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Septic Dosing Systems: float-controlled, gravity-controlled, siphon-controlled: this septic system design article defines, describes, and explains the operation of the types of effluent gravity dosing septic systems, including gravity dosing, using septic effluent tipping buckets, tipping pans, or dipping systems, bell siphons, or float-controlled (floating outlet) effluent dosing systems for septic system effluent final treatment and disposal. We explain how non-electric float or gravity-operated septic dosing systems are designed, installed, and how they work.
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How and When Septic Effluent is Moved Through a Septic System - Methods For Septic Effluent Distribution Using Gravity Systems
The page top image shows a sketch of a basic septic system design using gravity dosing, with effluent flowing from a septic tank to a dosing chamber and from there to a drainfield. Image: Indiana state health department. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted.
Readers interested in septic effluent pressure dosing systems should also see PRESSURE DOSING SYSTEMS.
Our sketch (left, source US EPA) shows a generic septic effluent pressure dosing system, combining a septic tank, a dosing tank, a diverter valve,and two septic effluent dispersal loops through a soil absorption field.
Septic effluent is distributed to a system final treatment and disposal using either gravity methods (which depend on terrain slope - see GRAVITY/SIPHON DOSING SYSTEMS) or pressure methods - PRESSURE DOSING SYSTEMS (which use a pump to move effluent to its destination treatment and disposal area). Effluent may be distributed for final soil absorption by several methods listed here:
How septic effluent dosing systems work
Septic effluent dosing systems have an effect on the drainfield much like flushing a toilet: the drainfield may be at rest, then it has to receive a surge of wastewater effluent. The size of the dosing tank or dosing mechanism determines the size of the effluent dose, and the controls on the system (electronic, float, tipping by gravity, etc) set the dosing interval.
To extend drainfield life and/or to accommodate soils with poor percolation rates, some dosing systems, particularly those using mechanical or gravity-operated dosing mechanisms, use alternating siphons or tipping dosers that can flush septic effluent into alternating septic drainfield areas.
Because some solids that flowed out of the septic tank may settle in the dosing tank, it is a good idea to periodically inspect and clean the dosing tank when the septic tank itself is pumped, cleaned, and inspected.
Wastewater effluent is distributed for final treatment over time either by uncontrolled, or controlled methods.
Uncontrolled septic effluent flow: A conventional gravity septic system and drainfield is "uncontrolled". When waste enters the septic tank, it forces the same volume of effluent out of the tank and into the leach field. Some experts call this a continuous or trickling septic system.
Conventional septic tank and drainfields use this approach. The timing of effluent movement or "trickle" into the absorption field is based simply on when people are using the building plumbing and thus based simply on when wastewater flows out of the building into the septic tank.
Controlled septic effluent flow: in controlled systems effluent is sent to the final treatment and disposal system such as an absorption field under either mechanical control such as a tipping or siphon system or under pump control, such as by use of a septic effluent pressure manifold or a septic effluent drip network.
In some large wastewater treatment systems with a significant if not uniformly continuous inflow, outflow of the system may be continuous in some designs. But many system use an intermittent effluent dosing method which operates by a pump controlled perhaps by a float in an effluent receiving chamber, or by a siphoning or tipping bucket mechanical system (gravity systems) which we discuss here.
Bell siphon effluent dispersal systems are a septic effluent dosing method that has been in use since about 1900. Bell siphon dosing systems use a bell-shaped cover over a vertical dosing chamber outlet pipe, combined with a vent pipe that lets air out of the bell chamber to control effluent flow.
Illustration at left, courtesy USDA, edited by DF
Effluent level in the dosing chamber rises in both the chamber and inside the bell (through the open bottom of the bell). As effluent rises, air inside the bell vents out through a small-diameter pipe.
When effluent in the dosing chamber reaches the level of the bell-vent pipe outlet (which is letting air out of the bell), liquid rising inside the bell slows (as no more air escapes the bell) and the remaining air trapped inside the bell begins to push out of the dosing chamber outlet pipe and trap.
When effluent reaches the maximum design level in the dosing chamber, air in the trap (at maximum pressure) is expelled through the dosing chamber trap and is followed by the dose of septic effluent, starting a siphon action.
The siphon action moves effluent from the dosing chamber to the absorption field, leach field or sand filter bed.
As effluent leaves the dosing chamber the effluent level in the chamber drops until it reaches the open bottom of the bell. At this point the siphon action is "broken" and siphoning of effluent out of the chamber stops.
Bell siphon effluent dispersal designs deliver a fixed effluent dose to the absorption system or drainfield at a frequency which will depend on the rate of usage of the septic system, or the rate of flow of wastewater into the system.
An example of a bell siphon effluent dosing system including a simple animation can be seen at Fluid Dynamic Siphons,. Contact Fluid Dynamic at 970-879-2494 or firstname.lastname@example.org - Steamboat Springs, Colorado.
See Float Control Dosing Systems just below for an updated approach to bell siphon septic system designs.
Also see pressure and gravity dosing discussions in Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
A septic tipping dosing system, also referred to as a bucket tipping or tipping pan, uses a bucket mounted on an axle or hinge inside of an effluent dosing chamber to first accumulate and then dispense septic effluent to the absorption system (referred to in some countries such as New Zealand and Australia as "filtration disposal trenches".
Effluent flows from the septic tank to the dosing chamber, filling the bucket. As the bucket is hinged near its center line, and because the front of the bucket is wedge shaped, as the bucket fills to a specified level or "dose" of effluent, the weight of the effluent causes the bucket to "tip" forward, spilling its contents into the dosing chamber.
An effluent drain connected at the bottom of the dosing chamber directs the tipped effluent to the drain field or soil absorption system.
The size of the bucket in gallons determines the dose of effluent that it will apply to the drainfield. The rate of fill of the dosing bucket depends of course on the in-flow rate of sewage to the septic tank.
Tipping bucket dosing mechanisms operate without requiring electricity and are an alternative to siphon dosing systems described above and float-controlled dosing systems described below.
Typically a conventional leach field or a raised sand bed filter is used but manufactured wetlands can also be utilized.
Float-controlled dosing systems operate without requiring electricity (unless a pump is used to move effluent up to the dosing chamber). Gravity powers the flow of effluent from the chamber to the soil absorption system.
The Flout™ may be left mothballed in place at say a vacation home. No extra steps are required to re-activate the system. The Flout never requires priming or recharging of the air bubble like a bell siphon
To handle various dosing volumes or flow rates, floating outlets can be arranged in a single float controlled outlet or alternatively in multiple outlet systems. Designs that alternate between two or more outlets (pictured at the top of this section) permit alternate drain field use, providing for longer rests between dosing cycles. Ganged outlets (double, triple or more Floats operating at once) to provide for larger dosing quantities and flow rates out of the same effluent chamber. Ganged Flouts also provide the only true parallel effluent distribution, insuring the same amount of effluent flows out of each outlet every time.
Effluent dosing quantities per cycle range from about 30 gallons to 2000 gallons, depending on the size and dimensions of effluent dosing chamber. The Flout™ can have a drawdown of 8" to 52" and should be sized to fit the chamber.
The manufacturer (see below) asserts that this design is easier to install than a bell siphon system. It tolerates out of level conditions better and never requires priming. Effluent pumping from the septic tank is only needed where the septic tank is lower than the dosing chamber, and simpler, less costly pumps are required than with other dosing systems. Certainly in comparison with pressure dosing systems (seelinks listed at the "More Reading" links at the bottom of this article ) this will be the case.
An example of a floating outlet design for dosing systems for septic effluent disposal can be seen in animation at Rissy Plastics FLOUT floating outlet for septic effluent dispersal.
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