Waste water treatment methods

Wastewater, both municipal and industrial, is all the water that has been used by humans for both home (municipal) or industrial tasks or has impacted on urban surfaces. The important thing to remember about wastewater is that it is roughly 99.97% water (H₂O). It is that 0.03% represents the nutrients, toxins and sediments our activities generate. Wastewater treatment helps to remove toxins in water waste before it is returned to the environment.

Types of Water waste

Wastewater is divided into several categories, each with important waste components and treatment needs.

• Brown water: It is the water that passes through our toilet and carries away human excrement and sanitary wastes.
• Grey water: It is derived from showers, dishwashers, and washing machines. It is basically any water with soap or detergent in it and is high in phosphorus, an important nutrient in plant and algae growth.
• Stormwater: It is water that runs off roofs, pavement, roads or any impervious surface and directly into waterways. In urban areas, roads and homes typically have stormwater drains to effectively drain water to prevent flooding on the road or property. Stormwater typically has high concentrations of motor oil, salt (in cold regions) and detergents (typically from windshield washing fluid).
• Industrial wastewater: This can be any of the above, but also includes water that has been warmed or cooled during processing, even if it is still uncontaminated water (imagine what would happen to the wildlife in a river if a powerplant released 60°C water into a river without cooling it first).

_{E.coli under microscope}

Wastewater treatment process

The wastewater treatment process has evolved over time and entails different processes.

For decades the old adage “Dilution is the solution to pollution” represented how wastewater was treated – wastewater was released into rivers, streams, and the ocean without any treatment and it was hoped that water currents and rainfall would wash away and dilute (but never eliminate) the toxins and pollutants.

Also, remember that Earth’s population was half of what it is today in 1970 and we can no longer depend on the water cycle to dilute and move wastes away from human concern. We also now understand that the environment that absorbs our wastes is impacted and degraded from pollution. Ecosystem services can eventually be overwhelmed by the sheer volume of wastewater and then no longer able to absorb or filter those wastes effectively and safely.

Today, the fundamental goal of wastewater treatment is to be able to release water (outfall) to the environment without negatively impacting it (including the water supply of downstream users, both human and environmental). Municipal and industrial wastewater are typically treated in four stages.

Diagram of Conventional Wastewater Processing – schematic (Click to expand)

Birdseye photograph of Conventional Wastewater Processing (map) (Click to expand)(modified from the Queensland Government Department of Environment and Science website)

Preliminary and Primary Treatment

Larger solids, such as plastics and other objects discharged into sewers, are removed when wastewater is passed through screens (the “Large Object Separator”) and then into trucks for transport to solid waste disposal facilities. Smaller particles, such as sand, are removed in sedimentation ponds and grit traps. Wastewater then flows into large tanks where solids settle and are removed as sludge (sludge processing) and may be combined with other materials for further processing to be used as fertilizer.

Secondary treatment

Secondary treatment uses micro-organisms (e.g. bacteria) to break down and remove the remaining dissolved wastes and fine particles in the wastewater. These micro-organisms and their wastes are incorporated in the sludge. Aerated lagoons are often used to increase the activity of aerobic bacteria while bioreactors may be used to improve anaerobic reactions and produce methane for power generation.

Advanced/Nutrient removal

This stage removes nitrogen and phosphorus nutrients that could cause algal blooms in our waterways and threaten aquatic life. Algal blooms can cause visual pollution, may use up dissolved oxygen essential for aquatic life leading to “fish kills”, and some forms of algae may be toxic. Nutrient removal is not available at all sewage treatment plants because it requires expensive specialised equipment.

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.

Disinfection

This process may take place at any stage in the wastewater treatment process after Preliminary, Primary or Secondary treatment. Disinfection removes disease-causing micro-organisms such as Escherichia coli (E. coli) using UV-light, Ozone (O₃) or chlorine. 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 into the environment.