Metal finishers are splitting their oil: acid, alum, and a DAF‑versus‑UF showdown

Hot‑dip galvanizing and electroplating shops run alkaline cleaners, often NaOH‑based surfactant baths, to degrease metal. Once spent, those cleaners become high‑pH, oil‑laden emulsions — frequently “thousands of mg/L” of oil and grease (mg/L denotes milligrams per liter). A pilot study put a typical metal‑finishing wastewater at about 0.5% oil/grease, roughly 5,000 mg/L (www.researchgate.net).

That kind of load dwarfs common discharge limits. Indonesian standards for plating/galvanizing set TSS (total suspended solids) ≤20 mg/L (www.karbonaktif.org); oil/grease is not explicitly listed in this category, but in practice must be removed to meet solids/clarity requirements. Unremoved emulsified oil harms downstream treatment and violates volatile‑organic guidelines.

In day‑to‑day operations, spent baths become unusable after only a few months as oil accumulates — forcing expensive disposal (www.filtsol.com). The upshot: pretreatment is critical; the oil needs to be separated before final treatment or discharge.

Chemical emulsion splitting (acid/alum)

To destroy the stable surfactant emulsion, plants deploy chemical splitting agents. Acidification (e.g., H₂SO₄ or HCl) neutralizes the alkaline surfactants; protonation breaks micelles and destabilizes the emulsion so oil coalesces into droplets. In effect, acid provides a positive charge that overcomes the negative zeta potential on oil droplets, releasing trapped oil. Roughly stoichiometric neutralization is needed — about ~0.5 mol H₂SO₄ per mol NaOH — plus extra to drive demulsification.

Alum (Al₂(SO₄)₃) hydrolyzes to Al³⁺ and OH⁻. The Al³⁺ cations adsorb onto oil droplets, neutralizing their charge and forming insoluble hydroxides, which coagulate oil into flocs. An EPA guidance for metal‑finishing oily waste notes emulsion‑breaking is typically done “by the addition of chemicals (alum and polymers) to accomplish coagulation and flocculation of the oily wastes” (nepis.epa.gov). A patent states that “positively‑charged alum at low pH neutralizes the negative stabilizing charge on the oil droplets … to cause demulsification and coagulation” (patents.google.com). In practice, a synthetic flocculant or polymer is often added alongside alum to strengthen floc formation; many facilities source such coagulants via coagulant programs.

Demulsifier dosages are tuned by jar tests. As a rule of thumb, chemical costs run on the order of a few dollars per m³ of wastewater. For example, treating oily produced water with ferric chloride was about ~$4/m³ (coagulant cost only) (www.mdpi.com). Spent alkali baths will typically need comparable or higher amounts of acid/alum to neutralize and drop the oil. The outcome of this step: the once‑stable emulsion is “broken,” oil coalesces, and it can be removed physically.

Physical separation: DAF and UF

Once demulsified, the liberated oil and solids are removed by physical means — typically dissolved air flotation (DAF) or ultrafiltration (UF), both common in physical separation systems.

In DAF (dissolved air flotation), the demulsified wastewater is pressurized with air, typically 3–6 bar, to create microbubbles. These bubbles adhere to oil droplets and flocs, buoying them to the surface for skimming. Flotation‑based systems have demonstrated on the order of ~90% removal of oil and grease and suspended solids in industrial service; a full‑scale refinery effluent treatment achieved ~90% O&G removal and 75% COD (chemical oxygen demand) removal using flotation (pmc.ncbi.nlm.nih.gov). A typical single‑pass DAF plant can reduce an influent oil level of ~5,000 mg/L by tenfold or more (depending on coagulant dosing) to a few hundred mg/L. DAF energy usage is modest: one study found flotation consumed ~0.091 kWh/m³ (versus ~0.86 kWh/m³ for a membrane bioreactor, MBR) (pmc.ncbi.nlm.nih.gov). A packaged DAF unit has relatively low capital cost — small industrial units can be ~$15–20k (evuchina.en.made-in-china.com) — and low operating cost, making it cost‑effective for moderate and high flowrates where full removal to near‑zero isn’t required. Trade‑offs: DAF leaves a residual oil concentration (often 5–10% of inlet) and produces a separated oily sludge (water plus concentrated oil) requiring disposal or further treatment.

Ultrafiltration (UF) uses a membrane — typically polymeric hollow‑fiber or tubular — to physically retain oil droplets and solids while passing water. As a polishing step, UF can reduce oil/grease levels almost to zero. In a pilot treating 0.5% O&G (5,000 mg/L) emulsified wastewater, UF produced permeate with <50 mg/L oil and <25 mg/L TSS, about ~97–98% oil removal with either charged or neutral UF membranes (www.researchgate.net). UF units require pre‑filtration of large particles and potential stabilization (e.g., pH control) but, once optimized, deliver high flux on the order of 80–100 L/m²·h in these cases (www.researchgate.net). The trade‑off is cost: UF has higher capital and operating costs (membrane modules, high‑feed pumps, periodic chemical cleaning), with energy on the order of 0.2–0.3 kWh/m³ or more and eventual membrane wear. However, UF does not require ongoing chemical dosing for separation (only for membrane cleaning) and generates minimal sludge — the concentrated retentate is typically a small fraction of the volume. Facilities specifying UF often consider packaged ultrafiltration units as a compact polishing option.

Performance deltas and maintenance

Empirical data highlight the gap: DAF with coagulant will normally leave oil in the hundreds of mg/L, whereas UF can achieve below 50 mg/L (www.researchgate.net). Numerically, DAF‑based pretreatment (API+DAF) removed 90% of organics (pmc.ncbi.nlm.nih.gov), while UF achieved ~98% oil rejection (www.researchgate.net). Maintenance profiles differ: DAF needs periodic sludge removal and chemical refills; UF requires membrane cleaning/CIP schedules.

Cost‑effectiveness and selection guide

Bulk removal vs. recycle: If the aim is to meet discharge limits (TSS, COD, etc.) at minimum cost, a single‑stage DAF with acid/alum pretreatment is often best. It has low capital and low energy demand — flotation energy at ~0.091 kWh/m³ (versus ~0.86 kWh/m³ for an MBR) (pmc.ncbi.nlm.nih.gov). Chemical cost is moderate — think ~$2–4/m³ for coagulant as a rule of thumb (www.mdpi.com). One example approach: neutralize with ~10–20 kg H₂SO₄ per m³ (bringing pH to ~6–8), add ~200–500 mg/L alum (with polymer), then send to DAF — yielding effluent oil in the few‑hundred mg/L range, often sufficient to meet permits after final polishing.

High purity or reuse needs: If very low oil levels are required (e.g., <50 mg/L) or water reuse is desired, UF becomes attractive despite higher cost. Industry experience shows ultrafiltration can prolong bath life 2–4× (www.filtsol.com), meaning fewer changeouts and hazardous waste disposals. In lifecycle terms, saving on raw cleaner and disposal ($100–$300 per ton for hazardous waste) can justify the NSF of UF.

Volume and scale: For very large flows, DAF scales economically. Small shops with moderate flow (<10 m³/day) might opt for packaged UF units (with feed pump and cartridge/tubular membranes) if water quality needs are stringent. UF tends to have a smaller footprint.

Hybrid approaches: Sequential processes can be used. An acid/alum breakup plus DAF can be followed by UF polishing of the DAF effluent — pushing oil down to <10 mg/L if needed. This doubles capital costs and is justified mainly when regulatory or reuse standards demand it.

Regulatory compliance: Indonesian effluent standards for plating/galvanizing (Permen LHK 5/2014) specify tight limits on metals and TSS (e.g., TSS ≤20 mg/L) (www.karbonaktif.org). While oil and grease aren’t explicitly set in this category, in practice any high oil load must be removed to avoid solids carryover and meet treatment objectives. Achieving near‑zero free oil (as UF does) essentially guarantees compliance, but a well‑designed DAF system typically also meets legal thresholds with much lower cost.

Summary comparison metrics

DAF+chemistry: Low CAPEX, low energy, leaves moderate oil in effluent. Example outcome: ~90% removal (e.g., 5,000→500 mg/L) (pmc.ncbi.nlm.nih.gov). Produces large sludge (~1–3% vol.).

UF: Higher CAPEX/OPEX, higher energy, yields very clean effluent (<50 mg/L O&G) (www.researchgate.net). Little sludge. Good for recycling.

Adoption notes and sources

In practice, a cost‑effective decision often favors DAF with acid/alum splitting for high‑volume or disposal‑focused scenarios, and UF (or UF polishing) when water reuse or extremely low oil levels justify the expense. One industry source notes that, driven by operating cost and regulation, many metal‑finishing plants have implemented UF bath‑recycling systems as the most economical approach over time (www.filtsol.com).

Key Data Points: Removing ~90% of oil with flotation (into the low‑hundred mg/L range) (pmc.ncbi.nlm.nih.gov). Achieving ~97–98% removal to ~<50 mg/L with UF (www.researchgate.net). DAF pretreatment energy ~0.091 kWh/m³ (flotation only) (pmc.ncbi.nlm.nih.gov). Coagulant costs on the order of $1–4/m³ (www.mdpi.com).

Sources: Guidelines and studies from industry and regulators support the above. The U.S. EPA metal‑finishing effluent development document models an “Option 1” system of alum+polymer breaking + skimming (nepis.epa.gov). Recent technical reviews report ~90% oil removal via flotation (pmc.ncbi.nlm.nih.gov) and ~98% via ultrafiltration (www.researchgate.net). These data, along with pilot cost analyses (www.mdpi.com) (pmc.ncbi.nlm.nih.gov), underpin the design choices outlined.