Sewage treatment process

The following information explains each stage of the treatment process.

1. Primary treatment

This treatment removes solid matter. Larger solids, such as plastics and other objects wrongly discharged to sewers, are removed when wastewater is passed through screens. Smaller particles, such as sand, are removed in grit traps.

Wastewater then flows into large tanks where solids settle and are removed as sludge. Grease and scum are skimmed from the surface.

2. Secondary treatment

This uses tiny living organisms knows as micro-organisms to break down and remove remaining dissolved wastes and fine particles. Micro-organisms and wastes are incorporated in the sludge.

3. Nutrient removal

This removes nitrogen and phosphorus nutrients that could cause algal blooms in our waterways and threaten aquatic life. Algal blooms can cause visual pollution and some forms may be toxic. In some circumstances, algal blooms use up dissolved oxygen, which is essential for aquatic life.

Nutrient removal is not available at all sewage treatment plants because it requires expensive specialised equipment. However, it is becoming more common in Queensland.

Clear liquid effluent produced after treatment may still contain disease-causing micro-organisms. If this effluent is released into waterways such as rivers or the sea, the micro-organisms will eventually die out. Until this happens, these waterways could be a health risk. Where people use these waterways, effluent needs disinfection to make it safe for release.

4. Disinfection

Disinfection removes disease-causing micro-organisms. Suitable and cost-effective disinfection methods for our Region include adding chemicals to effluent and irradiation with ultraviolet light. In less populated areas, effluent may be held in lagoons or ponds for several weeks, allowing micro-organisms to die off before the effluent is released.