Reverse Osmosis Concentrate: Water Quality Challenges and a Catalytic Ozonation Route
Reverse osmosis (RO) membrane separation is an advanced liquid purification method. Compared with conventional treatment it offers significant cost efficiency, simple operation and maintenance, no acid or alkali dosing and a compact footprint, and it is therefore widely used in seawater and brackish water desalination, ultrapure water production, and the concentration and recovery of chemical materials. As membrane products diversify, membrane performance improves and systems are optimised, the technology is increasingly applied to municipal wastewater reuse and industrial wastewater purification.
RO uses pressure, typically 1–10 MPa, to drive water molecules through a selective separation membrane, thereby separating, extracting, refining and concentrating materials. It not only removes salts and ionic species effectively but also retains organic matter, colloids, bacteria and viruses well. In operation an RO membrane converts roughly 80–85% of the feed into clean permeate, while all the retained substances collect in a concentrate that accounts for 15–20% of the feed volume, with pollutant concentrations typically 2 to 3 times those of the feed. If this high-salinity, high-organic concentrate is not handled properly, it not only wastes a valuable water resource but can also cause serious environmental pollution.
The water quality of RO concentrate depends mainly on the feed source. Concentrate from wastewater treatment systems usually has significantly higher conductivity, total dissolved solids and chloride content than the brine produced by seawater desalination. Its composition is dominated by dissolved inorganic salts and by small, soluble, poorly degradable organic molecules. The organic fraction may include hard-to-degrade chemicals from domestic wastewater such as pesticides, personal care products, pharmaceutical residues and endocrine disruptors; residues generated during treatment such as soluble microbial products, partially biodegraded organics and antiscalants; and various biomass including bacteria, viruses, oocysts and cell debris. All of these pose a potential threat to the ecosystem. The inorganic salts consist mainly of chloride, carbonate, bicarbonate, sulfate and nitrate, together with calcium, magnesium and sodium cations.
Given the large volume and high salt content of RO concentrate, and guided by the principles of zero liquid discharge and resource recovery, SINOKLE has introduced an innovative treatment route, high-efficiency catalytic ozonation, dedicated to the challenge of RO concentrate.
The process shown is SINOKLE's proprietary CDOF (Cyclonic Dissolved Ozone Flotation unit), which creatively combines advanced ozone oxidation, cyclone separation and dissolved gas flotation to achieve efficient, integrated removal of a wide range of refractory pollutants.
Catalytic ozonation has a short process flow and simple equipment. The treatment units are small and compact and the footprint is limited; few electrical devices are needed and power consumption is low, so operating costs stay low. Operation is convenient and can be automated, which simplifies on-site requirements. When flow or water quality changes, the process can be adjusted accordingly, giving strong adaptability and resistance to shock loads. It can also be combined with other technologies such as biological treatment, so that each plays to its strengths and overall treatment performance improves further.
Laboratory trial on RO concentrate
Test data
Test conclusion
With this process and an ozone dose of 200 mg/L, the effluent COD was 53.75 mg/L, the COD removed was 180 mg/L, and the ozone dosing ratio was 1.11.
