ACT wastewater treatment process

Step 1: Removal of large objects

Wastewater enters the Lower Molonglo Water Quality Control Centre (LMWQCC) via two sewer mains, the Ginninderra Outfall Sewer Tunnel and the Molonglo Valley Interceptor sewer.

The sewage is stored in a holding facility and regulated to maintain a constant flow through the plant, normally between 1,000 and 1,300 litres per second.

Sewage is first passed through fine screens to remove objects that could damage or interfere with the process and equipment. The screens remove all objects greater than 5 mm (millimetres) in diameter.

These objects, called screenings, are macerated, or softened, dewatered and then mixed with sludge to be centrifuged before being burnt in a furnace.

During the screening, a variety of interesting and unusual objects are recovered such as false teeth, plastic toys, garden connections, cans, bottles, metal objects and paintbrushes.

Step 2: Primary treatment

The sewage flows from the screens into the primary sedimentation tanks. Chemicals are added and the raw sewage is agitated, or mixed, to help the chemicals react.

This process is called flocculation and the end result is a ‘sewage sludge’ that is easier to separate from the wastewater later in the process.

The flocculation also removes some chemicals from the water such as phosphorus. This is crucial as phosphorous contributes significantly to the growth of toxic algae in rivers and reservoirs.

Mechanical scrapers are used to collect the sludge from the bottom of the primary sedimentation tanks and floating scum from the surface. They are collected separately in troughs that are emptied regularly. The sludge and scum are then treated separately.

Step 3: Removal of ammonia

Ammonia is removed through a biological reaction process. At high concentrations, ammonia is toxic to fish so it is essential to remove it before water is returned to river systems.

Ammonia also has an extremely pungent smell. You can recognise the smell of ammonia in cleaning products, cat urine, and even sweat!

Step 4: Secondary clarification

Following the ammonia removal, sludge particles are flocculated, or caused to form larger particles which are heavier and easier to separate.

The mixture of wastewater and flocculated bacteria is pumped to the secondary clarifiers where a separation process removes the solids, including the micro-organisms in the sludge.

Skip to Step 7 below to find out what happens to the separated sludge and scum.

Step 5: Filtration

A further chemical treatment removes most of the remaining traces of phosphorus, and then the wastewater is passed through the final set of filters.

These filters consist of finely crushed coal about one metre in depth, and then sand, about half a metre in depth. This filtering process removes solids that remain suspended in the wastewater, leaving a clear effluent, or liquid waste.

Step 6: Chlorination

Chlorine is added to the effluent to disinfect, or kill, any remaining microbiological pathogens. This process takes at least half an hour.

The effluent is then treated with sulphur dioxide gas to remove excess chlorine, as too much chlorine would be harmful to a river’s ecosystem.

The effluent is then ready for discharge into a river. The effluent is now very clean water. It is so clean that ActewAGL uses clear effluent from the treatment plant for fish tanks containing live native fish. Platypus also live in the water just below the effluent outlet.

Step 7: Treatment of sludge and scum

Sludge and scum, removed in earlier steps of the process, are now dealt with separately.

The sludge is stored in a sludge holding tank. Polymers are added to the sludge to help it thicken. The sludge is ‘dewatered’ by spinning it in a centrifuge to separate water from the solids. What remains is a thick, muddy substance known as "cake". (It’s not the kind of cake you would want to eat, though!)

The “cake” is moved to a furnace where it is incinerated at 750-1000°C to produce a fertiliser known as Agri-Ash. This is sold to farmers as a soil conditioner.

About 700,000 litres of sludge is treated each day to produce 47 tonnes of solids. After incineration, this becomes about 16 tonnes of Agri-Ash .

Scum is treated separately. It is stored in a heated tank to thicken by evaporating the water. It is then also moved to the furnace to be incinerated.

Step 8: Discharge of effluent into the Molonglo River

Typically, 80-100 million litres of wastewater is treated at LMWQCC every day. This is equivalent to 35-40 Olympic swimming pools full of water!

The treated wastewater is discharged into the Molonglo and Murrumbidgee Rivers. From there, it drains into the Murray-Darling Basin river system, ultimately discharging into the ocean south of Adelaide. This river system plays an important role in providing water to inland communities for domestic and agricultural purposes, such as drinking, irrigation and recreation.