Nitrogen Removal
- The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water.
- Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.
- Many sewage treatment plants use centrifugal pumps to transfer the nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle(IMLR) pumps.
Phosphorus Removal
- Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems
- Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20% of their mass). When the biomass enriched in these bacteria is separated from the treated water, these bio solids have a high fertilizer value
- Phosphorus removal can also be achieved by chemical precipitation, usually with ferric chloride, alum or lime
- Another method for phosphorus removal is to use granular laterite
Disinfection
- The purpose of disinfection in the treatment of waste water is to substantially reduce the number of micro organisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc.
- Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite