Vacuum Sewer Systems (cont)
The advantage of vacuum collection systems may include substantial reductions in water use, material costs, excavation costs, and treatment expenses. In short, there is a potential for overall cost effectiveness.
Specifically, the following advantages are evident:
• Small pipe sizes, usually 4”, 6”, 8” and 10” are used.
• No manholes are necessary.
• Field changes can easily be made as unforeseen underground obstacles can be avoided by going over, under, or around them.
• Installation of smaller diameter pipes at shallow depths eliminates the need for wide, deep trenches reducing excavation costs and potential dewatering costs.
• High scouring velocities are attained, reducing the risk of blockages and keeping wastewater aerated and mixed.
• Elimination of the exposure of maintenance personnel to the risk of H2S gas hazards.
• The system will not allow major leaks to go unnoticed, resulting in a reduced environmental damage from exfiltration of wastewater.
• Only one source of power, at the vacuum station, is required. No on-lot power demand exists at valve pits.
• The elimination of infiltration permits a reduction of size and cost of the treatment plant.
• Vacuum stations can be designed to blend with the surroundings more so than traditional lift stations.
• Valve pits are more concealable at the customer’s property than are grinder pump stations.
• A single source responsibility exists as one operating entity operates and maintains the entire system, including the on-lot valve pit and valve.
There are a number of disadvantages to vacuum systems. First, while the mains are small and require shallow burial (just below the frost line), they are limited to approximately 20 feet of head. The sewer lines must have a specific profile of pockets or running traps, so installation requires the same attention to grade as a gravity sewer main.
The biggest disadvantage is system size. The central vacuum stations require a large capital investment, so a system for less than 50 homes is not economically feasible.
Valve pits and sumps are needed to accept the wastes from the house. These may consist of one unit with two (2) separate chambers. The upper chamber houses the vacuum valve and the bottom chamber is the sump into which the building sewer is connected. These two chambers are sealed from each other. The combination valve pit/sump is usually made of fiberglass, and is able to withstand traffic loads. Buffer tanks are used for large customers or when a pressure/vacuum or gravity/vacuum interface is desired, as would be the case with a hybrid system.
The vacuum valve provides the interface between the vacuum in the collection piping and the atmospheric air in the building sewer and sump. System vacuum in the collection piping is maintained when the valve is closed. With the valve opened, system vacuum evacuates the contents of the sump. The valve is entirely pneumatic by design, and has a 3-in. opening size. Some states have made this a minimum size requirement, as this matches the throat diameter of the standard toilet.
A 4-in. air-intake is installed on the homeowner’s building sewer, downstream of all of the house traps. This air-intake is necessary to provide the volume of air that follows the sewage into the main resulting in the pressure differential that becomes the driving force. This also circumvents the problem of inadequate house venting which can result in trap evacuation. Some operating entities require the air-intake to be located near a permanent structure for aesthetic and protection reasons. In some instances, local ordinances may stipulate a minimum setback distance from the building structure.
The piping network connects the individual valve pits to the collection tank at the vacuum station. Schedule 40, SDR 21 or SDR 26 PVC pipe is used, with SDR 21 being the most common. Early systems used solvent-welded joints, but most recent systems use O-ring rubber gasketed pipe. Where gasketed pipe is used, the gaskets must be certified for use under vacuum conditions. Typical sizes include 3-in, 4-in, 6-in, 8-in and 10-in pipe. PVC pressure fittings are needed for directional change as well as for the crossover connections from the service line to the main line. These fittings may be solvent-welded or gasketed. The recent trend is to avoid solvent-welded fittings where possible, although there is a cost trade-off to consider, as the gasketed fittings typically are more expensive, but are less labor intensive than the solvent welded fittings. Lifts or vertical profile changes are used for to maintain shallow trench depths as well as for uphill liquid transport. These lifts are made in a saw-tooth fashion. A single lift consists of two (2) 45-degree fittings connected with a short length of pipe.
Division valves are used to isolate various sections of vacuum mains thereby allowing operations personnel to troubleshoot maintenance problems in a timely fashion. Both plug and resilient-wedge gate valves have been used, although most recent systems use gate valves. Some designs have included gauge taps installed just downstream of the division valve. This tap makes it possible for one person to troubleshoot without having to check vacuum at the station. This greatly reduces emergency maintenance expenses, both from a time and manpower standpoint.
Different pipe location identification methods have been used. These include magnetic trace tape in the top of the trench, metal-toning wires above the pipe during construction; utility frequency based electronic markers, and color-coding of the pipe itself.