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Impact of Post-Digestion Temperature on Biogas Production and Reactor Efficiency in Serial CSTRs

This study investigates the effects of three post-digestion temperatures (15°C, 37°C, and 55°C) on biogas production, effluent quality, process stability, methanogenic activities, and microbial community structure in serial Continuous Stirred Tank Reactors (CSTRs). The findings reveal that maintaining a post-digestion temperature of at least 37°C can enhance biogas production by 11% and significantly improve effluent quality by reducing Volatile Fatty Acids (VFA) levels by 45%. These results suggest that optimizing post-digestion temperature is critical for maximizing the efficiency of biogas plants.

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Impact of Post-Digestion Temperature on Biogas Production and Reactor Efficiency in Serial CSTRs

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  1. Effect of post digestion temperature on serial CSTR biogas reactorsKanokwan Boe, Dimitar Karakashev, Eric Trably and Irini AngelidakiInstitute of Environment & Resources DTU, Technical University of Denmark, Building 113, DK-2800, Kgs. Lyngby, Denmark

  2. Aim Study the effect of three post digestion temperatures (15 oC, 37 oC and 55 oC) on biogas production, effluent quality, process stability, methanogenic activities and microbial communities structure in serial CSTR reactors.

  3. Background Principal of serial CSTR: Two CSTR reactors connecting in series with long HRT in the first (main) digester and short HRT in the second reactor (storage tank, post digester ) Single CSTR Serial CSTR Why serial CSTR? - Most of centralized biogas plants in Denmark operate with a single CSTR. - Short-circuit decreases the efficiency of biogas production and pathogen reduction. - 5-10% of the total biogas potential is lost with the reactor effluent. Experimental results showed that serial CSTR configuration could improve biogas production and effluent quality compared to a single CSTR reactor.

  4. Serial digestion

  5. Serial CSTR system: Under steady state conditions, serial CSTR could obtain up to: 11% higher biogas production; 45 % decrease of VFA levels compared to a single CSTR.

  6. Effect of post-digestion temperature on process performance in serial CSTR Substrate Manure mixed with 25% organic waste. The substrate was obtained from a thermophilic full-scale biogas plant (Lemvig, Denmark) Figure 1. Experimental setup

  7. Table 1. Reactor operational data AnalysisTS, VS, lipids, pH, VFA, total- and ammonium N, methane, biogas production, methanogenic activity tests (utilization of acetate, propionate, butyrate, H2/CO2, glucose), microbial community structure (FISH, TGGE)

  8. Results Figure 2. Experimental results for biogas production, VFA and pH.

  9. Results Table 2. Operating parameters during steady state period (day 45-70)

  10. Results Methanogenic activity tests • higher methanogenic activities were noticed in R55 and R0 compared to R15 and R37; • no or very low methanogenic activity was noticed for R15

  11. Results Figure 4: Picture from CLSM of sample from R15. Figure 3: Picture from CLSM of sample from the main reactor Ro.

  12. Conclusions Postdigestion temperature affected: -level of extra biogas production; -level of VFA (quality of effluent); -methanogenic activities; -microbial community diversity and microbial abundance.

  13. Importance of the results obtained Results obtained in this study clearly showed that in full - scale biogas plants the operational temperature of the effluent storage tanks (postdigesters) should be keep at least 37oC. This could provide reasonable compromise between energy cost for postdigester heating and amount of additional biogas expected.

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