Determination of Suspended Solids in Sediment and Water Samples: Standards, Methods, and Application

Suspended solids determination is a core analytical procedure in environmental monitoring that quantifies the particulate matter retained on a filter from water, wastewater, and sediment-related samples. Accurate measurement of suspended solids and suspended sediment concentration is essential for discharge compliance, dredging supervision, and sediment transport studies. This article explains the testing principles, applicable standards, sampling requirements, and analytical methods used by accredited third-party laboratories for suspended solids and sediment determination.

Technical Overview

Suspended solids refer to the particulate matter in a water sample that is retained on a filter (commonly a glass fiber filter or a membrane with a nominal pore size of 0.45 micrometers) after filtration, expressed as mass per unit volume (mg/L) after drying at a defined temperature. In sediment-related programs, the corresponding parameter is often reported as suspended sediment concentration (SSC), which describes the mass concentration of mineral and organic particles transported in the water column. Because suspended particles scatter light, smother benthic habitats, carry adsorbed pollutants, and directly affect treatment plant performance, this parameter is one of the most frequently monitored indicators in surface water, wastewater, and dredging projects.

The determination is primarily performed by gravimetric analysis: a well-mixed sample is filtered, the retained residue is dried to constant weight at 103 to 105 degrees Celsius, and the mass gain of the filter is used to calculate the suspended solids concentration. Widely recognized method standards include GB 11901-89 (water quality - determination of suspended substance - gravimetric method), ISO 11923 (determination of suspended solids by flow-through centrifuge), ASTM D5907 (filterable and nonfilterable matter in water), ASTM D3977 (sediment concentration in water samples), and EN 872 (filtration through glass fiber filters). JIS K 0102 also specifies suspended solids testing within its industrial wastewater testing framework. Accredited third-party laboratories select the appropriate standard according to the sample matrix, regulatory context, and reporting requirements of the project.

Testing Samples

Surface Water and Groundwater Samples

River, lake, reservoir, estuary, and groundwater samples are collected in clean glass or polyethylene containers. Typical collection volumes range from 500 mL to several liters, depending on the expected solids concentration. Samples are kept at 4 degrees Celsius during transport and analyzed as soon as practicable, ideally within seven days, without chemical preservation.

Sediment Elutriates and Pore Water

For dredging, disposal, and capping assessments, sediment elutriates are prepared by mixing the sediment sample with site water at a defined ratio, allowing a standard mixing and settling period, and then filtering or decanting the supernatant. The suspended solids of the elutriate and of sediment pore water help evaluate the potential release of particulate-bound contaminants during resuspension events.

Municipal and Industrial Wastewater

Influent, aeration tank mixed liquor, and final effluent samples are tested for process control and discharge compliance. High-solids matrices such as mixed liquor suspended solids (MLSS) require smaller filtration volumes, while low-level effluents require larger volumes to accumulate a measurable residue on the filter.

Marine and Brackish Waters

Estuarine and seawater samples contain dissolved salts that remain on the filter if not removed, causing positive bias. Standard practice requires rinsing the filter, residue, and filtration apparatus with several portions of deionized water after filtration to flush dissolved salts through the filter before drying.

Testing Items

Typical reporting parameters for suspended solids and sediment determination programs include:

• Total suspended solids (TSS), expressed in mg/L, determined by filtration and drying at 103 to 105 degrees Celsius.
• Volatile suspended solids (VSS), the organic fraction lost on ignition at approximately 550 degrees Celsius.
• Fixed suspended solids (FSS), the inorganic ash residue remaining after ignition.
• Suspended sediment concentration (SSC) in mg/L, following whole-sample procedures suited to sediment-laden waters.
• Settleable solids, reported as mL/L, describing the volume of solids that settle under quiescent conditions over a timed interval.
• The volatile-to-fixed solids ratio, used to characterize the organic content of suspended matter in treatment and ecological studies.

Testing Methods

Gravimetric Filtration Method (Reference Procedure)

The reference approach follows GB 11901-89, EN 872, and ASTM D5907 in essential steps. A glass fiber or 0.45 micrometer membrane filter is preconditioned, dried, cooled in a desiccator, and weighed to establish a stable tare weight. A measured, well-mixed sample volume is filtered under vacuum or pressure; the suspended residue is then rinsed with deionized water, and the filter is dried at 103 to 105 degrees Celsius to constant weight. After cooling, the filter is reweighed, and the suspended solids concentration is calculated as the dried residue mass divided by the filtered sample volume. The filtered volume is selected so that the collected residue is large enough to exceed balance resolution, typically at least 1 to 2.5 mg, yet small enough to avoid clogging the filter.

Volatile and Fixed Solids Determination

To distinguish organic from inorganic particulate matter, the dried filter and residue from the TSS determination are ignited in a muffle furnace at 550 plus or minus 50 degrees Celsius. The mass lost during ignition represents volatile suspended solids, while the remaining ash represents fixed suspended solids. This fractionation is widely used to track biomass in biological treatment systems and to estimate the organic character of natural suspended matter.

Centrifugation Method

For samples containing oils, greases, or very high solids loads that blind filters rapidly, ISO 11923 describes separation using a flow-through centrifuge. The collected solids are dried and weighed analogously to the filtration method, and the result is reported with clear reference to the centrifugal procedure. Results from centrifugation and filtration may differ slightly because separation efficiency depends on particle size and density.

Suspended Sediment Concentration Procedures

ASTM D3977 provides techniques specifically intended for sediment-laden water, including filtration, evaporation, and wet-sieving followed by drying. These whole-sample methods are preferred in sediment transport studies because they capture the full particle size spectrum, whereas TSS subsampling can underestimate concentration when sand-size particles settle rapidly in the sample container.

Instrumental and Field Screening Methods

Turbidimeters, optical backscatter sensors, laser diffraction analyzers, and acoustic instruments provide rapid, continuous estimates of suspended particle levels. These surrogate measurements must be calibrated against gravimetric results for each site, since particle size, shape, color, and composition all influence the optical or acoustic response.

Quality Assurance and Quality Control

Reliable results depend on disciplined laboratory practice, including:

• Method blanks and filter blanks to monitor contamination of filters, rinse water, and glassware.
• Duplicate filtrations and duplicate dryings to verify precision and confirm constant weight criteria.
• Calibrated analytical balances with appropriate resolution, typically 0.1 mg, used in a controlled environment.
• Vigorous homogenization before subsampling, particularly for sand-bearing samples prone to rapid settling.
• Documentation of filtered volume, filter type, drying temperature, and any deviations from the referenced standard.

Applications

Suspended solids and sediment determination data support a broad range of engineering, regulatory, and scientific objectives:

• Compliance monitoring of wastewater discharge against permitted concentration limits for suspended solids.
• Dredging supervision and turbidity management, where suspended sediment levels around operating dredges are compared against action thresholds.
• Construction site stormwater monitoring and erosion control verification.
• Process control at water and wastewater treatment plants, including MLSS, MLVSS, and effluent quality tracking.
• Sediment transport and depositional studies in rivers, reservoirs, estuaries, and coastal waters.
• Environmental impact assessments and ecological studies linking turbidity, light penetration, and benthic habitat condition.
• Verification of sediment capping, disposal, and beneficial reuse programs through elutriate and pore water testing.

FAQ

What is the difference between TSS and suspended sediment concentration (SSC)?

TSS is determined by filtering a subsample drawn from a mixed container, while SSC procedures process the entire sample or a measured, well-homogenized portion, including sand-size particles. For water with significant sand loads, SSC results are typically higher and more representative of true particle concentration, whereas TSS remains the standard parameter for most discharge permits.

Why is a 0.45 micrometer filter commonly specified?

The 0.45 micrometer pore size provides an operational convention that separates particulate from dissolved fractions with good reproducibility. It is embedded in most national and international method standards, so results remain comparable between laboratories and over time, even though colloidal particles both smaller and larger can behave in complex ways near this cutoff.

How should samples be preserved, and what holding times apply?

Samples are collected in clean containers, cooled to approximately 4 degrees Celsius, and analyzed without chemical preservatives. Best practice is to complete the determination within seven days of collection, and as soon as possible for samples containing biodegradable organic matter or rapidly settling coarse sediment.

Can saline samples be tested without bias?

Yes, provided the filter, retained solids, and filtration apparatus are rinsed with several portions of deionized water after filtration. This removes dissolved salts that would otherwise remain as residue and inflate the reported suspended solids value.

What detection limit can be achieved?

The practical reporting limit depends on the filtered volume and the balance resolution. With a 0.1 mg balance and a one-liter sample, reporting limits on the order of 1 to 5 mg/L are routinely achieved, while lower concentrations require larger volumes or more sensitive weighing arrangements, which accredited third-party laboratories can define in their method validation documentation.