Potencialidade dos pré-tratamentos térmico solar e enzimático para produção de biogás a partir de microalgas oriundas de sistemas de tratamento de esgoto

Abstract

The microalgae biomass produced in wastewater treatment systems has a high potential for valorization through anaerobic digestion for biogas production. However, the hydrolysis of this biomass during anaerobic metabolism has certain limitations, mainly because of the high content of biopolymers of low biodegradability present in the cell wall of microalgae and the complexity of its structure. In this sense, the objective of this work was to evaluate the effect of two pretreatment techniques on the biodegradability of microalgal biomass from a high rate algae pond responsible for the post-treatment of the effluent from a UASB reactor. For this purpose, tests in batch operation mode were used to study the thermal and enzymatic pretreatments. The thermal pretreatment, the most consistent alternative according to the literature, has as a disadvantage the high energy consumption. Therefore, in order to reduce this consumption, the pretreatment was carried out using a low-cost solar heating system, composed of solar collector tubes, with different exposure times to the pretreatment being compared (4, 7 and 10 hours). Then, the biochemical methane potential test (BMP) was carried out to determine the optimal biodegradability condition from the different exposure times. Regarding the enzymatic pretreatment, it was conducted by the codigestion of microalgal biomass with rumen content in a BMP experiment, comparing the microalgae/rumen ratios of 0.5, 1, 3 and 5. The advantage of using rumen content is that, in addition to the widely reported hydrolytic capacity of its microorganisms, cocktails of natural enzymes are used instead of the use of commercial enzymes, which entails an elevated cost. The results showed that, regarding the thermal pretreatment, the peak temperature observed was 93ºC, after 4 hours of pre-treatment. The COD solubilization achieved was 14%, 22% and 24% after 4, 7 and 10 hours of pretreatment, respectively. The BMP tests that showed the highest biogas yield were after 7 and 10 hours of pre-treatment of the biomass (210.2 mL.gVS-1), with a 63% increase in biogas production compared to the control. In the case of enzymatic pretreatment, codigestion in the 0.5 microalgae/rumen ratio showed the highest methane yield (299 mLCH4.gVS-1),which corresponds to an increase of 17% when compared to the theoretical methane yield from the microalgal and rumen controls. Regarding the previous literature, the pre-treatments used showed promising results.