Questions and Answers about Septic Systems
by
Curt Kerns, M.S., R.P.Bio., C.F.S.
Incomplete wastewater treatment has become a serious problem in lower British Columbia. Over 98% of our intertidal waters are closed to shellfish harvests due to pollution. Whether it is a municipal plant that is overloaded during storm water run off, or back yard septic fields that cease functioning during our long, wet, cool winters, incompletely treated sewage is detracting from the esthetic quality that is central to the quality of life in beautiful British Columbia.
Incomplete treatment of wastewater discharged into watercourse that find their way to saltwater and especially direct discharge of untreated sewage into salt water has been implicated as the causal mechanism of red tides by scientists at the Scripps Oceanography Institute in California. Marine bacteria bloom, feeding on the dissolved compounds, thus creating the conditions optimal for the red tide organisms to flourish. The incidences of red tides have grown precipitously in the last century. Here in BC, the Straits of Georgia have recently seen blooms in areas never before visited, such as outbreaks in Baynes Sound (located near Courtenay) an area, which used to be free of the toxic blooms.
It has been the common practice to install the least costly wastewater treatment systems -- such as primary treatment only (septic tanks), especially in rural areas, throughout North America. A dense, anaerobic (without oxygen) bacterial mat grows eventually plugging soils especially in cold, wet climates. With central treatment plants being economically impractical in low density neighborhoods (as well as the problem of what to do with the effluent from central plants) advanced onsite wastewater treatment is recognized as being the most obvious solution. In many areas a report to the US Congress, the Environmental Protection Agency cites a number of advantages to using decentralized onsite treatment systems: protects public health and the environment, appropriate for low density communities and varying site conditions, and
typically much more cost effective than centralized plants.
Questions (click-on
for 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.
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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.
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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.
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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) and so must be pumped out of the treatment plant, usually once every 2 - 5 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%.
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Cross section of a bed showing ponded
effluent due to biomat formation |
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. Drying, even 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. 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.
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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
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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.
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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 humid 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.
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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 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 upon plants), incompletely digesting, excrete, treat in a secondary treatment (aeration) plant decomposing 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.
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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.
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Septic tanks, over time, fill with soil, artificial and natural fibers. How long depends upon the number of persons in your household, the usage patters it receives, and the size of your septic tank. See the table below for an approximation. Its a good idea to have your septic tank inspected at least once every three years, more often if you have a large family, and especially if you use an in-sink garbage disposal.
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CK Ventures Ltd
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Comox Valley 338-1181
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