► Cr(VI) removal by Fe
0 loading on MWCNTs nanocomposites was first studied. ► Fe
0 on the surface or in the network of MWCNTs was observed by SEM, TEM, and XRD. ► Optimal mass ratio of MWCNTs to Fe
...0 was found and the efficiency was compared. ► Kinetics under different conditions were tested using pseudo first-order model. ► Effect of ionic strength, pH, and foreign ions on Cr(VI) removal was investigated.
For the first time, nanoscale zero-valent iron (nZVI)–multiwalled carbon nanotube (MWCNT) nanocomposites were adopted to remove Cr(VI) from wastewater. Such composites were prepared through depositing nZVI particles onto MWCNTs by in situ reduction of ferrous sulfate and then characterized by TEM, SEM and XRD. The results showed that nZVI particles could disperse on the surface or into the network of MWCNTs. Compared to bare nZVI or nZVI-activated carbon composites, the nZVI–MWCNT nanocomposites exhibited around 36% higher efficiency on Cr(VI) removal. The mass ratio of nZVI to MWCNTs was optimized at 1:2, at ionic strength of 0.05
M NaCl. The reaction followed a pseudo first-order model under different initial Cr(VI) concentrations and pHs. Low pH and initial Cr(VI) concentration could increase both removal efficiency and rate constants. Anions, such as
SO
4
2
-
,
NO
3
-
and
HCO
3
-
, exhibited negative effects on the removal of Cr(VI), while the effects of
PO
4
3
-
and
SiO
3
2
-
were insignificant. Overall, nZVI–MWCNT nanocomposites offer a promising alternative material for the removal of Cr(VI) ions from wastewater.
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•A systematic evaluation of the uncertainties in SARS-CoV-2 prevalence estimation.•The excretion uncertainty is limited for a catchment with >10 cases.•In-sewer persistence is largely ...unknown with considerable uncertainties.•Analysis of SARS-CoV-2 needs to be improved to minimize analytical uncertainty.
Wastewater-based epidemiology (WBE) is a promising approach for estimating population-wide COVID-19 prevalence through detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA in wastewater. However, various methodological challenges associated with WBE would affect the accuracy of prevalence estimation. To date, the overall uncertainty of WBE and the impact of each step on the prevalence estimation are largely unknown. This study divided the WBE approach into five steps (i.e., virus shedding; in-sewer transportation; sampling and storage; analysis of SARS-CoV-2 RNA concentration in wastewater; back-estimation) and further summarized and quantified the uncertainties associated with each step through a systematic review. Although the shedding of SARS-CoV-2 RNA varied greatly between COVID-19 positive patients, with more than 10 infected persons in the catchment area, the uncertainty caused by the excretion rate became limited for the prevalence estimation. Using a high-frequency flow-proportional sampling and estimating the prevalence through actual water usage data significantly reduced the overall uncertainties to around 20–40% (relative standard deviation, RSD). And under such a scenario, the analytical uncertainty of SARS-CoV-2 RNA in wastewater was the dominant factor. This highlights the importance of using surrogate viruses as internal or external standards during the wastewater analysis, and the need for further improvement on analytical approaches to minimize the analytical uncertainty. This study supports the application of WBE as a complementary surveillance strategy for monitoring COVID-19 prevalence and provides methodological improvements and suggestions to enhance the reliability for future studies.
Chloride is reported to play a significant role in corrosion reactions, products and kinetics of ferrous metals. To enhance the understanding of the effects of soil environments, especially the ...saline soils with high levels of chloride, on the corrosion of ductile iron and carbon steel, a 3-month corrosion test was carried out by exposing ferrous metals to soils of six chloride concentrations. The surface morphology, rust compositions and corrosion kinetics were comprehensively studied by visual observation, scanning electron microscopy (SEM), X-Ray diffraction (XRD), weight loss, pit depth measurement, linear polarization and electrochemical impedance spectroscopy (EIS) measurements. It showed that chloride ions influenced the characteristics and compositions of rust layers by diverting and participating in corrosion reactions. α-FeOOH, γ-FeOOH and iron oxides were major corrosion products, while β-Fe
O
(OH)
Cl
rather than β-FeOOH was formed when high chloride concentrations were provided. Chloride also suppressed the decreasing of corrosion rates, whereas increased the difficulty in the diffusion process by thickening the rust layers and transforming the rust compositions. Carbon steel is more susceptible to chloride attacks than ductile iron. The corrosion kinetics of ductile iron and carbon steel corresponded with the probabilistic and bilinear model respectively.
Methane production from anaerobic digestion of waste activated sludge (WAS) is often limited by the slow degradation and poor substrate availability of WAS. Our previous study revealed that WAS ...pre-treatment using free nitrous acid (FNA, i.e. HNO2) is an economically feasible and environmentally friendly method for promoting methane production. In order to further improve methane production from WAS, this study presents a novel strategy based on combined FNA and heat pre-treatment. WAS from a full-scale plant was treated for 24 h with FNA alone (0.52–1.43 mg N/L at 25 °C), heat alone (35, 55 and 70 °C), and FNA (0.52–1.11 mg N/L) combined with heat (35, 55 and 70 °C). The pre-treated WAS was then used for biochemical methane potential tests. Compared to the control (no FNA or heat pre-treatment of WAS), biochemical methane potential of the pre-treated WAS was increased by 12–16%, 0–6%, 17–26%, respectively; hydrolysis rate was improved by 15–25%, 10–25%, 20–25%, respectively, for the three types of pre-treatment. Heat pre-treatment at 55 and 70 °C, independent of the presence or absence of FNA, achieved approximately 4.5 log inactivation of pathogens (in comparison to ∼1 log inactivation with FNA treatment alone), thus capable of producing Class A biosolids. The combined FNA and heat pre-treatment is an economically and environmentally attractive technology for the pre-treatment of WAS prior to anaerobic digestion, particularly considering that both FNA and heat can be produced as by-products of anaerobic sludge digestion.
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•FNA + heat pre-treatment is effective in enhancing anaerobic methane production.•FNA + heat pre-treatment improves hydrolysis rate and methane potential of the sludge.•FNA + heat pre-treatment attains higher methane production than FNA or heat alone does.•FNA + heat pre-treatment has the potential to achieve Class A biosolids.•FNA + heat pre-treatment is economically and environmentally attractive.
Nitrogen removal via nitrite (i.e. the nitrite pathway) is beneficial for carbon-limited biological wastewater treatment plants. This study presents a novel strategy for achieving the nitrite pathway, ...which involves recirculating a portion of the activated sludge through a side-stream sludge treatment unit, where the sludge is subject to treatment with free nitrous acid (FNA i.e. HNO2). The strategy is proposed based on a novel discovery reported in this work that in the concentration range of 0.24–1.35 mg HNO2−–N/L, FNA is substantially more biocidal to nitrite oxidizing bacteria (NOB) than to ammonium oxidizing bacteria (AOB). Two sequencing batch reactors (SBR) treating synthetic domestic wastewater were used to demonstrate the concept, with one serving as an experimental reactor and the other as a control. In the experimental system, 22% of the sludge from the SBR was transferred to the side-stream treatment unit each day, and was subject to FNA treatment at 1.35 mg N/L for 24 h and then returned to the SBR. The nitrite pathway was rapidly (in 15 d) established in the experimental reactor with an average nitrite accumulation ratio (NO2−–N/(NO2−–N + NO3−–N) × 100%) of above 80%. Fluorescence in-situ hybridization demonstrated that the NOB population in the experimental reactor was 80% lower than that in the control reactor, indicating that the majority of NOB were eliminated from the experimental reactor. The FNA-based strategy for establishing the nitrite pathway substantially improved total nitrogen removal, and did not increase N2O emission or deteriorate sludge settleability. The strategy can be easily integrated with a previously demonstrated strategy, which enhances methane production through pre-treatment of secondary activated sludge, to enable maximum energy recovery while achieving improved nitrogen removal.
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•FNA inactivates NOB to a much larger extent than it does on AOB.•Sludge treatment using FNA is effective in establishing the nitrite pathway.•Sludge treatment using FNA does not increase N2O emission or SVI.•FNA-based method could potentially improve nitrogen removal and methane production simultaneously.
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•Bi-metal-decorated amorphous bismuth oxide photocatalysts were fabricated.•The Bi metal could activate the amorphous bismuth oxide via the SPR effect.•The photocatalytic performance ...can be well tuned via content of Bi metal.•Bi metal/amorphous bismuth oxide displayed enhanced photocatalytic activity.•The photocatalysis mechanism was revealed with ESR and in situ DRIFT.
Amorphous semiconductors are seldom exploited as effective photocatalysts, as they are restricted by abundant bulk defects as carrier recombination centers. To activate amorphous bismuth oxide for efficient visible-light photocatalytic performance, a novel and facile strategy was developed. Plasmonic Bimetal-decorated amorphous bismuth oxide (Bi–BiO) was prepared by partial reduction with NaBH4. The content of Bi metal and the photocatalytic activity of the catalysts can be modulated by controlling the concentration of NaBH4 solution. Various techniques were employed to explore the structural features, optical properties, and active species during photocatalysis. The as-synthesized Bi–BiO catalysts were applied in photocatalytic removal of NO in air under and exhibited highly enhanced visible light photocatalytic activity. The significantly increased photocatalytic capability can be attributed to the combined effects of the enhanced visible light absorption and the improved separation efficiency of the charge carriers attributed to the surface plasmon resonance conferred by Bi metal. The advanced Bi–BiO catalysts also exhibited high photochemical and structural stability under repeated irradiation. Moreover, in situ DRIFT was carried out to reveal the time-dependent evolution of reaction intermediates during photocatalytic NO oxidation. A molecular-level photocatalysis mechanism was first proposed for Bi–BiO based on ESR and in situ DRIFT. This work could provide a new perspective in utilizing non-noble-metal Bi as a key activation factor to trigger the photocatalytic ability of amorphous semiconductors.
Improvement of sludge dewaterability is crucial for reducing the costs of sludge disposal in wastewater treatment plants. This study presents a novel conditioning method for improving waste activated ...sludge dewaterability by combination of persulfate and zero-valent iron. The combination of zero-valent iron (0-30g/L) and persulfate (0-6g/L) under neutral pH substantially enhanced the sludge dewaterability due to the advanced oxidization reactions. The highest enhancement of sludge dewaterability was achieved at 4g persulfate/L and 15g zero-valent iron/L, with which the capillary suction time was reduced by over 50%. The release of soluble chemical oxygen demand during the conditioning process implied the decomposition of sludge structure and microorganisms, which facilitated the improvement of dewaterability due to the release of bound water that was included in sludge structure and microorganism. Economic analysis showed that the proposed conditioning process with persulfate and ZVI is more economically favorable for improving WAS dewaterability than classical Fenton reagent.
Many studies of sewer corrosion are performed in accelerated conditions that are not representing the actual corrosion processes. This study investigated the effects of various factors over 3.5 years ...under controlled conditions simulating the sewer environment. Concrete coupons prepared from precorroded sewers were exposed, both in the gas phase and partially submerged in wastewater, in laboratory controlled corrosion chambers. Over the 45 month exposure period, three environmental factors of H2S concentration, relative humidity and air temperature were controlled at different levels in the corrosion chambers. A total of 36 exposure conditions were investigated to determine the long term effects of these factors by regular retrieval of concrete coupons for detailed analysis of surface pH, corrosion layer sulfate levels and concrete loss. Corrosion rates were also determined for different exposure periods. It was found that the corrosion rate of both gas-phase and partially-submerged coupons was positively correlated with the H2S concentration in the gas phase. Relative humidity played also a role for the corrosion activity of the gas-phase coupons. However, the partially-submerged coupons were not affected by humidity as the surfaces of these coupons were saturated due to capillary suction of sewage on the coupon surface. The effect of temperature on corrosion activity varied and possibly the acclimation of corrosion-inducing microbes to temperature mitigated effects of that factor. It was apparent that biological sulfide oxidation was not the limiting step of the overall corrosion process. These findings provide real insights into the long-term effects of these key environmental factors on the sewer corrosion processes.
Several recent studies showed that nitrite dosage to wastewater results in long-lasting reduction of the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms. In this study, we ...revealed that the quick reduction in these activities is due to the biocidal effect of free nitrous acid (FNA), the protonated form of nitrite, on biofilm microorganisms. The microbial viability was assessed after sewer biofilms being exposed to wastewater containing nitrite at concentrations of 0–120 mg-N/L under pH levels of 5–7 for 6–24 h. The viable fraction of microorganisms was found to decrease substantially from approximately 80% prior to the treatment to 5–15% after 6–24 h treatment at FNA levels above 0.2 mg-N/L. The level of the biocidal effect has a much stronger correlation with the FNA concentration, which is well described by an exponential function, than with the nitrite concentration or with the pH level, suggesting that FNA is the actual biocidal agent. An increase of the treatment from 6 to 12 and 24 h resulted in only slight decreases in microbial viability. Physical disrupted biofilm was more susceptible to FNA in comparison with intact biofilms, indicating that the biocidal effect of FNA on biofilms was somewhat reduced by mass transfer limitations. The inability to achieve 2-log killing even in the case of disrupted biofilms suggests that some microorganisms may be more resistant to FNA than others. The recovery of biofilm activities in anaerobic reactors after being exposed to FNA at 0.18 and 0.36 mg-N/L, respectively, resembled the regrowth of residual sulfate-reducing bacteria and methanogens, further confirming the biocidal effects of FNA on microorganisms in biofilms.
► Free nitrous acid showed strong biocidal effect toward microorganisms in biofilms. ► FNA, not nitrite or pH, is the actual biocidal agent. ► Limitation of mass transfer in the biofilm mitigated the biocidal effect. ► Some microorganisms are resistant to FNA. ► Intermittent FNA dosing is cost-effective to control sulfide and methane.
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