Combined Sewer vs. Separate Sanitary Systems in Older Texas Cities: Why Legacy Infrastructure Changes a Discharge Permit’s Monitoring Burden

What is the difference between a combined and a separate sewer?

Design intent. EPA's definitions at 40 CFR 35.2005 describe a combined sewer as one designed as both a sanitary sewer and a storm sewer. A sanitary sewer carries wastes from homes, businesses, industry and institutions, with only minor quantities of ground, storm and surface water that are not admitted on purpose.

True combined systems are mostly a Northeast and Great Lakes issue. EPA's map of combined sewer overflows counts about 700 US communities with combined sewers. Their permits must conform to EPA's 1994 CSO Control Policy under Clean Water Act §402(q), which requires nine minimum controls and a long-term control plan.

In an older Texas city, the common problem is a separate system that behaves like a combined one. Confirm which you have from the utility's records and its permit before choosing a fix. Our overview of types of sewer systems covers the basics.

Why does an older separate system act combined in wet weather?

Because decades of deterioration open paths for water that is not sewage. A well-maintained separate system keeps fairly steady flow regardless of rainfall. Older pipe goes through ground movement, settlement and root intrusion that degrade joints and structure.

After enough of that, a system built and permitted as separate can carry storm-driven flow in a big rain, even though its design and classification never changed.

How do inflow and infiltration get in?

Inflow comes in directly; infiltration seeps in from the ground. Under the federal definitions, inflow is water from sources such as roof leaders, yard and area drains, manhole covers and cross connections with storm sewers. Infiltration is groundwater entering through defective pipes, joints, connections or manholes.

Both add flow unrelated to wastewater generation. Both tend to be worst in the oldest parts of a system, and infiltration often rises with the water table after heavy rain.

What happens when wet weather flow exceeds capacity?

Overflows in the collection system or bypasses at the plant. TCEQ's SSO Initiative page defines a sanitary sewer overflow (SSO) as an unauthorized discharge of untreated or partially treated wastewater from a collection system or its components, such as a manhole, lift station or cleanout, before it reaches the plant. A bypass diverts part of the flow around treatment at the plant itself.

Both carry reporting duties. Under 30 TAC §305.125(9), noncompliance that may endanger human health, safety or the environment must be reported orally within 24 hours and in writing within five days.

The reporting format is changing. TCEQ's NPDES electronic reporting page lists sewer overflow and bypass event reports as electronic under federal Phase 2, effective December 21, 2025. It also says the federal tool was not ready for Texas facilities as of December 19, 2025, so keep using the Noncompliance Notification form (TCEQ-00501) until TCEQ says otherwise.

How does I&I show up in the permit's numbers?

As wet weather flow spikes in the monitoring data, and as capacity pressure. Flow reported on the plant's discharge monitoring reports (DMRs) jumps during and after rain in a way a tight system would not show. The plant has to treat that water too, which strains performance when flow is highest and margin is thinnest.

It can also pull expansion planning forward. Under 30 TAC §305.126(a), a domestic plant whose flow reaches 75% of permitted average daily flow for three consecutive months must start engineering and financial planning for expansion. At 90% for three months, it must get authorization to begin construction. Measured flow includes I&I, so a wet season can move a plant toward those triggers.

A plant sized on dry weather assumptions may look adequate on average daily flow and still be undersized for the conditions it periodically faces.

Where should flow metering go?

At points through the collection system, not just at the plant. Plant influent totals confirm a problem exists. Meters placed across the system and read through a range of rain events show how much extra flow enters and where.

That location data is what makes the next step, an I&I study, efficient.

What does an I&I study involve?

Metering plus field tests that find the specific sources. A formal study combines flow metering with smoke testing, dye testing and CCTV inspection of pipes and manholes. The result is a ranked list of the pipe segments, manholes and connections contributing the most extraneous flow.

That ranking lets a utility spend rehabilitation money where it removes the most wet weather flow, instead of spreading it evenly across the system.

Should you rehabilitate or separate?

Rehabilitate a separate system; separation is for a true combined one. For a separate system with I&I, the fixes are targeted:

• pipe lining or replacement for infiltration through deteriorated pipe
• manhole rehabilitation for damaged covers, frames and structures
• disconnecting and rerouting illegal or undocumented cross connections

Sewer separation means building a new storm drain system and taking stormwater out of the sanitary system entirely. It is a larger, more capital intensive job with a longer planning timeline. Build either into routine capital improvement planning rather than waiting for an overflow to force it.

What is TCEQ's SSO Initiative?

A voluntary program for systems that want to fix chronic overflows on a plan. TCEQ's SSO Initiative, started in 2004, lets eligible publicly owned systems commit to an SSO plan with annual progress reports. In return, they generally avoid formal TCEQ enforcement for overflows the plan addresses. Systems already under formal enforcement for SSOs are not eligible, and the program does not prevent EPA enforcement.

What should an infill developer check?

Real wet weather capacity, not nominal design capacity. A system with heavy I&I may have less room for new connections than its design numbers suggest. Ask the utility early about known constraints in the service area, as part of evaluating wastewater options before you close.

A quick readiness check for a legacy system or an infill site:

1. Do you know whether the system is combined or separate by design?
2. Do you have wet weather flow data, not just dry weather averages?
3. Have recent overflows or bypasses been identified and reported?
4. Is the plant's three-month flow trend checked against the 75% and 90% triggers?
5. For a new connection, has the utility confirmed capacity in wet weather?

If any answer is no, get the data before committing capital or a site.

Limits of this article. This is general education on Texas rules and federal definitions. Your permit's own conditions and TCEQ's current reporting instructions govern your system.

Frequently asked questions

How do we know if our system has a significant I&I problem?

Compare dry weather flow with flow during and after a big rain. A large jump against a stable dry weather baseline points to I&I worth a detailed study.

Is sewer separation always the right fix for wet weather problems?

No. Separation applies to a true combined system. A separate system with I&I usually needs targeted rehabilitation of the defects a study identifies.

Can I&I cause compliance problems even if the plant meets its permitted capacity on paper?

Yes. Permitted capacity reflects average or design flow, and a wet weather spike can push actual flow well past it, straining treatment and raising overflow and bypass risk.

I'm the public works director for a small Texas city, and our plant's flow hit 78% of permitted capacity during a wet spring. Does that start the 75% planning requirement?

It can. The trigger is flow at 75% of permitted average daily flow for three consecutive months, and measured flow includes I&I. Check your three-month record with your engineer, and look at an I&I study alongside expansion planning.

I'm developing an infill project in an older part of a Texas city. Should I worry about the city's I&I if fixing it isn't my responsibility?

Yes. Wet weather behavior decides whether there is real capacity for your connection, whoever owns the existing deficiencies. Ask the utility for capacity in wet conditions before you commit.

Related resources

• I&I Analysis: Finding and Fixing Your Worst Sections
• Sanitary Sewer Overflow Risk in Dallas's Older Watersheds
• How Inflow and Infiltration Impacts Your Collection System and Wastewater Treatment Plant
• Self Reporting Violations and TCEQ Enforcement Discretion

Sorting out wet weather flow in a legacy collection system?

We help cities and developers measure what an older system really carries in the rain, find where the water gets in, and tie the fixes to the permit.

Talk to an engineer about your system

We specialize in:

• Flow metering and wet weather monitoring programs
• Inflow and infiltration studies
• Collection system rehabilitation design
• Treatment plant capacity and loading analysis
• Capital improvement planning
• Infill capacity evaluations

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com