Suspended Solids in Water: Particle Size, the 0.45 um Cut-Off and Why the Measurement Matters
Look at a glass of river water and the haze in it is a mixture of things, but the part the eye actually registers is suspended solids: particles with diameters roughly between 0.1 um and 100 um. They consist mainly of silt, clay, protozoa, algae, bacteria, viruses and high-molecular-weight organic matter, and they stay suspended in flowing water. The turbidity of water is very largely caused by them. Particles fine enough to remain in suspension for a long time in seawater are sometimes called suspended solids or suspended colloids as well.
What They Are Made Of
Suspended matter splits into two fractions, organic and inorganic. The organic fraction is mostly detrital particles built from carbohydrates, proteins and lipids. The inorganic fraction includes terrigenous mineral detritus such as quartz, feldspar, carbonates and clay, and authigenic marine minerals such as precipitated silicates and carbon. Particle size in general ranges from a few to several hundred micrometers, and because of that, chemical analysis usually begins by separating the solids from seawater using filter membranes with a pore size of 0.45 um.
The organic components are largely biological remains, excreta and decomposition products: carbohydrates such as cellulose and starch, proteins, lipids, and chitin. The inorganic components are continental mineral detritus and silicates formed through chemical processes in seawater.
The Definition Turns on the Filter
There is a neat trick in the way suspended solids are defined: they are the solid matter that cannot pass through a filter. That makes the definition operational rather than abstract. A filter paper pre-dried to constant weight is placed in a Buchner funnel, the sample is filtered through medium-speed quantitative filter paper, and after drying at 103-105 degrees C to constant weight the total non-filterable residue is read off as the suspended-solids content.
The standard used for water-quality work refines this further. Suspended solids are the matter retained on a membrane filter with a pore size of 0.45 um and dried at 103-105 degrees C to constant weight. That single figure, 0.45 um, is a convention rather than a law of nature: the fraction passing through a 0.45 um filter is defined as "dissolved" and "soluble", and whatever is retained is called suspended solids. Change the pore size and you change the answer, which is why methods state it so deliberately.
Running the Test
Collected samples should be analysed as soon as possible. If they must be held, store them in a refrigerator at 4 degrees C for no longer than seven days. The apparatus is ordinary laboratory equipment plus an all-glass microporous membrane filtration set and a GN-CA membrane filter of 0.45 um pore size and 60 mm diameter.
Sampling containers - polyethylene or hard glass bottles - are washed with detergent, rinsed with tap water and then distilled water, and rinsed three times with the water to be sampled before filling. A representative sample of 500-1,000 mL is collected and the bottle capped tightly. Floating or submerged non-homogeneous solid matter is not classified as suspended solids and is removed from the sample. No preservative is added, so that the partition equilibrium between solid and liquid phases is not disturbed.
For the measurement itself, flat-tipped forceps place the membrane filter in a weighing bottle previously dried to constant weight. It goes into an oven at 103-105 degrees C for half an hour, then into a desiccator to cool to room temperature before weighing. Drying, cooling and weighing repeat until two successive weighings differ by no more than 0.2 mg. The membrane at constant weight is seated on the filter support, the matching funnel is fitted and clamped, and the membrane is wetted with distilled water under continuous suction.
A measured 100 mL of thoroughly mixed sample is then filtered under suction until all the water has passed, washed three times with 10 mL of distilled water each time, and left under suction to remove traces of water. The membrane bearing the solids goes back into its weighing bottle, into the oven at 103-105 degrees C for one hour, then to a desiccator to cool and be weighed, repeating until successive weighings differ by no more than 0.4 mg. A practical note: too much retained solid entrains water, prolongs drying and can make filtration difficult, in which case reduce the sample volume; too little raises weighing error and lowers accuracy, in which case increase it. A retained mass of 5-100 mg is the usual working range.
Why Anyone Should Care
Suspended solids are the most common pollutant in water and an important water-quality indicator, expressed as mass per unit volume, normally in milligrams per litre. In a boiler system they are actively harmful: they readily form scale, which causes metal corrosion and overheating and leads to foaming and carryover. In a surface water body they make the water turbid, reduce transparency, affect the respiration and metabolism of aquatic organisms, and can even block channels. In treatment works, solids also interfere with the operation of treatment and recovery equipment, and some fractions are toxic to a degree.
Because almost every wastewater contains suspended solids, removing them is a fundamental task, essentially a solid-liquid separation exercise. In primary treatment, physical methods - coarse and fine screens or mesh to intercept non-dissolved solids, air flotation, grit removal and sedimentation by gravity - take out most of the load. In biochemical and advanced stages, chemical flocculation and sedimentation dose coagulants so that colloidal solids and dispersed particles form larger, heavier flocs that settle. To push effluent quality further, filtration methods such as biofilm filtration and activated carbon adsorption can be applied as a final barrier.