Wastewater treatment process
Lower Molonglo Water Quality Control Centre
ActewAGL's treatment process at the Lower Molonglo Water Quality Control Centre (LMWQCC) ensures that water discharged into the Molonglo River will not affect the environment.
Treated water from the LMWQCC plays an important role in keeping local rivers flowing and supporting aquatic life — especially during dry periods — and reducing algae growth.
During dry periods, treated water from LMWQCC can provide up to 100% of the total flow of the Molonglo and Murrumbidgee Rivers. Due to this significant contribution to local river systems, maintaining water quality and the effectiveness of the treatment processes is a high priority.
Following is a brief outline of the steps in LMWQCC’s wastewater treatment process. You may also view a diagram of our treatment process.
Wastewater treatment process
Wastewater enters the sewer system from homes and businesses across the ACT and arrives at LMWQCC in two main inflow pipes from north and south Canberra.
As wastewater enters the plant it is screened through three millimetre aperture screens to remove objects that could damage or interfere with the process and equipment.
Sludge and screenings
The sludge and screenings are dewatered through spinning in a centrifuge to form a thick, muddy substance called cake.
Agri-Ash
The cake is conveyed to the furnace where it is incinerated along with the grit at 750 to 1000°C to produce Agri-Ash.
Agri-Ash is collected for use as a soil conditioner.
Regulating inflow
Screened wastewater can be moved to the bypass storage dam which is used to control the inflow of wastewater to the plant and avoid hydraulic overloading. This wastewater is allowed to continue through the treatment process when flows through the plant are at a more manageable level.
Excess inflows generally occur during extended periods of heavy rain or in heavy rain when there are illegal stormwater pipe connections, cracked pipes and manhole covers along the wastewater network.
The bypass storage dam can hold up to 147 million litres, approximately 1.5 times the LMWQCC’s normal daily intake of wastewater. If required, wastewater can also be diverted to the bypass storage dam at other stages of processing.
LMWQCC also has the ability to hold back up to 25 million litres of wastewater in the sewer system until the plant is ready to
treat it.
Screen wastewater is then aerated in the grit tanks to let grit, made up of heavy particles such as sand, sink to the bottom where it is removed.
Sludge and screenings
The sludge and screenings are dewatered again to form a substance called cake which is then converted into Agri-Ash (see Step 2).
Wastewater moves to the primary sedimentation tanks where organic material settles to the bottom of the tanks as sludge. Oils and grease float to the surface. The sludge, oils and grease are then collected and removed.
Sludge
The sludge is dewatered to form a substance called cake before being processed to produce Agri-Ash (see Step 2).
Wastewater moves to the anoxic biological reactors where micro-organisms naturally remove nitrogen, releasing it into the atmosphere as gas.
Nitrogen and phosphorus can act as fertiliser for algae and water-plants. As these can be harmful to river ecosystems, they are removed during the treatment process.
Wastewater moves to the biological reactor tanks where air is pumped through porous diffusers to mix it with returned activated sludge which contains micro-organisms. The pumped air provides oxygen to the micro-organisms which naturally oxodise ammonia.
Wastewater then moves to secondary clarification where the liquid is separated from sludge which still contains the active micro-organisms.
A portion of the activated sludge is returned to the biological reactor tanks to be re-used (see Step 6). The excess activated sludge is returned to the primary sedimentation tanks (see Step 4).
Wastewater then passes through tertiary filters, which remove suspended solids to produce a clear effluent. The filters consist of finely crushed coal and sand.
The clear wastewater then moves to the chlorine contact tanks where chlorine is added to disinfect any remaining microbiological pathogens. Sulphur dioxide gas it then used to remove excess chlorine, which would be harmful to the river ecosystem. The treated water is then ready to be re-used and discharged into the river.
Effluent discharge targets
LMWQCC meets the following effluent discharge targets to ensure that treated water will not be harmful to the environment. Treated water leaving the plant contains less than:
- 8 milligrams (mg) biochemical oxygen demand per litre
- 550mg of total dissolved solids per litre
- 0.4mg of total phosphorus per litre
- 2100 kilograms per day total nitrogen average daily load limit
- 10mg of suspended solids per litre
- 1.6 to 7.4mg of ammonia per litre, depending on the season
- 0.2mg total chlorine per litre
- 200 thermotolerant coliforms per 100 millilitres