Septic System Basics

Wastewater from our homes can contain a wide variety of contaminants, including disease-causing pathogens (e.g., bacteria, viruses), nutrients (e.g., nitrogen, phosphorus), heavy metals, as well as contaminants of emerging concern like pharmaceuticals and personal care products, among others. Wastewater must be effectively treated to protect both human and environmental health.

Septic systems treat and dispose of wastewater from sinks, laundry, showers, and toilets from homes or buildings in rural areas where centralized sewer systems are unavailable. Septic systems are also commonly called on-lot or on-site sewage treatment systems because they typically treat and dispose of wastewater within the same property where it is generated. About 26% of Pennsylvania households use septic systems to treat their domestic sewage.

Septic system components

A conventional or traditional septic system is the most commonly used type. As seen in Figure 1 below, the basic components include a septic tank, distribution box, and drain field.

1. Septic tank

The septic tank is an underground container that collects all the wastewater flow from the home or building. They have an inlet pipe that brings in wastewater from the residence or building and an outlet pipe where partially treated wastewater eventually exits the tank, see Figure 2. As the first stop for all the untreated sewage, the septic tank should be water-tight; therefore, most tanks are commonly made of concrete but can also come in other materials such as plastic or fiberglass.

Septic tanks are generally sized based on the number of bedrooms in the home because the number of bedrooms indicates household size and can thus be used to estimate wastewater volumes from the home. For example, a house with three bedrooms in Pennsylvania must have a 900-gallon or larger septic tank. The more bedrooms, the larger the septic tank.

Septic tanks can have one or two compartments (or chambers). One-compartment tanks were common in the past, but double compartments are becoming more common today. A double-compartment septic tank is divided into two sections by a dividing wall within the tank. The double-compartment tanks have become popular because they provide better wastewater treatment. As Figure 2 shows, septic tanks must also have inspection ports above both the inlet and outlet for checking the condition of the baffles and covered access ports to each chamber for pumping and cleaning the tank.

How the septic tank treats wastewater

Once wastewater flows into the first chamber of the septic tank, the treatment process begins. Solids separate and settle to the bottom of the tank while the oils and greases float to the top. There is no oxygen in septic tanks; therefore, naturally occurring bacteria that function without oxygen (or anaerobic bacteria) break down the solids. This breakdown process is known as anaerobic digestion. Over time, the solids form a layer of sludge at the bottom, the oils and greases form a layer of scum at the top, and partially treated wastewater stays in the middle.

As more wastewater flows into the first tank from the home or building, the partially treated wastewater in the middle of the tank is displaced and flows out of the first chamber into the second chamber of the tank.  While in the second chamber, additional separation of solids and floatation of scum happens, thus improving the quality of the wastewater. Eventually, treated wastewater, known as septic effluent, leaves the second chamber through a baffled outlet and towards the drain field. Double-compartment tanks provide better treatment because of the additional settling of solids in the second compartment before wastewater exits the tank. Depending on the tank size and overall wastewater flow from a household, wastewater in the tank can have at least 24 hours of settling and treatment before moving into the drain field.

The tank must have a baffle at both the tank inlet and outlet to prevent the wastewater from short-circuiting across the liquid surface and to keep the scum layer and suspended particles from flowing with the wastewater into the drain field (See Figure 2). The exit baffle or tank outlet should also be fitted with an exit filter to capture any solids or particulates in the tank effluent. This prevents solids and particulate matter from exiting and clogging the drain field.

Over time, the sludge at the bottom of the tank accumulates and requires pumping by a licensed professional. It is recommended that septic tanks be pumped every 3 to 5 years or when a system inspection indicates that the tank is more than one-third filled with solids. Pumping frequencies can vary depending on the tank size and the number of people in the household. Local regulations may require septic system inspection and/or pumping on a certain schedule (often every 3 years). Regular pumping is beneficial because it:  

• Ensures that there is sufficient space within the septic tank to hold wastewater and allow for solids to settle
• Prevents solids or sludge from accumulating up to levels that back up into the home
• Prevents sludge and solids from flowing from the tank and causing clogs in the drain field

2. Distribution box

The distribution box is also called the D-box. It is a small round or rectangular watertight containment made of concrete or plastic. The D-box is underground between the septic tank and the drain field piping. It is connected to the outlet pipe on the septic tank on one end and the series of pipes leading to the drain field on the other end.

The purpose of the distribution box is to receive septic tank effluent and evenly distribute it into the pipes that disperse wastewater in the drain field. Uniformly distributed wastewater in the drain field ensures that the drain field operates effectively and prevents one area from being overloaded with septic effluent.

3. Drain field

The drain field is also known as a leach field or absorption area. This is where the partially treated wastewater or septic effluent is disposed of for further treatment. It comprises of a network of perforated pipes laid on aggregate materials and a suitable soil layer. Once laid, the pipes are covered with additional aggregate material, geotextile material, and backfill soil. These pipes receive septic effluent through the distribution box and uniformly disperse to the underlying aggregate layer allowing it percolate through the suitable soil layers.

How drain fields provide additional wastewater treatment

Once wastewater is in the drain field, it trickles through the underlying aggregate and the suitable soil layer. As the wastewater moves downward, the soil within the drain field acts as a filter that traps particles and organic matter in septic effluent. Additionally, the effluent comes in contact with naturally occurring bacteria in soil that further break down pollutants in wastewater. Some of the minerals in the soil can also react with the chemicals in wastewater to help break them down. These processes help filter the wastewater naturally before eventually reaching and mixing with nearby groundwater or surface water bodies. 

Types of drain fields

There are different types of drain fields, and the type permitted for construction in a home varies based on site-specific factors. Some factors considered include lot size and shape, soil type and characteristics such as its ability to absorb water, topography and slope, underlying geology, depth to the groundwater table,  existing infrastructure (e.g., location of private wells), among other environmental factors and local regulations. These factors are evaluated to determine the most suitable type of drain field to effectively treat septic effluent while minimizing contamination of nearby groundwater and surface water resources. For example, based on Pennsylvania code, the top of a limiting zone, i.e., groundwater table or bedrock, should be a minimum of 4 feet below the bottom of the gravel aggregate layer of the drain field. This separation distance is beneficial as it ensures that there is sufficient suitable soil to filter septic effluent before mixing with groundwater. The types of drain fields include:

Standard trenches: This type of drain field consists of a network of perforated pipes laid in narrow (12 to 72 inches width) parallel trenches following land surface contours (See Figure 1).

Seepage bed: These rectangular excavations contain several perforated pipes to distribute septic effluent. The depth of aggregate materials in the seepage bed should be 12 to 36 inches.

Elevated sand mound: A sand mound is also commonly called a raised bed or a mound system. This type of drain field is a bed built above ground on top of a man-made sand plateau. Effluent is pumped and dispersed into the mound of sand. Sand mounds are used in areas where the soil is unsuitable or the water table is too high to have a conventional trench or seepage bed. The major difference between elevated sand mounds and conventional in-ground-absorption systems is the addition of sand to the top of the absorption area in order to increase the depth to the limiting zone to the required minimum of 4 feet.

Subsurface sand filter beds and trenches: According to the Pennsylvania Code, subsurface sand filters cannot be utilized on soils where the limiting zone is less than 6 feet below the soil surface. System design should meet the requirements of the standard trenches or seepage beds.  As the name suggests, they consist of a sand filter bed layered in an excavated area with a maximum depth of 5 feet. Pipes evenly dispersing effluent are laid on top of the sand bed and are covered with soil backfill.

Alternative drain fields: Newer designs for drain fields exist. Some include drip irrigation, individual residential spray irrigation systems (IRSIS), evaporation systems, and constructed wetlands, among many others.

Caring for your drain field

The drain field is the most expensive and difficult septic system component to correct or replace. Regular care and maintenance can help extend the lifespan and the entire septic system. Here are some things you can do:

• Have your septic system professionally inspected. Inspections help with early detection and remedy of any system issues. Inspections can also help determine if you need to have the septic tank pumped.
• Regular maintenance: A septic tank should be pumped every 3-5 years or when a septic system inspection finds that the tank is more than one-third filled with solids or scum. Regular pumping ensures maximum treatment within tanks and prevents solids from reaching the drain field. Solids that reach the drain field can clog pipes and the soil, thereby minimizing treatment efficiency.
• Divert excess water from roof drains and sump pumps away from the drain field. Excess water can flood your drain field and slow down or stop the wastewater treatment process.
• Keep heavy equipment, traffic, and trees away from the drain field. These can compact the soil, damage pipes, and cause the drain field to clog.