Two rival playbooks dominate mine wash-water cleanup: chemical treatment with dissolved air flotation (DAF), and ultrafiltration (UF) membranes. The data points to a pragmatic first choice—and a clear path when ultralow oil is non‑negotiable.
Heavy‑equipment wash water doesn’t just look dirty—it quantifiably is. Field studies report oil concentrations around 80–160 mg/L, with a broader range of 10–386 mg/L, and surfactants (surface‑active detergents) at 3–68 mg/L (researchgate.net) (researchgate.net). The solids problem is just as real: TSS (total suspended solids) can run high with silt and clay.
Indonesian coal‑wash discharge limits under KLH 113/2003 set pH at 6–9, total iron (Fe) ≤7 mg/L, manganese (Mn) ≤4 mg/L, and TSS ≤400 mg/L (greenchem.co.id). There’s no explicit oil limit, but oil and grease (OG) load COD/BOD (oxygen demand) and must be minimized before discharge or reuse. In practice, a treatment train has to knock down TSS and crack emulsified oil; typical targets are in the tens of mg/L for oil/grease and under 400 mg/L TSS.
Influent characterization and primary separation
One design truism stands up in the field: characterize the influent first. Where TSS and oils are extremely high (≫1,000 mg/L), a robust primary stage—sedimentation and skimming—before any advanced unit process is warranted. Simple front‑end options such as primary screens and oil removal and a dedicated free‑oil separator stabilize the load to downstream equipment.
Chemical coagulation/flocculation plus DAF
Coagulation/flocculation (chemical aggregation of fine particles and emulsified droplets) followed by DAF (dissolved air flotation; microbubbles lift floc to the surface) is the workhorse combination for gritty, oily wash water. With typical coagulant doses on the order of 100–500 mg/L, a single‑stage coag+DAF can usually push oil/grease to well under 50 mg/L and TSS removal above 90% (pmc.ncbi.nlm.nih.gov) (redalyc.org). In oil‑refinery pretreatment, an API (gravity oil‑water) separator plus DAF removed ≈90% of TSS and oil/grease and cut COD by 75% (pmc.ncbi.nlm.nih.gov).
The approach scales and travels: a mobile DAF pilot stripped 95% of oil from produced water (redalyc.org). In ship‑bilge tests, adding PAFC/PAC coagulants at 300–500 mg/L and microbubble flotation reduced 3,000–5,000 mg/L emulsified oil to under 15 mg/L—about 99.7% removal (pmc.ncbi.nlm.nih.gov).
Energy needs are modest. Flotation routes clocked in near 0.091 kWh/m³ for API+DAF, versus about 0.86 kWh/m³ for a membrane bioreactor (MBR) in comparable duty (pmc.ncbi.nlm.nih.gov). DAF systems have also become more compact and cost‑efficient than older coag‑sedimentation plants (redalyc.org).
Performance is chemical‑dependent. Without flocculant aid, DAF oil removal may be moderate (~77% oil, 59% TSS), rising to ~94% oil with polymer support (researchgate.net). Sludge handling is integral: DAF sludge is voluminous with water content often >95% (wet floc requiring dewatering). pH control—often with lime—plus skilled dosing are required for stable operation. Mines typically meter coagulants with a dosing pump and draw on polyaluminum coagulants and flocculant polymers to break emulsions before a DAF unit skims the floated layer.
Ultrafiltration membranes under oily loads
UF (ultrafiltration) uses polymer membranes with ~0.01–0.1 µm pores as an effective physical barrier to oil droplets and fine solids. In a carwash effluent study, the UF train reported “almost 100% removal of oil” with permeate turbidity around 0.2 NTU (nephelometric turbidity units; a clarity measure) (mdpi.com). Another industrial program kept UF effluent OG “consistently below 100 mg/L” with very low TSS (researchgate.net). Final permeate turbidity of 0.12–0.35 NTU was recorded after periodic cleaning in the wash‑water study (mdpi.com).
But the trade‑offs are operational. The same carwash study noted that “intensive fouling was noticed” under oily, surfactant‑rich loads, and UF units needed frequent CIP (clean‑in‑place) with alkaline detergent at about pH 11.5 to hold flux (mdpi.com) (mdpi.com). Energy/pressure is higher (typically ~1–3 bar feed), membranes can degrade under oil adsorption and repeated cleans, and upsets hurt flux if pretreatment slips.
Concentrate handling is different from sludge. One UF pilot accumulated a retentate of roughly 5% oil, further concentrated (to about 50% oil) and hauled offsite (researchgate.net). For projects pushing into reuse, modular skids such as ultrafiltration can deliver very high clarity with no added coagulant (beyond occasional cleaning reagents, for which plants draw on membrane cleaners), but expect higher capex and maintenance.
Cost, energy, and achievable outcomes
In practice, coagulation/flocculation plus DAF is more cost‑effective for heavy‑duty wash water with high fats, oils, and grease (FOG) and TSS—unless extremely high polishing is mandated. One refinery comparison found the flotation route (API+DAF) had about an order‑of‑magnitude lower energy use (0.091 vs 0.86 kWh/m³) and roughly one‑sixth the net present cost relative to an MBR (pmc.ncbi.nlm.nih.gov). Modern DAF designs are trending more compact and economical than legacy clarifiers (redalyc.org).
Removal rates with DAF routinely satisfy typical discharge targets: a ~90% TSS cut can drop raw loads from several thousand mg/L to a few hundred mg/L—meeting Indonesia’s 400 mg/L TSS limit (pmc.ncbi.nlm.nih.gov). UF delivers higher purity—including tiny emulsified droplets that might escape flotation—but at higher energy, cleaning frequency, and replacement risk. In one UF carwash test, operators ran 5–6 days of cyclic operation with daily caustic cleans; long‑term flux stabilized but only under harsh cleaning (mdpi.com). Hybrid schemes—DAF followed by UF—are common where ultralow OG is required.
Selection guide for mine wash bays
A coal‑mine equipment wash facility can use the following decision anchors, backed by pilot jar tests on actual wash water:
- Removal target. If discharge needs are in the “few–tens mg/L” OG range, DAF alone often suffices; if “nearly zero” OG is required for reuse or future limits, plan UF (or MF) after flotation.
- Flow rate and variability. High and variable flows favor the turndown and simplicity of DAF; smaller, steady flows can suit membranes.
- Footprint and energy. UF skids are compact but rely on pumped pressure; DAF tanks are larger but low‑pressure and energy‑light.
- Operating costs. DAF spends on chemicals (e.g., alum/iron salts and polymer—often 20–100 mg/L each, with total doses commonly 100–500 mg/L) and compressors/mixers; UF spends on electricity and frequent CIP. The refinery example reported DAF OPEX at roughly one‑sixth that of a membrane system for similar influent (pmc.ncbi.nlm.nih.gov).
- Sludge and concentrate disposal. DAF produces wet sludge with water content often >95% (~5–10% solids), requiring dewatering; UF produces an oily concentrate (a pilot saw ~5%‑oil retentate, further concentrated to ~50% oil for offsite hauling) (researchgate.net).
- Reliability and operations. DAF is mechanically simpler and more tolerant of feed swings; UF demands steady pretreatment (screens, sedimentation) and frequent cleaning. Upstream stability tools include an automatic screen and general wastewater ancillaries for flow equalization.
Putting the train together
For a coal‑mine wash bay targeting Indonesian TSS limits, coag‑floc with polymers (e.g., PAC or ferric coagulant plus PAM) and a DAF system is typically the most cost‑effective first step. Where ultralow OG is required for reuse or anticipated rules, add UF membranes downstream. Pilot testing with actual wash water—checking OG, TSS, and surfactants—is recommended to size reagents and predict fouling and sludge behavior. Mines generally source coagulants and polymers via coagulant programs and water‑treatment chemicals, meter them with a dosing pump, and cost out equipment and O&M to select the most economical, reliable mix. Where reuse quality is a future goal, teams also evaluate broader membrane systems roadmaps.
Source notes and data points
Reported OG in heavy‑equipment wash water: ~80–160 mg/L (range 10–386 mg/L); surfactants 3–68 mg/L (researchgate.net) (researchgate.net). Indonesian coal‑wash effluent limits: pH 6–9, Fe ≤7 mg/L, Mn ≤4 mg/L, TSS ≤400 mg/L (greenchem.co.id).
DAF outcomes: 90–95% TSS and O&G removal is routine; API+DAF showed ≈90% OG/TSS and 75% COD reduction (pmc.ncbi.nlm.nih.gov) (redalyc.org). A mobile DAF pilot removed 95% oil (redalyc.org). Ship‑bilge microflotation with PAFC/PAC (300–500 mg/L) cut 3,000–5,000 mg/L emulsified oil to <15 mg/L (~99.7%) (pmc.ncbi.nlm.nih.gov). Without polymer, DAF removal may be ~77% oil and 59% TSS; with polymer ~94% oil (researchgate.net). Energy: API+DAF ≈0.091 kWh/m³ vs MBR ~0.86 kWh/m³, and roughly one‑sixth the net present cost (pmc.ncbi.nlm.nih.gov).
UF outcomes: “Almost 100% removal of oil,” permeate turbidity ~0.2 NTU; final 0.12–0.35 NTU after periodic cleaning; “intensive fouling was noticed”; frequent alkaline CIP at pH ~11.5; 5–6 days cyclic runs with daily caustic cleans; flux stabilized under harsh cleaning (mdpi.com) (mdpi.com). UF effluent OG “consistently below 100 mg/L” in an industrial study; retentate ~5% oil concentrated to ~50% oil for hauling (researchgate.net).
