Aspectos ambientales relacionados con el uso de lodos de depuradora como enmienda de suelosestabilidad de la materia orgánica, biodisponibilidad de metales pesados y contribución al almacenamiento de carbono

Resumen

The production of sewage sludge has dramatically increased in the last decades. The application of sewage sludge to soil is a current practice for returning valuable nutrients and organic matter to reclaim degraded soils. However, this application raises management and environmental concerns related to the high levels of labile organic matter (typical from non-stable materials) water and microorganisms and the presence of heavy metals. For these reasons, it is necessary to develop studies related to the stability of sewage sludge and other aspects related to their management as well to assess environmental aspects derived from the application of these materials in agricultural soils: bioavailability of heavy metals and contribution to the C stock. These aspects are covered by this thesis by the development of various studies published in journals of impact. Composting is one of the best ways to minimize many of the problems associated to sewage sludge such as odours, water content, microbial presence, high levels biodegradability of the organic matter and presence of phytotoxic substances. A composting process developed in a full-scale facility was studied, obtaining mature and stable products after 40-50 days of composting with the microbial respiration, ash content and the absorbance at 665 nm as the most useful parameters to explain compost evolution. The industrial composting processes are normally expensive, requiring specific facilities. This can be seen as a drawback, especially for small and medium size wastewater plants. An alternative to reduce the water and microorganisms levels from sewage sludge, using a free and renewable energy, is the solar drying under greenhouse facilities but concern exists about the impact of this process on the agronomic value of sewage sludge. Three sewage sludge were solar dried and the process could be considered as positive since the fertilizer and amendment value of the sewage sludges were maintained, reducing the water content and odours (improving the management), using a free, renewable and clean energy. Since some of the parameters related to the stability (and analyzed in the previous studies) are laborious, expensive and time-consuming, there is a need to develop easy, inexpensive and rapid methods as near-infrared (NIR) spectroscopy for their assessment. The potential of this technique for the prediction of stability parameters in sewage sludge and derived compost has been demonstrated in this study. The main concern derived from the application of sewage sludge in soils is the presence of heavy metals. It is well recognized that the estimation of the heavy metal bioavailability in soil is preferable than the total content for determining potential risks derived from their presence. Consequently, considerable effort has been made during the last few decades to estimate heavy metal bioavailability and thereby predict their impact on the soil ecosystem, however, no single method is recognized universally. Thus, two studies were developed for the assessment of different methods to estimate heavy metal bioavailability. In the first study, and using soils amended with sewage sludge polluted with heavy metals, our results suggest that none of the extractants tested (CaCl2, LMWOAs y EDTA) can be used to provide an estimation of phytoavailability of Cu, Ni and Zn independent of soil properties. However, when soils were divided into acid and basic or when the soil properties were included by multiple regression, LMWOAs extraction resulted to be the most efficient method to estimate the phytoavailability of these metals. The inclusion of soil properties greatly improved the estimations of heavy metal phytoavailability provided by the single chemical extractants and texture was the most important property. In the second study, different methods (heavy metals in pore water, diffusive gradients in thin films (DGT), DTPA and total heavy metals in soil) were tested for assessing the heavy metal bioavailability to wheat in a variety of soils having a range of properties and heavy metal contents. The effect of the presence of heavy metals on selected biological properties (shoot yield and sulfatase activity) was also studied. The DGT was the most widely applicable method for the assessment of heavy metal bioavailability in the studied soils. This technique successfully predicted the Cu, Ni, Pb and Zn root concentrations, and it was suitable to distinguish between low and high Cr concentrations. All of the methods used in this study failed to determine the Cd concentrations in the plants. In the range of metals used, neither the shoot yield nor the sulfatase enzyme activity was affected by metal concentrations. Finally, the application of a single dose of sewage sludge in agricultural soils contributed to the short-term soil organic C pool, especially for soils with low initial levels of this parameter. However, this contribution was negative in soils having high initial levels due to the potential degradation of the native soil organic C.