Gravimetric Determination of Suspended Solids in Water

The gravimetric method, often referred to as the filtration‑weighing method, is the standard reference technique for measuring total suspended solids (TSS) in water. It is widely used for surface water, wastewater, and industrial effluents because of its directness and accuracy.

Principle. A well‑mixed water sample of known volume is filtered through a pre‑weighed glass fibre or membrane filter (typically 0.45 µm pore size). The residue retained on the filter is dried to constant weight at 103–105 °C. The increase in filter mass divided by the sample volume gives the concentration of suspended solids, expressed in mg/L.

Procedure. The filter is first conditioned by rinsing with deionised water, dried in an oven, cooled in a desiccator, and weighed on an analytical balance. A measured volume of the sample – usually 50–500 mL, depending on the expected solids load – is then passed through the filter under vacuum. After filtration, the filter is returned to the oven, dried again, cooled, and reweighed. The cycle repeats until the weight change is less than 0.5 mg or 4 % of the previous weight, ensuring constant weight. Blanks and duplicates are run for quality control.

Advantages. This method is a primary, direct measurement that does not rely on calibration curves or chemical assumptions. It is applicable to all types of water, regardless of particle colour, shape, or composition, and can measure solids from a few mg/L up to several thousand mg/L by adjusting sample volume. Because it is a standard reference method (e.g., USEPA 160.2, ISO 11923, GB/T 11901‑1989), results are legally defensible and accepted for regulatory compliance.

Limitations. The gravimetric method is labour‑intensive and time‑consuming; a single analysis can take several hours. It requires a stable oven, a precise balance, and a vacuum filtration setup. Very low concentrations (below 10 mg/L) are subject to significant weighing errors. Moreover, dissolved or colloidal matter that passes through the filter is not measured, while volatile solids lost during drying are excluded from the reported residue. The method cannot provide real‑time or online data, making it unsuitable for process control or early warning.