The purpose of the tank is two fold, first as a filter and second as a treatment device. Most tanks are 1,000-2,000 gallons.

Material Transfer

Americans on average produce about 75 gallons per day of waste water. If you have a house with 4 people and a 1500 gallon tank, the water would stay in the tank for about 5 days. During these 5 days things happen.

First is settling. Every thing heavier than water sinks to the bottom and forms the sludge layer (cellular debris, cellulose, silica, borax). Everything lighter floats to the top (fats, oils, grease) and forms the scum layer. What remains in between is called effluent and is the only acceptable discharge into the drainfield. The tank is a series of baffles and sumps that ensure only effluent can pass. If too much solid waste is allowed to accumulate in the tank it begins to pass through to the drainfield, eventually clogging it to the point of failure.

The bacteria in the septic tank digest most of the solid waste that flows into the tank, and converts it into liquid and gas. Some solids can't be digested fast enough by the bacteria, and they begin to collect in the tank. As the tank begins to fill up with undigested solids it effectively becomes smaller. Because of reduced tank capacity the retention time is reduced, as a result the waste strength is increased. Increased waste strength over extended periods of time ALWAYS results in reduced system capacity and life.

Treated waste water exits the tank with vastly reduced waste strength (TSS, BOD, nitrate, sulfate, phosphates) and passes through an outlet pipe to a distribution box. The distribution box is small, (compared to the septic tank) often about 12"-18" across. It's purpose is to maintain equal distribution of waste water to the leach lines or pits (dry wells).

The drainfield is the area of ground that absorbs the treated waste water and may contain leach lines, pressure lines, dry wells, a soil mound, or a bed style absorption area.

Leach lines are trenches in the ground. They are usually 1-3 feet wide and 2-3 feet deep (deep trench leach lines can be 8 feet and deeper). Most leach line trenches are filled with gravel and are lined with perforated pipe that can conduct wastewater the length of the trench. The gravel is covered with filter-fabric to prevent it from being loaded up with soil back fill, usually the top 6-12 inches of the trench. Waste water soaks into the trench floor and sidewalls, and percolates down through the soil and eventually, joins with other ground water to complete the cycle. During this process it is filtered and purified.

Dry wells are very similar, but they are vertical bore holes, usually 3-4 feet in diameter and 25-45 feet deep.

This is merely a brief overview of the basic concepts of conventional gravity fed systems. More challenging site and elevation conditions give rise to more complex systems that use pumps, electronic timer controls, electric valves, and exotic filters.

However the basics still apply to all...filtration, waste strength reduction, and leaching into the ground.

• AGE
  25-35 years is the common life expectancy for most drainfields. It is possible for some drainfields to operate for many years beyond this point.
  
  
• LACK OF MAINTENANCE
  Any septic system that is not pumped enough will have premature failure. There are no exceptions. In addition the tank can be inspected during pumping. If you can catch a problem early it can be fixed cheap, if it goes unprepared for a long time, it can turn into a major problem, causing MISERY and costing many thousands.
  
  
• ROOTS
  Roots love septic systems. Some species of trees are especially troublesome. Fast growing trees are the worst. Do not underestimate the ability of roots to find and invade a septic system. Roots will follow pipes and can reach out well beyond the drip-line of the tree. They can crush and plug pipes, fill distribution boxes, choke septic tanks, shift and break critical parts of the septic system, dooming it to premature failure for years before there is any noticeable impact on the performance system. In septic, roots are the "silent killers".
  
  
• OVERLOAD
  If you have a three bedroom house where eight people are living, the septic system is probably overloaded. Increased tank pumping can moderate the effect of the high waste load, but the system is still undersized and is vulnerable to failure. You might want to think about it like towing a very heavy trailer with a small truck. You can get away with it for a while but if you do it constantly that little truck is going to break.
  
  Additionally, and not withstanding the organic wasteload, if there is too much water passing through the system it will fail. A leaky flapper valve in a toilet will pass an astonishing amount of water over the course of a few months-tens of thousands of gallons!
  
  Surge loads can be troublesome. Surface drainage/grading (either dumping run-off water on the tank or drainfield) and even doing all your laundry in one day can cause surge loads. Surges can flood septic tanks defeating their ability to retain solid waste, sending solids down the line to clog your drainfield.
  
  
• DRAINFIELD ABUSE
  Anything that you do that could compact the soil in the drainfield, should be avoided. For example, corraling horses or cattle, driving heavy vehicles, building structures, paving, etc (drywells and deep-trench are immune from being compacted).
  
  Unfortunately, and very often at the point of failure, irreversible damage to the drainfield has occurred. Don't let this happen to your system. If it's been more than 4 years since your last service please call.