Enhanced anaerobic digestion of anaerobic granules by low-strength ultrasonication

Abstract

The steep increase in energy prices since the 1970s has reduced the attractiveness of aerobic waste treatment, contributing to the redirection of the research efforts towards energy saving alternatives such as anaerobic digestion (AD). AD is the bioconversion process consisting of hydrolysis, acidogenesis, acetogenisis, and methanogenesis. Its advantages are as follows: (1) reduced waste volume, (2) generating energy-rich gas in the form of methane (CH4), and (3) yielding nutrient-containing final products. However, the slow growth rate of the methanogens has been identified as a disadvantage coupled with the performance fluctuation due to their highly sensitive characteristics. In order to retain a highly concentrated biomass, various types of high rate anaerobic treatment systems such as the anaerobic contact reactor, anaerobic filter reactor, fluidized bed reactor, expended bed reactor, anaerobic membrane bioreactor, upflow anaerobic sludge blanket reactor (UASBr), etc. have been introduced. Among these reactors, the UASBr has been considered as the most powerful process with its successful operations and worldwide popularity being attributed to the dense sludge bed made from the granular sludge in the reactor bottom. However, as there is no mechanical mixing, the mass transfer between the granular sludge and substrate is often limited. To overcome the limitations of upflow anaerobic sludge blanket reactor (UASBr), i.e. limited mass transfer between the granules and substrate caused by the lack of mechanical mixing, in the present work, four vibrators were installed inside the UASBr to intermittently irradiate the methanogenic granules with expectations of enhanced microbial activity, mixing, and mass transfer rate by cavitation bubbles. Firstly, low-strength ultrasonication was applied at various ultrasonication densities (UD) (0 to 0.1 W/mL) and ultrasonication time (UT) (0 to 30 min) to methanogenic granules on the purpose of increasing their activity...