Alternative processes for hexavalent chromium (Cr(VI)) removal from drinking water continue to be of interest for utilities despite the existence of several established technologies. Stannous chloride (SnCl 2 ) can reduce Cr(VI) to trivalent chromium, but research has been limited, especially related to the filterability of total chromium (Cr(T)) following reduction. At the pilot scale, SnCl 2 was tested over a range of doses in three ground-waters with naturally occurring Cr(VI) concentrations ranging from 0.020 to 0.090 mg/L. Stannous chloride was found to be effective as a reductant at doses <2 mg/L and contact times <5 min. A tin-to-chromium molar dose ratio of 4 was sufficient for reducing Cr(VI) to below 0.010 mg/L. Cartridge filters were unable to practically remove Cr(T) following reduction, but a standard-design sand filter was able to remove Cr(T) to <0.010 mg/L.
For many water systems,
ion exchange is the best available technology
to meet nitrate, chromium, arsenic, and other inorganic removal
requirements. The principal economic, environmental, and operational
considerations relate to waste disposal. The salt saturator at
ion exchange (IX) installations provides an untapped chemical energy
source to reduce the IX waste brine volume that can be leveraged by
integrating forward osmosis (FO) between the saturator and waste brine
tank. The waste brine (FO feed solution) is concentrated by the salt
saturator solution (FO draw solution) as water permeates across the
FO membrane from the waste to the saturator. This study demonstrates
waste reductions of 85% for chromium and 65% for nitrate IX waste
brines. The augmented draw solution produced can be directly used
in a subsequent IX regeneration, and waste solids can be recovered
through precipitation. Due to the periodic generation of IX waste,
high fluxes and single-pass efficiency are not required from the FO
process. With the incorporation of FO into the IX process, significant
operational flexibility to optimize the recovery of water from the
waste brine exists, thereby decreasing the primary cost of IX operation.
Managing waste brine from strong base anion exchange processes used for hexavalent chromium removal is an important operational, environmental, and economic consideration. This study investigates the use of nanofiltration to recover excess regenerant salt and reduce the waste volume using brine collected from full‐ and pilot‐scale installations. Using a 2 N sodium chloride regeneration solution, divalent anions (i.e., sulfate and chromate) exhibited high rejections (>0.97), and monovalent anions (i.e., chloride and nitrate) exhibited low to negative rejections (−0.2 to 0.05), allowing preferential passage of excess regenerant salt. A batch concentration model was developed for a case study. Waste can be concentrated to 0.6 bed volume and a significant fraction of the regenerant salt can be recovered. This process would require about 20 m2 of membrane area per 1,000 L of resin to treat waste in 8 h, which could be implemented in a mobile treatment unit serving multiple decentralized systems.
Irrigation accounts
for 42% of the total freshwater withdrawals
in the United States. Climate change, the pressure of a growing population,
degrading water quality, and increased competition from other sectors
could constrain continuous supply to meet future agricultural water
demand. This study presents an evaluation framework to assess the
potential reuse of agricultural drainage water for crop irrigation.
Using a regional approach, we review the current state of agricultural
drainage treatment and reuse and the institutional, economic, and
other barriers that can influence the reuse decision. In the 31 eastern
states, agricultural drainage contains valuable nutrients that can
be reused for irrigation with minimal treatment, while the 17 western
states struggle with large volumes of saline drainage that can contain
constituents of concern (e.g., selenium), preventing reuse without
treatment. Using a new decision-support tool called WaterTAP3, a potential
treatment train for saline agricultural drainage was analyzed to identify
treatment challenges, research needs, and the potential implementation
at a larger scale. As demonstrated by our case study, desalination
of agricultural drainage is costly and energy intensive and will require
sizable investments to fully develop and optimize technologies as
well as manage the generated waste and brine.
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