Produção de metano em reator AnSBBR a partir de efluente de fecularia previamente acidificado: desempenho e viabilidade econômica

Abstract

Cassava starch industries have produced wastewater, which is their main environmental problem. Therefore, two-phase anaerobic digestion systems represent a newsworthy treatment alternative due to their efficiency in organic load removal and biogas production capacity. Thus, this study aimed at evaluating methane production from cassava starch wastewater in a sequential batch biofilm reactor (AnSBBR) as part of a two-phase system. The evaluation was composed of two main parts: i) analysis of a methanogenic reactor performance according to the increasing organic loads; and ii) the economic availability analysis regarding the reactor scaling-up for industrial scale. In the first part of this study, some previously acidified cassava starch wastewater was used to feed a methanogenic AnSBBR reactor on laboratory scale. The reactor has been submitted to increasing applied organic volumetric loads from 3.7 to 12.0 g COD.L-1.d-1, which were established by variation between affluent concentration and cycle time of a sequential batch reactor. The affluent concentration ranged from 2.8; 4.1 and 6.0 g COD-L-1, in sequential batches with cycle time of 6, 8 and 12 hours. The reactor was carried out at 30±1oC; it was stirred by a liquid phase recirculation and has been filled with support medium composed of polyurethane foam. The results of this first stage have shown that there was an increase in the organic loading rate (OLR) due to an increase on the affluent concentration and, mainly, a decrease on the cycle time duration showed an affect on the reactor performance. Methane productivity increased due to the increase in OLR and has reached 2.7 L CH4 L-1d-1 at OLR 12.0 g COD-L-1.d-1. COD removal has also been affected by OLR increase and ranged from 94 to 60%. And, the lowest values of COD removal were ascribed to the shortest cycle times. All the experimental conditions were constant. In the second part of this study, a two-phase anaerobic digestion system was designed on an industrial scale, based on the results previously obtained. Finally, an economic feasibility analysis was developed to settle down this project, which considered a twenty-year useful life term. The results of the second phase evinced that the simple payback was 8.4 years and the discounted payback was 15.9 years. Thus, it was recorded that this project can bring forth a profit similar to the net present value (R$ 1.9 million) at the end of the project lifetime, while internal rate of return was calculated in 11.1%. These results indicated that the two-stage system is economically viable to produce methane from starch wastewater.