Collection System
The collection system conveys wastewater from Metro’s connectors for treatment.
The collection system conveys raw wastewater for treatment. An interceptor sewer carries wastewater directly to the treatment facility or to another interceptor. Individual connectors maintain their systems and Metro Water Recovery typically takes ownership of the collection system when an interceptor segment carries more than one connector’s flow. The flow and loadings are metered at the transition of each connector’s systems to Metro’s system. The majority of Metro’s service area and collection system are serviced by gravity flow interceptors with lift station facilities where gravity flow is not possible. There is no treatment associated with the collection system, but there are opportunities to utilize the fairly stable temperatures within the collection system for heat recovery and heat rejection.
Metro Water Recovery’s service area
Metro’s service area covers approximately 805-square miles including Denver, Arvada, Aurora, Brighton, Lakewood, Thornton, Westminster and portions of Adams, Weld, Arapahoe, Douglas, and Jefferson counties. The service area is divided into seven drainage basins summarized in the table below.
| Drainage Basins | Tributary Treatment Facility |
|---|---|
| Central Denver North | RWHTF |
| Central Denver South | RWHTF |
| Cherry Creek | RWHTF |
| Clear Creek | RWHTF |
| Lower South Platte | RWHTF |
| Sand Creek | RWHTF |
| South Platte | NTP |
The South Platte basin
Prior to construction and startup of the NTP, the South Platte basin was formerly part of the Lower South Platte basin. The northern portion of the Lower South Platte basin was redefined when the South Platte Interceptor was created, which now sends flows to the NTP. The South Platte basin will be expanded following the construction of the Second Creek Interceptor. All of the flow from the Second Creek Interceptor will be treated at the NTP.
The facilities comprising the Metro Water Recovery Transmission System include:
- 43 Interceptors
- 234 miles of gravity pipe (8-inch to 90-inch diameter)
- 30 siphon structures
- 106 metering facilities
- 111 diversion structures
- 3,708 manholes
- Three lift stations and three miles of force mains
Diversion Structures
The diversion structures located throughout Metro’s Collection System are used for two purposes:
- Temporary flow diversions to perform line cleaning, physical and closed-circuit television inspections, maintenance, and construction
- Flow control between parallel or relief interceptors
All of the 111 diversion structures in the Transmission System use stop logs to regulate flow. Stop log height is varied to achieve the desired flow split between pipes.
Flow Metering Facilities
Metro uses flow metering facilities throughout the Transmission System to support the Annual Charge program and monitor interceptor flows, perform capacity evaluations, and support operations and maintenance needs. The data from these flow meters is also used to calibrate Metro’s Transmission System hydraulic model. Flow meters record flow on a short-term or continuous basis. Metro typically uses flumes (Parshall, Cutthroat, or Palmer Bowlus) to meter flow for gravity interceptors. Magnetic meters are used for pressurized pipes (force mains).
Annual Charge Program
Metro is considered to be a wholesale wastewater treatment provider. This means Metro does not directly bill residents or businesses. Instead, as defined by section 4 of Metro Water Recovery’s Rules and Regulations, Metro bills its connectors based on how much flow they send to the two plants and the contents of the flow (loadings). Annual charges are assessed annually to all of Metro’s connectors to cover the operating costs, debt service, and capital needs of Metro. Annual charges account for 75% of Metro’s total revenue in any given year.
The Environmental Services Sampling Team (EST) within the Environmental Services Department is responsible for collecting Annual Charge (AC) wastewater samples, for measuring flow, and for maintaining the associated equipment. The AC sampling schedule is developed annually to assure representative flow metering and sample collection.
Frequency of the sampling is based on the customer category established in R&R Section 4. Customer categories with higher flow and/or more variable pollutant loadings are monitored more frequently than categories with lesser contributions. The AC Sampling is conducted during the work week, throughout the weekends, and on holidays.
There are approximately 100 annual charge stations throughout the collection system. Each station is classified into a group (A-E) based on the amount of flow and loadings that are expected. The loadings are categorized by analytical data of TSS (Total Suspended Solids), BOD (Biological Oxygen Demand), and TKN (Total Kjeldahl Nitrogen).