Understanding Your Community’s Wastewater Collection System

A wastewater collection system is the buried network of service laterals, gravity sewers, manholes, lift stations and force mains that carries used water from every building to the treatment plant. It runs mostly on gravity and mostly out of sight. When it fails, the cause is usually grease or roots, clear water leaking in, or pipes and pumps too small for the flow.

What is a wastewater collection system made of?

Pipes that run on gravity, structures that give crews a way in, and pumps where gravity runs out.

EPA's 2005 guide to collection system capacity, management, operation and maintenance (CMOM) lists what a good system map shows. From the building out:

• Service laterals carry flow from each building to the public sewer. Part of a lateral is often on private property, and the utility's sewer ordinance sets the rules for it.
• Collector sewers take flow from the laterals, trunk sewers gather it, and interceptors carry it to the plant.
• Manholes and cleanouts give crews access to inspect and clean.
• Lift stations pump flow through a pressure pipe, the force main, where gravity cannot reach the plant. Sizing both is its own calculation, covered in our walkthrough of force main and pump sizing.

States set the design rules. In Texas, TCEQ's pipe design rule, 30 TAC §217.53, requires gravity pipe at least 6 inches in diameter, and its manhole rule, 30 TAC §217.55, requires a manhole at every change in direction, slope or pipe size, at every intersection and at the end of any pipe that may be extended.

How does wastewater move through the pipes?

By gravity, in pipes sloped steeply enough to keep solids moving.

The CMOM guide notes very little buildup at 2.0 feet per second. Below about 1.0 to 1.4 feet per second, grit and solids can settle and lead to a blockage. Texas requires slopes that give at least 2.0 feet per second flowing full, and pipe sized for the peak flow plus infiltration and inflow.

Steep ground creates the opposite problem. Under §217.53, a pipe that would run faster than 10 feet per second flowing full needs a design that protects against pipe and bedding displacement. On mountain streets, that is a real design condition.

What goes wrong in a collection system?

Blockages, clear water getting in, corrosion, and growth outrunning capacity.

The CMOM guide says stoppages in gravity sewers usually come from a structural defect, poor design or construction, buildup in the pipe (especially grease) or root intrusion. Service connections that protrude into the main can catch debris.

Clear water comes two ways. Under EPA's definitions in 40 CFR 35.2005, infiltration is groundwater that seeps in through defective pipes, joints, connections and manholes. Inflow enters through roof leaders, yard drains, manhole covers and cross connections with storm sewers. EPA New England's 2014 guide for estimating infiltration and inflow says infiltration peaks at seasonal high groundwater, which usually follows snowmelt and soil thaw. In a Colorado mountain town, that is snowmelt season. The guide says the extra water robs the system of capacity and shortens the life of the plant. See how an I&I analysis finds the worst sections.

Corrosion is the quiet one. Hydrogen sulfide builds where flow is slow, at pump stations and where force mains discharge. It forms sulfuric acid that eats concrete, steel and iron. Clay and plastic pipe resist it.

Growth is the last. The CMOM guide notes many systems built to industry standards now overflow in wet weather because clear water or population outran the design.

Why do backups and overflows matter?

Raw sewage ends up in homes, streets and streams, and the system owner answers for it.

EPA's sanitary sewer overflow page says overflows can contaminate waters and back up into homes, causing property damage and threatening public health. One that reaches waters of the United States is prohibited unless an NPDES permit authorizes it. TCEQ's sanitary sewer overflow page calls an overflow "a type of unauthorized discharge." Texas permittees report noncompliance that may endanger health, safety or the environment orally within 24 hours and in writing within five days, under 30 TAC §305.125(9).

How do utilities find problems before pipes fail?

Measure the flow, then look inside the pipes where the numbers point.

1. Flow monitoring. Permanent meters sit at pump stations and key junctions. Temporary meters, typically in place 30 to 90 days, move around to find the areas with high wet weather flow.
2. Smoke testing. The CMOM guide calls it a relatively inexpensive and quick way to find inflow from downspouts, yard drains and faulty service connections.
3. CCTV. A camera travels the cleaned pipe and leaves a permanent video record.
4. Manhole inspection, typically every one to five years, and more often for problem manholes.

How are failing sewers fixed without digging up the street?

Often through the existing manholes, with a liner or pipe bursting. One bad segment may still be an open-cut point repair.

The CMOM guide separates nonstructural repairs, which seal leaking joints in sound pipe, from structural repairs, which line or replace it. Trenchless options include cured-in-place pipe (CIPP) and pipe bursting. The right method depends on pipe size and material, location, flow and surface conditions.

Do not stop at the mains. The guide warns that laterals, manhole covers in the path of runoff and cracked manhole walls all let clear water in.

What does a good maintenance program include?

A current map, an inventory, a cleaning and inspection schedule, and records that decide what gets fixed next.

The CMOM guide frames it as knowing what the system is, where it is and what condition it is in, then planning work on condition and performance. Cleaning needs vary: a restaurant district may need it every six months, while new PVC pipe with no grease may go five years.

Texas puts this in rule. 30 TAC §217.69 requires a maintenance and inspection program, developed with an engineer and licensed operators, with schedules, condition and life expectancy estimates, and records.

What can residents and businesses do?

Flush only toilet paper, keep grease out of drains, and keep rainwater out of the sewer.

EPA's March 2020 release on flushing asks people to flush only toilet paper, because wipes can damage plumbing, sewers and septic systems. Restaurants belong on a grease trap program with regular pumping and inspection. Downspouts and yard drains tied into the sewer are inflow, and Texas design rules prohibit any connection between stormwater and wastewater collection systems. Report overflows and sewer odors to your utility.

Is your collection system ready for the next ten years?

It is ready for planning when you can answer yes to all six.

1. Do you have a current map of mains, manholes, lift stations and force mains, with pipe age and material?
2. Do you have a written cleaning and inspection schedule, with records?
3. Do you know your plant flow in dry weather and in your wettest season?
4. Do you know which areas send the most clear water during snowmelt or storms?
5. Does every lift station have a plan for a power outage?
6. Do you know how close your flow is to your permit? In Texas, three straight months at 75 percent triggers expansion planning under 30 TAC §305.126.

Six yeses mean you can plan from data. Any no is the first project.

Questions for your engineer: Where does our wet weather flow come from? Which pipes are most likely to fail next? What should we fix this year?

A note on limits: this is general education from EPA guidance and TCEQ rules, as of October 2026. Your permit, your state's rules and your engineer's review govern your system.

Frequently asked questions

What is the difference between a sanitary sewer and a storm sewer?

Under the federal definitions, a sanitary sewer carries wastewater from homes, businesses and industry. A storm sewer carries only stormwater, runoff, street wash water and drainage. A combined sewer is designed to do both.

Who is responsible for the sewer line from my house to the main?

Your utility's sewer ordinance decides. Ask where the utility's maintenance ends.

I manage public works for a small Colorado mountain town, and our plant flow jumps every spring during snowmelt. Where do we start looking?

Start with plant data. Average the nighttime flows, midnight to 6 a.m., over several dry days at high groundwater, less any large commercial night flows. The EPA New England guide treats the rest as mostly groundwater. Then move temporary meters through the system and put cameras on the worst areas first.

I sit on the board of a Texas utility district, and the same manhole has overflowed three times this year. What should we ask our operator and engineer?

Ask what the cleaning records say caused each stoppage. Dry weather points to grease, roots or a defect. Wet weather points to capacity or clear water. Ask for a camera run upstream and downstream, and confirm each overflow was reported as TCEQ requires.

Related resources

• Gravity Sewer vs. Pressure Sewer: Choosing the Right System for Your Community
• Types of Sewer Systems
• Optimizing Collection Systems: Advanced Solutions for I&I Challenges in Rural Communities
• Capital Improvement Planning for Small Municipalities: Where to Start When You Don't Know Where to Start

Not sure what shape your collection system is in?

We read your flow data, inspections and maps, and turn them into a ranked list of fixes and a plan for growth.

Talk to Modern Engineering Solutions about your collection system, or call (214) 833-6748.