“Sanitary waste flows downhill” is the first lesson taught in any plumbing class because it is a simple concept and is often understood as the only solution. However, the team designing a recent healthcare campus in Wisconsin had to challenge that notion.

This vacuum waste system project spotlight expands and illustrates concepts presented in two previous articles by John Gregory: September 2022, “The Mindful Designer,” http://bit.ly/3Fz7yF6; and January 2023, “The Case for Vacuum Waste Systems,” https://bit.ly/3Cob1Yf.

The project design started as any other healthcare project in Wisconsin. With the plumbing plans requiring review and approval from the Wisconsin Department of Safety and Professional Services, an early underground package was submitted to prepare for the first phase of construction. 

The finished space is approximately 85,000 square feet, including the hospital, ambulatory surgery center and medical office areas. The fixtures were set, the piping properly designed and sanitary, storm and water connections coordinated with the civil engineer. The underground plans were approved without a hitch.

Then came the problem in the form of a geotechnical clarification. In its previous life, the site had been home to a large wood door factory causing widespread soil contamination, and due to the widespread and deeply buried wood fill, the final geotechnical report noted the following regarding the utility construction:

“Due to the presence of relatively widespread wood fill (which is potentially highly compressible), it is recommended utilities be supported by the deep or intermediate foundation system, or that the buried wood be completely removed from beneath utilities, or that utilities be placed to bear below the wood fill. If utilities bear upon fill, buried topsoil and/or wood debris, a substantially increased settlement risk and the related potential for cracking, rupture or other distress (including failure) must be accepted.”

In addition, the soils caused the structural system to consist of a thickened slab paired with deep foundation piles. They required a piped methane mitigation system installed directly below the slab that extended through the roof. Typically, even when these deep structural foundation systems are required, the newly installed clean fill below the building footprint is acceptable to support the underground utilities. 

However, the geotechnical engineer confirmed this was not the case for this site. The risk of inaccessible underground piping below the structure failing is the last thing a client of a new construction project wants to own, so the project team went back to the drawing board.

Unsurprisingly, a suggestion to pick a new site from the lowly plumbing engineer was not heard too loudly, and removing the wood fill was not feasible. Hence, after multiple pricing exercises and deliberation, the most plausible strategies to support the underground plumbing were over-excavating to install additional engineered fill below the piping, supporting with helical piles from below, hanging the pipe from the slab above, or transitioning to a vacuum waste system.

Ultimately, the systems were redesigned to minimize underground piping. Then, a combination was implemented within the building footprint of hanging pipe from the slab where necessary (such as the storm, domestic water and certain sanitary piping) and a vacuum waste system for the bulk of the sanitary system. 

The hanging solution was only implemented where necessary due to the high cost of corrosion-resistant, stainless-steel hangers that were required to bear the weight of the filled pipe and the volume of fill above in case of complete soil failure below the pipe (see Figure 1). Outside the footprint, over-excavation was implemented for the buried site civil utilities and helical piles for the exterior decontamination waste storage tank, given there was no structure to support from.

At any rate, the vacuum waste system had officially entered the project scope. HGA partnered with Miron Construction, Tweet/Garot Mechanical, and AcornVac to implement the design.

What is a vacuum waste system?

Healthcare plumbing engineers are often familiar with medical vacuum systems used for various suction functions during medical procedures. A vacuum waste system takes advantage of the same universal vacuum principles by using vacuum pump equipment to transport sanitary waste overhead, through grinders and into storage tanks, which then discharge to the gravity building drain, eliminating the need for underfloor piping in most cases.

Niche and atypical rules exist with a vacuum waste system not found in traditional gravity systems. Sanitary pipe sizes, especially the mains, are generally smaller because the system is under constant negative pressure. However, the piping requires a 1/4 inch/foot slope. 

Why use a vacuum system if slope is still required like a gravity system? The primary benefit is the slope does not need to be continuous, meaning “slope make-ups” can be installed back up to a higher elevation and around obstacles, such as in Figures 2-4. It is important to abide by manufacturer-specified dimensions and instructions regarding slope make-ups to ensure proper functionality.

Some designers may be more familiar with these systems in the setting of supermarkets and convenience stores for freezer section condensate disposal or for penal institutions for increased security, but these systems are also gaining traction in the healthcare industry for multiple reasons.

Vacuum waste benefits in healthcare

Often, the primary benefit is how the system greatly reduces the scope of current construction or future renovations, as evident at this site. Not having to saw-cut the floor to relocate existing grade-level fixtures provides long-sought-after flexibility. 

However, this benefit is not limited solely to grade-level fixtures. Adding fixtures to the level above an existing operating room suite is a great example, as shutting down the suite for an extended period to install traditional gravity waste would cause enough disruption and cost to make the initial purchase of the vacuum waste equipment worthwhile.

Another system benefit is a reduction in water closet water usage and smaller domestic water pipe sizes as a result. With a discharge of 0.26 gallons/flush, the Wisconsin plumbing code goes as far as to designate a value of one water supply fixture unit (WSFU) per vacuum-flush water closet fixture. 

When compared to the 6.5 WSFU for a standard commercial flush valve water closet, the pipe size savings are exponential, with only a 1/2-inch water supply needed to serve each toilet room for this project. See John Gregory’s article for a larger case study focusing on the water-saving potential (https://bit.ly/3Cob1Yf).

In the same vein, with the sanitary waste being collected into storage tanks, the discharge to the building drain can be systematically controlled, which allows that pipe size to also be reduced compared to a traditional gravity system.

Additionally, a traditional venting system is not required due to negative pressure in the vacuum piping network. However, if a traditional venting system is not used, extending the waste receiver air intake piping to above the ceiling with a charcoal filter can be added at each fixture to allow air to enter and exit the receiver to mitigate odors. This project used a traditional venting system to alleviate safety and health concerns.

Finally, there is documented evidence that flushing a vacuum-flush water closet is more sanitary than a gravity-flushed fixture due to the suction action mitigating bowl content agitation (see Figures 5 and 6). For instance, NSF International’s test report comparing different water closet flushes determined that the overspray of AcornVac’s vacuum flush water closet was not detectable. (NSF International, 2012, https://bit.ly/4jElkZ4.)

System limitations

As with any system, there are also limitations to consider.

Being a mechanically driven system, space is required for the vacuum center equipment. The waste storage tanks and grinders can be in the same room as the vacuum equipment. However, with the system having been added so late in the design for this project, it was least impactful to the floor plan to separate the equipment. The vacuum equipment and some of the waste drainage valves also require electrical connections and controllers.

Beyond the equipment, it is important to properly brace the vacuum waste piping to resist thrust loads that exist in a vacuum plumbing system. A general solution used at this site was to route the mains at the sides of corridors for more effective sidewall bracing. Extraction valves, which require accessibility comparable to domestic water system valves, are also installed to properly operate each fixture. 

Proper accessibility to the water closet extraction valves is especially important because it is where the bulk of clogs occur in the vacuum plumbing system. However, the clog is limited to an individual fixture, not the waste piping downstream of the fixture, a problem that routinely plagues gravity waste systems.

Additionally, there were fixture selection limitations due to specific fixtures required by the vacuum waste manufacturer. Stainless-steel vacuum flush water closets were needed to provide bedpan lugs due to the manufacturer not having a porcelain option with bedpan lugs. Without the option of combining the bedpan washer with a standard water closet flush valve, recessed bedpan washers separate from the water closet were also needed.

Trench drains were easier to implement for showers, as a standard floor drain would require a larger slab depression and additional piping. Sink and lavatory fixtures discharge initially to in-wall box accumulators provided by the manufacturer, which collect the drainage until a sensor triggers the extraction valve to open, allowing the vacuum to extract the waste. 

There is also understandable hesitancy around the sound of the vacuum flush water closets, but it is not louder than a typical flush valve, only a different sound to get accustomed to.

Areas where high sanitary waste loads or atypical waste were expected also impacted the design at this site. The central sterile processing equipment had high sanitary waste volumes, which discharged quickly. These loads would have significantly increased the vacuum system size and expense with very little benefit to the facility. 

As a result, that area remained as a gravity system with stainless-steel hangers supporting the piping from the slab. The designated decontamination sanitary waste piping also remained as a gravity system to avoid acidic waste in the vacuum waste system.

A separate grease vacuum waste system was also needed to serve the kitchen, including separate storage tanks that discharged to a grease interceptor just upstream of the building drain.

For isolated larger extended flow demands that need to be on the vacuum waste system, such as the cart washer or emergency shower stations for this project, vertical pipe accumulators are required. These can be considered small sumps (often 8 inches or 10 inches in diameter) extended vertically below the slab that can collect enough waste to allow the vacuum waste system to function efficiently.

Finally, substantial heat is discharged by the vacuum waste pumps that needs to be considered. For this project, the vacuum center was located in the same room as the medical gas compressors, so the HVAC engineer only had a single space to consider a large cooling load.

Key project takeaways

• Clarify and coordinate with the geotechnical engineer early, paying specific attention to utility construction requirements.

• Implement a vacuum waste system as early in the design as possible to properly coordinate all requirements and nuances.

• While vacuum waste systems may not fix all sanitary piping dilemmas or be feasible in all projects, it is a worthwhile tool to add to any plumbing engineer’s toolbox.

Logan Stone, EIT, is a mechanical staff member at the HGA Architects and Engineers office in Milwaukee who specializes in designing plumbing infrastructure for healthcare campuses, as well as other market sectors across the country. He is an active member of the American Society of Plumbing Engineers and an adjunct professor at the Milwaukee School of Engineering, where he teaches plumbing design to students majoring in architectural engineering.