Simplifying Solids Tips to Improve Your Solids Analysis

Complete Article from Edition 58 APG eNewsletter

Article by Wade DeLong , Lab Manager at APG

Two of the most common analytical methods used in wastewater and drinking water analysis are total dissolved solids (TDS) and total suspended solids (TSS). While these analytical methods are easy to understand and require little instrumentation, they are technique sensitive. This article explores the background of solids analysis and suggests some tips to ensure more accurate and precise measurement of solids.

Total Dissolved Solids (TDS)
Total solids are the sum of the dissolved and suspended solids in a sample. Total dissolved solids (TDS) are the solids that will pass through a 2.0 um or smaller filter. TDS are both inorganic and organic in origin. Inorganic sources are minerals such as calcium, magnesium, and bicarbonate from rocks. Organic TDS can come from leaves, industrial waste, and sewage. TDS in a water sample comes from natural sources such as the bedrock and decaying plant material and introduced sources such as waste, urban runoff, and chemicals left over from water treatment processes. A constant level of TDS is essential to the health of aquatic organisms as the density of the TDS determines water flow in and out of cells. Elevated TDS levels are not generally health hazards for humans, but have an effect on the smell and taste of drinking water.

The EPA classified TDS as a Secondary Drinking Water Standard and has set a limit of TDS less than 500 mg/L for drinking water. High concentrations of TDS due to carbonates can lead to scale formation and a bitter taste to the water. If a water source tastes salty and an increase in corrosivity is observed, it may have a high TDS concentration due to chlorides. These effects are not necessarily dangerous but are unpleasant. Scale formation is a nuisance as it can be damaging to water heaters and plumbing.

	TDS	TSS
Standard Method	2540 C	2540 D
EPA Method	160.1	160.2
Description	Solids that pass through a 2.0 um filter	Solids that can be filtered out of a sample.
Effect of High Concentration	Scale Formation Bitter taste to water Salty taste to water Unpleasant smell to water	Clogging of fish gills Destruction of aquatic breeding grounds Reduction in light penetration in the water Increase in water temperature Difficulty in disinfecting the water supply
Effect of Low Concentration	Increased corrosivity	Toxic metals remain in the water instead of complexing with the TSS and settling out of the water supply
Health Hazard to Humans	Very little, effects are unpleasant, not hazardous	Makes disinfection of water supply more difficult
Aquatic Ecosystem Hazard	Effects cellular osmosis of aquatic organisms	Kills fish Reduces growth rate of fish and other aquatic life Reduces available light, limiting plants’ and algae’s ability to produce food and oxygen Changes the bottom of waterways, killing bottom dwellers and destroying breeding areas

Total Suspended Solids (TSS)
Total suspended solids (TSS) are the solids in a water sample that will not pass through the filter. They can often be seen as particles in the water. TSS can be inorganic in origin, resulting from soil particles in the water. It can also be organic, such as over populations of algae. High TSS levels are very destructive to aquatic ecosystems. High sedimentation can clog fish gills and inhibit growth rates. It can also limit the light penetration in a body of water. This leads to a reduction in algae and aquatic plants' ability to produce food and oxygen for other aquatic organisms. Sedimentation changes the bottom of the waterway and smothers bottom dwelling organisms. High TSS levels can interfere with coagulation, filtration, and disinfection in drinking water treatment processes. Turbidity is the clarity of the water. The higher the TSS levels, the more turbid the water. Higher turbidity means that more chlorine is required to disinfect the water supply. While there is no definite quantitative limit for TSS, levels greater than 80 mg/L are very destructive to the aquatic ecosystem.

Analytical Methods
TDS can be evaluated by Standard Method 2540C or EPA Method 160.1. The water sample is filtered through a glass fiber filter into a pre-weighed crucible. The filtrate is evaporated to dryness and then dried to a constant weight at 180 C. The difference in the weight of the crucible is the TDS.

The TSS analysis procedures can be found in Standard Method 2540 D and EPA Method 160.2. The water sample is filtered through the glass fiber filter. The filter is dried to a constant weight at 103 C to 105 C. The increase in the weight of the glass fiber filter is the TSS.

Analytical Transfer
Both the TDS and the TSS methods are technique sensitive. The key to methods that are technique sensitive is consistency. It is much easier to troubleshoot these types of methods if the sample is handled the same every time. When analyzing samples for TSS and TDS, analytically sound, consistent technique is the key to success. When preparing PT or QC samples, the first issue usually occurs when the powder is transferred from the vial to the one-liter volumetric flask. The sample may not be transferred completely.

It is referred to as analytical transfer when all of the material is completely transferred. The first step in analytical transfer is to wet the solid material. This will minimize the loss of material due to dusting. Begin by carefully rinsing the sides of the vial. Then pour the contents into the funnel. Then rinse the vial. It is important to rinse all of the following three times:

• The sides of the vial
• The vial's cap
• The funnel
• The stem of the funnel

Homogeneity of the Sample
Once all of the powder has been transferred, the vessel is brought to volume and mixed thoroughly. It is recommended that the sample is mixed for approximately fifteen minutes to ensure homogeneity. Vigorous shaking for at least thirty seconds will also mix the sample, but a magnetic stir bar will ensure a more homogeneous sample.

Subsampling
Subsampling is using small aliquots of the full volume sample in the analysis. It is recommended that a graduated cylinder be used to measure at least 250 mL of the one-liter sample. It is preferable to use 500 mL aliquots. It is very important to rinse the graduated cylinder thoroughly to make sure that no material is lost in the subsampling step. Once again, the rule of three times should be employed when rinsing the graduated cylinder.

Drying Time
The drying time required in the TDS method is sensitive to changes in the TDS sample. If the TDS sample being analyzed has a high mineral concentration, it can absorb moisture. This type of sample will require extended drying and must be weighed quickly to ensure that moisture from the air does not affect the result. If you know that your sample has a high concentration of minerals or you have difficulty drying your sample to a constant weight, you should be sure take the proper steps to ensure that the sample does not absorb moisture. Refer to Standard Methods for more information about this kind of interference. Samples that have a high concentration of bicarbonate should be dried longer at 180 C in order to convert the bicarbonate to carbonate. In order to establish if any of these interferences affect your sample, you should carefully monitor the drying, cooling, and weighing cycle for samples coming from different sources or after changes to your system. You may need to adjust the cycle based on this information.

Stability of PT and QC Samples
The solids PT or QC sample is stable for two years if it is in the unopened vial. Once diluted to volume, the sample is stable for about 24 hours. You will see an increase in TDS as the TSS is dissolved if you keep a full volume solids sample for a long period.

Periodic participation in solids PT studies and frequent use of independently prepared quality control materials can help you train analysts and find problems with equipment or analytical technique before your method fails. By carefully transferring the sample, properly rinsing all of the surfaces, and maintaining consistent technique, you can ensure accurate and precise measurement of solids.