Today’s stringent water quality legislation demands ever more accurate measurement to help protect the world’s aquatic environment. The potentially damaging consequences of high levels of turbidity in waterways, arising from natural or human-made causes, makes it a particularly critical parameter to measure. Jonathan Penn for ABB Measurement & Analytics explains why and looks at how new technology is helping to address some of the issues around obtaining an accurate measurement
The ability of light to travel through water has an important bearing on the health of aquatic ecosystems. Without an adequate supply of daylight, underwater plants are unable to photosynthesise effectively, and to produce the oxygen needed for aquatic life including fish, amphibians and waterborne insects.
Where high levels of dissolved or particulate matter are present in water, light can become scattered or even absorbed, with less of it available for photosynthesis. If it persists, this situation can result in the death of plant life, depleting oxygen levels in the water both through reduced photosynthesis and decomposition. This will also impact the aquatic food chain, affecting both the growth and survival of dependent organisms.
High levels of particulate matter can also elevate temperatures, with the particles absorbing more heat from the sun than water molecules. As the water becomes warmer, it is less capable of storing dissolved oxygen (DO), causing levels to fall and leading to stratification, where warmer and colder water do not mix, restricting the transfer of DO. This can starve the lower layers of a watercourse of the DO needed to sustain life.
Sources of turbidity can be attributed to ‘natural’ causes, such as erosion from riverbanks or sediment deposition due to flooding and those resulting from human activities.
In the case of human activities, pollutants can attach to the suspended particles and enter the receiving water. High levels of organic suspended solids, such as those found in municipal waste water, can have a high biological oxygen demand, or BOD, that will lower further the levels of dissolved oxygen in the receiving water and can potentially lead to profound effects on the water body itself, and the resident aquatic life. High nutrient levels can encourage the formation of harmful algal blooms that can further deplete DO levels.
The impact of suspended solids on turbidity
Particles that are larger than two microns are generally considered to be total suspended solids (TSS). Suspended solids include silt, sediment, bacteria, clay, algae and non-settleable solids, all of which can affect the passage of light through water. Although some will naturally settle over time, if flow conditions allow, some will stay suspended in the water.
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 by these particles, it can be used to estimate the total suspended solids level. However 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. This information is useful both for discharge compliance monitoring and as a means of assessing the operational efficiency of wastewater treatment processes. Â Â
Correlating turbidity and suspended solids
The most widely accepted method for measuring turbidity is to measure light scattered at a certain angle. This is normally 90 degrees in order to reduce the effect of stray light and absorption. The turbidity of a sample is measured as the intensity of light scattered by the material suspended in the sample. The scattered light intensity itself is proportional to the number of suspended particles, or suspended solids. Based on this relationship, it is possible to infer the mass of suspended particles for a given level of turbidity.
To measure the concentration of suspended solids in a sample of turbid water, all that is needed is to produce a calibration curve that relates the mass of suspended solids to the turbidity. In a typical installation, a grab sample will be taken from the process water and the turbidity reading recorded. The suspended solids content will then be measured using a laboratory-based gravimetric procedure in accordance with ASTM method D5907-10. In this procedure, the suspended solids are filtered from the water before being dried and weighed. The resulting value, which is the total suspended solids level in mg/l, is used to calculate a conversion coefficient for the relationship between the turbidity and the suspended solids concentration.
Although this procedure is useful in quantifying a relationship between total suspended solids and turbidity, a single sample is of limited use as an indicator of overall conditions. As a one-off measurement, a single sample will only ever be indicative of a given set of conditions for a given period of time, which may not apply universally.
It is also important to note that suspended solids levels can vary independently of the turbidity measurement. Turbidity, which is measured in Nephelometric Turbidity Units (NTU), is essentially the measurement of the scattering and / or absorption of light caused by suspended solids in water. Total suspended solids, on the other hand, is a quantitative measurement of the concentrations of suspended particles. As such, there is no single way of recognising the differences in the size or composition of the suspended particles or their impact on the turbidity measurement.
Constant changes in the conditions being measured will have a direct impact on the calculation of the coefficients in the calibration curve. If the composition of the sample varies, then a new calibration will be needed.
Added complications can also arise where different instruments and measurement and reporting methods are used. Many instruments use different techniques for measuring turbidity, for example, such that no two devices will produce exactly the same result for the same sample. The two main standards governing turbidity measurement – USEPA Method 180.1 and ISO 7027 – also stipulate different approaches, such as different wavelengths and allowable scattering angles for light detectors, which can lead to variation of results.
Developments in analytical measurement technology are helping to address the issues associated with obtaining a reliable measurement. ABB’s ATS430 turbidity sensor, for example, uses the latest advances in optical measurement technology to deliver precise and ultra-stable measurement of turbidity and suspended solids concentrations up to 4000NTU or 100,000 mg/l.
The sensor uses internationally-approved nephelometric measurement technology in accordance with the ISO 7027 method to measure both high level turbidity and total suspended solids (TSS) content in the sample. A beam of infra-red light is emitted by an LED directly into the sample. The light beam is scattered by particles in the sample, and the scattered light intensity is measured by the sensor’s photodetector positioned at 90 degrees to the light beam, with the resulting data then being relayed digitally to the transmitter. As there is a known relationship between the amount of solids in suspension and the turbidity of a sample, the turbidity reading can be used to provide a real time estimate of the level of suspended solids in the sample.
A key feature within the sensor is its adaptive TSS calibration function, which automatically updates the turbidity to TSS calibration coefficient every time an in-process calibration is performed.
Every time a new TSS calibration is completed, a new coefficient is calculated as a weighted average of the current coefficient plus the new coefficient. The advantage of this is that the calculated TSS values follow changes in the process whilst smoothing out any sudden jumps that are likely to be due to unrepresentative sampling in obtaining the laboratory sample or laboratory errors in determining the TSS content of that sample.
The impact of rising populations and growing pollution from industrial activity are making it more important than ever to keep a check on what’s entering the world’s watercourses. The potential damage that can be caused by depletion of DO levels caused by excessive turbidity and suspended solids means that effluent discharges from both municipal and industrial sources are subject to increasingly tight regulation, requiring careful monitoring to ensure limits are maintained.
