Chlorinated Bilge Water Storage & Tank Compatibility

Storing Chlorinated Bilge Water? Start Here

Chlorinated bilge water is not a single chemical — it is a shipboard waste stream. Bilge water collects in a vessel's lowest compartments and is a complex mixture of seawater, emulsified fuel, lubricating and hydraulic oils, detergents, solvents, rust and suspended solids. "Chlorinated" means the stream has been dosed with sodium hypochlorite (or electro-generated free chlorine) to control bacteria, odor and biofouling before treatment or discharge. The result is an oily, saline, oxidizer-bearing emulsion that is notoriously difficult to handle.

Materials of construction matter because two separate attack mechanisms act at once: chlorides and free chlorine drive metal corrosion, while the dispersed petroleum fraction permeates and swells common plastics. Bilge-water holding, slop and oily-water-separator feed tanks must therefore be specified for hydrocarbons and an oxidizing brine simultaneously — a combination that rules out standard polyethylene for long-term containment.

Is Polyethylene (HDPE / XLPE) Compatible with Chlorinated Bilge Water?

Honest verdict: not recommended (U) for long-term primary storage. The aqueous side of bilge water is actually friendly to polyethylene — HDPE handles seawater/brine and dilute sodium hypochlorite well, with little or no damage to HDPE reported after extended exposure to even concentrated hypochlorite. The problem is the oil. Bilge water carries emulsified diesel, fuel oil and lubricants, and petroleum hydrocarbons permeate the polyethylene matrix, causing swelling, plasticization, loss of stiffness/strength and accelerated environmental stress cracking (ESC). Published work shows polyethylene can absorb several percent of its weight in fuel with roughly a 50% drop in strength and modulus, and HDPE coupons fail rapidly under ESC test conditions in diesel/biodiesel.

For these reasons, treat poly as suitable only for short-term collection or transfer of bilge water, not for long-term holding. For permanent bilge/slop service choose internally lined or coated steel, vinyl-ester FRP, or stainless with chloride-pitting monitoring. XLPE improves ESC resistance over HDPE but does not overcome long-term hydrocarbon permeation, so it carries the same long-term caution.

Material compatibility at a glance

Chlorinated bilge water is a dual-threat stream: a salty, hypochlorite-dosed aqueous phase plus an emulsified petroleum (fuel/lube/hydraulic-oil) phase. Polyethylene tolerates the brine and dilute chlorine but is steadily attacked by the hydrocarbon fraction (permeation, swelling, stress cracking), so it is not suitable for long-term primary storage. Lined/coated steel slop tanks, vinyl-ester FRP, or stainless with corrosion monitoring are the appropriate materials; seals should be FKM/PTFE, not EPDM.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Oxidizer hazard: never mix chlorinated bilge water with acids or ammonia — it can liberate toxic chlorine or chloramine gas (EUH206).
• Flammable film: the entrained fuel/oil layer can ignite; keep ignition sources away and treat as a potential flammable-liquid surface even though the bulk is water.
• Irritant: hypochlorite and surfactant content cause skin and serious eye irritation; wear chemical goggles and gloves.
• Confined space: bilge and slop spaces accumulate vapors and are oxygen-deficient; follow confined-space entry and ventilation procedures.
• Environmental: oil and chlorinated by-products are harmful to aquatic life; discharge only via a compliant oily-water separator meeting MARPOL Annex I 15 ppm limits.
• Corrosion / failure risk: chloride + chlorine corrode unlined metal and hydrocarbons crack plastic — confirm tank MOC before use to avoid leaks.

Common questions

Can I store chlorinated bilge water in a poly (HDPE/XLPE) tank?

Only for short-term collection or transfer. Polyethylene handles the salty, chlorine-dosed water phase fine, but the emulsified fuel and lube oil permeate and swell the plastic and promote stress cracking over time. For long-term holding use lined/coated steel, vinyl-ester FRP, or monitored stainless.

Why is bilge water 'chlorinated' in the first place?

Sodium hypochlorite or electro-generated free chlorine is added to kill bacteria, suppress odor and control biofouling/slime in holding and slop tanks before the stream is treated or discharged. It is a disinfectant dose, typically a few ppm of free chlorine, not bulk chemical storage.

What actually attacks the tank — the chlorine or the oil?

Both, by different routes. Chlorides and free chlorine drive pitting/corrosion in metals, while the petroleum (fuel/lube) fraction permeates and swells plastics and cracks polyethylene. A bilge-water tank must be rated for an oxidizing brine and hydrocarbons at the same time.

What seal and gasket materials should I use?

Use FKM (Viton) or PTFE. EPDM resists the hypochlorite but is swollen and degraded by the oil fraction, so it is a poor choice for an oily bilge stream. FKM/PTFE handle both the hydrocarbons and the oxidizer.