Technology that uses domestic sewage for farming irrigation can help save water and reduce water consumption on farms, since farming is the economic activity with the highest water consumption. The production of one kilogram of rice, for example, demands 2,000 to 5,000 liters of water. A group of researchers from Esalq, the Luiz de Queiroz College of Agronomy of the University of São Paulo/USP in Piracicaba, São Paulo State, has made significant progress in this respect. In one of the experiments with sugarcane, the team managed to increase productivity by 50% by irrigating the crop with treated domestic wastewater. The results of the study, part of a theme project supported by FAPESP and coordinated by professor Adolpho José Melfi, from Esalq and from Nupegel, the Center of Research of Lithosphere Geochemistry and Geophysics, under the dean’s office of USP, were published in the August issue of the journal Scientia Agricola.
According to the researcher, the use of wastewater – another name for domestic sewage – to irrigate crops is safe and has several advantages. “One of the risks is having pathogenic organisms in sewage. However, before being used on crops, the treated sewage can be put through a disinfecting process that uses chlorine or ultraviolet radiation. This practically eliminates the sewage pathogens”, he points out. “In addition, there is detailed and constant monitoring of the quality of the wastewater, the soil, the groundwater and of the crop to avoid any kind of contamination”. According to Melfi, the high content of sodium in the sewage can affect soil structure because of clay migration, leading to a reduction of the soil’s aeration and hydraulic conductivity. Melfi adds that this problem can be remedied by adding phosphogypsum to the soil. “The phosphogypsum, in powder form, is put into the soil and neutralizes the effect of the sodium”, he states. As for the groundwater, the main concern is related to contamination by nitrogen, a micronutrient found in sewage that, if absorbed by plants, can infiltrate the soil and, in the form of nitrate, pollute the underground reservoirs of water.
Environmental quality is the main advantage of this new system. Besides reducing water consumption, domestic wastewater irrigation improves the quality of waterways, because they are no longer polluted by the treated sewage from cities. Although domestic sewage undergoes treatment, it is continues to be full of nutrients (especially nitrogen, phosphorus and potassium), which leads to proliferation of algae and affects the quality of the water, in a process called eutrophication. However, these wastewater nutrients are a major advantage when it comes to farming, as the nutrients are essential for plant growth. Therefore, farmers can use less fertilizer on their crops, which provides savings. “This is important, because fertilizers have a significant impact on farm production costs”, points out professor Célia Regina Montes, from Cena and Nupegel, who is also a member of the research group. In one of the experimental plantations, which involved hay, nitrogen fertilizer savings came to 80% during a year with less rainfall, when irrigation with effluents was more intense.
Ponds and reactors
According to the researcher, sewage treated by different systems can be used for irrigation, but only domestic sewage is being used in this theme project. “Industrial sewage has different characteristics and undesirable compounds, such as heavy metals. Farm areas in general are less industrialized and therefore the treated sewage is predominantly domestic”, she points out. Sewage treated by two biological processes was used in the study conducted at USP. One process resorts to stabilization ponds, whereas the other uses an anaerobic reactor called Upflow Anaerobic Sludge Blanket/UASB and an activated sludge process. Solar energy drives the sewage treatment process in the first case. This system consists of a set of ponds containing aerobic and anaerobic bacteria that degrade the organic matter in sewage. The sewage is kept in the ponds for a specific amount of time and when it is released it is free from most of the organic matter and can then be discarded into waterways. Stabilization ponds are the most commonly used system in small and medium size cities in the State of São Paulo and are run by Sabesp, the São Paulo State water supply and sewage company, in 319 municipal regions of the state. The anaerobic reactors are also based on a biological process, but the process requires electric power. The treatment uses anaerobic bacteria, after which the sewage goes to an aeration pond before flowing into the river. In both cases, the sewage is disinfected at the end of the treatment with either chlorine or ultraviolet radiation.
Any crop can be irrigated with domestic wastewater in theory, although the ideal measure would be to prioritize crops that are not consumed in a raw state and that are industrially processed. High processing temperature reduces the risk of contamination by pathogens. Experiments with corn, sunflower, coffee, hay and sugarcane were conducted as part of the FAPESP theme project. Researchers from USP’s Polytechnic School, School of Public Health and Geosciences Institute, from the Embrapa research center in São Carlos, from Inpe (the National Institute of Space Research) in São José dos Campos, and from Sud Toulon-Var university in France took part in the project. The aid experiments were conducted at the six-hectare experimental plantation in the city of Lins, State of São Paulo. In Piracicaba, the experiments are being conducted on orange and sugarcane crops grown on a two-hectare experimental plantation. Productivity gains were obtained in all the experiments, partly because of the irrigation process – some crops, such as sugarcane, are not usually irrigated – and partly because of the nutrients in the effluents. The effects of irrigation with wastewater on the productivity of sugarcane crops, reported in an article in the journal Scientia Agricola, were studied for 16 months at a plantation in the town of Lins. At the end of the study, the researchers found that productivity had increased by 50%.
The use of treated sewage is not new and has been in use for many years. The so-called sewage farms in Australia, Germany and France have been in existence for more than one century. This system has also been introduced in other countries such as the USA, Saudi Arabia, Chile and, more recently, Mexico. Farm crops – for example, the orange crop in some Florida plantations – irrigated in this manner are already on the market. In Brazil, the same group from Esalq started in-depth studies on this subject some 10 years ago. However, there are still no specific federal laws on the issue. In 2005, a resolution of Conama, the National Environment Council, set the guidelines for the use of treated sewage in five different circumstances: farm and forest irrigation, urban use, industrial use, recovery of degraded areas and aquaculture. Each of these items, however, needs specific resolutions to, among other issues, regulate the quality of effluents. “A group linked to the Science and Technology Chamber of the National Hydric Resources Council is working on the draft of a resolution for the regulation of farm and forest use”, says Célia Montes. “I am sure that – taking all the necessary precautions – we could have already adopted this system to a far greater extent in Brazil”.
According to the researchers involved in the project, the best way to irrigate crops is by reusing water, always employing the drip method, as this allows one to avoid dispersing aerosols with effluent particles. In the case of certain crops, such as hay, for which aspersion irrigation is used, the recommendation is to suspend irrigation if there are strong winds. “This is an added precaution”, says Adolpho Melfi. He also points out that, ideally, sewage treatment stations should be located on high grounds close to the plantations, making it easier for the treated sewage to flow out to a reservoir right next to the plantations. Melfi adds that “tanker trucks can also be used to carry the treated sewage.”
Researchers were also concerned about farmers? and consumers? acceptance of food crops irrigated with treated sewage. A survey was conducted last year with farmers from the Lins region, on their views of this new method. Most said they had no problem in adopting the system, provided that it had been validated by a research institution such as USP, for example. The only problem that they emphasized was the cost of implanting and running the system. This survey was part of roughly 10 master’s degree dissertations of students involved in this project since 2000, when the early studies were conducted and then included in the theme project, which began in 2005. The project is due to be completed next year. Another seven doctoral theses, eight post-doctoral research studies and 14 scientific initiation papers were produced as part of this project. “The aim of Fapesp’s theme project was to prove the feasibility of the system and to train human resources in this field”, Melfi emphasizes.
Use of sewage treated with biological processes (stabilization ponds) and UASB/slush-activated reactors on farmland (nº 04/14315-4); Modality Theme Project; Coordinator Adolpho José Melfi – USP; Investment R$ 1,055,509.45 and US$ 227,031.64 (FAPESP)
LEAL, R. M. P. et al. Soil exchangeable cations, sugarcane production and nutrient uptake after wastewater irrigation. Scientia Agricola. V. 66, n. 2, p. 242-49. 2009.
FONSECA, A. F. et al. Treated sewage effluent as a source of water and nitrogen for Tifton 85 bermudagrass. Agricultural Water Management. V. 87, p. 328-36, 2007.