Lignite addition during anaerobic digestion of ammonium rich swine manure enhances biogas production

Excess ammonia inhibits methanogenesis during anaerobic digestion. The addition of lignite removes ammonia/ammonium from wastewater. Different lignite addition rates (0, 10, 20, 40, 70, 100 g/L) to ammonium rich lab scale anaerobic digestions of swine manure were investigated. The optimum lignite rate was selected by assessing methane production potential, maximum methane production rate, and the lag phase time using the modified Gompertz and logistic models. Results showed that methane production at lignite addition rates of 70 g/L and 100 g/L, is significantly increased compared to all other rates. A digester with a lignite addition rate of 70 g/L of swine manure had the highest methane production (i.e. 31 % higher than the control), and a digester with a lignite rate of 100 g/ L showed the highest biogas production. The results indicate that addition of lignite at >= 70 g/L reduced ammonia inhibition of methanogenesis during anaerobic digestion of swine manure by lowering the free ammonia nitrogen (FAN) concentration below the threshold level (45 mg/L) and lowering the ammonia/ ammonium ratio, mainly through the lowering of pH.