Tank Washing Wastewater: Reaching GB 8978-1996 Tertiary Limits Without Gravity Separation Alone
Tank washing wastewater has been an environmental compliance headache at oil depots for years. Wash water carries oil, COD and BOD together, and that combination is awkward: a gravity oil separator removes the free oil and stops there, leaving the depot short of the tertiary discharge standard in the Integrated Wastewater Discharge Standard (GB 8978-1996). Building the answer around internationally pioneered integrated CDOF and CDFU units is what closes the gap, and the effluent from this route meets the integrated wastewater discharge standard consistently enough to have drawn praise from domestic and overseas customers.
Split the problem before picking equipment
Most of the difficulty evaporates once the effluent is divided into two separate jobs. Oil is a separation problem, so it should be handled physically. COD is an oxidation problem, so it needs advanced oxidation. Dropping either half is why single-technology upgrades keep disappointing depot owners. The core technical route follows straight from that split: two-stage CDFU cyclonic dissolved air flotation for purely physical separation, then CDOF ozone catalytic oxidation flotation for deep COD degradation, delivered as an integrated skid-mounted unit with stable compliant effluent.
First unit: two-stage CDFU, physical throughout
CDFU (Cyclonic Dissolved Gas Flotation Unit) is a patented technology independently developed by SINOKLE at an internationally advanced technical level, and it is a new-generation, domestically pioneered, high-efficiency, compact, enclosed, pressurized flotation technology. Three mechanisms are fused inside it. Cyclonic centrifugal separation uses the centrifugal force field to accelerate oil droplet coalescence. Ultrafine bubble generation produces micro-bubbles that adhere to oil droplets and lift them efficiently. Dissolved air flotation releases those bubbles uniformly in an enclosed, pressurized environment.
The targets are waste oil, emulsified oil and suspended solids, removed efficiently and rapidly, with the action purely physical so the oil stays recoverable. The two-stage design carries a specific logic: the first stage bears the main load and handles coarse separation, while the second stage works on difficult components such as emulsified oil. Gradient purification of that kind is what keeps oil-removal performance from decaying as the feed changes.
Second unit: CDOF, oxidation where separation runs out
CDOF (Cyclonic Dissolved Ozone Flotation Unit, an integrated ozone advanced catalytic oxidation and flotation device) is SINOKLE's patented technology, a domestically original and internationally leading high-efficiency integrated oxidation and flotation unit holding a PCT international patent. It is built from five systems, each with a clear function.
- Oxygen generation system: on-site oxygen production, self-sufficient feedstock.
- Ozone generation system: efficient ozone generation.
- Efficient ozone dosing system: precise dosing, improved utilization efficiency.
- Multi-catalytic reaction system: catalytic activation, multiplied oxidation capacity.
- Cyclonic dissolved air flotation system: oxidation and separation completed at the same time.
Six core technologies work in concert inside it: homogeneous and heterogeneous catalysis, cyclonic flow, dissolved air flotation, cavitation, supercritical catalytic oxidation and fully enclosed pressurized oxidation. The result is efficient removal of petroleum, suspended solids, colloids, COD, bacteria and other pollutants in one piece of equipment.
Gravity separation against the integrated route
Set side by side, the contrast is not subtle. Simple gravity separation removes only part of the free oil and has no oxidation capability at all; the two-stage CDFU plus CDOF route removes petroleum, emulsified oil, suspended solids, colloids, COD and bacteria, backed by ozone multi-catalysis, cavitation, supercritical and fully enclosed pressurized oxidation. One relies on scattered facilities with a large footprint, the other on an internationally pioneered integrated skid-mounted unit. One cannot reach the tertiary discharge standard; the other meets the integrated wastewater discharge standard stably. Only the integrated route carries CDFU and CDOF patents, with CDOF holding a PCT international patent.
Why an integrated skid changes the project, not just the layout
The skid matters more than it first appears. Depots are rarely short of land in the abstract, but the land next to the tank farm is usually taken, and a treatment plant that has to be built half a kilometre away needs pumping, power and operator time that never appear in the equipment budget. Putting two-stage CDFU and CDOF into one integrated skid keeps the whole train inside the existing footprint and shortens the installation window, which for a depot means fewer days with tanks out of service.
Enclosed, pressurized operation brings a second benefit that is easy to overlook. Wash water carries volatile compounds and odour, and an open gravity separator makes both of them the neighbourhood's problem. A closed vessel does not. For depots close to residential boundaries, that difference can decide whether a project gets permitted at all.
What the market has done with it
With CDOF and CDFU integrated units at its core, this overall solution has been implemented across oil-depot wastewater treatment, delivering stable compliant effluent and favourable feedback from domestic and overseas customers. An internationally pioneered equipment route, delivered as an integrated skid, is becoming a dependable choice for tank washing wastewater precisely because it answers both halves of the problem, oil and COD, without asking the operator to choose between them.