Simpler sewer system

July 2012 » Features » PROJECT CASE STUDY
Oak Island, N.C., a small coastal community, boasts one of the nation—€™s largest vacuum sewer systems.
Gene Kudgus, P.E.
An inconspicuous vacuum station and a portable generator (left) are practical reasons Oak Island chose vacuum sewer technology.

Any kid who digs a hole on the beach discovers an important geology lesson — when the hole gets deep enough, it begins to fill with water. The sandy soil, high water table, and flat terrain make deep digging difficult, even with a plastic pail and shovel.

Project
Vacuum sewer system, Oak Island, N.C.

Participants

Oak Island Public Utilities Department
Oak Island Public Works Department
Black & Veatch

Product application

An AIRVAC vacuum sewer system provides a less costly and simpler sewage collection system for a small coastal community

Engineers and public utilities directors know this, too. Installing underground utilities such as gravity sewers can create significant and expensive problems. Trenches are likely to fill with groundwater, creating numerous installation issues. Progress is often very slow, existing services are disrupted, and streets become inaccessible. In our case, numerous lift stations would be required, which would further delay completion and escalate costs. Everyone becomes annoyed and the complaints ultimately end up on the desk of the public utilities director and engineers who are directing the work.

Oak Island, N.C., avoided these problems by installing a vacuum sewer system by AIRVAC. We experienced less digging and disruption, and we got a modern sewer system that is easy to maintain and extremely reliable. Additionally, because the installation was much less disruptive than a gravity sewer would have been, we received far fewer calls from disgruntled homeowners. For a project that impacted more than 7,200 homes — making it one of the largest vacuum systems in the country — that’s a noteworthy achievement.

Search for an alternative
Oak Island has a population of about 7,800 people, but that can swell by several thousand tourists and visitors — as many as 35,000 people — during the summer months. It is a typical oceanfront community: The topography is flat and the water table is high. In 2001, our public utilities department began installation of a gravity sewer system to serve the commercial corridor of town, which is vital to the tourism that so many local businesses depend on. It was during the installation of that system that we became convinced that installing gravity sewers throughout the entire community would be extremely difficult and costly.

Based on our installation of the gravity system, we projected that we would need 70 to 80 lift stations to serve all of Oak Island with sewage collection. The cost of land, equipment, and materials would be staggering, and the effort and time required to dig trenches deep enough for gravity lines — as deep as 16 feet — would be enormous. The disruption, inconvenience, and cost associated with gravity sewers forced us to seek a better alternative.

During this time we became acquainted with vacuum sewer technology at an industry trade show. Not long afterward, a regional manager from AIRVAC, Frank Bland, came to talk with us. AIRVAC, which had installed a vacuum system in nearby New Bern, N.C., a few years earlier, encouraged us to talk with the public utilities department in New Bern and get their evaluation of vacuum sewers.

In New Bern, public utilities officials showed us their vacuum sewer and explained the numerous benefits surrounding installation and maintenance of the system. They explained that the installation process required relatively shallow trenches and that the system came together easily. We discovered that there are no manholes associated with vacuum sewers, and that a single vacuum station can serve a large number of connections. We also saw that routine maintenance is simple and that vacuum sewers are remarkably reliable.

System operators can remotely monitor vacuum station equipment and make adjustments as needed.
Installation of vacuum sewers replaced sometimes leaky septic tanks on Oak Island and helped restore local marshlands.

Our visit to New Bern convinced us that in our situation, AIRVAC sewers were an excellent alternative to a costly and complex gravity sewer installation.

Shallow excavation and lightweight materials, such as this valve pit, make AIRVAC installation faster, easier, and safer than gravity sewer installation.

Working with the civil engineering firm Black & Veatch and AIRVAC, we went about the process of designing a vacuum system for our city. We determined that nine vacuum stations were necessary — compared with the 70-plus pumping stations that would have been needed for a gravity system. Efficient vacuum stations occupy about the same space as a small house and are designed to look like the surrounding architecture so they blend into their surroundings.

The cost analysis prepared by Black & Veatch indicated that the total system cost for an AIRVAC system would be approximately 25 percent less than the cost of a gravity sewer installation when considering the expenses associated with deep trenches, dewatering, pumping stations, and all of the related costs. That fact, combined with the benefits of vacuum sewers we saw in New Bern, made our decision relatively easy.

Installation of the AIRVAC system went smoothly. Trenches for the vacuum collection lines range from 5 to 8 feet in depth, about half the depth of trenches for a gravity sewer. Shallower trenches meant that we could use smaller, and less, equipment. When we ran into unexpected underground obstacles, it was relatively easy to work around them. And since AIRVAC systems don’t require manholes — which are often a huge source of groundwater intrusion and maintenance — we avoided the cost and disruption of installing those, as well.

Figure 1: Components of a vacuum sewer system

Vacuum collection lines are made of PVC pipe that can easily be placed into the trenches by hand. Collection lines are laid in a “sawtooth” profile (see Figure 1), which helps maintain consistent pressure within the line. Valve pits were installed within the town’s right of ways near the street for easy maintenance access. Each valve pit typically serves one to four homes or one to two businesses. Oak Island also installed buffer tanks that accommodate larger-flow customers.

One of the important benefits of AIRVAC sewers is that it is a closed system — there is no infiltration or exfiltration. The elimination of groundwater and stormwater from the system keeps wastewater treatment costs at a minimum. This is especially important in an area such as Oak Island, where salt water can overload a leaky sewer system and create problems at the treatment plant. Meanwhile, preventing exfiltration protects the environment. If a leak in a vacuum collection line occurs, the vacuum pressure within the line keeps sewage in, preventing it from escaping into the environment. Before we installed vacuum sewers, we had approximately 7,000 homes and businesses with septic tanks in the town, some of them leaking sewage into the local marshlands. This was slowly killing the vegetation and aquatic life in the area. Now, the greenery is returning and the aquatic life is thriving.

Operation and maintenance
Three Oak Island Public Utilities Department employees received four days of training by AIRVAC on the system and have been largely responsible for most system maintenance. If a question comes up that we can’t answer, we simply call AIRVAC and they walk us through the issue. The vacuum stations are easily maintained; we visit each station daily to check the gauges and monitors and to perform routine maintenance. The working environment in a vacuum station is clean, unlike the maintenance issues typically associated with gravity sewers or grinder pump systems. We also test the valve pits around the town periodically to make sure they are firing properly. It’s important to note that our public utilities staff almost never comes in contact with raw sewage because vacuum sewers are completely contained.

Being a coastal community, we are always subject to the threat of hurricanes. Seven of our vacuum stations have portable generators, so in the event of a hurricane, we can take the generators inland until the storm has passed, then return and get the system operating again. This will help keep sewers operational even after a hurricane.

About 98 percent of our sewer system currently is installed, and we are happy with the results — benefiting homeowners and visitors for decades to come. Any city with similar issues should seriously consider vacuum sewers.

Gene Kudgus, P.E., is public services director, and David E. Kelly, II, is public utilities director, for Oak Island, N.C. (www.oakislandnc.com).


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