Dear Sir,

I would suggest to start with a mass balance for both sulfate and chloride as to assess the quantities for each resulting from production and wastewater neutralisation. Depending on the mass balance results you may consider several options such as (1) to improve/extend wastewater equalisation as to achieve auto-neutralisation of acid and alkaline wastewater, and/or (2) to replace mineral acids (sulfuric, hydrochloric, ...) by CO2 or organic acid (formic, acetic), and/or (3) remove sulfates in wastewater biologically.

We applied all these wastewater options successfully in several cases.

Last-but-not-least, you may consider to change your sodium hypochloride (NaOCl) production from chemical to electrochemical resulting in very little saline wastewater.

Dear Hopewell Molefe

Please use as a method with lowest cost such as: Selective precipitation and filtration subsequently.

Kind Regards

There is another option-electric field enhanced membrane filtration. Prepare one membrane with negative surface charge charge. There are various routes of this preparation; like, mixed matrix membrane with some metallic nano-particle etc. Now addition of some hydrophilic pore forming material in the membrane matrix, will improve flux/ throughput. Then apply some electric field across the membrane while doing membrane filtration. This way you can achieve high rejection while maintaining high throughput. This process is not much energy intensive.

Suphates you can easily remove by dosing some lime ( Ca(OH)2)to it. Calculated quantity of CaCO3 also can be added if waste water is very acidic. Chloride removal has to be done membrane filtration only because precipitation technique will not work. More specific answers can be given if you let me know secondary parameters like pH, general chemical composition, the source of waste water tc

Dear Mr Peeters ( Forgive me if I didn't honor you well here)

I will definitely conduct the mass balance, however we are still working on having the lab equipment to test for chlorides and sulfates, we just know from the municipality (they take samples from the discharge tank) that we are exceeding the limit. Thereafter I will assess the feasibility of replacing mineral acids with CO2 or Organic Acids. I will also look into the biological treatment to reduce sulfates. Thank you very much 

Jagadeesh, I hope the following will give the general idea:

The plant is divided into 2 sections i.e. The Acid-Alkali and Solvents Section. The liquid waste is generated from these 2 sections and we have 3 Sumps collecting liquid waste from the solvent section, this waste is sent to the treatment sump, then to the discharge tank we generally don’t have trouble with it except that it gets oily at times.

Then there are 2 Sumps collecting the Acid-Alkali waste which are the most problematic sumps in the plant. These 2 sumps really have their parameters depending on the activity of the day: The PH fluctuates from very acidic to very basic.

Source of waste in general: From Acid Alkali Section we offload and load, pack, blend, wash vessels and working with the following Chemicals: NaOH, HCL, H2SO4 (98%), HNO3, NH3, H3O4P (phosphoric Acid), Formalin, Battery Acid, NaOCl, De-ionized water, Hydrogen Peroxide.

Solvents Section: UF Resin, Pthalates (DOP & DBP), Chlorinated Paraffin, M.E.G., LAWS, Thinners 200, Vacsol Concentrate, Xylene/SB70/Toluene, Benzene, Shellsol X7B, Alosol PI , Methanol , Thinsol 160 Acetone, Propylol, Ethyl Acetate.

The municipality require that the following water quality parameters be not violated with the liquid discharge from the plant:

1. PH range: 5.5-12.0

2. Conductivity should not exceed 500mS/m

3. Both Chlorides and Sulphates should not exceed 1500mg/L

4. COD: 5000mg/L

Identify the sources and levels of generation of chloride ion and sulphate ions in different waste water stream. Acidic waste water stream can be neutralized with alkaline stream and vice versa where ever possible thus reducing the consumption of HCl and H2SO4. Other option can be using some amount organic acids if possible

Satish. Thanks for confirming my earlier recommendations: mass balance (sources, loads, discharge) and auto-neutralisation by full equalisation. Using CO2 is more economic than organic acids and will not increase the COD (ref. limit).

Currently we are doing PH equalization, often times even combining our organic waste stream with the inorganic to try and regulate the PH. Is this practice advisable? but it works for the PH but not so very sure with other parameters (like COD), but it does reduce/increase the PH, thus reducing the amount of Acid/Caustic added, thus also avoiding unnecessary addition of Sodium ions, Chlorides and sulfates (from HCl/H2SO4 and NaOH). 

I would like to extend my appreciation to everyone who contributed in answering my question. However you are still welcome to bring any helpful ideas. Many Thanks

Hello Hopewell,

Thanks. I agree with your approach. We applied a similar solution for a bulk chemicals distributor some 20 years ago. This included an advanced activated sludge (AAS) biological treatment after full equalization as to meet the water quality requirements for discharge into the river. If the organics would be biodegradable then both the COD and sulfates of the mixed wastewater can be removed.

Dear Hopewell Molefe

no activated sludge plant can solve the problem of sulfates or chlorides. The chlorides precipitate very well only with silver nitrate (too expensive!) While the sulfates would be possible to make them fall well with barium hydroxide, although the resulting mud (barium sulfate) is a bit 'sticky. The problem should be solved before machining with targeted interventions to neutralize the liquid alkalini for example with the fumes of the boiler (verry chip!)

Dear Paolo,

I am sorry to respectfully disagree with you about the biological removal of sulfates. We apply this successfully since 30 years. I would suggest to learn more about the sulfur cycle. You can start via Google. Wishing you success!

Without filtration, there is nothing that can be done with chlorides. Sulfates only can be removed with barium or biological removal. However, both of these techniques have problems as the barium will only reduce it so much and you are still above most regulatory levels. The biological system can produce H2S as a product and you can find yourself with a very restrictive air permit if not careful. In all the systems we have looked at neither option would reduce the sulfate enough for our purposes (water recycle and permits) and the requirement of an air permit stopped the biological system. In all the cases, the extra paid to just discharge it was a much lower cost.

Thank you Henry, I'm still yet to do proper cost analysis and assess all the restrictions and permits with my local municipality here. However I believe that the suggestion (by Bruno) about doing the mass balance and see how are my Sulphates and Chlorides content before and after the neutralization, then I can carefully review the use of the mineral acids, they might possibly be making a significant contribution to the so high level of these ions and be efficient with my 'equalization' before any additional treatment. The biological treatment seems to prevail most of the time, as far the research I've conducted is concerned and it might best suit my position (treating for COD and Sulphates reduction)  

Hi Molefe

 I agree with Paolo Broglio,you can removal the Sulphates  with barium and then removal the Chloride with  silver nitrate very well.

Hello Vahid,

Thank you for your input, barium does work indeed. However to try and precipitate the chloride would be quiet expensive for me, we've a lot of quantity to process (200 000 Lt per day on average). A filtration process will need some bit of assessment but has potential and the alternative for the use of mineral acids for regulating PH. Thank you very much people

Muhammad,

Thanks for your suggestions. do you have some good publications on some of those technologies?