Metodología para la optimización energética en plantas de tratamiento de aguas residuales

Abstract

This doctoral thesis compiles the studies and tests carried out at the San Pedro del Pinatar Wastewater Treatment Plant (Murcia, Spain) to reduce the energy demand in the biological aeration stage of the activated sludge, reduce operating costs and improve its economy, guaranteeing the operation and stability of the process. The organization of these procedures gives rise to a methodology for the energy optimization of any wastewater treatment facility, which is specified in three different phases: The first phase consisted of modifying the operating criteria of the biological treatment system for activated sludge and ultrafiltration membranes to minimize the oxygen requirements of the micro-organisms in the biological reactor at source. The results obtained allowed the establishment of optimum working conditions marked by the minimum concentration of suspended solids in the mixed liquor that the system can accept, depending on the contaminant load received, and a sludge age depending on the temperature, adapted to the expected performance. The second phase focused on selecting the optimum working conditions for the air generation and injection equipment, eliminating pressure losses in the air lines and reducing disturbances in the distribution of the flow within the biological reactor, even redesigning the biological aeration installation with a modular proposal. This makes it possible to adapt the installation to the real operating needs at any given time and, therefore, to minimize energy requirements by reducing aeration needs and over-consumption. This is achieved by managing the efficiency of oxygen transfer from the gas phase to the liquid phase using the off-gas hood measurement method and the use of modelling and simulation tools based on computational fluid dynamics (CFD) using Ansys v13.0. The third phase consisted of defining and implementing a new biological aeration control strategy with which to minimize the deviations between the target values and the real values of the parameters to be controlled. This is an adaptive predictive expert regulation system supported by the recording of measurements made by strategically installed sensors and mathematical algorithms based on models. It consists of: - Establish aeration cycles based on the measurement of the concentration of ammonium (N-NH4+) and nitrate (N-NO3-) ions contained in the water. - Establish the criteria for adjusting the optimum dissolved oxygen concentration demanded by the microorganisms in the oxic zones, in order to achieve the required water quality levels in terms of ammonium (N-NH4+) and nitrate (N-NO3-) concentration. - Implement the criteria for adjusting the working pressure in the air line, in order to achieve the required levels of dissolved oxygen in each biological aeration chamber by means of the automatic regulation of control valves installed in the air line. - Regulate the flow of air injected into the membrane bioreactor according to the flow of water required for treatment. The complete development of the methodology for the proposed optimization has made it possible to achieve a reduction of more than 40% in the total energy consumption of the installation, which means a saving in operating costs of 118,041 euros/year and a reduction in CO2 emissions of 1,078 kg/year, which contributes greatly to the European sustainable development strategies which promote, among other things, sustainable consumption and production.