Heavy Fuel Oil Tank Cleaning Wastewater: Why One Separator Cannot Clear GB 8978-1996
Heavy-fuel-oil storage-tank farms generate a cleaning wastewater that causes disproportionate trouble for its volume. Oil, COD and BOD arrive together, the composition is messy, and the concentration moves with whatever the tank last held. Conventional simple oil separation intercepts part of the floating oil and stops there, which leaves the effluent short of the tertiary limit in the Integrated Wastewater Discharge Standard (GB 8978-1996) and leaves the operator carrying a compliance risk that never quite goes away.
Three pollutants, three different problems
Oil. The forms are not equivalent. Floating oil floats well and is relatively easy to intercept. Emulsified oil sits in the water as fine droplets wrapped in an interfacial film, and a gravity separator cannot break that film. Dispersed oil falls in between. Until the oil is dealt with across all three forms, neither the appearance nor the numbers of the effluent will pass.
COD. Tank-cleaning water carries hydrocarbons and other organics in quantity. COD reflects the oxygen consumed by chemical oxidation and is the core indicator of organic-pollution load, so it has to be genuinely degraded rather than diluted.
BOD. This reflects the biodegradable fraction. High-BOD discharge strips dissolved oxygen from the receiving water and damages the aquatic ecosystem. BOD and COD together make up the dual organic pressure on the permit.
The weakness of simple oil separation is that it addresses one small part of one of these. It intercepts some floating oil; it does not break emulsion, and it does not reduce COD or BOD.
Stage one: CDFU breaks the emulsion
CDFU, the Cyclonic Dissolved Gas Flotation Unit, is a proprietary patented technology developed independently by SINOKLE. It combines cyclonic centrifugal separation, micro-fine bubble generation and dissolved air flotation into a new generation of efficient, compact, enclosed and pressurized flotation equipment.
Its mechanism works on two levels at once. Microbubbles bring the foul oil to the water surface, and the same microbubbles act as a demulsifier, destroying the interfacial film of emulsified oil and releasing oil that was effectively hidden. Cyclonic centrifugal force then drives the oil–water separation harder, so foul oil, emulsified oil and suspended solids come out quickly. This stage answers the question of how oil gets out of the water.
Stage two: coalescing fibre recovers the remainder
Water leaving CDFU still holds residual droplets. It passes into the JuJie coalescing oil remover, which achieves fine oil–water separation through specialised coalescing fibres that intercept what the flotation stage left behind.
That stage earns its place twice over. It makes the effluent cleaner and cuts the organic load reaching the oxidation step, and it collects the separated foul oil for reuse, turning a waste stream into a recovered product. It answers both how the residual oil is removed and what happens to it afterwards.
Stage three: CDOF degrades what separation cannot reach
Dissolved organics are not a separation problem, and no amount of flotation will lower COD on its own. CDOF, the Cyclonic Dissolved Ozone Flotation Unit, is an ozone advanced catalytic oxidation-flotation integrated unit combining ozone multi-catalytic oxidation, hydrodynamic cavitation and cyclonic dissolved air flotation. The development is domestically original, internationally leading, and protected by PCT international patents.
The unit brings together an oxygen-generation system, an ozone-generation system, an efficient ozone-dosing system, a multi-catalytic reaction system and a cyclonic dissolved-air flotation system. Inside a fully enclosed pressurized oxidation environment, homogeneous and heterogeneous catalysis, cyclonic flow, dissolved-air flotation, cavitation and supercritical catalytic oxidation act together, and highly active oxidants break down the refractory pollutants in the water. This is the stage that closes the COD gap and holds the effluent at specification.
Side by side with simple oil separation
- Treatment capacity: simple separation intercepts part of the floating oil only; the three-stage train delivers deep treatment and stable compliance.
- Oil removal: limited floating-oil interception with no answer to emulsion, against CDFU microbubble demulsification plus cyclonic separation handling both foul and emulsified oil.
- COD and BOD: essentially unchanged by simple separation, against CDOF catalytic ozone oxidation decomposing refractory pollutants and reducing COD.
- Oil recovery: none or practically difficult, against a coalescing stage that collects oil for reuse.
- Discharge standard: hard to meet GB 8978-1996 tertiary, against effluent that holds the tertiary limit.
- Equipment form: large open tanks with a big footprint, against compact enclosed pressurized units.
Why the combination outperforms the sum
The gain does not come from owning three pieces of equipment. It comes from the gradient: physical separation, then fibre recovery, then chemical degradation.
Division of labour is clean. CDFU handles how oil comes out, the coalescing remover handles how it is removed and recovered, and CDOF handles how COD is degraded. Each stage does only what it is good at, so none of them is spent on a job it does poorly. Load reduction is progressive: CDFU strips the bulk of the oil and reduces the downstream load, the coalescing stage finely intercepts and lowers the organic concentration, and CDOF is left to attack refractory substances with a lighter load — the better the upstream work, the more efficient the oxidation and the steadier the overall process.
Engineering deployment favours it too. CDFU's efficient compact enclosed pressurized design and CDOF's integrated build keep the footprint small and system integration high, with a short installation and commissioning cycle — a real advantage on a power-station site where space and outage time are both tight.
The short version
Tank-farm cleaning wastewater is hard because complex oil forms and a high organic load arrive in the same stream, and simple oil separation fails because it can only intercept floating oil. The three-stage route works through that gradient: CDFU demulsifies and removes oil, the coalescing remover finely recovers what is left, and CDOF catalytically degrades the dissolved organics. The result is an effluent that holds GB 8978-1996 tertiary consistently. For heavy-fuel-oil power stations, that is a deep-treatment answer that stands up from mechanism through to installation.