Basic Soil Concepts

Approximately three and one-half million North Carolinians use on-site systems to treat and dispose of their household wastewater, according to the 1990 U.S. Census. These systems generally rely on a wastewater receiving tank, or septic tank, and a treatment and disposal field for proper treatment and disposal of the sewage. Most of the purification of the wastewater occurs in the soil beneath the drainfield.

Using a soil absorption system for disposal and treatment of waste􀀴ater taJc:es advantage of the physical, chemical, and biological processes in the soil. Soil can absorb and treat the wastewater and its constituents.

Not all soils and sites will adequately treat and dispose of wastewater. Soil and site evaluations are necessary in order to locate on-site systems on appropriate sites. This section presents soil concepts that are required to conduct a soil and site evaluation.

The Use of Soil for On-Site Systems

There are many methods to treat and dispose of wastewater. On-site systems usually rely on the soil because it provides an inexpensive and reliable medium for wastewater treatment and disposal. The porous nature of soil and the biological activity in the soil are key characteristics in absorbing and treating wastewater.

Because of large variations in soil characteristics, not all soils can suitably treat and dispose of the waste. The challenge for those involved with on-site systems is to design and install wastewater systems that optimize a soil's treatment potential. The following concepts should be considered when selecting sites for on-site systems.

• Processes that purify wastewater include physical filtration by the soil particles, chemical treatment through ion exchange and transformation in chemical reactions, biological oxidation and decomposition by micro-organisms, and uptaJc:e of nutrients by plants.

• Soils can vary greatly over short distances, even from one side of a lot to the other. Because of their spatial variability, understanding soil becomes critical in the selection and evaluation of the sites where on-site systems will be located.

• All soils are composed of mineral matter, organic matter, and voids or spaces that can be filled with either water or air. Soil water and soil air are inversely related to each other because water and air compete for the same void space in the soils ( see Figure 4.4.1).

• A winding flow path through soil voids that is neither too rapid nor too slow provides for maximum treatment of wastes by natural soil processes.

Figure 4.4.1 The relationship between soil solids, soil wetness, and void space.

From the North Carolina Onsite Guidance Manual

Bacteria, Viruses, and Protozoa in On-Site Systems

Bacteria, viruses, and protozoa cause many human and animal diseases. Bacteria cause cholera, shigellosis, salmonella, and typhoid. Giardia and cryptosporidium are protozoa that cause dysentery. Hepatitis is caused by a viral contaminant. The comments listed below give some information about how biological contaminants are removed by on-site systems.

• Bacteria are removed primarily by filtration, adsorption, and natural die-off. The biomat provides a barrier to the transport of many bacteria into the soil.

  • If the soil is unsaturated, bacteria are not usually transported more than three feet. If, however, saturated flow occuI'$, bacteria can move further.

  • Saturated flow, high wastewater effluent loading rates, shallow depth to soil wetness conditions, or fractured bedrock contribute to bacterial con­tamination from on-site systems.

• Viruses, which are much smaller than bacteria, are removed by adsorption, filtration, precipitation, biological enzyme attack, and natural die-off.

  • Greater clay content, low soil pH, low soil moisture content, and low effluent loading rates are important factors that decrease the possibility of viral contamination to the ground water from on-site systems.

• Wastewater also contains other micro-organisms, such as protozoa, which can cause disease. Unfortunately, with the exception of bacteria and viruses, little is known about the behavior of pathogens in on-site systems and in the soil. Because there have been few reported outbreaks of disease caused by microbes other than bacteria and viruses from subsurface wastewater disposal systems, it appears that these biologic agents are retained in the soil, probably because of their relatively large size.

• Protozoa form cysts that can survive under a wide range of conditions and are very resistant to disinfectants usually employed in drinking water treatment. Protozoan cells and cysts are generally much larger than bacteria, which may mean that they can be filtered by the soil. Filtration by the soil of protozoan cysts has been shown in the case of Giardia lamblia cysts, as reported by Yates in 1987.

From the North Carolina Onsite Guidance Manual

Other Chemical Pollutants in On-Site Systems

Wastewater from various facilities can contain a wide variety of contaminants. Common domestic sewage has at least minor levels of the pollutants discussed below. Contaminants present in wastewater from other facilities, such as commer­cial and industrial establishments, vary widely depending on the type of activities taking place in the facility generating the wastewater.

• Chloride and sulfate occur commonly in wastewater. Both of these chemicals move readily through the soil. Soils cannot adsorb chloride and sulfate anions because these chemicals are repelled from the negative surface charge of the soils. Because chemicals such as chloride and sulfate leach into the ground water the primary mechanism for reduction is dilution. If the concentration of these chemicals in ground water is too high, then the density of on-site systems in an area may be restricted.

• Sodium cations are adsorbed to the soil aggregates, which are held together by organic matter and clay. If sodium levels in wastewater are too high, the sodium may disperse the organic matter and clay in the soil. Such soil dispersion changes the soil structure and reduces the rate of water movement through the soil, which can cause failure of an on-site system.

• Detergent surfactants are removed from wastewater effluent by adsorption to soil particles and by biodegradation. Aerated soil conditions enhance biodegra­dation and increase the treatment of surfactants. Adsorption of surfactants not only removes them from the wastewater, but it also increases the time for additional biodegradation to occur.

• Toxic organic compounds, such as pesticides and nonbiodegradable organic compounds, degrade slowly. Since these compounds usually are not adsorbed by the soil, they may leach and contaminate the ground water. The best way to minimize the impact of these chemicals is to keep them out of the on-site system.

• Heavy metals in high concentrations are usually toxic. Such metals can slow or stop the bacterial action in the septic tank and in the treatment and disposal trench. These metals should not be put into the on-site system.

From the North Carolina Onsite Guidance Manual

Phosphorous in On-Site Systems

Phosphorus can enter a wastewater system in a variety of forms. Organic and synthetic phosphorus are transformed by bacteria to the simple orthophosphate form. Because excess phosphorus can stimulate eutrophication, the excessive growth of algae and aquatic plants in streams, rivers, and lakes, it is important that phosphorus not enter water bodies in high concentration. Fortunately, orthophos­phate usually is immobilized by a number of processes in the soil.

• According to research by Bicki et al. (1985), if soil conditions below the on­site system treatment and disposal trenches are aerobic and unsaturated, phospho­rus concentrations can be reduced by 85% to 95%.

Phosphate immobilization processes in the soil include adsorbtion to the soil particles or biomat, precipitation in the soil (Figure 4.1.3), or biological uptake.

Phosphorus fixation in the soil

From the North Carolina Onsite Guidance Manual

Nitrogen in On-Site Systems

Nitrogen enters domestic on-site systems mainly as organic nitrogen, which means the nitrogen is part of a large biological molecule such as a protein. Bacteria and other microbes oxidize or mineralize the organic nitrogen to ammonium forms. The ammonium can be volatilized to the atmosphere, used by bacteria and plants, or adsorbed by the biomat or soil. Ammonium can also be converted under aerobic conditions to nitrate and nitrite in soils by Nitrosomonas and Nitrobacter bacteria. The nitrate form of nitrogen can be used by bacteria or plants. Under anaerobic conditions, nitrate can be transformed to nitrogen gas, a process known as denitrification. Figure 4.1.2 demonstrates the nitrogen cycle: gains and losses of nitrogen in the atmosphere and soil.

• Because nitrate is very soluble and is not absorbed by soil, it can move through the soil into the ground water and adjoining surface waters.

• Hthere are too many on-site systems in one area, nitrate levels in ground water may exceed the U.S. Environmental Protection Agency's Maximum Contaminate Level for nitrogen of 10 milligrams/liter (mg/I). Nitrogen levels above 10 mg/I may cause sickness or death to small babies and at higher levels can be harmful to adults.

• Denitrification is most likely to occur in anaerobic zones, such as wet soils, or as shallow ground water moves through riparian areas next to streams. However, denitrification is limited in a properly sited system because the aerobic soil conditions will not allow denitrification to occur.

• Research by Bicki et al. (1985) shows that of the total nitrogen produced from on-site systems, only 20% to 40% is adsorbed or removed during flow through unsaturated soils. Therefore, dilution and denitrification are the mechanisms that must be relied upon to reduce ground water nitrate concentrations.

Figure 4.1.2 The Nitrogen Cycle In Soll. (Sauchelli, Fertilizer Nitrogen -Its Chemistry and Technology, 1964. Reprinted with permission of Van Nostrand Reinhold Publishing Company, New York, NY.)

From the North Carolina Onsite Guidance Manual

Wastewater Movement Through the Soil

The rate of wastewater flow through the soil is critical to the ability of a soil to treat wastewater. If wastewater moves too rapidly through the soil, the chemical, physical, and biological reactions that must occur to retard, reduce, and transform the pollutants are impeded. Any soil condition that causes an increased rate of flow, such as a high water table, high hydraulic loadings, or shallow depth to seasonally high water tables, can potentially cause contamination of ground water because the wastewater has not been adequately treated.

Water moves though unsaturated soil more slowly than through saturated soil. The slower movement under unsaturated conditions provides more treatment and more protection of ground water than can be obtained in saturated soil.

In North Carolina, for soils in Groups II, III, and IV, 12 inches of unsaturated soil between the bottom of the trench and any soil wetness condition, ground water, or other unsuitable soil condition is required for proper treatment of wastewater.

• For Group I soils (sandy soils), an 18-inch separation distance is required between the trench bottom and any soil wetness conditions or the ground water. Because the rate of wastewater movement through Group I soils is faster than Group II, III, or IV soils, a greater separation distance is needed to properly treat the wastewater before it enters the ground water.

Reference

15A NCAC 18A.1955(m)

From the North Carolina Onsite Guidance Manual

Wastewater Treatment in Soils

The purpose of wastewater treatment is to reduce the pollutants in wastewater that can contaminate ground and surface water systems. Without proper treatment, wastewater can cause public health problems because of the potential spread of bacteria and viruses and can cause environmental degradation and contamination. Wastewater contains bacteria, viruses, nitrogen compounds, and toxic organic compounds which can cause disease in humans. Chemical constituents in wastewater that can adversely affect the environment are oxygen-demanding substances, nitrogen, phosphorus, chloride, sulfate, sodium, heavy metals, toxic organic compounds, detergent surfactants, and suspended solids.

In most on-site systems, the soil is used to treat and dispose of wastewater. Depending on a number of factors, the soil can remove or reduce these pollutants. This section describes the process of wastewater treatment in soils. The health and environmental impacts of the constituents that are released from on-site systems are also discussed in this section.

Treatment Processes In the Soil

Most of the organic solids in domestic sewage are removed by settling that takes place in the septic tank. Some of the solids will partially biodegrade in the tank. Wastewater that leaves the septic tank and enters the soil receives most of its treatment in the unsaturated aerobic regions under the treatment and disposal field.

Wastewater treatment in the soil can be broken down into three different types of processes: physica􀀂 chemica􀀄 and biological. Table 4.1.1 describes these processes.

• Physical processes include soil filtration, sedimentation in the soil profile, dispersion, and dilution.

• Chemical processes involve cation exchange, adsorption, organic residue complex formation, and precipitation.

• Biological processes consist of biological oxidation, nitrification, denitrifi­cation and plant uptake, inactivation, immobilization, and predation.

All of these processes may occur independently or together for any given wastewater constituent.

Aerobic Treatment

The most rapid treatment of wastewater occurs in an aerobic soil environment, where oxygen is present in the soil. Oxygen allows aerobic bacteria and other microorganisms to feed on the wastewater and break down the contaminants to simpler and less-harmful products. Because oxygen is a very powerful chemical, aerobic degradation proceeds much faster than similar anaerobic processes that occur in the absence of oxygen. The faster aerobic processes help increase the amount of treatment the wastewater receives before the wastewater enters ground or surface water.

• Aerobic conditions promote rapid die-off of some pathogenic, or disease­causing, bacteria that require anaerobic conditions to live.

• Additionally, oxygen in the soil favors the growth of aerobic bacteria and micro­organisms over the anaerobic organisms. In some instances, aerobic organisms may feed on the anaerobic populations, further reducing pathogen numbers.

Wastewater Treatment Processes in Soils

Biomat

A significant degree of treatment occurs at the biomat in the treatment and disposal trenches. The biomat is a biologically active layer that covers the bottom and sides of the trenches. It is formed from complex bacterial polysaccharides and accumu­lated organic substances as a result of wastewater moving through the trench into the surrounding soil. A biomat is vital in obtaining a high degree of treatment of the wastewater and to preventing pollution of ground water. However, biomats slow the flow of wastewater into the soil and may even clog the soil surface so that the trench can no longer absorb any wastewater. Figure 4.1.1 shows biomat formation.

Figure 4.1.1 Blomat formation over time. Blomat forms first at beginning of trench and progresses along the entire trench over time.

Biomat Formation Over Time

The following process describes biomat formation over time.

• Initially, when the soil absorption system begins operation, there is no biomat present in the trench. The rate of wastewater flow out of the trench is determined by the soil.

• As the on-site system is used, the biomat first forms in the trenches where the wastewater enters. Over time the biomat progresses down the trench to eventually cover the entire length of the trench.

• The formation of the biomat begins immediately but talces from three to eight years to form completely. The amount of time that it takes to form the biomat depends on a number of factors, including the hydraulic loading rate of the trench, the dosing schedule, the types of substances in the wastewater, and the tempera­ture. The upper portion of the biomat is anaerobic; the lower portion of the biomat grades from anaerobic at its top to aerobic at its bottom because the soil conditions below are aerobic.

• Once formed, the biomat physically, biologically, and chemically removes or reduces many wastewater constituents. Further, it limits the rate that water can move into the soil, helping in the purification process because the microbes have more time to treat the sewage.

• Because the biomat limits flow from the system into the soil, the amount of wastewater loaded into the system can be no greater than the amount of wastewater that can move through the biomat into the soil. When the amount of wastewater entering the system is the same or less than the amount of wastewater exiting through the biomat and into the soil, the on-site system is considered to be at equilibrium.

• A mature biomat is generally in equilibrium: the addition of organic matter to the mat occurs at the same rate as degradation of the organic matter by soil organisms.

From the North Carolina Onsite Guidance Manual

Site Evaluation and Field Testing

Introduction

Finding a suitable site and soil is essential to the placement and proper functioning of any on-site system. This chapter is designed to provide informa­tion on determining the proper site and soil for the placement of on-site systems and bas been written specifically for the use of environmental health specialists as a training guide.

• Section 4.1, Wastewater Treatment in Soils, describes the constituents of wastewater, the chemicals and human pathogens, and how these potentially harmful constituents are treated and absorbed by the soil.

• Section 4.2, Ground Water, presents an overview of ground water and how ground water can be affected by improperly sited or malfunctioning on-site systems.

• Section 4.3,SoilsandGeologyo/North Caroli.a, introduces the reader to the different soils and geology of North Carolina and describes how they affect the siting and functioning of on-site systems.

• Section 4.4, Basic Soil Concepts, presents soil and landscape position con­cepts necessary to conduct a site and soil evaluation. This section then relates these concepts to the placement and functioning of on-site system.

• Section 4.5, Site and SoilE11aluation Procedures, provides details on how to make a thorough site and soil evaluation. Site evaluation factors and classifica­tions are discussed in detail and the rules for determining the placement of an on­site system are described.

• Section 4.6, On-Site Wastewater Loading Rates, discusses the importance of calculating the proper on-site wastewater loading rates and then instructs the reader on making the calculations for both conventional, modified, and alternative on-site systems.

• Section 4. 7, Site Suitability: Matching the Site Characteristics to Appropri­ate Designs, introduces the reader to the Soil Site Evaluation for On-Site Wastewater System form used to permit on-site systems. Six Soil Site Evaluation for On-Site Wastewater System forms, which have been completed, are included to help the reader understand how the forms are used to determine site and soil StJitability for on-site systems.

From the North Carolina Onsite Guidance Manual

Principles of On-Site Wastewater Treatment and Disposal

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

Science of On-Site Wastewater Treatment and Disposal

During the early years of this century, on-site wastewater management was basically a trial-and-error process. Systems that failed were not of concern because on-site wastewater disposal was usually only used in rural areas with sparse population. There was little need for detailed knowledge; therefore, on-site wastewater treatment and disposal got little attention.

As rural electrification enabled more people to install indoor plumbing and as rural and suburban populations grew, a greater need arose for concise information about proper installation and operation of on-site systems. Almost 70 years have passed since Henry Ryon first suggested that the performance of an on-site system depended on the percolation rate of the soil. Since that time, many studies have been conducted on on-site system requirements and performance.

This section presents the important principles and guidelines learned about on-site wastewater treatment and disposal.

From the North Carolina Onsite Guidance Manual

History of On-Site Wastewater Management

The use of septic tanks to treat wastewater goes back to the middle of the nineteenth century. Frenchman J .L. Mouras first made a masonry tank to receive wastewater from a home in the town ofVesoul, France. After twelve years of operation, the tank was found to have only a small amount of solids in it. Mouras had expected that the tank would be very full, so he concluded that some process must be taking place that reduced the volume of solids. He and A. Moigno, a priest and scientist, experimented with the tank to learn more about processes taking place in the tank. Mouras patented the tank in 1881.

Use of septic tanks in the United States began about 1883 in Boston, Massachu­setts. There, Edward S. Philbrick designed a two-chamber, round, vertical­cylindrical tank with a dosing siphon.

Although these early developments showed promise, on-site wastewater disposal remained at a crude level well into the twentieth century in both Europe and the US. During the early part of this century, city-dwellers were served by large central collection systems and had no need for on-site wastewater disposal. Rural dwellers relied on privies and other simple waste disposal means because few farms had indoor plumbing.

Since the first quarter of this century, most development work on improving on­site systems has been done in the US. By the middle of the 1920s, Henry Ryon of the New York State Department of Health began to study methods to improve on­site system performance. He realized that the most critical part of the system is the treatment and disposal field. To help ensure adequate soil absorption, he developed the percolation test. This test has been widely used to help determine the level of soil absorption possible for an on-site system, although it has been more recently shown to provide inconsistent and unrealistic information.

The next big effort to improve on-site wastewater management occurred in the late 1940s. Until that time, only the percolation test and a few guidelines were used to determine soil and site suitability for on-site system installation. Rural electrification gave farm families indoor plumbing and the opportunity to install on-site wastewater disposal systems. Soldiers returning from World War II spawned a housing boom in suburban areas where on-site systems were the only choice for wastewater disposal. However, because of the lack of knowledge of on­site system operation, failures were common. The explosion in housing growth and the growing threat to public health brought about the first study of on-site systems by the US Public Health Service in 1946.

Since that landmark study, many studies have been conducted on conventional, modified conventional, alternative, innovative, and experimental on-site systems. The research has pointed out that the most critical part of the conventional on-site system is the treatment and disposal field. We now have better ways to determine the suitability of a site for an on-site system, and we know more about improving the performance of on-site systems. The next section presents some of the findings from the research done on on-site systems.

From the North Carolina Onsite Guidance Manual

Public Health Threats

Much of today's public health knowledge regarding on-site systems was obtained during the early part of this century. Until that time, many outbreaks of contagious diseases occurred because sources of disease ( drinking or coming into contact with contaminated water) were not yet known or understood. These contagious diseases are called water-borne diseases because they are spread by contaminated water. Other diseases were found to result when people came into contact with improperly disposed human wastes.

A basic principle learned in those early years was that to improve overall public health, sources of disease must be kept away from human contact. On-site systems use this principle by carrying human wastes deep into the soil and letting the soil absorb the wastewater so that the disease-causing organisms are in the soil, separated from humans. If on-site systems malfunction, the improperly treated and disposed wastewater becomes a potential source of disease and a genuine public health threat when humans come into contact with it.

Diseases carried in wastewater.

Improper disposal of human waste creates ideal conditions for outbreaks of many contagious diseases. Water-borne diseases include typhoid fever, cholera, dysen­tery, hepatitis, giardiasis, cryptosporidosis, hookworm, tapeworm, and other diseases that have plagued humankind since ancient times. Because we have developed proper means to treat and dispose of human wastes and wastewater, these previously common diseases no longer present a major problem.

How disease is spread.

From the public health point of view, there are two very dangerous types of on­site system failure. The first occurs when the wastewater does not infiltrate the ground. Instead, the wastewater ponds, or comes to the land's surface and forms a small pool or wet, mushy area. This unabsorbed wastewater may contain many disease-causing bacteria, viruses, and parasites.

There are three ways that humans could become sick when wastewater ponds in a treatment and disposal field.

1. Humans can come into contact with the pooled wastewater. Children are most likely to play in the pools or wet soil, but adults may have to walk through or work in the area. Once the wastewater is on the person's hands or body, the germs can spread to their mouth or nose where they are swallowed or inhaled.

2. Humans can drink contaminated water. Failing on-site systems can pollute wells, streams, rivers, and lakes, which may be used as water supplies. Wastewater from the pool can flow into a nearby stream and thereby contaminate the water bodies downstream.

3. Disease germs can be spread by insects or other animals to human food or drinking water. The animals that spread disease germs are called vectors. One of the best known vectors is the common housefly. It spreads disease by landing in or drinking pooled wastewater and then landing on food that humans later eat. The germs on the fly are then eaten by the humans.

A second type of on-site system failure occurs when an on-site system pollutes a well. This type of failure happens when the well is not properly constructed. In some cases, the on-site system may not be pooled wastewater on the surface, but the wastewater flows through cracks in the soil or underlying rock into the well.

If wastewater enters the well, people get sick by drinking water from the well. The on-site system or the well or both may have to be moved or rebuilt to ensure a clean supply of water.

Toxic chemicals.

Wastewater not only carries many diseases, it also contains chemicals that can cause poor health, cancer, or death. A range of information exists about the different chemicals found in the effluent from on-site systems.

Nitrate. One chemical, nitrate, has long been known to affect health, and we know a great deal about its origin and health effects. Nitrogen in wastewater is converted to nitrate by the bacteria in the septic tank and field during decomposition. The nitrate moves rapidly with the wastewater through the soil. If the wastewater gets into a drinking water well, the nitrate can be drunk by the site's residents or neighbors.

• Infants younger than six. months are most susceptible to nitrate poisoning. Bacteria that live in the digestive tracts of newborn babies convert nitrate to nitrite. Nitrite then reacts with hemoglobin, which carries oxygen in blood, to form methemoglobin. Methemoglobin cannot carry oxygen, thus'the affected baby suffers oxygen deficiency. The resulting condition is referred to as methemoglobinemia, or "blue baby syndrome." Most reported cases of blue baby syndrome due to contaminated water have ocurred with greater than 40 mg/I nitrate-nitrogen.

• The US EPA standard for nitrate is 10 mg/I as nitrogen in drinking water.

Artificial chemicals. Many artificial chemicals can be found in on-site wastewa­ter effluent. Many chemicals are not "biodegradable" or broken down by bacteria in the septic tank and field. Because these chemicals are not broken down, they can flow into ground water or surface water and eventually into drinking water.

• Because of the wide variety of artificial chemicals, it is difficult to say what types of problems the chemicals may cause. Some chemicals are toxic or poisonous, while others may cause cancer or other diseases.

Wastewater must be absorbed into the soil and have adequate contact time in the soil so that it does not spread disease germs or toxic chemicals. When the wastewater is held in the soil, and the soil is suitable for on-site wastewater treatment, neither humans nor animals can come into contact with it and it will not pollute streams or ground water. Thus, a properly operating on-site system protects public health.

On-site wastewater disposal can pose a threat to the environment. Presently, more than 50% of North Carolina housing units, representing about 3.5 million people, depend upon on-site wastewater disposal. Based on these figures, on-site systems distribute 360 million gallons of wastewater to the environment each day.

The large volume of wastewater being discharged into the environment can cause damage to both surface and ground water. Damage is caused by the way the wastewater is discharged in the environment and the type and amount of pollutants in the water.

Nonpoint source pollution.

Pollution from on-site systems is categorized as nonpoint source pollution. Nonpoint source pollution comes from activities that are spread over large areas ofland. Point source pollution, on the other hand, comes from a single point such as a pipe discharging industrial waste from a large factory.

Most nonpoint source pollution results from common activities and from land use. Examples of other nonpoint source pollution sources are:

• fertilizer and pesticides from farming;

• oil, grease and toxic metals from parking lots, roads, and automobiles;

• sediment from bare land, construction sites, and newly developed areas; and

• industrial and commercial chemicals from spills and leaks at industrial sites and commercial zones.

If on-site systems malfunction, the wastewater can contribute significant quanti­ties of raw sewage and bacteria to surface and ground water. In addition, the wastewater from on-site systems contains certain pollutants, such as nitrate and phosphorus, which are not biodegradable in the on-site system but act as pollutants in water bodies. Thus, even systems that appear to be functioning properly can contribute to pollution of streams, lakes, marshes, or ground water.

Environmental impacts.

Pollutants in the wastewater affect animals, plants, and their habitats. Because on­site systems continually contribute huge amounts of wastewater to the environ­ment, the long-term effects can be very serious. Some environmental impacts of wastewater are discussed below.

• Nitrate and phosphorus from on-site systems can cause eutrophication, an overgrowth of algae, plants, and bacteria in water bodies. Often, eutrophication appears as algae blooms in streams, rivers, estuaries, or marshes, and even sounds and bays on the ocean. Overgrowth causes fish kills and ruins the habitat for many types of plants and animals. Eutrophication occurs more often where the water moves slowly, such as in lakes, bays, and slow-moving rivers.

• In some areas, on-site systems are blamed for destroying shellfishing by releasing bacteria into the receiving water.

• Wastewater from on-site systems can cause certain parts of a stream to become anaerobic, which means there is no oxygen in the water. Pollutants in the on-site wastewater serve as food for bacteria and certain types of anaerobic animals. The bacteria and animals grow very rapidly and use up all the oxygen in the stream. This lack of oxygen suffocates fish, other animals,· and plants.

• Toxic and synthetic chemicals from on-site systems can enter the shallow ground water. This happens most often where the soil is sandy or the water table is very high. Under these conditions, the wastewater does not receive adequate treatment to biodegrade the pollutants in the wastewater.

Because so many on-site systems discharge so much wastewater, all on-site systems must be installed so that the wastewater receives the best treatment possible to protect the environment.

From the North Carolina Onsite Guidance Manual

Impact of On-Site Wastewater Pollution

Each year over 40,000 new on-site systems are installed in North Carolina for new housing, commercial, and industrial development, adding to the existing 1,440,000 on-site systems already in use in North Carolina. These systems contribute over 360 million gallons of wastewater to the environment every day.

Properly designed and located, on-site systems can be a permanent means of wastewater disposal that protects public health and has minimal effect on the environment. Most on-site systems function satisfactorily; however, a significant number of systems fail to perform as designed, and pressure is increasing to install on-site systems on unsuitable sites. The following statistics indicate the scope of the problem of failing on-site systems in North Carolina.

• At the time of this publication, approximately 12,600 systems in North Carolina are repaired each year because of failure. Additionally, about 11 % of improvement permit applications are denied, mainly because selected sites are unsuitable.

• Public health agencies process 62,000 applications per year for site evalua­tions for both new systems and the repair of failing systems. Environmental health professionals perform 136,000 consultative site visits per year, often to inspect a failing on-site system.

These failing on-site systems and unsuitable sites are major concerns.

From the North Carolina Onsite Guidance Manual