Residency & Law / Mexican Fosa Septica

Mexican Fosa Septica

Mexican Fosa Septica

5 October 2006 Real Estate FYI 99

We promised to explain why you should not put paper in a Mexican toilet, and we’re not going to let you down. There are actually several answers to this question and the most puzzling of these is: you can put paper in a Mexican toilet! Yes, you can. The toilet will not spit the paper back out like a wrinkled dollar from a vending machine. The toilet police won’t show up at your door. And there will be no immediate ill effect from your indiscretion.

But it would be rude.

In many tourist destinations in Mexico, especially hotels, where modern sewage treatment is available, you are encouraged to flush your paper, just like in Gringolandia. The hotel management may even post little signs to let their Mexican guests know that they are expected to dispose of their papel confort down the toilet. They have to do this because Mexicans are trained from birth to be very polite.

But away from the tourist hangouts, and especially in private homes, you will encounter a small, covered, plastic or plastic-lined wastebasket near the toilet. If you see one, then be a polite guest and put your paper in there, not down the toilet.

To understand what etiquette has to do with how you dispose of toilet paper in Mexico, we need a basic understanding of sewer systems. If you are reading this from your home or office in Gringolandia, chances are that your toilet (and anything else that drains from your house) is connected to your city’s public sewer system. Everything you flush flows through large concrete pipes to an industrial processing plant where the solids are separated from the liquids. The liquids are filtered and treated with chemicals and the result is released back into the environment in the least offensive way possible where nature finishes the process using evaporation and rain. The solids are also treated and refined, resulting either in trash or fertilizer. This immense infrastructure is quite expensive to install, operate and maintain. It also consumes a lot of energy. These are your tax dollars at work.

Gringos who flush outside the city limits use a private septic tank, called a fosa septica (septic pit) or sumidero (drain) here in Mexico. In Gringolandia, a septic tank is usually made of a durable plastic and has two chambers. The first is called the sediment chamber. This is where the wastewater enters. Most of the solids collect here at the bottom where anything organic is slowly digested by bacteria. The liquids continue on, passing through PVC pipe (and perhaps some baffles to prevent large solids from entering) into the second stage, called the clarifying chamber. Here, particles are allowed to settle while the clearest liquid exits the septic tank.

Unlike their northern neighbors, almost all Mexicans use a fosa septica, which is not much different than a Gringolandia septic tank. This technology is very old, so the process is the same. The only difference is the materials. Here in Mexico, many of the colonial houses and other buildings were constructed before the advent of plastics, so most fosa septica are built from plastered stone or concrete block. While plastic septic tanks have one or more manhole covers to permit inspection and cleaning, Mexican fosa septica are generally covered with a slab of concrete and sealed with plaster, like a tomb.

The important difference between a traditional, Mexican fosa septica and a plastic, Gringolandia septic tank is what happens when the clarified liquid is released back into the environment. In the plastic version, the liquid enters one or more perforated PVC pipes, which are buried in long trenches about four feet deep, filled halfway with gravel and covered with topsoil. This is called a drain field, and it’s where you want to plant your strawberries.

In the traditional, Mexican version, the liquid flows down into a filtro (filter), which is a concrete-lined pozo (well) filled with several inches of gravel on top, followed by several inches of charcoal in the middle, followed by a foot or two of sand at the bottom. Why use a filtro and not a drain field? One reason is because the filtro does not use plastics. Another is that this method takes up less space, which is a requirement in colonial urban zones.

But the filtro is the hurdle, so to speak, on the toilet paper trail. Even if much of the paper discarded in a Mexican toilet remains in the fosa septica’s sediment chamber, tiny bits do float past the clarifying chamber and into the filtro, so that over time a paper mache sludge builds up.

How much time does it take to clog a filtro? Nobody knows. It depends on how big the fosa septica and filtro are, how many people are using the toiliet and how much paper or other non-biodegradable stuff they’re flushing down there. It could take three years, or five, or ten. If you don’t flush any paper, it could take fifty or more.

As you probably know, all septic tank systems eventually fill up with sludge and non-biodegradable stuff and have to be pumped out. In Gringolandia, where most septic tanks are located under a lawn in the yard, this is not such a big deal: just dig for a few minutes, screw off the lid and pump away.

In Mexico, maintenance can be a bit more trouble. Many fosa septica are located under the patio, or they might be under the foundation of your house or even partially under your neighbor’s house, because many of these old colonial homes are the result of subdividing a larger mansion. Even when located in a back yard, the access is limited, which means the workers and their hoses will probably be coming through your front door. What’s more, most fosa septica are as old as the houses. Digging into them, like unearthing an ancient tomb, can be risky, leading to cracks or a complete collapse.

This is where famous Mexican thriftiness meets Mexican toilet training. No matter what their socio-economic station in life, Mexicans stretch their pesos and pretty much everything else. When the convenience of flushing paper down the toilet is at the expense of flushing pesos by cracking open a fosa septica, Mexicans would rather have the pesos. In this sense, it would be as rude to flush paper down your host’s toilet in Mexico as it would be to leave the door open on their refrigerator.

Most new homes and residential developments being built in Mexico today do use plastic septic tanks in their construction. When we were working on the design of our new home, we were offered the choice of a plastic septic tank or the traditional fosa septica. The plastic version, called a Septi-K, is billed as an environment-friendly version. It costs less than a fosa septica and has a cover you can remove to rinse the internal filter. The clarified liquids empty into a leach field or French drain. Every ten to 30 months, depending on use, you have to manually remove the lodo (uh... mud), which you can put in your yard as fertilizer or perhaps share with friends. And you can flush paper into it like a gringo.


When we visit Gringolandia, we now feel uncomfortable putting paper in the toilet. Is it because sorting recyclables by hand is planet-friendly? Is it because it feels like throwing money down the toilet? Or is it just force of habit? Hard to say. In the end, we chose the traditional fosa septica for our new house.

So now you know what to do when you visit our bathroom and why you are doing it. Thanks to you, we may never have to service our fosa septica. At least in this one small way, we have assimilated into Mexican culture.


  • joshua 7 years ago

    So who does the pumping on the coast here in yucatan, is there anyone with a contact number, or place? There has got to be some company?
    I think I have seen the truck pass me by.

  • Jack W. 7 years ago

    I find it amazing that this subject has generated more comment than nearly any that I have read on your site in over three years. What I find particularly interesting is that some people consider this information "ridiculous" and "nonsense". He obviously has never been around when it was necessary to pump a tank which had become clogged with paper from too many visiting gringos. My, what a stink. Like "working gringos" said, "you can flush it, just NOT IN OURS". I have lived in and around relatives and friends who have lived with septic systems ever since I was very young and I guess it has become my habit to look for the little trash can to see what's in it and whether I needed to be a good visitor too. Thanks for this very informative article. We are adding a bathroom to a three hundred year old house along with a kitchen and this has given me some ideas about where to put the sumidero , rather than UNDER the bathroom which is where I think my contractor intended to put it. Also, an excellent point to mention regarding systems near cenotes. I certainly would not want to swim in a cenote if the water indicated that someone's fosa septica was leaching into the water supply for the cenote. Job well done.

  • Jim 7 years ago

    Steve from South Africa...

    Is your "bioaugmentation" product available in Mexico? More specifically Baja California Sur?


  • CasiYucateco 7 years ago

    In the last picture in the article, the man on the left is using a hand pump made for the purpose. There are various types of devices used to pump the mud from the bottom.

    You could probably also ask the place where you buy the tank. They should be able to advise you.

  • Bart Borriello 7 years ago

    I live in San Cristobal and am building a house and will be using Roto-Plas septic tank. I have one question. How do you get the mud/waste out of the tank. I am not able to find a clear answer anywhere. I see the picture posted on this forum but don't quite understand the procedure....thanks

  • maurice 8 years ago

    I live in the UK but spent several of my earlieryears in Libya and Oman. At first I wondered why the Arabs all carried a tin of water with them into a toilet. Quite simple really! No toilet paper. They simply washed their posteriors after any action. Having been forced into employing the same method I soon realised how much more hygienic it was. Naturally, ones hands have to do the washing (the left of course, the right is for eating ), but then that is what we do in the shower, isn't it? Since returning to the UK I find using toilet paper quite disgusting (especially the cheaper brands ). I have now installed a flexible hose with a portable shower head beside all my 3 toilets. Toilet paper is simply no more. All you septic tank owners should seriously consider this "Do It Yourself" improved bidet method. You will be surprised how soon you become converted and how soon you will regard toilet paper users with disdain. By the way, I share a septic tank with my neighbour and we have a problem in that the final outflow is not soaking away due to the high level of clay below the top soil. I like one of your readers ideas whereby a stick of dynamite creates a sufficiently large subterranean soakaway crater. I might have a bit of a problem persuading my that this is the right route as the soakaway has to be in his garden.

  • Steve 8 years ago

    I live in South Africa where the inhabitants in the more rural areas make use of a waterless pit toilet. These smell, attract flies & mosquitoes (malaria), harbour waterborne diseases and get full. I market a "bioaugmentation" product (specially selected natural bacteria) that biodegrades & liquefies completely all organic matter, including paper, in the pit toilet and can empty naturally the contents within 10 to 20 days. It reduces the smell by over 85% within 48 hours and the waterborne diseases are neutralized in the pit. The effect on a septic tank is similar and will totally biodegrade and liquefy the solids and sludge residue normally found in the bottom of the tanks. This product will also open up the soakaway drainage rendering the system fully functional. You will not have to use a vacuum truck again. The product is extremely successful in the biodegrading of grease, fats and oils in grease traps (food prep & restaurants) and decongesting and liquefying the solid fats buidlup in "grey water" drainage systems. Solutions out of Africa can be used very successfully in the rest of the "other" world sewage systems.

  • JAYMO 8 years ago

    HI, I am assisting my parents in renovating a casa in san pancho, and we are at the septic tank stage. The house was partially completed when we purchased the house, and the septic system has not been installed. we are looking at a plastic tank system that will serve around 6 bedrooms. We just received a bid from our contractor for 95,000 pesos. Can anyone elaborate on whether or not this seems like an accurate cost for this system. in doing research it seems high compared to prices here in america, but then again that is a quick search on the internet.

    especially since an earlier post stated he got a concrete system for around 2000 grand.

  • Raymond 8 years ago

    I just got back from Merida, it was my first trip there and Mexico. I bought a house after being there 9 days after booking an appointment with Eric of Mexico International before I left Canada. I am from Burlington, Ontario, Canada. I fell in love with city and it's people at first site. I have learned a lot from this site and now know what they mean when I saw the signs in the toilets there and now understand what they meant. I thought they were referring to paper towels, not toilet paper. Where I live I pay property taxes of $2,000.00 per year and the city is building parks, theaters, beautiful botanical gardens, pools, community centres and conservation areas but they charge you for parking your car and charge you to use all of these facilities. Not everybody can afford to spend $10.00 to go to a conservation area or $75.00 to go to a theatre which was paid for through their taxes. Merida has provided these for all of it's citizens to use and enjoy, rich and poor. When I was there the National Ballet of Cuba was performing in front of the Cathedral for everybody, not just those who could afford to spend the $75.00 in a theatre which their tax dollars paid for. The city has something every day of the week for the people living there. It didn't matter where I went, the people were so kind and tolerant of me, even though I speak no Spanish. The policeman who told me my camera strap was hanging from my bag and the Mexican lady who took a picture of me in front of a monument on the Paseo Montejo with my camera. The wonderful peope I met in the plazas. I have been all over Europe and the U.S.A. and have never felt as comfortable as I did in Merida. I only wish I had found about Merida earlier in my life. I first found out about Merida when I was watching a TV show called "House Hunter's International" ... a couple from the city next to me was buying a house there, my realtor told me they sold 45 houses because of this show. On a lighter note, @ Paul Bowley, are you bragging when you said your body parts are hitting the water? Just kidding.

  • Paul Bowley 10 years ago

    I will accept the information given and have followed the general rule of no flush the toilet paper without much more than an esthetic problem.

    Why don´t they have toilet seats? Sitting on the porcelain is very uncomfortable. There should be a course describing how to use el bano. Sometimes, my body parts hit the water so to speak and I find this very disquieting...a yuchy experience!

  • Working Gringos 10 years ago

    Hola Juan,

    Our concrete fosa septica serves 5 toilets and cost around $1,800 USD to construct. Its design is much improved over the diagram in this article. The contractor and architect was Justiniano Cervera Mario Humberto, who also specialized in swimming pools. His telephone number is (999)947-5132.

    The so-called "gray water" from the kitchen and bathrooms also flows through the fosa septica.

    We grew up in California, but you probably know more about Whittier than we do!

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