Private Treatment: The Septic Tank

In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks.

A septic tank is simply a big concrete or steel tank that is buried in the yard. The tank might hold 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and leaves the tank at the other. The tank looks something like this in cross-section:

In this picture, you can see three layers. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.

Wastewater comes into the septic tank from the sewer pipes in the house, as shown here:

A septic tank naturally produces gases (caused by bacteria breaking down the organic material in the wastewater), and these gases don’t smell good. Sinks therefore have loops of pipe called P-traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a vent pipe instead — if you look at the roof of any house, you will see one or more vent pipes poking through.

As new water enters the tank, it displaces the water that’s already there. This water flows out of the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches filled with gravel. The following diagram shows an overhead view of a house, septic tank, distribution box and drain field:

A typical drain field pipe is 4 inches (10 centimeters) in diameter and is buried in a trench that is 4 to 6 feet (about 1.5 m) deep and 2 feet (0.6 m) wide. The gravel fills the bottom 2 to 3 feet of the trench and dirt covers the gravel, like this:

The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field is determined by how well the ground absorbs water. In places where the ground is hard clay that absorbs water very slowly, the drain field has to be much bigger.

A septic system is normally powered by nothing but gravity. Water flows down from the house to the tank, and down from the tank to the drain field. It is a completely passive system.

You may have heard the expression, “The grass is always greener over the septic tank.” Actually, it’s the drain field, and the grass really is greener — it takes advantage of the moisture and nutrients in the drain field.

Urban Wastewater Systems

In urban and suburban areas where people are packed closer together and where there is a lot more wastewater to treat, the community will construct a sewer system that collects wastewater and takes it to a wastewater treatment facility.

In the ideal case, a sewer system is completely gravity-powered, like a septic system. Pipes from each house or building flow to a sewer main that runs, for example, down the middle of the street. The sewer main might be 3 to 5 feet (1 to 1.5 m) in diameter. Periodically, a vertical pipe will run up from the main to the surface, where it is covered by a manhole cover. Manholes allow access to the main for maintenance purposes.

The sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In order to help gravity do its job, the wastewater treatment plant is usually located in a low-lying area, and sewer mains will often follow creekbeds and streambeds (which flow naturally downhill) to the plant.

Normally, the lay of the land will not completely cooperate, and gravity cannot do all the work. In these cases, the sewer system will include a grinder-pump or a lift station to move the wastewater up over a hill.

Once the water reaches the wastewater treatment plant, it goes through one, two or three stages of treatment (depending on the sophistication of the plant).

Here’s what each stage does:

• The first stage, known as primary treatment, does the same thing a septic tank does. It allows the solids to settle out of the water and the scum to rise. The system then collects the solids for disposal (either in a landfill or an incinerator).
  Primary treatment is very simple — it involves a screen followed by a set of pools or ponds that let the water sit so that the solids can settle out.

• Primary treatment might remove half of the solids, organic materials and bacteria from the water. If the plant does no more than primary treatment, then the water is chlorinated to kill the remaining bacteria and discharged.
• The second stage, known as secondary treatment, removes organic materials and nutrients. This is done with the help of bacteria — the water flows to large, aerated tanks where bacteria consume everything they can.

• The wastewater then flows to settling tanks where the bacteria settle out. Secondary treatment might remove 90 percent of all solids and organic materials from the wastewater.

• he third stage, known as tertiary treatment, varies depending on the community and the composition of the wastewater. Typically, the third stage will use chemicals to remove phosphorous and nitrogen from the water, but may also include filter beds and other types of treatment. Chlorine added to the water kills any remaining bacteria, and the water is discharged.