What is a Septic System and How Does it Work?
A septic tank system includes an underground tank and leach field A well designed and maintained concrete, fiberglass or plastic tank should last about 50 years. Because of corrosion problems, steel tanks may only last a decade or less. Most typical is a two compartment septic tank.
The size of the tank will vary depending upon local codes, but a typical tank for a family of four would have a liquid capacity of 1,500 gallons.
On the left is the input pipe from the dwelling, on the far right is the output pipe to the leach field. The tank itself is water tight and divided into two semi-compartments. This division allows for improved digestion of the waste materials. When the waste flows into the tank, the heavy solids (primarily feces) sink to -the bottom to form a layer of 'sludge'. Lighter materials (grease, fats, small food particles ' etc.) float on the surface forming a layer of "scum'. Between these two layers is a soup of suspended materials and water soluble chemicals (urea from urine and many household chemicals). The division into two chambers increases the efficiency of the system at removing suspended solids. The second compartment receives its "load", or liquid mixture, already substantially clarified (much of the solid material has settled out of the liquid). There is little turbulence in the second chamber either, because the load enters more slowly. Both of these factors allow settling of finer suspended solids than can occur in the first chamber where incoming material acts to churn up the chamber contents.
The process of digestion in the tank is carried out primarily by microbes excreted from our, gastrointestinal tracts (E. coli, for instance). Digestion is an anaerobic process, meaning -that oxygen is not required. Gases (hydrogen sulfide and methane) are produced and must be vented. Basically the same -thing that happens in a septic tank also occurs in our guts and in centralized waste water treatment plants, however a properly operating septic system is probably the most efficient of the three. In the septic system, the gases help to stir the sludge, scum and liquid layers which promotes further digestion of the solids. A properly functioning tank will convert the bulk of solids into liquid waste through the processes of digestion and hence, decomposition.
A septic system is well suited to breakdown human excrement, and a well designed, properly used and maintained system is one of the best choices for waste disposal in certain areas of the country. However, there are many potential problems with septic tanks. One of which is that people put a lot more than human waste down their drains. Even simple food items such as too much grease, cooking oil or fat may greatly reduce the efficiency of the system. Household cleaners, paints and other toxics are also toxic to the bacteria which make the system operate properly. Excesses of these chemicals may cause a severe disruption in the system.
Putting an excess load on the system when more people are in the house (flushing the toilet, taking showers and otherwise running more water into the system) then the system is designed for can result in materials moving through the system too quickly to be decomposed and contamination problems may result.
There are many considerations to be made before installing a septic tank system. In order- for it to function properly, it is important for the surrounding soil to have certain characteristics, the most important of which has to do with permeability. The water carrying capacity of the soil must be measured before a system can be approved for building and must be known before a proper system can be designed. Usually a percolation test is performed to determine the adequacy of the soil to support a septic system.
Another critical design consideration has to do with the height of the water table. The leach field must have a certain separation from the water table to prevent contamination from occurring. Likewise layers of impermeable "soil' must be a certain depth below the leach field.
There is an engineering modification known as the Wisconsin mound system which may allow the use of septic tank systems in areas previously considered to be unsuitable due to slowly permeable soils (percolation rates slower than 60 minutes per inch), thin soils over permeable bedrock, and permanent or periodically high groundwater tables. Basically the only differences are the addition of a pumping station to pump the tank effluent up to a leach field constructed in a mound on top of the natural soil surface. This system is definitely much more expensive than a traditional septic tank system, but offers a viable solution in regions where the soil characteristics preclude the use of a traditional system.
Because the proper functioning of the system is so heavily dependent upon the user, there is a tremendous problem particularly back East, with groundwater contamination as a result of inadequate design, use, and/or maintenance. This contamination is predominately microbiological. Microbes, both bacteria and viruses may remain viable much longer underground then when they are exposed to the elements. They are small enough that they may travel with the plume of percolating water from the leach field and contaminate drinking water sources, either groundwater or wells.
The average household of 3 uses 150,000 gallons of water per year, a family of five may use as much as an acre foot, or 325,900 gallons per year. Of this, approximately 1/2 is used indoors and thus goes down the drain into the septic system (the other- 1/2 is used out-of-doors). In other- words, between 200-400 gallons of water (plus wastes) goes into a family's septic tank DAILY. If the system is not properly designed to accommodate these flows, then the sewage will not be properly treated before flowing into the leach field. Problems will also arise if the leach field is in soils which can not absorb the level of flow generated, or- if the soil does not retain the liquid long enough for additional decomposition to occur.
The typical sources of waste water entering a septic system are toilets (approximately 38%), laundry (25%), showers/baths (22%) and sinks/other (15%). Therefore, the potential contaminants must all be introduced into the system from one of these sources. The principal contaminant-type of concern is microbiological (pathogenic bacteria and viruses).
Soils which are very permeable (have a rapid percolation rate),
also have a very small capacity to absorb effluent from the leach field and this capacity may be quickly exceeded if the system is not designed to take this into account. Not allowing for soils with little capacity to absorb moisture is a prime reason groundwater contamination occurs, because pollutants tend to move rapidly through the soil with little chance for decomposition.
The typical leach field will be perpetually wet (remember that several hundred gallons of liquid a day enter the tank and thus the field). This moisture encourages the growth of a "slime mat" composed of a variety of microscopic plants (algae) and animals (bacteria, etc.). This slime mat is the final clarifier of the waste water, pulling out left-over nutrients for their own use. They will also decompose, to varying degrees, certain synthetic organic chemicals such as some pesticides and solvents.
Many environmental factors (rainfall, soil moisture, temperature and pH, and availability of organic material in the soil) influence the movement and fate of microbes from the septic system through the soil to groundwater. Once out of the French drains in the leach field, pathogenic bacteria will have to compete for food with soil microbes and the microbes in the slime mat underlying the leach field.
Phosphorous, a contaminant introduced from many laundry detergents, typically is not a groundwater contamination problem because it is readily taken up by iron, aluminum and calcium naturally occurring in the soil. Urea is converted by the septic system flora into nitrite, nitrate and ammonium. Nitrate may be a groundwater contaminant particularly in soils which are very permeable. Nitrate moves readily through most soils dissolved in water.
Metals pose interesting problems. Possible contaminants include lead (from lead water pipes or lead solder- on water pipes), arsenic (found as a contaminant in phosphate detergents), iron, tin, zinc, copper and cadmium. They are not typically a concern in septic systems.
Movement of many organic contaminants such as solvents, cleaners, degreasers and pesticides, through soils is not well understood. There is certainly the possibility for organics, such as solvents, to move with water through the soil to groundwater. Also possible are adsorption onto soil, decomposition by soil microbes or uptake by microbes or plants. The environmental fate of most pesticides has been closely examined, but not from the prospective of subsurface introduction via a septic system.