Maintain water pH level 3

[louvreChE]

My client is planning to utilize the existing distribution system to convey the newly treated water with pH level of 6 to 7. Since the water is treted with RO and pH adjustment, its mineral content, including calcium, is very low. The distribution system is made up of cement-lined iron pipes, and it's concerned that the aggresive water will cause the leaching of lime from the cement lining and expedite it's failure. In addition, due to the high organic content of the current water flowing to the distribution line,a significant biofilm covers the inside walls of the distribution system pipes, and that the introduction of the newly treated water which is low-organic type of water will result in the sloughing of large masses of this biofilm that delivers to the customer.

The problem is once the water reached the consumer the pH spikes to level of 10 & above. Will there be any solution to solve this problem before it reach the consumer? What chemicals should be added and at what doses if the new treated water is 116,000 m3/day?

 

[bimr]

As for the lack of minerals, what you need to do is to remineralize the water. You have 2 options. You can blend in some of the raw water with the RO product water, or you can renmineralized similiar to the manner used for desalination. That would be to add lime and CO2. The lime will add the calcium and the CO2 will be used to adjust the pH. The goal is to treat the water such the finished water has a slight tendency to scale rather than corrode.

You also should clean out and sterilize the distribution system.

 

[dalcazar]

How big is this system? Sterilizing a large distribution system might be a very big challenge, if the system can be shut down then a series of chemicals could be used. Sterilizing might be the wrong choice because the only surefire way to do that is to fill the lines with steam and keep it circulating, something that seems very hard with distribution lines since it will cool and condense. Sanitation is probably a better term to use, Iodine is a good chemical if used in the right concentration, Ozone is also a good choice and something that can be used in intervals later to keep the lines clean without compromizing water quality.

Less environmentally friendly alternatives would include chlorine gas, sodium hypoclorite and Sodium hydroxide. Sodium hydroxide is cheap, somewhat easy to use and the pH shock would kill just about anything as well as contribute to removing the biofilm. Neutralization can be done with citric acid, which is mostly non-toxic.

No matter what the case is, you're going to have to purge the biofilm from the line and it's guaranteed to clog valves, faucets and any other downstream obstructions. This is obvious I know, but I just want to point out that you'll probably have to tear down some parts of the line or open them up to get this to work.

My suggestion would be a run with a sanitizing agent, a pH shock treatment and then a complete purge.

what size lines are we talking about here? could they be cleaned with a roto-rooter sort of device?

 

[louvreChE]

Hi dalcazar & bimr,

thanks for the info above.

The pipe size is about 300-350 mm lines. How often do you think we have to sanitize it? Will there be any treatment process we can do for the newly treated water before it steps in the distribution system considering that we have a regular washing of the line? What is that roto-rooter device? Do you have any catalougue, supplier or details? How does it work?

Appreciate your answers.

 

[dalcazar]

Since you will be working with RO'ed water, the sanitizing should only be carried on for a limited period of time to cleanse the lines since all water that comes in later will be free of nutrients or microbiological contaminants.

The device I reffer to is a common water jet action cleaner for sewer and/or stormwater drainage lines, the size of your lines makes it possible to used such a machine to blow away the biofilm. I would recommend using the machine first to remove the biofilm, then a series of passes with sanitizing agents. You should try out the machine first on a short section of pipe to test integrity of the cement liner on the pipes, you don't want the high pressure water jets to blast away the cement lining as well, this will happen easily with coarse grained cement. here is a very good site for all those applications

http://www.waterjettingdirectory.com/chemical.htm

Ozone is not as good an alternative in cleaning pipes because it will get knocked out of solution rather quickly when covering a long distance in pipes or hoses, however it is probably the best agent because of it's very high oxidizing power and because it will not leave a detectable residue behind (it is self-neutralizing) unlike chlorine or iodine which might taint the quality of the water once the system enters operation. Iodine is however the broadest-spectrum sanitizer, usually eliminating 99.99% of all organisms.

Hydrogen peroxide is an alternative I neglected to mention earlier, it can be expensive, must be applied in the concentration it comes in from the factory and it must be left in place to act, but like ozone it leaves behind no residue and breaks down into innocous by-products, it works on the same oxidizing principle as ozone and cannot fall out of solution. Remember that H2O2 will foam very aggresively and that the reaction is exothermic, factors that must be taken into account, it may blow out explosively and presents a safety hazard.

If ozone or peroxide are used to flush the system after cleaning then it's very possible all you would need to do would be run the water thru the system to remove any dead contaminants, the water itself would need only remineralization as bimr said.

sanitation after cleaning the lines should only be done every few months, microbiological analysis of the water will determine if this needs to be done or not and thus you may not have to invest in either an ozone generator or sanitizing compounds, Ozone cannot be stockpiled and H2O2 will lose potency over time. The lines may keep clean for a long time.

maybe someone else can add anything I've forgotten.

hope this clarifies things more

 

[louvreChE]

Dalcazar, how about the ozone process of the distribution line how does it work? Do I need to insert somekind a UV inside the pipeline to eliminate the biofilm?

 

[quark]

Now a days, ozone generators don't require any extra chemical to generate ozone. They generate ozone from electrolysis of water and this is in line with FDA requirement. But you may have to put multiple stations in the distribution line as the half life of ozone is maximum 20 minutes. Ozone generators are very expensive. You can alternately go for NaOCl generators that produce NaOCl from common salt. UV systems will be too costly (both capital and operational costs) and the efficiencies never match with that of chemical disinfection systems. Formaline is a good disinfectant but lethal.

You can reduce biofilm formation if the line velocities exceed 1.5m/s. You should better do the pH correction before RO system and run the system at higher recovery rates to take care of mineral issue.

 

[louvreChE]

I would like to thank all of you for your advice. All has been focusing to the distribution line cleaning.

Is there any advice focusing on the water itself before stepping into the distribution system other than remineralization? Water content is already on the standard pH, is there anything we can do trigger the pH so as not to spike on the latter end?

 

[dalcazar]

maybe you can provide more details as to why the pH is spiking? I can't recommend anything until the cause is known.

the ozone generators of choice today are Corona Discharge units which are more efficient and productive than UV units. you would probably only need to rent the unit. it works by difusing ozone into the water, this water is then injected directly into the distribution lines, ozone is the most powerful oxidizing compound known to man and thus it will kill any microorganisms it comes in touch with

 

[bimr]

You should disinfect the distribution system with chlorine per the method recommended in AWWA C652. If the chlorination will not clean out the distribution system, then the next step is pig the lines. After cleaning, the system will need to be flushed.

You should also plan on installing a chlorination system to keep the problem from coming back.

The pH problem is caused by the corrosion attacking the concrete pipe. To keep the problem away, a chemical treatment program is necessary. Orthophosphate, sulfate, and chloride salts of zinc have been found to be successful in slowing the mechanism of attack on the pipe. Note that waters with a pH below 8 are particularly destructive to concrete or cement lined pipes. You should to talk to a chemical supplier such as Betz or Nalco to set up the proper chemical treatment program.

 

[louvreChE]

You guys are magnificent!

bimr just give me a very good picture of what I need to do with my prolems. Thanks for all of you. Hope I'll be as expert as you guys someday.

 

[dalcazar]

it's experience only

 

[Linzx]

You can analyze the Langelier Index of you raw and treated water. It'll tell you how "aggresive" your water is and how much to raise the pH to. You may also want to know the LIs at various points of your distribution system.