Development of an oyster shell and lignite modified zeolite (OLMZ) fixed bioreactor coupled with intermittent light stimulation for high efficient ammonium-rich anaerobic digestion process

Abstract

A novel oyster shell and lignite modified zeolite (OLMZ) fixed bioreactor incorporated with intermittent light illumination process was developed for alleviating ammonia inhibition to improve efficiency of ammonium-rich anaerobic digestion (AD). The SEM morphology of the newly synthesized OLMZ showed porous structure and rough surface, with increased BET surface area compared to unmodified zeolite (UMZ). After modification with oyster shell and lignite, the metal cations content in OLMZ increased significantly and the ammonia adsorption capability of OLMZ improved by 1.3 folds compared to UMZ. Thus, the results suggested that OLMZ is a better adsorbent than UMZ for ammonia uptake. Furthermore, the pseudo-second-order kinetics and the isotherm results verified the adsorption of ammonia on OLMZ was dominated by ion exchange with high affinity for ammonia uptake. As for the anaerobic digestion, the light illuminated OLMZ fixed bioreactor process (OLMZ-I) resulted in the highest methane yield of 372 ± 30 mL/g-DOCremoved, which showed 300% increase compared to the control (124 ± 12 mL/g-DOCremoved). In addition, OLMZ-I showed higher ammonia removal capacity and stable pH which indicated that the ammonia inhibition was effectively mitigated. Correspondingly, metal cations supplementation and light stimulation in OLMZ-I showed increased ATP and coenzyme F420 implying high activation of methanogens leading to improved CH4 production. The large biomass quantity and SEM observation of microbes immobilized on the fixed OLMZ system aided by light further supported the enhanced performance of the reactor. Furthermore, the conductance of sludge in OLMZ-I bioreactor increased 3 folds compared to the control, reflecting a potential direct interspecies electron transfer (DIET) between the anaerobes was strengthened. The synergetic effects of ammonia removal, microbes immobilization, metal cations supplementation and accelerated electrical communication between methanogenic system combined with light stimulation on methanogen activation facilitated the enhancement of methanogenesis. Therefore, the OLMZ fixed bioreactor coupled with intermittent light stimulation shows great potential in practical application for bioconversion of ammonium-rich livestock waste.