Gray water trial will help determine feasibility for landscape irrigation
The second phase of a study on the use of gray water for landscape irrigation will soon be underway and its results may affect how municipalities view gray water as an alternative non-potable water source statewide and beyond, said the Texas A&M AgriLife Research scientist leading the study.
Dr. Raul Cabrera, AgriLife Research horticulturist at the Texas A&M AgriLife Research and Extension Center in Uvalde, said he is now in the process of starting the second phase of his study on the long-term effects of gray water irrigation on the growth, performance and aesthetics of several ornamental plant species.
"Gray water provides a potential alternative, non-potable water source available in practically every home," Cabrera said. "By definition, gray water is simply wastewater, basically soapy water coming primarily from washing machines, showers and bathtubs.
"The research is particularly useful for application in urban areas where as much as 50 percent of household water is used for landscape irrigation."
He said with many parts of the nation facing water restrictions, competition for water resources and drought, it is becoming more necessary to minimize our dependence on potable water supplies.
"Using native and adaptive plants, applying good management practices such as spreading mulch to retain moisture and the wise use of supplemental irrigation are vital in preserving the state's and nation's potable water resources," Cabrera said.
He said gray water accounts for about half of the wastewater from a typical household.
Cabrera said the use of gray water from washing machines alone has the potential for saving a significant amount of the state's potable water supply. He said if gray water from household laundry in Texas could be used for landscape irrigation, it could save about 400,000 acre feet of potable water annually. (Texas A&M AgriLife photo)
"It is estimated that the average family of four will produce 90 to 100 gallons of gray water per day. That's a lot of non-potable water that could be going toward supplemental landscape irrigation."
Cabrera said reuse of gray water produced from household laundry alone throughout Texas could save some 400,000 acre feet of water per year to the state, about 8 percent of its annual urban water use.
"However, only a few municipalities in Texas even have ordinances addressing the use of gray water, and many municipalities in this state and others may be hesitant to employ the use of gray water since they don't know about its potential impact to human health and their surrounding ecosystems," he said. "The results of these gray water studies could potentially be used to help bolster the case for their inclusion as a feasible alternative to potable water for landscape irrigation of ornamental plants and turfgrass."
The first phase of the gray water study was conducted in a greenhouse environment, whereas this phase is being conducted in the more 'real-world' environment of a half-acre exterior plot located on center grounds, Cabrera explained.
"We started work toward this second phase last year by planting small native and adaptive plants in replicated rows, then gave them a full year to establish, making necessary adjustments to the drip irrigation system based on individual plant species."
He said as with the greenhouse phase, this new phase will also employ three different water treatments. The control treatment will be irrigation with good-quality well water. The second will be irrigation using gray water containing fabric detergent and softener. The third will have these same ingredients plus a fabric bleaching agent.
"Because of a much lower water requirement for the small-scale greenhouse study, we were able to use an actual washing machine to make the gray water treatments and apply them to the plants through a pressurized mini-sprinkler irrigation system," Cabrera said.
He said the next phase will require far more water, so we are using pressure-driven injectors—proportioners that will apply the gray water ingredients through a calibrated drip irrigation system.
"The proportioners will allow the application of the same concentrations of detergent and softener surfactants and bleach as generated with the washing machine in the greenhouse study, where we used the detergent manufacturers' suggested rates for large laundry loads."
He said the second phase trial plot consists of 24 rows of plants with 13 different native, exotic and adaptive plant species mirrored in each row. Plants species include live oak, cenizo, almond verbena, lantana, rosemary, yaupon holly, cedar elm, Italian cypress, Mexican olive, crape myrtle, rose, plumbago and muhly grass.
"The plants in this second phase of the study will be given six irrigation treatments replicated four times throughout the plot," he said. "The three gray waters will be administered at two different application rates – one at a standard rate and another at a lower or deficit-irrigation rate. For the next three years, we will examine the effects of gray water on the growth, performance and aesthetics of various plant species, as well as its effect on the physical, chemical and biological characteristics of the soil."
The research plot also has a small weather station that will monitor variables such as temperature, rainfall, relative humidity, wind speed and direction, and dew point, and calculate daily evapotranspiration, which will provide a reference point for irrigation treatments.
"We will also be installing sensors to monitor the soil moisture content and temperature as well as the concentration of salts in the soil over time," Cabrera said.
He said though time and replication would yield objective results, one of the areas he would be paying particular attention to during the second trial phase will be the effects of gray water containing the bleaching agent, as this could negatively and severely affect plants and the biology of the soil due to the action of potentially high levels of free chlorine in this treatment.
"Our greenhouse study showed that for several plants, the use of gray water with a bleaching agent caused distress and even phytotoxicity," Cabrera said. "Free chlorine, a gaseous by-product of bleach is a powerful surface disinfectant at relatively low concentrations, but potentially toxic to plants and animals at higher levels, such as those found in freshly produced laundry gray water."
He said most municipalities in the U.S. that permit gray water for irrigation use do not allow its storage, so this water source must be used immediately.
"If bleach was used then its free chlorine is likely to be in a potentially harmful concentration for some plants when used right away," Cabrera said. "But what we have found is that if you store gray water containing bleach for a day, most of its free chlorine volatilizes, that is to say is lost as a gas, and its concentration drops to a plant-safe level. This is another aspect we hope to address in our research toward making the case for gray water use."
Cabrera said the results from the first greenhouse phase of the study have been encouraging and that he is interested in gathering and analyzing the data from this new field trial phase, which is intended to mimic what will happen in an actual urban landscape environment.