Biological Treatment Processes Research Articles

Removal of organic micropollutants in a drinking water treatment plant by powdered activated carbon followed by rapid sand filtration

Powdered activated carbon (PAC) is a promising technology to reduce organic micropollutants (OMPs) in drinking water treatment plants and combined with biological treatment processes such as rapid sand filters (RSFs), has the potential to remove a broad range of OMPs during water treatment. However, the removal of OMPs by a combination of two treatments (PAC-RSF) is not fully understood. 49 OMPs were spiked in PAC followed by RSF pilot plant treatment and the effluent was measured after 1, 4 and 7 day(s). Also, in a separate experiment 29 OMPs (out of 49) were dosed into the RSF column to assess the removal of OMPs by just RSF. Statistical analyses were performed and the OMPs were classified according to their charge and hydrophobicity. The results showed that OMPs were removed at the highest percentage after the first 24 h of dosing. The highest removal was observed for the neutral and hydrophobic compounds (80.0%–99.5%). The removal of neutral and hydrophilic compounds varied (0.14%–98.5%) depending on the possibility of forming π-π interactions and hydrogen bonds. Electrostatic repulsion between anionic OMPs and negatively charged PAC led to a decrease in the removal of anionic compounds. In contrast, electrostatic attraction led to an increase in the removal of cationic OMPs. Regarding OMPs removal by RSF, some compounds were removed by the sand filter depending partly on their physical-chemical properties and more dominantly on their functional groups. By comparing the OMPs removal by PAC-RSF with RSF, it was possible to determine the contribution of biological processes in the sand filter in the removal of some OMPs.

Simultaneous medium chain fatty acids production and process carbon emissions reduction in a continuous-flow reactor: Re-understanding of carbon flow distribution

Due to its wide application and high value, the production of medium chain fatty acids (MCFAs) from waste biomass has become one of the worldwide research hotspots. Increasing the carbon element participation from short-chain fatty acids to the form of MCFAs is also conductive to reduce the release of biogas from biological treatment process, because carbon is in the form of MCFAs instead of biogas which directly contribute to process carbon emissions reduction. However, many barriers limiting MCFAs production and application remain to be resolved. Aiming continuous MCFAs production from lactate-rich waste biomass, this study optimized the operation conditions and clarified the main limiting factors and possible mechanisms. The maximum caproic acid concentration of 2.757 g/L were obtained at the Upflow Velocity (ULV) of 1.15 m/h and pH 4.9–5.1. Caproiciproducens, Pseudoramibacter, norank_f_Eubacteriaceae, and Oscillibacter were identified to be the dominant microbial genus responsible for MCFAs production from lactate. The reduction of carbon emissions calculation was also studied in the present processes.

Coupling of (methane + air)-membrane biofilms and air-membrane biofilms: Treatment of p-nitroaniline wastewater

Membrane biofilm (MBf) technology is a promising biological water treatment process that combines membrane aeration with biofilms. To expand its application in the treatment of toxic organic wastewater, methane/air gas mixture-MBfs ((CH4 + Air)-MBfs) and air-MBfs were coupled to enhance the treatment of p-nitroaniline (PNA) wastewater. Based on exploration of the membrane permeability of methane and oxygen, a hybrid MBf reactor was constructed, and the degradation characteristics of PNA and the coupling effects of (CH4 + Air)-MBfs and air-MBfs were studied. The permeation flux of methane was found to be 1.114 g/(m2 d) when using a methane/air gas mixture at an aeration pressure of 10 kPa, and this result was better than that when methane was used as the aeration gas alone. Aeration with a methane/air gas mixture provided conditions for realizing aerobic methane oxidation; the aerobic methane oxidation that occurred in the (CH4 + Air)-MBfs promoted the reduction of PNA, and the intermediates of PNA degradation were further degraded by the air-MBfs. At an influent PNA membrane area load of 1.67 g/(m2 d), the PNA removal load reached 187.30 g/(m3 d). The coupling of MBfs took advantage of different matrix-based MBfs and promoted the degradation of PNA by utilizing the synergistic effects of various functional microorganisms.

Research Progress on Biological Treatment Process of Coking Wastewater and Influence of Flora on Pollutants

Research Progress on Biological Treatment Process of Coking Wastewater and Influence of Flora on Pollutants

Occurrence of antibiotics in the different biological treatment processes, reclaimed wastewater treatment plants and effluent-irrigated soils

Reclaimed wastewater is increasingly being used for agricultural irrigation to address water scarcity, particularly in arid and semiarid areas. In this study, the occurrence and removal of 24 antibiotics were investigated in two wastewater treatment systems (WWTSs) and their irrigation areas located in the arid northwestern region of China. The different WWTSs included absorption–biodegradation (AB) + anaerobic–anoxic–oxic (AAO) and AAO + AAO + membrane bioreactor (MBR). Nineteen antibiotics were detected, with concentrations ranging from low to 60.8 μg/L. The dominant antibiotic classes were quinolones (QNs) and macrolides (MLs) in winter, and MLs and sulfonamides (SAs) in summer. The AAO + AAO + MBR system exhibited better removal efficiencies for most antibiotics, improving their adsorption and biodegradation. Except for ofloxacin (OFC) and norfloxacin (NFC), the concentrations of the detected antibiotics were higher in summer than in winter. However, the antibiotic removal efficiency increased by 2–141% in summer compared to winter due to the increase in microbial activity with temperature. Even though reclaimed treatment water plants contributed to the removal of antibiotics, residual antibiotics still affected the soil environment after green irrigation. In irrigated soil, QNs were predominantly detected, with a maximum concentration of 150.9 ng/kg for NFC. An ecological risk assessment showed that OFC, NFC, enrofloxacin, and lomefloxacin presented low or medium ecological risks in at least two irrigated sites. Thus, more attention should be given to the reuse of reclaimed wastewater, as it may pose toxicity risks to organisms in the soil environment.

Advanced Oxidation Pretreatment for Biological Treatment of Reclaimer Wastewater Containing High Concentration N-methyldiethanolamine

A wastewater treatment configuration consisting of advanced oxidation pretreatment and biological wastewater treatment process (BWTP) was investigated to treat a reclaimer wastewater generated in a steel-making industry, which contained high concentration MDEA (N-methyldiethanolamine) of up to 20,548 mg/L and other pollutants such as formate, phenol, and thiocyanate. The Fenton, ozone, and peroxone methods were tested as candidates, and the peroxone method was chosen because it could selectively decompose MDEA resulting in the final MDEA and chemical oxygen demand (COD) removal efficiencies of 92.87% and 27.16%, respectively. Through the respirometer tests using the sludge of the BWTP, it was identified that the microbial toxicity of the peroxone-pretreated wastewater was negligible and the short-term biochemical oxygen demand (BOD) to COD ratio, indicating that the biodegradability of wastewater significantly increased from 0.103 to 0.147 by the peroxone pretreatment. Analysis of the oxygen uptake rate profiles also revealed that the microbial degradation rate of the pollutants present in the reclaimer wastewater was in the order of phenol > formate > thiocyanate > MDEA, which could be changed depending on the microbial community structure of the BWTP.

Influence of high-strength permethrin and transfluthrin on biological wastewater treatment

Pyrethroids are increasingly used in mosquito-repellent products, including laundry detergents and softeners. However, the effects of high concentrations of pyrethroids on the efficiency of conventional biological wastewater treatment processes are unclear. This study investigated the impact of high-strength permethrin (PM) and transfluthrin (TFL) on the performance of the activated sludge (AS)-based biological treatment process, specifically the removal of chemical oxygen demand (COD), total nitrogen (TN), and pyrethroids. Despite the presence of extremely high-strength PM and TFL (approximately 400 times the practical concentrations in municipal wastewater), the process exhibited good removal efficiencies (COD >90.9%, TN >77.3%) and effluent quality (<20 mg COD/L, <1 mg NH4+-N/L, <10 mg TN/L), which are within the standards. Over 99% of the PM and TFL were removed from the wastewater via biodegradation and adsorption by sludge. Adsorption increasingly contributed to the removal of both compounds, especially PM, as their loading concentrations increased. Under practical (low) loading concentrations, biodegradation accounted for 22.0%–40.0%, 27.4%–41.0%, and 95.0%–99.87% of the cis-PM, trans-PM, and TFL removals, respectively, while adsorption accounted for the remainder. The 16S rRNA gene amplicon sequencing revealed that the extremely high-strength PM and TFL induced considerable evolution of the microbial community, especially the denitrifying populations. Bioinformatics analyses further disclosed that Azoarcus and Thauera corresponded to the biodegradation of PM and TFL. Collectively, the experimental results corroborated that extremely high-strength PM and TFL will not deteriorate the efficiency of the AS-based biological treatment process, and more importantly, microbes in the sludge could alter the microbial community to enable pyrethroid biodegradation.

AHLS-pred: a novel sequence-based predictor of acyl-homoserine-lactone synthases using machine learning algorithms.

Acyl-homoserine-lactones (AHLs), as the major quorum sensing (QS) signalling molecules in Gram-negative bacteria, have shown great application potential in regulating biological nutrient removal process. The identification of AHLs synthases plays an essential role in in-depth research on QS mechanisms and applications of biological wastewater treatment processes. This work proposed the first prediction model for AHLs synthases based on machine learning algorithms, namely, AHLS-pred. The training dataset AHLS1400 and the independent testing dataset AHLS132 for AHLSs prediction were first established. Three sequence-based feature extraction methods are utilized to generate feature descriptors, namely, amino acid composition, dipeptide composition and G-gap dipeptide composition respectively. Subsequently, the optimal features were obtained based on the sorted feature descriptors (in F-score order) and the sequential forward search strategy. By comparing five different machine learning algorithms, the final prediction model is trained with support vector machine classifier on AHLS1400 in fivefold cross-validation with the best performance (ACC=99.43%, MCC=0.989, AUC= 0.997). The results show that AHLS-pred achieves an ACC of 94.70%, MCC of 0.894 and AUC of 0.995 on the independent testing dataset AHLS132. It demonstrates that AHLS-pred is a promising and powerful prediction method for accelerating the process of AHLSs computational identification.

A hybrid process for leachate wastewater treatment: Evaporation and reverse osmosis/sequencing batch reactor.

In this study, a hybrid process for leachate wastewater treatment including evaporation and reverse osmosis (RO) membrane or biological treatment systems was suggested. Experiments were performed on a real landfill leachate wastewater. The leachate was subjected to evaporation; as a result, a distillate was obtained containing less organic matter and less substantial amounts of other pollutants, as ammonium salts and total phenols were removed. Tests were carried out at different evaporation temperatures and times. The initial leachate pH was adjusted and optimized. For optimum conditions, each of chemical oxygen demand (COD), total phenol, and ammonium salt concentrations were reduced to 99.99%, 95.00%, and 83.00%, respectively. The distillate of the first stage of the proposed process was then exposed to RO membrane system, as a first study, under different transmembrane pressure of 20, 30, and 40 bar and at different pH values of 7, 8, and 9. As a second suggested treatment system, the distillate was subjected to a biological treatment process for 30 days as a retention time, pH = 6, and room temperature 25°C ± 1°C. At the end of the research study, a comparison was conducted between results obtained with RO membrane separation and biological treatment system as two distinct treatment systems proposed for leachate landfill wastewater treatment. Although both systems were effective for landfill leachate wastewater treatment, however, with the RO membrane separation system, COD removal efficiency reached 99.99%. In the other hand, with biological treatment process, COD elimination was as much as 90.00%. Certainly, evaporation and RO are not novel ways of landfill leachate treatment; however, few studies have attempted to use similar combined system for landfill leachate wastewater treatment and attained effective results of treated water. PRACTITIONER POINTS: A hybrid process of evaporation and RO membrane or biological treatment systems was suggested for leachate wastewater treatment. For optimum conditions, COD, total phenols, and ammonium salt reductions were achieved to 99.99%, 95%, and 83%, respectively, after the first evaporation stage. The distillate of the first stage of the proposed process was then exposed to RO membrane system and biological treatment system. Different transmembrane pressure and different pH values were optimized for RO.

Selective Proliferation of Antibiotic-Resistant Bacteria in the Biological Treatment Process at a Municipal Wastewater Treatment Plant in India

Selective Proliferation of Antibiotic-Resistant Bacteria in the Biological Treatment Process at a Municipal Wastewater Treatment Plant in India

Biological Treatment of Contaminants of Emerging Concern in Wastewater: A Review

Contaminants of emerging concerns (CECs) persist in the environment and are present in the air, water, and soil. They include various industrial, chemical, pharmaceutical, and personal care products (PCPs) and metallic elements, pesticides, and endocrine disruptors. CECs can cause adverse ecological and health impacts by passing into plants and human foods via different routes. Recently, different treatment processes have been investigated for the remediation of CECs that are present in wastewater, and a current review emphasized different biological treatments for the removal of a wide range of CECs. Based on these studies, endocrine disrupting compounds (EDCs) can be removed by a membrane bioreactor (MBR) and pharmaceuticals and pesticides can be effectively removed by biological activated carbon (BAC). Microalgae-based treatment can remove some types of CECs, to some extent, and the activated sludge process (ASP) and trickling filters (TFs) can remove an average amount of CECs from wastewater. Due to the deficiencies of these treatment processes when removing CECs in wastewater, various hybrid treatment systems have been explored. This study will focus on the different conventional and novel biological treatment processes for the removal of different CECs.

Comparative analysis of the removal and transformation of 10 typical pharmaceutical and personal care products in secondary treatment of sewage: A case study of two biological treatment processes

To reduce the occurrence of PPCPs in the environment, its removal in wastewater treatment plants (WWTPs) has attracted much attention. This study reported the removal characteristics of 10 typical PPCPs in two biological treatment processes: blast aeration and UNITANK. The results demonstrated that caffeine (CAF), bezafibrate (BZB), gemfibrozil (GFB), metoprolol (MET), sulfamethoxazole (SMX), and sulfamerazine (SMZ) were mainly removed through biodegradation. Among them, the removal efficiency of CAF was the largest (>85%), and MET was the lowest (<30%). Sulfadiazine (SDZ), naproxen (NPX), sulpiride (SUL), and ofloxacin (OFX) were removed through the combined effect of biodegradation and sludge adsorption. During biotransformation of sulfonamides (SAs), MET, and SUL, some of the conjugates were decomposed into their parent forms again. The results revealed that the adsorption of drugs was correlated with octanol-water partition coefficient (Kow) and electrostatic interaction. The biodegradation efficiency was affected by oxygen supply conditions. The facultative environment, longer sludge retention time (SRT) and hydraulic retention time (HRT) were positively correlated with the removal of most drugs (CAF, SUL, GFB, MET, NPX, OFX, SDZ, SMX). This paper summarized the similarities and differences of drugs’ removal characteristics under different processes and wish to provide useful suggestion for the removal technology of PPCPs in WWTP.

Novel Quantitative Evaluation of Biotreatment Suitability of Wastewater

The development of wastewater treatment industry has gradually entered the high-standard period and the wastewater treatment technology needs to be refined for different types of wastewater. Traditional water quality indicators are not able to explain new problems encountered in the current wastewater treatment process, especially the potential of removing pollutants via biological methods. This research proposed a new method of evaluating the biological treatment process by measuring the oxygen consumption in the biodegradation of pollutants on-the-go and describing the complete biological oxygen consumption process. The biodegradability of wastewater from an actual textile wastewater treatment plant was quantitatively evaluated by analyzing the proportion of different organic pollutions. Results showed that the hydrolytic acidification can improve the biodegradability of textile wastewater by increasing the content of biodegradable organic matter (growth of 86.4%), and air flotation has little effect on the biodegradability of the wastewater. Moreover, the biodegradability of the textile wastewater could be improved by increasing the nitrogen and phosphorus content, which could come from urea and K2HPO4. Concretely, nitrogen source mainly increases organic matter of rapid bio-treated and organic matter of easy bio-treated by 14.94% and 70.79%, and phosphorus source mainly increases the organic matter of easy bio-treated by 143.75%. We found that the optimum concentration of additional N and P to the textile wastewater was 35 mg/L and 45 mg/L, respectively. This approach holds great application prospects such as risk control, optimizing treatment technology, and management, due to its characteristics of being simple, easy to use, and rapid online implement action.

Characterizing the Chemical Contaminants Diversity and Toxic Potential of Untreated Wastewater From a Drug Rehabilitation Hospital: Understanding Impact on Downstream Environment

This study characterizes a total of 21 wastewater samples collected from Al Amal hospital, and aims to determine if untreated hospital wastewater may impose a potentially detrimental impact on the downstream municipal biological wastewater treatment process. By means of solid phase extraction and liquid chromatography with tandem mass spectrometry (LC-MS/MS), chemical contaminants in these wastewater samples were determined in a non-targeted manner. In-silico characterization for the mutagenicity and reactive oxygen species (ROS) producing capabilities was performed by checking against database and literature. However, majority of the chemical contaminants have no prior information available and remain uncharacterized for both traits. Instead, in-vitro mutagenicity tests by means of Ames test showed that majority of the samples were non-mutagenic except for 5 samples that imposed mutagenic effect at high concentrations of &amp;gt;×10. In-vitro tests to determine for intracellular ROS production further showed that one of the mutagenic samples collected on Jun-22 positively induce ROS production and subsequently increased horizontal gene transfer via natural transformation. The findings in this study suggest that a specialty hospital like Al Amal does not frequently contribute mutagenic compounds and ROS to the wastewater streams, and in instances where it contributed positively, would require a high concentration to do so. Hence in general, wastewater streams from a specialty hospital like Al Amal may be unlikely to significantly perturb the downstream environment.

Phosphorus removal from municipal wastewater through a novel Trichosporon asahii BZ: Performance and mechanism

A yeast BZ was screened from a laboratory-scale anaerobic/aerobic reactor and designated as Trichosporon asahii through 26S rDNA gene sequence analysis. The screened BZ abated over 70% of phosphorus in municipal sewage with 2–10 mg/L phosphorus in the appropriate conditions. The yeast BZ had strong adaptability to pH and the dissolved oxygen, but the cultivation temperature, carbon source, the ratio of C/P and the ratio of N/P had a critical influence on the phosphorus abatement performance of yeast BZ. The analysis of phosphorus concentration in the wastewater, cells, and extracellular polymeric substances (EPS) suggested that about 55%–66% of the removed phosphorus was in the yeast cells and 34%–45% in the EPS. The proposed probable metabolic mechanism of phosphorus in yeast BZ showed that EPS acted as a dynamic phosphorous transfer station, and most of phosphorus was transferred into yeast cells through EPS transfer station. These findings have crucial implications for the development of a promising stable and easy-operation biological phosphorus abatement process for municipal wastewater treatment.

Inhibition Effects of Free Ammonia (FA) on the Rates of Growth, Photosynthesis and Respiration of Chlorella vulgaris

The potential use of microalgae in advanced biological wastewater treatment processes has been actively studied due to their excellent nitrogen and phosphorus removal capabilities. This study investigated the inhibitory effects of free ammonia (FA) on microalgae under autotrophic, heterotrophic, and mixotrophic growth conditions, since the inhibitory effects in heterotrophic and mixotrophic modes has not been reported yet. Chlorella vulgaris were exposed to 3–20 mg-N L−1 of FA in the autotrophic, heterotrophic, and mixotrophic modes and the effects on growth, photosynthesis, and respiration were studied. The microalgal growth rate was 2–3 times greater in the mixotrophic mode than the other modes. Microalgal growth in the autotrophic mode was most vulnerable to FA and the growth inhibition was observed at FA concentration of ≥ 9 mg-N L−1. Similarly, the photosynthesis and respiration of C. vulgaris were inhibited at FA concentrations higher than 12 mg-N LT−1. The removal of carbon and ammonia was greater in the mixotrophic mode than the other modes. The ammonia removal by microalgae increased in proportion to the FA concentration unlike the carbon removal. This was attributed to the ammonium accumulation in microalgae due to the proton imbalance and differences in intracellular and external pH conditions, which were caused by the presence of excess FA in water. Overall, the results show that the mixotrophic microalgae have greater tolerance to FA, and hence, greater potential for use in treatment of wastewaters having high FA concentrations.

Application of machine learning techniques to model a full-scale wastewater treatment plant with biological nutrient removal

A full-scale biological nutrient removal wastewater treatment process was simulated using artificial intelligence. In wastewater treatment plants, adaptive machine learning models can reduce process disruptions and generate savings through optimized operation. Machine learning is also useful when simulating processes that are particularly complex and where the physio-chemical interactions are not well understood, such as biological nutrients removal. Current models in literature only focus on the prediction of a small number of effluent parameters using a direct input-output approach. This paper presents a machine learning ensemble model that combines artificial neural networks, adaptive neuro-fuzzy inference systems, and support vector regression to predict 15 process parameters that include biomass properties, operation parameters, and effluent characteristics. A historical dataset between 2010 and 2020 was used to develop and validate the model. The model features a six-stage modular model structure where each parameter was predicted using a separate model and based on the preceding predicted parameters. The average correlation coefficient, normalized root mean square error, and symmetric mean absolute error of 69%, 0.06%, and 7.5%, respectively. The ensemble approach improved the average prediction accuracy over individual base models by 5%. The model developed in this study was more versatile than other machine learning models in the literature and relatively reduced the ambiguity of black-box data-driven models.

Evaluation of the effect of peracetic acid solution on the performance of a continuous-flow biological nitrogen removal (BNR) system

• PAA addition directly to the anaerobic reactor did not affect BNR system performance. • PAA-bearing wastewater at 200 mg/L residual PAA fed to the anaerobic reactor resulted in BNR system upset. • The BNR system was more susceptible to residual PAA without the anaerobic reactor. • Fast, complete BNR system recovery within one HRT feeding with PAA-free wastewater. • Betaproteobacteriales, Sphingobacteriales and Rhodobacterales were affected by PAA. Peracetic acid (PAA) has been extensively used in poultry processing plants as an effective disinfectant alternative to chlorine-based products. However, PAA-bearing poultry processing wastewater, as part of the regular disinfection process and/or in case of accidental spills, can lead to severe upsets of biological wastewater treatment processes. The objective of this study was to assess the short- and long-term effect of PAA solution (PAA and hydrogen peroxide, H 2 O 2 ) on the performance of a continuous-flow biological nitrogen removal (BNR) system, fed with poultry processing wastewater. The system consisted of three reactors in series: anaerobic (R1), anoxic (R2), and aerobic (R3). Continuous, direct addition of PAA solution up to 200/28.6 mg/L PAA/H 2 O 2 (feed-based concentration) to the anaerobic reactor (R1) did not affect the performance of the BNR system, which achieved over 90 and 82% COD and N removal, respectively. However, both COD and N removal were negatively affected by continuous feeding with residual PAA-bearing wastewater at 80 and 200 mg/L. PAA was self-decomposed or by reaction with other wastewater components, such as proteins and transition metals, rather than being biodegraded. Fast recovery of COD and N removal was achieved within three retention times feeding with PAA/H 2 O 2 -free wastewater. The BNR system was more susceptible to residual PAA without the R1, i.e., two-reactor system. Complete recovery from the impact of residual PAA was achieved, suggesting the effect of PAA on system performance was fast reversed and not lasting. The impact on the BNR system was a combination of enzyme inhibition and cell lysis leading to biomass loss. The PAA impact in R1 (anaerobic) on Synergistales and Bacteroidales was reversed, but not on Lactobacillales and Pseudomonadales. Campylobacterales and Sphingomonadales were enriched when PAA was fed to the BNR system. Betaproteobacteriales, Sphingobacteriales and Rhodobacterales in R2 (anoxic) and R3 (aerobic) reactors were negatively affected by PAA, but the impact was fast reversed when PAA addition was ceased.

Response Surface Methodology for Optimization of Rotating Biological Contactor Combined with External Membrane Filtration for Wastewater Treatment.

A large amount of wastewater is directly discharged into water bodies without treatment, causing surface water contamination. A rotating biological contactor (RBC) is an attached biological wastewater treatment process that offers a low energy footprint. However, its unstable removal efficiency makes it less popular. This study optimized operating parameters in RBC combined with external membrane filtration (RBC-ME), in which the latter acted as a post-treatment step to stabilize the biological performance. Response surface methodology (RSM) was employed to optimize the biological and filtration performance by exploiting three parameters, namely disk rotation, hydraulic retention time (HRT), and sludge retention time (SRT). Results show that the RBC-ME exhibited superior biological treatment capacity and higher effluent quality compared to stand-alone RBC. It attained 87.9 ± 3.2% of chemical oxygen demand, 45.2 ± 0.7% total nitrogen, 97.9 ± 0.1% turbidity, and 98.9 ± 1.1% ammonia removals. The RSM showed a good agreement between the model and the experimental data. The maximum permeability of 144.6 L/m2 h bar could be achieved under the optimum parameters of 36.1 rpm disk rotation, 18 h HRT, and 14.9 d SRT. This work demonstrated the effective use of statistical modeling to enhance RBC-ME system performance to obtain a sustainable and energy-efficient condition.

Sewage-sludge derived activated carbon impregnated with polysulfide-sulfidated nZVI：A promising material for Cr(Ⅵ) reductive stabilization

This study aims to examine the reductive performance of a sulfidated iron-carbon composite synthesized using a sewage sludge from a typical biological wastewater treatment process. Sewage sludge was first carbonized under optimal conditions to prepare activated carbon (AC), followed by sequential treatment with nZVI and polysulfide to synthesize the iron-carbon composite S-nZVI/AC. Polysulfide was used to sulfurize nZVI due to its unique properties in stability, persistence and safety. Boehm titration and BET test showed that AC carried both acidic and basic sites in concentrations of 1.04 and 0.81 mmol/g per gram of material. The unmodified AC served mainly as a carrier to disperse nZVI grains from aggregation, although it also possessed a limited capacity of 3.5 mg/g for Cr(VI) sorption at neutral pH. Functionalized AC with nZVI boosted its capacity to about 29 times, while further treatment with polysulfide could further expand the material’s capacity by 36% to 136.0 mg/g. Sulfurizing the material with polysulfide also raised its reactivity as revealed by the kinetic experiments. Material characterizations suggested that multiple reductive species from S-nZVI/AC, including S(-Ⅱ) and Fe(Ⅱ) dissociated from FeS, Sx2-, and Fe0, might have participated in immobilizing Cr(VI). Specifically, S(-Ⅱ) and Sx2- were oxidized to SO42-, S2O32-, and S0, in decreasing order of concentration as inferred from XPS analysis.