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Conventional Septic Systems Aerobic Treatment Units
Conventional Septic Tank
Everything that goes down any of the drains in the house (toilets, showers, sinks, laundry machines) travels first to the septic tank. The septic tank is a large-volume, watertight tank which provides initial treatment of the household wastewater by intercepting solids and settleable organic matter before disposal of the wastewater (effluent) to the drain field.
FUNCTION OF THE SEPTIC TANK
While relatively simple in construction and operation, the septic tank provides a number of important functions through a complex interaction of physical and biological processes. The essential functions of the septic tank are to: receive all wastewater from the house separate solids from the wastewater flow cause reduction and decomposition of accumulated solids provide storage for the separated solids (sludge and scum) pass the clarified wastewater (effluent) out to the drain field for final treatment and disposal.
Primary Treatment
As stated, the main function of the septic tank is to remove solids from the wastewater and provide a clarified effluent for disposal to the drain field. The septic tank provides a relatively quiescent body of water where the wastewater is retained long enough to let the solids separate by both settling and flotation. This process is often called primary treatment and results in three products: scum, sludge, and effluent. Scum: Substances lighter than water (oil, grease, fats) float to the top, where they form a scum layer. This scum layer floats on top of the water surface in the tank. Aerobic bacteria work at digesting floating solids.
Sludge: The "sinkable" solids (soil, grit, bones, unconsumed food particles) settle to the bottom of the tank and form a sludge layer. The sludge is denser than water and fluid in nature, so it forms a flat layer along the tank bottom. Underwater anaerobic bacteria consume organic materials in the sludge, giving off gases in the process and then, as they die off, become part of the sludge. Effluent: Effluent is the clarified wastewater left over after the scum has floated to the top and the sludge has settled to the bottom. It is the clarified liquid between scum and sludge. It flows through the septic tank outlet into the drain field.
HOW LONG LIQUIDS MUST REMAIN IN TANK
Effective volume: The floating scum layer on top and the sludge layer on the bottom take up a certain amount of the total volume in the tank. The effective volume is the liquid volume in the clear space between the scum and sludge layers. This is where the active solids separation occurs as the wastewater sits in the tank. Retention time: In order for adequate separation of solids to occur, the wastewater needs to sit long enough in the quiescent conditions of the tank. The time the water spends in the tank, on its way from inlet to outlet, is known as the retention time. The retention time is a function of the effective volume and the daily household wastewater flow rate:
Retention Time (days) = Effective Volume (gallons)/Flow Rate (gallons per day). A common design rule is for a tank to provide a minimum retention time of at least 24 hours, during which one-half to two-thirds of the tank volume is taken up by sludge and scum storage. Note that this is a minimum retention time, under conditions with a lot of accumulated solids in the tank. Under ordinary conditions (i.e., with routine maintenance pumping) a tank should be able to provide two to three days of retention time. As sludge and scum accumulate and take up more volume in the tank, the effective volume is gradually reduced, which results in a reduced retention time. If this process continues unchecked-if the accumulated solids are not cleaned out (pumped) often enough-wastewater will not spend enough time in the tank for adequate separation of solids, and solids may flow out of the tank with the effluent into the drain field. This can result in clogged pipes and gravel in the drain field, one of the most common causes of septic system failure.
SOLIDS STORAGE
In order to avoid frequent removal of accumulated solids, the septic tank is (hopefully) designed with ample volume so that sludge and scum can be stored in the tank for an extended period of time. A general design rule is that one-half to two-thirds of the tank volume is reserved for sludge and scum accumulation. A properly designed and used septic system should have the capacity to store solids for about five years or more. However, the rate of solids accumulation varies greatly from one household to another, and actual storage time can only be determined by routine septic tank inspections.
ANAEROBIC DECOMPOSITION
While fresh solids are continually added to the scum and sludge layers, anaerobic bacteria (bacteria that live without oxygen) consume the organic material in the solids. The by-products of this decomposition are soluble compounds, which are carried away in the liquid effluent, and various gases, which are vented out of the tank via the inlet pipe that ties into the house plumbing air vent system.Anaerobic decomposition results in a slow reduction of the volume of
accumulated solids in the septic tank. This occurs primarily in the sludge layer but also, to a lesser degree, in the scum layer. The volume of the sludge layer is also reduced by compaction of the older, underlying sludge. While a certain amount of volume reduction occurs over time, sludge and scum layers gradually build up in the tank and eventually must be pumped out.
FLOW INTO AND OUT OF THE TANK
The inlet and outlet ports of the tank are generally equipped with devices such as baffles, concrete tees, or in more recent years, sanitary tees (T-shaped pipes with one short and one long leg).
Inlets
The inlet device dissipates the energy of the incoming flow and deflects it downwards. The vertical leg of the tee extends below the liquid surface well into the clear space below the scum layer. This prevents disturbance of the floating scum layer and reduces disruptive turbulence caused by incoming flows. The inlet device also is supposed to prevent short-circuiting of flows across the water surface directly to the outlet. The upper leg of the inlet should extend well above the liquid surface in order to prevent floating scum from backing up into, and possibly plugging, the main inlet pipe. The open top of the inlet tee allows venting of gases out of the tank through the inlet pipe and fresh air vents of the household plumbing. Outlets The outlet device is designed to retain the scum layer within the tank. A sanitary tee can be used with the lower leg extending below the scum layer. The elevation of the outlet port should be 2 to 3 inches below the elevation
of the inlet port. This prevents backwater and stranding of solids in the main inlet pipe during momentary rises in the tank liquid level caused by surges of incoming wastewater.
Typical inlet/outlet tees
Gas Deflection Baffle
Gases are produced by the natural digestion of sludge at the bottom of the tank, and particles of sludge can be carried upward by these rising gases. Some tanks have a gas deflection baffle, which prevents gas bubbles (to which solid particles often adhere) from leaving the tank by deflecting them away from the outlet and preventing them from entering the drain field.
THE EFFLUENT FILTER
In newer systems, there is often an effluent filter: one of the significant improvements in septic tank design in decades. They range from 4 to 18 inches in diameter. As we have described, the most serious problem with septic systems is the migration of solids, grease, or oil into the drain field, and the filter is effective in preventing this. A filter restricts and limits passage of suspended solids into the effluent. Solids in a filtered system's effluent discharge are significantly less than those produced in a non-screened system. FLOW BUFFERING
The septic tank also provides a buffering of flows between the house and the drain field. Large surges from the household, such as toilet flushing or washing machine drainage, are dampened by the septic tank so that the flows
leaving the tank and entering the drain field are at substantially lower flow rates and extend over a longer period of time than the incoming surges.
MICROBES IN SEPTIC TANKS DIGEST, DISSOLVE, AND GASIFY COMPLEX ORGANIC WASTES
In 1907, W. P. Dunbar conducted tests on the decomposition of vegetable and animal matter in septic tanks. He stated, "The author has investigated the subject by suspending in septic tanks a large number of solid organic
substances, such as cooked vegetables, cabbages, turnips, potatoes, peas, beans, bread, various forms of cellulose, flesh in the form of dead bodies of animals, skinned and unskinned, various kinds of fat, bones, cartilage, etc., and has shown that many of these substances are almost completely dissolved in from three to four weeks. They first presented a swollen appearance, and increased in weight. The turnips had holes on the surface, which gradually became deeper. The edges of the cabbage leaves looked as though they had been bitten, and similar signs of decomposition were visible
in the case of other substances. Of the skinned animals, the skeleton alone remained after a short time; with the unskinned animals the process lasted rather longer. At this stage I will only point out that the experiments were so arranged that no portion of the substances could be washed away; their disappearance was therefore due to solution and gasification."
Aerobic Treatment Units (ATU) These units are now required in most jurisdictions within Texas. The requirement is largely based on the soil type in your area and clay soil being the major factor. Clay does not readily absorb water which makes a conventional septic system not function properly and will cause frequent backup problems. The picture below is a typical 3-tank ATU which can be incorporated with either sprinkler nozzles or a sub-surface pressure-dosing system. Some ATU's come as one large unit which incorporates all the tanks into one. These systems require frequent inspection under the guidance of the Texas Commission on Environmental Quality (TCEQ). Current regulation states the homeowner is required to contract with a licensed service company to inspect the system 3 times annually. A typical home inspector cannot inspect these. More information can be obtained at: http://www.tceq.state.tx.us/compliance/compliance_support/regulatory/ossf/ossfmaintenance.html  HOW THEY WORK The platform mounted operations/control center contains the air pump, control panel and alarm horn and light for the unit. Wastewater enters the system at the Inlet and begins the treatment cycle in the Aeration Compartment. Air is pumped from the Air Pump through the Air Distribution Manifold, the Flexible Air Lines and out into the Aeration Compartment through the Anchored Diffuser Assemblies. Here the air mixes with the wastewater and the aerobic bacterial colony flourishes, treating the wastewater.
As the treatment cycle produces digested effluent, the mixing action produced by the diffusers causes the effluent to enter the Clarification Compartment. The conical shape of the Clarification Compartment causes most remaining suspended solids to drop out the bottom and re-enter the treatment cycle in the Aeration compartment.
As additional wastewater enters the system via the Inlet, clarified effluent is then pushed by hydraulic action out of the Clarification Compartment through the Outlet, where it enters to the effluent disposal system of choice.
The alarm system in the platform mounted operations/control center is activated by the High-level Float if the system becomes too full (volumetric overload) or by the control panel if air pressure drops to an unacceptable level. All Aerobic Wastewater Treatment Systems are suspended growth floating bio-mass systems designed to treat common household sewage only. In plain language, this means that the systems contain and depend on an aerobic (oxygen-requiring) bacterial colony which, when well cared for, digests and treats household sewage just as aerobic bacteria clean groundwater in nature. This is why aerobic systems have an air pump and air distribution piping – for supplying the aerobic bacteria with oxygen. Bacteria, however, can't eat just anything and everything. For a listing of things to avoid feeding to an aerobic bacterial system, see below.
Normal maintenance is required on all aerobic treatment units, as they are activated sludge sewage treatment systems just like large municipal water treatment plants. Factory-trained installers or service technicians regularly service these installations, checking to be sure they are in proper working order.
Aerobic systems are not designed for disposal of solid waste or chemicals. If homeowners stay within the guidlines listed below, their systems should function properly as sewage treatment systems and should not require unscheduled maintenance or frequent expensive pump-outs.
What NOT to put in an aerobic treatment system: Non-biodegradable items such as cigarette butts, match sticks, disposable diapers, feminine hygene products, condoms, hair, coffee grounds, rags, paper towels, bandages, etc. These are solid-waste items and should be disposed of in regular trash service.
Greases, fats or oils. This includes cooking oils and meat or meat grease. Pesticides, herbicides, photographic chemicals or other toxins. Paints, thinners or household chemicals, including many cleaning compounds, and mop-bucket water. Water softener unit backwash. Citrus products, oranges, lemons, grapefruit, etc. Home-brewery or winemaking wastes. Strong medicines, particularly antibiotics. Antibacterial soaps and antibacterial laundry detergents should be avoided. These are the current marketing rage, but overuse will only breed resistant strains of bacteria in the home, and kill the aerobic action in the treatment system. Strong disinfectants or bleaches, particularly products containing chlorine or ammonia. Commercial septic tank additives; they do more harm than good.
Kitchen garbage grinders (disposals) should be used sparingly, if at all. Dispose of food waste in a solid waste bin or compost heap. Food waste represents additional loading on the aerobic plant, possibly causing organic overloading and more frequent pump-outs of the system. Volumetric Overload of an Aerobic System Aerobic systems are rated for a maximum volume throughput per day, ie. 500 gallons per day (GPD) and only household wastewater (sinks, tubs, washing machines, toilets, etc.) should be allowed into the unit. 'Volumetric overloading' is simply a term used to describe putting more than the rated amount of wastewater through the system during a 24 hour period. To avoid volumetric overloading of our system, which can result in incomplete treatment conditions, observe the following:
Avoid multiple wash loads in one day; spread out your laundry during the week. Watch for leaking/flowing faucets or toilets and repair immediately. Use water flow reducing devices whenever possible. Be aware of any excessive water use of any kind.
Home Cleaning Products Guidelines: Recommended laundry detergents are: Powdered, low-sudsing, low-phosphate, non-antibacterial and bio-degradable.
Recommended household cleaning products are: Non-chlorine, non-ammonia, non-antibacterial, non-toxic and bio-degradable. |
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