Primary Treatment

Primary treatment redirects Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) in the liquid stream to the solids stream, redirects carbon to digestion for biogas production, and reduces capacity requirements for the secondary process.

After passing through preliminary treatment, the flow arrives at primary treatment. The purpose of primary treatment is to settle materials in a large tank referred to as a clarifier. This process uses gravity separation to remove readily settleable solids and organic material to reduce loading to the secondary treatment process. The primary contaminants removed in this process are BOD and TSS.

The settled solids are collected from the bottom of the clarifier and pumped to the Gravity Thickeners (GVT), while fats, oils, and grease (FOG) are skimmed from the surface. Heavier organic materials are settled and removed as sludge.

Metro utilizes circular primary clarifiers in the primary 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
Currently Removing 35%
Remaining 65%
TSS – Total Suspended Solids
Currently Removing 65%
Remaining 35%
N – Nitrogen
Currently Removing 3%
Remaining 97%
P – Phosphorus
Currently Removing 3%
Remaining 97%
E. coli
Currently Removing 3%
Remaining 97%

Any intermediate process solids removed from the primary 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

2022-2023 Primary Conduit Rehabilitation (Year 3) work at RWHTF under PAR 1363: $14.9M


Thin Sludge Pumping Optimization

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.

Odor Control Facilities

2034-2036 Investment: $23.3M

Collection and treatment of foul air released during the primary treatment process.

Upcoming Innovations

Long-term investigation into carbon redirection to increase biogas production and reduce carbon loading to secondary treatment.

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