Otimização das variáveis que influenciam a produção de hidrogênio por fermentação escura

Abstract

In this present study, the use of cheese whey permeate was studied aiming hydrogen production during dark fermentation using a microbial consortium in batch processes. Inoculum volume and substrate concentration were evaluated in the fermentative process in the range of 0.8 to 13.3 (v/v) and 10 to 80 g/L of lactose, respectively. Thereafter, three Central Composite Designs (CCD) were conducted in order to verify the influence of some conditions of fermentation. The results of the CCD s showed that the values that presented higher productivity and yield within the studied ranges were: pH equal to 7.0, 0.6 g/L of ferrous iron sulfate, 1.5 g/L of ammonium sulfate, 30ºC and 1.28 g/L of magnesium sulfate, given a productivity of 127.8 mmol H2/L.day and a yield of 4.84 mol H2/mol of consumed substrate. Fermentations in scale-up were performed after the preliminary tests in order to verify the hydrogen production behavior in relation to time. Fermentations with pH control were also studied at pH values of 5.5 and 6.5. The fermentation at pH in 5.5 favored the metabolic routes of butyric acid production (3.5 g/L) and acetic acid (2.5 g/L) that lead to hydrogen synthesis. Moreover, sequential batch processes were tested aiming to obtain higher hydrogen production for longer time. In this study, the volume of withdrawn medium with addition of new medium and the time of medium removal were evaluated. The highest time of production, which was 84 h, was observed when 466 mL of medium were withdrawn in each 24 h in the first 48 h of fermentation, and subsequently, in each 12 h. Two different thermal pretreatments (93 ºC/15 min and 50 ºC/15 days) were also applied to the inoculum aiming to select hydrogen producing bacteria. After the pretreatment, fermentations were conducted at 30, 50 and 70 ºC. The best results were achieved when fermentation occurred at 30 ºC with the inoculum treated at 93 ºC/15 min, resulting in productivity of 128.8 mmol H2/L day.