INVESTIGATION OF BIOGAS PRODUCTION FROM SOLID-STATE ANAEROBIC DIGESTION OF CATTLE MANURE

Inoculum for solid-state anaerobic digestion (SSAD) and SSAD reactor design

Dairy cattle manure from the solid/liquid separation of wastewater from the National Taiwan University (NTU) dairy farm was used to as the sole substrate for SSAD reactors. The sludge from the anaerobic digesters of the NTU dairy farm was used as the initial inoculum for enrichment of the SSAD reactors. After three batches of the enrichment process (about 90 days), the methanogenically activated (McA) mixture from the previous batches of SSAD were utilized as the inocula for the further SSAD experiments (Figure 2). The initial and final weight of dairy cattle manure and MeA mixture was 5 and 4.1-4.8 kg and the volume of initial and final liquid was 2.5 and 2.7-4.0 L, respectively for SSAD experiment. Acrylic anaerobic digester (19 cm inside diameterx 115 cm height) in triplicate with a working volume of 37 L was used in this study (Figure 3). A thermostatic recirculation water batch was equipped outside the digester to maintain the digester at 36±1°C. Each digester had an independent leachate recirculation magnet pump (Wee & Su 2019), which was used to recirculate the leachate from the side port at the bottom of the digester to the side port at the top of the digester through rubber tubes (18 mm outside diameter x 8 mm inside diameter). A digital programming timer controlled the leachate recirculation and the recirculation frequency was every 20 min for the 20 sec recirculation process (approximately 3.8 L leachate/recirculation).

Preliminary study of SSAD by fresh cattle manure

A preliminary study was performed in an acrylic anaerobic digester (19 cm i.d. x 115 cm Height) with fresh cattle manure (5 kg) as the sole feedstock. The preliminary results showed that two peaks of biogas yield occurred during Days 1 to 7 and Days 30 to 34 (Figure 4A). Nitrogen content in the biogas decreased from 63.8 to 19.2%. However, Methane content increased from 8.5 to 52.7% for the 37-d period. The results implied that denitrification was dominant in the anaerobic digester during Days 1 to 7 and methanogenesis became dominant after Day 16 (Figure 4B) (Wee & Su 2019).

Flow chart of the two-step biological treatment system (Wee & Su 2019)

Figure 2. Flow chart of the two-step biological treatment system (Wee & Su 2019).

Design of the SSAD reactor (Wee & Su 2019)

Figure 3. Design of the SSAD reactor (Wee & Su 2019).

Biogas production (A) and biogas contents (B) of the preliminary SSAD study (Wee & Su 2019)

Figure 4. Biogas production (A) and biogas contents (B) of the preliminary SSAD study (Wee & Su 2019).

Time course experiment of SSAD

In order to accelerate the SSAD process, various inoculation ratios (digestate fiber from previous experiments) were tested as well as the operation conditions. Theoretically, the more digestate fiber inoculate, the faster SSAD process (Figure 2). Two groups of initial pH (7.8 and 5.2-5.5) and three groups of inoculation ratios (IR) (50, 30, and 10%) were designed to evaluate efficiency of SSAD experiments. The group with 50, 30, and 10% inoculation ratio are referred to IR50, IR30, and IR10, respectively (Wee & Su 2019). Every time course experiment of SSAD was conducted for a 14-d period, daily biogas yield was collected and measured by applying water displacement method with a 6-L glass gas collector. The MeA mixture from the initial SSAD reactor as inocula were added manually into a laundry mesh bag (60 cm x 60 cm) by the inoculation ratios of 50, 30, and 10% (w/w) respectively, the total weight of the mixture was 5 kg.

Effect of pH adjustment and IR on daily and cumulative biogas yield

Biogas productivity was measured in terms of volatile solids destroyed (VStics) or VS loaded (VSload). Results showed that the group IR50 with pH adjustment had the largest biogas peak value, 21.7±0.2 L/kg VS)oad/day, after the peak of biogas yield on Day 3. the daily biogas yield decreased gradually to 5.8110.2 L/kg VS|oad/day on Day 14. While the peak value of daily biogas yield was 17.310.3 and 11.710.3 L/kg VS)oad/day for group IR30 and IR10 with pH adjustment, respectively. The peak value of daily biogas yield was 6.410.3, 6.910.2, and 4.5

10.4 L/kg VSioad/day for group IR50, IR30, and 1R10 without pH adjustment, respectively (Wee and Su, 2019). Experimental results implied that pH was a key parameter that influenced the peak sizes; indeed, the groups with pH adjustment had a larger peak size from 21.7 to 11.7 L/kg VSioad/day, where groups without pH adjustment had a small peak size from 6.9 to 4.5 L/kg VSioad/day. The cumulative biogas yield of groups with pH adjustment (124.615.8 -164.611.9 L/kg VSU)ad) w'ere all significantly higher than groups without pH adjustment (37.711.9-45.512.6 L/kg VSload) (Wee & Su 2019).

Effect of pH adjustment and IR on methane concentration

Methane productivity was measured in terms of VSdcs, VS]oad, or volume (Mollcr et al. 2004). Thus, theoretical methane yield (Bu) and ultimate methane yield (Bo) were defined in terms of either VS destroyed (L CH^kg VSdcs) or VS loaded (L CH4/kg VSioad) based on either the actually bio-degraded or total load VS contents of the substrate mixture, respectively, by the SSAD process. This result indicated the group IR50 with pH adjustment had a faster meth- anogenesis start-up and resulted in faster increase of methane concentration. The group IR30 with pH adjustment had the highest cumulative methane yield (96.812.0 L CH4/kg VS|oad) on Day 14 than the groups IR10"(69.213.7 L CH4/kg VSload) and IR50 (86.811.0 L CH4/kg VSload) with pH adjustment. However, the groups without pH adjustment had lower cumulative methane yield (12.112.5-16.011.7 L CH^kg VS]oad) on Day 14 compared to groups with pH adjustment regardless inoculation ratios (Wee & Su 2019).

The study of Mollcr ct al. (2004) showed that the theoretical methane yield and ultimate methane yield of dairy cattle manure w'ere 468161 L CH4/kg VSdcs and 148141 L CH4/kg VSload, respectively. The theoretical methane yield of the group IR30 with pH adjustment (626.1128.7 L CH4/kg VSdcs) wfas comparable to Mollcr’s study (468161 L CH4/kg VSdcs) (Wee & Su 2019, Mollcr ct al. 2004). While the ultimate methane yield of the group IR30 with pH adjustment (96.8112.0 L CH4/kg VSioad) was slightly low'cr than Moller’s study (148141 L CH^kg VSioad). The experimental results implied that the methane productivity of SSAD in this study was comparable to other SSAD studies (Wee & Su 2019).

 
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