Determination of Suspended Solids in Surface Water: Gravimetric Methods, Standards, and Applications

Suspended solids determination is one of the most fundamental tests in surface water quality monitoring, quantifying the concentration of particulate matter retained on a filter and dried to constant weight. The gravimetric filtration method, standardized in GB 11901, ISO 11923, EN 872, and ASTM D5907, provides reliable results for rivers, lakes, reservoirs, and coastal waters. This article explains the testing principle, sampling requirements, analytical procedure, quality control measures, and typical applications of suspended solids analysis in environmental laboratories.

Technical Overview

Suspended solids (SS), also referred to as total suspended solids (TSS), denotes the portion of particulate matter in a water sample that is retained on a standard glass-fiber or membrane filter with a nominal pore size of 0.45 micrometers and dried to constant weight at 103–105 degrees Celsius. In surface water, suspended solids originate from soil erosion, surface runoff, algal growth, resuspended sediments, atmospheric deposition, and municipal or industrial discharges.

Elevated suspended solids concentrations reduce light penetration and photosynthesis, smother benthic habitats and fish spawning grounds, clog fish gills, and serve as a carrier vehicle for adsorbed pollutants such as heavy metals, nutrients, pathogens, and hydrophobic organic compounds. For this reason, suspended solids serve as a core indicator in surface water quality classification, effluent compliance assessment, and watershed management programs.

The analytical principle is straightforward gravimetry: a well-mixed sample of known volume is filtered through a pre-conditioned, pre-weighed filter. The filter plus retained residue is dried at 103–105 degrees Celsius to constant weight, cooled in a desiccator, and reweighed. The increase in mass, divided by the filtered volume, yields the suspended solids concentration in milligrams per liter. Internationally recognized method standards include:

• GB 11901-89 – Water quality; determination of suspended substance; gravimetric method (Chinese national standard)
• ISO 11923 – Water quality; determination of suspended solids by filtration through glass-fibre filters
• EN 872 – Water quality; determination of suspended solids; method by filtration through glass fibre filters
• ASTM D5907 – Standard test methods for filterable and nonfilterable matter in water
• US EPA Method 160.2 – Residue, non-filterable (gravimetric, dried at 103–105 degrees Celsius)

Testing Samples

River and Stream Water

Flowing water bodies typically exhibit the widest concentration range of suspended solids, from below 5 mg/L in stable baseflow periods to several thousand mg/L during storm events. Samples should be collected from the centroid of flow at mid-depth, avoiding bank edges, backwaters, and bed disturbance. Because concentration changes rapidly with hydrological conditions, flow rate and recent rainfall data should be recorded alongside sampling.

Lake and Reservoir Water

Standing water bodies are subject to thermal stratification, seasonal algal blooms, and sediment focusing, all of which create vertical and spatial gradients in suspended solids. Depth-integrated or multi-point sampling at defined depths is recommended for representative characterization. Epilimnetic samples are generally collected 0.5 meters below the surface.

Estuarine and Coastal Surface Water

Brackish and saline samples introduce special analytical considerations because high dissolved salt content can bias gravimetric results if filters are not rinsed thoroughly after filtration. Tidal cycles strongly influence suspended load, so sampling times should be referenced to tidal stage.

Pond, Wetland, and Shallow Surface Water

Shallow systems with dense vegetation or organic detritus often contain heterogeneous mixtures of mineral grains and biogenic particles. Careful agitation before subsampling is essential to prevent settling bias during sample handling.

Sample Collection and Preservation

• Collect in clean polyethylene or wide-mouth glass containers with minimal headspace and zero field filtration.
• Refrigerate samples at 4 degrees Celsius in the dark; avoid freezing, which can aggregate or lyse particles.
• Analyze as soon as practicable, preferably within 24 hours and no later than 7 days, consistent with ISO and standard method hold-time requirements.
• Gently invert and homogenize the entire container immediately before aliquot withdrawal; never decant settled samples without resuspension.

Testing Items

A comprehensive suspended solids characterization of surface water typically encompasses the following determinations:

• Total suspended solids (TSS): residue retained on a 0.45 micrometer filter and dried at 103–105 degrees Celsius, reported in mg/L.
• Volatile suspended solids (VSS): the organic fraction of suspended solids, determined by igniting the TSS residue in a muffle furnace at 550 degrees Celsius; the mass loss represents volatile matter.
• Fixed suspended solids (FSS): the inorganic or mineral ash fraction remaining after 550 degrees Celsius ignition, useful for distinguishing sediment inputs from biological production.
• Settleable solids: volume of solids settling in one hour in an Imhoff cone, reported in mL/L, relevant to sedimentation behavior.
• Related supporting parameters: turbidity (NTU), total solids, total dissolved solids, and volatile/ash ratios used to interpret particle origin.

Testing Methods

The reference procedure is the gravimetric filtration method described below, harmonized across GB 11901, ISO 11923, EN 872, and US EPA 160.2.

Apparatus and Materials

• Glass-fiber filters (47 mm, nominal pore size 0.45–0.7 micrometers) or 0.45 micrometer membrane filters, or Gooch crucibles with glass-fiber discs.
• Vacuum filtration manifold with receiver flask and controlled suction.
• Forced-convection drying oven maintained at 103 plus or minus 2 degrees Celsius.
• Muffle furnace at 550 plus or minus 50 degrees Celsius for volatile solids determination.
• Desiccator with indicating silica gel.
• Analytical balance readable to 0.1 mg, calibrated and verified daily.
• Graduated cylinders or wide-bore pipettes of appropriate volume, Class A.

Procedure

• Condition the filter: rinse with distilled or deionized water, dry at 103–105 degrees Celsius for at least one hour, cool in a desiccator, and weigh repeatedly until consecutive masses differ by no more than 0.4–0.5 mg. Record the constant mass as the blank filter mass.
• Homogenize the sample by gentle but complete inversion; avoid vigorous shaking that fragments fragile aggregates.
• Measure and filter a suitable aliquot volume, selected so that the retained residue falls between approximately 2.5 mg and 200 mg; typical surface water volumes range from 100 mL to 1000 mL.
• Apply moderate vacuum during filtration and rinse the filter and filtration apparatus walls with three successive 10 mL portions of distilled water to remove dissolved salts.
• Dry the filter with residue at 103–105 degrees Celsius for at least one hour, cool in a desiccator, and weigh. Repeat the drying–cooling–weighing cycle until the mass change between consecutive weighings is within 0.4–0.5 mg.
• For VSS, transfer the weighed filter to a muffle furnace, ignite at 550 degrees Celsius for 15–20 minutes, cool in a desiccator, and reweigh; the mass loss is the volatile fraction.

Calculation

Suspended solids concentration is calculated as TSS (mg/L) = (A − B) × 10^6 / V, where A is the constant mass of the dried filter plus residue in grams, B is the constant mass of the blank filter in grams, and V is the filtered sample volume in milliliters. Volatile and fixed fractions are computed analogously from the pre- and post-ignition masses. Results are conventionally reported to two or three significant figures.

Quality Control and Interferences

• Analyze method blanks (distilled water filtered through a pre-weighed filter) with each batch to verify filter and rinse-water cleanliness.
• Perform duplicate determinations; the relative percent difference is typically expected to be below 10–20 percent for samples above 10 times the detection limit.
• Verify constant-weight criteria, balance calibration, oven and muffle furnace temperature uniformity, and desiccant condition as part of routine quality assurance in accredited third-party laboratories.
• Major interferences include high dissolved solids causing positive bias (mitigated by thorough rinsing), floating oils and grease retained on the filter, and sample heterogeneity from inadequate mixing.
• The practical detection limit is approximately 2–4 mg/L, governed by the filtered volume and balance readability; very clear waters require larger aliquots (1 L or more) for reliable quantification.

Applications

Suspended solids data from surface water testing support a wide spectrum of regulatory, operational, and scientific objectives:

• Surface water quality classification and compliance monitoring against environmental quality standards such as GB 3838-2002.
• Discharge permit verification for municipal and industrial effluents released to receiving water bodies.
• Performance evaluation of water and wastewater treatment processes, including clarification, coagulation–flocculation, and filtration efficiency.
• Erosion and sediment control programs at construction sites, mines, and agricultural catchments, including turbidity-to-TSS correlation studies.
• Lake eutrophication and harmful algal bloom investigations, where VSS/FSS ratios distinguish algal biomass from mineral sediment inputs.
• Hydrological research on sediment transport, reservoir siltation, and river morphodynamics.
• Calibration and validation of online turbidity sensors and surrogate models used in continuous automated monitoring networks.
• Ecological risk assessment, since suspended load directly governs light attenuation and benthic habitat condition.

FAQ

What is the difference between suspended solids and turbidity?

Suspended solids is a direct gravimetric measurement of particle mass per unit volume (mg/L), whereas turbidity is an optical measurement of light-scattering intensity (NTU or FNU). Turbidity responds to particle size, shape, and color as well as concentration, so the two parameters correlate empirically but are not interchangeable; a site-specific correlation must be established to convert between them.

Why is the drying temperature set at 103–105 degrees Celsius?

This temperature range removes free and bound water from the residue while minimizing the loss of volatile organic matter and crystallization water from certain minerals. It represents an internationally harmonized compromise that ensures comparability of results across GB, ISO, EN, and EPA methods.

Why is a 0.45 micrometer filter used as the operational boundary?

The 0.45 micrometer pore size is the conventional operational definition separating "suspended" from "dissolved" solids in water analysis. It is a practical convention rather than a true physical boundary, but using it consistently makes results comparable between laboratories and over time.

How much sample volume should be filtered?

The volume is chosen so the retained residue falls roughly between 2.5 mg and 200 mg. For turbid rivers, 50–100 mL may suffice; for clear lakes and reservoirs, 500–1000 mL or more may be needed. Insufficient residue magnifies weighing error, while excessive residue traps water and dissolved salts, both degrading accuracy.

What is the sample hold time for suspended solids analysis?

Samples should be refrigerated at 4 degrees Celsius and analyzed as soon as possible, ideally within 24 hours. Standard methods generally specify a maximum hold time of 7 days; biological activity, particle aggregation, and container-wall adsorption progressively alter the true concentration beyond this window.

What does a high volatile suspended solids percentage indicate?

A high VSS proportion (typically above 50 percent of TSS) indicates that the suspended load is dominated by organic matter, such as algal cells, detritus, or wastewater-derived solids. A low VSS proportion points to predominantly mineral sediment from erosion or resuspension, which helps identify pollution sources and guide management actions.