TSS/NTU Converter

Total suspended solids and turbidity measure related but physically different things. TSS is gravimetric: a known volume is drawn through a glass fiber filter of about 1.5 micrometer nominal pore size, dried at 103 to 105 degrees C, and weighed, giving milligrams per liter. Turbidity is optical, quantifying light scattered at 90 degrees from an incident beam and reported in nephelometric turbidity units calibrated against formazin suspensions. Because scattering depends on particle size, shape, refractive index, and color rather than mass alone, no universal conversion exists, and the linear relation TSS equals a times NTU plus b must be fitted for each site. Reported slopes for surface waters commonly fall between about 1 and 3 mg/L per NTU, with fine clay-dominated water at the low end and coarse sand-laden runoff far higher. A usable regression needs paired samples across the full flow range, ideally 20 or more spanning storm and baseflow conditions, and a coefficient of determination above roughly 0.8. The relation degrades where particle size distribution shifts seasonally, at very high turbidity where multiple scattering suppresses the signal, and where dissolved organic color absorbs light. Air bubbles and biofouling on an in-stream sensor produce false high readings, and the intercept b should sit near zero for a physically sensible fit.

Turbidity measurement began with visual comparison. The Jackson candle turbidimeter, in use by the late nineteenth century, required an observer to pour sample into a graduated tube until the image of a candle flame beneath it disappeared, with the depth converted to Jackson turbidity units. The method could not read below about 25 units, which made it useless for treated drinking water, and results depended on the observer. Formazin, a reproducible polymer suspension made from hydrazine sulfate and hexamethylenetetramine, was introduced as a calibration standard in the 1920s and remains the primary standard today. Photoelectric nephelometers displaced the candle method during the 1960s and 1970s, and the resulting nephelometric turbidity unit was codified by the EPA as Method 180.1, which specifies a white tungsten lamp and a detector at 90 degrees. ISO 7027, first published in 1984, instead specifies an 860 nanometer infrared source, whose readings are reported as formazin nephelometric units and are less affected by water color. Regulation drove precision downward: the Surface Water Treatment Rule of 1989 set a 0.5 NTU filtered water limit and the Interim Enhanced rule of 1998 tightened it to 0.3 NTU in 95 percent of readings. Use of continuous turbidity as a regression surrogate for suspended sediment was formalized in US Geological Survey guidance published in 2009.