|ENGINEERING DESIGN MANUAL -- TREATMENT AND DISPERSAL PACKAGES: RESIDENTIAL & COMMERCIAL
SOIL AND SITING CONSIDERATIONS
Loading rates should not be assigned based on textural classes alone. The characterization of a soil based receiver site involves a systematic evaluation by trained individuals. Factors to consider consist of a variety of topographic and soil conditions such as landscape position, slope, soil depth, depth to water table, depth to restriction, soil consistence, clay mineralogy, compaction, density, and site uniformity.
Subsurface disposal of septic tank effluent is intended to be an aerobic process but more often than not in conventional gravel systems, due to the anaerobic quality of the effluent, and localized overloading due to poor distribution and resulting in the lack of soil aeration, it remains anaerobic. Anaerobic conditions at the soil interface can cause biologic clogging due to slimes produced by anaerobic microbes and accumulation of untreated or inert solids. Generally, anaerobic decomposition causes an accumulation of waste byproducts at the soil interface to continue to slow the wastewater (sewage effluent) entering the soil.
Infiltration rate estimates based on texture essentially are a general status of the total amount of macro pores that would allow water to freely pass through the soil. The macro pores also provide conduits for gas exchange with the surface. The amount, depth, and distribution of macro pores within a soil matrix is very important for performance of any on site system for the soil to efficiently treat and dispose of effluent.
The loading rate and system configuration should consider a site and soil determination and be based on and of vertical and horizontal subsurface water movement estimation over a limitation. Landscape linear loading, that is the gallons per linear foot per day along the topographic contour should be minimized.
Secondary factors in void status such as a reduction by density or compaction, or enhancement by soil ped structure need consideration. Caution should be exercised when depending upon secondary soil void development to increase loading rates beyond the those that may be estimated by texture. The influence of the secondary voids may not extend deep enough to provide the reaeration necessary for efficient biological treatment of the organic constituent of the wastewater. Compaction by even minimum construction activity will always impact or could severely destroy soil structure. These considerations are very important in the case of the find sands and clayey textures and the characteristic shallow installations.
When properly sited, drip disposal provides aerobic unsaturated flow conditions at the contact with the soil interface. Saturation of the soil voids should be for brief periods during or directly after dosing. In a typical small flows situation, dose volumes per emitter per dose events are approximately .1 gallon per dose. At these small volumes, water movement may be primarily influenced by matric (capillary or suction) forces within the soil in addition to or preceding downward (or lateral) gravity flow . The result is the retainment of the effluent from the point of distribution, outward and upward such that a drip disposal system can be considered a method of surface disposal within the upper soil horizons. Coupled with time dosing at regular intervals, aerobic conditions are maintained and a nearly static environment is created for the microbial population. The soil treatment system essentially functions as a trickling filter with a film flow condition over the surface of the soil aggregate as the effluent moves within the soil column.
NON UNIFORM SHAPES
With the equal distribution as a fixed characteristic of the pressure compensating emitter, a variety of site configurations can be accommodated with drip disposal. Several suitable areas of varying sizes may be utilized maintaining the same gallons per linear foot loading rate in all areas. Absorption areas do not have to be exact squares or rectangles. Soil and landscape suitability may be maximized without the design constraints of conventional trench type systems. Runs of varying lengths are possible such as in the case of a triangular shape sites.
As a shallow or ultra shallow placed graveless system, with minimum or no excavation or site disturbance, drip disposal can be sited and installed within treed sites. Depending on the size and density of the vegetation and method of installation, some clearing and tree removal may be required. Preserve as many of the larger trees as possible. Remove all others by cutting flush with the ground surface and removing by hand. Stumps should be gently grubbed with a backhoe and removed with as little disturbance as possible. The resulting depressions should be filled with loamy material, evenly incorporated and graded by hand. A certain number of smaller aesthetically valued trees may be preserved as they can be easily removed later during installation if necessary.
In treed sites and with zones of unique configuration, systems should be feed and return from the header ditch side with no large trees located along the header ditch itself. Preferably keep trees form 15 to 20 feet from the header ditch side of the site.
Additional area will likely be required due to loss of installable area due to tree trunks and roots in the way of the drip line. Typically, long runs are installed above and below the larger trees with shorter runs installed beside the trees connecting the longer runs and providing for full coverage of the absorption area. In all systems care should be taken to insure that short runs are avoided at the lower portions of an individual zone to minimize increased loading due to gravity and resulting hydraulic system draindown. These above considerations and procedures sound more like construction guidelines and they are. However it is the initial site analysis by the evaluator that determines if adequate area is a available for installation and estimate the extent of tree preservation possible.
Soil and site evaluation is only slightly different than evaluations for conventional systems. The only difference is due to the enhanced ability to use treed sites and sites with complex topography. The systems ability to be easily designed into multiple zones of varying size lend it to general application in nearly any situation.
The layout in treed sites involves thinning the site of scrub trees and brush to clear a path for the vibratory plow to work in. Even with shallow hand installed sites, minimum clearing is still recommended to provide easier working area for installation and future inspections.
Determine the trees which are of benefit to save and clear the rest making sure the shallow soil is NOT compacted or significantly disturbed. Hand clearing is recommended if surface or shallow installation is specified. If the site is to be installed with a vibratory plow use a machine with equal or less track or tire weight to clear the underbrush. Do not drive any equipment on the site if the equipment leaves a tire track during operation. For shallow systems, working the site when it is very dry is important.
Determine the linear feet of tubing to be installed based on standard site evaluation methods for the type of site and soil. Install long runs adjacent to saved trees then fill in the tubing between the long runs to cover the area. Then fill in the short runs to provide equal size laterals in each zone. Adjust the spacing of the tubing from 1.5' on center and on up to whatever is necessary to provide the specified amount of tubing in the site. Closer spacing is possible down to 1' in certain situations.
Note the supply manifold is shown to have a lateral "jump" when incorporating short runs and long runs in the same lateral. These jumps should be connected using flex pipe to protect from future damage. It is easier for treed sites, as well as non uniform configurations, if the supply and return pipes are located on the same side of the field.
The more trees you want to save the less yield of tubing you will get in linear feet of tubing per unit of area. This will most likely increase the number of short runs you will need so the installation cost may increase. The designer must evaluate the cost to benefit for each site to determine the value of saving the maximum number of trees.
When selecting trees to save consult with a tree expert to determine if the remaining trees will be tolerant of the increased amount of moisture available. Also some trees are more sensitive to root damage during installation than others. It may be necessary to damage the tree to install the system. Care must be taken not to kill the tree or removal problems will be increased at a later date.
IRREGULAR SHAPED SITES
The above installation was a two zone drip distribution repair with a single tank sand filter provided due to the shallow installation and small area to service the home. The site was severely restricted due to the contour and closeness to the property lines. We were able to achieve a suitable installation by combining short runs and long runs to maintain nearly the same length laterals in each individual zone to cover the available area. All laterals in a zone are to be as close as possible in length to maintain acceptable field flushing.
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