Most treatment technologies will remove substances other than the target substances. For instance, biological treatment can effectively remove a certain amount of metal ions from wastewater. Because metal ions do not particularly like to be dissolved in water (they are hydrophobic), they are driven by the second law of thermodynamics to be adsorbed on the surface of solids just about any solids, including activated sludge solids.

This mechanism for removing metals from wastewater is often undesirable, since the presence of the metal ions in the waste sludge may render the sludge unsuitable for a desired disposal method. Composting with wood chips to produce a horticultural soil conditioner is an example. Another example is simple disposal in an ordinary municipal landfill. In other instances, the removal of metal ions from a wastewater that has very low concentrations of them may serve as a fortuitous polishing step. In any case, biological treatment is a principle technology for removal of organics from wastewaters, and its unintended removal of metal ions is a nonprinciple mechanism.