Effects of sludge retention time, carbon and initial biomass concentrations on selection process: From activated sludge to polyhydroxyalkanoate accumulating cultures

Bioaugmentation of Thauera mechernichensis TL1 for enhanced polyhydroxyalkanoate production in mixed microbial consortia for wastewater treatment

Polyhydroxyalkanoate (PHA) is a fully biodegradable bioplastic. To foster a circular economy, the integration of PHA production into wastewater treatment facilities can be accomplished using mixed microbial consortia. The effectiveness of this approach relies greatly on the enrichment of PHA-accumulating microorganisms. Hence, our study focused on bioaugmenting Thauera mechernichensis TL1 into mixed microbial consortia with the aim of enriching PHA-accumulating microorganisms and enhancing PHA production. Three sequencing batch reactors-SBRctrl, SBR2.5%, and SBR25%-were operated under feast/famine conditions. SBR2.5% and SBR25% were bioaugmented with T. mechernichensis TL1 at 2.5%w/w of mixed liquor volatile suspended solids (MLVSS) and 25%w/w MLVSS, respectively, while SBRctrl was not bioaugmented. SBR2.5% and SBR25% achieved maximum PHA accumulation capacities of 56.3 %gPHA/g mixed liquor suspended solids (MLSS) and 50.2 %gPHA/gMLSS, respectively, which were higher than the 25.4 %gPHA/gMLSS achieved by SBRctrl. The results of quantitative polymerase chain reaction targeting the 16S rRNA gene specific to T. mechernichensis showed higher abundances of T. mechernichensis in SBR2.5% and SBR25% compared with SBRctrl in the 3rd, 17th, and 31st cycles. Fluorescence in situ hybridization, together with fluorescent staining of PHA with Nile blue A, confirmed PHA accumulation in Thauera spp. The study demonstrated that bioaugmentation of T. mechernichensis TL1 at 2.5%w/w MLVSS is an effective strategy to enhance PHA accumulation and facilitate the enrichment of PHA-accumulating microorganisms in mixed microbial consortia. The findings could contribute to the advancement of PHA production from wastewater, enabling the transformation of wastewater treatment plants into water and resource recovery facilities.

Untargeted metabolomics elucidated biosynthesis of polyhydroxyalkanoate by mixed microbial cultures from waste activated sludge under different pH values

Waste activated sludge has been frequently used as mixed substrate to produce polyhydroxyalkanoate (PHA). However, insufficient research on microbial metabolism has led to difficulties in regulating PHA accumulation in mixed microbial cultures (MMCs). To explore the variation of functional genes during domestication and the effect of different pH conditions on metabolic pathways during PHA accumulation, MMCs were domesticated by adding acetate and propionate with aerobic dynamic feeding strategy for 60 days. As the domestication progressed, the microbial community diversity declined and PHA-producing bacteria, Brevundimonas, Dechloromonas and Hyphomonas, were enriched. Through bacterial function prediction by PICRUSt the gene rpoE involved in starvation resistance of bacteria was enriched after the domestication. The pH value of 8.5 was the best condition for PHA accumulation in MMCs, under which a maximum PHA content reached 23.50% and hydroxybutyric (HB)/hydroxyvaleric (HV) reached 2.22. Untargeted metabolomics analysis exhibited that pH conditions of 7 and 8.5 could promote the up-regulation of significant differential metabolites, while higher alkaline conditions caused the inhibition of metabolic activity. Functional annotation showed that pH condition of 8.5 significantly affected Pyrimidine metabolism, resulting in an increase in PHA production. Regarding the pathways of PHA biosynthesis, acetoacetate was found to be significant in the metabolism of hydroxybutyric, and the alkaline condition could restrain the conversion from hydroxybutyric (HB) to the acetoacetate to protect PHB accumulation in MMCs compared with neutral condition. Taken together, the present results can advance the fundamental understanding of metabolic function in PHA accumulation under different pH conditions.

Integrative Genome-Scale Metabolic Modeling Reveals Versatile Metabolic Strategies for Methane Utilization in Methylomicrobium album BG8

Methylomicrobium album BG8 is an aerobic methanotrophic bacterium with promising features as a microbial cell factory for the conversion of methane to value-added chemicals. However, the lack of a genome-scale metabolic model (GEM) of M. album BG8 has hindered the development of systems biology and metabolic engineering of this methanotroph. To fill this gap, a high-quality GEM was constructed to facilitate a system-level understanding of the biochemistry of M. album BG8. Flux balance analysis, constrained with time-series data derived from experiments with various levels of methane, oxygen, and biomass, was used to investigate the metabolic states that promote the production of biomass and the excretion of carbon dioxide, formate, and acetate. The experimental and modeling results indicated that M. album BG8 requires a ratio of ∼1.5:1 between the oxygen- and methane-specific uptake rates for optimal growth. Integrative modeling revealed that at ratios of  >2:1 oxygen-to-methane uptake flux, carbon dioxide and formate were the preferred excreted compounds, while at ratios of <1.5:1 acetate accounted for a larger fraction of the total excreted flux. Our results showed a coupling between biomass production and the excretion of carbon dioxide that was linked to the ratio between the oxygen- and methane-specific uptake rates. In contrast, acetate excretion was experimentally detected during exponential growth only when the initial biomass concentration was increased. A relatively lower growth rate was also observed when acetate was produced in the exponential phase, suggesting a trade-off between biomass and acetate production.

IMPORTANCE A genome-scale metabolic model (GEM) is an integrative platform that enables the incorporation of a wide range of experimental data. It is used to reveal system-level metabolism and, thus, clarify the link between the genotype and phenotype. The lack of a GEM for Methylomicrobium album BG8, an aerobic methane-oxidizing bacterium, has hindered its use in environmental and industrial biotechnology applications. The diverse metabolic states indicated by the GEM developed in this study demonstrate the versatility in the methane metabolic processes used by this strain. The integrative GEM presented here will aid the implementation of the design-build-test-learn paradigm in the metabolic engineering of M. album BG8. This advance will facilitate the development of a robust methane bioconversion platform and help to mitigate methane emissions from environmental systems.

Ecological insight into incompatibility between polymer storage and floc settling in polyhydroxyalkanoate producer selection using complex carbon sources

Polyhydroxyalkanoate (PHA) producer selection is a key step in mixed culture (MC) production. This study focused on incompatibility between PHA storage and floc settling of MCs in the selection process. In a selector using fermented waste activated sludge as substrate under varying organic loading, average maximum PHA content obtained in batch assays increased by ∼ 22 wt% and biomass concentration increased by ∼ 34% with the increasing of organic loading. However, poor floc settling occurred, causing decreased batch PHA production and costly downstream process. A flank community which can corporately use non-VFA organics existed in the selector. When organic loading increased, PHA producers had stronger negative interactions, but not cooperation with the flank community members. Thus, high PHA storage of MCs was bounded to the domination of core PHA producer. But the domination of Thauera bacteria under high organic loading indirectly induced a bloom of filamentous bacteria.

Sludge retention time impacts on polyhydroxyalkanoate productivity in uncoupled storage/growth processes

The process involving mixed microbial cultures (MMCs) and waste-based substrates emerged as an alternative solution to reduce the market price of polyhydroxyalkanoates (PHAs). The selection of an efficient MMC that displays a significant PHA accumulation potential and a high growth rate is considered a key factor for the MMC PHA production feasibility. This study used a pilot plant to investigate the dynamics of growth vs storage in a mixed culture fed with fermented fruit waste under uncoupled carbon and nitrogen feeding. Varying sludge retention times (SRTs) (2 and 4 d) and organic loading rates (OLRs) (from 2.6 to 14.5 gCOD.L^-1.d^-1) were imposed for this purpose. Results showed that, regardless of the OLR imposed, cultures selected at lower SRT grew faster and more efficiently using stored PHA. However, they had inferior specific storage rates and accumulation capacity, resulting in lower PHA productivity. Additionally, the polymer storage yield was independent of the SRT, and was directly linked with the abundance of putative PHA-storers in the MMC. The high PHA productivity (4.6 ± 0.3 g.L^-1.d^-1) obtained for the culture selected at 4 d of SRT was 80% above that obtained for the lower SRT tested, underlining the importance of achieving a good balance between culture growth and accumulation capacity to increase the viability of the PHA-producing process from wastes.

On-Line Control of Feast/Famine Cycles to Improve PHB Accumulation during Cultivation of Mixed Microbial Cultures in Sequential Batch Reactors

Production of polyhydroxyalkanoates (PHA) has generated great interest as building blocks for bioplastic production. Their production using mixed microbial cultures represents an interesting alternative, since it enables the use of organic wastes as a carbon source. Feast/famine strategy is a common way to promote selection of microorganisms with PHA accumulation capacity. However, when using waste sources, changes in substrate concentration are expected, that may affect performance and efficiency of the process. This study showed how the dissolved oxygen level can be used for online control of the cycle time, ensuring that the desired feast/famine ratio is effectively applied. An operation strategy is presented and validated, using sequential batch reactors fed with acetate as the carbon source. Production of polyhydroxybutyrate (PHB) was studied, which is the expected type of PHA to be synthetized when using acetate as substrate. Two reactors were operated by applying the proposed control strategy, to provide F/F ratios of 0.2 and 0.6, respectively. A third reactor was operated with a fixed cycle time, for comparison purposes. Results showed that the reactor that operated at an F/F ratio of 0.6 promoted higher biomass productivity and PHB content, as a result of a better use of available time, preventing unnecessary long famine times. The application of the tested strategy is a simple a reliable way to promote a better performance of feast/famine-based bioreactors involving mixed microbial cultures for PHB production.

Dependence of poly-β-hydroxybutyrate accumulation in sludge on biomass concentration in SBRs

The combination of wastewater treatment with polyhydroxyalkanoate production has attracted increasing interest in the context of the circular economy. Recent studies have thus attempted to optimize the conditions for polyhydroxyalkanoate accumulation in sludge when treating wastewater. The effects of biomass concentration and sludge morphologies in reactors on PHB storage, however, were neglected in the literature. Therefore, in this study settling time and organic loading rate were manipulated to adjust sludge morphology and biomass concentration in sequential batch reactors (SBRs) to investigate their influence on PHB storage in the feast phase. Our study shows that reducing settling times in SBRs from 10 to 0 min under organic loading rate of 3 g L^-1 d^-1 resulted in the decrease in biomass concentration at steady states from 4.2 to 1.0 g L^-1 and the change of sludge morphology from well-settled granules to poorly settled pinpoint flocs, but PHB content in sludge at the end of feast phase increased from 7.7 to 26.7%. The well-fitted regression lines between PHB content, SRT, feast/famine and food/microorganisms ratios and biomass concentration under different settling times suggest that PHB was highly dependent on biomass concentration but independent on sludge morphology. Under settling time of 0 min, the increase in OLR from 3 to 7.5 g L^-1 d^-1 resulted in an increased biomass concentration from 1.0 to 2.1 g L^-1 and an increase in PHB content from 26.7 to 33.8%. The batch and fed-batch experiments with different biomass concentrations also showed the influence of biomass concentration on PHB accumulation in sludge. The conclusion of the dependence of PHB content on biomass concentration under a fixed OLR and varied OLRs drawn from this study enables sludge PHB content as high as possible by adjusting biomass concentration in SBRs apart from the selective enriching strategies for PHB accumulating organisms when treating VFA-rich wastewater.

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.

Effects of chemical oxygen demand concentration, pH and operation cycle on polyhydroxyalkanoates synthesis with waste sludge

To reduce the cost of polyhydroxyalkanoate (PHA) production and dispose the amount of waste sludge simultaneously, chemical oxygen demand (COD) concentration, pH and operation cycle were investigated to find the optimal PHA synthesis conditions with waste sludge in this study. The maximum PHA content (31.3% of the cell dry weight (CDW)), as well as the highest PHA conversion rate (0.30 mg COD/mg COD) and PHA-specific synthesis rate (6.12 mg COD/mg CDW·h), was achieved with initial COD concentration, pH value and operation cycle: 6000 mg/L, 8.5 and 24 h. In order to further investigate the process of PHA synthesis, COD removal rate and CDW were also introduced. This study could provide valuable information for increasing the production of PHA with waste sludge.

Influence of inoculum variation and nutrient availability on polyhydroxybutyrate production from activated sludge

Carbon recovery through polyhydroxybutyrate (PHB) production can create a value-added waste management system. Activated sludge as inoculum enables PHB production using cheap and renewable carbons source, bringing PHB at par to conventional plastics. The PHB accumulating potential of activated sludge needs to be improved to realize the objective. The interaction between the origin of activated sludge, petroleum refinery sludge and brewery sludge, and nitrogen availability was explored to effect culture enrichment, improve PHB accumulation, and polymer characteristics through aerobic dynamic feeding. Consequently, nitrogen excess and limitation enrichment of both sludges produced mix microbial culture with adequate PHB storage of 7.8 ± 0.05%, 14.4 ± 0.04%, 14.4 ± 0.04%, 13.4 ± 0.02% respectively. Batch accumulation revealed higher PHB accumulation of 76.1 ± 0.03% and 71.7 ± 0.05% under nitrogen limitation for PRS and BS enriched under nitrogen excess condition compared to any other combination. The higher decomposition temperature of 285 °C, 293 °C, and a lower melting point of 168 °C, 165 °C with a higher molecular weight of 4.3x10⁵g/mol and semi-crystalline arrangement indicates the potential applications for extracted PHB. PHB production enhanced under nitrogen limited conditions with culture enriched under nitrogen excess condition. However, similar PHB storage, physiochemical property, and overlapping microbial community show an insignificant effect of sludge origin on PHB production.

Poly(3-hydroxybutyrate-3-hydroxyvalerate) production from pretreated waste lignocellulosic hydrolysates and acetate co-substrate

The purpose of this study was to explore the potential of producing Poly(3-hydroxybutyrate-3-hydroxyvalerate) (PHBV) by mixed microbial culture (MMC) with lignocellulosic hydrolysates and acetate co-substrate as feedstock. The addition of co-substrate acetate led to the introduction of HV monomer into the polyhydroxyalkanoate (PHA), and the initial mixed sludge suspension (MLSS) increased with the increase of acetate. Almost 1.91-fold increase in the yield of PHA was achieved with limited nitrogen medium (the carbon to nitrogen ratio (C/N) was 33) compared to the normal nitrogen medium (C/N = 20). Limiting nitrogen source and micro alkaline culture environment was more conducive to the accumulation of PHBV. PHA production achieved to the highest value of about 2308.45 mg/L under the condition of optimized technology. Acidovorax was the dominant genus of all bioreactors using co-substrate. Further, utilizing lignocellulosic hydrolysate and acetate co-substrate as feedstock in mixed microbial culture was a promising approach in a low-cost large-scale PHA production.

Recent developments in Polyhydroxyalkanoates (PHAs) production - A review

Polyhydroxyalkanoates (PHAs) are inevitably a key biopolymer that has the potential to replace the conventional petrochemical based plastics that pose jeopardy to the environment globally. Even then the reach of PHA in the common market is so restricted. The economy of PHA is such that, even after several attempts the overall production cost seems to be high and this very factor surpasses PHAs usage when compared to the conventional polymers. The major focus of the review relies on the synthesis of PHA from Mixed Microbial Cultures (MMCs), through a 3-stage process most probably utilizing feedstocks from waste streams or models that mimic them. Emphasis was given to the works carried out in the past decade and their coherence with each and every individual criteria (Aeration, Substrate and bioprocess parameters) such that to understand their effect in enhancing the overall production of PHA.

Production of polyhydroxyalkanoates and enrichment of associated microbes in bioreactors fed with rice winery wastewater at various organic loading rates

This study aimed to explore the production of polyhydroxyalkanoates (PHA) and selection of PHA-accumulating microorganisms in bioreactors fed with rice winery wastewater at various organic loading rates (OLRs). The substrate utilization, sludge properties, PHA synthesis and microbial community structure of three sequencing batch reactors were monitored. The results show the highest PHA yield (0.23 g/g) was achieved in one of the three reactors with an OLR of 2.4 g COD/L/d, in which Zoogloea was the most dominant PHA-accumulating microorganism. To quantify the PHA production and track the population changing profiles of the PHA-accumulating microorganisms in the long-term reactor operation, the Activated Sludge Model No. 3 was modified with two different heterotrophic microorganisms responding differently with the same substrate. The modeling results indicate that a moderate OLR (>2.4 gCOD/L/d) was beneficial for PHA production. The results are useful for understanding the PHA production from industrial wastewaters and selection of PHA-accumulating microorganisms.

Improving PHA production in a SBR of coupling PHA-storing microorganism enrichment and PHA accumulation by feed-on-demand control

With volatile fatty acids as substrates, the typical polyhydroxyalkanoates (PHAs) production by mixed culture always includes two steps: PHA-storing culture enrichment via aerobic dynamic feeding strategy and PHA accumulation under nutrient-limited condition. To simplify the PHA-production steps, the enrichment and accumulation step were coupled in a SBR. At start-up period, to investigate the effect of settling selection, one acetate-fed SBR was operated by settling selection-double growth limitation (SS-DGL) strategy, while the other was operated by DGL strategy. The results showed that the stable operation in SBR1 was obtained at about 21, 12 days faster than SBR2, implying the settling selection accelerated the start-up process. After omitting the settling selection under the stable operation, the SBR1 was run above 15 days. The results showed that the performance was not substantial altered. Therefore, the settling selection affected the start-up process but not the stable operation. At operational period, based on the sharp decreasing of oxygen uptake rate (OUR), the poly-β-hydroxybutyrate (PHB) content was improved 13%, from 70 to 83% by feed-on-demand control-double growth limitation (FD-DGL). And the harvested volumetric productivity was 5.0 gPHB/L/day, almost 1-folder improvement. That was to say, the PHB production in a SBR of coupling the enrichment and accumulation step was improved by feed-on-demand control. Meanwhile, the FD experiment can keep steady running for 10 SRTs. Therefore, the SS-DGL/FD-DGL strategy was a promising method for PHA production.

Insights into Feast-Famine polyhydroxyalkanoate (PHA)-producer selection: Microbial community succession, relationships with system function and underlying driving forces

The Feast-Famine (FF) process has been frequently used to select polyhydroxyalkanoate (PHA)-accumulating mixed cultures (MCs), but there has been little insight into the ecophysiology of the microbial community during the selection process. In three FF systems with well-defined conditions, synchronized variations in higher-order properties of MCs and complicate microbial community succession mainly including enrichment and elimination of non-top competitors and unexpected turnover of top competitors, were observed. Quantification of PHA-accumulating function genes (phaC) revealed that the top competitors maintained the PHA synthesis by playing consecutive roles when the highly dynamic turnover occurred. Due to its specific physiological characteristics during the PHA-accumulating process, Thauera strain OTU 7 was found to be responsible for the fluctuating SVI, which threatened the robustness of the FF system. This trait was also responsible for its later competitive exclusion by the other PHA-producer, Paracoccus strain OTU 1. Deterministic processes dominated the entire FF system, resulting in the inevitable microbial community succession in the acclimation phase and maintenance of the stable PHA-accumulating function in the maturation phase. However, neutral processes, likely caused by predation from bacterial phages, also occurred, which led to the unpredictable temporal dynamics of the top competitors.

Overall process of using a valerate-dominant sludge hydrolysate to produce high-quality polyhydroxyalkanoates (PHA) in a mixed culture

The overall process of polyhydroxyalkanoates (PHA) production in a mixed culture fed by thermophilic fermented valerate-dominant sludge hydrolysate with high-level soluble organics (proteins and carbohydrates) and nutrients (nitrogen and phosphorus) was investigated in this study. The valerate-dominant hydrolysate was fed to enrich a PHA culture with an increasing concentration, and the enriched culture displayed a strong PHA-producing capacity under feast-famine conditions. Valerate in the feedstock was preferentially utilized over acetate and butyrate, and its uptake correlated with the production of 3-hydroxyvalerate (3HV) and 3-hydroxy-2-methylvalerate (3H2MV). The maximum PHA content (42.31%) was highest to date in a mixed culture with complex feedstock, and the PHA consisted of 3-hydroxybutyrate (3HB), 3HV, 3H2MV at 68.4, 23.7, 7.9 mmol C%. PHA production was inhibited when the nutrients exceeded a certain limit. Microbial analysis revealed that valerate-dominant feedstock caused Delftia (53%) to become the prevailing group over other PHA-producing bacteria. For long-term operation, 75% of the biomass at the end of feast phase was collected for PHA recovery, and the entire process exhibited a potential to produce 5 g PHA from 1 kg sludge. These findings indicate that the complex valerate-dominant sludge hydrolysate can be used to stably produce PHA containing high 3HV and 3H2MV.