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.
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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

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

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

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.

    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
     Incorrect operation of the
    filtration system.
    Backwashing Cycle
     Improper frequency of
    Loss of media during
   .Clogging of backwash
   nozzles due to migration of
    Corrosion of backwash
 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
 Equalization facilities
ahead of filter should be
 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

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

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.
 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

  Tertiary  Granular  Filtration - Problems and  Remedies
  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

 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.