VFA Production

Samples from fermentation F4 was sent to NMBU for VFA analyzes. Measured parameters in this fermentation can be found in Table 9. The VFA distribution at the end of fermentation F4 is given in Figure 17. Almost 80 % of the VFA produced is acetic and propionic acid.

More than 50 % of the VFA is propionic acid. This is beneficial if the FPSS is used in EBPR, since propionate favors PAOs over GAOs (Oehmen, et al., 2005).

In addition, the mix of acetate and propionate will favor PAO. The biomass in the SBR has been enriched with only acetate. According to Oehmen, et al. (2005) this is beneficial since GAOs seems to have a slower reaction when carbon source is changed. When the PAOs have an advantage, the uptake of VFA and release of phosphate under anaerobic conditions is better. This is therefore beneficial for P-stripping.

Table 9: Measured parameters from Fermentation F4

Fermentation F4 HRT: 4 days Batches: 1 TS: 0,84%

Day sCOD [mg/L] PO4-P [mg/L] NH4-N [mg/L] VFA [mg sCOD/L]

0 161 3,3 15,69 75,98

1 452 6,39 21,55 313,99

2 526 6,82 18,94 392,96

3 648 6,48 14,4 464,58

4 584 5,98 7,36 442,27

Figure 17: The VFA distribution at the end of fermentation F4 (Day 4) with approximately 1% TS 36%

52%

3%

2%

4%

2%

Acetic acid Propionic acid Iso-butyric acid Butyric acid Iso-valeric acid Valeric acid

35

The production of VFA in percentage of the production of total sCOD is found by using the numbers in Table 9.

𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑠𝐶𝑂𝐷 = 584𝑚𝑔

⁄ − 161 𝑚 𝐿𝐿 ⁄ = 423 𝑚𝑔/𝐿 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑎𝑐𝑖𝑑𝑠 = 442,27𝑚𝑔

⁄ − 75,98𝐿 𝑚𝑔

⁄ = 366,3 𝑚𝑔/𝐿 𝐿

𝑉𝐹𝐴 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 = 366,3 𝑚𝑔/𝐿

423 𝑚𝑔/𝐿 ∙ 100% = 86,6%

This means that 86,6% of the total sCOD production in fermentation F4 is in the form of VFA. This is also very beneficial for use in EBPR processes, since PAOs need VFA for P- release.

Figure 18 shows how the production of acetic acid, propionic acid and all acids combined, changes over the timeline of fermentation F4. It illustrates at what times during the fermentation, the production of the VFAs are highest. The highest production of acetic acid is in the first 24 hours. Then the production is decreasing fast, to a point where the production is negative. Production of propionic acid does shows the same trend in the first 48 hours. After a small decrease there is a new increase in production, before dropping in the last 24 hours. The production of the total acids is highly influenced by the production of acetic and propionic acid since it makes up almost 90% of the total VFAs.

Figure 18: The production of acetic acid, propionic acid and the sum of all acids over the time of fermentation F4, with approximately 1% TS. Given as percentage of the total production of each acid

Calculated ratios between acetic acid and propionic acid are listed in Table 10. According to Lopez-Vazquez, et al. (2009) a ratio between acetate and propionate of 75:25 or 50:50 is favorable for PAOs in EBPR. The ratio in fermentation F4 is close to 50:50, but after 48 hours this ratio is moving more towards 40:60.

-40 -20 0 20 40 60 80 100 120

0 20 40 60 80 100

VFA production as %

Hours

Acetic acid Propionic acid Sum acids

36

The highest mass of VFA occurs after 72 hours. However, the ratio between acetic and propionic acid is worsening after 48 hours, and the production of acetic acid and the total sum of acids decreases after this time. It may therefore be more beneficial to stop the fermentation after 48 hours.

Table 10: Ratio between acetic acid and propionic acid through the fermentation with 1% TS

Time [hours]

Acetic acid [mg]

Propionic acid [mg]

Ratio

[acetic acid : propionic acid]

0 59,61 77,60 43:57

24 322,19 237,26 58:42

48 389,21 316,77 55:45

72 368,97 460,71 44:56

96 321,95 463,65 41:59

Figure 19 shows the VFA distribution at the end of the fermentation conducted by Svendby (2019). This fermentation started and ended at the same time as fermentation F4, and the primary sludge and wastewater was collected at the same time. The only difference between the fermentations were the TS percentage. In fermentation F4 the initial TS was around 1%, while the fermentation done by Svendby (2019) was around 2%. The production of acetic acid and propionic acid is similar in these fermentations. The main difference is the production of butyric acid. In fermentation done by Svendby (2019), 11%

of the production is butyric acid, while in fermentation F4 the production is 2%. Studies have shown that butyrate is equally good as acetate as a carbon source for P-removal (Rustrian et al., 1996: Mulkerrins, et al., 2004). Zeng, et al. (2006) observed that only acetic acid was produced when the solid concentration was low, during fermentation of primary sludge. The increase in production of butyric acid could therefore be the increase in TS concentration.

Figure 19: VFA distribution at the end of fermentation, with 2% TS, conducted by Svendby (2019) 29%

51%

3%

11%

4% 3%

Acetic acid Propionic acid Iso-butyric acid Butyric acid Iso-valeric acid Valeric acid

37

Figure 20 shows the production of acetic acid, propionic acid and the combined acids over time for fermentation by Svendby (2019). It shows similar trends as fermentation F4, where most of the production occurs in the first 24 hours of the fermentation.

Figure 20: The production of acetic acid, propionic acid and the sum of all acids over the time of fermentation, with 2% TS, by Svendby (2019). Given as percentage of the total production of each

acid

The accumulated mass of total VFA in fermentation F4 and fermentation by Svendby (2019) can be seen in Figure 21. The production of VFA in mass is higher in Svendby (2019), where the concentration of TS was around 2%, compared to fermentation 4 with around 1% TS. The rate of production is also higher in the fermentation with the highest TS concentration. This is in accordance with results obtained by Zeng, et al (2006), who found that an increase in solids concentration increased the VFA production rate quite substantially.

-20 -10 0 10 20 30 40 50 60 70 80

0 20 40 60 80 100

VFA production as % of total production

Hours

Acetic acid Propionic acid Sum acids

38

Figure 21: Accumulated VFA production in Fermentation F4 and fermentation by Svendby (2019)

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