How Sewer And Septic Systems Work

A brief overview of how septic tank and sewer treatment systems work.

A septic tank system is a high volume, leak-proof tank where the initial treatment of household waste takes place. Septic tanks have several important jobs. They receive the wastewater from the house and separate the solids from the water. The tank stores the accumulated solids and reduces the solids while sending the cleared wastewater to the drainfield. This system is typically found in rural and other areas that do not have access to a municipal sewer system.

Once the wastewater enters the septic tank, the primary treatment begins. The water inside the tank is very still and this allows for separation of the material to start. Heavy solids settle to the bottom, forming a sludge layer. Underwater anaerobic bacteria works to consume the sludge portion. The material that is lighter than water like oil and grease floats to the top forming a scum layer. Aerobic bacteria work at eating this layer. The middle layer in the tank is called the effluent. This clarified wastewater flows out of the tank into the drainfield.

The time that the wastewater remains in the tank for separation to occur varies with the size of the tank and how much volume comes from the house. Minimum retention time is about 24 hours but two to three days are considered best. As the sludge builds up at the bottom, the tank will lose volume. Regular pumping of the tank is recommended to prevent a situation where there is not enough space for separation and solids flow out into the drainfield. This occurrence is the main cause of clogged pipes and gravel, which causes failure in the septic tank system.



The drainfield is the other critical part of the septic tank system. This is where the effluent or wastewater goes when it leaves the tank. It typically consists of several narrow, gravel-filled trenches with a perforated pipe near the top. The pipe allows for an even distribution of the wastewater. From here, the effluent eventually makes it way into the soil where it is further filtered by the soil and biological activity.

In more populated areas, wastewater or sewage gets transported to the local treatment plants through sets of pipes. Local sewage pipes feed into interceptor sewer pipes, which then carry it to the treatment plant. Before sewage enters the treatment plant, it is screen for large objects like brick and logs.

Preliminary treatment involves a grit chamber. In this tank, the mud and sand settle to bottom and is later removed and taken to an environmentally safe disposal area. Then the sewage flows into primary settling tanks where about 60 percent of the solids settle down to form sludge. In treatment plants, which process very large amounts of wastewater, oxygen is added to the water to speed the growth of microorganisms. These microorganisms consume the waste and settle to the bottom of the secondary settling tanks. The remaining water is disinfected and released into receiving waters.

Most of the toxic chemicals in the effluent have been removed by this time. People need to be aware of dumping improper chemicals down the drain. There are also less toxic household cleaners available that should be used when possible.

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