In addition to the biogas yield, different parameters were monitored at the end of each cycle to assess the quality of the digestate and supernatant. The process performance parameters measured are summarised in Table 7-4 and Figure 7-4.
Table 7- 4: VS removal during sequential batch AD assays Reactors VS Removal %
1st Cycle 2nd cycle 3rd cycle 4th cycle 5th cycle 6th cycle R1 40.7±0.2 35.98±0.2 33.8±0.1 34.63±0.6 32.36±0.98 30.65±1.50 R2 36.25±0.2 36.32±0.3 42.60±0.1 41.78±0.2 45.62±1.25 44.96±1.44 R3 36.11±0.1 36.25±0.6 39.53±0.8 40.27±0.3 44.32±1.71 43.95±1.77 R4 33.99±0.4 35.68±0.1 49.58±0.9 50.12±0.5 55.20±1.27 55.69±1.18
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(a) (b)
(c) (d)
Figure 7- 4: Reactor performance during sequential AD- (a) TS removal (%), (b) pH, (c) VA (mg/L) and (d) ammonia
The VS removal at the end of each cycle is shown in Table 7-4. In all the reactors, the VS removal for the first 2 cycles (untreated substrates) was around 36.7±1.9%. It was observed that the VS removal under semi-continuous AD was 41.7% (Zahan et al., 2018b) and the VS removal of 42.8%, obtained using BMP assays at 4% TS feed (Zahan et al., 2018c) for CL ACoD with agricultural wastes and FW. As the reactors used acclimatised inoculum as anaerobic microbial source, therefore, the inoculum had already adapted to the different substrates-mix (feedstock), which could explain the TS and VS removal at these high TS of 15%
being comparable to the AD at 4% TS, though slightly less.
For reactor R1, the VS removal decreased to around 34.8%-30.65% with the continual operation of the reactors, which synchronised with the trend in biogas production. As shown in Figure 7-4, the inhibitory parameters (NH4 and VA)
0%
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 7.1
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6
0
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 0
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6
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increased in the reactors with times, causing less activity of the methanogenic bacteria, which led to 15.4% decrease in VS removal.
When either CL or WS was alkali pre-treated, VS removal of around 42.6% and 39.5% respectively were observed for reactors R2 and R3. The VS removal from AD reactors that received pre-treated WS mixture was around 2% higher compared to the treated CL mixtures, as WS has more cellulose and less lignin content than CL.
VS removal of around 50% was obtained when both the alkali pre-treated samples were fed into the reactor R4 in cycle 3 and 4. During the 5th and 6th cycle, when sequential alkali and acid pre-treatment were applied, the VS removal ranged between 44% to 55%, for reactors R2-R4, which is almost 20-50% higher than the initial removal. From Figure 7-4 (a), the TS and VS removal correlated with the biogas production trends. For reactor R1, the TS removal efficiency decreased with continual operation. The highest TS removal was around 47% for reactor R4 when feedstock that contains sequential alkaline and acid pre-treated CL and WS mixture was applied.
Pre-treatment also improved the AD process performance. Characteristics at the end of each cycle are shown in Figure 7-4. At the start, the pH was between 7.3-7.6 for cycle 1 and 2 (Figure 7-4 (b)), which increased to 7.7 with subsequent repeats of feeding for reactor R1. With each repeat, the reactors were overloaded with CL as indicated by the increase in pH, associated with the accumulation of ammonia from the uric acid and proteins, and this subsequently increased the pH level (7.7) in the reactor. In this way, inhibition in the form of decreased TS, VS removal was evident for reactor R1 (Table 7-4 and Figure 7-4). The pH level decreased (7.4-7.45) with pre-treated feed which led to an improvement in biogas production and VS removal. With sequential pre-treatment, the pH level decreased further within the optimal range for the high performing reactors which are an ideal situation for stable methanogenesis (Zahan et al., 2018b).
No accumulation of VAs was evident and the highest VA observed with untreated feed (1.55 g/L) which is well below the inhibition threshold (4 g/L) (Siegert and Banks, 2005). The VA accumulation threshold can vary from substrate to substrate depending upon several factors and tolerances of microbial source .The accumulation of VA can cause instability of the process by inhibiting acetotrophic
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methanogenesis (Zahan et al., 2018b). From Figure 7-4 (c), the VAs in the reactors decreased further with pre-treated substrates, indicating no inhibitory effect for VA and was associated with increased biogas production.
From Figure 7-4(d), for reactor R1, the NH4 increased with the continual cycle repeats feeding cycles. For wastewater, if the NH4 level is between 1.5–2.0 g/L, inhibition occurs (Zahan et al., 2016b). The ammonia level in reactor R1 was in the inhibition range and with each repeat the ammonia level increased. As acclimatised inoculum was used, it improved the tolerance for NH4 level and the AD reactor was continued to produce biogas. Eventually, however, inhibition occurred in the reactor, if no measure to stop inhibition was taken. For reactor R2, a decrease in NH4 was observed for cycles 3 and 4. Similar trend was observed for R3 which received feedstock including CL pre-treated samples. The pre-treatment improved the process performance, most likely through the increased water soluble organics as well as soluble organics which served to provide balance environment in terms of C/N and degradation products utilisation. From Figure 7-4(d), the NH4 level in reactor R3 is slightly lower than reactor R2. This is probably due to the pre-treatment of CL which removed some of the free ammonia in CL samples whereas in reactor R3, untreated CL was used. For reactor R4, where both CL and WS pre-treated samples were fed, best performance was observed, and the ammonia was below 1500 mg/L.
From the results it can be summarised that with each sequential repeat feed the removal of TS and VS decreased with increase in pH, and ammonia. This was very consistent with their biogas production trends. On the other hand, the pH, VA, ammonia level decreased with pre-treated feed with increased TS and VS removal.
The inhibition in the reactors occurred more of an occurrence of ammonia inhibition rather than VA accumulation. The specific biogas production and CH4 yield depended mostly on the origin of the substrates, composition, and operational conditions i.e. SRT, temperature for large reactors (Zahan et al., 2018b). Therefore, the results reported here should be applied to a continuous process and a small pilot scale continuously fed anaerobic digester before they are used in a large-scale scenario.
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