Prolonging the Life of on-Site Sewage Treatment Systems
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David Gustafson, PE
University of Minnesota
Civil Engineering Dept.
St. Paul, Minnesota
Every on-site system is made up of three parts. Each of these affects system performance, and each effect is cumulative. Performance of the tank, for example, is influenced by the performance of the plumbing. Performance of the soil treatment system is influenced by both tank and plumbing performances.
The plumbing portion includes all the water-using devices in the structure. This is the part of the system that starts the process and keeps us all busy. Each of these components impacts how the system will perform. The users of the system, the people operating and maintaining the plumbing system, are a critical key to system life, and they must be aware of their importance.
How water is used in the structure affects the entire system. Each system has been designed to handle a certain rate of flow, and systems will fail if overloaded. Periodic overloading definitely lessens the life expectancy of the system. Minimizing water use with low-flow fixtures will lengthen the life of the system by asking it to deal with less waste.
The overloading can also be caused by the organic strength of the waste. High BOD (>175 mg/l) out of the septic tank will cause problems with system downstream performance and is the result of problems earlier in the flow pattern.
Improper use, such as the disposal of hazardous waste or medicine can rack the system and be chronic problems for any system. The only treatment for these systems is the removal from the system all together. They should never be put into the system.
If the plumbing is not maintained, it can quickly lead to problems in the rest of the system. Leaky fixtures or clean water sources create problems all through the system.
the tank is the start of the treatment system. The three general categories of tanks are septic, aerobic and holding. Each has its own life expectancy and problems. The job of each is similar: to collect the solids and prepare the effluent for its final resting-place, which may be a typical soil treatment area, or something more exotic, such as a sand filter or constructed wetland. In any case, the separation of the chunks from the water begins in the tank.
A septic tank is the most common tank choice, because of the cost-effectiveness of the anaerobic treatment, and the ease of operation. The moving parts are none, and as long as it is sized properly (>36 hours detention time), the natural forces of gravity are an effective treatment mechanism. Appropriate maintenance and proper construction techniques govern the performance.
The material choices are typical concrete or plastic. The life expectancy of either one, starting with a good tank, is essentially equal. The importance of a good quality tank, though, is critical. If the concrete is not mixed properly, or if the tank is not properly cured (a “green” tank) the life of the tank can be seriously impacted. Certain concrete tank designs, too, can lead to trouble, for example two-piece tanks, which are likely to leak. Great care must be taken to assure that the tank is watertight and will stay that way for the long haul.
Plastic tanks have two potential problems. The first is they may not be strong enough for every application; in particular, they may collapse under the weight of the soil above them. The potential for collapse must be identified and addressed in the selection of the tank. The second is that they are lightweight, and should never be full of wastewater to weigh them down, so they may begin to float in conditions of high groundwater. Again, this potential problem must be addressed during the design and selection process.
The tank must be large enough for separation to take place. the amount of water used, the maximum flow and average daily flow, are used to calculate tank size. The presence of a pump in the basement or a garbage disposal may require additional capacity for proper tank performance.
The maintenance of the tanks is also critical for long system life. The frequency recommended for maintenance varies widely, but the one thing all experts agree on is that the tank must be pumped. So a tank can work forever, as long as solids are periodically removed. The actual time frame for each system will be different. A rule of thumb is once every three years without a garbage disposal, once every 18 months with a garbage disposal. The following chart is a user-friendly way to look at use and its impact on maintenance.
Most aerobic tanks are made out of plastic so the life of the tank is similar to a septic tank. The concerns for plastic septic tanks are also concerns for plastic aerobic tanks: strength and weight.
The life expectancy of the aeration equipment is the key to the life of the aerobic tank. Without the aeration system, the aerobic tank is just a poorly designed septic tank. This equipment must be in proper working condition: the air producer (blower or compressor) and the air deliveries (coarse or fine bubble diffusers) must all be operating properly. The typical life of the air-producing devices would be in the 3-7 year range. That does not mean that the system need only be checked once every three years, but gives a picture of replacement costs. The air diffuser life expectancy is much less consistent. The variables include waste strength, delivery pressure, and diffuser design.
The maintenance needs are the same, but the requirement that the aeration equipment be always operational means that the system may need to be inspected on an more frequent basis. It would be easy for the owner to turn off the air source so that the system did not operate as efficiently or at all.
A holding tank’s life can be long or short. With proper use, this system may last indefinitely. The problem is that with the purchase of sewage pump and illegal land application, the system can quickly short-circuited. The life expectancy is therefore related to the regulation not the system’s competent life.
3. Lift Station
The failures of the lift station (pump system) are typically related to life expectancy of pumps and controls. This portion of the system has been viewed as the weak link in the system because of the problems with breakdowns. The industry has made great strides in design and manufacture of lift station components, and life expectancy has risen dramatically. The potential for the floats or the pump to go bad is always present, but typically early failure here is typically related to poor choice of supplier, or to misapplication of a pump type.
4. Soil Treatment System Failures
The upstream portions of the system will directly affect the life of the soil system. Soil receiving typical waste from a septic tank has a design life of somewhere between 7 and 27 years at design loading rates. If the system is dosed with less water, it may last longer. If it’s absurd by more or dirty water, its lifespan will be significantly shorter.
There are three major types of soil system failure: organic, inorganic and design. The outcome is the same, but the causes and solutions are different.
*Overloading of Food (BOD)
The term “organic failure” means the over-feeding of the bacteria. As bacteria “feed” on organic matter, they form a layer at the soil surface, the biomat. The biomat helps retain water in the soil system longer, leading to proper treatment. But if the bacteria are over-fed, the biomat grows too much, restricting the flow of water so that the system fails due to lack of flow. This situation can be created by (among other things) lack of maintenance of the septic tank, high-strength waste being put into the system, and hydraulic overloading of the system.
Recovery from organic failure is possible.
Organic failure is directly related to the bacteria colonies on the soil surface within the soil treatment system. This failure can be reversed by changing the environment to allow the bacteria to “catch up”. Correction of whatever the problem was should be the first step in the recovery, or the problem will quickly return.
Resting of the system- The first method of recovery would be remove the source of food until the bacteria have consumed the excess. This means turning the flow of tank effluent off. Recovery will probably not be complete, but the system will operate again. The length of resting necessary be determined by the severity of the problem.Higher level of treatment- The second method would be the increase in treatment levels before wastewater reaches the soil system. If the amount of organic matter in the waste is reduced, the biomat may be reduced and the system should operate better. The reason for this improvement is being debated: is the lower level of food or the increase in oxygen the cure? At this point the reason is not clear but the result of a better-operating system are apparent.
Chemicals: NO!- the final method is the addition of chemicals. This is a bad idea. If the chemical is toxic to the bacteria in system, what will it do to the groundwater? Testing done on hydrogen peroxide showed that the impact on the soil was significant, and it actually decreasedthe ability of water to move through the soil.
The second type of failure is the overloading of the inorganic solids in the system. Unlike the organic solids, these particles will not break down and the system will be permanently plugged. These particles would be lint from clothes, the dirt particles, and any other solid object in the wastewater. These particles physically plug the pores in the soil. Resting will not help, but this plugging can be prevented, and there are methods for physically removing the particles.
The way to prevent this is by proper maintenance of the tank and plumbing system. This would include cleaning the tank and watching what is put into the system. Another way to retain a higher percentage of these solids in the tank would be through the use of an effluent filter or screen. These devices appear to do an excellent job at removing the suspended solids, increasing the life expectancy of the system.
Once the damage has been done, is there an easy fix? The answer is that no one is sure. A method that appears to have some potential is the Terra-Lift System. The proper application of this process (properly designed, systemized, and allowed to maintain the proper separation from the saturated soil or bedrock appears to either open new soil pores or to clear the old ones, allowing the system to operate properly again.
All to common are the mistakes made in the design of the system. These can take a number of directions but here are the major three.
Long term acceptance rate- The proper sizing of the soil treatment area is still an art. Because of the “magic” component, the ease with which site conditions are misapprehended or unnoticed, and the local variation of proper sizing application, the designer can come up with the wrong numbers. This will typically mean an under-designed system. This can also include the flow estimates. Adding a little safety factor to any system can greatly increase its life expectancy.
Poor siting- The location and siting of the system may be reviewed in the companion paper, “Selecting Sites for On-Site Sewage Treatment System.”
Screw up- Finally, there are all the potential construction mistakes available. The biggest, in respect to life expectancy would be use of improper materials and damage to the soil. Improper materials can take many forms. Poor selection of pipe can cause settling and breaking, dirty rock can directly reduce the amount of water the system can handle and can reduce the available system for treating the waste water, and dirty sand just won’t work at all and the system constructed with it will fail so fast the checks will barely clear.
Soil damage is similar. The destruction of the soil structure by compaction or smearing will take years off the system’s operation. The protection of the soil through the construction process is critical to long system life.
Don’t feel that it’s impossible to get long life out of an on- site system! It is possible. What it takes is an understanding of the system, and a partnership between the professionals and the regulatory authority. This partnership allows everyone to be a part of the solution. Education is the last ingredient to cement together this partnership.
Please address any questions to Dr. David Lindbo.