Summary
The performance evaluation of tertiary granular
media filtration systems indicates that mechanical
problems are common with these systems. These .
problems are primarily responsible for the impaired
quality of effluent from the filtration system and the
increased maintenance requirements. It is not
unusual for such problems to occur considering that
tertiary filtration of wastewater is a relatively recent
development as a treatment process. The
performance of tertiary filtration systems could be
significantly improved by incorporating certain
modifications in the design of the system, and
ensuring that systematic operation and maintenance
procedures are followed. A summary of the major
problems identified and suggested remedies is
presented in Table 1.
vvEPA
United States
Environmental Protection
Agency
August
1984
Tertiary
Granular
Filtration
Problems
and
Remedies
J/3
For Additional Information:
EPA Offices:
EPA-OWPO (WH-547)
401 M Street SW
Washington, DC 20460
(202)382-7365/7368
EPA-MERL (443)
26 West StClair Street
Cincinnati, OH 45268
(513)684-7613
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backwashing systems such as surface wash or air
scour do not operate properly, the bed will not be
thoroughly cleaned and will eventually become
clogged.
Mudballs are agglomerations of extraneous material
which accumulate in the bed over extended periods
of time due to improper backwashing. Consequently,
the effectiveness of the filtering and backwashing
operation is significantly reduced.
Loss of filter media is caused by excessively high
backwashing rates, uneven distribution of backwash
water at the bottom of the bed, and incorrect se-
quencing of the auxiliary backwash system. Media
loss increases operating costs for media replace-
ment. It also results in an increased rate of buildup
of head loss in the bed in dual media filters due to
the loss of the coarser media from the top layer of
the bed. Gravel mounding refers to disruption of the
support gravel at the bottom of the filter bed due to
excessively high backwash rates. Improper design
of the filter underdrain system causes migration of
the filter media and clogging of the backwash
nozzles. These nozzles and other parts of the under-
drain system can also be corroded due to galvanic
corrosion.
Suggested Remedies
In order to ensure that the filter is backwashed at the
appropriate time intervals, the frequency of back-
washing should be controlled on the basis of both
head loss and fixed time interval. The correct rate
and duration of backwashing are essential for
proper filter system operation. Manufacturer's speci-
fications and operating instructions should be
closely followed. Settings of the rate and timer
controls for backwashing should be regularly
checked. Maintaining the correct operational
sequencing is critical for effective backwashing. The
wash water troughs should be maintained at the
correct level to prevent short circuiting during the
backwashing cycle. At a minimum, a monthly visual
check should be made of each filter cell through a
complete backwash cycle to ensure that the correct
operational sequence is maintained. A maintenance
schedule should be established and followed to
ensure that the downtime due to mechanical
problems in the backwashing system is kept to a
minimum.
The use of backwashing nozzles fitted with a pro-
tective plate on top should be considered to avoid
clogging of the nozzles. In order to prevent electro-
lytic corrosion problems, compatible materials of
construction should be used for backwash nozzles,
underdrain support structure and the filter bed.
Problem
Filtration Cycle
• Frequent clogging of
media and buildup of
excessive head loss.
« Hydraulic surges in
influent flow to filters.
o Excessive filtration system
downtime due to equipment
problems.
• Incorrect operation of the
filtration system.
Backwashing Cycle
• Improper frequency of
backwashing.
• Loss of media during
backwashing.
•.Clogging of backwash
nozzles due to migration of
media.
• Corrosion of backwash
nozzles.
Remedy
« Multimedia filter should be
considered. Design filters
to operate either in parallel
or series. Improve quality of
influent to filter by incorpor-
ating modifications to pro-
cesses ahead of filter. Add
a disinfectant, usually
chlorine, to the filter influent
periodically. Ensure proper
rate and duration of
backwashing.
• Equalization facilities
ahead of filter should be
provided,
• An adequate maintenance
program should be estab-
lished and followed.
• Formal training of opera-
tors on filtration technology
and operation of the system
should be provided.
o Backwash frequency
should be controlled by
both predetermined head
loss and fixed time
interval.
o Excessive backwashing
rates should be avoided.
Correct operational
sequencing of backwash-
ing operation should be
ensured. Proper levels of
washwater troughs should
be maintained.
• Use of nozzles fitted with
a protective plate on top
should be considered.
• Use of compatible
materials of construction
for nozzles, underdrain
support structure, and filter
bed to avoid electrolysis
and galvanic corrosion.
Table 1 Tertiary Filtration Systems: Summary of Problems and
Suggested Remedies
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Tertiary Granular Filtration - Problems and Remedies
The major problems experienced with tertiary fil-
tration systems and the recommendations for im-
proving their performance are discussed separately
for the filtration cycle and the backwash cycle.
Start
Figure 3. Gravity Filter Backwash Cycle.
Common Problems - Filtration Cycle
One of the most common problems with the filtration
cycle is frequent clogging of the filter media. This
results in buildup of excessive head loss in the bed
within a short period of time and consequently in-
creases the frequency of backwashing. Media
clogging is caused by solids carryover from prior
treatment processes, microbial growth in the1 filter
bed, and high concentration of oil and grease in the
influent to the filter. Operating difficulties and poor
effluent quality are often caused by hydraulic surges
in the influent flow. Maintenance operations at many
treatment plants are not adequate to keep the filter
system functioning properly. Insufficient
maintenance can result in ineffective
instrumentation systems and inoperative valves,
pumps and other process control devices.
inadequate training of personnel for operation of
tertiary filtration systems is common in many plants..
This lack of knowledge adversely affects filter per-
formance since necessary process adjustments are
not made under critical operating conditions.
Suggested Remedies
The problems associated with the filtration cycle can
be reduced by implementing the following re-
commendations: (1) judiciously select the type of
filter media. Multi-media filters provide efficient
utilization of filter depth, better response to high
solids loadings and longer filter runs. Single media
filters cost less but generally require more frequent
backwashing than multimedia filters; (2) design the
filter system with the option to operate either in
parallel or series mode. In case of increased
suspended solids loading, series operation could
help to meet the requirements for effluent quality and
minimize the problem of frequent backwashing; (3)
incorporate modifications to the upstream clarifica-
tion process (e.g., polymer addition) to decrease
suspended solids loading in the clarifier effluent
flowing to the filter; (4) add a ballast tank to stabilize
the influent flow and solids concentration; (5) record
influent flows to filters and provide an automatic
controller or weir to ensure even flow distribution
among filters. When flow exceeds the filter's design
hydraulic capacity, provisions to divert the additional
flow to stand-by filters or surge tanks should be
considered; (6) apply a disinfectant, usually chlorine,
to filters taken off-line temporarily, to control
microbial growth in the filter bed; (7) provide pre-
treatment process to remove oil and grease in the
filter influent; (8) establish an adequate maintenance
program for regularly checking the filtration equip-
ment such as flow controller, media depth, valves
and pumps; and (9) provide formal training on
filtration technology and filter operation to all
operators who work with the filtration system.
Common Problems - Backwash Cycle
Several problems are associated with the
backwashing cycle of tertiary filtration systems.
Media clogging, mudball formation, media loss, and
gravel mounding and displacement are common
problems of the backwashing cycle.
Backwash systems designed to initiate operation
based on a single criterion (i.e., either fixed time
interval or head loss) can result in improper
frequency of backwash.Media clogging results from
solids buildup in the filter bed due to insufficient rate
or duration of backwashing. If the auxiliary
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Tertiary Granular Filtration - Problems and Remedies
Introduction
Tertiary granular media filtration is utilized in
wastewater treatment for the removal of suspended
and/or colloidal matter from effluents of biological
and chemical treatment processes. The use of
filtration systems for wastewater treatment is a
relatively recent development. However, they have
been used in water treatment for over 50 years. The
concept of utilizing filtration in wastewater treatment
came into practice to meet the more stringent re-
quirements for effluent not possible with conven-
tional secondary treatment. Tertitary filtration is
primarily aimed at removing the fine suspended
solids that are carried over in the effluent from the
final clarification process. The BOD associated with
the suspended solids is also removed during
filtration. The resulting filtered effluent is low in BOD
and suspended solids concentration and, in general,
meets the effluent standards for advanced waste
treatment.
A comprehensive evaluation of selected advanced
treatment (AT) facilities utilizing granular filtration.
systems was recently completed wittvllje objective
of identifying common problems assoBfated with
tertiary filtration systems. The primary Impacts
resulting from these problems are short filter runs,
increased frequency of backwashing, poor effluent
quality and high maintenance costs. Based on this
evaluation, suggested remedies to the problems
identified are offered.
Process Description
Filters are classified according to configuration of
filter media, media depth, direction of flow, type of
flow, and method of flow rate control. The three
types of filter media configurations used with
granular media filters are as follows: (1) single
medium — usually sand, (2) dual media — usually
anthracite and sand, (3) multimedia — usually an-
thracite, sand, and garnet.
A filter can be a "shallow" (2 to 3 feet) or "deep" (5 to
6 feet) bed type. Filters are designed for upflow or
downflow through the filter bed. Filters can be of the
gravity or pressure type, and can be operated at
either a constant rate of flow or declining rate of
flow. Typical granular media filter configurations are
shown in Figures 1 and 2. Filters are periodically
backwashed with clean water to remove the sus-
pended solids that accumulate in the filter media.
Backwash can be initiated either by a predetermined
head loss or by a timer set at a fixed time interval. In
the backwash cycle expansion of the bed occurs
due to fluidization, and the suspended solids that
accumulate on the grains of the media are scoured
and removed by the shearing action of the water and
interparticle collisions.
Figure 1. Typical Gravity Type (Downflow) Granular Media Filter.
Figure 2. Typical Pressure Type Granular Media Filter.
In order to improve the effectiveness of the back-
washing operation, auxiliary surface-wash or air-
scour systems are often used. Figure 3 shows a
typical backwash operation cycle for a gravity filter. .
In addition to the filter media and backwash
systems, several appurtenances are necessary for
operation of the filter, including the following: (1)
underdrain system, which collects,.the filtered
effluent and distributes the backwash water, (2)
w'ashwater troughs used to collect and remove the
backwash water from the filter, and (3) surface-wash
or air-scour systems, which are used as auxiliary
aids to backwash systems.
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