LOW Sewer System
How the Vacuum Sewer System Works in Lake of the Woods (LOW)
The sewage collection system starts with a gravity flow from the individual home to a collection tank normally located in the adjacent utility easement. Each collection tank has a valve that opens when the tank reaches a certain level. When the valve opens the sewage is sucked out by vacuum into a piping system that goes to one of the 13 vacuum pumping stations around LOW. When the level in the collection tank gets down to a low level (not completely empty) the valve shuts. The vacuum is created by pumps at the vacuum pump stations. Other pumps at these pump stations pump the sewage, under positive pressure, to the sewage treatment plant. Problems occur in the vacuum portion of the system between the individual collection tank and the vacuum pump station.
Problems most often occur due to mechanical failure of collection tank valves. If a valve fails to open, the individual collection tank will overflow. If a valve fails to close properly (due to a malfunction or as often happens due to being jammed by a rag, toy, or other article flushed down a toilet) the collection tank will be sucked completely dry and begin sucking air into the piping. Sucking air into the piping destroys the vacuum and will prevent or reduce sewage from being sucked out of other collection tanks in that branch of the piping.
The other common source of failure is the piping itself. In a gravity flow system, a small piping leak causes no or very little problem; any small sewage leakage just flows into the ground as it does in a septic system. In our vacuum sewage collection system, a small piping leak allows air into the pipes thus destroying the vacuum and the ability to suck sewage from individual collection tanks.
When a particular branch of piping has lost vacuum due to a failed valve or leaking pipe, there are usually some collection tanks more likely to overflow than others. These are likely to be the tanks furthest from the pumping station (which usually have the weakest vacuum), those in very low parts of the topography, or tanks that have the highest inflow due to the greatest water usage. That is why one individual may have never had a problem but may have had a neighbor who has frequent problems, or vice versa.
There are several steps that individual homeowners can take to mitigate sewage problems. First, if you notice an overflow coming from the collection tank on your property or anywhere, notify RSA immediately. Technicians are on-call seven days a week and will respond quickly. Also, if you notice an overflow, stop dumping sewage. Stop the dishwasher and washing machine and stop flushing the toilets since everything you dump will end up on your lawn. RSA will clean up any mess and will spread lime on the area to control any smell. If you notice any unusual sounds from your collection tank this could mean that a valve is stuck open, and you should notify RSA immediately.
History of the Lake of the Woods Sewer System
The vacuum sewer system was originally installed to replace failing septic tank systems constructed in the initial stages of development that occurred during the late 1960’s. Continuing sewer system construction almost exclusively employed the use of vacuum sewers, the design and construction of which do not generally conform to current industry vacuum sewer configuration standards. The system was owned and operated by several private companies until the Rapidan Service Authority (RSA) assumed ownership in late 1987.
The existing LOW vacuum sewer system has experienced a long history of operational problems resulting in numerous overflows and back-ups into residential structures. Overflow events reportedly reached a frequency of 50 to 60 events per day prior to RSA’s involvement. Upon transfer of the vacuum sewer system to RSA, RSA has undertaken significant capital improvements, as well as operation and maintenance (O&M) efforts to reduce overflow events to a frequency of less than 1 event per day.
In 2004, RSA, in conjunction with Virginia DEQ and as part of a Special Consent Order, had an engineering study conducted by Draper Aden Associates (DAA) of the vacuum sewer system serving the LOW community. As part of the study, DAA reviewed the system design, operations, and history of overflows. The system review performed by DAA revealed the following:
The initial construction of the LOW vacuum sewer system relied heavily on the use of concrete collection tanks, the design of which was not specifically intended for vacuum sewer applications. The deterioration of these tanks resulted in substantial infiltration and inflow (I&I) into the vacuum sewer system and their configuration makes O&M activities difficult to perform. RSA has initiated a program to replace these tanks with a standardized collection tank system that involves the use of a fiberglass 30-gallon capacity collection tank configured with a fiberglass barrel structure that houses the vacuum valve. Many of these tanks serve more than one home and there are approximately 3,700 of these collection tanks in use with upwards of 500 tanks around the large lake itself. The concrete tank replacement project still continues today.
Vacuum valves are connected to the vacuum sewer main and regulate both the admission of air and collected wastewater into the vacuum sewer system. Three different companies manufacture the vacuum valves used in the LOW vacuum sewer system as follows: Iseki, AIRVAC®, and ITT Grinnell. The Iseki and AIRVAC® valves are in typical use in most vacuum sewer systems in Virginia and reportedly perform well. Conversely, the ITT Grinnell valves are not in common use in Virginia and the type of valve used in the LOW vacuum sewer system is outdated. RSA adopted a policy to replace ITT Grinnell valves. Today we have less than 40 ITT Grinnell valves in the system, and they are being replaced with Iseki valves as they fail.
A review of the vacuum sewer mains, when viewed in an overall context, depict extensive use of small diameter vacuum mains over long conveyance distances, principally in areas adjacent to the main lake. These shoreline areas represent the lowest elevations served by the vacuum sewer system. In addition, there is a significant amount of dissected terrain (i.e., hills and valleys) that requires multiple system lifts. This review also revealed the wide-ranging use of U-shaped vacuum main profiles prior to 1987 (also referred to as “sag” profiles). The use of the sag profile had been abandoned for a considerable period of time since the sag areas regularly become waterlogged and prevented adequate vacuum from reaching upstream mains and collection tanks. Design in 2004 required the use of a saw-tooth profile that is constructed using precise grade tolerances. DAA estimated that a substantial portion of the LOW vacuum sewer system is comprised of vacuum mains with sag profiles. In addition, the extensive use of small diameter vacuum mains with sag profiles only exacerbates the waterlog/vacuum problem. This vacuum sewer main review revealed that in addition to the sag profile concern, several areas of the LOW vacuum sewer system require either high system lift or multiple system lifts (in some cases both conditions exist). Historical operational data from various installations and research performed by vacuum system manufacturers resulted in current (2004) lift parameters that commonly limit cumulative lift requirements to 13 feet. This parameter is exceeded in several instances throughout the LOW vacuum sewer system resulting in another system problem.
In addition to the general design configuration issues discussed above, it was determined that a substantial length of vacuum sewer main throughout the LOW community was not properly located to facilitate O&M activities. A portion of vacuum main situated along the shoreline of the main lake was installed prior to the filling of the lake. This condition has resulted in the following issues: insufficient workspace for repairs and maintenance, conflicts with private structures and landscaping, and poor accessibility due to wetlands or terrain-related obstructions.
The ability for O&M personnel to perform routine maintenance functions and respond to any emergency conditions can be severely hampered by the location of numerous vacuum mains. Relocation of vacuum mains is the only practical remedy for this situation and RSA has relocated many sewer lines to the front of various properties and replaced many old vacuum mains with new, larger mains. Additionally - where possible - vacuum sewer is being replaced with gravity sewer.
DAA’s review of the vacuum pump stations indicated from operational data that the LOW Vacuum Pump Stations were operating near maximum practical vacuum levels. As a result, vacuum sewer system problems appear to be due almost wholly to the configuration, age, and location of the vacuum sewer mains. As a result, RSA has been upgrading these station on a regular basis and continues to do so. Further, a new engineering study is currently underway to evaluate each of the 13 vacuum pump stations to determine the need for upgrades or replacements.
The principal finding of the DAA report was that the primary cause of any system overflow is overwhelmingly related to the configuration of the system prior to ownership and operation by RSA. Since 2009, RSA has spent over $5.4 million on vacuum sewer system improvements and continues to make more improvements annually.