Treating Nanofiltration Concentrate: Catalytic Ozone Oxidation and Swirl Flotation in One Unit

Nanofiltration has quietly become the workhorse between ultrafiltration and reverse osmosis: tight enough to retain divalent salts, organics, bacteria and colloids, loose enough to run at pressures well below RO - hence the industry nicknames low-pressure reverse osmosis and loose RO. The duties are broad: drinking and industrial water purification, wastewater purification, and concentration of valuable components in process fluids.

Where the membranes come from

Most nanofiltration membranes are derivatives of reverse osmosis membranes: cellulose acetate membranes, cellulose triacetate membranes, aromatic polyamide composite membranes and sulfonated polyethersulfone membranes. At pressures above the osmotic pressure, water molecules and a small amount of dissolved salt pass through the selective semi-permeable membrane, while other dissolved substances, colloids, organics, bacteria and microorganisms leave with the concentrate. That concentrate is the technology's accounting problem - it carries a high pollution load, and it must be properly treated to meet discharge standards before it can leave the plant.

The concentrate problem

Every cubic meter of permeate produces concentrate that is dirtier than the feed. Treating it by dilution or evaporation is expensive; discharging it untreated is increasingly not an option. The practical question is how to oxidize the retained organics - largely refractory COD - economically, at scale, inside a plant that already runs a membrane system.

Concentrate chemistry also gets worse as recovery rises. The organics nanofiltration rejects are, by selection, the ones that passed through biology or coagulation untouched - humic substances, scaling-tending multivalent ions, residual surfactants. So the stream feeding any post-treatment is simultaneously more concentrated and more refractory than the raw feed ever was. That is the design reality any concentrate process must beat, and it is why simple dilution-and-recycle loops tend to fail: the refractory fraction accumulates until the membrane train fouls.

The SINOKLE combined catalytic process

SINOKLE addresses the high-concentration nanofiltration concentrate challenge with its independently developed core technologies and proprietary heterogeneous catalysts, using a combined catalytic process that shortens reaction time significantly and degrades COD efficiently. The centerpiece is the CDOF unit - swirl ozone dissolved-air flotation - the company's patented technology integrating three processes in one device: multiple catalytic ozone oxidation, hydrodynamic cavitation and swirl flotation. It is the first of its kind in China and an internationally advanced integrated oxidation-flotation device, removing petroleum substances, suspended solids, colloids, COD, bacteria and other pollutants from wastewater rapidly and effectively. The system assembles an oxygen generation unit, an ozone generation unit, an efficient ozone dosing unit, a multiple catalytic reaction unit and a swirl dissolved-air flotation unit.

Catalyst innovations behind the performance

• Multiple transition metal oxides, precious metals included, as catalytic components, with material ratios adjusted through extensive experiments and engineering verification for adaptability across wastewaters and high catalytic activity;
• Multi-stage precise temperature-controlled sintering that preserves active components, reduces production energy consumption and cuts the catalyst loss rate during use, preventing secondary pollution;
• A high-activity carrier with a porous structure providing high specific surface area, doped with catalytic components that are not easily lost, giving high mechanical strength and long service life;
• Effective lowering of reaction activation energy for deep oxidation and maximum removal of organic pollutants;
• Acceleration of ozone self-decomposition in water, raising the hydroxyl radical concentration so oxidation efficiency runs 2-5 times higher than ozone oxidation alone;
• Atomic layer deposition modifying the catalyst so the carrier stays super hydrophilic - resisting pollution, scaling and clogging over long operating cycles.

Bench evidence

Laboratory testing on a landfill leachate sample - among the most refractory feeds available - showed the point plainly. After combined catalytic oxidation of the concentrate in the CDOF unit, water quality improved significantly, with a clear before-and-after comparison between water samples. The COD degradation effect was the most obvious: after combined catalytic oxidation with the new catalyst, COD fell rapidly and substantially. For a membrane plant operator, that result reads directly - the concentrate stream can be oxidized in-unit rather than hauled, diluted or evaporated.

Equipment count matters for operators too. Because the CDOF unit combines oxidation and separation, the footprint a plant allocates to concentrate treatment shrinks accordingly, and the automated, enclosed operation style matches the rest of a modern membrane hall. Where concentrate volume is large, multiple units run in parallel off the same oxygen and ozone generation skids, keeping turndown simple as the membrane plant's recovery changes with season and feed quality.

Integration logic

The sequence in practice: nanofiltration retains the load, the concentrate feeds the CDOF train, catalytic ozone oxidation breaks the refractory structures, and swirl flotation strips the flocculated organics and suspended solids in the same vessel. Oxygen and ozone generation are on-site, dosing is metered to the load, and the oxidation-flotation combination means one installation handles what would otherwise take separate oxidation and separation stages. For plants planning nanofiltration upgrades or fighting concentrate discharge limits, the evaluation path is the usual one - bench test on the actual concentrate, then pilot at design flux - but the destination is established technology rather than a research project.