Working Principles of Wastewater Evaporators
Wastewater evaporators realize the separation of water and contaminants in wastewater through thermal-driven phase transition, converting liquid water into vapor while concentrating solid or dissolved pollutants for centralized treatment. The core process involves four key steps:
1. Wastewater Pretreatment
Raw wastewater is first subjected to filtration, pH adjustment, and scale inhibition treatment to remove large suspended solids, adjust the water quality to a suitable range, and prevent scaling or corrosion of the evaporator’s heat exchange surface. For high-salt wastewater, anti-scaling agents are added to avoid crystal deposition that could block pipelines and reduce heat transfer efficiency.
2. Heating and Vaporization
Pretreated wastewater is pumped into the evaporator’s heat exchange chamber, where it is heated by a heat source (e.g., steam, thermal oil, industrial waste heat) to a temperature above its boiling point under the operating pressure. Most industrial wastewater evaporators adopt vacuum operation technology, which lowers the boiling point of wastewater to 40–70℃, significantly reducing energy consumption and avoiding thermal decomposition of organic pollutants. During heating, water molecules vaporize into low-pressure vapor, while non-volatile contaminants (salts, heavy metals, organic sludge) remain in the liquid phase and gradually concentrate.
3. Vapor-liquid Separation
The mixture of vapor and concentrated wastewater enters the separation chamber, where vapor rises to the top under the action of gravity and centrifugal force, while the concentrated liquid (brine or sludge) settles at the bottom. A demister is installed in the separation chamber to capture tiny liquid droplets entrained in the vapor, ensuring the purity of the generated vapor and preventing contaminants from entering the subsequent condensation system.
4. Vapor Condensation and Concentrate Treatment
The purified vapor is introduced into a condenser, where it is cooled into distilled water (reclaimed water) with low salt content and low COD. This reclaimed water can be reused in industrial production processes (e.g., cooling water, washing water) or discharged after reaching environmental standards. The concentrated liquid at the bottom of the evaporator is treated according to its composition: high-salt concentrate can be further crystallized to recover usable salts, while hazardous waste concentrate is transferred to qualified disposal units for safe incineration or solidification.
Wastewater evaporators are classified into different types based on heat transfer mode, circulation structure, and energy utilization efficiency, with each type adapted to specific wastewater characteristics:
1. Forced Circulation Evaporators
Equipped with a high-power circulation pump, this type forces wastewater to flow through the heat exchange tube at a high speed (2–3 m/s). The high flow rate scours the tube wall continuously, preventing scaling and fouling, making it suitable for treating high-salt, high-viscosity, and easily crystallized wastewater (e.g., electroplating wastewater, chemical wastewater). Its advantages include stable operation and strong adaptability to complex water quality; the main limitation is relatively high energy consumption due to the circulation pump.
2. Falling-film Evaporators
Wastewater is evenly distributed at the top of the heat exchange tube bundle and flows downward along the tube wall in the form of a thin film. The thin liquid film has a large heat transfer area and short residence time, enabling high-efficiency heat exchange. Falling-film evaporators are ideal for treating low-to-medium viscosity wastewater with low scaling tendency (e.g., food processing wastewater, pharmaceutical wastewater). They feature low energy consumption and compact structure, but require strict pretreatment to avoid blockage of the liquid distributor.
3. Rising-film Evaporators
Wastewater is heated at the bottom of the heat exchange tube; the generated vapor drives the liquid to rise upward along the tube wall, forming a thin film. The co-current flow of vapor and liquid enhances heat transfer efficiency, making this type suitable for treating wastewater with moderate viscosity and high heat sensitivity (e.g., printing and dyeing wastewater containing organic dyes). Rising-film evaporators have a simple structure and easy maintenance, but are not suitable for high-salt wastewater prone to scaling.
4. Scraped Surface Evaporators
Equipped with rotating scraper blades inside the heat exchange cylinder, the blades continuously spread wastewater into a thin film on the heated wall and scrape off any adhering scale or crystals. This design allows the evaporator to handle high-viscosity, high-solid-content, and severely fouling wastewater (e.g., sludge-containing wastewater, polymer-containing chemical wastewater). Its advantages include ultra-short material residence time and strong anti-fouling ability; the main drawback is high manufacturing and operating costs due to the mechanical scraping system.
5. Multi-effect Evaporators (MED)
Composed of 2–6 evaporator bodies connected in series, the secondary vapor generated by the first effect (high-temperature and high-pressure) is used as the heat source for the next effect (low-temperature and low-pressure). This cascade utilization of vapor significantly reduces the consumption of raw steam, cutting energy costs by 30–70% compared to single-effect evaporators. Multi-effect evaporators are widely used in large-scale wastewater treatment projects (e.g., desalination of industrial wastewater, treatment of landfill leachate).
6. Mechanical Vapor Recompression Evaporators (MVR)
Equipped with a mechanical compressor that compresses the low-pressure secondary vapor generated by the evaporator to increase its temperature and pressure, then feeds it back to the heat exchange chamber as a heat source. MVR evaporators achieve self-sufficiency in heat without requiring external steam input, with energy consumption only 1/5–1/3 of that of single-effect evaporators. They are suitable for small-to-medium scale wastewater treatment projects with stable water quality (e.g., electronic manufacturing wastewater, pharmaceutical intermediate wastewater) and are recognized as an energy-saving and environmentally friendly wastewater treatment technology.
Environmental Applications of Wastewater Evaporators
Wastewater evaporators play a crucial role in resource recycling and pollution control, and are widely used in various industrial sectors with high wastewater discharge:
1. Chemical Industry
Chemical plants generate large volumes of high-salt, high-COD wastewater containing organic solvents, heavy metals, and toxic pollutants. Evaporators separate water from contaminants, producing reclaimed water for reuse in production and concentrating hazardous pollutants for centralized disposal. For example, multi-effect evaporators are used to treat wastewater from pesticide and fertilizer production, reducing wastewater discharge by over 80% and recovering usable salts (e.g., ammonium sulfate) from the concentrate.
2. Electroplating Industry
Electroplating wastewater is characterized by high concentrations of heavy metals (chromium, nickel, copper) and cyanide, which pose severe risks to the environment. Evaporators treat this wastewater by vaporizing water, concentrating heavy metal ions into a small-volume sludge, which is then sent for metal recovery or solidification. The reclaimed water can be reused as rinsing water in electroplating processes, realizing closed-loop water circulation and reducing the consumption of fresh water.
3. Food and Beverage Industry
Food processing wastewater contains high concentrations of organic matter (sugars, proteins, fats) and suspended solids, with high BOD and COD values. Falling-film evaporators are used to concentrate this wastewater, converting organic matter into a nutrient-rich sludge that can be used as animal feed additives or biogas fermentation raw materials. The reclaimed water meets the standards for industrial cooling and flushing, reducing the environmental impact of wastewater discharge.
4. Pharmaceutical Industry
Pharmaceutical wastewater is complex in composition, containing residual drugs, intermediates, and biological contaminants, with strict requirements for discharge standards. MVR evaporators are preferred for treating this wastewater due to their low energy consumption and short residence time, which avoid the degradation of active pharmaceutical ingredients and ensure the stability of the treatment process. The concentrated wastewater is incinerated to eliminate toxic substances, while the reclaimed water is reused in pharmaceutical production to comply with GMP standards.
5. Landfill Leachate Treatment
Landfill leachate is a highly polluting wastewater with high COD, BOD, and ammonia nitrogen concentrations, and variable water quality. Multi-effect evaporators combined with MVR technology are used to treat leachate, separating it into reclaimed water and concentrated sludge. The reclaimed water can be discharged after advanced treatment, while the concentrated sludge is returned to the landfill for disposal, effectively solving the problem of leachate pollution in landfills.
6. Textile and Printing and Dyeing Industry
Printing and dyeing wastewater contains a large amount of dyes, auxiliaries, and salts, with high chromaticity and COD. Rising-film evaporators treat this wastewater by vaporizing water, reducing the volume of wastewater and concentrating dyes for recovery. The reclaimed water is reused in the dyeing process, reducing water consumption and the discharge of colored wastewater, which helps textile enterprises meet strict environmental emission standards.
