System that replaces secondary decanters reduced 95% of organic matter in tests
Study developed by environmental engineer Amanda Rodrigues Inácio, with a view to more efficient treatment of sanitary sewage, shows the feasibility of eliminating the extensive secondary decanters, used in the final phase of the activated sludge process and intended for the deposition of biomass, formed in the previous phase by action of microorganisms that consume organic matter. To this end, in the same aeration tank, where the degradation of organic matter occurs, it introduced geosynthetic blankets capable of retaining biomass with the same efficiency as the traditional process that uses decanters, with equivalent cost and space savings.
The tested system allowed a reduction of more than 95% of organic matter in sewage, higher than the minimum determined by legislation (80%). The same success was seen in relation to the clarification of treated sewage. For turbidity and suspended solids, the average removal efficiencies were, respectively, 98,5% and 99%.
The prolonged aeration activated sludge system, one of the most used in Brazil, involves grating that blocks the passage of larger solids; the sand trap in which particles with dimensions close to those of sand are deposited; the biological reactor in which oxygen is supplied for the development of microorganisms (aerobic action) that feed on organic matter; the deposition of sludge, made up of microorganisms, in the secondary decanter; and finally the release of treated sewage into waterways.
The work, carried out with the Department of Sanitation and Environment, of the Faculty of Civil Engineering, Architecture and Urbanism (FEC) at Unicamp, was supervised by professor Carlos Gomes da Nave Mendes. The sanitary sewage used in the research originates from some Unicamp facilities and presents characteristics typical of domestic origin.
The process
The researcher explains that the biological mechanisms of sewage treatment depend on the action of microorganisms present in the sewage itself, a process that occurs naturally in a body of water after the discharge of waste. One of the biological processes for treating domestic and industrial sewage, widely applied in Brazil and other countries, is the activated sludge system. Among its variants, this is a prolonged aeration system followed by the deposition of sludge in a large area reserved for secondary decantation, a procedure that demands time and space for implementation.
As an alternative to eliminating this space and accelerating the process, the use of MBR systems is established in Europe and the USA, Membrane Bioreactor, in which the secondary decanter is replaced by filtration modules, consisting of membranes, installed in the aeration tank itself or external to it.
It turns out that the widespread adoption of the MBR process comes up against the high costs of the polymeric membranes used, in the maintenance and operation of the equipment. With the idea of reducing the costs of the process, making it viable for use on a larger scale in countries with fewer resources, the researcher studied the possibility of replacing the secondary decanter with filtration with geosynthetic membranes manufactured in Brazil, which are easy to acquire, and already used in embankments and retaining walls.
For the study, filtration modules were built with geosynthetic blankets, installed in the aeration tank, so that the sewage filtration process could occur.
Although authors argue that any material that allows obtaining films with controlled porosity can be used to manufacture these membranes, the evaluation of the application of geosynthetic blankets in MBR systems for sanitary sewage treatment is still a challenge and there are few studies on the subject. . Amanda then proposed to study the feasibility of using geosynthetic blankets, which are cheaper and manufactured nationally, as an economically more viable alternative.
Comparing efficiencies
For comparative purposes, the efficiencies of filtration modules constructed with two polyester geosynthetic blankets, from two different manufacturers, were evaluated and three types of materials were used as spacers between them: anti-slip fabric made of polyester with PVC coating, three-dimensional geomat made of polyester filaments, thermowelded polypropylene and acrylic blanket. The spacers have the function of increasing the contact surface between the liquid and the blanket, improving the filtration process.
The assembly formed by the blanket-spacer-matte-spacer overlap is then folded and crossed by a perforated pipe and placed inside the blanket. A peristaltic pump connected to the pipe removes the filtrate using suction.
Combining the two chosen geosynthetic blankets with the three different spacers, six configurations of filtration modules were constructed with a view to comparing results. The final form of a filtration module constructed with a geosynthetic spacer blanket is shown in the figure.
As we saw, the focus of the research was to use geosynthetics as a filtration membrane in an activated sludge system, in order to replace the secondary clarifier, increase the system's efficiency and reduce the implantation area. It was verified, as shown by the results already mentioned, that in terms of final effluent quality, it is possible to replace the secondary decanter with geosynthetic blanket filtration.
For the author, operationalization of the system on a large scale also requires the development of a process that solves the problem of solids deposits (fouling) in the filtration modules, that is, from the encrustation of solids in the membrane, which over time slows down the filtration process and, if there is no periodic cleaning, leads to its loss.
