Dear members,
 I want to know whether any relationship exists between the total hardness and Total dissolved solids(TDS) of an effluent. In an electroplating industry, the effluent charcteristics shows that Total hardness(3500ppm) and TDS(2400ppm).Hardness>TDS.Is it possible? Turbidity is 28NTU, pH(5.7- 6), Chloride(350ppm).Is there any thumb rule for TDS & hardness relationship to comment an effluent.
Plz let me know that.Thanking you.
senthil

senthil,

You didn't specify how you were measuring these parameters. If you are estimating TDS by conductivity, the results could be skewed by the distribution of ions in the water. There is no way that hardness could be greater than TDS because "hardness" is part of the TDS. Hardness is estimated by titrating a sample with EDTA and is an approxmation of the calcium and magnesium ions in solution. My guess is that your sample contains water softener regenerant discharge which contains higher concentrations of calcium and magnesium ions and lower conductivity than a sodium chlotide solution.

The other option is to see how your testing procedures measure standard, blind and/or duplicate samples.

grrun    

I strongly regret to be in contradiction with Grrun.
In principle it is possible to have higher total hardness then total dissolved solids.
An example
when you have 1 mol of magnesium sulfide in water TDS is 56 g/l, total hardness (expressed as CaCO3) is 100 g/l.

Kind regards
T. Sobisch

TDS vs Hardness:

I think most of you have at least one point right.  Typical domestic wastewater TDS is normally higher than hardness.  Total dissolved solids is a measure of ionic (dissolved sugars, salts...) compounds present in solution.  At 180­°C oven, the water is evaporated and a residue is left behind which represents these dissolved compounds (CaCO3 is part of the TDS).  As Senthil, has mentioned.  We are dealing with an electroplating industry that has chloride of about 350 ppm.  If anyone takes the time to read Standard Methods, chloride can and will have a interference effect on the method.  Need to find out if any recirculated water is being used in the process and if constant weight is being attained in the TDS method.  At such high TDS concentrations, constant weight is a must specially if the residue is hygroscopic.  Standard Methods also mention that if the sample is "...high in bicarbonate it (sic) requires careful and prolonged drying at 180°C to insure complete conversion of bicarbonate to carbonate."

Since it is an electroplating industry, there are metals present in the sample which may also contribute to significant interference in the hardness analysis.by creating an indistinct end point.  The prefered method to determine hardness is by calculation:

Hardness, mg equivalent CaCO3/L=2.497 [Ca, mg/L] + 4.118 [Mg, mg/L]

Lastly, it is recommended that the dried residue that is yielded during the TDS process be between 2.5 to 200 mg. That is, use a sample volume that will yield between these two weights.  If interference is likely the culprit, why not spike the sample in the hardness and TDS samples?  This is nothing more than QA/QC that should be done in the normal course of laboratory procedures.

Just my two cents worth of comments.

Let me further explain; hardness is not an actual chemical compound. What is measured is the quantity of sodium EDTA required to complex the positive ions (calcium, magnesium, iron, nickel, etc.) which tend to form insoluble precipitates with the old fashioned soaps of years gone by, by rendering them ineffective for suspending other soils in water. To make the results of the titration meaningful, the results were converted to the most common source of water "hardness", calcium carbonate. Therefore, the test only measures the "hardness" that an equivalent amount of calcium carbonate would produce. The point that I'm trying to make is that "hardness" is not a chemical compound in the water. Calcium hydroxide would have a hardness to TDS ratio of 100:74. While having a water with hardness greater than TDS is not normal, it is not impossible because "hardness" is not a chemical compound, only a chemical characteristic of the water.

Flame photometry could be used to establish the concentration of calcium and magnesium in the water to demonstrate that these ions are not in concentrations greater than the TDS. Anyway, I hope that I have added some clarity to this discussion,

grrun    

Folks:

Total dissolved solids can be measured by conductivity utilizing the proper equipment (meter and conductivity cell).  Though this is not an approved method as specified by Standard Methods, it is more of a rule of thumb procedure to approximate a value!  As I stated above, most of you have made excellent points but we need to stick to the intial question.  Senthil has asked if it is possible to have hardness>TDS.  Sobisch has offered his educated guess as to a possible situation where this might occur.  His example is a excellent situation.  We all have expertises to offer in this forum and I personally can vouch for everyone that posts in this Forum just by reading through past postings. Please, keep it professional, we all can see what you comment.