Reflecting
the desire for equitable control of disinfection byproducts (DBPs) at the tap, the Stage 2 DBP rule brings a new focus to
differentiating among distribution system (DS) monitoring locations with respect to DBP
occurrence levels. However, studies showing that DBP concentrations can vary significantly
within a 24-hour period at both point of entry (POE) and DS sites raises important questions
related to sampling design and data interpretation. These findings suggest that observed
differences in DBP concentrations among DS locations could derive from the timing of sample
collection rather than from location attributes. The research presented here involved monitoring short-term
fluctuations in DBP concentrations at the POE of one Philadelphia Water Department
(PWD) treatment plant, and examined corresponding water quality and operational data in detail
to explain observed variations. This work was part of a broader study designed to determine the
prevalence and magnitude of diurnal variations in finished water DBPs from different
geographical regions in the U.S., to identify their causes, to evaluate their significance with
respect to monitoring programs, and to provide a foundation for further study of temporal
variation in DBPs at DS sites. The project involved 2 sampling rounds to capture seasonal water
quality variations. Each round included Trihalomethane (THM) and Haloacetic Acid (HAA) sample collection at the POE and at
selected locations through the treatment train every three hours, continuously over a five-day
period. Temporal variability patterns observed for THMs differed markedly from those for
HAAs. During both sampling events, THM4 concentrations transitioned dramatically between
low and high values in a seemingly random and binary pattern. These changes did not
correspond to any known shifts in source or treated water quality or in applied chemical doses. In
contrast, HAA6 levels exhibited only gradual changes in concentration over both sampling
periods and these changes corresponded with observed source water quality trends. Subsequent
investigation suggested post-filter pH adjustment practices as the cause of the THM fluctuations.
Follow-up controlled laboratory experiments confirmed this hypothesis. Operational changes
now being implemented as a consequence of the study findings are expected to result in
substantially lower and more consistent average finished water THM4 concentrations. This high-resolution
examination of DBP and treatment data at PWD yielded critical insights about
operational impacts on DBP formation that were not previously apparent. Includes tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 1.1 MB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 15 |
| Published : | 11/01/2008 |
