Arrested Development - Ex-Pats Mexico

Sparks, To answer your original question, you might want to try installing the drain field pipe using short pieces of terra cotta pipe about one foot long., they were called “drain tiles”. We used to use them in the 1950s and earlier before the perforated plastic pipes were developed for that purpose.

To install them, if you can find them, you prepare a bed of gravel about one foot deep in the bottom of the trench, lay the pipes in them on a downhill slope of about ¼ inch per foot with a gap of about ½ inch between them. We used to cover the gaps on the top of the pipes with strips of tarpaper to prevent anything from dropping into the pipes and partially clogging them. The drain field pipes were then covered with one foot of gravel before filling the trench with earth to grade level.

With the sandy soil on your lot, you will probably not need to prepare a gravel bed under the pipes of a full foot, but a few inches of gravel should definitely be used. Drain fields dispose of septic waste fluids by two methods, percolation (absorption), and evaporation of fluids that rise to the surface of the ground via capillary attraction, so the septic tank and drain field should be installed as shallow, or close to the ground surface as possible.

A septic tank is actually a digester; nothing will enter the drain field except clear, if nasty, fluids in a properly sized and designed septic system. In theory, a well-designed septic system will never require the septic tank to be pumped out. As a practical matter, a septic tank in a moderate climate will need to have accumulated sludge pumped out of it about every twenty years, probably annually in snow country.

If you cannot find any drain tiles here, and I doubt that you will, drilling holes in 4” plastic pipes will probably be necessary, since drain fields and true septic tanks are not widely used in México. If you use plastic pipes, couplings will not be necessary as long as you cover the gaps between pipes with a strip of plastic or other impermeable building paper, etc. If you do use couplings, I would not glue the pipes together, you want them to leak.

In my youth, as a licensed plumber, I have installed dozens of septic systems in California.

Rex

If they don't use a septic tank like used in the States what do they use? Does it just collect in a tank and evaporate?

I remember the old red clay pipes with a big mouth on one end about a meter long. The larger ones could be used as a sand filled ashtray with the big end up. Seen nothing like that down here. A French type drain with plastic is probably what I'll use

Speedy, they use what we would call a modified “crib” in most places here. They are usually a metal or plastic tank within another tank, both of them have holes in them to allow the liquids to leach out and hopefully drain into the soil. If the solids are disposed of, it is dumb luck. A true crib is just a wooden tank with no bottom. They are built like a log cabin, but with no notches in the logs, so there are big gaps in the walls. They are installed with no drain field piping, as are most of the waste tanks in México. The use of cribs is illegal as a health issue anywhere in The US where there is an effective and enforced building code, with inspectors, etc.

A true septic tank has at least two compartments in it. There is a baffle resembling a wall that ends about 6 inches above the bottom and top of the tank. The purpose of the baffle is to slow the passage of the wastewater so the naturally occurring bacteria will digest all solids, etc. Only clear water will pass into the drain field of a properly sized, and designed septic system. The size of the septic tank and the length of piping in the drain field are determined by the number and types of fixtures, and the maximum number of occupants for the house. If only two people live in a house with four bedrooms, then eight occupants are used when determining the properties that the septic system must have.

A septic tank has a precise definition. They are not simply a tank to receive sewage. With no enforced building codes, anything goes here in México. Builders here will tell you that you “don’t need” a true septic tank with a drain field, you “don’t need” a grounded electrical system, you “don’t need” plumbing vents, etc., etc., etc. Most of them are not trained or educated to work in the construction industry. They may be honest, good men, and hard workers, but most of them are unqualified.

No additives of any kind need to be used in a properly designed septic tank. The bacteria that do the digesting are naturally occurring. You are carrying a bunch of them around in your stomach.

Rex

Thanks Rex, very interesting. I own a hunting camp in Georgia and that sounds similar ( barrel in the ground ) to what was used years ago.

Speedy, I used the barrel in the ground method myself one time in Northern California, we were building a flood control dam and we had a camp for all of the pipe fitters. We were receiving daily per diem payments for providing our own housing. I had brought my travel trailer to live in. Our camp was right next to the river there on a sandy beach.

I cut some holes in a 55-gallon barrel and buried it in the sand on the beach. I connected the holding tank of the trailer using plastic pipe. I didn’t glue the fitting joints in the pipe; it was only going to be a two-year project.

It was illegal as hell, but I wasn’t concerned. It was a flood control project about 30 miles from the nearest paved road, and the water in the small river was not used as a water supply anywhere; and most of the workers on the project bathed in it upstream of our camp, I had a shower in my trailer.

It worked, but I wouldn’t recommend that method of wastewater disposal for a permanent house anywhere.

Rex

I suppose this is good enough for a basic septic .... size and depth are the only issues

Sparks, the drawing of a septic tank looks good, it is a little different from those that I installed in the past, the lower gap in the interior baffle was at the bottom of the tank in the ones that I installed. The gap in the depiction in the drawing is higher; it should work fine though. The purpose of the baffle is to delay the passage of the liquids until the digesting process has been finished.

As far as sizing goes, it has been about 50 years since I installed a septic system, and I don’t remember any specific sizing. In the early 1950s in northern California we used to use a state approved manual to determine the size of the tank and drain field required, based on the number and types of fixtures, and the number of maximum occupants for a house. No building lot smaller than 5 acres was allowed to have a septic tank system installed in Napa County.

Around 1960 the state requirements were changed. Use of the manual was discontinued, and only qualified civil engineers were allowed to determine the size of the septic tank and length of the drain field piping, the minimum lot size allowed for the installation of a septic system was reduced to one acre in Napa County. Health and legal specifications may have changed since I left California 40 years ago.

Among other things that the engineers did when making their calculations, was to have several test holes bored in the ground and they filled them with water. A stake was driven into the ground next to each borehole and a pencil mark was made at a known height above the water surface in each hole. The engineer came back to the jobsite for several days to measure the amount the water surface had dropped so they could calculate the absorption rate of the earth to make their calculations.

The plumbers installing the system no longer had to do the calculations; we just followed the specifications as determined by the engineer.

With the sandy soil that you have described on your lot, you probably won’t need to be concerned about the absorption rate.

In The US, we used to be able to buy pre-fabricated concrete septic tanks of various sizes and have them delivered to the jobsite, or form and pour tanks in place. At one time, metal septic tanks were available there. They were illegal to use in most counties because of corrosion they were short lived. Pre-fabricated tanks are probably not available here. Relying on my memory alone, I would build a tank at least 8 or 10 feet long, 8 feet wide and 6 or 8 feet deep.

Relying on memory again, I would install about 50 or 60 linear feet of drain tile for each occupant, and no less than about 150 feet of drain tile, more is better. The drain field does not need to be one long continuous run of pipe; it can be installed in more than one trench side by side with about 4 feet between them. If more than one run of drain tiles are used, they should all be joined in a small tank similar to the registros used by the plumbers down here. There should only be one outlet from the tank into that registro.

It is critical that if several pipe runs of drain tiles are connected into a registro, that they enter it all at the same level, so that no one pipe run will receive more effluent from the septic tank than any other drain pipe run.

The inspectors used to fill those little collecting tanks that I am calling registros here with water and they would look to see that the water entered all of the drain tiles simultaneously. If necessary, they would sometimes use a mastic gun and lay a bead of mastic on the bottom of one or more pipes until the water would enter all of them at the same time.

As to the depth of the system, in a moderate climate everything should be as shallow as possible, starting with the plumbing piping system in the house, so that some of the effluent will reach the earth surface via capillary attraction (wicking) and evaporate.

In snow country, the septic tank and drain field must be installed below the frost line. They will be deep, in Anchorage the frost line is 7 feet. For that reason, septic tanks in a cold climate must usually be pumped out every year. Large pipes are usually run from the top of the tank in cold climates to the earth surface to facilitate getting a hose to the bottom of the tank so it can be pumped out.

Sorry if this post turned out to be a book. Once I got started, I couldn’t stop.

Rex

Yes I've done 'perk tests' or percolation tests but don't think that will be a problem .... but all the drain fields I've worked with have been on acres ... not lots. No way will we have space for a field that would be legal up north. I'm thinking of only one run across the front of the property (maybe 10-12 meters) or a pit with no cement bottom