Why Tank Bottom Water Is a Different DAF Sizing Problem
Tank bottom water is the mixture pumped from the drain sumps of crude, intermediate, and slop storage tanks during periodic drawdowns — typically once or twice per month, sometimes weekly, when product levels fall and the water heel is pushed out. It carries a stratified free-oil layer on top, an emulsified oil "rag" layer in the middle, and a sediment slurry underneath, and the stream sent forward is a time-varying blend of all three. Oil content regularly lands between 200 and 10,000 mg/L depending on tank service, tank age, and how long the water heel has been sitting, with total suspended solids in the 200-3,000 mg/L range and a pH that drifts between 5.5 and 8.0 in most refinery and terminal sumps.
That profile is fundamentally different from the colloids-and-TSS feed that municipal DAF sizing guides are written for. Two operational facts make the difference: the draw is intermittent, so peak flow runs 3-5× the daily mean while a tank is being emptied, and the load is oil-dominated, so the design constraint is oil flux (kg oil/m²·h) rather than TSS or surface overflow rate. Generic guidance to size a DAF at 5-10 m/h surface loading assumes a steady colloidal feed and quietly under-specs equipment for an oil surge. The 2020 DUT mineral-oil thesis (Dladla, Optimization of DAF for separating industrial mineral oil from water, Durban University of Technology) explicitly treats industrial refinery oil-in-water as a distinct optimization case from municipal DAF, with pH 5 and air-to-water ratio of 8-12% identified as the performance-defining variables (source: DUT 2020).
The Five-Step Sizing Calculation Chain
A defensible DAF sizing for tank bottom water is a five-step chain, and skipping any step produces a unit that either oil-breakthroughs on the first draw or costs 30% more in CAPEX than the duty requires.
- Step 1 — Establish peak hourly flow Q (m³/h). Calculate from tank volume, expected draw rate (pump curve), and the number of tanks drawn simultaneously. Use peak, not daily mean. A 5,000 m³ tank drawn over 8 hours with the water heel in the last 5% produces a peak far higher than the monthly average. For a typical two-tank simultaneous draw the engineer should see Q between 15 and 60 m³/h.
- Step 2 — Characterize the feed and set oil flux. Measure free oil + emulsified oil + TSS. Design oil flux for oily tank bottom water sits in the 15-40 kg oil/m²·h band; below 15 the unit is under-stressed, above 40 the float blanket collapses and oil returns through the launder.
- Step 3 — Select surface loading rate and compute area. Use 5-15 m/h — lower bound for high oil, upper for low oil. Compute A = Q / SLR. For Q = 30 m³/h at SLR = 8 m/h, A = 3.75 m² of effective float surface.
- Step 4 — Size the saturator and recycle loop. Pressure 300-500 kPa, recycle ratio 8-12% per the DUT 2020 optimization, recycle flow = 0.08-0.12 × Q. For Q = 30 m³/h, recycle is 2.4-3.6 m³/h through a pressurized saturator sized for ~400 kPa operating pressure.
- Step 5 — Add 20-30% hydraulic margin for slug events, rag layer spikes, and cleaning cycles. This is what prevents the chronic oil breakthrough that kills most first-pass DAF designs on tank drawdowns.
Worked check: 30 m³/h × 1.25 margin = 37.5 m³/h design flow → 4.7 m² float area at SLR 8 m/h, with a saturator on 3.0 m³/h recycle at 400 kPa.
| Step | Parameter | Value / Range | Source / Note |
|---|---|---|---|
| 1 | Peak flow Q | 15-60 m³/h (typical two-tank draw) | Engineer calc from pump curve |
| 2 | Design oil flux | 15-40 kg oil/m²·h | Zhongsheng field data, 2026 |
| 3 | Surface loading rate (SLR) | 5-15 m/h | Oil-dependent, lower for high oil |
| 3 | Float area A = Q/SLR | e.g. 3.75 m² at 30 m³/h, SLR 8 | Calculated |
| 4 | Saturator pressure | 300-500 kPa | DUT 2020 optimum 350-425 kPa |
| 4 | Air-to-water recycle | 8-12% of Q | DUT 2020 optimum 10% |
| 5 | Hydraulic margin | +20-30% on Q | Slug + rag event headroom |
Pre-Treatment Chemistry and Floc Condition
DAF on emulsified oil without chemistry is a guaranteed oil-breakthrough. Emulsified oil droplets in the 5-20 μm range do not attach to micro-bubbles on their own — they need a coagulated or flocculated bridge. The DUT 2020 study confirmed pH 5 as the optimum for mineral-oil-in-water (source: DUT 2020), and that pH window of 5-7 is the working range most refinery jar tests land on before commissioning. Outside that window, either the cationic coagulant charge is suppressed (pH > 7.5) or the oil-surfactant complex restabilizes (pH < 4.5).
Coagulant selection is typically cationic inorganic (alum or ferric chloride at 30-50 mg/L) or a cationic polymer such as the Zetag-type organics the DUT 2020 work screened as effective. Mixing energy matters as much as dose: flash mix at 100-300 s⁻¹ for ~30 s disperses the coagulant, then slow mix at 20-50 s⁻¹ for 5-10 min grows large, low-density flocs that micro-bubbles can lift. Per the CRC Press chapter on floc size and density in DAF versus sedimentation (Edzwald, Algae Laden Water Treatment by Dissolved Air Flotation), DAF wants flocs that are larger and lower-density than the compact flocs that settle well — a different mixing target than gravity thickening, and a common reason operators "tune" the wrong way. Polymer make-up and dose trim should run on an automatic chemical dosing system with flow-paced control so dose tracks the slug.
Saturator and Air-to-Water Ratio Tuning
The saturator is the heart of a DAF and the most commonly mis-specified component on tank bottom water service. The operating envelope is well-bounded: 300-500 kPa saturator pressure and an 8-12% air-to-water recycle ratio per the DUT 2020 optimization, with 400 kPa and 10% as the practical mid-points for most refinery and terminal skids (source: DUT 2020). Higher pressure dissolves more air per unit recycle water, which is thermodynamically attractive, but compressor and pump cost scale with it, and above ~500 kPa the marginal air mass per kPa falls off. Below 8% recycle, bubble flux is insufficient to lift the oil load and you see partial floats. Above 12%, the float blanket destabilizes — bubbles shear through and the effluent starts carrying visible oil.
Recycle water should be clarified DAF effluent, not raw influent. Saturators fed on raw tank bottom water foul in weeks, not months — the sediment and rag layer coat the packing and choke air transfer. The bubble size that matters is the 10-100 μm window; these micro-bubbles are generated by depressurizing saturated recycle through needle valves or special release nozzles, and the air comes out of solution as a cloud of fine bubbles that attach to flocculated oil and rise at 0.5-2 m/min.
| Parameter | Operating Range | Typical Setpoint | Consequence of Out-of-Range |
|---|---|---|---|
| Saturator pressure | 300-500 kPa | 400 kPa | Low → poor bubble flux; high → diminishing return, higher cost |
| Air-to-water recycle | 8-12% | 10% | Low → partial floats; high → blanket destabilizes, oil shears through |
| Micro-bubble size | 10-100 μm | 30-60 μm | Outside range → poor oil attachment |
| Recycle water source | Clarified DAF effluent | Effluent | Raw influent → packing fouling, unstable bubble population |
| Flotation residence time | 5-15 min | 10 min | Short → rag carryover; long → over-sized tank |
Matching the Zhongsheng ZSQ DAF to Your Flow
Once Q, area, and saturator size are set, the next decision is which skid model fits. The Zhongsheng ZSQ series DAF system covers 4-300 m³/h across 13 standard catalog models, which sits squarely over the envelope of single-tank and two-tank terminal drawdowns. Selection logic is to pick the model whose rated flow sits at 70-85% of peak design flow — high enough that the unit is not chronically underloaded (which hurts floc-bubble contact time), low enough that there is margin for the slug event the operator has not yet seen.
For oily water service, the ZSQ configuration should include the oil-rated top skimmer with adjustable weir, a bottom scraper with cycle-timer for sediment removal, and an inclined-plate pack for enhanced oil-bubble contact. Pair the skid with a flow-paced polymer system on the automatic chemical dosing system so dose tracks the slug in real time rather than running at a fixed setpoint. For cross-stream sizing, engineers handling soapstock should also see the DAF sizing guide for edible oil soapstock water, and the related electronics-side case is covered in the DAF sizing for HF etch waste guide.
| Design Peak Flow (m³/h) | ZSQ Model Band | Loading Target | Notes |
|---|---|---|---|
| 4-10 | Small skid (lab/terminal pilot) | 70-85% of rating | Single small tank draw |
| 10-30 | Mid skid | 70-85% of rating | Typical single-tank refinery draw |
| 30-80 | Large skid | 70-85% of rating | Two-tank simultaneous draw |
| 80-300 | Multi-skid / parallel | 70-85% of rating | Tank farm or terminal header |
Commissioning and Troubleshooting Checklist
Most DAF commissioning failures on tank bottom water show up in the first 90 days and trace back to one of three modes:
- Oil breakthrough at the effluent launder. Check saturator pressure (target 400 kPa), recycle ratio (target 10%), and polymer dose. Run a jar flotation first to confirm chemistry is right before touching hydraulics — most breakthrough is chemistry, not hardware.
- Foaming in the float blanket. Residual polymer carryover and over-loaded oil flux are the usual causes. Drop the air-to-water ratio by 2-3 percentage points and re-check the skimmer withdrawal rate; foam that does not collapse into a stable oil layer is a hydraulic symptom, not a chemical one.
- Sediment accumulation in the cell. Verify the bottom scraper cycle and the sludge drain frequency. For tanks with heavy sediment (older crude slop tanks, resid service), add a pre-sediment step — a small lamella or cone tank ahead of the DAF — rather than asking the float cell to handle settleables it was not designed for.
Frequently Asked Questions
What flow range does a standard DAF skid cover for tank bottom water?
Standard packaged DAF units run from about 4 m³/h for small terminals up to 300 m³/h for refinery and tank-farm headers, which covers the typical single-tank and two-tank simultaneous draw envelope. Zhongsheng's ZSQ catalog spans this range across 13 models, so most tank bottom water duties fit without custom fabrication.
What oil concentration can a DAF handle on tank bottom water?
A properly sized and conditioned DAF handles 200-10,000 mg/L oil-in-water, with design oil flux held in the 15-40 kg/m²·h band. Above 10,000 mg/L the unit needs a pre-coalescer or two-stage DAF; below 200 mg/L the load is light enough that the upper end of the SLR range (10-15 m/h) is appropriate.
Is DAF alone sufficient for tank bottom water, or is pre-treatment required?
DAF alone is rarely sufficient for emulsified oil above 500 mg/L — pre-treatment chemistry is required. pH adjustment to 5-7, coagulant at 30-50 mg/L, and a polymer flocculant are standard, with a flow-paced dosing system to track the slug. For heavy sediment, add a pre-sediment step before the DAF.
How often does DAF float sludge need to be removed?
For typical refinery tank bottom water, the float layer should be scraped every 2-4 hours and the bottom sediment drained daily, with the scraper cycle adjustable based on visual float thickness. Heavy-sediment tanks may need bottom drain every shift; routine cycles are set during commissioning jar tests.
What air saturation pressure is typical for industrial DAF service?
300-500 kPa is the operating envelope, with 400 kPa the typical setpoint and the DUT 2020 mineral-oil study identifying 350-425 kPa as the performance optimum. Below 300 kPa bubble flux is too low; above 500 kPa the marginal air mass per kPa falls off and compressor cost is not justified.
