Precipitation of heavy metals
The use of lime to adjust pH levels (by making them higher) can precipitate most heavy metals in the form of hydroxides.
Precipitation of phosphates and sulphates
When waste water contains phosphates and sulphates, lime makes it easier to precipitate them as well as the heavy metals, in the form of insoluble salts - Ca3(PO4)2, CaSO4 - that are easy to collect.
The coagulant action of lime neutralises the negative charge on colloid particles contained in water. It also causes the flocculation of suspended impurities which makes decanting easier. Lime acts as the flocculating agent to which metallic salts and/or polymers can also be added.
Waste water purification separates out and concentrates the pollutants suspended in the water in the form of solid matter known as residual sludge. This sludge is a by-product which consists of organic and/or mineral matter, which can contain trace elements such as heavy metals.
Biological residual sludge contains organic material which can ferment and is a health hazard because it contains pathogens.