Application of purple phototrophic bacteria in a biofilm photobioreactor for single cell protein production: Biofilm vs suspended growth

Light/dark cycles and iron supplementation to enhance the simultaneous production of polyhydroxyalkanoates, 5-aminolevulinic acid, coenzyme Q10, and pigments through photofermentation

This study aimed to obtain 5-aminolevulinic acid (5-ALA), Coenzyme Q10 (CoQ10), polyhydroxyalkanoates (PHA), carotenoids, and bacteriochlorophylls (Bchl) through the photofermentation of residual wine lees. Light/dark cycles and iron supplementation were evaluated. Under 12-hour light/dark cycles, the production of 5-ALA, CoQ10, carotenoids, and Bchl increased by 1.7, 2.8, 1.7, and 2.4 times, respectively, compared to the continuous illumination control, while PHA production decreased from 511 to 445 mg/L. Combining light/dark cycles with iron supplementation enhanced the biomass production rate. The CoQ10 content increased by 4.9 times (reaching 8.8 mg/g-dw), carotenoids by 3.7 times (6.4 mg/g-dw), and Bchl by 6.4 times (17.9 mg/g-dw) compared to the control treatment, while maintaining 5-ALA at 5.3 µmol/L and PHA at 377 mg/L. The combination of light/dark cycles and iron provides a triple benefit: increased production of value-added substances, enhanced biomass production rate, and improved organic matter removal, making it an attractive option for winery effluent treatment and valorization.

Generative deep learning model assisted multi-objective optimization for wastewater nitrogen to protein conversion by photosynthetic bacteria

For decades, the photosynthetic bacteria (PSB)-based nitrogen treatment and valorization from wastewater have been explored. However, balancing nitrogen removal performance and resource recovery potential in PSB has remained a key unresolved issue for a long time. This study employed generative deep learning algorithms to achieve high-quality data generation, supporting multi-objective optimization in nitrogen removal, protein concentration, and nitrogen-to-protein conversion. In this study, the Variational Auto-Encoders model generated 5000 samples related to PSB nitrogen recovery, significantly enhancing the original dataset. The Elastic Neural Network (ENN) model showed better fitting results with the generated data. In single-objective evaluations, SHapley Additive exPlanations analysis identified the most important factors: carbon source, nitrogen source, and light type for total nitrogen (TN) removal; nitrogen source, nitrogen loading rate (NLR), and light type for protein concentration; nitrogen source, light type, and chemical oxygen demand (COD) for nitrogen conversion. Multi-objective optimization identified eight pareto front points, with the following input variable ranges: COD 3.42-7.48 g L-1, TN 0.22-0.37 g L-1, COD:TN ratio 9.28-33.22, hydraulic retention time 4.02-7.67 days, illuminance 967.71-1405.56 lx, and NLR 0.28-0.77 g L-1 d-1. The pareto solutions were mostly achieved under Near Infrared (NIR) light. Validation experiments further supported these findings, showing that NIR light achieved nitrogen-to-protein conversion reaching 44 % of the removed nitrogen. Additionally, NIR light significantly enhanced gene expression related to ammonia assimilation and protein translation processes compared to white light. The proposed generative framework provided an innovative solution for multi-objective optimization of wastewater nitrogen valorization under limited data conditions.

Effect of light intensity and wavelength on carotenoid profiles in mixed cultures of purple phototrophic bacteria

Purple phototrophic bacteria (PPB) have strong potential for applications such as protein-rich animal feed, with carotenoids providing added immune-nutritional value. This study investigated carotenoid profiles in PPB mixed cultures under varying light conditions, focusing on individual carotenoid quantification using advanced analytical techniques, rather than relying on total carotenoid estimates or spectral inference alone. Lycopene, 3,4-dihydrorhodopin, and rhodopin were consistently identified as the major carotenoids across conditions. Low-light (1.7 W m-2) increased both lycopene (5.1 mg gVS-1) and the total carotenoid content compared to higher intensities (2 mg gVS-1 at 33.1 W m-2). Infrared light alone promoted higher carotenoid production compared to its combination with other wavelengths. The capacity to influence both the quantity and composition of carotenoids opens new possibilities to optimise PPB biomass for targeted feed and biotechnology applications, where individual carotenoids provide distinct nutritional and functional benefits.

Single-cell protein production from photosynthetic bacteria wastewater treatment

The production of single-cell protein (SCP) from microorganisms holds significant importance due to its potential as an alternative protein source. Photosynthetic bacteria (PSB) wastewater treatment and resource recovery method stands out as an effective means to produce SCP, protein content is usually in the 40-60% range, thereby making it a highly valuable byproduct. This comprehensive review not only summarizes the current methods for the production and utilization of SCP but also traces the historical evolution of protein production from PSB wastewater treatment. It delves into the various factors that influence the yield of SCP, meticulously analyzing aspects such as the specific PSB strain employed, the type of wastewater processed, and the light-oxygen conditions under which the process occurs.While this technology has garnered increasing attention in recent years owing to its dual benefits of wastewater treatment and SCP production, the number of studies conducted in this field remains relatively scarce. Furthermore, the majority of these studies have primarily focused on the utilization of the Rhodopseudomonas genus for treating food wastewater treatment under light-anaerobic conditions. Despite these advancements, challenges to economic viability and limitations to industrial-scale production remain. At the conclusion of this review, we discuss the existing problems within the technology, such as the need for optimized conditions for different PSB strains and wastewater types, as well as the potential future prospects for its widespread adoption and commercialization.

Alum sludge-driven electro-phytoremediation in constructed wetlands: a novel approach for sustainable nutrient removal

In addition to their advantages as promising methods for wastewater treatment, CWs exhibit poor performance in terms of N and P removal efficiency in the effluent of wastewater treatment plants. By focusing on this issue, we designed CWs integrated with a biochar-doped activated carbon cloth (ACC) electrode and alum sludge from water treatment plants as a substrate to achieve concomitant organic matter and nutrient removal efficiency. Compared with the use of one layer of alum sludge in CWs (CWs-C3) with ACC electrodes inserted in two layers, which uses one layer of alum sludge, a significant improvement in removal efficiency was achieved (96% for COD; 89% for TN; and 77% for TP). The findings revealed that the application of potential accompanied by the insertion of a cathode ACC electrode into the first layer of alum sludge was beneficial for completing nitrification and facilitating denitrification in the cathode and anode regions, respectively, resulting in increased removal of organic matter and nutrients. Further evaluation revealed that the TN-TP synergetic removal mechanism was influenced by the use of Fe2+ as an electron donor and as a driving force for the development of autotrophic denitrifying bacteria to increase nitrate reduction. Additionally, the formation of FePO4 and AlPO4 and their adsorption through the interaction of FeOOH and AlOOH with phosphate constitute the main removal mechanism for TP in wastewater. Another reason for the increased removal efficiency in the CW-C3 reactor was the greater abundance and microbial diversity effectuated by the application of potential in the anode and cathode regions. In summary, a promising strategy for simultaneously promoting organic matter and nutrients and utilizing CWs on a large scale and in practical applications was proposed.

Purple non-sulfur bacteria for biotechnological applications

In this review, we focus on how purple non-sulfur bacteria can be leveraged for sustainable bioproduction to support the circular economy. We discuss the state of the field with respect to the use of purple bacteria for energy production, their role in wastewater treatment, as a fertilizer, and as a chassis for bioplastic production. We explore their ability to serve as single-cell protein and production platforms for fine chemicals from waste materials. We also introduce more Avant-Garde technologies that leverage the unique metabolisms of purple bacteria, including microbial electrosynthesis and co-culture. These technologies will be pivotal in our efforts to mitigate climate change and circularize the economy in the next two decades.

ONE-SENTENCE SUMMARY

Purple non-sulfur bacteria are utilized for a range of biotechnological applications, including the production of bio-energy, single cell protein, fertilizer, bioplastics, fine chemicals, in wastewater treatment and in novel applications like co-cultures and microbial electrosynthesis.

Current trends and possibilities of typical microbial protein production approaches: a review

Global population growth and demographic restructuring are driving the food and agriculture sectors to provide greater quantities and varieties of food, of which protein resources are particularly important. Traditional animal-source proteins are becoming increasingly difficult to meet the demand of the current consumer market, and the search for alternative protein sources is urgent. Microbial proteins are biomass obtained from nonpathogenic single-celled organisms, such as bacteria, fungi, and microalgae. They contain large amounts of proteins and essential amino acids as well as a variety of other nutritive substances, which are considered to be promising sustainable alternatives to traditional proteins. In this review, typical approaches to microbial protein synthesis processes were highlighted and the characteristics and applications of different types of microbial proteins were described. Bacteria, fungi, and microalgae can be individually or co-cultured to obtain protein-rich biomass using starch-based raw materials, organic wastes, and one-carbon compounds as fermentation substrates. Microbial proteins have been gradually used in practical applications as foods, nutritional supplements, flavor modifiers, and animal feeds. However, further development and application of microbial proteins require more advanced biotechnological support, screening of good strains, and safety considerations. This review contributes to accelerating the practical application of microbial proteins as a promising alternative protein resource and provides a sustainable solution to the food crisis facing the world.

Predicting photosynthetic bacteria-derived protein synthesis from wastewater using machine learning and causal inference

Causal inference-assisted machine learning was used to predict photosynthetic bacterial (PSB) protein production capacity and identify key factors. The extreme gradient boosting algorithm effectively predicted protein content, while the gradient boosting decision tree algorithm excelled in predicting protein production, protein productivity, and protein energy yields. Driving factors were identified, with suitable ranges: protein content (pH 6.0-7.5, hydraulic retention time (HRT) < 3.8 d), protein production (biomass > 1.7 g, organic loading rate (OLR) > 9.2 gL-1d-1, temperature 26.7-35.0 °C), protein productivity (HRT < 3.5 d, biomass > 1.6 g, OLR > 10.0 gL-1d-1), and protein energy yields (light energy 0.1-4.4 kWh, biomass 1.7-65.0 g, chemical oxygen demand (COD) 0.1-2.5 gL-1). Illuminance, dissolved oxygen, COD, and COD/total nitrogen ratio were causal factors influencing protein production. Two-dimensional partial dependence plot revealed the interaction between two driving factors. This study enhances information on PSB protein production and offers insights for wastewater treatment and sustainable resource development.

Light intensity effects on bioproduct recovery from fuel synthesis wastewater using purple phototrophic bacteria in a hybrid biofilm-suspended growth system

This research looked at how three different light intensities (1600, 4300, and 7200 lx) affect the biomass development, treatment of fuel synthesis wastewater and the recovery of valuable bioproducts between biofilm and suspended growth in a purple-bacteria enriched photobioreactor. Each condition was run in duplicate using an agricultural shade cloth as the biofilm support media in a continuously mixed batch reactor. The results showed that the highest chemical oxygen demand (COD) removal rate (56.8 ± 0.9 %) was found under the highest light intensity (7200 lx), which also led to the most biofilm formation and highest biofilm biomass production (1225 ± 95.7 mg). The maximum carotenoids (Crts) and bacteriochlorophylls (BChls) content occurred in the suspended growth of the 7200 lx reactor. BChls decreased with light intensity in suspended growth, while in biofilm both Crts and BChls were relatively stable between light conditions, likely due to an averaging effect as biofilm thickened at higher light intensity. Light intensity did not affect protein content of the biomass, however, biofilm showed a lower average (41.2 % to 43.7 %) than suspended biomass (45.4 % to 47.7 %). For polyhydroxybutyrate (PHB) the highest cell concentration in biofilm occurred at 1600 lx (11.4 ± 2.4 %), while for suspended growth it occurred at 7200 lx (22.7 ± 0.3 %), though total PHB productivity remained similar between reactors. Shading effects from the externally located biofilm could explain most variations in bioproduct distribution. Overall, these findings suggest that controlling light intensity can effectively influence the treatment of fuel synthesis wastewater and the recovery of valuable bioproducts in a biofilm photobioreactor.

Sustainable production of microbial protein from carbon dioxide in the integrated bioelectrochemical system using recycled nitrogen sources

Given the urgency of climate change, it is imperative to develop innovative technologies for repurposing CO2 into value-added products to achieve carbon neutrality. Additionally, repurposing nitrogen-source-derived wastewater streams is crucial, focusing on sustainability rather than conventional nitrogen removal in wastewater treatment plants. In this context, microbial protein (MP) production presents a sustainable and promising approach for transforming recovered low-value resources into high-quality feed and food. We assessed MP production by hydrogen-oxidizing bacteria (HOB) utilizing CO2 and various nitrogen sources. Specifically, we investigated MP production by two different HOB strains, Cupriavidus necator H16 and Xanthobacter viscosus 7d, within an integrated water-splitting biosynthetic system that generates in situ H2 via water electrolysis. The electroautotrophically produced MPs of C. necator H16 and X. viscosus 7d exhibited amino acid contents of 555 and 717 mg protein/g cell dry weight, with 243 and 299 mg essential amino acid/g cell dry weight, respectively. They could serve as viable alternatives to conventional food/feed sources like fishmeal or soybean protein. Ammonium-rich wastewater streams are preferable for MP production in integrated bioelectrochemical systems. This study provides valuable insights into sustainable, carbon-neutral MP production using CO2, water, renewable electricity, and recycled nitrogen sources.

Recent advances and challenges in single cell protein (SCP) technologies for food and feed production

The global population is increasing, with a predicted demand for 1250 million tonnes of animal-derived protein by 2050, which will be difficult to meet. Single-cell protein (SCP) offers a sustainable solution. This review covers SCP production mechanisms, microbial and substrate choices, and advancements in metabolic engineering and CRISPR-Cas. It emphasizes second-generation substrates and fermentation for a circular economy. Despite challenges like high nucleic acid content, SCP promises to solve the global nutrition problem.

Potential of enriched phototrophic purple bacteria for H2 bioconversion into single cell protein

Single cell protein (SCP) has emerged as an alternative protein source, potentially based on the recovery of carbon and nutrients from waste-derived resources as part of the circular economy. From those resources, gaseous substrates have the advantage of an easy sterilization, allowing the production of pathogen-free SCP. Sterile gaseous substrates allow producing pathogen-free SCP. This study evaluated the use of an enriched phototrophic purple bacteria (PPB) consortium for SCP production using H2 and CO2 as electron and C sources. The influence of pH (6.0-8.5), temperature (15-50 °C) and light intensity (0-50 W·m-2) on the growth kinetics and biomass yields was investigated using batch tests. Optimal conditions were found at pH 7, 25 °C and light intensities over 30 W·m-2. High biomass and protein yields were achieved (~ 1 g CODbiomass·g CODH2consumed-1 and 3.9-4.4 g protein·g H2-1) regardless of the environmental conditions, being amongst the highest values reported from gaseous streams. These high yields were obtained thanks to the use of light as a sole energy source by the PPB consortium, allowing a total utilization of H2 for growth. Hydrogen uptake rates varied considerably, with values up to 61 ± 5 mg COD·d-1 for the overall H2 consumption rates and 2.00 ± 0.14 g COD·g COD-1·d-1 for the maximum specific uptake rates under optimal growth conditions. The latter value was estimated using a mechanistic model able to represent PPB growth on H2. The biomass exhibited high protein contents (>50 % w/w) and adequate amino acid profiles, showing its suitability as SCP for feed. PPB were the dominant bacteria during the experiments (relative abundance over 80 % in most tests), with a stable population dominated by Rhodobacter sp. and Rhodopseudomonas sp. This study demonstrates the potential of enriched PPB cultures for H2 bioconversion into SCP.

Microbial community-based production of single cell protein from soybean-processing wastewater of variable chemical composition

The use of food-processing wastewaters to produce microbial biomass-derived single cell protein (SCP) is a sustainable way to meet the global food demand. Microbial community-based approaches to SCP production have the potential benefits of lower costs and greater resource recovery compared to pure cultures, yet they have received scarce attention. Here, SCP production from soybean-processing wastewaters using their existent microbial communities was evaluated. Six sequencing batch reactors of 4.5-L working volume were operated at 30 °C for 34 d in cycles consisting of 3-h anaerobic and 9-h aerobic phases. Four reactors received no microbial inoculum and the remaining two were amended with 1.5 L of a mixed culture from a prior SCP production cycle. Reactors produced more SCP when fed with wastewaters of higher soluble total Kjeldahl nitrogen (sTKN) content. The protein yield in biomass ranged from 0.53 to 3.13 g protein/g sTKN, with a maximum protein content of 50 %. The average removal of soluble chemical oxygen demand (sCOD) and soluble total nitrogen (sTN) was 92 % and 73 %, respectively. Distinct microbial genera were enriched in all six bioreactors, with Azospirillum, Rhodobacter, Lactococcus, and Novosphingobium dominating. The study showed that constituents in soybean wastewater can be converted to SCP and demonstrated the effect of variable influent wastewater composition on SCP production.

Resource recovery using enriched purple phototrophic bacteria in an outdoor flat plate photobioreactor: Suspended vs. attached growth

Purple phototrophic bacteria (PPB) can produce single-cell protein from wastewater at high yields. Growing in a biofilm vs suspended can improve product quality and consistency. This study compares suspended and attached growths of enriched PPB cultures in an outdoor flat plate photobioreactor treating poultry-processing wastewater. Attached growth had lower VFA removal efficiencies (95 ± 2.7 vs 84 ± 6.4 %) due to light limitations and low substrate diffusion rates. Nevertheless, similar overall treatment performances and productivities were achieved (16 ± 2.2 and 18 ± 2.4 gCOD·m^-2·d^-1 for attached and suspended) at loading rates of 1.2-1.5 gCOD·L^-1·d^-1. Biofilms had higher quality than suspended biomass, with lower ash contents (6.9(0.6)% vs 57(16)%) and higher PPB abundances (0.45-0.67 vs 0.30-0.45). The biofilm (20-50 % of the total biomass) might be used as feed and the suspended fraction as fertiliser, improving the economics of the process. Semi-continuous PPB growth outdoors as biofilm is technically feasible, obtaining a superior product without jeopardising performance.

Expanding mechanistic models to represent purple phototrophic bacteria enriched cultures growing outdoors

The economic feasibility of purple phototrophic bacteria (PPB) for resource recovery relies on using enriched-mixed cultures and sunlight. This work presents an extended Photo-Anaerobic Model (ePAnM), considering: (i) the diverse metabolic capabilities of PPB, (ii) microbial clades interacting with PPB, and (iii) varying environmental conditions. Key kinetic and stoichiometric parameters were either determined experimentally (with dedicated tests), calculated, or gathered from literature. The model was calibrated and validated using different datasets from an outdoors demonstration-scale reactor, as well as results from aerobic and anaerobic batch tests. The ePAnM was able to predict the concentrations of key compounds/components (e.g., COD, volatile fatty acids, and nutrients), as well as microbial communities (with anaerobic systems dominated by fermenters and PPB). The results underlined the importance of considering other microbial clades and varying environmental conditions. The model predicted a minimum hydraulic retention time of 0.5 d^-1. A maximum width of 10 cm in flat plate reactors should not be exceeded. Simulations showed the potential of a combined day-anaerobic/night-aerobic operational strategy to allow efficient continuous operation.

Biotechnological Production of Sustainable Microbial Proteins from Agro-Industrial Residues and By-Products

Microbial proteins, i.e., single-cell proteins or microbial biomass, can be cultivated for food and animal feed due to their high protein content and the fact that they represent a rich source of carbohydrates, minerals, fats, vitamins, and amino acids. Another advantage of single-cell proteins is their rapid production due to the growth rate of microorganisms and the possibility of using agro-industrial waste, residues and by-products for production through this renewable technology. Agro-industrial residues and by-products represent materials obtained from various processes in agriculture and agriculture-related industries; taking into account their composition and characteristics, as well as vast amounts, they have an enormous potential to generate sustainable bioproducts, such as microbial proteins. This review aims to summarize contemporary scientific research related to the production of microbial proteins on various agro-industrial residues and by-products, as well as to emphasize the current state of production of single-cell proteins and the importance of their production to ease the food crisis and support sustainable development.

Nitrogen recovery from wastewater by microbial assimilation - A review

The increased nitrogen (N) input with low utilization rate in artificial N management has led to massive reactive N (Nr) flows, putting the Earth in a high-risk state. It is essential to recover and recycle Nr during or after Nr removal from wastewater to reduce N input while simultaneously mitigate Nr pollution in addressing the N stress. However, mechanisms for efficient Nr recovery during or after Nr removal remain unclear. Here, the occurrence of N risk and progress in wastewater treatment in recent years as well as challenges of the current technologies for N recovery from wastewater were reviewed. Through analyzing N conversion fluxes in biogeochemical N-cycling networks, microbial N assimilation through photosynthetic and heterotrophic microorganisms was highlighted as promising alternative for synergistic N removal and recovery in wastewater treatment. In addition, the prospects and gaps of Nr recovery from wastewater through microbial assimilation are discussed.

Biogranulation process facilitates cost-efficient resources recovery from microalgae-based wastewater treatment systems and the creation of a circular bioeconomy

Energy self-sufficient wastewater treatment designs can reduce net energy consumption and achieve resources recovery. Microalgae are regarded as a promising candidate for developing a circular bioeconomy in wastewater treatment plants (WWTPs) due to its potential for simultaneous wastewater remediation and high value-added materials production. Much effort has been made to overcome the high production costs for microalgae; however, biomass harvesting still remains as the bottleneck for its large-scale application. In this study, the novel biogranulation system facilitating easier and faster microalgae harvesting was firstly compared with the conventional suspended culture for energy-efficiency and sustainability assessment on microalgae (Ankistrodesmus falcatus var. acicularis) cultivation using the synthetic anaerobic digestion liquor. Results demonstrated that the biogranulation system enhanced volumetric biomass productivity (223.17 ± 11.82 g/m³/day) by about 4.4 times compared to that from the suspended system (41.57 ± 2.08 g/m³/day) under the same environmental conditions. It was noticed that lipids, carbohydrates and proteins were accumulated in microalgae cells along with nutrients remediation, and the microalgae granules with much higher proteins content (313.28 ± 26.67 mg/g-VSS) could be easily harvested through 2 min gravity sedimentation with little impact on the contents of carbohydrates and lipids. In the whole cultivation and harvesting process, the biomass mass-based electricity consumption and footprint demand by the biogranulation system were reduced by 58% and 76%, respectively. Results from this study provide a cost-effective and sustainable approach for microalgae in the treatment of nutrients rich digestion liquor with simultaneous production of valuable biomaterials.

Outdoor demonstration-scale flat plate photobioreactor for resource recovery with purple phototrophic bacteria

To make purple phototrophic bacteria (PPB)-based technologies a reality for resource recovery, research must be demonstrated outdoors, using scaled reactors. In this study, a 10 m long PPB-enriched flat plate photobioreactor (FPPBR) with a volume of 0.95 m³ was operated for 253 days, fed with poultry processing wastewater. Different operational strategies were tested, including varying influent types, retention times, feeding strategies, and anaerobic/aerobic conditions in a novel mixed metabolic mode concept. The overall results show that regardless of the fermented wastewater fed (raw or after solid removal via dissolved air flotation) and the varying environmental conditions (e.g., light exposure and temperatures), the FPPBR provided effective volatile fatty acids (VFAs), N, and P removals (average efficiencies of >90%, 34-77%, and 28-45%, respectively). The removal of N and P was limited by the availability of biodegradable COD. Biomass (C, N and P) could be harvested at ∼90% VS/TS ratio, 58% crude protein content and a suitable amino acid profile for potential feed applications. During fully anaerobic operation with semicontinuous/day-only feeding, the FPPBR showed biomass productivities between 25 and 84 g VS m^-2 d^-1 (high due to solid influx; the productivities estimated from COD removal rates were 6.0-24 g VS•m^-2•d^-1 (conservative values)), and soluble COD removal rates of up to 1.0 g•L^-1•d^-1 (overall average of 0.34 ± 0.16 g•L^-1•d^-1). Under these conditions, the relative abundance of PPB in the harvested biomass was up to 56%. A minimum overall HRT of 2-2.4 d (1.0-1.2 d when only fed during the day) is recommended to avoid PPB washout, assuming no biomass retention. A combined daily-illuminated-anaerobic/night-aerobic operation (supplying air during night-time) exploiting photoheterotrophy during the day and aerobic chemoheterotrophy of the same bacteria at night improved the overall removal performance, avoiding VFA accumulation during the night. However, while enabling enhanced treatment, this resulted in a lower relative abundance of PPB and reduced biomass productivities, highlighting the need to balance resource recovery and treatment goals.

Naturally illuminated photobioreactors for resource recovery from piggery and chicken-processing wastewaters utilising purple phototrophic bacteria

Resource recovery from wastewater, preferably as high value products, has become an integral part of modern wastewater treatment. This work presents the potential to produce single cell protein (SCP) from pre-settled piggery wastewater (PWW) and meat chicken processing wastewater (CWW), utilising anaerobic purple phototrophic bacteria (PPB). PPB were grown as biofilm in outdoors 60 L, 80 L and 100 L flat-plate reactors, operated in sequential batch mode. PPB biofilm was recovered from reactor walls at a total solid (TS) content ∼90 g•L - 1, and the harvested biomass (depending on the wastewater) had a consistent quality, with high protein contents (50-65%) and low ash, potentially applicable as SCP. The COD, N and P removal efficiencies were 71±5.3%, 22±6.6%, 65±5.6% for PWW and 78±1.8%, 67±2.7% and 37±4.0% for CWW, respectively, with biofilm areal productivities up to 14 g TS•m - 2•d - 1. This was achieved at ammonium-N concentrations over 1.0 g•L - 1 and temperatures up to 55 °C and down to 6 °C (daily fluctuations of 20-30 °C). The removal performances and biomass productivities were mostly dependent on the bioavailable COD in the form of volatile fatty acids (VFA). At sufficient VFA availability, the irradiance became limiting, capping biofilm formation. Harvesting of the suspended fraction resulted in increased productivities and recovery efficiencies, but lowered the product quality (e.g., containing undesired inerts). The optimum between quantity and quality of product is dependent on the wastewater characteristics (i.e., organic degradable fraction) and potential pre-treatment. This study shows the potential to utilise sunlight to treat agri-industrial wastewaters while generating protein-rich PPB biomass to be used as a feed, feed additive or feed supplement.

Microbial community-based protein production from wastewater for animal feed applications

Single cell protein (SCP) derived from microbial biomass represents a promising source of protein for animal feed additives. While microbial community-based approaches to SCP production using nutrient-rich wastewaters incur lower costs than traditional single organism-based approaches, they have received little attention. This review focuses on SCP production using wastewaters with an emphasis on food-processing wastewaters. An elemental carbon-to-nitrogen ratio ranging from 10 to 20 is recommended to promote a high microbial biomass protein yield. Proteobacteria was identified as the most prevalent phylum within SCP-producing microbial communities. More research is needed to determine the composition of the microbial community best suited for SCP production, as well as its relationship with the microbial community in influent food-processing wastewaters. Remaining challenges are target protein and essential amino acids content, protein quantification and biomass yield assessment. The review presents bioreactor design considerations towards defining suitable operating conditions for SCP production through microbial community-based fermentation.

Polyhydroxyalkanoates from organic waste streams using purple non-sulfur bacteria

Many microorganisms can produce intracellular and extracellular biopolymers, such as polyhydroxyalkanoates (PHA). Despite PHA's benefits, their widespread at the industrial level has not occurred due mainly to high production costs. PHA production under a biorefinery scheme is proposed to improve its economic viability. In this context, purple non-sulfur bacteria (PNSB) are ideal candidates to produce PHA and other substances of economic interest. This review describes the PHA production by PNSB under different metabolic pathways, by using a wide range of wastes and under diverse operational conditions such as aerobic and anaerobic metabolism, irradiance level, light or dark conditions. Some strategies, such as controlling the feed regime, biofilm reactors, and open photobioreactors in outdoor conditions, were identified from the literature review as the approach needed to improve the process's economic viability when using mixed cultures of PNSB and wastes as substrates.

Purple phototrophic bacteria granules under high and low upflow velocities

The application of granular biomass has enabled energy efficient, high-rate wastewater treatment systems. While initially designed for high-strength wastewater treatment, granular systems can also play a major role in resource recovery. This study focused on the formation of purple phototrophic bacteria (PPB) granular biomass during synthetic wastewater treatment. Liquid upflow velocity was applied as the driving force for granulation. Separate reactors were operated at either low (2-5m h^-1) or high (6-9m h^-1) upflow velocities, with sludge retention times (SRTs) ranging from 5-15d. Reactors produced anaerobic, photo-granules within ~50d. The sludge volume index (SVI₃₀) of the granules was 10mL g^-1 and average settling rates were greater than 30m h^-1, both metrics being similar to existing granular technologies. Granule sizes of 2-3mm were recorded, however the particle size distribution was bimodal with a large floc fraction (70-80% volume fraction). The extracellular polymeric substance (EPS) and alginate-like extract (ALE) contents were similar to those in aerobic granular biomass. Fluorescence in-situ hybridisation (FISH) imaging identified PPB bacteria dispersed throughout the granules with very few methanogens and an active core. Outer layer morphology was substantially different in the two reactors. The high-upflow reactor had an outer layer of Chromatiales and an inner layer of Rhodobacteriales, while the low-upflow reactor had lower abundances of both, and limited layering. According to 16s gene sequencing, PPB were a similar fraction of the microbial community in both reactors (40-70%), but the high upflow granules were dominated by Chromatiales (supporting FISH results), while the low upflow velocity reactor had a more diverse PPB community. Methanogens were seen only in the low upflow granules and only in small amounts (≤8%). Granule crude protein content was ~0.60gCP gVS^-1 (~0.45gCP gTS^-1), similar to that from other PPB production technologies. The growth of a rapid settling and discrete PPB granular biomass on synthetic wastewater suggests methods for resource recovery using PPB can be diversified to also include granular biomass.