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Water monitoring


Maximising turbidity and total suspended solids measurement accuracy


Protecting the UK’s waterways against pollution is unrelenting, making continuous, accurate measurement of potential contaminants 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. In this article, Jonathan Penn, ABB’s Continuous Water & Gas Analyser global product manager, explains why and outlines some top tips for achieving maximum performance from turbidity measurement equipment


The clearer the water, the greater the ability of light to penetrate to aquatic plants which generate the oxygen needed for aquatic life. Controlling the level of turbidity and suspended solids in treated wastewater discharged to the environment is therefore vital in preventing damage caused by depletion of dissolved oxygen levels. For this reason, suspended solids concentrations of effluent discharges are tightly regulated and need to be carefully monitored to ensure that regulatory limits are maintained. In municipal and wastewater applications,


T


excessive levels of suspended solids 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


he clarity of water in a stream, river or ocean is a key determinant in fostering a healthy and balanced aquatic ecosystem.


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 indicative of a poorly operating treatment process. It is important to address this as residual suspended solids breaking through from sewage digestion processes will continue to have a biological oxygen demand 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.


The ImPACT of sUsPended solIds on TUrBIdITy 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. 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.


TeChnIqUes for meAsUrInG TUrBIdITy One of the challenges in measuring turbidity has been to define a unit of turbidity that can be used to compare measurements from different devices. As a measurement of quality rather than quantity, turbidity units have no physical value, making it difficult to compare values, especially where they are measured using different devices and different methods of measurement. Two standards aimed at helping the production


of turbidimeters that are comparable to each other are the EPA 180.11 method and the ISO7027 standard. The purpose of these standards is to ensure that results obtained with a turbidimeter that has been designed to


34 November 2018 Instrumentation Monthly


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