Alkalinity Increase

[ozfish]

I have ground water high in iron and low in Calcium and Mangnesium.

Iron approx 47mg/l
Ca approx 2mg/l
Mg approx 7mg/l

I presently use NaOH to raise the pH to precipitate the iron prior to filtering.

The resulting water is poorly buffered and used for aquaculture where hardness should be in the order of 100ppm.

I would like to increase the calcium levels and the hardness.

Any advice - ideally particular solutions that eliminate the NaOH in the process would be great.

Thank you

 

[bimr]

The use of NaOH is not going to work for you.

You need to remove the iron with conventional techniques. If there is no oxygen present, you can use a water softener because the iron is still soluble.

As for the lack of minerals, what you need to do is to remineralize the water 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.

 

[ivanhoe374]

Calcite filter perhaps? (for reminerilization)

What is the pH of the water before adding NaOH? if it is say above 5 all you may need to do is aerate to precipitate out the iron (would be much cheaper then dosing caustic i would guess!)

 

[ozfish]

pH before adding NaOH is 5.4
Aeration takes the pH up to around 6.4 max and then begins to drop as the iron oxide begins to precipitate. Calcite filter may work however I am concerned that the calcite may become clogged or establish a film after several weeks which will prevent it from working. - Do you know of any calcite filter designs?

Thanks

 

[ivanhoe374]

We recently designed a gravity flow calcite filter in a concrete tank. They can also be done with a standard FRP pressure filter as well. Can you not backwash off the "film" that you are worried about?

 

[bimr]

The iron concentration if correct, is probably too high for a calcite filter. If you are using this water only for agriculture, there may be no need to remove the iron.

The calcite filter is also probably not going to work unless there is CO2 acidity present. Another name for a calcite filter is a neutralizing filter. Calcite filters will raise the pH to only 7.3 or 7.3 which may not be adequate for your application. Calcite filters work by reacting with the CO2 and carbonate alkalinties and forming bicarbonate alkalinity.

Do you have a complete water analysis including the sodium and alkalinity? And what is the flow rate of the application? Also, what are the water specifications for your application?

CO2 + CaCO3 + H20 to Ca(HCO3)2

 

[ozfish]

Bimr thanks for your input.
The water is used for aquaculture - growing warm water finfish, hence all iron manganese must be removed.

Iron level has been confirmed by a lab.

I will send full water analysis later today.

I beleive the supply water has significant CO2. Taking the pH behond 7 would be ideal

 

[ozfish]

Hi BIMR
I have lab results.

Ca total = 2.4mg/l
Magnesium total = 22
Iron total = 22mg/l
Manganese total = 2.1
Alkalinity = total as CaCO3 = 25mg/l
Hardness Calcium as CaCO3 = 29mg/l
Hardness Magnesium = 64mg/l
Hardness Total = 93mg/l

I have also had some testing done on the fish being grown in the water and they are low in blood levels. I need to increase calcium levels in intake water. The hardness can increase safely by 100ppm.

I was thinking of using CaCl but in lieu of the Calcite reactor and the NaOH.

Have you used CaCl before and how does it compare price wise?

Regards
Ozfish

 

[bimr]

Calcium chloride can be used. It is inexpensive and will provide the calcium that you desire.

However, you will still need to add an alkali. If you want to oxidize the iron, you need to take the pH up to at least a pH of 7.5. You can do this with lime, soda ash, or caustic. The lime will provide calcium too, but of course is more difficult to handle. Soda ash may be your best option since it is easier to handle and safer, plus it will add alkalinity (and buffering) to the water. Additional alkalinty will cause the pH to be more stable in the pond.