Filter operations field guide

58 Pages·2006·0.326 MB·English

by  Nicholas G Pizzi

CHAPTER 1
Regulations and Theory
The purpose of this field guide is to provide water
treatment plant shift operators with the basic tools for
use at the job site. The guide is designed to be carried
in the shirt or hip pocket and is divided into general
sections describing the regulatory, theoretical, opera-
tional, inspection, and maintenance issues of filtra-
tion. The guide is primarily directed to operators
who employ granular filter media configurations.
Granular media filters are used in water treatment
plants (WTPs) to store particulate contaminants for
eventual waste disposal. These particulates are made
up of the naturally occurring suspended particles in
the source water as well as the particulates that are
created through the use of coagulants.
Operation and maintenance of filters is a pri-
mary pursuit of a plant operator with low turbidity
output being one of the goals. Periodic and judi-
cious use of these guidelines should provide the
operator with the ability to produce low turbidity
water on a consistent basis.
FILTER REGULATIONS
The regulatory framework for filtration will con-
tinue to evolve. This means that this guide may
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become out of date with respect to applicable regu-
lations. Additionally, local regulators may require
performance goals and reporting that go beyond
federal rules, and the operator is cautioned to check
with the local regulators for guidance.
The Interim Enhanced Surface Water Treat-
ment Rule (IESWTR) is the primary driver for
filter operation requirements. It states that com-
bined filter effluent (CFE) turbidity levels for con-
ventional and direct filtration plants must be less
than or equal to 0.3 nephelometric turbidity units
(ntu) in at least 95% of the samples analyzed each
month, and no samples shall exceed one (1) ntu.
Samples must be collected at 4-hr intervals or less.
CFE turbidity levels for slow sand filtration plants
must be less than or equal to 0.5 ntu in at least
95% of the samples analyzed each month, and no
samples shall exceed one (1) ntu.
The IESWTR also states that each filter in
production must be monitored continuously for
turbidity levels and that measurement must be
recorded at no less than 15-min intervals. These
results must be archived for three years. Operators
must report turbidity values that exceed 1.0 ntu in
two consecutive 15-min samples if they occur in
the first four hours of the filter run. Thereafter, the
recording of any two consecutive 15-min samples
that exceed 0.5 ntu will trigger a need for reporting.
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This pertains to filters in production, not those off-
line or in the process of backwash.
A graphical representation of the filter per-
formance (filter profile) is required when filters
produce poor quality water such as previously
described. The regulator will need to see the filter
profile as a function of turbidity (or particle counts)
versus time for the entire length of the run.
The IESWTR places an emphasis on timely fil-
ter ripening and the responsiveness of operators to
the problem. It is aimed at reducing the frequency,
the duration, and the magnitude of particle passage
(spiking) in filter effluent water.
THEORETICAL CONSIDERATIONS
FOR OPERATORS
Granular media filtration is a unit process that
is intended to remove particulate contamina-
tion from processed water and store the particles
for eventual disposal. The particulates that filters
remove are usually those found in the source water
along with particles created through coagulation,
softening, and oxidation processes. Filtration may
employ the concept of depth filtration, where par-
ticles are filtered by the mechanism of attachment.
Properly pretreated particles will attach or adhere
to the media grains if the filter is operating within
certain parameters.
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Gravity filter operation, which repeats continu-
ously, consists of filter start-up, ripening, unit fil-
ter volume production (during which head loss is
incurred), optimization, effluent turbidity mea-
surement, and perhaps particle count measurement.
Operation may also include goal-setting, terminal
breakthrough, and backwashing (or cleaning).
The important factors that operators can con-
trol are filter loading rates and run times, pretreat-
ment of the water that is applied to the filter, and
filter maintenance. In general, operators should
understand that filters may not perform well if they
are overloaded beyond their design capacity or if
they are loaded with poorly pretreated water (see
Chapter 2). Poorly maintained filters will also fail
in time.
The most important tasks related to filtration
revolve around these three factors: (1)Water applied
to filters must receive the proper conditioning (pre-
treatment); (2) filters must be operated at or near
the design or approved loading rate; and (3) filters
need to be maintained and inspected for problems
before they arise.
Most operationally induced turbidity spiking
events—those that result in noncompliance—can
usually be traced to one or more of these three factors.
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CHAPTER 2
Operations
OPERATIONAL TECHNIQUES
Filter operations depend on operators who can make
good judgments and who can perform hands-on
techniques in a timely fashion. A good understand-
ing of the workings of a filter allows an operator to
make these judgments. Plants that have few filters
should employ an operational strategy that provides
staggered filter runs so that no two washes come
due at the same time. Plants that have many filters
and adequate spent filter backwash water storage
can backwash with a more random schedule, but
they should understand the impact that any given
backwash event may have on other processes.
Every filter ought to be equipped with a contin-
uously recording turbidimeter even if not required
by regulation. Filters that need attention from the
operator may go unnoticed if they are not moni-
tored continuously. Operators should become
familiar with the control console at each filter
and with the SCADA readouts from the consoles
if present. Each filter will normally be equipped
with readouts and trending capability for hours of
service, effluent turbidity, loss of head, flow rate,
and the associated data for backwash.
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The filters should be operated at the design
capacity where possible, and operators should start
and stop them to achieve this. Operators should
understand the effect that placing filters into ser-
vice has on all upstream and downstream pro-
cesses. Operators should understand the effects
that upstream and downstream processes have on
filter operations. An operator should never make a
change to a plant flow rate, a chemical feed rate,
or a backwash rate without knowing what effect it
will have on the filters.
No maintenance of any sort should be sched-
uled until it is studied for its possible deleterious
effect on the filtration process.
LOADING RATES
The rate at which water flows through the filter bed
is called the loading rate and is usually expressed as
gallons per minute per square foot (gpm/ft2).
Example 1: A filter with a surface dimension of
10 ft by 12 ft is filtering 360 gpm. What is the
filter loading rate?
The surface area of a filter
12 ft × 10 ft = 120 ft2. The
360 gpm 10 ft flow rate through the filter is
360 gpm, and if it is divided by
the 120 ft2 surface area, it yields
12 ft a loading rate of 3 gpm/ft2.
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Filter loading rates should be determined and
recorded every day for every filter. Excessive load-
ing rates can lead to elevated turbidity output and
noncompliance. Loading rates that are too low can
increase the solids retention in the top media lay-
ers, which may cause poor performance. Therefore,
there is a range at which filters should be loaded:
these optimum loading rate ranges should be deter-
mined and maintained by all operators as closely as
possible. Operators at WTPs with many filters may
have an easier time accomplishing this strategy.
RATE OF FLOW CONTROL
The ability to control the rate that water flows
through a filter is important. In plants with mul-
tiple filters, head loss buildup can allow unequal
loading of filters. This can cause excessive filter
rates in some filters with the accompanied high
turbidity output.
Water treatment plants are usually equipped
with rate of flow control mechanisms in one of two
categories: equal rate filtration or declining rate
filtration.
Equal rate filtration is a misnomer in that it is
rarely seen. In this scheme, rate control valves are
placed on each individual filter effluent. The valve
modulates flow in an attempt to keep a relatively
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constant flow rate. All of the on-line filters are thus
kept at the preset rate. However, when one filter
is taken off-line for backwash, the rates of flow of
all of the other filters must increase to keep pace.
Rate of flow is influenced by the level of the water
coming into the filters. Therefore, WTPs are usu-
ally designed to keep a preset level in the channel
that feeds the filters. If this level is closely modu-
lated, the filter rate of flow control valves will do
an acceptable job of keeping pace. If there is a wide
tolerance (too much level fluctuation) in the chan-
nel level, the filter rate of flow control valves may
“hunt” or modulate over a wide range, and the fil-
ters will show some sloughing of floc.
Declining rate filtration allows the pretreated
water to enter the filter at a level below the nor-
mal water level in each filter and discharges to the
clearwell at a level above the filter media. In this
way, the filters operate with the same available head
at any instant. Therefore, the cleanest filters will
operate at the highest flows, and the dirtiest filters
will slow down. All filter rates decline in step-wise
fashion after each backwash. When a clean filter
is placed into service, it assumes the highest flow.
FILTER RUN TIMES
The length of time from the instant that a filter
is placed into service to the time that it is taken
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out of service for backwash is called the filter run
time. There should be no interruption in the filter
run time. That is, once a filter is put into service
it should not stop until backwashing is necessary
before returning to service. Filter run times should
be recorded for each filter and for each cycle. Short
run times indicate potential pretreatment difficul-
ties. Excessively long run times can lead to tur-
bidity and particle breakthrough. Optimum run
times should be established for each treatment
plant.
Some plants do not operate 24 hours per day.
This is often the case at smaller installations where
perhaps one or two shifts are sufficient to supply
enough drinking water for the users’ requirements.
These plants shut down at night and resume opera-
tions the next morning. The filters should be back-
washed before they go into service in the morning.
It is left to the operator to determine if they are best
washed at night before shutting the plant or in the
morning at startup. That decision is often a result
of the amount of clearwell volume available for
backwash supply and of the space required for the
spent filter backwash to be stored or discharged.
UNIT FILTER RUN VOLUME
The amount of water that is filtered during the
filter run time is called the unit filter run volume
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