Conventional wastewater treatment uses micro-organisms to decompose (or breakdown) organic matter in wastewater. If too much untreated wastewater or other organic matter is added to the aquatic environment, dissolved oxygen levels will drop to levels which may not be able to support aquatic life. Wastewater treatment systems are designed to digest much of the organic matter before the wastewater is released into the environment.
Conventional wastewater treatment systems generally use physical, chemical, and biological processes, including:
Preliminary Treatment - Physically removes large solids using screens (remove large objects such as stones or sticks that could plug lines or block tank inlets) and grit channels/chambers (slows down the flow to allow grit to fall out)
Primary Treatment - Physically remove solids using sedimentation tanks (settling tanks or clarifiers) where settleable solids settle out and are pumped away, while oils float to the top and are skimmed off.
Secondary Treatment - Promote the growth of bacteria and other microbes that break down the organic wastes (i.e. biological treatment processes). Microorganisms convert nonsettleable solids to settleable solids. Sedimentation typically follows, allowing the settleable solids to settle out.
Tertiary (Advanced) Treatment - Is used only where it is needed to protect the receiving waters from excess nutrients (phosphorus and/or nitrogen) or for the removal of chemical (e.g. carbon adsorption).
Disinfection - Kills disease-causing organisms, most commonly through chlorination (occasionally ozone or ultraviolet light).
Common technologies and processes utilised in wastewater treatment include:
Flow Metering
- Process monitoring and control.
Screening
- Removal of coarse and settleable solids by interception.
Communition
- Grinding of coarse solids to uniform size.
Flow Equalisation
- Equalisation of flow and mass loadings of BOD and suspended solids.
Mixing
- Mixing of chemicals/gas and maintaining solids in suspension.
Flocculation
- Promotes aggregation of small particles into larger particles to enhance their removal by gravity sedimentation.
Sedimentation
- Removal of settleable solids and thickening of sludges.
Flotation
- Removal of finely suspended solids and particles with density close to that of water.
- Also used for thickening of biological sludges.
Filtration
- Removal of fine residual suspended solids remaining after biological and chemical treatment.
Activated Sludge
- Microorganisms break down organic material with aeration and agitation.
Trickling Filters
- Beds of coarse media (often stones or plastic) over which wastewater is allowed to trickle and where microorganisms, attached to and growing on the media, break down the organic material in the wastewater.
Lagoons
- Slow, cheap, and relatively inefficient, but can be used for various types of wastewater and are reliant on the interaction of sunlight, algae, microorganisms, and oxygen.