Strategies for Using Tank Aeration for Stage 2 DBP Compliance

Strategies for Using Tank Aeration for Stage 2 DBP Compliance
David S. Briley, PE, Hazen and Sawyer
Tiffanie Hawley, Cape Fear Public Utility Authority
Allison Reinert, Hazen and Sawyer
Erik Rosenfeldt, PhD, PE, Hazen and Sawyer
ABSTRACT
Tank Aeration is gaining interest as lower cost alternative for Stage 2 DBP Rule compliance as compared
to other compliance strategies. This method utilizes gas transfer principals to physically strip volatile THMs
from finished drinking water and achieve reductions of 25 – 45 percent. Research as well as practical
applications have shown that aeration can be effective even for the brominated THM species. This is of
critical importance for water systems facing shifts to brominated species, and a resulting increase in mass
concentrations of TTHMs, due to the presence of bromide into the water supply.
But, tank aeration is certainly not a panacea. One critical consideration for implementation of aeration is
where to locate aeration treatment; at the water treatment plant or in tanks in the distribution system.
Aeration at the water treatment plant provides for TTHM reductions throughout the distribution system.
However, the size of the aeration system is larger, which affects capital and operating costs. Tank aeration
systems in the distribution system can be more cost-effective since the systems are treating a smaller flow.
However, aeration in a distribution system tank does not necessarily correspond with significant THM
reductions at all Stage 2 monitoring locations. By carefully considering system hydraulics aided by a
calibrated distribution system model, along with firm understanding of THM formation kinetics and the
benefits and limitations of in-tank aeration for THM stripping, a methodology for evaluating the true impact
of these technologies as Stage 2 DBP Rule compliance strategies has been developed.
This paper will present case studies evaluating THM removal for systems experiencing high levels of
predominantly brominated THMs. The approach included:
1. Development of an in-tank aeration performance model to assess design variables such as THM
speciation, formation kinetics, inlet THM levels, nozzle size, pumping rates, and water flow
2. Review of historical THM data to establish reduction goals.
3. Assessment of THM formation kinetics in the distribution system.
4. Evaluation of tank influence areas using a calibrated distribution system hydraulic model.
Ultimately, the work has resulted in development of effective strategies for Stage 2 DBPR compliance,
inclusive of and reliant upon the inclusion of in-tank aeration processes.
INTRODUCTION
Disinfection byproducts (DBPs) are formed when chlorine used for disinfection reacts with organic materials
naturally present in surface waters. The United States EPA has regulated several of these chlorination
disinfection byproducts for years, primarily Trihalomethanes (THMs) and Haloacetic acids (HAAs). DBP
reduction has conventionally been approached by limiting formation during treatment by delaying chlorine
addition, improving organic content removal efficiencies or adding treatment at the plant. The onset of the
Stage 2 Disinfectants / Disinfectant Byproducts Rule (D/DBPR) has found some utilities up against the
maximum contaminant limits for a variety of reasons. When options for preventing THM formation at the
plant or in the distribution system have been either exhausted or become prohibitively expensive, utilities
are turning to water quality improvements by removing THMs from the water at the treatment plan or in
the distribution system. Utilities have recently shown interest in evaluating THM removal by using tank
mixers and aerators in finished water storage tanks.
Removal of THMs after formation in water storage tanks can be an attractive solution that targets specific
locations with DBP compliance concerns. But is it too good to be true? It's not pure propaganda: adding
aeration systems to tanks can be effective at removing THMs. It may be a cost-effective step for compliance.
But, the systems are also certainly not a panacea. Before committing financial resources and introducing a
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
change to the system’s water quality, a holistic approach to water quality and distribution system operations
is required.
This article provides a case study of the Cape Fear Public Utility Authority (CFPUA) in Wilmington, NC.
CFPUA has observed increasing TTHMs in recent years along with higher levels of bromide in their source
water. The result was a shift of THM species from largely non-brominated species to mainly brominated
THMs, depending upon the bromide concentrations in the raw water.
CFPUA embarked on an evaluation of alternatives for DBP control to comply with the Stage 2 D/DBP Rule.
In particular, an evaluation of the effectiveness of aeration systems for THM control, even for brominated
THMs, was conducted. Hazen and Sawyer assisted CFPUA by using a holistic approach to water quality
and distribution system operations. First, potential THM reductions from aerating finished water in the
clearwell at the water treatment plant was assessed for meeting THM compliance goals. Also, a distribution
system hydraulic model was used to trace water originating from elevated storage tanks to provide
information about the expected impact of tank aeration techniques on various portions of the distribution
system and nearby compliance sites.
Aeration
Chemical contaminant removal via aeration has been used for decades. Packed air towers and draft
aeration at many water and waste water treatment plants strip contaminants in the liquid phase into the air
phase. Research and other case studies indicate that the relatively volatile nature of THMs make aeration
an attractive option for removing THMs but that HAA compounds are not amenable to removal by aeration.
The effectiveness of aeration processes are fundamentally defined by Henry’s Law which describes the
equilibrium partitioning of a particular chemical between air and water. Henry’s law constants are
compound specific and increase for compounds which are more easily removed from water through
aeration. Table 1 displays Henry’s Law constants for the four THMs regulated under the Stage 2 DBP Rule.
Chloroform displays the highest constant (and is therefore the most readily stripped from water). Henry’s
Law constant decreases with increased bromide substitution. This means that it can be more difficult to
remove brominated DBPs by aeration processes than chloroform.
Table 1: Henry’s Law Constants for Regulated THMs
Compound
Henry’s
Law
Constant at 25oC
Chloroform
0.127
Bromodichloromethane
0.076
Dibromchloromethane
0.035
Bromoform
0.0175
However, recent experience has shown that with spray aeration, high removal efficiencies can be achieved
for brominated THMs. This mainly due to the fact that spray aeration relies on small water droplets
surrounded by air, and therefore, achieves very high air:water ratios as compared to diffused aeration. In
spray aeration, water is pumped through nozzles in which small water droplets pass through the air (or
headspace in the tank) before landing back in the water. To realize the benefits of spray aeration throughout
the full volume of the tank, however, the top layer must be fully blended to provide consistent water quality.
Complete mixing is coupled with spray aeration to realize the water quality benefits of treatment throughout
the entire volume of the tank.
Bromide Changes Things
Small changes in the source water characteristics can impact THM speciation because the chloride ion is
much more abundant in natural water sources than bromide. Therefore, given a constant chloride
concentration, very small changes in bromide concentration have a significant impact on the Cl/Br mass
ratio. When water enters the treatment plant with elevated bromide concentrations, there are very few viable
treatment options for bromide removal so the elevated concentrations are maintained through the water
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
treatment plant, and remain in the treated drinking water. Upon chlorination, bromide is rapidly incorporated
into the THMs, leading to a larger proportion of brominated DBPs.
Total Trihalomethanes (TTHM) are the regulated parameter in the Stage 2 D/DBP Rule, but the reported
mass concentrations are the sum of four different THMs: trichloromethane (chloroform, CHCl3),
dichlorobromomethane (CHCl2Br), chlorodibromomethane (CHClBr2) and tribromomethane (bromoform,
CHBr3). The impacts of bromide incorporation into THMs have practical implications on DBP reporting
because bromide is a heavier atom than chlorine, and therefore results in heavier THMs. Since the MCL
is a mass concentration, utilities with greater bromide incorporation have a greater chance to exceed the
Stage 2 THM MCL.
When considering installation of an aeration system into a storage tank to remove THMs, utilities
experiencing high bromide incorporation must consider that the brominated THM species are not as
amenable to removal by aeration as chloroform. Therefore utilities with high bromide THM speciation do
not realize benefits from the diffused bubble aeration. But because the volume of air surrounding the water
droplets is so much larger than the water droplets themselves, spray aeration overcomes the air volume
constraint of diffused bubble aeration and can achieve higher THM removal rates more efficiently.
METHODOLOGY
Define THM Reduction Goals
Historical DBP records were analyzed to assess the required percent THM reductions at compliance sites.
The analysis used THM data collected by CFPUA in preparation for Stage 2 D/DBP Rule monitoring. As
shown in Table 2, most sample sites would have been below the LRAA MCL, with exception of Sample site
B07. Also, several locations have historically shown levels greater than the EPA recommended compliance
design level of 64 ppb (80 percent of MCL of 80 ppb). Historical data indicates that up to 26 percent
reductions in THMs are needed to achieve the design goal.
Table 2: THM Removal Requirements at Stage 2 DBP Monitoring Sites
THM Reduction To
Max THM LRAA
Location
Achieve Goal of 64 ppb
(ppb)
(%)
B01
62
0%
B02
72
11%
B03
75
14%
B04
77
17%
B05
69
7%
B06
74
14%
B07
86
26%
B08
69
8%
Modeling Aeration Performance
Next, a model of in-tank spray aeration systems was developed by Robin Collins, PhD from the University
of New Hampshire and used to assess the impact of installing aeration in a clearwell at the water
treatment plant. The model was also used to assess variation in THM reduction as a function of flow,
temperature, THM formation kinetics, and bromide concentration. Based on the model, upon aeration in
the 4 MG clearwell, THMs are expected to be reduced by approximately 25 percent with 60 percent of
flow recirculated throughout the clearwell aerators. After leaving the clearwell, however, TTHMs will
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
continue to form in the distribution system. An assumption was made that THM formation in the
distribution system would occur at the same rate. Theoretically, THM formation potential has been
reduced but practically, this means that the magnitude of THM reduction in the tank will translate through
the system. The estimated TTHMs at the Stage 2 DBP monitoring sites after aeration in the clearwell are
shown in Figure 1. This analysis demonstrated that clearwell aeration will help CFPUA achieve Stage 2
DBP compliance. However, several monitoring locations would still have exceeded the 64 ppb goal. One
site in particular exceeded the goal and is located in an area with the highest water age. Therefore,
aeration of distribution system tanks was assessed to confirm whether further THM reductions could be
achieved
Figure 1. Estimated TTHM LRAA Values
80 ppb LRAA MCL
64 ppb Goal
RESULTS
Relationship between Storage Tank and Compliance Site
To establish whether aeration in elevated storage tanks can improve THM reduction in a portion of the
distribution system, the distribution system hydraulic and flow patterns must first be understood to know
where this water goes after leaving the tank. Even if complete THM removal were possible in the tank,
DBP compliance may not be achieved if the treated water is conveyed to the desired areas. Public health
is only protected from improved tank water quality if the water from the tank reaches the customers.
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
Tank operations affect water quality in distribution systems, but the extent depends on many other things
related to the distribution system such as high service pump controls, booster pump controls, valve
operations at elevated tanks, and distribution system hydraulic conditions. To examine the influence of
storage tanks in the distribution system’s pressure districts as sources, a source tracing analysis was
conducted using the modeling software. The storage tanks were defined as source nodes and the software
calculates and reports the percent of water reaching any location in the distribution system that had its
origin at the selected source node over time. Percentages for this analysis are shown as an average over
24 hours to account for diurnal variability in operations and demand.
To estimate impacts of these findings on the expected THM levels at Stage 2 sites influenced by the storage
tanks, an analysis of the impact of the THM reduction system on the associated Stage 2 compliance sites
was performed. Figure 2 shows the source trace analysis for one of CFPUA’s elevated storage tank. This
analysis shows that Site B07, which was predicted to have TTHMs above the 64 ppb goal, receives
approximately 30 percent of its water from the closest tank over a 24 hour period.
Figure 2: Source Trace Analysis for Storage Tank and Impacts at DBP Compliance Sites
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
Based on the aeration performance modeling, it was determined that a 40 percent TTHM reduction could
be achieved in the elevated tanks. To estimate the expected TTHM reduction at Stage 2 DBP monitoring
sites, the tank influence on that area was also considered as shown in Table 3.
Table 3: Elevated Tank Aeration Impacts on Selected Stage 2 DBP Sites
Location of
Percent THM
Average Impact on the
Modeled Reduction
Aeration System
Reduction at the Tank
Stage 2 Monitoring
at Site with Tank
(Elevated Tank)
(Assumed)
Location
Aeration
(%)
(%)
17th Street
Dawson Street
40%
40%
26% (BO7)
18% (BO4)
10% (BO7)
7% (BO4)
The impact of elevated tank aeration in addition to clearwell aeration is shown in Table 4. The Stage 2
DBP sites with the highest TTHM levels (B04 and B07) are compared for several alternatives. Without
addition of aeration or any other process improvements, the maximum LRAA at those sires would have
been 77 ppb at B04 and 86 ppb at B07. A shown in Figure 2, clearwell aeration is expected to reduce
TTHMs at these sites below 80 ppb MCL but TTHM levels would still exceed the goal of 64 ppb. Table 4
illustrates that aeration in the clearwell at the WTP with aeration of elevated tanks can further reduce
TTHMs in these areas with the highest water age. For Site B04, TTHMs are estimated below the goal of
64 ppb, but TTHMs are still predicted to exceed the goal at Site B07. Although the TTHM goal is not
achieved, it is expected that the aeration systems will achieve Stage 2 D/DBP Rule compliance.
Table 4: Improvement of THM LRAA with Clearwell and Tank Aeration
Max THM LRAA (ppb)
Stage 2 Site
Predicted Without
Aeration
Elevated Tank
Aeration Only of 17th
St. and Dawson St.
Clearwell and
Elevated Tank
Aeration of 17th St.
and Dawson St.
B04
B07
77
86
72
77
62
68
CONCLUSIONS
To consider aeration as a DBP compliance strategy, a holistic approach to distribution system water quality
is needed. Mixing and aeration systems may be effective at removing TTHM levels in storage tanks, and
they may be a cost-effective step for compliance. But, they are certainly not a panacea.
Source water quality, tank turn-over and other operations certainly affect water quality and the effectiveness
of aeration systems. Aeration of clearwells at the water treatment facility are preferred for ensuring a THM
reduction throughout the distribution system. However, clearwell aeration alone may not achieve DBP
compliance depending upon source water quality, water age, and many other factors.
Aeration in distribution systems requires a thorough understanding of the hydraulic conditions and
operations of the distribution system. DBP compliance may not be achieved if the reduced TTHMs do not
reach compliance sites. It is critical to understand hydraulic patterns to assess how effective aeration of
storage tanks in the distribution system can be effective for Stage 2 DBP compliance. Likewise, public
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance
health is not protected from improved tank water quality if the water from the tanks does not reach the
customers. Distribution systems can be very complicated hydraulically. Each system is highly variable and
has its own limitations. Examining and changing distribution system operations can enhance THM removal
systems
Based on this evaluation, spray aeration of the clearwell as well as two elevated storage tanks appears to
be an effective strategy for Stage 2 DBP compliance and controlling TTHM levels (on an LRAA basis)
below the 80 ppb MCL for CFPUA.
Strategies for Using Tank Aeration
for Stage 2 DBP Compliance

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