Alkalinity problem for efficient coagulation

[Nesse]

Hello,

I am working on a drinking water treatment project design which takes place in an African country.

The chosen surface water (and only available) is soft and very low in mineral content (very low alkalinity). Alkalinity is not enough for the coagulant additiv to be effective (generaly Aluminum sulfate). By working with the "Hallopeau & Dubin carbonic graph", I realize adding soda ash (Na2CO3) or lime would not solve the problem as we quickly arrive on the vertical part of the curves where pH rises but no more the alkalinity.

If I do the calculation properly (using the graph), adding soda ash or lime would allow me to use effectivly 25 mg/l aluminum sulfate. All additional aluminum sulfate would not have alkalinity to consume. I know by experience that in rainy season, I could need to use until 80 to 100 mg/l aluminum sulfate to reach an effective coagulation, so 25 mg/l is not enough.

All the ferric coagulants consume also alkalinity so changing coagulant additiv to a ferric coagulant does not seem to me a good solution.

Another alternative is to add carbonic gaz CO2 jointly with lime or soda ash, but this requires:

- supply of carbonic gaz; in deep african part, probably difficult;
- refregirated storage of carbonic gaz; in an environment where there is often power outage, risky;
- highly qualified operators which lack in that area;

Therefore it seems to me unappropriate to implement such process considering the above difficulties and risks.

Finally, I read it was also possible to mineralize water through slow carbonate dissolution using marine limestone but it seems this process is adapted to small treatment plants (500 m3/d maximum) whereas the expected treatment plant capacity in this project is 11 000 m3/d.

I did not find an appropriate solution to solve the very low alkalinity problem of this water. I would really appreciate if someone could help me on this !

Here below are some caracteristics of the water measured end of february which corresponds to the end of the dry season:

- Water temperature: 25°C
- pH: 6
- Complete alcalimetric title (CAT): 0.2 °F and Calcium title (TCa): 0.4°F
- Turbidity: 20 NTU
- Dissolved suspended solids: 16 mg/l
- Color: 115 (Pt)
- Permanganate oxidability: 8.6 mg/l
- Total iron: 0.85 mg/l

Thank you very much in advance for your help !

Arnaud

 

[bimr]

You are correct, you need to increase the pH. You can use soda ash, sal soda, caustic soda, or lime to accomplish this. Carbon Dioxide will lower the pH and should not be used.

 

[BurnettSolutions]

Arnaud,

CO2 and Lime will complement each other to add alkalinity, improving the alkalinity around 40%. How many parts of alkalinity do you need to add? My company actually owns the patent on that process, US Patent No. 8,951,419 B2. Feel free to contact me at cburnett@burnett-inc.com for any specific questions you may have.
Could you put a generator at the plant in case power goes out? The CO2 unit only runs a couple hours a day. You could still possibly pull it off. Let me know.

 

[bimr]

Carbon dioxide is a weak acid and one fails to understand how adding an acid will cause the water to become more alkaline, thus raising the pH. Carbon dioxide is generally used to lower the pH.

If you look at the "Hallopeau & Dubin carbonic graph", one can see that the problem is caused by too much carbon dioxide. The solution is to either air strip the carbon dioxide or eliminate the carbon dioxide by reacting it with an alkaline material. The pH increases with either solution.

As to the OP's problem, the solution is to add enough alkalinity to raise the pH to a level that allows good coagulation and also to add enough alkalinity to mitigate the alkalinity loss caused by the coagulant.

The "Hallopeau & Dubin carbonic graph" is not really appropriate for what you are trying to accomplish.

If you look at the alkalinity equilibrium graph, you can see that if you add enough alkalinity so that the total alkalinity is 80% bicarbonate and 20% carbon dioxide, you would have a pH of 7, regardless of how much alkalinity is present.

Add any alkalinity that is economical, but do not add any more carbon dioxide. You already have too much.

 

[ScotCarl]

Have you considered using bicarb soda (plain old baking powder). This has a similar effect to soda ash in raising the alkalinity of the water, but with less effect on the pH.

 

[IRstuff]

Claim 1 of the cited patent says that adding CO2 and lime lower the pH

http://www.freepatentsonline.com/8951419.html

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[bimr]

Yes, Claim 17 also says it lowers and raises the pH.

"Claim 17. A method for treatment of water, comprising the steps of: lowering the pH of the water, increasing the pH, hardness, alkalinity, and turbidity of the water after said step of lowering the pH by adding calcium hydroxide to the water; and; decreasing the turbidity of the water after said step of increasing, said step of decreasing comprising mechanical agitation of the water with at least one impeller rotated at a speed of rotation sufficient to decrease the turbidity of the water, said decreasing step occurring without significantly affecting the pH, hardness, or alkalinity of the water."

The fact of the matter is no one would ever make use of the patent for a number of reasons.

1. Carbon dioxide equipment is very expensive and it is difficult to dissolve the carbon dioxide in water. Specialized equipment is necessary to dissolve and feed carbon dioxide. The majority of the carbon dioxide users are the beverage bottlers who need the fizz in the soda, not water treatment plants.

2. Lime handling equipment is also very expensive and lime is also difficult to dissolve in water.

3. Carbon dioxide and lime react to produce a precipitate. I can not think of a single application that would do this.

http://water.me.vccs.edu/exam_prep/limesodaash.htm

4. Hydrated lime has a purity of 93% meaning that there are a substantial inerts presents that will foul the water with suspended matter and turbidity.

http://www.atlanticpetro.com/index.php?option=com_content&view=article&id=85&Itemid=520

5. Carbon dioxide is not used to increase alkalinity; see the bottom of page 55.

[

https://books.google.com/books?id=V...lkalinity water treatment coagulation&f=false