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Removing Interior Sources of Inflow and Infiltration

Removing Interior Sources of Inflow and Infiltration

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    Without an air gap, lines to underground sump pump discharges could freeze or get clogged. Photo: RJN Group Inc.

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    Typical configuration for a storm sump connected to the sanitary sewer with a diverter valve. Drawing: RJN Group Inc.

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    Dye test to confirm the location of a sump pump's discharge point (here, it's the larger white PVC with metallic hanging strap). To identify a footing tile, auger drill along the foundation while dye testing or inspect the service lateral from the cleanout or main sewer with a launcher camera. Both methods can produce a false positive, but they're much more reliable than smoke testing. Photo: RJN Group Inc.

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    High-density residential areas with poor drainage impact options for backyard sump discharges. Image: RJN Group Inc.

 
 

Though it seems otherwise, the total annual volume of rain most metropolitan areas receive from year to year hasn't changed much over the last century. What has changed, though, is the increased frequency of intense storm events. Where pipelines haven't been rehabilitated to keep up with growth and deterioration, any additional volume — like infiltration and/or inflow (I/I) from stormwater and ground-water leaking into pipes — increases the likelihood of sanitary sewer overflows and basement backups.

Typically, about half the rain from a storm percolates through the soil to recharge groundwater, another 40% flows into storm sewers and receiving streams, and 15% evaporates. The remaining 5% enters public sanitary sewer systems. Utilities that have focused on eliminating infiltration/inflow by rehabilitating aging pipelines have a final source to address: private property.

Exterior sources include driveway drains, window well, stairwell and patio drains, cleanouts and service laterals, and are typically identified through smoke testing. Interior sources include sump pumps and foundation drains. In some cases, particularly when a utility has significantly rehabilitated the sewer system, the latter account for roughly half of remaining infiltration/inflow.

Determining the type and location of interior sources usually requires visually inspecting the lower level of a home, where trained inspectors can encounter a variety of sump pump configurations that builders have deployed through the years. In Chicago, for example, inspectors can find up to six:

  • Direct connection storm sump pump. Most common. Footing tile drains into sump pit through one or more pipes; sump discharge connects directly to the overhead sanitary sewer. No laundry or washroom connections discharge into the sump.
  • Storm sump pump with diverter valve. Same as above except a valve sends discharge to the sanitary sewer or overland (typically to the rear or side yard) when manually turned.
  • Storm sump discharge to laundry tub. Discharge is routed to the tub, which drains to a below-grade service lateral by gravity.
  • Combination sump pump with laundry tubs. Footing tile and tub discharge to the sanitary sewer. Sometimes has a diverter valve that can be turned to direct discharge to the sewer when tubs are operating and turned again to return flows to the rear/side yard when the laundry is finished.
  • Combination sump pump with washroom/toilet connection. Footing tiles and washroom/shower facilities connect to the sanitary sewer. Common in basement remodelings when owners try to avoid installing an expensive sanitary ejector sump and pit.
  • Unsealed sanitary sump pump. No footing tile. Instead, groundwater from around the foundation drains through the pump pit's gravel bottom directly into the sanitary sewer.

The other culprit: foundation drains

To relieve high groundwater pressure, many homes built before 1970 have footing tiles that connect directly to the sanitary sewer service lateral. Sometimes these drains are constructed with pipes made of clay or other materials that are directly connected with fittings to the lateral. In other cases, the foundation is bedded with stone and groundwater flows through intentionally open joints on the lateral.

Either way, peak flows approximate those of a sump pump connected to the sanitary sewer. Although it represents an unusual condition, we once found a footing tile that was still discharging at 25 gallons per minute (gpm) a week after a storm.

The most reliable ways to verify these connections is to auger drill along the foundation while dye testing or inspect the service lateral from the cleanout or main sewer with a launcher camera. Both methods can produce a false positive, but they're much more reliable than smoke testing.


Impact on flow rates

The peak wet-weather flow rates associated with sump pump and foundation drain sources are a function of storm duration and intensity, antecedent moisture conditions, and site-specific conditions including roof area, groundwater levels, basement wall cracks and seepage, drainage pattern away from the home, and — with the easy availability of low-cost equipment — sump pump capacity and depth.

Sump pumps rated at 50 gpm at a total dynamic head of 12 feet cost less than $200. Unaware that their sump pump connection can contribute to public sewer surcharging and overflows, homeowners often replace pumps at a lower level in the sump pit, increasing the frequency and duration of sump operation.

In the Upper Midwest, a 30-minute one-year-recurrence-interval storm (about 1 inch of rain) easily generates a runoff rate equal to 30 gpm from a 1,500-square-foot roof. In areas with high antecedent moisture and ground-water levels, it's not unusual for pumps rated at 30 gpm to 50 gpm to run continuously for more than an hour after a one-year storm event. During a heavy event, an 8-inch-diameter sewer laid at a minimum slope will be fully surcharged by 10 sump pump or foundation drain connections.

Four disconnection options

1) Direct storm sump pumps. Least expensive and simplest. Reroute discharge to the rear or side yard instead of the front yard because water may freeze on sidewalks and streets in months where the pump can operate with ambient temperatures below freezing.

Where lot sizes are small and localized rear yard ponding is a problem, some communities have installed rear yard storm sewer inlets (sometimes with restictors) especially where multiple homes with disconnected sump pumps are clustered together. If rear or side yard topography allows for a sump discharge to be routed underground, an air gap should always be provided. Rain gardens can absorb some of the discharge resulting from the disconnection and minimize localized rear or side yard flooding.

Sometimes homeowners reconnect the sump to alleviate ponding. The common use of PVC plumbing has increased the likelihood of these reconnections. To keep this from happening, reinspect homes at least every few years.

Cost: $250 to $500.

2) Laundry tub combination pumps.

Retrofit kits that mount underneath the tub simplify disconnection. The new laundry tub sump is routed to the sanitary sewer and the existing sump is routed to the rear or side yard.

Cost: $500 to $1,000.

3) Washroom/toilet combination pump. Usually requires what the homeowner was trying to avoid: installing a sump pit and ejector pump for the washroom/toilet and rerouting the combination sump to the back or side yard.

Cost: $2,000 to $3,000.

4) Direct foundation drains. Most expensive and politically fraught. Requires excavation to disconnect the drain tile from the service lateral and installing a new sump pit and pump that discharges to the back or side yard.

Cost: $5,000 to $10,000.

These aren't the final solution

To relieve homeowners of chronic basement backups, some communities install overhead sewers and check valve units that replace existing outside cleanouts.

Both are effective ways to buy time to implement a systemwide infiltration/inflow reduction program. But in addition to the high cost, installing a large number of either within a small area can cause backups in adjacent homes that hadn't experienced them before.

Overhead sewer programs should require foundation drains or sump pumps to be disconnected from the sanitary sewer. After it's installed, homeowners need to be prepared to maintain a sump pump — including dealing with power outages.

Retrofit check valves can't be installed on a home with either a foundation drain or sump pump connection to the sanitary sewer because the basement will flood as soon as the outside check valve activates.

Funding and legal issues

Homeowners often balk at disconnection efforts, even though most communities have ordinances that prohibit discharging groundwater infiltration and stormwater inflow into the sewer.

Some communities categorize foundation drains as legal but nonconforming because of the cost and disruption involved in disconnecting them. Combination sumps and unsealed sanitary sumps are sometimes treated this way as well.

Enforcement mechanisms range from fines to shutting off service and issuing a search warrant to preventing a seller from closing on a home until a property transfer inspection for clearwater sources into the sewer is conducted.

The courts have upheld the use of public funds to cover some or all disconnection costs. Some utilities pick up the entire cost, others require homeowners to foot the bill. Some communities opt to share funding, allowing homeowners to pay their portion over time as a supplemental charge on the sewer service bill.