Understanding TSS and Turbidity. Are they the same thing?

The Short Answer

Total Suspended Solids (TSS) and turbidity are closely related measurements used to assess the amount of suspended material in water, but they are not the same thing.

This distinction is particularly important when monitoring concrete wash water, construction-site runoff, settlement lagoons, rivers and wastewater discharges.

What is TSS?

TSS = Total Suspended Solids

TSS measures the actual mass of solid particles suspended in a water sample.

Typical particles include:

• Silt and clay

• Sand

• Soil particles

• Cement and concrete fines

• Organic matter

• Algae

• Other suspended particulate material

TSS is normally reported as:

mg/L (milligrams per litre)

For example:

A TSS result of 500 mg/L means that, under the specified laboratory method, approximately 500 mg of suspended material is present per litre of water.

What is turbidity?

Turbidity measures how much suspended material interferes with the passage of light through the water.

It is an optical measurement, rather than a direct measurement of the mass of solids.

Turbidity is usually reported in:

NTU – Nephelometric Turbidity Units

A turbidity meter shines light through the sample and measures the light scattered by particles.

Why TSS and turbidity don't give the same result

There is no universal conversion between NTU and mg/L.

For example, two water samples could both have:

Sample A TSS 100 mg/l Turbidity 100 NTU

Sample B TSS 20 mg/l Turbidity 100 NTU

Both can produce similar optical readings even though the amount of suspended material is very different.

This happens because turbidity is strongly affected by:

• Particle size

• Particle shape

• Particle colour

• Particle composition

• Refractive properties

• How particles are distributed in the water

Fine clay particles, for example, can produce considerable turbidity while contributing relatively little mass.

Why this matters for sampling discharges and water bodies

Effluents and surface water ater can contain extremely fine particles.

Consequently, you can have water that:

• looks relatively clear but still contains significant dissolved/colloidal material; or

• has high turbidity because of very fine particles even though the mass of suspended solids is not exceptionally high.

Turbidity is therefore useful for rapid operational monitoring, but it should not automatically be treated as a substitute for TSS.

For a waste water treatment system, a useful approach is often:

Turbidity → rapid, continuous/field monitoring

TSS → laboratory verification and compliance assessment

Can turbidity be converted to TSS?

Yes, but only after establishing a site- and water-specific correlation.

You can collect samples and measure both:

1. Turbidity in NTU

2. TSS in mg/L

You can then develop a calibration relationship such as:

TSS = a × turbidity + b

However, that relationship may change when the character of the suspended material changes.

For example, the relationship developed for soil/silt runoff should not automatically be applied to concrete wash water.

A particularly important point for environmental monitoring

If you're using turbidity to control a discharge, DON'T SIMPLY ASSUME:

1 NTU = 1 mg/L TSS

That relationship is generally not valid.

Instead, establish a correlation for the particular water being treated.

For example, for a concrete wash-water treatment system you could undertake a series of paired measurements:

Plotting TSS against turbidity can then establish whether turbidity is a reliable surrogate for TSS at that particular site.

TSS vs turbidity – simple summary