► Coconut shell based carbons were chemically treated by alkalis and acids. ► GAC modified by alkalis had better
o-xylene adsorption capacity. ► We analyzed textural and surface chemistry changes ...induced on the GAC. ► Reducing hydrophilic carbon surface favors abatement of hydrophobic VOCs. ► Reducing hydrophilic carbon surface results in less adsorbent regeneration.
In this study, coconut shell based carbons were chemically treated by ammonia, sodium hydroxide, nitric acid, sulphuric acid, and phosphoric acid to determine suitable modification for improving adsorption ability of hydrophobic volatile organic compounds (VOCs) on granular activated carbons (GAC). The saturated adsorption capacities of
o-xylene, a hydrophobic volatile organic compound, were measured and adsorption effects of the original and modified activated carbons were compared. Results showed that GAC modified by alkalis had better
o-xylene adsorption capacity. Uptake amount was enhanced by 26.5% and reduced by 21.6% after modification by NH
3H
2O and H
2SO
4, respectively. Compared with the original, GAC modified by acid had less adsorption capacity. Both SEM/EDAX and BET were used to identify the structural characteristics of the tested GAC, while IR spectroscopy and Boehm's titration were applied to analysis the surface functional groups. Relationships between physicochemical characteristics of GAC and their adsorption performances demonstrated that
o-xylene adsorption capacity was related to surface area, pore volume, and functional groups of the GAC surface. Removing surface oxygen groups, which constitute the source of surface acidity, and reducing hydrophilic carbon surface favors adsorption capacity of hydrophobic VOCs on carbons. The performances of modified GACs were also investigated in the purification of gases containing complex components (
o-xylene and steam) in the stream.
Wastewater treatment plants (WWTPs) are major sources of airborne bacteria, which could pose health risks to WWTP workers and surrounding residents. In this study, air samples were collected from ...various treatment facilities of a typical WWTP. Community compositions of airborne bacteria were identified by high-throughput sequencing technique. SourceTracker was used to determine the percentages of airborne bacteria from wastewater, sludge, ambient air, and other environment. Health risks associated with airborne bacteria were estimated based on the average daily dose rates (ADD) of exposure by inhalation and skin contact. Concentrations of airborne bacteria varied in a wide range of 23–4878 CFU/m3. The main emission sources of airborne bacteria were treatment facilities with aeration, mechanical agitation, and located indoors. For treatment facilities located indoors, higher percentages of airborne bacteria were associated with wastewater and sludge, while more airborne bacteria were originated from the ambient air for outdoor installations. Opportunistic pathogens such as Micrococcus, Bacteroides, Chryseobacterium, Pseudomonas, and Acinetobacter, were detected in airborne bacteria. Inhalation was the main pathway for on-site workers exposure to airborne bacteria. Due to the presence of opportunistic pathogens, strict control measures should be employed in WWTPs to reduce the infection risks.
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•High levels of airborne bacteria were found in a typical WWTP.•Characterization of outdoor and indoor airborne bacteria presented distinctions.•Sources of airborne bacteria were identified by SourceTracker method.•Higher proportion of indoor airborne bacteria originated from wastewater and sludge.•Pathogens were detected and inhalation was the main exposure pathway.
Aeration and mechanical agitation are the main drivers of aerosol generation in wastewater treatment plants (WWTPs). However, the effect of aeration mode on aerosol characteristics remains poorly ...understood. In this study, horizontal rotor aeration and fine bubble aeration in the same WWTP were selected to identify the effect on the emission, size distribution, microbial and chemical composition. For bacteria, fungi, Enterobacteriaceae, Staphylococcus aureus, and Pseudomonas aeruginosa in aerosols, the horizontal rotor aeration had higher contributions to the emissions than the fine bubble aeration. Horizontal rotor aeration generated a more coarse fraction (size > 7 μm) and a comparable respirable fraction (RF; size < 3.3 μm) compared with those of fine bubble aeration. More types of potential pathogens were generated by horizontal rotor aeration. The most easily aerosolized genera generated by horizontal rotor aeration and fine bubble aeration, were Trichosporon and Mycobacterium, with the aerosolization factors of 633.70 and 192.56, respectively. For Cl−, SO42−, NO3−, Zn, Ba, Cd, Sc, V, Rb, Ca, K, Ca, K, Mg, Na and Si in the aerosols, the contributions of fine bubble aeration were higher than those of horizontal rotor aeration. Due to the aerosol specialty from the different aeration modes, targeted manipulations should be employed to reduce the exposure risks.
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•Effect of aeration mode on aerosol characteristics was studied.•A full-scale WWTP with two biological processes (the OD and A2/O) was selected.•Aerosolization potentials of pathogens of the two aeration modes were different.•More potential pathogens were found in aerosols by horizontal rotor aeration.•Appropriate strategies should be recommended to control aerosols.
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.
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•Augmentation of SCFA yield from anaerobic sludge fermentation was investigated.•The SCFA yield increased with the studied fermentation pH stepwise strategy.•The studied strategy greatly promoted synergy of chemical and biological processes.•A conceptual model was proposed to better understand the underlying mechanism.
•Two TAD of food waste both had good performances at HRT 30 d in stable stage.•The VS destructions and methane yields of food waste in two TAD were comparable.•Two TAD recovered similar energy from ...the fed and destructed food waste.•Single stage TAD had higher energy recovery, rate and special rate of reaction for the four AD steps.•Two-stage TAD need to be adjusted its operational parameters to improve its AD efficiency.
The single-stage and two-stage thermophilic anaerobic digestion (TAD) in lab scale continuously stirred tank reactor systems fed semi-continuously with food waste (FW) were studied to compare their performance, energy balance and reaction processes. The experimental results showed the two TADs both had good performance at a sludge retention time (SRT) of 30 d. The VS destruction (83.22 ± 1.33%) of the single-stage TAD was comparable with the two-stage TAD (82.02 ± 1.25%) during the steady period. While the average biogas yield of the two-stage TAD (0.810 ± 0.13 L/g Added VS) was higher than that of single-stage TAD (0.775 ± 0.20 L/g Added VS) and the methane content of the former (59.1 ± 1.4%) was lower than that of the latter (61.6 ± 2.1%), the methane yields of the two TADs were similar. The single stage TAD had higher energy recovery, rate and specific rate of reaction for the four AD steps than the two-stage TAD. The two-stage TAD had to adjust its operational parameters to improve its AD efficiency.
•The damage to cell walls and cell membranes was evaluated.•The cell walls and membranes of sludge microorganisms were all damaged in the nine EPS extraction.•N-acetylglucosamine was suitable for the ...evaluation of cell wall damage.•The damage to cell membranes could be characterized by flow cytometry measurement.
Extracellular polymeric substances (EPS) are susceptible to contamination by intracellular substances released during the extraction of EPS owing to the damage caused to microbial cell structures. The damage to cell walls and cell membranes in nine EPS extraction processes of activated sludge was evaluated in this study. The extraction of EPS (including proteins, carbohydrates and DNA) was the highest using the NaOH extraction method and the lowest using formaldehyde extraction. All nine EPS extraction methods in this study resulted in cell wall and membrane damage. The damage to cell walls, evaluated by 2-keto-3-deoxyoctonate (KDO) and N-acetylglucosamine content changes in extracted EPS, was the most significant in the NaOH extraction process. Formaldehyde extraction showed a similar extent of damage to cell walls to those detected in the control method (centrifugation), while those in the formaldehyde-NaOH and cation exchange resin extractions were slightly higher than those detected in the control. N-acetylglucosamine was more suitable than KDO for the evaluation of cell wall damage in the EPS extraction of activated sludge. The damage to cell membranes was characterized by two fluorochromes (propidium iodide and FITC Annexin V) with flow cytometry (FCM) measurement. The highest proportion of membrane-damaged cells was detected in NaOH extraction (26.54% of total cells) while membrane-damaged cells comprised 8.19% of total cells in the control.
•Chemical and bacterial properties in sludge dewatering house were analyzed.•Levels of airborne bacteria and chemicals showed regional variations.•Bacterial population in bioaerosols also presented ...significant regional disparity.•Common potential pathogens were detected in bioaerosols from all SDHs.•RH and temperature were major parameters on bioaerosols to survive in SDHs.
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Municipal wastewater treatment plants (MWTPs) are regarded as sources of airborne microorganisms. Sludge dewatering house (SDH) is one of the most serious indoor bioaerosol pollution treatment sectors in MWTPs. In this study, properties of bioaerosols from SDHs of nine MWTPs were investigated in China. Results suggested that bioaerosols were generated mainly from the mixed liquor and will be promoted by the mechanical motion of belts of dewatering devices. They will accumulate in the SDHs due to the treatment devices placed inside. Levels of airborne bacteria and chemicals showed regional variations. In Hefei and Yixing, the emissions of total suspended particles (TSP) and airborne bacteria were higher than those in Beijing and Guangzhou. Results of bacterial population showed that bacterial species in bioaerosols from SDHs also presented significant regional disparity; these regional disparities were closely related with the source of bioaerosols in SDHs. Among these identified bacterial species, some common potential pathogens were detected in all SDHs, such as Aeromonas caviae, Flavobacterium sp., and Staphylococcus lentus. Relative humidity (RH) and temperature were the major parameters on bioaerosols to survive. As shown in this study, SDHs in wastewater treatment plants should be provided considerable attention for being an emission source of indoor bioaerosols.
The influence of hydraulic retention times (HRTs) reduction from 25 days to 15 days on the enhancement effects of two pretreatments (thermal pretreatment and alkaline-thermal pretreatment) on the ...continuous anaerobic digestion (AD) of sewage sludge was studied in a long-term experiment (196 days). The operation of the semi-continuous AD fed with raw sludge or pretreated sludge was stable at the three HRTs. The methane production increased from 70.6 to 165.8 ml/L·d to 75.2–172.6 ml/L·d and the methane yield decreased from 98.9 to 234.9 ml/g added volatile solid to 65.6–144.9 ml/g added volatile solid when the HRT reduced from 25 days to 15 days. The two pretreatments reduced the HRT of raw sludge AD by over 40%, and the effects of the alkaline-thermal pretreatment were greater than those of the thermal pretreatment. The reduction of HRT from 25 days to 15 days increased the enhancement effects of the two pretreatments on the removal of organic matter (4.7–15.9% for volatile solid), average hydrolysis ratio (36.9–116.4%), and specific hydrolysis rate (44.1–155.6%) but decreased the enhancement effects of the pretreatments on the methane production (0.9–4.6%) and yield (4.0–15.8%), average reaction ratios (0.4–8.2%), and specific rates of the last three AD steps (0.1–13.9%). The influence of HRT reduction on the enhancement effects of the alkaline-thermal pretreatment for sludge AD was slightly greater than on the enhancement effects of the thermal pretreatment.
•Thermal and alkaline-thermal pretreatments can enhance AD of sludge at three HRTs.•The enhancement effects of the latter were higher than those of the former.•Their enhancement effects on organic matter removals increased with HRT reduction.•Their enhancement effects on methane yield decreased with HRT reduction by 4.0–15.8%.•The influences of HRT reduction on the enhancement effect of the latter were higher.
The generation and emission of airborne bacteria from a biochemical reaction tank (BRT) for wastewater treatment was investigated by altering the aeration rate. The levels of bioaerosols increased ...from 715 ± 69 to 1597 ± 135 CFU/m3 (total airborne bacteria) and from 78 ± 6 to 359 ± 18 CFU/m3 (intestinal bacteria) as the aeration rate increased from 0.3 to 1.2 m3/h. Most airborne bacteria were attached to particles smaller than 4.7 μm at an aeration rate of 0.3 m3/h. They were found attached to larger particles (>4.7 μm) when the aeration rate increased to 1.2 m3/h. A similar phenomenon was observed for intestinal bacteria. The high-throughput sequencing technique was used to assay the microbial populations of the bioaerosols. Both microbial counts and diversity increased as the aeration rate increased. Brevundimonas (63.82%), Chryseobacterium (16.54%), and Micrococcaceae (12.37%) were the dominant intestinal bacteria at an aeration rate of 0.3 m3/h. Pseudochrobactrum (33.10%), Citrobacter (21.28%), and Yersinia (18.21%) were the dominant intestinal bacteria at an aeration rate of 1.2 m3/h. The level, particle size distribution, population structure, and diversity of the bioaerosols were all affected by aeration rate. The source tracker results indicated that water and the surrounding air were the two main bioaerosol sources. The contribution of water is greater at larger levels of aeration. Inhalation was the main pathway of microbial aerosol intake for people in the surrounding area. The exposure hazard quotients for adult males were generally higher than those for adult females. Necessary measures should be taken to ensure worker safety.
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•Generation and emission of airborne bacteria during microporous aeration•Source tracker indicated water and ambient air were sources of airborne bacteria.•Microbial counts and diversity increased as the aeration rate accelerated.•Intestinal bacteria presented in the bioaerosols.•Health risks of intestinal bacteria in bioaerosols are greater than general bacteria.
For the removal of phosphate（PO43-） from water, an adsorbent was prepared via carbonization of sewage sludge from a wastewater treatment plant： carbonized sludge adsorbent（CSA）. The mechanism of ...phosphate removal was determined after studying the structure and chemical properties of the CSA and its influence on phosphate removal. The results demonstrate that phosphate adsorption by the CSA can be fitted with the pseudo second-order kinetics and Langmuir isotherm models, indicating that the adsorption is single molecular layer adsorption dominated by chemical reaction. The active sites binding phosphate on the surface are composed of mineral particles containing Si/Ca/Al/Fe. The mineral containing Ca, calcite, is the main factor responsible for phosphate removal. The phosphate removal mechanism is a complex process including crystallization via the interaction between Ca2＋ and PO43-; formation of precipitates of Ca2＋, Al3＋, and PO43-; and adsorption of PO43-on some recalcitrant oxides composed of Si/Al/Fe.
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