Ever since the water crisis became more serious in mid-2014, it has become known that among all human activities using fresh water, agriculture consumes the most. According to the United Nations, 70% of the total available supply of water—which is not much, just 3% of the planet’s total water—is used for irrigation. In Brazil, this rate is 72%. Sugarcane alone, one of Brazil’s major crops for which São Paulo State is responsible for half of its national production, takes at least 1,500 liters per square meter of cultivated area per year. Given this situation and the worst drought the State has faced in recent years, and even before this problem increased so dramatically, researchers at the three state universities were looking into alternative approaches. The most promising is the use of treated domestic sewage for irrigation. Known as water reuse, this option increases sugarcane productivity and brings environmental benefits, because sewage is not released into rivers and other sources of water.
One such project is under development by the team of Edson Eiji Matsura, an agronomist and professor at the School of Agricultural Engineering (Feagri) of the University of Campinas (Unicamp). “Our main goal was to find out how we could produce sugarcane through sustainable irrigation, with a view toward the economic, environmental and social aspects, because the whole of society gains from reduced water consumption,” he says. The result indicated that the reduction in water use by an experimental sugarcane planting was 50%. “The main challenge was to use recycled water from treatment of domestic sewage from our own School.”
To do the experiment, the Unicamp team stored the School’s sewage, which was subjected to three types of treatment, the first in anaerobic reactors (closed tanks with bacteria), then with aquatic macrophytes, plants with large root systems that filter sewage. Finally, the liquid passed through sand filters. Then the wastewater was transported to the crop by means of pumps and pipes, where it was distributed to the sugarcane fields by drip lines buried at depths of 20 centimeters (cm) or 40 cm, a technique known as subsurface or groundwater irrigation. “It’s the safest way to irrigate with this material, because it is not harmful to people or the plants above the ground,” says Matsura. “In addition, it is more efficient because there is no loss through evaporation.”
The experiment was carried out by planting sugarcane in a one-half hectare area, equivalent to 5,000 square meters (m2), in an experimental field that is part of the School, which covers a total of about 10 hectares. Sugarcane growing starts with a planting, and 18 months later the crop is harvested. A small part of the cut plant is left behind in the field and it sprouts. Since then, three crops have been harvested. A good commercial planting can produce up to seven harvests. After that, the field should be replanted with new plants. With irrigation using wastewater, Matsura hopes to get up to 10.
His optimism stems from the efficiency of irrigation with sewage tested on blocks of the School’s growing area with nine types of treatment: without irrigation; irrigation with sewage applied at a depth of 20 cm with and without fertigation (term used when chemical fertilizers are added to the wastewater); sewage applied at a depth of 40 cm with and without fertigation; irrigation with surface reservoir water (lake or river) at 20 cm; and reservoir water at 40 cm. Each was assessed according to certain parameters, including yield, vegetative growth, gas exchange, nutritional assessment of leaves and stems, and soil fertility in the first and second crops,” says Ivo Zuiton, Matsura’s student, for his doctoral thesis, which he defended at the School in early February 2015.
Among the project’s principal findings was a marked decrease in the use of fertilizers. The nutrient concentration in the effluent allowed a reduction of up to 100% in the need for nitrogen and phosphorous in the second crop, and more than 50% in phosphorus, nitrogen and potassium in the first crop, compared with the irrigation treatments using reservoir water. As for productivity, it reached more than 200 metric tons per hectare on plots irrigated with sewage, double those irrigated only with rainwater.
According to Matsura, in all irrigated treatments, the water footprint, which is the total volume of water consumed directly and indirectly in the process of producing goods and services, was lower than in the non-irrigated. That is because the irrigated footprint proportionately used less water, since it produced more sugarcane. Thus, by dividing the yield by the amount of water consumed, the water footprint was lower in the irrigated. In the case of irrigation with sewage and fertigation, the reduction was more than 50% compared with the non-irrigated area. “The sugarcane water footprint in the irrigated crops was lower than the estimated footprint for non-irrigated crops, with reductions ranging from 35.3 cubic meters (m³) to 23.1 m³ per metric ton for treatments with sewage and fertigation and water from rivers and lakes without this additional fertilizer,” says Matsura. “The results confirmed the hypothesis that subsurface drip irrigation would reduce the water footprint in sugarcane production.”
Matsura’s work is a continuation of another study coordinated by Professor Adolpho José Melfi at the Luiz de Queiroz College of Agriculture (ESALQ) of the University of São Paulo (USP) in the city of Piracicaba. Between 2005 and 2010, researchers led by Melfi also tested the use of treated sewage to irrigate sugarcane (see Pesquisa FAPESP Issue No. 166). One of the main results was a 60% increase in crop yield. “In addition, we demonstrated that it is possible, depending on how the irrigation is managed, to provide a full supply of nitrogen, potassium and calcium as needed by the sugarcane,” says Melfi. “Irrigation with treated effluents proved to be a viable practice for agriculture both from an economic and environmental standpoint, by reducing the use of mineral fertilizers and by enabling better management of water resources,” says Célia Regina Montes, a researcher at the Center for Nuclear Energy in Agriculture (CENA/USP), who also worked on the study. “Another advantage is that in times of drought the sugarcane crop would not suffer from a lack of moisture, because sewage is continuously produced and can always be used.”
In a similar line of research, but with another plant, Rogério Faria, a researcher at the School of Agriculture and Veterinary Sciences on the Jaboticabal campus of São Paulo State University (Unesp), has studied the use of effluents with fertigation in the cultivation of Brachiaria brizantha, a perennial grass originally from Africa that has adapted to Brazil and is used in animal feed. “Concentrated population growth in urban areas significantly increases the amount of sewage produced,” says Faria. “Due to the high cost of water treatment systems, it is necessary to encourage the use of sewage.” Therefore, he believes, the use of this material for irrigation is an increasingly attractive alternative. Applying wastewater by means of fertigation meets the water needs of the crop, by irrigating and applying waste with a high ratio of micro and macronutrients. “In our study, the treated sewage effluent provided up to 1,000 kilograms per hectare (kg/ha) of nitrogen and up to 600 kg/ha per year of potassium, in addition to other nutrients essential for forage,” he says. This represents the amount farmers could save on mineral fertilizers, thereby reducing their production costs. The study also found an increase in gross revenue for the crop, since the Brachiaria increased forage production by about 60%.
Irrigation with treated sewage is not the only way to reduce water consumption in Brazilian agriculture, especially in the sugar-ethanol sector. This was shown by a study conducted by two researchers: Fábio César da Silva of Embrapa Agricultural Information Technology, a division of the Brazilian Agricultural Research Corporation, located in Campinas, and Alexei Barban do Patrocínio, at the São Paulo State School of Technology, located in Piracicaba. Conducted at four plants—three in São Paulo and one in Paraná—the research showed that some simple technologies such as the dry cleaning of sugarcane for straw burning in high-pressure boilers decreases water consumption by 11% to 13% compared with the usual wet wash. “Another measure, replacement of the spray cooling system by a cooling tower, would reduce losses from between 5% and 8% to between 1.5% and 3% of the total water balance,” says Silva.
With these actions, the four plants, which have a combined capacity to process 2,400 metric tons of sugarcane per hour, could save, in absolute numbers, 32,895 m³ of water per day. According to Silva, today’s criteria for international plant certification require that consumption be less than 20 liters per kilogram of sugar produced and 30 liters per kilogram of ethanol. “The values obtained in our study show that it is possible to obtain water consumption of less than 10 liters per kilogram of sugar,” he says. “Therefore, the research findings will help in the development of sustainable solutions for the production of ethanol and sugar.”
In addition to the economic advantages, these studies show the environmental benefits that the alternatives identified in the work can bring. One is the preservation of lakes, rivers and other reservoirs, in addition to groundwater, because the sewage used for irrigation ceases to be released into these sources of water. “That means using these sources for more noble purposes, such as human and animal consumption, since water resources are becoming scarcer.”
Despite the good results obtained experimentally so far and the prospects they open, there are still some obstacles to the large-scale use of treated domestic wastewater for irrigation, none of which are insurmountable. Melfi says, for example, that the effluents may contain heavy metals, pathogenic organisms, a high sodium content and nitrogen. “In the case of wastewater, however, heavy metals do not prevent its use for irrigation because the levels, when present, are under the restricted amounts imposed by law and by the World Health Organization,” he says.
With regard to the risks of pathogenic organisms, Melfi says they can be minimized if crops to be processed industrially are selected for irrigation, as is the case with sugarcane. For other crops there is the possibility of disinfecting the treated sewage with chlorine before its use for irrigation, for example. “As for the sodium, in our project we found that the high concentration of this element, which can cause negative effects on the physical properties of the soil, was naturally reversible after periods of rain,” he says. “Furthermore, it is possible, if necessary, to correct the soil with the application of gypsum. With respect to nitrogen, correcting the irrigation levels, that is, providing the exact amount required by the crop, mostly prevents nitrate from leaching into the water table.
A more serious obstacle is the lack of specific legislation governing the issue. What exists today is merely Resolution No. 375 of August 29, 2006, of the National Environmental Council (Conama), regarding domestic sewage sludge, which is the solid by-product of the treatment, in addition to Resolution 121/2010 of the National Water Resources Council (CNRH), which establishes guidelines and criteria for the direct reuse of non-potable water for agricultural and forestry purposes. “Brazil has little experience with using effluents in irrigation, so the law is not specific enough,” says Faria. He believes that these studies will serve as the basis for the development of specific legislation, as well as for the installation of prototypes in test cities.
“A prototype would be tested in sewage treatment plants of small and medium-sized cities, with around 500,000 inhabitants, preventing the flow of such water into other sources of water,” he says. “In medium-sized and small cities this issue can easily be solved by including the areas where the effluent will be applied in the design of future treatment plants, provided that the required area is small.” Thus, in a city of about 80,000, such as Jaboticabal in São Paulo State, the volume of effluents generated would irrigate 240-320 hectares, while in nearby Ribeirao Preto with about 600,000 inhabitants, the irrigated area would be 1,800-2,400 hectares.
1. Use of sewage effluents treated by biological processes (stabilization ponds and upflow anaerobic sludge blanket (UASB)/activated sludge reactors) in agricultural soils (No. 2004/14315-4); Grant mechanism Thematic Project; Principal investigator Adolpho José Melfi (USP); Investment R$1,055,509.45 and $227,031.64 (FAPESP).
2. Impact of the application of treated sewage via subsurface drip on the nutrition and physiology of sugarcane (No. 2012/03588-6); Grant mechanism Doctoral Scholarship (Ivo Zution Gonçalves); Principal investigator Edson Eiji Matsura (Unicamp); Investment R$135,512.52 (FAPESP).
3. Application of treated sewage via a subsurface irrigation system in the cultivation of sugarcane (No. 2011/07301-0); Grant mechanism Regular Line of Research Project Award; Principal investigator Edson Eiji Matsura (Unicamp); Investment R$158,751.89 and $65,387.10 (FAPESP).
4. Effects of the application of treated sewage effluent, via fertigation, on soil and on the Brachiaria crop (No. 2012/12923-3); Grant mechanism Regular Line of Research Project Award; Principal investigator Rogério Teixeira de Faria (Unesp); Investment R$67,686.75 (FAPESP).