7
% (Table 3) of the influent to the activated sludge tank, they
Aguilar of the City of San Francisco Public Utilities Com-
mission, for help with obtaining samples. This research was
supported by grants from the WateReuse Foundation and
the National Water Research Institute.
accounted for most of the total mass flow of precursors to
this unit. At the Central Contra Costa Sanitation District
WWTP, another plant where DMA-based polymers were used,
the centrate return flow contained 250 000 ng/ L ((52 000
ng/ L) of NDMA precursors. In contrast, a sample of the filtrate
of the filter press at the Dublin/ San Ramon Services District
WWTP, which uses an ADAMQUAT polymer, contained only
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(
1) United States Environmental Protection Agency (EPA). Inte-
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1
200 ng/ L ((280 ng/ L) of NDMA precursors.
The use of DMA-based polyacrylamides for solids treat-
ment can contribute a significant fraction of the overall
NDMA precursors in secondary effluent even at the relatively
low TSS concentrations typically observed (i.e., <30 mg/ L).
For example, 20 mg/ L of particles from the San Francisco
Southeast WWTP would contribute 1900 ng of NDMA
precursors/ L, which is approximately equal to the highest
concentration of dissolved NDMA precursors in any of the
secondary effluent samples. Other researchers also have
reported a relationship between polymer use and NDMA
formation. For example, use of a similar DMA-based polymer
to improve wastewater filtration enhanced NDMA formation
upon return of the backwash water to the secondary
treatment unit (52). Although chloramination of ADAMQUAT
polymers also resulted in NDMA formation (Table 1), the
NDMA concentrations formed were orders of magnitude
lower than those formed with DMA-based polyacrylamides.
Strategies for Preventing NDMA Form ation During
Wastewater Chlorination. Several practical strategies may
be used to minimize NDMA formation in conventional and
advanced wastewater treatment plants. The most appropriate
strategy will depend on plant configuration and planned uses
of the water.
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Application of a disinfectant other than chlorine will
minimize NDMA formation. If chlorine use is necessary,
NDMA formation also can be minimized by nitrification.
When hypochlorite was applied to aliquots of the same
samples from the Truckee Meadows Water Reclamation
Facility to which chloramine had been applied under similar
conditions (solid bars in Figure 4), NDMA formation was
(12) Jafvert, C. T.; Valentine, R. L. Environ. Sci. Technol. 1992, 26,
577-586.
(
(
(
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similar to that observed in the secondary effluent sample
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+
(
[NH
4
] ) 1.4 mM) dosed with monochloramine but de-
creased by over 95% in the nitrified and denitrified samples
+
(
[NH
4
] < 4 µM generally in plant effluent (53)). Mono-
chloramine likely was the predominant form of chlorine in
the secondary effluent sample but not in the nitrified and
denitrified samples. The absence of ammonia precludes
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1
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other, less expensive treatment methods are not viable.
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Acknowledgments
We thank the staff members at the wastewater treatment
plants at which sampling occurred, particularly Mrs. Azalea
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