Reverse Osmosis Concentrate: Characteristics, Reuse Paths and Advanced Ozone Catalytic Oxidation Deep Treatment Solution
Reverse osmosis membrane separation technology has been applied at scale in electric power, chemical industry, municipal water supply, seawater desalination and other industries thanks to its advantages: no phase change of materials, relatively low energy consumption, desalination rate above 99%, mature and reliable process, highly integrated equipment, convenient automated control and low O&M difficulty. However, the technology has a significant water utilization bottleneck - the conventional design recovery rate is only 75%, and under actual conditions such as influent quality fluctuations and membrane fouling the recovery is often lower, ultimately generating about 30% reverse osmosis concentrate.
This concentrate is not ordinary wastewater; its water quality characteristics are extremely complex:
On one hand, the total dissolved solids (TDS) concentration in the water is 3-4 times that of the raw water, rich in inorganic ions such as calcium, magnesium and sodium;
on the other hand, the RO membrane intercepts more than 90% of organics and colloids, causing refractory organics and suspended colloids to be greatly enriched in the concentrate; some concentrate also contains trace heavy metal ions.
If discharged directly into natural water bodies, the high salinity will destroy soil osmotic pressure, cause vegetation withering, pollute surface water and groundwater, and even affect the salinity balance of marine ecosystems;
if discharged to a municipal wastewater treatment plant, the high TDS will inhibit the metabolic activity of microorganisms in activated sludge, causing the biochemical treatment system's efficiency to drop sharply, and in severe cases may paralyze the system.
Therefore, RO concentrate needs to be treated in three coordinated dimensions: volume reduction (optimizing membrane system design and improving recovery), detoxification (reducing pollutant hazards) and resource utilization (achieving recycling), among whichresource reuseis the core direction that balances environmental protection and economic benefits.
In large water consumers such as power plants, RO concentrate reuse can effectively reduce fresh water consumption and wastewater discharge, but three technical problems must first be solved:
First, scaling tendency: because the RO membrane's carbon dioxide permeability is nearly 100%, the pH on the concentrate side rises to 8-9, while the calcium ion permeability is almost zero and its concentration rises to 4 times that of the raw water.
The combination of the two easily generates calcium carbonate deposits attached to the inner walls of equipment; second, fouling: the intercepted organics and colloids keep accumulating on the concentrate side, forming a viscous pollutant layer that blocks pipes and equipment;
third, biofouling: nutrients such as nitrogen and phosphorus enriched in the concentrate provide a breeding ground for microorganisms, easily forming biofilms that cause equipment corrosion.
To address these problems, power plants have developed mature reuse schemes:
Ash-flushing water: the ash and slag from coal combustion in power plants contains a large amount of silicate ions, which coagulate with calcium and magnesium ions in the concentrate to form stable calcium silicate and magnesium silicate precipitates; this not only shortens the settling distance of ash in the sedimentation tank but also reduces scaling and blockage of ash transport pipes and lowers pipe-cleaning frequency;
Circulating water make-up water: mixing RO concentrate with fresh water at a ratio of 1:2-1:3 to adjust the TDS of the mixed water below 1,500 mg/L can meet the water quality requirements of the power plant's circulating cooling water, replace part of the fresh water, and reduce water costs;
Greening water: the power plant has a large greening area; after the concentrate is diluted by mixing with rainwater or reclaimed water, or its salinity is reduced by simple filtration pretreatment, it can be used to irrigate salt-tolerant plants (such as tamarisk and Suaeda salsa), but soil salt content must be monitored regularly to avoid salt accumulation;
Coal yard spraying / ultrafiltration backwash water: after retention by the membrane system, the suspended solids concentration of RO concentrate is usually below 5 mg/L, fully meeting the water quality standards for coal yard dust-suppression spraying water (turbidity <= 30 NTU) and ultrafiltration system backwash water (turbidity <= 10 NTU), and can be directly reused without additional treatment.
In view of the characteristics of RO concentrate - large treatment volume (some enterprises generate thousands of tons per day), high salinity (TDS often exceeding 10,000 mg/L) and a high proportion of refractory organics - and in line with the national "zero discharge of brine" and resource recovery policy requirements, SINOKLE Technology has developed CDOF (Cyclonic Dissolved Ozone Flotation Unit) technology, providing a solution for deep treatment of the concentrate.
The core innovation of this process lies in the organic integration of three technologies:
first, ozone advanced oxidation technology, which doses ozone and activates it to generate hydroxyl radicals (-OH) to efficiently oxidize and decompose refractory organics in the concentrate (such as aromatic compounds and heterocyclic compounds);
second, cyclonic separation technology, which uses the centrifugal force generated by high-speed rotation to enhance the mixing contact between ozone and concentrate and improve oxidation reaction efficiency;
third, dissolved air flotation technology, which combines the microbubbles generated during oxidation with pollutant flocs to achieve solid-liquid separation through buoyancy, removing suspended pollutants and part of the oxidation products.
From practical application, this process has multiple advantages: in terms of process, no complex pretreatment is required - only two core units, "oxidation - separation"; the equipment is highly integrated, and a single unit occupies only 1/3 of the footprint of conventional processes; in terms of energy consumption, the main electrical equipment is the ozone generator and circulation pump, with power consumption of about 0.8-1.2 kWh/m3 of water treated, lower than UV advanced oxidation (1.5-2.5 kWh/m3); operating costs are controllable, the ozone dosage is flexibly adjusted according to COD concentration, and the automation system enables real-time response to fluctuations in water volume and quality, with strong shock load resistance; in addition, the process can be combined with biochemical treatment, membrane distillation and other technologies - for example, after ozone oxidation, the refractory organics in the concentrate are converted into readily biodegradable small-molecule organics, and subsequent connection to a biochemical system can further reduce COD, ultimately achieving compliant discharge or zero-liquid-discharge treatment of the concentrate.
Laboratory data on the treatment of RO concentrate from a power plant show: when the ozone dosage is 200 mg/L, the COD of raw water (233.75 mg/L) drops to 53.75 mg/L after treatment, with a COD removal of 180 mg/L and an ozone dosage ratio (ozone dosage to COD removal) of 1.11, far lower than that of conventional ozone oxidation (usually 1.5-2.0), proving that the process efficiently degrades organics while reducing chemical consumption, with good techno-economics.
