Desenvolvimento de um sistema de eletrocoagulação-flotação para o tratamento de água para abastecimento público

Abstract

Although essential for human life, health and dignity, there is a large proportion of people without access to services of water supply. Most of them are in regions with difficult access, low demographic density and/or high spatial dispersion, which makes unfeasible conventional solutions to serve them. Innovative solutions are also desired to complement public supply systems in face of the growing water scarcity and to supply the growing demand for water in a limited physical space. So, the objective of this study was developing an electrocoagulation-flotation system (ECF system), at pilot scale, for the surface water treatment for public supply and evaluating its performance. The ECF system was composed by two modular electrocoagulators followed by a helical tubular flocculator (HTF), with flowrate of 20 L∙min-1 . This configuration was developed for better use of the coagulants and microbubbles generated in the electrocoagulation by the formation of aerated flocs. The adopted conditions in the electrocoagulators were upward flow, aluminum electrodes, parallel monopolar connection, inter-electrodes distance of 3 mm, current density of 0.43-1.28 mA∙cm-2 , flow speed of 0.04 m∙s-1 , hydraulic detention time (HDT) of 51.0 s, and contact time with the electrodes of 43.3 s. The adopted conditions in the HTF were vertical orientation with upward flow, HDT of 6.36 min, speed gradient of 71.7-182.8 s -1 , Camp number of 27,366-69,770, and Reynolds number of 7,059. The study was performed at the water treatment plant of Santa Maria-RS, with water from the rivers Vacacaí-Mirim and Ibicuí-Mirim. The treatment was performed with pH adjustment of the water to 6.4. The ECF system was capable to provide turbidity of 1.3 ± 0.4 NTU and apparent color of 16.8 ± 5.8 uC to the treated water, which were appropriate for the further filtration step. Among the results of performance, the removal efficiency of total coliforms (95.6 ± 4.7%), E. coli (89.4 ± 8.8%), chlorophyll a (87.7 ± 10.9%), apparent color (73.0 ± 8.2%), and turbidity (79.9 ± 4.6%) were stand out, which shows the efficiency of the system in the water clarification and micro-organisms inactivation. The system had an energetic consumption of 0.02- 0.37 kWh∙m-3 , aluminum consumption of 3.0-9.0 mg∙L-1 , and hydrochloric acid of 2.5-21.5 mL∙m-3 . Operations of cleaning or electrodes change proved to be essential for keeping the energetic efficiency of the system. The energy consumption proved to be a good indicator for determining the moment to do it. The operation cost of the system was R$ 0.19 ± 0.07∙m-3 , where 49.1% referred to energy consumption, 44.1% to aluminum consumption, and 6.6% to hydrochloric acid consumption. The use of renewable energy sources can reduce the operational costs to R$ 0,10 ± 0,06∙m-3 . The ECF system showed potential of being diffused as an alternative technology for surface water treatment, compact, easily installable, operable, and automatable, especially attractive for decentralized supply systems (rural, riverside, peri-urban and/or hard-to-reach communities), as well as for coping with water scarcity and/or seasonal demands.