pH Reduction Advice

[dweiman]

Hi All,

I have a small (currently) private water system which consists of two wells which feed a concrete storage tank, which in turn feeds a series of pressurization tanks and the distribution system. The problem is that the water tends to be a bit corrosive (pH 5.6-6.0). I am getting high readings for copper and lead and think metals from the piping may be leaching into the system. The system has a peak demand of about 85gpm. I'd like to use some sort of liquid chemical feed to the storage tank for treatment. Any suggestions? Would it be of any use to dose the wells? Any advice would be greatly appreciated!

 

[semo]

The pH is only one factor of knowing whether or not the water is corrosive; but, with that low of a pH, I'm relatively sure that you are correct. The Langlier Index can help determine the corrosivity or scale forming of the water.

Now to your question. Lime and Caustic Soda (Sodium Hydroxide) are typically used to raise pH. Lime is typically cheaper in chemical costs; but, a lot more maintenance intensive. Lime is typically in a dry powder form and mixed with water to form a slurry. This is done with a mixer and tank (manually) or a lime slaker. Then it is fed into the water stream with a pump, injector, or by gravity. I've seen liquid metering pumps used for this; but, the lime has a tendency to fall out of suspension and scale onto the small feed lines and pumps thus clogging them up. Flow rates are vary erratic. Gravity with a slaker is the best option but more costly.

Caustic soda can be delivered in 55 gal. drums of 50% solution. It is then fed into the water stream. Liquid metering pumps such as LMI, Pulsitron, Prominent, W&T, etc. can be used to pump the fluid directly from the drum to the waterline.

If well flow is constant, controls can be simply modified with a relay to energize the pump/injector when the well comes on. This can be with small capital costs.

 

[hydrae]

Dweiman

For small systems, I suggest either soda ash (sodium carbonate) or baking soda (sodium bicarbonate). The products are easier to procure and have less handling difficulties. Both are powders and are available in 5 lb tubs or boxes to 100 lb bags. Both products are available NSF 60 but you may have to look for a particular brand. Dissolve the powder in water at a rate of no more than 1 lb per gallon and use liquid injection pumps as suggested by Semo.

As for dose rate, the simplest is trial and error, start at a low dose and increase the injection rate until the desired Langlier index is achieved. Or for a starting point, fill a 50 gallon drum with water and add teaspoons of product until the pH is at a good target, this will give you a rough dose rate that you then adjust by the speed and stroke of the pump or the concentration of the product

Do you have the total alkalinity of the water? Is the pH and alkalinity of each well the same? if not and the wells are operated individually the each well will require its own injection pump.
Caustic soda and lime will raise the pH with little effect on alkalinity.
Soda ash will raise both pH and alkalinity.
Baking soda will raise the pH less than soda ash but alkalinity more. It is also available at the local grocery store

Hydrae