Influence of operating conditions for volatile fatty acids enrichment as a first step for polyhydroxyalkanoate production on a municipal waste water treatment plant

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.

Implementing polyhydroxyalkanoates production to anaerobic digestion of organic fraction of municipal solid waste to diversify products and increase total energy recovery

A simple biorefinery aimed at producing both biomethane (CH₄) and polyhydroxyalkanoates (PHAs), was proposed to valorize the organic fraction of municipal solid waste (OFMSW). Anaerobic digestion (AD) was tested at different organic loading rates (OLR-I-II-III) (i.e. 3, 4 and 6 g L^-1 d^-1, respectively), producing biomethane and volatile fatty acids (VFAs)-rich digestate, the VFAs were then used to produce PHAs. Specific biogas and CH₄ production remained similar when adopting different OLRs (biogas of 522-600 NL kg^-1 VS and CH₄ of 64-67% v/v). VFAs concentrated with OLR increases and their patterns were modified. PHA production was in the range of 117-199 g kg^-1 OFMSW_TS with the lowest production being associated to different polymer composition. The net energy recovery of this simple biorefinery accounted for 64% of OFMSW energy content, and the PHAs produced represented over 30% of the total energy.

Application of chemo thermal coupled sonic homogenization of marine macroalgal biomass for energy efficient volatile fatty acid recovery

The present study aimed to employ energy efficient chemo thermal coupled sonic homogenization (CTSH) to obtain VFA from marine macroalgal hydrolysate, (Ulva fasciata). At first, chemo thermal homogenization (CTH) was applied on macroalgal biomass by adjusting the temperature, pH and treatment time from 60 to 90 ℃, 4-7 and 0-60 min, respectively. A higher organic matter solubilisation of 11.81% was obtained at an optimum pH of 6 at a temperature of 80 ℃ with 40 min of homogenization time. The results of CTSH implied that a higher organic matter solubilization of 26.4% was achieved by combined CTSH (sonic power & treatment time - 140 W & 14 min treatment time). CTSH considerably doubles the liquefaction in comparison with CTH. Based on OMS grouping, achieving 25% was sufficient for VFA production (2172.09 mg/L) and considered as economically feasible with net cost of 97.17 USD/ton of macroalgae.

Volatile fatty acid production from semi-synthetic milk processing wastewater under alkali pH: The pearls and pitfalls of microbial culture

Volatile fatty acids (VFA) are one of the most promising sustainable and environmentally friendly bioproduct owing to their wide usage area and high market demand. For this reason, in this study, the evaluation of VFA production from pure and mixed bacterial cultures was aimed. Three different mixed cultures with C. aceticum, C. butyricum and P. acidipropionici as pure cultures were used for inoculation of milk processing wastewater fermentation under pH 10 for 15 days. The mixed culture fermentation had the highest VFA production efficiency whereas the highest amount of acetic, butyric and propionic acid productions were obtained by C. aceticum, C. butyricum and P. acidipropionici, respectively. Also, the mixed cultures demonstrated faster pH regulation and acclimation than the pure cultures tested. Therefore, development of synthetic cultures may offer a useful approach to produce VFA mixtures with one-dominant acid type and with high production efficiency.

Volatile fatty acids production via mixed culture fermentation: Revealing the link between pH, inoculum type and bacterial composition

The aim of the study was to investigate the effects of operational parameters, inoculum type and bacterial community on mixed culture fermentation to produce one dominant acid type in the mixture of volatile fatty acids (VFA). The study was performed using three different inocula (large&small granular and slurry) with glucose under various initial pH. The VFA production efficiency reached to 0,97 (gCOD/gSCOD) by granular sludge. VFA composition was changed by initial pH: in neutral conditions, acetic acid; in acidic conditions, acetic and butyric acids, in alkali conditions butyric acid were dominated, respectively. The VFA production was positively affected by the high relative abundance of Firmicutes. On the contrary, a negative correlation was seen between VFA production and the relative abundance of Chloroflexi. The results revealed the physical sludge structure of inoculum was the key factor for production efficiency, whereas, pH was the most important parameter to affect VFA composition.

Effect of basic oxygen furnace slag addition on enhanced alkaline sludge fermentation and simultaneous phosphate removal

This study presents a promising approach that enhances the sludge fermentation by using basic oxygen furnace (BOF) slag as an alkaline source for the first time. BOF slag added to the reactors could maintain a stable alkaline condition due to continuous release of Ca(OH)₂ from slag. The reactor pH could be adjusted to a target value by the choice of the BOF slag dose. Concentrations of soluble chemical oxygen demand (sCOD) and short-chain carboxylates (SCCs) were substantially increased in the presence of BOF slag. At a BOF slag mass to sludge volume ratio of 1/10 g slag/L sludge, the reactor pH was maintained at 10 and the concentration of SCCs produced was the highest (i.e., 3510 mg COD L^-1 from 14,000 mg VS L^-1 of sludge mixture), followed by B/S ratios of 1/20, 1.50, 1/5, and 1/2.5 g slag L^-1 sludge with reactor pH of 9.4, 8.9, 10.5, and 11, respectively. Our data suggest that the pH value that best facilitates the degradation of sludge into SCCs and inhibit the conversion of SCCs into biogas is around 10. Interestingly, compositions of the accumulated SCCs varied greatly depending on the BOF slag dose. BOF slag showed phosphorus removal ability due to enhanced precipitation of Ca-PO₄^3--P complexes, which significantly lowered PO₄^3- concentration of the reactor effluent.

Determination of Microbial Maintenance in Acetogenesis and Methanogenesis by Experimental and Modeling Techniques

For biogas-producing continuous stirred tank reactors, an increase in dilution rate increases the methane production rate as long as substrate input can be converted fully. However, higher dilution rates necessitate higher specific microbial growth rates, which are assumed to have a strong impact on the apparent microbial biomass yield due to cellular maintenance. To test this, we operated two reactors at 37°C in parallel at dilution rates of 0.18 and 0.07 days-1 (hydraulic retention times of 5.5 and 14 days, doubling times of 3.9 and 9.9 days in steady state) with identical inoculum and a mixture of volatile fatty acids as sole carbon sources. We evaluated the performance of the Anaerobic Digestion Model No. 1 (ADM1), a thermodynamic black box approach (TBA), and dynamic flux balance analysis (dFBA), to describe the experimental observations. All models overestimated the impact of dilution rate on the apparent microbial biomass yield when using default parameter values. Based on our analysis, a maintenance coefficient value below 0.2 kJ per carbon mole of microbial biomass per hour should be used for the TBA, corresponding to 0.12 mmol ATP per gram dry weight per hour for dFBA, which strongly deviates from the value of 9.8 kJ Cmol h-1 that has been suggested to apply to all anaerobic microorganisms at 37°C. We hypothesized that a decrease in dilution rate might select taxa with minimized maintenance expenditure. However, no major differences in the dominating taxa between the reactors were observed based on amplicon sequencing of 16S rRNA genes and terminal restriction fragment length polymorphism analysis of mcrA genes. Surprisingly, Methanosaeta dominated over Methanosarcina even at a dilution rate of 0.18 days-1, which contradicts previous model expectations. Furthermore, only 23-49% of the bacterial reads could be assigned to known syntrophic fatty acid oxidizers, indicating that unknown members of this functional group remain to be discovered. In conclusion, microbial maintenance was found to be much lower for acetogenesis and methanogenesis than previously assumed, likely due to the exceptionally low growth rates in anaerobic digestion. This finding might also be relevant for other microbial systems operating at similarly low growth rates.

From sewage sludge and agri-food waste to VFA: Individual acid production potential and up-scaling

Volatile fatty acid (VFA) production through anaerobic fermentation may constitute an innovative solution for organic waste management within the context of circular economy. In the present study, the evolution of individual VFA during laboratory-scale fermentation of sewage sludge (SS), winery wastewater (W_ww) and meat and bone meal (MBM) was assessed, focusing on the effect of pH (5.5 and 10) and temperature (35 and 55 °C). Up-scaling of the fermentation process was evaluated in batch operation. The latter showed that specific VFA could be produced, giving similar individual evolution to lab-scale testing. To be precise, acetic acid percentage ranged within 30-65% and increased up to 5900 mg O₂ L^-1 during SS fermentation at 55 °C and pH 9. In addition, 60% butyric acid was reached during W_ww acid fermentation at 55 °C, which corresponded to 6670 mg O₂ L^-1 concentration in the fermentation broth. Regarding valeric acid, over 20% proportion and 2700 mg O₂ L^-1 were reached in MBM acid fermentation at 35 °C. Finally, iso-valeric maximum level ranged within 15-17% in SS alkaline fermentation at 55 °C, which represented a concentration close to 2000 mg O₂ L^-1. Interestingly, co-fermentation of agri-food waste and SS at thermophilic temperature and alkaline pH, boosted the VFA concentration 1.7-2 fold, which suggests that anaerobic co-fermentation of substrates from different nature could give promising outcomes in full-scale operation.

Selective VFA production potential from organic waste streams: Assessing temperature and pH influence

This study explored the volatile fatty acid (VFA) production potential of seven waste streams from urban and agroindustrial sources. For that purpose, batch assays were performed under acidic (pH 5.5) and alkaline (pH 10) conditions at both mesophilic (35°C) and thermophilic (55°C) temperature. Overall, the VFA yield was influenced by temperature, and it was positively affected by pH, ranging between 220 and 677mgCODg^-1COD_fed for liquid waste streams and between 127 and 611mgCODg^-1COD_fed for solid waste streams and urban sludge. The highest VFA concentration and highest VFA/sCOD ratio was obtained during the organic fraction of municipal solid waste (OFMSW) fermentation, with 8,320mgCODL^-1 and 94% at alkaline pH and mesophilic temperature. The results of this study suggest that selective VFA production, i.e. via propionic, butyric and acetic acid production, might be feasible for scaling-up purposes with specific waste streams by adjusting the process parameters.

Polyhydroxyalkanoate Production on Waste Water Treatment Plants: Process Scheme, Operating Conditions and Potential Analysis for German and European Municipal Waste Water Treatment Plants

This work describes the production of polyhydroxyalkanoates (PHA) as a side stream process on a municipal waste water treatment plant (WWTP) and a subsequent analysis of the production potential in Germany and the European Union (EU). Therefore, tests with different types of sludge from a WWTP were investigated regarding their volatile fatty acids (VFA) production-potential. Afterwards, primary sludge was used as substrate to test a series of operating conditions (temperature, pH, retention time (RT) and withdrawal (WD)) in order to find suitable settings for a high and stable VFA production. In a second step, various tests regarding a high PHA production and stable PHA composition to determine the influence of substrate concentration, temperature, pH and cycle time of an installed feast/famine-regime were conducted. Experiments with a semi-continuous reactor operation showed that a short RT of 4 days and a small WD of 25% at pH = 6 and around 30 °C is preferable for a high VFA production rate (PR) of 1913 mg_VFA/(L×d) and a stable VFA composition. A high PHA production up to 28.4% of cell dry weight (CDW) was reached at lower substrate concentration, 20 °C, neutral pH-value and a 24 h cycle time. A final step a potential analysis, based on the results and detailed data from German waste water treatment plants, showed that the theoretically possible production of biopolymers in Germany amounts to more than 19% of the 2016 worldwide biopolymer production. In addition, a profound estimation regarding the EU showed that in theory about 120% of the worldwide biopolymer production (in 2016) could be produced on European waste water treatment plants.

Potential for polyhydroxyalkanoate production on German or European municipal waste water treatment plants

Biopolymers, which are made of renewable raw materials and/or biodegradable residual materials present a possible alternative to common plastic. A potential analysis, based on experimental results in laboratory scale and detailed data from German waste water treatment plants, showed that the theoretically possible production of biopolymers in Germany amounts to more than 20% of the 2015 worldwide biopolymer production. In addition a profound estimation regarding all European Union member states showed that theoretically about 115% of the actual worldwide biopolymer production could be produced on European waste water treatment plants. With an upgraded biopolymer production and a theoretically reachable biopolymer proportion of around 60% of the cell dry weight a total of 1,794,656tPHAa or approximately 236% of today's biopolymer production could be produced on waste water treatment plants in the European Union, using primary sludge as raw material only.

Integrated production of polyhydroxyalkanoates (PHAs) with municipal wastewater and sludge treatment at pilot scale

A pilot-scale process was operated over 22 months at the Brussels North Wastewater Treatment Plant (WWTP) in order to evaluate polyhydroxyalkanoate (PHA) production integration with services of municipal wastewater and sludge management. Activated sludge was produced with PHA accumulation potential (PAP) by applying feast-famine selection while treating the readily biodegradable COD from influent wastewater (average removals of 70% COD, 60% CODsol, 24% nitrogen, and 46% phosphorus). The biomass PAP was evaluated to be in excess of 0.4gPHA/gVSS. Batch fermentation of full-scale WWTP sludge at selected temperatures (35, 42 and 55 °C) produced centrate (6-9.4 gCODVFA/L) of consistent VFA composition, with optimal fermentation performance at 42 °C. Centrate was used to accumulate PHA up to 0.39 gPHA/gVSS. The centrate nutrients are a challenge to the accumulation process but producing a biomass with 0.5 gPHA/gVSS is considered to be realistically achievable within the typically available carbon flows at municipal waste management facilities.

Polyhydroxyalkanoate production as a side stream process on a municipal waste water treatment plant

This work describes the production of polyhydroxyalkanoates (PHAs) as a side stream process on a municipal waste water treatment plant (WWTP) at different operation conditions. Therefore various tests were conducted regarding a high PHA production and stable PHA composition. Influence of substrate concentration, temperature, pH and cycle time of an installed feast/famine-regime were investigated. The results demonstrated a strong influence of the operating conditions on the PHA production. Lower substrate concentration, 20°C, neutral pH-value and a 24h cycle time are preferable for high PHA production up to 28.4% of cell dry weight (CDW). PHA composition was influenced by cycle time only and a stable PHA composition was reached.