Sieving of municipal wastewater and recovery of bio-based volatile fatty acids at pilot scale

Tannery sludge valorization through zeolite-assisted anaerobic process for short-chain fatty acids (SCFAs) production

Tannery sludge, a challenging waste, was utilized as a substrate for the production of Short-Chain Fatty Acids (SCFAs) through a series of six thermophilic Continuous Stirred-Tank Reactor runs. The sludge was subjected to a mild thermal pre-treatment and incorporated zeolites (chabazite in run II, and clinoptilolite in run III) in the acidification process. Results highlighted zeolites' impact on chromium concentration and the SCFAs/CODSOL ratio. Ammonia release remained consistent at around 47 % and 51 % for run I and II, respectively, but surpassed 60% in run III, suggesting limited zeolite effectiveness in NH4 absorption. Chromium release in the liquid fraction, due to thermal pretreatment, reached 335 mg/L. While in tests without zeolite, complete removal proved challenging, in zeolite-amended runs, complete removal was achieved, showcasing the materials' heavy metal absorption capacity. SCFA concentrations reached 20260 mgCOD/L, with acidification efficiency varying; runs I and III had ratios around 0.70 COD/COD, while run II showed substantial improvement (0.92) with chabazite. Anaerobic fermentation-digestion mass balance indicated a 41% reduction in landfill sludge mass, reducing its environmental footprint while yielding valuable byproducts like biogas and SCFAs. These findings underscore zeolites' potential in heavy metal absorption and acidification process enhancement, paving the way for applications with tannery sludge.

Valorization of organic carbon in primary sludge via semi-continuous dark fermentation: First step to establish a wastewater biorefinery

In this study, lab-scale, bench-scale, and pilot-scale experiments were carried out to optimize short-chain fatty acids production from primary sludge. Batch tests showed the requirement of short retention times and semi-continuous operation mode showed a plateau of maximum daily productivity at 36-hours hydraulic retention time with minimal methanation. Optimization from pH 5 to pH 10 at 36 h-hydraulic retention time under long-term semi-continuous operating mode revealed that production of short-chain fatty acids was pH dependent and highest yields could be achieved at pH 7 by establishing optimum redox conditions for fermentation. Pilot-scale experiments at 32 °C showed that daily productivity (3.1 g∙Lreactor-1∙dHRT-1) and yields (150 mg∙gVS-1; OLR = 21 gVS∙Lreactor-1∙dHRT-1; pH 7) of short-chain fatty acids could be significantly improved, specifically for acetic and propionic acids. From these results, a robust dark fermentation step for recovery of valuable products from the solids treatment step in a biorefinery can be achieved.

Primary filtration of municipal wastewater with sludge fermentation - Impacts on biological nutrient removal

Primary filtration is a compact pre-treatment process for municipal wastewater, which can lead to high removal of total suspended solids (TSS) if polymer is added prior to filtration. Extensive carbon removal with rotating belt filter (RBF) can be combined with filter primary sludge fermentation at ambient temperature, in order to produce volatile fatty acids (VFAs) as carbon source for biological nutrient removal (BNR). This process was implemented at large pilot-scale and operated for more than a year. The results showed that the RBF efficiently removed particles >10 μm, and that the TSS removal had a strong linear correlation to the influent TSS concentration. Fermentation of the sludge at ambient temperature and five days retention time and addition of the fermentate to the wastewater could nearly double the VFA concentration in the wastewater by adding 31 ± 9 mg VFA-COD/L. Meanwhile, an increase of 2 mg/L of ammonium nitrogen, and 0.7 mg /L of phosphate phosphorus would be added to the wastewater with the fermentate. Adding the fermented sludge to the wastewater stream and removing the particles with RBF makes it possible to utilize nearly all the produced VFAs for BNR, and the feasibility of this configuration was shown at pilot-scale. According to simulations of subsequent BNR, the pre-treatment would lead to lower effluent total nitrogen concentrations. Alternatively, the required BNR volume could be reduced by 11-18 %. The estimated total biogas production was similar for pre-treatment with primary settler and RBF with fermentation. RBF without fermentation gave the most favourable energy balance, but did not reach the same low effluent value for total nitrogen as RBF with fermentation.

Nitrite accumulation in activated sludge through cyclic anaerobic exposure with acetate

Achieving nitrite accumulation still remains challenging for efficient short-cut biological nitrogen removal in municipal wastewater treatment. To tackle the problem of insufficient carbon in incoming wastewater for biological nutrient removal, a return activated sludge (RAS) fermentation method has been proposed and demonstrated to enable producing supplemental volatile fatty acids (VFAs) and enhance biological phosphorus removal via sludge cycling between mainstream and a sidestream anaerobic reactor. However, the impacts of long anaerobic exposure with acetate on nitrifying bacteria, known as the aerobic chemoautotrophic microorganisms, remains unexplored. In this study, the activated sludge underwent a cyclic anaerobic treatment with the addition of acetate (Ac), the effects on nitrification rate, abundance and microdiversity of nitrifying communities were comprehensively assessed. Firstly, batch activity tests proved the direct addition of high acetate (above 1000 mg/L) could cause inhibition on the nitrification rate, moreover, the inhibitory effect was stronger on nitrite-oxidizing bacteria (NOB) activity than that of ammonia-oxidizing bacteria (AOB). Then, a sequencing batch reactor (SBR) was applied to test the nitrogen conversion performance for low-strength ammonium wastewater. Nitrite accumulation could be achieved via the cyclic anaerobic exposure with 1000-5000 mg Ac/L. The maximum effluent concentration of nitrite was 40.8 ± 3.5 mg N/L with nitrite accumulation ratio (NAR) of 67.6 ± 3.5%. The decrease in NOB activity (72.7%) was greater than AOB of 42.4%, promoting nitrite accumulation via nitritation process. Furthermore, the cyclic anaerobic exposure with acetate can largely reshape the nitrifying communities. As the dominant AOB and NOB, the abundance of Nitrosomonas and Nitrospira were both decreased with species-level microdiversity in the nitrifying communities. However, the heterotrophic microorganism, Thauera, were found to be highly enriched (from 0 to 17.3%), which may act as the potential nitrite producer as proved by the increased nitrate reduction gene abundance. This study can provide new insights into achieving mainstream nitrite accumulation by involving sidestream RAS fermentation towards efficient wastewater treatment management.

Seasonal variations in acidogenic fermentation of filter primary sludge

Primary treatment of municipal wastewater by rotating belt filtration followed by hydrolysis and acidogenic fermentation of the filter primary sludge (FPS) at ambient temperature was studied at pilot-scale during one year. The seasonal variations of volatile fatty acids (VFAs), nutrient release and soluble COD production as well as microbial community assembly were assessed, leading to novel findings for fermentation at ambient temperature. The reproducibility of VFA production performance was first established by operating the two fermentation reactors under the same conditions, showing similar results regarding VFA production and microbial community structure. One year of operation at 5 d retention time (RT) and 16-29 °C resulted in an average VFA yield of 180±35 mg COD/g VSin and soluble COD yield of 242±40 mg COD/g VSin. The VFA formation was temperature-dependent, with ϴ=1.033±0.005 ( [Formula: see text] . The seasonal variations of the acetic and propionic acid productions were pronounced, whereas the productions of VFAs with longer chains were more stable regardless of temperature. The community structure of the reactor microbiomes was also clearly affected by season and temperature and linked with the production spectrum of VFAs. The ammonium and phosphate releases were stable during the year, leading to a decrease in ratios of soluble COD to NH4+-N and PO43--P during winter. The soluble COD yield was 11% and 27% higher at 5 d RT compared to 3 and 2 d RT respectively, but the corresponding volumetric productivities were lower. The dissimilarities between microbiomes in influent FPS and fermenters were significant even at a short RT of 2 d, and increased with longer RT of 3 and 5 d, primarily caused by selection of bacteria within Bacteroidota in the fermentation reactors.

High-Efficiency Recovery of Acetic Acid from Water Using Electroactive Gas-Stripping Membranes

Recovery of carbon-based resources from waste is a critical need for achieving carbon neutrality and reducing fossil carbon extraction. We demonstrate a new approach for extracting volatile fatty acids (VFAs) using a multifunctional direct heated and pH swing membrane contactor. The membrane is a multilayer laminate composed of a carbon fiber (CF) bound to a hydrophobic membrane and sealed with a layer of polydimethylsiloxane (PDMS); this CF is used as a resistive heater to provide a thermal driving force for PDMS that, while a highly hydrophobic material, is known for its ability to rapidly pass gases, including water vapor. The transport mechanism for gas transport involves the diffusion of molecules through the free volume of the polymer matrix. CF coated with polyaniline (PANI) is used as an anode to induce an acidic pH swing at the interface between the membrane and water, which can protonate the VFA molecule. The innovative multilayer membrane used in this study has successfully demonstrated a highly efficient recovery of VFAs by simultaneously combining pH swing and joule heating. This novel technique has revealed a new concept in the field of VFA recovery, offering promising prospects for further advancements in this area. The energy consumption was 3.37 kWh/kg for acetic acid (AA), and an excellent separation factor of AA/water of 51.55 ± 2.11 was obtained with high AA fluxes of 51.00 ± 0.82 g.m^-2hr^-1. The interfacial electrochemical reactions enable the extraction of VFAs without the need for bulk temperature and pH modification.

Thermal hydrolysis pre-treatment has no positive influence on volatile fatty acids production from sewage sludge

The study compares the potential to produce volatile fatty acids (VFA) from sewage sludge, both raw and thermally pre-treated in two modes of operation. In batch mode, raw sludge at pH 8 obtained the highest maximum VFA yield (0.41 g COD-VFA/g CODfed) whereas pre-treated sludge achieved a lower value (0.27 g COD-VFA/g CODfed). The operation of 5-L continuous reactors showed that thermal hydrolysis pre-treatment (THP) did not have any significant influence on VFA yields, averaging 15.1 % g COD-VFA/g COD with raw sludge and 16.6 % g COD-VFA/g COD with pre-treated one. Microbial community analysis showed that phylum Firmicutes was predominant in both reactors and that the enzymatic profiles involved in VFA production were very similar regardless of the substrate fed.

Physico-chemical characteristics and biodegradability of primary effluent and particulate matter removed by microscreens

In this study, the physical, chemical, and biological characteristics of raw wastewater were compared with the liquid and solid streams generated by a primary clarifier (PC), a rotating belt filter (RBF, 350 μm), and a drum filter (DF, 60 μm) and series (SER) combination of an RBF with a PC or a DF using pilot-scale primary treatment units. The RBF removed about 36% of the influent total suspended solids. The DF and PC yielded an influent total suspended solid removal of 47% to 55% in both individual (parallel) and SER configurations. The size fractionation and chemical characterizations of the liquid fractions indicated a significant change in the wastewater composition in both filter configurations with no variation in the biodegradability of liquid fractions. The solids recovered by RBF had a higher total solids (TS) concentration and a higher volatile solids (VS) content (0.92 g VS/g TS) than that of DF and PC treatments (0.58 to 0.84 g VS/g TS). DF and PC sludge demonstrated a higher biodegradability rate (k₁ ; 0.11 d^-1  < k₁  < 0.20 d^-1 ) than solids recovered by RBF (0.09 d^-1 ). The retained solids in the SER configuration demonstrated a significantly lower theoretical biochemical methane potential than the parallel configuration, likely due to the presence of smaller particles with a significantly higher ratio of particulate chemical oxygen demand over volatile suspended solids (1.86 to 2.40 g chemical oxygen demand/g volatile suspended solids). These results indicated that the physical, chemical, and biological characteristics of liquid and solids from different filter configurations are required to determine design criteria to upgrade or retrofit water resource recovery facilities using an RBF or a DF. PRACTITIONER POINTS: A rotating belt filter (RBF) removed less solids than a drum filter (DF) or a primary clarifier (PC). A series configuration of an RBF with either a DF or PC resulted in an effluent with a lower proportion of slowly biodegradable organic matter than in a parallel configuration. Solids from an RBF, a DF, or a PC had similar theoretical biochemical methane potential.

Single CSTR can be as effective as an SBR in selecting PHA-storing biomass from municipal wastewater-derived feedstock

A key step for the production of polyhydroxyalkanoates (PHAs) from organic waste streams is the selection of a biomass with a high PHA-storage capacity (selection-step), which is usually performed in sequencing batch reactors (SBR). A major advancement would be to perform such selection in continuous reactors to facilitate the full-scale implementation of PHA production from municipal wastewater (MWW)-derived feedstock. The present study therefore investigates to what extent a simple continuous-flow stirred-tank reactor (CSTR) represents a relevant alternative to anSBR. To this end, we operated two selection reactors (CSTR vs. SBR) on filtered primary sludge fermentate while performing a detailed analysis of the microbial communities, and monitoring PHA-storage over long-term (∼150 days) and during accumulation batches. Our study demonstrates that a simple CSTR is as effective as an SBR in selecting biomass with high PHA-storage capacity (up to 0.65 gPHA gVSS^-1) while being 50% more efficient in terms of substrate to biomass conversion yields. We also show that such selection can occur on VFA-rich feedstock containing nitrogen (N) and phosphorus (P) in excess, whereas previously, selection of PHA-storing organisms in a single CSTR has only been studied under P limitation. We further found that microbial competition was mostly affected by nutrient availability (N and P) rather than by the reactor operation mode (CSTR vs. SBR). Similar microbial communities therefore developed in both selection reactors, while microbial communities were very different depending on N availability. Rhodobacteraceae gen. were most abundant when growth conditions were stable and N-limited, whereas dynamic N- (and P-) excess conditions favoured the selection of the known PHA-storer Comamonas, and led to the highest observed PHA-storage capacity. Overall, we demonstrate that biomass with high storage capacity can be selected in a simple CSTR on a wider range of feedstock than just P-limited ones.

Study on the preparation and feasibility of a novel adding-type biological slow-release carbon source

The development of slow-release carbon sources is an effective biological treatment to remove nutrients from wastewater with low carbon-to-nitrogen ratio (C/N). Most filling-type slow-release carbon could not fulfil the needs of current wastewater treatment plants (WWTPs) process. And most adding-type slow-release carbon sources were prepared using some expensive chemical materials. In this study, combining the advantages of the aforementioned types, a novel adding-type wastepaper-flora (AT-WF) slow-release carbon source was proposed, aiming to realise wastepaper recycling in WWTPs. The screening and identification of the mixed flora, AT-WF carbon source release behaviour, and denitrification performance were investigated. The results showed that through the proposed screening method, a considerable proportion of cellulose-degradation-related genera was enriched, and the cellulose degradation ability and ratio of readily available carbon sources of flora T4, S4 and S5 were effectively strengthened. AT-WF had significant carbon release ability and stability, with an average total organic carbon (TOC) release of 8.82 ± 2.36 mg/g. Kinetic analysis showed that the entire carbon release process was more consistent with the first-order equation. Piecewise fitting with the Ritger-Peppas equation exhibited that the rapid-release (RR) stage was skeleton dissolution and the slow-release (SR) stage was Fick diffusion. Denitrification efficiency can achieve a high average removal efficiency of 94.17%, which could theoretically contribute 11.2% more to the total inorganic nitrogen (TIN) removal. Thus, this study indicated that AT-WF could be utilised as an alternative carbon source in WWTPs.

Influent carbon to phosphorus ratio drives the selection of PHA-storing organisms in a single CSTR

Enriching a biomass with a high fraction of polyhydroxyalkanoate-storing organisms (PHA-storers) represents an essential step in the production of PHAs (bioplastics) from municipal wastewater using mixed microbial cultures. A major challenge is however to create selective growth conditions that are favourable to PHA-storers. Our study thus investigates to what extent the influent COD to phosphorus (COD:P) ratio can be used as a tool for the robust selection of PHA-storers in a single continuous-flow stirred-tank reactor (CSTR). Therefore, we operated five CSTRs in parallel, fed with synthetic wastewater (50% acetate - 50% propionate) with different COD:P ratios (200-1000 gCOD gP^-1), and performed a detailed analysis of the microbial communities over long-term (30-70 solid retention times). Our study demonstrates that efficient and robust selection of PHA-storers can be achieved in a single CSTR at high influent COD:P ratios. The selective advantage for PHA-storers increases with the influent COD:P ratio, but only if growth conditions remain limited by both C-substrate and P. In contrast, selection performance deteriorates when COD:P ratios are too high and growth conditions are limited by P only. At an optimal COD:P ratio of 800 gCOD gP^-1, a stable microbial community consisting of >90% PHA-storers and dominated by Pannonibacter sp. was selected in the long-term. Finally, our results suggest that high COD:P ratios provide a selective advantage to microorganisms with low cellular P requirements, explaining why different PHA-storers (i.e., Xanthobacter sp. vs. Pannonibacter sp.) were selected depending on the influent COD:P ratio (i.e., 200 vs. 800 gCOD gP^-1). Overall, our results provide relevant insights for the development of a new approach for selecting PHA-storers, based on the use of a single CSTR and control of the influent COD:P ratio.

Double-stage membrane-assisted anaerobic digestion process intensification for production and recovery of volatile fatty acids from food waste

The potential of organic waste streams (i.e., food waste) for the sustainable production of precursor chemicals such as volatile fatty acids (VFAs) using anaerobic digestion (AD) has received significant attention in the present days. AD-derived VFAs have great market appeal if the challenges with their recovery and purification from the complex AD effluent is overcome. In this study, a microfiltration immersed membrane bioreactor (MBR) was used for the production of VFAs from food waste and simultaneously in-situ recovery of VFAs. The MBR set-up was applied for 98 days, with a maximum yield of 0.2 gVFA/gVS_added at an organic loading rate (OLR) of 4 g VS/L/d. The recovered permeate was then subjected to further purification using a side stream ultrafiltration unit. It was found that the removal rates of total solids (TS), total suspended solids (TSS), dissolved solids (DS), volatile solids (VS) and volatile suspended solids (VSS) were above 70-80% in both membranes (10 kDa and 50 kDa), and Phosphorus (P), Total Kjeldahl Nitrogen (TKN), chemical oxygen demand (COD), and NH₄₊-N were also removed partially. Particularly, VFAs concentration (above 6 g/L) was higher for 10 kDa at pH 5.4 in ultrafiltered solution and permeate flux decline was higher for 10 kDa at pH 5.4. These results are also supported by the measurement of UV-Vis spectra of the solution and visual appearance, providing a promising approach towards building a VFAs-based platform.

Valorisation of residues from municipal wastewater sieving through anaerobic (co-)digestion with biological sludge

The Circular and Green Economy principles is inspiring new approaches to municipal wastewater treatment plants (MWWTPs) design and operation. Recently, an ever-growing interest is devoted to exploring the alternatives for switching the WWTPs from being able to 'simply' removing contaminants from water to biorefinery-like plants where energy and material can be recovered. In this perspective, both wastewater and residues from process can be valorised for recovering nutrients (N and P), producing value added products (i.e. biopolymers), energy vectors and biofuels (i.e. bio-H₂, bio-CH₄ and bioethanol). As an additional benefit, changing the approach for WWTPs design and operation will decrease the overall amount of landfilled residues. In this context, the present research is aimed at evaluating the CH₄ production potential of MWW screening units' residues. While such a stream is typically landfilled, the expected progressive increase of biodegradable matter content due to the ban on single-use plastic along with the boost of bioplastics makes the investigation of different biochemical valorisation routes more and more interesting from an environmental and economical perspective. Thus, a full-scale data collection campaign was performed to gain information on screening residues amount and properties and to analyse the relationship with influent flowrate. The most relevant residue properties were measured, and lab-scale tests were carried out to evaluate the bio-CH₄ potential.

Expanding ASM models towards integrated processes for short-cut nitrogen removal and bioplastic recovery

In next-generation water resource recovery facilities (WRRFs), it is becoming increasingly important to save energy costs and promote resource recovery of valuable products. One way of reducing the substantial aeration energy costs at WRRFs is to employ shortcut N removal, while polyhydroxyalkanoate (PHA) production and recovery as bioplastic is a promising means of recovering a valuable product from biosolids. Both objectives can be achieved simultaneously through the Short-Cut Enhanced Phosphorus and PHA Recovery (SCEPPHAR) process. However, current mathematical models have not previously been employed to describe the behavior of such a process, which limits engineering design and optimisation of process operation. This work focusses on extending the ASM3 model towards the description of short-cut nitrogen removal and simultaneous PHA recovery in a sidestream treatment process. The calibrated and validated model described very well the nitritation process coupled with the aerobic feast/anoxic famine process for the selection of PHA producing organisms at a pilot-scale facility operated in Carbonera, Italy, where the normalised root mean squared error (NRMSE) was consistently <20%. Furthermore, the model applied to the PHA selection stage could effectively describe the PHA accumulation stage without recalibration. A simulation study was performed using the modified ASM3 model to assess the relative benefits of the SCEPPHAR process strategy as compared to the fully aerobic selection process for mixed culture PHA production. While the level of PHA production was found to be 34% lower with SCEPPHAR, a 43% savings in volatile fatty acids (VFAs) demand, a 15% decrease in Total suspended solids (TSS) production and a 28% decrease in oxygen demand were also achievable, which could lead to savings in operational costs. This study facilitates the design and optimisation of WRRFs that integrate short-cut N removal with PHA production, saving aeration energy costs while achieving resource recovery.

Factors influencing pressure-driven membrane-assisted volatile fatty acids recovery and purification-A review

Volatile fatty acids (VFAs) are building block chemicals that can be produced through bioconversion of organic waste streams via anaerobic digestion as intermediate products. Purified VFAs are applicable in a wide range of industrial applications such as food, textiles, cosmetics, pharmaceuticals etc. production. The present review focuses on VFAs recovery methods and technologies such as adsorption, distillation, extraction, gas stripping, esterification and membrane based techniques etc., while presenting a discussion of their pros and cons. Moreover, a great attention has been given to the recovery of VFAs through membrane filtration as a promising sustainable clarification, fractionation and concentration approach. In this regard, a thorough overview of factors affecting membrane filtration performance for VFAs recovery has been presented. Filtration techniques such as nanofiltration and reverse osmosis have shown to be capable of recovering over 90% of VFAs content from organic effluent steams, proving the direct effect of membrane materials/surface chemistry, pore size and solution pH in recovery success level. Overall, this review presents a new insight into challenges and potentials of membrane filtration for VFAs recovery based on the effects of factors such as operational parameters, membrane properties and effluent characteristics.

The capture technology matters: Composition of municipal wastewater solids drives complexity of microbial community structure and volatile fatty acid profile during anaerobic fermentation

The production of volatile fatty acids (VFAs) represents a relevant option to valorize municipal wastewater (MWW). In this context, different capture technologies can be used to recover organic carbon from wastewater in form of solids, while pre-treatment of those solids has the potential to increase VFA production during subsequent fermentation. Our study investigates how VFA composition produced by fermentation is influenced (i) by the choice of the capture technology, as well as (ii) by the use of thermal alkaline pre-treatment (TAP). Therefore, the fermentation of solids originating from a primary settler, a micro-sieve, and a high-rate activated sludge (HRAS) system was investigated in continuous lab-scale fermenters, with and without TAP. Our study demonstrates that the capture technology strongly influences the composition of the produced solids, which in turn drives the complexity of the fermenter's microbial community and ultimately, of the VFA composition. Solids captured with the primary settler or micro-sieve consisted primarily of polysaccharides, and led to the establishment of a microbial community specialized in the degradation of complex carbohydrates. The produced VFA composition was relatively simple, with acetate and propionate accounting for >90% of the VFAs. In contrast, the HRAS system produced biomass-rich solids associated with higher protein contents. The microbial community which then developed in the fermenter was therefore more diversified and capable of converting a wider range of substrates (polysaccharides, proteins, amino acids). Ultimately, the produced VFA composition was more complex, with equal fractions of isoacids and propionate (both ~20%), while acetate remained the dominant acid (~50%). Finally, TAP did not significantly modify the VFA composition while increasing VFA yields on HRAS and sieved material by 35% and 20%, respectively. Overall, we demonstrated that the selection of the technology used to capture organic substrates from MWW governs the composition of the VFA cocktail, ultimately with implications for their further utilization.

Electro-fermentation: Sustainable bioproductions steered by electricity

The market of biobased products obtainable via fermentation processes is steadily increasing over the past few years, driven by the need to create a decarbonized economy. To date, industrial fermentation (IF) employs either pure or mixed microbial cultures (MMC) whereby the type of the microbial catalysts and the used feedstock affect metabolic pathways and, in turn, the type of product(s) generated. In many cases, especially when dealing with MMC, the economic viability of IF is hindered by factors such as the low attained product titer and selectivity, which ultimately challenge the downstream recovery and purification steps. In this context, electro-fermentation (EF) represents an innovative approach, based on the use of a polarized electrode interface to trigger changes in the rate, yield, titer or product distribution deriving from traditional fermentation processes. In principle, the electrode in EF can act as an electron acceptor (i.e., anodic electro-fermentation, AEF) or donor (i.e., cathodic electro-fermentation, CEF), or simply as a mean to control the oxidation-reduction potential of the fermentation broth. However, the molecular and biochemical basis underlying the EF process are still largely unknown. This review paper provides a comprehensive overview of recent literature studies including both AEF and CEF examples with either pure or mixed microbial cultures. A critical analysis of biochemical, microbiological, and engineering aspects which presently hamper the transition of the EF technology from the laboratory to the market is also presented.

Emerging electrochemistry-based process for sludge treatment and resources recovery: A review

The electrochemical process is gaining widespread interest as an emerging alternative for sludge treatment. Its potentials for sludge stabilization and resources recovery have been well proven to date. Despite the high effectiveness of the electrochemical process having been highlighted in several studies, concerns about the electrochemical sludge treatment, including energy consumption, scale-up feasibility, and electrode stability, have not yet been addressed. The present paper critically reviews the versatile uses of the electrochemical processes for sludge treatment and resource recovery, from the fundamentals to the practical applications. Particularly considered are the enhancement of the digestion of the anaerobic sludge and dewaterability, removal of pathogens and heavy metals, and control of sludge malodor. In addition, the opportunities and challenges of the sludge-based resource recovery (i.e., nitrogen, phosphorus, and volatile fatty acids) are discussed. Insights into the working mechanisms (e.g., electroporation, electrokinetics and electrooxidation) of electrochemical processes are reviewed, and perspectives and future research directions are proposed. This work is expected to provide an in-depth understanding and broaden the potential applications of electrochemical processes for sludge treatment and resource recovery.

Sewage sludge digestion beyond biogas: Electrochemical pretreatment for biochemicals

Low economic gains from biogas drive research on shifting to volatile fatty acid (VFA) production during anaerobic sludge digestion. pH control and methanogenesis inhibition are widely used strategies for VFA production via anaerobic digestion of sludge. However, these strategies require perpetual dosing of chemicals, increasing cost and operation complexity. Here, we applied electrochemical pretreatment (EPT) (12 V/30 min) for VFA production during anaerobic sludge digestion. The underlying mechanisms of the VFA production induced by EPT were explored systematically through analyses of the changes in the EPT operation parameters, the sludge characteristics, and the microbial community structure and functional enzymes involving in the subsequent sludge digestion. EPT with carbon-based electrodes selectively inhibited methanogenesis by down-regulating heterodisulfide reductase without affecting enzymatic acidogenesis and hydrolysis, resulting in accumulation of VFAs (up to 389±12 mg acetic acid equivalent/L). Propionate and acetate were, respectively enriched to 89 and 75% of the total VFAs after carbon- and graphite- EPT. Titanium-EPT produced lower levels of VFA; instead, biogas yield increased by ∼20%. We anticipate that EPT will advance VFA recovery from diverse organic wastes to meet the global challenge of resource supply and waste management.

Bioaugmented Mixed Culture by Clostridium aceticum to Manipulate Volatile Fatty Acids Composition From the Fermentation of Cheese Production Wastewater

Production of targeted volatile fatty acid (VFA) composition by fermentation is a promising approach for upstream and post-stream VFA applications. In the current study, the bioaugmented mixed microbial culture by Clostridium aceticum was used to produce an acetic acid dominant VFA mixture. For this purpose, anaerobic sequencing batch reactors (bioaugmented and control) were operated under pH 10 and fed by cheese processing wastewater. The efficiency and stability of the bioaugmentation strategy were monitored using the production and composition of VFA, the quantity of C. aceticum (by qPCR), and bacterial community profile (16S rRNA Illumina Sequencing). The bioaugmented mixed culture significantly increased acetic acid concentration in the VFA mixture (from 1170 ± 18 to 122 ± 9 mgCOD/L) compared to the control reactor. Furthermore, the total VFA production (from 1254 ± 11 to 5493 ± 36 mgCOD/L) was also enhanced. Nevertheless, the bioaugmentation could not shift the propionic acid dominancy in the VFA mixture. The most significant effect of bioaugmentation on the bacterial community profile was seen in the relative abundance of the Thermoanaerobacterales Family III. Incertae sedis, its relative abundance increased simultaneously with the gene copy number of C. aceticum during bioaugmentation. These results suggest that there might be a syntropy between species of Thermoanaerobacterales Family III. Incertae sedis and C. aceticum. The cycle analysis showed that 6 h (instead of 24 h) was adequate retention time to achieve the same acetic acid and total VFA production efficiency. Biobased acetic acid production is widely applicable and economically competitive with petroleum-based production, and this study has the potential to enable a new approach as produced acetic acid dominant VFA can replace external carbon sources for different processes (such as denitrification) in WWTPs. In this way, the higher treatment efficiency for WWTPs can be obtained by recovered substrate from the waste streams that promote a circular economy approach.

Wastewater Valorization: Practice around the World at Pilot- and Full-Scale

Over the last few years, wastewater treatment plants (WWTPs) have been rebranded as water resource recovery facilities (WRRFs), which recognize the resource recovery potential that exists in wastewater streams. WRRFs contribute to a circular economy by not only producing clean water but by recovering valuable resources such as nutrients, energy, and other bio-based materials. To this aim, huge efforts in technological progress have been made to valorize sewage and sewage sludge, transforming them into valuable resources. This review summarizes some of the widely used and effective strategies applied at pilot- and full-scale settings in order to valorize the wastewater treatment process. An overview of the different technologies applied in the water and sludge line is presented, covering a broad range of resources, i.e., water, biomass, energy, nutrients, volatile fatty acids (VFA), polyhydroxyalkanoates (PHA), and exopolymeric substances (EPS). Moreover, guidelines and regulations around the world related to water reuse and resource valorization are reviewed.

Review and perspectives of enhanced volatile fatty acids production from acidogenic fermentation of lignocellulosic biomass wastes

Lignocellulosic biomass wastes are abundant resources that are usually valorized for methane-rich biogas via anaerobic digestion. Conversion of lignocellulose into volatile fatty acids (VFA) rather than biogas is attracting attention due to the higher value-added products that come with VFA utilization. This review consolidated the latest studies associated with characteristics of lignocellulosic biomass, the effects of process parameters during acidogenic fermentation, and the intensification strategies to accumulate more VFA. The differences between anaerobic digestion technology and acidogenic fermentation technology were discussed. Performance-enhancing strategies surveyed included (1) alkaline fermentation; (2) co-digestion and high solid-state fermentation; (3) pretreatments; (4) use of high loading rate and short retention time; (5) integration with electrochemical technology, and (6) adoption of membrane bioreactors. The recommended operations include: mesophilic temperature (thermophilic for high loading rate fermentation), C/N ratio (20-40), OLR (< 12 g volatile solids (VS)/(L·d)), and the maximum HRT (8-12 days), alkaline fermentation, membrane technology or electrodialysis recovery. Lastly, perspectives were put into place based on critical analysis on status of acidogenic fermentation of lignocellulosic biomass wastes for VFA production.

Effects of the Sludge Retention Time and Carbon Source on Polyhydroxyalkanoate-Storing Biomass Selection under Aerobic-Feast and Anoxic-Famine Conditions

Polyhydroxyalkanoates (PHAs) are versatile biodegradable polymers produced by bacteria and are suitable for many downstream applications. They can be produced inexpensively from mixed microbial cultures under feast and famine conditions in the presence of biobased volatile fatty acids (VFAs). Here, we investigated the effect of changing the sludge retention time (SRT) and the addition of fermented cellulosic primary sludge (CPS) as a carbon source on the selection of PHA-storing biomass when applying the feast and famine strategy under aerobic and anoxic conditions, respectively. Increasing the SRT from 5 to 7-10 days enhanced PHA yields under feast conditions from 0.18 gCOD_PHA/gCOD_VFA (period 1) to 0.40 gCOD_PHA/gCOD_VFA (period 2). The use of fermented CPS as a carbon source (period 3) increased PHA yields to 0.62 gCOD_PHA/gCOD_VFA despite the presence of biodegradable non-VFA fractions. Microbial characterization by denaturing gradient gel electrophoresis and fluorescence in situ hybridization revealed high microbial speciation during the three experimental periods. In period 3, the dominant genera were Thauera, Paracoccus, and Azoarcus, which accounted for ∼95% of the total microbial biomass.

Biomass fuel production from cellulosic sludge through biodrying: Aeration strategies, quality of end-products, gaseous emissions and techno-economic assessment

This study assesses the technological, environmental and economic feasibility of biodrying to valorise cellulosic sludge as a renewable energy source. Specifically, three different aeration strategies were compared in terms of biodrying performance, energetic consumption, gaseous emissions, quality of end-products and techno-economic analysis. These strategies were based on different combinations of convective drying with biogenic heat produced. Two innovative biodrying performance indicators (Energetic Biodrying Index and Biodrying Performance Index) were proposed to better assess the initial and operational conditions that favour the maximum energy process efficiency and the highest end-product quality. The end-products obtained consistently presented moisture contents below 40% and lower heating values above 9.4 MJ·kg-1. However, the best values achieved were 32.6% and 10.4 MJ·kg-1 for moisture content and lower heating value, respectively. Low N₂O and CH₄ emissions confirmed the effective aeration of all three strategies carried out, while NH₄ and tVOCs were related either to temperature or biological phenomena. A techno-economic analysis proved the economic viability and attractiveness of the biodrying technology for cellulosic sludge in all the strategies applied.

Hydrophobic Deep Eutectic Solvents for the Recovery of Bio-Based Chemicals: Solid–Liquid Equilibria and Liquid–Liquid Extraction

The solid–liquid equilibrium (SLE) behavior and liquid–liquid extraction (LLX) abilities of deep eutectic solvents (DESs) containing (a) thymol and L-menthol, and (b) trioctylphosphine oxide (TOPO) and L-menthol were evaluated. The distribution coefficients (KD) were determined for the solutes relevant for two biorefinery cases, including formic acid, levulinic acid, furfural, acetic acid, propionic acid, butyric acid, and L-lactic acid. Overall, for both cases, an increasing KD was observed for both DESs for acids increasing in size and thus hydrophobicity. Furfural, being the most hydrophobic, was seen to extract the highest KD (for DES (a) 14.2 ± 2.2 and (b) 4.1 ± 0.3), and the KD of lactic acid was small, independent of the DESs (DES (a) 0.5 ± 0.07 and DES (b) 0.4 ± 0.05). The KD of the acids for the TOPO and L-menthol DES were in similar ranges as for traditional TOPO-containing composite solvents, while for the thymol/L-menthol DES, in the absence of the Lewis base functionality, a smaller KD was observed. The selectivity of formic acid and levulinic acid separation was different for the two DESs investigated because of the acid–base interaction of the phosphine group. The thymol and L-menthol DES was selective towards levulinic acid (Sij = 9.3 ± 0.10, and the TOPO and L-menthol DES was selective towards FA (Sij = 2.1 ± 0.28).

A hybrid dry-fermentation and membrane contactor system: Enhanced volatile fatty acid (VFA) production and recovery from organic solid wastes

Anaerobic dry-fermentation of food wastes can be utilized for the production of volatile fatty acids (VFA). However, especially for high load fermentation systems, accumulation of VFAs may result in inhibition of fermentation process. In this study, separation of VFAs from synthetic mixtures via a vapor permeation membrane contactor (VPMC) system with an air-filled polytetrafluoroethylene (PTFE) membrane was assessed at various temperatures and permeate solution concentrations. In addition, a pioneering integrated leach-bed fermentation and membrane separation system was operated with undefined mixed culture for the purpose of enhanced VFA production along with its recovery. Hybrid system resulted in 42% enhancement in total VFA production and 60% of total VFAs were recovered through the VPMC system. The results of this study revealed that integrated system can be exploited as a means of increasing organic loading to fermentation systems and increasing the value of VFA production.

Characterization of Polyhydroxyalkanoates Produced at Pilot Scale From Different Organic Wastes

Polyhydroxyalkanoates (PHAs) production at pilot scale has been recently investigated and carried out exploiting different process configurations and organic wastes. More in detail, three pilot platforms, in Treviso (North-East of Italy), Carbonera (North-East of Italy) and Lisbon, produced PHAs by open mixed microbial cultures (MMCs) and different organic waste streams: organic fraction of municipal solid waste and sewage sludge (OFMSW-WAS), cellulosic primary sludge (CPS), and fruit waste (FW), respectively. In this context, two stabilization methods have been applied, and compared, for preserving the amount of PHA inside the cells: thermal drying and wet acidification of the biomass at the end of PHA accumulation process. Afterward, polymer has been extracted following an optimized method based on aqueous-phase inorganic reagents. Several PHA samples were then characterized to determine PHA purity, chemical composition, molecular weight, and thermal properties. The polymer contained two types of monomers, namely 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) at a relative percentage of 92.6-79.8 and 7.4-20.2 w/w, respectively, for Treviso and Lisbon plants. On the other hand, an opposite range was found for 3HB and 3HV monomers of PHA from Carbonera, which is 44.0-13.0 and 56.0-87.0 w/w, respectively. PHA extracted from wet-acidified biomass had generally higher viscosity average molecular weights (M v ) (on average 424.8 ± 20.6 and 224.9 ± 21.9 KDa, respectively, for Treviso and Lisbon) while PHA recovered from thermally stabilized dried biomass had a three-fold lower M v .

Microbial biodiesel production from industrial organic wastes by oleaginous microorganisms: Current status and prospects

This review aims to encourage the technical development of microbial biodiesel production from industrial-organic-wastes-derived volatile fatty acids (VFAs). To this end, this article summarizes the current status of several key technical steps during microbial biodiesel production, including (1) acidogenic fermentation of bio-wastes for VFA collection, (2) lipid accumulation in oleaginous microorganisms, (3) microbial lipid extraction, (4) transesterification of microbial lipids into crude biodiesel, and (5) crude biodiesel purification. The emerging membrane-based bioprocesses such as electrodialysis, forward osmosis and membrane distillation, are promising approaches as they could help tackle technical challenges related to the separation and recovery of VFAs from the fermentation broth. The genetic engineering and metabolic engineering approaches could be applied to design microbial species with higher lipid productivity and rapid growth rate for enhanced fatty acids synthesis. The enhanced in situ transesterification technologies aided by microwave, ultrasound and supercritical solvents are also recommended for future research. Technical limitations and cost-effectiveness of microbial biodiesel production from bio-wastes are also discussed, in regard to its potential industrial development. Based on the overview on microbial biodiesel technologies, an integrated biodiesel production line incorporating all the critical technical steps is proposed for unified management and continuous optimization for highly efficient biodiesel production.

Characterization of preconcentrated domestic wastewater toward efficient bioenergy recovery: Applying size fractionation, chemical composition and biomethane potential assay

Domestic wastewater (DWW) can be preconcentrated to facilitate energy recovery via anaerobic digestion (AD), following the concept of "carbon capture-anaerobic conversion-bioenergy utilization." Herein, real DWW and preconcentrated domestic wastewater (PDWW) were both subject to particle size fractionation (0.45-2000 μm). DWW is a type of low-strength wastewater (average COD of 440.26 mg/L), wherein 60% of the COD is attributed to the substances with particle size greater than 0.45 μm. Proteins, polysaccharides, and lipids are the major DWW components. PDWW with a high COD concentration of 2125.89 ± 273.71 mg/L was obtained by the dynamic membrane filtration (DMF) process. PDWW shows larger proportions of settleable and suspended fractions, and accounted for 63.4% and 33.8% of the particle size distribution, and 52.4% and 32.2% of the COD, respectively. The acceptable biomethane potential of 262.52 ± 11.86 mL CH₄/g COD of PDWW indicates bioenergy recovery is feasible based on DWW preconcentration and AD.

Enhanced Acetogenesis of Waste Activated Sludge by Conditioning with Processed Organic Wastes in Co-Fermentation: Kinetics, Performance and Microbial Response

Aimed at the low ratio of carbon and nitrogen (C/N, approximately 7/1) of waste activated sludge (WAS), which would inhibit the acetogenesis process during anaerobic fermentation, this study introduced three brewing wastes, including vinegar (VR), stillage (SR) and soy sauce (SSR) residues, to promote acetogenesis by co-fermenting with WAS. Results showed that different brewing wastes contributed differently to the volatile fatty acids (VFAs) yield. The best performance was observed with SSR (4517 ± 367 mg COD/L), particularly rich in C2–C3 VFAs, corresponding to 40% and 52% higher concentrations than with SR and VR, respectively. Meanwhile, the hydrolysis rate constant peaked at 0.0059 h−1 in the SSR test, compared to the sole WAS test (0.0018 h−1). Furthermore, canonical correlation analysis reflected that the functional consortia, known to ferment saccharides/amino acids into C2–C3 VFAs (i.e., Proteiniclasticum, Petrimonas, Cloacibacillus and Gemmobacter), was related to the characteristics of the feedstock.

Wastewater treatment in 2050: Challenges ahead and future vision in a European context

•Wastewater treatment plants are slowly evolving.•In the next decade, successful implementation of technologies is focused intensification.•New technologies will be focused on gases and bio-based materials.•Academics need more freedom to develop novel concepts.•Collaborative platforms that enable demonstration of high-risk technologies is key.