Suspended solids measurement

The measurement of suspended solids is assuming greater importance in industrial applications, driven both by environmental issues and operating cost optimisation.

Authorities are increasingly demanding accurate and reliable suspended solids measurement as part of operating licences. In addition, the costs of chemical aids to separation processes can be significant. Since many of these additives are dosed in ratio to feed solids, accurate solids measurement facilitates optimal control and cost minimisation.

Pulp and paper unit operations are in general carried out in suspension. Specialist techniques have been developed to accu-rately measure pulp suspensions and these have application in many industries.

General measuring techniques

No inline transmitter measures the actual suspended solids content directly, it is only measuring a parameter that correlates with the true solids content. The true solids content of a sample can only be determined in a laboratory.

Suspended solids content is defined as follows: Total solids (%) = ((dry weight of sample) / (total weight of sample))*100

There is a wide range of different measuring principles for suspended solids measurement, with no one general method that works optimally for all applications. How ap-propriate a measuring technique is for any application depends on a range of process and installation issues. As a consequence, it is important to understand and consider the benefits and disadvantages of a measuring principle when selecting a transmitter for any application. Available measuring principles include (amongst others) a range of optical techniques, microwave measurement and the use of shear force.

Optical/peak method

The optical peak method is suitable for solids measurement across a wide range of applications. One reason being that it inherently compensates for the effect of particle size distribution on the optical measurement.

The basis for this method is that the suspension comprises two primary components - large and small particles. The large particles can be seen as a transparent network between which the small particles float around. If a light beam of small diameter penetrates the suspension, the intensity of the received transmitted light will vary.

The highest intensity (or peak value) will be obtained when there are no fibres/large particles at all in the path of the light beam, and the light is only damped by the small particles. The presence of large and small particles will result in a continuously varying transmitted light signal. From the transmitted light, the content of fine and large particles can be identified. Thus the content of large and small particles can be measured separately - and will together give a measure of the total solids content, independent of particle size.

This measuring technique is suitable for applications across a very wide range of solids contents. Traditional optical techniques tend to suffer from signal instability in the presence of suspensions with non-uniform size distribution. The peak method outlined above enables measurement of solids content under these conditions.

Optical reflection light

If a light beam penetrates a suspension, the light will be scattered by the suspended particles. The scattering is a measure of the solids content. This tech-nique is the basis of many process tur-bidity transmitters.

The scattering of light particles de-pends strongly on the particle's size, its shape, surface struc-ture and refractive index. These depend-encies make it diffi-cult to use optical reflective sensors in general, especially when suspension composition varies a lot, however they remain a cost-effective solution in homogeneous suspensions.

Microwave

Microwaves have a different velocity depending on material of propagation. This forms the basis of the microwave solids transmitter. A microwave signal is transmitted through the suspension.

The velocity change of the transmitted microwave signal depends on the solids content of the suspension and is measured by the receiving antenna. When using microwaves to measure suspended solids content, it is the different behaviour of the microwaves in water and in the suspended solids that is measured.

The transmitting antenna transmits a microwave signal through the suspension, and the velocity of the waves is measured. The propagation velocity (v) of microwaves in a material is determined by the following formula where c is the velocity of light in a vacuum, and e is the dielectric constant of the material:

The dielectric constant varies between different materials. For water and cellulose (fibre) there is a large difference in e, which is important to enable the measurement of the solids content of the slurry. The same applies to most solids suspensions.

The temperature of water changes microwave velocity and it is therefore essential to also measure suspension temperature to allow for temperature compensation.

The result is a very accurate technique which is particularly useful in higher solids slurries found in wastewater treatment plants and similar applications.

Shear force

For high solids applications, particularly in the wastewater treatment industry, shear force measurement can be used. There are a number of different ways in which shear force measurement is achieved.

In general, the shear force can be described as the force (F) induced when a mea-suring body is exposed to the flowing sus-pension. The amount of solids in suspension creates the main component of the force induced, but other factors such as the suspension network strength and the flow velocity V also influence measurement. The network strength depends on such factors as solids type, manufacturing process and treatment of the solid components.

A number of variants of the shear force principle are available, with the rotating shear force transmitter being the most widely applicable, particularly in waste sludge applications.

Shear force rotating sensor

This type of transmitter has been refined over the years, and its technique is still the dominant principle for accurate fibre consistency mea-surement in the pulp and paper industries to-day. Because of the higher unit cost, such transmitters are installed primarily in positions where measurement accuracy is considered vital, or where blade transmitters do not per-form the function for other reasons.

The rotation of the sensing element in the suspension net-work results in a torque, which retards the measuring shaft in relation to the drive shaft. This rotational displacement changes with solids content and is used to generate the internal signal that represents suspended solids. This technique allows an accurate and robust mea-surement of suspended solids, particularly in wastewater treatment sludges.

To ensure optimal performance and utility, it is important to select the correct method for any suspended solids measurement ap-plication. The return on investment realised with accurate measurement of suspended solids will generally far exceed the most stringent return on capital requirements.