Vacuum Sewers 101 Page 1

Vacuum Sewers 101

PDHengineer Course No. C-4028

Portions of this course use material excerpted from Alternative Sewer Systems, 2nd ed.; Manual of Practice No. FD-12 Reprinted with permission from Alternative Sewer Systems, 2nd ed.; Manual of Practice No. FD-12. Copyright © 2007 Water Environment Federation: Alexandria, Virginia.

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I. INTRODUCTION

A. History of Alternative Collection Systems (ACS) In the late 1960's, the cost of conventional gravity collection systems in smaller communities was found to dwarf the cost of treatment and disposal. The capital cost of conventional collection systems was averaging almost four times the cost of treatment, and operation and maintenance costs followed a similar pattern because of the greater number of lift stations required per unit length of pipe, owing to the increased lengths of pipe needed to service these less-densely populated areas. In response to these problems, efforts were initiated by the public and private sectors to develop low-cost collection systems that could serve the needs of rural communities that constituted over 80 percent of the communities in need of wastewater collection and treatment. At that time there were only two wastewater options available to rural populations (conventional sewers and centralized wastewater treatment or unmanaged individual home septic systems). The problem was approached from two directions. First, intensified research was initiated to develop and evaluate onsite wastewater systems that could overcome site and soil limitations. Second, new wastewater collection approaches that were less costly were also being developed and evaluated. Today, this array of low-cost sewers conveying wastewater to distributed treatment systems and various alternative and advanced onsite systems are known collectively as “decentralized wastewater technologies”. The collection systems typically are called “alternative collection systems” (ACS). One of these, vacuum sewers, is the focus of this course. Initial demonstration projects for ACS technologies were funded by the U.S. Environmental Protection Agency (USEPA) and its predecessor agencies and the Farmers Home Administration (now the Rural Utilities Service of USDA). Because of the promise shown by these technologies, they were given special status for increased federal cost sharing under the innovative and alternative (I&A) technology provisions of the Clean Water Act of 1977. Thus stimulated, these technologies flourished in small communities which were able to secure grants under this program. More than 500 alternative collection systems (ACSs) were installed under the I&A provisions, and a significant number have also been constructed with state, local, and private funding since that time. The federal Construction Grants Program was terminated and evolved into the State Revolving Fund (SRF) Program as the 1990s began. Although the inherent value of these systems in small communities and certain sections of larger urban areas was recognized by several practitioners, reductions in available grants and the slow rate of incorporation into engineering education programs caused a slowing in the rate of ACS applications. However, where communities had the foresight to compare the present worth of these technologies to conventional

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sewerage without federal grants, the economic benefits were obvious. The primary disincentives were the lack of familiarity by engineers with these technologies, public perception of these systems as “temporary substitutes for a real sewer”, and a paucity of grant funding for communities willing to adopt these systems. However, in 1997 the USEPA issued a report (EPA/832/R-97/001b) entitled “Response to Congress on Use of Decentralized Wastewater Treatment Systems” This report was directed by Congress in response to the same concerns over the high cost of conventional sewerage to solve wastewater problems and the efforts of the Rural Electric Cooperatives and other interested parties to undertake management of rural wastewater systems, thus allowing use of more advanced wastewater systems, including alternative collection systems, to better serve rural America’s needs. The report clearly stated the value of the decentralized approach in rural and peri-urban areas to make necessary improvements in wastewater infrastructure more affordable. It added the emphasis that these technologies must be properly managed in order to provide the level of performance required.` The decentralized approach to wastewater management was described to include onsite systems, cluster systems, and mixed solutions under a competent and empowered centralized management program, similar to what has historically been necessary for conventional sewer systems. More recently, the term “distributed wastewater management” has been adopted to better describe the need for centralized management of all wastewater (and even all water-related) systems. This report stimulated new federal programs, both within USEPA and through directed Congressional funding, to identify, investigate, and minimize the barriers to full consideration of this approach. The Water Environment Federation (WEF) has been involved in several of these efforts, and the creation of a new manual (WEF MOP FD-12) is part of those activities. Some noteworthy examples of barriers include local rules that prohibit use of any collection technology other than conventional gravity sewers by local agencies, engineering consultants that steer clients away from alternative collection systems (ACS) through ultraconservative cost estimating owing to their unfamiliarity with the technology, and federal and state funding programs whose rules penalize use of any unconventional technology by applicants. Because of their relative newness, all ACS types, including vacuum sewers, have suffered from some cases of mis-design, mis-installation, mismanagement, misuse and misapplication, as have all new technologies. Because they are still not generally included in mainstream engineering education, they are still dismissed by engineers who are fearful of new concepts with lower capital costs and operator/managers who have had no management training specific to these systems. The purpose of the WEF MOP FD-12 manual is to provide information that will help overcome these concerns and to facilitate consideration of these technologies to solve existing wastewater problems and reduce the cost of wastewater management for new developments.

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B. History of Vacuum Systems The use and acceptance of ACS have expanded greatly in the last 30 years. One of these, vacuum sewers, has been used in Europe for over 100 years, although this technology has only been in the United States since the late 1960’s. Vacuum sewer collection systems were patented in the United States in 1888. The first commercial applications of such systems were by the Liljendahl Corporation (now known as Electrolux) of Sweden in 1959. Several other companies have since entered and contributed to this market, both in the US and worldwide. Thirty years ago, vacuum sewers were regarded as “new” and only to be used as a system of last resort. Improvements in the technology later led to acceptance as “alternative” sewers, but still only to be used when significant savings would result. Now, vacuum sewers have become an acceptable alternative in the proper application and are providing efficient and reliable sewer service to communities all around the world. The 1991 EPA Manual, Alternative Wastewater Collection Systems, characterized vacuum sewers as lagging behind other collection types. This was a fair assessment at that time, but is no longer true as vacuum is now viewed on par with other system types. The lessons learned from the early systems resulted in better design and operation guidelines. Advancements in the technology coupled with system component improvements, have led to more reliable, efficient systems. Finally, awareness of the technology and its limits has been raised through the process of educational seminars, papers and magazine articles. All of these factors have led to an increased comfort level with vacuum technology.

C. Comparison to other system types Each section of the WEF MOP FD-12 manual is concerned with a specific ACS technology. Each cites a series of site conditions that favor that technology over conventional gravity sewers. Not surprisingly, that list is somewhat similar for all ACS types considering the variety of commonalities they share. Only a few site and management conditions clearly favor a given type of ACS over the others. The reason why one ACS technology has been chosen over the others for the case studies presented in that manual is rarely, if ever, due to careful and comprehensive evaluation of all ACS technologies and subsequent comparison. Usually the engineer involved has some familiarity with one type of ACS and attempts to do a comparison against conventional sewerage. Depending on how well that is performed, the availability and rules of financial assistance programs, and the municipality�s stated desires, a system is chosen. As incomplete as that appears to others, it is still well beyond the typical facility plan that compares only a few treatment options, with collection and dispersal already assumed.

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II. HOW IT WORKS

A. Theory of Operation Vacuum sewerage is a mechanized system of wastewater transport. Unlike gravity flow, vacuum sewers use differential air pressure to move the sewage. A central source of power to operate vacuum pumps is required to maintain vacuum (negative pressure) on the collection system. The system requires a normally closed vacuum/gravity interface valve at each entry point to seal the lines so that vacuum can be maintained. These valves, located in valve pits, open when a predetermined amount of sewage accumulates in collecting sumps. The resulting differential pressure between atmosphere and vacuum becomes the driving force that propels the sewage towards the vacuum station. The exact principles of operation of a vacuum sewer system are somewhat empirical by nature. An early concept centering on liquid plug flow assumed that a wastewater plug completely sealed the pipe bore during static conditions. The movement of the plug through the pipe bore was attributed to the pressure differential behind and in front of the plug. Pipe friction would cause the plug to disintegrate, thus eliminating the driving force. Therefore, reformer pockets were located in the vacuum sewer to allow the plug to reform by gravity and thus restore the pressure differential (Figure 1). In this concept, the re- establishment of the pressure differential for each disintegrated plug was a major design consideration.

Figure 1 Earlier Design Concept – Reformer Pocket

(Courtesy AIRVAC)

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In the current saw-tooth profile design concept, the reformer pockets are eliminated so that the wastewater does not completely fill or "seal" the pipe bore. Air flows above the liquid, thus maintaining a vacuum condition throughout the length of the pipeline (Figure 2). In this concept, the liquid is assumed to take the form of a spiral, rotating, hollow cylinder. The momentum of the wastewater and the air carries the previously disintegrated cylinders over the downstream sawtooth lifts. The momentum of each subsequent air/liquid slug and its contribution to the progressive movement of the liquid component of the previous slugs are the major design considerations. Both of the above design concepts are approximations and oversimplifications of a complex, two-phase flow system. The character of the flow within the vacuum sewer varies considerably. The plug flow concept is probably a reasonable approximation of the flow as it enters the system, whereas the progressive movement concept is more likely a better approximation of the flow throughout the vacuum main. The reformer pocket concept, used in all of the early U.S. systems, gradually gave way to the saw-tooth profile concept. Since the early 1980’s, virtually all systems in the U.S. have been designed using the saw-tooth profile.

B. The process – from the house to the vacuum station Figure 3 and the following discussion describe the vacuum sewer process of the wastewater’s travel from the house to the vacuum station. A more detailed description of the major system components is found later in this course.

Figure 2

Current Design Concept – Sawtooth Profile (Courtesy AIRVAC)

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Figure 3 How it Works

(Courtesy AIRVAC)

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House to valve pit As far as the homeowner is concerned, connecting to a vacuum system is similar to connecting to any other sewer system. Sewage flows by gravity away from the house through a small diameter PVC pipe to the point of connection of the public sewer system (Figure 4). In this case, the point of connection is the valve pit.

In the valve pit Vacuum created by vacuum pumps located at the vacuum station is transferred through the vacuum mains and to the valve pit. The valve pit is where the interface between gravity and vacuum occurs. Housed in the top chamber of the valve pit is an interface valve. This valve is normally closed in order to seal the vacuum mains. This ensures that vacuum is maintained on the piping network at all times. The lower chamber of the valve pit is a sump that receives the sewage from the house. When 10 gallons of sewage accumulates in the sump, the interface valve automatically opens. This is done without any electrical power being required. The valve opens and in 3-4 seconds, the contents of the sump are evacuated. The valve stays open for another 2 or 3 seconds to allow for atmospheric air to enter the system. This air comes from the air-intake located by the house.

Figure 4

House / Pit / Main Relationship (Courtesy AIRVAC)

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In the vacuum mains The resulting pressure differential between the positive pressure of atmosphere air and the negative pressure in the vacuum main becomes the driving force that propels the sewage towards the vacuum station. The pressure differential that exists at the normal operating vacuum levels provides the energy to propel the sewage at velocities of 15-18 fps. When the sewage enter the vacuum main it travels as far as its initial energy allows, until frictional forces cause it to come to rest. As other valves in the piping network open, additional slugs of sewage and air enter the system. Each subsequent energy input continues to move the sewage toward the vacuum station. Many view the vacuum pipeline as a “vacuum-assisted gravity sewer”. Like gravity sewers, vacuum sewers are installed with a positive slope toward the vacuum station. When vacuum mains start to become deep, a “lift” is used to return the main to a more acceptable depth. It is at these lifts that vacuum “assists” the sewage on its travel toward the vacuum station. The lifts are part of the saw-tooth configuration of the vacuum mains, which is a key feature of a vacuum system. The saw-tooth profile is used to keep an open passageway on the top of the piping network, thereby preventing the pipe from becoming sealed. By doing this, air flows above the liquid, and the vacuum that is created at the vacuum station can be transferred to every valve pit. This ensures that the maximum pressure differential, and hence, maximum energy, can be obtained at each valve pit.

At the vacuum station Eventually the sewage reaches the vacuum station. The vacuum station has 3 major components: the collection tank, the vacuum pumps and the sewage pumps (Figure 5). The vacuum pumps and the vacuum mains are connected to the top part of the collection tank. This part of the tank is kept open so that the 16 - 20 in. mercury vacuum that is created by the vacuum pumps can be transferred to the vacuum mains and ultimately to the valve pits.

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The vacuum pumps do not run continually, but rather in cycles. They run for a short period, usually 3 to 5 minutes, in order to establish the high level of 20 in. inches of mercury vacuum. When this level is achieved, they turn off. As valves throughout the system open and admit atmospheric air, vacuum levels gradually drop. When the vacuum level reaches 16 in. of mercury vacuum, the vacuum pumps come on again and run to re-establish the 20 in. of mercury vacuum. Sewage from the vacuum mains enters the collection tank and accumulates in the bottom part of the tank. When enough accumulates, the sewage pumps come on and pump the sewage out of the collection tank through a force main to the ultimate point of disposal.

Figure 5

Components of a Typical Vacuum Station (Courtesy AIRVAC)

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III. APPLICABILITY & ADVANTAGES

A. Applicability – ACS in general There is no reason that ACS systems should be characterized as “small community technologies”, even though most systems have been located in these settings. The reality is that ACS systems should be considered in suburban and even in certain urban locations that cannot be cost-effectively served conventional gravity sewers. By their nature, they should be cost effective in any area where there is substantial distance between sources or topographic, hydrologic, and geologic conditions that dictate multiple lift stations in a conventional sewer. Indeed, when one looks at the waterfront areas of most major cities, the potential value of shallow, watertight sewers instead of conventional deep collection pipes that are located in ground water, one realizes that their potential is clearly greater than their present application. Because of their inherent capital cost-saving potential, ACS systems should be considered and evaluated for all municipalities of 10,000 people or less, unsewered areas in metropolitan regions, and new developments. Existing communities of 3,500-10,000 population can likely provide a sustainable and effective management program for any ACS technology if it is empowered to collect user fees, make and enforce rules, and assure proper staff and/or practitioner training. Small existing communities with populations under 1,000, with no assistance from larger governmental or private management entities, are probably unable to provide proper management for any type of community system. Arrangements with county government, private entities or other nearby larger utilities may eliminate this O&M barrier for these smaller communities.

B. Applicability – Vacuum Sewers Experience has shown that for vacuum systems to be cost effective, a minimum of 75 to 100 customers (houses or equivalents) per vacuum station is generally required. The average number of customers per station in systems presently in operation is about 200 to 500, but that average is increasing every year. Vacuum systems are to a degree limited by topography. The most successful applications have been in relatively dense developments with moderate terrain changes. The vacuum produced by a vacuum station is generally capable of lifting sewage 15- 20 ft, depending on the operating vacuum level of the system. This amount of lift is often sufficient to allow the designer to avoid all or many of the lift stations that would be required in a conventional gravity system.

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The consulting engineer usually drives the community’s choice of collection system type during the planning stages of a wastewater facilities project. This choice is normally based on the result of a cost-effectiveness analysis. While gravity may appear to be less costly in situations where the terrain is favorable for gravity flow, many small factors considered collectively may result in a vacuum system being the proper choice. Below are the general conditions that are conducive to the selection of vacuum sewers.

• Unstable soil • Flat terrain • Rolling land with many small elevation changes • High water table • Restricted construction condition • Rock • New urban development in rural areas • Existing urban development where built-out conditions exist • Sensitive eco-system

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C. Advantages of Vacuum Systems 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.

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IV. EXTENT OF USE

A. Extent of use in the U.S. Currently, 3 companies are active in the U.S. vacuum sewer market.: AIRVAC, Roevac and Iseki. As shown in Table 1, almost all systems presently in operation in the United States are AIRVAC systems. For this reason, the remainder of this course focuses on that approach. This in no way represents any endorsement of that system, but merely reflects the present state of the art in vacuum sewerage in the U.S.

Table 1

Vacuum Sewers: Extent of Use in the U.S. As of Dec 31, 2006

Manufacturer # of operating

systems # states

w/systems* 1st operating U.S. system

Most recent system

AIRVAC

244

27

1972

2006

Iseki

5 3 1999 2002

Roevac

14 7 2002 2006

EVAC

5 4 1969 1985

Vac-Q-Tec

19 3 1969 1975

TOTAL 287 29

• Some states have systems by multiple manufacturers; therefore total # is not additive.

Figure 6 shows the distribution of operating residential vacuum sewer systems in the United States, as of December 31, 2006. This same information, including a breakdown of the states where a particular vacuum manufacturer has installed a vacuum system is provided in Table 2.

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Figure 6

States with Vacuum Systems – Dec 2006 (Courtesy AIRVAC)

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Table 2

# Operating Systems- U.S. As of Dec 31, 2006

AIRVAC

ISEKI

ROEVAC

Vac-Q-Tec

EVAC Period

active in US market

1969 to

2006

1992 to

2006

1995 to

2006

1969 to

1975

1969 to

1985 AL 1 AK 5 1 1 1 AR 4 CA 1 2 CT 1 FL 35 1 6 GA 2 IN 34 IL 1 KY 4 1 LA 1 MA 3 MD 19 15 MI 2 MO 2 2 MS 2 NC 15 NJ 1 NM 14 NY 9 1 OH 11 OR 1 1 PA 5 1 SC 1 2 TN 7 TX 6 VA 21 3 2 WA 13 WV 26

Total 244 5 14 19 5

NOTE: Projects are scheduled for design/construction in these states in the next several

years: UT, NV and NH. Other states may be contemplating systems as well.

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B. Extent of use worldwide There are also numerous worldwide installations using vacuum sewers, with operating systems in 35 foreign countries including many countries in Europe and Asia as well as Australia, Canada and Mexico.

Table 3

Countries with vacuum systems (Yr 2006)

Australia Mexico Bahamas Netherlands

Brazil New Zealand Brunei Oman

Canada Poland Czech Republic Portugal

England Puerto Rico France Qatar

Germany Scotland Greece Slovakia Hungary Slovenia Ireland South Africa

Italy Spain Japan Thailand Korea UAE

Lithuania United States Malaysia Wales

West Indies

The number of countries with vacuum systems continues to grow every year. Information on the number of vacuum systems worldwide is not readily available. However, AIRVAC alone has more than 500 international projects to add to their 250 US projects. Based on this, it is estimated that there are 1000+ vacuum systems worldwide.

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V. DESCRIPTION OF SYSTEM COMPONENTS A vacuum sewer system consists of three major components: the valve pit, the vacuum mains, and the vacuum station. Figure 7 shows the relationship of three components to the customer.

A. Valve Pits 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.

Figure 7

Major Components of a Vacuum System (Courtesy AIRVAC)

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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. B. Vacuum Mains 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.

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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.

C. Vacuum Station Vacuum stations function as transfer facilities between a central collection point for all vacuum sewer lines and a pressurized line leading directly or indirectly to a treatment facility. Figure 9 shows the major components of the vacuum station. Vacuum pumps are needed to produce the vacuum necessary for liquid/air transport. They may be either the liquid-ring or sliding-vane type, although most recent systems use the sliding vane type. Efficiency in the normal operating range is often cited as the reason for this. The optimum operating range is 16-20 in. of mercury (Hg). The vacuum pumps, however, should have the capability of providing up to 25 in. of Hg as this level is sometimes needed during emergency conditions and in the troubleshooting process. Redundancy is required, as design capacity must be met with one pump out of service. Discharge pumps are required to transfer the liquid that is pulled into the collection tank by the vacuum pumps to its ultimate point of disposal. Dry pit pumps have been used extensively, although submersible sewage pumps located on guide rails within the collection tank may be used as an alternative. The most frequently used pump has been the non-clog type. Redundancy is required, with each pump capable of providing 100 percent of the design capacity. The level controls are set for a minimum of 2 minutes pump running time to prevent excessive pump starting and related, increased wear. The pumps should have shutoff valves on both the suction and discharge piping to allow for removal during maintenance without affecting the vacuum level. Check valves are used on each pump discharge line or on a common manifold after the discharge lines are joined to it. Equalizing lines are to be installed on each pump. Their purpose is to equalize the liquid level on both sides of the impeller so that air is removed. This ensures that the impeller is filled with liquid, which allows the discharge pump to start without having to pump against the vacuum in the collection tank. Since this setup will result in a small part of the discharge flow being re-circulated to the collection tank, a decreased net pump capacity results.

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Discharge pumps are typically located at an elevation below the collection tank to minimize the net positive suction head (NPSH) requirement. In conjunction with NPSH requirements, pump heads (TDH) must be increased by 23 ft to account for collection tank vacuum. Both vertical and horizontal pumps can be used. Materials of construction for discharge pumps are commonly cast iron with stainless steel shafts. Cast aluminum, bronze, and brass should be avoided. Double mechanical seals, which are adaptable to vacuum service, should be used. An emergency (or standby) generator is a must. It ensures that on-lot flooding or backup will be prevented through the continuing operation of the system in the event of a power outage. Standard generators are available from a variety of manufacturers. The wastewater is stored in the collection tank until a sufficient volume accumulates, at which point the tank is evacuated. It is a sealed, vacuum-tight vessel made of carbon steel, fiberglass, or stainless steel. Fiberglass or stainless steel tanks are generally more expensive, but do not require the periodic maintenance of a carbon steel tank, which may require painting every 5 to 6 years. Vacuum, produced by the vacuum pumps, is transferred to the collection system through the top part of this tank. The part of the tank below the invert of the incoming vacuum collection lines acts as the wet well. A bolted hatch provides access to the tank should it be necessary. Most collection tanks are located at a low elevation relative to most of the components of the vacuum station. This minimizes the lift required for the sewage to enter the collection tank, since sewage must enter at or near the top of the tank to ensure that vacuum can be restored upstream. This may result in a deep basement required in the vacuum station. Vacuum switches located on the collection tank control the vacuum pumps. The usual operating level is 16-20 in. of Hg with a low level alarm of 14-in. of Hg. Seven (7) probes, one for each of the six (6) set points of the pumping cycle and one (1) as a ground, are located inside of the collection tank and control the discharge pumps. The vacuum system control panel houses all of the motor starters, overloads, control circuitry, and the hours run meter for each vacuum and sewage pump. The vacuum chart recorder, collection tank level control relays, and fault monitoring equipment are also normally located within the vacuum system control panel. Fault monitoring systems include telephone dialers or other telemetry equipment including radio based SCADA systems, digital or fiber optic based SCADA systems and telephone based SCADA communications systems.

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Vacuum gauges, required to allow the operator to monitor the system, are used on all incoming lines as well as on the collection tank. These gauges are very important in the troubleshooting procedures. Chart recorders for both the vacuum and sewer pumps are needed so that system characteristics can be established and monitored. It is standard practice in the U.S. for the vacuum station equipment to be supplied by the vacuum manufacturer who pre-assembles and tests the equipment and then ships it to the job-site on a skid(s). These skids can then be lifted into the building and connected to the incoming vacuum mains and the outgoing force main. The vacuum station equipment must be installed in a protective structure. Materials of construction are the choice of the consulting engineer and typically are selected to match the architecture of the surrounding community.

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VI. PRELIMINARY SYSTEM LAYOUT & COMPONENT SIZING This section provides a general overview pertaining to the design of the various components of a vacuum system. The reader is referred to the 2005 version of AIRVAC’s Design Manual for additional and more detailed design information.

A. Design Flows All of the major vacuum system components are sized according to peak flow, expressed in gallons per minute (gpm). Peak flow rates are calculated by applying a peaking factor to an average daily flow rate. Average Daily Flow (Qave) Based on the current Ten State Standards, sewage flow rates shall be based on one of the following: 1. Documented wastewater flow for the area being served. Water use records

are typically used for this purpose. 2. 100 gallons per person per day combined with home population densities

specific to the service area. Most approval agencies will accept published U.S. Census Bureau home density for this criterion.

Peaking Factor (PF) The peaking factor suggested by the design firm should be used, with one exception: the minimum peaking factor should never be less than 2.5. If not established by the consulting firm, regulatory agency or other applicable regulations, the peaking factor should be based on the following formula:

18 + 1000/POPULATION 4 + 1000/POPULATION

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For example, if the service area has a population density of 1200, the peaking factor would be: 2.118 + = 3.75

2.14 + Table 4 shows peak factors for various populations. Please note that these are not the exact figures that would be returned by the formula but rather are rounded figures for presentation purposes only.

Table 4

Peak Factors Based on Ten State Standards formula

Population Peak factor

100 4.25 500 4.00 1200 3.75 2500 3.50 5000 3.25 9000 3.00

Peak Flow (Qmax) Applying the peak factor to the average daily flow rate and converting to gpm will yield the peak flow to be used as the basis of design.

Qa /1440 x PF = Qmax where:

Qa = Ave daily flow (gpd) PF = Peak factor Qmax = Peak Flow (gpm)

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Example 1 If the design firm provides the average daily flow based on local water records and recommends a peaking factor, both should be used as the basis for design. Average daily flow rate: 75 gpcd Persons/house 3.0 # houses: 400 Peak factor: 3.5 Qa = 75 gpcd x 3.5 per/hse x 400 hses = 105,000 gpd PF = 3.50

Qmax = 105,000 gpd/1440 x 3.50 = 255 gpm

Example 2 If the design firm does not suggest average daily flow rates and peaking factors, then the Ten State Standards should be used for both. Average daily flow rate: 100 gpcd Population (3.0 x 400): 1200 Peak factor 3.75 Qa = 100 gpcd x 1200 persons = 120,000 gpd PF = 3.75

Qmax = 120,000 gpd/1440 x 3.75 = 313 gpm

Infiltration The vacuum system is a sealed system that eliminates ground water infiltration from the piping network and the interface valve pits. However, ground water can enter the system as a result of leaking house plumbing or as a result of building roof drains being connected to the plumbing system. While vacuum systems have some inherent reserve capacity, significant amounts of homeowner I&I can result is severe system operating problems. For this reason, it is recommended that designers consider methods of eliminating ground water from plumbing systems during the design phase of a project rather than adding a homeowner infiltration component to the design flow.

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B. Vacuum Mains: Geometry and Sizing The geometry of a vacuum sewer system is similar to that of a water distribution system. Rather than looped, however, it is normally designed in a tree pattern. The length of vacuum mains is generally governed by two factors. These are static lift and friction losses. The determination of these losses is beyond the scope of this course. This topic will be covered in a subsequent course: Vacuum Sewers: Design & Installation Guidelines. The reader is also referred to the WEF MOP FD-12 manual where details are provided. Due to restraints placed upon each design by topography and sewage flows, it is impossible to give a definite maximum line length (length from vacuum station to line extremity). In perfectly flat terrain with no unusual subsurface obstacles present, a length of 10,000 ft can easily be achieved. With elevation to overcome, this length would become shorter. With positive elevation toward the vacuum station, this length could be longer. As an example, one operating system has a line that, from the vacuum station to the line extremity, 16,500 ft in length. There are three (3) major items for the designer to consider when laying out a vacuum system: Multiple service zones: By locating the vacuum station centrally, it is possible for multiple vacuum mains to enter the station, which effectively divides the service area into zones. This results in operational flexibility as well as service reliability. With multiple service zones, the operator can respond to system problems, such as low station vacuum, by analyzing the collection system on a zone by zone basis to see which zone has the problem. The problem zone can then be isolated from the rest of the system so that normal service is possible in the unaffected zones while the problem is identified and solved. Minimize pipe sizes: By dividing the service area into zones, the total peak flow to the station is also spread out among the various zones, making it possible to minimize the pipe sizes. Minimize static loss: Static loss is generally limited to 13 ft. Items that result in static loss are increased line length, elevation differences, utility conflicts and the relationship of the valve pit location to the vacuum main. Vacuum sewer design rules have been developed largely as a result of studying operating systems. Important design parameters such as minimum distance between lifts, minimum slopes, slopes between lifts, etc. are contained in AIRVAC’s 2005 Design Manual.

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Based on in-house hydraulic testing and an adaptation of the Hazen-Williams equation, AIRVAC developed a table showing the recommended maximum flow rates that should be used for design purposes as well as the absolute maximum flow rate for a given pipe size. Table 5 shows these recommendations.

Table 5 Recommended & Absolute Maximum

Flow Rates for Various Pipe Sizes

Pipe Diameter

(in)

Recommended Maximum Design Flow Rate (gpm)

Absolute Maximum

Flow Rate (gpm) 4 40 55 6 105 150 8 210 305

10 375 545

Line size changes are made when the cumulative flow exceeds the maximum recommended design flow for a given line size. Most designers will make this transition at a logical geographic location such as a street intersection. The values in Table 5 should be used for planning purposes or as a starting point for the detailed design. In the latter case, estimated site-specific flow inputs along with AIRVAC’s friction tables should be used in the hydraulic calculations. A correctly sized line will yield a relatively small friction loss. If the next larger pipe size significantly reduces friction loss, the line was originally undersized. The maximum number of houses served by a given line size is shown on Table 6, which assumes the peak design flow for 1 house is 0.50 gpm.

Table 6

Maximum Number of Houses Served for Various Pipe Sizes (based on Rec’d Maximum Design Flow using

a peak design flow of 0.50 gpm/house)

Pipe Diameter (in)

Maximum Number of Homes Served

4 80 6 210 8 420 10 750

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C. Vacuum Mains: Routing An advantage to the use of vacuum sewers is that the small diameter PVC pipe used is flexible and can be easily routed horizontally around obstacles. The feature allows vacuum sewers to follow a winding path as necessary. In most cases, vacuum sewer mains are located outside of and adjacent to the edge of pavement and approximately parallel to the road or street, which reduces the expenses of pavement repair and traffic control. In areas subject to unusual erosion, the preferred location is often within the paved area. Some municipalities also favor installation within the paved area since subsequent excavation is less likely and more controlled (via permit application only), and therefore a location more protected from damage. However, community disruption potential during construction and maintenance for this approach increases substantially. With two or more houses sharing one valve pit, overall system construction costs can be significantly reduced, resulting in major cost advantage. In some circumstances, however, this approach may require the main line to be located in private property, typically in the back yard. There are two disadvantages to this type of routing. First, it requires permanent easements from one of the property owners, which may be difficult to obtain. Second, experience has shown that multiple house hookups can be a source of neighborhood friction unless the pit is located on public property. The designer should carefully consider the tradeoff of reduced costs to the social issues prior to making the final routing decision.

D. Valve Pits: House to pit sharing ratio IN AIRVAC’s valve pit, up to four separate building sewers can be connected to one sump, each at 90 degrees to one another. However, this is rarely done as property lines considerations and other factors may render this impractical. By far, the most common valve pit sharing arrangement is for two adjacent houses to share a single valve pit (AIRVAC, 2005a). Some have attempted to reduce costs by having additional houses sharing a single valve pit. Experience has shown that, while this may appear to be viable on paper, many times it is not achievable during construction. And, even if it is, the perceived cost savings does not always materialize. Longer runs of gravity laterals are required which results in deeper valve pits needed to accommodate this. Also, the additional 2 or 3 ft of excavation of not just the pit, but the gravity laterals as well, may result in extensive dewatering. In certain cases, such as the existence of a cul-de-sac or when small lots with short front footage exists, it may be possible to serve 3 or even 4 houses with a single valve pit; however, all other design factors must be considered.

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E. Vacuum Station: component sizing A detailed discussion of this topic is beyond the scope of this course; however, some general guidelines on how the vacuum station components are sized are provided in Table 7 below.

Table 7 Vacuum Station Component Sizing

Component How Sized

Collection Tank

To insure adequate operating volume to prevent

excessive sewage pumping cycles and to provide emergency storage volume.

Sewage Pumps

Based on total peak flow to the vacuum station or as necessary to maintain 2 ft/sec scouring velocity

within the force-main whichever is greater.

Vacuum Pumps

Based on 2 factors: 1) peak flow & length of line and 2) the total system piping volume.

These topics will be covered in a subsequent course: Vacuum Sewers: Design & Installation Guidelines. The reader is also referred to the WEF MOP FD-12 manual and the AIRVAC 2005 Design manual where details are provided.

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VII. DESIGN AND INSTALLATION CONSIDERATIONS A detailed discussion of these topics is beyond the scope of this course. These topics will be covered in a subsequent course: Vacuum Sewers: Design & Installation Guidelines. The reader is also referred to the WEF MOP FD-12 manual where details are provided. VIII. OPERATION & MAINTENANCE CONSIDERATIONS A detailed discussion of this topic is beyond the scope of this course. This topic will be covered in a subsequent course: Vacuum Sewers: O&M and System Management Considerations. The reader is also referred to the WEF MOP FD-12 manual where details are provided. IX. SUMMARY Like all the ACS alternatives, vacuum sewers have certain design, installation, and system management requirements. When these are satisfied, the system will perform with all the reliability of any other collection system. One of the vacuum system suppliers also offers design and construction assistance in order to assure that the system is properly installed, given the sensitivity of the technology to improper construction. In short, the vacuum system offers the same convenience as any other type of public sewer system with reference to the actual discharge from the home and meeting the needs of the particular locality. Indeed, there are some unknown or unresolved issues in ACS technology, but these are rapidly disappearing with time. None are considered serious enough to retard continued and expanded application of these systems. The WEF MOP FD-12 manual is intended to stimulate consideration of ACS technology and minimize its misuse where it is an inappropriate solution to a problem.