초록
<P><B>Abstract</B></P> <P>In this study, a three-stage upflow anaerobic sludge blanket (UASB) system was developed and tested for hydrogen (H<SUB>2</SUB>) and methane (CH<SUB>4</SUB>) production from cassava wastewater with an emphasis on CH<SUB>4</SUB> production. The experiment was carried out at mesophilic temperature (37 °C) at different chemical oxygen demand (COD) loading rates from 5 to 18 kg/m<SUP>3</SUP>d (based on total liquid holding volume) with a recycle ratio of the final effluent to both the first and second bioreactors at a constant 1: 1 flow rate ratio of feed: final effluent. The first bioreactor was maintained pH at 5.5 while those of the other two bioreactors were not controlled. At an optimum COD loading rate of 15 kg/m<SUP>3</SUP>d, the system provided the highest COD removal level (92.5%) and the highest H<SUB>2</SUB> and CH<SUB>4</SUB> yields of 0.43 mL H<SUB>2</SUB>/g COD applied, and 328 mL CH<SUB>4</SUB>/g COD applied, respectively. The very high productivity of CH<SUB>4</SUB> with the very low H<SUB>2</SUB> productivity resulted from the recycled methanogen sludge from the third bioreactor to the first and second bioreactors. The process performance of the three-stage UASB system in terms of optimum COD loading rate and total energy yield was much higher than those of single and two-stage anaerobic processes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The three-stage UASB provided a high optimum COD loading rate with a high energy yield. </LI> <LI> The high CH<SUB>4</SUB> yield with very low H<SUB>2</SUB> yield resulted from the recycled methanogenic sludge. </LI> <LI> The recycled effluent could provide proper pH values in both first and second bioreactors with a minimum NaOH consumption. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>