Suspended Solids Monitor – SoliSense®

How do you measure from low NTU all the way up to 8% solids? By using the SoliSense® from Pi. Using patented optical technology the self-cleaning, simple to use, low drift suspended solids sensor from Pi is a cost-effective way of measuring the solids in a range of applications including all those associated with municipal waste water plants and municipal water plants such as mixed liquor suspended solids (MLSS), return activated sludge (RAS), plant inlet and plant outlet, centrate, filter backwash and other monitoring requirements. Because of its patented technology that allows it to measure such a large range in a single sensor, the SoliSense® has integrated self-cleaning for a long maintenance intervals with calibration checking as little as once per year. Installation can be dipping into a tank or inserting into a pipe.

SoliSense® Suspended Solids Monitor 3D View

If you have any questions or would like to find out more about SoliSense® get in touch with one of our technical experts.

Focus On

Focus Ons are a series of short articles distributed by email providing technical information regarding instrumentation, process measurement in potable, waste, process and pool waters. If you would like to join the mailing list, please contact us.


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’.

Treating water and wastewater has two main focuses: the removal of organics and particles, and then disinfection. Having information about the level of suspended solids can be invaluable in informing treatment processes (such as tracking changes in suspended solids in raw water entering a drinking water plant or monitoring the efficacy wastewater at the outlet), and so these kinds of sensors are very common in many industries and applications.

Suspended solids are generally measured in mg/l. Sometimes, if they are present in large amounts, a percentage is used or g/l.

Suspended solids and turbidity are measured differently, but they are related. Suspended solids measurements are based on the mass of solids present in a certain volume of water (mg/l), whereas turbidity is measuring how ‘cloudy’ it is in NTU. A common conversion used is 1mg/l = 2 NTU, but different suspended materials can have a different relationship.

The qualities of the process water and the solids to be measured will affect the effective maximum level that can be measured. For a ‘clean’ application (such as silica in clean water), we could expect to be able to measure up to 50% solids (500,000 mg/l) reliably. On the opposite side of the spectrum, black sludge in a wastewater plant may have a reliable limit as low as 5% (50,000 mg/l). Experience shows that in most wastewater works the upper limit is around 8% solids (80,000 mg/l).

The SoliSense® has integrated temperature compensation as standard, and so changes in temperature will have a negligible effect upon the sensor. However, the SoliSense® is a rugged stainless steel sensor of significant mass; it can take time for the SoliSense® to reach thermal equilibrium, so rapid changes in temperature (>2°C per 10 min) could have an effect upon the reading.


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