**3. Feedstock changes in the anaerobic digestion process of organic solid waste**

This section reports information compiled from the literature on studies investigating the transitory states during the AD process when the type or composition of the feedstock fed has changed. A change in feedstock type would refer to a feed with different substrates, whereas a change in feedstock composition would refer to a feed where the



*AmD: anaerobic mono-digestion; AcoD: anaerobic co-digestion, where OLR, organic loading rate; VFA, volatile fatty acids; TA, total alkalinity; VS, volatile solids; TAN, total ammonia nitrogen; SELR, specific energy loading rate; and COD, chemical oxygen demand.*

#### **Table 1.**

*Type of feedstock change.*

percentage composition of the various substrates that compose the feedstock varies. In these works, to evaluate whether the AD process was finally able to adapt to the perturbations of the system, the parameters previously described in section 2 were assessed.

*Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*

#### **3.1 Type of feedstock**

This section includes all the studies found in the literature that evaluate the AD process stability when changing the feedstock type (**Table 1**). These studies mostly use seasonal wastes generated in agri-food industries, i.e. fruit or vegetable processing, as feedstock, such as fruit pulp waste by a juice-producing company, winery waste, olive pomace, or sugar beet pulp. All these studies agree that the seasonality of the fruit and vegetable processing industries and waste from different crops would complicate operating a digester under the same conditions over a long period, because the waste supply could be changed or discontinued frequently [7, 46]. Therefore, using a single digester, fed with multiple feedstocks generated in the same geographical area and strongly dependent on seasonality, would require a deep knowledge of the behavior of the AD process when exposed to the resulting feed changes.

Despite the limited literature on the field, there is a wide variety of approaches for assessing the effect of feedstock type change on the stability of the AD process. Feedstock type change has been evaluated in mono-digestion processes with sequential feeding [6] or two-stage processes [45, 47]. It has also been studied in the transition from mono-digestion to co-digestion by applying feedstock change in the latter case [7, 40] and co-digestion processes with sequential feeding [6] or multi-substrate [46].

All research that has monitored pH as a stability parameter has used substrates from similar origins and characteristics, so it has reported stable pH values between 7.0 and 8.0 [6, 40]. As for monitoring alkalinity, VFAs concentration, and VFA/TA ratio, variable results have been reported, all of them related to the varying composition of the feedstocks fed. According to Pellera et al. [6], who evaluated the sequential feeding of four agro-industrial feedstocks (CGW → WW → OP → JW; cotton gin was (CGW), juice industry waste (JW), olive pomace (OP), and winery waste (WW)), the VFAs concentration was higher during the first two stages, especially for the experiments that started with feeding the most biodegradable feedstocks, i.e. WW and JW. Then, the values decreased to stable levels until the end of the experimentation, while the TA showed an increasing trend. Similarly, the VFA/TA ratio followed the same trend that VFA, without exceeding the value of 0.4, thus corroborating the system's stability. In contrast, Fonoll et al. [7] stated that feedstock changes did not increase VFAs concentration. However, due to the different biodegradability of fruit wastes, methane production and digester alkalinity changed to a lesser extent. The VFA/TA ratio values showed stability while changing feedstock despite the observed alkalinity fluctuations. Carvalheira et al. [45] evaluated the feedstock change in a two-stage anaerobic monodigestion process, using fruit pulp waste by a juice-producing company as a substrate. During the monitoring of the acidogenic reactor, differences in the profile of fermentation products, i.e. VFAs, lactic acid, and ethanol, were identified and quantified when using other fruit pulp wastes. These results were attributed to carbohydrate concentration and OLR on the effluent composition. On the contrary, Mateus et al. [47], who also evaluated the feedstock change in a two-stage anaerobic mono-digestion process, reported a stable fermentation product profile, regardless of the different carbohydrate concentrations in the substrates and OLR changes. The difference between both studies could be attributed to the fact that the OLR range used by Carvalheira et al. [45] to apply the feedstock change was higher.

The evaluation of AD process stability when feedstock type changes through SELR was reported by Carvalho et al. [40]. The SELR values ranged between 0.22 and 0.33 d−1, without significant differences, keeping the values below 0.4 d−1 and ensuring that the digestor worked under stable conditions (section 2.2).

Concerning methane production and composition, available research reports stable production values and relates their differences to the characteristics and biodegradability of the feedstocks fed [6, 7, 40, 46]. However, one of the most remarkable results dealing with methane production was reported in the study by Pellera et al. [6], which evaluated a sequential feeding by mono-digestion and co-digestion. Methane production with the same feedstock fed in different feeding sequences had similar values, attributed to an immediate response of the microbial population to each substrate. In fact, after providing the digesters with four feedstocks (mono- or co-substrate) in sequential order, the last feeding was carried out with the feedstock that had been fed first in each assay. The results demonstrated that the final methane production values were higher than their first values in all cases (**Table 1**). As an explanation for these results, they suggested a positive level of microbial population adaptation, albeit also possible presence of higher amounts of degradable material in the reactors as it was fed on 14 times. On the other hand, Carvalheira et al. [45] showed an increase in fermentation product concentration in the effluent of the methanogenic reactor, with the change of substrate reaching a maximum of 6.565 g COD L−1, even after decreasing the OLR. There was a significant acetic and propionic acid accumulation, 2.44 and 1.44 g COD L−1, respectively. The decrease in OLR and biodegradable matter accumulation decreased methane production when peach pulp was replaced by apple pulp, from 4.33 to 3.38 g COD (L·d)−1, respectively (**Table 1**). The decrease in process efficiency indicated that the microbial community was affected by the influent change and could not treat the apple influent with a high OLR as efficiently as the previous peach influent. Despite influent variations, stable performance of the methanogenic stage was achieved, probably due to the buffering capacity of the acidogenic community at the initial stage. In contrast, Mateus et al. [47] reported differences in the biogas composition generated in the acidogenic step in evaluating the two-stage mono-digestion process. In this case, the difference in carbohydrate concentration seemed to mainly affect the gas production and composition in the acidogenic reactor. No hydrogen production was detected with the peach pulp waste but with the raspberry and white guava pulps waste, ranging from 4 to 34%.

Reviewed studies state that whether or not there was instability during the whole AD experiment when the feedstock type changes, the microbial population has acclimatized well to the change. Different authors have argued that an acclimatization period would not be necessary with each change of feed material, as the microbial community is already adapted to substrates of a similar nature. Studies assessing changes in the microbial population ensure that the reactors were abundant in archaeal methanogens, mainly *Methanosaeta*, responsible for acetoclastic methanogenesis, the most common process in AD processes involving the production of CH4 and CO2 from acetate. *Methanobacterium*, microorganisms responsible for hydrogenotrophic methanogenesis involving methane production from CO2 and H2, were also identified. The microbial community composition remained relatively constant over time in each experiment [45, 47].

#### **3.2 Composition of feedstock**

This section includes all the studies found in the literature that evaluate the AD process stability when changing the feedstock composition in the influent (**Table 2**). These studies mostly use a mixture of wastes whose composition is strongly dependent on seasonality, such as food waste, fruit and vegetable waste from wholesale markets, meat waste, or the organic fraction of municipal solid waste (OFMSW).

#### *Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*

All these studies aimed to evaluate changes in feedstock composition in the influent on the digesters' stability. However, some assays kept the organic loading rate (OLR) constant throughout the experimentation, despite the change in influent composition, to attribute the changes in reactor behavior to the change in composition [48, 49]. On the contrary, in other studies, by ignoring the intrinsic modification of the OLR due to the change in composition due to percentage (w:w or v:v) increase, they evaluated the combined effect of these two factors [2, 4, 23, 26, 34, 35, 43].

Unlike described in section 3.1, despite the limited literature in this field, not many different approaches have been studied to assess the effect of changing feedstock composition on the stability of the AD process. Feedstock composition change has been evaluated in the single- and two-stage mono-digestion process at the pilot scale [4, 34] and in the transition from mono-digestion to co-digestion by increasing the co-substrate percentage in the feed mixture [2, 23, 26, 35, 43, 48, 49]. Some studies have implemented changes in compositional percentages to improve methane production by adjusting the C/N ratio and the most optimal fruit and vegetable percentage.

Some research that monitored pH as a stability parameter by changing the feedstock composition in the influent has reported stable pH values between 7.0 and 8.0 and within the optimal range described in the literature for methanogenic bacteria [2, 26, 35, 43]. However, some other studies have reported fluctuations in these parameters. For example, Arhoun et al. [23], who evaluated the change in feedstock composition and seasonal variations, observed a very slight trend of decreasing pH with winter substrate. This slight acidification was related to the mixture's pH value, which was 3.5, lower than the other seasons, approximately 4.8. Masebinu et al. [4] and Scano et al. [34] have also reported a slight decrease in pH values with an increasing percentage of fruit in the feedstock composition, a higher percentage of citrus fruit, and fruits with a very high content of simple sugars, respectively. In addition, García-Peña et al. [49] have also described a quick drop in pH when feeding the reactors with FVW that was solved by adding buffer (NaOH 0.8 M) to supply the appropriate buffering capacity and avoid excessive pH drop under unbalanced conditions.

As indicated in section 3.1, regarding the monitoring of alkalinity, VFA concentration, and VFA/TA ratio, variable results have been reported, all of them related to the varying composition of the feedstocks fed and the specific stress situations performed during reactor feeding.

Some authors assessing alkalinity and VFAs and their corresponding ratios report stable values, and compliance with stability recommendations for VFA/TA and IA/PA ratios has been reported in the literature [23, 26, 35]. Tonanzi et al. [2] have reported a slight transient accumulation of acetic acid (60% of the soluble content) as a result of an increase in OLR up to 3.5 g VS (L d)−1, reflected in a decrease in methane production. Propionic acid remained at low levels. The robustness of the microbiome and buffering capacities ensured quick recovery, acetic acid was eliminated, and methane production reached a stable value of 0.29 NL3 CH4 g VS−1 (**Table 2**). Masebinu et al. [4] have observed two significant instabilities caused by reaching high OLRs (3.42 and 4.06 g VS (L d)−1), i.e. mixtures with high fruit concentrations. A high OLR causes the system to be susceptible to fluctuations in feed composition and operating parameters. Maintaining a high fruit fraction in the substrate mixture caused a decrease in pH, an increase in the VFA/TA ratio above the stable region (0.45 and 0.53), and an eventual reduction in biogas production. As the percentages of fruit in the feed mix were reduced, all improved and returned to stability with improved biogas yield. Both pH and VFA/TA immediately indicated instability for an exceptionally high fruit concentration.



*Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*



**Table 2.**

*Composition of feedstock change.*

*Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*

On the other hand, in the research carried out by Fonoll et al. [48] and Scano et al. [34], the digestion systems were subjected to stressful scenarios to compare the robustness of the process with respect periods of stability. Fonoll et al. [48] have reported that 15% and 30% replacement of biowaste (BioW) with waste paper (WP) did not affect VFA and alkalinity levels. However, for a replacement of 30%, acidification of the supernatant used to dilute the feedstock led to a rapid accumulation of VFA (2400 mg L−1), which decreased methane production. Recovery was carried out after a period without feeding and by re-establishing the feed supply using a new batch of supernatant. Scano et al. [34] observed an initial increase in VFA/TA, reaching values close to 0.65, corresponding to the increase in OLR due to a high percentage of fruit in the feed mixture. As a corrective strategy, the percentage of fruit in the mix was reduced, resulting in a corresponding reduction in VFA/TA. During the subsequent stages, VFA/ TA was mainly influenced by chemical composition differences of the feed substrate, changes in OLR, and the simple sugar content of the fruit waste. Experimental results reported that the AD process still performed well with well-balanced mixtures of fruit and vegetable wastes, even for VFA/TA of up to 0.5. The highly elevated VFA/TA values (above 1) were derived from specific stress tests performed by feeding the reactor with substantial quantities of substrates with high content of simple sugars. The substrate mixture's large melon (which contains large amounts of highly degradable sugars) caused significant instability. Furthermore, it was complicated to stabilize the process at the next stage, as the available VWFs were mainly composed of fruit waste.

The assessment of the stability of the AD process when the feedstock composition changes through the TAN concentration measurement was reported by Cabbai et al. [35]. Organic nitrogen from the feed substrates (SS-OFSMW and SwS) was metabolized by the biomass-producing ammonia, although the levels were safe for process stability. García-Peña et al. [49], who evaluated the co-digestion of FVW by varying the percentage of meat residue (MR), reported that the addition of MT (75:25), rich in protein, started to release ammonia from the hydrolysis of the protein, which favored an increase in the alkalinity of the medium and the pH drop regulation.

Regarding methane production and composition, all the researchers have reported stable production values and related their differences to the characteristics and biodegradability of the feedstocks fed, just as to the different OLRs evaluated (**Table 2**).

In cases of feedstock composition change evaluating the microbial population adaptation, changes have been observed in contrast to feedstock-type changes. Tonanzi et al. [2] and Cheng et al. [43], who evaluated co-digestion of activated sewage sludge with percentage changes of FW, detected hydrolytic bacteria growth, such as *Bacteroidales*, especially the *Prolixibacteriaceae* family, whose relative abundance increased linearly with FW percentage in the mixture composition. As for the archaeal populations, high diversity indices were found when the FW and activated sewage sludge percentages in the feedstock composition varied, suggesting that the archaeal biodiversity was affected by the reactor feed conditions. Most of the acetoclastic methanogens determined belonged to the order *Methanosarcinales*, mainly to the genus *Methanosaeta*. In contrast, the hydrogenotrophic methanogens identified belonged to the orders *Methanomicrobiales* and *Methanobacteriales*, mainly to the genus *Methanobacterium*. The combined relative abundances of the three methanogens did not show significant changes in the two investigations. However, it was clear that *Methanosaeta* competed over *Methanobacterium*, and the latter had advantages with increasing FW in the feedstock composition. Furthermore, Tonanzi et al. [2] stated that minimal activated sewage sludge addition (FW: WAS, 95:5) enriched the microbial community with *Methanospirilloun* and *Candidatus Methanophastidiosun*,

#### *Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*

which have a high H2-consuming capacity, avoiding thermodynamic bottlenecks and failure of the FW mono-digestion process by the drop in activity of acetoclastic microorganisms as a result of a dramatic accumulation of propionic acid.

On the other hand, García-Peña et al. [49] reported enrichment of the microbial population of Firmicutes (fermenting bacteria that degrade VFAs), just to Bacteroidetes (proteolytic bacteria probably involved in the degradation of MR) when the percentage of MR increased in the composition of the raw material feeding. As for the archaea population, it was reported that the hydrogenotrophic methanogenic genus *Methanobacteriales* represented more than 93% of the presence of archaea in the digester. The hydrogenotrophic methanogenic community dominated even though the digester was inoculated with cow manure, which commonly contains acetoclastic methanogens. Finally, it may be argued that when changes are carried out to the composition of the feedstock fed with substrates of very diverse origins and biodegradability, microbial populations adapt to the changes in the end.
