Anaerobic digestion of dissolved air floatation slurries: Effect of substrate concentration and pH

Abstract

Dissolved air floatation (DAF) slurries are an unconventional substrate for anaerobic digestion owing to their high COD concentrations, mainly consisting of lipids. Batch anaerobic digestion of DAF slurry was conducted to investigate the methane production through dilution of the slurry (with mineral salt medium, MSM) and by adjusting the initial pH (to 6.0, 5.5, 5.0 and 4.5) The performance of two different inocula, namely waste activated sludge (WAS) treating domestic wastewater and anaerobic granular sludge (AGS) treating dairy wastewater, was compared. The DAF slurry had a pH of 7.03 ( ± 0.07), conductivity of 6.69 ( ± 0.54) mS/cm, and a COD and ammonium concentration of 85.96 ( ± 8.32) g/L and 168.49 ( ± 1.79) mg N/L, respectively. DAF slurry without dilution produced a maximum methane yield of 58.60 ( ± 1.97) mL/g COD after 30 days of incubation. Dilution with MSM up to three folds, improved the methane yield by about 67%–105% and 67%–177% with WAS and AGS as the inoculum, respectively. Higher methane productivity was achieved at an inoculum to substrate ratio of at least 1.5 for WAS and 2.0 for AGS, respectively. AGS was more effective with a 20% increase in methane production from 3-fold diluted DAF slurry when compared with WAS. Adjustment of the initial pH of the DAF slurry to 6.0 resulted in a methane yield of 359.71 ( ± 32.72) mL/g COD, which was 23% higher than the control without pH adjustment. This study demonstrated that the methane yield from DAF slurries can be enhanced through substrate dilution and initial pH adjustment. • Methane yield enhanced by 23% upon adjustment of initial pH of the DAF slurry to 6.0. • 3-fold dilution increased methane production rate by 275% with granular sludge. • Anaerobic granular sludge is an efficient inoculum for methanogenesis of DAF slurry. • Lag phase duration was not shortened by dilution or pH adjustment of the DAF slurry. • Optimal inoculum to substrate ratio at least 1.5 and 2.0 for WAS and AGS, respectively.