All the water used in homes, such as in the bathroom, toilets, washing machines, dishwashers and sinks, that ends up in drains or in sewage system is referred to as wastewater. Industries and businesses frequently contribute a large volume of wastewater to sewage collection systems.
Wastewater is approximately 99 percent water by weight. Wastewater is normally called influent as it passes through the wastewater treatment facility. Wastewater is treated at the wastewater treatment plants before it is permitted to be released back to the environment. Wastewater treatment plants helps nature to defend water from excessive pollution.
Wastewater follows a determined treatment path in order to achieve water quality standards, regardless of using conventional treatment or advanced treatment systems. Most wastewater treatment plants consist of a primary treatment and a secondary treatment.
Figure 1. Wastewater Treatment Plant (Image source www.omafra.gov.on.ca)
Primary treatment involves the separation and removal of solid matter from liquid waste. These solid matters will either float or readily settle out by gravity. Physical processes such as screening, grit removal and comminution may be used during primary treatment.
Large objects such as sticks or stones which could block tank inlets or plug lines are removed during the screening process. Grit chamber is used to slow down the wastewater flow and the grit are allowed to fall out. Settleable solids are settled out in a sedimentation tank and are pumped away.
Wastewater is exposed to aerobic bacteria during secondary treatment. Aerobic bacteria are used to break down pathogens, other contaminants and suspended organic matter. Aerobic bacteria are naturally supplied in wetland habitats through bacteria and microbes found there. Baffles with a special coating of aerobic bacteria are often used by sewage treatment plants.
Municipal wastewater is normally disinfected with the use of chlorine or other disinfecting compounds. Occasionally, ultraviolet light or ozone can be used.
There is an increase in the number of wastewater treatment facilities that employ a tertiary treatment process. Tertiary treatment involves removing nutrients such as phosphorus and nitrogen from wastewater.
The removal of nutrients is an important step of restricting downstream effects such as algal blooms and eutrophication, which destroy ecosystems and habitats.
Last update 28th January 2008