Dear Gentlemen:
We are pleased to present herewith our preliminary
findings on the effects of microbial additives on sulfide
generation, the resulting release of hydrogen sulfide
gasses and the corrosive effects of those gasses within
the South Suburban Sewer Transportation Network (STN)
and South Wastewater Treatment Facility (STP) owned
and operated by the City of Baton Rouge, Parish of East
Baton Rouge.
While the study period, May through October 1998, has
not yet concluded, the data accumulated to date are,
in our opinion, sufficient in quantity and trend to
allow us to draw some preliminary conclusions with a
reasonable confidence that data assembled during the
remainder of the study period will further support the
preliminary findings.
As previously stated, the purpose of the study is to
determine the effects of a microbial product feed on
corrosion within the South STN and at the South Wastewater
Treatment Facility. It is an accepted fact that hydrogen
sulfide gas, generated within the wastewater stream
by sulfur reducing bacteria, is highly corrosive to
most metals. What is less widely known is that the bacterial
reduction of elemental sulfur to the sulfide ion and
hydrogen sulfide, and the subsequent conversion of these
products by bacteria living within the “slime”
layer on surfaces above the water line in sewer pipes
and headworks structures, produces sulfuric acid which
reacts with (is corrosive to) the cement used in the
construction of many components of a sewer system. Over
time this “corrosion” will eventually reduce
the structure’s wall thickness to the point where
it can no longer support the loads imposed on it. In
the case of concrete sewer pipes the interior wall above
the flow line will corrode and the pipe will collapse.
This same corrosion occurs wherever aged wastewater
accumulates in structures vented to atmosphere, such
as pump station wet wells and treatment plant headworks.
At the pumping stations within the south STN the evidence
of sulfide corrosion is minimal because the wastewater
is relatively fresh and the newer wet wells have been
provided with coatings to protect against attack. However,
even at these locations, corrosion can and will take
place albeit at a much slower rate and, should the protective
coating be damaged or incomplete in its application,
corrosion will concentrate at that point and spread
behind the coating with the same end result.
The nature of the STN system particularly aggravates
the problem of sulfide corrosion once the wastewater
is discharged to atmosphere at the treatment plants.
The age of the sewerage being transported, combined
with the anaerobic conditions present within the STN,
promotes the generation of sulfides and the subsequent
reactions and byproducts previously mentioned. In the
case of the STN, however, the final step leading to
actual corrosion—generation of sulfuric acid—is
not allowed to take place because the pipe is full at
all times. The generation of sulfide by the biomass
in the wastewater stream continues, however, resulting
in higher potential for corrosion at the discharge point,
as evidenced in the headworks at both the North and
South treatment plants. Discussions with City/Parish
personnel confirm that no known corrosion problems exist
within the piping networks comprising the North and
South STNs. In one instance a portion of the prestressed
concrete cylinder pipe near the North Treatment Plant
had to be replaced as the result of damage from adjacent
but unrelated construction. After removal of the damaged
section of pipe, City/Parish personnel discovered that
the manufacturer’s stenciling inside the pipe
was still evident after a number of years in service.
The one instance of force main corrosion that we are
aware of occurred in the collection system tributary
to the Central Wastewater Treatment Plant. The force
main exhibiting the effects of sulfide corrosion had
a continuous downward slope for some distance prior
to a free discharge. This downward slope and free discharge
resulted in a hydraulic gradient which was below the
top of the pipe which allowed hydrogen sulfide, off-gassed
from the turbulence at the discharge, to draft back
into the pipe, resulting in sulfide corrosion and the
need to replace a portion of the force main. The very
nature of the STNs, with their elevated discharges,
insures that the hydraulic grades remain above the tops
of the pipes, thus preventing entrance of corrosion
precursors. According to data collected by Global Pollution
Control, Inc., hydrogen sulfide (H2S) levels have, with
the exception of a few data points, remained constant
in the influent box at the South Treatment Plant headworks.
H2S levels in the exhaust and inside screening room
of the headworks show a decline based on linear regression
trend lines of the data available.
Data obtained from the Department of Public Works (DPW),
however, show H2S levels for the months of May, June
and July to be significantly higher in the headworks
influent box that the values for the same time period
in 1997. Furthermore, the values of H2S from DPW are
significantly higher than those from BIO3 by a factor
of 10 in some cases. This difference in these reading
may be attributable to 1.) hydrogen peroxide was being
fed into the STN immediately upstream of the STP in
1997, and 2.) the location within the influent box at
which the samples are taken may be different. Dissolved
sulfide levels measured in the headworks and at two
microbial feed points at the extremities of the South
STN have, according to BIO3 test results, declined.
The decline is most pronounced at the headworks, where
dissolved sulfide concentrations have declined from
an initial high of > 11.25 mg/l to approximately
2 mg/l and are expected to remain level near the 2 mg/l
figure. This value is consistent with a sulfide generation
model developed for the South STN.
An interesting phenomena was discovered in running
the sulfide generation prediction model. At 100% design
flow in the South STN the theoretical increase in sulfides
rose from an initial value of 0.1 mg/l ton only 2.4
mg/l. However, for the months of may through August,
the average flowrate in the South STN was only 10% of
the designed flowrate for which the sulfide generation
model shows a theoretica dissolved sulfides increase
from the initial value of 0.1 ppm to a value of 23.8
ppm.
Actual dissolved sulfide measurements, however, show
measured sulfide levels declining to approximately 2
ppm, thus indicating that the bacteria being fed in
the South STN are having a direct effect on the generation
of sulfides in the force main system.
The significant amount of data, some of which are attached
hereto, accumulated to date leads us to the preliminary
findings:
- Microbial feed in the South STN will reduce corrosion
at the South Treatment Plant headworks by 80-90% as
compared to an untreated waste stream.
- The reduced corrosion rates resulting from the microbial
feed will increase the concrete structures’
useful life by a factor of 4.33-5.77 times—possibly
as much as 11.88 times if Dissolved Sulfide levels
stabilize at 2.0mg/l.
- Corrosion in pump station wet wells at the extremities
of the transportation network, while minimal to begin
with, can be reduced by a factor of 10, thus increating
the structural life of the wet well by the same factor.
- Provided that the Air Release Values on the force
main system are maintained and no pockets of air are
allowed to accumulate, there should be little or no
corrosion to the interior of the force main piping.
As previously stated though the study period has not
yet run the full course the data and trends indicate
the microbial feed is having the beneficial effect of
reducing the generation of sulfides with a corresponding
reduction in predicted corrosion rates on concrete pump
stations and headworks structures. Flow rates into the
STP have been relatively consistent and low, 10% of
design flow, which provides a worst case scenario for
conducting the study. The remainder of the study period
will occur during the local “dry” season
so the impact of higher flows in the study results should
be minimal.
We would like to take this opportunity to thank the
Department of Public Works, Mitch O’Brien, Kent
Mudd and Dr. Yu for their cooperation and assistance
in making valuable information and data available for
use in preparation of this report.
It has been a pleasure working with BIO3 and In-Pipe
System Group and we look forward to the successful completion
of the study.
Should you have any questions, please do not hesitate
to call.
Very Truly Yours,
Chenevert-Songy-Rodi-Soderberg
J. Keith Shackelfored, P.E.
6767 Perkins Road, Suite 200
Baton Rouge, LA 70808
Phone (504) 769-0546
Fax (504) 767-0060
http://www.csrsonline.com/
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