Shipyards Are Engineering Hull Washwater to Near‑Zero Discharge

On modern hull‑treatment pads, every drop counts. The floor is impervious and sloped to sealed trenches and sumps—often covered to exclude rain—so 100% of rinse/runoff is captured and piped to a tank (paint booths pair leak‑proof sumps with curbs to ensure 100% capture, with no discharge to water). Initial screens or grit traps intercept larger fragments, and oil/water separation protects downstream stages. By capturing runoff at the source and matching conveyance to plant capacity, the facility averts uncontrolled discharges and no overflow.

There’s regulatory muscle behind the design. Indonesia’s “clean industry” mandate for plating/welding (Permen LHK 5/2014) places metals around ~0.5–1.0 mg/L (Cu, Zn, Ni, etc.) and total suspended solids (TSS) at ~20 mg/L, according to Ady Water. The treatment train below is built to reliably deliver under those targets.

Impervious collection and containment

Capture starts with engineered hardscape: sloped, sealed trenches and sumps feeding a central tank, with covered drains to exclude stormwater. Shipyards layer in primary barriers—screens and grit traps—to keep debris out of pumps, a use case for primary separation gear and a manual screen at the headworks. Where petroleum cuts or solvents are present, a dedicated stage for free‑oil removal shields the core process. The goal is simple and explicit: collect 100% of washwater (no discharge to water) and meter it to treatment with no overflow.

Metal‑salt coagulation parameters

Coagulation—adding metal salts and polymers to destabilize colloids—is the core removal step for fine particulates and dissolved heavy metals. Typical coagulants are ferric chloride (FeCl₃) or alum (Al₂(SO₄)₃). Empirical studies show FeCl₃ at ~30–50 mg/L plus a 0.2–1.0 mg/L organic polymer dose yields very high metal removal; for example, 40 mg/L FeCl₃ + 0.5 mg/L anionic polymer achieved ~95% Pb, 92% Cr, 79% Cu, and 57% Zn removal in spiked wastewater (ResearchGate).

In practical shipyard tests, combining basic lime (CaO) with FeCl₃ was even more effective: a lab/full‑scale study reported 99.0–99.9% removal of Cu, Zn, Pb—with similar rates for Cr and Fe—using CaO + FeCl₃ (ResearchGate). These results imply that heavy‑metal concentrations around 10 mg/L (Cu/Zn) can be driven into the µg/L range—well below Indonesia’s thresholds (e.g., Cu <0.5 mg/L: Ady Water)—and align with bench‑scale outcomes below. Dosing execution hinges on metering; plants typically use an accurate dosing pump with on‑site jar tests to verify the final setpoint via commercial coagulants and complementary flocculants.

Design parameters matter: rapid mix for ~20–60 s after coagulant addition, then slow mixing/flocculation for 5–15 min to grow flocs. Optimal pH is crucial—ferric/chloride systems work best near neutral to slightly alkaline (pH ~7–9) where metal‑hydroxides precipitate; chrome often needs pH ~8–9, nickel higher. Jar tests frequently show >80% TSS removal and >90% heavy‑metal capture at the optimum dose (ResearchGate). As a starting point, FeCl₃ 30 mg/L + polymer 0.5 mg/L can achieve >80–90% reduction of suspended solids and precipitated metals (dose adjustments via jar testing recommended; study reference repeated for clarity: ResearchGate).

Gravity clarification and flotation options

Flocculated water flows to clarification (gravity sedimentation or dissolved‑air flotation, DAF). In a properly designed clarifier (detention ~30–60 min), most floc and associated metals settle out; suspended‑solids removal commonly exceeds 70–90%. Johnson et al. assumed 80% TSS removal in their baseline (ResearchGate). After clarification, effluent TSS is typically <10–20 mg/L and turbidity often under 10 NTU (nephelometric turbidity units). For hard‑to‑settle flocs or oil‑rich streams, flotation can enhance separation. The combined chemistry + sedimentation ensures final effluent metals generally fall below strict limits.

Secondary clarification can add lamella plates or weirs to maximize floc capture; compact modules like a lamella settler increase surface area without expanding footprint, and a conventional clarifier or a targeted DAF unit can be selected per influent characteristics. In one design case, post‑treatment Cu, Zn and phenol were all below regulated thresholds, with metal levels described as “lower than necessary” in the clarified effluent (ResearchGate; standards context: Ady Water).

Multi‑media filtration polishing

Clarified water is polished with rapid sand or multi‑media beds, which remove >90% of remaining suspended solids and drive turbidity below 5 NTU (often <1 NTU). Intermittent sand filters have demonstrated up to 99% bacteria removal and effective fine‑particle capture (Water & Wastewater). In practice, a well‑managed filter produces effluent essentially free of visible turbidity; residual metal‑bearing particulates (<10 µm) are trapped, and periodic backwash returns solids to sludge handling.

For media selection, a dual‑layer bed pairing sand/silica with anthracite is widely used for fine colloid capture. If exceptionally low metal concentrations are needed for reuse, a final adsorption step using activated carbon or ion‑exchange resin may be added, but for hull‑scrubber runoff this is usually unnecessary once solids are removed.

Dewatering and B3 disposal

Clarifier underflow concentrates nearly all captured metals; thickened sludge typically measures ~2–5% solids. Dewatering—via plate‑and‑frame or recessed‑plate filter press, belt filter press, or centrifuge—reduces volume and haulage. A recessed‑plate filter press can take ~3% feed to ~20% cake solids, an 85% volume reduction; an EPA case reported 3% feed dewatered to 20% solids (EPA NEPIS). Belt presses typically achieve ~15–25% solids. The dewatered cake is semi‑dry, often <5% of treated water volume, but must be handled as hazardous B3 waste (Indonesia’s classification for toxic/hazardous materials) due to Pb, Cu, Hg, Zn content; Indonesian shipyard practice (e.g., PT PAL) confirms these metals in sludge (ResearchGate).

By law, such sludge must be packaged/neutralized (if required) and sent to a licensed hazardous‑waste facility. Disposal costs are significant: EPA studies note untreated sludge disposal ranges ~$0.20–0.75 per gallon, whereas dewatered sludge (at 20% solids) is about $0.20–0.50/gal (EPA NEPIS). Thorough dewatering (to 15–25%) and sludge minimization via good flocculation therefore reduce hauled volume and cost.

Measured outcomes and standards

A well‑designed system yields >80–90% removal of TSS and heavy metals, meeting or surpassing Indonesian standards (Ady Water). Bench‑scale treatment has lowered Cu and Zn to <0.05 mg/L and Pb to <0.02 mg/L (ResearchGate), echoing the earlier finding that CaO + FeCl₃ can deliver 99.0–99.9% removal for Cu, Zn, Pb with similar rates for Cr and Fe (ResearchGate).

The treated water can often be recycled for cleaning or safely reused in the yard; sludge production is on the order of kilograms per ton of metal processed (often <0.1 m³ per ton metal), requiring occasional B3 disposal. Full‑scale experience backs the flow sheet: one boat‑wash installation reduced Cu, Zn and Sn “to the regulated values” using FeCl₃ flocculation (ResearchGate), with metals in clarified effluent described elsewhere as “lower than necessary” (ResearchGate).