A septic tank is a water tight tank. That's it. There really isn't much to it -- wastewater goes in and comes out of it. The key is to think of septic tank systems, which involve more than just the septic tank. In the diagram below, the dirt that would normally cover all of this system is removed for a better view. We see waste moving from the house to the septic tank, and eventually making its way to an "absorption field." Wastewater is draining directly into the soil in the absorption field. In this absorption field, the soil microbes act to digest the wastewater. In other words, the real treatment is happening in the absorption field, not the septic tank. The key is for the wastewater to drain evenly so that the field doesn't turn into mud or, even worse than that, wastewater back up towards the house!
So what does the septic tank do? Taking a closer look below, we see that its fundamental purpose is to slow down the movement of wastes -- typically, it takes about 24 hours for wastes to travel through the septic tank. During this time, some of the solids may settle out and break up, but very little treatment is going on. In the cross section view given below, we see that sludge (with solids) settles to the bottom and scum flots on the top, but basically it is wastewater moving through the tank. However, if the solids have broken up in that 24 hour period, there is less chance for clogging further down the line in the absorption field.
Since the absorption field is where the digestion takes place, we now take a closer look in the cross section diagram given below. The "distribution pipe" shown below has holes in it -- we deliberately want the watewater to drain into the absorption field. We surround the distribution pipe with gravel and cover it with dirt (backfill), but the key is to to drain the wastewater evenly over a wide area.
Backing away from this cross section just a bit, we can see a more active view in the diagram below. Notice that we are deliberately leaking wastewater into the soil. As long as we do this evenly and do not overwhelm the soil with too much wastewater, the soil is able to digest the wastes.
This helps explain some of the concerns with septic tanks:
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