How Does A Septic Tank Work And Other Home Sewage Questions

What is a Septic Tank?

Many people think of septic systems as rare and ob

Many people think of septic systems as rare and obsolete. As many as one in five American homes boasts a septic tank, especially in rural areas. A septic tank is one big underground tank that holds and treats waste. It allows homeowners to safely dispose of wastewater from bathrooms, showers, dishwashing, etc., and take it outside. 

Septic tanks and their supporting hardware are located underground. Usually, they are installed at least ten feet away from the house for safety reasons. They also must be placed far enough from wells, so you don’t risk drinking water contamination.

A septic tank is most often made of concrete or plastic. Some newer models are made of high-quality polymers such as polyethylene. Septic tanks come in various sizes to suit different homes.  They generally hold between 1,000 and 1,500 gallons. 

What is a Septic Tank Used For?

Water that flows from your house is often contaminated, making it unsafe to drink or handle. Bathing, washing dishes, and doing laundry, also produces dirty water. 

Septic tanks use natural and mechanical processes to treat wastewater that passes through the system. It doesn’t matter where it comes from inside the home. It removes unwanted solids and organic matter before returning water to the ground. 

That’s where your septic system comes in. Septic systems treat wastewater when city sewer systems are not available. 


Drainfield Layout

Your drainfield layout will require a minimum of 2 trenches of equal size. The separation of the water flow into two, three or more lines is accomplished with a distribution box aka. D-box to divide the flow. The distribution box uses pipes equipped with flow control valves in the form of eccentric plugs that evenly split the flow between multiple drain lines. The effluent has to flow downhill from the tank outlet, through the distribution box and down the individual trenches. These trenches should be dead leveled so the water spills out onto the floor of each trench.

Septic tank components

Septic tanks are constructed of many materials, including reinforced concrete, fiberglass or metal. The tank may contain up to three compartments. Most important is that the septic tank must be watertight to prevent leakage of untreated sewage that might pollute groundwater and to prevent leakage of groundwater into the tank that could overload the absorption field.

Figure 2 Figure 2Typical components of a reinforced concrete septic tank. 

Reinforced concrete tanks, the most common, are built using one of three configurations: mid-seal, top-seal or monolithic cast. The mid-seal tank is cast in two nearly identical halves that are joined together with a sticky, tar-like mastic sealant. The top-seal tank is formed into a one-piece tank portion with a concrete lid secured to the open top with mastic sealant. The monolithic cast tank is factory-formed as one unit, making it more watertight but more expensive than the other two types of concrete tanks.

Fiberglass or composite plastic tanks offer a near-perfect watertight seal. These tanks may cost more than concrete tanks and require extra care when installing on sites with rocky fill material that could damage the tank.

Metal sewage tanks are not to be used unless specifically allowed by the regulatory agency on a case-by-case basis, and then should be used only if they are covered inside and outside with a bituminous coating. Even with such a coating, metal septic tanks are prone to corrosion and subsequent collapse.

In all septic tanks, the inlet and outlet pipes should be at least 4-inch diameter Schedule 40 PVC, cast-iron or other approved pipe and be protected by baffles or sanitary tees made of acid-resistant concrete, acid-resistant fiberglass or plastic. The baffles or tees ensure that floating scum from the tank doesn’t plug the absorption field or inlet pipe. Six-inch diameter inspection pipes should be located above the baffles or tees and extend to the top of the ground surface. These pipes are for checking solids levels and clogs and should be capped when not in use. A 20-inch or larger square or round manhole should be installed in the cover of the tank over each compartment to provide access for agitation and cleaning. Extending the manhole riser to the top of the ground surface will simplify cleaning operations. Manhole openings should be covered with tight lids of heavy metal or concrete for safety purposes.

A relatively new technology, septic tank filters, are basket-like screens that enhance treatment by trapping and retaining solids in the tank. They are included with some newer septic tank designs or can be retrofitted to work with older designs through use of added manhole openings. Regular maintenance is important: These screens must periodically be removed, hosed clean with water and replaced in the tank. These devices still need to be cleaned by a service professional.

A watertight, 4-inch diameter Schedule 40 PVC pipe should connect the septic tank to the plumbing drains of the home. Slope the pipe 1/4 inch per foot (1/8 inch per foot minimum) toward the tank. Install cleanouts (Figure 3) every 50 feet (100 feet minimum) along the pipe or wherever the pipe turns corners sharper than 45 degrees.

Figure 3a Figure 3aManhole cleanout for drain pipe. 

Figure 3b Figure 3bPVC pipe cleanout for drain pipe. 

Get your tank pumped…

Pumping removes the buildup of sludge and scum, which slows down bacterial action in the tank. Your tank may need pumping each year, but it’s possible to go two or three years between pumpings, depending on the size of your tank and the amount of waste you run through the system. Ask your inspector to make a rough recommendation for how often your tank should be pumped.

Your Drainfield Not Working Properly? Now What?

When your toilets back up or wastewater puddles up in your Drainfield area, chances are you will need to get it serviced or repaired. If roots have blocked your drainfield or kitchen grease, oils and fats have gotten into your lines, there may be nothing that can be done except to take out the old drainfield and install a new one. Sun Plumbing offers Free Estimates for your drainfield Repair or Replacement.

If roots or grease are not your problem, you may want to consider getting your Drainfield Cleaned. Over the years Sun Plumbing has perfected our cleaning process with amazing results. Our success rate of drainfield rejuvenation has consistently been above 95% of the Drainfields we have serviced. We can clean your Drainfield without disruption of water usage and with minimal disruption to your yard.

Our cleaning includes a warranty, and we offer credit toward a drainfield replacement by Sun Plumbing – if our cleaning fails. For larger failing Drainfields we may recommend adding Aeration to the septic tank. This is always an option for any drainfield that may have a biomat buildup at the absorbtion area.

If you do need a new Drainfield, there could be a chance you may have to install a pump tank with an elevated Drainfield for the new system. As a general rule, if your system needs to be replaced and it is older then 1992, the Environmental Health Department may determine your old system is to close to the ground water table. This means your new system will need to be installed higher, requiring the addition of a pump tank and pump.

Get an inspection

A thorough initial inspection by a pro will cost $300 to $500; after that, regular inspections cost less than $100 each. Your pro will be able to tell you how often your system should be inspected and how does a septic tank work.

Simple as a septic system may seem, evaluating its health really requires an expert. There are plenty of contractors who will gladly pump the sludge out of your tank, but in my experience many can’t fully answer how does a septic system work or how it should be maintained. I highly recommend looking for a contractor who has received some formal training in the science of septic systems. Some states have adopted certification programs for septic contractors—check with your Secretary of State’s office to see if yours is among them.

A complete inspection will determine whether your system is up to code (many are not) and the condition of the tank and drain field. A good inspector will also be able to tell you whether your tank is large enough for your household, and the maximum volume of water you can pass through it in a day.

You may be able to improve the performance of your system by adding bacteria with a product such as RID-X. Your pro should be able to tell you if your system will benefit from this treatment as you discover how does a septic tank work.

Before You Start Digging

It is always a good idea to make a proper evaluation of the job at hand before you start the actual construction work. Before you get your shovel out and start digging holes it would be a good idea to first get yourself a scale map of your home and property.

The best place to build a residential septic system is the backyard, underneath the garage, or any side adjacent to a street.

Should I Install A Septic Tank?

If you have neighbors, ask them about a septic tank. Your neighbors are on the same septic system. If you live alone in a rural area, your local septic contractors can help you. 

Don’t Overload the Septic System

There are certain things you shouldn’t flush down the drain when using a septic system. Septic tanks can only handle organic waste and septic-safe tissue. Things that may overload your septic system include:

  • Diapers and sanitary products
  • Disposable wipes
  • Paint and chemicals
  • Cat litter
  • Coffee grounds
  • Fabric and clothing
  • Bulky items

Before sending anything down the drain, it’s best to check if it’s septic-safe. Most types of toilet paper are septic safe, but biodegradable is best to not disturb the vital bacteria. Overloading your system could result in foul smells, backed-up toilets, and even sewage forcing its way up through the leach field.

Read More: How long do septic tanks usually last 

How big a septic tank?

The first thing we need to know is how big the tank should be. There are several ways of working this out but if you go to the charts at this will allow you to decide the size you will need.

Let us say we need a tank of 2.9 cubic metres (the amount of effluent the tank will hold) this is typical for an average 2 bathroom house with 4 people living in it. Refer to the construction drawing below for a 2.88 cu m septic tank designed for a typical household. To download the complete septic tank detail drawing you can go to

How many chambers?

A septic tank should have at least two chambers, 3 is better but 2 is enough. The second chamber can be around half the size of the first chamber.

In this tank the first chamber is 1.2m x 1.0m x 1.6m deep = 1.92 cubic metres liquid capacity. The second chamber is 0.6m x 1.0m x 1.6m deep = 0.96 cu m capacity = a total capacity of 2.88 cubic metres. Note that 1.6 metres is the depth of the effluent in the tank.

This tank is purposely designed to be quite deep while having a smaller footprint for use on smaller blocks of land.

Click on the image to download the full drawing

The walls and floor of the tank are 10 cms (4 inches) thick reinforced concrete providing sufficient strength to withstand water pressure and a reasonable amount of earth movement. The reinforcing steel is a mesh of 8 mm diameter reinforcing bars spaced 15 cms (6 inches) apart and built into a mesh box.

Septic Tank Design

Septic tank design example for home as follows,

The proper capacity of a septic tank is essential otherwise wastewater backflow toward the house. The septic tank design considering a future increase in wastewater generation design proper capacity septic tank which can last long for years.

Septic Tank Design Drawing
Septic Tank Design Drawing

Septic tank design example calculation for residential building is given below:

Let’s take one example of House5 Members

Daily Water Usage for a House5 Person

  • Cooking  10 Liters
  • Bathing & Toilet – 90 Liters/Person, So for 5 person  – 450 liters/Day
  • Washing clothes & Utensils – 35 Liters
  • Cleaning House – 15 Liters
  • Other 10 Litres

Total – 520 Litres/Day Approximately

In septic tank design, we consider detention time as 3 days. So the designed tank should have the capacity to retain the household wastewater for at least 3 days.

Total wastewater in 3 days – 520×3 = 1560 Liters

So, we take more than that about 2000 Liters capacity minimum for a home. For the Septic tank design, the depth of the tank should not be less than 1.8m.

Take sludge settled down per person 30 liters/year. So here we take sludge removal 2 years once.

Total Accumulated Sludge = 30 litres x 5 persons x 2 years = 300 Litres

Total Septic Tank Capacity = 2000+300 = 2300 Liters

We know that 1 cubic meter = 1000 Liters = 2300/1000 = 2.3 Cum

Area required @ 1.8m depth = 2.3/1.8 = 1.2 Sqm

We take Length and Width ratio of Septic Tank is 4:1 or 2:1

Length(L): Breath(B) ratio taken as 4 : 1

So, 4 B x B = 1.2 Sq. m where B=0.54m

(Note: Minimum width of tank should not be less than 750mm)

So that L = 4×0.75 = 3m

L – 3m; B – 0.75m;

Depth = 1.8+0.3 = 2.1m (free board should be provided at least 300mm)

Septic Tank capacity = 3 x 2.1 x 0.75 = 4.725 Cum

= 4.725x 1000

Septic tank design capacity = 4725 Liters

Read More: Estimation Of A Building With Plan

Septic Tanks Potential Problems

Sometimes excess wasting of cooking oils and grease can fill up the upper portion of the septic tank and can cause the inlet drains to block. It is a fact that grease and oil are often difficult to degrade and can cause odor problems and difficulties with periodic emptying.

Flushing material such as sanitary towels, cotton buds, etc. which are nonbiodegradable hygiene nature will rapidly fill or clog a septic tank and these materials should not be disposed of in this way.

Some people use a waste grinder to dispose of waste food will cause a rapid overload of the system and early failure.

Septic tank system damage due to some chemical contact such as pesticides, herbicides, materials with high concentrations of bleach, or any other inorganic materials such as paints, solvents, etc. Such materials inhibit bacterial function.

Roots of a tree growing above the tank and shrubbery or the drain field may clog and or rupture them.

Excessive water in the septic tank due to some plumbing leakage may lead to an overload of the septic tank system.

How Does A Septic Tank Work?

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Septic tank sewage is decomposed by bacteria, making it easier to transfer. Decomposed matter it known as “sludge,” which settles at the bottom of a tank. Waste that doesn’t decompose floats to the surface, which is called “scum.” Scum and sludge have a water layer between them. 

Septic tanks reduce the movement of raw sewage and waste. Without a septic tank, sewage would enter the surrounding soil. This is bad for the environment and human health.

Absorption Field

Excess water flows into the absorption field after passing through the septic tank. The soil needs to be percolated and then becomes solid and packed like clay. 

Loose soil like sand isn’t good because water can’t pass through it nor will it be absorbed. This is where the absorbtion field comes in. Another name for the absorption field is the drain field.


  • 19 CSR 20-3.060, Minimum Construction Standards for On-Site Sewage Disposal Systems, Missouri Department of Health, February 1996.
  • On-site Wastewater Treatment and Disposal Systems Design Manual, U.S. Environmental Protection Agency, 1980.
  • Pipeline Newsletter, volume 6, number 3, National Small Flows Clearinghouse, 1995. Pipeline Newsletter, volume 6, number 4, National Small Flows Clearinghouse, 1995. Septic System Owner’s Guide, PC-6583-S, Minnesota Extension Service, 1995.


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