A number of principles form the basis for on-site systems. These principles come from the many studies to determine the best ways to provide safe and reliable wastewater treatment and disposal.
Treatment and absorption of wastewater by soil.
The vast majority of on-site treatment and disposal systems depend on the soil for treatment and disposal of sewage. Although some on-site systems use surface discharge or land application to dispose of wastewater, such systems are relatively fe-:,v in number.
The research conducted over the last 40 years has shown that the treatment and disposal field is the most critical part of an on-site system.
Devices that receive sewage upstream of the treatment and disposal field pretreat the sewage to prevent clogging of the treatment and disposal field.
The focus of on-site wastewater treatment and disposal, and the principles listed here, concentrate on the treatment and disposal field. The principles and details are listed below.
First principle. On-site systems should ensure that the effluent is absorbed by the soil and does not come to the land surface or flow directly into streams, rivers, lakes, the ocean, or the ground waters.
Sewage carries many disease-causing bacteria or germs. As long as the sewage effluent stays in the soil, people are protected because the bacteria and viruses stay in the soil where there is no contact with humans. However, if the effluent comes to the ground surface, children and adults can pick up the bacteria and become ill or die. On-site systems that fail cause effluent to puddle or pool on the ground, which is dangerous to public health.
On-site systems not only dispose of sewage, but also treat the sewage to remove bacteria, other disease-causing organisms, and pollutants. The treatment of the wastewater takes place in the soil, so the wastewater must stay in the soil for the pollutants to be removed.
Second principle. On-site systems should maximize the aerobic treatment of the sewage.
Sewage undergoes aerobic treatment in soil layers that are not saturated with water. These soil layers are called the unsaturated zone or vadose zone because the soil is dry or damp but not completely wet. The unsaturated zone is aerobic because air and oxygen enter and help to remove bacteria and pollutants form the sewage.
Aerobic treatment is the fastest and most complete treatment the effluent can receive in the soil.
On-site systems should be located where the effluent must travel the farthest distance possible before getting to the water table or wet soil layers. Long travel distance helps prevent pollution of ground water.
Third principle. On-site systems should apply effluent to the soil only in a suitable and prepared treatment and disposal field.
A treatment and disposal field is an area of land where effluent flows through pipes with holes into specially prepared trenches or beds to be absorbed by the soil. The treatment and disposal field is where the main treatment of the effluent takes place and where all the liquid effluent is absorbed.’
Only certain soils and certain locations should be used as treatment and disposal fields. These areas are selected by environmental health specialists to provide the safest and most reliable place to absorb liquid effluent.
Septic tanks, pump tanks, or piping in areas other than the treatment and disposal field should not leak. Effluent leaks in areas outside the treatment and disposal field can and have resulted in contamination of ground water, wells, the land surface, and surface waters.
Fourth principle. Treatment and disposal field trenches should be as long and narrow as possible to maximize the effluent's contact with the soil, which increases treatment.
Short and wide field trenches may have the same amount of trench bottom area as a long, narrow trench, but the long, narrow trench has much more side wall area that can absorb effluent and spread the effluent out over more land.
Fifth principle. Treatment and disposal field trenches should have level bottoms and should be level along their entire length to distribute the effluent as evenly as possible.
Field trenches with slanted bottoms or trenches that slope along their length will make the effluent flow to the lowest area. All treatment and disposal of the effluent will have to take place in that one low area, which can cause early failure of the field and threaten public health if the effluent ponds on the land surface.
These five principles are the most important concepts in on-site wastewater treatment and disposal. The design and installation of all on-site systems should be guided by these principles.
Pre-treatment of sewage be/ ore soil absorption.
To protect the treatment and disposal field from clogging, some pre-treatment is necessary. The conventional on-site system uses a septic tank to pre-treat sewage before it flows to the field. Septic tanks operate on the following principles.
Septic tanks: First principle. Septic tanks remove solids suspended in sewage. The large volume of the septic tank slows the wastewater so that heavy solids can settle to the bottom and buoyant materials, such as oil and grease, can float to the top. Heavy solids form a layer of sludge on the bottom of the tank, while the oil and grease make a scum layer that floats on the wastewater. Various types of baffles, such as walls and outlet tees, are used to keep the settled and floating solids from moving out to the treatment and disposal field.
A septic tank that is working well removes about half of the pollutants in the sewage by either letting them settle out or float to the surface of the wastewater.
Septic tanks: Second principle. The second important function of the septic tank is to store solids. Because the solids are stored in the large volume of the septic tank, the tank has to be pumped out only every few years. The tank must be large enough to store the solids and still allow additional solids to settle out.
Septic tanks: Third principle. Some of the solids in the septic tank are digested by bacteria in the tank. Certain bacteria, called anaerobes because they live in areas where there is no oxygen, eat the sewage and produce various gases.
Considerable difference of opinion exists on how much digestion of solids takes place. Regardless of how much digestion occurs, beneficial effects of digestion are that the sludge volume and the strength of the wastewater are reduced by the bacteria. However, the gas produced by the bacteria rises through the wastewater and causes the sludge to be stirred up and possibly flow out to the treatment and disposal field. Gases produced by the bacteria are poisonous and can bum or explode, making the air inside a septic tank very dangerous. They are also highly corrosive and can deteriorate the tank and outlet tees.
Improving septic tank performance.
The following points can help make septic tanks work better.
To get the best settling, septic tanks should be much longer than they are wide. A longer length allows the water to flow along a long path, leaving plenty of time for the solids to settle. The tank should be at least twice as long as it is wide.
Shallow, flat tanks allow for better settling than deep and narrow tanks. Solids settle out faster in a shallow tank than in a deep tank.
Larger septic tanks work better than small tanks because they hold the wastewater longer for better settling and have more storage volume for sludge and scum.
Septic tanks with more compartments work better than septic tanks with one compartment, because more solids are trapped in the compartments.
Properly designed baffle walls keep the in-flowing sewage from stirring up the sludge and carrying solids out to the treatment and disposal field.
For best performance, the inlet and outlet of the septic tank must be separated by a long flow path for the wastewater. If the inlet and outlet are too close, the wastewater flows rapidly to the outlet before the solids can settle and the grease can separate from the water.
Outlets work best if they have a fitting to keep the scum from flowing out into the treatment and disposal field.
From the North Carolina Onsite Guidance Manual