Tertiary treatment graphic

Tertiary Treatment

Treatment polishing for removal of solids, nutrients, pathogens, and thermal pollution.

While a majority of treatment is conducted within the secondary processes, tertiary treatment is sometimes needed to remove additional contaminants, solids and pathogens to produce a better level of water quality to meet more stringent water quality requirements. Flocculation, sedimentation, and filtration are common processes employed to meet this objective when targeting solids and nutrients reduction. Tertiary processes can also include targeted constituents such as thermal pollution.

Robert W. Hite Treatment Facility

The RWHTF currently does not have tertiary treatment and secondary effluent is sent directly to disinfection.

Future stringent permit limits will require additional systems. Pilot experiments and studies will soon commence to investigate the alternatives for achieving anticipated water quality targets.

Northern Treatment Plant

The NTP has additional processes prior to disinfection for polishing of effluent by removing solids and nutrients.  These systems help reach enhanced nutrient removal levels while reducing the turbidity of the water to improve the subsequent ultraviolet (UV) disinfection.

The tertiary treatment involves flocculation, sedimentation, and coarse sand filtration. For enhanced phosphorus removal, the process includes the addition of aluminum sulfate (“alum”) as needed to convert the soluble orthophosphate to a bound particulate solid. Alum is dosed ahead of the flocculation-sedimentation basins, then is stirred very slowly and gently to allow the chemical to form flocs (small aggregates of particles). The flocs are then slowed further into the sedimentation area and allowed to roll slowly down hundreds of thin, tilted sheets of metal, called “lamella plates.”  The floc, which contains phosphorus and small solids particles, is then removed from the process to be handled in solids treatment.  To fully remove all fine particles, the water is then filtered through deep beds of sand before flowing to the disinfection 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 96%
Currently Removing 1%
Remaining 3%
TSS – Total Suspended Solids
Previously Removed 95%
Currently Removing 4%
Remaining 1%
N – Nitrogen
Previously Removed 77%
Currently Removing 1%
Remaining 22%
P – Phosphorus
Previously Removed 93%
Currently Removing 6%
Removed by previous processes 1%
E. coli
Previously Removed 34%
Currently Removing 1%
Remaining 65%

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 earlier.

Northern Treatment Plant

Capital Investments

None active


PAR 1416 – NTP Effluent Temperature

2022-2029 Investment: $22.0M

Provide a demonstration cooling tower system capable of treating up to three million gallons per day (MGD) that can be deployed on post-secondary effluent to determine feasibility and best practices associated with cooling tower technology. Based on the results of the demonstration, provide a full-scale cooling tower demonstration on post-tertiary effluent capable of treating up to 14.4 MGD that is expandable up to 28.8 MGD in a later phase.

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.

Tertiary System Facility

2030-2035 Investment: $579M

A major capital project will be designed and constructed to achieve enhanced levels of nutrient treatment prior to anticipated permit limits.

Effluent Temperature Compliance

2030-2035 Investment: $55.9M

In conjunction with tertiary facilities, provide final temperature reduction facilities in the post-tertiary effluent.

Upcoming Innovations

Capital Investments (15 years)

Tertiary Pilot (RWHTF)

2028 Investment: $1M

An evaluation of tertiary treatment alternatives will be conducted to determine the most technically, financially, and sustainably responsible means of employing tertiary treatment to achieve very stringent future regulatory limits. This study will include pilot experiments, modeling, and technology evaluations to help inform future capital investments.

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