How Can I Reduce Sulphates and Conductivity in Effluent

Welcome to the forum and congratulations on posting a question with relative information included.

As you describe, you really have two issues that may be interacting. TDS and conductivity.

My first question for clarification relates to the TDS. (as opposed to TSS) Do you know what the major constituents are?

I suspect that you need to remove adequate ions from solution to reach your final goal and the sulphates would seem a good target, since you know they are high and always present.

You might investigate the "iron" ions to bind the sulphate to precipitate, or "milk of Magnesia". Both these are used in normal sewer systems to balance pH and reduce sulphur reactions during movement of wastewater through the underground pipe networks.

If you have TSS, then the first suggestion is to remove as much as possible since these may be buffering the benefit of your sodium hydroxide and other treatments.

Ultimately, you need to get enough ions out of solution to get your conductivity down. Precipitation is well understood and you should be able to get local help by contacting the local sewer treatment facility.

From what I understand, you currently have a solution of Sulfuric Acid (H2SO4) and your end goal is to reduce conductivity / dissolved solids.

Current conductivity 10,000 mS/m = 100,000 uS/cm, and need to reduce down to approx 4% of current conductivity (remove 96% of the condy).

The problem as I see it is you are adding dissolved solids, expecting to be able to reduce the level of dissolved solids. Do you see the problem?

Sure some of the dissolved solids may drop out, as Engineer suggested the sulfates are readily insoluble. But you're adding something else at the same time (this defeats the purpose).

Your base solution is H2SO4 in water. This is the sourse of the Sulfates, but I'm assuming the water wasn't pure to start with, so likely some hardness (Mg & Ca), some alkalinity, and some anoins (possibly SO4) present also conttributing to your conductivity.

Cations include ...

H+ (as the water is acidic)

Mg++

Ca++

few others (small concentrations)

Anoins include ...

SO4 (and lots of it)

few others (small concentrations)

My advice, try using an ion exchange column. You need to use Anoin exchange resin. The resin is changed with Sodium Hydroxide, when charged only the OH- ions are held. When you pass your solution over the column, the hydroxide ions transfer and are displaced by heavier charged molecules (in this case SO4--).

This will have two impacts. you will reduce the SO4 (and other anion content), and increase the pH without adding additional ions into the solution.

But (the downside), there is alot of waste with this process. As the charging solutions, and the startup and end solutions are all put to waste. So this may not be the silver bullet, but it is the only way I can think of to achieve your goal.

Good luck.

Anthony

Have you considered using barium hydroxide to neutralise the sulphuric acid? It has reasonable solubility in water and has the advantage that barium sulphate is highly insoluble in water, hence drastically reducing the TDS.

Options:

1. Raise the temperature of the effluent before the final settlement and filtration steps. CaSO₄ has a negative solubility characteristic and will precipitate more readily at elevated temperatures.
2. Blend the final effluent stream with another with much lower salt levels before final discharge.

3. Dilute the effluent stream with water from another source.

4. Eliminate the sodium hydroxide. Sodium sulphate is highly soluble, contributing to both TDS and conductivity in the final effluent, and the lime will do the job instead producing an insoluble precipitate that can be easily separated before final discharge.

Hi, Thanks for all the info. I have not thought if barium sulphate, will def give that one a go. Unfortunately dilution is not the answer here :(, Yes an ion exchange may help, I was also thinking about a demin plant. There is no straight solution to this problem. Yes, the acid itself does contribute to the sulphates, and the water definitely has some ions in it. Will keep researching.