6.4 Sumps, Ejectors, and Backwater Valves

Sub-Sewer Drainage Sizing and Design

When plumbing fixtures are located on floors below the gravity sewer line (e.g., in a basement or sub-grade parking garage), their waste cannot flow out of the building by gravity. In these situations, the waste must discharge into an approved sump basin and be pumped up into the gravity drainage system.

Under IPC Section 712, sumps are divided into two categories:

1. Drainage Sumps (Sump Pumps): Collect clear water from subsoil drains, foundation drains, or clear-water waste.
2. Sewage Sumps (Sewage Ejectors): Receive and discharge sanitary sewage containing solid waste.

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Duplex Sewage Ejector Systems

In commercial installations, a pump failure is a critical health hazard that could shut down the entire building. Therefore, IPC Section 712.4.2 requires commercial sewage ejectors serving multiple fixtures to be duplex systems equipped with two pumps.

Duplex Operation Rules:

• Alternating Duty: The control system must alternate the operation of the pumps automatically to distribute wear evenly.
• High-Water Alarm: An audible and visual alarm must be installed to alert building maintenance if the liquid level in the basin rises above the normal pump-starting point.
• Secondary Pump Startup: If the primary pump fails or the inflow exceeds the capacity of a single pump, both pumps must operate simultaneously to handle the peak load.

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Venting and Basins (IPC Section 712.1 & 712.3)

Sewage sumps collect active solid waste, which generates hazardous and explosive gases (methane, hydrogen sulfide).

• Gas-Tight Covers: Sump basins must be constructed of concrete, fiberglass, steel, or plastic, and must be provided with a bolted, gas-tight lid.
• Vent Requirements: Every sewage sump must be vented to the atmosphere to relieve pressure changes when water enters or leaves the tank. The vent must be sized no smaller than 1-1/2 inches and must connect directly to the building's vent system.
• Air Admittance Valve (AAV) Prohibition: Under IPC Section 918.8, AAVs are prohibited from venting sewage sumps. Because AAVs only open under negative pressure, they cannot relieve the positive pressure created when waste rapidly enters the sump. This positive pressure would blow out the trap seals of nearby fixtures.

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Sump Sizing, Capacity, and GPM Calculations

Sewage ejectors and sump pumps must be sized to handle the anticipated flow rate. An undersized pump will cycle too frequently, leading to premature motor failure, or fail to keep up with peak demand, resulting in flooding.

• Flow Rate (GPM) Sizing: The pump capacity must be matched to the fixture unit load. A common engineering rule of thumb is to size the pump for a minimum flow rate of 20 GPM for small installations, or to calculate the peak flow rate based on the fixture demand. One DFU is roughly equivalent to 1 GPM of continuous flow, but for pump sizing, designers often use specific tables or curves to determine the required GPM.
• Macerating Toilet Systems: These systems are specialized sumps that collect waste from a single water closet, lavatory, and shower, grind the waste, and pump it through small-diameter piping (often 3/4 or 1 inch). They must comply with ASME A112.3.4.

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Discharge Piping and Valve Sequence (IPC Section 712.2)

The sizing and piping layout of pump discharge lines are critical to prevent clogging and allow for safe maintenance.

• Minimum Pipe Size: Standard sewage ejectors must have a minimum discharge pipe size of 2 inches to allow the passage of solids. Simplex grinder pumps (which chop solids into a fine slurry) are permitted to have a minimum discharge size of 1-1/4 inches.
• Required Valves: Each discharge line must be equipped with two valves:
  1. A check valve to prevent pumped liquid from draining back into the sump when the pump shuts off.
  2. A gate valve or full-port ball valve to isolate the pump and check valve for maintenance.
• Sequence: The check valve must be installed closest to the pump, with the gate/ball valve installed downstream (on the discharge side) of the check valve. This allows the check valve to be serviced without draining the entire vertical discharge stack.

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Backwater Valves (IPC Section 715)

A backwater valve is a check valve installed in a drainage line that allows water to flow out of the building but automatically closes to prevent sewage from flowing back into the building when the public sewer surcharges (floods).

When Are They Required?

Under IPC Section 715.1, a backwater valve is required on drainage piping serving fixtures whose flood level rims are below the elevation of the next upstream manhole cover of the public sewer system. The upstream manhole rim serves as the datum point because it represents the highest level water can reach in the public sewer before spilling onto the street.

Critical Installation and Inspection Rules:

• Location Limit: Backwater valves must only protect fixtures located below the surcharge level. They must not be installed on the main building drain serving upper floors. If a backwater valve is placed on the main line and closes during a sewer backup, any water used on upper floors will flow down, hit the closed valve, and back up through the lowest fixtures in the building, flooding the basement.
• Accessibility and Maintenance: Backwater valves must be installed in an accessible location (e.g., in a crawl space, floor vault, or behind an access panel) so they can be inspected and cleared of debris. The valve must be inspected annually, and any accumulated debris on the valve seat must be removed to ensure a tight seal.
• Access Sleeves: If a backwater valve is buried below a concrete slab, it must be provided with an access sleeve and cover plate extending to the finished floor level.