How to treat Wastewater with High salt concentration

Waste water with high salt concentration means the waste water contain organic matter and at least 3.5% (mass concentration) of total dissolved solids (TDS). Normally source from seawater desalination system and other chemical industries. With the rapid increase of economy, the discharge of wastewater with high salt concentration increase, thus here list several treatment methods for it.

I. Dosing coagulation-traditional pretreatment process of air floatation and precipitation

When the COD concentration of the salty wastewater is below 5000mg/L, and there is no requirement for the quality of the crystalline salt, the traditional process is to pretreat the wastewater through doing, air floatation and precipitation, then entering the evaporation concentrated crystal desalination system. This method require relatively lower investment and operating cost, but the crystalline salt quality is poor so hard to sell.

II. Fenton or electro-Fenton catalytic oxidation pretreatment process

Fenton reagent contains H2O2 and Fe2+, which has strong oxidizing ability to organic pollutants in wastewater, and the reaction speed is fast while investment relatively low, after precipitation purification the product water could be like water after pre-treatment.

But it has requirements for wastewater: pH value 2-4 and produce sludge containing more iron. When the pH value of waste water lower the system will be more cost-effective, or add chemicals will increase operation cost. If COD concentration is about 10000 mg/L this method will be okay, if much higher which require multi-stage oxidation purification treatment, this Fenton process will lose advantage.

III. Double membrane pretreatment process

Firstly to use a semipermeable membrane with a pore size of 20-2000Ao (10-6.5-10-4.5cm) for ultrafiltration to remove colloidal substances and macromolecular substances such as proteins, various enzymes and bacteria. As the permeated water capacity decrease which the salt content has not changed, so the salt concentration increase. At this time use a semipermeable membrane with a pore size of 1-20Ao (10-7.5-10-6.5cm) to remove inorganic salts, sugars, amino acids, BOD, COD, etc. Then the salt concentration increase again, then the final water can be used to evaporate and crystalize for removing salt.

The advantage of this method is to decrease the wastewater capacity need to be treated by evaporate and crystalize for removing salt. But the membrane need to be cleaned and replaced so the operation cost is high and the concentration water with higher pollution concentration will be accumulated.

For salt concentration higher than 5000mg/L waste water can be used to evaporate and crystalize directly but pre-treatment before that is necessary.

IV. Ozone/catalysis/coagulation composite pretreatment process

Ozone is used as a strong oxidant in combination with a catalyst and a coagulant to perform a sufficient cross-linking synergistic reaction in a specific environment, which can break the cyclic and long chains in the wastewater and improve the biodegradability of the wastewater. Create the suitable reaction condition can fully oxidize the dissolved organic pollutants in the wastewater, destroy the colloids, chromophores, and odors in the wastewater, and remove COD, BOD, SS, odor, and some colors in the wastewater, but can not remove salt and most ammonia nitrogen. This treatment method produce relatively less sludge so it is much simpler than the Fenton process, it can operate by multi-stage series to better ensure product water quality. For the wastewater pH value>5 with high COD concentration, and salt concentration>5000mg/L can choose this treatment method. If the salted wastewater has high chroma or high ammonia nitrogen, it must be decolorized and deaminated separately.

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