Abstract:
The removal of micropollutants (MPs) from secondary municipal wastewater by
an advanced oxidation process (AOP) based on UV irradiation combined with
hydrogen peroxide (UV/H2O2) has been assessed through pilot-scale
experiments incorporating microfiltration (MF) and reverse osmosis (RO). Tests
employed low concentrations of a range of emerging contaminants of concern,
and the water quality varied by blending of waters from different sources.
Under optimum H2O2 and lamp power conditions, the process achieved >99%
removal of N-nitrosodimethylamine (NDMA) and endocrine disrupting
compounds (EDCs) from all waters. Pesticide removal, in particular
metaldehyde, atrazine and 2, 4 5-T, was dependent on water transmittance
(UVT), and levels of Total Organic Carbon (TOC) and other hydroxyl radical
(HO.) scavengers. Chloroform, a trihalomethane (THM), was not readily
degraded (<10% removal in either stream), as was TOC removal.
Further analysis of metaldehyde removal identified UVT, reaction time, and
H2O2 dose to be influential parameters in determining degradation as a function
of UV dose. In comparison, the impact of H2O2 dose and UVT was negligible on
NDMA degradation; removal increased from 89 to >98% on increasing the UV
dose from 200 to 680 mJ cm-2 from the MF permeate. Nitrite by-products were
observed at elevated levels, promoted by low pH and high UV doses.
An operational cost assessment revealed energy consumption to account for
65% with lamp replacement contributing 25%. A comparison of three unit
process sequences, based on MF, RO, AOP and activated carbon (AC),
revealed MF-RO-AOP to be the most cost effective provided management of
the RO concentrate stream incurs no significant cost. Results demonstrated
AOPs to satisfactorily reduce levels of the more challenging recalcitrant MPs to
meet stringent water quality standards for wastewater reuse, but that practical
limitations exist and the cost penalty is significant.
