Fermentation liquor of CaO2 treated chemically enhanced primary sedimentation (CEPS) sludge for bioplastic biosynthesis

Iron driven organic carbon capture, pretreatment, recovery and upgrade in wastewater: Process technologies, mechanisms, and implications

Wastewater treatment plants face significant challenges in transitioning from energy-intensive systems to carbon-neutral, energy-saving systems, and a large amount of chemical energy in wastewater remains untapped. Iron is widely used in modern wastewater treatment. Research shows that leveraging the coupled redox relationship of iron and carbon can redirect this energy (in the form of carbon) towards resource utilization. Therefore, re-examining the application of iron in existing wastewater carbon processes is particularly important. In this review, we investigate the latest research progress on iron for wastewater carbon flow restructuring. During the iron-based chemically enhanced primary treatment (CEPT) process, organic carbon is captured into sludge and its bioavailability is enhanced through iron-based advanced oxidation processes (AOP) pretreatment, further being recovered or upgraded to value-added products in anaerobic biological processes. We discuss the roles and mechanisms of iron in CEPT, AOP, anaerobic biological processes, and biorefining in driving organic carbon conversion. The dosage of iron, as a critical parameter, significantly affects the recovery and utilization of sludge carbon resources, particularly by promoting effective electron transfer. We propose a pathway for beneficial conversion of wastewater organic carbon driven by iron and analyze the benefits of the main products in detail. Through this review, we hope to provide new insights into the application of iron chemicals and current wastewater treatment models.

Application of calcium peroxide in promoting resource recovery from municipal sludge: A review

The harmless disposal, resource recovery, and synergistic efficiency reduction of municipal sludge have been the research focuses for the last few years. Calcium peroxide (CaO2) is a multifunctional and safe peroxide that produces an alkaline oxidation environment to promote the fermentation of municipal sludge to produce hydrogen (H2) and volatile fatty acids (VFAs), thus realizing sludge resource recovery. This review outlines the research achievements of CaO2 in sludge resource recovery, improvement of sludge dewaterability, and removal of pollutants from sludge in recent years. Meanwhile, the mechanism of CaO2 and its influencing factors have also been comprehensively summarized. Finally, the future development direction of the application of CaO2 in municipal sludge is prospected. This review would provide theoretical reference for the potential engineering applications of CaO2 in improving sludge treatment in the future.

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Genome-centric metagenomics provides new insights into metabolic pathways of polyhydroxyalkanoates biosynthesis and functional microorganisms subsisting on municipal organic wastes

The microbial community of a sequencing batch reactor operated under feast and famine conditions for production of polyhydroxyalkanoates (PHAs) was characterized through high-throughput sequencing and metagenomic analysis. The fermented food waste and chemically-enhanced primary sludge was fed in this bioreactor. After acclimation, the PHA yield achieved as high as 0.60-0.69 g CODPHA/g CODS. The complete changes of microbial community structure were found during shifts of feedstock. A synthesis of SCL/MCL-PHAs pathway was established for PHA-producing bioreactor in this mixed-culture system. The structure-performance relationship of PHA-producing microbial community and feedstock composition was investigated. The results showed that microbial community tends to be decentralized and prefer team work for PHA synthesis to consume the multiple substrates and digest inevitable non-VFA contents in fermented liquor. This study also discovered unreported potential PHA producers (e.g., genera Tabrizicola, Nannocystis, Ga0077539, Ga0077559, JOSHI-001, SNC69-320 and UBA2334) subsisting on municipal organic wastes and expands the current knowledge about mixed-culture system that the PHA synthesis pathway is widely existed in activated sludge.

Enhanced volatile fatty acid production from waste activated sludge by urea hydrogen peroxide: performance and mechanisms

Anaerobic acidogenesis of waste activated sludge (WAS) presents significant potential for resource recovery and waste treatment. However, the slow hydrolysis of WAS limits the efficiency of this approach. In this study, we applied urea hydrogen peroxide (UHP) pretreatment to enhance WAS hydrolysis and investigated the effects of operating parameters on volatile fatty acid (VFA) production and the associated mechanisms. Results demonstrated that UHP significantly improved WAS hydrolysis and VFA production, with a three-fold increase in soluble chemical oxygen demand (SCOD) compared to the control group. UHP dosage emerged as the most critical factor for VFA production, with the maximum VFA concentration increasing from 1127.6 to 8800.9 mg COD per L as UHP dosage ranged from 0 to 6 mmol g-1 VSS (Volatile suspended solids). At an optimal UHP dosage of 4 mmol g-1 VSS, both the unit oxidant promotion efficiency (ΔVFAs/ΔUHP) and the maximum VFA concentration reached relatively high levels, at 35.3 mg COD per mmol and 7527.3 mg COD per L, respectively. UHP pretreatment generated alkaline conditions, H2O2, ·OH and free ammonia, which collectively disrupted the extracellular polymeric substances (EPS) structure, transforming unextractable EPS into extractable forms and promoting the release of organic matter during both the pretreatment and fermentation stages. Excitation-emission matrix (EEM) analysis revealed that UHP increased the concentration of easily utilizable organic matter, providing more substrates for acidogenic bacteria and enhancing VFA production. Furthermore, weak alkaline conditions and high free ammonia concentrations in the UHP group facilitated VFA accumulation by preventing rapid acidification and suppressing methanogen activity. This study offers valuable insights into the potential of UHP pretreatment for enhancing WAS hydrolysis and VFA production, with promising applications in wastewater treatment and resource recovery.

Ionic strength of the liquid phase of different sludge streams in a wastewater treatment plant

In a wastewater treatment plant (WWTP), several sludge streams exist and the composition of their liquid phase varies with time and place. For evaluating the potential for formation of precipitates and equilibria for weak acids/bases, the ionic strength and chemical composition need to be known. This information is often not available in literature, and even neglected in chemical model-based research. Based on a literature review, we proposed three ranges of concentration (low, typical and high) for the major constituents of the liquid phase of the different streams in a WWTP. The study also discusses the reasons for the concentration evolution, and the exceptional cases, to allow readers to consider the right range depending on their situation. The ionic strength of the different streams and the contribution of its constituents were calculated based on the ionic composition. The major contributors to the ionic strength for the wastewater-based streams (influent, effluent and mixed sludge) were Na⁺, Cl^-, Mg²⁺ and Ca²⁺, representing 50-70% of the ionic strength. For digestate, NH₄⁺ and HCO₃^- accounted for 65-75% of the ionic strength. Even though the ionic strength is recognized to impact several important wastewater treatment processes, its utilization in literature is not always adequate, which is discussed in this study.

Recovery of resources from industrial wastewater employing electrochemical technologies: status, advancements and perspectives

In the last two decades, water use has increased at twice the rate of population growth. The freshwater resources are getting polluted by contaminants like heavy metals, pesticides, hydrocarbons, organic waste, pathogens, fertilizers, and emerging pollutants. Globally more than 80% of the wastewater is released into the environment without proper treatment. Rapid industrialization has a dramatic effect on developing countries leading to significant losses to economic and health well-being in terms of toxicological impacts on humans and the environment through air, water, and soil pollution. This article provides an overview of physical, chemical, and biological processes to remove wastewater contaminants. A physical and/or chemical technique alone appears ineffective for recovering useful resources from wastewater containing complex components. There is a requirement for more processes or processes combined with membrane and biological processes to enhance operational efficiency and quality. More processes or those that are combined with biological and membrane-based processes are required to enhance operational efficiencies and quality. This paper intends to provide an exhaustive review of electrochemical technologies including microbial electrochemical technologies. It provides comprehensive information for the recovery of metals, nutrients, sulfur, hydrogen, and heat from industrial effluents. This article aims to give detailed information into the advancements in electrochemical processes to energy use, improve restoration performance, and achieve commercialization. It also covers bottlenecks and perspectives of this research area.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.

Novel CaO2 beads used in the anaerobic fermentation of iron-rich sludge for simultaneous short-chain fatty acids and phosphorus recovery under ambient conditions

A novel composite CaO₂ bead was prepared to improve total short-chain fatty acids (TSCFAs) production and phosphorus (P) recovery from iron-rich waste activated sludge (WAS) during ambient anaerobic fermentation. Results showed that CaO₂ mass percentage of 5% and CaCl₂:nylon66 = 1:1 (mass ratio) were the optimal prescription for the preparation of CaO₂ beads with porous structure, loose morphology, and sustained-release of CaO₂. The highest TSCFAs production (356 mg/g VSS) was observed and about 9% of P in sludge could be recovered on beads. The decrease of Fe-phosphate and Fe-oxides in the sludge were due to different mechanisms. Microbial community analyses showed that CaO₂ beads effectively enriched dissimilatory iron-reducing bacteria (DIRB) and promoted iron-reduction related genes. After fermentation, the P-rich beads are easy to separate from sludge for further P recovery, and the supernatant carrying abundant acetate and Fe²⁺ can be returned to the wastewater treatment line to improve nutrient removal.

Utilizing the crop waste of date palm fruit to biosynthesize polyhydroxyalkanoate bioplastics with favorable properties

Polyhydroxyalkanoate (PHA), a family of biodegradable and renewable biopolymers that could potentially play a significant role in bioeconomy. In this study we investigated the potential of date waste (DW) biomass as feedstock to produce PHA by the halophilic archaeon Haloferax mediterranei. The concentration of essential trace elements for H. mediterranei cells during growth and PHA biopolymer accumulation was optimized. A maximum cell dry mass of (CDM) (12.8 g L^-1) and PHA concentration of (3.20 g L^-1) were achieved in DW extract media that was not supplemented with trace elements, indicating that DW is a promising source for trace elements. The cultivation was scaled-up to fed-batch bioreactor fermentations under non-sterile conditions and resulted in CDM and PHA content of 18.0 g L^-1 and 25%, respectively. The produced PHA was confirmed to be poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with high 3-hydroxyvalerate (3 HV) content of 18.0 mol%. This 3 HV molar percent was achieved without the addition of expensive precursors. The PHBV is of high molecular weight (746.0 kDa) and narrow polydispersity (PDI = 1.5), and displayed reduced melting at 148.1 °C. The X-ray diffraction (XRD) analysis showed that the PHBV has amorphous nature which increases the degradation rates and workability of the biopolymer. The isotopic ratio ¹³C/¹²C (δ ¹³C) for PHBV was found to be - 19.1‰, which indicated that H. mediterranei prefers lighter bonds to break and uses the lighter atoms for the biosynthesis of PHBV.

Effects of individual volatile fatty acids (VFAs) on phosphorus recovery by magnesium ammonium phosphate

Volatile fatty acids (VFAs) are a major component of dissolved organic matter in alkaline fermentation supernatants. In this study, effects of different VFAs (acetate, propionate, and butyrate) on phosphorus recovery, as magnesium ammonium phosphate (MgNH₄PO₄·6H₂O, or MAP), were studied. Results showed that optimal pH was 9.5 and MAP purity was ∼70% in VFA-free wastewater. With VFA addition, MAP purities of precipitates were higher, reaching 75%-85%. The crystalline characterization of precipitates suggested that VFAs had a weak complexation ability with Mg²⁺ and NH₄⁺. Further, pH changes during the MAP crystallization process were monitored and results indicated VFAs only contributed to the alkalinity condition, which, in turn, improved the MAP crystallization process. These data provide for a better understanding of P recovery by MAP precipitates from VFA-rich fermented supernatants.

Metagenomic characterization of the enhanced performance of anaerobic fermentation of waste activated sludge with CaO2 addition at ambient temperature: Fatty acid biosynthesis metabolic pathway and CAZymes

This study investigated the chemical and genetic mechanisms of anaerobic fermentation of waste activated sludge (WAS) with CaO₂ addition at ambient temperature. The microbial community structures, key microorganisms, functional profiles and related carbohydrate-active enzymes were further revealed according to metagenomic sequencing combining with 16S rRNA gene amplicon sequencing. Results showed that the prolonged period of alkaline condition generated from CaO₂ contributed significantly to the continuous destruction of WAS, and the oxidative environment caused by CaO₂ further enhanced flocs dissolution. This synergistic effect also significantly changed the microbial community. Oxidation contributed more than the alkaline condition to the decline of microbial diversity, while the effect of alkaline condition was greater than that of oxidation in the change of microbial community structure. The key enhanced genes associated with fatty acid biosynthesis pathways with CaO₂ addition were highlighted. Three kinds of high-abundance acetyl-CoA carboxylase genes and eleven kinds of synthetase, hydrolase, lyase and oxidoreductase genes promoted by CaO₂ were distributed throughout each branch of fatty acid biosynthesis pathway (ko00061). Moreover, carbohydrate binding modules (CBMs) and glycoside hydrolases (GHs) were the top two carbohydrate-active enzymes (CAZymes) improved by CaO₂ addition. CaO₂ can also effectively promote the function of lysozyme and the metabolism of several monosaccharides. This work provides a deep insight into the advantage of CaO₂ in promoting sludge solubilization and acidification at the genetic levels, thus expanding the application of CaO₂ in sludge treatment and resource recovery.

Effect of the method of falling water aeration-reflux on nitrogen removal and applicability in a novel upflow microaerobic sludge reactor treating low carbon-to-nitrogen ratio wastewater

A novel falling water aeration-external reflux upflow microaerobic sludge reactor (UMSR) was designed to treat wastewater with the low chemical oxygen demand (COD) to total nitrogen (TN) ratio. The result showed the concentration of dissolved oxygen (DO) in the reactor could be accurately controlled by adjusting the reflux ratio of oxygenated water. The higher aeration efficiency in pollutant removal could be obtained by the reoxygenation mode of the small height falling water. At the reflux ratio of 5:1, the ammonium, nitrite and nitrate nitrogen concentrations in the effluent of UMSR were 6.0, 0.4 and 6.1 mg/L on average, respectively. The removal efficiency of ammonium nitrogen and total nitrogen reached 90.53% and 80.77%, respectively with the influent COD/TN as being 1.0. The structure of the microbial community confirmed the existence of partial-denitrification/anaerobic ammonium oxidation (anammox) bacteria, autotrophic and heterotrophic denitrifiers contributed to nitrogen and carbon removal in UMSR.

Electro-fermentation of iron-enhanced primary sedimentation sludge in a two-chamber bioreactor for product separation and resource recovery

Iron (Fe)-based chemically enhanced primary sedimentation (CEPS) is an effective process used to remove organic and phosphorus (P) pollutants from wastewater into sludge. In this study, electro-fermentation (EF) technology was developed to treat the organic- and P-rich Fe-sludge for the purposes of sludge reduction and resource recovery. Using a two-chamber bioreactor with a cation exchange membrane and an exterior voltage (0.5-1.0 V) for sludge treatment, the EF system enabled product accumulation and separation during the fermentation process. Compared with the conventional fermentation in a single-chamber reactor, the EF treatment of Fe-sludge significantly improved the efficiency of P dissolution from the sludge from 8% to 56% after 4 d. Meanwhile, about 70% of released ammonium ions and 50% of Fe in the sludge were driven by the current from the sludge suspension into the cathode chamber for potential recovery. With a similar yield of volatile fatty acids (VFAs) but less ammonium remaining, the COD_VFA/N_NH4 of the sludge supernatant from the EF reactor could reach 67.7, much higher than that from the single-chamber fermenter at 13.0. The protease activity was effectively promoted by the EF treatment, suggesting that both electrochemical and biological hydrolysis effects contributed to the increased P release from the sludge. Microbial community analysis showed that the electrical stimulation increased the relative abundance of Firmicutes and facilitated the growth of Acidobacteria. Overall, electro-fermentation was developed as an effective biotechnology for processing Fe-based CEPS sludge with the benefits of recovering organics, phosphorus, ammonium and iron resources.