Designing and Building in Central and East Africa

From Sky to Earth: The Path of Water Through the Global Model Earthship

In my previous post, From Sky to Earth: The Path of Water in the Simple Survival, I brought us through the most basic path of water for a single studio sized “Earthship.” This was the most ‘bare bones’ method of providing sufficient water storage and reuse for a single person or couple living “off the grid.”

earthship-global-modelWhile this may be sufficient for the most resourceful homeowners (or for those of us with tight budgets), it’s important to know that some of off-grid housing projects can supply all of the amenities required for modern living. The Earthship team has developed such a design which they have titled “The Global Model”, named with the intention that such a design can be built anywhere in the world. While alternative designs should be considered for extreme climates, the path of water employed in the Global Model represents a top-of-the-line strategy to both provide filtered, potable drinking water, reuse grey water for the growth of crops, and incorporate standard septic details to ensure code compliance in formal building areas.

afritekt-earthship-gm-00The global model is designed almost purely in section. The slope of the front “greenhouse” glass and solar panel facades are designed to
maximum efficiency according to geographic latitude. The roof is also designed to a minimum, consistent slope in response to the winter solstice. This ensure that the roof receives sunlight at all times of the year, enabling snow to melt more rapidly. With the exception of operable skylights to assist in warm weather ventilation, the single surface roof also minimizes the potential for leaks.

afritekt-earthship-gm-01The path of water harvesting from the roof is much the same as the Simple Survival. While any standard gutter system could be used, on-site cut and formed gutters allow for a custom transition from gutter to scupper to storage. The current Earthship strategy includes a small dam at the end of the scupper, and gravel within a perforated industrial sized salad bowl (literally a salad bowl, as in cooking supply store, not hardware). The water then passes through an inverted toilet flange. This system must be bolted (with at least two bolts) to the water tank below.  If the project is being built in an area with regular snow fall, a non-toxic snow melting system can be installed that distributes heated glycol through copper pipes. This is an expensive option, but could be the difference maker when waiting for April rains.

afritekt-earthship-gm-02As is the case with the simple survival model, the water storage tanks are located within the earth berm and behind the tire retaining wall. It is essential that whatever tanks are installed are designed for operating below ground. (Most water tanks are designed for surface storage, and will rupture or otherwise deteriorate in subsurface conditions.) For a large scale home like that of
the global model, it can be assumed that more than one water tank would be required (precipitation rates and total number of occupants are determining factors). The connections between the tanks must be done with 2” flexible PVC pipe. The 2” is then reduced through a T to 1” semi-flexible pipe before entering the interior of the home at a single location. Everything viewed at right is buried below ground.

afritekt-earthship-gm-03While traditional homes may consider the fireplace as the hearth the organizes the plan of the home, the Water Organizing Module (WOM) is this the comparable element in an Earthship.  This system, comprised of a number of filters and connections, the effective hub of the entire water distribution system. Located just below the entry point from the water storage, the WOM begins with 50 filter, followed by a pressure activated 24 Volt DC pump. After passing through a 500 filter, water for washing is distributed and stored within a pressure tank. The pressure tank may be anywhere between 10 and 50 gallons, depending on your number of occupants and fixtures. Finally, an additional 1000 filter and ceramic potable water system distribute drinking water to a few points throughout the home.

afritekt-earthship-gm-04The Global Model Earthship seeks to provide all of the amenities of modern everyday life. This includes laundry washing machines, hot and pressured water for showers, and toilets that flush like normal. It should be made clear however, that this interconnected system will not work if it is not consistently used. Because grey water is drawn from the planters to flush the toilets, the system will come to stand still if the residents do not use the kitchen sinks and take regular showers. The human resident therefore becomes an integral part of this ecosystem; activating it and sustaining it.

After the initial use, the grey water is directed to a system of “greywater planters”. These basins filter the water for repeated use while providing needed moisture for indoor crops.  While the width of a planter is afritekt-earthship-gm-05typically determined by the 3 meter roll of EPDM that the Earthship team uses, meaning that for a needed depth of approximately 36”, the width also ends up being the same. (Depending on the strength of the terrain and the quality of construction, these channels could be made from reinforced concrete as well – though it would be harder to repair.)  At each transition moment in the planter, an inspection chamber is needed to ensure that water flows consistently through base of rock.  It must also be noted that a recirculation pump is required to run for a number of hours each day.  This bilge pump is typically linked directly to its own solar panel that provides power during hours of sunlight.

afritekt-earthship-gm-06The grey water from the kitchen sink and shower enter the grey water cells through a simple worm box. This composite box with perforated copper base makes an ideal home for red wiggler worms when kept closed and at a fairly consistent temperature. With only an initial base of wood filings, egg shells and/or newspaper clippings, the worms can convert the incoming grey water from the kitchen sink into natural fertilizers for the grow beds.  With the box sitting over a rock bulb, the nutrient rich grey water will be able to immediately pass deep into the planter.

Whenever the greywater is transferred from one planter to another or extracted for reuse, a rock bulb, inspection chamber, and EPDM skirt must be installed to ensure unobstructed flow. A transfer from one planter to another – pictured at right – must be installed so that both tanks are equalized; meaning that the water stay level and does not overflow from one to another. Also, any puncture to the EPDM must be done so precisely, cutting a 1” hole for the 2” pipe.



The end of the planter must create a filtered condition so that a pair of bilge pumps can regularly recirculate the greywater and extract water for toilet flushing. It is therefore essential that in the meter before approaching the rock bulbs, a pete-moss filter is placed.  This is essentially a perforated off-the-shelf bag that can fit snuggly into the channel.  A large rock build then supports two inspection chambers that house DC powered bilge pumps.


With water being reused from the greywater planters, a toilet flush afritekt-earthship-gm-09essentially consumes no water. With a pressure activated pump and a small 50 filter, the water is clear and no one is the wiser.  Upon flushing in a Global Model, the blackwater then heads to a standard, often prefabricated septic tank. These large concrete structures are designed to break down solid material and to be exhausted on a regular basis. These structures commonly overflow into a leach field safely underfoot. Earthships however, guide this black water into what the team has termed an “Evapo-Transpiration Cell” or E.T. Cell.  This outdoor system follows much the same standards as the greywater cells inside, though uses every bit of nutrients before reconnecting to a leach field.

Understanding this path of water and all of the details associated with it was one of the main reasons why I sought a place at the Earthship Academy. In light of the projected water crisis that will affect many parts of the world (including the high desert of Taos, New Mexico), it’s it extremely concerning that such decentralized water systems have not become more standard practice.  I expect that in the coming decade professionals will be looking towards these systems found in the Global Model as examples that can begin to question established building codes that require wasteful consumption of valuable resources.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s