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Table 2 Parameters monitored during operation of the laboratory stirred-tank anaerobic digesters (R1–R6)

From: Effluent solids recirculation to municipal sludge digesters enhances long-chain fatty acids degradation capacity

  Daysa R1 R2 R3 R4 R5 R6
Average specific biogas production (ml g−1VSsubstrate day−1)b 0–84 510 ± 95 590 ± 100 570 ± 110 610 ± 100 610 ± 90 570 ± 90
85–171 510 ± 75 590 ± 80 590 ± 80 525 ± 85 940 ± 310 950 ± 310
172–200 515 ± 65 745 ± 400 821 ± 480 590 ± 80 1540 ± 175 1315 ± 310
Cumulative residual biogas (ml)c 201–255 16,340 108,930 85,240 25,280 73,360 97,130
Cumulative residual biogas from oleate (ml)d 201–255 0 92,590 68,900 0 48,080 71,850
Methane (% of biogas) 0–84 61 ± 2 60 ± 2 64 ± 1 60 ± 1 61 ± 1 64 ± 1
85–171 61 ± 1 62 ± 3 64 ± 1 60 ± 1 61 ± 2 66 ± 1
172–200 61 ± 2 63 ± 1 65 ± 1 60 ± 2 64 ± 3 66 ± 2
Gaseous H2S (ppm) 0–84 < 20 < 20 50 ± 35 < 20  < 20 50 ± 30
85–171 < 20 < 20 190 ± 80 < 20  < 20 70 ± 20
172–200 < 20 < 20 320 ± 200 < 20  < 20 150 ± 15
pH 0–200 7.4 ± 0.1 7.4 ± 0.1 7.5 ± 0.1 7.6 ± 0.2 7.6 ± 0.2 7.7 ± 0.2
Total solids (% of total weight) 0–84 2.1 ± 0.3 1.9 ± 0.3 2.4 ± 0.1 2.6 ± 0.2 2.4 ± 0.5 2.8 ± 0.4
85–171 2.0 ± 0.4 2.3 ± 0.4 2.9 ± 0.4 3.5 ± 0.7 3.6 ± 0.4 4.1 ± 0.7
172–200 2.3 ± 0.4 3.5 ± 0.2 3.9 ± 0.8 4.6 ± 0.2 5.0 ± 0.5 5.5 ± 0.3
Volatile solids (% of TS) 0–84 56 ± 2 58 ± 2 53 ± 2 56 ± 2 56 ± 2 53 ± 3
85–171 53 ± 2 54 ± 3 50 ± 2 47 ± 4 48 ± 3 45 ± 3
172–200 50 ± 1 61 ± 5 61 ± 6 43 ± 1 48 ± 2 43 ± 2
  1. aDay 0–84: start-up phase; day 85: start of semi-continuous oleate feeding to R2, R3, R5, and R6; day 172: start of oleate pulse feeding; day 201: end of PASS and oleate feeding
  2. bCalculation of specific biogas production is based on the influent VS content of fresh substrate, does not include the recirculated VS
  3. cCumulative biogas production between days 201 and 255 after the PASS and oleate feeding was stopped
  4. dCumulative residual biogas production from oleate calculated by subtracting the cumulative biogas production in control digester, R1, from the values obtained for oleate-amended R2 and R3 and subtracting the cumulative biogas production of control digester with effluent recirculation, R4, from the values obtained for R5 and R6
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