I&I Tracking Dashboard Reference Guide

Overview

The I&I Tracking Dashboard provides facility-level estimates of Inflow and Infiltration (I&I) for wastewater treatment plants across the United States. I&I refers to water that enters the sewer system from sources other than legitimate wastewater — primarily stormwater running into the system through cracks or illegal connections (inflow), and groundwater seeping in through deteriorating pipes (infiltration).

By comparing a facility's flow during dry weather against its overall average flow, the dashboard estimates what share of total throughput is driven by I&I rather than actual wastewater. Facilities with high I&I are often candidates for infrastructure rehabilitation, as excess flow increases treatment costs, can trigger permit violations, and reduces available capacity.

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Filters

Facilities 

Monitoring

Distance From Facility

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Table Columns

NPDES ID

The unique permit identifier assigned by the EPA under the National Pollutant Discharge Elimination System. Each facility has one NPDES ID that can be used to look up additional information in the EPA's ECHO database.

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Facility Name

The name of the wastewater treatment facility as registered with the EPA.

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Design Flow

What it is: The rated treatment capacity of the facility in millions of gallons per day (MGD), as reported to the EPA. This is the maximum daily volume the facility was designed to handle under normal operating conditions.

Note: Not all facilities report a design flow value. Where this column is blank, the value was not available in the EPA's facility registry.

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Capacity

What it is: The percentage of design flow currently being used, calculated as:

Capacity % = Average Daily Flow ÷ Design Flow × 100

How to read it: A facility at 52.9% capacity is using just over half of its rated treatment volume. Facilities consistently operating above 80–90% capacity may be at elevated risk during high I&I events, as wet-weather inflow can push total flow toward or beyond design limits. Facilities with no recorded Design Flow value will show a blank Capacity.

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Average Daily Flow (ADF)

What it is: The average of all monthly flow readings submitted by the facility to the EPA for the selected year, measured in MGD. This represents the facility's total average daily throughput, including both legitimate wastewater and any I&I contribution.

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Dry Weather Flow (DWF)

What it is: An estimate of the facility's baseline flow during periods of low precipitation. The DWF is the flow expected in the absence of significant I&I influence, measured in MGD.

How it is calculated: The dashboard identifies dry months at the nearest qualifying NOAA weather station (months with ≤ 1.50 inches total rainfall and ≤ 5 rainy days). It then takes a weighted average of the facility’s dry for that year, anchoring the estimate toward true dry-weather conditions. If a facility does not have any dry months reported, the value will be the lowest MGD value recorded in a single month.

How to read it: A DWF close to ADF means the facility's flow is relatively stable regardless of weather. A DWF significantly lower than ADF means the facility experiences substantial wet-weather-driven flow increases.

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I&I %

What it is: The proportion of a facility's average annual flow estimated to come from inflow and infiltration, expressed as a percentage.

Formula: I&I % = 1 − (DWF ÷ ADF)

How to read it:

These ranges are indicative. Facility size, age, location, and infrastructure type should always be considered alongside the percentage.

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I&I Diff

What it is: The absolute volume of flow attributable to I&I, in MGD.

Formula: I&I Diff = ADF − DWF

How to read it: I&I Percentage tells you the relative impact; I&I Diff tells you the actual volume.

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I&I Cost

What it is: The estimated annual cost of treating the excess I&I flow, expressed in US dollars.

Formula: I&I Cost = I&I Diff (MGD) × $2,583 per million gallons × 365 days

This multiplies the daily excess I&I volume by a treatment cost rate of $2,583 per million gallons, then by 365 days to produce an annualised figure. The cost rate represents the assumed average cost to treat one million gallons of wastewater through the facility.

Example: A facility with an I&I Diff of 0.30 MGD would carry an estimated annual I&I cost of:
0.30 × $2,583 × 365 = $282,898.50

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Dry Month Count

What it is: The number of months in the selected year classified as "dry" at the matched weather station. A month is determined to be a dry month when the following are true:

• ≤ 1.50 inches total rainfall

• ≤ 5 rainy days

  • A rainy day has > 0.1 inches of precipitation

• Precipitation recorded for >80% of days in the month

How to read it: More dry months provides a more robust DWF estimate. If a year had very few dry months (e.g. an unusually wet year), the DWF may be based on a smaller sample and should be interpreted with greater caution. A dry month count of 0 means no dry months were identified at the matched station for that year, in these cases a minimum of 1 month is used as a floor for the Dry Weather Flow value.

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Trend Chart

The chart in the top right of the dashboard shows two trend lines across available years for all facilities matching the current filters:

• Average I&I Percentage (blue): the mean I&I percentage across all filtered facilities for each year

• Average MGD Usage % (yellow): the mean capacity utilization (ADF as a share of Design Flow) across filtered facilities for each year

How to read it: This chart provides a macro view of system-wide I&I trends. Rising I&I percentage over time may indicate deteriorating infrastructure across the filtered set. The MGD Usage % line contextualizes whether facilities are approaching capacity constraints.

Tip: The chart is most informative when filters are applied to the dashboard. For example, looking at single facility will visually display the trends for that facility going back several year.

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Facility Map

The facility map plots all filtered facilities geographically, with each bubble representing one facility.

• Bubble size: determined by I&I Difference (MGD). Larger bubbles indicate a greater volume of excess flow attributable to I&I.

• Bubble colour: determined by I&I Percentage. Darker blue indicates a higher proportion of total flow driven by I&I.

Hovering over a bubble displays the facility's NPDES ID, estimated I&I Cost, I&I Difference, and I&I Percentage.

The most concerning facilities are those that are both large and dark, high I&I volume and a high proportion of weather-driven flow.

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Facility Weather Data Aggregation

I&I metrics are calculated using precipitation data from the nearest qualifying NOAA weather station within 20 miles of each facility. Priority is given to stations that reported data for all 12 months of the year with at least 80% daily coverage. If no such station exists, the threshold drops to 70% coverage. Where neither pass finds a qualifying station, a fallback method is applied where the DWF calculation defaults to the two lowest DWF values.

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Data Sources 

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Frequently Asked Questions

Why does a facility have a blank Design Flow or Capacity?
Not all facilities report a design flow value to the EPA. Where this field is blank, no value was available in the EPA's facility registry for that permit.

Can I compare the same facility across multiple years?
Yes, use the I&I Yearly Breakdown table to see year-over-year trends for individual facilities. Consistent increases in I&I percentage may indicate ongoing infrastructure deterioration. Select a facility row in the Facility Summary table to see the values for the facility highlighted in the dashboard tables and charts.

Why might two nearby facilities show very different I&I percentages?
I&I is highly dependent on the specific condition of each facility's collection system. Age, material, maintenance history, and local geology all affect how much stormwater and groundwater enters the pipes serving a given facility, even within the same city.

What does a very low Dry Month Count mean for reliability?
In unusually wet years, fewer months may qualify as dry, which means the DWF estimate is based on a smaller sample. Treat these values with greater caution and consider cross-referencing with adjacent years.

Are there any Outlier Exclusions?

Two filters are applied before a facility appears in the dashboard:

• Any individual monthly flow reading that exceeds 9.9× the facility's calculated average flow is excluded as an anomalous spike.

• Any facility-year where ADF is 4× or more than DWF is removed entirely, as this typically indicates a data quality issue rather than a genuine I&I signal.