Review on stabilization/solidification methods and mechanism of heavy metals based on OPC-based binders.

Stabilization/solidification (S/S) with ordinary portland cement (OPC)-based binders is a suitable method to remediate heavy metal (HM)-contaminated soil and reuse resources of industrial wastes. In industrial wastes, alkaline wastes such as red mud (RM), soda residue (SR), pulverized fly ash (PFA), and alkalinity granulated blast furnace slag (GGBS) can immobilize HM ions (Pb2+, Zn2+, Cd2+, Cr3+, and Cu2+) by precipitation. However, some HM ions (such as AsO43-) would redissolve within the strong alkali environment. In this case, PFA, GGBS, metakaolin (MK), and incinerated sewage sludge ash (ISSA) which have low pH, can be used to immobilize HM ions or added to the OPC-based binders to adjust the pH in the soil products. Moreover, the calcium silicate hydrate (CSH), calcium aluminum silicate hydrate (CASH), ettringite (AFt), and calcium monosulfoalumiante hydrates (AFm) generated during the pozzolanic reaction can also immobilize HM ions by adsorption on the surface, ion exchange, and encapsulation. SR and GGBS can be used to immobilize the HMs (such as CrO42- and AsO43-), which are mainly affected by AFt and AFm. For those not affected by AFt and AFm but related to immobilization by precipitating (such as Mn2+), other wastes except SR and GGBS are suitable for treating contaminated soil. Nevertheless, the formation of AFt is also instrumental for soil product strength. There are several factors affecting soil product strength. In the future, the influence of different hydration products on the S/S effects, competitive adsorption of HM ions, effects on long-term HM stabilization, and novel materials are worth being explored by researchers.

A novel reactor combining anammox and Fenton-like reactions for the simultaneous removal of organic carbon and nitrogen at different organic carbon to nitrogen ratios.

A Fenton-like reaction and anaerobic ammonium oxidation (anammox) were combined to construct a novel process named FenTaMox for removing nitrogen (N) and organic carbon (measured as chemical oxidation demand (COD)). Two columns were packed with iron-manganese-sepiolite, a catalyst that uses hydrogen peroxide (H2O2) to catalyze Fenton-like reactions, and inoculated with marine anammox bacteria. During the start-up, marine anammox medium was fed into both columns to acclimate the marine anammox bacteria to iron-manganese-sepiolite. Batch experiments revealed that the marine anammox bacteria were not affected by 60 mgL-1 of H2O2. Next, medium containing glucose and H2O2 was fed into one column as the FenTaMox treatment, while medium containing glucose but no H2O2 was fed into the other column as the control. At a COD/N of 4, FenTaMox exhibited higher removal efficiencies of N and COD compared with that of the control, suggesting the application of FenTaMox for organic carbon- and N- removal.

Natural attenuation and remobilization of arsenic in a small river contaminated by the volcanic eruption of Mount Iou in southern Kyushu Island, Japan.

The active volcano Mount Iou, in the southern part of Japan, erupted in 2018 for the first time in approximately 250 years. Geothermal water discharged from Mount Iou had high concentrations of toxic elements, such as arsenic (As), which could seriously contaminate the adjacent river. In this study, we aimed to clarify the natural attenuation of As in the river through daily water sampling for approximately eight months. The risk of As in the sediment was also evaluated using the sequential extraction procedures. The highest As concentration (2000 µg/L) was observed upstream but typically remained below 10 µg/L downstream. Dissolved As was the main form in the river water on non-rainy days. Arsenic concentration in the river naturally decreased through dilution and sorption/coprecipitation with Fe, Mn, and Al (hydr)oxides during flow. However, peaks in As concentration were frequently observed during rainfall events, possibly due to sediment resuspension. Furthermore, the range of pseudo-total As in the sediment was 4.62-14.3 mg/kg. Total As content was highest upstream before decreasing further along the flow. When using the modified Keon method, 44-70% of the total As existed as more reactive fractions associated with (hydr)oxides.

Effects of structural vulnerability of flat-sheet membranes on fouling development in continuous submerged membrane bioreactors.

The relationship between fouling development in a continuous laboratory-scale membrane reactor (MBR/Lab) and the membrane material was investigated using flat-sheet membranes prepared from four materials (polyvinylidene difluoride (PVDF), polyethersulfone, chlorinated polyvinyl chloride, and polytetrafluoroethylene). Further, the characteristics of the suspension liquid in MBR/Lab were compared with those of samples from actual wastewater treatment plants. It was found that, in addition to the membrane material's own characteristics, the structural vulnerability of the membranes had a determining effect on fouling development. The PVDF membrane showed the highest transmembrane pressure during MBR operation and its surface experienced significant damage because of the shearing stress caused by aeration, resulting in the penetration of the membrane by the fouling compounds. The characteristics of suspension liquid in MBR/Lab were almost similar to those in the MBR at a night-soil treatment plant and the aeration tank of a sewage treatment plant.

Changes of nitrogen-removal performance and that of the bacterial community in a mixed culture comprising freshwater and marine anammox bacteria under averaged environmental condition.

Nitrogen-removal processes using anammox bacteria are expected to achieve high-rate removal while remaining economical, and their practical applications have been investigated. However, anammox bacteria still have unfavorable characteristics for practical use, including susceptibility to a change in environmental conditions. In this study, with an aim of exploring the adaptability of mixed anammox bacteria to environmental conditions, the shift of nitrogen-removal performance and bacterial community in a mixed culture comprising freshwater anammox bacteria (FAB) and marine anammox bacteria (MAB) were investigated by a continuously stirred tank reactor (CSTR). The CSTR inoculated with the mixed anammox bacteria was operated for 180 days under an averaged condition between freshwater and marine conditions with a temperature of 27.5 °C and a synthetic medium with 15 g/L NaCl was used. Nitrogen-removal performance became stable after 114 days and more than 90% of nitrogen that was loaded into the reactor was removed in the range of nitrogen loading rate 0.07-0.42 kg N/m3/d. After operating at 0.42 kg N/m3/d for one month, a biomass sample was taken and its bacterial community was analyzed by clone-library analysis using a partial sequence of 16S rRNA. Among the clones of anammox bacteria that were made by an anammox-bacteria-specific primer, 97% of them were MAB and only 3% were FAB. These results indicate that the bacterial community including anammox bacteria was evidently changed due to environmental conditions and that the averaged condition in this study was suitable for marine bacteria rather than freshwater bacteria.

Understanding nitrate contamination based on the relationship between changes in groundwater levels and changes in water quality with precipitation fluctuations.

There are growing concerns about nitrate contamination in Kumamoto City, where >700,000 people completely depend on groundwater as a source of drinking water. We found that some groundwater samples showed considerably different nitrate concentrations although their sampling locations were close to one another, and we speculated that this phenomenon was due to the differences in subsurface geological properties. In order to verify this hypothesis, we carried out temporally intensive long-term monitoring of the groundwater levels and water qualities at three of the closely related sampling wells, and the results revealed that the changes in water level and water quality were different at each well. The water level at well T1, where nitrate concentrations ranged from 12 to 26 mg N/L, showed a significantly sensitive and unique response to heavy rain, which indicated that the subsurface at this site might be highly permeable; this would have allowed for the influent water to easily reach the groundwater aquifer over a short period. However, wells T2 and T3, which were located within 0.6 and 1.9 km from well T1, respectively, had nitrate concentrations that were lower than that in well T1 (4.5-8.0 mg N/L) and showed only gradual responses to heavy rain. These observations suggest that the highly permeable subsurface properties in the vicinity of well T1 contributed to the more serious nitrate contamination in well T1 than those at wells T2 and T3. This study demonstrates the importance of temporally intensive, long-term monitoring for capturing changes in groundwater level and water quality with precipitation fluctuations, and we showed how this approach can lead to a better understanding of the nitrate contamination situation.

High growth potential and nitrogen removal performance of marine anammox bacteria in shrimp-aquaculture sediment.

Anaerobic ammonium oxidation (anammox) bacteria were enriched in continuous packed-bed columns with marine sediment. One column (SB-C) was packed with only marine sediment collected from a shrimp-aquaculture pond, and another column (SB-AMX) was inoculated with marine anammox bacteria (MAB) as a control. These columns were continuously fed with natural or artificial seawater including ammonium (NH4+) and nitrite (NO2-). The SB-AMX showed anammox activities from the beginning and continued for over 200 days. However, the SB-C had no nitrogen removal performance for over 170 days. After adding a bicarbonate solution (KHCO3) to the sediment-only packed column, anammox activity was observed within 13 days. The column exhibited a nitrogen removal efficiency (NRE) of 88% at a nitrogen loading rate (NLR) of 1.0 kg-N·m-3·day-1, which was comparable to the control one. A next-generation sequencing analysis revealed the predominance of MAB related to "Candidatus Scalindua spp.". In addition, the co-occurrence of sulfur-oxidizing denitrifiers was observed, which suggests their symbiotic relationship. This study suggests the applicability of MAB for in-situ bioremediation of nitrogen-contaminated marine sediments and reveals a potential microbial interaction between anammox and sulfur-oxidizing communities responsible for nitrogen and sulfur cycling in marine aquaculture systems.

Application of a modified conceptual rainfall-runoff model to simulation of groundwater level in an undefined watershed.

Groundwater level simulation models can help ensure the proper management and use of urban and rural water supply. In this paper, we propose a groundwater level tank model (GLTM) based on a conceptual rainfall-runoff model (tank model) to simulate fluctuations in groundwater level. The variables used in the simulations consist of daily rainfall and daily groundwater level, which were recorded between April 2011 and March 2015 at two representative observation wells in Kumamoto City, Japan. We determined the best-fit model parameters by root-mean-square error through use of the Shuffled Complex Evolution-University of Arizona algorithm on a simulated data set. Calibration and validation results were evaluated by their coefficients of determination, Nash-Sutcliffe efficiency coefficients, and root-mean-square error values. The GLTM provided accurate results in both the calibration and validation of fluctuations in groundwater level. The split sample test results indicate a good reliability. These results indicate that this model can provide a simple approach to the accurate simulation of groundwater levels.

One-stage partial nitritation and anammox in membrane bioreactor.

Partial nitritation and anammox (PN/A) was applied in a lab-scale membrane bioreactor (MBR) to investigate its technical feasibility for treating ammonium-rich wastewater with low C/N ratio. The bacterial community was analyzed by molecular cloning and 16S rRNA sequence analysis. Partial nitritation (PN) was first realized in MBR by seeding aerobic activated sludge. With dissolved oxygen control, a steady effluent mixture with NO2 (-)-N/NH4 (+)-N ratio of 1.13 ± 0.08 was generated from the PN process. Subsequently, the MBR was seeded with anammox biomass on day 59. After running 300 days, the one-stage PN/A achieved a maximum nitrogen removal rate of 1.45 kg N/m(3)/day at the nitrogen removal efficiency of 89.5 %. Microbial community analysis revealed that Nitrosomonas sp. HKU and Nitrosospira sp. YKU corresponded to nitritation; meanwhile, Candidatus Brocadia TKU sp. accounted for nitrogen removal of the PN/A system. Specifically, Nitrosomonas sp. were enriched in the reactor at the PN/A phase and then conquered Nitrosospira sp. to be the predominant ammonia oxidizers. Nitrite oxidizers and denitrifiers were detected in symbiosis with aforementioned microbes. Denitrification promised potential plus nitrogen depletion. The present one-stage PN/A process allows a significant decrease in operational costs compared with classical nitrification/denitrification.

Nitrogen removal properties in a continuous marine anammox bacteria reactor under rapid and extensive salinity changes.

Salinity tolerance is one of the most important factors for the application of bioreactors to high-salinity wastewater. Although marine anammox bacteria (MAB) might be expected to tolerate higher salinities than freshwater anammox bacteria, there is little information on the effects of salinity on MAB activity. This study aimed to reveal the nitrogen removal properties in a continuous MAB reactor under conditions of rapid and extensive salinity changes. The reactor demonstrated stable nitrogen removal performance with a removal efficiency of over 85% under salinity conditions ranging from 0 to 50 g/L NaCl. The reactor performance was also well maintained, even though the salinity was rapidly changed from 30 to 50 g/L and from 30 to 0 g/L. Other evidence suggested that the seawater medium used contained components essential for effective MAB performance. Bacterial community analysis using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) showed that planctomycete UKU-1, the dominant MAB species in the inoculum, was the main contributor to anammox activity under all conditions. The PCR-DGGE using a universal bacterial primer set showed different DNA band patterns between the reactor biomass sample collected under conditions of 75 g/L NaCl and all other conditions (0, 30, 50 and freshwater-medium). All DNA sequences determined were very similar to those of bacterial species from marine environments, anaerobic environments, or wastewater-treatment facilities.