From Emulsified Oil to Marine Discharge Limits: Combining CDFU, KHC, CDOF and KFM by Objective
Water quality is local. One site hands you oil and heavy emulsification; the next is defined by COD, colour and smell; a third carries oil, suspended solids, colloids and refractory organics all at once. A single vessel, however well built, addresses a slice of that. Long-term stable performance usually comes from a sequence, and the sequence has to be chosen against the pollutant form rather than against a product brochure.
When oil and emulsification dominate
The first objective is blunt: get the oil number down fast so downstream units are not fighting a load they were never sized for. SINOKLE generally starts with a coalescing deoiler ahead of CDFU, or a high-efficiency coalescing deoiler ahead of two-stage CDFU where the inlet is severe.
The coalescing deoiler drives collision, coalescence and growth of fine droplets, doing a front-end pre-separation. CDFU then takes over with swirl separation, micro-nano bubble flotation and coalescing demulsification to lift out fine droplets, emulsified oil and suspended solids. At a petrochemical plant in Xinjiang handling desalter and tank-farm oily water, the inlet ran at ≤20,000 ppm oil; after a high-efficiency coalescing deoiler plus two-stage CDFU the outlet sat at ≤150 ppm with oil-in-water at ≤30%.
When the oil is extreme and the emulsion is stable
Coking acidic water and its neighbours defeat single-stage flotation. Concentration is high, and the droplet population includes a strongly emulsified fraction that will not respond to gravity or to one pass of bubbles.
The CDFU plus KHC high-efficiency coalescing deoiler route splits the work: CDFU removes most floating oil, dispersed oil and suspended solids, and the KHC performs deep coalescing separation on the fine emulsified residue. A Sinopec coking acidic-water project in Henan treated ≤150,000 ppm influent down to ≤100 ppm at the outlet, which is the kind of ratio that separates a combined route from a standalone unit.
When oil is only part of the problem
Oilfield produced water, fracturing flowback fluid and drilling waste liquid come with turbidity, SS, iron and a refractory fraction. Deoiling alone will not clear the discharge specification.
CDFU + CDOF + KFM is the configuration for that. Front-end CDFU deoils and removes solids; mid-stage CDOF runs ozone multi-catalytic oxidation; back-end KFM filters to hold the effluent steady. The Jiayou oilfield project in Congo (Brazzaville), Africa, fed gathering-station wastewater, flowback fluid and drilling waste liquid at 4,600 m³/d and met the marine-discharge standard after the train — petroleum ≤0.5 ppm, turbidity ≤3 NFU, SS ≤3 mg/L and iron ≤0.15 mg/L.
Why a standalone unit keeps disappointing
The most common failure in the field is not a bad unit, it is a good unit asked to do two jobs. A flotation vessel sized for bulk deoiling will always look poor on fine emulsified droplets, and a polishing filter asked to carry a high oil load will blind in weeks rather than months. That is what makes the choice of inlet condition so important: the same CDFU produces a very different outlet depending on whether anything upstream has already taken the free oil off the top.
It also explains why pilot data from one site transfers so badly to another. Influent oil concentration, droplet size distribution and the age of the emulsion all move the answer, and two streams with identical oil numbers can behave completely differently across the same equipment.
When the tail needs polishing
KFM is the unit that plays the final safeguard. Its media is modified to be super-hydrophilic, giving high filtration precision, low resistance, strong anti-fouling behaviour, easy cleaning and long service life. At ≤50 ppm oil and ≤50 mg/L SS on the inlet, the outlet reaches ≤6 ppm oil, ≤2 mg/L SS and a median particle size of ≤1.5 μm — suitable for produced water, refinery and chemical effluent where oil and solids still need another step down.
When COD, colour and odour are the constraint
For streams where the binding limits are COD, chromaticity and odour rather than oil, the integrated CDOF ozone advanced-oxidation flotation unit carries the load. It combines ozone advanced oxidation, swirl-enhanced mass transfer, dissolved-air flotation and catalytic oxidation in one shell, and is used for refractory organic removal, decolorization, deodorization and standard-upgrading retrofits.
Against single ozone oxidation, CDOF puts the emphasis on ozone utilisation and reaction efficiency, using a closed pressurized reaction environment and a multi-catalytic system to hold COD reduction and effluent stability. It also removes COD, suspended solids, colloids, bacteria and viruses, which makes it usable as pretreatment or advanced treatment on landfill leachate, refining, metallurgy and pharmaceutical wastewater.
The selection logic in one pass
Read the objectives in order. High oil — coalescing deoiling and CDFU for rapid reduction. Highly emulsified and extremely concentrated — add KHC for deep coalescing. Oil plus solids plus refractory pollutants — CDFU, CDOF and KFM together. Effluent needing refined assurance — KFM at the end. COD, colour and odour core — CDOF for deep oxidation.
That is not equipment stacking. It is a sequence set by pollutant form and treatment target: front-end load reduction, mid-stage intensification, back-end assurance. Run that way, oil, suspended solids, COD, chromaticity and odour improve together, which is what petroleum refining, oilfield development, landfill leachate and power-plant oily wastewater projects actually need.