Sequential Anaerobic-Aerobic Treatment Enhances Sulfamethoxazole Removal: From Batch Cultures to Observations in a Large-Scale Wastewater Treatment Plant.

Sulfamethoxazole (SMX) passes through conventional wastewater treatment plants (WWTPs) mainly unaltered. Under anoxic conditions sulfate-reducing bacteria can transform SMX but the fate of the transformation products (TPs) and their prevalence in WWTPs remain unknown. Here, we report the anaerobic formation and aerobic degradation of SMX TPs. SMX biotransformation was observed in nitrate- and sulfate-reducing enrichment cultures. We identified 10 SMX TPs predominantly showing alterations in the heterocyclic and N4-arylamine moieties. Abiotic oxic incubation of sulfate-reducing culture filtrates led to further degradation of the major anaerobic SMX TPs. Upon reinoculation under oxic conditions, all anaerobically formed TPs, including the secondary TPs, were degraded. In samples collected at different stages of a full-scale municipal WWTP, anaerobically formed SMX TPs were detected at high concentrations in the primary clarifier and digested sludge units, where anoxic conditions were prevalent. Contrarily, their concentrations were lower in oxic zones like the biological treatment and final effluent. Our results suggest that anaerobically formed TPs were eliminated in the aerobic treatment stages, consistent with our observations in batch biotransformation experiments. More generally, our findings highlight the significance of varying redox states determining the fate of SMX and its TPs in engineered environments.

Biodegradation of petroleum hydrocarbons and changes in microbial community structure in sediment under nitrate-, ferric-, sulfate-reducing and methanogenic conditions.

In the present study, the biodegradation behaviors of petroleum hydrocarbons under various reducing conditions were investigated. n-Alkanes and polycyclic aromatic hydrocarbons (PAHs) were degraded with NO3-, Fe3+, SO42-, or HCO3- as terminal electron acceptors (TEAs), which link to four typical reducing conditions (i.e., nitrate-reducing, ferric-reducing, sulfate-reducing and methanogenic conditions, respectively) in sediment. The fastest degradation rates were achieved under sulfate-reducing conditions with half-lives of 49.51 days for n-alkanes and 58.74 days for PAHs. For short-chain n-alkanes and low-molecular weight (LMW) PAHs, relatively higher removal efficiencies were achieved under nitrate- and ferric-reducing conditions. The degradation of long-chain n-alkanes and high-molecular weight (HMW) PAHs coupled to methanogenesis was the most favored as compared with other reducing conditions. Carboxylation was found to be the principle mechanism for regulating n-alkane degradation coupled to denitrification, while the activation of n-alkanes by the addition of fumarate was the principle mechanism for the n-alkane degradation under sulfate-reducing conditions. The anaerobic metabolism of n-alkanes may not proceed via fumarate addition or carboxylation under ferric-reducing and methanogenic conditions. Illumina HiSeq sequencing revealed dissimilar structures of the microbial communities under various reducing conditions. It is hypothesized that the utilization of different TEAs for n-alkane and PAH degradation resulted in distinct microbial community structures, which were highly correlated with the varied degradation behaviors of petroleum hydrocarbons in sediment. The current results may provide reference value on better understanding the biodegradation behaviors of n-alkanes and PAHs in association with the induced microbial communities in sedimentary environments under the four typical reducing conditions.

Effect of water treatment additives on lime softening residual trace chemical composition--implications for disposal and reuse.

Drinking water treatment residues (WTR) offer potential benefits when recycled through land application. The current guidance in Florida, US allows for unrestricted land application of lime softening WTR; alum and ferric WTR require additional evaluation of total and leachable concentrations of select trace metals prior to land application. In some cases a mixed WTR is produced when lime softening is accompanied by the addition of a coagulant or other treatment chemical; applicability of the current guidance is unclear. The objective of this research was to characterize the total and leachable chemical content of WTR from Florida facilities that utilize multiple treatment chemicals. Lime and mixed lime WTR samples were collected from 18 water treatment facilities in Florida. Total and leachable concentrations of the WTR were measured. To assess the potential for disposal of mixed WTR as clean fill below the water table, leaching tests were conducted at multiple liquid to solid ratios and under reducing conditions. The results were compared to risk-based soil and groundwater contamination thresholds. Total metal concentrations of WTR were found to be below Florida soil contaminant thresholds with Fe found in the highest abundance at a concentration of 3600 mg/kg-dry. Aluminum was the only element that exceeded the Florida groundwater contaminant thresholds using SPLP (95% UCL = 0.23 mg/L; risk threshold = 0.2 mg/L). Tests under reducing conditions showed elevated concentrations of Fe and Mn, ranging from 1 to 3 orders of magnitude higher than SPLP leachates. Mixed lime WTR concentrations (total and leachable) were lower than the ferric and alum WTR concentrations, supporting that mixed WTR are appropriately represented as lime WTR. Testing of WTR under reducing conditions demonstrated the potential for release of certain trace metals (Fe, Al, Mn) above applicable regulatory thresholds; additional evaluation is needed to assess management options where reducing conditions may develop.

Fate of sulfamethoxazole in wetland sediment under controlled redox conditions.

Redox condition is an important controlling factor for contaminant removal in constructed wetlands; however, the redox-sensitivity of antibiotic removal in wetland sediments under controlled conditions with specific electron acceptors remains unclear. Here, using a 14C radioactive tracer, we explored fate of sulfamethoxazole (SMX) in a wetland sediment slurry under oxic, nitrate-reducing, iron-reducing, and methanogenic conditions. In the sterile treatment, unlike the comparable SMX dissipation from the water phase under four redox conditions, non-extractable residues (NERs) of SMX was highest formed in the sediment under oxic condition, mainly in sequestered and ester/amide-linked forms. Microorganisms markedly promoted SMX transformation in the slurry. The dissipation rate of SMX and its transformation products (TPs) followed the order: oxic ≈ iron-reducing > methanogenic >> nitrate-reducing conditions, being consistent with the dynamics of microbial community in the sediment, where microbial diversity was greater and networks connectivity linking dominant bacteria to SMX transformation were more complex under oxic and iron-reducing conditions. Kinetic modeling indicated that the transformation trend of SMX and its TPs into the endpoint pool NERs depended on the redox conditions. Addition of wetland plant exudates and sediment dissolved organic matter at environmental concentrations affected neither the abiotic nor the biotic transformation of SMX. Overall, the iron-reducing condition was proven the most favorable and eco-friendly for SMX transformation, as it resulted in a high rate of SMX dissipation from water without an increase in toxicity and subsequent formation of significant stable NERs in sediment. Our study comprehensively revealed the abiotic and biotic transformation processes of SMX under controlled redox conditions and demonstrated iron-reducing condition allowing optimal removal of SMX in constructed wetlands.

Distribution and hydrogeochemical behavior of arsenic enriched groundwater in the sedimentary aquifer comparison between Datong Basin (China) and Kushtia District (Bangladesh).

A prompt growth in research on arsenic occurrence and behavior in the environment has occurred over the last decade or so. High arsenic (As) in groundwater has become a major global concern due to its widespread occurrence. A comparative hydrogeochemical study was performed on the occurrence of high As groundwater in Datong Basin, China, and Kushtia District, Bangladesh. A total of 132 groundwater samples (83 from Datong Basin and 49 from Kushtia District) were collected to analyze the major hydrogeochemical components and trace elements in groundwater of both areas. Factor analysis (FA) was applied on the hydrochemical data to identify the major hydrogeochemical processes in sedimentary aquifers. High As groundwater was observed in the low-lying central parts of Datong Basin, which are composed of the Holocene alluvial and lacustrine aquifers. The elevated As concentrations ranged from 0.31 to 452 µg/L and distributed in depths between 20 and 45 m. As-enriched groundwater is mainly Na-HCO3 type water and characterized by higher pH value, high Na+, low Ca2+, SO42-, and NO3- along with moderate TDS. The alkaline and reducing subsurface environment facilitate the leaching of As in sedimentary aquifers. The release and distribution of As in aquifers are resulted from the reduction of As-carrying crystalline iron (Fe) oxide/hydroxides and oxidation of organic matter. The aquifers of Kushtia District, Bangladesh, are unconsolidated, alluvial in nature, and developed from Holocene floodplain and Pleistocene deposits. High As (6.04-590.7 µg/L) groundwater occurs mainly in shallow aquifers. The Ca-HCO3 type groundwater is distinguished by circum-neutral pH, medium-high EC, high HCO3-, and low content of NO3-, SO42-, K+, and Cl-. The reductive suspension of MnOOH increases the dissolved As loads and redox responsive elements such as SO42- and pyrite oxidation act as the main mechanisms for As release in groundwater. As is mobilized by anaerobic leakage from the brown-clay and gray-sand into the sediment. Infiltration from irrigation return and surface wash water are the potential factors that remobilize As. The weak loading of Fe suggests that the release of Fe and As is decoupled in sedimentary aquifers of Kushtia District.

Reduction of adsorbed As(V) on nano-TiO2 by sulfate-reducing bacteria.

Reduction of surface-bound arsenate [As(V)] and subsequent release into the aqueous phase contribute to elevated As in groundwater. However, this natural process is not fully understood, especially in the presence of sulfate-reducing bacteria (SRB). Gaining mechanistic insights into solid-As(V)-SRB interactions motivated our molecular level study on the fate of nano-TiO2 bound As(V) in the presence of Desulfovibrio vulgaris DP4, a strain of SRB, using incubation and in situ ATR-FTIR experiments. The incubation results clearly revealed the reduction of As(V), either adsorbed on nano-TiO2 or dissolved, in the presence of SRB. In contrast, this As(V) reduction was not observed in abiotic control experiments where sulfide was used as the reductant. Moreover, the reduction was faster for surface-bound As(V) than for dissolved As(V), as evidenced by the appearance of As(III) at 45h and 75h, respectively. ATR-FTIR results provided direct evidence that the surface-bound As(V) was reduced to As(III) on TiO2 surfaces in the presence of SRB. In addition, the As(V) desorption from nano-TiO2 was promoted by SRB relative to abiotic sulfide, due to the competition between As(V) and bacterial phosphate groups for TiO2 surface sites. This competition was corroborated by the ATR-FTIR analysis, which showed inner-sphere surface complex formation by bacterial phosphate groups on TiO2 surfaces. The results from this study highlight the importance of indirect bacteria-mediated As(V) reduction and release in geochemical systems.

Effects of aerobic and anaerobic biological processes on leaching of heavy metals from soil amended with sewage sludge compost.

The risk from leaching of heavy metals is a major factor hindering land application of sewage sludge compost (SSC). Understanding the change in heavy metal leaching resulting from soil biological processes provides important information for assessing long-term behavior of heavy metals in the compost amended soil. In this paper, 180days aerobic incubation and 240days anaerobic incubation were conducted to investigate the effects of the aerobic and anaerobic biological processes on heavy metal leaching from soil amended with SSC, combined with chemical speciation modeling. Results showed that leaching concentrations of heavy metals at natural pH were similar before and after biological process. However, the major processes controlling heavy metals were influenced by the decrease of DOC with organic matter mineralization during biological processes. Mineralization of organic matter lowered the contribution of DOC-complexation to Ni and Zn leaching. Besides, the reducing condition produced by biological processes, particularly by the anaerobic biological process, resulted in the loss of sorption sites for As on Fe hydroxide, which increased the potential risk of As release at alkaline pH.