Integração de Wetlands construídos e células de combustível microbianas para o tratamento de efluentes urbanos com potencial geração de bioenergia.

Abstract

Given the necessity and potential application of sustainable and decentralized treatment technologies for urban wastewaters, the Constructed Wetland (CW) systems have been widely implemented and investigated around the world. Despite being an already consolidated technology, with the advances of analytical monitoring and of the concept of circular economy in sanitation, new research and improvements are necessary to optimize the treatment performance, promote the recovery of resources and landscape integration, as well as increase the treatment systems lifespan. In this context, the main objective of the present research was to develop pilot scale treatment systems combining an anaerobic reactor/biofilter unit, different configurations of CWs with microbial fuel cells and units filled with adsorbent reactive material (light expanded clay aggregate, or LECA, and broken bricks). The treatment system was constructed and operated in the University of Santa Cruz do Sul wastewater treatment plant (WWTP), from 2018 to 2021, with the units being fed with the wastewater generated in the university campus. The developed treatment system was composed of an anaerobic biofilter (1,450 L), 4 CW units with different configurations (each of 1000 L) and a filter unit (200 L) filled with reactive media. The first CW unit was designed as a Floating Treatment Wetland (FTW), in which a buoyant support kept the plant under hydroponic conditions, acting similar to a decanter. The two vertical flow CW units, filled with gravel n. 0, n. 1 and n. 2 and pebbles, were vegetated with different plant species, one of the units without substrate saturation and the other unit 80% saturated. The horizontal subsurface flow CW was also filled with gravel and pebbles and vegetated with different plant species, but this unit had a compartment filled with broken bricks aiming for P adsorption. In the first doctorate year, a review study was carried out regarding Floating treatment wetland (FTW) systems, as well as the conception, project and construction of the experimental treatment units. The system operation can be divided in two phases of approximately 1 year each: a first period of operation under batch flow (2019) and later with pulse feeding (2021). During 2019, the treatment systems were operated under batch flow, with weekly sampling and feeding with wastewater, with loading rates of 13,6 L/m2 day. During this period, the integrated systems efficiently promoted the removals of organic matter (82% for BOD5 and 81% for TOC), eutrophication potential (87%, 68% and 93% for total P, total N and ammonia nitrogen, respectively), and other monitored parameters, such as turbidity (94%) and total coliforms (4 log units or >99.99%) and suspended solids (100%). However, in the following years, the unit’s operation was changed to automated (with 3 to 6 short pulses everyday), therefore approaching the operation conditions usually adopted in full-scale WWTPs. During 13 months of operation, three different application rates in the treatment systems were performed: 20.5 L/m2 day (for 5 months), 41 L/m2 day (for 5 months) and 61.6 L/m2 d (for 3 months), considering the superficial area of the CWs. In this second monitoring period, the obtained mean efficiencies concerning the treatment performance were lower than those previously verified, mainly due to the increase in BOD5, COD and nutrients loadings. Under the best performance conditions, for the FTW + CW-MFC and Vertical Flow CW + Horizontal Flow CW, average removal efficiencies of 92% and 89% were obtained for BOD5, considering mean areal application rates of 4.1 and 2.1 gBOD5/day, respectively. Regarding bioenergy generation, different electrode materials were tested (graphite, granular activated carbon and stainless steel – AISI 304), besides different electrode spacing in a vertical flow CW and in FTW systems. Although high voltage values between the electrodes were verified (with peaks of 687 mV with granular activated carbon as electrode and maximum of 0.003 mW/m2 considering the WC superficial area and 0.57 mW/m2 , based on the anode area) the current power output generated by the systems is still limited. It was observed that factors such as the electrode surface area, the water level (substrate saturation) and plant roots influenced the monitored voltage values. Therefore, the obtained results demonstrated that the developed combined systems were efficient in the treatment of urban wastewaters. However, concerning bioenergy generation, for examples to power biosensors or LED lights (with necessary powers of a few mWs), further studies must be carried in order to better understand and improve the mechanisms of bioelectricity generation, without compromising the application of the CW-MFC units in pilot and real-scale treating real wastewaters.