**4.5 Hydraulic retention time (HRT)**

HRT can be described as the average length of time that the substrate and biomass remain inside the reactor [6]. HRT should be long enough to promote the hydrolysis and the acidogenic fermentation steps, which depends on the type of FW [6]. In theory, high HRT is advantageous for VFA production since the microbial population has more time to convert the substrate. However, a very high HRT reduce the quantity of waste to be treated per day and can favor the methanogens activity, if suitable pH is applied [5, 6]. Moreover, high HRT can lead to VFA yield stabilization due to feedstock limitation [5]. The optimal HRT can vary even for the same feedstock [7]. Teixeira et al., [21] studied the effect of HRT (19 and 41 days) on the production of VFA at OLR of 4.3 gTS/(L.d). Due to the kind of substrate (solid and complex substrate without pre-treatment), high HRT was applied. The increase of HRT boosted the production of VFA (from 11.2 gFP/L to 24.4 gFP/L), since a longer contact between microorganisms and substrate was promoted. The prevailing acid produced was propionic acid, followed by acetic, butyric and valeric acids. However, their content was similar for both tested HRT, showing that HRT had no significant impact on the VFA composition. In another study, using a mixture of sewage sludge with cheese whey as feedstock, a sequential increase of HRT (from 10 to 20 days) was investigated [27]. The change of HRT increased the acidification degree (from 27 to 45%) with a similar ratio of VFA/sCOD (85 and 89%). The increase of HRT also promoted a slightly change on the VFA composition, with a slightly increase of iso-butyric and butyric acids (from 51 to 55%) and a decrease of acetic acid (from 33 to 24%). HRT depends not only on the type of substrate, but also on other operational parameters. As most of the studies are performed in batch reactors, the information about HRT effect on the VFA production is scarce.

## **4.6 Inoculum**

The type of microorganisms present in the mixed microbial cultures may affect the acids production. It is necessary a careful selection of the microbial population present in the acidogenic fermentation process. If inadequate, a disparity on microbial populations can delay or limit the fermentative reactions and pathways, lowering the process yields [13]. Anaerobic inoculum from anaerobic sludge digesters obtained higher FW hydrolysis and VFA yield than aerobic inoculum from activated sludge process [13]. Atasoy et al. [40] investigated the effect of three types of inoculum with two different physical sludge structure (small and large granular sludge and anaerobic digester sludge) on VFAs production and composition. The highest VFA production was obtained with large granular sludge (1.99 ± 0.06 gCOD/L), followed by anaerobic digester sludge (1.14 ± 0.07 gCOD/L) and small granular sludge (1.06 ± 0.12 gCOD/L). As for VFA production, large granular sludge led to the highest VFA yield (0.97 gVFA/gSCOD). For small granular and anaerobic digester sludge a similar yield was obtained (0.36 and 0.38 gVFA/gSCOD, respectively), indicating that VFA production efficiency changed with the inoculum type.

Moreover, this study showed the ability of granular sludge to attain high VFA production efficiencies instead of biogas production, which is the usual application for granular sludge. Butyric and propionic acids were the main VFAs produced using large and small granular sludge, respectively, while acetic and propionic acids had similar content with anaerobic digester sludge. Wang et al. [23] studied the effect of pH (4.0, 5.0, 6.0 and not controlled) using two types of inoculum (anaerobic and aerobic). The highest VFA concentrations were obtained at pH 5.0 and 6.0, independently of the inoculum. Moreover, when anaerobic inoculum was used, it was obtained a slightly higher VFA production when compared with aerobic inoculum, which could be related to a higher acidogenic bacteria content present in the anaerobic inoculum and higher microbial activities under anaerobic conditions. Acetic and butyric acids were the main compounds (representing 90% of total VFAs) in all experiments except at pH 4.0 and not controlled pH using aerobic inoculum, where acetic and propionic acids were the major acids produced. Another study [13] evaluated the effect of inoculum source (mesophilic anaerobic sludge from biosolids digester of a municipal wastewater treatment plant (35 °C), thermophilic anaerobic sludge treating flour residues (55 °C) and hyperthermophilic anaerobic sludge treating microalgae) on the production of VFAs under mesophilic, thermophilic and hyperthermophilic conditions (70 °C). The mesophilic and thermophilic reactors were also operated at 70 °C. The hydrolysis efficiency was similar for all reactors, ranging between 27 and 40%. The reactor operation obtained under thermophilic conditions led to the highest fermentation yield (0.44 gCOD/gVSS-CODadded), followed by mesophilic (0.33 gCOD/gVSS-CODadded) and hyperthermophilic conditions (0.08 gCOD/gVSS-CODadded). Moreover, the fermentation yield at 70 °C using mesophilic and thermophilic were lower than that obtained at standard conditions (0.30 gCOD/gVSS-CODadded and 0.28 gCOD/gVSS-CODadded, respectively). VFAs accounted to ca. 60–71% of the solubilized matter at mesophilic and thermophilic conditions, with acetic acid the major compound (70%) at mesophilic temperature and butyric acid (60%) the major compound at thermophilic temperature. The higher production of VFA at 35 °C and 55 °C revealed the importance of inoculum source in the improvement of acidogenic activity. Thus, different inoculum types present a variability in microbial populations which lead to a distinct performance of hydrolytic and acidogenic processes.

### **4.7 Pre-treatment**

The pre-treatment will make the complex compound of FW more accessible for the hydrolysis, being the rate-limiting step of fermentation process.

Pre-treatments can be divided in physical, chemical and biological categories [41]. Physical pre-treatment present the high efficiency in terms of degradation but present high costs related to high energy consumption. Chemical pre-treatment is a cheap and efficient process but is not environmentally appealing and the chemicals used may cause fermentation inhibition. Biological pre-treatment presents several advantages (natural process, environmentally friendly, non-toxic for fermentation, economic) but are slower than physical and chemical processes.

Physical pre-treatment comprises heat, mechanical and radiation processes to change the structure and/or composition of FW. In terms of chemical pre-treatment the most commonly used are alkaline or acidic solutions, hydrogen peroxide and ozone. Alkaline and acidic pre-treatments break the cell wall promoting the solubilization. However, these pre-treatments may cause equipment corrosion and interfere with the fermentation pH. Ozone pre-treatment is safer but expensive. Hydrogen peroxide is toxic for the environment and causes cell growth inhibition, even though it presents a high solubilization degree [41]. Biological pre-treatment

*From Food Waste to Volatile Fatty Acids towards a Circular Economy DOI: http://dx.doi.org/10.5772/intechopen.96542*

comprises microbial (inoculated microorganisms as single culture or consortia and microorganisms from matured compost) and enzymatic (single or mixed enzymes) pre-treatment. Enzymatic pre-treatment is faster than microbial pre-treatment. However, it can be a costly treatment due to the operating costs involved in the production and extraction processes [41].

Guo et al., [18] evaluated the effect of different pre-treatments (sulfuric acid, acetic acid, aqueous ammonia, sodium hydroxide and steam explosion) of corn stalk on the production of organic acids. Steam explosion was the most suitable process for microbial growth and VFAs production, achieving a total of 2.98 g/L. The lowest production of VFAs was achieved using the acetic acid pre-treatment. Shen et al. [20] investigated the effect of hydrothermal pre-treatment (160 °C and 30 minutes), on tofu and egg white and assessed the production of VFAs using the pre-treated and untreated feedstock, observing that the pre-treatment improved the VFA production from tofu but not from egg white. Treated tofu reached a maximum VFA concentration of 21.07 g/L and a yield of 0.46 g/gVS, while with treated egg white a maximum VFA concentration of 11.45 g/L and a VFA yield of 0.20 g/gVS were achieved. Contrarily to the VFA yields, the VFA composition was not affected by the hydrothermal treatment. Yin et al., [24] also assessed the effect of hydrothermal pre-treatment (140, 160, 180 and 200 °C and 30 minutes) on the production of VFAs from FW. The hydrothermal pre-treatment of FW enhanced the VFA production. The optimal hydrothermal temperature was at 160 °C, where a VFA yield of 0.908 g/gVSremoval and a VFA concentration of 34.1 g/L (increase of 47.6% compared with control) was reached. Independently of pre-treatment temperature, butyric and acetic acids (between 38.6–41.2% and 31.1–35.2%, respectively) were the dominant acids, followed by propionic acid (about 20%) and valeric acid (about 8%, except for pre-treatment at 200 °C (14.51%)). Pre-treatment of FW is one option to achieve high solubilization, improving the FW biodegradability and consequently enhance the VFAs production.

#### **4.8 Other factors**

Other factors, such as total solids (TS) [15, 42], co-digestion [43] and substrate shift [16, 17] also affect the VFA production.

The initial total solid (TS) concentration can limit the mass transfer between the substrate and the microorganisms [42]. Wang et al., [42] tested four initial TS concentrations (40, 70, 100 and 130 g/L) observing a slower VFAs production at higher TS concentration, although a higher VFA concentration was obtained (62.24 gCOD/L). The increase of TS content, from 40 to 130 g/L, led to a maximum VFA concentration of 26.10, 39.68, 59.58 and 62.64 gCOD/L. On the other hand, the increase of TS content led to a decrease of VFA yield (0.799, 0.644, 0.604 and 0.467 gCOD/gVSfed) and acidification degree (48.2%, 42.7%, 41.2%, and 35.8%). Propionic acid accounted to 30.19–34.86% of the total VFAs and was not affected by the TS concentration, while a higher content of butyric acid and a lower content of acetic acid were achieved at higher TS content. Bermúdez-Penabad et al., [15], also observed an increase of VFA concentration with the increase of TS content (from 2.5 to 8%TS (w/v)) and obtained the higher VFA yield and acidification at lower TS concentration (0.73 gCODVFA/gCODwaste and 73%, respectively). The lower yields at higher TS content could be related to inhibition at high VFA concentration. Independently of TS content, acetic acid was the dominant acid produced, followed by butyric, iso-valeric and propionic acids.

Co-digestion consists in the simultaneous treatment of two or more substrates. Although the mono-digestion of FW is suitable, co-digestion presents several advantages and benefits, like as an improvement of nutrients balance, synergistic

effects between microorganisms, dilution of potential toxicity, increase of digestion rate [10, 26]. FW can be mixed with different other wastes to improve the system performance [10]. In Feng's study [26] an improvement in VFA production was obtained by the addition of FW to waste activated sludge fermentation. The production of VFA with only waste activated sludge or FW was 971.7 and 1468.5 mgCOD/L, respectively, while in the co-digestion increased to 8236.6 mgCOD/L. Moreover, the VFA composition obtained from waste activated sludge or FW fermentation was different than that obtained in the co-digestion, although acetic and propionic acids were the prevalent acids produced. Another study [27] also showed that the increase of cheese whey content on the sewage sludge digestion increased the production of VFAs. Using only sewage sludge as substrate the maximum concentration obtained was 1507 mgCOD/L, while the addition of cheese whey, in a ratio of 25:75, allowed to increase the VFA concentration up to 3226 mgCOD/L. The addition of cheese whey also led to a changed in the VFA profile, with acetic, propionic and butyric acids being the main acids produced, while acetic, propionic and iso-valeric acids were the prevalent acids produced in sewage sludge fermentation.

Most of the wastes are seasonal which can affect the continuous production of the VFAs. So, two possible solutions may be used: (1) feedstock storage or (2) feedstock shift. In the first option, the storage might require huge buffer tanks or facilities, besides the possible feedstock degradation during the storage. In the second option, the change between different feedstock could led to a different VFA composition and production, besides could affect the robustness of the continuous operation. Duque et al., [17] studied the feedstock shift from cheese whey to sugar cane molasses to cheese whey, observing an immediate response to the feedstock shift by the change of fermented products profile. The highest fermented products concentration (13.2 gCOD/L) was achieved using sugar cane molasses, being propionic and valeric acids the most prevalent compounds. During the operation with cheese whey, a maximum concentration of fermented products of 9.7–10.6 gCOD/L was achieved, being acetic and butyric acids the dominant compounds. Although the feedstock shift from cheese whey to sugar cane molasses changed the fermented products concentration and composition as well as the kinetic parameters, the shift from SCM to cheese whey demonstrated the process robustness since the system responded similarly to the first cheese whey. Mateus et al., [16] assessed the effect of the feedstock shift and operational conditions (pH and OLR/HRT) of three pulp waste (peach, raspberry and white guava) on acidogenesis. The authors observed that, independently of pulp composition, the fermented products profiles were similar and stabilized over a short period of time after each operation change (feedstock shift or operational conditions), showing the robustness of the system. Butyric, acetic and valeric acids were the main acids produced in all the conditions tested. Generally, the latter studies showed the ability of the microbial community to deal with FW feedstock shift with no need to stop the operation, representing an important advantage at full scale operation.
