Three
different ultraviolet-advanced oxidation processes (UV-AOPs) (UV/hydrogen
peroxide, UV/free chlorine, and UV/supplemental monochloramine) used
in full advanced treatment (FAT) for potable reuse were evaluated
at pilot scale using reverse osmosis (RO) permeate from a FAT facility.
Oxidant concentrations were varied, and each AOP was assessed for
its ability to remove N-nitrosodimethylamine (NDMA)
and NDMA precursors. For all AOPs, NDMA was removed well (to near
1.2 ng/L). However, UV/free chlorine exhibited less apparent NDMA
destruction, and this level of destruction decreased as the free chlorine
concentration increased. Removal of NDMA precursors was variable with
no oxidant showing superior performance, ranging widely from 1% to
84% removal. The level of NDMA precursors increased in only two of
38 UV-AOP pilot tests, suggesting that formation of NDMA precursors
by UV-AOP is limited. Adjustment of the UV-AOP pH was evaluated as
a strategy for controlling NDMA precursors. Increasing the RO permeate
pH prior to UV-AOP from 5.5 (ambient pH) to ∼7.0 using NaOH,
to decrease the dichloramine concentration, decreased the level of
destruction of NDMA precursors (i.e., increased the level of NDMA
formation) in the UV/HOCl AOP (likely due to an increased level of
radical scavenging by OCl–). Increasing the pH to
∼8.5 via the addition of lime decrease the level of NDMA destruction
in the UV/H2O2 AOP due to the lower NDMA quantum
yield at higher pH.
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