What Inflow and Infiltration Look Like From the Building Side

Why this matters

A sanitary system is sized for the water people use. Rain is not in that number. When stormwater finds a way into sanitary piping it arrives as a load nobody planned for, and it arrives at the same minute for every building on the street, which is why wet-weather backups cluster instead of scattering. Two different defects put it there, they get fixed by different people using different methods, and the one that causes the backup is not the one that moves the most water over a year. Separating them from the building side, before anyone digs, is the whole job of this card.

The two words, and the single gate between them

Inflow is stormwater with a direct, open path into sanitary piping: a roof leader tied into a sanitary line, an area drain or yard drain plumbed the wrong way, a foundation drain or sump discharge landing in the sanitary lateral, a cracked or missing cleanout cap sitting in a puddle, a manhole in a low spot. The path is open pipe or an open hole, and water goes in as fast as it lands.

Infiltration is groundwater entering through the pipe's own defects: open or displaced joints, circumferential cracks, root-opened seams, a failed lateral connection at the main. The path is soil, and water arrives at the rate the ground gives it up.

The gate that separates them is time, not place:

Does the flow rise inside the same hour as the rain and fall back within hours of it stopping, or does it rise over a day or more and stay up for days after the sky clears?

Fast up and fast down is inflow. Slow up, slow down, and a peak that can arrive after the storm has ended is infiltration. Most older buildings have some of both, so the useful question is which one dominates the number you care about.

What the building side can actually see

You are not going to walk the main. From inside the property line you have four honest sources of evidence, and every one of them is a time series rather than a snapshot:

• A flow record on the lateral, if the owner or the utility has metered it. This is the strongest evidence and the rest of this card assumes you can get at least a rough one.
• Pump run-hour or cycle-count logs on any sump, ejector or lift station tied to the system. A cycle counter is a flow meter with a coarse resolution, and it is often the only recorded data an older building has.
• The complaint log itself, timestamped against local rainfall records. Three backups in two years, all within four hours of a rainfall event over about an inch, is a finding.
• Standing water where it should not be: a wet cleanout box, a manhole with a debris ring above the pipe crown, a floor drain that weeps when it has not been used.

Read these before touching anything. A rodding or jetting run rearranges the debris ring, which is one of the few pieces of physical evidence that records how high the line surcharged.

Before anyone reads a structure

Everything below is read from data and from the surface. A manhole, a wet well and any large interceptor is a permit-required confined space, and the killer in them is the atmosphere rather than the water. Entry happens under a written permit program with continuous atmospheric monitoring, an attendant outside and non-entry retrieval rigged first: 29 CFR 1910 Part 1910.146 in general industry, 29 CFR 1926 Subpart AA on construction work. Name which Part your job falls under before the permit is written, because the documents are not identical.

Hydrogen sulfide deadens the sense of smell well below the concentrations that incapacitate, so a technician's own nose is the least reliable instrument on site and smelling less of it as you work is a warning rather than reassurance. If a lid has to come off to place a meter, that is a rigged lift with a proper hook and feet clear of the swing, and the crew that opens it works the permit; you read what their instrument recorded.

The correction that has to be applied before any excess is attributed

Raw peak flow is not excess flow. Subtract the dry-weather value for that clock hour on that day of the week, not the daily average. Sanitary flow has a strong diurnal shape, so a 2 pm peak compared against a 24-hour average charges the morning-and-evening pattern of ordinary occupancy to the rain. That single substitution is the most common way a wet-weather number gets inflated, and it always inflates.

Two more corrections, both of which run against you rather than for you:

• The dry-weather baseline itself may already contain infiltration. A lateral below the water table takes groundwater in every season. A baseline collected in a wet month therefore hides part of the defect inside the number you are subtracting, and your reported infiltration comes out low. Take the baseline in the driest stretch you can get and say in the report which month it came from.
• Meter error needs its basis and its character before it enters the arithmetic. A stated plus or minus 2 percent of reading shrinks as the reading shrinks, unlike a percent-of-full-scale figure that does not. If both numbers in a subtraction come from the same instrument, that error is a fixed systematic offset and it largely cancels, leaving about 2 percent of the difference. If they come from two different instruments the two errors are independent spreads and combine in quadrature, which is a bigger number than either one alone.

Worked example: one storm, two buildings, one gate

Both buildings sit on the same street and took the same 1.1 inches of rain over three hours. Both were metered on the lateral with the same instrument, specified at plus or minus 2 percent of reading, for fourteen days.

Building A, single-storey retail with a flat roof.

• Dry-weather average across ten dry days: 4.0 gpm.
• Dry-weather value at the 2 pm clock hour, weekday: 9.0 gpm.
• Peak during the storm: 26.0 gpm, reached about 40 minutes after the heaviest rainfall.
• Five hours after the rain stopped: 8.8 gpm, which is the ordinary value for that hour.

Correction applied: excess = 26.0 - 9.0 = 17.0 gpm. Using the 4.0 daily average instead would have given 22.0 gpm and charged the lunch rush to the storm.

Meter correction: both readings came from one instrument, so the offset cancels in the subtraction and what remains is 2 percent of the difference, 0.02 x 17.0 = 0.34 gpm. Had the peak and the baseline come from two different meters, the independent spreads would combine in quadrature: 0.02 x 26.0 = 0.52 and 0.02 x 9.0 = 0.18, giving the square root of (0.52 squared plus 0.18 squared), about 0.55 gpm. Same instruments, different answer, purely because of how the errors combine.

So Building A carries an excess of 17.0 gpm, about 0.3 either way, which is 1.9 times its normal flow for that hour arriving on top of it. Rise inside the hour, fall inside five. Inflow.

Building B, four-storey walk-up apartment, older clay lateral.

• Dry-weather average: 6.0 gpm. Dry-weather value at 2 pm: 11.0 gpm.
• Peak during the storm, same clock hour: 14.5 gpm, so excess = 14.5 - 11.0 = 3.5 gpm.
• Two days later, same clock hour, no rain since: 17.0 gpm, so excess = 17.0 - 11.0 = 6.0 gpm, larger than on the day it rained.
• Flow stays above 14 gpm for six days and decays gradually.

Rise over a day, peak after the storm, decay over most of a week. Infiltration.

Now the part that reverses the intuition. Building A's excess is the bigger number and it is the one that surcharges the street. Building B moves more water. Approximating the area under each curve as a triangle, which is what these two shapes roughly are:

• A: about 8.0 gpm average excess over roughly 4 hours = 8.0 x 240 = 1,920 gallons.
• B: about 3.0 gpm average excess over 6 days = 3.0 x 1,440 x 6 = 25,920 gallons.

That is about 13 times the volume from the defect with under half the peak (6.0 gpm against 17.0). Those two average-excess figures are trapezoid estimates of a curve, not measurements, and they are labelled that way here because someone will otherwise quote them as data.

The two numbers answer two different questions. If the complaint is a basement backing up during a thunderstorm, A's 17.0 gpm peak is the finding. If the complaint is a treated-volume charge, a lift station burning run hours, or a septic field that never dries out, B's 25,920 gallons is the finding, and chasing A's roof leader will not move it.

What flips the reading

• A sump pump discharging to the sanitary line produces a stepped trace, square-edged on and off cycles rather than a smooth curve. In daily totals it looks like infiltration because it runs for days. In mechanism it is inflow: one discrete connection, fixable in hours. Read the shape of the trace, not the duration.
• A flat trace with a wet-weather backup anyway means you are seeing the street, not the building. The lateral is fine and the main is surcharging into it, which is a different card and a different fix.
• Both signatures on one meter is the normal case in a building over about fifty years old. Rank by which number the customer is actually paying for before recommending anything.

Who owns which fix

This is the reason the separation is worth the trouble.

Inflow work is the better first move on almost every building, not because it is the larger defect but because the certainty is high and each fix is provable. Infiltration work is a program, not a job.

How to verify you separated them correctly

1. Re-read your peak against the right baseline. Take the excess figure in your report and confirm the number you subtracted is the same clock hour and same day type. If it is a daily average, the excess is overstated and you do not yet know by how much.
2. Name the season your baseline came from, in the report. If it is a wet month, state in the same sentence that the infiltration figure is a lower bound and write it with one inequality sign rather than a plus-or-minus interval.
3. Check the decay tail against the rainfall record, not memory. The single most persuasive line in an I and I report is a flow that peaked two days after the rain stopped, and it is also the easiest to get backwards.
4. Confirm the direction words. If you wrote "rose and held", find the two readings that show a hold. Verify it against the printed series, including whichever reading spoiled the pattern, and report that one too.

References

• 29 CFR 1910 Part 1910.146, permit-required confined spaces, general industry; 29 CFR 1926 Subpart AA, the construction counterpart
• Trade-standard practice for wet-weather flow monitoring and dry-weather baseline development
• The model plumbing and drainage codes as adopted and amended by your local jurisdiction, which is what governs whether a roof leader, foundation drain or sump discharge may lawfully connect to a sanitary lateral
• See related: Why a Pressure Test and a Smoke Test Find Different Faults; How to Run a Smoke Test Without Creating a Complaint; Why a Sewer Is a Confined Space and What That Changes