Process flow - step 1

Preliminary treatment bar screen

Removal of large objects

As wastewater enters LMWQCC, it is screened through bar screens for the removal of objects that could damage or interfere with the process and equipment. The bar screens are parallel bars placed at an angle to enable sewage to pass through while large objects are caught on the bars.

During this screening process, a variety of interesting objects are recovered such as false teeth, plastic toys, garden connections, cans, bottles, metal objects and paint brushes. All of these objects make their way into the sewer system down toilets and sewer manholes.

Chemical addition

Ferrous chloride (FeCl2, an industrial by-product), and lime are added to the sewage before it enters the grit tanks, to improve the subsequent chemical treatment process.

Ferrous chloride precipitates phosphorous. Removing phosphorous is crucial as it contributes significantly to the growth of toxic algae in rivers and reservoirs. This process of using ferrous chloride to remove phosphorous was developed by ACTEW and is called Ferraq^®.

Lime is added to raise the pH level. This helps the ferrous chloride remove phosphorous and other material from the solution. At a later stage, polymer is added to assist flocculation (binding of particles). This helps the sedimentation process.

Grit removal

The effectiveness of the chemicals added in the pre-treatment stage is improved by extra mixing in the grit tanks, and by introducing compressed air into the tanks (aeration).

The sewage is held in the aerated grit chamber for a time to trap the grit by air rotation. In the grit chamber the movement of the liquid is slowed down to about 0.3 metre per second or slower. This allows the heavier inorganic materials to settle out. The air flow is adjusted to create a velocity near the floor low enough for the grit to settle.

There are two grit chambers at LMWQCC. The LMWQCC workers are able to clean and maintain the chambers regularly by alternating their use.

The grit (approx 1.5 tonnes/day) is then pumped to a grit classifier where the wastewater is separated, then further treated.

Next: Step 2

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