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

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

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 % 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. Experimental results showed that serial CSTR configuration could improve biogas production and effluent quality compared to a single CSTR reactor.

Serial digestion

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.

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

Table 1. Reactor operational data Analysis TS, 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)

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

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

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

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

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

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 37 o C. This could provide reasonable compromise between energy cost for postdigester heating and amount of additional biogas expected.