Ship Bilge Water in Three Stages: CDFU, KFM and CDOF Against Grade 3 Limits

The ocean is the foundation of the shipping industry, and ship oily wastewater — engine room bilge water and tank washing water in particular — sits squarely on top of the marine ecosystem. It is heavily emulsified and complex in composition, and conventional processes have two chronic weaknesses: low deoiling efficiency and a tendency to create secondary pollution. Holding compliant discharge is therefore a core challenge for operators rather than an administrative detail.

Shenzhen SINOKLE addresses it with a combined process of CDFU pre-treatment deoiling, KFM filtration and CDOF catalytic ozone oxidation. Three stages, each doing a different job, together meeting the Grade 3 requirements of the Integrated Wastewater Discharge Standard.

Stage one: CDFU takes the oil load down

CDFU, a compact swirl dissolved-air flotation unit, is the front-end workhorse. It combines centrifugal separation with ultra-fine bubble technology to generate 5-30 μm bubbles whose specific surface area is 10 times that of conventional bubbles. With a dual-zone flotation structure, single-stage deoiling efficiency exceeds 90% and suspended solids removal exceeds 85%.

The unit demulsifies purely physically, so no chemical is needed and no oily sludge — which is hazardous waste — is produced. Recovered oil returns to use as a resource, which balances the environmental and economic sides of the operation. Equipment is skid-mounted and takes only a third of the footprint of a conventional flotation unit, with a hydraulic retention time of just 1-5 minutes, which suits the limited space and large operating swings of a ship.

Stage two: KFM fine filtration

Pre-treated water then enters the KFM activated filter media filter. The medium is a super-hydrophilic modified material prepared through 60 process steps, carrying a negative surface charge that adsorbs pollutants efficiently. It is self-inhibiting against bacteria, resists compaction and backwashes easily.

Filtration precision is where it earns its place. Wastewater with influent oil content ≤ 50 ppm and suspended solids (SS) ≤ 50 mg/L is treated to oil content ≤ 6 ppm, SS ≤ 2 mg/L and a median particle size ≤ 1.5 μm. That protects the downstream oxidation stage from clogging and takes pressure off terminal equipment, which reduces overall O&M cost.

Stage three: CDOF as the purification terminal

CDOF, the integrated catalytic ozone oxidation and flotation unit, couples catalytic ozonation deeply with dissolved-air flotation. A proprietary catalyst activates ozone to generate highly oxidative hydroxyl radicals that mineralise refractory organics without selectivity. COD removal exceeds 85%, and colour, odour and toxic substances come out with it. Built-in flotation then separates the flocculent material the oxidation produces.

Against conventional ozonation the differences are practical: reaction time is only 10-30 minutes and operating costs are 20%-30% lower. The package runs fully enclosed with no odour leakage, which on a vessel matters as much as the numbers do.

Why three stages rather than two

Two stages are tempting on a vessel where space is the scarcest resource, but the droplet populations involved do not overlap enough to allow it. CDFU removes the bulk of the oil and solids yet leaves a fine residue, and a filter alone would blind on that residue long before its media was exhausted. Putting CDOF last means the oxidation stage receives water already clean enough for the catalyst to work on refractory organics rather than on oil, which is the only way its COD removal figure holds up over time.

Each stage therefore protects the next. CDFU shields the KFM media from excessive oil loading, and the KFM filter shields the CDOF catalyst from the solids and fine oil that would otherwise foul its surface. Removing any one of the three does not simply reduce performance; it changes which component fails first.

Monitoring follows the same logic and stays deliberately simple. Rather than instrumenting every stage, operators track the indicators that actually predict failure — differential pressure across the filter and effluent oil content after the oxidation stage — which keeps the maintenance load within what a ship's crew can absorb.

The compliance arithmetic

The train has been verified on real projects. For ship oily wastewater with influent oil content of about 1,000 mg/L and suspended solids of about 1,000 mg/L, final effluent stably meets the Grade 3 requirements of the Integrated Wastewater Discharge Standard, GB 8978-1996 — COD ≤ 400 mg/L, petroleum substances ≤ 10 mg/L and SS ≤ 50 mg/L — with key indicators better than the standard limits. Shock-load tolerance is strong as well, so swings in oil type and concentration do not destabilise the effluent.

Built for shipboard conditions

• Environmental performance. Mainly a purely physical process: no chemicals, no secondary pollution, and a 90% reduction in sludge generation, which lowers hazardous waste disposal costs substantially.
• Simple O&M. A PLC/DCS control system supports remote monitoring and unattended running, with few moving parts and maintenance costs at only one third of conventional processes.
• Scenario fit. The fully enclosed pressurised design prevents leakage of hydrogen sulphide and other oil vapours, copes with ship motion and restricted space, and can be retrofitted online.

For an operator weighing compliance cost against voyage risk, the deciding factor is usually whether the equipment keeps working without an engineer standing next to it. That is what the three-stage CDFU + KFM + CDOF arrangement is designed around.