Sulphates remouval problem

[BTJCM]

Hi all, I make a lot of test with different type of coagulants, and the conclusion from all the experiments is that the coagulant(aluminum oxychloride) which we are using right now doesn't remove very well SO4 from the waste water, when we are using a little bit more than usual(which happen 1-3 days in week) hydrosulfide. The other coagulant which I tested CFS(coagulant-flocculant-sorbent(PACL)) who have 10-11% Al2O3 or 20-23% w/w PACL, SG = 1.18 50% basicity and 10,5% w/w Cl pH = 6.8-7.2, remove better SO4, but is not so efficient removal for COD, BOD5 and sulphides. So my question is: can I mix them and acquire better results, or I need to search for other solution. I can increase a little bit the quantity of coagulant dosing from 0.5kg coagulant/m3 waste water to 0.6-0.65 kg/m3, that will slightly improve my situation but wont solve it. Aluminum oxychloride have 10-13% AL2О3, or 10-13% w/w, SG = 1.13 pH = 4-5. The pH of waste water before treatment is between 7.5-13.5. The flow rate of waste water is 63 l/min.

Thank you in advance.

Best Regards,
George

 

[ashtree]

Rather than mix them , you could try dosing both of them individually and together. It is difficult to say whether or not it will improve things substantially.
This method may not even be the best way to remove the COD, BOD or sulfates and i certainly cannot be certain about that without knowing all the details of the water quality.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Chemical removal of sulfate is not practical.

Please expand on what you are trying to accomplish.

 

[BTJCM]

Hi bimr, I'm trying to reduce sulfates, from waste water. They are around 400-535 mg/l , I want to reduce them below 370 mg/l. My limit for sulfates are 400 mg/l. That's what I want to accomplish. There is no biological treatment in our ETP, there is only aeration, coagulation and precipitation. In most cases I'm in the limits, but the values of sulfates are between 330-535 mg/l, but when we use a lot of hydrosulfite the value for sulfates is between 400-465 in most cases.
Some of indicators value from last testing: pH = 7.17, TSS = 6 mg/l, SO4 = 445 mg/l, Sulfides = 0.05 mg/l, BOD5 = 122 mg/l, COD = 857 mg/l.

Best Regards,
George

 

[ashtree]

Without having a full chemical analysis of the water you are treating its difficult to be able to recommend anything with any degree of certainty. Likewise with out all the information on your treatment process it is also difficult to make absolute recommendations. However a dose of lime (100mg/l) may allow you to form a Calcium sulfate precipitate which will remove some of the sulfates and get you below the limit you need. However you will have to be able to handle the higher pH and the extra sludge generated caused by the lime dose. This is likely to be an order of magnitude higher than what you have now. The higher pH may interfere with the coagulants being used and cause an effluent quality exceedance. You would also need to determine if the higher pH impacts on how the hydrosulfite reacts in and degardes in the water at the higher pH.

I would sugggest you do some jar testing and sampling before you rush into doing this.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[BTJCM]

Hi ashtree, when pH is higher more sulfides are formed, when is lower more sulfates are formed.

 

[springrise]

What is the source of alkalinity in your wastewater (pH up to 13.5 ..)?
Is the hydrosulfite the only source of sulfur in your wastewater?
How do you adjust pH of the wastewater?

Getting below 370 mg/L is tough, lime won't do it neither, because calcium sulfate solubility is still rather high. Messing with two coagulants is tricky, especially if your wastewater has variable quality over time (which probably is, given the pH range that you mentioned). Some options that you might have:
1. equalizing your wastewater: this can help if your "overall average" wastewater quality is below 400 mg/L sulfates
2. isolating the stream that contains high concentrations of sulfite/sulfate at the source, treating it separately (by precipitation of sulfates and separation of the sludge) and returning the resulting wastewater back to the "main" stream. You can even consider disposing via external company if the volume of the high-S stream is low. Knowledge of all contributing wastewater streams is required.
3. crazy idea: use barium salt

Also, be aware of the relationship between sulfides and sulfates (sulfides get oxidized to sulfates rather easily) and between sulfides and BOD/COD (sulfides increase the values, and for sulfites I'd expect the same behaviour).

 

[BTJCM]

Hi springrise, I will make detail explanation. I'm working in thread dyeing factory. The only think I can tell you about dyes is that they are organic, dispersive, and they are not classified as harmful, hazardous and so on. In the process of dying we are using NaOH(from where the high pH come from), we also using lubricant(from where we have petroleum products in the waste water), there are some dyes(not more than 2-3 dyes which we don't use very often which contain sulfur) but the main source for sulfides and sulfates is hydrosulfide. One of the chemicals that we don't use very often, but when we use it, it mess a lot with local limits values and that is SUNSOLT, when we use SUNSOLT, COD, BOD5, SO4, sulfides increase between 10-20%. We neutralize the pH environment with CO2(gas).Every day my waste water have different chemical composition, because it's depending on orders in production area(almost every day they are different). Sometimes we can use 1.9-2.3 kg hydrosulfide and 2-3 kg NaOH for day, but we have days when we use 18-19 kg hydrosulfide, and 16-18 kg NaOH.

Best Regards,
George

 

[bimr]

One would think that your problem would go away if you had a reliable pH control system.

Suggest you discard the carbon dioxide neutralization as it is not economical, is hard to control, and will generate excess solids.

Note that hydrogen sulfide gas is extremely dangerous and your pH control system should be failsafe to avoid acidic conditions.

 

[BTJCM]

Hi bimr, pH controller which we have control pH = 8. The controller regulate through electrode how much CO2 need to dose so we can have pH = 8 in our waste water. I can say CO2 neutralization its probably the most economical option that I have. Because the expense for CO2 for just 1 month is not more than 30-40 euro. The lowest pH that I measured was 6.58 and that was in emergency situation. I don't want to use dangerous chemicals in waste water treatment process, because one of our politic is to reduce the dangerous chemicals which we can use in the entire factory.


Best Regards,
George