Comparison of Evaporation Crystallization Processes for Ammonia Nitrogen Wastewater

When using evaporation crystallization for high-ammonia nitrogen wastewater, the mainstream processes can be categorized into three types: multi-effect evaporation (MEE), mechanical vapor recompression (MVR), and low-temperature vacuum evaporation. All three can convert ammonium salts from the liquid phase to the solid phase, but they differ significantly in energy consumption, steam dependence, ammonia escape control, investment intensity, and operational flexibility.

1.Multi-effect Evaporation Crystallization for Ammonia Nitrogen Wastewater

MEE relies on fresh steam for staged heat transfer, producing 0.25-0.35 tons of steam per ton of water, resulting in the highest energy consumption. The system is simple and has good tolerance to water quality fluctuations, making it suitable for scenarios with self-contained power plants and high electricity prices. However, high-temperature evaporation (90-110℃) causes a large amount of free ammonia to escape, requiring a multi-stage acid scrubbing tower; otherwise, the ammonia concentration in the tail gas is prone to exceeding standards. Simultaneously, the heat exchange surfaces are prone to scaling, requiring a cleaning cycle of typically 7-10 days, resulting in a higher maintenance frequency than the other two processes.

2.MVR Evaporation and Crystallization of Ammonia Nitrogen Wastewater

MVR uses a compressor to heat secondary steam to 18-25℃ for reuse. It consumes only 25-35 kWh of electricity per ton of water, requiring almost no continuous steam replenishment, resulting in operating costs 20-30% lower than quadruple-effect evaporation. Evaporation temperatures can be as low as 70℃, reducing ammonia partial pressure by an order of magnitude, and simultaneously reducing exhaust gas volume and scrubbing load. It boasts a high degree of automation, enabling one-button start/stop via frequency converters and software monitoring. The disadvantage is that the compressor has strict requirements for the cleanliness of the secondary steam. If the wastewater has high COD and strong foaming properties, defoamer must be added and online cleaning must be implemented; otherwise, the impeller is prone to coking, increasing maintenance costs.

3.Low-Temperature Evaporation and Crystallization of Ammonia Nitrogen Wastewater

Low-temperature vacuum evaporation operates at 37-45℃, with minimal impact from boiling point rise, making it particularly suitable for high-COD, high-viscosity, and easily coking ammonia nitrogen mother liquor. The nearly 100℃ heat transfer temperature difference allows the use of low-grade hot water or waste heat from the plant as a heat source, with power consumption only 30-40% of that of MVR. Due to the low temperature, ammonia escape is negligible, resulting in a longer equipment lifespan. However, the saturated vapor pressure is low at low temperatures, leading to low evaporation intensity. A single unit's processing capacity is typically ≤3 t·h⁻¹, requiring multiple units in parallel for scale-up. Furthermore, the vacuum system is complex, with a one-time investment comparable to MVR, making it more suitable for small-volume, high-value-added ammonium salt recovery or deep mother liquor reduction scenarios.

In summary, for projects with a water volume ≥5 t·h⁻¹, moderate electricity prices, short process requirements, and small footprint, MVR is the preferred choice. For projects with inexpensive waste heat, a water volume <3 t·h⁻¹, high COD, and less stringent requirements for crystal color, low-temperature vacuum evaporation is more economical. If there is a large surplus of low-pressure steam on-site and expanding the electrical load is difficult, multi-effect evaporation can be considered, but additional space must be reserved for ammonia washing and frequent cleaning. In practical engineering, MVR is often connected in series with a low-temperature dryer to form a "main concentration + terminal drying" combination, which not only retains the low energy consumption advantage of MVR, but also utilizes the low temperature section to completely solidify the mother liquor, achieving near-zero discharge of ammonia nitrogen wastewater.