Anaerobic Digestion of Secondary Tannery Sludge: Optimisation of Initial pH and Temperature and Evaluation of Kinetics
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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 non-monotonic. 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.
KeywordsAnaerobic digestion Secondary tannery sludge Optimisation Temperature Influent pH Kinetics
The authors would like to thank the Water Research Commission (WRC) for funding this project and the NATSURV K5/2490 project team. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and therefore the WRC does not accept any liability in this regard thereto.
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