Agronomical-environmental viability of treated sewage effluent disposal in a soil-pasture system.

Author(s)

Adriel Ferreira da Fonseca

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2005-07-05

Examining board members

Adolpho Jose Melfi; Luis Reynaldo Ferracciú Alleoni; Wagner Bettiol; Yves Marie Pierre Henri Lucas; Fabio Poggiani

• Advisor: Adolpho Jose Melfi

Abstract

Treated sewage effluent (TSE) utilization for irrigation represents an antique, popular and attractive alternative to the common disposal of effluent to watercourses and includes three main purposes: effluent complementary treatment, water and nutrient source to the soil-plant system. However, because in Brazil no experiences in wastewater recycling exists consequently few scientific studies were carried out despite the importance of the subject. For this study an experiment was conducted for two years in Lins (São Paulo State, Brazil). The main objectives were (i) to test the feasibility of secondary-treated sewage effluent (STSE) utilization as an alternative water and nitrogen source for Tifton 85 Bermudagrass; (ii) to evaluate concentrations of nutrients and toxic elements in potable water (PW), STSE, plant, soil and soil solution samples; (iii) to verify the behavior of dispersed clays in water. A randomized complete block design was used with four replications. Five treatments were applied: (i) T1 (control) - irrigation with PW plus 520 kg ha-1 year-1 nitrogen as mineral fertilizer (NMF); (ii) T2, T3, T4 and T5 - irrigation with STSE plus 0, 171.6, 343.2 and 520 kg ha-1 year-1 of NMF, respectively. Water and effluent characteristics were monthly monitored, and aboveground dry matter (DM) was measured bimonthly. Plant (leaves and culms + sheaths), soil and soil solution (up to 100 cm) were analyzed during 18 months, to monitor elements concentrations over the experimental period. It was verified that STSE can replace efficiently the PW used for irrigation for the production of hay. This replacement can provide economical benefits, enhancement of forage quality, and mitigation of soil acidity. The magnitude of grass productivity as well as of NMF economy is dependent on rain and the used irrigation rates. Considering the complete substitution of PW for irrigation by STSE, the necessary dose of NFM for obtaining high yields could be reduce by 32.2 to 81.0% without negative effects on nutrient contents in the grass and soil fertility. On the other hand, also by non-consideration the effluent nutrients in pasture fertilization management, it may occur increasing dry matter yields and element contents (inclusive sodium), enhancement of forage quality, and, moreover without deleterious environmental effects. Because of high sodium concentrations in PW (control) and the STSE, it is suggested that the pasture-soil system will not tolerate the high sodium input, a future conditioning of the soil will be necessary. The nutrient inputs to the system associated with an adequate soil moisture tension to the grass may increase soil organic matter mineralization followed by decrease of carbon and nitrogen stocks. In short and medium term periods it is assumed that the irrigation with domestic TSE will not negatively influence the soil-pasture system by high concentrations of boron and heavy metals. (AU)