CaCO3 Scaling in the Distribution System

Published on by in Technology

I have a problem with progressive encrustation or scaling of CaCO3 within a distribution system in an underdeveloped area in central Africa.

The raw water source is a relatively warm (~70 deg F, ~21°C), from a volcanic lake that is alkaline and highly mineralized. Raw water pH is about 11 and raw and finished water LSIs are about 1.3.

There is no centralised treatment. The only treatment is coarse screening at each raw water pump station and the addition of calcium hypochlorite for disinfection. Mixing and dosage of calcium hypochlorite are poorly controlled.

Can calcium from the calcium hypochlorite be binding with carbonates and bicarbonates from the lake water?

LSIs for both raw and finished water are high. How significant of a role does the CaCO3 play in scaling?

One option to consider is to switch to sodium hypochlorite. Could this solve the scaling problem?
Given the lack of centralised treatment and supply chain issue, pH treatment is not possible in the near future.

Taxonomy

8 Answers

  1. Dear Mr. Chip Ullstad

    What is the value of alkalinity & Hardness (Alkaline & Non alkaline). 1.3 LSI indicates that water has scale causing tendency. Use of Calcium Hypochlorite makes the condition favorable for scaling. Sodium hypochlorite will increase pH and may precipitate out calcium and magnesium.  Increase in pH will increase LSI. Gaseous chlorination will be better option. Reduction of pH with Hydrochloric acid will also reduce scaling. For further details you may contact me on sarafrv@virajenvirozing.com

    Prof. Rajendrakumar V Saraf

  2. Hi Ullstad

    You can calculate the SI (solubility index) to give you an idea of the precipitation potential. As you can see the pH has a huge impact on the precipitation potential. Without lowering the pH you will always have a problem. Just check the following source for more info:

     

    https://www.drink-water-eng-sci.net/6/115/2013/dwes-6-115-2013.pdf

     

  3. Having noticed that the flowing water has a high pH which almost decreases the disinfection efficacy of chlorine. The higher pH, the lower disinfection of chlorine due to presence of hypochlorite ions which are less disinfection efficiency than hypochlorous acid so, I assume that you are dosing high amount of calcium hypochlorite.

    I do recommend to reduce the pH of distributed water up to 7-7.5 then add disinfectants to minimize the biological growth on the inner wall of pipeline with taking in consideration the value of LSI, I am always keen to keep LSI in the range (+0.5 - +0.75) 

    As mentioned before there are different factors affecting on the LSI such water specification (Hardness, Alkalinity, TDS and water temperature as well.

  4. A non-toxic and biodegradable descaler chemical my company has developed and manufactured may be of interest to you. This may be more cost-effective than trying to balance the water chemistry or add additional inhibitors which in my experience have failed to successfully manage the problem.

    We have been supplying to Asia and Australia for the past 5 years to address this specific issue.

    I understand the lack of centralized treatment and supply chain is an issue. I have dealt with this in the past to remote locations in the South West Pacific Region and I am happy to work with to see if our solution is viable in your situation.

    Please email me at andrew@waterandoilsolutions.com.au

    1. By reducing pH

    Reducing pH means acidifying the water. This can be done by adding hydrochloric or sulphuric acid to the water to lower pH below 6.0 units. Acidifying the water aims to keeps scale forming calcium and magnesium ions in solution as the water moves through the pipeline. Be aware that reducing pH could cause corrosion in metal pipework.

    1. Sequestering agent.

    Adding sequestering agents (scale inhibitors) such as sodium hexametaphosphate to water may inhibit the precipitation of scale. They may also form a lining on the pipe wall to deter the deposition of scale. One such agent, sodium hexametaphosphate, is marketed under the trade name of ‘calgon’

    1. Pipeline operation

    To avoid sedimentation and Precipitation of scale forming compounds can be accelerated if there are constrictions in the pipeline. A change in the flow pattern can mean a change in pressure or velocity that will trigger precipitation in susceptible waters. You should ensure that unnecessary bends or constrictions such as a change in pipe diameter, are eliminated. A change to a pipe of larger diameter could be considered if flow velocities are high. Calcium carbonate may also precipitate out if there is a change in water temperature. In situations where the pipeline is near to or at ground surface, it can absorb sufficient heat to cause deposition. This is particularly so where flow is intermittent and long periods of no flow in the hot sunlight are likely.

  6. Thanks for your time helping with this!

  7. Concerning the water is for human consumption, the biggest problem is disinfection. The chlorine does not act as biocide at pH higher than 8.

    So, firstly you have to lower pH to 7,0-7,2 by HCl addition.

    This way also LSI will be much lower and CaCO3 scaling will diminuished. 

  8. The alkalinity of the water according to the literature is due to hydroxides, carbonates and bicarbonates (calcium, magnesium, sulfate, phosphates, etc.). What is probably co-occurring is the precipitation of the calcium, magnesium and sulfate carbonate present in the lake water, favored by the water temperature. It is important to conduct a water analysis of the lake, to have a better diagnosis of the water. The treatment of chlorination with calcium hypochlorite also favors this precipitation (fouling in the pipeline), which can cause the pipe to rupture and cause an accident.
     The alkalinity is mainly due to the presence of bicarbonates, carbonates and hydroxides. The most common compounds are as follows:
    - calcium or magnesium hydroxides;
    - calcium or magnesium carbonates;
    - bicarbonates of calcium or magnesium;
    - bicarbonates of sodium or potassium.
    Even waters with a pH of less than 7.0 (5.5 for example), can, and in general, have alkalinity, as it usually contains bicarbonates.
    Depending on the pH of the water, the following compounds can be found:
    - pH values ​​above 9,4: hydroxides and carbonates (caustic alkalinity);
    PH values ​​between 8.3 and 9.4: carbonates and bicarbonates;
    - pH values ​​between 4.4 and 8.3: only bicarbonates.

    Treatment processes - Suggestion:
    Ionic process of zeolites:

    Zeolites are complex silicates of sodium and aluminum, which has the property of changing the sodium of its composition by other ions, such as those of calcium and magnesium, retaining these elements that cause the hardness. They are, therefore, ion exchangers.

    A softening facility of this type comprises zeolite beds, similar to the quick filters, through which the hard water to be treated passes.

    In summary, the reduction of hardness by the zeolites consists of exchanging the calcium and magnesium ions, responsible for the hardness of a water, by sodium ions supplied by the exchangers. After the zeolites have yielded all their sodium ions to the water, the process must be reversed by subjecting the bed of exchangers to contact with a concentrated solution of common salt for regeneration.

    In contact with the brine, the zeolites make a new ionic exchange, retaining the sodium again and releasing the calcium and magnesium ions in the washing water, which is discarded.

     

    Process with Ion Exchange Resin:

    The reduction of the hardness is effected through the passage of water through a bed of cationic resin, in a process of slowing down and / or demineralization.
    As this process is described in another technical document (slowdown), it will not be detailed here.

    1 Comment

    1. Thanks for your time helping with this!