Water Conservation

The Watson Wick Flush Toilet System

During a recent visit to a friend´s organic farm in Costa Rica, I made my way up the steps to his composting toilet system surrounded by bamboo posts and palm-thatched roofing. As I sat there listening to the birds, I noticed a small poster hanging on the door. It was a comic strip with two drawings. The first drawing showed two extraterrestrials staring into a toilet with question marks over their heads. The second drawing had one of the aliens saying to the other: “No wonder they went extinct; they shat in their own drinking water!”

Every day, we collectively waste billions of gallons of water in the process of taking our own waste as far away as possible from our homes. “Out of sight and out of mind” is the guiding framework for why we have so willingly come to accept flush toilets as the only “sanitary” way to depose of our own waste.

While several unique designs of composting toilets are beginning to gain acceptance among everyday people, there is still a large segment of the population that wants nothing to do with shoveling their own (composted) manure out of a compost bin in order spread it around their fruit trees. For people who have an absolute aversion to composting toilet systems, the Watson Wick septic tank is an attempt to make some use out of the thousands of gallons of black water that either get sent to the sewer system or anaerobically buried in a useless underground septic tank.

A Few Basics

The water from your flush toilet must be treated differently than gray water because of the dangerous pathogens that it contains. While waterless composting systems are perhaps the most environmentally friendly way to deal with your human waste, not everyone is going to be open to composting their feces. Flush toilets have come to be considered synonymous with “hygiene.”

The Watson Wick system is an innovative way to get the most out of your flush toilets. Whereas almost all flush toilets either send black water to the municipal sewers and water treatment plants or to a septic system which leach the black water far beneath the soil (and out of the reach of most plants´ roots), the Watson Wick System allow the nutrients that are in black water to be utilized by plants while completing eliminating any sort of risk associated with the pathogens that our waste contains.

Flush toilets use extreme amounts of water. A normal flush toilet uses an average of 1 gallon per flush. For a family of four, the toilet gets flushed an average of 16 times per day. Over the course of a year, more than 5,000 gallons of water are wasted just to send your human waste (packed with very usable nutrients and fertility) into an unusable water treatment plant.

How the Watson Wick System Works

A Watson wick septic system uses pumice (or coarse sand) as a wick to filter, clean, and decontaminate black water. It also differs from normal septic systems in that the leach field is close enough to the surface of the ground so that plants´ roots can access the nutrients in the black water. To build a Watson Wick system, you´ll need to dig a 2.5-foot deep trench and lay 18-24 inches of pumice covered by 6-12 inches of topsoil.

The length of your trench will depend on the size of your family and how much usage the system will receive. On one side of the trench, a simple infiltrator “box” that is left without a floor can be made out of blocks or brick and will help to capture the waste and filter it through the system. On top of the system, water-loving plants should be planted. Alternatively, you could also plant fruit trees, flowers, grape arbors or other plant species that will benefit from the excessive nutrient load while not posing any sort of risk to pathogen transmission. For example, you wouldn´t want to plant potatoes, but apples would be fine!

The pumice acts as an infiltrator. The solid waste turns into underground compost while the roots of the plants suck up the liquid which uses the nutrients and transpires the excess water. In this way, the 5,000 gallons of water you flush down the toilet yearly is converted in nutrients for your orchard and then transpired into the air.

If too much water enters the system, the pumice acts as a filter and filtered water drains out of an exit pipe placed on the other end of the system. Were too much water to accumulate, the system would quickly go anaerobic and smell horrible. Because the liquid is used up by the plant roots or filtered before exiting the system, however, the system remains aerobic, completely sanitary, and puts to good use your nutrient-rich waste.

Some people have successfully experimented with placing a Watson Wick system underneath a greenhouse. The excess nutrients and liquid maintain the soil moisture while providing all the fertility for the plants grown in the greenhouse. You might even be able to plant some dwarf citrus or banana trees if your winters are mild enough.

With a little bit of design and forethought, almost any home can have their plumbing systems restructured to send their black water into a Watson Wick system that will allow our human waste, along with the excess water from all that flushing, to go to good use.

Tobias Roberts

After working in the development industry for over a decade, Tobias decided it was time to stop advising Central American farmers how to do things if he didn´t have a piece of land to live coherently with what he taught. Together with his family he runs a small agro-forestry farm, tourism cooperative, and natural building collective in the mountains of El Salvador.


  1. A variation in a sand filter or horizontal flow Reed bed. Great idea, but these systems are very prone to blockage….

  2. Eight years ago we installed a Watson Wick on our property in the state of Yucatan, Mexico, which to this date works nicely. The use of a coarse aggregate, 3/8-1/2 diameter, helps to avoid blockage of the system allowing for the water to wick while providing aeration for the roots.

  3. A plan – or a link to a web site with a good plan and design – would help, but I got the idea and thank you for sharing it. What I plan to do is to install a composting toilet and convert the existing one into a “Watson Wick” one.

  4. This is an acceptable alternative for people in a warm climate and in un-incorporated areas. Properly build they can last for years. They all silt up after a while. We had a similar system in Vista Ca. We had great roses, avocados and pomegranates.
    However, when it stopped working, the liquids bubbled to the surface and ran across the yard in smelly stream every-time the toilet flushed.

    The whole thing had to be removed. The new zoning required a holding tank with a mounded leach bed. Regular clean out was required for the solids.

    Today zoning does not permit either of these types of system.
    Septic systems or sewers are the norm in most populated areas

    Milwaukee drys and sells the treated solids as Malorganite. It is a good product that keeps the city green.

  5. Here on our farm in Uruguay we are installing a Watson Wick bed as tertiary (final treatment) for our septic system. Primary treatment is a 1500 gallon polyethylene septic tank where solids settle. This will require pumping, but given the large size, it should be 5-10 year intervals. The liquid then flows to the existing “pozo negro” which is a concrete block leach tank that is surrounded by gravel. The existing tank did not work well as a leach pit due to the high clay content of the surrounding soil, so the effluent from it will now flow into a wick bed that is 10 feet x 30 feet x 3 feet in size. It will contain 2 feet of gravel and 8-10 inches of soil. The primary and secondary settling, anaerobic treatment tanks. will in theory, prevent clogging of the gravel bed by removing solid material and silt. Both tanks have access hatches so they can be pumped out at the same time.

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