The workings of a Septic System

This little slideshow is intended to demostrate how a septic system works. For information on how to design a septic system, check with resources in your region, as optimal specifications differ. The components in blue are the initial parts of this system. The blue rectangle is the septic tank, and the bule gravel hatched section is the field line.

Looking over the edge of the deck there is a patch of gravel. The septic tank is located below this gravel. Here in Japan we cannot purchase inexpensive ready made septic tanks, and there is high seismic activity, so this one was constructed out of concrete.

The floor was poured first, which included re-bar that extended up the full height of the walls. So each re-bar is one continuous length which spans the height of one wall, bends 90 degrees and runs along the floor and then bends another 90 up the opposite wall.

One side is plumb with the edge of the deck. It is not advisable for any part of the tank to be obstructed by any structure. The inlet will be located close to this corner, but the interior piping will drop the sewage at the middle of that far wall.

Here's how a septic tank works; Try a little experiment if you will. Take a drinking glass and fill it with water. This represents (I will watch my language here, and use 2 Portuguese words) all of the water that went down the drains and toilet, including the Pipi that went down the toilet. Then dump a few raisins into the glass. This represents the Coco and other solids that went down the toilet. You will see those raisins eventually sink to the bottom. So it is in a septic tank. Next, pour in a little oil. Cooking oil, olive oil whatever. It forms a layer at the top. This represents any grease or fat that went down the drains. Our little glass experiment has just perfomed the first stage of sewage treatment, using no energy except the force of gravity!

As new sewage is constantly pouring into our septic system, we need to let out some of the water, which was separated from the solids and the oils; it must exit the tank for further treatment downstream. We will then locate the exit pipe on that back wall by the deck stairs. The liquid that exits there will be free from solids, fats and grease. Meanwhile, back in the tank, the Pipi, Coco and everything else continue to get separated.

The walls and ceiling have been done in one pour. The inlet hole is barely visible at the top of the left wall near the ceiling and near the corner. This is where the raw sewage enters the tank. A septic system is a wonderful invention that is the most environmentally friendly way to treat sewage, and return clean water to ground. Furthermore, it is a system where we can take harmful pathogens, (those horrible dirty germs!) and harness them to work for us! Not only do we make them work, we quarantine them underground so that they cannot infect us. There, they all eventually die, and the next ones come down the pipe, clean them up, and continue the work.

These are the 2 walls that don't have a pipe penetration. Just as in our little experiment, here in this tank the solids accumulate on the bottom, and the fats and oils float and accumulate on the surface of the water near the top of the tank. In this tank, millions of Aerobic bacteria (which arrive daily from the house drains) Do their work. They eat the solids which accumulate from the bottom of the tank. They also eat the fats which float on the surface. AND, because they are AEROBIC bacteria, they need and consume oxygen, which they get from the water.

On that back wall, you can see the exit hole. The surface of the liquid in the tank will be just slightly above the bottom of that hole. Notice that there is some distance from the surface of the liquid, to the ceiling of the tank. That gives room for the fats and grease to accumulate on the surface. Finally much gases are produced here including methane and hydrogen. These are what cause the odors, and carry the odors to our nose (if we dumped our Pipi and Coco on the street). But with a septic system, we don't have to smell them! These gases go back up the pipes, and exit through the vent stacks on the roof. Some septic systems employ another vent at the tank, or at the end of the field line.

So what keeps the fats and greases from just flowing out of the tank? On both the entrance and exit, there are Tee fittings set up like this. This is the entry. Here, the pipes keep the entering sewage from splashing down through and interrupting the grease layer. A properly sized and built system which is properly used will not need to be pumped. As long as the anaerobic bacteria are consuming the solids and fats, the sludge layer will not over fill. In fact, I have opened and reused tanks that were completely empty of sludge! There was only clear water. In such cases, if there is a period of vacancy, the bacteria eat it all up, run of food, and they die too.

And this is the exit. The fats and greases cannot get into this pipe, so they are blocked from exiting. When you buy a septic tank, these pipes and fittings are already set up in the tank. So where does the partially treated liquid go after it exits the tank?

As mentioned previously, the tank lies beneath this gravel patch. After exiting the tank, the liquid flows along a leach line where it seeps into the ground. Here, one of the leach lines terminates at the blue wheel barrow.

The entire length of the field line must be level, which means the bottom of the trench, the pipe run, and the gravel level, must all be level. However, the ground above the field line does not need to be level. On the left, by the power pole, you can see a scope builders level setup. A laser level will work fine. However, you cannot rely on a carpenters level, as seen in the trench, it was used here only between taking sights.

This entire septic system was dug by hand, with a pick and shovel. Here we can see the new trench extending along the same level as the existing trench.

Sights are taken from the level, and new sections of the trench are excavated. The excavated area at the left has nothing to do with the septic system; it is the construction of an access road to the parcel behind this parcel. However, the soil that is being removed has tons of stones and broken cement tiles. The soil is carted to that wooden box with a steel grate on it. This road construction project is also being done with a pick and shovel.

After the soil is sifted through the grate, each batch of gravel is dumped into this pile. Only in this project. Usually gravel is purchased and delivered to the site.

On a heavy day of household water usage, the level is slightly higher, but just below the bottom of the perforated pipe. But, this is too close for comfort, thus the extension project was undertaken.

Finally, the extension pipe is connected. The joint is not at the yellow tape, but a few rings further down. On this day, the water level is lower. Here, in the gravel and surrounding soil of the leach bed, ANAEROBIC bacteria live and work. They work in a much drier environment. They eat the smaller water borne particles that were not separated by the septic tank. They continue to eat whatever remaining waste products there are in the water. The water seeps into the gravel bed, where it is stored, while the much slower process of water seeping into the surrounding soil occurs.

The perforated pipe is laid level along the gravel bed which has been carefully brought to the same level along its entire length. In this project, I just used left over corrugated electrical conduit and ran it along a table saw to cut the 2 lines of holes. In North America corrugated pipe designed for field lines is cheap and readily available.

The pipe is laid so that there are no holes directly at the bottom. The goal is to get a water way along the botom 1/4 of the pipe diameter that DOES NOT leak water. THEN when the entire length of the pipe water level reaches that point, water begins seeping from all the holes on both sides of the pipe SIMULTANEOUSLY. This will happen very slowly. We don't want gushes or torrents of water, as these will erode and seal up the soil; which in turn will impede the seepage processly immensely.

Prior to backfilling, some method of protecting the gravel from soil incursion is employed. Tar paper is one method. Hay, straw, or swamp grass has been used in some locations. Depending upon the region, soil, regulations, etc, the goal here is to keep soil from infiltrating the voids between the stones. In one sense, this gravel bed acts as a holding tank, since absorption into the soil could take hours or even days in some conditions such as heavy rainfall.

In road or foundation construction, we want to compact the gravel as much as possible, But it is the OPPOSITE in drain line construction. We are not trying to support weight here, we WANT some air space, or "voids" among the stones to route and retain water. When done by hand, backfilling can be gentle; avoiding unnecessary compaction of the gravel.

Since the new pipe section had already been connected, water was already flowing to 1/4 the diameter of the entire length, and seeping into the new gravel bed. And further; you can clearly see how the water has seeped into the surrounding soil, fanning out from the gravel bed. The moist soil and the dry soil boundary can clearly be seen here. At this edge of the property the ground is so low that one can observe this. However, The ground level will be filled and raised here into a mound or plateau.

During part of the construction, a small drainage trench was made to keep the water level low. Even though at this point, the water looks clean, it is not pure; it still contains millions of pathogens! Later it was filled in, and the termination of the field line was mounded. Depending on the number of occupants, land area and soil, a septic system can be designed. A septic system is truly one of the best environmental and health technologies of our time.

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