Sewage Treatment - How it Works Today
There are three modern lagoon systems at the Western Treatment Plant.
A modern lagoon is typically made up of 10 large ponds, each of which can hold around 600 million litres of water. Sewage flows slowly through these ponds, allowing bacteria to break down the organic material. The water gets cleaner and cleaner as it flows through each of the ponds.
Flow chart: How the modern lagoon systems work
- Raw sewage flows into lagoon systems at the Western Treatment Plant.
- Massive covers suppress the odour, halve greenhouse gas emissions and capture methane gas.
- The methane gas is used to power the aerators and other parts of the plant.
- Aerators dissolve oxygen into the wastewater.
- The activated sludge system removes nitrogen from the wastewater.
- After 30 to 35 days in the lagoon system, the treated effluent is either recycled or is discharged to Port Phillip Bay.
- Recycled water is supplied to a range of offsite customers, including Melbourne Water’s Hoppers Crossing pumping station, and used onsite for irrigation and conservation.
There are two main types of ponds used in lagoon treatment - anaerobic (without oxygen) and aerobic (with oxygen). Anaerobic and aerobic ponds produce different types of bacteria – and we need both types of bacteria to break down the sewage.
The first stage of lagoon treatment is anaerobic. An anaerobic pond has no oxygen because the oxygen has been consumed by the bacteria that breaks down the pollution in the sewage.
Under anaerobic conditions, bacteria produce strong, unpleasant smells and release dangerous greenhouse gases. These ponds are covered with plastic covers that capture the unpleasant smells and greenhouse gases. We use the captured gases (called biogas) as fuel to generate electricity to run the plant.
As sewage becomes more treated, more oxygen becomes available in the water. In the first pond however, we need to use aerators to pump air (or oxygen) into the water to reduce the unpleasant smellls. In remaining ponds, oxygen is available naturally. As ponds become more aerobic, smells become less of a problem.
By the tenth pond, sewage is known as treated effluent, and is ready for either recycling or for discharge into Port Phillip Bay.
It usually takes around 30 to 35 days for sewage to be processed through each of the plant’s three modern lagoons.