Microbial insights of enhanced anaerobic conversion of syngas into volatile fatty acids by co-fermentation with carbohydrate-rich synthetic wastewater

Table 1 Summary of the experimental performances

	G5	GS5	G15	GS15	P5	PS5	P15	PS15	S
CO consumption (mL/day)	–	57.4 ± 2.8	–	51.8 ± 2.5	–	26.1 ± 1.4	–	21.9 ± 2.3	45.6 ± 2.1
H₂ consumption (mL/day)	–	61.4 ± 3.2	–	48.2 ± 2.7	–	27.5 ± 2.2	–	21.1 ± 2.7	42.4 ± 2.8
CO conversion efficiency (%)^a	–	71.7 ± 3.5	–	64.7 ± 4.0	–	32.6 ± 1.8	–	27.3 ± 2.9	57.6 ± 2.8
H₂ conversion efficiency (%)^a	–	76.7 ± 1.2	–	60.3 ± 0.9	–	34.4 ± 0.7	–	26.4 ± 2.1	53.0 ± 1.7
Acetate conversion efficiency from syngas (%)^b	–	90 ± 2		69 ± 1	–	153 ± 3	–	212 ± 5	96 ± 2
VFAs conversion efficiency from organic (%)^c	84 ± 1	92 ± 4	81 ± 2	84 ± 2	95 ± 3	93 ± 2	94 ± 4	93 ± 5	–
Glucose/BSA (mg/L)	230 ± 19	227 ± 29	1256 ± 136	1194 ± 211	725 ± 37	794 ± 71	3584 ± 203	3346 ± 309	296 ± 19
Total COD (mg COD/L)^d	5300	6822	15,900	17,422	7100	8622	21,300	22,822	1522
VFAs/total COD (%)^d	82 ± 1.2	86 ± 2.3	75 ± 0.9	78 ± 2.5	82 ± 1.0	70 ± 2.1	64 ± 2.7	63 ± 3.2	55 ± 1.3
Residual syngas/total COD (%)^d	–	6 ± 0.5	–	4 ± 0.3	–	10 ± 0.8	–	5 ± 0.4	40 ± 1.2
Residual organic/total COD (%)^d	5 ± 0.2	3 ± 0.1	9 ± 0.4	8 ± 0.7	11 ± 1.4	10 ± 0.8	24 ± 1.9	22 ± 2.2	–
Unbalanced COD/total COD (%)^d	12 ± 2.5	4 ± 0.1	15 ± 2.2	11 ± 3.1	6 ± 0.5	9 ± 1.3	11 ± 1.6	10 ± 0.9	5 ± 1.3
NH₄⁺-N (mg/L)	80 ± 5	87 ± 17	78 ± 4	80 ± 5	935 ± 46	916 ± 49	1608 ± 91	1623 ± 63

^aThe P-values of CO/H₂ conversion efficiency among all the reactors were calculated by Duncan analysis and shown in Additional file 1: Table S1
^bOne mole acetate was produced from 4 mol H₂ or 4 mol CO (4CO + 2H₂O → CH₃COOH + 2CO₂ and 4H₂ + 2CO₂ → CH₃COOH + 2H₂O), and the theoretical acetate production from H₂ and CO was calculated according to the above two stoichiometric equations. The acetate conversion efficiency from converted CO and H₂ was calculated: the measured acetate production/ the theoretical acetate production from H₂ and CO
^cThe theoretical VFA production (mgCOD/L) from organic was calculated by subtracting the COD of residual organic from the COD of added organic. The VFAs conversion efficiency from organic was calculated: the measured VFAs production/the theoretical VFAs production from organic
^dCOD mass balance in all bottles at steady state. Total COD was the sum of organic and syngas. COD was calculated based on the following: CO: 0.57 gCOD/g, H₂: 8 gCOD/g, ACETATE: 1.07 gCOD/g, propionate: 1.51 gCOD/g, butyrate: 1.81 g COD/g, valerate: 2.04 g COD/g, carbohydrate: 1.06 g COD/g, protein: 1.42 g COD/g, lactate: 1.06 gCOD/g. Unbalanced COD was calculated by subtracting the COD of VFAs, residual syngas, residual organic from total COD

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