The Deciding Factors! The What, How, and Why of Project Chemistries – Total Dissolved Solids – December 2024

Understanding the Chemistry of Water

Water is often called the “universal solvent” due to its polarity, which enables it to dissolve a vast range of materials; however, suspended or dissolved solid compounds can negatively impact water quality in several ways. For example, drinking water with large amounts of dissolved solids may adversely affect its color, taste or smell, or can cause problems, such as pipe corrosion. Water with large amounts of suspended solids may be unappealing for hygienic purposes, such as showering or bathing. Additionally, high mineral content may render it ill-suited for industrial use.

That’s why it’s important to properly assess solid content in water sources to ensure it’s safe for drinking and use in agriculture and industrial processes, as well as compliant in meeting remediation and wastewater limits. In this article, we explain the common methods for analyzing solid compounds in water, and feature insights from our laboratory and chemistry experts.

Exploring the Latest Techniques in Solid Content Measurement for Water Quality

According to Standard Method (SM) 2540 (described below), total suspended solids (TSS) and total dissolved solids (TDS) in a water sample are delineated using a 2.0-µm pore size glass-fiber filter. TDS particles are small enough to pass through the filter, while TSS cannot. TDS is a straightforward analysis used to gain an understanding of water quality without distinguishing which compounds comprise the sample. In the laboratory, TDS is measured using a simple gravimetric analysis. For verification of laboratory results, the correctness calculations described in SM 1030E can be used to calculate TDS using the conductivity of the water sample and/or the sum of its main components (sodium, potassium, calcium, magnesium, chloride, sulfate, and alkalinity).

Method SM 2540C – Measurement of TDS dried at 180°C.

o   A volume of stirred sample is pipetted through a vacuum filtration apparatus with 2.0-µm pore filter, with the diameter of the filter being dependent on the amount of dried residue (between 2.5 and 200 mg) yielded within 10 minutes. Filter size (40- to 60-µm fritted disk) and sample volume is adjusted to reach this dried residue amount by increasing the filter size (diameter) and/or decreasing the sample volume. After the sample elutes through the filter, an additional three successive washes of 10-mL reagent grade water are poured through the filtration apparatus, with suction applied for approximately 3 minutes. The filtrate is then transferred to a weighed evaporating dish (ED) and evaporated dry using either a steam bath or drying oven. The sample is then dried for at least 1 hour in an oven at 180 ± 2°C. After drying, the ED is cooled in a desiccator and weighed. The drying cycle should be repeated until a constant weight is achieved or a weight change < 4% of the previous weight or 0.5 mg (the lesser of the two). A minimum of 10% of all samples are evaluated as duplicates, and duplicate calculations must be “within 5% of their average weight.” TDS is then calculated as follows:

mg of TDS/L = ([weight of dried residue + dish, mg – weight of dish, mg] × 1000 / sample volume, mL)

Method SM 2540D – Measurement of TSS dried at 103-105°C.

o   A volume of stirred sample is pipetted (midpoint of container not in vortex) through a weighed standard glass-fiber filter with the diameter of the filter being dependent on the amount of dried residue (between 2.5 and 200 mg) yielded within 10 minutes. In order to meet the required dried residue amount, sample volume may be increased up to 1 L; if filtration takes longer than 10 minutes, an increase in filter diameter or decrease in sample volume may be necessary. After the sample elutes through the glass-fiber filter, an additional three successive washes of 10-mL reagent grade water is poured through the filtration apparatus and suction is applied for approximately 3 minutes. The filter is then transferred to a weighed aluminum dish, or the crucible adapter is removed if performed using a Gooch crucible and filter. The aluminum dish or crucible is then dried for at least 1 hour at 103-105°C. After drying, the aluminum dish or crucible is cooled in a desiccator and weighed. The drying cycle should be repeated until a constant weight is achieved or a weight change < 4% of the previous weight or 0.5 mg (the lesser of the two). A minimum of 10% of all samples are evaluated as duplicates, and duplicate calculations must be “within 5% of their average weight.” TSS is then calculated as follows:

mg of TSS/L = ([weight of filter + dried residue, mg – weight of filter, mg] × 1000 / sample volume, mL)

Data Validation Issues

For TDS results, data validation issues arise when specific requirements outlined in SM 2540C are not met. Error is introduced when residue amounts are outside the specified 2.5-mg to 200-mg limits, when constant weight was not achieved, if the wrong drying temperature was used, and/or when the wrong filter size was used. For instance, for residues > 200 mg, a water-trapping crust can form, thereby preventing all moisture from being fully removed from the sample. In this case, the data validator would need to ask if the laboratory prepared the lowest possible volume. Alternatively, if the residue amount was < 2.5 mg for TDS, the data validator would need to ask if the highest possible volume was prepared. If so, the data validator would need to consider if constant weight was achieved, and potentially, compare the TDS result to historical data or perform the correctness calculation. As an additional example, if a TDS result falls within the normal range but constant weight was not achieved or the wrong temperature was used, then the data validator would need to decide if qualification should be applied.

To gain some insight into addressing biases in TDS analyses both in the laboratory and as a data validator, we spoke with Environmental Standards’ Brittany VanVelkinburgh and Ammie Martin.

Laboratory Analyst Perspective

We asked Environmental Standards QA Chemist, and former laboratory Analyst, Brittany VanVelkinburgh some questions about TDS and how impactful it is in decision-making. Here is what Brittany had to say:

• What information does TDS analysis provide during a project?

It depends on the project the laboratory is doing. Consulting laboratory – not that important, have limits to reach; wastewater programs, maximum contaminant levels (MCLs) for drinking water (500 mg/L for most states); easy in and out test; more relevant; Just because it’s dissolved in the water doesn’t mean you know what it is. We don’t know what those dissolved compounds actually are. TDS are dried at a low temp, no furnace. So, there is a possibility of other contaminants being there.

• On a scale of 1-10, how would you rate the importance of this analysis? (How impactful is TDS to decision-making?)

Around a 7, 10 being highest priority, because 90% of projects that come in have TDS and TSS together, to see what the total solids content is. TSS + TDS gives you total solids. Additionally, the higher the TDS is, the higher the conductivity of the water is, which poses some risks.

• Do you have any example(s) of the TDS analysis causing a tremendous headache? Example(s) of the opposite (being greatly helpful)?

One of biggest headaches is that everything must be dry, with absolutely no moisture in the leftover residual. In one sample, unknown interferences happened. There was an oily film, and the oil didn’t dissipate at the drying temp of 103-105 degrees. Conductivity causes boiling point to increase. You don’t want the product to boil over. The oily substance wouldn’t allow the water to cook off. Then when it went into the 180-degree oven, it caused problems. There’s no method guidance for handling situations like that. If not fully dry, that creates a bias.

Another issue with TDS is the vessel used to dry the liquid. If using pre-weighed bags, you have to consider the static of those bags because static will affect the weight. The bag must be run through a static guard. For something like drinking water, you want to be as accurate as possible, but static friction interacts with the elements and creates inconsistent weights, which causes problems with accuracy.

• Is there a program you feel this analysis is most importantly tied to? Remediation, construction/development, mining, ?

Quality programs – Drinking water is #1 because it has an MCL. Not everything has an MCL. There is also monitoring wells and wastewater treatment. Wastewater treatment is important because you want to prevent contamination of other (water) bodies, for instance treated wastewater is being used for irrigation/agriculture. 

TDS is also important for remediation. You don’t want high TDS because it will mix into soil and reduce crop yield. Don’t want high TDS in soil if used for agriculture. TDS is not run on soil, it’s run on water (like run off, or monitoring well, etc.).   Farmers pull the water from the ground. If the groundwater has high TDS which come from the wastewater plants discharging that, it’s a cycle. 

• Is there a preferred method of analysis?

SM 2540C is the preferred method.

• Are there resource bottlenecks for this method (SM 2540C)?

Yes. In a production laboratory, hundreds of samples are coming in. For NPDES programs, the majority of samples get BOD [biochemical oxygen demand], COD [chemical oxygen demand], TDS, etc. For TDS, there is a 7-day holding time. At the time the lab receives the samples, they try to run them; it takes roughly an hour to do 1-2 batches at a maximum of 40 samples per hour. That’s a lot of productivity allocated to that analysis. To do all the steps, it takes a good chunk of time. For some samples, you’re looking at historical trends to know if you should move forward with the results or if you need to do a reanalysis. Much of the time, the laboratory can’t do reanalysis within holding time, and that costs the client more as well.

• What is the typical cost for a TDS analysis?

Generally, on the cheaper side.

• How does the laboratory troubleshoot TDS issues?

1. Check the conductivity, because that’s a good way to back-calculate to figure out if your TDS is in line without re-running it;
2. Re-run it. You can re-weigh and run through static guard (if using bags);
3. Check historical trends.

Those are the three first go-to troubleshooting strategies for TDS issues. 

• Any other thoughts?

TDS is the most straight-forward of the gravitational analyses. You filter the sample, the filtrate is collected in a bag, you put it in 103°C oven for a few hours to reduce its volume so it doesn’t boil over, then you move it to a 180ºC oven. Then let it cool and weigh it.

Data Validation Consultant Perspective

We also asked Environmental Standards Associate Chemist Ammie Martin about TDS and how impactful it is in decision-making. Here is what Ammie had to say:

• What information does TDS analysis provide during a project?

Not sure on the global why do field consultants run it scale, but I have used it to determine if the laboratory knew what they were doing and if numbers made sense. I have used the calculation formula before to find errors in the reporting of the components of the calculation. I have also used the values to determine if the data reported made sense based on knowing the effect high TDS has on some analyses.

• How impactful is TDS to decision-making?

I would say that is very dependent on what you are doing. It provides an overall picture of water quality, so if you just want to generally know if the water quality is good, then it is very important. If you need to have information on what is in the water if you have TDS, then additional tests are required to get actual information.

• Do you have any example(s) of the analysis causing a tremendous headache? Example(s) of the opposite?

Yes, if TDS is too high you will have a difficult time getting a representative sample. They have to serial dilute and use a very low volume to meet the residue requirements, so this adds error. In addition, some laboratories just cannot accurately analyze TDS (filter seating issues, sample switches, unclean balance, exceeding max residue, overdiluting/wrong sample volumes, incorrect weights, not constant weight, wrong temperature, etc.). This can be seen when SM 1030E is used to calculate the TDS, and the calculation is compared to the gravimetric result. Alternatively, this calculation can help find errors in the component results, such as incorrect dilution factors.

• Is there a program you feel this analysis is most importantly tied to? Remediation, construction/development, mining, etc.?

Drinking water analysis, possibly NPDES or anywhere water might be released to the environment, possibly remediation or construction if near a waterbody that might be affected by the activities.

• Is there a preferred method of analysis?

NOT US EPA. The best option would be SM or maybe ASTM, although I do not have a lot of experience with ASTM.

• Any other thoughts?

It is deceptively simple, and a lot can go wrong with the analysis. People should, at a minimum (if the some or all of the other parameters are available), perform the calculation for a sanity check. If the test is important, they should have the data validated and laboratory audited (accredited does not automatically equate to good data). Sample splits or double-blind PE [performance evaluation] samples are also options if very important.

Key Takeaways

Understanding and measuring TDS and TSS is key to assessing water quality. To spot potential errors in TDS analyses and decide if data qualification is needed, data validators should check whether the laboratory took all necessary steps to fix any biases. It’s also important to use any available supplemental data, such as the correctness calculations from SM 1030E, along with historical data, to ensure clients get the right information. By following these standards, we can ensure safe water quality for drinking, agriculture, and industrial use, which helps protect public health and the environment.