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Wastewater Treatment Levels
Wastewater from the sewerage system flows to sewage treatment plants which are located near rivers or on the coast. Here the wastewater is treated before being discharged into rivers or oceans.
The discharges from Sydney Water's treatment plants must comply with licenses issued by the Environment Protection Authority. These licences specify the quantity and quality of treated wastewater that can be released to rivers and the ocean. The effect of wastewater treatment discharges on water quality and aquatic life is monitored every week.
To minimise the impact of treated wastewater discharged to waterways, Sydney Water is upgrading treatment processes and investigating recycling opportunities. Sydney Water is also educating the community about simple things we can all do to help improve treated wastewater.
The level of treatment ranges from high-rate primary to tertiary.
Primary is the initial stage of sewage treatment and involves removing solid particles from the sewage. Sewage is filtered through fine screens to take out solid matter such as paper, cotton tips and plastic. Heavy particles like sand sink to the bottom and are removed. The sewage then flows into the primary sedimentation tanks. Here human waste, called sludge, settles to the bottom and oils and grease float to the top where they are collected.
High-rate primary treatment occurs at high flow rates and removes fewer solids.
Secondary treatment removes dissolved and suspended organic and inorganic solids, through bacterial decomposition which breaks down this material. The primary effluent flows into tanks where the bacteria grows and naturally treats the wastewater. The liquid flows into sedimentation tanks where more particles settle on the bottom for collection.
Tertiary treatment further removes inorganic compounds, and substances such as the plant nutrients nitrogen (N) and phosphorus (P).
Disinfection can be applied at any treatment level to inactivate disease-causing micro-organisms such as bacteria, viruses and parasites.
During periods of heavy rain, when there are very high flow rates in the wastewater system, sewage flows may be diverted around some processes at the plants (a partial bypass).
