Uso de esgoto tratado na irrigação de algodoeiro e efeitos nos atributos químicos do solo.

Abstract

The shortage of irrigation water is an obstacle to agricultural and social development in arid and semiarid regions. Reuse of treated sewage water in agriculture is a suitable alternative for such regions, since it not only increases water availability, but also supplies organic matter and nutrients present in these effluents for plant nutrition. Despite the potential benefits, reuse of domestic wastewater for agricultural purposes is not a risk free practice, considering that the presence of salts, organic pollutants and pathogens could cause damages to the environment, human health, soil, ground water and irrigated crops. For this reason, it is important to study the efficiency of wastewater treatment methods in obtaining effluents suitable for disposal on cultivated lands. Based on the above considerations, the present research aimed to evaluate the potential of domestic sewage treated by anaerobic processes and post treated in intermittent sand filters for cotton irrigation. The implications of disposing of these effluents in terms of soil chemical properties were also investigated. For this purpose, cotton plants were grown in pots containing haplustalfs soil submitted to the treatments which resulted from the factorial combination between four sources of irrigation water - (i) freshwater, (ii) effluent treated by an UASB reactor, (iii) effluent treated by an UASB reactor and post treated by 2 intermittent sand filter in series, and (iv) effluent treated by a septic tank and post treated in an intermittent sand filter - and five levels of nitrogen fertilization - 0, 45, 90, 135 e 180 kg ha-1. Plants were irrigated for 123 days, and their growth and leaf nitrogen accumulation were measured. At the end of the crop cycle, plant yield and nutritional status were determined. The soil post cultivation was also evaluated to verify the chemical changes caused by the cotton cultivation. According to the results, irrigation with treated wastewater is beneficial for both cotton growth and nutrition, resulting in good crop yields without affecting the quality of fiber. Treated wastewater is also suitable for replacing the conventional nitrogen fertilization of cotton plants, which is reflected in greater financial savings for the producers. Besides the fact of obtaining agronomic advantages, the use of secondary effluents post treated by intermittent sand filters provides better health security, since this treatment removes the helminth eggs which are an important cause of human illness. However, it was found that irrigation with wastewater was not effective in restoring nutrients exported by cotton crop and that it increased sodium levels in soil after cultivation. Considering this, it is highly recommended to perform effective soil management of agricultural lands irrigated with treated wastewater in order to maintain soil fertility and to prevent soil salinization and sodification.