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Suspended solids and other solid substances in wastewater
All natural waters contain solid substances. Such substances include suspended solids and dissolved solids, and in total, they’re called total solids (TS).
Total solids are the total amount of left-over residues after water gets evaporated without any filtration. If the water sample was filtered, the dried residues left on membrane filter are called total suspended solids (TTS), when the particles obtained from evaporated filtrate are called total dissolved solids (TDS).
Definition of total suspended solids?
Suspended solids generally refer to organic or inorganic particles that can be seen suspended in water. When concentration of suspended solids in water gets higher, water turbidity increases and photosynthesis decreases. It can result in low concentration of oxegen and death of aquatic organisms. Industrial wastewater is one of the main pollution sources of surface water due to its large amount and complex pollutants. Suspended solids concentration in industrial wastewater can easily reach up to 30,000 mg/L. Therefore, total suspended solids are an important test item for wastewater quality analysis, and can be used as an indicator to determine degree of pollution.
Difference between total suspended solids and turbidity
Turbidity refers to the degree of light being scattered when irradiates into water. It can be observed as transparency or clarity. Turbidity sources may include suspended solids, particles, fine organic matters, algae, plankton or microorganisms. Low water transparency indicates high turbidity, which may affect the photosynthesis of aquatic plants and growth of aquatic organisms. High water turbidity may also disturb disinfection and make water purification less effective. Based on the principle of light scattering, turbidity is generally measured by a nephelometer, and its commonly used unit is Nephelometric Turbidity Unit (NTU)
Both turbidity and total suspended solids measure the transparency of water bodies, but there are slight differences between these two measuring methods. Turbidity is usually measured directly on site with a portable device, and the data can be obtained immediately. In other hand, total suspended solids are analyzed in the laboratory by experimental methods such as filtration and drying. Therefore, turbidity measures the degree of light scattering in a water body; total suspended solids is a quantitative measure of fine particles such as suspended solids in a water body. Particles that affect turbidity and total suspended solids are also different, as shown in the following figure:
How to measure TSS? Example of EPA Method 160.2
Gravimetric analysis is the most common method of measuring total suspended solids in water. It’s a quantitative method that weighs the residues collected from glass-fiber or cellulose ester membrane filters after filtration.
- This method is applicable to drinking, surface, and saline waters, domestic and industrial wastes.
- The practical range of the determination is 4 mg/L to 20,000 mg/L.
Sample Handling and Preservation
Non-representative particulates such as leaves, sticks, fish, and lumps of fecal matter should be excluded from the sample if it is determined that their inclusion is not desired in the final result.
Preservation of the sample is not practical; analysis should begin as soon as possible. Refrigeration or icing to 4°C, to minimize microbiological decomposition of solids, is recommended.
Preparation of glass fiber filter disc
Place the glass fiber filter on the membrane filter apparatus or insert into bottom of a suitable Gooch crucible with wrinkled surface up. While vacuum is applied, wash the disc with three successive 20 mL volumes of distilled water.
Remove all traces of water by continuing to apply vacuum after water has passed through.
Remove filter from membrane filter apparatus or both crucible and filter if Gooch crucible is used, and dry in an oven at 103-105°C for one hour.
Remove to desiccator and store until needed. Repeat the drying cycle until a constant weight is obtained (weight loss is less than 0.5 mg). Weigh immediately before use.
After weighing, handle the filter or crucible/filter with forceps or tongs only.
Selection of Sample Volume
For a 4.7 cm diameter filter, filter 100 mL of sample. If weight of captured residue is less than 1.0 mg, the sample volume must be increased to provide at least 1.0 mg of residue.
If other filter diameters are used, start with a sample volume equal to 7 mL/cm2 of filter area and collect at least a weight of residue proportional to the 1.0 mg stated above.
Process of filtration
- Assemble the filtering apparatus and begin suction. Wet the filter with a small volume of distilled water to seat it against the fritted support.
- Take the sample vigorously and quantitatively transfer the predetermined sample volume selected in 7.2 to the filter using a graduated cylinder. Remove all traces of water by continuing to apply vacuum after sample has passed through.
- With suction on, wash the graduated cylinder, filter, non-filterable residue and filter funnel wall with three portions of distilled water allowing complete drainage between washing. Remove all traces of water by continuing to apply vacuum after water has passed through.
- Carefully remove the filter from the filter support. Alternatively, remove crucible and filter from crucible adapter. Dry at least one hour at 103-105°C. Cool in a desiccator and weigh. Repeat the drying cycle until a constant weight is obtained (weight loss is less than 0.5 mg).
- Calculate non-filterable residue.
Membrane & pore size used in different countries
Country | Method | Glass-fiber Membrane | Pore Size |
USA | SM 2540 D | Pall A/B Type | 1 um |
EPA 160.2 | Millipore AP-40 | 0.7 um | |
Reeves Angel 934-AH (Whatman grade 934 AH) | 1.5 um | ||
Gelman type A/E (or Pall type A/E) | 1 um | ||
Taiwan | NIEA W210.58A | Whatman grade 934 AH | 1.5 um |
Pall type A/E | 1 um | ||
Millipore Type AP-40 | 0.7 um | ||
E-D Scientific Specialties grade 161 | 1.1 um |
In this research, Analytical Methods (2018), the water samples were filtered by glass fiber membrane filter and cellulose ester membrane filter with an effective filter membrane pore size ranging from 0.4 to 3 um. The results showed no significant differences in the total suspended solids obtained.
When selecting a filter membrane, choose appropriate pore size according to the physical properties of the water sample itself and its particle size. It is suggested to choose a filter with smaller pore size for water body that is clear with lower turbidity and vise versa to avoid rapid fouling of the membrane.
