Suspended solids
Suspended solids, also known as total suspended solids (TSS) in water quality assessments, are fine particulate materials dispersed in water or other fluids that do not dissolve and remain suspended rather than settling rapidly.[1] These particles include inorganic sediments like clay and silt, as well as organic matter such as algae, bacteria, and detritus, and are a key indicator of sediment load and pollution in aquatic environments.[2] The measurement of suspended solids involves filtering a known volume of well-mixed water sample through a standardized glass fiber filter with a nominal pore size of 1.5–2.0 micrometers, followed by drying the retained residue at 103–105 °C to constant weight and calculating the concentration as milligrams per liter (mg/L).[3] This procedure, outlined in EPA Method 160.2 and Standard Methods 2540 D, distinguishes suspended solids from dissolved solids by capturing only the non-soluble fraction.[4] Variations in filter pore size or drying temperature can influence results, but these methods ensure comparability across monitoring programs.[5] Suspended solids play a critical role in water quality, as elevated concentrations contribute to turbidity, reducing light availability for photosynthesis and disrupting aquatic plant growth.[6] These particles adsorb pollutants such as heavy metals, phosphorus, and pathogens, facilitating their transport and bioavailability, which can lead to eutrophication, toxicity to fish, and habitat degradation.[7] High TSS levels also warm surface waters by absorbing sunlight, lower dissolved oxygen capacity, and increase costs for water treatment and fisheries management, making them a common cause of impaired water bodies under regulatory frameworks like the Clean Water Act.[8][9]Definition and Fundamentals
Definition
Suspended solids (SS), also referred to as total suspended solids (TSS) in many regulatory and monitoring contexts, are small solid particles dispersed within a liquid medium, primarily water, that remain in suspension rather than settling out quickly. These particles stay afloat due to mechanisms such as Brownian motion for very fine particles, turbulence in flowing water, or their inherently small size, which prevents rapid gravitational settling.[10][11] This dispersion distinguishes them from larger sediments that settle readily and from dissolved substances that integrate molecularly into the liquid. SS occur commonly in natural waters like rivers and lakes, as well as in wastewater and industrial effluents, where they serve as a critical parameter for assessing water quality and pollution levels.[12][13] In these contexts, elevated SS concentrations can indicate erosion, organic decay, or human activity impacts, influencing clarity, oxygen levels, and ecosystem health. The quantification of suspended solids originated in early 20th-century water treatment studies, with standardized methods developed through editions of Standard Methods for the Examination of Water and Wastewater to guide treatment processes and regulatory compliance.[14][15] Unlike dissolved solids, which form a homogeneous solution at the molecular level and cannot be removed by simple filtration, suspended solids exist as discrete particles that scatter light and can be physically separated. Conceptually, this can be illustrated in a diagram showing a water column with visible specks of particles randomly distributed and jostled by motion, in contrast to a clear solution where no such particles are discernible.[16]Distinction from Related Solids
Suspended solids in water are distinguished from settleable solids primarily by their settling behavior and particle size. Settleable solids consist of larger particles, typically greater than 0.1 mm, that settle out under the influence of gravity within a defined period, such as one hour, as measured by methods like the Imhoff cone test.[17] In contrast, suspended solids encompass a broader range of particles, including those that do not settle readily and remain dispersed in the water column. Unlike dissolved solids, which are molecular or ionic solutes that pass through standard filtration media, suspended solids are particulate matter retained on standard glass fiber filters with nominal pore sizes of 1.5 µm or smaller, as per common implementations of Standard Methods 2540 D. Dissolved solids, often quantified as total dissolved solids (TDS), include substances like salts and minerals that do not contribute to turbidity or settleability and are measured after filtration by evaporating the filtrate.[17][18][19] This filtration-based separation highlights the operational distinction: suspended solids are the fraction captured during the process, while dissolved solids pass through. Suspended solids also overlap with colloidal solids but differ in terms of size range and stability. Colloidal solids are fine particles, generally 1 nm to 1 µm in diameter, that remain suspended due to electrostatic repulsion and Brownian motion, resisting settling without coagulation.[20][21] However, suspended solids include both settleable fractions and non-settleable colloidal fractions, allowing for a more inclusive categorization that captures particles up to larger sizes.[17][12] A key metric for partitioning these categories is the relationship between total solids (TS), suspended solids (SS), and dissolved solids (DS), expressed as: TS = SS + DS This equation reflects the basic division after filtration and drying, where settleable solids form a subset of SS rather than a separate additive term.[17][22]Characteristics and Properties
Particle Size and Types
Suspended solids are classified by particle size into non-settleable and settleable fractions, which determine their behavior in aquatic environments. Non-settleable solids, typically smaller than 0.1 mm (often in the range of 1-100 μm), include fine particles such as clays, silts, and organic matter that remain suspended due to their small size and low settling velocity.[12][10] In contrast, settleable solids range from 0.1 mm to 1 mm and consist of larger particles that can gravitate to the bottom under quiescent conditions, such as coarse sediments or aggregates. This size-based distinction is crucial for understanding suspension dynamics, as smaller particles contribute to long-term turbidity while larger ones affect short-term sedimentation.[12] The composition of suspended solids further categorizes them into inorganic, organic, and anthropogenic types, each with distinct origins and suspension characteristics. Inorganic suspended solids, such as silt, sand, and metal oxides, originate from geological erosion and industrial processes, often exhibiting higher densities that promote eventual settling.[12][10] Organic types include biological materials like algae, bacteria, and detritus from decaying plant and animal matter, which are lighter and more prone to prolonged suspension due to their lower density and potential for flocculation.[12] Anthropogenic suspended solids encompass human-derived pollutants, including plastics and industrial residues; microplastics, recognized as a significant environmental concern since the 2010s, exemplify these particles that persist in suspension due to their engineered buoyancy and resistance to degradation.[10][12] The suspension and settling behavior of these particles is primarily governed by Stokes' law, which describes the terminal settling velocity of spherical particles in a viscous fluid under laminar flow conditions. The formula is given by: v = \frac{2}{9} \frac{(\rho_p - \rho_f) g r^2}{\mu} where v is the settling velocity, \rho_p is the particle density, \rho_f is the fluid density, g is the acceleration due to gravity, r is the particle radius, and \mu is the dynamic viscosity of the fluid.[23] This equation highlights how settling velocity increases quadratically with particle radius and depends on the density difference between the particle and fluid, while being inversely proportional to viscosity; assumptions include low Reynolds numbers (laminar flow, typically Re < 1), non-interacting spherical particles, and negligible inertia or turbulence.[24] Smaller particles (e.g., 1-100 μm non-settleable solids) exhibit near-zero velocity in water, remaining suspended for extended periods, whereas larger settleable fractions (0.1-1 mm) settle more rapidly, influencing their distribution in water bodies.[23] In recent years, emerging anthropogenic types such as nanoplastics—particles smaller than 1 μm—have been identified in polluted waters, complicating traditional classifications and suspension models. Post-2020 environmental studies have documented nanoplastics in suspended solids across aquatic systems, noting their high mobility and persistence due to sizes below typical filtration thresholds, often derived from the fragmentation of larger microplastics.[25] These nanoplastics, detected in concentrations up to thousands of particles per liter in contaminated surface waters, underscore the evolving impact of human activities on suspended solid dynamics.[26]Physical and Chemical Properties
Suspended solids exhibit a range of physical properties that influence their behavior in aquatic environments. The density of these particles typically ranges from 1.1 to 2.6 g/cm³, with lower values for organic matter (around 1.27 g/cm³) and higher densities for mineral components like siliceous clays (up to 2.65 g/cm³).[27] Particle shape varies from spherical to irregular or fibrous forms, such as curved chrysotile fibers or straight amphibole structures, which affect hydrodynamic drag and settling dynamics.[28] Fine suspended solids, particularly colloids and clays, possess high specific surface areas, often reaching up to 100 m²/g, enabling extensive interactions with surrounding water and solutes.[29] Chemically, suspended solids often carry a surface charge determined by their zeta potential, which is frequently negative for clay particles due to their mineral composition, leading to electrostatic repulsion and enhanced stability in suspension.[28] This surface charge facilitates adsorption of pollutants, including heavy metals and organic compounds, following the Langmuir isotherm model, where the fractional surface coverage \theta is given by \theta = \frac{K C}{1 + K C} with K as the adsorption equilibrium constant and C as the equilibrium concentration of the adsorbate; this monolayer adsorption is particularly pronounced on high-surface-area particles like humic acid-coated solids.[28] Aggregation tendencies of suspended solids are governed by environmental factors such as pH and ionic strength, which modulate particle stability through compression of the electrical double layer.[30] At higher ionic strengths, repulsion decreases, promoting van der Waals attractions and initial aggregation, while trivalent ions like Al³⁺ induce flocculation by neutralizing charges and forming hydroxide bridges, especially effective in water treatment at pH values around 6.4–7.5 where Al solubility is minimized.[31] Lower pH can enhance cationic flocculant performance by reducing polymer ionization, further aiding floc formation.[30] Recent studies highlight the role of biofilms in enhancing microbial attachment to suspended solids, where initial reversible adhesion of planktonic cells to particle surfaces progresses to irreversible binding via extracellular polymeric substances, increasing cell densities to approximately 10⁵ cells/mm² within days.[32] Surface roughness and hydrophobicity of the particles promote this biofilm maturation, altering particle properties and facilitating nutrient and pollutant transport in aquatic systems.[32]Measurement and Analysis
Standard Measurement Methods
The standard measurement of suspended solids relies on gravimetric methods, which quantify the mass of particulate matter retained on a filter after processing a water sample. These protocols ensure consistency in environmental monitoring, wastewater treatment, and regulatory compliance by providing a direct measure of total suspended solids (TSS) concentration.[19] The primary gravimetric procedure involves filtering a well-mixed sample through a pre-weighed glass-fiber filter disk with a nominal pore size of 1.0 to 2.0 μm, such as Whatman Grade 934-AH, to capture particles larger than the pore size. The filter and retained residue are then dried at 103–105°C to constant weight, defined as two consecutive weighings differing by less than 0.5 mg or 4% of the net weight, whichever is smaller. The TSS concentration is calculated using the formula:\text{TSS (mg/L)} = \frac{(A - B) \times 1000}{V}
where A is the weight of the dried filter plus residue (mg), B is the weight of the dried filter alone (mg), and V is the sample volume filtered (mL). Sample volumes typically range from 100 to 1000 mL, adjusted to yield 2.5 to 200 mg of residue for accuracy. This method, outlined in EPA Method 160.2 and APHA Standard Method 2540 D, is applicable to potable, surface, saline, and wastewater samples with TSS concentrations up to 20,000 mg/L.[3][17] To differentiate total suspended solids from the organic fraction, volatile suspended solids (VSS) are determined by igniting the dried TSS residue at 550°C for 15–20 minutes in a muffle furnace, followed by cooling and reweighing. The mass lost represents the volatile (primarily organic) portion, with the inorganic fraction as fixed suspended solids (FSS). The VSS/TSS ratio serves as an indicator of the organic content and potential biodegradability in treatment processes, where higher ratios suggest greater biological treatability. This extension is detailed in APHA Standard Method 2540 E.[17][33] Laboratory standards like EPA Method 160.2 and APHA 2540 D emphasize controlled conditions, including analysis within 7 days of collection (ideally 24 hours) and preservation by cooling to 4°C. Field sampling requires careful techniques to maintain representativeness, such as collecting from mid-depth without introducing turbulence that could resuspend settled particles or dissolve volatiles. These protocols originated in the mid-20th century, with the gravimetric TSS method first formalized in 1946 by the American Public Health Association for wastewater analysis, evolving from earlier volumetric and evaporative approaches. Standardization intensified in the 1970s following the Clean Water Act of 1972, which established TSS thresholds for effluent discharges to protect water quality.[3][17][34][35]