A traditional anaerobic septic system has soil absorption area which consists of an area of ground which has been usually tested for a percolation rate, soil type, and location and found suitable for the absorption up to a fixed quantity of water per day. Oxygen in the soil is not tested for and may be the most important component of a soil absorption area. If it is not one of the most important components, it is the least understood.

While all of the components are important, each component plays a role in a functioning soil absorption area. Proper preparation of the soil, the components which distribute the water, proper depth, and ground cover each play a significant part. Proper sizing of the septic tank to allow for retention time, baffles, and an outlet filter are also important. In addition, pumping the septic tank on a regular schedule, at least every three years, can help ensure that the septic system continues to function and absorb water in an environmentally acceptable manner.

The soil absorption area is the most expensive component of the septic system. It consists of a method to distribute the water from the septic tank to the ground to be absorbed. It must accept water almost every day and dispose of it in order to repeat the process the next day. This soil absorption system is a very complex biological and chemical system. It is also susceptible to upsets and can deteriorate over time.

A traditional septic system drainfield has both aerobic and anaerobic bacteria. In this system, when water containing nutrients flows into the soil absorption area, a biologically active film develops. This slime layer, or commonly called the “biofilm or biomat”, is composed of bacteria and their waste products. This slime is actually a chemical compound secreted by the bacteria to anchor themselves to the bottom and sidewalls of the aggregate and the soil interface and to each other to prevent being washed away by the water flow through the septic system. The bacteria serve a vital purpose in a septic system by absorbing the waste and breaking it down and into soluble byproducts. Without these beneficial bacteria, the septic system would not work.

As this film forms and grows, the outside of this film is composed of aerobic bacteria and the inside of anaerobic bacteria. The water coming from the septic tank has already been depleted of oxygen and is thus an anaerobic environment. It is oxygen from the soil that allows the activity of the aerobic bacteria. This aerobic activity is important to the digestion of waste from the septic tank. Aerobic bacteria digest waste faster and more efficiently with different by-products than anaerobic bacteria.

Recent evidence has shown that the biofilm is composed of both aerobic and anaerobic bacteria. As the biofilm thickens, which results from use of the system, the anaerobic bacteria can grow underneath the aerobic bacteria. This protects them from oxygen. These anaerobic bacteria use the sulfur from the waste as their source of energy. The byproducts of these bacteria include a form of sulfur called sulfides. One of the characteristics of sulfides is the “rotten egg smell” which we associate with septic or sewer systems. Sulfides also have a characteristic in which they react with metals, mostly iron in soil and water, to a form a black, very insoluble, and very sticky material which is detrimental to the environment and health of the septic system. This black material is mainly iron sulfide. In the presence of oxygen, these sulfides slowly react with oxygen to form soluble sulfates which wash down into the soil. This is the same process which takes place if the system is allowed to rest for an extended period.

Why does the rate of water absorption change with use? What may cause the long term water absorption rate to be less than the initial perc test rate and then continue to decrease
over time? The answers to these questions lie in the characteristics of the soil, the oxygen requirements of the waste from the septic tank, and the amount of water added to the system.

First the long term acceptance rate of the soil. We all notice that soil which is constantly moist and wet does not absorb water as fast as new soil or soil that has been dry for a period of time. This can be due to more anaerobic biological activity and the resulting insoluble and clogging by-products. Since this activity can slow the absorption of water, this is one reason why a drainfield should be larger that might be assumed from a perc test alone.

What can affect the drainage of water into the soil? The main thing which can affect the long term water absorption properties of a drainfield is the availability of oxygen. Why is this important?

Oxygen is used in several ways in the drainfield. One way is to provide an essential material for the aerobic bacteria in the soil. The efficient aerobic bacteria use oxygen in their digestion process to break down the waste. Remember the biofilm is aerobic on the outside, but below the surface it consists of an anaerobic layer. These anaerobic bacteria use the sulfur from the waste as their source of energy. As we have seen, the byproducts of these bacteria include a form of sulfur called sulfides. Sulfides have a characteristic in which they react with metals, mostly iron in soil and water, to a form black, very insoluble, sticky material which is mainly iron sulfide. In an oxygen rich environment, these sulfides slowly react with oxygen to form soluble sulfates which wash down into the soil. This demand for oxygen competes with the aerobic bacteria for the available oxygen. If the system is in balance, there is enough oxygen for the aerobic bacteria and the constant removal of the sulfides. This balance helps determine the long term absorption rate for the drainfield.

Maintaining an oxygen balance in a septic system is crucial for its overall function. First, the bacteria that serve to breakdown the waste depend on oxygen to survive. If the oxygen is depleted in a system, these bacteria will actually convert to anaerobic bacteria. In addition, the sulfide by-product of the anaerobic bacteria cannot be broken down without oxygen. Therefore, oxygen is required to deal with the anaerobic biofilm by-products.

What can cause the system to become unbalanced? The removal of available oxygen is the main reason that this can occur. If the waste coming from the septic tank has a high biological oxygen demand, then the oxygen from the soil may not be enough to supply both the aerobic bacteria and the removal of sulfides. This causes the sulfides to build-up with the result that the anaerobic bacteria activity increases and more sulfides are produced. This thickening of this biological film then helps slow the transfer of water and digestion of waste from the septic tank. The system becomes more anaerobic with the result of more sulfides. These sulfides build up on the biomat and at the interface where the drainfield media and the soil meet. This slows the transfer of water into the ground.

Another way of producing this situation is to hydraulically overload the system. Hydraulic overloading means adding water faster that the system can dispose of it into the ground. This results in standing water in the aggregate. This overloading of anaerobic water prevents oxygen from getting to the biomat and the ground below. As water stands in the drainfield, anaerobic conditions become predominate. This condition increases the production of sulfides and a resulting thickening of the biomat and buildup of insoluble sticky black sulfides in the soil.

How can this condition be fixed? One method is to stop adding water to the system. When a system rests, oxygen in the soil to slowly oxidizes the sulfides and allows the clogging mat to decompose. When the system is started up again, the process starts over. Another method is to reduce the amount of water and waste in the water going to the drainfield. Proper maintenance of the septic tank, a regular pumping schedule, and reducing the organic waste entering the system may help. This organic waste is not only food and human waste, but also soaps, detergents, and things such as shampoo and cream rinses. If the amount of water entering the system is consistently more and faster that can be absorbed into the soil in a period of time, the drainfield will become more anaerobic and develop problems.

The characteristics of the soil and plant coverage over a drainfield and depth of the laterals can also affect the amount of oxygen which can penetrate the soil.

What can be done to maintain or improve the water absorption in a drainfield? There are several techniques which can be used. These may be regulated in some states. If the drainfield is flooded, that is standing water above the laterals, soil fracturing may be used in some areas. This technique blasts compressed air into the drainfield to open paths through the clogging mat for the water to flow through. This may help the long term situation because more oxygen may be available and help reduce the sulfide build-up. The system may be rested for a period of time which may be as long as six to twelve months. This is probably not possible unless the system was installed with alternating drainfields connected by a valve to divert water from one to the other. Another way is to replace the soil absorption area. There are also companies offering kits to convert the septic tank from a traditional anaerobic system to an aerobic system. This would over time have the effect changing the chemistry and biology of the drainfield and eliminating the sulfides from the soil absorption area. Each of these methods is generally expensive.

Septic-ScrubTM drainfield flow restorer is a chemical treatment to oxidize the sulfides and release oxygen into the drainfield. This is essentially the same way that letting the system rest does, except much faster. The product is added between the septic tank and the drainfield. The use of the system does not need to be restricted during the treatment. However for best results the drainfield should be dewatered down to the soil interface either by resting or by pumping the drainfield. It is also environmentally friendly and does not add any materials to the environment which are not naturally present. Treatment of seepage pits and cesspools requires special techniques for best results.

Septic-Scrub can not only be used to try to restore flow to a slowed system, but can be used effectively to help prevent the system from building up sulfides to the point that the water absorption is severely slowed and builds up in the aggregate. Preventative maintenance can help the system before major problems develop.