Impact of ITHP and THP configurations on lipids biodegradation

Figures 7.5 (A and B) shows the average lipid degradation kinetics obtained from three of the experimental runs conducted. The individual data points and graphs of lipid degradation kinetics from the three batch digestion runs collected from the ITHP and THP configurations are presented in Appendix C. Figures 7.5, shows the stepwise reduction of lipid concentration with time in the ITHP and THP configurations.

Figure (7.5) Average lipids degradation kinetics during three runs of batch anaerobic

digestion of sewage sludge using THP and ITHP configurations.

A is a lipid degradation kinetics – THP configuration and B is a lipid degradation kinetics - ITHP configuration.

A

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For clarity Figures 7.6 A and B show the lipid degradation trends in the three runs per digestion configuration studied.

Figure (7.6) Summary of lipids degradation kinetics of THP and ITHP configurations from

combined batch runs.

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The data showed that the overall lipids concentration removal characteristic in the ITHP configuration was similar to that of the THP configuration. As expected the THP configuration had a faster initial removal rate and had two staged lipids removal trend.

During first stage, the lipids concentration was reduced from 257 mg/l to 24 mg/l within 172.3 hours (circa 7.2 days) of digestion time, this is circa 91% lipid removal rate, whereas, during the second stage the lipids concentration was reduced from 24 mg/l to 16 mg/l within remaining 200 hours (8.3 days), which accounted a 3.1% lipid removal rate based on the initial lipid concentration.

Similarly, in the ITHP configuration, the lipid removal showed two stages, faster during first stage and relatively slower second stage. During first stage, the lipids concentration was reduced from 291 mg/l to 86 mg/l within 43 hours of digestion time, which represented 70% lipid removal rate, whereas during the second stage the lipids concentration was reduced from 86 mg/l to 21 mg/l (within 248 hours or 10.3 days of digestion time) which is 22% lipid removal rate based on the initial lipid concentration. The data from individual runs are presented in Appendix C.

The remaining lipids residue in the digestates obtained from both configurations were higher than the minimum lipids threshold value reported in literature. According to Gonzalez (2006) when lipids concentration in the digesting sludge was higher than 1 mg g-1, it was the sludge constituents most easily degraded. The finding from this research configuration is in agreement with Gonzalez’s finding (Gonzalez, 2006).

Similar to carbohydrate and proteins biodegradation process described in sections 7.4.1 and section 7.4.2 , the lipids stepwise biodegradation rate constants (k values) were derived from the gradients of the equations fitted to different sections of the graphs as shown in Figure 7.5, A to B above. Figure 7.6, A and B are the visual representation of anaerobic lipids biodegradation process during repeated runs of batch anaerobic digestion process.

Table 7.4 shows the average rate constants calculated from all 3 runs used during this research work and the raw data used to produce this average values is presented in Appendix C (Table 11.3 C, table 11.4C and table 11.5C).

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Table (7.4) Summary of selected values of average lipids degradation (decay) rate constants.

Lipids degradation rate constants

THP ITHP MAD

Kinetics Rate constant

(per day) r 2 Rate constant (per day) r 2 Rate constant (per day) r 2 First Order k1 -0.025 0.93 k1 -0.057 0.97 k - 0.04 0.94 k2 -0.001 0.31 k2 -0.006 0.97 - - Average k value -0.013 0.62 Average k value -0.032 0.97 - -

Reference This work Ercoskum and Ozkal, (2011)

During the ITHP process the lipids degradation rate constant obtained ranged from -0.006 to -0.057 per day, with average k value of - 0.032 per day (Table 7.4). The initial faster degradation of lipids followed the first order kinetics with k values of 0.057 per day, and the second stage also followed a first order kinetics with k value of -0.006 per day.

Likewise, during the THP process the lipids degradation rate constant obtained ranged from -0.001 to -0.025 per day, with average k value of - 0.013 per day. In the THP configuration, the faster initial lipids degradation rate followed a first order kinetics with k values of - 0.025 per day and the second section rather followed a pseudo first order kinetics with a k of 0.001 per day. When the lipids degradation rate constants obtained from the ITHP and THP configurations were compared, the data suggested that the lipid degradation in the ITHP configuration was faster than that of the THP configuration.

The r2 values in Figure 7.5 indicated that the data displayed a good fit. THP yielded r2 values ranging from 0.31 - 0.93 with an average r2 value of 0.62 and ITHP produced r2 of 0.97 during both stages of lipid degradation with an average r2 value of 0.97. The literature value reported by Ercoskum and Ozkal (2011) for MAD is much higher than the values found in this research work for the ITHP and THP configurations (Table 7.4).

During this anaerobic batch sludge digestion process, changes in rate constants in the THP configuration were observed between 6 to 172 hours after setting up the anaerobic digestion experiment, whereas in the ITHP configuration similar changes were observed between 6 to 60 hours, but extended changes in lipids concentration continued up to 324 hours (13.5 days) after setting up of the anaerobic digestion experiment. In both configurations a fast initial decrease in lipids concentration demonstrated a high substrate consumption often followed by a slower rate of consumption (Figure 7.5, 1A and 3F).

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Table 7.4 shows that the ITHP configuration produced higher initial k value (k1) to that of

THP which suggested that the ITHP configuration was able enhance the anaerobic digestion process performance when utilising an already digested sludge and making digested sludge lipids constituents digestible.

Although, the overall lipids degradation patterns in both configurations was similar; and the lipids concentration between the repeated three runs in each configurations was slightly different, indicating changes in sludge composition over time. This means sludge can vary in its composition due to varying amount of primary and SAS sludge input, but its degradation characteristics stay unchanged (Figure 7.6).

This apparent digested sludge degradation and improved ITHP configuration performance was attributed to the combined impact of thermal hydrolysis on sewage sludge and prolonged anaerobic sludge digestion processes used in the ITHP configuration, which allowed the solubilisation of the particulate organic matter and faster biochemical reaction.

As mentioned earlier, the reproducibility of the data between repeated digestions runs was difficult to achieve due to the changing nature of the sludges used. However, the overall lipids degradation kinetics profile and trends were consistently reproducible (Figure 7.6).

In both cases, during this experimental period, it was noticed from the data obtained that the overall sludge digestion process was faster and there was no evidence of long chain fatty acid (LCFA) related inhibition observed, neither aceticlastic nor hydrogenotrophic methanogens were affected as reported by Koster and Cramer (1987); Lalman and Bagley (2001) cited in Jiang et al. (2012).

The data obtained from this work rather suggested that lipids are utilised by the bacteria as one of their most significant substrates during the anaerobic digestion process (Gonzalez, 2006). The hydrolysis products provide the main carbon source for bacterial growth (Hanaki et al., 1981 cited in Gonzalez, 2006) and lipids hydrolysis produced short-chain VFAs, CO2,

and H2 which are the main end-products of acidogenesis (Horan, 1990). This finding suggests

that lipids degradation is faster in the ITHP configuration than in the THP configuration. Therefore, this finding is slightly in agreement with reports in literature that described lipids as being difficult to degrade substrate (Hanaki et al., 1981 cited in Gonzalez, 2006; Sousa et al., 2009) during conventional digestion process including the THP configuration. Even some authors recommended limiting the amount of lipids input in the anaerobic digesters (Batstone

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et al., 2000 cited in Gonzalez, 2006), but the use of ITHP configuration made the sludge lipids content highly amenable to anaerobic digestion process.

7.5 Summary

The sludge composition showed that the THP configuration digested sludge consisted of 39% carbohydrates, 22% proteins, 2% lipids and 4% fibre contents, whereas, ITHP configuration digested sludge consisted of 36% carbohydrates, 26% proteins, 2% lipids and 1% fibre contents. The high proportion of carbohydrate and protein fractions measured in the ITHP digested sludge indicated the existence of lysed biomass, i.e., extra-cellular polymeric substance (EPS). The carbohydrates degradation kinetics showed stepwise change in the carbohydrates concentration over time in both configurations.

The data showed that, in both THP and ITHP configurations, the fast and steep carbohydrate degradation step was followed by much slower steady carbohydrate degradation. However, comparing the kinetics degradation data in THP and ITHP configurations showed that the overall degradation of carbohydrate measured in the ITHP configurations was, as expected slower than that of the THP configuration in all three runs.

Overall, the carbohydrate degradation rate constants in both configurations studied varied throughout the digestion time confirming the multi-stage nature of bacterial activity. A fast initial decrease in carbohydrate concentration demonstrated a high substrate consumption often followed by a slower decline.

The proteins degradation kinetics showed that a fast initial decrease in protein concentration, demonstrating a high substrate consumption that often followed by a slower decline. Particularly, the ITHP configuration produced slightly higher initial k value (k1) than

the k1 value of THP which indicates a slightly faster degradation reaction in the ITHP

configuration due to combined effects of thermal and biological processes involved.

The overall degradation profile of protein measured in the ITHP configuration was similar to that of the degradation profile obtained from the THP configuration.

The lipid degradation kinetics pattern in both configurations also showed stepwise degradation. During the ITHP process the faster initial degradation of lipids followed the first order kinetics, whereas in the THP configuration, the faster initial reduction in lipids concentration also followed a first order kinetic reaction.

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CHAPTER 8