Thermophilic anaerobic membrane bioreactor for pulp and paper primary sludge treatment: Effect of solids retention time on the biological performance

Lignocellulosic biomass is a promising renewable feedstock for the production of bioenergy and platform chemicals; however, its use would not be economically viable without adopting effective transformation technologies. In this work, long-term anaerobic treatment (370 days) of pulp and paper primary sludge (PS) was performed by a thermophilic anaerobic membrane bioreactor (AnMBR) for the first time to investigate the effect of various solid retention times (SRTs) on biogas production and solids reduction ratios. The tested STRs (32-55 days) have shown varying biogas productivity, and it can be concluded that the longer the SRT, the higher is the biogas yield. At the optimum solids retention time of 55 d, an organic loading rate of 2.15 +/- 0.10 kg-TSS/L.d, and hydraulic retention time of 5 d, average biogas yield of 106.4 +/- 8.5 m(3) biogas/tonne TSS (added) (approximately 72.7 m(3) CH4/t VSadded) was achieved with an average methane content of 56 +/- 4% and the solids reduction ratio ranged between 47% and 54.9%. Besides biogas yield, the quality of the permeates, solids reduction, and digestates characteristic/dewaterability changed by changing the SRT. The mixed liquor suspended solids concentrations and solids reduction increased with increasing SRT, while the effluent COD concentration and sludge dewaterability decreased. The Fourier Transform Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) findings exhibited that the digestate contains more lignin and cellulose than the other substances, while the nitrogen and carbon concentration decreased with increasing SRT.

Automated summary

This study investigated the long-term anaerobic treatment of pulp and paper primary sludge using a thermophilic anaerobic membrane bioreactor. Results showed that increasing the solid retention time (SRT) led to higher biogas yield, as well as changes in digestate characteristics and dewaterability. The findings suggest that the presence of lignin and cellulose increased in the digestate with longer SRTs, while nitrogen and carbon concentrations decreased.