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.

Aeration basins at RWHTF

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.

Previously removed pollutants from prior processes

Amount of pollutants actively being removed by the current process

Remaining amount of the pollutant

BOD – Biochemical Oxygen Demand
Previously Removed 35%
Currently Removing 61%
Remaining 4%
TSS – Total Suspended Solids
Previously Removed 65%
Currently Removing 30%
Remaining 5%
N – Nitrogen
Previously Removed 3%
Currently Removing 78%
Remaining 22%
P – Phosphorus
Previously Removed 3%
Currently Removing 93%
Remaining 7%
E. coli
Previously Removed 3%
Currently Removing 31%
Remaining 66%

Any intermediate process solids removed from the aeration basins or secondary clarifiers during cleaning or maintenance that cannot be fed into the solids processing system are characterized and disposed of as solid waste. These solids are also subject to the TENORM regulations discussed in preliminary treatment.

Robert W. Hite Treatment Facility

Capital Investments

None active.


PAR 1336 – North Secondary Pilot Program

2019-2022 Investment: $250K

A promising new process promotes the densification of microorganisms through the application of biological and physical pressure. The increased settleability of the microorganisms in the clarifiers allows for an increase in the population in the process, enabling more treatment capacity within the existing footprint. This process is currently being studied at the RWHTF and hopes to inform the forthcoming secondary improvement project.

PAR 1357 – Blower No. 8 Replacement

2019-2022 Investment: $1.9M

When a blower failed at the RWHTF, Metro took the opportunity to install and demonstrate a prototype, high-speed turbo blower. This blower brings the direct-drive technology to a scale appropriate at large facility. The blower has shown increased efficiency which will result in reduced electricity use in the highest energy area of treatment.

Energy Optimization – PAR 1390 – Aeration Control Strategy Review and Implementation (RWHTF & NTP)

2019-2022 Investment: $250K

Since aeration is the most energy-intensive of the treatment processes and vital to effective operation, the automation and control of this process is a constant area of study and optimization. An evaluation of aeration strategies is occurring to select the best means of control across the RWHTF North and South Complexes and the NTP secondary process. This study will investigate the latest instrumentation and control systems available for implementation.

PAR 1411 – North Secondary Upgrades & Intensification

2022-2028 Investment: $206M

A large portion of the infrastructure in the North Secondary Process at the RWHTF is some of the oldest at the facility. A project is launching to replace assets which are reaching the end of useful life. The project is also looking to incorporate demonstrated new technologies and processes such as the demonstrated magnetic bearing high-speed blower and will install advanced aeration control. Metro is also ensuring that the project and process will support future regulatory drivers. Intensification through densification will allow for full nitrification within the existing basin footprint in support of near-term future goals.

Robert W. Hite Treatment Facility

Capital Investments (15 years)

Projects are based on the current anticipated regulatory requirements but are subject to change without notice. Projects are planned and are subject to change without notice including the cost estimates.

South Secondary Expansion and Improvements

2028-2031 Investment: $94M

Future effluent water quality goals will necessitate an increase in treatment capacity of the secondary processes at the RWHTF. A project is planned to expand the South Secondary complex by two aeration basins, two secondary clarifiers, and one sidestream basin to provide the additional treatment. The work will also include rehabilitation of various assets to improve the operations and maintainability of the system.

Treatment Process Map Learn more about biosolids Learn more about the collection system Learn more about the influent Learn more about the sources