Waterguide - TDS and TSS

What are TDS and TSS?

TDS stands for “total dissolved solids” and indicates the amount of dissolved substances in a liquid medium. The dissolved substances can be both inorganic and organic. The concentration is always specified in ppm (parts per million).

TSS stands for “total suspended solids” and indicates the quantity of suspended solids in the water, and does not include dissolved substances.

How are the TDS value and TSS value measured?

The TDS value can be measured directly using the gravimetric method or it can be estimated using a factor to convert the conductivity measurement into a TDS value. As the conductivity-based method is much easier and faster than the gravimetric method, the former has become a common way of estimating the TDS.

Gravimetric method: To determine the TDS value using the gravimetric method, a certain amount of water is heated to 105 °C, which causes it to evaporate. This leaves behind residue consisting of solid substances. The solid substances are weighed, which provides the TDS value. This is the most reliable and accurate method, even though it is slightly more time-consuming.

To determine the TSS value, water is filtered and the filter containing the residue is dried. The difference in weight between before and after the filtration determines the quantity of suspended solids.

Conductivity measurement: In most waters, there is a relatively consistent correlation between electrical conductivity and dissolved substances. This makes it quite easy to measure the conductivity using a handheld conductivity meter. The measuring device converts the conductivity (measured in µS/cm) into the TDS (as ppm or mg/l) by multiplying the value with a conversion factor (usually 0.5 – 0.7).

What are these values used for?

In the context of feedwater, the TDS value serves as a guideline for roughly designing the reverse osmosis if no water analysis is available. Herco reverse osmosis units are normally configured to a feedwater TDS value of 1,000 mg/l (based on the conductivity threshold of drinking water).

The feedwater of a reverse osmosis unit should not contain any suspended particles, and so the TSS value should be almost zero. However, if there does happen to be a certain concentration of suspended substances, pre-treatment using filtration or ultrafiltration is required.

TDS values in water

Fresh water: < 1,000 ppm
Brackish water: 1,000 to 10,000 ppm
Salt water: 10,000 to 35,000 ppm
Hypersaline: > 35,000 ppm

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