Formazin Attenuation Unit (FAU): High-Range Turbidity Measurement
The formazin attenuation unit (FAU) quantifies turbidity for samples with high suspended solids, where scattered light measurements saturate. FAU relies on measuring the decrease in transmitted light intensity through a defined path length, using formazin polymer suspensions as calibration standards.
Environmental laboratories pair FAU readings with the NTU scale to cover the full turbidity range, ensuring wastewater permits and drinking water regulations remain in compliance.
Definition and Measurement Principle
FAU readings derive from attenuation, not scatter. Instruments emit light through the sample and measure transmitted intensity; turbidity increases as suspended particles absorb and scatter light out of the path. The unit is calibrated so that a 400 FAU suspension reduces light transmission in a standard cell according to ISO 7027 guidelines.
Because attenuation responds linearly at high solids concentrations, FAU is ideal for industrial effluents, mining discharges, and sludge streams where NTU meters lose sensitivity. Modern meters often report both FAU and NTU, automatically switching ranges based on detected signal strength.
History and Standards
Formazin-based turbidity standards emerged in the mid-20th century as a reproducible alternative to kaolin suspensions. ISO 7027 and EPA Method 180.1 describe how to prepare formazin stock solutions and calibrate both nephelometric and attenuation meters.
Utilities adopted FAU reporting as high-rate clarification and filtration systems began handling increasingly turbid source waters. Industrial pretreatment programs now reference FAU in discharge permits, especially for facilities with variable solids loads.
Sampling and Instrumentation
Sample Handling
Mix samples gently to resuspend settled solids without introducing air bubbles, which can bias attenuation measurements. Analyse promptly or store at 4 °C to minimise biological growth and particle settling.
Calibration
Prepare calibration standards by diluting formazin stock solutions to 40, 100, 400, or higher FAU values. Verify instrument linearity across the expected measurement range and document calibration dates as part of quality assurance protocols.
Interferences
Colour, dissolved organic matter, and air bubbles can absorb or scatter light, leading to false high readings. Use appropriate wavelength filters (typically infrared at 860 nm) and degassing steps to mitigate these interferences.
Applications
Wastewater treatment. Primary clarifiers, sludge thickening, and combined sewer overflow monitoring rely on FAU to capture extreme solids swings. Operators feed FAU trends into the stormwater runoff volume planner to size equalisation basins and avoid permit exceedances during peak events.
Mining and industrial discharge. Tailings pipelines and wash plants produce highly turbid flows. FAU meters confirm whether flocculation and settling units meet permit limits before discharge to receiving waters.
Drinking water treatment. FAU supplements NTU monitoring when extreme storm events or algal blooms increase raw water turbidity, guiding chemical dosing and filter backwash scheduling.
Data Reporting and Integration
Document sample IDs, collection times, holding conditions, and calibration logs. Convert FAU data to estimated solids concentrations using site-specific correlations, and compare against gravimetric TSS measurements for validation.
Share turbidity data with watershed models, stormwater control plans, and sustainability dashboards. Linking FAU trends to calculators such as the water efficiency savings tool helps organisations connect water quality improvements with conservation initiatives.