1. Polluted water arrives as wastewater through sewers from homes, factories and via Septic Tank Operators from the rural community.

What is wastewater and where does it come from?
Wastewater (or sewage) is dirty, used water that goes down the drains and toilets of homes, schools, businesses and factories.

Where does wastewater go?
Wastewater is collected in a network of pipes called sewers (or via the Septic Tank Operators) that carry it to wastewater treatment works. At these works, the water is cleaned before it is put into a river.

Why must wastewater be cleaned?
Firstly, the wastes in wastewater are dangerous and can threaten the health of humans and the environment. These dangerous wastes must be taken out of the water and treated and disposed of safely. Secondly, wastewater is made up of 99.9% water and only 0.1% wastes. The large amount of water is only used for transporting the wastes. Water is a precious resource, and must be returned to the natural water cycle.

2. Wastewater passes through SCREENS to remove rags, plastic bags etc.

What is screening?
When wastewater first arrives at a wastewater treatment works, it passes through metal bar screens. These screens trap large objects (like rags, papers and plastics), but allow the rest of the wastewater to pass through.

Why is this step needed?
It is important to remove the large objects because they can cause damage to the pipes, pumps and tanks in the rest of the works.

What happens to the screenings?
The screenings (objects caught by the screens) are removed and buried in a landfill.

3a. Then passes into the DEGRITTER. Here the sand and grit are removed.

How is the grit removed?
After passing through the bar screens, the grit (stones, sand and sail) is removed in the grit chamber. Here the water slows down slightly, allowing the heavy grit to sink to the bottom

Why is grit removed?
The grit could damage pumps, pipes and tanks in the rest of the works.

3b. GRIT TO BURIAL

What happens to the grit?
The grit is taken to a landfill and buried.

4. The PRIMARY SETTLING TANK is where the raw sewage is separated further. The solid matter settles along the bottom while the suspended and disolved matter flows over the top of the tank. Oil on the surface is removed.

What happens in the primary settling stage?
The wastewater flows very slowly in large tanks. This allows the fine solids (from human wastes) and some of the germs to settle to the bottom of the tank forming a thick grey layer of sludge (primary sludge). Light substances (like oils) float to the surface.

What happens to the oils?
They are scraped off the surface by a scraper and buried in a landfill.

Why must the primary sludge be treated?
Primary sludge is untreated solid human waste containing tiny organisms including germs. The sludge is therefore very smelly and dangerous and must be removed from the tanks and treated.

5. Then goes to the biological system, known as the `activated sludge process' where the liquid is biologically oxidised. Air is beaten in by mechanical aerators and the bacteria purify the liquid in the presence of oxygen.

What is biological treatment?
This is a process that uses a special mixture of tiny organisms to feed on the dissolved nutrients in the wastewater. This takes place in large tanks called bioreactors. Some of the germs in the wastewater die at this stage of the process because they cannot survive the conditions in the bioreactor.

Which dissolved nutrients are removed during this stage?
Carbon (C) (from human faeces), nitrogen (N) (from human urine), and phosphorous (P) (from soaps).

What happens to the nutrients?
The tiny organisms store some of the nutrients inside themselves and convert some to carbon dioxide gas (CO₂), nitrogen gas (N₂), oxygen gas (O₂) and water (H₂O).

Why must the dissolved nutrients be removed?
If lots of dissolved nutrients were allowed to flow into a river, they would pollute the river and reduced the oxygen levels that are vital for the health of a river system.

6. The SECONDARY SETTLING TANK is where further settling takes place, as well as the separation process between the solids and the liquids. The organisms sink to the bottom to form secondary sludge.

What happens in the secondary settling stage?
The tiny organisms that feed on the nutrients in the bioreactors, and some of the germs from human wastes naturally cluster together and form heavy clumps. In large tanks, the water slows down allowing the clumps of tiny organisms to sink to the bottom forming a brown sludge layer (secondary sludge). This leaves a clear layer of water in the top section of the tanks.

What happens to the clear water layer?
It is send for disinfection.

Why must the secondary sludge be treated?
The sludge consists of the tiny organisms that were used in the biological treatment stage, and a few germs from human wastes. These organisms can be a health threat to humans and the environment.

What happens to the secondary sludge?
It is removed from the bottom of the tank, and is dried, killing the germs. The dried sludge is then ploughed into the land as a fertilizer, or used to make compost.

7a. The liquid from the Secondary Settling Tanks is then fed into MATURATION PONDS. The water is now clean enough to provide a natural environment for many local animals such as Pelicans, Yellow Billed Ducks, Malachite Kingfishers and many others.

7b. Some sludge is returned to the Activated Sludge Process (5) to aid in the bacterial process. The rest goes on to the Solids Treatment Phase.

8. In the CHLORINE ROOM a small amount of Chlorine is added to kill any remaining bacteria.

What is disinfection?
This involves the use of chemicals like chlorine or ozone gas, or exposure to ultra violet (UV) light, to kill tiny living organisms in water. Chlorine gas is the most commonly used disinfectant. It is added to the water in a disinfection tank.

Why is disinfection necessary?
The treated water leaving the secondary settling tanks is clear and may look safe, but it still contains a few tiny organisms. These include germs from human wastes and those used in the bioreactor. These tiny organisms could spread disease if they are allowed to flow into a river system.

9. THE RIVER

Where does the water finally go?
After disinfection, the water is put into a river. In this way it is returned to the natural water cycle.

10. In the SLUDGE THICKENER, the sludge is thickened to an approximate 4 - 6% solid and then pumped into the Digester.

11a. In the PRIMARY DIGESTER, the sludge is kept at body temperature (37⁰) to allow bacteria to break down the raw sludge. This is released as Methane gas.

11b. This gas is stored in the GAS HOLDER where it is used as a fuel to heat the sludge in the Primary Digester.

11c. In the SECONDARY DIGESTER, the sludge containing some matter not yet broken down, continues to digest and then thickens in the bottom of the tanks.

12. The thick digested sludge is then fed to a Belt Press (the de-watering system) where the liquid and solids are further separated.

13. The solids from the BELT PRESS are fed into a DRIER. The DRIER circulates very hot air through a fluidised bed system while breaking up the sludge to form small dry pellets.

14. These pellets are stockpiled and utilised as compost.

11a. In the PRIMARY DIGESTER, the sludge is kept at body temperature (37⁰) to allow bacteria to break down the raw sludge. This is released as Methane gas.