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

Influence of inorganic carbon on purple phototrophic bacteria polyhydroxyalkanoates production under high reductive stress environment

Fermented wastes rich in reduced compounds challenge anaerobic purple phototrophic bacteria (PPB) systems by causing redox imbalances. This study evaluated polyhydroxyalkanoates (PHA) production and CO2 fixation by PPB mixed cultures as means to balance internal redox, under four inorganic carbon (IC) availability conditions in a sequencing batch reactor. Culture selection under permanent IC presence promoted higher microbial diversity, but, strongly dependent on IC to balance internal redox, even when more oxidized substrates were supplemented. Increasing IC limitation favoured Rhodopseudomonas palustris, revealing its redox balancing capability independently of IC fixation. PHA contents of 20-29% gPHA/gVSS were achieved across all IC availabilities, however, when IC was present, growth was promoted over PHA production, indicating a preferable electron balance through CO2 fixation. Nevertheless, cultures selected under feast-famine IC exhibited a preference for PHA accumulation under IC limitation, suggesting IC tuning as a potential growth-controlling strategy to boost phototrophic PHA production using reduced feedstocks.

VFA extraction through silicone membrane fosters PHA production from nutrient-rich biowaste

This study presents a novel four-stage process for polyhydroxyalkanoates (PHA) production from nutrient-rich sheep cheese whey (CW). The key advancement was the integration of a volatile fatty acid (VFA) extraction stage into the conventional three-stage PHA production process. Application of membrane separation to fermented cheese whey resulted in the generation of a "retentate" stream containing both organic acids and nutrients, suitable for microbial culture selection, and a VFA-rich but nutrient deprived "permeate" stream, ideal for PHA accumulation. Thus, the carbon-to-nitrogen (C/N) ratio was optimized for both the selection and accumulation stages, which is crucial for efficient PHA production and for eliminating the need for exogenous nitrogen addition. The integrated process resulted in significantly higher yields (0.55 vs 0.26 gC-PHA gC-OA-1) and PHA content (37% vs 28%) than the control, where fermented cheese whey was directly used as feedstock for the accumulation stage. The results highlight the potential of this approach for optimizing PHA production from sub-optimal, nutrient-rich substrates.

Polyhydroxyalkanoates production from cheese whey under near-seawater salinity conditions

Treating saline streams presents considerable challenges due to their adverse effects on conventional biological processes, thereby leading to increased expenses in managing those side streams. With this in consideration, this study explores into the potential for valorizing fermented cheese whey (CW), a by-product of the dairy industry, into polyhydroxyalkanoates (PHA) using mixed microbial cultures (MMC) under conditions of near-seawater salinity (30 gNaCl/L). The selection of a PHA-accumulating MMC was successfully achieved using a sequential batch reactor operated under a feast and famine regime, with a hydraulic retention time of 14.5 h, a variable solids retention time of 3 and 4.5 days, and an organic loading rate (OLR) of 60 Cmmol/(L d). The selected culture demonstrated efficient PHA production rates and yields, maintaining robust performance even under high salinity conditions. During PHA accumulation, a maximum PHA content in biomass of 56.4 % wt. was achieved for a copolymer P(3HB-co-3HHx) with a 3HHx content of 7 %. Additionally, to asses the capacity of the culture to produce polymers with different compositions, valeric acid was supplemented to the real fermented feedstock which resulted in the production of terpolymers P(3HB-co-3HV-co-3HHx) with varied monomeric content and a higher maximum PHA content of 62 % wt. Additionally, this study highlights the potential utilization of seawater as alternative to freshwater for PHA production, thereby enhancing circular economy principles and promoting environmental sustainability.

PHA and EPS production from industrial wastewater by conventional activated sludge, membrane bioreactor and aerobic granular sludge technologies: A comprehensive comparison

The present study has focused on the mainstream integration of polyhydroxyalkanoate (PHA) production with industrial wastewater treatment by exploiting three different technologies all operating in sequencing batch reactors (SBR): conventional activated sludge (AS-SBR), membrane bioreactor (AS-MBR) and aerobic granular sludge (AGS). A full aerobic feast/famine strategy was adopted to obtain enrichment of biomass with PHA-storing bacteria. All the systems were operated at different organic loading (OLR) rate equal to 1-2-3 kgCOD/m3∙d in three respective experimental periods. The AS-MBR showed the better and stable carbon removal performance, whereas the effluent quality of the AS-SBR and AGS deteriorated at high OLR. Biomass enrichment with PHA-storing bacteria was successfully obtained in all the systems. The AS-MBR improved the PHA productivity with increasing OLR (max 35% w/w), whereas the AS-SBR reduced the PHA content (max 20% w/w) above an OLR threshold of 2 kgCOD/m3∙d. In contrast, in the AGS the increase of OLR resulted in a significant decrease in PHA productivity (max 14% w/w) and a concomitant increase of extracellular polymers (EPS) production (max 75% w/w). Results demonstrated that organic carbon was mainly driven towards the intracellular storage pathway in the AS-SBR (max yield 51%) and MBR (max yield 61%), whereas additional stressors in AGS (e.g., hydraulic selection pressure, shear forces) induced bacteria to channel the COD into extracellular storage compounds (max yield 50%) necessary to maintain the granule's structure. The results of the present study indicated that full-aerobic feast/famine strategy was more suitable for flocculent sludge-based technologies, although biofilm-like systems could open new scenarios for other biopolymers recovery (e.g., EPS). Moreover, the AS-MBR resulted the most suitable technology for the integration of PHA production in a mainstream industrial wastewater treatment plant, considering the greater process stability and the potential reclamation of the treated wastewater.

Environmental sustainability assessment of biodegradable bio-based poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from agro-residues: Production and end-of-life scenarios

In the context of a circular bio-based economy, more public attention has been paid to the environmental sustainability of biodegradable bio-based plastics, particularly plastics produced using emerging biotechnologies, e.g. poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or PHBV. However, this has not been thoroughly investigated in the literature. Therefore, this study aimed to address three aspects regarding the environmental impact of PHBV-based plastic: (i) the potential environmental benefits of scaling up pellet production from pilot to industrial scale and the environmental hotspots at each scale, (ii) the most favourable end-of-life (EOL) scenario for PHBV, and (iii) the environmental performance of PHBV compared to benchmark materials considering both the pellet production and EOL stages. Life cycle assessment (LCA) was implemented using Cumulative Exergy Extraction from the Natural Environment (CEENE) and Environmental Footprint (EF) methods. The results show that, firstly, when upscaling the PHBV pellet production from pilot to industrial scale, a significant environmental benefit can be achieved by reducing electricity and nutrient usage, together with the implementation of better practices such as recycling effluent for diluting feedstock. Moreover, from the circularity perspective, mechanical recycling might be the most favourable EOL scenario for short-life PHBV-based products, using the carbon neutrality approach, as the material remains recycled and hence environmental credits are achieved by substituting recyclates for virgin raw materials. Lastly, PHBV can be environmentally beneficial equal to or even to some extent greater than common bio- and fossil-based plastics produced with well-established technologies. Besides methodological choices, feedstock source and technology specifications (e.g. pure or mixed microbial cultures) were also identified as significant factors contributing to the variations in LCA of (bio)plastics; therefore, transparency in reporting these factors, along with consistency in implementing the methodologies, is crucial for conducting a meaningful comparative LCA.

Ammonia impact on the selection of a phototrophic - chemotrophic consortium for polyhydroxyalkanoates production under light-feast / dark-aerated-famine conditions

Phototrophic polyhydroxyalkanoate (PHA) production is an emerging technology for recovering carbon and nutrients from diverse wastewater streams. However, reliable selection methods for the enrichment of PHA accumulating purple phototrophic bacteria (PPB) in phototrophic mixed cultures (PMC) are needed. This research evaluates the impact of ammonia on the selection of a PHA accumulating phototrophic-chemotrophic consortium, towards the enrichment of PHA accumulating PPB. The culture was operated under light-feast/dark-aerated-famine and winter simulated-outdoor conditions (13.2 ± 0.9 °C, transient light, 143.5 W/m2), using real fermented domestic wastewater with molasses as feedstock. Three ammonia supply strategies were assessed: 1) ammonia available only in the light phase, 2) ammonia always present and 3) ammonia available only during the dark-aerated-famine phase. Results showed that the PMC selected under 1) ammonia only in the light and 3) dark-famine ammonia conditions, presented the lowest PHA accumulation capacity during the light period (11.1 % g PHA/g VSS and 10.4 % g PHA/g VSS, respectively). In case 1), the absence of ammonia during the dark-aerated-famine phase did not promote the selection of PHA storing PPB, whereas in case 3) the absence of ammonia during the light period favoured cyanobacteria growth as well as purple sulphur bacteria with increased non-PHA inclusions, resulting in an overall decrease of phototrophic PHA accumulation capacity. The best PHA accumulation performance was obtained with selection under permanent presence of ammonia (case 2), which attained a PHA content of 21.6 % g PHA/g VSS (10.2 Cmmol PHA/L), at a production rate of 0.57 g PHA/L·day, during the light period in the selection reactor. Results in case 2 also showed that feedstock composition impacts the PMC performance, with feedstocks richer in more reduced volatile fatty acids (butyric and valeric acids) decreasing phototrophic performance and leading to acids entering the dark-aerated phase. Nevertheless, the presence of organic carbon in the aerated phase was not detrimental to the system. In fact, it led to the establishment of a phototrophic-chemotrophic consortium that could photosynthetically accumulate a PHA content of 13.2 % g PHA/g VSS (6.7 Cmmol PHA/L) at a production rate of 0.20 g PHA/L·day in the light phase, and was able to further increase that storage up to 18.5 % g PHA/g VSS (11.0 Cmmol PHA/L) at a production rate of 1.35 g PHA/L·day in the dark-aerated period. Furthermore, the light-feast/dark-aerated-famine operation was able to maintain the performance of the selection reactor under winter conditions, unlike non-aerated PMC systems operated under summer conditions, suggesting that night-time aeration coupled with the constant presence of ammonia can contribute to overcoming the seasonal constraints of outdoor operation of PMCs for PHA production.

Production of short-chain fatty acids (SCFAs) as chemicals or substrates for microbes to obtain biochemicals

Carboxylic acids have become interesting platform molecules in the last years due to their versatility to act as carbon sources for different microorganisms or as precursors for the chemical industry. Among carboxylic acids, short-chain fatty acids (SCFAs) such as acetic, propionic, butyric, valeric, and caproic acids can be biotechnologically produced in an anaerobic fermentation process from lignocellulose or other organic wastes of agricultural, industrial, or municipal origin. The biosynthesis of SCFAs is advantageous compared to chemical synthesis, since the latter relies on fossil-derived raw materials, expensive and toxic catalysts and harsh process conditions. This review article gives an overview on biosynthesis of SCFAs from complex waste products. Different applications of SCFAs are explored and how these acids can be considered as a source of bioproducts, aiming at the development of a circular economy. The use of SCFAs as platform molecules requires adequate concentration and separation processes that are also addressed in this review. Various microorganisms such as bacteria or oleaginous yeasts can efficiently use SCFA mixtures derived from anaerobic fermentation, an attribute that can be exploited in microbial electrolytic cells or to produce biopolymers such as microbial oils or polyhydroxyalkanoates. Promising technologies for the microbial conversion of SCFAs into bioproducts are outlined with recent examples, highlighting SCFAs as interesting platform molecules for the development of future bioeconomy.

Producing volatile fatty acids and polyhydroxyalkanoates from foods by-products and waste: A review

Dairy products, extra virgin olive oil, red and white wines are excellent food products, appreciated all around the world. Their productions generate large amounts of by-products which urge for recycling and valorization. Moreover, another abundant waste stream produced in urban context is the Organic Fraction of Municipal Solid Wastes (OFMSW), whose global annual capita production is estimated at 85 kg. The recent environmental policies encourage their exploitation in a biorefinery loop to produce Volatile Fatty Acids (VFAs) and polyhydroxyalkanoates (PHAs). Typically, VFAs yields are high from cheese whey and OFMSW (0.55-0.90 g_{COD_VFAs}/g_COD), lower for Olive Mill and Winery Wastewaters. The VFAs conversion into PHAs can achieve values in the range 0.4-0.5 g_PHA/g_VSS for cheese whey and OFMSW, 0.6-0.7 g_PHA/g_VSS for winery wastewater, and 0.2-0.3 g_PHA/g_VSS for olive mill wastewaters. These conversion yields allowed to estimate a huge potential annual PHAs production of about 260 M tons.

Subcritical Water as a Pre-treatment of Mixed Microbial Biomass for the Extraction of Polyhydroxyalkanoates

Polyhydroxyalkanoate (PHA) recovery from microbial cells relies on either solvent extraction (usually using halogenated solvents) and/or digestion of the non-PHA cell mass (NPCM) by the action of chemicals (e.g., hypochlorite) that raise environmental and health hazards. A greener alternative for PHA recovery, subcritical water (SBW), was evaluated as a method for the dissolution of the NPCM of a mixed microbial culture (MMC) biomass. A temperature of 150 °C was found as a compromise to reach NPCM solubilization while mostly preventing the degradation of the biopolymer during the procedure. Such conditions yielded a polymer with a purity of 77%. PHA purity was further improved by combining the SBW treatment with hypochlorite digestion, in which a significantly lower hypochlorite concentration (0.1%, v/v) was sufficient to achieve an overall polymer purity of 80%. During the procedure, the biopolymer suffered some depolymerization, as evidenced by the lower molecular weight (M_w) and higher polydispersity of the extracted samples. Although such changes in the biopolymer’s molecular mass distribution impact its mechanical properties, impairing its utilization in most conventional plastic uses, the obtained PHA can find use in several applications, for example as additives or for the preparation of graft or block co-polymers, in which low-M_w oligomers are sought.

From Organic Wastes and Hydrocarbons Pollutants to Polyhydroxyalkanoates: Bioconversion by Terrestrial and Marine Bacteria

The use of fossil-based plastics has become unsustainable because of the polluting production processes, difficulties for waste management sectors, and high environmental impact. Polyhydroxyalkanoates (PHA) are bio-based biodegradable polymers derived from renewable resources and synthesized by bacteria as intracellular energy and carbon storage materials under nutrients or oxygen limitation and through the optimization of cultivation conditions with both pure and mixed culture systems. The PHA properties are affected by the same principles of oil-derived polyolefins, with a broad range of compositions, due to the incorporation of different monomers into the polymer matrix. As a consequence, the properties of such materials are represented by a broad range depending on tunable PHA composition. Producing waste-derived PHA is technically feasible with mixed microbial cultures (MMC), since no sterilization is required; this technology may represent a solution for waste treatment and valorization, and it has recently been developed at the pilot scale level with different process configurations where aerobic microorganisms are usually subjected to a dynamic feeding regime for their selection and to a high organic load for the intracellular accumulation of PHA. In this review, we report on studies on terrestrial and marine bacteria PHA-producers. The available knowledge on PHA production from the use of different kinds of organic wastes, and otherwise, petroleum-polluted natural matrices coupling bioremediation treatment has been explored. The advancements in these areas have been significant; they generally concern the terrestrial environment, where pilot and industrial processes are already established. Recently, marine bacteria have also offered interesting perspectives due to their advantageous effects on production practices, which they can relieve several constraints. Studies on the use of hydrocarbons as carbon sources offer evidence for the feasibility of the bioconversion of fossil-derived plastics into bioplastics.

Effect of the organic loading rate on the PHA-storing microbiome in sequencing batch reactors operated with uncoupled carbon and nitrogen feeding

Over the last years, in a search for sustainable and biodegradable alternatives to petrol-based plastics, biotechnological applications turned to the potentialities of mixed microbial cultures (MMC) for producing polyhydroxyalkanoates (PHAs). Under a feast and famine regime, an uncoupled carbon (C) and nitrogen (N)-feeding strategy may be adopted by dosing the C-source at the beginning of the feast and the N-source at the beginning of the famine in order to stimulate a PHA storage response and microbial growth. Even though this strategy has been already successfully applied for the PHA production, very few information is to date available regarding the MMC operating in these systems and the influence of Organic Loading Rate (OLR) on their selection and enrichment. To fill the gap, this study investigated the effect of the OLR on the selection of PHA-accumulating microorganisms in a sequencing batch reactor (SBR) operated with an uncoupled C and N feeding strategy. The SBR cycle length was set at 12 h and four OLRs values (4.25, 8.50, 12.75 and 18 gCOD L^-1 d^-1) were tested by changing the concentration of the feeding solution, made of a synthetic mixture of acetic (85% of the overall COD) and propionic (15%) acids. The PHA-storage yield increased by increasing the OLR (up to 0.69 COD/COD at 12.75 gCOD L^-1 d^-1) but significantly decreased (0.27 COD/COD) at 18 gCOD L^-1 d^-1 concomitantly with a longer feast phase and a lower PHA content in the biomass at the end of the feast phase. The selective pressure induced by the applied OLRs strongly influenced the microbiome composition revealing a high content of putative PHA-storing bacteria, such as Rhodobacter, Thauera and Paracoccus, in the SBR operated at OLRs 4.25, 8.50 and 12.75 g COD L^-1 d^-1 (up to 97.4% of total reads) and a low content (5.4%) in the SBR at 18 g COD L^-1 d^-1where the predominance of genus Nitrinicola was instead observed.

Polyhydroxyalkanoate bio-production and its rise as biomaterial of the future

The first observation of a polyhydroxyalkanoate (PHA) aggregate was in 1888 by Beijenrinck. Despite polyhydroxybutyrate (PHB) being the first type of PHA discovered, it was not extracted and characterized until 1925 by Maurice Lemoigne in France, even before the concept of "macromolecules" was known. After more than 30 years, in 1958, Wilkinson and co-workers rediscovered PHB and its metabolic role in the cells as storage compound. PHB started to be appealing to the industry in the 1980s, when a few companies started to commercialize microbially produced PHAs. During the 1990 s, the focus was on reducing production costs to make PHA production economically feasible, for instance by genetically modified microorganisms and even plants. Since then, many advances have been made: diverse wastes as feedstock, different production processes, and tailored design of biopolymers. This paper summarizes the scientific and technological development of PHAs from their discovery in 1888 until their latest applications and current commercial uses. Future perspectives have been devised too based on the current bottlenecks.