High-Salinity Wastewater: Distillation Desalination Versus Biological Treatment

Salt-loaded industrial streams are rarely a single problem. Besides dissolved salts they carry suspended solids, organic matter, heavy metals, residual nutrients and traces of process chemicals, and that combination is precisely why high-salinity wastewater resists simple treatment: every unit operation has to survive a corrosive, dense and biologically hostile medium. The main sources are chemical plants and the extraction and processing of oil and natural gas.

Practitioners choose between a thermal route, distillation desalination, and a biological route. They answer different questions, and the deciding factors are salt composition and concentration rather than novelty. Getting that choice wrong is expensive, because the two routes impose completely different material, land and operating requirements.

What Makes a Saline Stream Hard to Handle

Three properties drive design. Density: dissolved salt raises liquid density, slows gravity settling and forces larger clarifier surface areas. Ionic interference: calcium, sodium and chloride disturb floc formation and microbial metabolism in different ways, so a process tuned for one salt is often wrong for another. Scaling: carbonate and phosphate species precipitate readily, and those precipitates block aerators, packing and pipelines long before the plant reaches its design capacity.

The Thermal Route: Four Distillation Configurations

Distillation is the oldest and most widely deployed desalination family, and industrial distillation units grew directly out of seawater desalination. Saline water is heated to boiling and the vapour condensed as fresh water. The advantages are a simple structure, straightforward operation and good product quality, which is why the approach has survived a century of competing technologies.

Multi-effect evaporation

Several evaporators are connected in series, and the vapour produced in one vessel becomes the heat source for the next before finally being condensed. Low-temperature multi-effect distillation ranks among the most energy-efficient options, which explains its rapid development: unit capacities are rising, cheaper materials are replacing exotic alloys, and operating temperatures and heat-transfer efficiency are being pushed upward.

Multi-stage flash (MSF)

Feed brine is heated and then admitted to a flash chamber held below the saturation vapour pressure corresponding to that temperature. The brine enters superheated, partially flashes off and cools itself; the vapour is condensed as product water. MSF strings several chambers of successively lower pressure together, so evaporation and cooling proceed stage by stage while the brine concentrates, stopping before its temperature drops to that of the source water. Developed to overcome the severe scaling of early multi-effect units, MSF has become one of the most mature branches of the industry: simple and reliable equipment, high operational safety, good anti-scaling behaviour, wide operating flexibility and the ability to run on low-grade or waste heat. It suits large and very large plants and is widely deployed in the Gulf region.

Vapour compression condensation

Brine is preheated, partially evaporated, and the secondary steam is compressed to raise its pressure before being returned to the heating side of the evaporator, where it condenses as product water and gives up its heat. Applied to circulating cooling water, the process concentrates and discharges harmful components while recovering more than 95% of the blowdown as condensate for reuse as make-up water in circulating systems and boilers. The price is high, demanding materials of construction and heavy heat consumption, so it is normally limited to power plants in severely water-scarce regions.

The Biological Route: Matching the Flow to the Salt

The biological train for saline wastewater resembles an ordinary one, comprising equalization, aeration, secondary clarification, sludge return, excess sludge dewatering and nutrient dosing, but every step needs different sizing and different equipment.

Equalization and aeration

The equalization tank is designed around salt concentration swings rather than flow alone, covering both shock loads and planned dilution. In the aeration tank the salt type decides the strategy. Where CaCl2 dominates, conventional aeration works well: calcium ions strengthen activated-sludge floes, and high CaCl2 can lift the sludge ash content to 40%-50% and allow the mixed liquor concentration to exceed 5,000 mg/L. Aeration methods with greater lifting force, including conventional, deep-shaft and fluidized-bed systems, are then preferable, together with coarse diffusers. Fine-bubble and variable-orifice aerators are best avoided because inorganic salts plug their pores and kill agitation in the tank. For smaller flows, jet aeration can also be used when the flow is less than 1,000 m3; it transfers oxygen efficiently and resists clogging. Aeration intensity also runs above ordinary practice, in the region of 10 m3/(m2·h), or a central draft tube can be added to boost lifting and mixing. Under high salinity the faster oxygen transfer actually supports high sludge concentrations, and provided the zoogloea stays intact the sludge will not float and wash out even if filamentous organisms appear. Phosphorus dosing points need attention, because calcium phosphate precipitate both harms performance and scales pipelines.

SBR and biofilm decisions

In an SBR, aeration and settling share one vessel, so settling time must be designed with real margin, particularly for high sodium salt streams, which settle poorly. Extending the settle phase and reducing decant depth limits disturbance of the settled blanket. Where salinity fluctuates widely, an equalization tank remains necessary regardless.

Biofilm processes are a strong fit for saline wastewater, and biological aerated filters and contact oxidation are the usual choices. With high calcium salt content, packing and media selection matters, and a biological aerated filter needs a higher backwash intensity and longer duration. Contact oxidation tanks should use higher-void-ratio packing arranged for easy removal and washing, otherwise calcium carbonate formed during operation will clog it. For high NaCl streams the biofilm route is generally preferable: sludge ash content is lower than with CaCl2, the liquid is denser, and when bulking occurs or the aeration tank is shocked, the zoogloea floats and washes out more readily.

Clarification and sludge dewatering

Secondary clarifier surface loading should carry a margin, because denser liquid retards settling, most noticeably with NaCl. Larger flows, and CaCl2 streams in particular, are better served by a peripheral-driven scraper able to handle concentrated, dense sludge. With conventional activated sludge on high-CaCl2 water, raising the return sludge rate cushions fluctuation and improves stability. Because the inorganic fraction of the sludge is high and its organic removal capacity relatively low, a low loading rate delivers better removal than a high one; extended aeration, however, suits saline wastewater poorly, since long sludge age and long retention time age the activated sludge and degrade its flocculation performance.

Dewatering is comparatively easy. Excess sludge from CaCl2 treatment carries so much calcium salt that it releases water without any flocculant. After thickening, the sludge concentration can exceed 50 g/L, and the excess sludge quantity and design parameters stay close to ordinary practice.

Where the Research Is Heading

Biological limits are also being attacked directly. In May 2025, Dai Junbiao, a guest researcher at the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, working with the team of Professor Tang Hongzhi of Shanghai Jiao Tong University, constructed an engineered bacterial strain able to degrade five organic pollutants simultaneously. Verified with real industrial wastewater samples, the strain degraded combined pollutants efficiently in high-salinity water, offering a domestic answer to the salt inhibition that constrains conventional microbial treatment. In the same year the Ministry of Industry and Information Technology set task lists for difficult problems including the treatment and reuse of high-salinity wastewater and complete technology and equipment for recovering waste salt from high-salinity concentrate.

Reading the Choice Correctly

Distillation is the answer when salt load is extreme, the reuse value of water is high and waste heat is available. The biological route is the answer when organics dominate and salinity stays within a range that microbes can tolerate. The frequent mistake is to treat the two as competitors. In practice the strongest designs put thermal concentration and biological polishing in the same flow sheet, sized so that each stage removes what it is genuinely good at and leaves the rest to its partner.