Suspended solids determination is one of the most fundamental and widely performed analyses in water quality monitoring, serving as a key indicator of pollution levels, treatment efficiency, and regulatory compliance. This technical article provides a comprehensive overview of water quality suspended solids testing, covering sampling requirements, analytical parameters, gravimetric and instrumental methods, applicable standards such as GB 11901-89, ISO 11923, and ASTM D5907, and practical applications across environmental and industrial sectors.
Technical Overview
Suspended solids (SS), also referred to as total suspended solids (TSS) or non-filterable residue, are the particulate matter retained on a standard filter when a water sample is passed through it under specified conditions. In most standard methods, suspended solids are operationally defined as the fraction of matter captured by a glass fiber filter or a membrane filter with a nominal pore size of approximately 0.45 micrometers, dried to constant weight at 103–105 degrees Celsius.
Suspended solids typically include silt, clay, organic detritus, plankton, microorganisms, and fine inorganic particles. Elevated concentrations of suspended solids can reduce light penetration in natural water bodies, impair photosynthesis, smother benthic habitats, transport adsorbed pollutants such as heavy metals and hydrophobic organic compounds, and increase treatment costs in downstream processes. For these reasons, suspended solids serve as a core parameter in discharge permits, environmental quality standards, and process control programs worldwide.
Reliable suspended solids determination requires strict adherence to standardized procedures for sample collection, preservation, filtration, drying, and weighing. Accredited third-party laboratories routinely perform this analysis in accordance with national and international standards to ensure data comparability and legal defensibility.
Testing Samples
Surface Water
Rivers, lakes, reservoirs, and estuaries are monitored for suspended solids to evaluate turbidity conditions, sediment transport, ecological health, and compliance with environmental water quality standards. Sampling depth, flow conditions, and seasonal variability must be considered during sample design.
Municipal Wastewater
Influent, aeration basin mixed liquor, secondary effluent, and final discharges from wastewater treatment plants are analyzed to assess treatment performance, calculate sludge volume indices, and verify compliance with discharge limits for total suspended solids.
Industrial Wastewater
Effluents from industries such as pulp and paper, mining, food processing, textiles, chemicals, and power generation often carry high particulate loads. Suspended solids testing supports process optimization, pretreatment verification, and regulatory reporting.
Groundwater and Drinking Water Sources
Although naturally low in suspended solids, groundwater and source water for potable supply are tested to detect well deterioration, contamination events, and filter performance issues.
Stormwater and Runoff
Construction site runoff, urban stormwater, and agricultural drainage are monitored for suspended solids to control sediment discharge into receiving waters.
Reclaimed and Recycled Water
Reused water destined for irrigation, industrial cooling, or landscape applications must meet strict suspended solids limits to prevent clogging of distribution systems and fouling of equipment.
Testing Items
Comprehensive suspended solids analysis programs typically include the following measurable parameters:
Total suspended solids (TSS / SS): the mass of filterable particulate matter per unit volume, expressed in milligrams per liter (mg/L).
Volatile suspended solids (VSS): the organic fraction of suspended solids determined as the mass loss after ignition at approximately 550 degrees Celsius in a muffle furnace.
Fixed suspended solids (FSS): the inorganic or mineral residue remaining after ignition, also known as the ash fraction.
Settleable solids: the volume of solids that settle under quiescent conditions over a defined period, typically measured with an Imhoff cone and reported in milliliters per liter (mL/L).
Total solids (TS) and total dissolved solids (TDS): complementary parameters that characterize the overall solids profile of the sample.
Turbidity: an optical surrogate parameter, reported in NTU, often correlated with suspended solids concentrations for continuous monitoring purposes.
Testing Methods
Gravimetric Method (Reference Method)
The gravimetric filtration technique is the internationally recognized reference method for suspended solids determination. The general procedure involves the following steps:
Preparation of the filter: a glass fiber filter or 0.45 micrometer membrane filter is rinsed, dried at 103–105 degrees Celsius to constant weight, desiccated, and weighed to establish the tare mass.
Sample filtration: a well-mixed, representative aliquot of known volume is filtered through the prepared filter under vacuum or pressure. The volume is selected to yield a measurable residue loading, typically within an optimal range recommended by the applicable standard.
Rinsing: the filter and retained residue are rinsed with distilled water to remove dissolved salts and ensure complete transfer of solids.
Drying: the filter containing the residue is dried in an oven at 103–105 degrees Celsius until constant weight is achieved, usually requiring at least one to two hours with repeated weighing cycles.
Weighing and calculation: after cooling in a desiccator, the filter is reweighed, and the suspended solids concentration is calculated as the mass difference divided by the filtered sample volume, reported in mg/L.
Volatile and Fixed Solids Determination
To differentiate organic from inorganic particulate matter, the dried filter residue from the TSS determination is ignited in a muffle furnace at approximately 550 degrees Celsius. The mass loss corresponds to volatile suspended solids, while the remaining residue represents fixed suspended solids. This distinction is particularly valuable in wastewater process control, where the volatile fraction approximates the biomass content of activated sludge.
Applicable Standards
Suspended solids determination is performed in accredited third-party laboratories according to recognized national and international standards, including:
GB 11901-89: the Chinese national standard for water quality determination of suspended solids by the gravimetric method.
ISO 11923: the international standard specifying determination of suspended solids by filtration through glass fiber filters, including flow-through centrifuge procedures for specific applications.
ASTM D5907: the ASTM standard test methods for filterable and nonfilterable matter in water, covering both dissolved and suspended residue.
US EPA Method 160.2: the United States Environmental Protection Agency procedure for non-filterable residue dried at 103–105 degrees Celsius.
Complementary national and sector-specific standards governing settleable solids, sludge indices, and effluent discharge limits.
Alternative and Instrumental Methods
While gravimetry remains the definitive technique, several supplementary approaches are used for screening and continuous monitoring:
Turbidimetric methods: nephelometric turbidity measurements provide rapid, real-time estimates that can be empirically correlated with TSS for a given water matrix.
Optical suspended solids sensors: online probes using infrared absorbance or scattering enable continuous process monitoring in treatment plants.
Centrifugal separation: flow-through centrifugation offers an alternative separation mechanism specified in ISO 11923 for matrices where filtration is impractical.
Particle size analysis: laser diffraction techniques characterize the size distribution of suspended particulates, supporting advanced treatment design.
Quality Control Considerations
Method blank filters, duplicate determinations, matrix spikes, balance calibration, oven temperature verification, and controlled desiccation are essential quality assurance elements. Samples should be collected in clean containers, preserved by cooling at approximately 4 degrees Celsius, and analyzed promptly—commonly within seven days—since particulate matter may settle, aggregate, or degrade during storage. Homogenization by gentle inversion before subsampling is critical for representative results.
Applications
Suspended solids determination data support a broad range of environmental, industrial, and regulatory functions:
Wastewater treatment plant operation: monitoring of primary clarification, secondary settling, and tertiary filtration performance; sludge age and mixed liquor suspended solids (MLSS) process control.
Regulatory compliance: verification of effluent discharge against permit limits established under national discharge standards and local receiving water requirements.
Environmental impact assessment: baseline and post-project monitoring of rivers, lakes, and coastal waters affected by construction, dredging, or mining activities.
Industrial process optimization: control of clarification, flotation, thickening, and filtration circuits in industries such as pulp and paper, minerals processing, and food production.
Reclaimed water management: assurance of water quality suitable for irrigation, cooling towers, boiler feed pretreatment, and urban reuse schemes.
Research and remediation: evaluation of sediment control measures, erosion control effectiveness, and ecological restoration outcomes.
Product and equipment certification support: verification of filtration system performance and membrane integrity in water treatment applications.
FAQ
What is the difference between suspended solids and turbidity?
Suspended solids is a direct gravimetric measurement of particulate mass concentration in mg/L, while turbidity is an optical measurement of light scattering reported in NTU. The two parameters are related but not interchangeable; their correlation depends on particle size, shape, color, and composition, and must be established empirically for each water matrix.
What type of filter is used for TSS determination?
Standard methods specify either a borosilicate glass fiber filter or a membrane filter with a nominal pore size of approximately 0.45 micrometers. The filter type must be consistent with the requirements of the applicable standard, as results can vary with filter material and pore size.
Why is the drying temperature set at 103–105 degrees Celsius?
This temperature range removes free and bound water from the residue without significantly decomposing organic matter or driving off crystalline water from minerals, providing a reproducible and standardized measurement basis across laboratories.
What do volatile suspended solids indicate?
VSS represents the organic portion of suspended solids lost upon ignition at approximately 550 degrees Celsius. In biological wastewater treatment, VSS is widely used as an approximation of active biomass concentration in aeration systems.
How should water samples for suspended solids analysis be stored?
Samples should be collected in clean, inert containers, kept cool at approximately 4 degrees Celsius, protected from light, and analyzed as soon as practical, typically within seven days. Gentle but thorough mixing before subsampling is essential because particles settle rapidly.
What is a typical detection limit for the gravimetric method?
Practical reporting limits depend on sample volume, filter diameter, and balance sensitivity, but commonly fall in the range of approximately 1 to 5 mg/L. Filtering larger volumes of clean samples improves sensitivity, while overloaded filters introduce positive bias and should be avoided.
Can suspended solids analysis be performed on-site?
Field kits and portable optical sensors can provide rapid on-site estimates, but legally defensible and standards-compliant results require laboratory gravimetric analysis performed by accredited third-party laboratories under documented quality control procedures.
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