Turbidity, Suspended Solids, and Color

Why Turbidity, Suspended Solids, and Color Matter

Water leaving a treatment plant must be clear, safe, and aesthetically acceptable. Turbidity, total suspended solids (TSS), and color are three closely related but distinct measurements that tell operators how well the plant is removing particles and dissolved matter. Regulators set limits on these parameters because high turbidity can shield pathogens from disinfection, making it a direct public-health concern. Understanding how to measure each parameter accurately—and what the numbers mean—is a core operator skill.

Turbidity

Turbidity is a measure of how much light is scattered by particles suspended in water. It is expressed in Nephelometric Turbidity Units (NTU). The measurement is made with a nephelometer (also called a turbidimeter), which shines a beam of light through the sample and detects light scattered at a 90-degree angle to the beam. The more particles present, the more light is scattered, and the higher the NTU reading.

Turbidity is not the same as TSS. A sample can have high turbidity from very fine colloidal particles that pass through a filter, while TSS captures only the particles retained on the filter.

Calibrating and Using a Bench-Top Nephelometer

1. Allow the instrument to warm up per the manufacturer's instructions.

2. Calibrate using traceable standards—either formazin primary standards or commercially prepared StablCal® standards—at the ranges you expect to measure. Always start with the lowest standard and work upward.

3. Wipe the sample cell with a lint-free cloth to remove fingerprints; oils and scratches scatter light and cause false high readings.

4. Fill the cell to the marked line, cap it, and insert it in the same orientation each time (align the index mark).

5. Record the reading in NTU after the display stabilizes.

Acceptable accuracy at the low range (≤ 1 NTU) is critical because drinking-water regulatory limits are set in that range. A reading within ±0.5 NTU of the known standard value is the target for this lab.

Regulatory Limits for Filtered Public Water Supplies

The Safe Drinking Water Act directs the U.S. Environmental Protection Agency (EPA) to set maximum contaminant levels (MCLs) and treatment technique requirements. For filtered surface water systems operating under the Surface Water Treatment Rules, turbidity limits apply at the filter effluent. The specific numeric limits are found in federal and state regulations; your regulatory reference handout contains the current values your state has adopted. As a general principle, turbidity in filtered drinking water must be kept very low—well under 1 NTU in most cases—because:

• Particles can physically shield Giardia cysts, Cryptosporidium oocysts, and bacteria from UV light and chlorine disinfection.
• High turbidity signals that filtration may be failing, prompting immediate investigation.
• Consumers associate turbid water with contamination, creating public-trust issues even when water is technically safe.

Operators must monitor filter effluent turbidity continuously and log exceedances. Surpassing a regulatory trigger value requires specific follow-up actions defined in your state rules.

Total Suspended Solids (TSS) by Gravimetric Method

TSS measures the dry mass of particles retained on a standard glass-fiber filter, reported in milligrams per liter (mg/L). This is a gravimetric method because it depends on weighing—not on optical properties.

Procedure Overview (Standard Methods 2540 D)

1. Prepare the filter. Use a glass-fiber filter with the appropriate pore size (nominally 0.45 µm or as specified). Place it in an aluminum pan and dry it in an oven at 103–105 °C for at least one hour. Cool in a desiccator, then weigh on an analytical balance. Record this as Tare Weight (W1).

2. Filter the sample. Measure a known volume of well-mixed sample (e.g., 100 mL or 1,000 mL depending on expected solids concentration) through the prepared filter under vacuum. Rinse the graduated cylinder with a small amount of distilled water and pass it through the filter.

3. Dry the filter. Transfer the filter and retained solids to the oven at 103–105 °C for at least one hour. Cool in a desiccator.

4. Weigh. Record this as Final Weight (W2).

5. Calculate TSS. See the worked example below.

Avoiding errors: do not let the filter dry out during filtration (causes cracking), do not touch the filter with bare hands (oils add mass), and ensure the oven temperature does not exceed 105 °C (volatile solids may be lost at higher temperatures, biasing the result low).

Color

True color is measured after filtering out turbidity and is caused by dissolved organic matter (humic and fulvic acids from decaying vegetation), iron, or manganese. It is expressed in Platinum-Cobalt Units (PCU) or Color Units (CU). High color can indicate organic precursors that react with chlorine to form disinfection byproducts (DBPs) such as trihalomethanes. Color monitoring guides coagulation chemical doses.

Apparent color includes both dissolved and suspended contributions; true color is measured on filtered samples. This distinction matters when diagnosing treatment problems.

Safety in the Lab

• Safety glasses are required whenever you handle water samples, standards, or chemical reagents.
• Gloves protect against unknown sample constituents; wastewater samples may contain pathogens.
• Formazin stock solution is a suspected carcinogen—work in a ventilated area and dispose of according to your facility's chemical waste procedures.
• The drying oven reaches over 100 °C; use insulated gloves or tongs when removing items. Allow items to cool in the desiccator before handling.
• Wash hands before leaving the lab even if gloves were worn.
• Broken glass-fiber filters or cracked sample cells should be disposed of in a sharps/glass waste container, not the regular trash.