Wastewater is water that has been contaminated and must undergo treatment before being released into other bodies of water. If not treated, wastewater can lead to further contamination. Anything that is flushed down the toilet or rinsed down the drain is considered wastewater. However, wastewater can also come from other sources, such as agriculture.
Rainwater and runoff, which may contain various pollutants, are channelled to wastewater treatment facilities through street gutters. Farms can also produce a large amount of wastewater. This agricultural waste also contains pesticides and fertilisers.
Both domestic and agricultural wastewater require treatment before being reintroduced to freshwater sources. However, agricultural wastewater is more difficult to treat than domestic wastewater. This is because it often contains higher levels of pollutants, such as pesticides, fertilisers, and animal waste, which are more challenging to remove. Agricultural wastewater is also typically spread over a larger area.
If wastewater is not handled properly, it can harm the environment and many living organisms. Some of these negative impacts include:
The sewage treatment process includes the following steps:
1. The sewage either flows or is pumped to a treatment facility.
2. During the screening and grit removal process, large materials like leaves and silt are removed.
3. During sedimentation, sewage settles in large tanks. This allows heavier solids to sink to the bottom and form sewage sludge. The lighter liquid that remains at the top is called effluent.
4. Anaerobic bacteria digest the sludge, producing biogas that can be burned to generate electricity. The digested sewage sludge can also be used as fertiliser.
5. Aerobic bacteria treat the effluent by digesting organic molecules within the water. This process involves pumping air into an aeration tank to facilitate bacterial digestion (aeration).
6. Once treated, the effluent is released into the sea or nearby rivers.
For wastewater containing toxic substances, additional treatment stages may be necessary, such as: