Questions and Answers about Septic Systems
What does a septic tank do?
What does a leach field do?
What does a leach field do with aerobic treatment?
How is wastewater treated?
Why do septic systems fail?
Should hydrogen peroxide be used to clear a failed field?
Do septic tank additives work?
What is secondary treatment?
What is tertiary treatment?
I hear of all the problems with domestic wastewater. Can it be disposed of safely?
How often do I need to have my septic tank pumped?
Can septic tanks be pumped too often? We normally do ours every two years.
Can I reduce my septage disposal fees while I repair my septic system?
Cities in Europe have been using sewers for a long time. Do you think they make sense?
Question: What does a septic tank do?
Answer: A septic tank is a chamber of concrete or other material that has one or more chambers that receives wastewater. Grease, oils, fats, and wood fiber (from paper products) float forming a scum layer. The scum layer slowly liquefies by microbial action. Grit, diamond rings, and heavier solids sink to the bottom, as do larger organics. The settled solids also slowly liquefy due to anaerobic bacterial activity. (Oxygen is required to break down plant and animal materials contained in wastewater.) So very little actual treatment occurs as the only oxygen present is that which is dissolved in the influent, the oxygen demand is about 20-30 times as much. So a septic tank allows fats to float, and settles solids, slowly liquefying a portion passing it out into the leach field. Virtually all that passes into a septic tank either stays or passes into the leach field.
Question: What does a leach field do?
Answer: It accepts dissolved compounds and dissipates liquids. In a primary treatment system (septic tank only) the effluent is about 75% of full strength sewage. (The remainder is lodged in the septic tank.) The effluent passes into the leach field where the actual treatment occurs in several steps. First an anaerobic (without oxygen) microbial community forms degrading (moving to a lower energy level) various compounds. Then at the surface of the bacterial mat where oxygen is present the real decomposition occurs.
Question: What does a leach field do with aerobic treatment?
Answer: In a properly operating secondary treatment system, the leach field acts simply to dissipate water, some 95%+ of the decomposition having occurred in the treatment tank.
Question: How is wastewater treated?
Answer: Conceptually when a plant grows, water, minerals, and carbon dioxide are taken in and through the energy from sunlight coupled with photosynthesis, carbon molecules are combined with the minerals to form tissue. When plant (or animal) materials decompose just the opposite must occur. Oxygen is necessary to support an aerobic microbial community. If sufficient oxygen is present in a solution rich in carbohydrates, nitrogen , phosphorus and other trace minerals, decomposition is complete with water, carbon dioxide and minerals being the end products. Some humus-like material does remain (as in a compost pile or with peat in wetlands) and so must be pumped out of the treatment plant, usually once every 5-25 years for the average family. When decomposition is complete there are no odors, and the effluent is clear, no longer supports microbial growth, and the enteric bacterial population has been reduced by 99-99.99%.
Cross section of a bed showing ponded effluent due to biomat formation
Question: Why do septic systems fail?
Answer: Septic systems were used by the Romans, but probably originated long before that as people began to build towns. In warm, dry climates wastewater soon soaks into soil. Evaporation and liquids percolating downwards draws in oxygen. In higher latitudes (or altitudes) cold, wet soil slows decomposition when oxygen is lacking. Anaerobic organisms proliferate. Some can turn sulfates to sulfides, which then form insoluble compounds with metallic ions. That plugs soils. The optimal temperature for anaerobic activity is 37° C (98° F), so anaerobes act very slowly at temperatures found in Canada. After the leach field infiltrative surfaces clog with biomat, effluent has no where to go so backs up into the septic tank, and maybe into the house. Supply oxygen (say by drying the field out) and the metals dissolve again and aerobic organisms consume the anaerobes, opening up the soil spaces. Another cause of problems is a build up in solids in the septic tank that pass into the field, again plugging up the soil spaces.
Question: Should hydrogen peroxide be used to clear a failed field?
Answer: No, while it seemed to be a viable option for treating clogged drain fields, is not. Hydrogen peroxide (H2O2), has been proven to actually decrease field permeability. It destroys the structure of soils, especially finer textured soils, and appears to create an impermeable barrier due to soil "boiling" during treatment. Although not all sandy soils are affected by H2O2 treatments, it is not recommended for use on sandy soils either. To read the original research conducted by The Small Scale Waste Management Program at the University of Wisconsin at Madison, see http://www.estd.wvu.edu/nsfc/SFQ/questionanswer.html
Question: Do septic tank additives work?
Answer: The State of Washington has released a pamphlet on septic tank additives, The Truth About Septic System Additives (Washington State Department of Health, Community Environmental Health Program, PO Box 47826, Olympia, WA 98504-7826, (360) 586-1249 or Fax (360) 664-3071). They offer a list of products that are unlikely to cause harm to a septic tanks operation. But they caution "approved additives" will not necessarily improve the function of your septic tank. Additives are costly, and typically do not positively contribute to the natural process of decomposition. While it is difficult to make an unequivocal statement about additives due to the paucity of scientifically valid testing, the problem is that septic systems are best suited to warm, dry climates. Secondary treatment such as extended aeration is necessary to properly breakdown wastewater before discharging into the environment for further treatment.
Question: What is secondary treatment?
Answer: Removal of biochemical oxygen demand (BOD). That is accomplished by aeration, be it by air compressor or by a trickling filter. An environment rich in oxygen allowing aerobic organisms to flourish is required. Aerobes consume the dissolved compounds removing the energy fixed by photosynthesis, leaving water, minerals, and carbon dioxide plus a little recalcitrant organics such as humic substances (a constituent of peat and the building block of soil). Decomposition occurs in the treatment plant, not in the field such as with just primary treatment. Effluent is 95%+ treated, bacterial load reduced by 1,000 - 1,000,000 - fold, its odorless, and no longer supports bacterial growth so your field life is dramatically extended. Aerobic treatment by Nayadic plants can restore failed fields, in fact.
Question: What is tertiary treatment?
Answer: Nutrient removal. Can be done by various means, physiochemical and biological. For small flows such as domestic households or businesses, the most economical method is to allow either terrestrial or aquatic plants (in a constructed wetlands) to uptake. Large quantities of carbon such as peat or organically enriched soils can also remove nutrients. A pressure (pumped) field insures roots will not plug up drain line perforations. Tertiary treatment using plants completes the ecological cycle. We consume plants (or animals fed by plants), incompletely digesting, then excrete the remaineder. Treatment in a secondary treatment (aeration) plant breaksdown the compounds to water, carbon dioxide, and minerals. The minerals are taken up by other plants so the cycle is complete. We have acted in an ecologically sustainable manner.
Question: I hear of all the problems with domestic wastewater. Can it be disposed of safely? Shouldn't we all have sewers?
Answer: Not without treatment first. It was once thought simply putting it into the ground was sufficient. We are now finding serious problems result with nitrate contamination of ground water occurring. Domestic wastewater cannot be "disposed" of without harming public health or the environment, it must be treated to remove the contaminants including nutrients as well as pathogens. Research demonstrates contaminants from domestic wastewater can be found two meters under a sand layer. High organic material top soil is much more effective than sand at removing contaminants due to its higher surface area to volume consequently higher biological activities. The problem with sewers is that even with centralized treatment, many contaminants remain. Typically municipal treatment plants discharge to surface waters, which results harmful ecological effects. Properly designed, installed and operated onsite wastewater management systems are perminate and do not pollute the environment as the contaminants are safely broken down in soil.
Question: How often do I need to have my septic tank pumped?
Answer: Septic tanks over time fill with soil, and artificial fibers. How long depends upon the number of persons in your household, the usage patterns it receives, and the size of your septic tank. Water is a terrible medium to treat wastes in as there is so little oxygen present. In septic tanks there is typically no free oxygen, that’s why they are called septic. It’s a good idea to have your septic tank inspected at least once every 5 years, more often if you have a large family, and especially if you use an in-sink garbage grinder. It is only by measuring the thickness of the scum layer on top and the sludge layer on the tank bottom can it be determined when it is necessary to have a septic tank pumped. Pumping septic tanks that are followed by a secondary treatment plants is much less critical.
Question: Can septic tanks be pumped too often? We normally do ours every two years.
Answer: Yes, septic tanks should only be pumped when testing shows the floating scum layer on top, and settled sludge layer on the bottom occupy more than 2/3rds of the volume of the tank. Do not go by a calendar. Septic tanks are populated by anaerobic bacterial groups. The optimum temperature for anaerobic decomposition is 37⁰ C, which is body temperature. Temperature of wastewater coming from homes averages about 16⁰C, less if cold water laundries are frequent. At higher latitudes and higher altitudes where ambient temperatures are lower, decomposition slows further. So at the latitude of SW British Columbia it takes on average about 24 months for the anaerobic population to equilibrate. So during that period, solids are not liquefied as well as later on. As a consequence more solids are discharged to your field, thus shortening its life. So over pumping is not only expensive but also can shorten field life, which is really expensive.
Question: Can I reduce my septage disposal fees while I repair my septic system?
Answer: An owner can apply for a temporary reduction in the septage disposal fee while their septic system is under repair. Temporary reductions are available for a maximum period of 90 days and required an "authorised person" to sign the application form confirming that the system has failed.
Go to http://www.rdn.bc.ca/cms/wpattachments/wpID1868atID2792.pdf for an application, and return the completed form to:
Regional District of Nanaimo
6300 Hammond Bay Road
Question: Cities in Europe have been using sewers for a long time. Do you think they make sense?
Answer: "Those whose job is to select and design appropriate systems for the collection and treatment of sewage ... must bear in mind that European and North American practices do not represent the zenith of scientific achievement, nor are they the product of a logical and rational process. Rather, [they] are the product of history, a history that started about 100 years ago when little was known about the fundamental physics and chemistry of the subject and when practically no applicable microbiology had been discovered.... These practices are not especially clever, nor logical, nor completely effective--and it is not necessarily what would be done today if these same countries had the chance to start again."
This quote can be found in the World Bank publication Sanitation and Disease Health Aspects of Excreta and Wastewater Management, Edited by Richard Feacham.