Acidogenic fermentation of iron-enhanced primary sedimentation sludge under different pH conditions for production of volatile fatty acids

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.

Biological bromate reduction coupled with in situ gas fermentation in H2/CO2-based membrane biofilm reactor

Bromate, a carcinogenic contaminant generated in water disinfection, presents a pressing environmental concern. While biological bromate reduction is an effective remediation approach, its implementation often necessitates the addition of organics, incurring high operational costs. This study demonstrated the efficient biological bromate reduction using H2/CO2 mixture as the feedstock. A membrane biofilm reactor (MBfR) was used for the efficient delivery of gases. Long-term reactor operation showed a high-level bromate removal efficiency of above 95 %, yielding harmless bromide as the final product. Corresponding to the short hydraulic retention time of 0.25 d, a high bromate removal rate of 4 mg Br/L/d was achieved. During the long-term operation, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed, which can be regulated by controlling the gas flow. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms underpinning the efficient bromate removal, showing that the microbial bromate reduction was primarily driven by the VFAs produced from in situ gas fermentation. Microbial community analysis showed an increased abundance of Bacteroidota group from 4.0 % to 18.5 %, which is capable of performing syngas fermentation, and the presence of heterotrophic denitrifiers (e.g., Thauera and Brachymonas), which are known to perform bromate reduction. Together these results for the first time demonstrated the feasibility of using H2/CO2 mixture for bromate removal coupled with in situ VFAs production. The findings can facilitate the development of cost-effective strategies for groundwater and drinking water remediation.

Effect of FeCl3 concentration in chemically enhanced primary treatment on the performance of a conventional wastewater treatment plant. A case study

The effect of coagulant dosage in a chemically enhanced primary treatment (CEPT) on the performance of a conventional wastewater treatment plant (WWTP) has been investigated. Lab-scale experiments simulations were carried out in order to evaluate the effect of coagulant addition on the primary settling performance. In these experiments, FeCl3 was used as coagulant. Later, the WWTP was theoretically simulated using a commercial software (WEST®) to evaluate the effect of coagulation/flocculation on the global system, based on the results obtained at lab-scale. According to these results, the CEPT modifies the organic matter balance in the WWTP, decreasing the contribution of readily (SS) and slowly (XS) biodegradable fractions of COD to the aerobic biological process up to 27.3% and 80.8%, respectively, for a dosage of FeCl3 of 24 mg L-1. Consequently, total suspended solids in the aerobic reactor and the secondary purged sludge decreased up to 33% and 13%, respectively. However, the influence on effluent quality was negligible. On the contrary, suspended solids concentration in the sludge to be treated by anaerobic digestion increased, mainly regarding the Ss and Xs fractions, which caused an 8.1% increase in biogas production potential, with approximately 60% of CH4 concentration.

Effects of microbial deodorizer on pig feces fermentation and the underlying deodorizing mechanism

Microbial deodorization is a novel strategy for reducing odor in livestock and poultry feces. Herein, 12 strains of ammonia (NH3) and 15 hydrogen sulfide (H2S) removing bacteria were obtained with a removal efficiency of 65.20-79.80% and 34.90-79.70%, respectively. A novel bacteria deodorant named MIX (Bacillus zhangzhouensis, Bacillus altitudinis, and Acinetobacter pittii at a ratio of 1:1:2) were obtained. MIX can shorten the temperature rising stage by 2 days and prolong the thermophilic stage by 4 days. The ability of MIX to remove NH3, H2S, and volatile fatty acids (VFAs) and the underlying removal mechanism were analyzed during pig feces fermentation. MIX can significantly reduce the concentrations of NH3 and H2S by 41.82% and 66.35% and increase the concentrations of NO3--N and SO42- by 7.80% and 8.83% (P < 0.05), respectively, on the 25th day. Moreover, the concentrations of acetic, propionate, iso-valerate, and valerate were significantly reduced. The dominant bacteria communities at the phylum level were Firmicutes, Proteobacteria, Bacteroidetes, and Spirochaetes. B. zhangzhouensis and B. altitudinis could convert NH4+-N to NO3--N, and A. pittii could transfer H2S to SO42-. This study revealed that bacteria deodorant can reduce the concentrations of NH3, H2S, and VFAs in pig feces and increase those of NH4+, NO3-, and SO42- and has excellent potential in deodorizing livestock and poultry feces composting.

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.

Enhancing the production of volatile fatty acids by potassium permanganate from wasted sewage sludge: A batch test experiment

Recovering resources from wastewater treatment is vital for the transition from a linear to a circular economy model in the water sector. Volatile Fatty Acids (VFAs) are valuable products among the possible recovered resources. This study investigates the influence of potassium permanganate (KMnO4) addition during acidogenic fermentation of waste activated sludge for enhancing VFAs production. Specifically, different fermentation batch tests with and without KMnO4 addition were carried out using two distinctive sewage sludges as feedstocks. Results showed that KMnO4 addition increased the VFAs yield up to 144 and 196 mgCOD/g VSS for the two sludges. When KMnO4 was used as pre-treatment, 55 % of sCOD were VFAs. This latter result was mainly debited to the recalcitrant organics' disruption promoted by the oxidative permanganate ability.

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.

Seasonal variations in acidogenic fermentation of filter primary sludge

Primary treatment of municipal wastewater by rotating belt filtration followed by hydrolysis and acidogenic fermentation of the filter primary sludge (FPS) at ambient temperature was studied at pilot-scale during one year. The seasonal variations of volatile fatty acids (VFAs), nutrient release and soluble COD production as well as microbial community assembly were assessed, leading to novel findings for fermentation at ambient temperature. The reproducibility of VFA production performance was first established by operating the two fermentation reactors under the same conditions, showing similar results regarding VFA production and microbial community structure. One year of operation at 5 d retention time (RT) and 16-29 °C resulted in an average VFA yield of 180±35 mg COD/g VSin and soluble COD yield of 242±40 mg COD/g VSin. The VFA formation was temperature-dependent, with ϴ=1.033±0.005 ( [Formula: see text] . The seasonal variations of the acetic and propionic acid productions were pronounced, whereas the productions of VFAs with longer chains were more stable regardless of temperature. The community structure of the reactor microbiomes was also clearly affected by season and temperature and linked with the production spectrum of VFAs. The ammonium and phosphate releases were stable during the year, leading to a decrease in ratios of soluble COD to NH4+-N and PO43--P during winter. The soluble COD yield was 11% and 27% higher at 5 d RT compared to 3 and 2 d RT respectively, but the corresponding volumetric productivities were lower. The dissimilarities between microbiomes in influent FPS and fermenters were significant even at a short RT of 2 d, and increased with longer RT of 3 and 5 d, primarily caused by selection of bacteria within Bacteroidota in the fermentation reactors.

Pilot-scale hydrolysis of primary sludge for production of easily degradable carbon to treat biological wastewater or produce biogas

Organic compounds in wastewater are required for the biological removal of nitrogen, but they can also be used for biogas production. Distribution of the internal organic carbon at the plant is therefore critical to ensure high quality of the treated water, reduce greenhouse gas emissions, and optimize biogas production. We describe a wastewater treatment plant designed to focus equally on energy production, water quality, and reduced emissions of greenhouse gases. A disk filter was installed to remove as much carbon as possible during primary treatment. Primary sludge was then hydrolyzed and centrifuged. The hydrolysate centrate contained volatile fatty acids and was used either for the secondary wastewater treatment or to produce biogas. The yield during hydrolysis was 30-35 g volatile fatty acid per kg dry material or 40-65 g soluble COD per kg total solid. The specific denitrification rate was 20-40 g/(g·min), which is on the same order of magnitude as that for commonly used external carbon sources. Hydrolysis at around 35 °C and pH 7 gave the best results. The hydrolysate centrate can be stored and added to the biological treatment to improve water quality and reduce emissions of nitrous oxide or it can be used to produce biogas to optimize the operation of the plant.

Anaerobic co-digestion of kitchen waste with hyperthermophilically pretreated grass for biohydrogen and biomethane production

Anaerobic digestion of kitchen waste with grass after hyperthermophilic pretreatment was performed in semi-continuously operated reactors. The greatest methane yield of 293 NmlCH4/gVS (volatile solids) was reported for the mixture of both substrates at 55 °C with a solids retention time of 30 d and the corresponding organic lading rate of 1.72 kgVS/m3/d. In contrast, pretreated grass subjected to thermophilic digestion produced only 131 NmlCH4/gVS. However, when mesophilic conditions were applied, the digestion process turned into dark fermentation, especially visible for the mixture. Metagenomic analysis revealed the dominance Ruminococcaceae, Atopobiaceae and Lactobacillaceae at a family level in mesophilic processes, whereas Petrotogaceae, Synergistaceae, Hungateiclostridiaceae, Planococcaceae and two methanogens Methanosarcinaceae and Methanothermobacteriaceae were the most frequent microbes of thermophilic digestion. Kitchen waste can successfully be co-digested with hyperthermophilically pretreated grass at high loading rates, however the digesters must be operated at thermophilic temperatures.

Thiosulfate-assisted Fe2+/persulfate pretreatment effectively alleviating iron dose and enhancing biotransformation of waste activated sludge into high-value liquid products

Ferrous-based acidogenic fermentation (AF) as a means to treat waste activated sludge (WAS) and produce short-chain fatty acids (SCFAs) has drawn increasing attention, but the massive amount of "iron sludge" that it produces not only significantly increases costs and difficulty of disposal but also poses risks to the environment and human health. This study explored a novel approach to not only reduce the iron dosage required by AF but also to improve its performance by introducing a thiosulfate (TS)-assisted Fe²⁺/persulfate (TAFP) pretreatment. Effects of the TAFP pretreatment on WAS disintegration and biodegradability, SCFA production, and microbial community structure with different persulfate-Fe²⁺-thiosulfate molar ratios at 4:4:0 (R1), 4:3:1 (R2), 4:2:2 (R3) and 4:1:3 (R4) were investigated. The results showed that the TAFP pretreatment by a remarkable margin promoted the disintegration of WAS as well as the biodegradability of the organics released, owing to the production of robust free radicals (SO₄•^- and •OH) triggered by the thiosulfate and Fe²⁺ cycles. 48-day AF tests further showed maximum SCFA production, ranging roughly between 1283 and 1395 mg COD/L in the TAFP pretreated samples, much higher than Control (<120 mg/L) and R1 (around 593 mg COD/L). At the meantime, the Fe²⁺ dosage was reduced by 50% in R3 than that of R1. However, a prolonged lag phase of SCFA generation was observed between days 7 and 25, which was ascribable to the acidic conditions (pH < 4.5) closely related to impaired metabolic activities as well as electron transfer efficiencies and limited activities of acidogenic enzymes (i.e., pyruvate-ferredoxin oxidoreductase). Despite such lag phase, the economic and environmental assessment of AF of TAFP-pretreated WAS had a higher net SCFA yield and less "iron sludge" than either without any pretreatment or with Fe²⁺/persulfate-only pretreatment.

Volatile fatty acids production from waste streams by anaerobic digestion: A critical review of the roles and application of enzymes

Volatile fatty acids (VFAs) produced from organic-rich wastewater by anaerobic digestion attract attention due to the increasing volatile fatty acids market, sustainability and environmentally friendly characteristics. This review aims to give an overview of the roles and applications of enzymes, a biocatalyst which plays a significant role in anaerobic digestion, to enhance volatile fatty acids production. This paper systematically overviewed: (i) the enzymatic pathways of VFAs formation, competition, and consumption; (ii) the applications of enzymes in VFAs production; and (iii) feasible measures to boost the enzymatic processes. Furthermore, this review presents a critical evaluation on the major obstacles and feasible future research directions for the better applications of enzymatic processes to promote VFAs production from wastewater.

Biomethane production from waste activated sludge promoted by sludge incineration bottom ash: The distinctive role of metal cations and inert fractions

Sludge incineration bottom ash (SA), a solid waste generated by incineration of waste activated sludge (WAS), has been demonstrated as an inexpensive additive to increase biomethane production from anaerobic digestion (AD) of WAS. However, how SA improved methanogenic performance of a WAS digester remains elusive. Here, we addressed this question by fractionating the SA into accessible leachate (SA-L) and inert residue (SA-R) and investigating their individual effects. The cumulative biomethane production was increased by 6.7%, 20.2% and 39.6% with addition of SA-L, SA-R and SA, respectively. Mechanistic study showed that release of organic-binding metals (Ca and Fe) from SA dissolution suppressed volatile fatty acids production by increasing the apparent activation energy (AAE) and decreasing the surface binding sites for hydrolytic/acidogenic enzymes during WAS hydrolysis-acidogenesis, while trace elements in SA-L promoted metabolism of methanogens (Methanothermobacter and Methanosarcina). In contrast, the gypsum/silicate-cored SA-R facilitated hydrolysis-acidogenesis with reduced AAE but drastically inhibited methanogenesis due to competition of sulfate-reducing bacteria Thermodesulfovibrio. The comparative analysis of KEGG-based functional genes indicated that the enhanced methane metabolism and reductive CO₂ fixation pathways with SA addition could result from the release of trace elements to support key enzyme activities.

Enhanced Cr(VI) reduction in biocathode microbial electrolysis cell using Fenton-derived ferric sludge

Hexavalent chromium (Cr(VI)) is one of the major concerns for water environment and human health due to its high toxicicity, while ferric sludge produced from Fenton processes is also a tough nut to crack. In this study, the synergetic impact of ferric sludge derived from the Fenton process on the bioreduction of Cr(VI) in biocathode microbial electrolysis cell was investigated for the first time. As a result, Cr(VI) reduction efficiency at biocathode increased by 1.1-2.6 times with 50 mg/L ferric sludge under different operation conditions. Besides, the Cr(VI) reduction enhancement decreased with the increase of pH and initial Cr(VI) concentration or increased with the increase of ferric sludge dosage. Correspondingly, relatively higher power density (1.027 W/m³ with 100 mg/L ferric sludge while 0.827 W/m³ for control) and lower activation energy and resistance were also observed. Besides, the presence of ferric sludge increased biomass protein (1.7 times higher with 100 mg/L ferric sludge) and cytochrome c (1.4 times higher with 100 mg/L ferric sludge). The evolution of microbial community structure for a higher abundance of Cr(VI) and Fe(III)-reducing microorganisms were exhibited, implying the enhancement of Cr(VI) reduction was due to the formation of Fe(II) from the reduction of ferric sludge. These findings provide insights and theoretical support for developing a viable biotechnology platform to realize waste treatment using waste.

Efficient methane production from waste activated sludge and Fenton-like pretreated rice straw in an integrated bio-electrochemical system

Integrated microbial electrolysis cell-anaerobic digestion (MEC-AD) systems have demonstrated potential advantages for methane production in the presence of small amounts of residual inhibitors. In this study, a series of tests were conducted to analyse the acidification and methanogenesis performance of pretreated rice straw (RS) in anaerobic digestion (AD) and MEC-AD systems after the addition of Fenton-like reagents. The results indicated that the short-chain acids (SCFAs) accumulations reached 2284.64 ± 21.57 mg COD/L with a dosage ratio of 1/4 (g RS/g VSS sludge) in the MEC-AD system and that methane production increased by 63.8% compared with that of an individual AD system. In the interim, the net energy output reached 1.09 × 10³ J/g TCOD, which was 1.23 times higher than that of the AD system. The residual Fe³⁺/Fe²⁺ in the pretreatment reagent was capable of promoting acidification and methanogenesis in sludge and RS fermentation. The RS hydrolysis products could constrain methanogenesis, which can be mitigated by introducing an MEC. The microbiological analyses revealed that the MEC strongly increased the enrichment of hydrogenotrophic methanogens, especially Methanobacterium (61.16%). Meanwhile, the Syntrophomonas and Acetobacterium abundances increased to 2.81% and 2.65%, respectively, which suggested the reinforcement of acetogenesis and methanogenesis. Therefore, the enhanced hydrogenotrophic methanogens might have served as the key for enhancing the efficiency of methanogenesis due to the introduction of an MEC.

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

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

Bacterial survival strategies in sludge alkaline fermentation for volatile fatty acids production: Study on the physiological properties, temporal evolution and spatial distribution of bacterial community

This study investigated the dynamics of ATP synthase activity, phospholipid fatty acid (PLFA) profile, and temporal evolution and spatial distribution of bacterial community to analyze bacterial survival strategies in sludge alkaline fermentation (SAF) for volatile fatty acids (VFAs) production. The results revealed a significant increase in ATP synthase activity at pH 9 and 10 (p < 0.05), which could contribute to proton entry into cells and benefit bacterial survival. PLFA analysis indicated that the unsaturated fatty acids content increased with the increase of pH. Firmicutes were the dominant microorganisms in the running stage of the pH 10 reactor (35.81-62.34%) and might have been the key microbes that influenced VFAs production. Further analysis of the spatial distribution of microbial community suggested that Firmicutes mainly lived inside flocs during SAF. These findings provide an understanding for bacterial survival strategies in SAF, which could help to develop methods to further improve VFAs yield.

Biorefinery potential of chemically enhanced primary treatment sewage sludge to representative value-added chemicals - A de novo angle for wastewater treatment

Chemically enhanced primary treatment (CEPT) is an emerging sewage treatment strategy due to its high efficiency and small land requirement. CEPT sludge can be easily dewatered and used for energy recovery through incineration. However, with large amount of reusable nutrients (40% organic carbon, 23% lipids, and 17% protein), the value of CEPT sludge may have been underestimated. In this study, the biorefinery potential of CEPT sludge has been proven via production of 28.9 g/L ethanol or 50.3 g/L lactic acid (LA) or 1.43 filter paper unit (FPU)/mL cellulase from 10 g of CEPT sludge experiment. Inhibition on cell growth and potential inhibitors from plasticizers, pharmaceuticals, and surfactants were determined. Nevertheless, production titer was not affected or performed even better than the non-inhibitors controls. CEPT sludge showed significant potential in biochemical conversion, and the related products may offer an opportunity to support wastewater treatment toward sustainability and carbon neutrality.

Dark fermentative hydrogen production from hydrolyzed sugar beet pulp improved by nitrogen and phosphorus supplementation

The effect of nitrogen and phosphorous addition on hydrogen production from hydrolyzed Sugar beet pulp (SBP) was investigated using (NH₄)₃PO₄, NH₄Cl and K₃PO₄ as the supplements. In batch tests, the maximal hydrogen production of 279 dm³/kgVS was observed for K₃PO₄, which was added to SBP in a dose of 1 g/dm³. In semi-continuous experiments, the greatest hydrogen production of 36 dm³/kgVS was reported for the same supplement, and this value was twice higher than that of the control run. The analysis of microbiota revealed that the majority of bacteria was affiliated to the orders Clostridiales, Lactobacillales and Coriobacteriales. Moreover, a noticeable methane production was associated with the activity of Methanosphaera sp., which could grow in a low pH environment of dark fermentation.

Wastewater Valorization: Practice around the World at Pilot- and Full-Scale

Over the last few years, wastewater treatment plants (WWTPs) have been rebranded as water resource recovery facilities (WRRFs), which recognize the resource recovery potential that exists in wastewater streams. WRRFs contribute to a circular economy by not only producing clean water but by recovering valuable resources such as nutrients, energy, and other bio-based materials. To this aim, huge efforts in technological progress have been made to valorize sewage and sewage sludge, transforming them into valuable resources. This review summarizes some of the widely used and effective strategies applied at pilot- and full-scale settings in order to valorize the wastewater treatment process. An overview of the different technologies applied in the water and sludge line is presented, covering a broad range of resources, i.e., water, biomass, energy, nutrients, volatile fatty acids (VFA), polyhydroxyalkanoates (PHA), and exopolymeric substances (EPS). Moreover, guidelines and regulations around the world related to water reuse and resource valorization are reviewed.

Short-chain fatty acids resource recovery potential from algal sludge via anaerobic fermentation under various pH values

The harvesting of algal sludge from eutrophic lakes, including the large quantity of organic matters, has the potential to be used as valuable products through the process of resource recovery. This study investigates the fatty acid production potential from algal sludge via anaerobic fermentation under different pH values. The results indicated that the recovery of short-chain fatty acids (SCFAs) was the highest (3269.25 ± 32.89 mg·COD/L) at pH 11 after 7 days of fermentation. The SCFAs concentration at pH value 11 was 6.24, 1.27, 4.90, and 0.53 times higher compared with that at pH value 3, 5, 7, and 9, respectively. The SCFAs production was continually increased from day 1 to day 7 at pH value 7, 9, and 11. Much fewer middle- and long-chain fatty acids were produced compared with SCFAs. Gross. fatty acid production was the highest at pH 11. The concentrations of soluble protein and polysaccharide were the highest at pH 11, implying that the disruption of algal cells could have a high value at pH 11. The polysaccharide concentration was the lowest at pH 7. The fluorescence excitation-emission matrix profile implied that the disruption of algal cells was the greatest at pH 11. Methane production was greatest at pH 7 and 9. Overall, the results of this study revealed that a pH of 11 was optimal for the recovery of SCFAs from algal sludge due to the higher cell disruption, suitable ORP condition for SCFAs production and inhibition of methanogens.

Continuous waste activated sludge and food waste co-fermentation for synchronously recovering vivianite and volatile fatty acids at different sludge retention times: Performance and microbial response

A practical approach of synchronously recovering vivianite and volatile fatty acids (VFAs) by food waste (FW) and waste activated sludge (WAS) co-fermentation in continuous operation was investigated. Approximately 82.88% P as high-purity vivianite (95.23%) and 7894 mg COD/L VFAs were finally recovered. The simultaneous addition of FW and FeCl₃ contributed to the fermentation conditions by adjusting pH biologically and increasing the concentration of organic substrates, which enhanced the Fe³⁺ reduction efficiency and microbial activities (e.g., hydrolases and acidogenic enzymes). Microbial analysis found the functional bacteria related to Fe³⁺ reduction and VFAs generation were further enhanced and enriched. Besides, results indicated that the efficiencies of Fe²⁺ and P release and VFAs recovery were highly linked to SRT, the satisfactory fermentation performance was obtained at SRT of 6 d. This research would provide a practical waste recycling technology to treat FW and WAS simultaneously for recovering vivianite and VFAs synchronously.

Dark fermentative hydrogen production from hydrolyzed sugar beet pulp improved by iron addition

This study evaluated the impact of three different iron compounds (Fe₂O₃, FeSO₄, FeCl₃) on hydrogen production via mesophilic dark fermentation (DF) of hydrolyzed sugar beet pulp (SBP). In batch tests, the maximum hydrogen yield of over 200 dm³H₂/kgVS was achieved with the addition of 0.1 gFe₂O₃/dm³, which was twice greater than the control. In semi-continuous experiments, the highest hydrogen production of 52.11 dm³H₂/kgVS combined with 19.4 dm³CH₄/kgVS methane yield was obtained at a dose of 1 gFe₂O₃/dm³. Acetic, lactic and caproic acids were the main metabolic products of DF. Microbiological studies showed some balance between hydrogen producing microorganisms from the order Clostridiales and lactic acid producers (LAB) affiliated with the orders Lactobacillales and Coriobacteriales. Moreover, the presence of methanogens affiliated to the genera Methanobrevibacter and Methanosphaera was also documented. An interesting finding was the appearance of rare bacteria from the genus Caproiciproducens, which was responsible for increased caproic acid production.

Short-chain fatty acids recovery from sewage sludge via acidogenic fermentation as a carbon source for denitrification: A review

Wastewater treatment plants face the problem of a shortage of carbon source for denitrification. Acidogenic fermentation is an effective method for recovering short-chain fatty acids (SCFAs) as a carbon source from sewage sludge. Herein, the most recent advances in SCFAs production from primary sludge and waste activated sludge are systematically summarised and discussed. New technologies and problems pertaining to the improvement in SCFAs availability in fermentation liquids, including removal of ammoniacal nitrogen and phosphate and extraction of SCFAs from fermentation liquids, are analysed and evaluated. Furthermore, studies on the use of recovered SCFAs as a carbon source for denitrification are reviewed. Based on the above summarisation and discussion, some conclusions as well as perspectives on future studies and practical applications are presented. In particular, the recovery of carbon source/bioenergy from sewage sludge must be optimised considering nutrient removal/recovery simultaneously.

Removal of emerging contaminants from wastewater during chemically enhanced primary sedimentation and acidogenic sludge fermentation

A novel wastewater treatment process, which couples chemically enhanced primary sedimentation (CEPS) of sewage with acidogenic fermentation of sludge in tandem, has recently been developed to improve the removal of pollutants and nutrients, and recover valuable resources such as phosphorus and organics. This study represented the first laboratory-based examination on the level and removal of the emerging contaminants, including retinoids (i.e., retinoic acids (RAs) and their metabolites) and oestrogenic endocrine disrupting chemicals (EDCs; e.g., 4-nonylphenol, bisphenol A, etc.), in sewage, sludge and its supernatant during this novel wastewater treatment process. The results showed that 65% of retinoids and 73% of EDCs were removed from sewage after aluminum (Al) based CEPS, while 80% of retinoids and 72% of EDCs were removed after iron (Fe) based CEPS. After acidogenic fermentation of the CEPS sludge, 50% and 58% of retinoids, and 50% and 47% of EDCs were further removed in the supernatants of Al-sludge and Fe-sludge, respectively. While there were comparable removals for these two classes of emerging contaminants during Al- and Fe-based CEPS and sludge fermentation, Fe-based CEPS of sewage and sludge fermentation should be preferentially considered, given the relatively lower production of Fe-sludge and lower accumulation of retinoids in Fe-sludge. The levels of retinoids and EDCs in the supernatant and sludge changed during acidogenic fermentation of Fe-sludge. The removals of at-4-oxo-RA (i.e., the dominant retinoid) and bisphenol A (i.e., the dominant EDC) in the supernatant followed the pseudo first-order reaction model, with a half-life of 1.62 days (in the first two days) and 1.55 days (in the whole experiment of seven days), respectively. The results demonstrated the effective removal of emerging contaminants from the sewage and the supernatant during the CEPS and acidogenic sludge fermentation.

Long-term application of Swedish sewage sludge on farmland does not cause clear changes in the soil bacterial resistome

The widespread practice of applying sewage sludge to arable land makes use of nutrients indispensable for crops and reduces the need for inorganic fertilizer, however this application also provides a potential route for human exposure to chemical contaminants and microbial pathogens in the sludge. A recent concern is that such practice could promote environmental selection and dissemination of antibiotic resistant bacteria or resistance genes. Understanding the risks of sludge amendment in relation to antibiotic resistance development is important for sustainable agriculture, waste treatment and infectious disease management. To assess such risks, we took advantage of an agricultural field trial in southern Sweden, where land used for growing different crops has been amended with sludge every four years since 1981. We sampled raw, semi-digested and digested and stored sludge together with soils from the experimental plots before and two weeks after the most recent amendment in 2017. Levels of selected antimicrobials and bioavailable metals were determined and microbial effects were evaluated using both culture-independent metagenome sequencing and conventional culturing. Antimicrobials or bioavailable metals (Cu and Zn) did not accumulate to levels of concern for environmental selection of antibiotic resistance, and no coherent signs, neither on short or long time scales, of enrichment of antibiotic-resistant bacteria or resistance genes were found in soils amended with digested and stored sewage sludge in doses up to 12 metric tons per hectare. Likewise, only very few and slight differences in microbial community composition were observed after sludge amendment. Taken together, the current study does not indicate risks of sludge amendment related to antibiotic resistance development under the given conditions. Extrapolations should however be done with care as sludge quality and application practices vary between regions. Hence, the antibiotic concentrations and resistance load of the sludge are likely to be higher in regions with larger antibiotic consumption and resistance burden than Sweden.

In-situ ammonia stripping with alkaline fermentation of waste activated sludge to improve short-chain fatty acids production and carbon source availability

Alkaline fermentation of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs) has been proved to be promising to develop internal carbon source for denitrifying processes in municipal wastewater treatment plants. However, a large amount of ammoniacal nitrogen also releases during fermentation, resulting in inhibition of acidogenic process and reduction of carbon source availability. Alkaline fermentation of WAS combined with in-situ ammonia stripping was proposed to improve SCFAs production and carbon source availability simultaneously. The results showed that a maximal SCFAs production of 308.7 ± 4.8 mg chemical oxygen demand/g volatile suspended solid was achieved under the pH = 10 + In-situ ammonia stripping, which was 21.7% and 141.5% higher than that of the pH = 10 and the control, respectively. Most of the ammoniacal nitrogen was stripped out, and the WAS-derived SCFAs availability as carbon source increased by 103.2%. These findings indicate in-situ ammonia stripping could make alkaline fermentation of WAS more practical.

Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation

This study presented an innovative method for phosphorus (P) recovery as vivianite from waste activated sludge (WAS) via optimizing iron dosing and pH value during anaerobic fermentation (AF). The optimal conditions for vivianite formation were in the pH range of 6.0-9.0 with initial PO₄^3- >5 mg/L and Fe/P molar ratio of 1.5. Notably, FeCl₃ showed advantages over ZVI for the simultaneous release of Fe²⁺ and PO₄^3- during WAS fermentation, especially in acidic conditions. The FeCl₃ dosing at pH 3.0 could contribute to 78.81% Fe²⁺ release and 85.69% of total PO₄^3- release from WAS. They were ultimately recovered in the form of high-purity vivianite (93.67%). Clostridiaceae (40.25%) was the predominant bacteria in FeCl₃-pH3 reactors, which played key roles in inducing dissimilatory iron reduction for Fe²⁺ formation. Therefore, P recovery as vivianite from WAS fermentation might be a promising and highly valuable approach to relieve the P crisis.

Carbon source and phosphorus recovery from iron-enhanced primary sludge via anaerobic fermentation and sulfate reduction: Performance and future application

Anaerobic fermentation and sulfate reduction (AF-SR) was firstly used for recovery of carbon sources and phosphorus from Fe-enhanced primary sludge (Fe-sludge). With FeCl₃ dosage of 30 mg Fe/L, 63.0% of the chemical oxygen demand (COD) and 97.3% of the phosphorus were concentrated from sewage into Fe-sludge. Batch anaerobic fermentation tests of Fe-sludge with and without sulfate addition (AF-SR and control) were performed. The results showed that volatile fatty acid concentrations of the control and AF-SR were 211.0 and 270.2 mg COD/g volatile suspended solids, respectively. Furthermore, 33.2% (control) and 56.2% (AF-SR) of the total phosphorus in Fe-sludge was released. The recovery performances of carbon source and phosphorus were calculated based on struvite precipitation. The available carbon source of the AF-SR system was 44.5% higher than that of the control. A novel integrated wastewater and sludge treatment process based on chemically enhanced primary sedimentation and AF-SR is proposed for future application.

Enhanced production of short-chain fatty acids from waste activated sludge by addition of magnetite under suitable alkaline condition

Alkaline fermentation technology is an effective method for resource recovery, e.g., short-chain fatty acids (SCFAs), but the acidification process needs to be further enhanced. To improve the release and acidification of organic matters in waste activated sludge (WAS) fermentation simultaneously, a novel method was proposed with magnetite addition under alkaline condition in this study. Compared with the control, SCFAs and acetic acids yields increased by 21.2% ± 3.1% and 31.0% ± 1.2% in the 0.6 g/gVSS magnetite-based system, respectively. Besides, the activities of α-glucosidase, protease and acetate kinase (AK) were enhanced with magnetite addition, and the abundance of acidogenic microbes was improved obviously. Furthermore, magnetite reduced the release of PO₄^3--P significantly via the precipitation reaction. Of all, a novel side-stream WAS hydrolysis and acidification process based on magnetite addition under suitable alkaline condition is proposed, realizing the efficient recovery of carbon and phosphorus resources along with wastewater and WAS treatment.

Performance and bacterial community of moving bed biofilm reactors with various biocarriers treating primary wastewater effluent with a low organic strength and low C/N ratio

A laboratory-scale sequencing batch reactor (SBR) and two moving bed biofilm reactors (MBBRs) with different types of biocarriers were operated to treat the effluent of chemically enhanced primary sedimentation (CEPS). Due to the low organic strength and low carbon/nitrogen ratio of the CEPS effluent, COD and NH₄⁺-N were effectively removed by the MBBRs but not by the SBR. Of the two MBBRs, MBBR2 filled with LEVAPOR biocarrier cubes performed even better than MBBR1 filled with K3 polystyrene biocarriers. The continuous decline of the sludge concentration in the SBR and the high and stable biomass content in MBBR2 contributed to their performances. High-throughput sequencing analysis showed that the reactors had selective effects on the bacterial community structure. Principal coordinate analysis indicated the different dynamic successions in the three reactors. Network analysis showed different community composition and diversity that were highly suggestive of different bacterial interactions among the three bioreactors.

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.

In-situ biogas upgrading by a stepwise addition of ash additives: Methanogen adaption and CO2 sequestration

Biogas from anaerobic digestion (AD) of waste activated sludge (WAS) limited its utilization due to low value-added. In this study, an innovative addition mode for ash known as stepwise addition was developed to enhance methane production and improve CO₂ scavenge from AD of sludge. Experimental results confirmed stepwise addition of ash improved methane content to 79.4%, compared to control group (69.1%). Compared to Pulse addition and Control, the cumulative CH₄ production was promoted by 39.2% and 35.4%, respectively. Investigation of the mechanism indicated that stepwise addition of ash could decrease hydrolytic and acidifying enzyme activities but increase activity of coenzyme F420, compared to pulse addition group. Furthermore, stepwise addition of ash not only increased the abundance of Methanomassiliicoccus (34.48%), but also promoted amounts of CO₂ capture. This method ameliorate utilization availability of sludge ash for sludge anaerobic digestion through promoting cumulative methane production and increasing CO₂ storage capacity.

Stepwise pH control to promote synergy of chemical and biological processes for augmenting short-chain fatty acid production from anaerobic sludge fermentation

Although sludge-converted short-chain fatty acids (SCFAs) are promising feedstocks for biorefineries, it remains challenging to maximise SCFA production by enhancing synergies between chemical/biological hydrolysis and acidogenesis processes while employing a balanced composition of microbial communities to counteract methanogenesis. Herein, stepwise control of fermentation pH and chemical/microbiological composition analysis of fermented sludge were used to probe the underlying mechanisms of SCFA production. Fermentation at pH 11 during the first three days promoted both chemical and microbial hydrolysis of sludge proteins and provided a niche for Anaerobrancaceae sp. to transform soluble protein into SCFAs. When pH was decreased from 11 to 9, Acinetobacter, Proteiniborus, Proteiniclasticum, and other acetogens became predominant and stayed significantly more active than during first-stage fermentation at pH 11, which benefited the acidification of hydrolysed substrates. Further assays indicated that early-stage sludge fermentation at pH 11 decreased the total amount of methanogenic archaea and hence reduced the amount of SCFAs consumed for methane production. Thus, the use of stepwise pH control for sludge fermentation allowed one to establish process synergies, facilitate chemical and biological hydrolysis, inhibit methanogens, and promote the growth of acidifying bacterial communities, which resulted in efficient SCFA production from sludge.

A critical review of volatile fatty acids produced from waste activated sludge: enhanced strategies and its applications

This paper reviews the recent achievements in the enhanced production of volatile fatty acids (VFAs) from waste activated sludge (WAS). The enhanced strategies are divided into two approaches. The first strategy focuses on the regulation of carbon-to-nitrogen (C/N) ratio by co-digestion of WAS with carbon-rich substrates, including municipal solid wastes (MSW), marine algae, agricultural residues, and animal manures. The other strategy is to enhance the solubilization and hydrolysis of WAS or inhibit the methanogenesis by applying various pretreatments, such as mechanical, chemical, enzymatic, and thermal pretreatment. Finally, the applications of WAS-derived VFAs are discussed. The future researches in enhancing VFAs production and wide application of the VFAs from both technical and economic perspectives are proposed.

Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation

Phosphorus release and recovery from Fe-based chemically enhanced primary sedimentation (CEPS) sludge via alkaline fermentation was investigated. The coagulation results showed that 78% of organic matter and 95% of phosphorus were concentrated from sewage into sludge with the optimum dosages of 25 mg/L FeCl₃. The batch fermentation results revealed that 69.35% of the phosphorus in the Fe-sludge can be released and the maximum phosphorus concentration was 20.57 mg/L at pH 11. In the recovery stage, 90% of the P released in the fermented sludge supernatant was precipitated at a 2:1 ratio of magnesium to phosphorus and pH 11. The result of X-ray diffraction indicated that magnesium ammonium phosphate (MAP) was the major component of the precipitated solids. Thus, the present study provides an alternative option for phosphorus release and recovery as MAP from CEPS sludge via alkaline fermentation.

The effect of ISR on OFMSW during acidogenic fermentation for the production of AD precursor: kinetics and synergies

Acidogenic fermentation of organic fraction of municipal solid waste (OFMSW) and it's components (food waste and paper wastes) was studied in batch percolator reactor without artificial pH adjustment.

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

Chemically enhanced primary sedimentation (CEPS) technology has been widely applied in Hong Kong, exhibiting excellent performance in contaminants removal from sewage. The generated CEPS sludge contains abundance of organics which could be recovered as volatile fatty acids (VFAs) by fermentation for further utilization. In this work, the effect of calcium peroxide (CaO₂) on the fermentation of FeCl₃ based CEPS sludge was investigated. The feasibility of utilizing the fermentation liquor as substrate for polyhydroxyalkanoates (PHAs) biosynthesis was also evaluated. Results demonstrated that CaO₂ addition facilitated the disintegration of CEPS sludge and enhanced VFAs production. The maximum VFAs yield of 455.8 mg COD/g VSS was obtained with the dosage of 0.1 g CaO₂/g SS, improving by 44.7% compared with the control sludge. Acetic and propionic acid were the predominant components of the VFAs. Microbial analysis indicated that CaO₂ induced microbial reduction of Fe(III), accelerating the initial disintegration of FeCl₃ based CEPS sludge. Microbial communities with hydrolysis and acidogenesis functions were enriched effectively. CaO₂ treatment had no significant influence on the release of ammonia nitrogen (NH₄⁺-N), while reduced the concentration of orthophosphate (PO₄^3--P) and ferrous (Fe²⁺) in fermentation liquor, that was beneficial to the further utilization as substrate for PHAs biosynthesis. The VFA-rich fermentation liquor was proved to be a suitable substrate for PHAs biosynthesis. After cultivation, the PHAs content in activated sludge reached 22.3%, which was comparable to those obtained using waste materials as carbon source. This integrated technology could be a superior alternative of realizing sludge disposal and bioplastic production simultaneously.