Preliminary Treatment

Preliminary treatment removes large debris, trash, rags, and grit to protect downstream equipment and prevent inert materials from entering the treatment processes.

Preliminary treatment consists of removing non-degradable solids and abrasive grits to protect downstream processes. At Metro Water Recovery (Metro), this process consists of two separate systems: screening and grit removal.

Screening uses machines, called bar screens (or step screens), to remove large, untreatable, or unwanted materials, such as wood, plastics, paper, and “flushable” wipes that can clog pumps, mixers, and other equipment in the downstream treatment processes.

In the grit removal basins, the water is slowed down to allow dense particles and solids to settle. Due to their abrasive nature, sand and silt must be removed to protect pumps and pipes. Grit can also settle in aeration basins and digesters which can hinder treatment and take up valuable treatment volume.

The materials collected in screening and grit removal are washed, dewatered, and hauled to landfill for disposal.

Robert W. Hite Treatment Facility

The Robert W. Hite Treatment Facility (RWHTF) removes an average of six tons of grit and debris per day. The lighter organic particles remain in suspension and flow to the primary treatment process. The RWHTF preliminary treatment facilities are gravity-fed.

Northern Treatment Plant

The Northern Treatment Plant (NTP) requires an influent pump station prior to preliminary treatment due to the hydraulic profile of the interceptor and plant. The NTP collects and treats the foul air released during preliminary treatment, while the RWHTF does not.

Barscreens removing rags at NTP Headworks

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
Remaining 100%
TSS – Total Suspended Solids
Remaining 100%
N – Nitrogen
Remaining 100%
P – Phosphorus
Remaining 100%
E. coli
Remaining 100%

The non-degradable solids and grits are disposed of as solid waste under the resource conservation and recovery act (RCRA) requirements. Metro currently has contracts in place with multiple local landfills for these materials to be accepted. The waste profiles are updated and renewed every three years.

The grit waste is subject to Colorado’s Board of Health Commission’s Hazardous Materials and Waste Management Division’s radiation control regulations. These regulations cover technologically enhanced naturally occurring radioactive material, or TENORM. Metro’s completed waste profile evaluations and has determined that both RWHTF and NTP’s grit TENORM levels are below the exemption level and are not subject to additional waste handling requirements. The Regulatory Affairs Division maintains that characterization annually to ensure any process changes or changes in influent are captured. Bar screen rags and other debris are not subject to these regulations.

Capital Investments

None active


Metro has adopted sluicing of screened materials over conveyance, which has frequent operation and maintenance complications

Tipping troughs used to evenly load the dewatered and compacted screenings into the roll-off dumpsters were Serial No. 1 and 2 under PAR 1116. Metro has standardized tipping troughs across the two screening facilities at the RWHTF

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: $19.3M

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

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