Dark Fermentation Process Research Articles

Influence of initial pH on the production of volatile fatty acids and hydrogen during dark fermentation of kitchen waste

ABSTRACT The purpose of this work was to determine the effect of initial pH on the production of volatile fatty acids (VFA) and hydrogen (H2) in the dark fermentation processes of kitchen waste. The study was conducted in batch bioreactors of working volume 1 L for different initial pH in the range from 5.5 to 9.0. The dark fermentation processes were carried out for 4 days at 37°C. Initial organic load of the kitchen waste in all bioreactors amounted to 25.5 gVS/L. Buffering of pH during the fermentation process was carried out with the use of ammonia contained mainly in digested sludge. The optimal conditions for the production of VFA and H2 were achieved at the initial pH of 8. Production of VFA and H2 in these conditions was, respectively, 13.9 g/L and 72.4 mL/gVS. The main produced components of VFA were acetic and butyric acids. The production of ethanol and lactic acid was at very low levels due to the high ratio of the volatile fatty acids to total organic content of 0.86. With the optimal initial pH of 8 the yield of CO2 production was 0.30 gC/gC. High initial pH value (above 8) extended the lag phase duration in the course of H2 production. The dominant groups of micro-organisms at the most favourable initial pH of 8 for the production of VFA and H2 were Bacteroidetes, Firmicutes, Spirochaetes and Waste Water of Evry 1 (WWE1) at the phylum level.

Hydrogen Production by Dark Fermentation Process: Effect of Initial Organic Load

The treatment of residual biomass through the dark fermentation (DF) process is presented as a sustainable alternative, where bio-hydrogen is produced, which has a high energy potential, and bio-based sub-products. Consequently, this work aimed to determine the maximum organic load for the production of bio-hydrogen, from residual biomass available in Colombia. An experimental design was constructed to determine the biochemical hydrogen potential (BHP) of mixtures composed of three substrates: pig manure (PM), cocoa mucilage (CCM), and coffee mucilage (CFM). The design managed two independent variables, the organic load (10 gVS/l, 20 gVS/l, 30 gVS/l, 40 g VS/l, and 50 gVS/l), which were determined according to the physicochemical characterization of the substrates, and the S/X evaluated on two levels 1:1 and 1:2. The experiments were carried under mesophilic conditions (±35°C), a C/N ratio of 35, and a pH of 5.5 was fixed. As a result, five mixtures were evaluated, plus five targets. The reactors were erlenmeyers of 250mL, hermetically sealed, heated, and stirred in a hot plate. The tests were allowed to run until the detection of CH4; daily monitoring with a Biogas 5000® portable gas analyser did this. Following the determination of the physicochemical characteristics of the effluent, total solids (TS), volatile solids (VS), proteins, and chemical oxygen demand COD were analysed for each of the reactors. The results show that the organic load of 10gSV/L and the S/X of 1 has the highest production rate with 232 mL of H2, with a production of volatile fatty acids (VFA) of 4040mg COD/L, and a VS removal percentage of 84%. The removal of VS and the VFA allows suggesting secondary processes associated with bio-refinery schemes, for a more significant elimination of volatile solids and the obtaining of other value-added by-products.

Biohydrogen production from fruit and vegetable waste, sugar beet pulp and corn silage via dark fermentation

Dark fermentative hydrogen production was investigated using sugar beet pulp (SBP), fruit and vegetable waste (FWV) and corn silage (CS). The highest hydrogen yield of 52 cm3/gVS was achieved for FVW treated at an organic loading rate (OLR) of 17 gVS/m3•d. The hydrogen production from hydrolyzed SBP varied from 17 to 37 cm3/gVS, however the dark fermentation (DF) process was unstable due to the large amounts of lactic acid secreted by lactic acid bacteria. Moreover, in the final stage of the digester operation with SBP, a noticeable methane production of 5 cm3/gVS was observed. The production of hydrogen from corn silage was relatively low (24–27 cm3/gVS) due to operating problems and small sugar content of 7.69 g/l in this substrate. Microbiological analyses revealed the prevalence of lactic acid producing bacteria dominated by Lactobacillaceae, Coriobacteriaceae and Bifidobacteriaceae at a family level, which constituted as much as 40–60% of all detected microbes. The main hydrogen-producing bacteria were affiliated with the families Mogibacteriaceae and Ruminococcaceae. However, the most important finding was the activity of methanogens, which could grow at pH below 6 and produce methane thus lowering hydrogen yield, which was particularly visible in SBP experiments.

Hydrogen production by dark fermentation process from pig manure, cocoa mucilage, and coffee mucilage

The aim of this study was the evaluation of the influence of temperature, initial organic load, and C/N ratio in the biochemical hydrogen potential (BHP) by dark fermentation (DF) of mixtures composed of residual biomass derived from the Colombian agro-industry (pig manure, cocoa mucilage, and coffee mucilage) to contribute to residue management and the development of a bio-based economy in Colombia. A Box-Behnken experimental design was constructed, where the variables were as follows: three initial organic loads (2, 5, and 8 g COD/l), three C/N ratios (25, 35, and 45), and three different coffee and cocoa initial COD ratios (coffee mucilage (CFM):cocoa mucilage (CCM)) (1:3, 2:2, and 3:1). The experiments were carried out at thermophilic environments of 55 °C and 45 °C. Sludge from an anaerobic digester pretreated by thermal shock was used as an inoculum. The results of the experimental designs were used (one per temperature) to model the system mathematically. Also, a mathematical model MARS (multivariate adaptive regression splines) was used as an alternative method to corroborate the optimal points obtained. In both models, it was evidenced that at 55 °C and 45 °C, the best mixing conditions indicated a C/N ratio of 45 and the lowest CFM:CCM with a value close to 1:3. Likewise, it was evidenced that at the temperature of 55 °C, the optimum organic load is 8 g COD/l, while 45 °C is 2 g COD/l. Moreover, it was determined that CFM:CCM has less influence than the other independent variables studied, a factor that favors the country’s scope, due to the availability of these two substrates is not permanent throughout the year.

Integrated System Technology of POME Treatment for Biohydrogen and Biomethane Production in Malaysia

In recent years, production of biohydrogen and biomethane (or a mixture of these; biohythane) from organic wastes using two-stage bioreactor have been implemented by developing countries such as Germany, USA and the United Kingdom using the anaerobic digestion (AD) process. In Thailand, biohythane production in a two-stage process has been widely studied. However, in Malaysia, treating organic and agricultural wastes using an integrated system of dark fermentation (DF) coupled with anaerobic digestion (AD) is scarce. For instance, in most oil palm mills, palm oil mill effluent (POME) is treated using a conventional open-ponding system or closed-digester tank for biogas capture. This paper reviewed relevant literature studies on treating POME and other organic wastes using integrated bioreactor implementing DF and/or AD process for biohydrogen and/or biomethane production. Although the number of papers that have been published in this area is increasing, a further review is needed to reveal current technology used and its benefits, especially in Malaysia, since Malaysia is the second-largest oil palm producer in the world.

Addition of electron shuttling compounds and different pH conditions for hydrogen production by a heat-treated sludge

Abstract In the present work, the effect of anthraquinone-2-sulfonate (AQS) and its reduced form anthrahydroxyquinone-2-sulfonate (AH2QS), and different pH values were evaluated during a dark fermentation process using heat-treated anaerobic granular sludge for hydrogen production. The highest maximum hydrogen production (1.99 mmol) was obtained by adding the reduced electron shuttling compound (AH2QS) at pH 6. This experimental condition also reflected the highest substrate consumption efficiency and the lowest lag phase in relation to the AQS and the control (only sludge). In contrast, the lag phase and maximum hydrogen production were not impacted by AH2QS addition at initial pH of 6.4 and 7.8. Regardless of the initial pH value, the hydrogen production rate was increased after the addition of the electron shuttling compounds, which could be attributable to the performance of either the oxidized and reduced form of the electron shuttling compound. These findings demonstrate the feasibility of AH2QS addition during dark fermentation at pH 6.4 for improving the hydrogen production rate. Exoelectrogenic microorganisms identified in the inoculum could be responsible to improve the dark fermentation process in assays with electron shuttling compounds.

Enhancement of hydrogen production and energy recovery through electro-fermentation from the dark fermentation effluent of food waste

To enhance hydrogen production efficiency and energy recovery, a sequential dark fermentation and microbial electrochemical cell (MEC) process was evaluated for hydrogen production from food waste. The hydrogen production, electrochemical performance and microbial community dynamics were investigated during startup of the MEC that was inoculated with different sludges. Results suggest that biogas production rates and hydrogen proportions were 0.83 ​L/L d and 92.58%, respectively, using anaerobic digested sludge, which is higher than that of the anaerobic granular sludge (0.55 ​L/L d and 86.21%). The microbial community were predominated by bacterial genus Acetobacterium, Geobacter, Desulfovibrio, and archaeal genus Methanobrevibacter in electrode biofilms and the community structure was relatively stable both in anode and cathode. The sequential system obtained a 53.8% energy recovery rate and enhanced soluble chemical oxygen demand (sCOD) removal rate of 44.3%. This research demonstrated an important approach to utilize dark fermentation effluent to maximize the conversion of fermentation byproducts into hydrogen.

Essential Nutrients for Improving the Direct Processing of Raw Lignocellulosic Substrates Through the Dark Fermentation Process

The dark fermentation process is a base platform in which lignocellulosic substrates are biologically pretreated producing a gas mixture of H2/CO2, volatile fatty acids as intermediate chemicals, and solids enriched in cellulose. Optimal amounts of nutrients are required by mixed cultures to hydrolyze and ferment the substrate at once requiring optimal amounts of nutrients. In this study, two statistical approaches were used to screen essential nutrients for the direct conversion of lignocellulosic substrates into hydrogen (the chosen response variable) and to optimize such variable. First, a Plackett-Burman design of seven factors at two levels screened the main nutrients for the direct fermentation of the substrate. Urea, CaCl2, and KH2PO4 present a significant effect. The low levels of these nutrients in batch fermentation with 2% of total solids (90, 2, and 37 mg/L, respectively) improved 4-fold the hydrogen production in comparison with the control treatment containing no nutrients. Then, a central composite design with three factors at five levels predicted that the optimum levels of nutrients for the highest hydrogen production of 504 mL/L were 1018 mg/L of urea (a C/N ratio of 17.5), 109 mg/L of KH2PO4 (a C/P ratio of 338), and 411 mg/L of CaCl2. The usefulness of this optimized medium formulation was demonstrated using three lignocellulosic substrates where the hydrogen production increased 1.2- to 3.5-fold in comparison with the control treatments containing no nutrients.

Fermentative Conversion of Two-Step Pre-Treated Lignocellulosic Biomass to Hydrogen

Fermentative hydrogen production via dark fermentation with the application of lignocellulosic biomass requires a multistep pre-treatment procedure, due to the complexed structure of the raw material. Hence, the comparison of the hydrogen productivity potential of different lignocellulosic materials (LCMs) in relation to the lignocellulosic biomass composition is often considered as an interesting field of research. In this study, several types of biomass, representing woods, cereals and grass were processed by means of mechanical pre-treatment and alkaline and enzymatic hydrolysis. Hydrolysates were used in fermentative hydrogen production via dark fermentation process with Enterobacter aerogenes (model organism). The differences in the hydrogen productivity regarding different materials hydrolysates were analyzed using chemometric methods with respect to a wide dataset collected throughout this study. Hydrogen formation, as expected, was positively correlated with glucose concentration and total reducing sugars amount (YTRS) in enzymatic hydrolysates of LCMs, and negatively correlated with concentrations of enzymatic inhibitors i.e., HMF, furfural and total phenolic compounds in alkaline-hydrolysates LCMs, respectively. Interestingly, high hydrogen productivity was positively correlated with lignin content in raw LCMs and smaller mass loss of LCM after pre-treatment step. Besides results of chemometric analysis, the presented data analysis seems to confirm that the structure and chemical composition of lignin and hemicellulose present in the lignocellulosic material is more important to design the process of its bioconversion than the proportion between the cellulose, hemicellulose and lignin content in this material. For analyzed LCMs we found remarkable higher potential of hydrogen production via bioconversion process of woods i.e., beech (24.01 mL H2/g biomass), energetic poplar (23.41 mL H2/g biomass) or energetic willow (25.44 mL H2/g biomass) than for cereals i.e., triticale (17.82 mL H2/g biomass) and corn (14.37 mL H2/g biomass) or for meadow grass (7.22 mL H2/g biomass).

Biological hydrogen production via dark fermentation: A holistic approach from lab-scale to pilot-scale

Biohydrogen is considered as fuel of future owing to its distinctive attribute for clean energy generation, waste management and high energy content. Suitable feedstock play important role for achieving high rate hydrogen production via dark fermentation process. In this regard, different organic wastes such as cane molasses, distillery effluent and starchy wastewater were examined as potential substrates for biohydrogen production by Enterobacter cloacae IIT-BT 08. Groundnut deoiled cake (GDOC) was considered as additional nutritional supplement to enhance biohydrogen yields. The maximum hydrogen yield of 12.2 mol H2 kg−1 CODremoved was obtained using cane molasses and GDOC as co-substrates. To further ensure reliability of the process, bench (50 L) and pilot scale (10000 L) bioreactors were customized and operated. The pilot scale study achieved 76.2 m3 hydrogen with a COD removal and energy conversion efficiency of 18.1 kg m−3 and 37.9%, respectively. This study provides an extensive strategy in moving from lab to pilot scale biohydrogen production thereby, providing further opportunity for commercial exploitation.

Improvement of Digestate Stability Using Dark Fermentation and Anaerobic Digestion Processes

This paper assessed the effect of dark fermentation, the fermentative phase in a two-stage anaerobic digestion system, in terms of digestate biostabilization efficiency. The digestates analyzed in this study were obtained from a pilot-scale system in which two different substrates were used in order to simulate both the digestion and co-digestion process. Biostabilization performances were evaluated by measuring the specific oxygen uptake rate (SOUR) of the outgoing digestates. This index allowed us to define the degree of effectiveness in terms of stabilization of organic matter, between the traditional anaerobic digestion process and the two-stage configuration. Considering the traditional process as a reference scenario, the results highlighted an increase in biological stability for the two-stage co-digestion process, consisting of a dark fermentation stage, followed by an anaerobic digestion one. Digestates biostabilization efficiency increased up from 6.5% to 40.6% from the traditional one-stage configuration to the two-stage one by improving the anaerobic digestion process through a preliminary fermentative stage. The advantages of the two-stage process were due to the role of dark fermentation as a biological pre-treatment. Considering the partial stability results related to the second stage, biological stability was improved in comparison to a single-stage process, reaching an efficiency of 42.2% and 55.8% for the digestion and co-digestion scenario respectively. The dark fermentation phase allowed for a higher hydrolysis of the substrate, making it more easily degradable in the second phase. Results demonstrated better biostabilization performances of the outgoing digestates with the introduction of dark fermentation, resulting in more stable digestates for both the digestion and co-digestion process.

Inhibition by the ionic strength of hydrogen production from the organic fraction of municipal solid waste

Composition of the Organic Fraction of Municipal Solid Waste (OFMSW) in organic compounds and inorganic ions is highly variable and might impact the microbial activity in dark fermentation processes. In this study, the effect of the total amount of inorganic ions on fermentative hydrogen production was investigated. Batch experiments were carried out at pH 6 and under a temperature of 37 °C. A freshly reconstituted organic fraction of municipal solid waste (OFMSW) was used as model substrate. At low concentrations in ammonium or chloride ions (2.9–5.1 g L−1, respectively), the hydrogen yield reached a maximum of 40.8 ± 0.5. mLH2.gVS−1 and 25.1 ± 5.6 mLH2.gVS−1. In contrast, at high total ionic concentrations of ammonium and chloride (11.1–35.5 g L−1 respectively), a strong inhibition of the fermentative microbial activity and more particularly hydrogen production, was observed. When considering the ionic strength of each ion, the effects of ammonia, chloride or a mixture of different ions (Na+, K+, H+, Li+, NH4+, Mn2+, NH4+, Mg2+, Cl−, PO43−, Br−, I−, SO42−) showed very similar inhibitory trends regardless the type of ion or the composition of the ionic mixture. A threshold inhibitory value of the ionic strength was estimated at 0.75 ± 0.13 M with a substantial impact on the fermentative activity from 0.81 ± 0.12 M, with hydrogen yields of 18.1 ± 3.3 and 6.2 ± 4.1 mLH2.gVS−1, respectively. Microbial community composition was also significantly impacted with a specific decrease in relative abundance of hydrogen-producing bacteria from the genus Clostridium sp. at high ionic strength.

Enhanced hydrogen production by a sequential dark and photo fermentation process: Effects of initial feedstock composition, dilution and microbial population

Two-stage process of dark fermentation (DF) and photo fermentation (PF), using fruit and vegetable waste (FVW) and cheese whey powder (CWP), was used as an approach to enhance the hydrogen (H2) production. FVW and CWP at C/N ratios of 34, 39, 60, 71 and 82 were tested as substrates for DF. Dilution (1:2, 1:5, 1:10) of the DF effluents was used as a coupling strategy. DF effluents with low-butyrate and high lactate concentrations were obtained as a function of an increased C/N ratio, which results in high H2 production during the PF. Maximum overall H2 yields of 793.7 and 695.4 mLH2/gChemical Oxygen Demand (COD) were obtained using a 1:10 dilution, at a C/N ratio of 60 and 70, respectively. These H2 yields were higher than those obtained with the individual processes. The C/N ratio at the DF stage regulate not only H2 production but also the distribution and concentrations of by-products. These metabolites, in turn, control the H2 production during the PF. Predominant microbial population for both processes (DF: C/N = 34 Acetobacter lovaniensis, Clostridium butyricum; C/N = 39 C. butyricum, Enterobacter sp, Bifidobacterium; C/N = 82 Lactobacillus casei; PF: Rhodopseudomonas palustris) were in accordance with the final metabolic products.

Optimization of the medium composition for the improvement of hydrogen and butanol production using Clostridium saccharoperbutylacetonicum DSM 14923

The dark fermentation process promises a sustainable route for biohydrogen production. But the low substrate conversion efficiency hinders its commercial feasibility. The present study aims towards simultaneous hydrogen and butanol production to enhance the net energy recovery. Process parameters optimization revealed that pH of 6.5, temperature of 37 °C and inoculum size of 7% (v/v) were suitable for obtaining maximum hydrogen and butanol yields by C. saccharoperbutylacetonicum. Starch and xylan were observed to be preferred carbon sources suggesting effective utilization of C5 and C6 sugars. The maximum hydrogen yield of 264.3 mL g−1 starch and 216 mL g−1 xylan and butanol yield of 0.27 g g−1 starch and 0.24 g g−1 xylan were obtained with overall energy recovery of 85.61% (starch) and 75.22% (xylan), respectively. The present research work indicates the potency of bi-phasic fermentation to boost energy recovery from starch/xylan based feedstock.

Dark fermentative hydrogen production from food waste: Effect of biomass ash supplementation

This work aimed to investigate the effects of supplementing two distinct types of ash, namely fly ash (FA) and bottom ash (BA) on the dark fermentation (DF) process of food waste (FW) for H2 production. Both types of biomass combustion ash (BCA) were collected in an industrial bubbling fluidized bed combustor, using residual forest biomass as fuel. Results indicated that adding BCA at different doses of 1, 2 and 4 g/L could effectively enhance H2 generation when compared to the control test without BCA addition. This stimulatory effect was attributed to the crucial role of metal elements released from BCA such as sodium, potassium, calcium, magnesium, and iron in the provision of buffering capacity and inorganic nutrients for the functioning of hydrogen-forming bacteria. The highest H2 yield of 169 mL per g of volatile solids (VS) were obtained by adding only a small amount of BA (1 g/L) to the reactive system, representing a significant increment of 1070% compared to the control reactor. Furthermore, a significant decrease in the bacterial lag phase time from 26 h to 2.7 h, as well as about a 12-fold increase in the energy recovery as H2 gas was observed at BA dosage of 1 g/L in comparison with the control reactor. Overall, this study suggested that a proper addition of BCA could promote the DF process of FW and enhance biohydrogen production.

Production of carbohydrate enrich microalgal biomass as a bioenergy feedstock

Microalgal biomass is considered as a propitious feedstock for bioenergy generation. The present research work aimed to maximize the biomass and carbohydrate productivity of Scenedesmus obliquus UTEX 393 by optimizing physico-chemical parameters using single and multi-parameter optimization strategies. Individual and interactive effects of different parameters (temperature, pH, carbon source concentration and nitrogen source concentration) were investigated. Multi-response optimization study resulted in maximum biomass and carbohydrate productivity of 491 mg L−1 d−1and 270 mg L−1 d−1, respectively at the initial pH 6.69, temperature 27.65 °C, glucose concentration 3.33 g L−1 and urea concentration 126.77 mg L−1. Effect of photobioreactor configuration on biomass and carbohydrate production was also investigated. The biomass and carbohydrate productivity were enhanced to 560 mg L−1d−1 and 309 mg L−1 d−1, respectively using airlift photobioreactor. In a biorefinery framework, before subjecting the biomass for fermentative biohydrogen production, the lipid present (22.6% w/w) in the microalgae was extracted and transesterified for biodiesel production. The potentiality of defatted microalgal biomass (rich in carbohydrate) as a substrate for the dark fermentation process was evaluated using acidogenic mixed consortia. Cumulative hydrogen production of 68.9 mL g−1 DCW was achieved, which suggested the suitability of deoiled algal biomass as a biohydrogen feedstock.

Changes in microbial community structure during dark fermentative hydrogen production

Microbial community structure plays a significant role in the efficiency of dark fermentative hydrogen production using mixed culture. However, the detailed evolutions in microbial community structure during dark fermentation process are still unclear. This study investigated the detailed evolution patterns of microbial community structure during dark fermentation process by high-throughput pyrosequencing. Results showed that microbial community structure changed significantly over time in dark fermentation. Microbial diversity showed a constant decreasing trend during the fermentation process. The analysis of microbial community composition showed that Clostridium sensu stricto 1, Paraclostridium, Romboutsia and Paeniclostridium, which were all rarely existed in the inoculum, dramatically became dominant genera in the system after 6 h fermentation, with total relative abundance of more than 99%. This interesting result revealed that how quickly hydrogen-producing genera overwhelmed the microbial community in dark fermentation. Spearman correlation analysis showed that Clostridium sensu stricto 1 contributed the most to hydrogen fermentation performances.

Simultaneous treatment of agro-food and asbestos-cement waste by the combination of dark fermentation and hydrothermal processes

The inadequate management of asbestos-cement products (ACP) and/or wastes (ACW) generates hazardous airborne dusts. For this reason, the EU is promoting the removal of ACP from utilities, public and private buildings and is looking for innovative ACW treatments alternative to the landfilling. The simultaneous treatment of ACW and orange pulp or cheese whey was attempted with a mesophilic dark fermentation (DF) process followed by a 12 h hydrothermal (HT) treatment carried out with the addition of lactic or oxalic acid at 100 °C and ambient pressure. The DF of orange pulp or cheese whey in the presence of 5 g L−1 ACW lasted 171 h and led to the production of 4 LH2 and 5 LH2 per litre of solution, respectively. During the DF, the dissolution of the ACW matrix was almost completed, and the partial collapse of the asbestos fibres was observed. Both the organic acids that were added during the HT tests at a concentration ≥2.5 g L−1 completely destroyed the asbestos fibres. However, when 1.25 g L−1 lactic or oxalic acid was added before the HT tests, few fibre fragments were found, and their chemical composition differed from that of chrysotile.

An efficient dark fermentative hydrogen production by GMV control of pH

This study proposes that the on-line pH control via a model-based adaptive controller markedly improves the dark fermentative hydrogen production. According to the dynamic behavior of the dark fermentation process, pH, which rapidly declines with the beginning of the biogas production, should be precisely controlled around its optimal value in a narrow range. The success of on-line pH control was guaranteed by performing the preliminary simulation studies by experimental data obtained from dynamic analysis to determine ARMAX model order with Recursive Least Squares parameter estimation method and then to control the pH with Generalized Minimum Variance (GMV) controller. On-line control of pH at the optimal value of 6.0 during the 25 h dark fermentation process resulted in 5.4 times higher biogas production, 6.2 times higher biogas production potential, nearly doubled the duration of fermentation, and 18.4% biogas production rate increment in comparison with the uncontrolled pH case.

Food wastes and sewage sludge as feedstock for an urban biorefinery producing biofuels and added‐value bioproducts

AbstractThe updated Bioeconomy Strategy document “A sustainable bioeconomy for Europe: strengthening the connection between economy, society and the environment”, which was issued by the European Commission in October 2018, encourages the exploitation of organic wastes according to a pyramidal hierarchy in which the extraction of valuable biomolecules, which will be used as they are or as precursors of high‐added‐value compounds, is a priority in biofuel production. This review considers a biorefinery platform in which food waste and sewage sludge are adopted to produce volatile fatty acids (VFAs) through a dark fermentation process. VFA fermentation is optimized by slightly acid pH (6–7), short hydraulic retention time (1–7 days) and high organic load rate (more than 10 gTS L−1 d−1). Attention has been focused on VFA exploitation for polyhydroxyalkanoate (PHA) production via a ‘feast and famine’ strategy performed in sequencing batch reactors. The obtained PHA yields are around 0.4–0.5 gPHA gCOD−1. Moreover, VFAs allow for the production of biofuels, such as hydrogen and methane, through single‐ or double‐staged anaerobic digestion. Innovative bioelectrochemical upgrade strategies for biogas helps producers to obtain biomethane for the automotive sector. Moreover, biogas has recently been tested for the production of polyhydroxybutyrate, a biodegradable and biocompatible thermoplastic made by microorganisms from C1 carbon sources (CO2 and CH4). Digestates from anaerobic bioreactors are still rich in nitrogen and phosphorus compounds. These latter compounds have been identified as critical raw materials due to their low availability in the European Union and to increasing demand from the growing global population. Thus, nutrient recovery from digestate allows users to close the loop of the ‘circular economy’ approach. © 2019 Society of Chemical Industry