DRAINFIELD DISTRIBUTION SYSTEMS
Parallel drainfleld distribution via distribution box
There is a distribution box between the tank and drainfleld. Effluent flows to all trenches more or less equally. Note how distribution here differs from that above.
Effluent flows from the septic tank into the drainfleld through a system of watertight pipes which ultimately lead to the perforated pipes within the trenches. Various pipe connections maybe included, such as tees, wyes, elbows, distribution boxes, or drop boxes. This is the drainfleld distribution system.
There are two types of distribution systems: parallel and serial. With parallel distribution, effluent flows to all trenches at roughly the same time. With serial distribution, effluent flows initially to the first trench, then to the second, and so on, to each trench in sequence. The disadvantage of the latter system is that the first trench is overworked and often remains saturated, while the remaining trenches receive correspondingly less effluent. This can lead to septic tank products sequential failure of the trenches. One engineer refers to serial distribution as "designed system failure."
Serial drainfield distribution on slope
Note how riser pipes at end of distribution pipes cause effluent to flow to lower pipes only when upper pipes are full.
The drainfield provides both disposal and treatment of the septic tank effluent. Effluent flows from the septic tank to the drainfield through a watertight pipe and is then distributed within the drainfield trenches through perforated pipes in the gravel. The effluent flows through the gravel filling and then seeps (infiltrates) into the soil beneath and beside the trench. Here the main purification of the wastewater takes place through filtration and biological activity as it infiltrates through the biological mat (seep. 19) on the side- walls and at the bottom of the trench and then percolates through unsaturated soil. (However, in seasoned fields, there is practically no percolation through the trench bottom.) The purified liquid then eventually evaporates, is taken up by plants, or percolates into the groundwater.
Once the effluent leaves the septic tank, it flows:
• to the trenches through a watertight distribution system
• into the trench gravel through perforated pipes
• into the soil through the infiltrative surfaces (gravel-soil interface)
• through unsaturated (or saturated) soil beside and beneath the trench
Finally, it is either taken up by plants or percolates to groundwater.
Effluent gets treated in the drainfield:
• as it infiltrates into the soil: the biomat is the tool. (See adjacent column.)
• as it percolates through the soil
The gravel in the drainfield maintains the structure of the trenches and helps distribute the effluent to the infiltrative soil surfaces. Also, the porosity of the gravel provides temporary storage capacity during peak flows.
Engineer Randy May believes there are a number of things wrong with the use of gravel in drain- fields. Although the gravel maintains the structure of the trenches, it also is filling % of the available storage capacity in the drainfield with rock. The gravel itself provides little treatment of effluent and, moreover, may cause compaction in the trench bottom due to its weight and fines (if present) in the gravel. (See p. 22 regarding shallow drainfields, which use no gravel, and p. 23 regarding Infiltrator" Chamber Leaching Systems.)
GRAVITY RULES 1
there are two brilliant things about a properly functioning gravity-fed septic
system:
•the power is gravity, the earth's magnetic force — pulling water and wastes downward. no pumps, no electricity.
•the cleansing agent is the earth, where microorganisms, naturally present in the soil filter, feed on and purify septic tank effluent, including disease- causing organisms.
it is an elegant design, working silently underground, and requires only a minimum of maintenance to keep it functioning. it is practical, functional, and ecological. it's worth knowing how to be an intelligent steward of this living system.