What Goes Down the Sink, and What Absolutely Does Not

Dilute acids and alkalis can go to the drain once they are neutralised to roughly pH 6 to 9 and flushed with plenty of water. Check with indicator paper rather than assuming — neutralising is easy to overshoot in either direction. Neutralise slowly, in a large vessel, with stirring, and expect heat and gas. Adding bicarbonate to acid foams vigorously, and a beaker filled to the top delivers that foam onto the bench. Salt solutions, dilute sugar, and the water you rinsed glassware with are all fine. The test is whether what you are pouring would be a problem in a river at the concentration it arrives at — which, after the drain and the works, is very dilute indeed, but not zero.

HEAVY METAL solutions — lead, mercury, cadmium, chromium, and copper in quantity — go to a labelled waste container. Treatment works cannot remove them and they accumulate in the sludge that is spread on land. ORGANIC SOLVENTS go to a labelled solvent waste container, never to the drain. Many are flammable, most are not treatable, and a trap full of solvent vapour under a bench is a hazard that waits. SOLIDS go in a bin or a solid waste container, not washed down — filter them out first. And OILS and greases set in the cold part of the pipe and block it. Wipe greasy glassware with paper before it goes near a sink.

Keep separate containers for aqueous metal waste, organic solvent, and halogenated solvent. Mixing streams makes all of them harder and more expensive to deal with, and some combinations react. Label the container with what is in it as you add — not at the end. A bottle of mixed unknown liquid is the single most awkward object a workshop can produce, because nobody can treat it without first finding out what it is. Leave headspace, keep the cap on between additions, and stand the container in a tray. A full bottle with a loose cap on an open bench is how a small waste problem becomes a large spill.

A procedure that ends at "filter and collect the product" is not finished. Add the line that says what happens to the filtrate, the rinsings and the spent reagents, with the same precision as the rest. That line is what makes the method repeatable by somebody else. Without it, the next person guesses — and the guess is almost always the sink, because it is the nearest thing. Record quantities as well. Knowing that a run produces 400 mL of dilute copper solution lets the next person have a container ready, rather than discovering the problem with a full beaker in each hand.