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Building and retrofitting a septic tank effluent filter
Septic systems are biological on-site waste water treatment systems. People living in cities tend to
think that they are inferior to municipal water treatment systems. That is not true. They can be more
effective and pollute rivers less than municipal water treatment plants. The only reason they are not used
in cities is building density. You need a bit of space.
One big advantage of a septic system is that you have full control. You know that it is your own responsibility to maintain it. You can never have sewage water flowing reverse into your house because some city official failed to maintain and expand the public sewer.
I have a system that is from the 1950's and it's been working without any problems. One has to just
keep in mind that this is a biological treatment system relying on bacteria to do the job.
Bacteria metabolize solids, converting waste into water, carbon dioxide, dead cell mass, and mineral ash.
you must not poor anything into the sink or flush down the toilet that will kill or overwhelm those little
creatures. These bacteria do a very important job for you.
The two physical components of the septic system are a septic tank and a leaching bed (also called a tile bed, septic field, septic drain field).
The septic tank looks like this:
septic tank, concrete, older design, 1950's
This is a tank from the 1950's when plastic pipes did not really exist. Modern tanks work the same way but
the chambers would be interconnected by plastic tee-pipes. The chambers in my tank are separated
by wall sections at different heights. Light material such as e.g oils and soaps accumulate
at the top and heavier materials at the bottom. Sewage water goes through all these chambers where
a lot of bacteria live and as a result you get almost clear water coming out of the pipe on the right.
For every unit of water that flows on the left into the tank the same amount flows out on the right.
It can however happen that gases form in the heavy stuff at the bottom and then parts of it float up.
You can therefore have a small amount of solids going out of this type of tank. New septic tank designs have
therefore something called "effluent filter". It's a kind of a mesh with holes the size similar to mosquito nets.
Modern tanks have besides the effluent filter also an inlet baffle to keep the floating scum layer from plugging the inlet pipe however in this older model this can not happen since the inlet pipe sits much higher than the outlet. The inlet pipe is in this old tank always above the scum layer even if that layer has grown a bit. It is not necessary to add such an inlet baffle. An effluent filter is however a good upgrade for an older tank.
septic tank second chamber after 2 years, just before it was pumped/cleaned
You can see in the above photo that some scum floats on top behind the last wall in
the area near the exit pipe. The layer is very thin but it could potentially flow out of the pipe
into the septic drain field and clog it over time.
I can't install a standard effluent filter because all the different models of filters that I could find are too long and could
not be taken out for cleaning due to the manhole design of this tank. I did therefore build
my own filter and I used a fine stainless steel mesh for this.
Note that most metal mosquito nets are made from aluminum and therefore not suitable for building such a
The filter sits in a tee-pipe that is extended a bit to the bottom. The filter could be longer than the bottom pipe section of the tee. The filter is just a role that is closed at the bottom. The role needs to be about 1 inch smaller than the
inner diameter of the tee-pipe such that there is least 1/2 inch of space around the filter. Here are a few
photos of the filter that I designed.
The green "seal" at the top of the filter is made of a microfiber cloth. It seals the space between filter and the inside of the pipe near the top. One has to use fully synthetic materials and threads to sew this because natural fibers would start to break down quickly.
Everything is made from stainless steel, even the handle to pull the filter out. The filter is 12 inch long and the diameter is just a bit over 3 inch.
I used the opportunity to repair as well parts of the cement wall. The cement wall was already deteriorating
and there was a danger that it would eventually fail. After the tank was pumped and the water level was lowered
I used a special re-surfacing cement. I did also apply cement glue to the wall before putting the re-surfacing
cement on. This is to ensure proper bonding.
The tank is in good shape despite its age. The outgoing pipe was originally made from ceramic but it had
been replaced in 1997 by a modern 4-inch plastic pipe. It was therefore easy to install this plastic tee
with the filter.
6 month later, Oct. 19, 2019
Six month later I opened the tank again to see how the filter is working. The filter is slightly shorter then
commercial filters but I had no chance to install a longer filter because the tank lid is not directly above the filter.
This meant that the filter had to be short enough to be able to insert it and pull it out again. The capacity seems however just enough to run for less than 6 month. The filter
was quite "dirty" but it was still letting water pass. However the water level in the
tank was about 1/2 inch higher than the exit. I noticed that there was suddenly more
water flowing as soon as I pulled the filter. In other words a little bit of water pressure
was needed to let water pass through the filter. It was not free flowing.
This very small filter requires more frequent cleaning intervals or I need a filter
with a bigger surface.
The filter was designed completely in stainless steel however the handle bar at the top was a different stainless steel
than the filter mesh. The mesh looked like new after cleaning but the handle bar had rusted. The handle bar
was made from a stainless steel cooking utensil. There seem to be quite different grades
of stainless steel.
The black colored particles on the filter might be organic matter covered in black anaerobic bacteria.
To clean the filter I pull it out and wash it with a garden hose.
The system is working as expected and it is protecting the septic field. I had hoped for longer service intervals but it is hard to use a bigger filter given the location of the lid. I might design a filter that can bent a bit during insertion and removal. Such a flexible filter could be longer than the current filter.
Update, Nov. 24, 2019
I have designed a new filter with a flexible section in the middle. This allows me to make it longer despite the location of the tank lid. Here are the old and the new filter side by side. The longer filter should work with much longer cleaning cycles.
We have already some snow but it was still possible to open the septic tank.
Left: old filter, 22nd of November, one month after previous cleaning. Right: new long filter and old filter side by side.
You can see that the older (shorter) filter did not accumulate a lot of organic material in this short time period. I don't know if the amount of particles increases linearly after the tank was pumped or if it has a seasonal dependency. I have no idea but I will investigate that over the next few month. Now I have a filter that is twice as long and should be big enough to get me comfortably trough the winter because there is no easy way to get to the tank during the next 4 month. It will be covered in 3-5 feet of snow.
Update, March 28, 2020
There is still some snow in the backyard but the area around the tank is free of snow and
it is a warm day with +3'C. This means I can use the garden hose and I can open the tank
to see how the longer filter is doing.
Good news: the new filter did not cause even the slightest increase in water level. This means it is not obstructing the flow. It was winter and the biology in the tank seems to have changed a bit.
The dirt on the filter is no longer black but brown. It will be interesting to see if it becomes again black during the summer. I used some 100% waterproof glue to secure the thread that holds the bottom cover. There seems to be some chemical reaction with stuff
in the water or the glue is actually not waterproof. It's still holding but it changed from transparent to yellow/white.
I am really happy with this result. I will now try longer maintenance intervals and see how
long I can go with this filter size.
Update, Apr 11, 2020
I have been trying to figure out how much water is running through this filter because that should be directly proportional to the required filter size. During the winter month there is no
water used in the garden. Therefore the amount of freshwater consumed is equal to the amount of water running
through this filter. We are a 4 person household and the total amount of water consumed per day is
108 Gal/day = 408L/day. This is 102 Liter per day and person. It's a little bit below the average household consumption
Thus if you want to build a similar filter you can estimate if it should be bigger or smaller based on your
Update, Sep. 26, 2020
The filter is working perfectly. I am cleaning it today because I will soon remove the garden hose for the winter. The water level in the tank is not elevated compared to the outlet. I did not think that I had uneven grass growth over the septic field but now that I have this filter for almost 2 years I can say that the grass growth is more even than before. In other words the pipes in the field might have contained some organic sludge and the field has now recovered. It means that such a filter makes a difference and does indeed extend the life of a septic system.
Update, Oct. 26, 2021
The tank had been pumped in the spring of this year. I did not fully clean the filter at that time and just sprayed it a bit with the garden hose from the side while it was still in the tank. Now it's October and I am cleaning the filter before the start of the winter. It's quite clogged. The water level was a bit elevated (1 1/2 inch) and the stuff on the filter is black in color. Proper cleaning of the filter every 6 month is definitely required even with this bigger size. I will make a filter that has 3 sections and it will be about 1m long but I think 6 month maintenance cycles will still be required.
Why is it black? The filter was also black in 2019 and that year I had the tank pumped too. Is it related to the fact that the tank was pumped in spring? I think the black vs brown has something to do with the biology in the tank. It changes somehow.
Other factors that could play a role: My tank has concert lids. They are not fully air tight unless I cover them with some earth. I don't remember what I did in 2019 but this year I put some composted leaf and sawdust mixture
on the lid before covering it with decorative stones. This seals it and oxygen will not enter the tank. Last year I did not do that because I wanted to be able to open it easily to monitor the filter. Maybe some oxygen is drawn into the tank and that changes the biologic process. I don't know. It's a theory. I will keep sealing it and see if the color is still black next year (no tank pumping next year).
I am still happy with the filter. It seems to keep the field clean.
© 2004-2023 Guido Socher