Inflow and infiltration

# Inflow and infiltration

> I&I is water your system never should have collected. Most of it comes in through cracks, bad joints, and manhole covers you can actually go find.

Why the exam tests this

Every wastewater collections exam asks you to tell infiltration from inflow, and to read a flow record well enough to say which one is driving a problem. You are expected to judge a system's I&I condition and pick a response, not just define terms.

The exam leans on vocabulary that sounds alike but means different things: infiltration, inflow, base sanitary flow, dry weather flow, wet weather flow, peaking factor. Miss which one a question is asking about and your math can be right while your answer is still wrong. You also need the screening logic utilities use before they spend money on a full study, the gpdpp test, and the two ways I&I gets called excessive.

What you need to know

A sanitary sewer carries three flow components: base sanitary flow (BSF), groundwater infiltration (GWI), and rainfall derived inflow and infiltration, usually just called inflow. Every system has some I&I. Small amounts are normal and expected.

Infiltration is water other than sanitary wastewater that enters through defective pipes, pipe joints, connections, or manholes. It does not include inflow. Infiltration tracks with groundwater, so it runs even in dry weather and shifts with the season rather than with a single storm.

Inflow is water other than sanitary wastewater that enters from roof leaders, cellar and foundation drains, yard and area drains, springs and swampy ground, manhole covers, cross connections between storm and sanitary sewers, and catch basins. It does not include infiltration.

Inflow itself splits into two kinds. Direct inflow comes from connections like catch basins and roof leaders that respond within minutes of a storm. Delayed inflow comes from sources like sump pumps and foundation drains, and it builds and fades slowly.

Rainfall-induced infiltration is the short-term jump in infiltration a storm causes. It cannot be told apart from delayed inflow on a flow record. You count it inside delayed inflow.

Reading a flow record takes a handful of terms. Dry Weather Flow (DWF) is sanitary flow plus infiltration, measured over 7 to 14 days with no rain. Average Dry Weather flow (ADW) is that same dry period's average, timed for seasonal high groundwater so it also catches peak infiltration.

Wet weather flow is the highest daily flow during and right after a storm. It includes sanitary flow, infiltration, and inflow together. Peaking factor is the ratio of peak hourly flow to average daily flow.

You can estimate base sanitary flow (BSF) two ways. With flow meter data, find your ADW flow and subtract groundwater infiltration: BSF equals ADW minus GWI. With billed water usage, a system on universal water metering can use winter billing, when outdoor use is low, as a stand-in for base sanitary flow.

GWI itself comes from averaging nighttime flow, midnight to 6 a.m., over several dry-weather days at high groundwater. Subtract any significant industrial or commercial nighttime users first. As a rule of thumb, that GWI rate tracks close to the maximum weekly infiltration, with maximum daily infiltration running higher and maximum monthly infiltration running lower.

Utilities screen for excessive I&I with a gallons-per-day-per-person (gpdpp) test: divide treatment plant flow by sewered population. State standards for what counts as excessive land between 100 and 150 gpdpp. The older EPA figure of 120 gpdpp dates to 1985, before low-flow toilets became standard.

The 1992 Energy Policy Act dropped new toilets to 1.6 gallons a flush. A reading near 120 gpdpp is a bigger flag in a newer service area than in an older one, because newer plumbing already uses less water per person.

I&I counts as excessive under either of two tests. It causes overflows or bypasses, which makes it excessive by definition. Or the cost to transport and treat it exceeds the cost to fix it, judged by a cost-effectiveness analysis.

Even when I&I passes both of those tests, if it creates a health or environmental risk, you still have to act.

Beyond the screening test, flow monitoring for I&I means placing meters to isolate specific sub-basins. You are expected to compute average annual I&I rate and annual I&I volume from a flow record, not just describe them. The full unit-handling routine for that math lives in the wastewater collection math module.

A system needing more than a screening answer builds an RDII model. That model separates groundwater infiltration, base wastewater flow, and rain-driven RDII, then predicts how the system responds to storms it never measured directly. Five estimation methods feed those models: a constant unit rate, a percentage of rainfall volume (the R-factor), multiple linear regression, a synthetic unit hydrograph, and hydrologic surface runoff modeling.

Regulators have found no single one of those five methods works for every system, so the exam expects you to know they exist and roughly how each one works, not to run one yourself.

Worked examples

A Class II system's meters run through a dry stretch in early April, seven days with no rain. Snowmelt keeps groundwater high that week, and the ADW flow for the period reads 285,000 gallons per day.

The average of the midnight-to-6-a.m. flows over that same week, with no significant industrial users on the line, comes out to 95,000 gallons per day; that is GWI. BSF is ADW minus GWI: 285,000 minus 95,000 equals 190,000 gallons per day. Check your work with the calculator at the practice tool.

A utility filing an SRF application runs the gpdpp screen. The plant logs an average annual flow of 612,000 gallons per day, and the sewered population is 5,100. Flow divided by population is about 120 gpdpp, right at the old EPA benchmark.

Most of that utility's service area was built after 1992, when new toilets dropped to 1.6 gallons a flush. A 120 gpdpp reading here is a stronger flag for infiltration than the same number would be in an older town, where higher-flow fixtures are still common.

A system's average annual flow rate is 0.62 MGD, and its base sanitary flow rate, worked the same way as the first example, is 0.41 MGD. Average annual I&I rate is average annual flow minus BSF: 0.62 minus 0.41 equals 0.21 MGD.

Annual I&I volume is that rate times 365 days: 0.21 million gallons a day times 365 is about 76.7 million gallons a year. That is water moving through the plant that never needed to be there. Run the same steps yourself at the practice tool.

Common traps

• Calling groundwater seeping through a cracked pipe "inflow." That is infiltration; inflow needs a defined entry point, like a roof leader or an open manhole cover.
• Forgetting that rainfall-induced infiltration gets counted inside delayed inflow on the exam's accounting, not as extra infiltration.
• Picking a dry weather window that is dry but not at high groundwater, which understates infiltration and throws off your BSF number.
• Skipping the industrial or commercial nighttime flow subtraction when estimating GWI, which inflates the infiltration reading.
• Treating 120 gpdpp as a fixed cutoff instead of a dated benchmark that needs adjusting for low-flow fixtures.
• Assuming "not cost-effective to fix" means "no action needed," when an overflow or a health risk still forces a response.
• Mixing up peaking factor, a ratio, with peak flow, a rate, when a question hands you one and asks for the other.

Practice

The BSF, gpdpp, and annual I&I rate formulas above get a second pass on the wwc-1 test, worked at /tools/practice. Solve each one by hand before you check the answer, and drill the flow-term vocabulary alongside the math.

When a question gives you a flow record instead of a single number, practice pulling ADW, GWI, and BSF out of it the way you would from real plant data. Retake the calculation questions until unit handling, MGD versus gpd, gpdpp versus a raw population count, stops tripping you up.

Quick reference

• Three flow components: base sanitary flow (BSF), groundwater infiltration (GWI), rainfall derived inflow and infiltration (inflow, RDII).
• Infiltration: groundwater through defective pipes, joints, or manholes. Does not include inflow. Runs even in dry weather.
• Inflow: stormwater through roof leaders, drains, manhole covers, cross connections, catch basins. Does not include infiltration.
• Direct inflow responds within minutes of a storm. Delayed inflow builds and fades gradually and includes rainfall-induced infiltration.
• Dry Weather Flow (DWF): sanitary flow plus infiltration, measured over 7 to 14 dry days.
• Peaking factor: peak hourly flow divided by average daily flow.
• BSF equals ADW flow minus GWI.
• GWI: average nighttime flow, midnight to 6 a.m., during dry weather at high groundwater, minus significant industrial or commercial flow.
• gpdpp screen: plant flow divided by sewered population; states set the excessive threshold between 100 and 150 gpdpp.
• I&I is excessive if it causes overflows or bypasses, or if it costs more to transport and treat than to fix.
• Average annual I&I rate equals average annual flow minus BSF. Annual I&I volume equals that rate times 365 days.
• Five RDII estimation methods: constant unit rate, percentage of rainfall volume (R-factor), multiple linear regression, synthetic unit hydrograph, hydrologic surface runoff modeling.