7.3 Combined & Separated Sewer Connections

Combined & Separated Sewer Connections

1. Separated vs. Combined Drainage Infrastructure Principles

In municipal civil engineering and building plumbing design, wastewater and stormwater collection networks fall into two structural classifications:

1. Separated Sewer Systems: Municipalities operate two distinct underground utility main networks: a Sanitary Sewer Main (conveying domestic sewage and industrial liquid waste to a wastewater treatment plant) and a Storm Sewer Main (conveying surface rainwater runoff directly to rivers, lakes, or retention basins without chemical treatment).
2. Combined Sewer Systems: Older urban infrastructures utilize a single underground main pipe—a Combined Public Sewer—that collects both domestic sanitary sewage and surface stormwater runoff in a shared municipal conduit.

Mandatory Interior Building Separation Rule

Regardless of whether the public street utility sewer is separated or combined, both the International Plumbing Code (IPC 1101.2) and Uniform Plumbing Code (UPC 1101.3) enforce a strict, mandatory rule:

Sanitary drainage piping and storm drainage piping inside a building MUST remain completely separate.

Connecting storm vertical leaders, roof drains, or area drains into interior sanitary stacks or branches is strictly prohibited. Combining flows inside the building structure creates severe public health hazards: during torrential storms, heavy roof runoff fills sanitary pipes, blowing out fixture trap seals and backing up raw human sewage into lower-floor sinks, showers, and floor drains.

 INTERIOR BUILDING PIPING (MUST REMAIN SEPARATE)

  SANITARY WASTE STACK                 STORM LEADER STACK
   (Soil / Waste Fixtures)              (Roof Drains / Scuppers)
          │                                    │
          ▼                                    ▼
 ┌─────────────────┐                  ┌─────────────────┐
 │ Building Drain  │                  │ Building Storm  │
 │ (Sanitary)      │                  │ Drain           │
 └────────┬────────┘                  └────────┬────────┘
          │                                    │
 ═════════╪════════════════════════════════════╪════════════ Foundation Wall Boundary
          │                                    │
          │    EXTERIOR POINT OF COMBINED      │
          │    CONNECTION (OUTSIDE WALL)       │
          │    ┌───────────────────────────┐   │
          │    │ Building Storm Drain Trap │◄──┤ (Trap Sized for Storm GPM)
          │    └─────────────┬─────────────┘   │
          ▼                  ▼                 │
     ┌─────────────────────────────────────────┴─┐
     │ COMBINED BUILDING SEWER                   │
     └───────────────────┬───────────────────────┘
                         ▼ To Municipal Combined Main

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2. Building Storm Drain Trapping Requirements on Combined Sewer Connections

When a building storm drain discharges into a public combined sewer or a combined building sewer outside the foundation wall, a unique atmospheric hazard arises. Public combined sewers carry foul sewer gas, volatile organic compounds, and hydrogen sulfide. Without a barrier, sewer gases would travel up exterior storm leaders and discharge through roof drains, re-entering building air intakes or contaminating roof areas.

Building Storm Drain Trap Specifications

Under IPC 1111 and UPC 1101.12, when a storm drain connects to a combined sewer, an approved Building Storm Drain Trap must be installed on the horizontal storm drain line prior to its connection to the combined sewer:

• Trap Location: The trap must be located outside the building wall or inside the building near the point of exit, positioned in an accessible location.
• Trap Seal Depth: The storm trap must maintain a liquid seal depth of not less than 2 inches (51 mm) and not more than 4 inches (102 mm).
• Cleanouts: The trap must be equipped with accessible cleanouts on both the upstream (building) and downstream (sewer) sides of the trap seal to permit mechanical snaking.
• Air Relief / Venting: Traps installed on combined storm lines must be vented on the building side or connected to leaders that provide adequate relief to prevent trap seal siphonage during surge flows.

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3. Backflow Prevention & Backwater Valve Mandates for Storm Outlets

During extreme weather events, municipal combined sewers frequently become surcharged (filled above crown elevation under hydrostatic pressure). When a combined street sewer surcharges, liquid sewage backs up through lateral connections toward connected buildings.

Sub-Grade Storm Inlets & Backwater Valves

Any storm drainage inlet, area drain, driveway drain, or window well drain connected to a combined sewer system whose rim elevation is located below the elevation of the nearest upstream public sewer manhole cover must be protected against sewer surcharge.

                      UPSTREAM STREET MANHOLE
                      ══════════════════════ Rim / Cover Elevation
                               │
  DRIVEWAY AREA DRAIN          │ Surcharged Hydraulic Grade Line
  (Low-Elevation Rim)          │ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑
        │                      │
 ┌──────┴──────┐               │
 │ Drain Grate │               │
 └──────┬──────┘               │
        │                      │
 ┌──────┴──────────────┐       │
 │ BACKWATER VALVE     │       │
 │ (Normally Open Flap)│       │
 └──────┬──────────────┘       │
        ▼                      ▼
 ┌──────────────────────────────────────────────┐
 │ COMBINED SEWER LATERAL                       │
 └──────────────────────────────────────────────┘

• Backwater Valve Mandate: An approved backwater valve (complying with ASME A112.14.1) must be installed on the horizontal branch line serving low-level storm drains.
• Operational Mechanics: The backwater valve contains a balanced, swing-check flap mechanism. Under normal gravity flow, the flap hangs open to permit drainage. When main sewer surcharge forces water backward into the lateral, the reversed flow lifts the flap tightly against its seat, preventing sewage from flooding the driveway or lower floor drains.
• Accessibility: Backwater valves must be housed in an accessible pit or provided with an access cover for routine inspection and removal of debris.

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4. Converting Storm Flow (GPM) and Sanitary Fixture Units (DFU) for Pipe Sizing

When sizing a combined building sewer outside a building, plumbers must combine the sanitary hydraulic load (measured in DFUs) and the storm hydraulic load (measured in GPM) into a single equivalent hydraulic load.

Code Conversion Equivalence

Model plumbing codes establish a standard equivalence factor between Drainage Fixture Units and Stormwater GPM:

• 1 GPM of Stormwater Flow = 2 Drainage Fixture Units (DFUs) (for sizing combined horizontal building sewers).

Sizing Procedure for Combined Building Sewers

1. Calculate the total Sanitary DFU load of all fixtures in the building.
2. Calculate the peak Stormwater GPM load from projected roof and paved areas ($Q = A \times I \times 0.0104$).
3. Convert Stormwater GPM to equivalent DFUs ($GPM \times 2 = \text{Storm DFUs}$).
4. Add Sanitary DFUs + Storm DFUs = Total Combined DFUs.
5. Size the horizontal combined building sewer using standard DFU sizing tables (IPC Table 710.1(1) / UPC Table 717.1) at the designated slope (1/8 or 1/4 in/ft).

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5. Environmental Discharge Standards: Clear Water vs. Contaminated Runoff

Modern environmental standards enforced by the EPA National Pollutant Discharge Elimination System (NPDES) and local plumbing codes differentiate between clear-water waste and contaminated runoff:

1. Unpolluted Clear-Water Waste: Water from air conditioning condensate drains, refrigeration coils, ice machines, and commercial drinking fountains contains no organic pathogens. Model codes require clear-water waste to discharge into storm sewers, drywells, or non-potable recycling systems, forbidding discharge into sanitary sewers where municipal capacity is limited.
2. Industrial & Oil-Contaminated Stormwater: Runoff from commercial parking garages, auto repair bays, and gas station forecourts carries petroleum hydrocarbons and heavy metals. This runoff must pass through an approved Oil-Water Separator / Interceptor before discharging to a combined sewer or storm main.

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6. Worked Example: Combined Sewer Connection Sizing & Trapping Layout

Scenario

A commercial building has a sanitary load of 120 DFUs. The building roof measures 8,000 square feet in a location with a 100-year rainfall intensity of 2.5 inches per hour. The storm drain and sanitary drain run separately inside the building and exit through the foundation wall to connect into a single combined building sewer pitched at 1/4 in/ft (2.08%) slope.

• Step 1: Calculate Stormwater Flow Rate (GPM): $Q = A \times I \times 0.0104$ $Q = 8,000 \text{ sq ft} \times 2.5 \text{ in/hr} \times 0.0104 = 208 \text{ GPM}$

• Step 2: Convert Storm Flow GPM to Equivalent DFUs: $\text{Storm DFUs} = 208 \text{ GPM} \times 2 = 416 \text{ DFUs}$

• Step 3: Calculate Total Combined DFU Load: $\text{Total Combined Load} = 120 \text{ Sanitary DFUs} + 416 \text{ Storm DFUs} = 536 \text{ DFUs}$

• Step 4: Size Horizontal Combined Sewer at 1/4 in/ft Slope:

  • Consult Horizontal Building Sewer Sizing Table at 1/4 in/ft slope:
    • 4-inch sewer capacity = 216 DFUs (Insufficient: $536 > 216$).
    • 5-inch sewer capacity = 480 DFUs (Insufficient: $536 > 480$).
    • 6-inch sewer capacity = 840 DFUs (Sufficient: $536 \le 840$).
  • Selection: 6-inch combined horizontal building sewer.

• Step 5: Trapping Mandate Check:

  • Install a 6-inch building storm drain trap equipped with dual cleanouts on the exterior storm line immediately before it joins the 6-inch combined sewer pipe.