Production of lactic acid from thermal pretreated food waste through the fermentation of waste activated sludge: Effects of substrate and thermal pretreatment temperature

Bioprocesses for lactic acid production from organic wastes toward industrialization-a critical review

Lactic acid (LA) is a crucial chemical which has been widely used for industrial application. Microbial fermentation is the dominant pathway for LA production and has been regarded as the promising technology. In recent years, many studies on LA production from various organic wastes have been published, which provided alternative ways to reduce the LA production cost, and further recycle organic wastes. However, few researchers focused on industrial application of this technology due to the knowledge gap and some uncertainties. In this review, the recent advances, basic knowledge and limitations of LA fermentation from organic wastes are discussed, the challenges and suitable envisaged solutions for enhancing LA yield and productivity are provided to realize industrial application of this technology, and also some perspectives are given to further valorize the LA fermentation processes from organic wastes. This review can be a useful guidance for industrial LA production from organic wastes on a sustainable view.

Temperature-Driven Activated Sludge Bacterial Community Assembly and Carbon Transformation Potential: A Case Study of Industrial Plants in the Yangtze River Delta

Temperature plays a critical role in the efficiency and stability of industrial wastewater treatment plants (WWTPs). This study focuses on the effects of temperature on activated sludge (AS) communities within the A2O process of 19 industrial WWTPs in the Yangtze River Delta, a key industrial region in China. The investigation aims to understand how temperature influences AS community composition, functional assembly, and carbon transformation processes, including CO2 emission potential. Our findings reveal that increased operating temperatures lead to a decrease in alpha diversity, simplifying community structure and increasing modularity. Dominant species become more prevalent, with significant decreases in the relative abundance of Chloroflexi and Actinobacteria, and increases in Bacteroidetes and Firmicutes. Moreover, higher temperatures enhance the overall carbon conversion potential of AS, particularly boosting CO2 absorption in anaerobic conditions as the potential for CO2 emission during glycolysis and TCA cycles grows and diminishes, respectively. The study highlights that temperature is a major factor affecting microbial community characteristics and CO2 fluxes, with more pronounced effects observed in anaerobic sludge. This study provides valuable insights for maintaining stable A2O system operations, understanding carbon footprints, and improving COD removal efficiency in industrial WWTPs.

Lactic acid fermentation of food waste in a semicontinuous SBR system: influence of the influent composition and hydraulic retention time

Fermentation processes have been shown to be a good approach to food waste (FW) management. Among the commodities that can be bioproduced by using FW as an organic substrate and exploiting its biodegradability, there is lactic acid (LA). LA has gained the interest of research because of its role in the production of polylactic acid plastics. In this study, the influence of the HRT (2-5 days) used during the fermentation of the liquid fraction (∼12-13 g COD/L) of FW on LA yield and concentration was investigated. Moreover, the changes in the chemical composition (in terms of carbohydrates and organic metabolites concentration) of the influent occurring in the feeding tank were monitored and its influence on the downstream fermentation process was examined. High instability characterized the reactor run with the optimal production yield obtained on day 129 at an HRT 2 days with 0.81 g COD/g COD. This study shows the importance of the fluctuating composition of FW, a very heterogeneous and biologically active substrate, for the LA fermentation process. The non-steady state fermentation process was directly impacted by the unstable influent and shows that a good FW storage strategy has to be planned to achieve high and constant LA production.

Review in anaerobic digestion of food waste

Due to the special property of food waste (FW), anaerobic digestion of food waste is facing many challenges like foaming, acidification, ammonia nitrogen and (NH4+-N) inhibition which resulted in a low biogas yield. A better understanding on the problems exiting in the FW anaerobic digestion would enhance the bio-energy recovery and increase the stable operation. Meanwhile, to overcome the bottle necks, pretreatment, co-digestion and additives is proposed as well as the solutions to improve biogas yield in FW digestion system. At last, future research directions regarding FW anaerobic digestion were proposed.

Enzyme enhanced lactic acid fermentation of swine manure and apple waste: Insights from organic matter transformation and functional bacteria

Anaerobic co-fermentation is a favorable way to convert agricultural waste, such as swine manure (SM) and apple waste (AW), into lactic acid (LA) through microbial action. However, the limited hydrolysis of organic matter remains a main challenge in the anaerobic co-fermentation process. Therefore, this work aims to deeply understand the impact of cellulase (C) and protease (P) ratios on LA production during the anaerobic co-fermentation of SM with AW. Results showed that the combined use of cellulase and protease significantly improved the hydrolysis during the enzymatic pretreatment, thus enhancing the LA production in anaerobic acidification. The highest LA reached 41.02 ± 2.09 g/L within 12 days at the ratio of C/P = 1:3, which was approximately 1.26-fold of that in the control. After a C/P = 1:3 pretreatment, a significant SCOD release of 45.34 ± 2.87 g/L was achieved, which was 1.13 times the amount in the control. Moreover, improved LA production was also attributed to the release of large amounts of soluble carbohydrates and proteins with enzymatic pretreated SM and AW. The bacterial community analysis revealed that the hydrolytic bacteria Romboutsia and Clostridium_sensu_stricto_1 were enriched after enzyme pretreatment, and Lactobacillus was the dominant bacteria for LA production. This study provides an eco-friendly technology to enhance hydrolysis by enzymatic pretreatment and improve LA production during anaerobic fermentation.

Two birds with one stone: The multiple roles of hydrothermal treatment in dewatering municipal sludge and producing value-added products

Sludge dewatering and resource recovery are key steps in the sustainable treatment of municipal sludge (MS) owing to the high levels of moisture and nutrients. Among the treatment options available, hydrothermal treatment (HT) is promising to efficiently improve dewaterability and recover biofuels, nutrients, and materials from MS. However, hydrothermal conversion at different HT conditions generates multiple products. Integrating the characteristics of dewaterability and value-added products under different HT conditions facilitates the application of HT for the sustainable management of MS. Therefore, a comprehensive review of HT for its multiple roles in MS dewatering and value-added resource recovery is conducted. First, the impact of HT temperature on sludge dewaterability and key mechanisms are summarized. Then, this study elucidates the characteristics of biofuels produced (combustible gases, hydrochars, biocrudes, and H2-rich gases), nutrient recovery (proteins and phosphorus), and value-added materials under a wide range of HT conditions. Importantly, along with the integration and evaluation of HT product characteristics under different HT temperatures, this work proposes a conceptual sludge treatment system that integrates the different value-added products in different HT stages. Furthermore, a critical evaluation of the knowledge gaps in the HT for sludge deep dewatering, biofuels, nutrients, and materials recovery is provided along with recommendations for further research.

Thermo-alkaline pretreatment of excess sludge: Effects of temperature on volatile fatty acids accumulation and microbial community

In order to explore the role of thermal-alkaline pretreatment temperatures (TAPT) in sludge fermentation and the microbial characteristics, five groups (100, 120, 140, 160 °C and control group) were set up and the results showed that the increasing TAPT promoted the dissolution of soluble chemical oxygen demand (SCOD) and VFAs, but had slight influence on the release of NH₄⁺-N and PO₄^3--P. What's more, when it was 120 °C, the SCOD dissolution was comparable to that at 160 °C. Overall, 120 °C was the optimal condition, corresponding to the fact that the maximum release of SCOD was 8788.74 mg/L (2.63 times of the control group), the maximum dissolution of VFAs was 4596 mg/L (about 1.28 times of the control group). The trend of C/N was not significant. High-throughput sequencing showed that Firmicutes and Actinobacteriota were enriched with the temperature increasing, while Proteobacteria and Chloroflexi did not change significantly. Firmicutes was in a stable dominant position. Temperature conditions brought about significant changes in microbial interspecific interaction. Carbohydrate and amino acids had the highest metabolic abundance, especially at 120 °C group. The change rule of amino acid metabolism was similar to that of lipid metabolism, and the abundance of energy metabolism gradually increased with temperature. The protein metabolism was greatly affected by temperature. This study revealed the effect of microbial mechanism of TAPT on the sludge acid production efficiency.

The dissolution of polysaccharides and amino acids enhanced lactic acid production from household food waste during pretreatment process

A large amount of household food waste (HFW) is produced yearly, resulting in environmental problems and financial burdens. Bio-production of lactic acid (LA), a high value-added platform chemical, from HFW by anaerobic fermentation is a promising way of resource recovery. However, the LA production yield from HFW is low. This paper compared several pretreatment methods (hydrothermal pretreatment, chemical pretreatment, and combined hydrothermal and chemical pretreatment) to improve LA production from HFW. The result showed that the combined pretreatment (alkali-thermal pretreatment at pH 10 and 120 °C) significantly increased the LA production than single hydrothermal and chemical pretreatment. The pretreatment process promoted the dissolution of organics, especially the polysaccharides and amino acids, and further influenced the LA production by Lactobacillus rhamnosus ATCC 7469. Among the amino acids, aspartic acid (Asp), threonine (Thr), glutamic acid (Glu), glycine (Gly), alanine (Ala), cystine (Cys), valine (Val), isoleucine (Ile), arginine (Arg), and proline (Pro) significantly correlated with LA concentration.

Effect of thermal and ultrasonic pretreatment on lactic acid fermentation of food waste

Food waste (FW) generation has become one of the largest environmental concerns for human society. Thanks to its chemical features and its high biodegradability, FW can be used as starting platform to produce biocommodities. Lactic acid (LA) is one of those chemicals that is gaining the attention of industry and research for its important role in polylactic acids production. To exploit better the organic content of FW, several FW pretreatments have been proposed in the literature, though none of them were aimed at influencing LA fermentation. Thermal and ultrasonic pretreatment effects on solubilization rates and LA production yields have been investigated in this batch study. The highest solubilization rate was achieved with 30 minutes ultrasonic pretreated FW resulting in a 15% increment in soluble COD (sCOD). The highest LA yield was obtained after 90-minute thermal pretreatment at 80 and 100°C at a yield of 0.49 g LA•g COD^-1. This study shows that ultrasonic pretreatment generally performed better than thermal pretreatment when considering the increase in sCOD but caused a reduction in LA concentrations and yields after fermentation with high production of ethanol. The opposite trend was recorded in the thermal pretreated incubations, in which LA was present for 50% of the sCOD with higher LA concentrations of 2.90 g COD•L^-1.

Highly efficient oriented bioconversion of food waste to lactic acid in an open system: Microbial community analysis and biological carbon fixation evaluation

The valorization of organic solid waste to lactic acid (LA) in open fermentation systems has attracted tremendous interest in recent years. In this study, a highly efficient oriented LA bioconversion system from food waste (FW) in open mode was established. The maximum LA production was 115 g/L, with a high yield of 0.97 g-LA/g-total sugar. FW is a low-cost feedstock for LA production, containing indigenous hydrolysis and LA-producing bacteria (LAB). Saccharification and real-time pH control were found to be essential for maintaining LAB dominantly in open systems. Furthermore, microbial community analysis revealed that Enterococcus mundtii adapted to complex FW substrates and dominated the subsequent bioconversion process. The oriented LA bioconversion exhibited the capacity for biological carbon fixation by reducing CO₂ emissions by at least 21 kg per ton of FW under anaerobic conditions.

Microwave Pre-Treatment of Model Food Waste to Produce Short Chain Organic Acids and Ethanol via Anaerobic Fermentation

As an alternative to conventional anaerobic digestion for methane production, anaerobic fermentation (AF) of organic matter can produce short chain organic acids (SCOAs) in a sustainable way. This study investigated the effect of microwave (MW) pre-treatment on the AF of model food waste to SCOAs and ethanol. The MW pre-treatment was investigated at three temperatures (120, 150 and 180 °C) and residence times (2, 5 and 8 min). The MW treatment gave a significant reduction in the pH and volatile suspended solids (VSS). The largest reduction in the VSS was 20%, indicating solubilisation of the organic matter. The latter was also confirmed by the increase, although it was not statistically significant, in the soluble chemical oxygen demand (COD) and soluble carbohydrates. In the fermentation batch tests, the total product yield was higher (17.5% COD COD−1) than for the untreated substrate (11.1% COD COD−1). An electricity price of GBP 0.06 kWh−1 would correspond to the market value of the additional SCOAs produced with the pre-treated substrate. Although this price is lower than the current business price of electricity in the UK, the MW pre-treatment could become economically feasible with scale-up effects and by using free excess electricity coming from renewable resources.

Lactic acid production from mesophilic and thermophilic fermentation of food waste at different pH

It is promising to recover lactic acid (LA) from fermentation of food waste (FW). In this study, pH and temperatures were investigated comprehensively to find their effects on LA fermentation, and microbial analyses were used to take insight to the variation of LA production. The results showed that mesophilic fermentation benefited hydrolysis and acidification, leading to a high yield of LA, while thermophilic conditions restricted other producers at low pH, leading to a high purity of LA. Lactobacillus amylolyticus was the main LA producer under thermophilic conditions, but Thermoanaerobacterium thermosaccharolyticum boomed at pH 5.0-6.0 and it converted LA partly to butyric acid. Simultaneously, Bacillus coagulans also increased and improved the optical purity (OP) of L-LA. From a series of this study, an operational condition of pH 5.5 and temperature of 52 °C would be potentially suitable for lactate fermentation of FW with high purity of 89%, while a stable LA production with an OP of 68% was achieved at 55 °C and pH 6.0.

Microbial production of lactic acid from food waste: Latest advances, limits, and perspectives

A significant amount of food waste (FW) is produced every year. If it is not disposed of timeously, human health and the ecological environment can be negatively affected. Lactic acid (LA), a high value-added product, can be produced by fermentation from FW as a substrate, realizing the concurrent treatment and recycling of FW, which has attracted increasing research interest. In this paper, the latest advances and deficiencies were presented from the following aspects: microorganisms involved in LA fermentation and the metabolic pathways of Lactobacillus, fermentation conditions, and methods of enhanced biotransformation and LA separation. The limitations of the LA fermentation of FW are mainly associated with low LA concentration and yield, the low purity of L(+)-LA, and the high separation costs. The establishment of biorefineries of FW with lactic acid as the target product is the future development direction, but there are still many research studies to be done.

Meta-fermentation system with a mixed culture for the production of optically pure l-lactic acid can be reconstructed using the minimum isolates with a simplified pH control strategy

Meta-l-lactic acid fermentation from non-treated kitchen refuse was reconstructed using a combination of isolated bacterial strains under several pH control strategies. The meta-fermentation system was successfully reconstructed using a combination of Weizmannia coagulans MN-07, Caldibacillus thermoamylovorans OM55-6, and Caldibacillus hisashii N-11 strains. Additionally, a simplified constant pH control strategy was employed, which decreased fermentation time and increased production. The optimum pH (6.5) for the reconstructed meta-fermentation was favorable for the respective pure cultures of the three selected strains. The l-lactic acid production performance of the reconstructed meta-fermentation system was as follows: concentration, 24.5 g L^-1 ; optical purity, 100%; productivity, 0.341 g L^-1 h^-1 ; yield, 1.06 g g^-1 . These results indicated that constant pH control was effective in the reconstructed meta-fermentation with the best performance of l-lactic acid production at pH optimal for the selected bacterial growth, while the switching from swing pH control would suppress the activities of unfavorable bacterial species in un-isolated meta-fermentation.

Optimization of lactate production from co-fermentation of swine manure with apple waste and dynamics of microbial communities

Co-anaerobic fermentation (co-AF) of swine manure (SM) and apple waste (AW) has been proved to be beneficial for lactic acid (LA) production. In order to further improve the LA production, three important parameters, namely AW in feedstock, temperature, volatile solids (VS) of feedstock, were evaluated using Box-Behnken design and response surface methodology. The quadratic regression model was developed and interactive effects was found between the three parameters. Results showed that the maximum concentration, 31.18 g LA/L (with LA yield of 0.62 g/g VS), was obtained under optimum conditions of 60.4% AW in feedstock, 34.7 ℃, and 5.0% VS. At the optimum conditions, the solubilization of organic matter was enhanced compared with mono-fermentation of SM. Microbial community structure of the reactor diverged greatly with fermentation time. Clostridium and Lactobacillus were dominant bacteria in the fermentation process, resulting in a remarkably LA accumulation.

Fermentation liquid production of food wastes as carbon source for denitrification: Laboratory and full-scale investigation

Nitrogen removal is often limited in municipal wastewater treatment due to the insufficiency of carbon source, and using food wastes fermentation liquid as carbon source could cut down the cost of operating and recycle food wastes. Food wastes fermentation liquid production and application as external carbon source were explored in the laboratory and full-scale system in this study. In the laboratory scale, lactic acid and VFAs were the main components of fermentation liquid, and the highest total chemical oxygen demand (TCOD) production was obtained with activated sludge as inoculum. The yield of TCOD was around 794.5 mg/g TS_fed and NH₄⁺-N was 3.5 mg/g TS_fed. The denitrification rate with fermentation liquid was slightly lower than acetic acid and butyric acid, but higher than lactic acid and starch. In the full-scale investigation, the TCOD concentration in fermentation liquid was in the range of 6.9-12.8 g/L and the ratio of TCOD/inorganic nitrogen was 210.5-504.5:1. NO₃^--N removal increased from 52.1% to 94.2% after fermentation liquid addition, confirming the potentiality of food wastes fermentation liquid replace the commercial carbon source in wastewater treatment plants.

Impact of storage duration and micro-aerobic conditions on lactic acid production from food waste

Food waste (FW) is an abundant resource with great potential for lactic acid (LA) production. In the present study, the effect of storage time on FW characteristics and its potential for LA production was investigated. The largest part of sugars was consumed during 7 to 15 days of FW storage and the sugar consumption reached 68.0% after 15 days. To enhance the LA production, micro-aerobic conditions (13 mL air/g VS) and addition of β-glucosidase were applied to improve polysaccharides hydrolysis, resulting to increase of monosaccharides content to 76.6%. Regarding fermentative LA production, the highest LA titer and yield of hydrolyzed FW was 32.1 ± 0.5 g/L and 0.76 ± 0.01 g/g_-sugar, respectively. Furthermore, L-LA isomer was higher than 70% when FW was stored for up to 7 days. However, attention should be paid on controlling the FW storage to approximately one week.

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

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

Effect of ultrasonic pretreatment on chain elongation of saccharified residue from food waste by anaerobic fermentation

Converting biowaste into value-added products has raised the researchers' interests. In this study, bioconversion was applied to produce chain acids from food waste by anaerobic fermentation. To improve the caproic acid production, different pretreatments (i.e., ultrasonic, hydrothermal, and alkaline-thermal) were used for investigating their effects on the acidogenic production and microbial communities. The results showed that ultrasonic and hydrothermal pretreatments (207.8 and 210.1 mg COD/g VS, respectively) were very efficient for enhancing the caproic acid production, compared to the alkaline-thermal pretreated samples and control samples (72.6 and 97.5 mg COD/g VS, respectively). The ultrasonic pretreatment was beneficial for reducing volatile fatty acids (VFAs) during the caproic acid production, resulting in converting more lactic acid to caproic acid by adding the hydrothermal pretreatment. The microbial community analysis showed that the acidogenic bacteria Caproiciproducens dominated the fermentation in this bioconversion process of food waste into chain acids. The Caproiciproducens mainly degraded the proteins and carbohydrates from the saccharified residues of food waste to produce caproic acids through chain elongation procedure. The investigation and optimized method may help develop the bioconversion technology for producing VFAs products from food wastes.

Salinity enhances high optically active L-lactate production from co-fermentation of food waste and waste activated sludge: Unveiling the response of microbial community shift and functional profiling

Lactic acid (LA), a versatile platform molecule, can be fermented from organic wastes, such as food waste and waste activated sludge. In this study, an efficient approach using salt, a component of food waste as an additive, was proposed to increase LA production. The LA productivity was increased at 10 g NaCl/L and optical pure L-lactate was obtained at 30 g NaCl/L. The enhancement of LA was in accordance with the increased solubilization and the critical hydrolase activities under saline conditions. Moreover, high salinity (30-50 g NaCl/L) changed the common conversion of LA to volatile fatty acids. In addition, the key LA bacteria genera (Bacillus, Enterococcus, Lactobacillus) were selectively enriched under saline conditions. Strong correlations between salinity and functional genes for L-LA production were also observed. This study provides a practical way for the enrichment of L-LA with high optical activity from organic wastes.

Assessment of Integration between Lactic Acid, Biogas and Hydrochar Production in OFMSW Plants

Biological treatments such as anaerobic digestion and composting are known to be the most widespread methods to deal with Organic Fraction of Municipal Solid Waste (OFMSW). The production of biogas, a mix of methane and carbon dioxide, is worth but alone cannot solve the problems of waste disposal and recovery; moreover, the digestate could be stabilized by aerobic stabilization, which is one of the most widespread methods. The anaerobic digestion + composting integration converts 10% to 14% of the OFMSW into biogas, about 35–40% into compost and 35–40% into leachate. The economic sustainability could be rather increased by integrating the whole system with lactic acid production, because of the high added value and by substituting the composting process with the hydrothermal carbonization process. The assessment of this integrated scenario in term of mass balance demonstrates that the recovery of useful products with a potentially high economic added value increases, at the same time reducing the waste streams outgoing the plant. The economic evaluation of the operating costs for the traditional and the alternative systems confirms that the integration is a valid alternative and the most interesting solution is the utilization of the leachate produced during the anaerobic digestion process instead of fresh water required for the hydrothermal carbonization process.

Lactic acid production - producing microorganisms and substrates sources-state of art

Lactic acid is an organic compound produced via fermentation by different microorganisms that are able to use different carbohydrate sources. Lactic acid bacteria are the main bacteria used to produce lactic acid and among these, Lactobacillus spp. have been showing interesting fermentation capacities. The use of Bacillus spp. revealed good possibilities to reduce the fermentative costs. Interestingly, lactic acid high productivity was achieved by Corynebacterium glutamicum and E. coli, mainly after engineering genetic modification. Fungi, like Rhizopus spp. can metabolize different renewable carbon resources, with advantageously amylolytic properties to produce lactic acid. Additionally, yeasts can tolerate environmental restrictions (for example acidic conditions), being the wild-type low lactic acid producers that have been improved by genetic manipulation. Microalgae and cyanobacteria, as photosynthetic microorganisms can be an alternative lactic acid producer without carbohydrate feed costs. For lactic acid production, it is necessary to have substrates in the fermentation medium. Different carbohydrate sources can be used, from plant waste as molasses, starchy, lignocellulosic materials as agricultural and forestry residues. Dairy waste also can be used by the addition of supplementary components with a nitrogen source.

Enhanced production of volatile fatty acids by adding a kind of sulfate reducing bacteria under alkaline pH

Anaerobic digestion could make sludge stable and harmless, and the volatile fatty acids (VFAs) produced from it. The objective of this study was to reduced sludge production and realize the resource utilization of VFAs through enhance anaerobic sludge fermentation by adding sulfate reducing bacteria (SRB) under alkaline pH. Under the neutral and alkaline pH, SRB was added into the sludge fermentation liquid with sole stock solution and sterilization treatment respectively, while the liquid without any additives was used as control. The results indicated that obvious increase of the production of VFAs was observed after adding SRB under alkaline pH. And, more protein and polysaccharide were obtained which were the main substrates for the production of VFAs. The concentration of ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) were also increased with the addition of SRB. So, a high yield production of VFAs could be achieved through the addition of SRB + alkaline pH.

Recent advances on the sustainable approaches for conversion and reutilization of food wastes to valuable bioproducts

The increasing amounts of food wastage and accumulation generated per annum due to the growing human population worldwide often associated with environmental pollution issues and scarcity of natural resources. In view of this, science community has worked towards in finding sustainable approaches to replace the common practices for food waste management. The agricultural and food processing wastes rich in nutrients are often the attractive substrates for the bioconversion for valuable bioproducts such as industrial enzymes, biofuel and bioactive compounds. The sustainable approaches on the re-utilization of food wastes as the industrial substrates for production of valuable bioproducts has meet the goals of circular bioeconomy, results in the diversify applications and increasing market demands for the bioproducts. This review discusses the current practice and recent advances on reutilization of food waste for bioconversion of valuable bioproducts from agricultural and food processing wastes.

Stabilizing lactate production through repeated batch fermentation of food waste and waste activated sludge

Bio-valorization of organic waste streams, such as food waste and waste activated sludge, to lactic acid (LA) has recently drawn much attention. It offers an opportunity for resource recovery, alleviates environmental issues and potentially turns a profit. In this study, both stable and high LA yield (0.72 ± 0.15 g/g total chemical oxygen demand) and productivity rate (0.53 g/L•h) were obtained through repeated batch fermentation. Moreover, stable solubilization and increase in the critical hydrolase activities were achieved. Depletions of ammonia and phosphorus were correlated with the LA production. The relative abundance of the key LA bacteria genera (i.e., Alkaliphilus, Dysgonomonas, Enterococcus and Bifidobacterium) stabilized in the repeated batch reactor at a higher level (44.5 ± 2.53%) in comparison with the batch reactor (26.2 ± 4.74%). This work show a practical way for the sustainable valorization of organic wastes to LA by applying the repeated batch mode during biological treatment.

Advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge

Low acid production and acid-forming process instability are becoming the major issues to limit the popularization of anaerobic fermentation to produce volatile fatty acid. Considerable research efforts have been made to address these problems, from studying the microorganisms that are primarily responsible for or detrimental to this process, to determining their biochemical pathways and developing mathematical models that facilitate better prediction of process performance to identify the mechanism and optimization of process control. A limited understanding of the complex microbiology and biochemistry of anaerobic fermentation is the primary cause of acid production upset or failure. This review critically assesses the recent advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge from micro to macro scale, particularly relating to the microbiology, biochemistry, impact factors, and enhancement methods. Previous results suggest that further studies are necessary to substantially promote the efficiency and stability of acid production. One of the promising directions appears to be integrating the existing and growing pretreatment technologies and fermentation processes to enhance metabolic pathways of acetogens but inhibit activities of methanogens, which this study hopes to partially achieve.

Road to full bioconversion of biowaste to biochemicals centering on chain elongation: A mini review

Production of biochemicals from waste streams has been attracting increasing worldwide interest to achieve climate protection goals. Chain elongation (CE) for production of medium-chain carboxylic acids (MCCAs, especially caproate, enanthate and caprylate) from diverse biowaste has emerged as a potential economic and environmental technology for a sustainable society. The present mini review summarizes the research utilizing various synthetic or real waste-derived substrates available for MCCA production. Additionally, the microbial characteristics of the CE process are surveyed and discussed. Considering that a large proportion of recalcitrantly biodegradable biowaste and residues cannot be further utilized by CE systems and remain to be treated and disposed, we propose here a loop concept of bioconversion of biowaste to MCCAs making full use of the biowaste with zero emission. This could make possible an alternative technology for synthesis of value-added products from a wide range of biowaste, or even non-biodegradable waste (such as, plastics and rubbers). Meanwhile, the remaining scientific questions, unsolved problems, application potential and possible developments for this technology are discussed.

Activated sludge and other aerobic suspended culture processes

The fields in the process model of activated sludge, the characteristics and species of microbial communities, dynamics and mechanism in the process, the influence of different xenobiotics on activated sludge, anaerobic digestion on waste activated sludge, and design and operation for activated sludge are reviewed in 2018. Contrast with the past reviews, several new highlights such as waste activated sludge treatment, antibiotic and heavy-metal xenobiotic, and pretreatment for anaerobic digestion are mentioned in 2018, which indicated that the research tendency of activated sludge from wastewater treatment to waste sludge treatment in the retrieved literature is developing. PRACTITIONER POINTS: None.

Efficient l-lactic acid production from corncob residue using metabolically engineered thermo-tolerant yeast

Lactic acid is an important industrial product and the production from inexpensive and renewable lignocellulose can reduce the cost and environmental pollution. In this study, a Kluyveromyces marxianus strain which produced lactic acid efficiently from corncob was constructed. Firstly, two of six different lactate dehydrogenases, which from Plasmodium falciparum and Bacillus subtilis, respectively, were proved to be effective for l-lactic acid production. Then, five single genetic modifications were conducted. The overexpression of Saccharomyces cerevisiae proton-coupled monocarboxylate transporter, K. marxianus 6-phosphofructokinase, or disruption of K. marxianus putative d-lactate dehydrogenase enhanced the l-lactic acid accumulation. Finally, the strain YKX071, obtained via combination of above effective genetic engineering, produced 103.00 g/L l-lactic acid at 42 °C with optical purity of 99.5% from corncob residue via simultaneous saccharification and co-fermentation. This study first developed an effective platform for high optical purity l-lactic acid production from lignocellulose using yeast with inexpensive nitrogen sources.

Deep exploitation of refractory organics in anaerobic dynamic membrane bioreactor for volatile fatty acids production from sludge fermentation: Performance and effect of protease catalysis

Volatile fatty acids (VFAs) production from waste activated sludge fermentation could be improved in anaerobic dynamic membrane bioreactor (ADMBR) by retaining residual organics within the reactor and prolonging their reaction time. However, the accumulation of refractory organics made it operate unstably. Therefore, protease catalysis was adopted to deeply exploit those refractory organics in sludge. By combining with dynamic membrane retention, protease catalysis indeed presented a good performance. VFAs yield was further enhanced by over 40% in ADMBR. Membrane fouling was slightly relieved due to protein and polysaccharide degradations in the sludge of dynamic membrane. It was also interestingly found that not only protease activity of sludge was improved from 5 to 21 U/ml, but also β-GLC activity was enhanced from 13 to 20 μmoL/L/h. Microbial community analysis showed protease addition could reduce bacterial richness and evenness in sludge, and accelerate the growth of polysaccharides-hydrolyzing bacteria, as well as inhibit some proteolytic bacteria.

Copper (II) addition to accelerate lactic acid production from co-fermentation of food waste and waste activated sludge: Understanding of the corresponding metabolisms, microbial community and predictive functional profiling

Bio-refinery of food waste and waste activated sludge to high value-added chemicals, such as lactic acid, has attracted particular interest in recent years. In this paper, the effect of copper (II) dosing to the organic waste fermentation system on lactic acid production was evaluated, which proved to be a promising method to stimulate high yield of lactic acid (77.0% higher than blank) at dosage of 15 μM-Cu²⁺/g VSS. As mechanism study suggested, copper addition enhanced the activity of α-glycosidase and glycolysis, which increased the substrate for subsequent acidification; whereas, the high dosage (70 μM-Cu²⁺/g VSS) inhibited the conversion of lactic acid to VFA, thus stabilized lactic acid concentration. Microbial community study revealed that small amount of copper (II) at 15 μM/g VSS resulted in the proliferation of Lactobacillus to 82.6%, which mainly produced lactic acid. Finally, the variation of functional capabilities implied that the proposed homeostatic system II was activated at relatively low concentration of copper. Meanwhile, membrane transport function and carbohydrate metabolism were also strengthened. This study provides insights into the effect of copper (II) on the enhancement of lactic acid production from co-fermentation of food waste and waste activated sludge.