In this study, a systematic comparison between methanogenic and acidogenic potential tests of five waste streams from urban and agro-industrial origin was investigated. Methanogenic potential tests were performed under neutral pH (7.0) and thermophilic temperature (55°C). Additionally, acidogenic potential tests consisted of mono-fermentation tests at acidic pH (5.5) and co-fermentation tests performed at alkaline pH (9.0), under thermophilic temperature (55 °C). The methanogenic yield increased up to 0.54 gCH4 g−1 CODfed while the acidogenic yield ranged within 0.04–0.24 g VFA g−1 CODfed. The VFA (volatile fatty acids) yield was boosted when adding co-substrates that complemented the optimal C/N balance, i.e. 0.27–0.36 gVFA g−1 CODfed. Herein, highest total volatile fatty acid (tVFA) concentration was attained by microalgae biomass (MB), with 8769 mg COD L−1. During mono-fermentation tests under acid pH, butyric acid was promoted as main fermentation product, which varied between 1526 and 6114 mg COD L −1, whereas shifting to alkaline pH in co-fermentation tests promoted acetic production, from 3387 to 5415 mg COD L−1. The results of the current research revealed a significant potential of organic waste to enrich the carboxylic platform (e.g. acetic and butyric acids), which have higher industrial applicability and economic potential than methane.
Statement of Novelty
This study compares the methanogenic and acidogenic potential of five agro-industrial waste streams, namely, the organic fraction of municipal solid waste (OFMSW), artichoke (ART) and apple pulp (AP) waste; and two liquid effluents: sewage sludge (SS) and microalgae biomass (MB). These waste streams were selected since there is gap of knowledge to recover their full potential and they pose a handling challenge at local scale. In addition, the proposed methodology enabled a throughout comparison between anaerobic digestion and acidogenic fermentation processes. It should be highlighted that the manuscript gathers results that would give light to narrow the gap between laboratory anaerobic fermentation and industrial application.
Anaerobic digestion (AD) has been commonly used as an alternative to final disposal on landfill or incineration processes , taking advantage of the methane contained in the biogas. Due to the current situation of biogas production in some EU countries, it is necessary to reconsider alternative options to manage organic waste, which minize economic costs and maximize the environmental and social benefits. An promising strategy is valorizing the biodegradable organic waste through biorefinery platforms , which include the recovery of bioproducts and energy, while promoting landfill cost savings and new business opportunities . With regards to organic waste biorefinery technology, acidogenic fermentation has drawn an increased attention due the potential to synthesize value-added bio-based products such as volatile fatty acids (VFA) [4, 5].
The VFA production depends on fossil fuels and non-renewable petrochemical products. For example acetic acid production generates g 3.3 t CO2eq./t . However VFA can also be synthesized as intermediate products during the acidogenic stage of conventional anaerobic digestion processes, and they serve as substrates for the subsequent transformation of organic matter into a mixture of methane, carbon dioxide and hydrogen [7, 8].
During the acidogenic stage, a large variety of organic compounds (lipids, proteins and carbohydrates) are broken down into simple molecules, and the effluent formed consists of a mixture of short and medium-chain carboxylic acids, such as acetic, propionic, butyric and valeric acid or caproic acid , and other fermentation products such as lactic acid [10, 6], ethanol or succinate [11, 12]. Many competing microorganisms, biokinetics, variable substrates sources, catalysts and intermediate syntrophic reactions coming from mixed microbial cultures, participate in this anaerobic process . For example, species such as Clostridium aceticum, Clostridium propionicum, Clostridium butyricum and Clostridium aminovalericum affect acetate, propionate, n-butyrate and valerate formation, respectively [14, 15]. Recent studies which aimed to control the acidogenesis and acetogenesis stages to direct the reaction flows towards a specific end-product [16, 17]. In addition, a considerable amount of reseachers have focused on evaluating the influence of operation parameters, such as the pH, the substrate complexity, the temperature and the organic loading rate on VFA composition and concentration [18, 19]. In addition, the downstream processing to obtain concentrated or purified VFA streams has also been a focus of study .
Nowadays, VFA are used for multiple industrial applications such as food, pharmaceutical, petrochemical, chemical, and cosmetics industries . Promising applications in the use of VFA are (i) as a carbon source for the production of biopolymers, i.e., polyhydroxyalkanoates or PHA, due to high biodegradability, sustainability and environment-friendly properties [22, 23], (ii) external carbon source for biological removal of nutrients (e.g. phosphorus and nitrogen) in urban and industrial wastewater treatment plants (WWTP) [24, 25] and (iii) to generate electricity in microbial fuel cells (MFCs) from VFA-rich broths .
First attempts of fermentation process up-scaling were performed in recent years. Morgan-Sagastume et al.  and Tamis et al.  achieved a biopolymer conversion yield up to 0.38 g PHA/gsubstrate and 0.37 g PHA/gsubstrate at pilot-scale, respectively. Furthermore, a study by Liu et al.  described a full-scale operation of VFA production related to biological nutrient removal in WWTP.
The biobased production industry is increasing, with a 5–sixfold increase in 2020 compared to 2010 . In Addition, some authors have suggested a change in focus from conventional biogas production to anaerobic fermentation. For example, Bastidas-Oyanedel et al.  showed the high profitability of food waste fermentation compared to anaerobic digestion with aproximately 6–15 higher market value. Similarly, Perimenis et al. , reported that agro-industrial waste (i.e., fruit pulps and brewery residues) produced higher profits related to anaerobic fermentation.
However, this field requieres further study and clarification. Currently, there is a scarcity of studies dedicated to comparing the methanogenic and acidogenic conversion yield of organic waste. Among the few examples, the work of Tampio et al.  assessed the potential of biogas (CH4/g VSfed) and VFA (mg VFA/g VSfed) production of food waste (FW) and cow slurry (CS) using the anaerobic biogas plant inoculum treating the corresponding substrates, while Perimenis et al.  compared methanogenic (mLCH4/gCOD_substrate) and acidogenic (gCOD_tVFA/kgML) potential of agro-industrial residues.
The aim of this study is to perform a comprehensive comparison of the acidogenic and methanogenic potential of five waste streams from urban and agro-industrial origin, which pose a handling challenge at the Spanish and the European agro-industrial sector (e.g. cider production and artichoke canning industry). This study evaluated (i) the impact of the type of substrate on the methane and VFA production and (ii) the influence of co-substrates in the acidogenic fermentation yield for recalcitrant residues (e.g. apple pulp, artichoke residues and microalgae biomass). It should be highlighted that this study gives further insight regarding waste management and decision making from the Circular Economy approach.
Materials and Methods
Substrates and Inoculum
Methanogenic and acidogenic potential tests were performed with urban and agro-industrial waste. Herein, the waste streams consisted of three solid wastes: organic fraction of municipal solid waste (OFMSW), artichoke (ART) and apple pulp (AP) waste; and two liquid effluents: sewage sludge (SS) and microalgae biomass (MB).
The OFMSW was collected from Tecnun-University of Navarra canteen (Campus San Sebastian), which gathers the activity of more than 1,000 people. Artichoke (ART), Cynara scolymus, was collected from a canning plant (Villafranca, Navarra, Spain). Apple pulp (AP) was acquired from a cider producing industry (Astigarraga, Gipuzkoa, Spain). Sewage sludge (SS) was obtained from the thickener of an urban wastewater treatment plant (WTTP), where a high load biological process is utilized (Loyola WWTP, San Sebastian, Gipuzkoa, Spain). Microalgae biomass (MB) was provided by the University of Valladolid (Spain) and it was obtained from two 180-L high rate algae pond (HRAP), from outdoor and indoor conditions. Microalgae were cultivated in mineral medium using biogas. Characterization of MB showed a predominance of Mychonastes homosphaera (59.5%), followed by Pseudoanabaena sp. (39.5%), Navicula sp. (0.5%) and Scenedesmus sp. (0.5%). Those microalgae were harvested using Chemifloc.
Anaerobic digested sludge from a 6000 m3 CSTR mesophilic reactor of the above-mentioned urban WWTP was used as inoculum. Both substrates and inoculum were stored among 4 and 5 °C before use. The main characteristics of the substrates and inoculum used in the present study are shown in Table 1 (methanogenic potential) and Table 2 (acidogenic potential).
Bioreactor Set-Up for Methanogenic and Acidogenic Potential Tests
Methanogenic potential tests were carried out using a batch-mode set in triplicate by a period of 20-day assays. Pyrex bottles of 500 mL nominal volumes (350 mL working volumes) with a rubber joint that ensured hermetic seal were used. The pH was adjusted to neutral conditions at the beginning of the assays (pH 7.0) and temperature was increased to the thermophilic range (55 °C) with the purpose of comparing with the acidogenic potential tests. In order to guarantee neutral pH, bicarbonate salt was added in a concentration of 5 g NaHCO3 L−1. Quantities of each substrate were adjusted in the reactors to an initial g VS (volatile solids) concentration of 5 g VS L−1, whereas the inoculum concentration was set to 1.5-fold substrate, in terms of g VS L−1.
Acidogenic potential tests were performed in duplicate during 10-day batch assays in 500 mL Pyrex bottles (working volumes of 350 mL); except for SS + OFMSW and SS + ART co-fermentation, which were assessed into 2 L bottles (working volumes of 1.8 L). Mono-fermentation and co-fermentation tests were performed at acid (pH 5.5) and alkaline (pH 9) conditions, respectively both at thermophilic temperature. The volume of substrate and inoculum was adjusted to 1:1 (v/v) in the reactors, whereas the co-substrate represented 80% total volume of the substrate. These concentrations are based on pilot-scale fermentation tests performed by Esteban-Gutierrez et al. . The substrate-to-inoculum (S/I) ratio was higher compared with methanogenic potential to assure the presence of enough digestible fraction and inhibit the development of methanogenic bacteria [30, 33]. The pH was controlled daily and adjusted manually, by dosing HCl 4 N and NaOH 4 N to keep an optimum range during fermentation tests.
Control tests were performed to calculate the net biogas production, methane and VFA production of the organic waste streams, with sole inoculum as substrate. Nitrogen gas (99%) was flushed for at least one minute before sealing the bioreactor.
The analysis were performed directly over the raw samples for the analysis of the total fraction. Contrarily, the soluble parameters were determined by sample centrifugation (i.e. 12,000 rpm, 5–10 min), followed by filtration of the supernatant (Millipore 0.7 µm filters). tCOD (total chemical oxygen demand), sCOD (soluble chemical oxygen demand), TS (total solids), VS (volatile solids), TKN (total Kjeldahl nitrogen), TAN (total ammonia nitrogen), TA (total alkalinity) and pH were analysed according to Standard Methods . The C/N ratio was calculated through the ratio of total carbon and total nitrogen detected by element analyser LECO CS-200 and LECO TC-400, respectively.
Biogas production was manually quantified using a pressure sensor (IFM electronic, PN 20,279). Biogas pressure was converted to volume, under standard temperature and pressure conditions as suggested by Angelidaki et al. , i.e., to 0 °C and 1 atm. Biogas composition (N2, O2 CH4 and CO2) was measured using a GC-TCD HP6890 equipment (column SUPELCO 60/80 Carboxen, Ref. 10,001–2390-U).
VFA concentration and composition, i.e., acetic acid (HAc), propionic acid (HPr), butyric acid (HBut), isobutyric acid (Iso-But), valeric acid (HVal), isovaleric acid (Iso-Val), were analysed using gas chromatography (Agilent GC-6890 gas) equipped with a FID (Flame Ionization Detector) and a capillary column (DB-FFAP, 30 m × 0.25 mm i.d., 0.25 μm film, Agilent J&W ref. 122–3232E). The method consisted of treating the filtered fraction via liquid–liquid with an organic solvent as TBME (tert-butyl-methyl-ether). Pivalic acid and helium were used as an internal standard solution and carrier gas, respectively.
Methanogenic Conversion Yield
CODCH4 represents the amount of COD equivalents converted into methane gas, by considering the ideal ratio per gCODfed is 0.35 L CH4 .
Acidification Extent and Acidogenic Conversion Yield
A mode of measuring the correspondence between solubilized organic matter and the VFA produced is through acidification extent, which represents readily fermentable matter in the soluble fraction of the final mixture (sCOD), as shown in Eq. (2).
Regarding the acidogenic conversion or VFA yield, it is expressed as the total VFA production (COD equivalents) per gram of CODfed into the reactor.
The amount of VFA in Eqs. (2) and (3) is represented in COD equivalent of the final fermentation broth. The following stoichiometric COD factors were used: 1.07 gCOD g−1 acetate, 1.51 gCOD g−1 propionate, 1.81 gCOD g−1 butyrate and 2.04 gCOD g−1 valerate .
Results and Discussion
The methanogenic and acidogenic potential of different substrates and a comparison between both are presented as valorization options. Both VFA and methane potential were evaluated as two separate processes for the same waste fraction.
The methanogenic potential of five waste streams, namely, SS, OFMSW, ART, AP and MB; in terms of gVSfed and gCODfed, and is shown in Fig. 1 and Table 3. OFMSW showed the highest methane potential, generating a maximum production 376.79 ± 21.38 NmL/gVSfed.This result was related with the high VS removal, up to 51.64%, and an adequate C/N balance of the substrate (22.81%), which is in line with the results reported by Ponsá et al. . The vegetable and fruit waste yields (ART and AP), presented similar methane potential curves with a negligible production after day 14. Such behaviour might be related to fragments of lignin and cellulose, resistant to the microbial enzymatic action as previously suggested by Zhang et al. .
SS produced less methane compared to the other waste streams (193.15 ± 1.47 NmL/gVSfed), which might be related to the high increase in ammonium content at the end of the test (80.38%) . Interestingly, the highest methane content in the biogas (Table 3) was achieved by MB (61.17%). This value is in accordance with Mussgnug et al.  and Passos and Ferrer , who obtained 62% and 68% CH4, respectively, during the anaerobic digestion of microalgae. Despite MB showed a notable percentage of methane accumulated, a low methanogenic potential (gVSfed) was achieved (235.36 ± 0.25 NmL/gVSfed). In accordance with Solé-Bundó et al. , microalgae AD is limited by nitrogen inhibition associated with a high protein content and a low carbon-to-nitrogen (C/N) ratio. Therefore, for an optimum AD, balancing the C/N is required, which could be attained by employing and carbon-rich co-substrate.
In line with the aforementioned results, the methane production expressed in gCODfed (Fig. 1b) showed the same trend in kinetics of methanogenic conversion in values that range between 157.37 ± 1.20–295.70 ± 16.78 NmL/gCODfed.
VFA production was assessed through two operation scenarios: mono-fermentation of SS, OFMSW, ART, AP and MB under acid pH (5.5) and co-fermentation tests, i.e. SS + OFMSW, SS + ART, SS + AP and SS + MB under alkaline pH (9) and thermophilic temperature (Table 4). Alkaline conditions (e.g. pH 9) were considered based on the optimal VFA production results obtained during the fermentation of SS.
VFA evolution for each organic waste stream was expressed in terms of CODeq L−1, in order to standardize and compare the acidification extent and VFA yields in relation with organic matter converted.
Figure 2 (first row) shows the evolution of tVFA (total VFA) concentration and pH during mono-fermentation tests. Overall, VFA production was influenced by the particle size of substrate, total ammonia nitrogen accumulated, temperature and the pH evolution over time as previously reported by Atasoy et al. , Strazzera et al.  and Arslan et al. . A similar VFA concentration was obtained by agro-industrial wastes: ART, AP and MB, with values up to 8,559, 7,853 and 8,769 mg COD L−1, respectively, and an average pH value above 5.5.
The production of VFA from MB is relatively high compared to previous studies using fresh microalgal biomass as substrate. A possible explanation is given by Xia et al.  and Magdalena et al. , who reported that a boost of substrate concentration around 5–40 g/L and 3–12 g COD·d−1·L−1, respectively, lead to an increase in VFAs production and a significant drop of pH. For this study, a high substrate concentration (25.59 g/L) was used. However, the VFA production profile was different; an initial fast phase of acidogenic conversion was attained during the first 2 days and MB fermentation test, which accounted for 90.34% of the final VFA production. In addition, a considerable decrease on VFA production was observed during the subsequent period, from day 2 until day 7. An excessive ammonium accumulation (2,940 mg N L−1) might have inhibited the acidogenic step  thus limiting the VFA production. This behaviour correlates with a sudden pH drop from 6.64 to 5.64 (Fig. 2, first row) during these days. This is in line with the results of previous studies, which reported the fermentation at neutral conditions (pH 7.0) performed better for microalgae biomass compared to acid and alkaline conditions .
ART and AP waste showed a tendency to increase the VFA production until the end of the test. OFMSW and SS showed lower VFA production. OFMSW reached up to 5206 mg COD L−1 and an increase of 68.54% of the ammonium content. The final VFA production was similar to the values reported by Garcia-Aguirre et al.  who reached an final VFA concentration of 4981 mg COD L−1. On the other hand, SS reached a final VFA production of 5263 mg COD L−1 with a maximum VFA concentration on day 7. These results are in agreement with those presented by Moretto et al. , indicating that mesophilic and alkaline conditions were optimal for VFA production from OFMSW and SS.
Co-fermentation Tests of SS with Waste Streams
Co-fermentation tests (Fig. 2, second row), enabled to enhance the VFA production (except MB waste) and to obtain a different fermentation composition compared to the mono-fermentation tests (see Sect. 3.2.3). The final VFA concentration of SS + OFMSW was highest (11,124 mg COD L−1), which means a 2.14-fold increase compared to OFMSW mono-fermentation test. However, the maximum VFA concentration was observed on day 4, which decreased moderately the following 3 days. Apparently, the contribution of a liquid co-substrate in alkaline conditions induced a fast hydrolysis and solubilisation of OFMSW during the first days, contributing to the final VFA concentration compared to the sole substrate fermentation. Li et al. , provided a similar conclusion and stated that that co-digesting SS and OFMSW could enhance the hydrogen content and VFA production, In line with previous results, ART and AP waste co-fermentation with SS improved the final VFA concentration with values up to 10,355 and 9955 mg COD L−1, respectively. Apparently, alkaline environment enhanced the hydrolysis and solubilisation of carbohydrates and proteins, with a higher release of readily fermentable solubilized organic matter . In contrast, SS + MB co-fermentation was affected by co-substrate and alkaline conditions of the medium. Herein, a high increase of ammonium nitrogen (40%) and 17% of final VFA concentration reduction was observed. The high ammonium release might have induced a partial inhibition of acidogenesis from the 2nd day until 9th. This might be related to the high protein content of MB and SS mixture which led to high amounts of ammonia nitrogen, inhibiting the physiological metabolism of acidogens . Contrarily, at acidic conditions the ammonia remains in the ionized form (NH4) which is less toxic, since the pka value for ammonia at 37 °C is 9.25 .
VFA Composition of Mono-fermentation and Co-fermentation Tests
The VFA composition depends on factors affecting acidogenic fermentation process, such as the type of microbiota present in the inoculum, concentration and characteristics of the substrate, pH, redox potential, among others [52, 53]. According to Fig. 3a, b, the pH had a marked effect on the distribution of the fermentation broth until the last day (day 10) and it also affected by the type of substrate and the co-substrate in a lower extent.
It is important to highlight that under acidic conditions and thermophilic temperature (Fig. 3a), acidogenic conversion in mono-fermentation test was directed towards the metabolic pathway of butyric acid, followed by acetic acid. In addition, it was appreciated a low proportion of valeric acid, which fits with the results of Jiang et al. , who suggested that temperature also plays an important role in the distribution of bioproducts. Herein, acetate and propionate routes were the most prevalent VFA generated at 35 and 45 °C, accounting for 70% of total VFAs, whereas butyrate was the main product at 55 °C, accounting for 81% of all products, followed by acetate and propionate.
A notable butyric acid production was attained by OFMSW and AP up 4,172 and 6,114 mg COD L−1, respectively. Interestingly, ART waste shows a considerable production of acetic acid, up to 50.26% of the fermentation mixture, which could be due to a small part conversion of the propionate, butyrate, alcohols and CO2 to extra acetate through proton-reducing acetogenic pathway or homoacetogenic pathway . With regards to the overall outcome, the results obtained reproduced other studies performed by Feng et al. , Dosta et al. , who concluded that fermentation of carbohydrate rich waste stream is dominated by butyric acid metabolic pathway at acidic pH and thermophilic temperature.
A bioproduct shift from butyric to acetic acid was observed for alkaline pH (Fig. 3b) and co-fermentation with SS. According to previous work, e.g. Luo et al. , Garcia-Aguirre et al.  and Dahiya et al. , alkaline conditions might reduce the butyrate metabolic pathway. In this research, VFA spectrum of SS + OFMSW, SS + ART, SS + AP and SS + MB co-fermentation was dominated by acetic acid up to 38.36%, 50.66%, 54.39% and 46.52% of the fermentation mixture, respectively. As previously suggested, setting optimal pH conditions could lead the fermentation products towards target chemicals .
Evaluation of Acidification Extent and VFA Yield in Mono-fermentation and Co-fermentation Tests
Operation factors, such as pH and temperature, resulted in an increase of sCOD in all tests studied. Generally, sCOD formed is converted to carboxylic acids or VFA (acidification extent), alcohols and a gaseous phase consisting of H2 and CO2, when the methanogenic activity of microorganisms is adequately inhibited .
According to Table 4, the acidification extent of solid waste was lower than in liquid-type waste during mono-fermentation tests. Herein, high acidification extent was obtained into MB mono-fermentation with 54%, followed by SS with 53%, whereas a moderate acidification extent was observed during ART and AP mono-fermentation around 33–39% of sCOD. Perimenis et al.  suggested that a slight decrease could be due to substrates with a higher lignocellulosic fraction content (vegetable and fruits) are more recalcitrant to conversion acidogenic of the solubilized organic matter. Nonetheless, during OFMSW mono-fermentation, acidic conditions and thermophilic temperature turned out to be detrimental to the proliferation of acidogenic microorganisms, reaching low VFA/sCOD ratio of 9%. According to Fig. 2 (first row), a low pH (4.32–5.26) was toxic to acidogenic microorganisms. Hence, in order to survive at low pH condition, a neutral substance is promoted (i.e., ethanol which product of a metabolic pathway shift toward alcoholic fermentation) to prevent pH reduction . It is hypothesized that the lower substrate and inoculum ratio (S/I) influenced in the acidification extent in their research lead to the difference.
Co-fermentation with SS under acidic conditions improved the VFA/sCOD ratio. It is remarkable the acidification extent of SS + OFMSW co-fermentation that reached up to 42%, namely, it enhanced in 4.63-fold higher than OFMSW mono-fermentation. This fermentation performance is in agreement with Moretto et al. , who stated that, both neutral and alkaline pH, could be eligible working conditions in order to reach high VFA concentrations or high acidification yields in a mixture of OFMSW and WAS (SS), especially related to the solubilized COD (sCOD). Optimum acidification degrees were obtained during SS + ART fermentation, and SS + AP co-fermentation, with 55% and 46%, respectively. Contrarily, the acidification degree of MB was lower (42% of sCOD). As previously explained, the release of high amounts of ammonium under alkaline conditions affected the metabolism of acidogenic bacteria.
Overall, alkaline pH and the presence of a co-substrate enhanced the VFA yield (Table 4). This is in line with several studies that demonstrated that under alkaline pH conditions, such as at pH 9.0 or 10.0, the VFA yield was over 3–4 times of that at pH 5.0 or under uncontrolled neutral pH conditions . According to Garcia-Aguirre et al. , alkaline environment results beneficial for the solubilisation of fats, and degradation of soluble proteins and carbohydrates, as well as for avoiding the growth of both hydrogenotrophic and acetoclastic methanogens, thus enriching alkali tolerant conditions.
During mono-fermentation of OFMSW, a low conversion of CODfed to VFA was observed, which barely reached 39 mg VFA g−1 CODfed. This might be due to a high proportion of COD in the substrate in comparison with COD in the inoculum. Regarding the VFA yield achieved, by, Arslan et al.  and Jiang et al. , a low VFA yield is related with an overloading of organic substrate fed which lead to accumulating unutilized solid food waste in the reactor. In contrast, SS, ART, AP and AP substrates, the VFA yield was notably high around 129 mgVFA g−1 CODfed to 262 mgVFA g−1 CODfed.
An increase of VFA yield for all mixtures of SS with substrates is appreciated. SS and OFMSW co-fermentation boosted the VFA yield registered in 269 mg VFA g−1 CODfed. Similarly, in SS + ART, SS + AP and SS + MB waste fermentation, the VFA yield was high and accounted for 360, 353 and 311 mgVFA g−1 CODfed, respectively. According to Zhou et al. , the scenario of co-fermentation constitute a suitable supplement of nutrients and balances the C/N ratio. Furthermore, potential toxic compounds in one substrate could be diluted by adding another substrate [52, 59].
Comparison of Methanogenic and Acidogenic Potential Tests
A comparative of biogas and methane production among methanogenic potential tests and acidogenic potential tests for each substrate is shown in Supplementary Material Table S1. The methane produced during methanogenic potential tests was higher than during the acidogenic potential tests, in terms of gSVfed, since methanogens were successfully inhibited in the latter. Concretely, the methane percentage in acidogenic conditions remained low, around 1–17%.
According to Fig. 4, the conversion yields of the substrates into methane or VFA are expressed in terms of gCOD (considering conversion until day 10). During the experiment, the methanogenic conversion yield was significantly higher than the acidogenic conversion yield, except for SS. A plausible explanation might be related to the initial overload of substrate for acidogenic potential tests. Arslan et al.  stated that levels above 40 gCODfed L−1 may cause an overload or product inhibition. In this study, the initial substrate concentration for OFMSW, ART and AP was above this limit level, around 60–133 gCODfed L−1; whereas it was lower, 20 gCODfed and 37 gCODfed L−1 for SS and MB, respectively. Therefore, it is suggested a moderate conversion yield compared to the methanogenic conversion yield, related to the impact of the amount substrate.
The highest methanogenic conversion yield was obtained for OFMSW and AP (both up 54%). A similar conversion was reported by Perimenis et al.  with AP, who obtained around 46–67% substrate-to-methane COD conversion yield for this waste stream. SS, ART and MB attained a low or moderate methane COD conversion yield up 15, 44 and 25%, respectively. According to Pellera et al. , a low substrate to inoculum ratio (SIR),.i.e. 0.25–1.0 (on a VS basis) could led to a high specific methane yield on solid agro-industrial wastes. In this study, a low SIR (< 0.7) was employed, which had no inhibitory effect on the process.
During acidogenic fermentation tests, the acidogenic yield was in the range of previous literature studies and below 30% [30, 61], which is lower than the methanogenic tests. Higher values above 30% were obtained in a work by Eryildiz et al. , from controlled pH at 6 and low substrate (ISR 1:1). In line with the latter, Esteban-Gutiérrez et al.  reported a raise on the VFA yield, which ranged between 20–40% for SS and 35–49% from agro-industrial waste, at 55 °C, and when shifting the acid pH (5.5) towards alkaline pH (9).
In order to improve the acidogenic conversion yield, further study and optimization of the anaerobic fermentation process is required. Recent attempts have focused on the inoculum acclimatization or inoculum pretreatment, as a strategy to achieve a high VFA yield through deactivating the methanogens [63, 64]. Moreover, the substrate-related issues for optimum acidogenic conversion could be improved from the intensification of hydrolysis, i.e., pretreatment of complex substrates (e.g. physical, chemical, or biological means), which contain inhibitors as melanoidins formation from food waste , several flavour compounds from fruits (e.g. D-limonene) or recalcitrant materials (e.g. lignocellulosic wastes) [8, 63, 66].
The carboxylate platform has shown a market interest and high value, due to the industrial applicability of the biochemicals (i.e. liquid fuels and high-value chemicals) and the wide spectrum of market opportunities that arise . As the literature has pointed out, the market price for propionic acid, butyric acid and caproic acid are between 1500–2500 USD/t, which are more profitable than the methane price range, i.e., 400–900 USD/t [5, 67]. Despite the lower bioconversion of acidogenic fermentation tests, this study highlights the high potential of waste to bioproduct conversion which can be directed towards desired chemicals (e.g. butyric acid, acetic acid and propionic acid) or optimized with additional substrate addition [8, 68].
Despite the higher yield of methanogenic conversion (up to 54%) and the methane potential (193.15 ± 1.47–376.79 ± 21.38 NmL/gVSfed) obtained during the conventional anaerobic digestion of different agro-industrial waste streams, this study has shown a notable conversion of waste into value added bio-based products (VFA) which might be desirable in terms of profitability and the market price of bulk chemicals vs biofuels,. During the study, butyric acid (29–80%), acetic acid (19–50%), propionic acid (3–19%), were obtained from mono-fermentation of OFMSW, ART, AP, SS and MB. In addition, co-fermentation tests enabled to increase the solubilisation rate and VFA production in ranging 7,281–11,124 mg COD L−1, which could be a favourable scenario for a carboxylic platform. Overall, in a biorefinery scenario, this study broadens the overview on the choice of alternatives for the conversion of waste-to-resource, either by means of energy recovery (methane) or by means of bio-based products (VFA).
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Ceit would like to thank Gipuzkoa Provincial Council- Department of Environment and Hydraulic Works for its financial support and all the staff from the WWTP of San Sebastian. Furthermore, authors would like to thank Raul Muñoz, from the University of Valladolid, for kindly providing the microalgae biomass used in the present research.
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Cerdán, J.M.A., Tejido-Nuñez, Y., Aymerich, E. et al. A Comprehensive Comparison of Methane and Bio-Based Volatile Fatty Acids Production from Urban and Agro-Industrial Sources. Waste Biomass Valor 12, 1357–1369 (2021). https://doi.org/10.1007/s12649-020-01093-3
- Organic waste
- Volatile fatty acid