Prepared by Mohammad al-Asad and Majd Musa in association with Richard Brittain, 2002
One issue discussed following Brittain's presentation was how storing graywater for extended periods of time affects its quality. Brittain mentioned that graywater becomes cleaner the longer it is stored in tanks. This is because of the process of "anaerobic digestion" in which the particles found in the graywater keep digesting themselves. The same applies to rainwater in a storage tank. Rainwater is dirtiest immediately after rainfall since debris and bird droppings go into the tank. But with time, the litter settles down, the process of anaerobic digestion takes effect, and the water becomes cleaner. Brittain added that in both Casa del Agua and Desert House, the drip irrigation systems that distributed rainwater or graywater were never clogged.
A follow up question was whether graywater was unhealthy, and therefore should be treated through processes such as aeration or chlorination. Brittain answered that the plants seem to thrive on the nitrogen and the nutrients found in graywater, and that graywater therefore seems to function as a natural fertilizer. He added that the research he and his colleagues undertook also showed that soaps did not cause any problems. They tried to avoid using soaps containing borax and similar materials that may harm the plants. However, the researchers did not notice any negative signs resulting from using the graywater for irrigating plants during the operation years for the two projects.
Another question enquired as to why mixing graywater with rainwater had not been tried in Casa del Agua and Desert House. Brittain answered that this was because each project included ornamental plants, a herb garden, and a vegetable garden. The project team wanted to control the quality of the water being applied to each of these plant groups. Graywater is best suited for shrubs, trees, and lawn. On the other hand, rainwater is of a higher quality in terms of cleanliness, and therefore was applied to vegetables and herbs. Brittain added, however, that additional research should be carried out regarding the use of graywater for watering certain vegetable crops.
Brittain was asked whether one should provide two cisterns for the harvested rainwater so that when cleaning one cistern, its content of water can be drained to the second cistern rather than being discharged to the sewer system. Brittain agreed that the idea of having two cisterns for the harvested rainwater could be of use, but did not believe it necessary. He added that such tanks rarely need cleaning. In fact, he has crawled down into the rainwater storage tanks of Casa del Agua and Desert House, and has found them to be adequately clean. He did find silty deposits at the bottom of the tanks, but water is pumped out of those tanks from about 160 mm above the bottom, which is above the layer of silt. Brittain added that he crawled into a rainwater cistern that had been in operation for about sixty years, and there was hardly anything in it that really needed cleaning. Still, he agreed that when more than a single storage tank is available, one has the option of draining the content of one cistern into the other if a leak or another problem occurs.