Iron (iron)

Iron (Fe) is the most abundant industrial metal and is an essential nutrient, so its effluent concern is more about nuisance and water quality than acute toxicity. At elevated concentrations dissolved iron causes brown/orange staining of water, fixtures and laundry, imparts a metallic taste, promotes iron-bacteria growth (slime), and contributes to turbidity. Its effluent discharge limit is 3 mg/L across most discharge modes.

Iron enters effluent from steel and metal processing, pickling and acid-cleaning of steel, mining, and any process handling ferrous materials. It is also commonly introduced deliberately as a treatment chemical — iron salts (ferric chloride, ferrous sulphate) are widely used as coagulants and for phosphate and sulphide precipitation — so iron can appear in treated effluent from the treatment process itself if overdosed.

For recyclers, iron is ubiquitous given that ferrous metal is the largest-volume recycled material. Steel and iron scrap recycling, the ferrous fraction of e-waste, ELVs and mixed waste, and tyre bead wire all involve iron. Acid pickling or cleaning of ferrous scrap, and any wet handling of rusty or ferrous material, can put iron into the effluent. Iron is also relevant in the CBG sector, where iron-oxide media are used for H₂S scrubbing.

The practical relevance is that iron is usually an easily managed effluent parameter — it is removed by oxidation (aeration converts soluble ferrous iron to insoluble ferric) followed by settling/filtration, the same aeration-and-settle approach that is the simplest of all metal-removal steps. The main practical notes are to avoid overdosing iron-based coagulants (which would itself raise effluent iron), and to recognise that while iron is not highly toxic, visibly iron-stained discharge draws complaints and inspector attention just as suspended solids do. Meeting the 3 mg/L limit is generally straightforward, making iron one of the less troublesome metals in recycling effluent.