How Your Sewer Drain Works – And Everything Else

Image Credit: City of Waco

Understanding how your sewer drain works is essential for maintaining proper sanitation and wastewater disposal in your home. Here’s a brief summary of how a sewer drain system functions:

  • Wastewater Collection: In your home, wastewater from sinks, toilets, showers, and appliances (like washing machines and dishwashers) is collected and channeled into a network of pipes.
  • Gravity Flow: Most residential sewer systems rely on gravity to move wastewater. Pipes are sloped downward, allowing water and waste to flow naturally toward the main sewer line.
  • Main Sewer Line: All the individual drainage lines from different parts of your home converge into a single, larger pipe known as the main sewer line. This main line carries all the wastewater away from your house.
  • Ventilation: To ensure smooth drainage, the sewer system includes vent pipes that extend from the drainage pipes to the roof of your home. These vents allow air to enter the system, preventing vacuum formation and helping wastewater flow freely.
  • Clean-Outs: Sewer systems often include clean-out access points at strategic locations. These access points allow plumbers to inspect, clean, and clear blockages in the sewer line if needed.
  • Municipal Sewer or Septic Tank: The main sewer line connects to either the municipal sewer system or a septic tank, depending on whether you are connected to a public sewer system or have a private septic system.
  • Treatment and Disposal: If you are connected to a municipal sewer system, the wastewater is transported to a wastewater treatment plant for processing. In a septic system, wastewater is treated and filtered within the septic tank and then gradually released into the surrounding soil through a drain field.
  • Preventing Backflow: To prevent backflow of sewage into your home, sewer systems are equipped with traps and one-way valves, such as P-traps, which hold water to create a seal against sewer gases.
  • Routine Maintenance: Regular maintenance, including cleaning and inspection, is crucial to keep the sewer system functioning correctly. Tree roots, debris, and grease can accumulate in pipes and cause blockages if not addressed.

How Does a Domestic Sewage System Work?

To fully understand how your domestic sewage system works, we will start with the inside of the house where the fresh water is first utilized until the wastewater exits your property.

Before that, let us look at a house drainage system diagram.

House Drainage System Diagram

Image courtesyNations Home Inspection, Inc

1. Drain Lines

Your house has several fixtures which uses water and discharges it out as wastewater. These are toilets, sinks, tubs, showers, washing machines, dishwashers etc.

Each of these fixtures has a separate drain line which is responsible for draining away the waste water. A small part of the drain called a P-trap or drain trap is usually U-shaped to prevent sewer gases from coming up through the drains.

2. Main Drain Line

Your house’s main drain line is a vertical pipe and which is usually larger in diameter than the other’s fixtures’ drain lines. Above the fixture on your house’s highest level, the drain line is connected to the plumbing vent which runs through the roof of the house.

A plumbing vent removes sewer gases from the drainage system and also introduces in air so that the fixtures can drain fast and toilets can flush strongly.

At the bottom is where the main drain line is connected to the sewer line. Wastewater from each fixture drain line flows into the main drain line before leaving the house via the sewer line buried in your yard.

If you are on a septic system, the sewer line drains into the septic tank where the solids settle at the bottom while liquids flow out into the leach field. Of course this is after bacteria in the septic tank have broken down the waste.

3. Sewer Line

A sewer line is usually a 4 to 6-inch pipe (in diameter) connecting your house drains to the public sewer line or septic tank. There 2 sections of a private sewer line. These are the upper lateral and the lower lateral.

Old sewer lines were made of clay or cast iron. In modern plumbing however, PVC pipes are the most preferred materials for sewer lines since they are not infiltrated by tree roots.

As I had earlier mentioned, sewer lines are usually installed strictly sloping down towards the street or septic tank. This is so that the wastewater can flow out via gravity.

The recommended sewer line slope is 2%. This means that every foot of pipe has to drop by about ¼-inch. The slope can however be increased to ½-inch for every foot of pipe if the sewer line is too long of if it curves through the yard.

The Upper lateral

A sewer line’s upper lateral is the section of pipe from the house to the property line. This is usually close to the sidewalk.

The sewer line is installed in as straight line whenever possible but if there are obstructions in the desired path/route it has to take corners.

One important part of the upper sewer lateral is the sewer cleanout. A sewer cleanout is a 4-inch pipe which sticks a few inches from the ground usually located very close to the house.  It has a square nut on top of the cap.

In some houses, the cleanout is found inside the house usually in the lowest level. That would be the basement for most people.

The function of a sewer cleanout is to provide access to the sewer line. If your sewer line is clogged, it is backing up or you just want it inspected or cleaned that is where the plumber will insert their tools from.

In older houses which may not have a sewer cleanout, a plumber will have to pull a toilet to access the sewer line or do so from the plumbing vent on the roof of the house. You will however pay more for that.

The lower Lateral

The lower lateral is the section of the sewer line which connects the upper lateral to the public sewer line.

But why not connect the upper lateral to the sewer line directly you ask?

You see, the city’s sewer line is not installed along your property line. It is a few feet away and most importantly it is deeper than your sewer line.

You therefore need another section of pipe to connect the upper lateral to the city’s sewer line. And that is where the lower lateral comes in.

Most homeowners are usually confused on when they are responsible for the sewer line and when the city is responsible in as far as the lower lateral is concerned. This is because the lower lateral is usually outside the homeowner’s property line.

You will be surprised to learn that you are responsible for both the upper and lower sewer laterals. If any of these sections of pipes needs to be repaired, you will foot the bill from your pocket.

Instead of taking my word for it, you can actually grab a phone and call your city to find out. The rules might be different where you live but in most areas that is how it goes.

Even when you are responsible for the lower sewer lateral, you will need to inform your city when carrying out repairs especially when they involve excavations as they impact public utilities like roads and sidewalks.

Sewer Line Inspection and Cleaning

Sewer line inspection involves the use of a camera to check the conditions of the inside of the pipe. A camera connected to a flexible cable is inserted in the sewer line through the cleanout and a monitor on the surface will generate a video of everything being captured by the camera.

It is a very cost-effective and non-intrusive method of inspecting sewer lines. The camera inspection will tell if the pipe is corroded (cast iron), cracked, clogged or if there are tree roots growing inside the sewer line.

Depending on the results of the cameral inspection, the plumber will recommend the best method to clean the sewer line. This often involves using specially designed blades which are attached to the front of a motorized drain snake or hydro jetting.

If the sewer line is bellied/sagging or collapsed, the plumber will recommend the trenchless method of sewer line repair. In that method a sewer line will be replaced without digging a trench to remove the old one.

You should have your sewer line inspected and cleaned at least once every 2 to 5 years. This makes sure that you prevent it from clogging or even sewage backing up to your house.

It costs between $200 and $1000 to have your sewer line inspected and cleaned depending on the scope of work as well as your location.

It is costly to inspect and clean sewer lines but the cost is nowhere close to what it would cost if you had sewage backing up in your house. And remember homeowners insurance does not cover sewer lines unless you pay extra for that.

What Causes Sewer Drains to Clog

The following are the main causes of clogs and backups in sewer lines:

1. Tree roots

Tree roots will ordinarily grow towards sources of water and nutrition. And that is what domestic waste provides.

If there is a crack on your sewer line (happens in clay and cast iron pipes) tree roots will force themselves through the crack and grow rapidly once inside the sewer line.

 As they multiply, the roots trap solid waste from the wastewater and ends up clogging the pipe. What follows after that is sewage backing up from the drains in your house’s lowest levels which is usually the basement.

2. Grease

Never be tempted to dump grease down the kitchen sink drain after cooking. I know many folks say that it is ok to do so provided you follow it up with boiling water but just don’t do it.

The oil will cool down and solidify and when it combines with other substances in the wastewater it will clog the pipe badly. As you know, grease is quite sticky so the clog will be very nasty as well.

To properly dispose off grease just wait for it to cool then pour it in a sealable container before trashing it with the other home waste.

If you happen to accidentally pour grease down the drain check out this post on what to do next.

3. Flushing What You Shouldn’t

I always say that if it is not human waste or toilet paper then you should not flush it. Don’t use your toilet like trash bin. All that solid waste will end up accumulating inside the sewer line and slowly start to clog.

Even the so called ‘flushable wipes’ do not break down that easily and will still clog sewer line. Using too much toilet paper is another way that sewer lines end up clogged.

And that is how the sewage system works in a house. I hope this guide was of help to you.

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