An illustration of an influent channel


Metro’s discharge permits require that we collect and analyze samples of the wastewater entering (influent) at both RWHTF and NTP for various contaminants.

In Colorado’s discharge permits, the requirements associated with collecting and analyzing samples are called monitoring requirements. In addition to specifying what contaminants need to be monitored, the permits also specify:

  • how frequently samples must be collected
  • the type of samples that must be collected
  • which analytical methods can be used
  • the minimum quantitation levels that each analyses must meet

The monitoring requirements applicable to the influent of each plant are generally intended to provide the state regulators, the Water Quality Control Division (WQCD) within the Colorado Department of Public Health and Environment (CDPHE) and the EPA with information about the contaminant levels in the wastewater that we need to treat, information about federal and local contaminants of concern that are or may need to be regulated by Metro’s IPP, and part of the information that they need to understand how well we are treating certain contaminants.

arial view of influent

Each of Metro’s discharge permits also include a specified “design capacity” rating or amount of flow and contaminant loadings that each plant can treat.

Colorado law requires that the design capacity for POTWs be determined by WQCD engineers. The WQCD engineers base their determination upon technical information that the POTW provides for its facilities and the WQCD’s specific engineering criteria for POTWs.

The design capacity in discharge permits for POTWs is expressed in terms of hydraulic flow (in million gallons per day, MGD) and organic loading (in pounds or tons of 5-day biochemical oxygen demand (BOD5) per day or carbonaceous biochemical oxygen demand (cBOD) per day).

Metro’s specified design capacity for the RWHTF is 220 MGD and 212 tons of BOD5 per day and for NTP is 28.8 MGD and 53,300 lbs. of BOD5 per day.

If the flow and/or organic loading in the wastewater influent exceeds 85% and 90% of the permit specified design capacity Metro’s permits for both RWHTF and NTP include required steps to expand the capacity of the treatment facility. Metro’s discharge permits also include treatment removal requirements for CBOD5 and Total Suspended Solids (TSS). The treatment removal requirement for CBOD5 and TSS for both RWHTF and NTP is 85%; this means that Metro must remove, through its treatment processes, 85% of the CBOD5 and TSS that enters each plant.

NTP headworks from above

Metro is required to conduct the following influent monitoring:

Influent monitoring to provide information about flow and organic loadings received at RWHTF and NTP and how well we are treating certain contaminants:

  • Flow
  • % influent flow is of rated design capacity
  • CBOD5
  • BOD5
  • % influent BOD5 is of rated design capacity
  • TSS

Influent monitoring to provide information about contaminants that may be of federal concern or local concern for Metro’s IPP: 

  • Antimony (RWHTF only)
  • Arsenic
  • Beryllium (RHWTF only)
  • Cadmium
  • Chromium
  • Copper
  • Lead
  • Mercury
  • Molybdenum
  • Nickel
  • Selenium
  • Silver
  • Thallium (RWHTF only)
  • Zinc
  • Cyanide
  • Phenols

There is also required influent monitoring for several specific toxic contaminants that are required under federal regulations under 40 CFR 122, Appendix D.

Metro also conducts process control monitoring for the following constituents to track and understand water quality throughout the treatment processes:

BOD is the rate at which organisms use the oxygen in water or wastewater while stabilizing decomposable organic matter under aerobic conditions. In decomposition, organic matter serves as food for the bacteria and energy results from its oxidation.

BOD is the biological method used for the measurement of the total amount of dissolved oxygen (DO) used by microbes. Both aquatic life and microorganisms require oxygen to survive.

Microbes utilize DO in water to break down complex organic compounds present in the water such as sugars, cellulose, and other convertible synthetic substances. Marine animals such as fish use DO for respiration. However, the balance of DO consumption is quite delicate. When a large number of organic compounds are present in water, microbial activity will proliferate, putting a strain on the aquatic ecosystem. Critical DO reduction will ultimately harm marine life.

Total Suspended Solids are solid contaminants that either float on the surface of or are suspended in wastewater primarily due to their small size. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage.

High concentrations of suspended solids can cause many problems for stream health and aquatic life.

High levels of TSS can increase water temperatures and decrease DO levels. This is because suspended particles absorb more heat from solar radiation than water molecules will.

Nitrogen in wastewater is originally present in the form of organic nitrogen and ammonia. Natural biochemical processes slowly convert the organic nitrogen into ammonia, which is the form of nitrogen best able to be utilized as a nutrient by microorganisms in the treatment process. Treatment plants that have a Nitrogen limit will usually need to nitrify and denitrify in order to achieve the Nitrogen limit. Fully nitrified wastewater will have little or no organic nitrogen.

Nitrogen is an essential nutrient for plants and animals.

However, an excess amount of nitrogen in a waterway can lead to low levels of DO and negatively alter various plant life and organisms.

Influent wastewater is high in dissolved phosphorus. Dissolved phosphorus is converted into particulate phosphorus either chemically or biologically. These particulate solids are settled in a clarifier or filtered. Particulate solids are removed from process, reducing the total phosphorus concentration in the water.

Phosphorus removal during wastewater treatment is vital to ensuring public safety and protecting the environment.

Removal of this nutrient from waste streams is key to preventing eutrophication – a natural process inciting algae blooms.

Escherichia coli (E. coli) are bacteria found in the environment, foods, and intestines of people and animals. E. coli are a large and diverse group of bacteria. Although most strains of E. coli are harmless, some can make you sick.

E. coli in water is an indicator of pathogens such as bacteria and viruses.

Standard disinfection with chlorine, hypochlorite, chlorine dioxide, peracetic acid (used at RWHTF), ozone and ultraviolet (UV) (used at NTP) are effective in minutes in low turbidity water to remove E. coli.

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