8.3 Sewage Ejectors, Sumps & Macerating Systems

8.3 Sewage Ejectors, Sumps & Macerating Systems

Much usable floor space sits below the building sewer: basements, lower-level tenant spaces, fitness centers and mechanical rooms. Wastewater from fixtures there cannot reach the sewer by gravity. IPC Section 712 governs the sumps, pumps, ejectors and macerating systems that lift it into the gravity drainage system, and IPC 906.5 sizes their vents.

For plans examiners, pumped drainage is a frequent source of corrections: undersized pumps, missing valves, discharge lines tied into the wrong place and sumps vented incorrectly.

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Subdrains and What a Sump May Receive (IPC 712.1)

• Building subdrains that cannot discharge to the sewer by gravity discharge into a tightly covered and vented sump. From there, the liquid is lifted into the building gravity drainage system by automatic pumping equipment or another approved method.
• No gravity-capable piping: In other than existing structures, the sump shall not receive drainage from any piping capable of discharging by gravity to the building sewer. Upper floors must not drain through the pump.
• Below-sewer venting (906.5.1): Drainage piping below sewer level is vented in the same manner as a gravity system.
• Storm versus sanitary: Storm subdrains and sumps are covered separately in IPC 1112 and 1113. Clear-water and storm systems are usually kept out of sanitary ejector pits, and the local sewer authority often prohibits clear water in the sanitary sewer.

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Sump Pit Construction (IPC 712.3.2 and 712.3.4)

┌────────────────────────────────────────────────────────┐
│             SUMP PIT REQUIREMENTS (IPC 712.3.2)         │
├───────────────────────┬────────────────────────────────┤
│ Feature               │ Requirement                    │
├───────────────────────┼────────────────────────────────┤
│ Size                  │ >= 18 in. diameter, >= 24 in.  │
│                       │ deep, unless otherwise approved│
│ Materials             │ Tile, concrete, steel, plastic │
│                       │ or other approved materials    │
│ Bottom                │ Solid; permanently supports    │
│                       │ the pump                       │
│ Cover                 │ Gastight, removable, not more  │
│                       │ than 2 in. below grade/floor   │
│ Flow                  │ All drainage enters by gravity │
│ Venting               │ Per Chapter 9                  │
└───────────────────────┴────────────────────────────────┘

• Access and gravity entry: The pit is provided with access and located so all drainage flows into it by gravity.
• Cover loads: The cover must be adequate for the anticipated loads where it is located.
• Maximum effluent level (712.3.4): Level controls must keep the effluent from rising to within 2 inches of the invert of the gravity drain inlet into the sump, so the inlet never backs up.
• Buoyancy: Where groundwater is high, a plastic or fiberglass basin can float. Manufacturers' anti-flotation anchors or concrete collars address this, though the code itself states only the performance requirement.

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Pump and Ejector Capacity (IPC 712.3.1 and 712.4.2)

Solids Handling

Under IPC 712.4.2, a sewage pump or ejector must have the capacity and head for the application, and:

• Pumps or ejectors receiving the discharge of water closets must handle spherical solids up to and including 2 inches in diameter.
• Other pumps or ejectors must handle spherical solids up to and including 1/2 inch.

Minimum Capacity by Discharge Pipe Size (IPC Table 712.4.2)

These minimums keep the force main moving fast enough to carry solids (about 2 feet per second in the listed pipe sizes). Exceptions: Grinder pumps or grinder ejectors receiving water closet discharge may have a discharge opening as small as 1-1/4 inches, and macerating toilet assemblies serving a single water closet as small as 3/4 inch.

Total Dynamic Head (TDH)

IPC 712.3.1 requires sump pump capacity and head to suit the anticipated use. Examiners check the pump curve against the system's total dynamic head: $TDH = H_{\text{static}} + H_{\text{friction}} + H_{\text{residual}}$

• $H_{\text{static}}$ is the lift from the pump's low water level to the highest point of the discharge line.
• $H_{\text{friction}}$ is the friction loss in the discharge pipe, fittings, check valve and shutoff valve at design flow.
• $H_{\text{residual}}$ is any pressure needed at the outlet, usually near zero for a gravity drain connection.

Simplex or Duplex?

The IPC requires only that capacity and head suit the application. It does not mandate duplex pumps or a high-water alarm for every commercial occupancy. Many designers specify duplex alternating pumps and alarms for reliability in commercial and multifamily buildings, and some jurisdictions or owners require them. A plans examiner should cite the code accurately and treat duplex pumps as a design choice unless a local amendment requires them.

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Valves, Discharge Piping and Connection (IPC 712.2, 712.3.3 and 712.3.5)

            REQUIRED VALVES ON THE DISCHARGE LINE (IPC 712.2)

    To gravity drainage: wye into the TOP of horizontal piping,
    at least 10 pipe diameters from any stack base or fixture drain
                        ▲
                        │ [Full-open valve] downstream of the check valve
                        │
                        │ [Check valve]
                        │
    ====================╪==================== [GASTIGHT SUMP COVER]
                        │  Valves above the cover, or in an access pit
                        │  outside the sump where the discharge is below grade
                   [PUMP OUTLET]

Required Valves (IPC 712.2)

A check valve and a full-open valve on the discharge side of the check valve must be installed in the discharge piping between the pump or ejector and the gravity drainage system. Access must be provided to both. They are located above the sump cover, or, where the discharge pipe is below grade, outside the sump in an access pit with a removable cover.

With this arrangement, closing the full-open valve holds back the water standing in the force main and gravity system, so the check valve and pump can be serviced without draining that column into the pit.

Discharge Pipe Materials and Ratings (712.3.3)

Discharge pipe and fittings are copper or copper-alloy, CPVC, ductile iron, PE or PVC, rated for the maximum operating pressure and temperature, compatible with each other, and suitable for burial where buried.

Connection to the Gravity System (712.3.5)

Pumps connect to a building sewer, building drain, soil stack, waste stack or horizontal branch drain. Where the discharge connects to horizontal drainage piping, it enters through a wye fitting into the top of the pipe, and the wye is located not less than 10 pipe diameters from the base of any soil stack, waste stack or fixture drain. Side or bottom connections let gravity flow wash back into the force main.

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Sump Venting (IPC 906.5) and Air Admittance Valves (IPC 918.8)

Sump Vent Sizing (IPC 906.5.1 and Table 906.5.1)

Vents for sumps with sewage pumps or ejectors (other than pneumatic) are sized from Table 906.5.1 by the pump discharge capacity (gpm) and the developed length of the vent. Fifty percent of the developed length may be added for fittings where a more precise value is not available. Selected values:

Pneumatic ejectors (906.5.2): The air pressure relief pipe connects to an independent vent stack terminating like a roof vent extension, sized to relieve the ejector to atmospheric pressure but not less than 1-1/4 inches.

Air Admittance Valves on Sumps (IPC 918.8)

Under IPC 918.8, air admittance valves shall not be used to vent sumps or tanks except where the vent system for the sump or tank has been designed by an engineer.

┌────────────────────────────────────────────────────────┐
│      WHY AN ORDINARY AAV DOES NOT VENT A SEALED SUMP   │
├────────────────────────────────────────────────────────┤
│ 1. An AAV is a one-way valve: it opens only under      │
│    negative pressure to admit air.                     │
│ 2. Water entering a sealed sump DISPLACES air and      │
│    raises the pressure inside the basin.               │
│ 3. The AAV stays closed under positive pressure, so    │
│    displaced air cannot escape through it.             │
│ 4. That pressure can push gas back through fixture     │
│    traps or slow the inflow to the pit.                │
└────────────────────────────────────────────────────────┘

Without an engineered design, the sump needs an open vent sized from Table 906.5.1.

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Macerating Toilet Systems (IPC 712.4.1)

Where breaking a slab for a sump is impractical, the IPC recognizes macerating toilet systems complying with ASME A112.3.4/CSA B45.9, installed per the manufacturer's instructions.

System Anatomy

• A water closet that discharges to a compact reservoir with a cutter (macerator) and pump.
• Auxiliary inlets for a lavatory, shower or bathtub where the product allows.
• A small pressurized discharge line routed to a gravity drain. For a macerating assembly serving a single water closet, the discharge opening may be as small as 3/4 inch (712.4.2 Exception 2).

Plan Review Points

1. Standard: Listed to ASME A112.3.4/CSA B45.9.
2. Venting: Vented as the manufacturer requires and as Chapter 9 requires for the connected fixtures.
3. Fixture counts: A macerating water closet counts toward the fixtures required by Table 403.1 like any other water closet. Check that the product suits the expected use.

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Plan Examiner Review Checklist: Sumps and Ejectors

1. Gravity-capable piping: Do only fixtures that cannot drain by gravity discharge to the sump (712.1)?
2. Pit: At least 18 inches in diameter and 24 inches deep, with a gastight removable cover no more than 2 inches below the floor (712.3.2)?
3. Solids and capacity: Does a pump receiving water closets pass 2-inch spheres, and does its capacity meet Table 712.4.2 for the discharge size?
4. Valves: Is there a check valve with a full-open valve downstream, accessible above the cover or in an access pit (712.2)?
5. Connection: Does the discharge enter the top of horizontal piping through a wye at least 10 pipe diameters from any stack base or fixture drain (712.3.5)?
6. Vent: Is the sump vent sized from Table 906.5.1, and is any AAV on the sump supported by an engineered design (918.8)?