Plumbing Blog
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Understanding DMV And Septic Systems

While on the subject of clogs in residential plumbings, let’s look at how the drain, waste, and vent (DWV) system works in a home.

This information is presented to give homeowners a basic understanding of what’s involved and what they can do working alongside a plumber.

How a Drain, Waste & Vent (DWV) System Works

Every home has a drain, waste, and vent system known as DWV. This drainage system carries away waste water while preventing the flow of nasty sewer gasses into your home.

(In rural areas, some homes have a gray water system that has a drain line for sinks, showers, dishwasher and laundry and a separate black water line for toilets. The gray water can be used for irrigation and toilet flushes. We’re not concerned with these systems in this post.)

  • Water flows downhill. The entire DWV system is built on this principle, and most drain systems are gravity flow.
  • Each drain in a home travels down to a larger branch drain.
  • All the branch drains connect to a waste stack, a vertical pipe that carries water to the main drain or sewer line.
  • A large pipe leaves your home and leads to the city sewer or your own septic system. All of this is downhill, all the way to the sewer or septic.

In some cases, a home’s sewer line will be below the level of the city sewer, or a basement bathroom is below the home’s main sewer line and so pump equipment must be used to move the waste out to the main sewer or septic.

  • All fixtures and appliances have a trap. A trap keeps a few ounces of water at a low spot in the pipe to seal the drain. This plug of water prevents sewer gas from traveling through the pipes and into your home.
  • An example of a trap is the “P-trap” found under a sink. The water runs through the drain and through the trap. The last few ounces remain behind in that trap. It fills the pipe and so prevents sewer gas from getting into your home. Toilets have there own trap built right into the fixture.

Along with all the drain and waste pipes, there is a system of vent pipes integrated into the drain system.

  • These vent pipes allow sewer gas to be vented out above your home where it can quickly mix with the air and dissipate.
  • The vents also serve the important function of preventing a vacuum or siphon from occurring. Because water traveling through the pipes would create a siphon effect, the water in the traps would be pulled along leaving the traps empty, allowing sewer gas to enter the home.
  • If a vacuum occurs, the draining water slows down much like how water gurgles out when poured from a bottle. This slower moving water results in the greater likelihood of clogging.

A vent system is necessary for the proper operation of a drain and waste system.

Each fixture must be properly vented and so a vent line branches off of the drain line, to either join up to the main vent or vents directly up through the roof.

Where the System Fails

The gravity system works well, unless something impedes the flow of draining water. Swiftly moving water carries waste away. However, if water cannot move swiftly, then the whole system starts to fail.

Waste builds up, further slowing water flow, until nothing is moving.

  • The initial problem that results in this cascade effect may be simply inappropriate waste being sent down the drain. It can result from dips developing in the drain line as the result of earth settling.
  • Deterioration of the drain pipes can result in rough surfaces that snag toilet paper, which builds up, eventually clogging the line.
  • Tree roots grow through pipe fittings and snag waste.
  • Another problem is putting grease, sand, coffee grounds and other “coagulants” down the drain. While coagulants is not a plumbing term, it gives you a visual image of what is happening. Things like sand and grease tend to rest on the bottom of the pipe and aren’t easily washed away. They coagulate and build up over time.

The DWV system is really pretty simple. It has some important refinements and the system has to be put together well, with attention to details, which is what you expect the builder and plumber or plumbing contractor to do when the house is built.

However, if you need to make basic repairs and retrofits, these are things you can do with a simple understanding of how residential plumbing works. In the end, it’s still a bunch of pipes running downhill.

It’s when you get into serious renovation projects, re-plumbing bathrooms, adding or moving showers, tubs, and sinks that it’s best to call a plumber.

How Does a Septic System Work?

A septic system is an alternative to a municipal sewer system. Homes that are not connected to a municipal sewer must instead use an underground septic system to handle the sewage and waste water from a home.

Septic systems are made from two main parts:

  • the septic tank
  • the leaching or drain field

The waste water and solids from toilets, sinks, showers, laundry and kitchen are all carried away from the home and deposited into a septic tank.

  • A septic tank can be a metal, plastic, fiberglass or precast concrete tank, or it can be constructed on site with concrete or other materials.
  • The tank has an inlet near the top that connects to the home’s sewer line.
  • The tank also has an outlet that connects to the drain field.
  • A septic tank has one or more manholes and view ports for service and inspection.
  • As waste water flows into the tank, the solids sink to the bottom.

Three distinct layers develop in a properly maintained septic system.

  • At the bottom is the sludge
  • Clear water
  • At the top is a scum layer.

How Septic Systems Operate

  • Anaerobic microorganisms digest the waste and break it down into tiny organic particles which can then be carried away with the clear water. These organisms do not require oxygen and thus can function effectively in the oxygen starved water of a septic tank. Some disease causing germs or bacteria which are associated with sewage, are actually inhibited by the lack of oxygen in the water.
  • The water level in the tank rises as water from showers, laundry and regular household activities flows out from the house.
  • As the solids are broken down, they mix with the clear water. When the water level reaches the outlet, water flows into the drain field.
  • The drain field is typically an array of buried pipes with holes along their length. The pipes are surrounded by gravel or similar permeable material to allow the free flow of water.
  • A second stage of water treatment occurs as, this time, aerobic organisms digest the remaining organic material and neutralize the remaining disease causing germs. The water seeps out of the drain lines and into the ground where natural filtration also occurs in the soil.

For a septic system to work, several things must happen.

  • First, the septic tank must be large enough to handle the effluent delivered daily. If more gallons of water are delivered over a period of time than the system can dissipate, the sewage may overflow the tank and flow into the drain field. This can lead to solids clogging the drain field, which then leads to even less waste water being dissipated, exacerbating the problem. A septic tank should have a minimum of 250 gallons of capacity per person for each of the members of the household.
  • Secondly, the system relies on active colonies of both aerobic and anaerobic organisms. Use of drain clearing chemicals and bleach both have harmful effects on these helpful bacteria. If too much chemical waste enters the septic system it can damage the colonies which then slows down the waste water treatment process. If the colonies are completely destroyed, the septic system will fail until new colonies can be established.
  • The drain field must have permeable soil and a supply of oxygen. Paving over the field will reduce the oxygen levels in the soil and thus it will not support a large colony of useful bacteria and reduce the effectiveness of the system. Allowing vehicles to drive or park over the field will result in soil compaction which reduces the ability of the soil to take up oxygen or to dissipate water.
  • Finally, inorganic solids such as plastic and rubber must not be flushed into the septic system because they can lead to blockages. Just like with excessive water flow into the system, inorganic waster can clog the drain field and then reduce the effectiveness of the entire system. Never flush trash or anything that will not dissolve or break down in water.
  • Some of the solid material in the tank cannot be digested by the bacteria. Over time this layer of undigested material will build up and must be pumped out. Failure to periodically pump out the septic tank will result in reduced handling capacity, which then leads to increasingly frequent overflows and back ups in the home.

If there are issues with a septic system, call a plumber or plumbing company with experience working with septic systems or a specialist that deals exclusively with septic systems.

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Benjamin Franklin Plumbing® offers customers on-time plumbing services, replacement and repairs done right the first time with courtesy, convenience, cleanliness, competence and character.


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Ben Franklin Plumbing Southeast
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Opelika, AL 36801
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