Volatile fatty acid production from saline cooked mussel processing wastewater at low pH

Liquid Side Streams from Mussel and Herring Processing as Sources of Potential Income

The seafood industry generates significant amounts of process waters which can generate value upon recovery of their nutrients. Process waters from the herring marination chain and cooking of mussels were here characterized in terms of crude composition, volatile compounds, and nutritional and potentially toxic elements. Protein and total fatty acid contents of herring refrigerated sea water (RSW) reached 3 and 0.14 g/L, respectively, while herring presalting brine (13%) reached 16.3 g/L protein and 0.77 g/L total fatty acid. Among three herring marination brines vinegar brine (VMB), spice brine (SPB), and salt brine (SMB), SPB reached the highest protein (39 g/L) and fatty acids (3.0 g/L), whereas SMB and VMB at the most had 14 and 21 g protein/L, respectively, and 0.6 and 9.9 g fatty acids/L, respectively. Essential amino acid (EAA) in marination brines accounted for up to 59% of total amino acid (TAA). From mussel processing, cooking juice had more protein (14-23 g/L) than the rest of the process waters, and in all water types, EAA reached up to 42% of TAA. For all process waters, the most abundant nutritional elements were Na, K, P, Ca, and Se. The content of all potentially toxic elements was mostly below LOD, except for As which ranged from 0.07 to 1.07 mg/kg among all tested waters. Our findings shed light on liquid seafood side streams as untapped resources of nutrients which can be valorized into food/feed products.

Denitrification for acidic wastewater treatment: Long-term performance, microbial communities, and nitrous oxide emissions

Acidic nitrogenous wastewater often requires alkali pretreatment before biological treatment, which results in increased system complexity and operating costs. The demonstration of denitrification under acidic conditions would provide a theoretical basis for the direct treatment of such wastewater. In this study, the denitrification performance, microbial community, and nitrous oxide (N₂O) emissions under acidic conditions were investigated using a sequencing batch reactor. When the influent pH decreased from 5.5 to 4.5, the sequencing batch reactor removed 99.8 ± 0.2% of the nitrate and 92.5 ± 1.6% (n = 171) of the chemical oxygen demand, and the production efficiency of N₂O increased significantly to 11.45%. This was 2.6-fold higher than that observed at pH 5.5. The long-term denitrification treatment of acidic wastewater (pH 4.5) led to the formation of granular sludge, and Thauera, Allorhizobium-Neorhizobium-Parararhizobium-Rhizobium, and Diaphorobacter became the dominant microbes with a collective abundance of 81.3%. More importantly, only 0.25% of the nitrate was denitrified as N₂O, and the batch test revealed that the emissions of N₂O decreased with the increase in sludge size. These results indicate that denitrifying granular sludge formed under acidic conditions and denitrifying bacteria capable of N₂O reduction proliferated, which both resulted in a significant reduction in the release of N₂O.

Initial pH Conditions Shape the Microbial Community Structure of Sewage Sludge in Batch Fermentations for the Improvement of Volatile Fatty Acid Production

Conversion of wastewater treatment plants into biorefineries is a sustainable alternative for obtaining valuable compounds, thus reducing pollutants and costs and protecting the environment and human health. Under specific operating conditions, microbial fermentative products of sewage sludge are volatile fatty acids (VFA) that can be precursors of polyhydroxyalkanoate thermoplastic polyesters. The role of various operating parameters in VFA production has yet to be elucidated. This study aimed to correlate the levels of VFA yields with prokaryotic microbiota structures of sewage sludge in two sets of batch fermentations with an initial pH of 8 and 10. The sewage sludge used to inoculate the batch fermentations was collected from a Sicilian WWTP located in Marineo (Italy) as a case study. Gas chromatography analysis revealed that initial pH 10 stimulated chemical oxygen demands (sCOD) and VFA yields (2020 mg COD/L) in comparison with initial pH 8. Characterization of the sewage sludge prokaryotic community structures—analyzed by next-generation sequencing of 16S rRNA gene amplicons—demonstrated that the improved yield of VFA paralleled the increased abundance of fermenting bacteria belonging to Proteobacteria, Bacteroidetes, Chloroflexi, and Firmicutes phyla and, conversely, the reduced abundance of VFA-degrading strains, such as archaeal methanogens.

Upgrading the value of anaerobic fermentation via renewable chemicals production: A sustainable integration for circular bioeconomy

The single bioprocess approach has certain limitations in terms of process efficiency, product synthesis, and effective resource utilization. Integrated or combined bioprocessing maximizes resource recovery and creates a novel platform to establish sustainable biorefineries. Anaerobic fermentation (AF) is a well-established process for the transformation of organic waste into biogas; conversely, biogas CO₂ separation is a challenging and expensive process. Biological fixation of CO₂ for succinic acid (SA) mitigates CO₂ separation issues and produces commercially important renewable chemicals. Additionally, utilizing digestate rich in volatile fatty acid (VFA) to produce medium-chain fatty acids (MCFAs) creates a novel integrated platform by utilizing residual organic metabolites. The present review encapsulates the advantages and limitations of AF along with biogas CO₂ fixation for SA and digestate rich in VFA utilization for MCFA in a closed-loop approach. Biomethane and biohydrogen processes CO₂ utilization for SA production is cohesively deliberated along with the role of biohydrogen as an alternative reducing agent to augment SA yields. Similarly, MCFA production using VFA as a substrate and functional role of electron donors namely ethanol, lactate, and hydrogen are comprehensively discussed. A road map to establish the fermentative biorefinery approach in the framework of AF integrated sustainable bioprocess development is deliberated along with limitations and factors influencing for techno-economic analysis. The discussed integrated approach significantly contributes to promote the circular bioeconomy by establishing carbon-neutral processes in accord with sustainable development goals.

Polyhydroxyalkanoates Production by Mixed Microbial Culture under High Salinity

The fishing industry produces vast amounts of saline organic side streams that require adequate treatment and disposal. The bioconversion of saline resources into value-added products, such as biodegradable polyhydroxyalkanoates (PHAs), has not yet been fully explored. This study investigated PHA production by mixed microbial cultures under 30 gNaCl/L, the highest NaCl concentration reported for the acclimatization of a PHA-accumulating mixed microbial culture (MMC). The operational conditions used during the culture-selection stage resulted in an enriched PHA-accumulating culture dominated by the Rhodobacteraceae family (95.2%) and capable of storing PHAs up to 84.1% wt. (volatile suspended solids (VSS) basis) for the highest organic loading rate (OLR) applied (120 Cmmol/(L.d)). This culture presented a higher preference for the consumption of valeric acid (0.23 ± 0.03 CmolHVal/(CmolX.h)), and the 3HV monomer polymerization (0.33 ± 0.04 CmmolHV/(CmmolX.h) was higher as well. As result, a P(3HB-co-3HV)) with high HV content (63% wt.) was produced in the accumulation tests conducted at higher OLRs and with 30 gNaCl/L. A global volumetric PHA productivity of 0.77 gPHA/(L.h) and a specific PHA productivity of 0.21 gPHA/(gX.h) were achieved. These results suggested the significant potential of the bioconversion of saline resources into value-added products, such as PHAs.

Volatile Fatty Acids (VFA) Production from Wastewaters with High Salinity—Influence of pH, Salinity and Reactor Configuration

The hydrocarbon-based economy is moving at a large pace to a decarbonized sustainable bioeconomy based on biorefining all types of secondary carbohydrate-based raw materials. In this work, 50 g L−1 in COD of a mixture of food waste, brine and wastewater derived from a biodiesel production facility were used to produce organic acids, important building-blocks for a biobased industry. High salinity (12–18 g L−1), different reactors configuration operated in batch mode, and different initial pH were tested. In experiment I, a batch stirred reactor (BSR) at atmospheric pressure and a granular sludge bed column (GSBC) were tested with an initial pH of 5. In the end of the experiment, the acidification yield (ηa) was similar in both reactors (22–24%, w/w); nevertheless, lactic acid was in lower concentrations in BSR (6.3 g L−1 in COD), when compared to GSBC (8.0 g L−1 in COD), and valeric was the dominant acid, reaching 17.3% (w/w) in the BSR. In experiment II, the BSR and a pressurized batch stirred reactor (PBSR, operated at 6 bar) were tested with initial pH 7. The ηa and the VFA concentration were higher in the BSR (46%, 22.8 g L−1 in COD) than in the PBSR (41%, 20.3 g/L in COD), and longer chain acids were more predominant in BSR (24.4% butyric, 6.7% valeric, and 6.2% caproic acids) than in PBSR (23.2%, 6.2%, and 4.2%, respectively). The results show that initial pH of 7 allows achieving higher ηa, and the BSR presents the most suitable reactor among tested configurations to produce VFA from wastes/wastewaters with high salinity.

Strategies for the valorisation of a protein-rich saline waste stream into polyhydroxyalkanoates (PHA)

Saline Mussels Cooking Wastewater was valorised to produce PHA with Mixed Microbial Cultures (MMC). Due to the high protein content (1.8-5.7 g COD_PROT/L), PHA accumulating capacity was below 10%, so several strategies were tested. In the acidification unit, Na(HCO₃) was added, increasing protein conversion into Volatile Fatty Acids (VFA) from 10.3% to 69.2% and subsequent PHA accumulation from 6.9 to 14.7%. In the enrichment unit, the incorporation of a settling stage after the feast phase provoked a shift in the proteins' oxidation from the feast to the famine phase, where the nitrogen released in the famine is used by the MMC for growth. This increased the biomass concentration and the tolerated COD (from 1.6 to 4.2 g VSS/L and from 2.2 to 4.38 g COD/L). Finally, varying the proteins/VFA ratio for MMC acclimation to proteins allowed increasing PHA accumulation from 8.8 to 41.5%.

Effect of different vegetable wastes on the performance of volatile fatty acids production by anaerobic fermentation

Volatile fatty acids (VFAs) are intermediates of anaerobic fermentation with high value and wide range of usage. VFA production from vegetable wastes (VW) is an effective way to dispose of wastes and recover resources. The organic matter composition of the substrate influences VFA yield and distribution, which is related to the separation and purification of the downstream steps and the application of the product. Hence, potato peels, carrots, celery, and Chinese cabbage were selected to investigate the effect of VW types on the performance of the VFA production in a batch anaerobic fermentation reactor with continuous stirring at 37 °C, total solid (TS) of 4.5%. A VFA yield of 452 mg COD/g VS_feed (chemical oxygen demand (COD); volatile solids (VS)) was achieved from potato peels, which was 40.1%, 21.5%, and 124.9% higher than that of carrots, celery, and Chinese cabbage, respectively. The rapid acidification of carrots caused a sharp decline in pH and led to inhibition of VFA production. The acidification of celery started slowly, and the yield of hexanoic acid increased rapidly in the later stage of fermentation. The VFA yield of Chinese cabbage was inhibited due to the low initial pH, but the ethanol concentration reached 7577.04 mg COD/L. According to the VFA profile, the fermentation of potato peels, carrots, celery, and Chinese cabbage can be classified as propionate-type, butyrate-type, mixed-acid type, and ethanol-acetate type metabolic pathway, respectively. The results of this study suggest that a suitable combination of vegetable waste types is important for selective VFA production.

Recovery of Polyhydroxyalkanoates from Cooked Mussel Processing Wastewater at High Salinity and Acidic Conditions

Polyhydroxyalkanoates (PHA) are biodegradable polymers that can be intracellularly produced by microorganisms valorizing organic-rich wastes. In the present study, a PHA production system was fed with mussel cooker wastewater after acidogenic fermentation. Besides low pH (4.0 ± 0.3) and high salt (21.7 ± 2.9 g NaCl/L) concentrations, this wastewater also contained nitrogen concentrations (0.8 ± 0.1 g N/L), which were previously reported to be a challenge to the PHA accumulating bacteria enrichment. Bacteria with a PHA storage capacity were selected in an enrichment sequencing batch reactor (SBR) after 60 days of operation. The enriched mixed microbial culture (MMC) was mainly formed by microorganisms from phylum Bacteroidetes, and genera Azoarcus, Comamonas and Thauera from phylum Proteobacteria. The MMC was able to accumulate up to 25 wt% of PHA that was mainly limited by the wastewater nitrogen content, which promoted biomass growth instead of PHA accumulation. Indeed, when the presence of nutrient was limited, PHA stored in the accumulation reactor increased to up to 40.9 wt%. This work demonstrated the feasibility of the enrichment of a MMC with a PHA storage ability valorizing the fish-canning industrial wastewater at low pH, which is generally difficult to treat in wastewater treatment plants.

Environmental assessment of complex wastewater valorisation by polyhydroxyalkanoates production

Polyhydroxyalkanoates (PHA) are biodegradable polymers with renewable origin that are expected to substitute conventional petrochemical plastics. However, before they are commercialized, life-cycle environmental validation is needed, to prove that there is an actual benefit with the replacement of non-renewable plastics with PHA. Nowadays, environmental evaluations assessing bioplastics production at full-scale are scarce due to the lack of data, so experimental results were used to evaluate the feasibility of PHA production employing high load wastewater. A three-stage PHA production system utilising a mixed microbial culture (MMC) was successfully operated for two years employing complex wastewater from a fish-canning industry. The results obtained were scaled-up to define and compare a circular economy scenario performance, with PHA production, with the current linear approach (i.e. effluent generation, treatment and discharge). To the best of our knowledge, this is the first time that the environmental performance of a MMC-based full-scale PHA production system using saline wastewater is evaluated. Results show an average improvement of ca. 25% for nine out of ten studied categories if the circular economy approach is implemented. The sludge management strategy was a key factor for the environmental validation of the process, and if composting is applied instead of anaerobic digestion, the improvement is reported in eight categories. When a more conservative replacement yield of fossil-based plastic was tested, the circular economy approach was the preferable option in 8 out of 10 categories. The significance of the downstream process was also confirmed by this study, although it was not a barrier to show the feasibility of producing added-value bioproducts under a circular economy approach. Finally, this work proposes new process integration strategies to reduce the environmental burdens of PHA production and increase the body of knowledge on MMC-based processes, an area where LCA case studies are still scarce.