Turbidity testing during the sedimentation process is a critical quality-control and process-optimization practice in water and wastewater treatment, quantifying how effectively suspended solids are removed by gravity separation after coagulation and flocculation. This technical article reviews the underlying principles, sampling strategies, measurable parameters, standardized test methods, and industrial applications of sedimentation turbidity testing, with reference to internationally recognized standards such as ISO 7027 and ASTM D7315.
Technical Overview
Turbidity is an optical characteristic of water that describes the degree to which suspended, colloidal, and sub-microscopic particles scatter and absorb light. During sedimentation—the gravity-driven clarification stage that follows coagulation and flocculation in a conventional treatment train—turbidity serves as the most direct and responsive indicator of clarification performance. As flocs settle toward the basin floor, the supernatant becomes progressively clearer, and the residual turbidity of the settled water predicts both the hydraulic loading on downstream filters and the quality of the final treated water.
Turbidity testing in this context involves measuring the light-scattering properties of water samples drawn from defined points in the sedimentation process, typically using a nephelometer calibrated against formazin standards. Results are expressed in Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU). Well-operated sedimentation basins typically reduce influent turbidity by 80–95%, delivering settled water with turbidity values commonly below 2–5 NTU prior to filtration. Deviations from expected values signal problems such as under-dosing of coagulant, floc break-up caused by excessive mixing energy, temperature-driven density currents, short-circuiting, or excessive sludge blanket carryover.
Because turbidity responds immediately to process changes, it is one of the few water quality parameters suitable for both high-frequency laboratory verification and continuous online monitoring, making it a cornerstone of treatment plant control strategies and regulatory compliance programs.
Testing Samples
Raw Water (Influent) Samples
Influent samples collected ahead of the coagulation stage establish the baseline turbidity load. These values are essential for calculating sedimentation removal efficiency and for adjusting coagulant dosing when source water quality changes due to rainfall, seasonal turnover, algal events, or upstream industrial discharge.
Coagulated and Flocculated Water Samples
Samples taken after rapid mixing and after the flocculation basins reveal whether floc formation is adequate before water enters the clarifier. Flocs that are too small or too fragile at this stage will not settle effectively, and their turbidity signature provides early warning of coagulation chemistry problems.
Settled Water (Clarifier Effluent) Samples
The primary test sample is supernatant withdrawn from the clarified-water launders or weirs of the sedimentation basin. Sampling depth, location, and timing must be standardized because settled water turbidity varies across basin width and depth, and stratification can produce misleading grab results.
Sludge Blanket and Interface Samples
Profiles collected at incremental depths through the basin characterize the height and density of the sludge blanket. A rising blanket increases the risk of solids carryover into the settled water, so interface turbidity data guide sludge withdrawal schedules.
Industrial Process and Effluent Samples
In industrial settings, samples may include thickener overflow from mining or aggregate operations, precipitation reactor effluents from chemical and metals processing, primary and secondary clarifier effluents from pulp and paper mills, and stormwater retention basin discharges.
Testing Items
A comprehensive sedimentation turbidity evaluation typically includes the following measurable items:
- Settled water turbidity, reported in NTU or FNU, compared against plant targets and regulatory limits.
- Turbidity removal efficiency across the sedimentation stage, expressed as percent reduction from influent to effluent.
- Residual turbidity as a function of settling time, used to construct settling-velocity curves and determine the required basin detention period.
- Optimal coagulant dose, determined through jar-test correlations between coagulant concentration and settled residual turbidity.
- Sludge blanket height and compaction behavior, assessed through depth-profile turbidity measurements.
- Supernatant quality of sludge thickening and dewatering streams, including centrate and filtrate return loads.
- Ancillary parameters that influence turbidity results, including pH, temperature, alkalinity, and color, which affect coagulation chemistry and light-scattering behavior.
Testing Methods
Nephelometric Turbidity Measurement
The dominant laboratory technique is the nephelometric method, in which a light source illuminates the sample and a detector positioned at 90 degrees to the incident beam measures scattered intensity. International standard ISO 7027-1 specifies an 860 nm near-infrared light source, which minimizes interference from natural water color, while instruments following EPA Method 180.1 conventionally use a tungsten broadband source. Results are traceable to formazin or stabilformazin calibration suspensions, and accredited third-party laboratories maintain documented calibration chains covering the full working range.
Standardized Reference Methods
Sedimentation turbidity testing is performed under recognized methodologies including ISO 7027-1, ASTM D7315 (low-to-moderate turbidity by laser nephelometry), ASTM D1889, and the Chinese national standard GB 13200 for turbidity determination in water. Selection among these methods depends on the expected turbidity range, matrix color, and the data-quality objectives of the monitoring program.
Jar Test Procedure
The bench-scale jar test simulates the full coagulation–flocculation–sedimentation sequence. Identical raw water aliquots receive graduated coagulant doses, undergo controlled rapid-mix, slow-mix, and quiescent settling phases, and supernatant turbidity is measured at fixed settling intervals. The dose producing the lowest settled turbidity at acceptable cost defines the plant setpoint, and the settling curves reveal the minimum settling time required to achieve target clarity.
Online and Inline Monitoring
Continuous turbidity analyzers installed on clarified-water lines provide real-time data at intervals of one minute or less. Inline probes located within the basin can map blanket movement and detect density currents. Online data are periodically verified against laboratory nephelometer measurements to confirm calibration stability and compensate for fouling of optical surfaces.
Sample Handling and Quality Assurance
Careful technique is essential because turbidity samples are unstable: particles continue to settle or aggregate after collection. Best practice requires the following:
- Collect samples in clean, scratch-free glass or optical-grade containers with minimal headspace.
- Invert gently but thoroughly to resuspend settled particles immediately before measurement, avoiding air bubbles.
- Analyze within the shortest practical time, ideally within the holding window specified by the applicable standard.
- Verify instrument calibration with certified reference suspensions at concentrations bracketing the expected sample range.
- Replicate measurements and apply documented acceptance criteria for precision.
Applications
Sedimentation turbidity testing supports decision-making across a broad range of sectors:
- Drinking water treatment: verifying clarifier performance ahead of filtration, optimizing coagulant and polymer dosing, and protecting filtered-water compliance targets.
- Municipal wastewater treatment: monitoring primary and secondary clarifier efficiency, controlling return-activated and waste-activated sludge flows, and preventing solids washout during storm events.
- Mining and mineral processing: evaluating thickener and tailings pond performance, confirming that overflow water meets reuse or discharge criteria.
- Chemical and metals manufacturing: assessing precipitation, neutralization, and settling steps that remove heavy metals and reaction by-products.
- Pulp and paper, food and beverage processing: controlling fiber recovery, clarification of process liquors, and pretreatment of effluent before biological treatment or discharge.
- Stormwater and construction site management: demonstrating that sediment basins and settling facilities achieve required turbidity reductions before controlled release.
- Research and process design: using laboratory settling-column turbidity data to size clarifiers, thickeners, and lamella separators, and to validate computational settling models.
FAQ
What turbidity level should settled water achieve before filtration?
Most well-run conventional plants target settled water turbidity below 2–5 NTU. Lower settled turbidity extends filter run times and reduces disinfectant demand; values consistently above target usually indicate a coagulation, hydraulic, or sludge-withdrawal problem that warrants investigation.
What is the difference between NTU and FNU?
Both units describe turbidity measured at a 90-degree scattering angle. NTU is conventionally associated with instruments using a white or broadband light source, while FNU is associated with ISO 7027 instruments using an 860 nm infrared source. For most settled-water applications the values are comparable, but colored samples can show meaningful differences between the two measurement geometries.
Why does settled turbidity rise after heavy rainfall?
Storm runoff increases raw-water particle load and often shifts raw-water chemistry, destabilizing the established coagulation regime. Cooler, denser inflow can also create density currents that short-circuit the basin. Reinforced jar testing and adjusted coagulant dosing during such events help restore performance.
Can turbidity testing replace suspended solids testing?
No. Turbidity is an optical surrogate that correlates with suspended solids concentration only within a given water matrix and particle population. Periodic paired measurements of turbidity and total suspended solids are recommended to maintain a valid correlation, and gravimetric solids testing remains necessary for sludge and solids-balance calculations.
How frequently should sedimentation turbidity be tested?
Continuous online monitoring is preferred for operational control, supplemented by laboratory verification at least once per operating shift in treatment plants, and daily or per-batch testing for industrial settling processes. Jar tests are typically repeated whenever raw water quality changes significantly or on a routine weekly-to-monthly schedule for dose confirmation.