Refineries use about 2.5 gallons of water for every gallon of crude processed, and what returns as cooling-tower blowdown carries oil traces, zinc, and phosphate. An engineered pretreatment train—API/DAF, pH adjustment, clarification, and filtration—cuts those loads before the water hits the main effluent plant.
Cooling towers concentrate whatever the refinery puts in. Typical blowdown shows total dissolved solids (TDS, dissolved salts) around ~1,300 mg/L, phosphate ~5–7 mg/L, zinc ~1–2 mg/L, dissolved organic content (COD, a proxy for oxidizable organics) around ~100 mg/L, and small oil traces (mdpi.com) (mdpi.com). Refineries are major water users—roughly 2.5 gal of water per gal of crude processed (wateronline.com)—so blowdown volumes and contaminant loads add up fast.
Chemical programs often put the culprits there: standard cooling-water dosing runs ~0.5–2.5 mg/L Zn and 5–10 mg/L phosphate, which then appear in blowdown (Buecker and Post, 2019; chemengonline.com). Proposed U.S. guidelines aim for Zn ≤1 ppm (power-eng.com), and many receiving waters are labeled “phosphorus‑impaired,” effectively pushing phosphate discharge toward near‑zero (power-eng.com). In Indonesia, plant effluent must meet national standards (typically Zn ~0.5–1 mg/L, phosphate O(1 mg/L), O&G ≲5–10 mg/L, pH ~6–9).
Oil/water separation and flotation
The first move is to strip free and dispersed oil. An API‑gravity separator (API: a gravity-based oil/water separator; a plate coalescer is a common variant) followed by dissolved‑air flotation (DAF, a process that uses microbubbles to lift flocs to the surface) is recommended. Gravity separators alone remove ~25–65% of oil (nepis.epa.gov), so coagulant‑aided DAF is used to exceed ~80% removal★ (same source).
Hardware choices skew industrial. Primary removal can sit within screens and oil-removal primary systems to keep debris and free oil from reaching downstream tanks. For refining services, packaged oil removal units help reduce O&G to ≲5 mg/L before polishing. Many plants standardize on dissolved air flotation skids for that >80% uplift cited by EPA benchmarks.
pH adjustment and metal/phosphate precipitation
After oil removal, pH adjustment and co‑precipitation (forming insoluble compounds that settle) tackle dissolved species. Raising pH to ~8–9 starts carbonate/hydroxide precipitation; zinc in particular begins to precipitate as Zn(OH)₂ around pH 9–10. A two‑step chemical program is suggested: (a) dose a coagulant—ferric chloride or alum—to bind phosphate (forming FePO₄/AlPO₄) and co‑precipitate trace metals; (b) add lime or caustic to elevate pH and precipitate zinc and remaining hardness.
Dosing benchmarks are explicit. Ferric chloride around 10–20 mg Fe per L can remove ≳90% of ~5–6 mg/L PO₄. Raising pH to ~10 then precipitates zinc efficiently, targeting Zn below effluent limits. For consistency and control, chemical feeds often run through a metering dosing pump to maintain the ~8–10 pH window stepwise. Coagulant selection aligns with available coagulant formulations for refinery wastewater.
Clarification and solids handling
Coagulation‑flocculation tanks typically run ~30–60 minutes detention, producing metal hydroxide and phosphate flocs. A clarifier or tube settler then removes these solids, with >90% TSS removal typical from this stage. For a 20 m³/h blowdown carrying 6 mg/L PO₄ and 1.2 mg/L Zn, precipitating essentially all P and Zn yields ~0.12 kg/h solids (mostly Fe/Al phosphates and Zn hydroxide). The solids are dewatered separately.
Settling equipment varies by footprint. A conventional clarifier handles the 0.5–4 hour detention common to sedimentation. Where compactness matters, a tube settler can increase clarifier capacity 3–4×, reducing footprint significantly.
Filtration and final neutralization
Polishing follows. A sand or multimedia filter, or ultrafiltration (UF, a membrane step that sieves out fine particles), can capture fines and residual emulsified oil. Filtration targets a final suspended solids level <5 mg/L and typically removes >90% of any remaining O&G. After filtration, oil & grease levels should come in at <5 mg/L and TSS ≲5 mg/L (media‑dependent).
Material choices tend to be robust. Dual‑media beds based on sand/silica filtration are common for refinery blowdown. Plants seeking tighter polish across variable loads often add ultrafiltration modules downstream of clarification. For very fine polishing, facilities sometimes enclose elements within cartridge filters to catch sub‑10‑micron particles.
Because precipitation elevates pH, a final neutralization step is standard. The treated stream often exits the train at pH ~9–10; an acid or CO₂ contactor then trims pH into the 6.5–8.5 range before the main effluent plant (Indonesian standards typically allow pH 5.5–9.0). For tight control during neutralization, a dosing pump maintains acid addition and protects downstream biological units.
Performance metrics and regulatory context
The integrated train is designed to remove roughly 90%+ of Zn and PO₄ and >95% of oil and TSS. For example, with influent Zn at 1.2 mg/L and PO₄ at 6.6 mg/L (mdpi.com), post‑treatment levels could drop to ~0.1–0.2 mg/L Zn and <1 mg/L PO₄—compliant with strict limits (power-eng.com) (power-eng.com). Oil & grease reductions typically run from perhaps 20–50 mg/L down to <5 mg/L.
EPA data anchor the oil stage: gravity separators alone remove ~25–65% of oil, improving to >80% with chemical flotation (nepis.epa.gov). Cited guidelines (e.g., NPDES) suggest Zn limits ≈1 mg/L (power-eng.com). Literature reviews report electrochemical methods can remove ~99% P and Zn, and a simpler precipitation route can conservatively achieve ≥90% for moderate loads (mdpi.com) (mdpi.com) (note: electrocoagulation studies report ~99% metal removal, but conventional precipitation can also achieve >90% for moderate loads).
Two load snapshots illustrate scale. Processing 20 m³/h of blowdown (≈480 m³/day) and removing 6 mg/L PO₄ corresponds to ~2.9 kg P removed per day; the precipitant chemical (FeCl₃) added might be on the order of 30–50 kg/day to achieve that, yielding sludge to be handled. Separately, if blowdown is 20–50 m³/h, then Zn load is ~20.0–60.0 mg/h (0.48–1.44 g/day) and PO₄ is ~120–330 mg/h (2.8–7.9 g/day). Removing 90% leaves small residual loads; failing to pretreat would otherwise send ~480–1200 mg/day Zn and ~2.9–7.9 kg/day PO₄ to the effluent plant.
The design premise is straightforward and data‑driven: refineries are heavy water users (wateronline.com), typical blowdown carries Zn ≈1.2 mg/L and PO₄ ≈6.6 mg/L (mdpi.com) (mdpi.com), and regulatory context pushes Zn ≲1 mg/L and phosphorus effectively “to zero” (power-eng.com) (power-eng.com). Parkson (2018) details refinery water usage (wateronline.com), while Buecker and Post (2019) underscore the chemical origins of Zn and phosphate in blowdown (chemengonline.com).
