When wastewater enters the plant, it is screened to remove large objects from the wastewater stream, after which the ‘grit', or small abrasive particles, are settled out. Following grit removal, the wastewater is directed to large settling basins, or ‘clarifiers' in which oil and grease floats to the top and is skimmed off. The heavy sludge component settles to the bottom of the basin and is pumped to the anaerobic digesters, where biological treatment occurs.
The next phase of treatment is biological in nature, where microorganisms are maintained under optimal conditions that allow them to use the waste material dissolved in the wastewater as food and break the complex wastewater molecules into more elemental particles, thus cleaning the water. Microorganisms attached to the rock media in the 20 trickling filters reduce the strength of the wastewater, measured by the ‘Biochemical Oxygen Demand' test in the laboratory. Further biological treatment and ammonia removal is carried out in the aeration basins, where oxygen is supplied to microorganisms living in the ‘activated sludge' maintained in the basins. Again, the wastewater serves as food for the ‘bugs', which digest the waste portion of the wastewater, leaving cleaner water behind. Each biological process is followed by another physical treatment process, producing cleaner wastewater in each step.
Throughout the process, many chemicals are used to help purify the water. The chlorine in sodium hypochlorite destroys pathogenic bacteria in the wastewater that would make people sick. Hydrogen peroxide is used to oxidize waste particles and remove foul-smelling sulfides from the water. Ferrous chloride prevents hydrogen sulfide from becoming an atmospheric threat in the digesters. Sodium bisulfite, in turn, removes residual chlorine that would otherwise poison the wastewater effluent receiving stream, which is the Great Miami River.