# Water/wastewater problems posted by Shay

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I agree with Mooyepoo 100% that Shay's post is obviously a case of a student not reading the course material provided. The questions he posted are very basic water/wastewater treatment questions that don't require any sophisticated knowledge or any math beyond simple arithmetic. If Shay had read the course material he would have encountered the conversion factors that would have enabled him to work these problems. The thread is locked (appropriately), so I won't repeat the questions he posted.

I would suggest to Shay that he actually read the course material provided. Pay attention to such things as: 1 L=1000 ml, 1 ppm=1000 mg/L, 1 ppb=1000 ppm, ~0.43 psi=1 ft of H2O (conversely, ~2.31 ft of H2O=1 psi), a BOD bottle contains 300 ml, and the dilution factor (for BOD's) is the volume of the bottle divided by the volume of the sample. Many texts put definitions such as these in bold print.

There are some calculations used in water/wastewater treatment that might confuse a student and, thus, warrant a pointer or two to clarify an error that a student might be making. The questions that Shay posted do not fall into this category. They only require that the student to know arithmetic and a few definitions such as those given above.

Shay presented no work in his post for the simple reason that he didn't read the text and doesn't know the definitions of the terms used in the questions.

Chris

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How can 1 ppm = 1000mg/L if different compounds have different molecular weights and therefore different molar amounts?

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It is counter-intuitive give the name, but that is how it is measured.

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How can 1 ppm = 1000 mg/L if different compounds have different molecular weights and therefore different molar amounts?

I apologize for the error in my post : 1 ppm does not = 1000 mg/l, 1 ppm = 1 mg/L

Parts per million as used in water/wastewater treatment is a measurement of the concentration of a substance (in a solute, usually) by weight. Thus, one part per million is the same as 1 mg/1 million mg, 1mg/L (of water), 1 lb/ Mlb, etc. Calculations involving water/wastewater usually asume a specific gravity of one. Calculations involving sludges usually include the specific gravity of the sludge.

Chris

Edited to correct errors.

Edited by csmyth3025
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I agree Psycho - it sounds like rubbish; but with a little digging Chris and HyperV are shown to be completely correct

From the wikipage on parts per notation

Parts-per notation is often used describing dilute solutions in chemistry, for instance, the relative abundance of dissolved minerals or pollutants in water. The unit “1 ppm” can be used for a mass fraction if a water-borne pollutant is present at one-millionth of a gram per gram of sample solution.
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I was just wondering if this abnormality could have any real world effect or are the differences in number irrelevantly small for any real world application, or are levels of toxicity also measured in the same way to cancel out the effect?

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I think analytical chemists either just like being difficult or invented a new set of units to give themselves something to do that doesn't involve making up a heap of serial dilutions . Analytical chem is literally the only area I've ever had to use those types of calculations.

In terms of toxicity, there is data on toxicity levels for things like arsenic and whatnot in ppm, etc. It's also used to show concentrations of volatile/gaseous substances in the air. You'll often find things like the concentration at which we can detect smells, etc. expressed this way.

That's not to say that ppm, etc. is not completely daft. I really and truly think that it is, or is at the very less, incredibly deceptive. For some reason, chemists are afraid of having to look at numbers in which they have to use scientific notation. Don't ask me to explain it, because I really don't understand it myself. I'm hoping it's one of those grand conspiracies that will get bestowed upon me when I finish my PhD, but I won't be holding my breath.

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The parts power million units are not really that useful in my opinion either. mol/L, "molarity", is used for for the equations anyway to make things balance out with standard units and constants.

Maybe ppm is used because it is more comprehensible to the public, most of whom don't know what a mole is.

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The parts power million units are not really that useful in my opinion either. mol/L, "molarity", is used for for the equations anyway to make things balance out with standard units and constants.

Maybe ppm is used because it is more comprehensible to the public, most of whom don't know what a mole is.

As far as I know, the use of ppm concentrations is mainly restricted to calculations and parameters associated with process control and EPA specified limitations as they apply to water/wastewater treatment and, in most cases, levels of gases such as oxygen and hydrogen sulfide.

These units are generally used by technicians. They're not intended to be extremely precise because the monitoring equipment, sampling protocols, chemical dosage equipment and water/wastewater characteristics are imprecise and variable.

As an example, the solids in sludges generated by treatment processes is usually given in % solids (to the nearest %) rather than ppm because of the factors mentioned above. A 1% solids content (as a process control parameter) is equal to 10,000 ppm. This degree of uncertainty reflects the imprecise nature of the sampling techniques and the variability of the material being sampled. Nonetheless, this level of precision is sufficient to effectively control the processes to which they are applied.

Chris

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Oddly, nobody seems to complain about percentages. They get used a lot for strengths of drinks etc.

well, 1 ppm is 1% of 1% of 1%. There's no difference in principle (it's also 1‰ of 1‰ or one per mil of one per mil, but that's a bit obscure.)

If you are looking at , for example, the amount of lead in water to see if it's drinkable then moles don't really have anything to do with it. However if you divide the amount of lead that's tolerable by the volume of water you drink then you get an acceptable concentration. That concentration comes out in units of mass per volume or mass per mass. With water the numbers are the same for both. Unless there's toxicity data for LD50 in moles per Kg bodyweight somewhere then converting the concentration to moles would be pointless, you would have to convert it back again.

Also, for things like As. Pb and Cd there's relatively information the counter ion so the molecular mass is poorly defined. Is the lead in the water present as PbCl2 or PbSO4? If you don't know you can't assign a proper molar mass so you can't convert to moles anyway.

Similarly, if you tell a mining company that some ore body contains 1ppm of gold then they know it's 1 gram per tonne.

Why would they care about moles?

Moles are useful if you are doing reactions where the stoichiometry is important, but otherwise they don't help much.

It's no more a matter of precision of measurement or sampling than it would be for a percentage. If I buy a bottle of wine labelled as 13.2% I expect that to be an accurate measurement of the alcohol content. If they chose to write it as 132000 ppm the accuracy would be just the same.

The idea that these units are used by "technicians" is at best patronising.

There are plenty of legal standards where, for example, 10 ppm is legal and 10.1 is not.

There may be difficulties sampling and measuring to that precision, but it's not got anything to do with the units.

Incidentally, the other common usage of ppm is for gases where it refers to ppm by volume which is equivalent ( to a good approximation) to ppm by mole ratio.

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...The idea that these units [ppm] are used by "technicians" is at best patronising...

I didn't intend my comment to be patronizing towards technicians - especially since I am, myself, a technician.

There are those who deal with chemical equations who don't find units of ppm meaningful for their purposes. As you've pointed out, there are practical applications in regulatory definitions and industrial applications where ppm units are both meaningful and very practical for the calculations to which they're applied.

My reference to ppm units being used mainly by tehnicians refers to the fact that they're used mostly for technical applications rather than theoretical chemistry.

Chris

Good job!

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