Inflow & Infiltration Analysis: An Important Component of Sewer Flow Monitoring Programs
Inflow and infiltration (I&I) consist of stormwater and groundwater that enter the sanitary sewer system through pipe defects and improper storm drainage connections. While almost every sanitary sewer system will have inflow and/or infiltration, small amounts have historically been expected and even tolerated. When I&I causes sewer overflows or bypasses, or the cost to transport and treat exceeds the cost to locate and mitigate, I&I sources must be identified and resolved.
What is Inflow & Infiltration?
Inflow Definition:
Storm water inflow is defined as water discharged into the sewer system, including private sewer laterals, from direct connections such as downspouts, yard and area drains, holes in manhole covers, cross-connections from storm drains, or catch basins.
Inflow impacts the sewer system by creating a peak flow problem that often dictates the required capacity of downstream pipes and pumping stations. Because the response and magnitude of inflow are tied closely to the intensity of the storm event, the short-term peak instantaneous flows may result in surcharging and/or overflows within the sewer collection system. Severe inflow may result in sewage dilution which results in the upsetting of the biological treatment (secondary treatment) at the downstream treatment facility.
Inflow locations are typically easier to locate and less expensive to correct than infiltration sources. Generally, the costs to identify and remove sources of inflow are low compared to the benefits of public health and safety or the costs of building new infrastructure and/or facilities to convey and treat the instantaneous peak flows.
Infiltration Definition:
Infiltration is defined as water entering the sanitary sewer system through defects in pipes, pipe joints, and manhole walls, which may include cracks, offset joints, root intrusion points, and broken pipes.
Infiltration is further subdivided into three components as follows:
1. Groundwater Infiltration: Groundwater infiltration depends on the depth of the groundwater table above the pipelines as well as the percentage of the system submerged. The variation of groundwater levels and subsequent groundwater infiltration rates are seasonal by nature. On a day-to-day basis, groundwater infiltration rates are relatively steady and will not fluctuate greatly.
2. Rainfall-Dependent Infiltration: This component occurs as a result of stormwater and enters the sewer system through pipe defects, as with groundwater infiltration. The stormwater first percolates directly into the soil, then migrates to an infiltration point. Typically, the time of concentration for rainfall-dependent infiltration may be 24 hours or longer, but this depends on the soil permeability and saturation levels.
3. Rainfall-Responsive Infiltration is stormwater that enters the collection system indirectly through pipe defects, but normally in sewers constructed close to the ground surface such as private laterals. Rainfall-responsive infiltration is independent of the groundwater table and reaches defective sewers via the pipe trench in which the sewer is constructed, particularly if the pipe is placed in impermeable soils and bedded and backfilled with a granular material. In this case, the pipe trench serves as a conduit similar to a French drain, conveying storm drainage to defective joints and other openings in the system. This type of infiltration can have a quick response and graphically looks very similar to inflow.
Infiltration typically creates a long-term annual volumetric issue that impacts the cost of pumping and treating the additional volume of water.
Sources of infiltration are usually harder to locate and more expensive to correct than inflow sources. Infiltration sources include defects in deteriorated sewer pipes or manholes that may be a widespread issue throughout a sanitary sewer system; therefore, corrective action may require significant infrastructure improvement projects.
Possible sources for Inflow & Infiltration
Graphical Identification of I&I during Flow Monitoring Study
During the data analysis phase of a comprehensive flow monitoring study, inflow is usually recognized graphically by large-magnitude, short-duration spikes immediately following a rain event. Infiltration is often recognized graphically by a gradual increase in flow after a wet-weather event. The increased flow typically sustains for a period after rainfall has stopped and then gradually drops off as soils become less saturated and as groundwater levels recede to normal levels. During a flow monitoring study where rainfall data is collected, the real-time flows are plotted against the Average Dry Weather Flow (ADWF) to analyze the I&I response to rainfall events. The graphic below shows a sample of how this analysis is conducted and some of the measurements used to distinguish infiltration and inflow.
Sample Infiltration and Inflow Isolation Graph
I&I Analysis Metrics
After differentiating I&I flows from ADWF flows, various calculations can be made to determine which I&I component (inflow or infiltration) is more prevalent at a particular site and to compare the relative magnitudes of the I&I components between the drainage basins and between storm events:
Inflow – Peak I&I Flow Rate: Inflow is characterized by sharp, direct spikes occurring during a rainfall event. Peak I&I rates are used for inflow analysis.
Groundwater Infiltration (GWI): GWI analysis is conducted by looking at minimum dry weather flow to average dry weather flow ratios and comparing them to established standards to quantify the rate of excess groundwater infiltration.
Rainfall-Dependent Infiltration (RDI): RDI analysis is conducted by looking at the infiltration rates at set periods after the conclusion of a storm event. Depending on the particular collection system and the time required for flows to return to ADWF levels, different periods may be examined to determine the basins with the greatest or most sustained rainfall-dependent infiltration rates.
Total I&I: The combined inflow and infiltration is measured in gallons per site and per storm event. Because it is based on total I&I volume, it is used to identify the overall volumetric influence of I&I within the monitoring basin.
Normalization Methods
There are three ways to normalize the I/I analysis metrics for an “apples-to-apples” comparison amongst the different drainage basins:
per-ADWF: The metric is divided by the established average dry weather flow rate and is typically expressed as a ratio. Peaking Factors are examples of using ADWF to normalize data from different sites.
per-IDM: The metric is divided by the length of pipe (IDM [inch-diameter mile]) contained within the upstream basin. Final units typically are gallons per day (gpd) per IDM. Municipalities often have a groundwater infiltration allowance for new construction expressed in terms of IDM (for example, 250 gallons per day per IDM allowable groundwater infiltration).
per-ACRE: The metric is divided by the acreage of the upstream basin. Final units typically are gallons per day (gpd) per ACRE. R-Value is a commonly used metric for I&I studies and collection system modeling efforts that are based on the per-ACRE method.
In comprehensive studies, the infiltration and inflow indicators can be normalized by one or all methods described above. Weighting criteria can be applied to each metric to prioritize I&I reduction and mitigation by basin. For example, since construction repairs for I&I reduction are typically priced on a pipe length basis, the per-IDM metric could be given a higher weighting if future construction will be required.
A comprehensive flow monitoring program includes rainfall monitoring with inflow and infiltration analysis. If study results indicate that inflow and/or infiltration is an issue either system-wide or for specific basins, then smoke testing can be performed to quickly detect sources of I&I. You can read more about sewer system defect detection through smoke testing in this blog article.