Navajo Nation Improves Environmental Health with Vacuum Sewer System
In 2004, Shiprock’s community leadership requested that the Indian Health Service (IHS) Division of Sanitation Facilities Construction (DSFC) program plan a community sewer system to eliminate the use of drain fields in the Mesa Farm area. The IHS DSFC program exists to improve the health of Native American people by improving access to sanitation facilities like water and wastewater infrastructure.
The IHS project team began planning a gravity sewer system. However, the team soon discovered many obstacles to a gravity sewer system in the Mesa Farm area. Flat topography required sewer depths exceeding twenty feet in some areas. Narrow roads and existing utilities, like high voltage power lines and aging asbestos cement water lines, provided little space for excavating the proposed deep trenches. The water table was near the surface in some places and the soil was unstable for trenches. The team determined that construction costs for a gravity sewer were too expensive.
The IHS team investigated other types of sewer systems and determined that a vacuum sewer system was more cost effective than a gravity sewer system. Vacuum sewer systems assist wastewater movement using vacuum pumps and air valves. A vacuum sewer system usually allows for more line placement options, shallower trenches, and easier field alignment changes. Therefore, vacuum sewer capital costs can be significantly lower where high groundwater, unstable soils, congested utilities, flat or difficult terrain, right-of-way restrictions, or other challenges exist. Operations and maintenance costs may be higher for vacuum sewer systems unless one vacuum station can replace several proposed gravity lift stations. A vacuum sewer system may be the most economically feasible solution when site constraints make gravity sewer prohibitively expensive.
The Vacuum Sewer Solution
Vacuum sewer systems assist wastewater flow by using vacuum pumps and air valves to maintain a pressure difference between outside and inside the collection system. Wastewater flows from each home by a gravity sewer service line. This service line connects to a community valve pit shared by up to four homes. The valve pit has two chambers. The upper chamber is always at atmospheric pressure and is not exposed to wastewater. Inside the upper chamber, an air valve separates the collection system piping, which is always under vacuum, from the lower chamber, which is normally at atmospheric pressure. When the wastewater level in the lower chamber reaches approximately ten gallons, a pneumatic controller opens the air valve for 3 to 5 seconds. The difference in pressure between outside and inside the collection system forces a mixture of air and wastewater from the lower chamber into the system at high velocity. Once in the system, both gravity and the pressure differential created each time a valve opens move the wastewater downstream.
Each time a valve opens, however, the system vacuum level diminishes. To maintain a sufficient level of vacuum in the collection system, vacuum pumps remove air from the collection line pipes. They maintain the system vacuum level between 16 and 20 inches of mercury. The pumps are located at the lowest point in the system in a vacuum station. A tank collects wastewater in station and sewage pumps send to the wastewater through a force main to the wastewater treatment plant. Air removed from the collection lines is typically exhausted into a biofilter to remove odor.
Because of the high velocity of air and wastewater in the system, vacuum sewer collection lines can be laid at 0.2% slope regardless of pipe diameter. This is a significantly shallower slope than allowed for gravity sewer design. The lines must be installed at grade but no manholes or cleanouts are necessary anywhere in the entire collection system because of the high velocity of the air and wastewater mixture. Vacuum collection line layout can be altered in the field without a complete redesign of the vacuum collection system. Gravity collection systems require extensive redesign for line layout alteration. Vacuum sewer installation is therefore less expensive and more flexible than gravity sewer installation.
A vacuum sewer system can even raise the sewer pipe elevation in small increments using “lifts”. A lift uses two 45° vertical bends to raise the sewer pipe invert 1 to 1.5 feet. Wastewater collects in the upstream pipe at the bottom of the lift but still allows the vacuum pumps to remove air from the entire system. When a valve opens on the upstream side of a lift, the incoming air and wastewater mixture lifts the collected wastewater from the bottom to the top of the lift where it can continue flowing downstream. The energy used at each lift determines how many lifts can be installed in a system. Lifts can also provide vertical separation between sewer lines and existing utilities encountered during construction.
A Successful Project
The IHS team finalized a design in August 2013 and the contractor, the Navajo Engineering and Construction Authority, began construction the same month. The Indian Health Service purchased the vacuum and sewage pump skids, control panel, air valves, and valve pits from AIRVAC of Rochester, Indiana. AIRVAC was also instrumental in providing IHS with technical support throughout the design, construction, and startup of the system.
The Navajo Engineering and Construction Authority, began construction of Phase I in August 2013 and finished in August 2015. They installed approximately 12,200 feet of 4” polyvinyl chloride (PVC) pipe and 16,300 feet of 6” PVC pipe vacuum sewer main to connect 83 homes to the Shiprock Wastewater System. They constructed a vacuum station containing two 25 horsepower vacuum pumps, two 25 horsepower sewage pumps, and a 2,000 gallon storage tank. From the station, wastewater travels to the Shiprock Wastewater Treatment Plant via 5,300 feet of 6” high density polyethylene pipe sewer force main. The Navajo Engineering and Construction Authority installed 57 vacuum valve pits and approximately 3,500 feet of 3” PVC vacuum sewer service line connecting the valve pits to the main lines. They also installed approximately 8,000 feet of 4” gravity PVC house sewer service line to connect each home to a valve pit.
The Navajo Tribal Utility Authority began operating and maintaining the system after a final inspection was conducted with IHS. The project successfully eliminated 83 on-site septic tank and drain field systems. There are still several hundred homes in the Mesa Farm area waiting for subsequent phases of the sewer project. The Indian Health Service is seeking funding for these additional phases.
The Shiprock Mesa Farm area sewer system is the first vacuum sewer system on the Navajo Nation. This successful project will serve as a case study for other communities on the Navajo reservation seeking to extend community sewer services into areas where gravity sewer systems may not be feasible.