Secondary Treatment
Secondary treatment biologically removes carbon, nitrogen, and phosphorus. It then settles the intentionally grown microorganisms, either by returning microorganisms to the process or by sending them to the anaerobic digestion process.
Secondary treatment utilizes microorganisms in the form of bacteria, fungi, and protozoa in a process called activated sludge to reduce carbon and nitrogen and sustainably capture phosphorus. Managing the types and quantities of the microorganisms is vital for ensuring the success and efficiency of the process. This process takes place in a large tank referred to as an Aeration Basin (RWHTF 18, NTP 4). Oxygen is added to specific areas of this basin or “zones” to target the aerobic oxidation of ammonia and carbon while other zones reduce nitrate and phosphorus in unaerated volumes.
Clarifiers downstream of the aeration basins separate the microorganisms from the treated water through gravity separation. A portion of the settled solids are returned to the basins to maintain the required microbial population while the excess microorganisms are thickened and sent to the anaerobic digesters for conversion to sustainable methane.
Secondary Effluent
At the RWHTF, the secondary effluent continues to the disinfection process. At NTP, the effluent enters the tertiary treatment process.
Water Quality
Hover over the bar graphs below to see how much of the pollutant was removed during a previous treatment process, how much was removed during this treatment process, and how much is left to be removed in a future treatment process.
Key:
Previously removed pollutants from prior processes
Amount of pollutants actively being removed by the current process
Remaining amount of the pollutant