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.

Polyhydroxy butyrate production by Acinetobacter junii BP25, Aeromonas hydrophila ATCC 7966, and their co-culture using a feast and famine strategy

The study aimed to evaluate biopolymer production using two bacterial strains, Acinetobacter junii BP25 and Aeromonas hydrophila ATCC 7966, and their co-culture. Batch experiments were evaluated using acetate and butyrate as carbon sources in feast and famine strategy. Feast phase was studied using carbon, nitrates and phosphate in the ratio of 100:8:1 and famine phase was limited with the phosphate and nitrates. Co-culture resulted in highest specific growth rate (0.30 h^-1) in the feast phase and the famine phase accounted the maximum polyhydroxybutyrate (PHB) accumulation (2.46 g PHB/L), followed by Acinetobacter junii BP25 (0.25 h^-1 and 1.82 g PHB/L) and Aeromonas hydrophila ATCC 7966 (0.17 h^-1 and 1.12 g PHB/L). Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR) structural analysis confirmed as PHB. PHB production using the co-culture could be integrated with biohydrogen process using volatile fatty acids (VFA) as a carbon source in the biorefinery framework.

High-rate hydrogen production from galactose in an upflow anaerobic sludge blanket reactor (UASBr)

High-rate hydrogen production from galactose and rapid granule formation were achieved in a mesophilic (37 °C) upflow anaerobic sludge blank reactor (UASBr).

Mixed-culture H2 fermentation performance and the relation between microbial community composition and hydraulic retention times for a fixed bed reactor fed with galactose/glucose mixtures

This study examined the mesophilic continuous biohydrogen fermentation from galactose and glucose mixture with an initial substrate concentration of 15 g/L (galactose 12 g/L and glucose 3 g/L) as a resembling carbon source of pretreated red algal hydrolyzate. A fixed bed reactor was fed with the sugar mixture at various hydraulic retention times (HRTs) ranging 12 to 1.5 h. The maximum hydrogen production rate of 52.6 L/L-d was found at 2 h HRT, while the maximum hydrogen yield of 2.3±0.1 mol/mol hexose_added, was achieved at 3 h HRT. Microbial communities and species distribution were analyzed via quantitative polymerase chain reaction (qPCR) and the dominant bacterial population was found as Clostridia followed by Lactobacillus sp. Packing material retained higher 16S rRNA gene copy numbers of total bacteria and Clostridium butyricum fraction compared to fermentation liquor. The finding of the study has demonstrated that H₂ production from galactose and glucose mixture could be a viable approach for hydrogen production.

Kinetic modeling and microbial community analysis for high-rate biohydrogen production using a dynamic membrane

This study investigated the kinetic parameters of high-rate continuous performance and biofilm layer formation in a H₂-producing dynamic membrane bioreactor, composed of a continuously stirred tank reactor along with an external module containing polyester mesh with a pore size of 100 µm. A maximum H₂ production rate of 48.9 L/L-day and hydrogen yield of 2.8 mol/mol glucose_added were attained at a hydraulic retention time of 3 h. The maximum specific growth rate and Monod constant were estimated as 14.92 d^-1 and 1.02 g COD/L, respectively. During the entire operation without backwashing, the transmembrane pressure remained below 1.7 kPa, while tightly bound extracellular polymeric substances increased as the dynamic membrane was developed. Fluorescent in situ hybridization and quantitative polymerase chain reaction assays revealed that Clostridium butyricum was dominant in all samples; however, the biofilm had a higher proportion of Prevotella spp. than the fermentation liquor.

Recovering hydrogen production performance of upflow anaerobic sludge blanket reactor (UASBR) fed with galactose via repeated heat treatment strategy

This study evaluated the effect of repeated heat treatment towards the enhancement of hydrogen fermentation from galactose in an upflow anaerobic sludge blanket reactor with the hydraulic retention time of 6h and the operation temperature of 37°C. The hydrogen production rate (HPR) and hydrogen yield (HY) gradually increased up to 9.1L/L/d and 1.1mol/mol galactose, respectively, until the 33rd day of operation. When heat treatment at 80°C for 30min was applied, hydrogen production performance was enhanced by 37% with the enrichment of hydrogen producing bacteria population. The HPR and HY were achieved at 12.5L/L/d and 1.5mol/mol hexose, respectively, during further 30 cycles of reactor operation. The repeated heat treatment would be a viable strategy to warrant reliable continuous hydrogen production using mixed culture.

Effects of anti-foaming agents on biohydrogen production

The effects of antifoaming agents on fermentative hydrogen production using galactose in batch and continuous operations were investigated. Batch hydrogen production assays with LS-303 (dimethylpolysiloxane), LG-109 (polyalkylene), LG-126 (polyoxyethylenealkylene), and LG-299 (polyether) showed that the doses and types of antifoaming agents played a significant role in hydrogen production. During batch tests, LS-303 at 100μL/L resulted in the maximum hydrogen production rate (HPR) and hydrogen yield (HY) of 2.5L/L-d and 1.08mol H2/mol galactoseadded, respectively. The following continuously stirred tank reactor operated at 12h HRT with LS-303 at 100μL/L showed a stable HPR and HY of 4.9L/L-d and 1.17mol H2/mol galactoseadded, respectively, which were higher than those found for the control reactor. Microbial community analysis supported the alterations in H2 generation under different operating conditions and the stimulatory impact of certain antifoaming chemicals on H2 production was demonstrated.

Effect of 5-hydroxymethylfurfural (5-HMF) on high-rate continuous biohydrogen production from galactose

This study investigated the effect of 5-hydroxymethylfurfural (5-HMF) on high-rate continuous fermentative H₂ production in a lab-scale fixed bed reactor (FBR) inoculated with mixed culture granules and fed with 15g/L galactose at a hydraulic retention time of 6h and at 37°C. During the 83days of operation, 5-HMF up to 2.4g/L was spiked into the feedstock. The maximum hydrogen production performance of 26.6L/L-d and 2.9mol H₂/mol galactose_added were achieved at 5-HMF concentration of 0.6g/L. 5-HMF concentration exceeding 0.9g/L not only inhibited hydrogen production but also affected the biofilm structure and microbial community population. However, when 5-HMF was eliminated from the feedstock, the performance and microbial community population were rapidly recovered.

Enhancement of biomethane recovery from batch anaerobic digestion by exogenously adding an N-acyl homoserine lactone cocktail

Biomethane recovered through anaerobic digestion (AD) is a renewable, sustainable, and cost-effective alternative energy source that has the potential to help address rising energy demands. Efficient bioconversion during AD depends on the symbiotic relationship between hydrolytic bacteria and methanogenic archaea. Interactions between microorganisms occur in every biological system via a phenomenon known as quorum sensing (QS), in which signaling molecules are simultaneously transmitted and detected as a mode of cell-to-cell communication. However, there's still a lack of understanding on how QS works in the AD system, where diverse bacteria and archaea interact in a complex manner. In this study, different concentrations (0.5 and 5 μM) of signaling molecules in the form of an N-acyl homoserine lactone cocktail (C6-, C8-, C10-, and 3-oxo-C6-HSL) were prepared and introduced into anaerobic batch reactors to clearly assess how QS affects AD systems. It was observed that the methane yield increased with the addition of AHLs: a 5 μM AHL cocktail improved the methane yield (341.9 mL/g-COD) compared to the control without AHLs addition (285.9 mL/g-COD). Meanwhile, evidence of improved microbial growth and cell aggregation was noticed in AHLs-supplemented systems. Our findings also show that exogenously adding AHLs alters the microbial community structure by increasing the overall bacterial and archaeal population counts while favoring the growth of the methanogenic archaea group, which is essential in biomethane synthesis.

Biohydrogen production from glucose using submerged dynamic filtration module: Metabolic product distribution and flux-based analysis

A lab scale bioreactor with the submerged polyester mesh of pore size 100 μm, was used for biohydrogen production under mesophilic condition (35 °C). The reactor was continuously fed with glucose (15 g/L) for 90 days with a hydraulic retention time (HRT), ranging from 12 to 1.5 h. Peak hydrogen yield (HY) was achieved at 3 h HRT as 3.22 ± 0.22 mol H₂/mol glucose _added and the hydrogen production rate was achieved at 2 h HRT as 54.07 ± 3.69 L H₂/L-d, respectively. When HRT was reduced to 1.5 h, the hydrogen yield decreased to 1.04 ± 0.44 mol H₂/mol glucose _added. Washout of the hydrogen producing population and metabolic flux shift to non-hydrogen producing at 1.5 h HRT might have attributed to the lower performance of the bioreactor.

Role of quorum sensing and quorum quenching in anaerobic digestion: A scoping review

Anaerobic digestion (AD) is a biological process that employs anaerobic microorganisms to degrade organic material, yielding biogas and biofertilizers. Understanding quorum sensing (QS) signaling in mixed microbial systems provides valuable insights into microbial behavior and functions. This review aims to examine recent studies on the roles of QS and QQ in the AD processes. A QS signal molecule, N-acyl homoserine lactone (AHL), induce the production of extraceluller polymers, promoting biofilm formation and bacterial aggregation, thereby the efficiency of AD process. QS-assisted granule formation fosters syntrophy between acetogens and methanogens, leading to increased organic removal and methane production. Specific AHLs were shown to be correlated with the abundance of hydrolytic bacteria and acidogens, further benefiting methane production. QQ was shown to effectively control membrane fouling in anaerobic membrane bioreactors, yet its impact on methane productivity remains unclear. This review shed lights on the existing literature gaps regarding the mechanisms of QS and QQ in AD systems, which will play a vital role in advancing AD applications in the future.

Inhibiting Biofilm Formation via Simultaneous Application of Nitric Oxide and Quorum Quenching Bacteria

Membrane biofouling is an inevitable challenge in membrane-based water treatment systems such as membrane bioreactors. Recent studies have shown that biological approaches based on bacterial signaling can effectively control biofilm formation. Quorum quenching (QQ) is known to inhibit biofilm growth by disrupting quorum sensing (QS) signaling, while nitric oxide (NO) signaling helps to disperse biofilms. In this study, batch biofilm experiments were conducted to investigate the impact of simultaneously applying NO signaling and QQ for biofilm control using Pseudomonas aeruginosa PAO1 as a model microorganism. The NO treatment involved the injection of NONOates (NO donor compounds) into mature biofilms, while QQ was implemented by immobilizing QQ bacteria (Escherichia coli TOP10-AiiO or Rhodococcus sp. BH4) in alginate or polyvinyl alcohol/alginate beads to preserve the QQ activity. When QQ beads were applied together with (Z)-1-[N-(3-aminopropyl)-N-(n-propyl) amino]diazen-1-ium-1,2-diolate (PAPA NONOate), they achieved a 39.0% to 71.3% reduction in biofilm formation, which was substantially higher compared to their individual applications (16.0% to 54.4%). These findings highlight the significant potential of combining QQ and NO technologies for effective biofilm control across a variety of processes that require enhanced biofilm inhibition.

Potential of biofuel production from leather solid wastes: Indian scenario

India is one among the major leather-producing countries in the world which shares close to one-fourth of the world's leather solid wastes and most of these wastes are not effectively utilized. These wastes are rich in protein and lipids that could be a potential feedstock for biofuel production, i.e., biogas, biodiesel, etc. Among the 150,000 tons of daily leather solid wastes in India, approximately 87,150 tons are shared by pre-tanning operations (i.e., raw trimmings, fleshing, and hair wastes) while the rest of the 62,850 tons are shared by tanning, post-tanning, and finishing operations (i.e., wet blue trimmings, chrome splits, shavings, buffing dust, crust trimming wastes). This review article shows that there is considerable bioenergy potential for the use of leather solid wastes as a green fuel. The biogas potential of leather solid wastes is estimated to be 40,532.9 m3/day whereas the biodiesel potential is estimated as 15,452.6 L/day. The bio-oil and bio-char potential of leather solid wastes is estimated to be 80,513.0 L/day and 45.8 tons/day, respectively. Several factors influence the biofuel process efficacy, which needs to be taken into consideration while setting up a biofuel recovery plant. The overall biofuel potential of leather solid wastes shows that this feedstock is an untapped resource for energy recovery to add commercial benefits to India's energy supply. Furthermore, in addition to the economic benefits for investors, the use of leather solid wastes for biofuel production will yield a positive environmental impact.