Special Systems in the Code: Grease Interceptors, Gray Water, and Medical Gas References

Objective. Students will locate and summarize the code requirements governing at least three special plumbing systems (grease interceptors, gray-water reuse, and storm drainage) and explain where the plumbing code ends and a referenced specialty standard or code begins.

Special Systems in the Code: Grease Interceptors, Gray Water, and Medical Gas References

The International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) cover far more than household drains and supply lines. Both codes include chapters and appendices devoted to special plumbing systems — systems that involve unusual waste streams, water reuse, or connections to life-safety equipment. In each case, the plumbing code sets baseline requirements and then points you toward a referenced standard that governs the technical details the code itself does not fully cover. Knowing where one document ends and another begins is a core job skill.

Grease Interceptors

Commercial kitchens, restaurants, and food-processing facilities discharge fats, oils, and grease (FOG) into the drainage system. If FOG reaches the public sewer unchecked, it solidifies, restricts flow, and can cause sanitary sewer overflows — an environmental and public-health hazard. Both the IPC and UPC require grease interceptors (also called grease traps) to capture FOG before it enters the building drain.

Key code requirements address two things: sizing and access.

Sizing is based on the fixtures connected to the interceptor — typically sinks, floor drains, and dishwasher pre-rinse stations in commercial kitchens. The codes provide sizing tables that account for drainage flow rate, the number of fixture units, and in some cases the peak discharge period. Passive hydromechanical grease interceptors (the small under-sink units) and large exterior gravity grease interceptors (in-ground tanks) are sized by different methods. The Plumbing and Drainage Institute (PDI) standard PDI-G101 is the referenced document for hydromechanical interceptor sizing; the codes point you there for engineering details.

Maintenance access is non-negotiable. The code requires that interceptors be accessible for inspection and cleaning. Large exterior interceptors must have access risers or manholes at grade. The code does not specify a pumping schedule — that is typically set by the local authority having jurisdiction (AHJ) or the publicly owned treatment works (POTW) permit — but the physical access must be built in from the start.

Safety note: When inspecting or working near grease interceptors, be aware that decomposing FOG generates hydrogen sulfide and methane. Both are toxic and flammable. Follow confined-space entry procedures if you must enter an exterior interceptor vault, and never work alone. Proper PPE includes chemical-resistant gloves and, for entry, an air-monitoring device and a supplied-air or SCBA respirator.

Gray-Water Reuse

Gray water is wastewater from lavatories, bathtubs, showers, and laundry — water that has not contacted toilet waste. Reclaiming gray water for toilet flushing or subsurface irrigation can significantly reduce potable-water use.

The IPC and UPC do not mandate gray-water systems, but both codes include provisions for them — sometimes in appendices that a state or jurisdiction must specifically adopt to make them enforceable. This is a critical distinction: if your state has not adopted the gray-water appendix, there is no code permission to install a gray-water system, and the AHJ may prohibit it entirely. Some states have adopted their own gray-water regulations independent of the model code appendix.

Where an adopted gray-water appendix applies, typical requirements include:

Gray water must be collected separately from black water (toilet and kitchen sink waste).
Storage tanks must be covered, vented, and labeled "NON-POTABLE WATER — DO NOT DRINK."
Potable-water makeup connections must include an air gap or approved backflow preventer to protect the supply.
Gray water used for toilet flushing must be disinfected or treated to a specified quality standard before entering the building's flush supply.
Subsurface irrigation systems must meet minimum soil-cover depths to prevent human contact.

The backflow protection requirement is especially important: improperly installed cross-connections between gray-water systems and potable supply lines are a serious public-health hazard and a common inspection failure.

Storm Drainage

Storm drainage — the system that removes rainwater from roofs, area drains, and paved surfaces — is governed by the plumbing code but operates entirely separately from the sanitary system. The IPC dedicates a full chapter to storm drainage, covering roof drain sizing, leader (downspout) sizing, and the prohibition against connecting storm drainage to the sanitary sewer except where a combined sewer system is specifically authorized by the AHJ.

Sizing storm drainage requires the local design rainfall rate (measured in inches per hour), which comes from local meteorological data, not the plumbing code itself. The code provides sizing tables but instructs the designer to insert the correct local rainfall intensity. This is another example of the code pointing outward to an authoritative external source.

Safety note: Roof drain work involves fall hazards. Always follow OSHA fall-protection requirements when working at elevation.

Medical Gas: Where the Plumbing Code Ends

Hospitals and healthcare facilities use piped medical gas systems — oxygen, nitrous oxide, medical air, and medical vacuum — that are connected to patient care areas. The plumbing code may acknowledge medical gas piping and set some general installation requirements, but it explicitly defers to NFPA 99, Health Care Facilities Code for the design, installation, testing, and maintenance of these systems. NFPA 99 establishes the risk categories, pipe material requirements, pressure testing protocols, and labeling standards that govern medical gas.

Similarly, pressure vessels (such as expansion tanks and receivers) are governed by ASME Boiler and Pressure Vessel Code rules, not the plumbing code. HVAC connections and mechanical equipment rooms fall under the International Mechanical Code (IMC) or equivalent. A journeyman plumber needs to know where to stop and call in the right code authority or specialty contractor.

The Key Principle: Code Boundaries

In every special system, the plumbing code does three things: it sets the performance objective, it establishes the plumber's scope of work, and it references the standard that governs the technical details beyond that scope. Your job is to follow the plumbing code requirements precisely, recognize when you have reached a boundary, and flag any work that falls under a referenced standard for the appropriate review or specialty license.

Key terms10

Grease interceptorA plumbing device installed in the drainage system of a commercial kitchen or food-service facility to capture fats, oils, and grease before they enter the sanitary sewer.

Hydromechanical grease interceptorA compact, under-counter or in-line grease interceptor that uses water flow dynamics to separate grease; sized using PDI-G101 or equivalent referenced standard.

Gray waterWastewater from lavatories, tubs, showers, and laundry that has not contacted toilet or kitchen sink waste; may be reused for non-potable purposes where the applicable code or appendix permits.

Authority Having Jurisdiction (AHJ)The organization, office, or individual responsible for enforcing the requirements of a code — typically the local building or plumbing inspection department.

Referenced standardAn external document (such as NFPA 99 or PDI-G101) that a code adopts by reference, making that document's requirements enforceable as part of the code within its defined scope.

NFPA 99The Health Care Facilities Code published by the National Fire Protection Association; governs the design, installation, testing, and maintenance of medical gas and vacuum systems in healthcare occupancies.

Storm drainageThe plumbing system that collects and conveys rainwater from roofs, area drains, and paved surfaces to an approved point of disposal, kept completely separate from the sanitary drainage system unless a combined sewer is specifically authorized.

Code appendixA supplemental section of a model plumbing code that is not automatically enforceable; it takes effect only when a state or local jurisdiction formally adopts it.

Cross-connectionAny physical connection or arrangement between a potable water system and a non-potable source (such as a gray-water system) that could allow contamination to enter the drinking water supply.

Design rainfall rateThe local storm intensity value, expressed in inches per hour, used with plumbing code sizing tables to determine the required capacity of a storm drainage system; obtained from local meteorological data, not from the code itself.

In class: Guided code reading: students read the grease interceptor and interceptor sizing sections of the IPC, then summarize sizing criteria and maintenance access requirements in their own words. · Gray-water comparison: students compare gray-water reuse provisions in states that have adopted appendix gray-water chapters versus those that have not, using the code appendix and instructor-provided state adoption status list. · Boundary mapping exercise: students create a simple diagram showing where the IPC/UPC end and where NFPA 99, ASME pressure-vessel rules, or mechanical codes take over for medical gas, pressure vessels, and HVAC connections.
Materials: IPC and UPC special-systems chapters and appendices, state gray-water adoption status list (instructor-prepared from public sources), boundary diagram template.
Check: Written summary of three special systems: each summary must correctly state at least two code requirements for that system with section references — pass requires all three summaries meeting this standard.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.