No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Food waste valorization by purple phototrophic bacteria and anaerobic digestion after thermal hydrolysis
Abstract
Increased demand for effective waste management strategies, along with the need for a transition from a fossil fuel-based economy to a bio-based economy sharpens the need for synergies and scientific innovations. Food waste (FW) is an essential component of municipal solid waste, and its accumulation has become a global concern. This work discusses a closed-loop integrated biorefinery. It couples thermal hydrolysis with anaerobic digestion and photofermentation for the recovery of bioenergy resources and the production of value-added products. Thermal hydrolysis yielded up to 40.4% solids solubilization, allowing the separation of an organic-rich hydrolysate. This pre-treatment also improves anaerobic digestibility of the solid fraction, thus increasing biogas production, which can feed a combined heat and power plant. This approach makes the process sustainable and energy-efficient while decreasing the total volume of the disposal waste by 78.6%. Phototrophic treatment of the hydrolysate through a purple phototrophic bacteria-based mixed culture resulted in biomass growth with high protein content (65% wt.). The system also produced polyhydroxyalkanoates (PHA) and hydrogen, accounting for a total valorization of 16.9% of the initial total solids of the raw food waste. This variety of possible products allows setting a seasonal production in a biorefinery, depending on the composition of the debris and the market demand. Modulation of the nitrogen composition of the food waste can help to choose the best option, where low Nitrogen drives PHA and hydrogen production. In contrast, high Nitrogen leads to increased protein production.
... In this sense, there is a clear need to develop biorefineries capable of producing several products at the same time, eliminating uncertainties of seasonal variability and feedstock heterogeneity (Alibardi et al., 2020). For example, the technical feasibility of the production of PHA together with hydrogen and biogas in a photobiorefinery with PPB from food waste (Allegue et al., 2020), lignocellulosic waste or the organic fraction of municipal solid waste (OFMSW) (Allegue et al., 2022) have recently been studied. Regarding environmental performance, it is necessary to ensure that novel bioplastics avoid larger environmental pressure than that produced by fossil plastics. ...
Bioplastics offer a promising sustainable alternative to petroleum-based plastics due to their biodegradability as well as favourable thermal and mechanical properties. Among different types of biobased polymers, the production of polyhydroxyalkanoates (PHA) using purple phototrophic bacteria (PPB) and low-value substrates has gained increasing interest. Despite the momentum, challenges regarding the scalability and environmental feasibility of this biopolymer production pathway remain. In response, this study employs an exploratory LCA approach to quantitatively assesses the potential environmental implications of PHA production in powder form and the joint management of the organic fraction of municipal solid waste (OFMSW) through a novel photobiorefinery system that uses PPB mixed cultures. Environmental impacts were tested under multiple improvement scenarios and benchmarked against the production of conventional fossil-based granulate or unprocessed plastics, including low density polyethylene (LDPE), polyethylene terephthalate (PET) and polyurethane (PU). The photobiorefinery stage was found to have the greatest contribution to the impact categories, particularly due to direct emissions, consumption of electricity and production of extractive chemical agents used. These factors accounted for over 70% of the photobiorefinery impact in all cases. Avoided impacts provided net favourable outcomes in terms of carbon footprint and fossil resources when comparing PHA production to conventional plastics, especially PET and PU, with impact reductions ranging from 30% to 60%, respectively. However, when considering other impact categories like eutrophication, this situation was less favourable. The exploration of alternative scenarios offered significant impact reductions, especially when renewable electricity or an environmentally friendly extraction agent is used. Moreover, minimizing methane losses or co-producing hydrogen in the photobiorefinery had a notably positive effect on the carbon footprint, reducing the impact by more than 2 t of CO2 eq per t of PHA powder compared to the base case. Therefore, the implementation of feasible improvement measures in the short term can position PHA produced by mixed cultures as a sustainable alternative to petroleum-based plastics.
... Samples were blended, homogenized, and stored in a cold chamber at 4ºC until further use. The OFMSW was thermally hydrolyzed in advance at 150ºC for 38 min, according to previous results [34], being the liquid fraction of the hydrolysate separated by centrifugation and used in this work. The DWW came from the outlet of the primary settlers of two domestic wastewater treatment plants (WWTP) located in Estiviel, Toledo (Spain), and La Gavia, Madrid (Spain). ...
This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV with a photoperiod of 12/24 h and fed during the light Stage only, so the results can be extrapolated to outdoors open ponds operation by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and P values (<125mg/L <2 mg/L), respectively, and punctually on values in N (<15mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N and 66% of P. Furthermore, increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N and 93% of P despite low presence of PPB due to more oxidative conditions, which anyway step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scale up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system. This novel urban biorefinery allows reducing treatment costs of two urban wastes and embed them in the circular bioeconomy through water recycling and valorizing PPB biomass as a source of carbon, nitrogen and phosphorus.
... Samples were blended, homogenized, and stored in a cold chamber at 4 • C until further use. The OFMSW was thermally hydrolyzed in advance at 150 • C for 38 min, according to previous results [36], being the liquid fraction of the hydrolysate separated by centrifugation and used in this work. The DWW came from the outlet of the primary settlers of two domestic wastewater treatment plants (WWTPs) located in Estiviel, Toledo (Spain), and La Gavia, Madrid (Spain). ...
This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV
with a photoperiod of 12/24 h and fed during the light stage only so the results can be extrapolated to outdoor open pond operations by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and p-values (<125 mg/L; <2 mg/L), respectively, and punctually on values in N (<15 mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N, and 66% of P. Furthermore, by increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N, and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying an HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N, and 93% of P despite the low presence of PPB due to more oxidative conditions, which step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scaling up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system.
... acetate and cheese whey) (Guida et al., 2022;Matassa et al., 2022). The rest of the available scientific studies used inorganic gaseous substrates such as CO 2 and H 2 or organic gaseous CH 4 and biogas to produce their microbial feed/food product (Allegue et al., 2020;Khoshnevisan et al., 2020;Khoshnevisan et al., 2019;Oesterholt et al., 2019;Pan et al., 2021;Pihlajaniemi et al., 2020;Tsapekos et al., 2020;Yang et al., 2022;Yang et al., 2021;Zha et al., 2021). However, using these gaseous substrates for MP production can give rise to high land area requirements, for instance in the case of CO 2 for microalgae cultivation or dedicated crops for anaerobic digestion to produce CH 4 and CO 2 (Ciliberti et al., 2016;Majid et al., 2014;Ritala et al., 2017), high capital investments, safety concerns (i.e. ...
In contrast to traditional agriculture, microbial protein (MP) production is highly efficient in nitrogen (N) usage and can be employed to valorize a variety of recovered resources, thereby increasing the overall sustainability of food production. The present study aimed to establish the potential of seven recovered N sources originating from different waste streams for MP production using ethanol and acetate as growth substrates. The evaluation was based on specific growth rate, biomass yield, nutritional quality (i.e. macromolecular composition, amino acid (AA) and lipid profile) and food safety (i.e. concentration of heavy metals, polyaromatic hydrocarbons (PAH), pesticides and antibiotics) of the MP. The majority of the recovered N sources did not affect the kinetics and had a minor impact on the biomass yield, compared to their commercial equivalents. The nutritional content of the biomass was similar to soy flour and did not show major variations in AA and lipid profile for the different recovered N sources. Considering the heavy metal content, an average-weighing adult should not consume >53-213 g of the microbial biomass produced on recovered N per day due to its high copper content. A substantial amount of PAH were also found in the biomass. A daily consumption of 20 g/person/day would impose 2.0-2.8 times higher dietary exposure than the mean PAH exposure through nutrition in the EU, indicating a potential concern for human health. On the other hand, the biomass was free of antibiotics, and the traces of pesticides found did not raise any major concern for food applications. Based on the results of this work, no evidence was found to restrict the application of microbial biomass produced on recovered nitrogen as food.
... 150 In an integrated biorefinery approach, thermal hydrolysis, AD, and photofermentation were tied to produce bioenergy as well as value-added products (such as polyhydroxyalkanoates) from FW. 25,152,153 Thermal hydrolysis solubilized about 40.4% of solids in FW, facilitated the extraction of rich hydrolysate and improved the AD process, and reduced about 78.6% volume of the disposal. 154 The phototrophic treatment with purple bacteria enhanced protein-rich biomass growth. The authors also noted that the composition of FW played a vital role in producing value-added products from the integrated biorefinery approach. ...
Valorization of food waste (FW) is instrumental for reducing the environmental and economic burden of FW and transitioning to a circular economy. The FW valorization process has widely been studied to produce various end-use products and summarize them; however, their economic, environmental, and social aspects are limited. This study synthesizes some of the valorization methods used for FW management and produces value-added products for various applications, and also discusses the technological advances and their environmental, economic, and social aspects. Globally, 1.3 billion tonnes of edible food is lost or wasted each year, during which about 3.3 billion tonnes of greenhouse gas is emitted. The environmental (-347 to 2969 kg CO2 equiv/tonne FW) and economic (-100 to $138/tonne FW) impacts of FW depend on the multiple parameters of food chains and waste management systems. Although enormous efforts are underway to reduce FW as well as valorize unavoidable FW to reduce environmental and economic loss, it seems the transdisciplinary approach/initiative would be essential to minimize FW as well as abate the environmental impacts of FW. A joint effort from stakeholders is the key to reducing FW and the efficient and effective valorization of FW to improve its sustainability. However, any initiative in reducing food waste should consider a broader sustainability check to avoid risks to investment and the environment.
... Few works consider the full circularity concept, with the reintroduction of output streams in the same chain, what CE call as "closed-loops," or the use of these outputs to another productive cycles, what CE call as "open-loops." Reuse of water through wastewater treatment [30], use of bagasse [31] or other biomasses for energy generation [32][33][34][35] as well as replacement of volatile solvents by greener ones [36][37][38], are other CE alternatives proposed in literature for food production in the CE context. ...
Food processing, from agricultural production to domestic consumption, is responsible for generating great amounts of waste per year, resulting in soil, water, and air pollution. These pollutants, together with the uses of petrochemical process inputs such as solvents, additives, or fuels, increase the food chain’s environment impacts resulting in wasted resources. In response to this scenario, the circular economy (CE) theory is presented in literature as a liable alternative for the design of more sustainable production chains. In this context, this work was aimed at evaluating the literature’s approach on the CE concept within the food processing and food waste management. The works show the centrality of “food waste” as a focus for the application of the CE. However, despite the relevance of management, reuse, or valuation of food waste, particularly due to its contribution to carbon footprint and decrease of food safety, studies have found other strategies for improvement of CE in the food chain. In this case, works in literature were allocated within the framework presented by the Ellen Macarthur Foundation called ReSOLVE, with proposals for modification of production chain to promote the CE. Among the proposals, one should highlight: modification of productive systems for mitigation of environmental impacts and greenhouse emissions, processes optimization for decreasing the use of natural resources and wastes, use of 4.0 Industry such as IoT, big data, or machine learning techniques for improvement of the whole supply chain, development of collaborative platforms for production and market, use of residues or co-products by design of intra- or inter-chain loops, and exchange of process or inputs with high environmental impacts for greener ones.
... The annual municipal solid waste (MSW) production will rise from 1.3 billion tonnes now to 2.2 billion tonnes by 2025, of which around 46 wt% belongs to the organic fraction of MSW (OFMSW) (Campuzano and González-Martínez, 2016;International Energy Agency, 2018). Food waste is a significant part of OFMSW (Allegue et al., 2020). Food waste has been viewed as energy-, carbon-and nutrient-rich resources (Dahiya et al., 2018;Battista et al., 2020), which have been underutilised. ...
Rapid urbanisation has marked recent human history with more than 50% of the world's population now living in urban areas while the percentage and the overall population are still growing. Resource consumption is conglomerated in urban settlements which depend primarily on externally supplied and often fossil-based products. With the aim to transform cities sustainably from resource sinks to regenerative hubs, the concept of the Urban Bioeconomy poses a potential alternative to the current economic system. While the concept has been in discussion among practitioners and a few researchers, its meaning is not well established. Therefore, this study proposes a definition of the Urban Bioeconomy based on a systematic literature review, which focused on identifying its components, impacts and potential synergies within bioeconomic sectors, coined by the ‘Urban Bio-Symbiosis’. This work highlights existing and emerging bioeconomic components within cities such as urban farming, biowaste valorisation methods and green infrastructure techniques. It outlines opportunities and challenges of the Urban Bioeconomy by presenting potential positive and negative environmental, economic, social and health impacts. Based on these results, it identifies the potential of the Urban Bio-Symbiosis and Resource Circularity as promising solutions to bring about synergistic effects between different urban bioeconomic components themselves and with the other parts of the economy. By proposing a definition of the Urban Bioeconomy, this work sets the ground for further research in this field.
... Anaerobic fermentation of pretreated food hydrolysate increased biogas production. Furthermore, the phototropic fermentation using purple phototrophic bacterial culture resulted in overall biomass production and polyhydroxyalkanoate production (Allegue et al., 2020). In another study, sequential HCl-enzymatic pretreatment showed a significant improvement (42%), which on fermentation with Saccharomyces cerevisiae resulted in 0.42 g g − 1 of ethanol yield (Hafid et al., 2017). ...
Organic waste has increased as the global population and economy have grown exponentially. Food waste (FW) is posing a severe environmental issue because of mismanaged disposal techniques, which frequently result in the squandering of carbohydrate-rich feedstocks. In an advanced valorization strategy, organic material in FW can be used as a viable carbon source for microbial digestion and hence for the generation of value-added compounds. In comparison to traditional feedstocks, a modest pretreatment of the FW stream utilizing chemical, biochemical, or thermochemical techniques can extract bulk of sugars for microbial digestion. Pretreatment produces a large number of toxins and inhibitors that affect bacterial fuel and chemical conversion processes. Thus, the current review scrutinizes the FW structure, pretreatment methods (e.g., physical, chemical, physicochemical, and biological), and various strategies for detoxification before microbial fermentation into renewable chemical production. Technological and commercial challenges and future perspectives for FW integrated biorefineries have also been outlined.
... It was also ensured that the substrate did not degrade by keeping the feed bottle in a cooler at 4 • C. During the first 5 days of operation, the MPBR was fed with a synthetic substrate consisting of HAc:HPr:HBu:EtOH on a 1:1:1:1 COD basis and nutrients from the Ormerod growth medium described elsewhere [39] for acclimatization. Afterward, the liquid fraction of the fermenter effluent was fed, and the operating conditions were varied in 7 stages, as depicted in Table 1, which can be classified as Start-up (S0), operation under stable biomass growth (S1 and S2), first carbon overload (S3), biomass recovery (S4 and S5) and second carbon overload (S6). ...
The production of polyhydroxyalkanoates (PHA) with purple phototrophic bacteria (PPB) has been limited due to low yields and limited knowledge regarding the diverse routes used for carbon biosynthesis. The present study increases PHA accumulation yields using urban organic waste pretreated by steam explosion and acidogenic fermentation as substrate. Throughout the PPB-based photoheterotrophic process in an anaerobic membrane photobioreactor, the organic loading rate (OLR) was modified to increase the amount of PHA and biomass in the reactor. A maximum PHA accumulation of 42% (gPHA gBiomass⁻¹) on a dry basis was achieved and maintained for 10 d for an OLR of 1 gCOD L⁻¹ d⁻¹, and hydraulic and sludge retention times of 2 and 6 d, respectively. This PHA accumulation capacity is the maximum obtained using a mixed culture of PPB fed with waste. Also, a medium-chain PHA (polyhydroxyhexanoate) has been quantified, enhancing the physicochemical properties and diversifying their industrial applications. Furthermore, we show novel alternatives to PHA accumulation: carbon storage as glycogen and extracellular polymers while deriving the excess electrons into hydrogen. Finally, a statistical study of microbial communities has settled the environmental variables with the most significant influence on these communities' variability. This work demonstrates the importance of acquiring a thorough understanding of carbon accumulation and electron allocation strategies of PPB under stressful conditions and shows promising results for a larger scale implementation of a PPB-based photobiorefinery, which could valorize urban organic waste to produce different high added-value products within the context of the circular bioeconomy.
The application of the biorefinery concept has contributed to the reduction of the volumes of food waste (FW), considered an economic and environmental problem. The reuse of these residues has made it possible to give them added value and take advantage of them in the production of biofuels and other value-added products. However, during the FW conversion process, there are limitations that can hinder its correct development and that can be solved through energy integration and different bioprocesses. In this sense, hydrothermal pretreatment stands out in FW processing. Therefore, the aim of the chapter is to propose a methodology for the energy integration of hydrothermal pretreatment in food waste biorefinery terms.Key wordsBioenergyBioprocessesEfficiencyEnvironmentValue-added products
The growing global population and climate change threaten the availability of many critical resources, and have been directly impacting the food and agriculture sector. Therefore, new cultivation technologies must be rapidly developed and implemented to secure the world's future food needs. Closed-loop greenhouse agriculture systems provide an opportunity to decrease resource reliance and increase crop yield. Greenhouses provide versatility in what can be grown and the resources required to function. Greenhouses can become highly efficient and resilient through the application of a closed-loop systems approach that prioritizes repurposing, reusing, and recirculating resources. Here, we employ a text mining approach to research the available research (meta-research) and publications within the area of closed-loop systems in greenhouses. This meta-research provides a clearer definition of the term “closed-loop system” within the context of greenhouses, as the term was previously vaguely defined. Using this meta-research approach, we identify six major existing research topic areas in closed-loop agriculture systems, which include: models and controls; food waste; nutrient systems; growing media; heating; and energy. Furthermore, we identify four areas that require further urgent work, which include the establishment of better connection between academic research to industry applications; clearer criteria surrounding growing media selection; critical operational requirements of a closed-loop system; and the functionality and synergy between the many modules that comprise a closed-loop greenhouse systems.
Climate change and a growing global population pose ongoing threats to critical resources. As resources required by the agriculture sector continue to diminish, it is critical to leverage the emerging technologies and new solutions within the sector. New cultivation practices have emerged over the years, allowing food to be grown within urban areas. Greenhouses are versatile in the resources needed for their operation, as well as the foods that can be grown. While greenhouses provide a potential for a more constant food supply, there is a lack of optimization between the components. There are benefits to having modular components of a greenhouse, allowing for adjustments or repairs to singular pieces. However, there is inefficiency in the entire system, since each component functions without considering the others. To improve greenhouse efficiency, a closed-loop system can be introduced. A greenhouse is a closed system, and by repurposing, reusing, and recirculating resources, a greenhouse can evolve to have a closed-loop system. This enables the components of a system to share resources more effectively, communicate any systems changes that are required, and minimize waste outputs.
This research explores the current technology in the space of agriculture and computer science to create a fully closed-loop system. The most noticeable system components are food waste, nutrient systems, water systems, growing media, and heating and energy. Not all components within a greenhouse can leverage the same artificial intelligence methods and techniques based on existing findings. There are methods in place that allow the components to interpret data gathered from the greenhouse and alter its operational patterns. There remains a lack in communicating this information to other aspects of the system to have it make informed data-driven decisions as well. One can optimize singular components thereby reducing resource reliance, to a certain threshold until it impacts the plant’s development and yield. When all the systems components’ resource needs and outputs converge the functionality of the system can be optimized to utilize resources at a higher efficiency. Results are indicative of very siloed and isolated research, exploring closed-loop systems within greenhouses, but not leveraging its full capabilities.
Aviation industry has a vital role in the economic growth and social development of the world. But, it shows negative impacts on the environment and thus this industry is looking for sustainable and clean fuel. Hydrogen energy is trusted as a promising fuel in the aviation industry, owing to the remarkable advantages. Different techniques, driven by thermal, light and electrical energies were developed for hydrogen production. Among others, light assisted hydrogen production techniques have been considered in this review. Here, we have briefed the significance of photofermentation, photochemical methods (biophotolysis, photocatalytic water splitting and photocatalytic reforming) and light assisted electrochemical methods (light assisted electrolytic cell, microbial fuel cell and photocatalytic fuel cell) in hydrogen production. This review is also particularly focused on the potential use of wastes as feedstock/substrate in the above mentioned methods. Importance of nanomaterials in light assisted hydrogen production techniques has been discussed. Advancements in the improvement strategies for the production of hydrogen with high rate and yield have also been discussed. Based on the literature review, current challenges have been proposed for advancing the research in this field.
The world's population is expected to grow at an increasing rate, leading to increased food consumption and waste production. Even though food waste represents one of the most challenging economic and environmental issues of the 21st century, it also provides a vast array of valuable resources. To address the challenge, this study uses resource recovery from food waste to close the supply chain loop, which is the cornerstone of a circular economy. By applying the bibliometric review technique, trends and patterns in food waste and circular economy were studied. The analysis of frequent keywords in the field provided insights into further research directions. A Boolean search of the keywords in the Scopus database resulted in 288 articles, published between 2015 and 2021. Further screening of titles, keywords, and abstracts resulted in 155 journal articles. Biblio-metric coupling, including authors' co-citation data, co-occurrence, and the occurrence of keywords, was graphically mapped using VOSviewer software. From the analysis of the publications, eight broad themes emerged: (1) anaerobic digestion of food waste for circular economy creation; (2) food waste systems and life cycle assessments for circular economy; (3) bio-based circular economy approaches ; (4) consumer behavior and attitudes toward circular economies; (5) food supply chains and food waste in a circular economy; (6) material flow analysis and sustainability; (7) challenges, policies, and practices to achieve circularity; and (8) circular economy and patterns of consumption. Based on the eight themes, we emphasize an urgent need to promote the collaboration of governments , the private sector, educational institutions, and researchers, who should combine efforts to promote, integrate and accelerate acceptance of circularity, which will potentially mitigate greenhouse emissions associated with food loss and waste. We also highlight an opportunity to encourage consumer acceptance of upcycled food in the food waste hierarchy. In addition, we deduce that there is a need to quantify food waste and emissions of greenhouse gases due to this waste along the food value chain; this is important as it is one pathway of examining the 'food leaks' along the food supply chain. This can then inform optimal strategies targeting specific areas of the food supply chain experiencing food leaks. Lastly, food wastage affects the entire globe; however, future studies and funding need to be channeled towards investigating the possibility of implementing circularity in developing countries.
Polyhydroxyalkanoates (PHA) are a group of biopolymers produced naturally by microorganisms with properties similar to various petroleum-based plastics. However, to date their commercial production has remained uncompetitive due to substrate, sterilization, aeration and processing costs. Purple non-sulfur bacteria (PNSB) are a group of anoxygenic photoheterotrophic bacteria that have the ability to accumulate PHA under unbalanced conditions in anaerobic environments and constant feeding with high conversion ratios. Such characteristics could potentially overcome some of the bottlenecks of conventional chemoheterotrophic PHA production. Yet these organisms have received relatively limited attention. This review explores the factors involved in the PHA accumulation process from PNSB, highlighting the differences to conventional PHA production and the areas yet to be optimized. The roles of fermentation systems, carbon substrate, feeding conditions, nutrients, pH and various aspects of light are reviewed to understand their role in PHA accumulation in PNSB.
In this study, two different organic fractions of municipal solid waste (OFMSW) served as raw material in a novel treatment process that combines thermal hydrolysis (TH) pretreatment at different times, followed by anaerobic digestion of the solid fraction and photo-fermentation of the liquid fraction. The results indicate that both wastes performed similarly, and no statistically relevant differences stand out on the overall performance regarding TH times. The thermal pretreatment improves the biodegradability of the solid fraction during anaerobic digestion compensating the loss of the organic matter in the liquid fraction. The produced biogas may feed a combined heat and power (CHP) system, making the process energetically positive in all studied scenarios. In addition, the combination of TH and anaerobic digestion decreased the volume of the waste to be disposed by 59-61%, which is 5-11% higher than that obtained with the traditional treatment of anaerobic digestion process. Specific phototrophic activity tests were performed on the liquid phase using a mixed culture of purple phototrophic bacteria (PPB) that consumed up to 80% of the soluble organics. The assays yielded an average 52% efficiency on specific phototrophic activity (kM) and 62% on biomass yield (YX/S), compared to an optimized growth medium. PPB was also capable of producing polyhydroxyalkanoates, bioH2 and single-cell protein without optimization. Apart from methane, the overall mass balances showed yields up to 150 g of high added-value products per Kg of initial total solids on this proof-of-concept platform.
This study examined the impact of thermal hydrolysis process (THP) pre-treatment on anaerobic co-digestion of wastewater sludge and household waste and assessed whether THP was vital to achieve higher process capacity. Performance data were collected for both industrial- and laboratory-scale digesters and response in microbial community structure was evaluated by Illumina sequencing. Implementation of THP at the industrial-scale plant increased methane yield by 15% and enhanced substrate degradability. Possibility to extend the sludge retention time due to a higher solid content of the substrate, sanitisation of the digestate and improved fertiliser quality of the digestate were other industrial-scale benefits of THP installation. Continuously-fed laboratory-scale digesters were fed THP-treated or untreated substrate at an organic loading rate (OLR) of 5 g volatile solid (VS)/L/day, a feeding rate necessary at the corresponding industrial-scale plant to meet the estimated population increase within the municipality. The results indicated that the plant could have increased the capacity with unimpaired stability independently of THP installation, even though the retention time was significantly shortened during operation with untreated substrate. Microbial community analyses revealed increased contribution of the Clostridia class after THP installation in industrial-scale digesters and positive correlation between Firmicutes:Bacteriodetes and methane yield in all digesters. Differentiated profiles in laboratory-scale digesters indicated that a temperature increase from 37 to 42 °C in association with THP installation and altered substrate composition were strong determining factors shaping the microbial community. Overall, these findings can assist industrial-scale plants in choosing management strategies aimed at improving the efficiency of anaerobic digestion processes.
Aquaculture is the fastest growing animal food production industry, now producing 50% of all food fish. However, aquaculture feeds remain dependent on fishmeal derived from capture fisheries, which must be reduced for continued sustainable growth. Purple phototrophic bacteria (PPB) efficiently yield biomass from wastewater with high product homogeneity, a relatively high protein fraction, and potential added value as an ingredient for fish feeds. Here we test bulk replacement of fishmeal with PPB microbial biomass in diets for Asian sea bass (Lates calcarifer), a high value carnivorous fish with high protein to energy requirement. Mixed culture PPB were grown in a novel 1 m ³ attached photo-biofilm process using synthetic and real wastewater. Four experimental diets were formulated to commercial specifications but with the fishmeal substituted (0%, 33%, 66%, and 100%) with the synthetic grown PPB biomass and fed to a cohort of 540 juvenile fish divided amongst 12 tanks over 47 days. Weight and standard length were taken from individual fish at 18, 28, and 47d. No significant difference in survival was observed due to diet or other factors (94–100%). There was a negative correlation between PPB inclusion level and final weight (p = 5.94 × 10 ⁻⁵ ) with diet accounting for 4.1% of the variance over the trial (general linear model, R ² = 0.96, p = 1 × 10 ⁻⁶ ). Feed conversion ratio was also significantly influenced by diet (p = 6 × 10 ⁻⁷ ) with this factor accounting for 89% of variance. Specifically, feed conversion ratio (FCR) rose to 1.5 for the 100% replacement diet during the last sample period, approximately 1.0 for the partial replacement, and 0.8 for the nil replacement diet. However, this study demonstrates that bulk replacement of fishmeal by PPB is feasible, and commercially viable at 33% and 66% replacement.
Contributing to environmental pollution and resources depletion, food waste represents a considerable inefficiency of the global food system. Within the United Nations Sustainable Development Goal 12.3, countries committed to halve per-capita food waste generated at retail and consumer levels and to decrease food waste along the food supply chain by 2030. Reliable and detailed information on food waste is of utmost importance for the actors of the food supply chain, organizations and governments willing to implement and monitor effective reduction strategies. The present paper is a review of existing studies on food waste generation at the global and European scales and aims primarily at describing and comparing the approaches adopted, and secondarily at analysing their potential in supporting food waste related European interventions and policies. Ten studies were selected among relevant scientific papers and grey literature and their underlying quantification methodologies were systematically analysed. Methodological elements discussed in the paper include type of waste streams captured by estimations, distinction between edible and inedible food waste along the agro-food supply chain, reported units of measure, overall inefficiencies of the food system, and uncertainty of data. Current estimations of food loss and waste generation range between 194-389 kg per person per year at the global scale, and between 158-298 kg per person per year at the European scale. However, further efforts are needed to improve their level of detail and reliability and to foster their support to food loss and waste-related strategies.
This critical review outlines a roadmap for the conversion of chemical oxygen demand (COD) contained in sewage to commodities based on three-steps: capture COD as sludge, ferment it to volatile fatty acids (VFA), and upgrade VFA to products. The article analyzes the state-of-the-art of this three-step approach and discusses the bottlenecks and challenges. The potential of this approach is illustrated for the European Union’s 28 member states (EU-28) through Monte Carlo simulations. High-rate contact stabilization captures the highest amount of COD (66-86 gCOD person equivalent⁻¹ day⁻¹ in 60% of the iterations). Combined with thermal hydrolysis, this would lead to a VFA-yield of 23-44 gCOD person equivalent⁻¹ day⁻¹. Upgrading VFA generated by the EU-28 would allow, in 60% of the simulations, for a yearly production of 0.2-2.0 megatonnes of esters, 0.7-1.4 megatonnes of polyhydroxyalkanoates or 0.6-2.2 megatonnes of microbial protein substituting, respectively, 20-273%, 70-140% or 21-72% of their global counterparts (i.e.. petrochemical-based esters, bioplastics or fishmeal). From these flows, we conclude that sewage holds a strong potential as biorefinery feedstock, although research is needed to enhance capture, fermentation and upgrading efficiencies. These developments need to be supported by economic/environmental analyses and policies that incentivize a more sustainable management of our resources.
Purple phototrophic bacteria (PPB) have been recently proposed as a key potential mechanism for accumulative biotechnologies for wastewater treatment with total nutrient recovery, low greenhouse gas emissions, and a neutral to positive energy balance. Purple phototrophic bacteria have a complex metabolism which can be regulated for process control and optimization. Since microbial processes governing PPB metabolism differ from traditional processes used for wastewater treatment (e.g., aerobic and anaerobic functional groups in ASM and ADM1), a model basis has to be developed to be used as a framework for further detailed modelling under specific situations. This work presents a mixed population phototrophic model for domestic wastewater treatment in anaerobic conditions. The model includes photoheterotrophy, which is divided into acetate consumption and other organics consumption, chemoheterotrophy (including simplified fermentation and anaerobic oxidation) and photoautotrophy (using hydrogen as an electron donor), as microbial processes, as well as hydrolysis and biomass decay as biochemical processes, and is single-biomass based. The main processes have been evaluated through targeted batch experiments, and the key kinetic and stoichiometric parameters have been determined. The process was assessed by analyzing a continuous reactor simulation scenario within a longterm wastewater treatment system in a photo-anaerobic membrane bioreactor.
The effect of different oxidation processes at mild conditions including the coupled-Fenton (sono-Fenton, photo-Fenton and sono-photo-Fenton) and their blank systems (ultrasound, ultraviolet, zero valent iron and Fenton) on anaerobic digestion of the sludge for biogas production was investigated. Ultrasounds led to the highest organic matter solubilization (3.8 up to 5.2 g COD/L, for the raw and treated sludge, respectively), while for the rest, organic matter transformation was observed resulting in an almost soluble COD net balance. Results indicated that for the most oxidative processes, the released organic matter was probably mineralized by the hydroxyl radicals produced during the treatments. It is interesting to remark that even if the biochemical methane potential was barely enhanced by the different methods applied, all the methods demonstrated to enhance the overall kinetics of the biomethanation processes, increasing the rapidly biodegradable fraction of the sludge.
Anaerobic digestion of autoclaved (160°C, 6.2bar) and untreated source segregated food waste (FW) was compared over 473days in semi-continuously fed mesophilic reactors with trace elements supplementation, at organic loading rates (OLRs) of 2, 3, 4 and 6kg volatile solids(VS)/m(3)d. Methane yields at all OLR were 5-10% higher for untreated FW (maximum 0.483±0.013m(3) CH4/kgVS at 3kgVS/m(3)d) than autoclaved FW (maximum 0.439±0.020m(3) CH4/kgVS at 4kgVS/m(3)d). The residual methane potential of both digestates at all OLRs was less than 0.110m(3) CH4/kgVS, indicating efficient methanation in all cases. Use of acclimated inoculum allowed very rapid increases in OLR. Reactors fed on autoclaved FW showed lower ammonium and hydrogen sulphide concentrations, probably due to reduced protein hydrolysis as a result of formation of Maillard compounds. In the current study this reduced biodegradability appears to outweigh any benefit due to thermal hydrolysis of ligno-cellulosic components.
Treatment of poultry industry effluents produces wastewater sludge with high levels of organic compounds and pathogenic microorganisms. In this research, the thermal pre-treatment of poultry slaughterhouse sludge (PSS) was evaluated for low temperatures in combination with different exposure times as a pre-hydrolysis strategy to improve the anaerobic digestion process. Organic compounds solubilization and inactivation of pathogenic microorganisms were evaluated after treatment at 70, 80 or 90°C for 30, 60 or 90 min. The results showed that 90°C and 90 min were the most efficient conditions for solubilization of the organic compounds (10%). In addition, the bacteria populations and the more resistant structures, such as helminth eggs (HE), were completely inactivated. Finally, the thermal pre-treatment applied to the sludge increased methane yield by 52% and reduced hydraulic retention time (HRT) by 52%.
Food waste in the global food supply chain is reviewed in relation to the prospects for feeding a population of nine billion by 2050. Different definitions of food waste with respect to the complexities of food supply chains (FSCs)are discussed. An international literature review found a dearth of data on food waste and estimates varied widely; those for post-harvest losses of grain in developing countries might be overestimated. As much of the post-harvest loss data for developing countries was collected over 30 years ago, current global losses cannot be quantified. A significant gap exists in the understanding of the food waste implications of the rapid development of 'BRIC' economies. The limited data suggest that losses are much higher at the immediate post-harvest stages in developing countries and higher for perishable foods across industrialized and developing economies alike. For affluent economies, post-consumer food waste accounts for the greatest overall losses. To supplement the fragmentary picture and to gain a forward view, interviews were conducted with international FSC experts. The analyses highlighted the scale of the problem, the scope for improved system efficiencies and the challenges of affecting behavioural change to reduce post-consumer waste in affluent populations.
The application of anaerobic digestion technology is growing worldwide because of its economic and environmental benefits. As a consequence, a number of studies and research activities dealing with the determination of the biogas potential of solid organic substrates have been carrying out in the recent years. Therefore, it is of particular importance to define a protocol for the determination of the ultimate methane potential for a given solid substrates. In fact, this parameter determines, to a certain extent, both design and economic details of a biogas plant. Furthermore, the definition of common units to be used in anaerobic assays is increasingly requested from the scientific and engineering community. This paper presents some guidelines for biomethane potential assays prepared by the Task Group for the Anaerobic Biodegradation, Activity and Inhibition Assays of the Anaerobic Digestion Specialist Group of the International Water Association. This is the first step for the definition of a standard protocol.
Three differently metabolically engineered strains, 2 single PHA- and Hup- mutants and one double PHA-/Hup- mutant, of the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides RV, were constructed to improve a light-driven biohydrogen production process combined with the disposal of solid food wastes. These phenotypes were designed to abolish, singly or in combination, the competition of H2 photoproduction with polyhydroxyalkanoate (PHA) accumulation by inactivating PHA synthase activity, and with H2 recycling by abolishing the uptake hydrogenase enzyme. The performance of these mutants was compared with that of the wild-type strain in laboratory tests carried out in continuously fed photobioreactors using as substrates both synthetic media containing lactic acid and media from the acidogenic fermentation of actual fruit and vegetable wastes, containing mainly lactic acid, smaller amounts of acetic acia, and traces of higher volatile acids. With the lactic acid-based synthetic medium, the single Hup- and the double PHA-/Hup- mutants, but not the single PHA- mutant, exhibited increased rates of H2 photoproduction, about one third higher than that of the wild-type strain. With the food-waste-derived growth medium, only the single Hup- mutant showed higher rates of H2 production, but all 3 mutants sustained a longer-term H2 photoproduction phase than the wild-type strain, with the double mutant exhibiting overall the largest amount of H2 evolved. This work demonstrates the feasibility of single and multiple gene engineering of microorganisms to redirect their metabolism for improving H2 photoproduction using actual waste-derived substrates.
Purple non-sulfur bacteria (Rhodospirillaceae) have been extensively employed for studying principles of photosynthetic and respiratory electron transport phosphorylation and for investigating the regulation of gene expression in response to redox signals. Here, we use mathematical modeling to evaluate the steady-state behavior of the electron transport chain (ETC) in these bacteria under different environmental conditions. Elementary-modes analysis of a stoichiometric ETC model reveals nine operational modes. Most of them represent well-known functional states, however, two modes constitute reverse electron flow under respiratory conditions, which has been barely considered so far. We further present and analyze a kinetic model of the ETC in which rate laws of electron transfer steps are based on redox potential differences. Our model reproduces well-known phenomena of respiratory and photosynthetic operation of the ETC and also provides non-intuitive predictions. As one key result, model simulations demonstrate a stronger reduction of ubiquinone when switching from high-light to low-light conditions. This result is parameter insensitive and supports the hypothesis that the redox state of ubiquinone is a suitable signal for controlling photosynthetic gene expression.
Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a “removal and disposal” approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g CODbiomass·g CODremoved−1)), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9, 0.3–2.2 and 0.1–0.3 $·kgdry biomass−1. At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5–2.0 $·kg−1) appears as a promising valorisation route.
Propagation of emerging pollutants (EPs) in wastewater
treatment plants has become a warning sign, especially for novel
resource-recovery concepts. The fate of EPs on purple phototrophic
bacteria (PPB)-based systems has not yet been determined. This work
analyzes the performance of a photo-anaerobic membrane bioreactor
treating a low-N wastewater contaminated with 25 EPs. The chemical oxygen
demand (COD), N and P removal efficiencies were stable (76±8, 62±15 and
36±8%, respectively) for EPs loading rate ranging from 50 to 200 ng L-1
d-1. The PPB community adapted to changes in both the EPs concentration
and the organic loading rate (OLR) and maintained dominance with >85% of
total 16S gene copies. Indeed, an increment of the OLR caused an increase
of the biomass growth and activity concomitantly with a higher EPs
removal efficiency (30±13 vs 54±11% removal for OLR of 307±4 and 590±8
mgCOD L-1 d-1, respectively). Biodegradation is the main mechanism of EPs
removal due to low EPs accumulation on the biomass, the membrane or the
reactor walls. Low EPs adsorption avoided biomass contamination,
resulting in no effect on its biological methane potential. These results
support the use of PPB technologies for resource recovery with low EPs
contamination of the products.
This study evaluated the performance of purple phototrophic bacteria (PPB) at temperatures of 22 °C and 11 °C in a membrane photobioreactor. PPB performance and Monod parameters were evaluated at 11 °C and were compared to PPB performance at 22 °C. At 22 °C, the optimum HRT with respect to meeting target limits of TCOD < 50 mg/L, TN < 10 mg/L, and TP < 1 mg/L was 9 h with ethanol supplementation at 300 mgCODL−1 noting that ethanol was only needed to meet the TN limit. However, at 11 °C, the photobioreactor achieved effluent TCOD, TN, and TP concentrations of 70 mg/L, 10 mg/L, and 1.2 mg/L respectively at 9 h HRT and 300 mgCOD/L ethanol addition. Monod kinetic parameters Ks, K, Y, and Kd were determined to be 20 mgCOD/L, 1.8 mgCOD/(mgVSS.d), 0.54 mgVSS/mgCOD, and 0.09 d−1 at 22 °C and 31 mgCOD/L, 1.2 mgCOD/(mgVSS.d), 0.51 mgVSS/mgCOD, and 0.07 d−1 at 11 °C respectively.
Biological removal of organics, nitrogen and from saline wastewaters is adversely impacted by high salinity, which can be a major concern for treatment of industrial or domestic saline wastewater. In anaerobic treatment systems, sulfidogensis, especially when treating sulfate-rich saline wastewaters (e.g. seawater has 930 mgSO4-S L-1, or 2800 mg L-1 as SO42-) can cause additional biological, operational, and safety issues, due to H2S toxicity. Here, the use of anaerobic purple phototrophic bacteria (PPB) is tested as mediator to treat high salinity domestic wastewater (NaCl), and marine wastewater (Red Sea Salt - high sulfate, potassium, etc.) in a continuous anaerobic infra-red photo bioreactor, operated over 372d. Saline adapted PPB simultaneously removed COD, nitrogen and phosphorus with biomass yields of 0.8 gCOD gCOD-1. Batch activity tests found a broad optimum peak for saline adapted PPB between 30 and 70 mS cm-1, and 50% reduced activity at 140 mS cm-1 (3.5x seawater). For marine wastewater, high sulfate influent concentrations (770 mgSO4-S L-1) did not result in substantial H2S production (<1.6 mgS L-1) over 80 d. When irradiation was removed, sulfide rapidly rose to >90 mgS L-1 and the process failed. The results indicate rapid adaptation to high-salt conditions (both NaCl and marine), and the capacity for PPB to form a combined wastewater treatment/resource recovery process, particularly for salty industrial wastewater.
Polyhydroxyalkanoates (PHAs) production with phototrophic mixed cultures (PMCs) has been recently proposed. These cultures can be selected under the permanent presence of carbon and the PHA production can be enhanced in subsequent accumulation steps. To optimize the PHA production in accumulator reactors, this work evaluated the impact of 1) initial acetate concentration, 2) light intensity, 3) removal of residual nitrogen on the culture performance. Results indicate that low acetate concentration (<30 CmM) and specific light intensities around 20 W/gX are optimal operating conditions that lead to high polyhydroxybutyrate (PHB) storage yields (0.83 ± 0.07 Cmol-PHB/Cmol-Acet) and specific PHB production rates of 2.21 ± 0.07 Cmol-PHB/Cmol X d. This rate is three times higher than previously registered in non-optimized accumulation tests and enabled a PHA content increase from 15 to 30% in <4 h. Also, it was shown for the first time, the capability of a PMC to use a real waste, fermented cheese whey, to produce PHA with a hydroxyvalerate (HV) content of 12%. These results confirm that fermented wastes can be used as substrates for PHA production with PMCs and that the energy levels in sunlight that lead to specific light intensities from 10 to 20 W/gX are sufficient to drive phototrophic PHA production processes.
Nutrient losses in our food chain severely surpass our planetary boundaries. Resource recovery can contribute to mitigation, for instance through converting wastewater resources to microbial protein for animal feed. Wastewater typically holds a complex mixture of organics, posing a challenge to selectively produce heterotrophic biomass. Ensuring the product's quality could be achieved by anaerobic generation of volatile fatty acids (VFAs) followed by photoheterotrophic production of purple non-sulfur bacteria (PNSB) with infrared light. This study aimed to determine the most suitable PNSB culture for VFA conversion and map the effect of acetate, propionate, butyrate and a VFA mixture on growth and biomass yield. Six cultures were screened in batch: (i) Rhodopseudomonas palustris, (ii) Rhodobacter sphaeroides, (iii) Rhodospirillum rubrum, (iv) a 3-species synthetic community (i+ii+iii), (v) a community enriched on VFA holding Rb. capsulatus, and (vi) Rb. capsulatus (isolate ‘v’). The VFA mixture elevated growth rates with a factor 1.3–2.5 compared to individual VFA. Rb. capsulatus showed the highest growth rates: 1.8–2.2 d−1 (enriched) and 2.3–3.8 d−1 (isolated). In a photobioreactor (PBR) inoculated with the Rb. capsulatus enrichment, decreasing sludge retention time (SRT) yielded lower biomass concentrations, yet increased productivities, reaching 1.7 g dry weight (DW) L−1 d−1, the highest phototrophic rate reported thus far, and a growth rate of up to 5 d−1. PNSB represented 26–57% of the community and the diversity index was low (3–7), with a dominance of Rhodopseudomonas at long SRT and Rhodobacter at short SRT. The biomass yield for all cultures, in batch and reactor cultivation, approached 1 g CODBiomass g−1 CODRemoved. An economic estimation for a two-stage approach on brewery wastewater (load 2427 kg COD d−1) showed that 0.5 d SRT allowed for the lowest production cost (€ 10 kg−1 DW; equal shares for capex and opex). The findings strengthen the potential for a novel two-stage approach for resource recovery from industrial wastewater, enabling high-rate PNSB production.
Biogas utilization in fuel cell technology and hydrogen generation is a modern and economically viable approach. A pretreatment step prior to anaerobic digestion (AD) is obligatory to increase the hydrolysis, solubilize the complex matter present in organic fraction of municipal solid waste (OFMSW) and to achieve higher yield of biogas. This study was intended to find out the effects of thermal, chemical and thermochemical pretreatments on the properties and structure of OFMSW and also on biogas production. There was an increase in chemical oxygen demand of 6.87, 1.61 and 11.60% for thermal, chemical and thermochemical pretreatments, respectively. Also, the content of volatile solids was reduced by 2.36% by thermochemical pretreatment. FTIR, XRD and SEM analysis revealed that these pretreatments also caused chemical and morphological changes on the substrate, as a result reduced its crystallinity and enhanced the rate of hydrolysis. A significant increase of 54% in biogas yield was achieved after thermochemical pretreatment in comparison to untreated OFMSW sample.
Engineering microorganisms capable of simultaneously accumulating multiple products are economically attractive for biotechnology. Polyhydroxyalkanoates (PHA) or microbial bioplastics are promising as biodegradable plastics to address environmental concerns resulted from plastic wastes accumulation. Unfortunately, PHA production is still limited and cannot compete with the chemically synthesized plastics due to their high production cost. Efforts have been devoted to reduce PHA production cost by employing PHA co-production with other valuable chemicals. Successful co-productions of PHA have been demonstrated with amino acids, proteins, alcohols, hydrogen, biosurfactants, exopolysacharides and several fine chemicals. The strategy allows recovering PHA from the cells and other value-added products from the no-cells broths. Numerous successful strategies have been developed for minimizing the substrate cost and improving the product yields. This paper reviews the recent strategies developed in PHA co-production with other compounds, discusses the challenges and prospective during the scale up of the co-production strategies.
The growing amount of food waste (FW) in China poses great pressure on the environment. Complex solid organics limit the hydrolysis of FW, hence impairing anaerobic digestion. This study employed hydrothermal pretreatment (HTP) to facilitate the solubilization of FW. When HTP temperature was increased from 100 to 200 °C, soluble carbohydrate content first increased to a peak at 140 °C and then decreased, whereas total carbohydrate content was negatively correlated with increasing temperature due to the enhanced degradation and Maillard reactions. Protein solubilization was dramatically promoted after HTP, whereas protein degradation was negligibly enhanced. The hydrogen and methane yields from hydrothermally pretreated FW under the optimum condition (140 °C, 20 min) through two-stage fermentation were 43.0 and 511.6 mL/g volatile solids, respectively, resulting in an energy conversion efficiency (ECE) of 78.6%. The ECE of pretreated FW was higher than that of untreated FW by 31.7%.
A Rhodobacter capsulatus strain and a photoheterotrophic culture (IZT) were cultivated to produce hydrogen under different light-dark cycles. A dark fermentation effluent (DFE) was used as substrate. It was found that IZT culture had an average cumulative hydrogen production (Paccum H2) of 1300 ± 43 mL H2 L⁻¹ under continuous illumination and light-dark cycles of 30 or 60 min. In contrast, R. capsulatus reduced its Paccum H2 by 20% under 30:30 min light-dark cycles, but tripled its poly-β-hydroxybutyrate (PHB) content (308±2 mgPHB gdw⁻¹) compared to continuous illumination. The highest PHB content by IZT culture was 178±10 mgPHB gdw⁻¹ under 15:15 min light-dark cycles. PCR-DGGE analysis revealed that the IZT culture was mainly composed of Rhodopseudomonas palustris identified with high nucleotide similarity (99%). The evaluated cultures might be used for hydrogen and PHB production. They might provide energy savings by using light-dark cycles and DFE valorization.
A key future challenge of domestic wastewater treatment is nutrient recovery while still achieving acceptable discharge limits. Nutrient partitioning using purple phototrophic bacteria (PPB) has the
potential to biologically concentrate nutrients through growth. This study evaluates the use of PPB in a continuous photo-anaerobic membrane bioreactor (PAnMBR) for simultaneous organics and nutrient removal from domestic wastewater. This process could continuously treat domestic
wastewater to discharge limits (<50 mgCOD L-1, 5 mgN L-1, 1.0 mgP L-1). Approximately 6.4±1.3 gNH4-N and 1.1±0.2 gPO4-P for every 100 gSCOD were removed at a hydraulic retention time of 8 - 24 hours and volumetric loading rates of 0.8 - 2.5 COD kg m3 d-1. Thus, a minimum of 200 mg L-1
of ethanol (to provide soluble COD) was required to achieve these discharge limits. Microbial community through sequencing indicated dominance of >60% of PPB, though the PPB community was highly variable. The outcomes from the current work demonstrate the potential of PPB for
continuous domestic (and possibly industrial) wastewater treatment and nutrient recovery. Technical challenges include the in situ COD supply in a continuous reactor system, as well as efficient light delivery. Addition of external (agricultural or fossil) derived organics is not financially nor environmentally justified, and carbon needs to be sourced internally from the biomass itself to enable this technology. Reduced energy consumption for lighting is technically feasible, and needs to be
addressed as a key objective in scaleup.
The purpose of this study was to optimize the alkaline, ultrasonication, and thermal pretreatment in order to enhance the solubilization of food waste (FW) for the production of volatile fatty acids, hydrogen, and methane in thermophilic batch anaerobic digestion. Initially, the effect of pretreatment techniques in the acidogenic phase was studied, and the optimal combinations of different conditions were determined. It was found that each pretreatment technique affected food waste solubilization differently. Alkaline pretreatment increased hydrogen yield in the acidogenic sludge by four times over control. COD solubilization was increased by 47 % when FW pre-heated at 130 °C for 60 min. Ultrasonication at 20 kHz and 45 min reduced processing time to 38 h from the 60–80 h needed in normal operation. Response surface methodology (RSM) was used to optimize a combination of alkaline, ultrasonication, and thermal pretreatment. Optimized conditions were applied to methanogenic single-stage thermophilic AD process, and their impact on biogas production was monitored. Results showed that FW heated at 130 °C for 50 min geminates biogas production compared to control experiment. In conclusion, a short thermal pretreatment regime could significant affect biogas production in single-stage thermophilic AD.
The increase in the world population, vulnerability of conventional crop production to climate change, and population shifts to megacities justify a re-examination of current methods of converting reactive nitrogen to dinitrogen gas in sewage and waste treatment plants. Indeed, by up-grading treatment plants to factories in which the incoming materials are first deconstructed to units such as ammonia, carbon dioxide and clean minerals, one can implement a highly intensive and efficient microbial re-synthesis process in which the used nitrogen is harvested as microbial protein (at efficiencies close to 100%). This can be used for animal feed and food purposes. The technology for recovery of reactive nitrogen as microbial protein is available but a change of mindset needs to be achieved to make such recovery acceptable.
Microalgal biomass grown in wastewater treatment raceway ponds may be valorised producing bioenergy through anaerobic digestion. However, pretreatment techniques seem to be necessary for enhancing microalgae methane yield. In this study, hydrothermal pretreatment was studied prior to batch and continuous reactors. The pretreatment increased organic matter solubilisation (8-13%), anaerobic digestion rate (30-90%) and final methane yield (17-39%) in batch tests. The highest increase was attained with the pretreatment at 130 degrees C for 15 min, which was attested in a laboratory-scale continuous reactor operated at a hydraulic retention time of 20 days with an average organic loading rate of 0.7 g VS/L center dot day. The methane yield increased from 0.12 to 0.17 L CH4/g VS (41%) in the pretreated digester as compared to the control. Microscopic images of microalgal biomass showed that pretreated cells had unstructured organelles and disrupted cell wall external layer, which may enhance the hydrolysis. Indeed, images of the pretreated reactor digestate showed how cells were more degraded than in the control reactor.
The uncontrolled discharge of large amounts of food waste (FW) causes severe environmental pollution in many countries. Within different possible treatment routes, anaerobic digestion (AD) of FW into biogas, is a proven and effective solution for FW treatment and valorization. The present paper reviews the characteristics of FW, the principles of AD, the process parameters, and two approaches (pretreatment and co-digestion) for enhancing AD of food waste. Among the successive digestion reactions, hydrolysis is considered to be the rate-limiting step. To enhance the performance of AD, several physical, thermo-chemical, biological or combined pretreatments are reviewed. Moreover, a promising way for improving the performance of AD is the co-digestion of FW with other organic substrates, as confirmed by numerous studies, where a higher buffer capacity and an optimum nutrient balance enhance the biogas/methane yields of the co-digestion system.
Biological hydrogen production has been known for over a century and research directed at applying this process to a practical means of hydrogen fuel production has been carried out for over a quarter century. The various approaches that have been proposed and investigated are reviewed and critical limiting factors identified. The low energy content of solar irradiation dictates that photosynthetic processes operate at high conversion efficiencies and places severe restrictions on photobioreactor economics. Conversion efficiencies for direct biophotolysis are below 1% and indirect biophotolysis remains to be demonstrated. Dark fermentation of biomass or wastes presents an alternative route to biological hydrogen production that has been little studied. In this case the critical factor is the amount of hydrogen that can be produced per mole of substrate. Known pathways and experimental evidence indicates that at most 2–3mol of hydrogen can be obtained from substrates such as glucose. Process economics require that means be sought to increase these yields.
The effects of temperature and pressure on the Maillard reaction between ɛ-polylysine and dextran in subcritical water were investigated. Browning index was determined at 420 nm, and degree of graft was estimated by O-phthaldialdehyde (OPA) method. The formation of conjugates by Maillard reaction was testified by UV–vis spectrum and SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The emulsifying activity of the conjugates was greatly improved with the rise of temperature. The antimicrobial activity of the conjugates formed below 110 °C remained the same as ɛ-polylysine, and then decreased obviously when the reaction temperature was above 120 °C. The reaction pressure (0–10 MPa) had no effect on the emulsifying and antimicrobial activities, but higher pressure could effectively inhibit the formation of browning compounds.
An economic assessment of thermal hydrolysis as a pretreatment to anaerobic digestion has been achieved to evaluate its implementation in full-scale plants. Six different solid wastes have been studied, among them municipal solid waste (MSW). Thermal hydrolysis has been tested with batch lab-scale tests, from which an energy and economic assessment of three scenarios is performed: with and without energy integration (recovering heat to produce steam in a cogeneration plant), finally including the digestate management costs. Thermal hydrolysis has lead to an increase of the methane productions (up to 50%) and kinetics parameters (even double). The study has determined that a proper energy integration design could lead to important economic savings (5€/t) and thermal hydrolysis can enhance up to 40% the incomes of the digestion plant, even doubling them when digestate management costs are considered. In a full-scale MSW treatment plant (30,000t/year), thermal hydrolysis would provide almost 0.5M€/year net benefits.
The organics and nutrients in industrial and domestic wastewater are increasingly being regarded as a valuable resource for energy and nutrient recovery. Emerging concepts to redesign wastewater treatment as resource recovery systems include the use of different bacteria and algae to partition carbon and nutrients to the particulate phase through assimilation or bio-accumulation. This study evaluates the use of purple phototrophic bacteria (PPB) (also known as purple non-sulphur bacteria or PNSB) for such a biological concentration process through a series of batch tests. The key objectives are to (a) demonstrate consistent selection and enrichment of PPB using infrared light in a non-sterile medium, and (b) achieve effective partitioning of soluble organics, ammonium and phosphate into the PPB culture. PPB were successfully enriched from pre-settled domestic wastewater within 2-3 days and identified as members of the order Rhodobacterales. Under anaerobic conditions with infrared irradiation the enrichment culture was able to simultaneously remove COD (63 ± 5%), NH4-N (99.6%-0.12 ± 0.03 mgN L(-1)) and PO4-P (88%-0.8 ± 0.6 mgP L(-1)) from primary settled domestic wastewater in 24 h. In this experiment, acetate was added as an additional carbon source to demonstrate the maximal nitrogen and phosphorous elimination potential. Almost all the COD removed was assimilated into biomass rather than oxidised to CO2, with the total COD actually increasing during the batch experiments due to phototrophic synthesis. NH4-N and PO4-P were also assimilated by the biomass rather than removed through destructive oxidation or accumulation. The process offers the opportunity to concentrate organics and macronutrients from wastewater in one solids stream that can be anaerobically digested to generate energy and recover nutrients from the concentrated digestate. Technical challenges include the design of a continuous reactor system, as well as efficient delivery of electrons, either through light or chemical sources.
For Abstract see ChemInform Abstract in Full Text.
This review deals with the main mechanisms and parameters affecting design and performance of trickling filters in aquaculture. Relationships between nitrification rates and easily accessible process parameters, like bulk phase concentration of TAN, O2, organic matter (COD), nitrite, temperature, HCO3−, pH and the hydraulic loading of the trickling filter, are discussed in relation to the design and operation of such filters. Trickling filter design procedures are presented and one of them, a model describing the nitrification performance of trickling filters by plug-flow characteristics, is discussed in greater detail. Finally, practical aspects in relation to filter design and operation are presented.
Organic matter hydrolysis prior to anaerobic digestion has been shown to improve biogas production (30-50%) and reduce solids (20-60%) by ultrasound, chemical, conventional heating, and microwave pretreatments. Numerous studies have been performed to determine the extent of digestion improvement but few focus on financial feasibility of these processes. A comprehensive model was created using Microsoft Excel and its Visual Basic Assistant to evaluate pretreatment permutations for conventional wastewater treatment plants. The four above-mentioned processes were evaluated for energetic and financial demands. Well-established energy equations and wastewater characteristics, both average and high, were used. Average and high flows were 460 and 750×10(3) m3/d, respectively. Net costs per influent flow for ultrasound, chemical, conventional heating, and microwave were 0.0166, 0.0217, 0.0124, 0.0119 $/m3 and 0.0264, 0.0357, 0.0187, and 0.0162 $/m3 for average and high conditions, respectively. The average cost increase from results excluding pretreatment use for all processes was 0.003 and 0.0055 $/m3 for average and high conditions, respectively. No matter the permutation, pretreatments requiring more energy to achieve required hydrolysis levels were costlier. If energetic recoveries are substantial, dewaterability is positively affected, and solids meet environmental constraints to be handled and disposed at lower costs, pretreatments can be viable.
In this study the comparative destruction of municipal biosolids using thermal hydrolysis (140 or 165°C) and wet oxidation (220°C) was followed by biological degradation via mesophilic anaerobic digestion (36°C). Wet oxidation (WO) destroyed more than 93% of the VSS, while thermal hydrolysis (TH) at 140 and 165°C destroyed 9% and 22%, respectively. Combined TH and anaerobic digestion resulted in approximately 50% VSS destruction. The ultimate methane potential of the combined fractions from the thermal hydrolysis at 140 and 165°C improved by 12-13% relative to the untreated control sample. Methane production from the WO material was 53% of the control yield and wholly attributable to soluble organic carbon in the liquid fraction, indicating that the WO destroyed all putrescible carbon from the solids fraction. Point sampling during the BMP assay revealed that methanogenic development, not solids hydrolysis, was the kinetic barrier during anaerobic digestion in this study.
Five different pre-treatments were investigated to enhance the solubilisation and anaerobic biodegradability of kitchen waste (KW) in thermophilic batch and continuous tests. In the batch solubilisation tests, the highest and the lowest solubilisation efficiency were achieved with the thermo-acid and the pressure-depressure pre-treatments, respectively. However, in the batch biodegradability tests, the highest cumulative biogas production was obtained with the pressure-depressure method. In the continuous tests, the best performance in terms of an acceptable biogas production efficiency of 60% and stable in-reactor CODs and VFA concentrations corresponded to the pressure-depressure reactor, followed by freeze-thaw, acid, thermo-acid, thermo and control. The maximum OLR (5 g COD L(-1) d(-1)) applied in the pressure-depressure and freeze-thaw reactors almost doubled the control reactor. From the overall analysis, the freeze-thaw pre-treatment was the most profitable process with a net potential profit of around 11.5 € ton(-1) KW.
Commingled household waste (HW) that had a controlled composition was autoclaved at elevated pressures in the presence of saturated steam for one hour at the nominal temperature levels of 130 degrees C, 160 degrees C and 200 degrees C. The focus of this study was the impact of temperature/pressure on hydrolysis of organic matter during autoclaving and the extent of its hydrolysis. The pH decreased with autoclaving temperature with which it had a linear relationship, and ranged from 7.4 and 6 in floc, and 6.7 and 3.6 in steam condensate. Overall, organic matter solubilisation, as indicated by dissolved organic carbon, biological and chemical oxygen demands, and total dissolved solids, increased with temperature. Lignin did not appear to hydrolyse. Hemicellulose hydrolysed and degraded the most, followed by cellulose. The highest recoveries of hemicellulose and cellulose in solution were achieved at 160 degrees C, although the latter could be due to experimental error. The largest losses of hemicellulose and cellulose were recorded at 200 degrees C. The performance of the system in respect to hydrolysis was inferior compared to other hydrothermal systems, particularly those employing wet oxidation.
A two-stage fermentation process, consisting of a simultaneous saccharification and fermentation (SSF) stage and a dry methane fermentation stage, was developed to utilize garbage for the production of fuel ethanol and methane. Garbage from families, canteens and concessionaires was used for the study. Saccharification method was studied and the results indicated that the liquefaction pretreatment and the combination of cellulase and glucoamylase was effective for polysaccharide hydrolysis of family garbage with a high content of holocellulose and that SSF was suitable for ethanol fermentation of garbage. Ethanol productivity could be markedly increased from 1.7 to 7.0 g/l/h by repeated-batch SSF of family garbage. A high ethanol productivity of 17.7 g/l/h was achieved when canteen garbage was used. The stillage after distillation was treated by dry methane fermentation and the results indicated that the stillage was almost fully digested and that about 850 ml of biogas was recovered from 1 g of volatile total solid (VTS). Approximately 85% of the energy of the garbage was converted to fuels, ethanol and methane by this process.
The recycling concept under consideration is based on the process of Thermal Hydrolysis (TDH) followed by an anaerobic digestion. By increasing pressure and temperature the organic part of the waste is split up in a first step into short-chain fragments that are biologically well suited for microorganisms. The following fermentation runs much faster and more complete than in conventional digestion processes and the biogas yield is increased. Left is just a small amount of a solid residue that can be easily dewatered and utilized as surrogate fuel for incineration or as compost additive. The thermal hydrolysis process allows a complete energy recovery from organic waste. During the total procedure more energy sources are produced than are needed for running the plant. The procedure is especially suited for wet organic waste and biosolids that are difficult to compost, such as food scraps, biological waste from compact residential areas and sewage sludge. As a complete disinfection is granted due to the process temperatures the procedure is also suited for carcasses.
The degradation kinetics of normal and branched chain butyrate and valerate are important in protein-fed anaerobic systems, as a number of amino acids degrade to these organic acids. Including activated and primary wastewater sludge digesters, the majority of full-scale systems digest feeds with a significant or major fraction of COD as protein. This study assesses the validity of using a common kinetic parameter set and biological catalyst to represent butyrate, n-valerate, and i-valerate degradation in dynamic models. The i-valerate degradation stoichiometry in a continuous, mixed population system is also addressed, extending previous pure-culture and batch studies. A previously published mathematical model was modified to allow competitive uptake of i-valerate, and used to model a thermophilic manure digester operated over 180 days. The digester was periodically pulsed with straight and branched chain butyrate and valerate. Parameters were separately optimized to describe butyrate, i-valerate, and n-valerate degradation, as well as a lumped set optimized for all three substrates, and nonlinear, correlated parameter spaces estimated using an F distribution in the objective function (J). Each parameter set occupied mutually exclusive parameter spaces, indicating that all were statistically different from each other. However, qualitatively, the influence on model outputs was similar, and the lumped set would be reasonable for mixed acid digestion. The main characteristic not represented by Monod kinetics was a delay in i-valerate uptake, and was compensated for by a decreased maximum uptake rate (k(m)). Therefore, the kinetics need modification if fed predominantly i-valerate. Butyrate (i- and n-) and n-valerate could be modeled using stoichiometry consistent with beta-oxidation degradation pathways. However, i-valerate produced acetate only, supporting the stoichiometry of a reaction determined by other researchers in pure culture. Therefore, lumping i-valerate stoichiometry with that of n-valerate will not allow good system representation, especially when the feed consists of proteins high in leucine (which produces i-valerate), and the modified model structure and stoichiometry as proposed here should be used. This requires no additional kinetic parameters and one additional dynamic concentration state variable (i-valerate) in addition to the variables in the base model.
Bacterial Respiration And Photosynthesis
- C W Jones
C.W. Jones, Bacterial Respiration And Photosynthesis, Van Nostrand Reinhold Inc.,
U.S, 1982.