Prepared by Mohammad al-Asad and Majd Musa in association with Richard Brittain, 2002
Sources of graywater at Casa del Agua were the washing machine, the bathroom sinks, the showers, and bathtubs. Graywater drawn from these sources flowed by gravity via the modified drain system below the slab to a 208 liter sump tank located below the floor level of the house. The tank had an overflow valve that connected to the municipal sewer, and had a pump inside it that pumped the graywater through the various experimental treatment systems and into the graywater storage tanks. The pump would automatically operate as the container in which it is submersed fills up with water. The pump serves to reduce the level of the water so that it would not overflow onto the rest of the equipment. Brittain noted that in the context of this project the rainwater pump and pressure tank were separated from the graywater pump and pressure tank. Moreover, each of those systems served separate drip irrigation zones.
Brittain added that the calculations made for the volume of the expected harvested rainwater for the project indicated that storage tanks with a total capacity of about 51 cubic meters were needed. The project team decided to harvest at least half of the annual rainfall that fell on the roof surface and direct that into storage tanks so that the harvested water would be used for irrigation up to the next rain season. Consequently, two large tanks were first assigned for the harvested rainwater. However, after the first two years of the project's operation, it was found that the house was located in a "rain shadow" area, and therefore was not getting the expected amount of rainfall, and the rainwater storage tanks were not getting filled. Consequently, one of those tanks was converted for use as a second graywater storage tank so that graywater could be stored during the wintertime when the plants did not need the graywater for irrigation. The graywater, however, would be used during the arid pre-summer peak demand season (April through June).
Brittain described the underground rainwater storage tanks and their installation process (figure 3). He mentioned that the project team realized that it is costly to excavate and install the tanks. The two tanks were well compression tanks with approximately 16-mm steel walls. Tucson Water had retired a number of these tanks because they were no longer suited for pressurized conditions. The project team welded manways onto the tanks, provided them with lockable lids, and provided the necessary piping work including the inlets, overflows, and suction pipes from the pump. The manway provides access to the tank, which is especially needed for maintaining the foot valve, which retains the pump prime.
Figure 4 shows the area located above the underground rainwater tanks as it appeared after the project was landscaped in 1985. A mesquite tree (Prosopis velutina) was placed in the area over the tanks. Mesquites are known to have tap roots, so this particular tree was meant to grow between the two tanks to encourage its growing roots to go down into the ground. The heaviest watering needs would be for the small turf area that appears in figure 4 and that was provided with a drip irrigation system. The two potted plants that also appear in figure 4 are placed right on the access manways. Those can be removed whenever access to the tanks is needed. The manways also allow for monitoring the content of the tanks by inserting a calibrated dipstick into holes plugged with corks.