How to Size MBR for Compressor Oily Condensate (2026 Specs)

What Makes Compressor Oily Condensate a Different Design Problem

Compressor oily condensate is the aqueous discharge from inter-stage separators, after-coolers, and air-receiver drains on reciprocating and screw compressors, contaminated with lubricating oil carryover, condensed hydrocarbons, and trace rust inhibitors. It is not refinery desalter water (which has lower free oil but higher dissolved salts) and it is not food-industry FOG (which carries high suspended solids and emulsified fats). The defining traits are a low flow (typically 5–500 m³/day for a single plant), an intermittent duty tied to compressor runtime, and a dual oil phase — free oil at 50–500 mg/L that can spike above 1,000 mg/L on compressor startup, plus emulsified oil at 200–2,000 mg/L that does not separate by gravity alone.

Typical characterization envelope (Zhongsheng field data, 2025–2026): COD 5,000–25,000 mg/L, BOD₅ 1,500–8,000 mg/L, TSS 100–600 mg/L, total O&G 250–2,500 mg/L, pH 6.5–8.5, temperature 30–60 °C. The COD:BOD₅ ratio of roughly 3:1 indicates a stream that is biodegradable but oil-laden — well within the operating envelope of an aerobic MBR, provided the free oil is removed upstream. The MDPI 2022 review of biological processes in MBRs treating industrial oily wastewater (Bhattacharyya et al., J. Mar. Sci. Eng., 10(9):1229, published 2022-09) confirms aerobic MBR as a proven configuration for hydrocarbon-bearing petrochemical streams, but the review explicitly notes that oil-and-grease loading and emulsified fractions are the primary drivers of membrane fouling and must be controlled before the membrane tank.

Intermittency is the second axis that breaks generic oily-water MBR guides. Condensate flow is governed by compressor duty cycle, ambient humidity, and intercooler condensation rate; flow can swing 3–5× across a single shift, and on Monday-morning cold-start the free-oil fraction can triple before stabilizer rings seat. A biological tank sized only on average flow will be overloaded on peaks and starved on standby; the design must absorb this with equalization and a generous safety factor on the membrane area.

The 5-Step Sizing Method for a Compressor-Condensate MBR

The sizing method below is the order to follow on a P&ID, in a Basis-of-Design memo, or in a permit submittal. Each step is anchored to a measurable parameter so the result is defensible in front of an EPC reviewer or regulator.

Step 1 — Characterize. Collect one 24-h composite and a minimum of eight grab samples across one operating week, including one cold-start and one peak-load event. Report free oil (hexane-extractable), emulsified oil, total O&G, COD, BOD₅, TSS, temperature, pH, conductivity, and a flow profile versus compressor runtime. Without this, every downstream number is a guess.

Step 2 — Pretreat. Route through a corrugated plate interceptor (CPI) for free-oil skimming, then a DAF to drop total O&G below ~50 mg/L before the bioreactor. Unscreened oil films the membrane surface and inhibits oxygen transfer in the aeration tank; a 0.1 μm PVDF membrane will foul in days, not months, if O&G is not reduced first. An integrated MBR membrane bioreactor system should be specified only after this pretreatment envelope is locked.

Step 3 — Equalize. Size the equalization tank at 8–24 h of average flow, with mechanical mixing and gentle aeration to keep emulsions suspended and prevent anaerobic odor generation. The 8–24 h window covers the 3–5× flow swing typical of compressor duty cycles without overflowing on a cold-start spike.

Step 4 — Bioreact. Aerobic tank at HRT 18–36 h, MLSS 8,000–12,000 mg/L, SRT 25–40 d, F/M 0.05–0.15 kg BOD/kg MLSS·d, DO 2–3 mg/L, pH 7.0–7.5. Seed with hydrocarbon-acclimated sludge from a refinery or produced-water WWTP if available; otherwise seed from a municipal plant and ramp MLSS over 4–6 weeks while feed O&G is held below 100 mg/L.

Step 5 — Membrane-filter. Select a submerged PVDF flat-sheet module at flux 8–12 L/m²·h (40–60% below municipal MBR norms), with relaxation-plus-backflush cycles every 8–12 min and dedicated scour aeration at 0.3–0.5 m³ air per m² membrane area per hour. A DF-series PVDF flat-sheet membrane module in the 80–225 m² per-unit envelope is the standard selection for this duty.

MBR Design Parameters for Oily Condensate (2026 Reference Table)

The table below consolidates the working envelope for a compressor-condensate MBR. Use it as the MBR section of the process datasheet or the PFD callout block.

The flux envelope of 8–12 L/m²·h is intentionally 40–60% below the 20–25 L/m²·h typical of municipal MBRs; this is the price of running an MBR on a stream that carries residual emulsified oil even after DAF. The conservative flux is consistent with membrane-foulant characterization work on hypersaline oily wastewater, which identified oil–protein–polysaccharide gel layers as the dominant fouling mechanism at higher fluxes (Elsevier Chemical Engineering Journal, 2010, 10.1016/j.cej.2010.12.053, study published 2010-12, findings still cited as the baseline envelope in 2026 oily-MBR design). DF-series modules cover 80–225 m² per unit at 32–135 m³/day output with 0.1 μm PVDF flat sheet, which lets a 20–200 m³/day plant select one or two modules without overbuilding.

Pretreatment Train: Why CPI and DAF Are Non-Negotiable

Skimping on pretreatment is the single most common cause of premature membrane fouling and biological crash in oily-condensate MBRs. Free oil must be reduced to <50 mg/L (preferably <20 mg/L) before the bioreactor: oil films block 0.1 μm membrane pores within hours and coat biomass flocs, suppressing oxygen transfer and dropping SRT-effective biomass. Two unit operations handle this reliably.

Corrugated plate interceptor (CPI). Sized at 0.5–1.0 m³/m²·h surface loading with 5–15 min residence time, with an automatic oil-skimming weir on the outlet. The CPI removes bulk free oil that would otherwise overwhelm the DAF and consume excessive polymer.

Dissolved air flotation (DAF). Sized at 4–6 m³/m²·h hydraulic loading for oily streams, air-to-solid ratio 0.02–0.05 kg air/kg TSS, and polymer dose 2–10 mg/L (coagulant plus flocculant) to break oil-in-water emulsions. A ZSQ dissolved air flotation system in the 4–300 m³/h range covers the typical 5–200 m³/day condensate envelope in a single skid.

A chemical-dosing skid is the third leg of the train. Polymer selection and pH adjustment are not optional on condensate; pH must be lifted to 7.0–7.5 before biology, and coagulant feed (typically PAC 50–150 mg/L plus anionic polymer 2–5 mg/L) is what breaks the emulsified fraction that gravity alone cannot drop. An automatic chemical dosing skid with PLC-controlled injection keeps these setpoints stable across the 3–5× flow swing. For plants already running DAF sizing for white water or other oily streams, the same DAF envelope applies; only the polymer dose and pH target shift.

Sizing the Membrane: Flux, Area, and Module Count

The membrane-area calculation for a submerged MBR is straightforward, but only if the inputs are honest. The formula is:

Membrane area (m²) = Daily flow (m³/day) × 1000 / (24 × design flux, L/m²·h)

Worked on a 20 m³/day average flow with a 1.5× peak factor (so 30 m³/day peak) and a 10 L/m²·h design flux: required area = 30,000 / (24 × 10) = 125 m². At a design flux of 8 L/m²·h the same duty needs 156 m², which is the lower bound during winter operation at 15 °C when viscosity rises. The DF-series module family at 80 m² or 225 m² per unit means a 20 m³/day plant selects two 80 m² units (160 m² installed, ~28% redundancy) or one 225 m² unit with headroom for future load growth.

Temperature derating matters for outdoor or uninsulated equalization tanks: flux drops roughly 1.5% per °C below 20 °C as water viscosity rises. A 30 °C condensate stream needs no derating; a 15 °C winter stream needs 7–8% additional area. Always install at least one spare module in parallel so any single unit can be isolated for chemical cleaning-in-place (CIP) with 1,000–2,000 mg/L NaOCl or 0.5–2% citric acid without shutting down the whole MBR. The integrated MBR membrane bioreactor system skid envelope of 10–2,000 m³/day accommodates the full 5–500 m³/day compressor-condensate range covered by this article. A complete area and module selection is built around the DF-series PVDF flat-sheet membrane module envelope, which is the only 0.1 μm submerged PVDF unit family sized for this exact duty class in 2026.

Worked Numerical Example: 20 m³/day Compressor Condensate MBR

The example below shows the full method end-to-end on a realistic duty: three 200 kW screw compressors at 70% average loading, producing 20 m³/day of condensate with a 1.5× peak on hot afternoons. A 5 mm rotary bar screen on the CPI inlet protects downstream plates from rust scale, per the duty envelope of the GX-series rotary mechanical bar screen.

The effluent envelope of COD ≤50 mg/L, O&G ≤5 mg/L, TSS ≤1 mg/L is sufficient for industrial reuse (cooling-tower makeup, scrubber feed) or sewer discharge subject to local sewer-use ordinance limits. Engineers sizing zero-liquid-discharge trains downstream of the MBR for a similar oily stream can follow the same logic in the ZLD sizing for stamping press oily water guide.

Frequently Asked Questions

What flux should I use for an MBR on compressor oily condensate?

Design at 8–12 L/m²·h, which is 40–60% below the 20–25 L/m²·h norm for municipal MBRs. The conservative envelope accounts for residual emulsified oil that survives DAF and builds a gel layer on the membrane. At 30 °C no temperature derating is needed; at 15 °C derate by 7–8% (1.5% per °C below 20 °C).

Can I skip the DAF and feed the MBR directly after the CPI?

No. The CPI removes bulk free oil but leaves emulsified oil in the 200–2,000 mg/L range, which fouls 0.1 μm PVDF membranes within days and inhibits oxygen transfer in the bioreactor. A ZSQ dissolved air flotation system sized at 4–6 m³/m²·h with 2–10 mg/L polymer is required to drop total O&G below 50 mg/L before the MBR.

How do I handle cold-start oil spikes on Monday morning?

Size the equalization tank at 8–24 h of average flow with mixing and gentle aeration, then operate the DAF on a level-controlled batch mode so a 1,000+ mg/L free-oil spike is captured by CPI skimming and DAF polymer dose adjustment before it reaches the bioreactor. Peak flow factor on the MBR should be 1.3–2.0×; 1.5× is the typical selection for compressor condensate.

What MLSS and SRT work for hydrocarbon-acclimated biomass on condensate?

Operate at MLSS 8,000–12,000 mg/L and SRT 25–40 days, with F/M 0.05–0.15 kg BOD/kg MLSS·d. Long SRT favors the slow-growing hydrocarbonoclasts that biodegrade lube-oil carryover; an F/M above 0.20 typically indicates insufficient equalization or insufficient membrane flux. The MDPI 2022 review (Bhattacharyya et al., J. Mar. Sci. Eng., 10(9):1229) supports this envelope for petrochemical oily wastewater.

Is the same sizing method valid for related oily streams like white water or soapstock?

The structure is the same — characterize, pretreat (CPI/DAF), equalize, bioreact, membrane-filter — but the parameters shift. White water carries higher TSS and lower COD, so the pretreatment load moves upstream; the parallel method is in the MBR sizing for white water guide. Compressor condensate is unique in the combination of low flow, high intermittency, and dual free-plus-emulsified oil phases.