Effect of natural zeolite on methane production for anaerobic digestion of ammonium rich organic sludge

Abstract

The effect of an inorganic additive on the methane production from NH 4 +-rich organic sludge during anaerobic digestion was investigated using different kinds of inorganic adsorbent zeolites (mordenite, clinoptilolite, zeolite 3A, zeolite 4A), clay mineral (vermiculite), and manganese oxides (hollandite, birnessite). The additions of inorganic materials resulted in significant NH 4 + removals from the natural organic sludge ([NH 4 +]=1, 150 mg N/l), except for the H-type zeolite 3A and birnessite. However, an enhanced methane production was only achieved using natural mordenite. Natural mordenite also enhanced the methane production from the sludge with a markedly high NH 4 + concentration (4500 mg N/l) during anaerobic digestion. Chemical analyses of the sludge after the digestion showed considerable increases in the Ca 2+ and Mg 2+ concentrations in the presence of natural mordenite, but not with synthetic zeolite 3A. The effect of Ca 2+ or Mg 2+ addition on the methane production was studied using Na +-exchanges mordenite and Ca 2+ or Mg 2+-enriched sludge. The simultaneous addition of Ca 2+ ions and Na +-exchanged mordenite enhanced the methane production; the amount of produced methane was about three times greater than that using only the Na +-exchanged mordenite. In addition, comparing the methane production by the addition of natural mordenite or Ca 2+ ions, the methane production with natural mordenite was about 1.7 times higher than that with only Ca 2+ ions. The addition of 5% and 10% natural mordenite were suitable condition for obtaining a high methane production. These results indicated that the Ca 2+ ions, which are released from natural mordenite by a Ca 2+/NH 4 + exchange, enhanced the methane production of the organic waste at a high NH 4 + concentration. Natural mordenite has a synergistic effect on the Ca 2+ supply as well on the NH 4 + removal during anaerobic digestion, which is effective for the mitigation of NH 4 + inhibition against methane production.