Integrating the selection of PHA storing biomass and nitrogen removal via nitrite in the main wastewater treatment line

Integrated selection of PHA-storing biomass and nitrogen removal via nitrite from sludge reject water: a mathematical model

One of the most recent innovations to promote a circular economy during wastewater treatment is the production of biopolymers. It has recently been demonstrated that it is possible to integrate the production of biopolymers in the form of polyhydroxyalkanoates (PHA) with nitrogen removal via nitrite during the treatment of sludge reject water. In the present study, simulation of a new process for bioresource recovery was conducted by an appropriate modification of the Activated Sludge Model 3. The process consists of the integrated nitrogen removal via nitrite from sludge reject water and the selection of PHA-storing biomass by inducing a feast and famine regime under aerobic and anoxic conditions. According to the results, it is anticipated that simulation data matched very satisfactorily with the experimental data and confirmed the main experimental observation, showing that during the famine period the PHA depletion was almost complete due to the availability of nitrite as the electron acceptor. Simulation results indicate that the selection of the volumetric organic loading rate and of the relative duration of the aerobic feast/anoxic famine duration is critical in order to allow for the effective denitritation of the internally stored PHA during the famine phase.

Mechanisms of persistence and impact of ordinary heterotrophic organisms in aerobic granular sludge

The stability of granules, contaminant removal and microbial structure of an aerobic granular sludge (AGS) process were investigated with a focus on ordinary heterotrophic organisms (OHOs). Long-term stable granules and high removals of COD (97 %), NH₄⁺ (98 %), P (85 %) and total N (77 %) were achieved. Sequencing analyses identified 6.6 % of phosphorus-accumulating organisms in the sludge, concordant with the observed bio-P removal capacity. However, OHOs were the most abundant bacteria in the sludge (70-93 %) without resulting in unstable aggregates. Under current dogmas of microbial competition in activated sludge, it seemed contradictory that OHOs could persist in the long term in the AGS where COD was depleted beginning in the anaerobic phase. Microbial analyses showed that OHOs could survive in granules by micropredation, proteolysis, fermentation and EPS consumption. Heterotrophic-nitrification/ aerobic-denitrification was an active pathway in the AGS. These findings contribute to a better understanding of microbial competition in AGS and its stability.

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.

Redirecting carbon to recover VFA to facilitate biological short-cut nitrogen removal in wastewater treatment: A critical review

Wastewater treatment plants (WWTPs) are facing a great challenge to transition from energy-intensive to carbon-neutral and energy-efficient systems. Biological nutrient removal (BNR) can be severely impacted by carbon limitation, particularly for wastewater with a low carbon-to-nitrogen (C/N) ratio, which can significantly increase the operational costs. Waste activated sludge (WAS) is a valuable byproduct of WWTPs, as it contains high levels of organic matter that can be utilized to improve BNR management by recovering and reusing the fermentative volatile fatty acids (VFAs). This review provides a comprehensive examination of the recovery and reuse of VFAs in wastewater management, with a particular focus on advancing the preferable biological short-cut nitrogen removal process for carbon-insufficient municipal wastewaters. First, the method of carbon redirection for recovering VFAs was reviewed. Carbon could be captured through the two-stage A/B process or via sludge fermentation with different sludge pretreatment and process control strategies to accelerate sludge hydrolysis and inhibit methanogens to enhance VFA production. Second, VFAs can support the metabolism of autotrophic N-cycling microorganisms involved in wastewater treatment, such as AOB, NOB, anammox, and comammox bacteria. However, VFAs can also cause inhibition at high concentrations, leading to the partition of AOB and NOB; and can promote partial denitrification as an efficient carbon source for heterotrophic denitrifiers. Third, the lab- and pilot-scale engineering practices with different configurations (i.e., A²O, SBR, UASB) were summarized that have shown the feasibility of utilizing the fermentate to achieve superior nitrogen removal performance without the need for external carbon addition. Lastly, the future perspectives on leveraging the relationships between mainstream and sidestream, nitrogen and phosphorus, autotrophs and heterotrophs were given for sustainable and efficient BNR management.

Achieving robust and highly efficient nitrogen removal in a mainstream anammox reactor by introducing low concentrations of readily biodegradable organics

In this study, an anammox reactor was operated to treat low-strength (NH₄⁺ + NO₂^-, 25-35  mg/L) wastewater without (phase I) or with (phase II) readily biodegradable chemical oxygen demand (rbCOD). In phase I, although efficient nitrogen removal was achieved at the beginning, nitrate accumulated in the effluent after long-term operation (75  days), resulting in a decrease in the nitrogen removal efficiency to 30%. Microbial analysis revealed that the abundance of anammox bacteria decreased from 2.15 to 1.78%, whereas that of nitrite-oxidizing bacteria (NOB) increased from 0.14 to 0.56%. In phase II, rbCOD, in terms of acetate, was introduced into the reactor with a carbon/nitrogen ratio of 0.9. The nitrate concentration in the effluent decreased within 2 days. Advanced nitrogen removal was achieved in the following operation, with an average effluent total nitrogen of 3.4 mg/L. Despite the introduction of rbCOD, anammox pathway still dominated to the nitrogen loss. High-throughput sequencing indicated that high anammox abundance (2.48%) further supports its dominant position. The improvement in nitrogen removal was attributed to the enhanced suppression of NOB activity, simultaneous nitrate polishing through partial denitrification and anammox, and promotion of sludge granulation. Overall, the introduction of low concentrations of rbCOD is a feasible strategy for achieving robust and efficient nitrogen removal in mainstream anammox reactors.

Consistency between the metabolic performance of two aerobic granular sludge systems and the functional groups of bacteria detected by amplicon sequencing

Two sequential batch reactors (R1 and R2) of aerobic granular sludge (AGS) were inoculated with activated sludge of different origins. The objective was to investigate the granulation and the consistency between the structure of the microbial communities (16S rRNA amplicon sequencing) in each reactor and their metabolic performance (removal of C, N, and P). Both reactors were fed with acetate-based synthetic wastewater, targeting an anaerobic-aerobic cycle reputed to favor the phosphorus- and glycogen-accumulating organisms (PAO and GAO). Stable granulation was achieved in both reactors, where, instead of PAO, the dominant genera were ordinary heterotrophic organisms (OHO) such as Thauera, Paracoccus, and Flavobacterium known for their high capacity of aerobic storage of polyhydroxyalkanoates (PHA). Generally, there was good consistency between the metabolic behavior of each reactor and the bacterial genera detected. Both reactors showed high removals of C and complete nitrification (Nitrosomonas and Nitrospira detected) but a low level of simultaneous nitrification-denitrification (SND) during the aerated phase. The latter causes that nitrates were recycled to the initial phase, in detriment of PAO selection. Meanwhile, the study showed that selecting slow-growing OHOs (with aerobic storage capacity) favors stable granulation, revealing an alternative AGS technology for C and N removal.

Ordinary heterotrophic organisms with aerobic storage capacity provide stable aerobic granular sludge for C and N removal

The study investigated the mechanisms and microbial communities underlying the long-term stability and removal performances shown by aerobic granular sludge (AGS) reactor involving polyhydroxyalkanoates (PHA) aerobic-storing bacteria. The characteristics of the sludge, removal performances and bacterial community structure were determined. The prevailing metabolic phenotype was similar in the parent conventional activated sludge (CAS) reactor and its upgraded AGS version, showing high COD and NH₄ uptake, versus low P and N reduction. Polyphosphate and glycogen accumulating organisms, PAO and GAO, were not enriched in the reactors despite initial targeting of anaerobic-aerobic cycle. Instead, PHA-aerobic storing bacteria (Thauera and Paracoccus) were dominant, but revealing a stable AGS system for BOD and N removal. The PAO/GAO failed selection and Thauera overgrowth were analyzed for beneficial use in developing alternative AGS technology for BOD and N removal applications.

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.

Effects of different pre-treatments on the properties of polyhydroxyalkanoates extracted from sidestreams of a municipal wastewater treatment plant

The paper deals with effects of two different widespread extraction methods (conventional extraction and Soxhlet extraction) and four different pre-treatments (homogenization with pressure and with blades, sonication, and impact with glass spheres) on the extraction yields and properties of polyhydroxyalkanoate (PHA) extracted from biomass coming from an innovative process (short-cut enhanced phosphorus and PHA recovery) applied in a real wastewater treatment plant. The results show that the two different extraction processes affected the crystallization degree and the chemical composition of the polymer. On the other hand, the extractive yield was highly influenced by pre-treatments: homogenization provided a 15% more extractive yield than the others. Homogenization, especially at high pressure, proved to be the best pre-treatment also in terms of the purity, visual appearance (transparency and clearness), thermal stability, and mechanical performances of the obtained PHA films. All the PHA films begin to melt long before their degradation temperature (Td > 200 °C): this allows their use in the fields of extrusion or compression moulding. SYNOPSIS: Optimizing the extraction of PHAs from municipal wastewater gives a double beneficial environmental impact: wastewater treatment and circular bio-based carbon upgrade to biopolymers for the production of bioplastics and other intersectoral applications.

Biological nutrient removal enhancement using fermented primary and rotating belt filter biosolids

This research compared the impact of two primary treatment options (i.e. primary clarification and rotating belt filtration (RBF)) on biological nutrients removal (BNR) process, using sludge fermentation liquid (SFL) as a carbon source. The liquid fraction of both fermented primary and RBF sludges comparably enhanced BNR. Despite the significant contribution of the unpurified SFL to the sharp increase in nutrient levels; i.e. 47%-64% (primary effluent; PE), and 45%-53% (RBF) of the soluble nitrogen and phosphorus loads; readily biodegradable COD and volatile fatty acids (VFAs) fractions of the combined feed increased significantly (2.5-6.1 times), compared to the original feed by additional SFL. Removal efficiencies in the reactors reached 57% (total nitrogen) and 92% (total phosphorus) after addition of SFL. Effluent nitrogen and phosphorus of the two reactors were close in the range of 15 ± 6 mg N/L, and 0.5 ± 0.3 mg P/L, respectively. Kinetics studies showed denitrification rates of 1.3, and 1.13 kg NO₃-N/m³.d for primary effluent and RBF effluent-fed reactors, respectively. Phosphorus release rates were 11.7 and 9.7 mg PO₄-P/g VSS.h, for primary, and RBF effluents, respectively; showing 20%-22% lower rates in the RBF SFL. Incorporating experimental data into a plant-wide model for a 100 MLD facility receiving typical medium strength wastewater, showed that although primary treatment enhanced the biogas production by 96% (primary clarification) and 62% (RBF) trains; combined fermentation and anaerobic digestion was effective to enhance the biogas production by 59% on average, compared to the base scenario without primary treatment. Additionally, if primary clarification exists, then the addition of fermentation results in additional revenue of C$1890/d in the plant, considering additional revenue of C$2230/d due to VFA generation in contrast to only C$340/d loss due to the reduced methane production.

Strategies for the valorisation of a protein-rich saline waste stream into polyhydroxyalkanoates (PHA)

Saline Mussels Cooking Wastewater was valorised to produce PHA with Mixed Microbial Cultures (MMC). Due to the high protein content (1.8-5.7 g COD_PROT/L), PHA accumulating capacity was below 10%, so several strategies were tested. In the acidification unit, Na(HCO₃) was added, increasing protein conversion into Volatile Fatty Acids (VFA) from 10.3% to 69.2% and subsequent PHA accumulation from 6.9 to 14.7%. In the enrichment unit, the incorporation of a settling stage after the feast phase provoked a shift in the proteins' oxidation from the feast to the famine phase, where the nitrogen released in the famine is used by the MMC for growth. This increased the biomass concentration and the tolerated COD (from 1.6 to 4.2 g VSS/L and from 2.2 to 4.38 g COD/L). Finally, varying the proteins/VFA ratio for MMC acclimation to proteins allowed increasing PHA accumulation from 8.8 to 41.5%.

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.

Polyhydroxyalkanoate (PHA) production via resource recovery from industrial waste streams: A review of techniques and perspectives

The problem of waste generation in the form of wastewater and solid wastes has caused an urgent, yet persisting, global issue that calls for the development of sustainable treatment and resource recovery technologies. The production of value-added polyhydroxyalkanoates (PHAs) from industrial waste streams has attracted the attention of researchers and process industries because they could replace traditional plastics. PHAs are biopolymers with high degradability, with a variety of applications in the manufacturing sector (e.g. medical equipment, packaging). The aim of this review is to describe the techniques and industrial waste streams that are applied for PHA production. The different enrichment and accumulation techniques that employ mixed microbial communities and carbon recovery from industrial waste streams and various downstream processes were reviewed. PHA yields between 7.6 and 76 wt% were reported for pilot-scale PHA production; while, at the laboratory-scale, yields from PHA accumulation range between 8.6 and 56 wt%. The recent advances in the application of waste streams for PHA production could result in more widely spread PHA production at the industrial scale via its integration into biorefineries for co-generation of PHAs with other added-value products like biohydrogen and biogas.

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

Assessing the potential of waste activated sludge and food waste co-fermentation for carboxylic acids production

This study investigated waste activated sludge (WAS) and food waste (FW) co-fermentation in batch assays to produce carboxylic acids. Three mixtures (50%, 70% and 90% WAS in VS basis) were studied under different conditions: with and without extra alkalinity, and with and without WAS auto-hydrolysis pre-treatment. All tests were carried out at 35 °C, without pH adjustment and without external inoculum. Experimental results showed that co-fermentation yields, including volatile fatty acids and lactic acid, were always higher than WAS and FW mono-fermentation yields (ca. 100 and 80 mgCOD/gVS, respectively). Co-fermentation yields increased as the proportion of FW in the mixture increased, indicating that the improvement was primarily due to a higher FW degradation under co-fermentation conditions. The maximum co-fermentation yield was on average 480 mgCOD/gVS for the WAS/FW_50/50 mixture. The importance of pH on co-fermentation performance was evident in the experiments carried out with extra alkalinity, which showed that the proportion of WAS in the mixture should be high enough to keep the pH above 5.0. However, fermenters operational conditions should also prevent the enrichment of acetic acid consuming microorganisms. WAS auto-hydrolysis pre-treatment did not enhance co-fermentation yields but showed minor kinetic improvements. Regarding the product profile, butyric acid was enriched as the proportion of FW in the mixture increased and the concomitant pH decreased to the detriment of propionic acid. Propionic acid prevailed under neutral pH in the WAS mono-fermentation and the WAS/FW_90/10 mixture.

A review of biopolymer (Poly-β-hydroxybutyrate) synthesis in microbes cultivated on wastewater

The large quantities of non-degradable single use plastics, production and disposal, in addition to increasing amounts of municipal and industrial wastewaters are among the major global issues known today. Biodegradable plastics from biopolymers such as Poly-β-hydroxybutyrates (PHB) produced by microorganisms are potential substitutes for non-degradable petroleum-based plastics. This paper reviews the current status of wastewater-cultivated microbes utilized in PHB production, including the various types of wastewaters suitable for either pure or mixed culture PHB production. PHB-producing strains that have the potential for commercialization are also highlighted with proposed selection criteria for choosing the appropriate PHB microbe for optimization of processes. The biosynthetic pathways involved in producing microbial PHB are also discussed to highlight the advancements in genetic engineering techniques. Additionally, the paper outlines the factors influencing PHB production while exploring other metabolic pathways and metabolites simultaneously produced along with PHB in a bio-refinery context. Furthermore, the paper explores the effects of extraction methods on PHB yield and quality to ultimately facilitate the commercial production of biodegradable plastics. This review uniquely discusses the developments in research on microbial biopolymers, specifically PHB and also gives an overview of current commercial PHB companies making strides in cutting down plastic pollution and greenhouse gases.

Vapor-phase conversion of aqueous 3-hydroxybutyric acid and crotonic acid to propylene over solid acid catalysts

Vapor phase conversion of 3-hydroxybutyric and crotonic acid to propylene in a continuous-flow reactor over silica–alumina and niobium catalysts demonstrates a new strategy for producing renewable fuels and chemicals from wastewater carbon.

Substrate strategy optimization for polyhydroxyalkanoates producing culture enrichment from crude glycerol

Crude glycerol is by-product produced from biodiesel industry and can be converted directly by mixed microbial culture (MMC) into polyhydroxyalkanoates (PHAs). This study investigated the effects of the reverse (SBR_A) and positive (SBR_B) glycerol gradient substrate strategy on PHA-accumulating culture enrichment and the maximum PHA accumulating stability under substrates with different glycerol and volatile fatty acid (VFA) proportion. The results showed that crude glycerol was mainly used for PHA production rather than biomass growth in SBR_A. The maximum q_PHA was 0.65 g COD/g X^-1·h^-1 under sole crude glycerol condition in SBR_A, which was 2.41 times higher than that of SBR_B. Moreover, the PHA accumulating ability of the biomass from SBR_A was more stable than SBR_B. Saccharibacteria_genera_incertae_sedis was for the first time found to be the dominant genus using crude glycerol for PHA production. This research provides an insight into enrichment strategy to effectively enrich PHA-accumulating culture from crude glycerol.

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.

Recent advances in polyhydroxyalkanoate production: Feedstocks, strains and process developments

Polyhydroxyalkanoates (PHAs) have been actively studied in academia and industry for their properties comparable to petroleum-derived plastics and high biocompatibility. However, the major limitation for commercialization is their high cost. Feedstock costs, especially carbon costs, account for the majority of the final cost. Finding cheap feedstocks for PHA production and associated process development are critical for a cost-effective PHA production. In this study, waste materials from different sources, particularly lignocellulosic biomass, were proposed as suitable feedstocks for PHA production. Strains involved in the conversion of these feedstocks into PHA were reviewed. Newly isolated strains were emphasized. Related process development, including the factors that affect PHA production, fermentation modes and downstream processing, was elaborated upon.

Potential of endogenous PHA as electron donor for denitrification

The use of wastewater streams to obtain polyhydroxyalkanoates (PHA) as high added-value products is widely studied. However, nitrogen removal is not well integrated into this process. In this study, the optimal conditions to track the specific endogenous denitrifying activity (SEDA) driven by PHA as carbon source were selected as: sludge concentration of 0.5-2 g VSS/L, COD_PHA/N ratio higher than 5.4 g/g and between 40 and 60 mg NO₃^--N/L. The seeding biomass used to perform the activity tests was collected from two sequencing batch reactors and was able to store up to 69% wt/wt of PHA. SEDA values of 0.26-0.39 g N₂-N/(g VSS_act d) were achieved, which proved the potential of PHA-accumulating mixed microbial cultures to be used in nitrogen removal processes. The results indicated that there is not a preference in the consumption of hydroxybutyrate over hydroxyvalerate and that PHA concentrations lower than 5% wt/wt do not allow the obtainment of the maximum SEDA value. Finally, N₂O gas production was not detected in the SEDA experiments.

Review on biological wastewater treatment and resources recovery: attached and suspended growth systems

The treatment of wastewater for reuse is a potential solution to meet ever increasing urban, industrial, agricultural, and environmental demands across the world, where clean water availability is scarce. There are several traditional wastewater treatment processes that offer varying degrees of effectiveness in addition to presenting environmental, economic, and social disadvantages. Development of promising and inexpensive technologies to provide the reusable water in needful amounts using wastewaters as a cheap source of key nutrients and organic matter is required. Wastewater treatment by biological methods is becoming more important in the light of recovering value-added plant nutrients, heavy metals, biosolids, and bioenergy resources. Different types of solid contaminants in effluents can be removed simultaneously by pure cultures or mixed microbial consortia. Based on the structural organization of microbial biomass, biological treatment systems are classified into two types: dispersed growth system and attached growth system. Biological treatment methods associated with fixed-film growth have been recognized as highly effective and more energy efficient than suspended growth systems. This review discusses the recent breakthroughs in advanced biological wastewater treatment using both the systems, and also focuses on key energetic resources recovery driven by biological technologies.

Polyhydroxyalkanoates (PHA) production from fermented thermal-hydrolyzed sludge by PHA-storing denitrifiers integrating PHA accumulation with nitrate removal

Polyhydroxyalkanoates (PHA) production from fermented thermal-hydrolyzed sludge was conducted by mixed microbial cultures (MMCs) in the study. An MMC enriched in the species Brachymonas_denitrificans (60.18%) was selected under an aerobic feast/famine regime, which is capable of denitrification and accumulating PHA. To take advantage of the PHA-storing denitrifiers, an aerobic-feast/anoxic-famine regime was applied to integrate culture selection with denitrification. The results showed that cultures enriched under the regime exhibited a PHA storage capacity with PHA yield on VFA of 0.47 gCOD/gCOD and well denitrification performance achieving nitrate removal of 98%. Moreover, the aerobic-feast/anoxic-famine regime could originate a comparable maximum PHA content to the complete aerobic feast/famine regime (49.7 wt% versus. 47.1 wt%, respectively), yet reduce aeration energy input by 79% in the culture selection process. Finally, this study investigated the accumulation of nitrite and nitrous oxide during PHA based denitrification and the feasibility of integrating the process with wastewater treatment.

Microbiome dynamics and phaC synthase genes selected in a pilot plant producing polyhydroxyalkanoate from the organic fraction of urban waste

This study analyses the bacterial population dynamics of a mixed microbial community (MMC) selected in a pilot plant producing polyhydroxyalkanoate (PHA) from the fermentation of the organic fraction of urban waste (OFMSW) and sewage sludge (SS). 16S rRNA gene high-throughput sequencing revealed the occurrence of a variety of PHA accumulating bacteria that ensured a stable PHA production in an open system operating with real substrates and without temperature control. The Volatile Fatty Acids (VFA) changes in the feed and the temperature variation affected the dynamics of the PHA-accumulating bacteria over the plant operation. Remarkably, the higher PHA content was associated to a MMC largely comprising of Hydrogenophaga species during the operation at higher working temperature. The involvement of a heterogeneous PHA-accumulating MMC was associated with a high phaC synthase genes biodiversity confirming the occurrence of a functional redundancy.

Biorefinery of cellulosic primary sludge towards targeted Short Chain Fatty Acids, phosphorus and methane recovery

Cellulose from used toilet paper is a major untapped resource embedded in municipal wastewater which recovery and valorization to valuable products can be optimized. Cellulosic primary sludge (CPS) can be separated by upstream dynamic sieving and anaerobically digested to recover methane as much as 4.02 m³/capita·year. On the other hand, optimal acidogenic fermenting conditions of CPS allows the production of targeted short-chain fatty acids (SCFAs) as much as 2.92 kg COD/capita·year. Here propionate content can be more than 30% and can optimize the enhanced biological phosphorus removal (EBPR) processes or the higher valuable co-polymer of polyhydroxyalkanoates (PHAs). In this work, first a full set of batch assays were used at three different temperatures (37, 55 and 70 °C) and three different initial pH (8, 9 and 10) to identify the best conditions for optimizing both the total SCFAs and propionate content from CPS fermentation. Then, the optimal conditions were applied in long term to a Sequencing Batch Fermentation Reactor where the highest propionate production (100-120 mg COD/g TVS_fed·d) was obtained at 37 °C and adjusting the feeding pH at 8. This was attributed to the higher hydrolysis efficiency of the cellulosic materials (up to 44%), which increased the selective growth of Propionibacterium acidopropionici in the fermentation broth up to 34%. At the same time, around 88% of the phosphorus released during the acidogenic fermentation was recovered as much as 0.15 kg of struvite per capita·year. Finally, the potential market value was preliminary estimated for the recovered materials that can triple over the conventional scenario of biogas recovery in existing municipal wastewater treatment plants.

Recent developments in biohythane production from household food wastes: A review

Biohythane is a hydrogen-methane blend with hydrogen concentration between 10 and 30% v/v. It can be produced from different organic substrates by two sequential anaerobic stages: a dark fermentation step followed by a second an anaerobic digestion step, for hydrogen and methane production, respectively. The advantages of this blend compared to either hydrogen or methane, as separate biofuels, are first presented in this work. The two-stage anaerobic process and the main operative parameters are then discussed. Attention is focused on the production of biohythane from household food wastes, one of the most abundant organic substrate available for anaerobic digestion: the main milestones and the future trends are exposed. In particular, the possibility to co-digest food wastes and sewage sludge to improve the process yield is discussed. Finally, the paper illustrates the developments of biohythane application in the automotive sector as well as its reduced environmental burden.

Challenges of scaling-up PHA production from waste streams. A review

The search for new materials that replace fossil fuel-based plastics has been focused on biopolymers with similar physicochemical properties to fossil fuel-based plastics, such as Polyhydroxyalkanoates (PHA). The present paper reviews the challenges of scaling-up PHA production from waste streams during the period from 2014 to 2016, focusing on the feasibility of the alternatives and the most promising alternatives to its scaling-up. The reviewed research studies mainly focus on reducing costs or obtaining more valuable polymers. In the future, the integration of PHA production into processes such as wastewater treatment plants, hydrogen production or biodiesel factories could enhance its implementation at industrial scale.

Ecosystem activation system (EAS) technology for remediation of eutrophic freshwater

Ecosystem activation system (EAS) was developed to create beneficial conditions for microbiome recovery and then restore and maintain the ecological integrity (microbial community, phytoplankton, zooplankton) for eutrophic freshwater rehabilitation. A 30 day’s filed test of EAS indicated that over 50% of contaminant was removed and the algae visibly disappeared. EAS treatment 2.5-fold increased the diversity of microbial community and changed the microbial community structure (e.g., two and three-fold decrease in the amount of Flavobacterium and Pseudomonas, typical abundant species of eutrophic freshwater, respectively). Further, the diversity of phytoplankton and zooplankton of treated water suggested that these species were diverse. Representative phytoplankton of eutrophic freshwater, Chlorella and Chlamydomonas were undetectable. The possible mechanism of EAS is restoring the trophic levels of the water body via bottom-up approach by microbial community.

Recent Advances and Challenges towards Sustainable Polyhydroxyalkanoate (PHA) Production

Sustainable biofuels, biomaterials, and fine chemicals production is a critical matter that research teams around the globe are focusing on nowadays. Polyhydroxyalkanoates represent one of the biomaterials of the future due to their physicochemical properties, biodegradability, and biocompatibility. Designing efficient and economic bioprocesses, combined with the respective social and environmental benefits, has brought together scientists from different backgrounds highlighting the multidisciplinary character of such a venture. In the current review, challenges and opportunities regarding polyhydroxyalkanoate production are presented and discussed, covering key steps of their overall production process by applying pure and mixed culture biotechnology, from raw bioprocess development to downstream processing.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.