Electroplating effluent contains $\mathrm{Cr}{}^{3+}$ and $\mathrm{Ni}{}^{2+}$ ions. A standard treatment step is to

1. Abubble oxygen through it until the ions evaporate
2. Badd sugar so that bacteria convert the ions to gases
3. Craise the pH with lime so that the metal hydroxides precipitate
4. Dlower the pH to below 1 to dissolve the ions further