Plumbing

The wastewater side of household plumbing collects used water from fixtures and appliances and delivers it to the treatment system(s).

Septic Tank

The septic tank is a solid watertight tank, or series of tanks, that receives waste water. It separates the solids from the liquids and stores the solids until they are decomposed or removed. The liquid, called effluent, is delivered to the soil treatment system.

Inlet and outlet baffles trap the floating solids in the tank. Inspection pipes allow monitoring of the tank and the manhole facilitates cleaning.

The size of the septic tank is based on the home’s potential water use volume and the type of appliances used. In aerobic tank systems, pumps and other mechanisms are necessary to deliver air to the tank.

Soil Treatment Area

The soil treatment part of the typical septic system is a network of perforated pipes or tubes surrounded by small rock and soil. Some designs use large plastic tubes or chambers instead of rock to disperse effluent from the tank into the surrounding soil.

The design of the treatment area (trench, mound, etc.) is based on the soil condition. The soil in the treatment area must not be saturated with water for extended periods of time during the year. Three feet of unsaturated soil below the system is necessary to complete the treatment process.

The size of the soil treatment area needed depends on the volume of water to be treated and the type of soil on the site. For example, a much larger soil area is needed for a large home or a home on clay soil than for a small home or one on sandy soil.

Pumps and a lift station may be components of a system where gravity flow doesn’t work. For example, in mounds and drip irrigation systems a pump is required to provide pressurized flow for distribution of effluent.

Enhancements

Sometimes enhancements, known as pretreatments, are added to septic systems. Some of the options are aerobic tanks, single pass or recirculating sand or peat filters, and constructed (lined) wetlands. These are located between the septic tank and the soil treatment system to improve the performance of the system or provide treatment in difficult soil conditions (for example, shallow bedrock or high water tables). These may require additional pumps and control devices.

Separation technology systems may require containers in the home that collect and compost solid organic wastes. Other devices may collect and store wastewater for delivery to a soil treatment or dispersal unit.

How Is Sewage Treated?

In the typical system, raw sewage is collected by the plumbing in the home and delivered to the septic tank. There the light solids float to the top, forming a scum layer, and the heavy solids sink to the bottom, forming sludge.

In the tank, organic solids such as food particles and human waste are decomposed by millions of naturally occurring bacteria.

The septic tank delivers the partially treated liquids, or effluent, to the soil treatment area. Effluent contains pathogens (disease-causing organisms), nutrients, and some fine solids. A thin layer of fine solids, dead bacteria, and soil bacteria, called a biomat, forms naturally where the effluent enters the soil. The biomat restricts the flow sufficiently to keep the soil beneath unsaturated.

The unsaturated soil contains oxygen which allows aerobic bacteria to live and destroy pathogens. These air spaces also force nutrients such as phosphorus and sodium to come in direct contact with soil particles to which they become attached. A portion of the nitrogen passes through into the groundwater. After passing through the unsaturated soil, the now harmless water evaporates into the air or returns to the soil and groundwater system. In regular septic tanks, the bacteria are anaerobic, that is, they live without air in the liquid. In aerobic tanks, the bacteria are aerobic and require air to live.

Source:  University of MN Onsite Sewage Treatment Program