Use an API separator for large free-oil droplets where space is available; use CPI for the same gravity-separation duty when footprint is tighter and solids will not foul the plate pack; use DAF for dispersed oil or emulsions that can be destabilized chemically. Many plants need API/CPI followed by DAF rather than one unit for every form of oil.
PT Beta Pramesti Asia integrates physical separation, treatment chemistry, and industrial wastewater systems in Indonesia. Selection starts by measuring oil form, droplet-size distribution, TSS, density, viscosity, surfactants, peak flow, and the downstream water-quality requirement before naming equipment.
API, CPI, or DAF selection by oil form
API and CPI units use density difference and gravity. They work when oil is genuinely free, droplets are large enough to rise, and flow is calm. DAF attaches microbubbles to droplets or floc so their apparent density falls and they rise; a stable emulsion normally still needs pH adjustment, coagulant, and/or polymer ahead of flotation.
| Wastewater condition | API separator | CPI separator | DAF |
|---|---|---|---|
| Visible free-oil layer and large droplets | Primary bulk-removal candidate | Primary candidate where plot area is limited | Potential next barrier, not always the first one |
| Free oil with variable flow and load | Needs equalization and surge control | More sensitive to maldistribution and plate fouling | Still needs hydraulic and solids-load control |
| Fine droplets produced by pumps, valves, or mixing | Performance falls sharply | Coalescing may help if droplets merge and plates stay clean | Better candidate when flotation testing proves separation |
| Surfactant- or detergent-stabilized emulsion | Does not break the emulsion | Does not chemically destabilize the emulsion | Candidate after emulsion breaking/coagulation is demonstrated |
| High grit, TSS, and heavy solids | Needs hopper/scraper and cleaning access | High risk of plate-pack blockage | Needs grit removal; DAF does not replace heavy-solids separation |
| Sticky float or sludge | Separate oil skimmer and sludge scraper | Plate-cleaning access is critical | Skimmer torque, beach, and sludge route control operability |
| Protecting biology or membranes | Removes bulk free oil upstream | Compact bulk-oil removal | Polishes FOG/TSS before biological or membrane treatment |
Total oil and grease alone cannot select the process. Two samples at 500 mg/L may behave completely differently: one forms an oil layer within minutes, while the other stays cloudy for hours because surfactants keep sub-visible droplets stable.
Oil sampling must capture slugs without losing analyte
Oil results are easily biased low because oil adheres to sampling tubes, vessel walls, and transfer tools. Use grab samples from a well-mixed point and collect a series across the shift to capture washdown, CIP, bath dumping, rainfall, or tank-drain slugs.
U.S. EPA Method 1664B (2010) specifies collecting approximately one liter in a glass bottle that is not pre-rinsed with sample. If analysis will be delayed by more than four hours, the method calls for acidifying to pH <2 and refrigerating at 0–6°C. Follow the applicable laboratory method and project sampling plan; do not take a small aliquot from a composite drum and assume oil adhering to the vessel was represented.
| Sampling input | Required form | Decision affected |
|---|---|---|
| Flow | Average, 15-minute/hourly peak, peak duration, batch volume | Equalization, bypass risk, separator area |
| HEM/oil and grease | Grab series, median, P90, representative maximum | Oil mass load and float/sludge duty |
| Non-polar material/TPH where relevant | Method, fraction, source stream | Distinguishes hydrocarbons from other extractable matter |
| Quiescent separation test | Free-oil depth, separation time, rag layer, water clarity | Tests whether gravity is viable |
| Droplet size | Distribution on fresh samples before/after pumps | Checks API/CPI basis and shear damage |
| Oil and water density | At minimum/maximum process temperature | Changes Stokes rise velocity |
| Viscosity and temperature | Shift/season range | Cold or viscous water separates more slowly |
| TSS, grit, settleable solids | Concentration, size, density, stickiness | Sets grit removal, scraper duty, CPI fouling risk |
| pH, surfactants, salinity, chemicals | Product, dose, injection point, contact time | Explains emulsion stability and frames jar tests |
| Total/dissolved COD and BOD | Before and after separation | Shows the load a separator cannot remove |
Record where each sample point sits relative to pumps, equalization, and chemical injection. A centrifugal pump or large pressure drop can create smaller droplets, so an upstream sample may not represent the feed entering the separator.
API separators remove large free-oil droplets
An API separator is an open rectangular basin with inlet distribution, a calm separation zone, oil skimming, baffles, sludge collection, and outlet control. Gravity calculations use Stokes’ law: rise velocity increases with droplet diameter squared and density difference, but falls as water viscosity increases.
The historical API Publication 421 covers conventional gravity and parallel-plate separators for refineries. API now lists it as a historical publication, so the project must confirm the owner’s current specification and applicable standards. A U.S. Department of Energy/OSTI technical report (2022) summarizes the API design basis as targeting oil droplets of about 150 µm; that is a free-oil screening basis, not a guaranteed outlet concentration.
A useful screening relationship is:
Rise velocity ∝ (water density − oil density) × diameter² ÷ viscosity
At equal density and viscosity, a 75 µm droplet rises at roughly one-quarter the velocity of a 150 µm droplet because diameter is squared. This is why a large gravity separator can still fail after high-shear pumping or emulsification. Final design must check flow distribution, short-circuiting, inlet turbulence, surface loading, retention, sludge volume, oil-layer control, vapor management, and peak flow.
CPI reduces footprint but needs a clean feed
A CPI, or corrugated/parallel plate interceptor, installs inclined plate packs so droplets travel a shorter vertical distance and gain surfaces on which to coalesce. It can provide substantial effective separation area in a compact envelope, but it does not turn a stable emulsion into free oil.
Use CPI when tests show droplets can coalesce, grit and TSS are controlled, and the plate pack can be cleaned. Review plate spacing, angle, material, lifting access, flushing, inlet distribution, sludge hopper, and isolation for maintenance. A plate pack blinded by solids or wax has lost its separation function even when flow remains below the catalogue rating.
Practical CPI gates are:
- Fresh wastewater separates free oil without aggressive chemical dosing.
- Heavy TSS and grit can be removed before the plate pack.
- Minimum temperature will not solidify oil or wax on the plates.
- Feed pumping and mixing do not continually create fine droplets.
- Operators have safe access, draining, flushing water, and a cleaning procedure.
DAF separates dispersed oil after chemistry is proven
DAF is suited to dispersed oil, light TSS, protein, fibers, and chemical floc that do not settle well. EPA describes DAF as releasing pressurized, air-saturated water so fine bubbles attach to oil and solids. An EPA manual gives 25–70 psig (about 1.7–4.8 barg) saturation pressure and 0.5–3 minutes in the pressure-retention vessel as historical ranges. Treat them as mechanism checks, not universal operating set points (U.S. EPA Innovative and Alternative Technology Assessment Manual).
DAF does not remove COD as one undifferentiated parameter. It removes the COD associated with separable oil, solids, or floc; dissolved COD continues to biological treatment, adsorption, oxidation, or membranes. Measure both total and dissolved COD in the feed and clarified subnatant.
Use the industrial wastewater DAF sizing guide for hydraulic loading, recycle, air-to-solids, and skimmer checks. This guide addresses train selection; final sizing begins only after oil form and chemical response are understood.
Jar and pilot tests must reproduce flotation
Emulsion breaking and coagulation should be tested on fresh samples at process temperature. A jar that produces settling floc does not prove that the same floc will capture microbubbles, rise, form a removable float, and create pumpable sludge.
- Run an untreated blank and record free separation, rag layer, and time.
- Test a pH range compatible with materials and downstream biology.
- Compare coagulants by active dose, not product volume alone.
- Add polymer after coagulation while avoiding floc-damaging shear.
- Use a bench flotation cell or pressurized recycle representative of DAF.
- Record HEM/FOG, TSS, total/dissolved COD, turbidity, float volume, rise rate, and subnatant clarity.
- Repeat normal feed, maximum slug, minimum temperature, and high-surfactant conditions.
- Select the recipe using both water quality and sludge volume/behavior.
PT Beta Pramesti Asia supplies wastewater coagulants and flocculants for floc formation. The dose must come from testing and then track actual flow and loading. When the package needs separate metering hardware, Watermart’s industrial dosing pumps provide the relevant equipment handoff.
Build a staged treatment train instead of one box
A reliable train removes each pollutant in the form in which it exists. Screening and grit removal protect downstream assets; API/CPI recovers bulk free oil; equalization absorbs slugs; coagulation-DAF removes dispersed/emulsified oil and TSS; biology removes biodegradable BOD/COD; polishing follows the reuse or discharge target.
| Test finding | Train worth testing | Reason |
|---|---|---|
| Free oil dominates, droplets are large, TSS is controlled | Screen/grit → API or CPI | Avoid chemical cost for oil that can be recovered physically |
| High free oil plus washdown emulsion | API/CPI → equalization → coagulation-DAF | Remove bulk oil before it increases chemical and DAF sludge demand |
| Emulsion dominates, little free oil | Equalization → emulsion break/coagulation → DAF | Gravity does not destabilize a stable emulsion |
| High grit and heavy solids | Screen/grit → sedimentation/API → DAF if needed | Heavy solids foul plates and do not reliably float |
| Dissolved COD remains high after DAF | DAF → biological/advanced treatment | Separators do not finish the dissolved fraction |
| Effluent feeds MBR or RO | API/CPI if relevant → DAF → biology → membrane | Multiple barriers reduce oil and solids loading on membranes |
Use wastewater physical separation to define headworks and oil-removal solutions to review oil behavior. A Betaqua Dissolved Air Flotation unit is the commercial handoff when testing proves flotation is the right barrier.
Sludge, oil recovery, and safety control operability
API/CPI systems can produce recovered oil, rag layer, and bottom sludge. DAF produces float containing water, oil, solids, coagulant, and polymer. Measure hourly/daily volume, percent total solids, density, pumpability, free oil, contaminants or heating value where relevant, and filtrate quality before selecting storage or dewatering.
Do not mix recovered oil with sludge when recovery is still practical. Provide interface control, high-high level protection, overflow containment, venting, skimmer access, sampling, closed drains, and lockout procedures. Wastewater containing volatile hydrocarbons also needs a hazardous-area and emissions assessment based on actual composition.
Test DAF sludge before choosing a Betaqua Screw Press. Float that looks concentrated may still be difficult to pump or may release oil to the filtrate. Size dewatering from kg dry solids/day and operating hours, not DAF influent flow.
Indonesian limits and compliance points are project-specific
Government Regulation PP No. 22 of 2021 governs Technical Approval for wastewater standards and verification before an Operational Feasibility Certificate (SLO). Parameters and limits depend on the activity, receiving or reuse route, and project approval. Check Minister of Environment Regulation No. 5 of 2014, including valid amendments, for sector standards rather than applying one oil-and-grease target to every industry.
Minister of Environment and Forestry Regulation No. 5 of 2021 includes treatment and sludge design, capacity, flow measurement, and compliance points in technical verification. Flowmeters, sampling ports, inspection access, the sludge route, and data logging are therefore design deliverables, not commissioning accessories.
API, CPI, and DAF procurement checklist
Issue suppliers with a stream balance, PFD, grab-series results, droplet distribution, density/viscosity, temperature, TSS/grit, surfactants, chemical inventory, effluent targets, jar/flotation results, hazardous-area basis, materials, and sludge tests. Require all assumptions and guarantee boundaries to be written.
Compare packages on effective area, inlet distribution, peak hydraulic and solids loads, oil/sludge removal, redundancy, turndown, cleaning access, instruments, energy, chemical consumption, performance testing, spares, and operator training. Contact the Beta Pramesti team with this dataset so the discussion begins with a provable treatment train.
Oily wastewater separation FAQ
Is CPI always better than an API separator?
No. CPI provides high effective area in a compact footprint, but the plate pack is more vulnerable to grit, sticky solids, wax, and maldistribution. An open API separator can be easier to inspect and clean in heavy service.
Can DAF break every oil emulsion?
No. DAF floats droplets or floc; a stable emulsion must first be destabilized. Jar/flotation testing on actual wastewater determines pH, coagulant, polymer, and whether the target is realistic.
Why can oil and grease fall while COD stays high?
Separation removes oil and solids that can be physically captured, while dissolved COD remains in the water. Total and dissolved COD results show the load that needs biological or advanced treatment.
When is a pilot justified?
Pilot work is justified when emulsions change by product, droplet data are uncertain, surfactants are high, sludge is difficult, downstream limits are strict, or failure would foul an MBR/RO. Test normal and peak conditions and close water, oil, solids, chemical, and sludge balances.