United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-083 May 1984
&ERA Project Summary
Water Filtration at Duluth
Frank X. Schleppenbach
A wide variety of studies were con-
ducted at a new water filtration plant in
Duluth, Minnesota, and in the distribu-
tion system. The plant was originally
constructed for removing asbestos fi-
bers from drinking water. Studies were
undertaken after the treatment plant
began operations. Work included moni-
toring the quality of raw, filtered, and
distribution system water; evaluating
monitoring methods; studying the
chemical conditioning of raw water be-
fore filtration; evaluating filtered water
quality during rate changes (filter
stress); evaluating bacteria removal by
filtration; studying sludge dewatering by
natural freezing; and modifying the cor-
rosive nature of the water caused by
alum coagulation.
Results showed that the filtration
plant can remove asbestiform fibers
from drinking water on a day-to-day
basis. Filtered water turbidity was also
consistently reduced below 0.1 ntu. The
various filter rate changes studied did
not have a continuous detrimental ef-
fect on the effluent quality at the filter
plant. Rusty water problems were alle-
viated by use of zinc orthophosphate
and sodium tripolyphosphate. Coliform
bacteria were removed by filtration in all
but one case, but numbers of bacteria
in the influent were low to begin with.
Alum sludge was effectively disposed of
by natural freezing in lagoons. Static
freezing in deep lagoons was not ef-
ficient.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
On October 16, 1975, the City of Duluth
received a demonstration grant to construct
a water filtration plant for removal of
asbestos fibers from drinking water. The
grant provided partial support for the plant
construction and full support for a wide
variety of studies to be carried out at the new
filtration plant and in the Duluth distribution
system. This report describes the studies that
were undertaken after the treatment plant
began operation. [Work funded at Duluth in-
cluded monitoring raw, filtered, and distribu-
tion system water quality; evaluating a
number of monitoring methods; studying
chemical conditioning of raw water before
filtration; evaluating filtered water quality
during rate changes (filter stress); evaluating
bacteria removal by filtration; studying
sludge dewatering by natural freezing; and
modifying the more corrosive nature of the
water caused by alum coagulation.
Water Quality Monitoring
The Lakewood Filtration Plant was built
to remove asbestos fibers from drinking
water. When the plant was built, the only
method for quantifying amphibole fibers in
water was the transmission electron micro-
scope (TEM) method. Arrangements were
made to perform TEM asbestos fiber count-
ing at Duluth because this reduced the time
needed for fiber counting; data could be ob-
tained from the electron microscope labo-
ratory as quickly as from the X-ray diffraction
laboratory. Nonetheless, the method was
slow and expensive, and other means of
monitoring water quality had to be evalu-
ated. Effective control of the filtration pro-
cess requires monitoring techniques with
rapid response time. Methods tested in-
cluded X-ray diffraction, particle counting,
and turbidimetry.
X-ray diffraction was used during the ini-
tial Duluth pilot plant research to measure
-------
amphibole mass and thus give an indication
of amphibole fiber removal. The method
does not give actual fiber counts, and
estimating fiber numbers on the basis of
mass is very uncertain, especially when fiber
diameters are variable. In addition, as the
filtered water quality improved, most am-
phibole mass determinations were below the
detectable limit (BDL). BDL values could not
be mathematically correlated to other water
quality data, so amphibole mass measure-
ment ceased.
Particle counting was done at Duluth with
an HIAC* particle counter and with a special-
ly fabricated optical detector that could dif-
ferentiate between rods (amphiboles) and
spheres (clays). This detector was developed
and tested by the University of Minnesota-
Duluth and described in "Optical Detection
of Fiber Particles in Water," (EPA-600/
2-79-127, U.S. Environmental Protection
Agency, Cincinnati, Ohio 45268).
The HIAC optical particle detector, pur-
chased by the City of Duluth Water Division,
has been used as an in-line detection device.
Typical particle counts in the 1- to GCKm size
range would be 1000 particles/L when the
plant is operating properly and in-line count-
ing is used. When grab samples are collected
and taken to the counter for analysis, counts
are higher. This result suggests that some
atmospheric or sample container contamina-
tion may occur in the grab sample pro-
cedure. Particle counting is a sensitive
monitoring method even though it is not
specific for asbestos fibers.
Turbidity has been monitored with a Hach
2100A bench model for grab samples and
Hach 1720 continuous flow turbidimeters for
on-line monitoring. An in-line Sigrist Photo-
meter (Model UP52B2) was also used to
measure turbidity of treated water from one
dual-media filter and one mixed-media filter.
The plant operation is very closely moni-
tored and carefully controlled at Duluth. As
a result, turbidity is routinely reduced from
the typical 1 nephefometric turbidity unit
(ntu) raw water value to 0.04 or 0.05 ntu in
filtered water. This high level of treatment
can be attained even when the raw water
temperature is near freezing, a situation that
usually makes effective coagulation more dif-
ficult.
Monitoring of amphibole asbestos fiber
counts was done using the TEM method.
Asbestos fibers in water are so small that
most of them can be seen only with the aid
of an electron microscope. The procedure
is slow and expensive (a few hundred dollars
per sample), so it is suitable only for confirm-
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
ing the efficacy of treatment as indicated by
cheaper, faster monitoring methods. More
than 500 asbestos samples were submitted
for analysis during this project. Some of
these were routine monitoring samples,
whereas others were collected during special '
studies.
Results of routine monitoring showed that
the filtration plant could dependably, day
after day, reduce the amphibole asbestos
fibers from counts on the order of 10 million
to 1 billion fibers/L in the raw water to levels
below 0.10 million fibers/L. Many filtered
water samples contained fewer than 0.01
million fibers/L.
Duluth has about 3 days of storage capac-
ity in its distribution system. As the treat-
ment plant was being completed, the Water
Division conducted a program of main
flushing and reservoir cleaning to remove the
debris that had accumulated during decades
of unfiltered water usage. After a year and
a half of filter plant operation, the amphibole
fiber counts in the distribution system were
similar to those in filtered water at the
Lakewood Filtration Plant.
Chemical Conditioning and
Corrosion Control
Chemical conditioning of water before
filtration is very important for all rapid-rate,
granular media filters, and it is essential for
asbestos fiber removal. Chemical condition-
ing was studied at Duluth using both the
10-gpm pilot plant and the full-scale plant.
Early in the operation of the Lakewood Filtra-
tion Plant, pilot- and later full-scale filtration
results showed that anionic polymers were
not needed for effective removal of am-
phibole fibers. Monitoring data showed no
chrysotile in raw water, so use of anionic
polymer was discontinued.
For effective filtration of Lake Superior
water, careful control of pH was shown to
be necessary. The proper operating pH
varies with temperature, ranging from pH 6.8
at 50° F to pH 7.35 at 32° F.
Other chemical conditioning research
showed that fluoride should not be added
before filtration when aluminum sulfate
(alum) is the coagulant. Fluoride can form
a soluble complex with aluminum, and pilot
plant results showed that when 1 mg/L of
fluoride was added before filtration, an extra
4 mg/L of alum (14 versus 10 mg/L) was
needed for effective filtration. This effect
should be considered when fluoridation is
practiced.
Pilot plant studies of the treatment of tur-
bid water (4 to 10 ntu) were conducted by
injecting a native clay slurry into the pilot
plant influent water. Alum doses of 15 to 25
mg/L were adequate to control turbidity and
amphibole fiber count in the filter effluent.
Soon after the filtration plant began op-
erating, the City of Duluth Water Division
began receiving complaints about rusty
water. Problems existed with galvanized iron
or steel plumbing. In response to this prob-
lem, zinc orthophosphate was fed to the
distribution system. A zinc concentration of
0.13 mg/L as Zn seems to be adequate for
control of corrosion in the distribution
system. To reduce problems associated with
corrosion of plumbing,-a sequestering agent
(sodium hexametaphosphate) was added at
a dosage of about 1 mg/L. This treatment
seems to have been satisfactory. After more
research, sodium tripolyphosphate was de-
termined to be a more economical sequester-
ing agent, and it is now being used.
Bacteria Removal by Filtration
For a 3-week period at Duluth, pre-
chlorination was discontinued and chlorine
was added to the filter effluent channel
downstream from the bacterial sampling
points. Bacterial counts were made on raw
and filtered water. Results showed that coli-
form bacteria were successfully removed in
all but one case. The numbers of bacteria
naturally present in the raw water were low,
however, so the filtration plant was not
challenged by large numbers of microor-
ganisms that might be present at other
locations.
Effects of Filter Rate Changes
on Water Quality
When granular-media filters are operated
at a constant rate, shear forces slowly in-
crease within the filter bed as coagulated par-
ticles are removed from the water and
gradually fill the pore spaces between the
media grains. The result of this gradual clog-
ging is seen as slowly increasing head loss.
When filtration rates change quickly, the
shear forces, or stresses, within the bed also
change quickly. Rapid rate changes can
cause deterioration of filtered water quality.
At Duluth, three rate change or stress con-
ditions and two filtration rates were in-
vestigated. The plant is operated at 20 or 30
mgd (3.2 or 4.5 gpm/ft2), so changes to or
from these rates were studied. The first
stress was caused by stopping plant opera-
tions during a filter run and then starting
plant operations again approximately 6 hr
later. This procedure is referred to as filter
restart. Since filters had not reached the ter-
minal headloss or suffered turbidity break-
through, the filters were restarted at the end
of this shutdown period in a partially used
condition. The filtration rate changed from
0 to 3.2 gpm/ft2 or from 0 to 4.9 gpm/ft2 at
the higher plant rates in approximately 15
min.
-------
The second stress was caused by remov-
ing one of the four filters for backwashing.
The filtration rate of the three remaining
filters was increased from 3.2 to 4.3 gpm/ft2
or from 4.9 to 6.5 gpm/ft2 within 60 sec. A
slight buffering of the increase occurs
because the water level over the filters in-
creases.
The third stress was caused by placing a
clean filter back into service. The filtering rate
increased from 0 to 3.2 gpm/ft2 and from 0
to 4.9 gpm/ft2. No predeposited material ex-
isted to be sloughed off during this filter rate
increase. During the setup or filter ripening
time of approximately 1 hr, the effluent tur-
bidity dropped from its startup high (less
than 0.10 ntu) to the normal operating rate
of less than 0.04 ntu.
Studies of the effect of filter stress or rate
change were made using both the dual-
media filter and one of the three mixed-
media filters installed at the Lakewood Filtra-
tion Plant. Water quality was assessed both
by asbestos fiber analysis and turbidity mea-
surements in a total of 44 filter runs.
Results generally showed that when the
coagulation process is effective, rate
changes of the magnitude experienced at
Duluth did not cause substantial deteriora-
tion of filtered water quality. In two tests in-
volving restart of dirty filters, dual-media and
mixed-media filters were started when they
were sufficiently clogged to have 9 ft of head
loss. In these instances, amphibole fiber
count in the filtered water ranged from 2 to
4 million fibers/L during the first 10 min of
the run. Fiber count dropped rapidly after
this time. Fiber counts in the range of 1 to
2 million fibers/L were observed twice with
dual-media filters when clean filters were
returned to production. Once during a back-
wash-induced rate increase, the dual-media
filter had 5 to 15 million fibers/L in the ef-
fluent. In most of the runs tested, asbestos
fiber concentrations did not exceed 1 million
fibers/L even during the first 5 min after the
stress or rate change had occurred.
Sludge Handling and Disposal
Disposal of alum coagulation sludge like
that produced at Duluth is generally difficult.
Because Duluth's water filtration plant
sludge contained asbestos fibers, the Min-
nesota Pollution Control Agency imposed
stringent ultimate disposal requirements on
the Water Division. To minimize the amount
of sludge requiring ultimate disposal, a study
of sludge freezing was undertaken.
When the Lakewood Filtration Plant was
built, three sludge lagoons were provided
(342-ft long, 90-ft wide, 8 ft deep, the maxi-
mum sludge depth). The original plan for
using the lagoons was to permit sludge in
one lagoon to freeze for an entire winter
season while newly produced sludge was
added to one or both of the other lagoons.
This freezing concept is referred to as static
freezing. Experience showed that the static
freezing concept was not the most effective
method available.
When natural freezing of sludge is prac-
ticed to dewater the sludge and condition it
for disposal, the static freezing process has
some important limitations. Because the at-
mosphere is below freezing but the soil at
the base of the lagoon is not, freezing pro-
ceeds from top to bottom. Thus, snow cover
cannot be allowed to remain on the partially
frozen sludge. To promote faster freezing,
snow cover must be removed from the la-
goon as soon as it can be safely done. At
Duluth, static freezing with good snow
removal practice yielded ice formation at a
reasonable rate until ice was about 2 ft thick.
After this point, the rate declined substan-
tially.
Because sludge freezes faster when it is
close to subfreezing air, sludge pumping was
studied. Unfrozen sludge was added to the
lagoon on top of previously frozen sludge.
Newly produced sludge or unfrozen sludge
pumped out from under the ice was spread
on the lagoon surface. Adding unfrozen
sludge to the top of the ice helped maintain
higher freezing rates. A comparison of freez-
ing rates observed at Duluth appears in Table
1.
Natural sludge freezing has been very ef-
fective at Duluth. After the sludge in the
lagoon freezes completely and thaws in the
spring, the clear supernatant water can be
decanted. The thin layer of remaining solids
has the appearance of instant coffee, and the
sludge volume is reduced so much that the
residue is about an inch deep. Sludge vol-
ume reduction has been so complete that
after 5 years, the lagoons have not had to
be cleaned.
Conclusions
Filter Plant Operation
The filtration plant has successfully
demonstrated that asbestiform fibers can be
removed from drinking water on a day-to-
day basis. Asbestos fiber counts have been
consistently below 0.1 million fibers/L since
the plant started operation.
With proper chemical treatment, filtered
water turbidity can consistently be reduced
to below 0.1 ntu, generally to 0.04 or 0.05
ntu.
The various rate changes studied did not
have a continuous detrimental effect on the
effluent quality of the filter plant. Changes
occurred, but these were generally
short-term.
As a result of these studies, filters are no
longer shut down with a high headloss. If a
filter has not reached the run termination
headloss of 8.0 ft prior to plant shutdown,
it is backwashed if the headloss is 7.5 ft or
greater. This procedure lessens the chance
of a major breakthrough of flocculated ma-
terials when the plant is restarted.
Alum dose varies with suspended solids
and ranges from 10 to 20 mg/L. Asbestiform
fibers are the last solids to be removed in the
process. The alum dose must satisfy all of
the coagulant needs of the other incoming
suspended solids before fibers will be
removed.
Optimum pH for fiber removal depends on
raw water temperature and varies from 6.85
when the raw water is relatively warm to 7.35
when it is colder.
The chlorine injection point was moved
from the rapid mix effluent to rapid mix
chamber #1 to eliminate a change in pH after
the coagulant is added.
Sodium hydroxide application was moved
to rapid mix #1 for pH control.
Because fluoride complexes alum, the
point of addition was moved from the filter
influent flume to the filter effluent flume.
Water Quality Monitoring
Techniques
Continuous-flow turbidimeters can be
used to monitor filter behavior. No direct cor-
relation exists between the results from the
HIAC particle counter and the results from
the electron microscope analysis, but the
particle counter can be used as an indicator
of water quality. Ninety percent of the
samples with particle counts of less than
1000 particles/100 mL contained less than
Table 1. Effect of Lagoon Operation of Sludge Freezing
Type of Operation
Freeze Degree Days*
to Freeze One Inch of Ice
Static freezing, 66% snow cover
Static freezing, 50% snow cover
Static freezing, good snow removal
tup to 25 in, of ice formed)
Layer freezing on top of ice
Layer freezing, good snow removal
117
98
38
35
28 to 34
*The number of freeze degree days for one day is defined as that day's average temperature subtracted
from 32°F. Total freeze degree days equals the sum of the freeze degree days for all days involved.
-------
20,000 amphibole fibers/L. Since the parti-
cle counter gives a direct measurement of
participates present in the water, it is more
useful than turbidity as a measurement of
water quality.
Corrosion Control
Because Lake Superior raw water is
saturated with air and is a very soft water,
rusty water problems did occur and were ap-
parently aggravated by introduction of alum
coagulation. Two corrosion control chem-
icals, zinc orthophosphate and sodium tri-
polyphosphate, have alleviated most of the
rusty water problems in domestic galvanized
iron piping.
Bacteria Removal
Coliform bacteria were successfully
removed by the filter plant in all but one case.
Note, however, that the filter plant was not
challenged by large numbers of microor-
ganisms, as might be the case at other
locations.
Backwash Sludge
Considerations
The data gathered in this project indicate
that with appropriate control, the natural
freezing of alum sludge is highly successful
in reducing sludge volume. Static freezing
in deep lagoons is not efficient for several
reasons. Freeze degree days can be deter-
mined from existing climatological data and
can be used as a guideline to design lagoons
for sludge volume reduction by natural freez-
ing. During freezing weather, sludge should
be delivered by spraying onto already formed
ice.
At Duluth, sludge is distributed to the
lagoons by spraying from the hydrants with
nozzles. The lagoon inlet structures are not
used. This method of oepration solved a
sludge distribution problem by giving even
distribution of sludge and a smooth surface
for winter freeze operation and snow
removal.
After 5 years of operation, no sludge solids
have had to be removed from the lagoons
because of the very effective dewatering at-
tained in the sludge freezing process.
The full report was submitted in fulfillment
of Grant No. S-804221 by the Duluth Depart-
ment of Water and Gas under the sponsor-
ship of the U.S. Environmental Protection
Agency.
Frank X. Schleppenbach is with the Duluth Department of Water and Gas, Duluth,
MN 55802.
Gary S. Logsdon is the EPA Project Officer (see below).
The complete report, entitled "Water Filtration at Duluth," (Order No. PB 84-177
807; Cost: $14.50. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
US GOVERNMENT PRINTING OFFICE 1984 — 759-015/7709
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAI
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
.
LOU W ULLEY
RtGION V EPA
S.
CHICAGO IL 60&04
------- |