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Chapter 4: Treatment Processes and Systems The relief lines are simple overflow lines that connect one trench to the adjacent lower trench. They are solid -wall pipes that connect the crown of the upper trench distribution pipe with the distribu- tion pipe in the lower trench. Successive relief lines are separated by 5 to 10 feet to avoid short- circuiting. This method of distribution makes full hydraulic use of all bottom and sidewall infiltration surfaces, creates the maximum hydrostatic head over the infiltration surfaces to force the water into the surrounding soil, and eliminates the problem of dividing flows evenly among independent trenches. However, because continuous ponding of the infiltration surfaces is necessary for the system to function, the trenches suffer hydraulic failure more rapidly and progressively because the infiltration surfaces cannot regenerate their infiltrative capacity. Drop box Drop box distribution systems function similarly to relief line systems except that drop boxes are used in place of the relief lines. Drop boxes are installed for each trench. They are connected in manifolds to trenches above and below (figure 4-10). The outlet invert can be placed near the top of each trench to force the trench to fill completely before it dis- charges to the next trench if a serial distribution mode of operation is desired. Solid -wall pipe is used between the boxes. The advantage of this method over serial relief lines is that individual trenches can be taken out of service by attaching 90 degree ells to the outlets that rise above the invert of the manifold connec- tion to the next trench drop box. It is easier to add additional trenches to a drop box system than to a serial relief line network. Also, the drop box system may be operated as an alternating trench system by using the 90 degree ells on unused lines. With this and the serial distribution system, the designer must carefully evaluate the downslope capacity of the site to ensure that it will not be overloaded when the entire system or specific trench combinations are functioning. Gravelless wastewater dispersal systems Gravelless systems have been widely used. They take many forms, including open -bottomed cham- bers, fabric -wrapped pipe, and synthetic materials such as expanded polystyrene foam chips (fig- ure 4-11). Some gravelless drain field systems use large -diameter corrugated plastic tubing covered with permeable nylon filter fabric not surrounded by gravel or rock. The area of fabric in contact with the soil provides the surface for the septic tank effluent to infiltrate the soil. The pipe is a mini- mum of 10 to 12 inches (25.4 to 30.5 centimeters) in diameter covered with spun bonded nylon filter fabric to distribute water around the pipe. The pipe is placed in a 12- to 24 -inch (30.5- to 61 -centime- ter) -wide trench. These systems can be installed in areas with steep slopes with small equipment and in hand -dug trenches where conventional gravel systems would not be possible. Reduced sizing of the infiltration surface is often promoted as another advantage of the gravelless system. This is based primarily on the premise that gravelless systems do not "mask" the infiltration surface as gravel does where the gravel is in direct contact with the soil. Proponents of this theory claim that an infiltration surface area reduction of 50 percent is warranted. However, these reductions are not based on scientific evidence though they have been codified in some jurisdictions (Amerson et al., 1991; Anderson et al., 1985; Carlile and Osborne, 1982; Effert and Cashell, 1987). Al- though gravel masking might occur in porous medium applications, reducing the infiltration surface area for gravelless systems increases the BOD mass loading to the available infiltration surface. Many soils might not be able to support the higher organic loading and, as a result, more severe soil clogging and greater penetration of pollutants into the vadose zone and ground water can occur (University of Wisconsin, 1978), negat- ing the benefits of the gravelless surface. A similar approach must be taken with any con- taminant in the pretreatment system effluent that must be removed before it reaches ground water or nearby surface waters. A 50 percent reduction in infiltrative surface area will likely result in less removal of BOD, pathogens, and other contami- nants in the vadose zone and increase the presence and concentrations of contaminants in effluent plumes. The relatively confined travel path of a plume provides fewer adsorption sites for removal of adsorbable contaminants (e.g., metals, phospho- rus, toxic organics). Because any potential reduc- tions in infiltrative surface area must be analyzed in a similar comprehensive fashion, the use of 4-20 USEPA Onsite Wastewater Treatment Systems Manual