Potentially Dangerous Cross Connection
If you are licensed as a potable water distribution and/or a wastewater collection system operator, you are probably familiar with the department’s design requirements and guidelines that specify the horizontal and vertical separation requirements of potable water mains from sanitary sewers and other sources of contamination. The Water Quality Control Division’s Safe Drinking Water Policy 5, Section 8.8 clearly establishes design guidelines for potable water systems that specify separation distances from potable water distribution lines under a variety of circumstances (horizontal separation in parallel pipes, vertical separation in parallel pipes, crossings, exceptions, sewer force mains).
In addition, Chapter 3.2.12 of Water Pollution Control Policy WPC-DR-1 (State of Colorado Design Criteria for Domestic Wastewater Treatment Works) also specifies separation distances between potable water lines and sewer installations. These requirements should be periodically reviewed by distribution and collection system operators and applied to reviews performed by these operators either in the field or in the planning phase. These two policies basically say the same thing: keep contamination sources away from potable water lines.
Safe Drinking Water Policy 5, Section 8.8.6 also clearly states that water pipes must not pass through or come in contact with any part of a sewer manhole. It can be tempting in tight right-of-ways to place water and sewer in the same narrow trench, using the vertical separation requirements so that you can save time and money by not digging a separate trench. When you get to a manhole, some may think “Just leave the water line secure well above the sewage flow. The pipe is sealed and the sewage is way down at the bottom of the manhole. Doesn’t that constitute an air gap for backflow purposes?” No, it does not. Don’t do it or allow contractors to do it.
Do we have this issue in Colorado?
Yes! During some sanitary surveys and collection system inspections, department inspectors have observed manholes that have potable water lines passing through them (Figure 1).
Figure 1 - Potable Water Line in Sanitary Sewer Manhole
Potable water lines that pass through sewer manholes constitute an uncontrolled cross connection since surging manholes could submerge the potable water lines, which could lead to sewage contaminating the potable water distribution system during a backflow event. This could cause serious contamination in the drinking water supply and cause a waterborne disease outbreak in the community that could be harmful to public health and very costly. If you observe or know of conditions like this in your distribution system, they must be immediately assessed as uncontrolled cross connections in your cross connection control program and controlled within 120 days of discovery or an extension must be requested.
Appropriate controls may include:
- Re-routing the water supply line around the manhole
- Removing the manhole
- Removing the water supply line
Note: providing encasement of the water supply line is not considered to be a permanent solution.
Be sure to document what action you have taken in your cross connection control program documentation and be prepared to discuss it during your next sanitary survey. If you need an extension to the 120 day limit, you can apply for an extension on the department’s website at Drinking Water Cross Connection Control Program and select Appendix C.1: 120 Day Cross-connection Control Extension Application. If you have any questions please contact the department’s Field Services Section at: cdphe_wqcd_fss_questions@state.co.us.
➽ Andy Poirot, PE
