Wastewater treatment and activated sludge
What is wastewater?
Wastewater is a mixture of sewage, water from household use—such as showers and washing machines—and water used in commercial and industrial applications.
Wastewater contains a variety of biological and chemical pollutants which make it unsafe for humans and the environment.
The objective of wastewater treatment is to remove or modify these pollutants, so that it can be safely discharged into the environment.
How is wastewater treated?
Treatment of wastewater typically involves primary, secondary, and sometimes tertiary steps. Secondary treatment is entirely dependent upon the activity of microbes such as Acidovorax.
1. Primary treatment
This involves physical removal of debris and contaminants. Large pieces of debris are removed by screening, and suspended particles are removed from the water through settling processes.
2. Secondary treatment
This describes the elimination of organic and inorganic contaminants, through the effect of microbiological activity upon wastewater material. The most widely used and effective methods of secondary treatment involve activated sludge.
3. Tertiary treatment
This involves removal of further nutrients and pathogens, using methods such as filtration, passage through wetlands, or disinfection by treatment with chlorine, ozone or ultraviolet light. Tertiary treatment is not always included in wastewater treatment systems.
The role of activated sludge
Activated sludge is a mixture of wastewater material, bacteria, protozoa, and microscopic animals such as rotifers. The microbes in activated sludge convert organic and inorganic contaminants in wastewater into harmless molecules.
In activated sludge treatment processes, incoming wastewater is first mixed with air and a quantity of pre-existing activated sludge. The air allows aerobic microbes in the activated sludge to proliferate throughout the wastewater.
Heterotrophic bacteria degrade and mineralize organic and inorganic material, and nitrifying bacteria convert ammonia—a toxic by-product of degradation of organic material—into non-toxic nitrate.
After the required treatment time has elapsed, the mixture is introduced into settling tanks. The solids, or sludge—which consists mainly of bacteria and should contain most remaining contaminants—is settled out from the water.
Some of the sludge is diverted back to the beginning of the process, where it will be mixed with the incoming wastewater. The remaining sludge is removed for further treatment in digesters. The water may be discharged to the environment, or may undergo a tertiary treatment step.