Performance optimization for Pb(II) -containing wastewater treatment in constructed wetland-microbial fuel cell triggered by biomass dosage and Pb(II) level.

Three identical sets of constructed wetland-microbial fuel cells (CW-MFCs) fabricated with biomass carbon source addition were constructed and underwent the short- and long-term experiments. For this, the efficacy of biomass dosage and Pb(II) concentration towards Pb(II) removal and concurrent bioelectricity production of CW-MFCs were systematically explored. From the perspective of integrated capabilities and economic benefits, the solid biomass carbon sources equivalent to 500 mg/L COD was regarded as the optimal dosage, and the corresponding device was labeled as CW-MFC-2. For the short-term experiment, the closed-circuit CW-MFC-2 produced maximum output voltages and power densities in a range of 386-657 mV and 1.55 × 103-6.31 × 103 mW/m2 with the increasing Pb(II) level, respectively. Also, Pb(II) removal up to 94.4-99.6% was obtained in CW-MFC-2. With respect to long-term experiment, Pb(II) removal, the maximum output voltage, and power density of CW-MFC-2 ranged from 98.7 to 99.2%, 322 to 387 mV, and 3.28 × 102 to 2.26 × 103 mW/m2 upon 200 mg/L Pb(II) level, respectively. The migration results confirmed the potential of substrate and biomass for Pb(II) adsorption and fixation. For the cathode, Pb(II) was fixed and removed via binding to O. This study enlarges our knowledge of effective modulation of CW-MFCs for the treatment of high-level Pb(II)-containing wastewater and bioelectricity generation via adopting desirable biomass dosage.

A novel filter-type constructed wetland for secondary effluent treatment: Performance and its microbial mechanism.

A novel filter-type constructed wetland was constructed by combining plastic fillers and mineral fillers for secondary effluent treatment. Findings showed that TN, TP and COD removal in the constructed wetland with composite fillers (CFCW) was 3.9%, 8.0% and 3.5% higher than that of constructed wetland with ordinary gravel fillers (CW) in the stable phase, respectively. CFCW showed better pollutants removal when dealing with higher influent concentrations and hydraulic loading. The main functional bacteria in two systems were significantly different (p < 0.05). Composite fillers could change the dominant genera, enhance genera activity and increase genera quantity. Denitrification (e.g., Pseudorhodobacter, Zoogloea, Pseudarthrobacter), nitrification (e.g., Devosia, Nitrospira), heterotrophic nitrification-aerobic denitrification (e.g., Paracoccus) and partial denitrification (e.g., g__Simplicispira) in CFCW provided diverse nitrogen metabolism pathways, resulting in higher nitrogen removal. The novel filter-type constructed wetland is suitable for the advanced treatment of sewage treatment plant effluent with enhanced pollutants removal and exuberant microorganisms.

Enhanced nickel removal and synchronous bioelectricity generation based on substrate types in microbial fuel cell coupled with constructed wetland: performance and microbial response.

In this study, an attempt was made to clarify the impact of substrates on the microbial fuel cell coupled with constructed wetland (CW-MFC) towards the treatment of nickel-containing wastewater. Herein, zeolite (ZEO), coal cinder (COA), ceramsite (CER), and granular activated carbon (GAC) were respectively introduced into lab-scaled CW-MFCs to systematically investigate the operational performances and microbial community response. GAC was deemed as the most effective substrate, and the corresponding device yielded favorable nickel removal efficiencies over 99% at different initial concentrations of nickel. GAC-CW-MFC likewise produced a maximum output voltage of 573 mV, power density of 8.95 mW/m2, and internal resistance of 177.9 Ω, respectively. The strong adsorptive capacity of nickel by GAC, accounting for 54.5% of total contaminant content, was mainly responsible for the favorable nickel removal performances of device GAC-CW-MFC. The high-valence Ni2+ was partially reduced to elemental Ni0 on the cathode, which provided evidence for the removal of heavy metals via the cathodic reduction of CW-MFC. The microbial community structure varied considerably as a result of substrates addition. For an introduction of GAC into the CW-MFC, a remarkably enriched population of genera Thermincola, norank_f__Geobacteraceae, Anaerovorax, Bacillus, etc. was noted. This study was dedicated to providing a theoretical guidance for an effective regulation of CW-MFC treatment on nickel-containing wastewater and accompanied by bioelectricity generation via the introduction of optimal substrate.

Denitrification performance, bioelectricity generation and microbial response in microbial fuel cell - constructed wetland treating carbon constraint wastewater.

For the deep reduction of nitrogen, the microbial fuel cell-constructed wetland (MFC-CW) was conducted for treating carbon constraint wastewater. Results indicated that nitrogen removal decreased from 94.96% to 24.96% with influent COD/TN (chemical oxygen demand/total nitrogen) from 4 to 0. MFC-CW was seriously affected by low organic wastewater. Wetland plants contributed to denitrification, with TN removal increasing from 46.13% to 64.87%. The bioenergy output showed a linear relationship with influent COD, and the maximum power density of 1.17 mW/m2 was obtained. Correlation analysis indicated that functional genera of Paenibacillus, Trichococcus, norank_KD4-96, norank_OLB14 played a crucial role in nitrogen removal. Influent COD and wetland plants affected carbon and nitrogen metabolisms, and key genes related to denitrification were more abundant in the cathode. Findings illustrated the nitrogen metabolism in MFC-CW with carbon constraint wastewater and will extend the application of MFC-CW in secondary effluent treatment from wastewater treatment plants.

Recycled utilization of ryegrass litter in constructed wetland coupled microbial fuel cell for carbon-limited wastewater treatment.

To solve wetland plant litter disposal and improve the nitrogen removal of carbon-limited wastewater, the integration of microbial fuel cell (MFC) and recycled utilization of ryegrass litter planted in constructed wetland (CW) may be effective. CW and MFC-CW with periodical ryegrass litter addition (10 days one cycle) were constructed to study the effects of ryegrass litter on nitrogen removal, electricity production and microorganism community. The results showed that total nitrogen removal of CW and MFC-CW after ryegrass litter addition reached 80.54 ± 10.99% and 81.94 ± 7.30%, increased by 22.19% and 17.50%, respectively. Three-dimensional excitation emission matrix fluorescence spectroscopy results revealed that the soluble organic matters produced by the hydrolyzed ryegrass litter were mainly tryptophan, tyrosine and fulvic acid, which promoted the growth of microorganisms and denitrification. The dosage of 200 g m-2 did not cause the rise of refractory organic matter in the effluent. The ryegrass litter addition promoted the average voltage and power density slightly in MFC-CW, but the internal resistance also increased temporarily. Compared to the sole CW, current stimulation caused by MFC not only helped to increase the denitrification, but also accelerated the biomass hydrolysis. MFC could contribute to the enrichment and growth of functional microorganisms related to denitrification and organic degradation, such as Vogesella, Devosia, Thermomonas and Brevibacterium. The bacterial genera involved in the ryegrass litter degradation were mainly Thermomonas, Propionicimonas, TM7a, Clostridium_sensu_stricto_1 and so on. This study provided a promising way for practical applications of MFC-CW in the treatment of carbon-limited wastewater, especially in small ecological facilities.

Simultaneous removal of heavy metals and bioelectricity generation in microbial fuel cell coupled with constructed wetland: an optimization study on substrate and plant types.

A microbial fuel cell coupled with constructed wetland (CW-MFC) was built to remove heavy metals (Zn and Ni) from sludge. The performance for the effects of substrates (granular activated carbon (GAC), ceramsite) and plants (Iris pseudacorus, water hyacinth) towards the heavy metal treatment as well as electricity generation was systematically investigated to determine the optimal constructions of CW-MFCs. The CW-MFC systems possessed higher Zn and Ni removal efficiencies as compared to CW. The maximal removal rates of Zn (76.88%) and Ni (66.02%) were obtained in system CW-MFC based on GAC and water hyacinth (GAC- and WH-CW-MFC). Correspondingly, the system produced the maximum voltage of 534.30 mV and power density of 70.86 mW·m-3, respectively. Plant roots and electrodes contributed supremely to the removal of heavy metals, especially for GAC- and WH-CW-MFC systems. The coincident enrichment rates of Zn and Ni reached 21.10% and 26.04% for plant roots and 14.48% and 16.50% for electrodes, respectively. A majority of the heavy metals on the sludge surface were confirmed as Zn and Ni. Furthermore, the high-valence Zn and Ni were effectively reduced to low-valence or elemental metals. This study provides a theoretical guidance for the optimal construction of CW-MFC and the resource utilization of sludge containing heavy metals.

Higher nitrogen removal achieved in constructed wetland with polyethylene fillers and NaOH-heating pre-treated corn stalks for advanced treatment of low C/N sewage.

Advanced processing of low C/N sewage faces the carbon sources shortage, while quantities of agricultural biomass wastes need to be disposed. This study investigated the potential of quantitative modified biomass addition in constructed wetlands (CWs) filled with polyethylene fillers. Results showed that the lignin in NaOH-heating pretreated corn stalks (NH-CSs) was destroyed, and the wrinkles on the stalks increased and became more soft after pretreatment, which was more conducive to the utilization of carbon sources and attachment of microorganisms. Compared with glucose and sodium acetate, the denitrification with mixed carbon source (glucose and NH-CSs) had the highest effective utilization percentage (61.37%) and NH-CSs were expected to become stable and fast-release carbon sources. After adding 30 g NH-CSs to the rear unit of CW with polyethylene fillers (CW-A), TN removal efficiency was increased by 18.21%, and the average removal efficiency of COD, NH4+-N, TN, and TP reached 54.83%, 89.95%, 64.11%, and 45.04%, respectively. Compared with the traditional CW (CW-B), CW-A had a significant denitrification advantage (P < 0.05), but the removal efficiency and effluent stability of phosphorus were inferior to CW-B. These results indicate that the biomass carbon sources such as corn stalks and polyethylene fillers have a good potential to improve the denitrification in CWs.

Temporal variation of vegetation at two operating landfills and its implications for landfill phytoremediation.

Landfilling is a popular method to dispose of solid waste around the world, but this practice is associated with inevitable negative consequences such as air and water pollution and potential health hazards. Phytoremediation is an economical and promising technology to address these problems. To select suitable species for restoring landfills in China, the vegetation characteristics and their physiological activities were investigated in two different landfills. The results showed that the vegetation coverage of the two landfills both increased with the closure time. The species diversity decreased with the closure time in the Chongming landfill, whereas it showed the opposite trend in the Jiangcungou landfill. Five dominant species (Conyza canadensis, Setaria viridis, Sesbania cannabina, Suaeda glauca, and Phragmites australis) and four dominant species (Festuca elata Keng, Amaranthus retroflexus, Lolium multiflorum, and Kochia scoparia) were identified in the Chongming landfill and Jiangcungou landfill, respectively, and these dominant species were mainly annual plants. In addition, the net photosynthetic rate of P. australis and K. scoparia was the highest in the Chongming landfill and Jiangcungou landfill, respectively. Their photosynthetic rates were influenced significantly by stomatal conductance. Based on the overall results, among the dominant species identified, P. australis and K. scoparia represent potentially well suited plants to use in the remediation of landfills in China, because of their strong adaptability to landfill environments and natural occurrence in landfills.

Enhanced denitrification and power generation of municipal wastewater treatment plants (WWTPs) effluents with biomass in microbial fuel cell coupled with constructed wetland.

A microbial fuel cell-constructed wetland (MFC-CW) with water hyacinth is established to remove the nitrogen and organics from municipal wastewater treatment plants (WWTPs) effluents. Because insufficient carbon sources in influent might decrease pollutants removal efficiency and electricity generation, this research aimed to select high-quality and low-cost biomass as additional carbon source to improve the performance of MFC-CW. Cellulose and hemicellulose (xylan) were chosen as the biomass. Results indicated that xylan displayed a higher nitrate removal (above 92%) compared with cellulose (10.9%). With xylan as carbon source, the anode packing removed nitrate above 80%, while the cathode packing only removed around 50%. With glucose as sole carbon source, the maximum total nitrogen (TN) removal of MFC-CW was 87.66 ± 4.23%, which was higher than that of MFC (85.58 ± 4.14%). The chemical oxygen demand (COD) and TN in the effluent of MFC-CW were maintained below 25 mg/L and 1.5 mg/L, respectively, with the COD/TN ratio around 5.4 and hydraulic retention time (HRT) at 48 h. The TN removal reached the maximum efficiency of 88.78 ± 3.98% when glucose and xylan ratio was in 40%:60% as composite carbon sources, and COD and TN in the effluent were below 20 mg/L and 1.5 mg/L, respectively. In addition, xylan as the additional carbon source significantly promoted the power density compared with sole glucose. Microbial community diversity in the MFC-CW was significantly higher than that in the single MFC or CW. Proteobacteria and Cyanobacteria_norank were relatively more dominant in the MFC-CW than those in the single MFC or CW, which accounted for high nitrogen removal and power generation. Findings in this study proved that MFC-CW with biomass addition enhanced nitrogen removal and power generation.

Methylmercury levels in cover soils from two landfills in Xi'an and Shanghai, China: Implications for mercury methylation potentials.

Landfills are considered important sources of mercury for surrounding ecosystems. Methylmercury (MeHg) levels in waste layers have been studied extensively; however, the levels of MeHg in cover soils remain undefined. Here, total mercury (THg) and MeHg concentrations in surface cover soils and soil cores from two landfills in China and possible factors affecting Hg methylation were studied. The mean MeHg concentration in surface cover soils from both landfills was 0.048 ng g-1, suggesting that cover soil layers are not active sites of MeHg production. Soil MeHg concentrations in both landfills were affected little by closure time. In the Jiangcungou landfill, no correlations between MeHg concentration and the measured environmental factors (e.g., THg, soil pH, organic matter (OM), and S) were observed that indicated that these parameters might have indirect effects on MeHg concentration. However, in the Laogang landfill, significant correlations were found between MeHg concentration and the measured environmental factors. The results showed that MeHg concentration in the surface cover soil from area D of the Laogang landfill is regulated mainly by soil pH, OM, and S, and that its vertical distribution in areas C and D is regulated mainly by soil pH and soil OM, respectively. These findings fill a knowledge gap regarding MeHg levels in cover soils and they advance our understanding of Hg cycling in landfills, presenting positive implications for landfill management and risk assessment of MeHg.

Exchange pattern of gaseous elemental mercury in landfill: mercury deposition under vegetation coverage and interactive effects of multiple meteorological conditions.

Landfill is known as a potential source of atmospheric Hg and an important component of the local or regional atmospheric Hg budget. This study investigated the gaseous elemental Hg surface-air fluxes under differing conditions at a typical municipal solid waste landfill site, highlighting the interactive effects of plant coverage and meteorological conditions. The results indicated that Hg fluxes exhibited a feature represented by diel variation. In particular, Hg deposition was observed under a condition of Kochia sieversiana coverage, whereas emission that occurred after K. sieversiana was removed. Hg emission was the dominant mode under conditions of Setaria viridis coverage and its removal; however, the average Hg emission flux with the S. viridis coverage was nearly four times lower than after its removal. These findings verified that the plant coverage should be a key factor influencing the Hg emission from landfills. In addition, Hg fluxes were correlated positively with solar radiation and air/soil temperature and correlated inversely with relative humidity under all conditions, except K. sieversiana coverage. This suggested that the interactive effects of meteorological conditions and plant coverage played a jointly significant role in the Hg emission from landfills. It was established that K. sieversiana can inhibit Hg emission efficiently, and therefore, it could potentially be suitable for use as a plant-based method to control Hg pollution from landfills.