decades, these metals have accumulated
on the pipes within the PCMC distribution system. Two release events were
documented in 2007 and 2010, resulting
in temporary excursions of As, Mn, Tl, and
mercury in certain areas
of the distribution system.
PCMC has maintained
comprehensive monitoring and flushing programs
to address these issues, but
more research into accumulation and release risk
factors and TIC behavior
was needed, in addition to
evaluation of more aggressive main-cleaning strategies.
Objectives and Approach
THE GOAL OF this project was to enhance
industry understanding of chemical,
physical, and biological mechanisms of
accumulation and release. More specifically, the project team sought to develop
guidance to help utilities assess their vulnerability to TIC accumulation and release.
Based on this vulnerability assessment, the
report also helps utilities select and implement prevention and mitigation strategies.
The project approach included an
intensive year-long monitoring program;
collection and analysis of pipe and flush
deposits; controlled cleaning trials comparing unidirectional flushing (UDF), ice
pigging, and swabbing, and assessing
associated costs; laboratory studies of
metals desorption and biofilm uptake/
release; and identification of key accumulation and release risk factors.
ANALYSIS OF 1,040 TIC samples showed
that despite PCMC’s known inventory
of legacy pipe deposits, the distribution
system is under good control. Only 22
releases (defined as ≥ 100% of background concentrations) were identified
during the year-long monitoring program.
This suggests that nearly 98% of results did
not indicate that a release had occurred.
Of the 22 TIC releases, 15 were particulate in nature, while seven were soluble.
Eleven of the particulate releases were
associated with Fe and/or Mn release,
indicating TIC co-occurrence with these
substrate metals. Of the
seven soluble releases, all
were associated with one
or more chemistry shifts.
Soluble thallium release
strongly related to a drop in
chlorine residual below 0.2
mg/L. Only three excursions
out of 1,040 samples were
measured in the distribution system.
Assessment of Cleaning Technique
TECHNIQUES were tested
at each of two sites, both
involving cement-lined pipe.
These techniques were UDF
(at sequentially applied
velocities of 3 and 6 ft/s), ice
pigging, and foam swabbing.
None of the techniques was
100% effective at remov-
ing accumulated deposits,
as confirmed by residual
deposit mass on pipe speci-
mens removed after each
cleaning test. Based on
the pipe area-normalized
particulate mass removed (mg/ft2), two-
pass swabbing was the most aggressive
technique. Ice pigging performed similarly
to single-pass swabbing, and both tech-
niques were significantly more aggressive
than UDF at 6 ft/s.
In terms of composition, legacy solids
removed by the least aggressive technique (UDF) were not a simple subset of
the overall accumulated mass. Rather, at
both sites, Fe and As were preferentially
removed by UDF relative to total particulate mass removed. This indicated that the
relatively small surficial mass capable of
being mobilized by UDF was concentrated
with Fe-As co-precipitates. More aggressive cleaning techniques were needed to
remove the deeper films with cohesive,
metals-rich solids (Figure 1).
Analysis of Main
WHEN COSTS ARE used as
a consideration for process
selection, only those techniques with comparable
relative to conditions and
cleaning goals should be
included. For example, ice
pigging and swabbing
with regard to removal
of cohesive deposits and
films on non-scale-forming
tested on cement-
lined pipe: UDF,
ice pigging, and
RISK FACTORS of
and release can
vary from system
to system, and
can even vary
within a system
for zones that are
fed by different
sources, have dif-
ferent pipe mate-
rials, and/or have
of source loading
or main cleaning