3.6 Sump Pumps, Sewage Ejectors & Backwater Valves

Sump Pumps, Sewage Ejectors & Backwater Valves

Sub-Grade Drainage: Gravity vs. Mechanical Lift

Whenever plumbing fixtures (water closets, showers, laundry trays, floor drains) are installed in basements or sub-grade levels where their flood level rims lie below the invert of the public sewer main or building sewer, their waste cannot drain by gravity.

Under UPC Section 710.0, all sub-grade fixtures must discharge into an approved liquid-tight and gas-tight sump basin, where an automatic sewage ejector pump lifts the waste and discharges it into the gravity building drain.

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Sump Pumps vs. Sewage Ejectors

It is vital to distinguish between clear-water sump pumps and sewage ejectors:

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Sewage Ejector Installation & Piping Rules

                    [ To Outdoor Vent Termination ]
                               ▲
                               │  Min 2" Vent Pipe
   To Gravity                  │
 Building Drain  ◄───[ Fullway Gate/Ball Valve ] (Shutoff Valve)
                       ▲
                       │
                 [ Swing Check Valve ] (Prevents Backflow into Sump)
                       ▲
                       │  Min 2" Discharge Line
            ┌──────────┴───────────────────────────┐
            │   GASKETED GAS-TIGHT COVER           │
      ──────┴──────────────────────────────────────┴────── Basement Floor
      │  ┌──────────────────────────────────────┐  │
      │  │                                      │  │
      │  │        SEWAGE EJECTOR PUMP           │  │
      │  │       (Handling 2" Solids)           │  │
      │  └──────────────────────────────────────┘  │
      └────────────────────────────────────────────┘

Basin and Venting Standards (UPC Section 710.2)

1. Basin Cover: Sumps receiving sewage must be enclosed in a gas-tight, liquid-tight pit with a bolted, gasketed cover.
2. Vent Sizing: Sumps must be vented to prevent air locking when sewage enters and to exhaust foul gases. The vent pipe must be at least 2 inches in diameter (or sized per UPC Table 710.1 based on pump GPM and pipe length).
3. Prohibition of AAVs: Air Admittance Valves (AAVs) rely on gravity to close and cannot handle positive air displacement when a pump rapidly discharges. AAVs are strictly prohibited on sewage ejector sumps.

Discharge Piping and Valve Requirements (UPC Section 710.4)

1. Discharge Diameter: The discharge pipe serving a non-grinder sewage ejector receiving water closet waste must be at least 2 inches in diameter.
2. Valve Sequence: Every ejector discharge line must be equipped with two valves, installed in a specific sequence moving away from the pump toward the building drain:
   • Step 1 (Closest to Pump): An approved swing check valve or quiet check valve to prevent backflow of liquid from the vertical stack back into the sump when the pump stops.
   • Step 2 (Downstream of Check Valve): An accessible fullway gate valve or ball valve installed downstream of the check valve. This shutoff valve allows isolation so the check valve can be serviced without draining the entire horizontal building drain system.

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Sump Pump Sizing Calculations

Pump capacity is calculated based on total connected DFU load converted to peak GPM, plus required dynamic head pressure.

Peak Flow Rate Formula

For residential ejector applications serving water closets, the pump must deliver a flow rate sufficient to achieve self-scouring velocity (2.0 fps) in the 2-inch discharge line:

$\text{Min Velocity} = 2.0 \text{ fps in 2" pipe} \implies \text{Min Pump Rate} \approx 21 \text{ GPM}$

Standard sewage ejector pumps are specified to deliver 20 to 30 GPM at a minimum 15 to 20 feet of Total Dynamic Head (TDH).

• Duplex Systems: In commercial buildings, public facilities, or multi-family dwellings, duplex pump systems (two pumps installed in a single basin) are mandatory. The control panel automatically alternates pump duty and activates both pumps simultaneously if a high-water alarm is triggered.

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Backwater Valves (UPC Section 710.1)

A backwater valve is a mechanical check valve installed in a drainage branch to prevent public sewer surcharges from backing up into basement plumbing fixtures during torrential rainstorms or municipal main blockages.

              STREET LEVEL MANHOLE COVER ELEVATION
                 ═════════════════════════════
                               │
     FLOOD LEVEL RIM           │ PUBLIC SEWER SURCHARGE LEVEL
     OF BASEMENT SINK          │
     ─────────┬───────         ▼
              │           (Sewer Surcharge Hazard Zone!)
              ▼
  ┌──────────────────────┐
  │ Basement Fixture     │ ───► [ BACKWATER VALVE ] ───► Public Sewer Main
  │ (Protected Branch)   │      (Flap Closes on Backflow)
  └──────────────────────┘

The base rule and the Iowa amendment (UPC Section 710.1)

The model UPC rule is familiar: drainage piping serving fixtures whose flood level rims are below the elevation of the cover of the next upstream manhole of the public sewer must be protected from backflow.

[!IMPORTANT] Iowa rewrites the trigger and the hardware. Rule 481—425.4(5)"b" adds to Section 710.1: "The requirement for the installation of a backwater valve shall apply only when determined necessary by the Authority Having Jurisdiction based on local conditions. When a valve is required by the Authority Having Jurisdiction, it will be a manually operated gate valve or fullway ball valve. An automatic backwater valve may also be installed but is not required."

Two exam-relevant consequences follow. First, in Iowa the below-manhole geometry alone does not make a backwater valve mandatory — the AHJ's determination does. Second, the device the Iowa code names is a manual gate or fullway ball valve; the spring- or gravity-operated automatic flap valve is optional additional protection, not the required item. Answers written from the unamended UPC will read "an approved automatic backwater valve is required," and on an Iowa exam that is the wrong answer.

Construction and Operation

• Backwater valves feature a smooth internal body containing a weighted, floating, or spring-assisted mechanical flap (gate).
• Under normal forward flow, wastewater pushes the flap open.
• When the public sewer surcharges and wastewater flows backward, hydrostatic head forces the flap tightly against its seat, sealing off the building interior.

[!CAUTION] Upper-Floor Isolation Rule (UPC 710.1): Fixtures located above the upstream manhole cover MUST NOT drain through the backwater valve! They must bypass the backwater valve and connect directly to the building drain downstream. If upper-floor fixtures drain through a closed backwater valve during a storm, the upper-floor discharge will overflow out of the lowest basement fixtures, completely defeating the valve's purpose.