What is the appropriate concentration for treating wastewater with a low-temperature evaporator?

For conventional low-temperature evaporators (non crystalline models, only for concentration reduction), the recommended TDS for inlet water is 5000mg/L to 100000mg/L (0.5% to 10%), with an optimal range of 1% to 5% (10000 to 50000mg/L). RO concentrated water, cutting fluid, and electroplating cleaning wastewater mostly fall within this range, with stable operation, low scaling, and moderate energy consumption.

1、 How to understand the concentration of incoming water

Less than 5000mg/L (0.5%) can also be treated, but it is not cost-effective. Low salt content, high water content ratio, evaporation of a large amount of clean water, high cost per unit of hazardous waste reduction, priority should be given to membrane reuse rather than direct low-temperature evaporation.

The optimal range is 1% to 5% (10000-50000mg/L) RO concentrate, machining cutting fluid, and ordinary electroplating cleaning waste liquid. This concentration is the most stable for operation, and the concentration ratio is generally 5-10 times. The quality of condensed water is good, and the risk of scaling is controllable.

5% to 10% (50000 to 100000 mg/L) can be processed, but to strictly control the concentration endpoint, conductivity/refractometer online monitoring must be installed to automatically discharge the concentration when it reaches the set concentration, in order to prevent pipeline blockage caused by supersaturation crystallization; Hard water and sulfate containing wastewater are more prone to scaling. It is recommended to use front-end pretreatment and scale inhibitors.

Ordinary low-temperature concentration evaporators with concentrations greater than 10% (>100000 mg/L) are not recommended to operate continuously for a long time as they are prone to crystallization and blockage of the heat exchange tubes; This high mother liquor requires a scraper low-temperature evaporation crystallization machine, which can concentrate to a salinity of 20% to 30% or even higher, and precipitate solid salts.

2、 Concentrate solution (discharge mother liquor) to control concentration

Conventional scraper free low-temperature evaporator: It is recommended to control the TDS of the concentrated mother liquor within 15% and avoid saturation to prevent crystallization from blocking the heat exchanger and concentration outlet.

Low temperature evaporation crystallization equipment with scraper/scraping wall: The mother liquor can be concentrated to 25% to 35%, directly precipitating salt residue and reducing hazardous waste more.

3、 COD concentration reference (organic waste liquid, cutting fluid/emulsion)

COD ≤ 80000mg/L is more stable;

COD is too high and contains a large amount of surfactants, which can easily cause foaming and material leakage, polluting the condensate water. The front-end needs to be demulsified, oil floating removed, and equipped with defoamers.

4、 Key points for selecting and operating to avoid pitfalls

It's not just about TDS, the ionic components are more critical: calcium and magnesium, sulfate ions, and phosphate ions are prone to form hard scales; If the chloride ion is high, 316L/titanium material should be selected for anti-corrosion.

For working conditions with large concentration fluctuations, it is recommended to set up buffer tanks to stabilize the inflow concentration and avoid sudden fluctuations in equipment load.

Prioritize conducting small-scale tests: For the same TDS, the solubility of different salts varies greatly. It is best to take samples for evaporation small-scale tests to determine the safe concentration endpoint.