Real Estate FYI / 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.


  • W. Carter 4 years ago

    In rural coastal areas, are septic tanks used to gather and hold excrement? Are septic tanks flushed out periodically?

  • daryl 4 years ago

    Up here in victoria, british columbia the city still pumps a million gallons per day of raw sewage into the pretty harbour. Mexico has septic pits as in rural parts of canada and somehow mexico is the dirty rat ????? Kelowna discharges and has the water intake close to this and beaches get closed due to e-coli as well.

  • Ann 5 years ago

    I went to Mexico for the first time in my life and was shocked to know that the place I was staying at, which was not a big resort, did not allow us to flush the toilet paper down the toilet. Many times I had to 'fish' out the toilet paper from the toilet because I have been accustomed to flushing my toilet paper down the toilet. I now understand why people have always said that a person should only drink bottled water...if the waste water is not filtered properly and the water is not purified, it's no wonder a person would get sick. I wish that I had read this article before going to Mexico because it was a 'rude awakening' for me. I now have a greater appreciation for my own country because I used to take things like that for I appreciate our sewage and water systems and realize how fortunate I am to live in a country that has proper sewage and water systems.

  • Alex 6 years ago

    We live in the Sonoran desert in San Carlos . All the fosas here are basically a 10' x8' x 7' deep hole in the ground. The bottom is dirt and the sides are staggered bricks to allow effluent to bleed off. There is no drain field just the hole in the ground. These work well here but need to be flushed and pumped every 15-20 years. We do not throw toilet paper in the toilet. I like the simplicity but feel that these may be too simplistic. Would like any comments and or suggestions

  • wpcoe 6 years ago

    I've lived in Thailand for 13 years, and for the most part, local Thais (especially in rural Thailand) do not flush toilet paper down the commode.

    However, most toilets have a hand held water sprayer hanging on the wall where most of us have toilet paper rolls hanging. It is identical to the vegetable sprayers many US kitchens have at the sink. All it takes is a T-junction to be able to divert the water from the pipe in the wall to two directions: one to the commode and one to the sprayer.

    Took me a few uses to master the art of spraying my backside without unduly wetting the entire bathroom, but I swear by the thing now. Wiping with paper just seems unsanitary. Ditto for using your left hand and a bottle of water.

    Even better are the "washlets" of Japan, which are toilet seats with a built-in bidet function. A telescoping tube comes out the correct distance and rinses the appropriate area -- a separate setting is used for women's personal hygiene. Some models have a heating element for areas (like Japan) where the water might get too cold in winter to spray at your derrière.

    I still use a little toilet paper to dry myself off, but I know many (most?) people just pull up their pants/skirts and air dray naturally. Some of the fancier Japanese washlets have a fan to blow you dry with heated air.

  • Antonio Suarez 6 years ago

    Brilliant detailed explanation! I agree how we feel when having the possibility to flush when we are used to throw paper in to bags. What I have decided is to flush the first one or two papers I use, those contain 99% of shit while I throw the rest in the classic Mexican basket bag. I have a biodigestor SIRDO which is known for is environmental compromise.

  • Rodrigo 6 years ago

    This is nonsense. I'm a Mexican, and I have lived in Mexico City all my life. I've never heard this argument for not flushing toilet paper before. It wouldn't damage the septic tanks and even if it does it's an exaggeration to say that almost all Mexicans have septic tanks under their houses. It is actually around 3/10 people and most of them living in urban areas. The rest of the country has county sewage systems. I invite you all Americans to come to Mexico and get to know our country a little better before you start talking nonsense.

  • Working Gringos 6 years ago

    In the grand scheme of things, it probably isn’t that big a problem, but if we were you and since the lid is off anyway, why not have a septic tank pumping service take this opportunity to clean out the tank? Oh, and put the lid back on before somebody’s dog disappears. Thanks.

  • jose 6 years ago

    I have a crazy question, crazy but true. We were doing some construction nearby our septic tank, so we removed the cover. Unfortunately the Mexican workers allowed some dirt to fall into the tank, but I figured it will be okay. The next day a dead cat appeared on the property, so the owner throws the cat into the tank, and in doing so he inadvertantly kicked his shoes off and they fell into the tank as well. I am the only gringo here and all the Mexicans think this is not going to be a problem in the future. Who is correct? Me or them? By the way, the cover remains off the tank till this day. It is simply covered with some corrugated metal. I am thinking the cat will bring rats, mice, etc., as well as disease. Will the cat get flushed away? There is no backup as of yet, but it has only been 5 days since the genius put the cat and shoes in there. Thanks for any thoughts or replies.

  • Septic tank 7 years ago

    The fosa septica would be banned in Europe because of spot contamination of groundwater. Someone drinks the water that you contaminate.

  • 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.

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