**4.4 Organic loading rate (OLR)**

OLR corresponds to the amount of substrate, in this case FW, fed to the reactor per day and per unit of working volume and can be expressed in terms of COD, total solids (TS), volatile solids (VS), volatile suspended solids (VSS) or dissolved organic carbon (DOC) [2]. The OLR increase promotes the production of VFAs, since more substrate is available [5]. However, the presence of inhibiting substances in the FW is a key factor that must be considered when operating at high OLR, as might affect the hydrolytic and acidogenic bacteria, as well as methanogens [7]. Several studies on the effect of OLR on VFA production from FW are summarized in **Table 3**.

Carvalheira et al. [1] observed the increase of VFA production (from 19.9 ± 5.0 gCOD/L to 34.4 ± 5.0 gCOD/L) and the change of VFA composition with the increase of OLR (from 21.2 ± 3.2 gCOD/(L.d) to 51.1 ± 8.8 gCOD/(L.d)) at HRT of 1 day. Under the lowest OLR, acetic and propionic acids were the major compounds (19.65% and 19.11%, respectively), while under the highest OLR propionic acid was the prevailing VFA (10.07%) (**Table 3**). Similarly, Jiang et al. [36] also observed the increase of VFA concentration with the increase of OLR. However, it was observed a decrease of VFA after 12 days of operation, under the highest OLR, indicating that an OLR of 11 gTS/(L.d) is more appropriate for VFA production from FW. Acetic and butyric acids were the dominant compounds accounting 60–65% of the total VFAs in all OLR applied. Acetic and valeric acids increased with the OLR, while propionic and butyric acids decreased with the OLR (**Table 3**). Teixeira et al., [21] studied the effect of OLR on VFA production using raw brewers' spent grain as feedstock and observed an increase of VFA concentration in about 2.5 times (**Table 3**). Regardless of the OLR, no significant effects on the VFA composition was observed, being propionic acid the dominant acid produced (**Table 3**).


**Table**

 **3.** *Effect of OLR on VFA production using FW as substrate.*

From the fermentation of different fractions of FW, the main compounds were acetic, caproic and butyric acids (**Table 3**), observing an increase of butyric and propionic acid and a decrease of acetic and caproic acids with the increase of OLR (**Table 3**). Moreover, the high production of acetic and butyric acids can be related to the high OLR applied, since these acids have been related to reactor overloading during anaerobic digestion processes [22].

OLR also influences the composition of VFAs, however it has to be correlated with other factors, such as pH and HRT, which also affects the VFA composition [2].
