Wastewater Treatment Process
Wastewater treatment plants use a combination of physical and biological processes to purify wastewater. These treatment processes are broken down into five major phases:
- Preliminary treatment
- Primary treatment
- Secondary treatment
- Disinfection treatment
- Sludge treatment
The preliminary treatment is a physical process of using large bars or screens to remove large pieces of garbage from the incoming wastewater (influent). By removing large trash in this initial step, this phase protects the main sewage system and equipment from potential damage.
As wastewater enters the sedimentation tanks (settling tanks) of the primary treatment phase, the flow of water is slowed. This permits heavier solids to settle to the bottom of the tank, while lighter particles float to the top. The settled solids (primary sludge) are pumped to another area for additional processing. The floating materials are skimmed off. The remaining partially-treated wastewater then moves to the next phase in the treatment facility.
The secondary treatment (activated sludge process) is a biological phase. Air is introduced into the wastewater aeration tanks. The air flow stirs the wastewater and sludge, but it is the transfer of oxygen from its gaseous state to a liquid state that actually stimulates the growth of bacteria and other beneficial organisms that naturally exist in the wastewater. By encouraging this bacterial growth, these microorganisms breakdown and consume a large portion of the organic materials. The aerated wastewater moves into final settling tanks, where additional particles can sink to the bottom (secondary sludge) and be physically removed. The remaining wastewater now passes into the disinfection phase.
The disinfection treatment introduces chlorine (sodium hypochlorite) into the semi-treated wastewater, for the purpose of killing harmful, disease-causing organisms. Upon completion of this process, the treated wastewater is now referred to as “effluent” and is approved for release into local waterways (streams, rivers, lakes).
This treatment phase combines the primary sludge and secondary sludge, which are concentrated using two additional stages: Thickening and Digestion.
The thickening stage allows further separation of water and solids. The water is reintroduced into earlier stages of the wastewater treatment process. The solids are moved into the “digestion” stage, which involves more time and steps to make it safer for the environment. The digestion tanks are oxygen-free (anaerobic). The sludge is heated to stimulate growth of anaerobic bacteria, which converts the thickened sludge into water, carbon dioxide and methane gas. NOTE: If fuel cells are installed at the facility, the carbon dioxide and methane gas can be converted into energy (as heat and electricity) and repurposed at the plant or sold.
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