Grey Water (Greywater) Storage & Tank Compatibility

Storing Grey Water (Greywater)? Start Here

Grey water (greywater) is the aqueous wastewater generated by showers, baths, bathroom and kitchen sinks, dishwashers and washing machines — every domestic source except toilets. It is overwhelmingly water (typically more than 99%), carrying a mixed load of surfactants from soaps and detergents, fats, oils and grease (FOG), suspended solids, organic matter, trace salts and microbes. It is not a single chemical with a fixed formula; composition swings with the source, so no single CAS number or SDS describes it.

Because it can make up the majority of household wastewater volume, grey water is widely captured for irrigation and non-potable reuse, which means it must be stored, settled and sometimes treated. Material of construction matters less for chemical attack here — the stream is near-neutral and dilute — and more for long-term durability against a wet, biologically active, surfactant- and grease-bearing environment that can stress-crack or corrode the wrong material over years of service.

Is Polyethylene (HDPE / XLPE) Compatible with Grey Water?

Yes — polyethylene is compatible (rating S) and is the standard tank material for grey water. The stream is a dilute, near-neutral aqueous liquid, which sits comfortably within polyethylene's broad resistance to water, mild bases and dilute aqueous mixtures. HDPE and crosslinked polyethylene (XLPE) tanks are the most common and economical vessels for grey-water collection, settling and non-potable reuse, and they are immune to the biological corrosion that attacks bare steel in this duty.

The one real caveat is environmental stress cracking. Surfactants, soaps and FOG are classic ESCR-reducing agents: combined with sustained mechanical stress, they can initiate cracks at fittings, welds and high-stress points well below the resin's tensile strength. Specify a high-ESCR polyethylene grade, keep fittings properly supported, and avoid storing hot grey water at elevated temperature for long periods. With those precautions, poly is the right and proven choice.

Material compatibility at a glance

Grey water is a dilute, near-neutral aqueous wastewater, so the dominant compatibility driver is simply water service with mild surfactant and FOG content — not aggressive chemical attack. HDPE and XLPE polyethylene are the standard, cost-effective choice; specify a high-ESCR resin because surfactants and grease can accelerate stress cracking at fittings and welds. Stainless steel suits pumps and process equipment. Bare carbon steel needs lining against biological corrosion.

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

• Biological / microbial load: grey water contains bacteria and pathogens; treat it as a hygiene hazard — avoid skin/eye contact and ingestion, and do not store untreated grey water more than ~24 hours before it turns septic.
• Septicity & odor: stored grey water depletes oxygen and can generate foul, potentially hazardous gases (including hydrogen sulfide) under anaerobic conditions; ventilate enclosed tanks.
• Slip / spill: soapy, FOG-laden liquid is slippery; contain spills and clean promptly.
• Variable pH: laundry sources can push pH toward 9 - 10 from bleach and caustic soaps — check before reuse on plants/soil.
• Additive-dependent hazards: the SDS of any concentrated cleaner, bleach or solvent that enters the stream governs the real hazard; do not assume grey water is inert.
• Regulatory: reuse is governed by local plumbing/health codes; confirm permitted uses and storage limits.

Common questions

Can I store grey water in a standard polyethylene water tank?

Yes. Grey water is a dilute, near-neutral aqueous stream and polyethylene (HDPE/XLPE) is the standard, economical tank material for it. Choose a high-ESCR resin and a dark/opaque tank to inhibit algae, and do not hold untreated grey water for long periods before it turns septic.

Does grey water have an NFPA 704 rating or SDS?

Not as a defined product — it is a variable mixture, not a single chemical, so there is no manufacturer SDS. A representative composite is roughly Health 1, Flammability 0, Reactivity 0 (non-flammable, low reactivity). Always defer to the SDS of any concentrated additive in the stream.

Why is environmental stress cracking a concern for poly grey-water tanks?

Surfactants, soaps and grease in grey water are known ESCR-reducing agents. Combined with mechanical stress at fittings and welds, they can initiate cracks below the resin's normal strength. Specifying a high-ESCR polyethylene grade and supporting fittings properly prevents this.

How long can grey water be stored before it becomes a problem?

Untreated grey water typically should not be stored more than about 24 hours; it goes anaerobic (septic), develops odor and can release hazardous gases. For longer storage, treatment (filtration/disinfection) is required and reuse must follow local code.