Effect of Acetate, Propionate and pH on Aqueous Concentration and Gaseous Methane and Hydrogen Production in Continuous Culture

Abstract Text:

Four continuous culture fermenters were used to determine the effect of varying volatile fatty acid concentrations and pH on hydrogen and methane production. The experiment constituted 4 treatments applied in 4 periods. Treatments were:  control, 20 mmol/d acetate infusion (INFAC), 7 mmol/d propionate infusion (INFPR), and low pH (LOWPH). In LOWPH buffer flow was adjusted to lower pH by 0.5 units compared to control. The fermenters were fed 40 g of a pelleted 50:50 alfalfa: concentrate diet once daily. One week of adjustment occurred each period before sampling. Filtered liquid effluent (20 ml) from the fermenters was sampled at 0, 2, 4, 6, 8, 12, 16, and 22 h after feeding for measurement of aqueous hydrogen and methane concentrations. Daily cumulative gas production and hourly production rate were analyzed for headspace methane and hydrogen. The effects of treatments are shown in Table 1. Headspace methane production was lower (P <0.05) in LOWPH compared to other treatments.  Methane production rate in LOWPH was lower (P <0.05) than other treatments from 2 to 9 h, and was lower (P <0.05) compared to INFAC and INFPR at 10 h. Aqueous methane concentrations were higher (P <0.05) in LOWPH compared to control and INFAC, whereas in INFPR it was higher than control (P <0.05). The differences were significant (P <0.05) at 16 h. Headspace hydrogen production was higher in INFAC, followed by INFPR and control, and LOWPH (P <0.05). Production rate of headspace hydrogen from INFAC was higher (P <0.05) than others from 2 to 7 h. The effect of treatment on aqueous hydrogen concentration was not significant (P>0.05).

These results reveal that headspace methane and hydrogen was lowered by low pH, and acetate infusion increased the headspace hydrogen in a continuous culture. Aqueous hydrogen was unaffected by the treatments, whereas aqueous methane was increased by low pH and propionate infusion.

Table 1. Effect of low pH, acetate infusion, and propionate infusion on gaseous production and aqueous concentration of methane and hydrogen in continuous culture

Gas production and concentration	Control	INFAC1	INFPR2	LOWPH3	SE	P value
Aqueous methane (uM)	120a	130a,b	176b,c	187c	18	0.02
Headspace methane (umol/d)	9775a	11266a	9998a	5823b	2220	<0.001
Aqueous hydrogen (uM)	2.25	1.74	2.01	1.47	0.31	0.35
Headspace hydrogen (umol/d)	65.6a	142.6b	76.9a,c	18.8d	29.7	<0.001

¹INFAC: Acetate infusion (20mmol/d); ²INFPR: Propionate infusion (7mmol/d); ³LOWPH: buffer adjusted to lower pH by 0.5 units.

Keywords: Fermenter, hydrogen, methane