Water Quality

[mack75]

Does anybody have any good information on water softening, specifically how much lime and soda ash is required.

I would appreciate anybodys help.

Thanks in advance.

 

[Casey]

I found this site a while back. MIT course notes available for download.

Check out lectures 8 and 9. They cover water softening... Not a whole lot, but it might get you going in the right direction.

http://ocw.mit.edu/OcwWeb/Civil-and-Enviro...Notes/index.htm

Also, testmasters had a set of environmental engineering notes available for download. It is no longer there, but if you want a copy let me know where I can send it... it is around 2 megs.

Regards,

Casey

 

[RIP - VTEnviro]

Does anybody have any good information on water softening, specifically how much lime and soda ash is required.
I would appreciate anybodys help.

Thanks in advance.

I recall it's a series of 4 chemical reactions you work your way through and sum up the lime and soda later. They explained it pretty well in my senior level water/ww class. I wrote it up in some personal notes I used for the exam.

They're at home but I'll try to scan and post that sheet when I got home. It's really straightforward.

 

[IlPadrino]

A week or so ago I took a stab at starting some Water Treatment notes. Maybe some can help flesh them out some more.

PE Notes - Water Treatment

 

[Guest]

A week or so ago I took a stab at starting some Water Treatment notes. Maybe some can help flesh them out some more.
PE Notes - Water Treatment

I have attached my own summary below. I will help with some fleshing as soon as I can make some time.

Is anyone interested in example types of problems? I can post a few from my AWWA Water Quality Treatment book.

JR

 

[mack75]

I have attached my own summary below. I will help with some fleshing as soon as I can make some time.


Is anyone interested in example types of problems? I can post a few from my AWWA Water Quality Treatment book.

JR

Could you please place some of your sample problems?

 

[Tido]

Could you please place some of your sample problems?

I think JR put an excellent page. That's the reaction you need to know. The idea is knowing what are the different hardness and how much soda and lime you need. Most importantly that you need the Co2 fulfilled before anything else. Here I have a question for you if you want to practice:

Q: Determine the chemical dosage for softening the following water to the practical limits.

C02-------9.6 mg/l

Ca2+----95.2mg/l

Mg2+----13.5mg/l

Na+------25.8mg/l

Alkaliknity-----198mg/l as CaC03

Cl----------67.8

S042+----------76.0mg/l

Best

 

[Guest]

Could you please place some of your sample problems?

From American Water Works Association (AWWA) - pg. 663

A groundwater was analyzed and found to have the following composition

pH = 7.0

CA2+ = 210 mg/L as CaCO3

Mg2+ = 15 mg/L as CaCO3

Alkalinity = 260 mg/L

Temperature = 10 C

Estimate the lime dose required to soften the water.

Potter, et al (1996)

1. A lime softening plant removes 135.0 mg/L as CaCO3 of calcium hardness and 29.9 mg/L as CaCO3 of magnesium hardness by the addition of 237.5 mg/L as CaCO3 of lime. The specific gravity of the softening solids is approximately 2.75 and the sludge is generated at about 10% solids. If the design flow of the plant is 0.10 m3/s and the sludge is dewatered to 60% solids, estimate the annual volume of sludge generated.

2. Determine whether or not the following water is stable (e.g. is it corrosive or will it deposit calcium).

Ca = 25.00 mg/L as CaCO3

Alkalinity = 40.00 mg/L as CaCO3

pH = 8.00

Temperature = 10 C

TDS = 80.00 mg/L

Answers provide after first response ....

JR

 

[Tido]

From American Water Works Association (AWWA) - pg. 663
A groundwater was analyzed and found to have the following composition

pH = 7.0

CA2+ = 210 mg/L as CaCO3

Mg2+ = 15 mg/L as CaCO3

Alkalinity = 260 mg/L

Temperature = 10 C

Estimate the lime dose required to soften the water.

Potter, et al (1996)

1. A lime softening plant removes 135.0 mg/L as CaCO3 of calcium hardness and 29.9 mg/L as CaCO3 of magnesium hardness by the addition of 237.5 mg/L as CaCO3 of lime. The specific gravity of the softening solids is approximately 2.75 and the sludge is generated at about 10% solids. If the design flow of the plant is 0.10 m3/s and the sludge is dewatered to 60% solids, estimate the annual volume of sludge generated.

2. Determine whether or not the following water is stable (e.g. is it corrosive or will it deposit calcium).

Ca = 25.00 mg/L as CaCO3

Alkalinity = 40.00 mg/L as CaCO3

pH = 8.00

Temperature = 10 C

TDS = 80.00 mg/L

Answers provide after first response ....

JR

I am going to try the first one

TH = 210 + 15 = 225mg/l

I am going to assume the alkalinity came from HC03, Neglet pH since it 7, otherwise it would matter

Since alkalinity is > TH, reset alkalinty to TH.

Therefore you need just the 225 mg/l lime as CaC03

Is that right?

 

[Guest]

I am going to try the first one
TH = 210 + 15 = 225mg/l

I am going to assume the alkalinity came from HC03, Neglet pH since it 7, otherwise it would matter

Since alkalinity is > TH, reset alkalinty to TH.

Therefore you need just the 225 mg/l lime as CaC03

Is that right?

Your calculation for for total hardness is correct; however, typically you don't need to consider magnesium removal if the concentration < 40 mg/L.

So, my answer for TH = 210 mg/L as CaCO3

Now, when considering the total addition of lime, you also need to consider the buffering capacity of your water since carbonate forms a diprotic acid (e.g. two stages of dissociation, two Ka values, or more on a chemistry level - two titration points). Therefore, you need to also estimate the carbonic acid concentration [H2CO3*] in terms of mg/L CaCO3.

Do you know how to do that?

Final answer: Lime dose for straight lime process = carbonic acid + calcium carbonate hardness = 155 mg/L as CaCO3 + 210 mg/L as CaCO3 = 365 mg/L as CaCO3

JR

 

[Tido]

Your calculation for for total hardness is correct; however, typically you don't need to consider magnesium removal if the concentration < 40 mg/L.
So, my answer for TH = 210 mg/L as CaCO3

Now, when considering the total addition of lime, you also need to consider the buffering capacity of your water since carbonate forms a diprotic acid (e.g. two stages of dissociation, two Ka values, or more on a chemistry level - two titration points). Therefore, you need to also estimate the carbonic acid concentration [H2CO3*] in terms of mg/L CaCO3.

Do you know how to do that?

Final answer: Lime dose for straight lime process = carbonic acid + calcium carbonate hardness = 155 mg/L as CaCO3 + 210 mg/L as CaCO3 = 365 mg/L as CaCO3

JR

Is the 155mg/l comes by setting the Langelier index zero?

Please advice. Thanks for your help.

 

[Guest]

Is the 155mg/l comes by setting the Langelier index zero?
Please advice. Thanks for your help.

You are on the right track, but this is how I break it down ...

1. At pH = 7.0, the alkalinity is primarily in the bicarbonate (HCO3-) form.

So, this means [HCO3-] = 260*(61/50)*(1/1000)*(1/61) = 5.2E-03 mol/L

2. Now we need to find the dissociation constant as a function of temperature (K):

Use equations,

K1 = 10^[(14.8435 - 3.404.71)/(T - 0.032786*T)]

K2 = 10^[(6.498 - 2909.39)/(T - 0.02379*T)]

K1 (10 C) = 3.47E-07 and K2 (10 C) = 3.1E-11

3. Now, we calculate the ionization fraction (alpha):

alpha1 = { [H+]/K1 + 1 + K2/[H+] } -1

alpha1 = 0.77

4. Not, we consider the TOTAL carbonic species concentration:

alpha1 = [HCO3-]/CT

CT = 6.75E-03 mol/L

5. So, going back to the mass balance equation CT = [H2CO3*] + [HCO3-] + [CO32-], we solve for carbonic acid:

[H2CO3*] = CT - [HCO3-] = 6.75E-03 - 5.2E-03 = 1.55E-03 mol/L

Which is equal to 155mg/L as CaCO3

Can you tell that I was the chem nerd in college??

Let me know if you need any part further explained.

JR

 

[Tido]

You are on the right track, but this is how I break it down ...
1. At pH = 7.0, the alkalinity is primarily in the bicarbonate (HCO3-) form.

So, this means [HCO3-] = 260*(61/50)*(1/1000)*(1/61) = 5.2E-03 mol/L

2. Now we need to find the dissociation constant as a function of temperature (K):

Use equations,

K1 = 10^[(14.8435 - 3.404.71)/(T - 0.032786*T)]

K2 = 10^[(6.498 - 2909.39)/(T - 0.02379*T)]

K1 (10 C) = 3.47E-07 and K2 (10 C) = 3.1E-11

3. Now, we calculate the ionization fraction (alpha):

alpha1 = { [H+]/K1 + 1 + K2/[H+] } -1

alpha1 = 0.77

4. Not, we consider the TOTAL carbonic species concentration:

alpha1 = [HCO3-]/CT

CT = 6.75E-03 mol/L

5. So, going back to the mass balance equation CT = [H2CO3*] + [HCO3-] + [CO32-], we solve for carbonic acid:

[H2CO3*] = CT - [HCO3-] = 6.75E-03 - 5.2E-03 = 1.55E-03 mol/L

Which is equal to 155mg/L as CaCO3

Can you tell that I was the chem nerd in college??

Let me know if you need any part further explained.

JR

Yes I can tell. You are the man, man!

thanks a bunch!