Recently there was a discussion within the ASPE Connects Open Forum regarding “Venting Vertical Offsets in Sanitary Drainage Piping.” As part of that discussion, Mr. James Richardson, plumbing inspection supervisor for the City of Columbus, Ohio, threw out an interesting point to the discussion. It is a consideration that many design professionals may have not considered in their designs as plumbing engineers generally think in terms of design within the building envelope.
So where does a sanitary plumbing system begin and end? As design professionals, our first and primary obligation is the protection of public health, safety and welfare above all other concerns. When one thinks of a sanitary plumbing system, it is generally confined to the Drainage, Waste, and Vent (DWV) system within the limits of a 30-inch boundary around the building envelope. Yes, 30-inches, although we normally use 5 feet to transition between the “building drain and building sewer.” The “5-foot rule” is little more than a “gentlemen’s agreement” that has evolved through the years within our industry. The typical code-in-force defines the transition between the drain and sewer as 30-inches. It is also normal industry practice to separate the interior plumbing system from the site, civil system at that same 5-foot point. This separation point also separates the plumbing consultant from the civil consultant.
While the plumbing consultant develops their design based on sound engineering principles; at the same time considering the minimum requirements of the plumbing code, remember that the code is a minimum standard and does not necessarily apply to all designs. The code is a reasonable guide or starting point, but sound engineering practices must drive the design. Using the minimum design criteria of the code can — and many times does — result in unintended issues with the sanitary system design.
Many of these issues can be traced back to when the code started to introduce the concept of reduced-size venting, which continues to be used in today’s designs. The problem with this approach is that the plumbing system venting must service much more than just the building sanitary system. This is because the building's sanitary system connects to an exterior waste distribution system; municipal sewer or private disposal system; sewer gases do not recognize the differences between the interior and exterior portions of the sanitary system. These sewer gases must be vented to maintain a neutral air pressure within the system, in an effort to maintain the 2-inch water column trap seal mandated by the code.
Although, as a design professional I must utilize code within the building, my obligations do not stop at the separation between interior and exterior systems. My obligation to the public requires one to understand the “overall” sanitary system; collection, distribution and processing/disposal. The civil consultant performs their work under accepted engineering practices and established standards. As they are not normally concerned with maintaining a “trap seal,” there is little to no concern about venting sewer gases generated within their systems. In the past, venting of the “public” system was accomplished via “holes” within manhole covers. These holes allowed sewer gases to escape from the system, minimizing air pressure variation within those systems.
However, over the years, the EPA (Environmental Protection Agency) came to recognize that a significant volume of “stormwater” was being allowed to enter the sanitary system increasing the volume of effluent that needed to be treated. Hence, the EPA mandated all new and replacement manhole covers should have a positive seal in an effort to eliminate stormwater infiltration into the sanitary sewer system. While this has helped to reduce the treatment volume, in cities like Columbus, Ohio, there remains a significant amount of combined sewage systems; sanitary and storm.
These combined sewer systems are slowly being separated into sanitary and storm in an effort to reduce the effluent treatment as well as minimize the pollution of our natural waters. This process will take many years and significant funds to totally eliminate combined sewers. Combined sewers do offer a better means of venting sewer gases through catch basins, curb inlets, etc. They also emit noxious sewer gas odor near street level; which is normally objectionable to the public. But, combined sewers do assist in maintaining neutral air pressures within the system during normal operations. The same is not true during major rain events when positive pressures can become sufficient to exhaust large volumes of sewer gases as well as expel combined sewage onto the street or into a building (need for backwater valve).
As a plumbing engineer, one needs to know and understand the complete system that they will be connecting into. This is because the exterior sanitary systems — sanitary or combined — are not designed with venting considerations in mind. Within the municipal systems, maintaining a neutral air pressure is not necessarily a concern. The volume of these large municipal systems can accommodate pressure surging within the exterior system environment. The municipal system operators and the civil consultants rely on the connection to building sewers and building drainage to vent their systems. These exterior systems see a significant range of positive and negative pressures (the pressures within these systems are seldom if ever neutral) that are reaching back into the buildings and their DWV systems.
So, it becomes incumbent on the plumbing engineer to be aware of the potential issues that can arise because of the pressure fluctuations generated within these exterior systems. As engineers and designers, we must protect the trap seals within our building from exterior-generated air pressure surges and fluctuations that exceed the code-mandated 2-inches of water column trap seal. So, it becomes a responsibility that is not spelled out in the minimum requirements of the plumbing section of the building codes. But a requirement nonetheless that needs to be evaluated and considered in our designs.
While we have no control over the exterior conditions being developed within exterior distribution and disposal systems, they are real and can have a negative impact on our design. So, while the codes have made efforts to reduce the vent sizing, from a restricted point of view, within the building environment, that environment is not isolated from the things to which it connects. As design professionals, we must be aware of the overall environment and consider how our system design contributes to the overall operation of the inter-connected systems; building and municipal or private disposal. One thing to consider, take the time to have the building/public sewer pressures recorded and evaluated. Depending on that evaluation, one may need to provide additional venting to negate the pressure fluctuation injected by the exterior sewer connection.
So, it becomes incumbent on the plumbing engineer to be aware of the potential issues that can arise because of the pressure fluctuations generated within these exterior systems. As engineers and designers, we must protect the trap seals within our building from exterior-generated air pressure surges and fluctuations that exceed the code-mandated 2-inches of water column trap seal.
This brings us back to the question of, “Where does the plumbing system begin and end?” Given the design professionals’ obligation to protect public health, safety and welfare, remove the “blinders” and think outside of the “foundation” box. The plumbing system is a component of a complex system of plumbing, sewers and treatment processes. Our system must work with the other parts of the total processes that make up the “system.” Sometimes, it may be necessary to increase the venting within the building to accommodate the pressure surges and fluctuations within a component of the total system over which we have no control.
At one time, the code required a running building trap with a building sewer main vent. This vent was intended to provide the venting of the municipal/private sewer systems and act as a disconnect between the exterior and interior air pressure conditions. As the EPA continues to “seal” the systems from stormwater infiltration, it may become necessary to isolate the sewer from the building drainage systems. And, as part of the isolation/separation, create venting for each system as was covered by the code previously. Just remember our obligation to protect the public through the design of our sanitary system. So, plumbing is not isolated to just the building or the code that covers that building. Plumbing involves the total environment in which it operates, regardless of the limiting factors of the code.