Why Tank Bottom Water Needs a Different DAF Design
DAF sizing for tank bottom water starts from peak hourly flow and oil flux, not municipal surface-loading defaults. Peak draw is typically 3–5× the daily mean; oil runs 200–10,000 mg/L and TSS 200–3,000 mg/L. Design oil flux stays in the 15–40 kg oil/m²·h band, with surface loading 5–15 m/h, saturator pressure 300–500 kPa, and 8–12% recycle of clarified effluent.
Tank bottom water is the mixture pumped from drain sumps of crude, intermediate, and slop storage tanks during periodic drawdowns. Draws typically run once or twice per month, sometimes weekly, when product levels fall and the water heel is pushed out. The stream carries a stratified free-oil layer on top, an emulsified oil "rag" layer in the middle, and a sediment slurry underneath. What reaches the DAF is a time-varying blend of all three layers. Oil content regularly lands between 200 and 10,000 mg/L depending on tank service, tank age, and heel residence time. Total suspended solids sit in the 200–3,000 mg/L range, and pH drifts between 5.5 and 8.0 in most refinery and terminal sumps.
That profile differs from the colloids-and-TSS feed that municipal DAF guides assume. Two operational facts drive the gap. The draw is intermittent, so peak flow runs 3–5× the daily mean while a tank is emptied. The load is oil-dominated, so the design constraint is oil flux (kg oil/m²·h) rather than TSS or surface overflow rate alone. Generic guidance to size a DAF at 5–10 m/h surface loading assumes a steady colloidal feed and under-specs equipment for an oil surge. The 2020 DUT mineral-oil thesis by Dladla (Optimization of DAF for separating industrial mineral oil from water, Durban University of Technology) treats refinery oil-in-water as a distinct case from municipal DAF. That work identifies pH 5 and an air-to-water ratio of 8–12% as the performance-defining variables (source: DUT 2020).
DAF Sizing for Tank Bottom Water: Five-Step Calculation
A defensible DAF sizing for tank bottom water is a five-step chain. Skipping any step produces a unit that oil-breakthroughs on the first draw, or costs about 30% more 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, Q usually lands 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. Use 300–500 kPa pressure and an 8–12% recycle ratio 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 about 400 kPa.
- 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. That yields 4.7 m² float area at SLR 8 m/h, with a saturator on 3.0 m³/h recycle at 400 kPa. Most plants we size for two-tank terminal draws land near the lower half of the 15–60 m³/h peak band until simultaneous draws are confirmed on the operating schedule.
| 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 | HydropureWater 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
Emulsified oil without chemistry produces oil breakthrough on DAF. 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). The pH window of 5–7 is where most refinery jar tests land before commissioning. Outside that window, cationic coagulant charge is suppressed above pH 7.5, or the oil-surfactant complex restabilizes below 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 screened in DUT 2020. Mixing energy matters as much as dose. Flash mix at 100–300 s⁻¹ for about 30 s disperses the coagulant. Slow mix at 20–50 s⁻¹ for 5–10 min then grows large, low-density flocs that micro-bubbles can lift. Per Edzwald's CRC Press chapter on floc size and density in DAF versus sedimentation (Algae Laden Water Treatment by Dissolved Air Flotation), DAF wants larger, lower-density flocs than gravity thickening. That different mixing target is 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 300–500 kPa saturator pressure and an 8–12% air-to-water recycle ratio per DUT 2020. For most refinery and terminal skids, 400 kPa and 10% are the practical mid-points (source: DUT 2020). Higher pressure dissolves more air per unit recycle water, but compressor and pump cost scale with it. Above about 500 kPa the marginal air mass per kPa falls off. Below 8% recycle, bubble flux cannot lift the oil load and partial floats appear. Above 12%, the float blanket destabilizes as bubbles shear through and effluent carries visible oil.
Recycle water should be clarified DAF effluent, not raw influent. Saturators fed on raw tank bottom water foul in weeks, not months. Sediment and rag coat the packing and choke air transfer. The bubble size that matters is the 10–100 μm window. Those micro-bubbles form when saturated recycle depressurizes through needle valves or release nozzles. Air comes out of solution as 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 HydropureWater ZSQ DAF to Your Flow
Skid selection follows after Q, float area, and saturator size are set. The Dissolved Air Flotation (DAF) System in the HydropureWater ZSQ series covers 4–300 m³/h across 13 standard catalog models. That band covers single-tank and two-tank terminal drawdowns. Pick the model whose rated flow sits at 70–85% of peak design flow. That keeps floc-bubble contact time from collapsing under chronic underload, while leaving 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 a fixed setpoint. Engineers comparing oily streams can cross-check the DAF sizing guide for edible oil soapstock water. The related electronics-side case is covered in the DAF sizing for HF etch waste guide. Where a second product reference helps procurement, the same Dissolved Air Flotation (DAF) System catalog band is the starting shortlist for 15–60 m³/h peak draws.
| 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 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 is better than asking the float cell to handle settleables it was not designed for.
Before freezing the model, walk this selection checklist. Confirm peak Q from the pump curve and simultaneous-tank count. Capture free + emulsified oil and TSS from the lab. Hold design oil flux inside 15–40 kg/m²·h. Choose SLR for the oil band and compute A = Q/SLR. Set the saturator at 300–500 kPa with 8–12% clarified recycle. Add 20–30% hydraulic margin. Prove chemistry in a jar at pH 5–7 before hydraulic trim.
Who This Is For and Next Step
Tank bottom water DAF sizing fits refinery, terminal, and tank-farm engineers specifying intermittent oily drawdowns where oil flux — not municipal SLR — is the binding constraint. Plants with continuous low-oil process water, or free oil already below about 200 mg/L after a well-sized API/CPI, can often stay on simpler gravity or coalescer trains. Those duties should look elsewhere first. When peak Q, oil flux, and saturator duty are drafted, send the numbers through a DAF sizing review for tank bottom water. That check confirms the skid band and recycle package against the same five-step chain.
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. That band covers typical single-tank and two-tank simultaneous draws. HydropureWater's ZSQ catalog spans this range across 13 models, so most tank bottom water duties fit without custom fabrication. Select so rated flow sits at 70–85% of peak design flow after the 20–30% hydraulic margin.
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 upper end of the SLR range (10–15 m/h) is appropriate. Oil flux above 40 kg/m²·h usually collapses the float blanket on the first heavy draw.
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. Use a flow-paced dosing system to track the slug. For heavy sediment, add a pre-sediment step before the DAF. Jar flotation should confirm attachment before hydraulic setpoints are locked.
How often does DAF float sludge need to be removed?
For typical refinery tank bottom water, scrape the float layer every 2–4 hours and drain bottom sediment daily. Adjust the scraper cycle from visual float thickness. Heavy-sediment tanks may need bottom drain every shift. Set routine cycles during commissioning jar tests. Scraping too slowly lets the blanket thicken and release oil back into the effluent launder.
What air saturation pressure is typical for industrial DAF service?
300–500 kPa is the operating envelope, with 400 kPa the typical setpoint. The DUT 2020 mineral-oil study identifies 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. Keep recycle on clarified effluent so packing does not foul within weeks.
