Water Treatment > Quality Water Solutions > Coagulation Control > Turbidity Measurement

Turbidity is measured by a light passing through a sample of water. The amount of light scattered to photocells perpendicular to the beam is the turbidity or cloudiness. While this does not directly indicate potential coagulant demand, historical data at a plant can correlate it to dosage. Also, rapid changes in turbidity can signal raw water quality excursions.

Older, online turbidimeters use a single light beam and photocell. In addition to light passing through flowing water, some of these bounce light off a meniscus in a chamber or pass light through a stream of falling water.

Turbidity measurement is an active field in which there are frequent advances in optics, light sources, detectors and analysis methods. Such improvements are essential if utilities are to meet 0.1 to 0.2 NTU levels. Newer technology uses multiple light sources and detectors. In one system, two beams at 90? to each other are read by two detectors 90° to each source. Such a crossbeam device increases homogeneity and gives better reproducibility. Newer devices are generally read rapidly with a logic circuit.

Turbidimeters are generally placed on the raw and finished waters. The trend is to gain more detail by placing turbidimeters after specific process units, such as after filters, preclarifiers or clarifiers.

If a turbidity reading is well above previous readings, determine the cause as soon as possible. To begin with take a second reading and, if the value is still high, recalibrate the instrument following factory-authorized protocols or the U.S. EPA approved method. This should take about five minutes to complete. Look for any system-based cause of turbidity, e.g., a dirty sample line or, if the sample is on filtered water, filter breakthrough or a problem with filter integrity.

If the reading still appears valid, do jar testing or evaluate zeta potential to define how much more coagulant is needed. Jar testing simulates the environments that raw water and chemicals encounter during mixing and settling. If the situation calls for immediate adjustment, modify dosage a modest amount and refine the change once jar testing results are available.

Turbidity issues can be evaluated by less-direct methods, e.g., by streaming current analysis or pilot filters placed on-line just after coagulation. These filters give an early warning of coagulant demand shifts and can indicate if the turbidity applied to the filters is adequately conditioned so it can be successfully removed.