The reason that wastewater needs to be purified is because it contains a lot of substamces as phosphorus, organic substances, bacteria, viruses and environmental toxins. It is not desirable for such substances to be returned to the sea. lakes, streams or rivers. Discharges of these substances will be very damaging for the life in the ocean.
Wastewater can be purified mechanic, biological or by a combination of this methods. Chemical treatment plant are built especially to purify chemicals as mercury, cadium, chrome, iron, gold, copper and phosphorus, while the biological method are mainly made to remove organic materials.