which attenuation increases until the signal is zero.

This means that a meaningful measure of TSS can be made going up the slope (measurement zone A) and going down the slope (measurement zone B). The problem with this is that the different samples will have a ‘hump’ where measurements aren’t possible in different places.

For most manufacturers, the solution is to quite tightly define the range at which the sensor can operate. Pi’s SoliSense^®s uses two detectors of different path lengths to measure the widest possible range of TSS in a single sensor.

Using long wavelength emitters decreases the effect of colour

Devices that use shorter wavelengths (blue, green, white) are more affected by the absorption of those wavelengths by the solids in the sample. Effectively some of the signal is lost to absorption rather than being available to scatter. The SoliSense^® uses an emitter at 860nm to minimise the absorption of the emitted light left to sample.

Using multiple emitter intensities allows for no zero, and the elimination of the effects of ambient light

Each time the SoliSense^® takes a measurement it isn’t simply measuring the amount of backscattered light (which includes an element of ambient light). It changes the amount of light emitted to 25%, 50%, 75% and 100%. From there it records a gradient for each sample point. As it does this very quickly it can be assumed that the ambient light is constant and is therefore absent from the gradient (compensated out). In addition, doing this during a calibration allows the SoliSense^® to back calibrate any zero offsets and remove these. This capability is patented by Pi and is unique in the marketplace.

Conclusion

The SoliSense^® from Pi uses patented technology to

• give the widest range of any currently available optically based TSS sensor
• minimises the effect of colour on the measurement of TSS
• compensates for zero drift and ambient light.

All of which means that the SoliSense^® is an extremely effective technology for continuously measuring Total Suspended Solids in industrial and municipal waste water treatment plants.

Sensor Fouling

What is the solution?

Process Instruments’ AutoClean and AutoFlush Systems

A solution for each application

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