Enhanced bio-methane production from ammonium-rich waste using eggshell-and lignite-modified zeolite (ELMZ) as a bio-adsorbent during anaerobic digestion

In this study, a calcium-rich natural material utilized bio-adsorbent, composed of eggshell-and lignite-modified zeolite (ELMZ) was developed for enhancing methane production at high ammonia concentrations. The ELMZ showed a maximum NH4+-N adsorption capacity of 55 mg/g, which was 5.9 folds of the natural zeolite (9.2 mg/g) and 1.4 folds zeolite modified by calcium chloride (40 mg/g), respectively. The ammonium adsorption process well fitted the pseudo-second-order kinetic model and the Freundlich isotherm. It indicated that its process was an ion-exchange process. The high abundance of cations, especially high Ca2+ in ELMZ supported the ion-exchange, which stimulated the anaerobic digestion process. The anaerobic digestion with ELMZ as a fixed-bed (initial NH4+-N concentration: 4.1 g/L), exhibited the best performance at methane yield of 250 mL CH4/g-DOCremoval, which is 7 folds higher than that achieved with natural zeolite (32 mL/g-DOCremoval) at the same ammonia level. As a bio-adsorbent, ELMZ contributes to enhance methane production and further benefits to a sustainable process since the eggshell waste is re-circulated.