The Bin Wash Station: What Happens to Ninety Litres of Water per Bin

A depot washing 60 bins a day uses about 90 litres per bin. That is 5,400 litres a day, 1.3 million litres a year, carrying food solids, fat and detergent. In most depots it goes to a drain with a silt trap that gets cleaned when somebody remembers. The bins come out clean and the drain does not, and the trade effluent consent that covers the wash bay usually has a suspended solids limit that nobody has ever tested against.

Wash water is the stream nobody doses, because it is not food waste and it is not quite wastewater. This article covers what is actually in it, the order of operations that keeps solids out of the drain, and how to size a settle-and-dose system for a 60-bin depot.

Wash Water Is the Stream Nobody Doses

Ninety litres per bin sounds like a lot until you watch the process: a pre-rinse, a scrub with detergent, a final rinse, and a pass with a squeegee. The first rinse carries most of the solids, perhaps 90 percent of them in the first 20 litres. The rest is detergent and dilute organic matter, which is why the first rinse is the one worth capturing and the last two are usually fine to drain.

That single observation cuts the problem by two-thirds. Capture 20 litres per bin instead of 90, and a 60-bin depot is dealing with 1,200 litres a day rather than 5,400.

Order of Operations: Empty, Rinse, Settle, Decide

Pre-rinse over a collection tray, not over the drain. The tray drains into a 1,000-litre settle tank with a sloped floor and a weir, where solids drop out over about four hours. The supernatant goes to drain if your consent allows it; the sludge at the bottom, perhaps 150 to 300 litres a day, is what gets dosed at 20 grams per litre and sent out with the food waste.

Do not dose the whole 5,400 litres. At 15 grams per litre that would be 81 kilos of polymer a day, which is more than most depots spend on the entire food waste stream. Settle first, then dose the fraction that actually needs it.

Sizing a Settle-and-Dose System for 60 Bins

Three tanks: a 500-litre collection tank under the wash tray, a 1,000-litre settle tank, and a 200-litre dose barrel. The dose barrel takes the sludge draw-off twice a day, 100 to 150 litres at a time, at 20 grams per litre, so 2 to 3 kilos per draw and 5 to 6 kilos a day. That is about 19 dollars a day at typical pricing, or roughly 5,000 a year.

The alternative is a tanker. At 0.12 dollars a litre for 300 litres of sludge a day, you are looking at 36 dollars a day, or 9,400 a year, plus the risk that a tanker cannot come on a Saturday. The polymer route is cheaper, and it keeps the material inside the waste stream you already pay to have collected.

The Drain Is Not a Bin

Three parameters decide whether your wash water is acceptable to drain: suspended solids, pH and fats. Detergent pushes pH up, food pushes it down, and the two together swing it around neutral, which is usually fine. Fats are the risk, because they congeal in the pipe and are what causes the call-out at 6 a.m. on a Monday. A 5-micron bag filter on the drain line costs 200 dollars and removes most of the solids that carry the fat.

Read your consent before you change anything. Most consents specify a maximum volume per day and a maximum concentration for suspended solids and fats, and discharging a new stream without declaring it is the kind of thing that gets discovered during a site visit and charged for retrospectively.

A tray with a lip, a settle tank with a visible sludge line, a dose barrel with a paddle and a marked 20-gram-per-litre card, and a log that records the daily draw and the kilos used. Four items, all cheap, and the whole system fits in a corner of the yard behind the wash bay.

If your wash bay is sending solids to drain and nobody has looked at the consent since it was signed, start with the pre-rinse tray. Send us your bin count, water use and sludge volume and we will size the dose barrel and the daily draw; the grades suited to detergent-bearing sludge are on the SAP powder products page.