Stage 2 - Secondary treatment

Once the wastewater has passed through primary treatment, most of the rubbish and solid waste material has been screened out.

The wastewater is now directed to secondary treatment.

Secondary treatment removes nutrients and remaining solids through bacterial decomposition. This treatment uses naturally occurring biological processes.

The level of oxygen in the wastewater is changed at different stages - to produce aerobic and anaerobic environments.

These two environments cause different bacterial communities to thrive.

These different 'communities' of bacteria remove different 'pollution' components from the water during the treatment processes.

The Penrith plant has two types of secondary treatment: the biological reactor and the Intermittently Decanted Aerated Lagoon, also known as IDAL.

In the 1980s, the plant was expanded to meet the demands of the growing population. It was upgraded with advanced biological treatment facilities.

A second process line using an IDAL system was built in 2003.

Both the biological reactor and IDAL processes treat wastewater to the same quality – but in slightly different ways.

First, let's take a look at the biological reactor.


The biological reactor processes up to 23 million litres of wastewater a day.

This section of the plant uses different types of bacteria to treat the wastewater.

There are 5 steps, each designed to remove something different, for example, phosphates or nitrates.

This step involves moving some of the wastewater from the primary distribution structure into the primary sedimentation tanks where 60% of all solids are removed.

In these tanks, the smaller solids and organic material that got past the screening process during primary treatment settle to the bottom of the tank where it becomes what we call ‘sludge’.

This sludge is pumped to fermenter tanks, while the wastewater flows directly to the anaerobic tanks.

In the fermenter, which is an anaerobic or oxygen poor environment, the solids from the sedimentation tanks are broken down. This produces an organic food supply for bacteria in the later stages of secondary treatment and helps remove phosphorus from the wastewater.

Phosphorous is a nutrient. When it enters waterways in high concentrations, it can cause algal blooms, which affect water quality and aquatic life.

That's why it's better to use phosphate-free, eco-friendly detergents or cleaning products.

In the anaerobic zone, there's no oxygen and no nitrate.

In this step, water from the primary sedimentation tanks and solids from the fermentation tank, are gravity fed into the anaerobic zones.

Bacteria absorb the carbon from wastewater into their cells, releasing phosphates as waste products.

During this stage there is still no oxygen. However, nitrates are present in the wastewater.

Since there is no oxygen available for the bacteria, they use carbon in the organic matter as a food source and convert the nitrates to nitrogen gas; which is released to the atmosphere.

In this zone, an oxygen rich environment is created by pumping air through fine bubble diffusers.

This raises the dissolved oxygen level.

Increased oxygen encourages the growth of bacteria, which consume and breakdown the complex organic compounds.

In the aeration zone, some of the organic matter will be used to grow new bacteria and some will be oxidised and released as carbon dioxide.

The bacteria now reduce the amount of phosphorus in the wastewater.

Some bacteria communities also convert ammonia into nitrates and water by oxidation – this process is called nitrification.

After the aeration zone, the mixed liquor is drained into the secondary clarifiers where the biological sludge settles in the tank and is returned to the anaerobic zone for further treatment.

The clear wastewater is sent to tertiary treatment.

Intermittently Decanted Aerated Lagoon

We've seen how the biological reactor works. What about the Intermittently Decanted Aerated Lagoon, or IDAL?

It performs the same functions as the biological reactor – but the processes – sedimentation, biological treatment, and clarification - all happen within the same tank.

Pretty clever huh? C'mon, I'll show you how it works.

The screened and degritted wastewater is gravity fed from the primary distribution structure into the IDAL: Anaerobic zone.

Some settled solids from the IDAL tanks are returned to the anaerobic zone to provide seeded sludge.

The wastewater moves into the IDAL tank.

Air is pumped into the IDALs through diffusers.

The air works with microorganisms in the tank to break down ammonia into nitrates and water.

It also breaks down organic matter which reduces the Biological Oxygen Demand (BOD).

Biological Oxygen Demand is a measure of the oxygen used by microorganisms to decompose waste. If there is a high concentration of organic waste in the water, there will also be more bacteria present working to decompose this waste.

After aeration, the wastewater is allowed to settle and the water becomes still.

No longer supplied with oxygen, the bacteria use carbon in the organic matter as a food source, converting nitrates to nitrogen gas; which is released to the atmosphere.

After settling, the IDAL is decanted.

Decanting involves separating liquid from solids.

The clear water flows over weirs from the top of the lagoon into an equalisation basin.

This basin controls the flow to the tertiary treatment process.

Flow equalisation prevents short bursts of wastewater entering tertiary treatment as it can overwhelm the system.

The stored wastewater is combined with the wastewater from the secondary clarifiers before entering the tertiary treatment process.

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