Florida village replaces septic tanks with vacuum collection system.
"However, the area to be served consisted of long, narrow, older, residential streets with major roadways on two sides of the proposed service area. The flat terrain, high groundwater table., and many poorly mapped, buried utilities caused us to begin thinking outside of the box," Eckler continued. "A conventional gravity sewer system would require deep excavations, at least two pump stations, and a long, expensive, disruptive construction process.
Design of the third-vacuum sewer collection system to replace septic tanks within the village of Palm Springs, Florida, is currently underway. "Seven years ago when we were asked to design a sanitary sewer collection system for the village of Palm Springs, we thought of a traditional gravity sewer system," says Donald A. Eckler, P.E., president, Eckler Engineering, Inc.
"These concerns led us to investigate both low pressure and vacuum sewer collection systems. Low pressure collection systems presented difficulties with permitting, ownership, and maintenance. The problems with low-pressure systems arise with the operation and maintenance of each individual pump station since they are located on private property. A vacuum sewer collection system does not have these same permitting issues. We researched vacuum sewer systems and settled on Airvac, Inc. (www.airvac.com). The permitting authorities were not enthusiastic about vacuum sewer systems either, but as they became more familiar with their features and benefits, they approved the installation of this small system which would initially have only 76 connections."
We were not sure that a small vacuum system would be cost effective, but when we looked at the alternative of installing a gravity sewer collection system, the cost difference was dramatic. The difference in pipeline depth alone made a significant difference in cost from both an excavation and restoration standpoint. The vacuum mains are installed at a fairly constant depth of between three and four ft, while a gravity sewer system would be buried between four and 14 ft deep. When you consider the installation of the dewatering equipment and stockpiling of the excavated material, you have now impacted the entire width of the narrow streets in this residential neighborhood. This is not only a slow, but expensive process.
Another major difference between a traditional gravity system and a vacuum system is that the gravity system would typically be laid in the middle of the street, whereas a vacuum system could be installed outside the edge of pavement in the grassy right-of-way. This would reduce the impact on the residents and the restoration costs. The collection system would also have to cross 10th Avenue North, which is a heavily traveled county road. The installation of the vacuum system would require significantly less time to construct and, consequently, minimize the impact on the traffic.
The cost of the installation of a vacuum sewer system is also impacted by the speed with which the system can be installed. The vacuum system can be installed at a rate of between 700 and 900 ft per day, similar to water main installation rates. Typical gravity sewer collection systems are installed between 50 and 300 ft a day when you consider deep excavations and the need for dewatering. Production rates significantly impact the cost of the project.
When all of the estimates and installation criteria were compared, it was easy to see that a vacuum sewer collection system is significantly less expensive than a conventional gravity sewer collection system. One of the biggest benefits is the reduced construction impact on the businesses and residents.
HOW THE VACUUM SYSTEM WORKS
Airvac's vacuum sewer system is surprisingly similar to a traditional gravity-flow system. In fact, everything on a customer's property is exactly the same, except for the addition of an air intake in the service line. The next change occurs at the property line where the customer's service line connects to a sump in a fiberglass valve pit. Up to four homes may be connected to a single valve pit.
The upper chamber of the valve pit contains a vacuum interface valve that is pneumatically-operated, so no external power is required. When a preset amount of liquid has accumulated in the sump, usually 10 gallons, the vacuum interface valve opens for a few seconds and the contents of the sump are pulled into the vacuum collection system. The air used in the evacuation enters the sump through the air intake installed in the customer's service line.
The vacuum sewer mains are installed in a unique, "sawtooth" pattern that takes advantage of both gravity flow arid vacuum assist. Small-diameter (four-in, to 10-in.) PVC mains are laid with a slight slope toward the vacuum station. When the depth of the main has increased about a foot, two 45-degree elbows separated by a short length of pipe are inserted to bring the main back to its earlier elevation. Since these risers are inserted about every 500 ft, the over profile of the main assumes a sawtooth pattern.
Sewage is pulled through the mains at a rate of 15 to 16 fps by a vacuum station. One vacuum station can serve up to 1,200 homes through mains up to 10,000 ft long in level terrain. Sewage pulled into the vacuum station empties into a sealed collection tank where it is held until it is pumped out to a force main or gravity line, which takes it to a treatment plant. The Airvac system has facilities to constantly monitor its performance automatically alert service personnel if necessary, and keep the system operating during power outages.
ONE SUCCESS CALLS FOR ANOTHER
"After we had some operating experience with the first small system, we decided to utilize the vacuum sewer collection system for a much larger area," says Bill Davis, assistant public services director, village of Palm Springs. "The larger area has between 250 and 300 residential and commercial connections which were using septic tanks."
This second vacuum sewer system for the village consisted of approximately 28,000 linear ft of pipe, 200 vacuum pits, and one vacuum pump station. The configuration and size of this collection system would have required four pump stations if a conventional gravity sewer system were constructed. The system covers more than one square mile and took nine private pump stations serving small commercial establishments out of service. "The health department was really pleased with getting rid of those private pump stations," says Davis.
The total construction cost for this second vacuum sewer collection system was $2.586 million. The vacuum pits and pump station equipment were approximately $700,000 and the piping and installation was $1.9 million. The engineer's estimate to service the area with a conventional gravity sewer system was $3.4 million, an extra cost of approximately $800,000.
"Our maintenance costs have been very low," Davis says. "So far, we haven't had to replace any major components. We have fewer pumps than we would have had with a gravity system and they are in a perfectly clean environment where they are easy to service. We expect to get between 10 and 15 years of continuous duty service without having to replace any of the pumps.
"I like the fact that I don't have any manholes or wet wells. In fact, there are no confined-space entry concerns with a vacuum sewer system. We have no need for hydrogen sulfide gas testing and the men in the field don't have to come into contact with sewage on a regular basis."
OPERATION AND MAINTENANCE
"We rarely have a problem with either of our two existing vacuum systems," says Bob Campbell, Palm Springs' vacuum sewer technician. "One of the systems has been online for a year and a half, and the other for four years. It's rare that I ever have to change out a vacuum interface valve. Most problems are the result of something like a piece of silverware or a toy getting into the system and holding the vacuum interface valve open.
"Of course, I have a schedule of regular maintenance. Some things are done daily, while others recur weekly, monthly, biannually, or annually. For instance, I go out each day and check each vacuum station to make sure everything is running properly. I spend about a half-hour in each station just checking operational liquid and vacuum levels, looking at indicators and listening to the sounds. Periodically, we change the oil in the vacuum pumps, clear the equalization lines, and change the chart recorder paper.
"Four of my fellow workers and I were trained at the vacuum system manufacturer's factory. During a week-long training school, we learned how to maintain and repair the system if something should go wrong. We learned how to isolate sections of the system to quickly locate an open vacuum interface valve. We also learned how to properly test and rebuild the interface valves and controllers. As a. result, we can quickly replace a malfunctioning valve or controller in the field, and take it to our centralized facility and rebuild it at our convenience. We don't have to send any of that back to the factory for service.
"The system can be programmed to dial up to four different telephone numbers, should something unexpected happen. Once the dialer reaches someone and gets a proper reply, it will not dial the other numbers. However, if it does not reach someone, it will keep dialing through the rotation every ten minutes until someone answers. We received a few calls while we were initially bringing the system online, but once we got the kinks out, the calls stopped."
"From start to finish, the job went really well," says Mike Johnson, project representative, Palm Springs. "Boys Engineering, Inc. and AKA Services, Inc., the companies which installed the systems, did a very good job on the pipe work, even though they had never installed a vacuum system before.
"The manufacturer had one of their employees on the job full-time to give technical guidance and to be sure the system was installed properly. They insisted that all of the installed pipe be vacuum-tested at the end of each day. As a result, we could all be assured there were no buried vacuum leaks in the systems.
"We had three or four crews working most of the time. One or two were busy building the vacuum station while another installed the vacuum mainlines and vacuum service laterals up to the customer's property line. Another crew installed the fiberglass vacuum service pits in the public right-of-way at the property line.
"These vacuum service pits are about six ft deep and divided into two sections. The bottom section is a water-tight sump which holds wastewater from up to four residences (typically two). The top section holds the vacuum interface valve and controller. The two sections are separated by an airtight partition. The valve pit and all of its contents will operate properly, even if they are completely submerged in water.
"We stubbed a line out of the valve pit for each customer and capped it at the property line. The customers were to be required to disconnect from their septic tanks and connect to the new vacuum sewage collection system within 90 days.
"I like the vacuum system. It went in quickly and it's environmentally safe. If you somehow get a leak, nothing is going out into the environment because of the suction. We'll get a signal at the vacuum station that there is a problem, and when we find the leak, it can be fixed temporarily with a piece of duct tape. I don't know of any other system that can make that claim," says Johnson.
THE THIRD SYSTEM
A third vacuum sewer collection system with 200 connections is being designed for the village, and there is a good possibility that another 200 connections will be added to this system at a future time. "The village likes the features and benefits of vacuum sewer systems as much as we do," says Margaret Blank, E.I., project engineer, Eckler Engineering, Inc. "Otherwise, we wouldn't be installing the third vacuum sewer system and talking about an additional phase with 200 more connections. Vacuum sewer collection systems are the correct choice for these well-established neighborhoods."
Doug Hammann, P.E.
Mr. Hammann is Project Manager, Eckler Engineering, Inc., Coral Springs, Florida.
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|Title Annotation:||Palm Springs, Florida|
|Date:||Oct 1, 2002|
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