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Watson Wicks: An Extremely Simple, Low Cost Alternative Septic System


Watson Wicks, also known as pumice wicks, are a very simple, inexpensive septic system alternative; shallow, subsurface bio swales designed to reuse the nutrients and water in blackwater for irrigating and feeding plants.


Tom Watson
Tom Watson —the guru. He's available for consulting at PO Box 8, Embudo, NM 87531 or (505) 579-4001.

Forget everything you think you know about conventional wastewater system design. Crazed genius Tom Watson has come up with a design which breaks every rule in the book.

You could call it a shallow cesspool which receives rainwater as well as sewage, and has an overflow to daylight (that sound you hear is the rules breaking).

Or, you could call it a subsurface bioswale which provides all the advantages of a flush toilet, septic system, and composting toilet, with lush plants around it and few of the disadvantages.

Tom has installed numerous systems at 9,000 feet in New Mexico. The oldest systems are pushing thirty years old. Some are working perfectly, some are not.

Something highly significant is definitely going on here. If you ask Tom how they work, you'll get an answer that sounds like the the sound of one hand clapping. Tom really knows his stuff, but he thinks so far out of the box that it is hard to grok what he is saying sometimes.

What this technology really needs is a grad student to to a thesis on it. Each system should be inventoried, photographed, drawn, measured, and characterized as to it's context and performance. (if you think you might be this grad student,  Email us  and perhaps we'll be your outside advisor.)

Here's Tom Watson on Watson Pumice Wicks:

"People want answers, not questions, but every system is different..."

"With an understanding of first principles, you can derive the system design for any given context on your least in theory. It is an art as much as a science."

"Keep pulling yourself away from thinking of it as an object, think of it as a process."

"Treat wastewater as landscape architecture opportunity."

"Mainly people have to go out and do it themselves. Learn first principles, then go do it themselves..."


Image from Sustainable Sources Building


The rest of this is my notes on conversations with Tom, collated with my experiences building a few wicks myself, and a few of my thoughts in [square brackets].

CAUTION: these are just unchecked notes which we may someday check, refine, expand, and turn into a book or a chapter in a book. While many of these are working flawlessly, a significant fraction have failed. I've had issues with gophers filling similar systems for greywater only with dirt in Santa Barbara (gophers love that they can dump dirt in the chamber without exposing themselves to predators on the surface).


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Design goals

First principles

Understand what soil organisms do

Improving soil quality over time

In response to the action of roots and soil organisms, the soil around properly designed and and installed Watson Wick becomes more permeable, higher organic matter, and of better tilth as the system ages. This is in contrast to a conventional septic system, where the permeability and tilth of the soil declines. [this is certainly true for mulch basins. Not sure how this would work in a wick, with salt from urine potentially lowering the permeability of clay soil, and no way to get mulch in there...but I suppose it's possible].

Provide for feedback loops

Use wastewater as a resource

Keep the Wick as close to the source as possible

Do as little plumbing as possible

Simpler is better.

Keep the Wick close to the home

So the Oasis of green it nourishes adds to the ambiance right around the home.

Plants are the main monitors of the system

Real monitors of system are plants and how they are doing.

Soil needs air as well as water

Paths for free air circulation are provided by pumice.

Provide access for inspection/ service

Ideally the entire top would be removable.[currently available infiltrators do not facilitate this.]

Three modes of water transport

Failure issues and responses

The most common failures are from failure to provide for a drain to daylight, digging the system too deep, or using too fine of aggregate.

No drain to daylight

A pumice filled trench which has at least fifteen feet of aggregate to "open water" inside an infiltrator is a crucial but often omitted feature. This drain should be from the level of the *bottom* of the infiltrator, not the top. [This short circuits progressive failure, see Long-term acceptance rate.]

Without this drain, a peak load can jump the system from an equilibrium with a 1/2"-1" deep anaerobic "bathtub" to a deepening bathtub death spiral. When the bathtub edges reach the top of the infiltrator, the system functions more like a septic tank and has much lower capacity.

With the drain, surges from peak loads can pass through harmlessly, discharging treated water to the surface instead of initiating progressive system failure. [some would question the harmlessness of surface discharge, but it's probably not too noxious after passage through the infiltrators and 15 feet of aggregate/ plant root hairs]

With pump-out tank don't need trench to daylight.

System too deep

The system has to be in the top foot or two of soil, where plant roots and soil organisms are most active. 21" is the typical max excavation depth, with 1-6" of soil over the top of a 16" infiltrator.

House drains often exit too deep for optimum Watson Wick depth.

If it is necessary to place a system deeper, air can be introduced deep into the soil column by backfilling with coarse pumice, with only a thin layer of soil on top.

Just be sure you have a drain to daylight as per above.

Too fine of aggregate against the infiltrator

Pumice fines, fine soil, or other fine aggregate can cause clogging at the infiltrator louvers. This kind of failure can be identified by a high water level visible inside the infiltrator inspection port and lower level or no water in on inspection of the surrounding pumice.

This kind of failure can be avoided by using coarse aggregate; over 3/8" in diameter, over 1/2" preferred.

Another option is to use more infiltrators, in which case the backfill can be native soil.

Low perk soil

Requires a much bigger system.

Climate-Season of precipitation

[Not much is known about performance of these systems outside the Southwest.]

Systems approach

The design must be considered in a whole systems context, or you'll have a higher probability of failure.

Redundant backup

Back up experiments with a redundant system; failure of the experiment can be relieved by switching to the backup (the system capacity recharges on giving the soil a rest). The backup could be an alternate infiltrator set, or another kind of toilet, or a toilet in an alternate location.

Building codes

Codes must ultimately adapt to solutions rather than visa versa. Be allies with the code people to help them adapt codes to solutions.

Some Watson Wicks have been code approved under experimental permits in New Mexico. [Note that model codes are national, adapted by the state; locals don't have much latitude to change the codes themselves, though they are the Gods of enforcement and interpretation. See Legalize sustainability.]

Testing results & observations

No wastewater plume

Tom found little or no free water in the soil space 8-18" below the infiltrators in established Wicks in New Mexico, using ceramic soil tensiometer probes which fill with water.

More roots in pumice than in soil

It was frequently observed that there were more roots in the pumice than the soil, perhaps due to the presence of more air. [I've had greywater-only subsurface systems fill completely with roots, necessitating a system overhaul.]

Fecal gorp is self-leveling

The wastewater mix coming into infiltrators seeks a common level, even across a long distance. I asked what Tom expected the wastewater level difference to be across a thirty foot length of one foot wide infiltrators fed from one end and he felt there would be little or none.

Design particulars and tricks

Always provide for a drain to daylight.

Place underneath a "lawn tray" [if you're collecting rain]

A dead flat expanse of lawn with a raised berm and hardened overflow is the perfect receptacle for rain water to flush the system and keep plants growing well.

Flat ground promotes rain infiltration, which is good for infiltrators in dry climates. [Also good for mulch basins for stormwater infiltration.]

Place infiltrators flat

Since the floors seal with a low-permeability anaerobic mat, you can make the most of each infiltrator by placing it flat, possibly stair-stepping down hills, with an overflow 1" from the bottom of the uphill infiltrator cascading overflow to the inlet of the next infiltrator in series, and so on. The Wick which I've monitored most closely is built on a steep slope (not advisable if you can help it) and it is loaded intermittently. I've never seen evidence of generalized anaerobic conditions or a mat inside.

Place infiltrators on lawn edges or under paths

Part of the artistic camouflage to minimize the visible difference in turf growth from areas underlain with pumice, infiltrator, or native soil.

Pumice over the top of an infiltrator, while it seems it would dry out the turf above, actually helps wet the overlying soil by allowing for vapor transport of moisture from below.

As little as 1" of soil will filter out sewage odors.

Salts don't disappear, get diluted and redistributed.

Anaerobic mat bathtub should optimally be 1/2"-1" deep

A "bathtub" lined with an anaerobic bacterial mat of low permeability typically forms in the bottom of the infiltrators.

The depth of this tub will rise and fall with the balance between the aerobic and anaerobic bacteria at the upper edge of the tub.

More water will enable the anaerobes to build the wall up, which will contain still more water, and so on.

Less water leaves the edge dry, so aerobic bacteria can eat it, dropping the water level further.

Tom occasionally jump starts the anaerobic slime mat by smearing the bottom of the infiltrators with pig manure.

Anaerobic mat depends on absence of oxygen, presence of creatures. Creatures are designed to get their mat more extensive; it reduces the permeability by about a factor of 100. This makes a feedback loop: water at 1", mat up to 1.25"...water level rises, etc.

The system size, overflow height will determine the dynamic balance and water level in system.

If the system isn't designed right, anaerobic mat will get ahead of aerobes and system failure will result.

The chamber can be anything which allows room for composting:

The inside of the chamber is so wet and warm that material will compost as if it were inside of a compost pile.

Surge capacity of the system:

Ability of system to absorb surges without going into destructive positive feedback loop depends on its size and the presence of a drain to daylight.

Intermittent loading

No problem with intermittent loading. Drying out is perfect conditions for complete breakdown.

Can accommodate a septic tank upstream but this is not necessary


Doesn't seem to be a problem in practice (see example at 9,000' elevation, below) If frost line is four feet, two foot deep infiltrators don't seem to be a problem.


(OK grad student-we need you to flesh out this section!)


A system at a bed and breakfast at 9,000' in New Mexico.

Experimental permitted system in Santa Fe

It performed perfectly for many years. At some point the inlet pipe separated from the infiltrator, and roots blocked the inlet, an easily-remedied problem.

Rural couple

Never had to take gorp out.

Inspection & Maintenance


Design whatever is easy to build.

If you're hand-digging, do it easy to hand dig.

If back hoeing, do it easy to backhoe—but don't dig too deep.

Do perk test.

If slower than 60 min per inch.

If faster than 5 min, then use 3 infiltrators, 50 gal 1" deep, will soak in through mat.


Just use more infiltrators if you don't want to surround infiltrators with external aggregate.

Rather than rise & clog, liquid will spread through aggregate on bottom of excavation.


Do three infiltrators then add more if needed.

Mature system would be equally wet throughout.

When digging, when trench is done and appropriately sized, roughen walls, add hydrogen peroxide to walls to roughen, if smeared smooth by digging tools..

Adding compost on construction unnecessary.

Inspection port/ overflow at inlet pipe, mid or end of infiltrator.


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