Creating an earthship black water is simple. It only requires a lot of digging dirt. However, the area where we installed the black water system was loose fill and had not been compacted, so it was very easy to remove.
The principal concept of the black water system is to separate solid material from liquid, allow the liquid to gravity filter though rock, gravel, and sand, then provide nutrients to plants growing on the surface. The solid material is contained in 6-7 stacked large truck tires in the middle. As new material is fed in, the solids slowly erode away and feed the aggregate filtering trench. All black water material is retained by large rubber pond liner material.
A horizontal hole was drilled from the inside the greenhouse area to a large cylinder shaped basin outside in front of the earthship. This hole was just wide enough to fit a three inch diameter PVC pipe, which would exhaust toilet water into the septic tank outside.
We spent one whole day digging the six foot wide basin as deep as we could and making the bottom flat. Next, rubber pond liner was laid on the bottom and side walls of the six foot wide basin. Then, we stacked 7 seven large tubeless truck tires into the well and packed random sized rocks around the tires. While preparing the earthship black water septic tank, a rebar and lath lid was created to be placed on top of the tire tank. The principal concept here is that the tires would hold the solids and allow the liquids to leach through to two other lined trenches on either side of the tire septic tank. As the solids age, the earthship septic tank becomes a time released nutrient feeder to the outdoor planters. The rebar cap (some called it a bird cage lid) was set into place over the truck tires. Then a thick gritty mix of mortar was then spread out over the top of the bird cage and let dry. There was one rectangular whole made in the top of the bird cage that was not covered in mortar. supposedly the opening was there in case the tank needed to be serviced at some later date. The rectangular hole was instead covered with a board. After that, the entire septic system was covered with the back fill dirt and we never saw it again.
Next, two long, narrow, and deep trenches were to be dug running parallel to the front of the earthship. The bottom of these trenches had to be one meter (36-39 inches) below the septic tank exhaust. Because the earthship was built on the edge of a cliff and the soil under the building was uncompressed fill dirt, digging the trenches became very precarious. It was easy to dig the loose soil, but as we dug deeper the side walls became more fragile. Just rubbing slightly against the trench walls would bring down a lot of loose material into the hole you just dug. It was like three steps forward and two steps back. In addition, you had to throw the soil out of the ditch up over the outer ridge of the trench; the deeper you dug, the higher you had to throw. We were in a hole about 7-8 feet deep at one point.
Then, after spending a whole day digging the trench to the proper depth, the next morning we found out that we needed to dig another foot deeper. However, it rained the previous night, the side walls got wet. The inner wall supporting the house collapsed into the trench, back filling about two feet. The situation became unsafe for the house as well as the trench diggers.
One side of the trenches we were able to complete. The process was to simply place a very large piece of rubber liner in the bottom and on the side walls of the trench. To hold up the liner on the side walls we punched large nails into the liner material and loose fill walls. Then large rocks were placed around a 3 inch PVC pipe coming out of the septic tank. Once the pipe was covered, a three foot (1 meter) layer of one inch gravel was placed in the trench inside the rubber liner. A layer of sand about three inches thick was placed on top of the gravel. Then we back filled the previous excavated loose fill material back into the trench. That side of the earthship black water system was complete. The owners would only need to filled in the top with planting mix and begin sowing locally adapted plants.
Tags: Earthships, Alternative Building Materials, Tire Walls, Portland Cement, Recycled Cans, Off-grid Living, Thermal Mass, Greenhouse, Grey Water, Black Water, Ventilation, Cistern, Solar Panels, Rainwater Catchment