Why Tank Bottom Water Breaks Generic MBR Sizing
Tank bottom water — the segregated aqueous phase drained from crude and product storage tanks, desalter effluent, and slop-oil emulsions — runs 3,000-15,000 mg/L COD, 500-5,000 mg/L TSS, 200-2,000 mg/L oil and grease, and 5,000-80,000 mg/L TDS when produced water is co-mingled. Generic MBR sizing copied from municipal texts fails on this stream because free oil coats the membrane surface and permanently blinds 0.1 µm pores, while emulsified hydrocarbons raise mixed-liquor viscosity and suppress oxygen transfer in the scour zone. Per the academic literature, an MBR is "an integrated treatment system of microfiltration (MF) or ultrafiltration (UF) membranes with a biological reactor" (Membranes, 2020-02), and the membrane component is what makes fouling control — not biology — the design driver on oily feeds. The defensible rule of thumb: tank bottom water must be pre-treated to <50 mg/L O&G (preferably <20 mg/L) before entering the MBR tank, a figure anchored to standard refinery pre-treatment practice and to UF membrane integration guidance (API 421 / MF-UF oily-water practice). Municipal MBR fluxes of 20-25 LMH are physically impossible to sustain on a raw tank bottom feed — flux must be derated to 10-15 LMH the moment emulsified oil enters the mixed liquor, regardless of how clean the upstream separator looks on a slip-stream sample.
Step 1 — Characterize the Tank Bottom Water Influent
Nine parameters must be characterized before a single sizing number is written on the data sheet: flow (m³/day), peak factor, total and soluble COD, BOD₅, TSS, oil and grease (free vs. emulsified), TDS/salinity, temperature, and pH. Realistic tank bottom water ranges are COD 3,000-15,000 mg/L, BOD₅/COD ratio 0.3-0.6 (often 0.25-0.40 for crude storage where hydrocarbons resist biodegradation), TSS 500-5,000 mg/L, O&G 200-2,000 mg/L, TDS 5,000-80,000 mg/L, and temperature 25-55 °C depending on whether the source is a heated desalter or an ambient storage drain. Peak factor selection is not a guess: apply 1.5-2.0× for routine batch tank drainage, and 2.5-3.0× for storm-on-tank events or pump-out campaigns when a whole tank heel is dropped to the sewer in a few hours. Composite sampling must run at least 7-14 days with diurnal grabs, because composition swings with rainfall ingress, product changeover, and heel-water removal — a single 24-hour composite will understate the O&G slug that kills a membrane cassette.
| Parameter | Typical range | Design value (basis) | Notes |
|---|---|---|---|
| Flow, average | 50-500 m³/day | Site-specific | Per tank farm audit |
| Peak factor | 1.5-3.0× | 1.75× routine, 2.5× storm | Drainage / pump-out profile |
| COD (total) | 3,000-15,000 mg/L | 5,000-8,000 mg/L | Soluble fraction 40-70% |
| BOD₅/COD | 0.25-0.60 | 0.40 | Lower for crude storage |
| TSS | 500-5,000 mg/L | 1,500 mg/L | Includes iron sulfide from desalters |
| Oil & Grease | 200-2,000 mg/L | 800 mg/L | Free vs. emulsified split matters |
| TDS / salinity | 5,000-80,000 mg/L | Site-specific | Drives SRT and chloride on SS |
| Temperature | 25-55 °C | 35 °C | Higher T lowers O₂ solubility |
| pH | 5.5-9.0 | 7.0-7.5 | Adjust upstream of MBR |
Step 2 — Select the Pre-Treatment Train (DAF + Equalization)
The minimum upstream train for protecting an MBR on tank bottom water is: coarse screening → API or corrugated-plate oil-water separator → equalization tank with low-shear mixing → dissolved air flotation (DAF) → pH and temperature trim. The DAF is non-negotiable — it is the unit operation that drops total O&G from 200-2,000 mg/L to <50 mg/L and TSS to <150 mg/L, removing the bulk of free and emulsified oil before the mixed liquor ever sees a membrane. A ZSQ series dissolved air flotation system in the 4-300 m³/h capacity range covers the typical segregated tank bottom water flow envelope from a single tank farm. Equalization must run 24-48 hours of HRT to dampen the 1.75-2.5× peak factor down to a manageable hydraulic load on the membranes; submersible mixers in the EQ tank must be low-shear (tip speed <3 m/s) to avoid re-emulsifying oil that the API separator has already coalesced. Skipping equalization is the single most common reason an otherwise correctly sized MBR runs out of membrane area within a month of start-up.
Step 3 — Set Biological Design Parameters (MLSS, F/M, HRT, SRT)
Size the aeration tank before touching the membrane tank — sizing the cassette first and back-solving biology is how membranes get chronically fouled. Target MLSS is 8,000-10,000 mg/L for oily tank bottom water, deliberately below the 10,000-12,000 mg/L typical of municipal MBR, because emulsified hydrocarbons raise mixed-liquor viscosity and above 12,000 mg/L the aeration tank foams and α-factor oxygen transfer collapses. F/M ratio runs 0.05-0.15 kg COD/kg MLSS·day; lean to 0.08-0.10 for refractory petroleum hydrocarbons. HRT is 18-36 hours in the aeration tank and 1-2 hours in the membrane chamber. SRT must be 30-60 days — long enough to retain slow-growing hydrocarbon-degrading biomass (Pseudomonas, Rhodococcus, Acinetobacter) and short enough to avoid excess EPS that would glue oil droplets to the membrane. DO setpoint: 1.5-2.5 mg/L in the aeration basin, 3-5 mg/L in the membrane scour zone where cross-flow is highest. pH control between 6.5-8.0 is handled by an automatic chemical dosing system tied to the equalization tank outlet.
| Parameter | Tank bottom water design | Municipal MBR (for contrast) | Driver |
|---|---|---|---|
| MLSS | 8,000-10,000 mg/L | 10,000-12,000 mg/L | Viscosity & α-factor |
| F/M | 0.05-0.15 kg COD/kg MLSS·d | 0.10-0.20 | Refractory hydrocarbons |
| HRT (aeration) | 18-36 h | 4-8 h | Slow hydrocarbon kinetics |
| HRT (membrane) | 1-2 h | 1-2 h | Cross-flow velocity |
| SRT | 30-60 d | 15-30 d | Biomass specialization |
| DO (aeration) | 1.5-2.5 mg/L | 1.5-2.0 mg/L | Scour zone needs more |
| DO (scour) | 3-5 mg/L | 2-4 mg/L | Air-scour O₂ demand |
| pH | 6.5-8.0 | 6.5-8.0 | Nitrification window |
Step 4 — Select Design Flux and Membrane Area
Flux selection is where oily-feed MBR design diverges most aggressively from generic texts. Apply 10-15 LMH for oily/emulsified feed and 15-20 LMH only when the upstream DAF has driven O&G below 20 mg/L; 20-25 LMH — the textbook municipal number — is not sustainable on tank bottom water regardless of how clean the upstream looks on a grab sample. The sizing formula is Membrane area (m²) = Q_peak (m³/h) ÷ [flux (LMH) × 0.001] × safety factor (1.10-1.20). Submerged PVDF flat sheet modules are preferred over hollow fiber for oily feeds: flat sheet geometry allows rigid air-scour across the entire panel face, and oil shears off the flat surface during relaxation, whereas oil lodges permanently in hollow fiber lumens and requires recovery cleaning. A DF series flat sheet module in the 0.1 µm pore size range, packaged as an integrated MBR membrane bioreactor system at 80-225 m² per cassette, produces 32-135 m³/day per module. For worked-example scale, take a design flow of 200 m³/day average × peak factor 1.75 = 350 m³/day = 14.6 m³/h peak; at 12 LMH design flux: 14.6 ÷ (12 × 0.001) × 1.15 = ~1,400 m², or 7-8 DF-series 225 m² cassettes. Compare this to a municipal MBR at the same 200 m³/day, which would specify ~250-350 m² — the oily-feed design carries 4-5× the membrane area to absorb the fouling load.
| Feed condition | Design flux (LMH) | Safety factor | CIP interval (weeks) |
|---|---|---|---|
| Oily/emulsified, O&G 50-100 mg/L | 10-12 | 1.20 | 2-4 |
| Pre-treated, O&G <20 mg/L | 15-20 | 1.15 | 4-8 |
| Municipal reference | 20-25 | 1.10 | 4-12 |
Step 5 — Size Scour Aeration and Recirculation
Scour aeration is the second-largest energy load after pumping, and it is the variable that controls whether the membrane runs two weeks or two months between cleans. Scour air requirement is 0.3-0.5 Nm³/m² membrane area·h for flat sheet modules, delivered through dedicated blowers sized at 1.2× nameplate to absorb altitude and fouling derating. For the 1,400 m² worked example, that is 420-700 Nm³/h of dedicated scour air — a non-trivial blower. Recycle from the membrane tank back to the aeration basin runs 3-5× permeate flow to keep MLSS distributed evenly and to prevent sludge blanket from settling on the membrane panels; specify an end-suction centrifugal pump in ductile iron or 316SS with a mechanical seal rated for hydrocarbon service (HNBR or FKM elastomers). Sludge wasting at 1-2% of influent flow is routed to a plate and frame filter press for dewatering to 18-22% DS cake — the waste biological solids carry adsorbed hydrocarbons, so the press must be in a classified area and the filtrate returned to the head of the DAF.
Step 6 — Pilot Test Protocol and CIP Frequency
No full-scale MBR should be committed on tank bottom water without an 8-12 week pilot, ideally spanning a full turn-around or seasonal swing so the worst-case O&G slug is captured. Log TMP, flux, temperature, MLSS, and scour air flow every 15 minutes via the pilot skidded PLC; trigger CIP when TMP rises to 30-40 kPa (vs. 5-10 kPa clean baseline). CIP chemicals: 1,000-2,000 mg/L NaOH + 200-500 mg/L NaOCl for organic/foulant recovery, followed by 1-2% citric acid for inorganic scale (calcium, iron sulfide from desalters). Standard CIP sequence: alkaline soak 2 h → backwash → acid soak 1 h → backwash → return to service, with each transition logged for ΔTMP. Expected CIP frequency on oily feed is every 2-8 weeks, versus 4-12 weeks on municipal feed — this delta is a direct cost-of-ownership input (NaOH, NaOCl, citric acid, downtime) and must be priced into the lifecycle OPEX, not absorbed as a surprise maintenance call. A pilot that delivers >6 weeks between cleans at design flux is the engineering go/no-go gate for full-scale procurement.
| Fouling symptom | Likely cause | Corrective action |
|---|---|---|
| TMP rises 5-10 kPa in <7 days | O&G breakthrough from DAF | Check DAF air:oil ratio; verify polymer dose |
| TMP plateaus then jumps | Biofilm / EPS accumulation | Alkaline+NaOCl CIP; review SRT |
| TMP spikes during rain event | Peak factor exceeded design | Increase EQ HRT; throttle forward flow |
| Flux decay at constant TMP | Gel-layer polarization | Increase MLSS wasting; check viscosity |
| Irreversible ΔTMP after CIP | PVDF oxidation / irreversible fouling | Module replacement; audit NaOCl strength |
Frequently Asked Questions
What flux should be used for MBR on tank bottom water?
10-15 LMH for oily/emulsified feed; 15-20 LMH only when upstream DAF holds O&G below 20 mg/L. The 20-25 LMH municipal default is not sustainable on tank bottom water.
What MLSS is appropriate for oily tank bottom MBR?
8,000-10,000 mg/L — below the 10,000-12,000 mg/L municipal range, because emulsified hydrocarbons raise mixed-liquor viscosity and above 12,000 mg/L aeration transfer collapses.
How much pre-treatment is required before the MBR?
API separator → 24-48 h equalization → DAF to <50 mg/L O&G (preferably <20 mg/L) → pH/temperature trim. The DAF is the unit operation that protects the membrane from blinding.
How often will the MBR need CIP on tank bottom water?
Every 2-8 weeks on oily feed versus 4-12 weeks on municipal feed. Trigger CIP when TMP reaches 30-40 kPa, using NaOH+NaOCl for organics and citric acid for scale.
What is the cost impact of using RO after the MBR?
MBR effluent typically runs 10-50 mg/L COD and <1 NTU — adequate feed for RO, but TDS at 5,000-80,000 mg/L means RO sees osmotic pressures of 1-15 bar, and antiscalant selection must handle residual silica and sulfate from the upstream DAF.
Related Equipment
- DF series flat sheet membrane module — specifications, capacity range, and technical data
