Effects of sewage treated by different technologies on the functioning of the receiving water bodies: nutrient retention, aquatic metabolism and gas emissions

Abstract

The Wastewater Treatment Plants (WTPs) significantly contribute for attenuating the problems associated with wastewater discharges into aquatic systems, such as rivers and streams. However, depending on the technology that is used in the WTPs, the removal of nitrogen (N) and phosphorus (P) may not occur (or occur only partially). Such nutrient inputs can cause changes in several ecosystem processes of the receiving water bodies and alter their self-depuration capacity. The main objective of this research will be evaluating the influence of treated sewage inputs on the retention of N and P in reaches of different tropical streams and correlating this process to aquatic metabolism and gas emissions. Nitrate, ammonium and phosphate retention rates will be quantified in three streams located downstream the effluents of WTPs (using UASB reactors, stabilization ponds or activated sludge process) and four reference streams to assess their self-depuration capacity. These results will be associated with estimates of metabolism (primary production and ecosystem respiration rates) and with carbon dioxide and methane emissions by the streams. In addition to the expected differences in impacted versus reference streams, different hypotheses will be tested related to the following issues: 1) the influence of environmental conditions (e.g., light and nutrient availability, residence time, size of transient storage zone) on metabolic rates and the N and P retention capacity; and 2) the relative importance of ecosystem respiration and methanogenesis for CO2 and CH4 emissions by the aquatic systems. Currently, there is a lack of such studies in tropical environments and it is expected that the results of this research will especially aid in the definition of criteria for the release of sewage into rivers and streams and in technology improvements in WTPs to maximize the self-purification capacity of the receiving water bodies. The proposal, if approved, will also allow the training of human resources (at least three PhD students from PPG/SHS/EESC/USP are already involved in the project) and will enable the publication of articles in peer-reviewed journals with high impact factors. (AU)