The feasibility of biologically removing nitrate from groundwater was tested by using cyanobacterial cultures in batch mode under laboratory conditions. Results demonstrated that nitrate-contaminated groundwater, when supplemented with phosphate and some trace elements, can be used as growth medium supporting vigorous growth of several strains of cyanobacteria. As cyanobacteria grew, nitrate was removed from the water. Of three species tested, Synechococcus sp. strain PCC 7942 displayed the highest nitrate uptake rate, but all species showed rapid removal of nitrate from groundwater. The nitrate uptake rate increased proportionally with increasing light intensity up to 100 mol of photons m ؊2 s ؊1, which parallels photosynthetic activity. The nitrate uptake rate was affected by inoculum size (i.e., cell density), fixed-nitrogen level in the cells in the inoculum, and aeration rate, with vigorously aerated, nitrate-sufficient cells in mid-logarithmic phase having the highest long-term nitrate uptake rate. Average nitrate uptake rates up to 0.05 mM NO 3 ؊ h ؊1 could be achieved at a culture optical density at 730 nm of 0.5 to 1.0 over a 2-day culture period. This result compares favorably with those reported for nitrate removal by other cyanobacteria and algae, and therefore effective nitrate removal from groundwater using this organism could be anticipated on large-scale operations.
Microalgae can be cultured in photobioreactors to sequester carbon dioxide and produce potentially valuable biomaterials. The goal of the present study was to identify and utilize microalgal strains that are capable of tolerating up to 20% CO2 (gas phase) concentrations under variable light or flue-gas blend conditions and reactor configurations to produce biomass. Scenedesmus sp. and Chlorella sp., both cultured from a Sonoran desert mineral spring, grew well and tolerated exposure to a gas mixture containing up to 20% CO2 applied continuously in batch reactors to the culture. Experiments were conducted with simulated coal-powered acidic flue gases containing SOx/NOx at concentrations of 200 to 350 ppmV. Microalgae did not grow well without pH control, and high levels (>250 mM) of nitrite or sulphite in the liquid media inhibited algal growth. Pseudo steady-state experiments were also conducted using helical tubular and flat-plate photobioreactors with continuous flow (water and gas) and with artificial or natural sunlight. With a 2 d hydraulic residence time (HRT), the helical tubular photobioreactor produced 0.50 +/- 0.11 g C d(-1) (0.056 +/- 0.012 g C L(-1) d(-1)) dry-weight cell mass during continuous fluorescent-lamp irradiance and 0.048 +/- 0.018 g C L(-1) d(-1) during 12 h light/darkness cycling. The flat-plate photobioreactor (2 d HRT) produced 0.42 +/- 0.28 g C L(-1) d(-1) with artificial lighting and with natural sunlight; a 4 d HRT produced 0.14 +/- 0.02 g C L(-1) d(-1). Reactor modelling indicated that a threshold of reactor size (i.e., HRT) an d reactor depth (path-length of light) exists based upon the optical density of the cells in the water column and their growth rates.
To determine if there are potential concerns related to the environmental end-of-life impacts of photovoltaic (PV) or quantum-dot display (QD) technologies, the goal of this study was to assess the magnitude of heavy metal leaching using simulated landfill methodologies from devices in an attempt to forecast the lifecycle environmental impacts of subsequent generations QD-enabled PV technologies. The underlying hypotheses are (H1) existing PV and QD thin-film technologies do not release heavy metals at concentrations exceeding RCRA or State of California regulatory limits, and (H2) the disposal of PV and QD thin-film technologies does not exceed Land Disposal Restrictions (LDR). Three task-oriented objectives were completed: (O1) Five representative PV panels and two representative thin-film displays with QD technology were obtained from commercial sources. (O2) RCRA Toxicity Characteristics Leaching Procedure (TCLP) tests and California Waste Extraction Tests (WET) were conducted in addition to microwave-assisted nitric acid digestion. (O3) Results were compared to the existing regulatory limits to examine the potential environmental end-of-life concerns. The heavy metal concentrations obtained from PV panels and QD thin-film displays when exposed to simulated landfill environments and extreme case leaching scenarios were generally several orders of magnitude lower than the promulgated standards and probably not of major concerns related to end-of-life safe disposal of these commercially available products. With exception to the findings for lead under the RCRA rules, the results confirmed that PV and QD thin-film technologies do not release heavy metals at concentrations exceeding RCRA or State of California characteristic hazardous waste regulatory limits. However, lead, mercury, and potentially other heavy metal releases have to be monitored to ensure that the disposal of this type of waste is in compliance with RCRA's LDR requirements and universal treatment standards because the second underlying hypothesis could not be completely supported for the leaching of these heavy metals. It could be anticipated that newer and more sophisticated soldering materials and approaches in the next generation of PV panels would significantly reduce the use of RCRA heavy metals or nanomaterials. However, although the generated data is limited to these representative PV and QD technologies and as such should not be considered applicable to the entire gamete of present-day technologies, these findings suggest that their release from future PV QD technologies would likely be greater from non-end-of-life processes than from traditional land disposal routes.
Hardness removal processes are very pH-dependent, especially for removal of magnesium (Mg) and silica (Si). Bench-scale tests were conducted with a groundwater that was supersaturated with carbon dioxide and contained calcium (Ca), Mg, and Si. The purpose of this work was to assess and optimize several softening processes to reduce chemical use (i.e., sludge production) and improve turbidity removal. Optimal dosing of lime and soda ash (OLSA) removed 79% of Ca and Mg hardness and 23% of Si hardness. Iron salt addition during OLSA improved the rate of turbidity removal, had minimal effect on Ca or Si removal, but decreased Mg removal unless the pH was readjusted to offset the pH decline that resulted from iron hydroxide formation. Sodium aluminate addition during OLSA improved the rate of turbidity removal and increased settled sludge viscosity, but did not affect Ca, Mg, or Si removal. In separate semibatch aeration softening tests (without lime or soda ash addition), Ca removal increased as a function of aeration rates; Mg and Si were not removed.The addition of a nuclei seed increased dissolved Ca removal from 60% without the nuclei seed to > 80% in the presence of 3 g/L of nuclei seed. Results indicate that the aeration softening process would remove hardness, decrease chemical consumption, and reduce sludge production. alcium (Ca) and magnesium (Mg ) are abundant alkaline earth metals that can significantly affect water quality, treatability, sludge production, and the economics of using a water supply for domestic or industrial applications (Batchelor et al, 1991). Ca and Mg are divalent cations present in igneous rock minerals as silicates (e.g., feldspar, olivine), in sedimentary rock as carbonates (e.g., calcite, dolomite), or in sandstone and detrital rock as cement between particles. Weathering of these rock types results in mineral dissolution and solubilization of Ca, Mg, silica (Si), and carbonate species (Hem, 1992). C
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