Septic Systems

How Septic Systems Work Septic systems are wastewater treatment systems that collect, treat, and dispose of wastewater generated by homes or businesses. The wastewater is treated onsite, rather than collected and transported to a centralized community wastewater treatment plant. If properly designed, installed, and maintained, a septic tank system can effectively treat household wastewater for up to 20 years or more. There are several variations of the basic septic system design in use today. While many systems are individually designed or adapted for a specific site, most work using the same basic principles. A septic system consists of two main parts - a septic tank and a drainfield, also known as a leachfield, disposal field, or a soil absorption system. The entire system is connected by pipes, and a sewer pipe connects the home or business to the septic system. The main function of the septic tank is to collect household wastewater, which includes water from the toilet, referred to as blackwater, and water from the bathtub, showers, sinks, and laundry, which is known collectively as graywater. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. Treatment begins when the household wastewater flows from the home to the septic tank through the sewer pipe. A baffle (an internal flap) or tee (a T-shaped pipe) at the inlet slows the flow of wastewater going into the tank and directs it downward toward the middle of the tank. The wastewater is then retained for a day or more in the tank to allow the solids in the wastewater to separate from the liquids. Inside the tank, solids lighter than water - such as greases, oils, and other solid materials like toilet paper - float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater. An outlet baffle in the septic tank is positioned to allow only the partially treated liquid waste in the middle layer to flow out of the tank for further treatment. The layers of scum and sludge remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. This process takes place anaerobically, or without the presence of oxygen, and gases produced from the decaying solids are vented back through the sewer line and released, usually through a plumbing vent located on the roof of the house. The sludge and scum that cannot be broken down is retained in the tank until the tank is eventually pumped. After the wastewater is allowed to settle and separate in the septic tank, the partially treated liquid from the middle layer flows through the outlet baffle or tee to the drainfield.In a conventional septic system, the wastewater flows by gravity from the septic tank to the drainfield or to a distribution device, which helps to uniformly distribute the wastewater flow in the drainfield. The drainfield or soil absorption field provides the final step in the wastewater treatment process. A standard drainfield is a series of trenches or a bed lined with gravel or coarse sand and buried one to three feet below the ground surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater. The drainfield treats the wastewater by allowing it to slowly trickle from the pipes out into the gravel and down through the soil. The gravel and soil in a drainfield act as biological filters. As the wastewater percolates (moves through the soil) to the groundwater below, the filtration process and organisms in the soil work together to remove toxics, bacteria, viruses, and other pollutants from the wastewater. Soil particles, particularly clay, chemically attract and hold sewage nutrients, metals, and disease carrying organisms. This process can effectively treat the wastewater to an acceptable level that will not contaminate the groundwater. Certain toxics, such as paints, thinners, pesticides, waste oils, and other hazardous chemicals, cannot be treated by the drainfield and should never be disposed of through a septic system. Some of these chemicals also kill the bacteria found in the septic tank, temporarily disrupting the natural treatment process that occurs in the septic tank. The size of the drainfield is determined by the amount of wastewater flow anticipated and the quality of the soil below. Soil type and the position of the water table also help determine how deep trenches should be. A thorough site evaluation should be conducted when the septic system is still in the planning stages. Septic system failures are often caused by poorly sited drainfields.

What Homeowners Need to Know About Septic Tank Design

Septic tanks are usually made of precast concrete, fiberglass, or plastic, and come in a variety of shapes and sizes. In order for septic tanks to work properly, they must be watertight and resistant to corrosion - for this reason, metal tanks are not recommended.

Most septic tanks are single-compartment tanks. Tanks with two or more compartments use the same processes to treat the wastewater, but often retain the wastewater in the tank longer allowing for additional settling time for the solids, and thus providing additional treatment before releasing it to the drainfield. Some states recommend or require two or more compartments for septic tanks that hold 1,00 gallons or more, or two or more septic tanks used in a series to provide additional treatment.

Septic tank filters, screen- or basket-like devices that trap and retain solids, are another way to enhance treatment inside septic tanks. A relatively new technology, septic tank filters are included with some newer septic tank designs, or can be retrofitted to work with older designs. Homeowners should check with their local health departments to see if septic tank filters are required or recommended.

Septic tacks are often rectangular, oval, or round. The overall shape of the septic tank has little to do with its performance, but tank size is a very important factor. Septic tanks must be large enough to accommodate the needs of the household.

The size of a septic tank is usually determined by the number of bedrooms (not bathrooms) in a home. One way to estimate the size of septic tank necessary for an average household would be to multiply 150 gallons per bedroom per day, and then multiply this number by two to allow for two days retention time in the tank.

While there are several formulas available for estimating septic tank size, it is most important for homeowners to know the specific regulations for septic tank size and design in their state or area. In most areas of the US, there are laws regulating the construction of the septic tank itself, the size of the tank, the number of tank compartments required, the construction and design of the drainfield, and maintenance of the system. Mort's experts provide proper site evaluation to meet with state and local regulations.

How Much Do Septic Systems Cost?

The cost of installing and maintaining a septic system varies greatly depending on its location and design. In order to accurately estimate what a septic system will cost, homeowners should contact their local health department for more information about the costs of septic systems in their area.

In most areas in the US, conventional septic systems cost from $2500 to $7500 to install. While certain site conditions or alternative drainfield designs can make installation more expensive, this is a general range for standard septic tank and soil absorption systems. Alternative septic systems requiring pumps or specially constructed drainfields can be considerably more expensive.

As a general rule of thumb, septic systems are most cost-effective in communities where houses are spaced widely apart, and where connection to a sewer system is not an option. When the cost of operation and maintenance of a centralized treatment plant is factored in, residents in small rural communities may pay many times more per household for a centralized sewer system than residents in more densely populated areas. In certain communities, a centralized sewer system would be so expensive to install and maintain that costs per household could exceed property values.