Anaerobic digestion of secondary tannery sludge: optimisation of initial PH and temperature and evaluation of kinetics

Abstract

The feasibility of retrofitting and operating a low cost anaerobic mesophilic batch reactor for the reduction of secondary tannery sludge (STNS) produced in tannery wastewater treatment plants at varying initial reactor pH ( pHin) using acclimated inoculum was demonstrated using the bio-methane potential test protocol. Temperature had a significant effect on process indicators whilst the effect of pHin was negated by the STNS’ high alkalinity and its buffering capacity offered by ammonia/ ammonium speciation, eliminating the need for pH control during digestion. Furthermore, there was an insignificant and significant variation in the biodegradability ( Bo) and gas yields of STNS, respectively. This nullified the need for reactor heating if gas yields were not prioritised. Nonetheless, optimum conditions (35.5–37.0 °C and pHin 6.5) yielded 200–217 mL biogas/gVS (≈ 55% CH4), 108–118 mL CH4/ gVS and 69–74% VS, 54–57% TS and 47–48% COD reduction. Cumulative CH4 generations were accurately modelled using the modified Gompertz, Logistic, First order, Cone (0.828 ≤ Adj R2 ≤ 0.986) and process kinetics were determined. However, the correlations between kinetics and process parameters were mostly nonmonotonic. The positive effect of temperature on A (r = 0.84) coincided with a decrease in methanogenesis rate (k) and ratio of hydrolysis and methanogenesis (K/μm) at optimum pHin signifying an inhibited steady state reactor condition.