Water Journal : Water Journal December 2014
WATER DECEMBER 2014 66 Technical Papers destroyed and theoretical biogas yield per kg COD equivalents from primary sludge (determined from stoichiometry), and adding that figure to an equivalent calculation for the glycerol. The COD of the glycerol was 1.67 million mg/L. These calculations predicted biogas increases of 80% and 110% for 1% and 2% glycerol added respectively, compared to the measured values of 50% and 90% increase. The first assumption was that the theoretical biogas yields for sludge were incorrect. The glycerol figure was considered correct as it was pure and the figure calculated from stoichiometry. The figure for primary sludge was determined as 1.19 m3/kg VS destroyed. This figure was recalculated in order to make the gas data fit. The new yields dropped by 25% to 40% for tests 1 and 2 respectively to make the biogas data coincide. However, adjusting the yield figures did not account for the large discrepancies noted in the results. Secondly, it was proposed that there were issues with the actual measurements of the biogas. In all instances theoretical determinations were higher than measured data. However, the differences in measured versus predicted results were inconsistent. If there were an issue with measurement, it is assumed that the measurements would be consistently different. Also, biogas measuring equipment was calibrated during the experiment. Therefore, it was assumed that biogas measurements were not the cause of the mismatch. Another attempt to address the inconsistencies was to make the COD results balance by adjusting the feed glycerol COD concentration as, unlike the sludge, this was not measured during the tests. In order to make the results match, the feed glycerol concentration would have to reduce to 316,000 and 258,000 mg/L for 1% and 2% addition rates respectively. These reductions were not considered realistic, so this hypothesis was rejected. Another calculation set was based on the assumption that not all the glycerol was being degraded, which could explain why measured biogas production was lower than predicted values. To check this proposal, COD destruction was determined for sludge alone based on the control digester. The COD destruction rates were 42% for the first, and 53% for the second control. These results were then used in the test digesters to determine the fraction of biogas attributable to the sludge. The COD destruction of the glycerol was then back-calculated so that the theoretical total biogas production was identical to the measured results. If the assumption is that the sludge degradation is identical in both control and test, then the back- calculation implies that only approximately 60% of the COD due to glycerol was being degraded. This figure aligns with work reviewed by Viana and co-workers (2012), who found waste glycerol degradation rates between 60% and 85% depending on the source of the glycerol. Potential reasons for incomplete degradation may be due to chloride content (which can be five times higher than required to inhibit gas production), sulphate contamination (from the use of sulphuric acid in the transesterification process), or the production of long-chain fatty acid intermediates (LCFA). These materials cause inhibition by adhering to bacterial cells, thus preventing passage of nutrients, and by having low density, causing the biomass to float (Viana et al., 2012). Another potential reason to explain the anomalies was that the glycerol was inhibiting the biogas production from the sludge. Based on the previous calculations it was possible to determine the fraction of biogas attributable to sludge and glycerol separately. As the conversion of COD to biogas is fixed, the quantity of biogas from each component entering the digester should be consistent with its load. Calculations showed that, for 1% glycerol addition, the sludge contributed 72% of the load into the digester but only accounted for 63% of the biogas. When repeated for 2% glycerol, the sludge contributed 60% of the load but only half of the gas. These results imply that the addition of glycerol is reducing the biogas production from the sludge itself, even though not all of the glycerol appears to be consumed. This could be due to the production of LCFAs as described above. Additionally, analysis of alkalinity data showed a linear decrease with increasing glycerol addition. Reduction rates of approximately 220 mg/L alkalinity as CaCO3 were observed per percentage point glycerol added as volume. Starting alkalinity was 1750 mg/l and the reductions noticed were equivalent to 14% and 24% for the increasing additions of glycerol. pH was also measured and it dropped by greater than a simple weighted average analysis based on sludge pH (which was pH 4) and glycerol. The results from this study suggest that the gas measurements appear lower than what is potentially achievable. Based on the above analysis, this appears to be most likely due to incomplete degradation of glycerol at 20 days HRT, with about a third exiting in the effluent and, also, potential suppression of biogas production from the sludge itself. Sankey diagrams were drawn (Figure 4) to show the flow of COD from digestion of sludge and glycerol. It is clear that adding glycerol to a sludge digester produces additional biogas. However, analysis of the results from this Figure 3. Impact of glycerol addition on biogas production. Blue bars = measured biogas; red bars = theoretical biogas production based on COD destroyed.
Water Journal November 2014
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