• Energy Research

The ecological floating bed (EFB) coupled with zero-valent iron (ZVI) is proposed to treat low carbon-to-nitrogen ratio water. However, the application of ZVI is limited by low electron transfer efficiency. Coupling ZVI with carbon materials may improve the performance. In this study, the EFB with ZVI coupled plant biomass (IB-EFB) was established to enhance denitrification performance and compared to the EFB with ZVI coupled activated carbon (IC-EFB). The results showed that higher denitrification rate was observed in IB-EFB (68.8%) than that in IC-EFB (54.40%), which attributed to the synergistic effect of ZVI and plant biomass. Plant biomass also promoted the electron transfer of ZVI which enhanced the Fe(II)-mediated denitrification. High-throughput sequencing analysis revealed that IB-EFB enriched iron-related denitrifying bacteria more effectively than IC-EFB, and obtained high abundance of phototrophic Fe(II)-oxidizing bacteria Rhodopseudomonas (19.26%). Thus coupling ZVI with plant biomass has a potential for enhanced nitrogen removal in EFB.

The capability of 14 zeolites synthesized from different fly ashes (ZFAs) to sequestrate Cr(III) from aqueous solutions was investigated in a batch mode. The influence of pH on the sorption of Cr(III) was examined. ZFAs had a much greater ability than fly ash to remove Cr(III), due to the high cation exchange capacity (CEC) and the high acid neutralizing capacity (ANC) of ZFAs. The mechanism of Cr(III) removal by ZFAs involved ion exchange and precipitation. A high-calcium content in both the fly ashes and ZFAs resulted in a high ANC value and, as a result, a high immobilization capacity for Cr(III). The pH strongly influenced Cr(III) removal by ZFAs. Inside the solubility range, removal of chromium increased with increasing pH. Hydroxysodalite made from a high-calcium fly ash had a higher sorptive capacity for Cr(III) than the NaP1 zeolite from medium- and low-calcium fly ashes. On the other hand, at pH values above the solubility range, the efficiency of chromium removal by the ZFAs approached 100% due to the precipitation of Cr(OH)3 on the sorbent surfaces. It is concluded that ZFAs and high-calcium fly ashes may be promising materials for the purification of Cr(III) from water/wastewater.

Aquatic plants litters from constructed wetlands might become pollutants without proper treatment. Due to its high carbon and low nitrogen contained, Iris pseudacorus litters have potential to be used as carbon source to enhance denitrification process in advanced treatments of secondary effluent from wastewater treatment plants. This study investigated the characteristics of carbon release form Iris pseudacorus litters and its performance on enhancement of nitrogen removal. The batch experiment showed that the organic carbon release process can be simulated by combining dissolution and hydrolysis process, and it was found that dissolved organic matters mainly consisted of 60% sugar and 35% humic acid-like compounds from the neutral detergent solution and hemicellulose of litters. The long-term operation of lab-scale constructed wetlands revealed a high nitrogen removal of 78.81-90.39% in treating the synthetic wastewater treatment plants effluent with the equivalent dosage of 25-150 g litters m-2 d-1. Furthermore, it is possible to establish an Iris pseudacorus self-consumed constructed wetland to reuse all of the litters produced during the operation. These findings can contribute to the understanding of the dynamics of carbon release from Iris pseudacorus litters and recycled utilization of plant biomass in the constructed wetlands.

In-situ light-availability control is commonly used to suppress Microcystis blooms in nutrient-rich water resources. It has been suggested that the reduction of column cyanobacterial biomass could mostly be attributed to the inhibition of photosynthesis. However, sinking loss may be another factor influencing the column cyanobacterial biomass. To further investigate the mechanism of this reduction, a mixing-static water column experimental apparatus was designed to simulate the reduction of Microcystis biomass under light-availability control. Under light-shading plus mixing, the reduction of Microcystis in the water column was attributed to both intrinsic biomass loss and sinking loss. Comparatively, under light-shading without mixing, the Microcystis accumulated in surface water, maintaining a continuously increase of intrinsic biomass. Meanwhile, the sinking loss increased as the water column became static, even exceeding the increase of intrinsic biomass, suggesting that sinking loss was the main mechanism for the reduction under light-shading. Further investigation indicated that both intrinsic growth rate and sinking loss rate varied in response to available light. Accordingly, a hypothesis is represented that the loss of column biomass and the shift in dominant species under light-availability control are primarily attributed to the combined effects of intrinsic biomass change and sinking loss, which both respond to available light.

Five simulated salt marsh wetlands with reed were constructed to investigate the effect of salinity on denitrification efficiency and its enhancement by reed biomass addition. It was found that the salinity of 7 ‰ and 10 ‰ could promote the organic carbon release of reed biomass. Results showed that the nitrate removal was highest at the salinity of 7 ‰, and would be further enhanced from 54.06 ± 12.46 % to 74.37 ± 11.53 % after the addition of reed biomass. Meanwhile, the lowest nitrous oxide emission flux was also achieved, with 0.23 mg/(m2 h) at this salinity. Microbiological analysis showed that salinity changed the microbial community. The increasing salinity increased the relative abundance of Chloroflexi and Actinobacteria, but decreased that of Proteobacteria. Main functional genera of denitrification changed from Desulfuromonas to Azoarcus and Anaeromyxbacter when the salinity increased to 15 ‰. These results will help to understand the nitrogen removal capacity of salt marsh wetlands with reed biomass addition.

In developing an algal treatment system, selenium (Se) removal efficiency by Chlorella vulgaris was evaluated under various conditions such as Se concentration, algal density, temperature and pH. A maximum removal efficiency plateau of ∼90% was observed between 1000-3000 μg Se/L while the tolerance of Se toxicity was found at 6000 μg Se/L. C. vulgaris of 0.75 g DW/L showed the highest removal efficiency (84%), and volatilization was dominant below 1.37 g DW/L. Se volatilization was two times higher at 25 °C than at 20 °C in the first 24 h. Moreover, the highest removal efficiency (77%) was obtained at pH 8.0, compared to 66.5% at pH 6.5 and 40% at pH 10.0. To prevent ecotoxicity, Se laden algae were further burned to ashes or filtered out by Anodonta woodiana. After burning, biomass Se was reduced by 99%, with organo-Se entirely converted into inorganic Se, lowering Se bioavailability. A. woodiana removed 54% of Se in 24 h, leading to Se bioaccumulation in soft tissues, which may serve as dietary Se supplements for human health. Our results suggest the cleanup of Se-contaminated water from either agricultural runoff or industrial discharge could be achieved using an algal treatment system with minimum potential ecotoxicity.

This study investigated the interaction between plant biomass and iron scraps and their influence on nitrogen (including nitrate and ammonia) and phosphorus removal in the subsurface flow constructed wetland. The results showed that with the addition of 0.5 g L-1 of plant biomass and 5.0 g L-1 of iron scraps, the nitrate, total nitrogen and total phosphorus removal were simultaneously improved. During 35 days of continuous operation, the plant biomass played main effect on the enhanced denitrification, accounting for about 57%, while iron scraps enhanced the other 43% of nitrogen removal and most phosphorus removal through precipitation inside the wetlands. Iron scraps could benefit the degradation of cellulose into low molecular carbohydrates by 10%, and the biomass could promote the oxidation of iron and increase the total phosphorus removal by 15%. Plant biomass coupled with iron scraps also improved simultaneously the richness, diversity and evenness of microbial community and promoted the abundance of Nitrospira (17.37%) and Thiobacillus (8.46%) in wetlands. In practice, putting iron scraps as matrix and placing plant biomass in the influent region would be a better choice. This research would provide a new method for effective utilization of plant biomass and iron scraps and further treatment of low-polluted wastewater in the wetlands.

Chromite Ore Processing Residue (COPR) is the byproduct of chromate production process, which contains a large amounts of Cr(VI). The present work developed a new technique to treat COPR, and the process involved mixing the COPR with rice straw followed by pyrolysis. It was found that the gaseous organic fraction generated during pyrolysis of straw was beneficial to Cr(VI) reduction. In the study, process variables, such as the amount of straw added to COPR, heating temperature and particle size, were systematically varied, and their influences on the Cr(VI) reduction in COPR were all investigated. After pyrolysis, Cr(VI) decreased greatly, from 3400 for untreated COPR to less than 30 mg kg(-1) for treated COPR at 600 degrees C.

Despite the fact that, in practice, enhanced biological phosphorus removal (EBPR) is often combined with biological nitrogen removal (BNR), the feasibility of nitrite (an intermediate in BNR) utilization, by denitrifying poly-P accumulating organisms (DPAO) is still unclear. In this study, the effect of nitrite on the EBPR was evaluated by investigating the dynamic response of DPAO to the gradual change of electron acceptor from nitrate to nitrite in an anaerobic/anoxic sequencing batch reactor (SBR). Under the nitrate condition, excellent EBPR was achieved, characterized by the DPAO biomass dominating in the sludge (51%-72% of VSS). Of the DPAO, 34% was the DPAOnitrate that could only use nitrate as electron acceptor, and 66% was the DPAOnitrite that could use nitrate/nitrite as electron acceptor. Nitrite in excess of 12 mg N/L had inhibition effect on the anoxic phosphorus uptake. As the electron acceptor changed, EBPR degraded slowly due to the gradual exclusion of the DPAOnitrate out of the reactor. The DPAOnitrite, therefore, became the single type of DPAO under the nitrite condition, and finally occupied only 33%-49% of the total sludge of VSS. Simultaneously, glycogen accumulating organisms (GAO), the competitor to the DPAO for substrate in the anaerobic phase, was significantly enhanced and grew to dominance in the reactor (46-62% of VSS). Also, the periodical exposure of sludge to the increased nitrite concentration elevated the inhibition level of nitrite for the anoxic phosphorus uptake (80 mg N/L). More attention should be paid to nitrite accumulation at full-scale wastewater treatment plants from the perspective of the stability of EBPR performance.