Salt used to make brines for regeneration of ion exchange (IX) resins is the dominant
economic and environmental liability of IX treatment systems for nitrate-contaminated
drinking water sources. To reduce salt usage, the applicability and environmental benefits
of using a catalytic reduction technology to treat nitrate in spent IX brines and enable
their reuse for IX resin regeneration were evaluated. Hybrid IX/catalyst systems were
designed and life cycle assessment of process consumables are used to set performance
targets for the catalyst reactor. Nitrate reduction was measured in a typical spent brine
(i.e., 5000 mg/L NO3
and 70,000 mg/L NaCl) using bimetallic PdeIn hydrogenation catalysts
with variable Pd (0.2e2.5 wt%) and In (0.0125e0.25 wt%) loadings on pelletized activated
carbon support (PdeIn/C). The highest activity of 50 mgNO3
/(min gPd) was obtained with
a 0.5 wt%Pde0.1 wt%In/C catalyst. Catalyst longevity was demonstrated by observing no
decrease in catalyst activity over more than 60 days in a packed-bed reactor. Based on
catalyst activity measured in batch and packed-bed reactors, environmental impacts of
hybrid IX/catalyst systems were evaluated for both sequencing-batch and continuous-flow
packed-bed reactor designs and environmental impacts of the sequencing-batch hybrid
system were found to be 38e81% of those of conventional IX. Major environmental impact
contributors other than salt consumption include Pd metal, hydrogen (electron donor), and
carbon dioxide (pH buffer). Sensitivity of environmental impacts of the sequencing-batch
hybrid reactor system to sulfate and bicarbonate anions indicate the hybrid system is
more sustainable than conventional IX when influent water contains