Every compressor that runs a piston or a screw element pulls in ordinary workshop air, and workshop air always carries water vapour. Compression heats that air and packs the water molecules closer together, so once the air cools again, on the way through the aftercooler, in the receiver, or in the pipework, some of that vapour has nowhere to go and turns back into liquid. The temperature at which this starts to happen is called the pressure dew point: below it, water drops out of the air stream as condensate. A workshop running a compressor most of the day, most days of the year, produces this liquid continuously.

Why the water is never clean

The condensate collecting in a receiver or a filter bowl is not just water. A lubricated piston or screw compressor uses oil to seal and cool the compression element, and a fraction of that oil carries over into the compressed air. As the water condenses out, it picks up this oil, along with any dust or pipe scale that has made its way into the system. The result is an oil-water mixture, and it behaves as one: some of it will separate out if left standing, but shaking, pumping or simply the turbulence of a working system keeps enough oil suspended or emulsified that pouring the liquid down a drain sends oil with it. Even an oil-free compressor, which uses water or air to seal the element instead of oil, is not automatically exempt: gearbox lubricants, aftercooler residues or a badly maintained inlet filter can still put trace oil into the condensate, so the assumption that no oil is present needs checking against the specific machine rather than the compressor type on the nameplate.

Where it collects

Condensate forms and settles wherever the air stream slows down or the temperature drops, which in practice means:

• the aftercooler, where hot air from the compression element is cooled before the receiver
• the receiver itself, at its lowest point, because a receiver runs cooler than the air entering it
• any line filter or dryer with a drain point
• low points in pipework runs, particularly where a line dips or runs outside

Each of these points needs a way to remove the liquid without losing compressed air with it, and what happens to that liquid once it is out of the system is a matter of trade effluent consent and waste disposal.

The sizing arithmetic behind how much air a system needs to produce in the first place is covered in the quotes guide, and the trade terms used here, including free air delivery and duty cycle, are defined in the glossary.

Environmental permitting for trade effluent and waste is devolved, so a compressor room in Cardiff or Belfast can face a different regulator, a different consent process and a different set of forms than one in Manchester or Glasgow doing the same job. The Pressure Systems Safety Regulations 2000 apply across Great Britain regardless, but what happens to the oily water once it leaves the drain is decided nation by nation.

Scotland

Discharges of trade effluent to the public sewer in Scotland are consented and regulated by the Scottish Environment Protection Agency (SEPA), and any discharge to a watercourse or to ground needs its own authorisation under the Water Environment (Controlled Activities) (Scotland) Regulations. A workshop consigning separated oil off site works with a waste carrier registered with SEPA, and the paperwork trail sits in SEPA's own systems.

Wales

In Wales, trade effluent consent for a sewer discharge is issued by the sewerage undertaker in the normal way, but Natural Resources Wales is the body responsible for permits covering discharges to controlled waters and for regulating waste carriers and waste transfer. A workshop sending oily condensate for disposal needs a carrier registered with Natural Resources Wales.

Northern Ireland

Northern Ireland runs under its own pressure systems legislation, so the written scheme and competent-person duties are worth checking against the Northern Ireland regulations directly. Trade effluent consent and waste carrier registration in Northern Ireland are handled through the Northern Ireland Environment Agency and NI Water, separately from arrangements in Great Britain.

What stays the same

Wherever the workshop sits, condensate from a lubricated compressor carries oil and cannot go straight down a surface water drain or into a watercourse, and a separator or a registered waste route is needed before disposal. The name of the regulator changes at each border; the underlying question, whether the water leaving the workshop is clean enough to discharge and who has agreed that it can, does not.

None of this substitutes for checking the current consent and waste carrier requirements with the regulator for the nation the workshop is in, since permitting conditions and registration details are set and updated by those bodies directly.