Inflow and infiltration studies and collection system design in Texas

What it covers

A collection system is the network of gravity sewers, manholes, lift stations and force mains that carries wastewater from buildings to the plant. Inflow and infiltration (I&I) is clear water that gets into it. Infiltration is groundwater that seeps in through cracked pipe, failed joints and leaking manholes. Inflow is storm water that pours in through manhole lids, broken cleanouts and roof or area drains tied to the sewer.

We do two kinds of work under this line. For an existing system, we measure the clear water, find where it gets in and plan the repairs. For a new or rehabilitated system, we design sewers that carry the peak flow and keep clear water out from the start.

Why I&I matters in Texas

TCEQ defines a sanitary sewer overflow as an unauthorized discharge of untreated or partially treated wastewater from a collection system, at a manhole, lift station or cleanout, before it reaches the plant. TCEQ names significant inflow and infiltration among the causes, along with poor operation and maintenance and too little capacity.

Clear water also counts against the plant. Under 30 TAC §305.126, a plant whose flow reaches 75% of its permitted flow for three consecutive months must start engineering and financial planning to expand. At 90% for three months, it needs TCEQ's authorization to build. Measured flow includes I&I, so a wet season can move a plant toward those triggers. A study shows how much flow repairs can realistically remove before you size an expansion.

TCEQ's Sanitary Sewer Overflow Initiative, a voluntary program started in 2004, lets eligible public systems address overflows under an SSO plan with annual progress reports. Systems already under formal enforcement for overflows cannot join, and the program does not prevent EPA enforcement.

When to call

• Your plant flow jumps after storms, or your lift stations run hard in wet weather.
• Manholes overflow or customers report backups when it rains.
• Your plant is near the 75% planning trigger and you need to know how much of the flow is clear water.
• You are buying an infill site in an older part of a Texas city. Wet weather flow, not a dry day snapshot, decides how much capacity your connection can count on.

What we do

1. Start with the records you have. Twelve months of daily plant influent flow, rainfall for the same period, a sewer map with sewershed boundaries, an overflow and backup log, and lift station flow or pump run times. Together they show whether you have an infiltration problem, an inflow problem or both.
2. Narrow it to a few sewersheds. We plan the flow metering: meters at the points where sewersheds discharge to a trunk, read through several storms in the wet season. Then we review the data and rank the sewersheds on normalized numbers, so the biggest basin does not top the list just for its size.
3. Find the sources. We plan the field tests for the sewersheds that rank worst: smoke and dye testing for inflow connections, CCTV of the pipe and manhole inspections for infiltration. Then we review the results, the CCTV footage and the inspection reports to place each defect.
4. Model the network. A model shows where pipes surcharge at peak wet weather flow, how much capacity the clear water takes and what removing it would free up. Under 30 TAC §217.53, data from the existing system can serve as the design basis.
5. Rank the repairs. Overflows and backups come first. Then each repair ranks by the clear water it removes for the money. The fixes are lining, manhole rehabilitation, pipe replacement where lining will not work, and disconnecting roof drains and other illegal connections. Most lining is trenchless, so streets stay open.
6. Design and submit. We prepare the lining plans, plans and specifications for the repairs, and cost estimates the owner can budget against. Under 30 TAC §217.6, each collection system project, rehabilitation included, goes to TCEQ with a summary transmittal letter sealed by the design engineer. Routine maintenance does not. A city that holds TCEQ approval authority reviews collection systems inside its jurisdiction instead.

Metering the same sewersheds after the work, through a comparable wet season, shows whether the repairs removed the flow they were meant to.

New sewers built tight

Chapter 217 sets the rules that keep new I&I out. Under §217.53, a gravity collection system must be designed for the peak flow plus infiltration and inflow, and the design must keep I&I to a minimum. Pipe joints need rubber gaskets, and building laterals need a manufactured fitting that keeps out infiltration and roots. A gravity sewer is at least 6 inches in diameter, with enough slope for 2 feet per second flowing full.

Under §217.57, new gravity pipe must pass a low pressure air test or an infiltration or exfiltration test. Exfiltration may not exceed 10 gallons per inch of diameter per mile of pipe per 24 hours, and pipe laid below groundwater gets an infiltration test instead. Under §217.58, every manhole must pass a leakage test. We put those tests in the specifications. See our water and sewer utility design and lift station design pages for the rest of the Texas design rules.

Work like this

• Milford, TX. A 15,000 gallon per day lift station for an RV park, sized for all three phases of the park, with TCEQ and City of Milford coordination and site construction services during the install.
• Lindsay, TX. The water line extension and the sewer system for a 48-lot subdivision, with the sewer designed to local codes.

We also do collection system and I&I work in Colorado. See our collection systems and inflow and infiltration pages.

What affects time and cost

The records you already keep set the starting point. A year of daily influent flow and rainfall can size the problem at a desk. Without them, metering comes first.

Metering has to catch real storms, so it usually runs months, not weeks, and it runs longer in a dry year. The number of sewersheds, the length of pipe to inspect and access to manholes set the field work. Budgets rarely cover every repair at once, so the ranking lets the owner fix the worst sewersheds first and spread the rest over later budgets.

TCEQ decides whether to review a collection system submittal, so we never promise an approval date. TCEQ staff outlined planned changes to Chapter 217 in October 2025, so we design to the rule in force when you build.