June 2022 – TD 320 OPERATING MANUAL – TURBIDITY SENSOR 4296
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8.2
TSS
Total suspended solids (TSS) are the main cause of turbidity. The most accepted method of
measuring suspended solids is by weight. To measure TSS, a water sample is filtered, dried, and
weighed. This method is the most accurate technique for measuring total suspended solids,
however it is also more difficult and time-consuming.
8.3
Total Suspended Solids Measurement Methods
Due to the correlations between turbidity and total suspended solids (TSS), turbidity
measurements are often substituted for suspended sediment concentrations and TSS
measurements. However, when possible, it is more accurate to measure total suspended solids by
weight. Turbidity readings can be affected by colored dissolved organic matter (CDOM), while
sediment and TSS measurements are not. While the most accurate and accepted method for
measuring suspended sediment and TSS is by weighing a sample, the USGS has begun using
acoustic Dopplers for in-situ sediment monitoring.
8.3.1
Total Suspended Solids and Suspended Sediment Concentration by Weight
Measured in milligrams per liter of water (mg/L), suspended sediment concentration and total
suspended solids readings are based on filtered and dried water samples. TSS encompasses any
particles larger than 2 microns in diameter. Any particle smaller than this is considered a dissolved
solid.
The main difference between total suspended solids (TSS) and suspended sediment
concentration (SSC) is in the amount of water analyzed. After filtering a water sample with a 2-
micron filter, the particles are dried and weighed to determine suspended solids. When an entire
sample is filtered, dried and weighed, the American Society for Testing of Materials considers the
measurement to be the suspended sediment concentration. If a water sample is further
subsampled, the subsequent mass measurement will be the TSS measurement. This can be done
by shaking/stirring and pouring from the sample bottle (EPA method) or by stirring and collecting a
sample with a pipette (APHA method). The EPA method is considered more consistent than the
pipette method.
A water sample is filtered, dried, dried and weighed to determine suspended sediment
concentrations. Photo Credit USGS
For samples with fine suspended particles (less than 53 microns), the total suspended solids
measurement and the suspended sediment concentration will be both precise and accurate
relative to the true concentration and each other. However, if larger (medium-coarse) particles are
included in the sample, sub-sampling can often introduce error into the TSS measurement. The
larger the particle, the more likely that it will not be included in the sub-sample. Coarse sediment,
such as sand, is often not included in the TSS measurement. This is due to the rapid settling of
larger particles. On the other hand, regardless of particle size, the SSC is usually within 5% of the
true particle concentration.