Wastewater
nitration
Design Considerations
-------
-------
EPA-625/4-74-007a
WASTEWATER FILTRATION
Design Considerations
I$91
sr^
&
STZ?
PrO^
5SEZ
ui
0
ENVIRONMENTAL PROTECTION AGENCY • Technology Transfer
July 1974
-------
ACKNOWLEDGEMENTS
This seminar publication contains materials prepared for the U.S.
Environmental Protection Agency Technology Transfer Program and has
been presented at Technology Transfer design seminars throughout the
United States.
The information in this publication was prepared by John L. Cleasby,
P.E., Professor of Civil Engineering, and E. Robert Baumann, P.E.,
Anson Marston Distinguished Professor of Engineering and Professor of
Civil Engineering, Iowa State University, Ames, Iowa.
NOTICE
The mention of trade names or commercial products in this publication is for
illustration purposes, and does not constitute endorsement or recommendation for
use by the U.S. Environmental Protection Agency.
Revised July 1977
-------
CONTENTS
Page
Chapter I. Introduction 1
Chapter II. Contrasts with Potable Water Experience 3
Chapter III. Filter Design — General Considerations 5
Wastewater Filtration Flow Schemes 5
Minimum Acceptable Filter Run Length 5
Filter Configurations 8
Headloss Development 12
Chapter IV. Filter Design —Detailed Considerations 14
Pilot Scale Testing 14
Optimization Considerations 17
Selection of Filtration Rate and Terminal Headloss 18
Selection of Filter Media 20
Methods of Filter Flow Control 26
Chapter V. Backwashing of Wastewater Filters 33
Surface Wash to Assist Backwashing 33
Air Scour to Assist Backwashing 34
Backwashing Recommendations 35
Chapter VI. Summary 39
References 40
Appendix A — Performance Data for Wastewater Filtration from the Literature 43
Reported Efficiencies for Direct Filtration of
Trickling Filter Plant Effluents 44
Reported Efficiencies for Direct Filtration of
Activated Sludge Plant Effluents 47
Sources 48
iii
-------
Chapter I
INTRODUCTION
Wastewater filtration is but one of the design engineer's alternatives which can be considered
in waste treatment flow schemes to meet specified effluent quality objectives. He should consider
it along with other alternatives, finally reaching a decision as to which of the several alternatives
is cost effective. This publication presents the questions which must be asked in wastewater filtra-
tion, the alternatives available in answering the questions, and the design procedures involved in
those alternatives.
It is presumed that the reader is familiar with granular media filtration from potable water
experience or from study of textbook sources1. Therefore, this publication stresses special aspects
related to wastewater filtration.
The first and most important question the designer must ask is whether filtration can meet the
specified effluent quality goals. If the goal is to upgrade the effluent of an existing secondary treat-
ment works, one must first evaluate the present performance and the reasons for that performance.
For example, what portions of the present effluent BOD are of soluble and suspended origin?
The filter can remove only a portion of the suspended BOD. If the effluent contains highly soluble
BOD, the only solution may be to upgrade the secondary treatment. If the effluent contains
primarily suspended BOD, effluent filtration or upgrading the secondary settling will be possible
alternative solutions.
The expected performance of the granular filters can be estimated from performance at similar
plants elsewhere, or by pilot studies at the plant in question. Appendix A presents a summary of
operating results at a number of plants in the U.S. and the U.K. Similar compilations with more data
from U.S. activated sludge plants are available in recent EPA design manuals2'3. The mean and
range of the performance data from these two sources is summarized in table 1-1.
The data in table 1-1 and the sources from which it was derived indicate that a marginal secon-
dary effluent could easily be upgraded to a 30-30 standard, and probably to a 20-20 standard, by
tertiary filtration, i.e., without chemical treatment. A good secondary effluent which already meets
the 30-30 standard may approach a 10-10 goal by tertiary filtration. If the effluent quality goal is
less than 10-10, some form of chemical treatment will be needed in the secondary or in a tertiary
stage prior to filtration.
After considering the effluent quality goals and the ability of granular filtration to achieve
those goals, and if filtration is still one viable alternative, the following design questions must be
considered in arriving at a successful installation:
• What are the appropriate flow schemes?
• What minimum filter run length is acceptable?
• What filter configurations are appropriate for wastewater?
• Is pilot scale testing needed, and if so, how should it be conducted?
1
-------
• What filtration rate and terminal headloss should be provided?
• What filter media size and depth should be provided?
• Should gravity or pressure filters be provided?
• What system of flow control should be used?
• What backwash provisions are needed for each filter media alternative being considered to
ensure effective backwashing in the long term?
• What underdrain system is appropriate for the media and backwash regime intended?
Each of these questions will be discussed in the following sections. But first, some contrasts
between wastewater and potable water filtration will be presented.
Table 1-1.—Median and range of performance of wastewater filters, combined data from Appendix A and
Reference 2. The data below give the range of mean values and the median of the means
including all filtration rates from 2 to 6 gpm/sq ft (inclusive) and media sues ^ 1mm effective
size
Filter influent type
Tertiary filtration
of trickling filter
plant effluent
n = number of
observations
Tertiary filtration
of activated sludge
plant effluent
Suspended solids (mg/l)
Influent Effluent
n = 31
Range Median Range Median Range Median
20-51 29 5-13 9 23-35 30
BODr (mg/l)
Influent Effluent
Range Median
n = 31
n = 6
7-55 16 2-10 6 No data
n = 23 n = 23
10-14
12
n = 6
No data
Filtration after 6-16 10 1-8 1.5 No data Nodata
chemical treatment
of secondary effluent n = 7 n = 6
2
-------
Chapter II
CONTRASTS WITH POTABLE WATER EXPERIENCE
Most of the published information on the design and operation of granular media filters has
been derived from experiences in potable water filtration. Such filters have been used in potable
water production for the removal of solids present in surface waters pretreated by coagulation and
sedimentation, for the removal of precipitates resulting from lime or lime/soda-ash softening, and
for the removal of iron or manganese found in many underground water supplies. The design and
operating experience in these applications has been carried over into wastewater applications,
sometimes disastrously. In evaluating waterworks filtration experience for application in waste-
water-filtration situations, several very important differences need to be emphasized.
With built-in raw and filtered water-storage capacity, water filters can be and generally are
operated at constant filtration rates for long periods, and steady operating conditions will prevail.
Thus, plant design can be based on the maximum day demand, not on hourly demand. In waste-
water filtration, however, the plant must be designed to handle a continuously varying and highly
unpredictable rate of flow, with variations from a nighttime dry weather flow to peak hourly flows
in stormwater runoff periods. As a result, the potential effects of peak flow rates must be con-
sidered in the filter-plant design.
In waterworks experience, the water filtered is much more consistent in both the level of solids
present and in their filtering characteristics (for example, iron-removal plants). Even with pretreated
surface water, the solids to be removed by filtration consist of low levels (usually under 5-10 FTU)
of floe carryover, with some attached colloidal solids contributing to the original raw water turbidity.
The filtering characteristics of solids that are mainly inorganic are more predictable than the filter-
ing characteristics of the inorganic-organic solids found in typical wastewaters to be treated. Even
in pretreated surface water filtration, the daily variations in solids levels are less than those en-
countered in wastewater. Considerable data are available to demonstrate that in raw wastewater,
the suspended solids levels will vary directly with the flow. Furthermore, treatment processes are
least efficient under peak-load operating conditions. Therefore, high suspended solids may leave
the final settling tanks of secondary treatment during peak-flow periods and be delivered to tertiary
filters. Thus, applied to filters, the wastewater presents its highest solids concentration to be re-
moved during the highest flow-rate periods. Even in well-operated plants, suspended solids loadings
to tertiary treatment filters during peak-flow periods can reach 30-50 mg/1 (15-25 FTU). Such loadings
contribute to high headloss and a consequent potential for very short filter runs. Thus, the critical design
condition to be considered occurs under peak-flow operating conditions.
With more uniform suspended solids levels and filtering characteristics, water-treatment filter
efficiency is a function mainly of the filtration rate and the influent suspended solids concentration.
In wastewater filtration, however, filtrate quality is less dependent on rate and influent suspended
solids levels.
As explained previously, the operation of secondary biological-treatment plants results in wide
variations in solids levels carried over to tertiary filters. The filtering characteristics of such solids
are affected by the solids retention times maintained in the biological reactors. As a result, filtering
characteristics are highly variable. Such solids, however, are much more "sticky" than water-plant
solids, and are much more difficult to remove effectively during filter backwashing. If chemical
3
-------
treatment of the wastewaters is practiced, the floe carryover to the filters assumes more of the
character of the chemical floe. Thus, a wastewater which has received a high degree of chemical
pretreatment will filter more like the comparable floe of potable water experience.
4
-------
Chapter III
FILTER DESIGN - GENERAL CONSIDERATIONS
WASTEWATER FILTRATION FLOW SCHEMES
In advanced wastewater treatment, filtration (figure III-l) is used for:
• Removal of residual biological floe in settled effluents from secondary treatment by
trickling filters or activated-sludge processes—the primary emphasis of this presentation. It
will be referred to as "tertiary filtration."
• Removal of precipitates resulting from alum, iron, or lime precipitation of phosphates in
secondary effluents from trickling filters or activated-sludge processes. The suspended
solids to be filtered can be substantially different from those in normal secondary effluent.
• Removal of solids remaining after the chemical coagulation of wastewaters in physical-
chemical waste-treatment processes, i.e., following lime treatment of raw wastewater and
before adsorption removal of soluble organics in carbon columns. Again, the solids to be
filtered can be substantially different from normal secondary effluent solids.
Filters may be used as the fined process of wastewater treatment (polishing secondary or
tertiary effluents) or as an intermediate process to prepare wastewater for further treatment (for
example, before downflow carbon adsorption columns or clinoptilolite ammonia exchange columns).
In either case, the required filters should be designed to provide a quality of filtrate equal to or
better than the desired effluent-quality goal at all times. Achieving this quality may require a pilot
study to evaluate the flow characteristics and solids characteristics of the water to be filtered. In
the absence of a pilot study, the design must be based on experience with similar filter influent
waters at other installations.
The problem of potential short filter cycles during peak-load periods requires that a flow
equalization tank be considered as part of the plant flow scheme. It may be installed near the plant
entrance to benefit the entire treatment operation, or it could be installed just prior to the filters.
Figure III-l indicates that at this point, wastewater quality is such as to present no odor or mixing
problems. Fifteen to twenty percent of mean daily-flow storage capacity would permit constant-
rate flow to the filters for a 24-hour period. One hundred percent of mean daily-flow storage
capacity would permit constant flow and nearly constant solids loads to the filters. Neither practice
is widespread today, and the benefits to filtration alone may not jusitfy the costs for such
provisions.
MINIMUM ACCEPTABLE FILTER RUN LENGTH
Since the capital cost of a filter is chiefly a function of the area of filter provided, the use of a high
filtration rate is usually preferred. In general, filter design should seek to maximize the net water pro-
duction per square foot of filter consistent with filter operating feasibility. Useful relationships between
net water production and run lengths obtained at different filtration rates are shown in figures III-2
and III-3. There are two alternatives. The first is shown in figure III-2 and it occurs when filtered water
is used for backwashing as all potable water filtration and most wastewater filtration plants. The second
5
-------
Pretreated
Backwash water recycle
raw
wastewater
Backwash
water
storage
Final
clarification
Primary
clarification
Biological
treatment
Final sludge recycle
Backwash
water
Waste sludge
Gr anular
media
filters
Chlorine
contact
Discharge
and/or
backwash
to river
storage
tank
Backwash water recycle
Final
Discharge
Pretreated'
raw
wastewater
to river
cation,
Packed
bed
reactors
Granular
media
filters
Final sludge
recycle
Primary
clarification
Backwash
water
storage
NH3 >
'educ-
tion ,
l/^SI
'Biological'
^treatment^
Chlorine
contact
a nd /o r
backwash
storage
tank
Waste sludge
Backwash water recycle
Anaerobic
packed bed
reactor
Backwash
water
storage
Organic
carbon
'Biological1
NO3
reduc
tion
nh3
reduc-
Pretreated
Primary
clarif 1
raw
wastewater
clarification
treatment
cation
Aerobic
packed bed
reactor
Final sludge
recycle
Granular
media
filters
Chlorine
contact
and/or
Discharge
backwash
to river
storage
tank
Figure 111-1. Granular media filters for tertiary wastewater treatment: (a) following biological
secondary treatment for carbonaceous BOD removal; (b) following biological secondary
and biological tertiary (packed-bed reactors) treatment for carbonaceous BOD and
ammonia reduction; (c) following biological secondary and biological tertiary (packed-
bed reactors, both aerobic and anaerobic) for carbonaceous BOD, ammonia, and nitrate
reduction. (Phosphorus levels may also be reduced by adding ferric or aluminum salts
and a polymer feed to solids contact units located ahead of the granular-media
filters.)
6
-------
E
a.
a
z
O
I—
o
D
Q
O
X
a.
(-
Z
<
Filter cycles per day
Conditions
30 Minute down time/wash
100 Gal/sq ft /wash
No wash water recycle
2 4 6 8
NEEDED FILTRATION RATE (gpm/sq ft)
Figure 111 -2. Effect of number of filter cycles per day on filtrate production when using filtered water for
backwashing.
Filter cycles per day
Conditions-
30 Minute down time/wash
100 Gal/sq ft/wash
Wash water recycled at equalized rate
10
NEEDED FILTRATION RATE (GPM/SQ FT) DURING
ACTUAL OPERATING TIME
Figure III-3. Effect of number of filter cycles per day on filtrate production when using unfiltered water
for backwashing. Plant production is the average plant output over the full 24 hour day.
7
-------
case, shown in figure III-3, occurs when unfiltered water is used in backwashing. However, it is not
generally recommended because of potential clogging of underdrain strainer or orifice openings.
The data for both figures was calculated assuming 30 minutes total down time per backwash
to allow for draindown time, auxiliary scour time, actual backwash time and start-up time to reach
normal rate. Also, the 100 gal/ft2 total wash water per backwash is typical of volumes adequate for
most filtration situations. In the case of recovered wash water, it is assumed that dirty wash water
would pass through a holding tank to permit flow equalization of the recirculated water. A sample
calculation for figure III-2 is shown below:
Backwashes per day (6-hour cycles) = 4
Downtime per backwash = 30 min
Actual filtration time (1,440) - (4 x 30) = 1320 min
Plant production = 4 gal/min/ft2 x 1,440 min/day = 5,760 gpd/ sq ft
Backwash water used = 100 x 4 = 400 gal
Needed filtration rate (5760 gal + 400 gal)/1320 = 4.67 gal/min/ft2
Backwash water as percent of production = (400/5760) x 100 = 6.9%
Figure III-2, which is appropriate for most wastewater filtration, would indicate little loss of pro-
duction if the number of backwash cycles per day per filter is limited to four or less, i.e., 6-hour
filter cycles or longer. Thus, under the peak-flow and suspended-solids load conditions predicted for
the design year, the cycles should be at least 6 hours. Considering typical flow and solids load varia-
tions, this should result in 24-hour cycles under average design year loads.
One must keep in mind the conditions selected to construct figures III-2 and III-3. Some
filters require more than 30 minutes to complete a backwash cycle, especially if complete gravity
draindown is essential or desired. Some require more than lOOgal/sq ft/ wash. If the downtime
and water use for a particular type of filter are expected to deviate significantly from those used
above, then the figures should be reconstructed and the cycle length decision reconsidered.
Different backwashing routines are discussed in more detail in a later section of this manual.
FILTER CONFIGURATIONS
A filter configuration must be selected for wastewater filter which is appropriate for the
higher influent solids anticipated as discussed in the previous sections. A granular media filter is
intended to filter in depth, i.e., it is intended that solids removal take place within the filter, and
not primarily at the entering surface.
8
-------
A number of alternate filter configurations have been developed to accommodate the higher
solids loads described above and to encourage filtration in depth. The alternates were developed
because the conventional sand filter of United States design is not well-suited to handle high influent
solids loads. This is due to the fact that the media is backwashed at a rate sufficient to expand
the bed, and stratification of the media occurs—the smaller sand grains tending to collect near the top
of the bed and the larger grains at the bottom. Since the conventional sand filter is operated with
downward flow, the solids encounter the finest media first, and filtration in depth is not encouraged.
The configurations developed to encourage filtration in depth, and thus to achieve longer filter
cycles with higher influent solids loads, are illustrated in figure III-4. The most common method in
the United States is the double-layer filter bed composed of a coarse layer of crushed anthracite coal
over a layer of finer sand. Coal has a lighter specific gravity than sand, and when the proper size, it re-
mains on top during backwashing if the backwash rate is sufficient to achieve full fluidization and some
expansion of the bed. The coarse coal removes the major portion of the sediment, allowing significant
depth of penetration, while the fine sand layer polishes the water to provide an effluent of good quality.
The crushed coal is angular in shape and has a higher porosity than sand (about 0.50 versus 0.40).
Therefore, the coal has a greater storage capacity for solids removed by filtration. One manufacturer
carries the concept one step further with another layer of finer and heavier garnet or illmenite under
the silica sand. The three layers are sized to encourage some intermixing between the layers.
European practice has included deeper filters of coarse sand4 (hereinafter referred to as single
media unstratified filters) backwashed without expansion using air and water simultaneously so that
stratification does not occur.
European practice also includes upflow and middle outlet sand filters. The upflow filter achieves
its benefit because of the graded gravel support at the bottom which acts as a roughing filter prior
to the deep bed of unstratified coarse sand above. However, there is a danger of lifting part of the
filter bed during the filtration cycle, which is seriously detrimental to the filtrate quality. This is
overcome with a retaining grid of steel bars on 4- to 6-inch centers which resists the lifting
tendency during the filtration cycle. This type of filter has received considerable attention in Great
Britain for wastewater filtration because of its high solids capacity5-6. Several full scale plants are
in service. The lifting problem is overcome by a different approach in the center outlet "Bi-Flow"
filter which has an inlet at both top and bottom. The flow automatically distributes itself so that the
upward and downward forces balance each other, eliminating the uplift danger of the upflow filter.
The extent of full-scale use of the "Bi-Flow" design is not known to the authors.
The advantages and disadvantages of these various configurations are partly related to the back-
washing requirements and are discussed in a general way below. Further details on backwash design
are presented later.
The advantages of the dual media filter include the fact that the practice is well established
in U.S. potable water practice, and the increased solids handling capacity of the coal layer is achieved
with a relatively shallow media because the sand layer is provided to protect the filtrate quality.
The shallow media reduces structural costs. One disadvantage is that full fluidization is essential
during the water backwash, to ensure that the coal will remain on top after backwash. This require-
ment may dictate excessive wash rates if a coarse coal is selected to lengthen filter cycles. Another
disadvantage is incurred if media-retaining underdrain strainers are provided to eliminate the use of
supporting graded gravel. The fine sand of the lower layer requires very fine openings in the
strainers, and clogging problems may result.
The triple media filter has similar requirements, advantages and disadvantages as the dual media
filter. It is a proprietary design and less experience is available than with dual media. In some appli-
cations, the third media may not improve filtrate quality as will be shown later. These filters are
usually supported on graded gravel, thus avoiding the dangers of strainers with fine openings.
9
-------
Influent
Coarse
Underdrain system
Conventional
sand
Influent
v u\\V > w»> w \\\\\\\\\
•\ vNw\»\w^ \\\\\\\\\\
* Wwv\\o.O>\o.
XVSand^;
Dual media
Coarse
Fine
Influent
Coarse
Silica Sand^o
Finer
Finest
Eff uent
Triple (mixed)
media
Influent
Coarser sand
unstratified
Effluent
~V~
f V
8 w
o w
f
llllllllllllllllll
Sand size
range
1-2 mm
unstratified
2-3 mm
10-15 mm
20-30 mm
*
Effluent
Retaining
grid
Influent
Effluent
r
Effluent
collector
""""UMIIII
Sand
Influent
Single media
unstratified
filter
Upflow
"Immedium"
filter
Bi-flow
filter
Figure III-4. Schematic diagrams of filter configurations for granular media filtration.
10
-------
The advantages of the single media unstratified filter are:
• The simplicity of a single media.
• The ready availability of the media.
• That coarser media may be used to achieve longer filter cycles without excessive backwash
rates since these filters are backwashed with air and water simultaneously without bed
expansion (i.e., below the water fluidization velocity).
• That larger underdrain strainer openings may be used and thus, less danger of strainer clogging.
• That transport of heavy solids released from the filter during backwash to the overflow is more
effective with simultaneous air and water wash than with water alone.
The disadvantages of the single media unstratified filter are:
• That a deeper bed is needed to try to compensate for the coarser media resulting in higher
structural costs.
• That the deeper bed may not necessarily ensure equivalent filtrate quality compared with a
dual or triple media bed as will be shown later.
• Precautions are necessary to ensure that filter media is not lost during the air plus water
backwashing operation.
The advantages of the upflow filter are:
• Unfiltered water is usually used for backwashing without danger of strainer clogging since
strainers are not used in the underdrain.
• There is no need to provide substantial water depth above the media for head development
as is desired in downflow gravity filters to avoid negative head development.
• Low backwash water rates are used because part of the wash cycle is with air and water
simultaneously.
The disadvantages of the upflow filter include:
• The total bed depth is large adding to the structural costs.
• Media loss during the air and water backwash has exposed the hold down grid in some
plants, leading to uplift difficulties at high headlosses.
• The backwash routine including draindown is fairly long, about 45-60 minutes depending
on the details.
• The initial filtrate after backwash can be no better than the backwash supply quality which
can be quite poor at times.
The uplift difficulty can be solved by retopping the filter with sand to keep the grid submerged
below the fixed bed sand surface, but such details are sometimes neglected.
11
-------
In addition to the configurations shown in figure III-4, there are various proprietary filter
configurations which are in use in wastewater filtration which depend on other methods to over-
come the high headloss characteristic of wastewater filtration. They are not described here because
of their proprietary nature and because they have been described in another EPA publication2.
Their absence herein represents neither a criticism nor an endorsement of those filters.
HEADLOSS DEVELOPMENT
Figure III-5 shows several examples of headloss development during solids separation by filtration.
Granular media filters remove suspended solids in one of the following ways:
• By removal of the suspended solids at the surface by the finer media at the top of the filter,
which forms a relatively thin layer of deposited solids at the surface.
• By depth removal of the suspended solids within the voids of the porous media—the better
the distribution of the solids throughout the depth of the filter media, the better the use
of the head available.
• By a combination of surface removal and depth removal, which is the usual case in filtration
of secondary effluents.
Solids removal may be predominantly at the surface if the filter media is too small or if the
filtration rate is too low. Surface removal of a compressible solid results in a headloss curve that is
exponential (fig. III-5a). Increasing the terminal headloss does not increase production per filter
run significantly with this type of headloss pattern. With surface-cake filtration of this type, the
filtration is dominantly achieved by the cake itself, and filtrate quality is constant throughout the
run.
On the other hand, if removal occurs entirely within the filter media, a headloss pattern such
as that in figure III-5b will result. Increasing the filtration rate increases the initial headloss. Since
the headloss curves are essentially parallel, increasing the filtration rate slightly decreases production
to any particular terminal headloss. Increasing the terminal headloss increases both the run length
and the production per run since the curves are nearly linear. Depth removal of this type may be
experienced using larger-size surface media and the various filter designs that provide coarse-to-fine
filtration. It is the most common pattern in potable water filtration and is observed in some waste-
water filtration.
Q = Constant
(a) VOLUME OF FILTRATE (O • t>
Q = 4
Q = Constant
(b) VOLUME OF FILTRATE (Q * f)
Q- 3 Q'6
Terminal
driving force
Optimum rate
(c) VOLUME OF FILTRATE (O • f)
Figure 111-5. Headloss development during filtration: (a) surface removal of compressible solids; (b) depth
removal of suspended solids; (c) depth removal of suspended solids with surface cake.
12
-------
When the solids are partly removed on the surface and partly in the depth of the filter, surface
removal will predominate at low filtration rates, and the headloss generated is characteristic of
surface removal headloss (e.g., fig. III-5a at the lower rates). With higher rates, the solids are carried
deeper into the granular medium and more filtrate is produced before the surface cake forms
(Q = 3 and 4, fig. III-5c). The rate may become high enough to prevent surface cake formation and
the headloss will then be controlled only by depth filtration. In figure III-5c, therefore, a flow rate
of 5 would be the optimum production rate since it produces the most filtrate per run. No sub-
stantial surface cake forms with filtration rates of 5 or higher, resulting in parallel headloss curves
above that rate. Filtration of secondary effluents tends to involve both surface and depth filtration,
and may thus behave as in figure III-5c7. Nearly linear headloss curves have been observed in filtra-
tion of alum-treated secondary effluent8.
Thus, if an exponential headloss curve is observed, production per filter run can be increased
by using higher filtration rates, or alternatively, the top media size can be enlarged by skimming or
replacement of the coal media if necessary.
In the operation of tertiary wastewater filters, plots of headloss versus time or volume of filtrate
can yield valuable information on the design of the filter media or the choice of a filtration rate.
13
-------
Chapter IV
FILTER DESIGN - DETAILED CONSIDERATIONS
The objective of filtration is to produce the desired quality and quantity of filtrate at the least
cost per unit of filtrate produced. The designer must choose between the various pretreatment al-
ternatives and various performance variables discussed below in reaching a final design. The various
alternatives must be tested against the basic objective.
The variables which affect performance fall in two categories: (1) the influent suspended
solids variable, such as the type, amount and filtrability of the solids, and (2) the physical filtration
variables such as the rate of filtration, terminal headloss provided, and the size, depth and type of
filter media.
PILOT SCALE TESTING
When new types of waters are to be filtered containing solids of unfamiliar filtrability, pilot
testing may be necessary to arrive at the proper design. Pilot testing on various wastewaters has
become increasingly common as such filters are needed in process flow schemes.
The pilot filtration apparatus should have three or more filters which can be run in parallel.
This is necessary because the influent solids may change from day to day (even hour to hour) so that
various design or operating variables must be compared in parallel rather than sequentially. The
three pilot filters can be operated in a series of experiments to evaluate the effect of media size,
media depth, media type (single, dual or multi media) and filtration rate on filtrate quality and
headloss generation. The filters should be equipped with pressure taps at intervals through the
depth so that the extent of depth filtration can be ascertained. The influent and effluent should
be monitored for suspended solids, turbidity and other parameters of interest so that the ability
to achieve filtrate quality goals can be determined, and the relation of solids load to headloss de-
velopment can be approximated. The pilot experiments should cover the full range of the variables
that may be used in the plant design, e.g., filtration rates of 2 to 8 gal/min/ft2 and terminal head-
losses to 30 feet. A good deal about expected performance can be learned by studying the results
of other investigators who have filtered similar influent solids. Substantial data of this type is pre-
sented for wastewater filtration in Appendix A and in other sources2'3.
Data for particular media can be presented as shown in figure IV-1, assuming that filtrate
quality goals are achieved for all variables presented. On such curves, it will be noted that the
influent solids load is the product of influent concentration (CQ) and the run length (t) and filtra-
tion rate (u). The three curves can be normalized roughly into a single curve by plotting the
average product of vt versus CQ. The shape of the curve is approximately hyperbolic indicating
an equation of the form x'y = constant, i.e., CQ • t • v = constant. This fact has led to an approxi-
mate approach for predicting headloss on the basis of solids capture per unit headloss increase for
a particular media size and type and concentration of influent solids. Some typical data of this
type are presented in table IV-1.
14
-------
While the data in table IV-1 are limited, especially for activated sludge, several things should
be noted:
There is substantial variability from plant to plant illustrating the uncertainty of using data
from one plant in the design of another.
There is little variation in solids capture over the common range of filtration rates, 2 to 6
gal/min/ft2, but variation is evident at extremely high rates as evidenced by the Cleveland
data9, and some Ames data at 8 gal/min/ft2 15.
12 in. — 1,84mm Anthracite
12 in. — 0.55mm Sand
tO 20 30
INFLUENT SOLIDS CONCENTRATION C0, mg/f
40
50
Figure IV-1. Run length vs influent SS concentration at various flow rates'
15
-------
Table \V-\.-Solids capture per foot of head/oss increase in direct filtration of secondary effluents
Solids
o . IXi Filtration Media capture
Secondary effluent Mode of ,, r,,,.
gal/min/ft2 °Perati°"b headloss L°Cati°n Reference
increase
TF
Full scale sand)
2.5-4
C
0.85-1.7C
TF
Pilot dual media)
2-6
C
1.84 ESd
TF
Pilot sand)
2-6
C
0.55 ES
TF
Full scale sand)
2.3
-
0.55 ES
TF
Pilot dual media)
2.1
C
1.03 ES
TF
Pilot sand)
2.1
c
2.0-3.6C
TF
Pilot dual media)
3.2
c
1.03 ES
TF
Pilot sand)
3.2
c
2.0-3.6c
TF
Pilot sand)
3.2 - 3.8
-
1-2.06°
TF
Pilot sand)
2
c
2.31 ESe
TF
Pilot sand)
2
c
1.82 ESe
TF
Pilot sand)
2
c
1.49 ESe
TF
Pilot sand)
2
c
0.97 ES®
TF
Pilot sand)
4
c
2.31 ES®
TF
Pilot sand)
4
c
1.82 ESe
TF
Pilot sand)
4
c
1.49 ESe
TF
Pilot sand)
4
c
0.97 ESe
TF
Pilot sand)
8
c
2.31 ESe
TF
Pilot sand)
8
c
1.82 ESe
TF
Pilot sand)
8
c
1.49 ES®
TF
Pilot sand)
8
c
0.97 ESe
AS
Pilot dual media)
16
c
1.78 ES
AS
Pilot dual media)
24
c
1.78 ES
AS
Pilot dual media)
32
c
1.78 ES
AS
Pilot dual media)
16
D
1.78 ES
AS
Pilot dual media)
22.2
D
1.78 ES
AS
Pilot dual media)
27.6
D
1.78 ES
AS
Pilot upflow)
2-5
C
1-2C
AS
Pilot dual media)
5.1
C
1.08 ES
AS
Pilot dual media)
5.1
C
1.45 ES
TF
Pilot dual media)
4.24
D
1.28 ES
AS
Pilot dual media)
4.24
D
1.28 ES
Luton, Eng.
Ames, Iowa
Ames, Iowa
Pretoria
Ames, la. Parallel
operation
Ames, la. Parallel
operation
Finham, Eng.
Ames, Iowa
Parallel
operation
Ames, Iowa
Parallel
operation
Ames, Iowa
Parallel
operation
Cleveland, Oh.
Parallel
operation
Cleveland, Oh.
Parallel
_ operation
W. Hertford-\
shire, Eng. J
Nevada, la.
Marshalltown, la.
10& 11
7
12
13
8
8
14
15
15
15
16
17
17
18
18
aTF = trickling-filter-ptant final effluent; AS = activated-sludge-plant final effluent
bC ¦ constant rate; D = declining rate
cMedia size range, unstratified due to backwash provisions
^Effective size of media. In dual media, only the ES of the top coal layer is presented
eUnstratified due to backwash provisions
16
-------
• The solids capture is quite low for media sizes about 1 mm and below, about 0.04 to
0.08 lb/sq ft/ft of headloss increase.
• Substantially higher solids capture is possible with larger media as illustrated by the parallel
studies at Ames8 >15.
• The low solids capture value with the coarse dual media7 is not in harmony with other ob-
servations at Ames for media of this size for unknown reasons.
Additional data of this type would be desirable, especially for tertiary filters at activated sludge
plants, and for filters at plants practicing various types of chemical treatment. Until such data are
collected, if one must design without the benefit of pilot studies, one must select conservative solids
capture values of about 0.04 form media of 1 mm ES and perhaps 0.12 for media of 2 mm ES with
gradual variation for intermediate sizes. These values should be appropriate in the common range
of filtration rates from 2 to 6 gal/min/ft2.
Chemical treatment produces weaker floe, and somewhat higher solids capture values can be
expected although the authors have data from only one study to present. A solids capture value of
about 0.04 lb/sq ft/ft headloss increase was calculated for a chemically treated secondary effluent8.
In this case, trickling filter plant effluent at Ames, Iowa, was treated with 200 mg/1 of alum for
phosphorous reduction, flocculated and settled before filtration. The filtration rate was 2 gpm/sq ft
and the dual media pilot Alters had 0.94 mm ES coal. The low value indicates that the media size
was too small.
In view of the low solids capture for media of 1 mm ES or below, such sizes are not recom-
mended for wastewater filtration. A size of at least 1.2 mm ES is suggested as a minimum, and
coarser sizes are preferred if appropriate backwashing is provided.
OPTIMIZATION CONSIDERATIONS
When one considers physical filtration variables, it is apparent that the engineer has a large
number of filter design options. Many combinations of filtration rate, media size and depth, and
terminal headloss may achieve the desired filtrate quality with acceptable filter run length. Higher
rates of filtration will require deeper beds and/or finer media for equal filtrate quality. Coarser filter
media will require deeper beds and/or lower filtration rates.
The various feasible design alternatives which will meet filtrate quality goals can be compared
on a capital and operating cost basis. A particular combination of the physical variables may result
in the filter effluent quality reaching its upper limit of acceptability at the same time that the total
headloss reaches a selected limit. Such a combination constitutes an optimum19, or more precisely
an operational optimum 20. A number of operational optimums are possible with a given influent
water and filtrate quality goal, but only one would yield water at least cost, i.e., at the economic
optimum. In recognition of these concepts, attempts are being made to optimize filter design20-21 >22.
17
-------
As one considers the available options, it should be cautioned that real life suspensions to be
filtered do not always fit idealized mathematical models of filtration. For example, it is not always
possible to offset higher rates of filtration, or coarser media sizes, with increased bed depth.
A good example of this occurs in direct filtration of secondary effluent wastewater. It appears
that a portion of the influent suspended solids are easily filtered from water regardless of the media
size, depth, or rate of filtration within the normal ranges of these variables. Another portion is not
removed regardless of the choices made for these variables. This may be due to the bimodal distri-
bution of particle sizes observed in secondary effluent of an activated sludge process 23 with one
group of very small particles (0 to 10 Aim with mode at about 5 nm) and another group of much
larger particles (20 to 160 jim with the mode at about 80 ixxrx).
In recognition of the many options available and the limitations discussed in the preceding
paragraphs, current practice with regard to these physical filtration variables will be discussed. This
is done with recognition of the possibility that current practice does not necessarily reflect optimum
design and that other designs may be equally good.
SELECTION OF FILTRATION RATE AND TERMINAL HEADLOSS
Wastewater solids may generate rapid headloss development due to the high solids concentra-
tions in the filter influent and the strong surface removal tendency of the solids. This is especially
true in the tertiary filtration of secondary effluents where filtrate quality is not appreciably de-
teriorated by filtration rates as high as 5 or 6 gal/min/ft2 using media with effective sizes up to
about 2 mm with media depths appropriate to the size. Nevertheless, average rates of 2 to 3 gal/
min/ft2 and peak rates of 5 gal/min/ft2 are common to achieve run length objectives2. Thus, in
wastewater filtration, the rate of filtration is dictated more by run length considerations than by
filtrate quality considerations.
Modeling of the filtration process has not yet progressed to the point where it is possible to
determine precisely what economic filtration rate and terminal headloss should be provided for a
granular-media filter. Huang and Buamman 21 found that the most economic terminal headloss for
filtration of iron on unisized-sand filters ranged between 8 and 11 feet at all filtration rates from 2
to 6 gal/min/ft^. Normal American water treatment practice would use a terminal headloss of 8 to
10 feet when using gravity filters. The filtration rate and terminal head should not be so high as to
result in failure of the filtration process by solids breakthrough. However, solids breakthrough does
not generally occur in the filtration of secondary effluents. A fraction of the solids pass through the
filter during the entire run, but further deterioration does not usually occur as the run progresses.
Studies indicate that pressure drops of as much as 30 feet of water could be used in filtration
of trickling filter effluents7'24 and in activated sludge effluents^.*8 through dual-media filters without
solids breakthrough. Economic considerations, however, may dictate pressure filters if such terminal
headlosses are to be provided.
The selection of the filtration rate and terminal headloss to be provided in design involves con-
sideration of a number of interrelated questions.
• What are the desired minimum and maximum filter run lengths? As discussed earlier, run
length should be at least 6 to 8 hours to avoid excessive backwash water use, but less than
about 36 to 48 hours to reduce anaerobic decomposition within the filter and possible
detriment to the effluent BOD. The desired run length can be achieved by selecting either
the terminal headloss or the filtration rate or both.
18
-------
• Will the backwash operation be automated to avoid manpower costs if short filter runs
occur? Automatic backwash is commonly provided in wastewater filtration plants.
• Is pressurized discharge desired to a subsequent treatment unit or to an effluent force main?
Pressurized discharge would tend to favor the use of pressure filters. In such cases, higher
rates and/or higher terminal headlosses may be economically feasible where they would not
be with gravity filters.
• Is the hydraulic profile of the existing secondary plant such that tertiary filters could be
added without repumping by limiting the terminal headloss?
• What is the size of the plant, the capital available, and the space available for tertiary filters?
A large plant with adequate capital resources may prefer multiple gravity filters, at lower
filtration rates and lower terminal headloss, using a more-or-less conventional water plant
design. A smaller plant, or one with limited capital or space, may prefer pressure filters
operated at higher filtration rates.
• Are there any regulatory agency policies which require gravity filters or prohibit pressure
filters, or does the client insist upon gravity filters for easier maintenance?
• What variations in influent flow rate and suspended solids concentration are expected, and
how will they be handled? If influent flow equalization is provided, this concern is partially
eliminated. If 24-hour minimum filter runs are the goal, the hourly variations in load will
balance out over the day and become of less concern. On the other hand, if 6-hour minimum
cycles are selected, peak 6-hour loads would be of concern.
To answer these questions rationally, some method of predicting run length as a function of
filtration rate, terminal loss, media size, and influent suspended solids is needed. As discussed earlier,
pilot plant studies at the plant in question yield the most reliable prediction. In their absence, the
designs can be based on a conservative value of solids capture per unit headloss.
If pilot plant data such as figure IV-1 are collected for different filter media and different
terminal headlosses, the data can be used to select several alternative design combinations of media,
filtration rate, and terminal headloss. These can be compared on the basis of capital and operating
costs. Furthermore, if the flow and solids load variations are predicted, the operational consequences
of those variations can be analyzed. One must be sure to limit the design alternatives to those that
have been shown to produce acceptable filtrate quality.
To illustrate the use of figure IV-1, assume that the minimum desired run length is selected to
be 8 hours, the maximum 8-hour influent solids concentration is estimated to be 40 mg/1, and the
terminal headloss is limited to 10 feet by one of the factors discussed above. From figure IV-1, the
peak 8-hour filtration rate must then be limited to 6 gal/min/ft^. if the average annual flow rate is
one third the peak, and the average influent solids is predicted to be 20 mg/1, then from figure IV-1,
the average run length could be 42 hours. It would be desirable under such loads to wash on a maxi-
mum 24-hour override to prevent anaerobic conditions in the filter.
If the design must be based on an assumed solids capture per unit headloss, then alternative
designs can be selected as illustrated below.
19
-------
Assume that a value of 0.07 lb/sq ft/ft headloss has been estimated for a trickling filter effluent
and a media size of 1.2 mm ES from table IV-1. This value can be used to estimate the terminal
headloss that must be provided to achieve a desired filter run length using an estimated secondary
effluent suspended solids concentration. For example, find the needed terminal headloss to achieve
24-hour average filter runs under the following conditions:
Average filtration rate = 3 gal/min/ft2, with range of 2 to 4.5 during the day
Average secondary effluent suspended solids = 30 mg/1
Average effluent suspended solids = 5 mg/1
Average suspended solids capture = 25 mg/1
Top media size = 1.2 mm
Calculate solids capture per square foot per run:
8 33
25 mg/1 removed x 3 gal/min/ft2 x 1,440 min/filter run x"~^g = 0.90 lb/ft2/run
Terminal headloss increase = „ = 13 ft/run
0.07 lb/ft2/ft headloss
Thus, a terminal headloss increase of about 13 feet would be required to meet the 24-hour filter
run. The initial headloss must be added to this figure to obtain the total terminal headloss. This
total is above the normal headloss provided for gravity filters and suggests either that pressure filters
be considered, or that the average filtration rate be reduced to 2 gal/min/ft2.
The filter runs could become substantially shorter during periods of poorer secondary treat-
ment plant performance. For example, if the secondary effluent suspended solids climbed to 50
mg/1, the run length would drop to 13.3 hours, other conditions being unchanged. Peak flows could
prevail for such a run length, further accentuating the solids load and reducing the run to 8.9 hours.
When filter cycles get this short, the backwash water being returned through the plant becomes sub-
stantial and further increases the load on the filters shortening the filter runs.
SELECTION OF FILTER MEDIA
The selection of the size and depth of filter media and the appropriate filtration rate are inter-
related. In general, filtrate quality is improved by the use of finer media, greater media depth or
lower filtration rates. Similarly, headloss generation rate is increased by finer media, greater media
depth and higher filtration rates. With some influent suspensions, these generalizations are not
demonstrated significantly. For example, in filtration of secondary effluents, filtration rate has
little effect upon filtrate quality over the usual range of rates employed—2 to 5 gal/min/ft2—and in-
creased media depth may not compensate for coarser media in achieving filtrate quality. As evidence,
tables IV-2a and b show that a dual media and a triple media filter provided slightly better filtrate
quality than an unsatratified coarse sand filter of 46-inch depth. Furthermore, table IV-2c shows
that changing the depth of the unstratified coarse sand filter had little effect on performance at the
filtration rate of 3 gal/min/ft2. However, greater depth is of benefit in maintaining filtrate quality
at higher filtration rates15.
20
-------
Table I V-2a.—Performance of a dual media, triple media and unstratified coarse sand filter when filtering
secondary effluent from the trickling filter plant at Ames, /owa8. Results are the mean values
from periodic composite samples collected during 8 weeks of operation in 1974 at 2.1 gal/min/
Filtered effluent
Influent
Dual
Triple
Unstratified
media8
mediab
mediac
Suspended solids
37.49
6.84
6.31
7.92
(mg/l)
n = 14d
0= 12.03®
o = 3.23
o = 3.87
o = 5.80
Turbidity (FTU)
16.38
2.38
2.20
2.89
n = 16
cj= 4.31
o = 0.97
o = 0.56
a= 1.10
BOD5 (mg/l)
14.61
3.73
4.11
4.73
n = 13
o — 6.00
0= 1.72
o = 2.03
0 = 2.56
Soluble BODg
3.88
1.97
2.20
2.34
(mg/l)
n = 10
o = 1.79
0 = 0.96
o = 0.99
o= 1.11
aDual media: 15 in. of 1.03 mm ES coal, 1.57 UC
9 in. of 0.49 mm ES sand, 1.41 UC
^Triple media: Same as above plus 3 in. garnet with 0.27 ES and 1.55 UC
cUnstratified sand: 46 in. of 2.0 mm ES sand (2 to 3.6 mm size range, 1.52 UC)
dn * number of composites averaged, each representing one filter run
®0 = standard deviation
The selection of the filter media also determines the required backwash regime, and thus, the
backwash requirements become an integral part of the media decision. Those requirements have
been discussed briefly in the prior section on filter configuration and will be discussed in more de-
tail later in the section on backwashing.
Granular filter media commonly used in water and wastewater filtration include silica sand,
garnet sand, and anthracite coal. These media can be purchased in a broad range of effective sizes
and uniformity coefficients. (The term "effective size" indicates the size of grain (in millimeters)
such that 10 percent (by weight) of the particles are smaller and 90 percent larger than itself.
"Uniformity coefficient" designates the ratio of the size of grain which has 60 percent of the sample
finer than itself, to the effective size which has 10 percent finer than itself.) The media have spe-
cific gravities approximately as follows:
• Anthracite coal, 1.35 to 1.75; most U.S. anthracite, 1.6 to 1.75; U.K. anthracite, 1.35 to 1.45.
• Silica sand, 2.65.
• Garnet sand, 4 to 4.2.
The detrimental effects of the strong surface removal tendency previously discussed
for wastewater filtration must be counteracted by selecting a media size where the flow enters
the media which will ensure that the bulk of the suspended solids removal does not occur at the
21
-------
Table IV-2b. —Performance of a dual media, triple media and unstratified coarse sand filter when filtering
secondary effluent from the trickling filter plant at Ames, Iowa8. Results are the mean values
from periodic composite samples collected during 9 weeks of operation in 1974 at 3.2
gal/min/ft2
Filtered effluent3
Influent
Dual
media
Triple
media
Unstratified
media
Suspended solids
(mg/l)
34.08
7.05
6.82
9.46
n = 14b
<7= 16.87c
ff= 4.27
o= 3.10
0= 4.53
Turbidity (FTU)
17.60
4.80
6.78
4.66
n = 15
o= 6.18
o= 2.28
a= 3.01
o = 2.12
BOD5 (mg/l)
30.38
12.68
12.99
14.46
n = 15
o= 14.52
o= 6.88
o = 6.82
o = 6.56
Soluble BOD5
(mg/l)
9.67
7.21
7.27
7.78
n = 15
ff= 3.76
<7= 3.72
o = 3.61
o= 3.57
aFilter media same as in table IV-2a except coal depth in dual and mixed media increased to 17 in.
= number of composites averaged, each representing one filter run
cO = standard deviation
entering surface. Pilot testing of different media is desired if time and budgets permit. If it is not
feasible, the following information will assist in selecting the media size or sizes.
For the tertiary filtration of secondary effluents, media size of at least 1.2 mm ES is required,
and coarser media is preferred if appropriate backwash is provided. Benefits to filter run length
accrue at least up to 2.3 mm ES as shown in the prior solids capture data in table IV-1.
For the filtration of chemically treated secondary effluents, a media size of not less than 1.0
mm has been suggested2. However, benefits of coarser media should also occur here, and the
sparsity of data makes pilot testing even more important.
Once the size of the media at the entering surface has been selected, the rest of the media
specification is dependent thereon. For example, the uniformity coefficients, the size of the sand
in dual media, and the depth of each media must be selected.
Low uniformity coefficients (UC) are desired to achieve easier backwashing. This is especially
true where fluidization of the media is required during backwashing as with dual and triple media
filters. This is true for dual and triple media because the entire media should be fluidized to achieve
restratification; therefore, the greater the UC (i.e., less uniform size range), the larger the backwash
rate required to fluidize the coarser grains thus provided. A UC of less than 1.3 is not generally
practical because of the sieving capabilities of commercial suppliers. A UC of less than 1.5 can be
obtained at a cost premium and is recommended.
A UC of less than 1.5 has the advantage that it will ensure that the coarser grain size in the
media (such as the 90 percent finer size, dgy) is not excessively large, requiring a large backwash
rate. Sieve analyses of filter media will usually plot linearly on either log-probability or
22
-------
Table \\!-2c.—Performance of three unstratified coarse sand fitters of different depth when filtering secondary
effluent from the trickling fitter plant at Ames, Iowa8. Results are the mean values from
periodic composite samples collected during 5 weeks of operation in 1975. Sand size was
2.5 to 3.7 mm size range
Filtered effluent3
Filter
influent
24 in.
depth*5
47 in.
depth
60 in.
depth
Suspended solids
(mg/l)
31.3
5.9
6.4
5.7
II
9.7
2.1
2.3
1.8
Turbidty (FTU)
12.6
3.30
3.38
3.14
ct (/? = 11)
3.14
1.21
1.14
1.14
BOD5 (mg/l)
15.6
6.5
7.1
6.6
a (n= 11)
4.7
2.8
2.5
2.5
Soluble B0D5
(mg/l)
5.3
3.9
4.0
3.8
a (n= 11)
1.7
1.3
1.3
1.3
aFiltration rate, 3.0 gal/min/ft^
'-'Filtrate from 24 inches of sand and 12 inches of supporting gravel
cO= standard deviation
n = number of composites averaged, each representing one filter run
arithmetic-probability paper. The ratios of d90/dio for media with a UC of 1.5 is 2.0 for the log
probability distribution and 1.83 for the arithmetic probability distribution. These ratios are use-
ful in estimating the dgo grain size which can then be used to determine the needed backwash rate.
An alternate method of specifying filter media which is used in the U.K. is to specify the range
of size within which the media must fall. For example, a 1.4 to 2.4 mm size range woyld fall be-
tween a U.S. standard 14-mesh and 8-mesh sieve. Some tolerance must be allowed at either end to
allow for the sieving capabilities of the suppliers. A 10 percent tolerance at each end is suggested,
i.e., 10 percent by weight could be smaller than 1.4 mm and 10 percent coarser than 2.4 mm. This
system of specification has the advantages that the effective size could be no smaller than the lower
end of the range, and the coarser media is more precisely limited which is of importance in select-
ing the needed backwash rate.
DUAL MEDIA
For dual media filters, the sizes of the sand layer must be selected to be compatible with the
coal which has been selected. The bottom sand (e.g., the 90 percent finer size) should have ap-
proximately the same or a somewhat lower flow rate required for fluidization than the bottom
coal to ensure that the entire bed fluidizes at the selected backwash rate.
To assist in the selection of the required backwash rate, and to assess the compatibility ques-
tion raised above, empirical data on the minimum fluidization velocity of coal, sand and garnet
sand at 25°C are presented in table IV-3, as well as empirical correction factors to be applied for
other water temperatures. The temperature correction factors agree substantially with data pre-
sented by Camp2 5
23
-------
Table IV-3.—Minimum fluidization velocities for various uniform sized media to achieve 10percent expansion
at 25° C, observed empirically8
Between U.S.
standard sieves
Mean
size
mm
Flow rate to achieve
10% expansion at 25°C, gpm/ft2
Passing
(mm)
Retained
Coal
Sand
Garnet
7
2.83
8
2.59
37
8
2.38
10
2.18
30
10
2.00
12
1.84
24
41
12
1.68
14
1.54
20
33
14
1.41
16
1.30
15.7
27
49
16
1.19
18
1.09
12.5
21
40
18
1.00
20
0.92
9.9
16.4
32
20
0.841
25
0.78
8.4
12.6
27
25
0.707
30
0.65
7.0
9.0
22
30
0.595
35
0.55
6.3
18.0
35
0.500
40
0.46
5.4
13.7
40
0.420
45
0.38
4.0
11.3
50
0.297
60 (0.25mm)
0.27
6.3
Specific gravity 1.7 2.65 4.1
Temperature correction — The following are approximate correction factors
to be applied for temperatures other than 25°C.
Temperature Multiply 25°
C° value by
30 1.09
25 1.00
20 0.91
15 0.83
10 0.75
5 0.68
24
-------
The effective size of the sand for a dual media filter should be selected to achieve the goal of
coarse-to-fine filtration without causing excessive media intermixing. If the coal density is in the
typical range of 1.65 to 1.75 g/cm^, a ratio of the 90 percent finer coal size to the 10 percent finer
sand size equal to about 3 will result in a few inches of media intermixing at the interface2 5. A
ratio of these sizes of 4 will result in substantial media intermixing, whereas a ratio of 2 to 2.5 will
cause a sharp interface. Choosing media sizes to achieve a sharp interface will mean that the benefits
of coarse-to-fine filtration will be partly lost. Therefore, a size ratio of about 3 is recommended.
The use of table IV-3 and the foregoing recommendation can be illustrated with an example.
Assume a coal of 1.2 mm ES has been selected with a UC less than 1.5 (size range of 1.2 to
2.2 mm, 8- to 16-mesh range). The sand should have an effective size of about 0.7 mm to be
one third of the coarse coal size. A sand size range of 0.7 to 1.4 mm could be specified (14-
to 25-mesh range), or one with an ES of 0.7 mm. The backwash rate for the coarse end of the
coal (2.38 mm) is 30 gal/min/ft2 at 25°C and the coarse sand (1.4 mm) is 27 gal/min/ft2.
Thus, they are compatible. If the peak expected operating temperature is expected to be 15°C,
the required backwash rate would be 30 X 0.83 = 25 gal/min/ft2.
It should be noted that no harm would be done if the coarser sand grains' were smaller than
1.4 mm. They would merely reach fluidization before the coarser coal gains. There is no danger
of inversion of the coal and sand layers during backwashing or complete intermixing as there is
with sand and garnet sand. The intermixing behavior of coal and sand, and sand and garnet sand
has been experimentally demonstrated26.
In addition to specifying the gradation of filter media used, the depth of media must be es-
tablished. At present, there is no reasonable method - other than pilot-plant operation — that can
be used to determine the optimum depth of filter media. Huang7 >24 established that, for filtration
of tricking filter plant effluent, a depth of at least 15 inches of 1.84 mm ES coal was desirable.
Theoretical considerations would indicate that media depths should increase with media size. For
practical designs based on a minimum of available information, the following minimum media
depths are recommended for dual media filters:
• Anthracite coal, 15 inches minimum to 20 inches.
• Silica sand, 12 inches minimum to 15 inches.
It should be emphasized that the media design illustrated by the foregoing example is one ap-
propriate design for tertiary filtration but it is not the only possibility. A coarser coal would yield
longer filter runs but require higher backwash rates. Nor is the example media design necessarily
best for chemically pretreated wastewaters, or where polyelectrolytes are to be used as filter aids.
In the latter case, a coarser top size may be desired (1.2 to 1.5 mm).
In dual or triple media filters, after each media layer is installed in the filter, it should be back-
washed and skimmed to remove unwanted fine sand before installing the next layer. This step can
be important, for example, because the sand may collect a low density coating after a number of
filter cycles. In one case, using alum coagulation of secondary effluent, these coatings caused the
fine sand to migrate to the coal surface where it formed a blinding surface layer2?.
UNSTRATIFIED SINGLE MEDIA
Single media filter beds comprised of unstratified coarse sand are also being used for wastewater
filtration. Sand depths of 4 to 5 feet and size ranges of 1.5 to 2.5 mm, 2 to 3 mm, and 2 to 4 mm
are being used. These filters offer the advantage of using a coarser media size and thus achieve
greater solids capture per unit headloss as shown previously in table IV-1. However, the provision
of adequate backwashing is essential.
25
-------
Because of the coarse sand sizes, backwash by fluidization and bed expansion in the usual U.S.
fashion would require excessive wash rates and is not feasible. Therefore, these filters are back-
washed with air and water simultaneously at rates just sufficient to cause a pulsing and a slow cir-
culation of the sand in the bed. This is followed by a short water wash at a rate below fluidization
to expel some air from the bed.
The overflow level during backwashing must be high enough above the sand surface to prevent
excessive loss of sand during the simultaneous air-water backwash. Even though the bed is not
fluidized, grains of sand are thrown above the fixed bed surface by the violence of the combined
air-water action. A vertical distance to overflow of 24 inches is recommended for the sand sizes
mentioned above based on laboratory, pilot and plant scale observations8. The common wash
routines for these sand sizes are presented in table IV-4.
METHODS OF FILTER FLOW CONTROL
If the design decisions made thus far are favorable to the use of gravity filters, the authors
would urge that the control system for the filters be achieved without an effluent flow controller.
Two preferable alternatives are presented on the following pages.
Before discussing these alternatives, a fundamental fact about filtration should be recalled,
namely, that in any filtration operation, the rate of flow through a filter may be expressed as
rate of flow - Eguatlon 1
filter resistance
The rate of flow through a water filter is usually expressed in gallons per minute per square
foot. The driving force refers to the pressure drop across the filter, which is available to force the
water through the filter. At the start of a filter run, the filter is clean and the driving force need
only overcome the resistance of the clean filter medium. As filtration continues, the suspended
solids removed by the filter collect on the filter surface, or in the filter medium, or both, and the
driving force must overcome the combined resistance of the filter medium and the solids removed
by the filter.
The filter resistance refers to the resistance of the filter medium and the solids removed by
the filter medium to the passage of water. The filter resistance increases during a filter run because
of the accumulation of the solids removed by the filter. The filter resistance also increases as the
pressure drop across the cake increases, because the solids already removed compress and become
more resistant to flow. Hence, as the filter resistance increases, the driving force across the filter
must increase proportionally to maintain a constant rate of flow.
There are three basic methods of operating filters that differ primarily in the way that the
driving force is applied across the filter. These methods are referred to as "constant pressure filtra-
tion", "constant rate filtration", and "variable declining rate filtration".
CONSTANT PRESSURE FILTRATION
In true constant pressure filtration the total available driving force is applied across the filter
throughout the filter run. At the beginning of the filter run, the filter resistance is low and the rate
of filtration is very high. (High driving force/low filter resistance = high rate of flow.) As the filter
clogs with solids, filter resistance increases, and, because the driving force remains constant, the flow
rate decreases. This method provides true declining rate filtration. Some pressure filters are
operated using this mode of operation.
26
-------
Table \\/-4.—Unstratified sand filter designs for wastewater with appropriate backwash routines
Media
Simultaneous wash
Water wash
Source3
Size range
Depth
Air rate
Water rate
Dur.
Rate
Dur.
(mm)
(ft)
(cfm/
ft2)
(gal/min/
ft2)
(min)
(gal/
min/ft2)
(min)
1.5-2.5
3
2.7
5.5
10
11
5
1
2-3
4-6
6-8
6-8
15
8
5
2
2.5-3.7
4 & 5
7
15
10
15
3
3
a1 — Successfully operating full scale plant in tertiary filtration at activated sludge plant in England observed by authors
2 — Manufacturers suggested media and wash routine in the U.S. for 2-3 mm and 2-4 mm sand. Provided acceptable
wash of 2-3.6 mm sand in tertiary filtration study at Ames, Iowa
3 — Successful wash routine in pilot scale study at Ames, Iowa, in tertiary filtration of trickling filter plant effluent8
CONSTANT RATE AND CONSTANT WATER LEVEL FILTRATION
The constant pressure method of filtration is seldom used with water or wastewater gravity fil-
ters because it requires a relatively large volume of water storage on the upstream side of the filter.
Current practice, therefore, has tended to the use of constant rate or constant water level for gravity
filters. The constant rate method is equally appropriate for pressure or gravity filters. In constant
rate and constant water level filtration, a constant pressure is supplied across the filter system and
the filtration rate or water level is then held constant by the action of a manually operated or auto-
matic effluent flow control valve. At the beginning of the filter run, the filter is clean and has little
resistance. If the full driving force were applied across the filter only, the flow rate would be very
high. To maintain a constant flow rate or water level, some of the available driving force is consumed
by an effluent flow control valve.
At the start of the filter run, the flow control valve is nearly closed in providing the additional
resistance needed to maintain the desired flow rate or water level. As filtration continues, the filter
becomes clogged with solids and the flow control valve opens slowly. When the valve is fully open
the run must be terminated, since any further increase in filter resistance will not be balanced by a
corresponding decrease in the resistance of the flow control valve. Thus, the ratio of driving force
to filter resistance (Equation 1) will decrease, and flow rate will decrease (water level also increases
on gravity filters).
The disadvantages of effluent control constant rate operation include the following:
• The initial and maintenance costs of the fairly complex rate control system are high.
• The filtered water quality is not as good using gravity granular media filters as that obtained
using declining rate filter operation in potable water filtration28-29. This disadvantage,
however, is not important in wastewater filtration.
• The rate or level control systems frequently do not function properly causing sudden
changes in rate which can have a bad effect in filtrate quality. In many existing works,
the control systems are non functional.
27
-------
INFLUENT FLOW SPLITTING
A number of other alternative methods of flow control are coming into use that will supplant
the effluent flow control valve28 for gravity filters. For example, some plants have been constructed
so as to split the flow nearly equally (influent flow splitting) to all the operating filters, usually by
means of an influent weir box on each filter. A schematic diagram of such a gravity filter is shown
in figure IV-2. The advantages of this system include the following:
• Constant rate filtration is achieved without rate controllers if the total plant flow remains
constant.
• When a filter is taken out of service for backwashing or returned to service after backwashing,
the water level gradually rises or descends in the operating filters until sufficient head is
achieved to handle the flow. Thus, the rate changes are made slowly and smoothly without
the abrupt effects associated with automatic or manual control equipment, causing the
least harmful effect to filtered water quality in potable filtration experience3 0 >31. The
importance of this factor in wastewater filtration has not been studied. It would tend to be
most important when filtering wastewaters pretreated with alum or iron salts for coagulation
or phosphate reduction.
• The headloss for a particular filter is evidenced by the water level in the filter box. When
the water reaches a desired maximum level (the desired terminal headloss), backwashing of
that filter is required.
• The effluent control weir must be located above the sand to prevent accidental dewatering
of the filter bed. This arrangement eliminates completely the possibility of negative head
in the filter and the well-known and undesirable problems (air binding due to gases coming
out of solution) that sometimes result from it.
The only disadvantage of the influent flow splitting system is that additional depth of filter
box is required owing to the raising of the effluent control weir. The depth of filter box above
the effluent weir must be high enough to provide the design terminal headloss.
VARIABLE DECLINING RATE FILTRATION
Variable declining rate operation is similar to influent flow splitting, and is another desirable
method of operation for gravity filters. Variable declining rate operation achieves all the influent
flow splitting advantages and some additional ones, without any of the disadvantages. Despite the
merits of this method, however, it has not received enough explanation or attention28.
Figure IV-3a illustrates the desirable arrangement for new plants designed for variable de-
clining rate operation. Great similarity exists between figures IV-2a and IV-3a, the principal dif-
ferences being the location and type of influent arrangement and the provision of less available
headloss.
The method of operation is similar to that described for figure IV-2a, with the following ex-
ceptions. Figure IV-3b illustrates the typical water level variation and headloss variation observed
with this mode of operation. The filter influent enters below the wash trough level of the filters.
When the water level in the filters is below the level of the wash trough, the installation operates
as an influent flow splitting constant rate plant. When the water level is above the level of the wash
trough, the installation operates as a variable declining rate plant. In general, the only time the
filter water level will be below the wash trough level will be when all filters are backwashed in
rapid sequence or after the total plant has been shut down, with no influent, so that the water
level drops below the wash trough. In most cases, the clean filter headloss through the piping,
28
-------
High-water level
Weir box
Influent
Available
head I oss
Wash trough
Effluent
- ^.weir^
Waste
Low-water level
Backwash
Media
Clear
well
Underdrain
Drain
>— Constant rate —
/ . I \
10
4 -
Backwash
Headloss due to solids removed by filter
Headloss in clean
filter media
Headlost in underdrain* and piping
I I I L
_L
(b)
TIME DURING TWO FILTER RUNS
Figure IV—2. Influent flow splitting filtration: (a) typical filter and clear well arrangement;
(b} filtration rate, water level, and headloss during two filter runs.
29
-------
Influent valve or gate
to each filter
— High-water level
Common influent header
pipe or channel
Wash trough
Available
headloss
0-.y«i«.ty»vX-T;uv.
Low-water level
j Wastt
Wrr
*>¦ •
* * w «
Backwash
Media
Underdrain
Clear
well
Drain
"¦
Rate indication &
limitations
Backwash No. 1
Rate for fitter 1 only
"A.
Backwash
No. 1
Rise in water level during wash -
Water level
lame In all
filters
Solids removal
by filter
Clean media
wiooii iiieuia i •
^ |
Underdrain*
and piping
ii
v- 5
o
<8
Backwash
No. 1
(b)
TIME DURING ONE FILTER RUN
Figure IV-3. Variable declining rate filtration: (a) typical filter and clear well arrangement; (b) filtration rate,
headloss, and water level during one filter run in a plant having four filters.
30
-------
media, and underdrains will range from 3 to 4 feet and keep the actual low water level above the
wash trough. The water level is essentially the same in all operating filters at all times; this is achieved
by providing a relatively large influent header (pipe or channel) to serve all the filters, and a rela-
tively large influent valve or gate to each individual filter. Thus, headlosses along the header or through
the influent valve are small and do not restrict the flow to each filter. The header and influent valve
will be able to deliver whatever flow each individual filter is capable of taking at the moment. A flow
restricting orifice or valve is recommended in the effluent pipe to prevent excessively high filtration
rates when the filter is clean and to indicate the approximate clean bed filtration rate.
Each filter will accept at any time that proportion of the total flow that the common water
level above all filters will permit it to handle. As filtration continues, the flow through the dirtiest
filter tends to decrease the most rapidly, causing the flow to redistribute itself automatically so that
the cleaner filters pick up the capacity lost by the dirtier filters. The water level rises slightly in the
redistribution of flow to provide the additional head needed by the cleaner filters to pick up the de-
creased flow of the dirtier filters. The cleanest filter accepts the greatest flow increase in this redis-
tribution. As the water level rises, it partly offsets the decreased flow through the dirtier filters; as
a result, the flow rate does not decrease as much or as rapidly as expected.
This method of operation causes a gradually declining rate toward the end of a filter run. Filter
effluent quality is affected adversely by abrupt increases in the rate of flow — here, the rate in-
creases occur in the cleaner filters where they have the least effect on filter effluent quality30.
Rate changes throughout the day due to changes in total plant flow, both upward and downward
(in all of the filters, dirty or clean), occur gradually and smoothly without any automatic control
equipment.
The advantages of declining rate operation over constant rate operation are as follows28-29.
• For waters that show effluent degradation toward the end of the run, the method provides
significantly better filter effluent quality than that obtained with constant rate (or constant
water level) filter operation.
• Less available headloss is needed compared with that required for constant rate operation
because the flow rate through the filter decreases toward the end of the filter run. The head-
loss in the underdrain and effluent piping system therefore decreases (with the square of the
flow rate) and becomes available to sustain the run for a longer period than would be pos-
sible under constant rate operation with the same available head. Similarly, the head dissi-
pated through the clogged portions of the filter media decreases linearly with the decreasing
flow rate.
For the foregoing reasons, declining rate filters are considered to be the most desirable type of
gravity filter operation, unless the design terminal headloss is quite high (e.g., greater than 10 feet).
Then constant level control or pressure filters may be a more economical choice. A bank of pressure
filters can also operate using variable declining rate filtration; however, any rate changes imposed on
the plant cause sudden changes in filtration rates with pressure filters.
Some of the concerns and questions raised about variable declining rate filtration are as follows:
• It appears to be an uncontrolled system with little available operator manipulation. This is,
in fact, an attribute which prevents operational abuse of the delicate filtration mechanisms.
31
-------
• If the rate limiting device is sized for design year peak loads, it will permit higher than neces-
sary filtration rates in the early plant life. This is true unless one limits the headloss utilized
during the early plant life, i.e., backwashes at lower water levels.
• What is the total available headloss to be provided? This is a difficult question but no more
difficult than it has been in the past for constant rate filtration plants. It is best guided by
past experience at the plant in question, or by pilot testing. In the absence of these, one must
resort to an assumed solids capture per unit headloss design as discussed previously to select
terminal headloss, and make adjustments downward for the headloss recovery discussed
previously.
Surprisingly, the water level fluctuation in plants operating on this system is not as great as
anticipated. Typical variations of 1.5 to 2 feet (0.5 to 0.7 m) have been reported in potable water
plants32'33.
32
-------
Chapter V
BACKWASHING OF WASTEWATER FILTERS
The principal problems in filter operation are associated with maintaining the filter bed in good
condition. Inadequate cleaning leaves a thin layer of compressible dirt or floe around each grain of
the media. As pressure drop across the filter media increases during the subsequent filter run, the
grains are squeezed together and cracks may form in the surface of the media, usually along the
walls first.
The heavier deposits of solids near the surface of the media break into pieces during the back-
wash. These pieces, called mudballs, may not disintegrate during the backwash. If small enough
and of low density, they float on the surface of the fluidized media. If larger or heavier, they may
sink into the filter, to the bottom, or to the sand-coal interface in dual media filters. Ultimately,
they must be broken up or removed from the filter or they reduce filtration effectiveness, or cause
shorter filter runs by dissipating available headloss.
In wastewater filters, slimes can reduce the average density of the filter grains and can cause
more loss of filter media during backwashing, or migration of fine sands in dual media higher into
the coal layer. Filamentous growths can cause blinding of the surface layers which shorten filter
runs.
Dirty filter media may be chemically cleaned in place as a temporary expedient short of re-
building the filter bed. Various chemicals have been used, including chlorine, copper sulfate, acids
and alkalies. Chlorine may be used where the material to be removed includes living and dead or-
ganisms or their metabolites. Copper sulfate is effective in killing algae growing on the walls or
medium. Alkalies can be effective on greasy deposits on the filter grains.
However, rather than attempting to correct dirty filter problems after they occur, the back-
washing system should be designed to prevent them from the onset.
Potable water filter backwashing practice in the U.S. has used the high velocity wash with sub-
stantial bed expansion (20 to 50%). This method does not solve all problems with dirty filters, and it
has created problems with shifting of the finer supporting gravel layers. The provision of a surface
wash system which introduces high velocity water jets before and during the backwash has largely
solved the problem of dirty filter media for potable water filters, but has not solved the problem
of shifting gravel. The growing use of wastewater filtration has further demonstrated the weakness
of water fluidization backwash. Backwashing is substantially more difficult and problems of
agglomerates and filter cracks are prominent.
The problem of shifting gravel and the more difficult backwashing of a wastewater filter has
stimulated renewed interest in the air scour method of auxiliary agitation, which has continued in use
in European practice. There is also interest in the use of underdrain systems with fine strainers
that do not require gravel, a system which was abandoned in the U.S. in the early twentieth cen-
tury due to clogging and corrosion problems.
SURFACE WASH TO ASSIST BACKWASHING
Evidence of the benefits of surface wash led to its widespread adoption for potable water
filters in the United States. Surface wash is introduced at pressures of 45 to 75 psig through
33
-------
orifices on a fixed piping grid or on a rotating arm, located 1 to 2 inches above the fixed bed. Sur-
face wash flow rates are about 1 in/min for the rotary type and 3 to 6 in/min for the fixed nozzle
type. The desired operating sequence involves draining the filter to the wash trough level or below,
applying the surface wash flow with no concurrent backwash flow for 1 to 2 minutes to break up
surface layers on top of the media, then continuing the surface wash with concurrent backwash
flow for several minutes until the backwash water begins to clear up. The concurrent application
may be at two rates, first a low rate to barely immerse the surface wash jets in the media followed
by a period with normal bed expansion. The surface wash is then terminated and water fluidization
backwash alone follows for 1 to 2 minutes to stratify the bed, a provision which is only important
in dual or triple media filters.
AIR SCOUR TO ASSIST BACKWASHING
Air scour consists of the distribution of air over the entire filter area at the bottom of the filter
media so that it flows upward through the media. It is used in a number of fashions to improve the
effectiveness of backwashing, and/or to permit the use of lower backwash water flow rates. The air
may be used prior to the water backwash or concurrently with the water backwash. When used
concurrently during backwash overflow, there is legitimate concern over potential loss of filter
media to the overflow due to the violent agitation created by the air scour. When air is used alone,
the water level is lowered 6 to 8 inches below the overflow level to prevent loss of filter media dur-
ing the air scour.
Air scour may be introduced to the filter through a pipe system which is completely separate
from the backwash water system, or it may be added through the use of a common system of noz-
zles (strainers) which distribute both the air and water, either sequentially or simultaneously. In
either method of distribution, if the air is introduced below graded gravel supporting the filter media,
there is concern over the movement of the finer gravel. This may occur as air is expelled by the
water at the onset of the water wash, or especially by air and water used concurrently by intention
or accident. This concern has led to the use of media retaining strainers in some filters which
eliminate the need for graded support gravel in the filter. However, these strainers may clog with
time causing decreased backwash flow capability or, possibly, structural failure of the underdrain
system. Such failures have occurred27. This clogging may be due to fine sand or coal, which leaks
through the strainer during downflow filtration and later is lodged forcefully in the strainer slots
during water backwash. The underdrain plenum below the strainers must be scrupulously cleaned
before the strainers are installed to prevent construction dirt or debris from later clogging the
strainers during backwash.
In view of the concerns expressed above and the renewed interest in air scour in the United
States, a summary of European air scour practice (in potable water treatment primarily) may be
worthwhile because it has been used there since the beginriing of rapid filtration.
Current potable water practice in the U.K. uses air first followed by water backwash. Plastic
strainers with 3 mm slots are used in the underdrains covered by 2 or more layers of gravel to sup-
port the media. Single media sand filters are common with size range of 0.6 to 1.2 mm, but dual
media filters are becoming more common since about 1970. For single media sand filters, the water
wash rate is intended to just reach minimum fluidization velocity with only 1 to 2 percent bed
expansion. Air is introduced for 3 to 5 minutes through the gravel layers at rates of 1 to 1.5
scfm/sq ft*, sometimes up to 2 scfm/sq ft, followed by water at 12 in/min (7.5 gal/min/ft^). Prob-
lems with gravel movement have occurred but only in a few cases. The absence of such problems
must be attributed to the low water and air flow rates which are presumably not sufficient to move
the fine gravel, and the fact that air and water are not used simultaneously. The preference for strain-
ers with 3 mm slots and gravel is based on prior experience with clogging of strainers with 0.5 mm slots.
~Standard cubic feet per minute (at 70°F and 14.7 psia in U.S. practice).
34
-------
The backwashing practice on the European continent as described below is obtained from four
sources4- 34> 35• 36, and therefore, probably does not reflect the diversity of the continental prac-
tice. The sources describe several points about the continental practice which differ substantially
from both U.S. and U.K. practice.
• Deeper beds of coarser sand are used and the backwash of these sands is at low rates, with
little or no expansion of the bed.
• Backwash is with air and water simultaneously at low water rates followed by water alone.
The air rate is 2 to 4 cfm/sq ft and the water rate is 10 in/min (6.2 gal/min/ft2) for the smaller
sand sizes (1 to 2 mm size range) and 6 to 8 cfm/sq ft air and 10 to 12 in/min (6.2 to 7.5 gal/
min/ft2) water for the coarser sizes with size ranges such as 2 to 3 mm, 2 to 4 mm. In fact,
the use of air and water simultaneously is considered absolutely essential for proper back-
washing. The danger of media loss is acknowledged if the simultaneous air-water backwash
is continued during overflow. It is suggested that if loss is observed, the backwash water
rate be reduced during the simultaneous air water backwash35. Mudballs are unknown in
Europe using this type of bed design and backwash system.
• Supporting gravel is sometimes used but is of the double reverse graded gravel arrangement,
i.e., coarse to fine to coarse in gradation36. Otherwise, media retaining strainers are used but
the dangers of clogging and underdrain failure are acknowledged34.
The renewed use of air scour in the United States has been patterned more after the British
practice of using air scour alone first, followed by water backwash. U.S. air rates have been typically
3 to 5 scfm/sq ft for 3 to 5 minutes, and the subsequent water wash is above fluidization velocity to
expand and restratify the dual media bed, typically 24 to 36 in/min (15 to 22.5 gal/min/ft2). The filters
are usually equipped with media retaining underdrain strainers without graded gravel support for the
media. Because of the fine sand media used in the dual media beds (0.5 mm to 0.6 mm effective
size), the strainer openings are very small (0.25 to 0.5 mm) and the strainer clogging problems and some
underdrain failures have occurred therefrom. Because of this problem, a reconsideration of the U.S.
air scour design practice may be appropriate.
BACKWASHING RECOMMENDATIONS
In view of the difficulty of backwashing wastewater filters, and the various filter media and
backwash routines available, a research study was conducted to compare the various alternatives as
applied to wastewater filtration. Various granular media filters were studied including single, dual
and triple media. Various methods of backwashing were compared including:
• Water fluidization only
• Air scour followed by water fluidization
• Surface wash and subsurface wash before and during water fluidization backwash
• Surface wash and subsurface wash before and during water fluidization backwash
• Simultaneous air scour and subfluidization water backwash.
35
-------
Some of the conclusions of that study are important to the design of wastewater filters and are,
therefore, quoted below8:
• The cleaning of granular media filters by water backwash alone to fluidize the filter bed is
inherently a weak cleaning method because particle collisions do not occur in a fluidized
bed and thus abrasion between the filter grains is negligible.
• The weakness of water fluidization backwash alone was clearly demonstrated during waste-
water filtration studies where a dual media filter which was washed by water fluidization
alone developed serious dirty filter problems such as floating mud balls, agglomerates at the
walls and surface cracks. These problems were observed when filtering either secondary ef-
fluent or secondary effluent which had been treated with alum for phosphorous reduction.
• The heavy mud ball and agglomerate accumulations caused higher initial headlosses and
shorter filter cycles. They may also cause poorer filtrate quality in some cases, although
such detriment was not demonstrated in this research.
• Simultaneous air scour and subfluidization backwash of unstratified coarse sand filters
proved to be the most effective method of backwash. However, this method should not
be used for finer filter media such as the coals and sands of the typical sizes used in duBl
and triple media filters because loss of media will occur during backwash overflow. The
choice of simultaneous air and water flow rates must be appropriate for the sand being
used and should result in some circulation of the sand for effective backwashing.
• The other two methods of improving backwashing, namely air scour followed by water
fluidization backwash, and surface (and subsurface) wash before and during water fluidiza-
tion backwash, proved to be comparable methods of backwash which can be applied to
single, dual and triple media filters. These two methods did not completely eliminate all
dirty filter problems, but both auxiliaries reduced the problems to acceptable levels so
that filter performance was not impaired.
• The use of some form of air scour auxiliary, or some form of surface wash auxiliary, is
essential to the satisfactory functioning of wastewater filters comprised of deep beds
(2 to 5 ft) of granular material which are backwashed after several feet of headloss develop-
ment. The auxiliary and the backwash routine must be appropriate to the filter media.
For example, subfluidization wash is limited to single media filters because stratification
is not essential (or even desired) for such filters. Fluidization capability is essential for
dual or triple media filters to permit restratification of the layers in their desired positions
at the end of the backwash. Air scour and water backwash simultaneously during overflow
is primarily useful on coarse sand filters because finer media will be lost due to the violence
of the combined air and water action. However, the simultaneous use of air and water can
be useful on dual and triple media filters prior to the onset of backwash overflow. The
above conclusion is not intended to apply to all types of wastewater filters such as the
various proprietary filters with their special backwashing provisions. Such filters and pro-
visions were not studied.
• The use of graded gravel to support the filter media is not recommended where the simul-
taneous flow of air scour and backwash water can pass through the gravel by intention, or
by accident, due to the danger of moving the gravel and thus upsetting the desired size
stratification of the gravel.
• Media retaining underdrain strainers with openings of less than 1 mm are not recommended
for wastewater filters due to the danger of progressive clogging.
36
-------
• The filter influent feedwater (e.g., secondary effluent) is not recommended as a backwash
water source because of the danger of progressive clogging of underdrain strainers and/or
gravel. The advantages of using feedwater do not justify the risks that result therefrom.
• Air scour is compatible with dual or triple media filters from the standpoint of minimal
abrasive loss of the anthracite coal media. However, the backwash routine must be con-
cluded with a period of fluidization and bed expansion to restratify the media layers after
the air scour.
The authors urge you to use the foregoing conclusions as design guides. In addition, the fol-
lowing design suggestions concerning the backwashing provisions should also be considered.
First, consider the use of air scour as applied to dual or triple media filters backwashed with
fluidization capability. In this case:
• Provide operational flexibility in the period of air scour between 2 and 10 minutes so the
operator can select the period he deems most appropriate.
• If supporting gravel is not used, provide the capability for simultaneous air and water
backwash. This technique requires provisions to allow for rapid draining of the filter to
near the filter media surface, followed by the brief simultaneous air and water backwash
until the water reaches within 6 to 8 inches of the wash troughs. The simultaneous wash
is then stopped, and either air alone or water alone may be continued. The water rate
during the simultaneous air water wash should be below fluidization velocity to extend
the time duration of that action to the maximum.
• Provide a backwash water volume of at least 100 gal/ft2 of filter per wash. This is based
on the observation that when backwashing at rates above the fluidization velocity for the
media, the total wash water required for effective cleaning is about the same regardless of
the backwash rate—about 75 to 100 gal/ft^ of filter. This observation is for typical U.S.
wash trough spacing with the trough edges about 3 feet above the surface of the filter
media. Larger spacing between troughs, or greater height of trough above the media,
would increase the wash water requirements. No economy of total wash water use is
achieved by adopting lower backwash rates (above fluidization), because the length of re-
quired backwash must be increased proportionately.
Second, consider the use of air and water backwash simultaneously without fluidization capa-
bility. In this case:
• Provide a backwash water volume of about 150 to 200 gal/ft2 of filter per wash. This is larger
than the prior case because less experience is available.
• Because of the effective solids transport capability of air and water used simultaneously,
wash troughs can be eliminated in favor of a single overflow trough along the length of the
filter if the transport distance is limited to about 12 feet.
Third, consider the use of surface wash auxiliary in dual or triple media filters backwashed with
fluidization capability.
• Provide a subsurface washer (as well as the surface washer) to attack the mudballs that sink
to the interface between the coal and the sand. The subsurface jets should be located at the
expected depth of the expanded interface. The writers have no information on the ability
of full scale rotary subsurface washers to remain operational in the long term due to the
greater drag they encounter, and the hostile environment. The pilot rotary subsurface
37
-------
washer used in the foregoing research was not a good model of a full scale unit, and con-
siderable difficulty was encountered in keeping it operational.
Two additional backwashing problems are of importance in wastewater filter plant design.
• Where does one get the water for backwashing?
• What is done with the dirty backwash water?
The best source of water for backwashing will be the effluent from the filters. If disinfection
of the plant effluent is practiced, the chlorine or ozone contact tank should provide sufficient ca-
pacity to permit drawing backwash water from this tank. If disinfection is not provided, then a
special backwash storage tank should be provided, through which all filter effluent should be di-
rected before final discharge. The backwash water storage tank should normally have sufficient
capacity to store all the water needed to backwash at least three filters in succession with the
volumes suggested above.
The dirty backwash water must be returned to the plant influent for further treatment. Be-
cause of the nonuniform scheduling of filter backwashing, the backwash water presents a signifi-
cant sludge load on the primary or secondary treatment facilities if returned to them at the rate of
backwashing. For that reason, dirty backwash water should be sent to a dirty backwash storage
tank and delivered from there at a nearly constant rate to the plant influent or secondary influent.
If flow equalization is not being practiced at the plant, it would be desirable to return the back-
wash wastewater during the low flow period of the day. This would entail a larger wastewater
storage tank and return pumping capability, but it would assist in flow balancing.
38
-------
Chapter VI
SUMMARY
The key questions involved in the proper design of granular filters for wastewater filtration
have been discussed in the foregoing sections, and design recommendations have been presented.
These recommendations are summarized as follows:
• The variable hydraulic and suspended solids load in secondary effluents must be considered
in the design to avoid short filter runs and excessive backwash water requirements.
• A filter that allows penetration of suspended solids is essential to obtain reasonable filter
run lengths. The filter media on the influent side should be at least 1.2 mm for tertiary
filtration, and preferably larger if appropriate backwash is provided.
• The filtration rate and terminal headloss should be selected to achieve a minimum filter
run length of 6 to 8 hours if flow equalization is not provided. Estimates of headloss de-
velopment and filtrate quality preferably should be based on pilot scale observations at the
particular installation. If such studies are not feasible, headloss development should be
based on past experience on the suspended solids capture per foot of headloss increase from
other similar installations.
• The effect of recycling of used backwash water through the plant on the filtration rate and
filter operation must be considered in predicting peak loads on the filters and resulting run
lengths.
• High filtration rates (3 gal/min/ft2 or higher at average load) and/or high influent suspended
solids to the filters (30 mg/1 or higher at average load) will cause high terminal headlosses
and may favor the use of pressure filters over gravity filters, especially for smaller plants
with limited capital resources.
• Lower filtration rates or lower influent suspended solids may permit the economical use of
gravity filters, especially in larger plants where multiple filters will be needed. At least two,
and preferably four, filters should be provided. If only two filters are provided, each should
be capable of handling peak design flows to allow for one filter to be out of service for
backwashing or repair. If four or more gravity filters are provided, the variable declining
rate method of operation is strongly recommended.
• The success of the wastewater filtration plant depends upon the provision of an effective
backwash system which is appropriate for the media selected. Details of backwashing re-
quirements for dual and triple media Alters and for unstratified coarse sand filters are pre-
sented.
39
-------
REFERENCES
1W. J. Weber, Jr., Physicochemical Processes for Water Quality Control, Wiley Interscience,
New York, N.Y., 1972.
2 "Process Design Manual for Suspended Solids Removal," U.S. Environmental Protection
Agency, EPA-625/l-75-003a, Washington, D.C., Jan. 75.
3 "Process Design Manual for Upgrading Existing Wastewater Treatment Plants," U.S. Envi-
ronmental Protection Agency, Washington, D.C., Oct. 1974.
4 H. Jung and E.S. Savage "Deep Bed Filtration," J. Amer. Water Works Assoc., 66:73-78,
1974.
5 J. R. Joslin and G. Greene, "Sand Filter Experiments at Derby," Water Pollution Control,
69:611-622,1970.
6 A. E. Naylor, S. C. Evans, and K. M. Dunscombe, "Recent Developments on the Rapid Sand
Filters at Luton," Water Pollution Control (British), pp. 309-320,1967.
7 J. Y. C. Huang, "Granular Filters for Tertiary Wastewater Treatment," unpublished doctoral
thesis, Iowa State University Library, Ames, la., 1972.
8 J. L. Cleasby, "Backwash of Granular Filters Used in Wastewater Filtration," Final Report,
EPA Project R802140, Jan. 1976. (ISU-ERI-Ames 76114.)
9 R. Nebolsine, I. Pouschine, Jr., and C. Y. Fan, "Ultra High Rate Filtration of Activated
Sludge Plant Effluent," U.S. Environmental Protection Agency, Office of Research and Monitoring,
EPA-R2-73-222, Apr. 1973.
10 S. C. Evans and F. W. Roberts, "Twelve Months' Operation of Sand Filtration and Micro-
Straining Plant at Luton," J. Proc., Inst. Sewage Purif., pt 4:333-341,1952.
11S. C. Evans, "Ten Years of Operation and Development at Luton Sewage Treatment Works,"
Water Sewage Works, 104:214-219,1957.
12 K. J. Merry, "Tertiary Treatment of Domestic Wastewater by Rapid Sand Filtration," un-
published master's thesis, Iowa State University Library, Ames, la., 1965.
13 N. P. Nicolle, "Humus Tank Performance, Microstraining and Sand Filtration," J. Proc.,
Inst. Sewage Purif., pt 1:19,1955.
14 A. E. S. Pettet, W. F. Collett, and T. H. Summers, "Mechanical Filtration of Sewage Ef-
fluents. I. Removal of Humus," J. Proc., Inst. Sewage Purif., pt 4:399-411, 1949.
15M. F. Dahab, "Single-Media, Unstratified-Bed Filtration of Secondary Effluent," un-
published master's thesis, Iowa State University Library, Ames, la., 1976.
40
-------
16 R. Wood, W. S. Smith, and J. K. Murray, "An Investigation Into Upflow Filtration," Water
Pollution Control (British), 67:421-426, 1968.
17 G. Tchobanoglous, "Filtration Techniques in Tertiary Treatment," J. Water Pollution Control
Fed., 42:604-623,1970.
18 D. A. Willis, "Variable Declining-Rate Filtration of Secondary Effluent," unpublished master's
thesis, Iowa State University Library, Ames, la., 1976.
19 J. Y. C. Huang, "Least Cost Sand Filter Design for Iron Removal," unpublished master's
thesis, Iowa State University Library, Ames, la., 1969.
20 E. R. Baumann, Least Cost Design—Optimization of Deep Bed Filters, The Scientific Basis
of Filtration, K. J. Ives, Editor, Noordoof International Publishing, Leyden, The Netherlands, 1975.
21 J. Y. C. Huang and E. R. Baumann, "Least Cost Sand Filter Design for Iron Removal,"
J. Sanit. Engr. Div., Amer. Soc. Civil Engr., 97:SA2,171-190, Apr. 1971.
22 K. J. Ives, "Optimization of Deep Bed Filters," Proceedings, First Pac. Chem. Engr. Cong.,
Soc. Chem. Engr., Japan, pt. 1:99-107, Oct. 1972.
23 G. Tchobanoglous and R. Eliassen, "Filtration of Treated Sewage Effluent," J. Sanit. Engr.
Div., Amer. Soc. Civil Engr., 96:243-265, 1970.
24 E. R. Baumann and J. Y. C. Huang, "Granular Filters for Tertiary Wastewater Treatment,"
J. Water Pollution Control Fed., 46:8,1958-1973, Aug. 1974.
25 T. F. Camp, "Discussion-Experience with Anthracite Filters," J. Amer. Water Works Assoc.,
53:1478-1483,1961.
26 J. L. Cleasby and C. F. Woods, "Intermixing of Dual Media and Multi-Media Granular
Filters," J. Amer. Water Works Assoc., 67:4,197-203, Apr. 1975.
27 J. L. Cleasby, E. W. Stangl, and G. A. Rice, "Developments in Backwashing Granular
Filters," J. Env. Eng. Div., Amer. Soc. Civil Engr., 101:EE5, 713-727, 1975.
28 J. L. Cleasby, "Filter Rate Control Without Rate Controllers," J. Amer. Water Works Assoc.,
61:4,181-185, Apr. 1969.
29H. E. Hudson, Jr., "Declining Rate Filtration," J. Amer. Water Works Assoc., 51:11,1455,
Nov. 1959.
30 J. L. Cleasby, M. M. Williamson, and E. R. Baumann, "Effect of Filtration Rate Changes on
Quality," J. Amer. Water Works Assoc., 55:7, 869-880, July 1963.
31 J. Tuepker, "Filter Performance Under Varying Operating Conditions," Proceedings of Con-
ference on Water Filtration, University of Missouri, Rolla, Mo., 1965.
41
-------
32 J. Arboleda, Jr., "Hydraulic Control Systems of Constant and Declining Rate in Filtration,"
J. Amer. Water Works Assoc., 66:87-94,1974.
33 J. L. Cleasby, "New Ideas in Filter Control Systems," In the Proceedings of a Symposium
"Procesos Modernos De Tratamiento De Agua" XIII Congreso Interamericano de Ingenieria Sani-
taria, Published by the Pan American Health Organization, Aug. 1972.
34 L. Huisman, "Rapid Filtration, Part 1," Delft University of Technology, Department of
Civil Engineering, mimeographed book of lectures in English, 1974.
35 S. A. Degremont, Water Treatment Handbook, Third English Edition, distributed by Hugh
K. Elliot, Ltd., 2a Russell Gardens Mews, Kensington, London W14, England, 1965.
36 E. S. Savage (Dravo Corp.), personal communication, February 19, 1975, describing media
and backwash provisions used by Pintsch Bamag (Germany) and Dravo (U.S.), 1975.
42
-------
Appendix A
PERFORMANCE DATA
FOR
WASTEWATER FILTRATION
FROM
THE LITERATURE
43
-------
Table A-1.—Reported Efficiencies For Direct Filtration Of Trickling Filter Plant Effluentsa
Location
Source
Media
Filter
Nc
Suspended solids (mg/l)
BODg (mg/l)
Type
Sizeb
Depth
Rate
gpm/sq ft
Influent
Effluent
Influent
Effluent
%
(mm)
(in.)
(U.S.)
Range
Avg
Range
II
Range Avg
Range Avg
Removal
Luton, England (1949)
1
Lab study
sand
.852.0
21
2.0
5
34 77
53
1-20
6
coal
1.0-2.0
21
2.0
7
41-67
51
4-13
7
sand
0.5-0.85
21
2.0
2
40-59
50
1-2
2
sand
0.5-1.0
21
2.0
2
49
49
0
0
Pilot study
sand
0.85-1.7
24
2.33
3/77
20-37
28
2-5
3
24-40 30
8-15
11
sand
0.85-1.7
24
2.66
1/77
20
20
3
3
31 31
15
15
sand
0.85-1.7
24
2.83
5/77
15-25
19
2-5
3
17-28 21
6-9
7
coal
0.85-2
18
1.7
2m
20-37
29
3-5
4
27-40 34
10-14
12
2-5
10
2.8
1m
15-28
21
2-5
3
17-28 23
6-13
8
Finham, England
1
sand
1.0-2.0
24
1.6
m
32
5
26
10
Pilot study
sand
1.0-2.0
24
1.9-2.4
m
40
6
31
10
<1949)
sand
1.0-2.0
24
2.9-3.2
m
35-36
7-8
22-25
10-13
sand
1.0-2.0
24
3.2-3.8
m
34-35
7-9
27-38
11-14
coal
1.0-2.0
24
1.4-1.9
m
43
10
32
14
2-5
3
2.0-2.3
m
38
5
27
10
2.4-2.8
m
32
11
35
14
2.9-3.2
m
34
8
28
13
3.2-3.7
m
34-35
7-8
30-37
14
Luton, England
Full scale (1957)
2
sand
2.5
12/77
7-18
13
1-7
6-15 9
2.8
4
Full scale (1967)
3
sand
0.85-1.7
36
3.4
3m
28-35
9-10
9-10
3-4
sand
0.85-1.7
36
5
3m
13
8
5
3
Pilot scale Upflow
sand
0.85-1.7
60
3.4
3/77
29-35
5
9-10
3
sand
0.85-1.7
60
4.8
3/77
13
6
5
3
Derby, England
4
sand
1.2-2.3
24
2.0
4
25-35
29
7-14
10
Pilot study (1970)
3.0
2
29-31
30
10-13
12
34-45
4.0
5
27-31
29
12-14
13
11-39
8.0
2
24-29
27
16-18
17
sand
1.2-1.7
24
2.0
4
26-35
29
7-15
10
45
3.0
2
29-31
30
9-13
11
15-47
4.0
5
24-32
28
11-14
13
8.0
2
23-29
26
16-18
17
-------
Table A-1 .—Reported Efficiencies for Direct Filtration Of Trickling Filter Plant Effluents? (continued)
Location
Source
Media
Filter
Rate
gpm/sq ft
(U.S.)
N°
Suspended solids (mg/l)
BODg (mg/l)
Type
Sizeb
(mm)
Depth
(in.)
Influent
Range Avg
Effluent
Range Avg
Influent
Range Avg
Effluent
Range Avg
%
Removal
Triple media "a"
coal
1.4-2.3
8
2.0
2
25-35
29
6-12
8
sand
1.2-1.4
8
4.0
4
27-28
27
11-13
12
33
garnet
0.7-0.85
8
8.0
3
23-29
26
15
15
Upflow sand "a"
sand
0.7-2.3
24
2.0
2
28-37
33
9-15
12
4.0
3
27-30
27
13-14
13
8.0
2
23-29
26
16-17
17
Triple media "b"
coal
1.4-2.3
8
3.0
2
29-31
30
7-11
9
38
sand
0.85-1.0
8
4.0
2
29-32
31
10
10
38
garnet
0.7-0.85
8
Upflow sand "b"
sand
0.85-2.3
24
3.0
2
29-31
30
10-14
12
4.0
2
28-32
30
11-14
13
Derby, England
4 sand
1.2-2.3
24
3.0
2
22-25
24
9-10
10
31
Pilot study after
4.0
5
20-24
22
7-10
9
50-64
trickling filter
6.0
2
19-26
23
10-11
11
35
improvement
sand
1.2-1.7
24
3.0
2
22-25
24
9-10
10
54
4.0
4
21-24
23
8-10
9
53-65
Triple media "b"
triple (see above)
3.0
1
22
22
8
8
4.0
5
20-24
22
6-9
8
6.0
2
19-26
23
9
9
Upflow sand "b"
sand (see above)
3.0
2
22-25
24
8-10
9
69
4.0
4
21-24
23
8-11
10
59-71
Upflow sand
sand
1.2-2.3
36
6.0
4
19-26
23
9-11
10
43-67
Ames, Iowa
Pilot study
5 sand
0.55 ES
24
2.0
15
11 49
20
1-15
6
38-115 56
13-49
24
(1965)
2.36 UC
4.0
15
ia58
19
1-24
6
29-130 53
15-65
23
6.0
15
8-60
18
2-27
6
25-132 50
13-74
24
-------
Table A-1 —Reported Efficiencies for Direct Filtration Of Trickling Filter Plant Effluentsa (continued)
Location
Source
Media
Type
Sizeb
(mm)
Depth
(in.)
Filter
Rate
gpm/sq ft
(U.S.)
Nc
Suspended solids (mg/l)
BOD5 (mg/l)
Influent
Effluent
Influent
Effluent
Range Avg Range Avg Range Avg Range Avg Removal
Pilot study (1973)
unpublished
dual media
coal 0.9 ES 12
sand 0.4 ES 12
1.7 12
21-75
33
1-8
39-85 57
6-19 13
Prepared by Gary A. Rice and John L. Cleasby, Iowa State University, March. 1974.
aBlank spaces in table due to data missing or not presented in manner needed for table, e.g., for averaging. All mg/l values rounded to nearest 1 mg/l.
bRange in size given, British practice, or ES (effective size) and UC (uniformity coefficient), U.S. practice.
C/V = number of values reported in the range and average presented. N generally represents individual filter runs unless followed by the letter m which indicates the number of
average monthly values presented, m without numeral means average of several months data (unspecified duration).
-P>-
Os
-------
Table A-2.—Reported Efficiencies For Direct Filtration Of Activated Sludge Plant Effluents3
BODg (mg/l)
Influent Effluent %
Range Avg Range Avg Removal
West Hertfordshire,
England (1968)
6
gravel
40-50
6
2.16
10-89
44
1-2
22
58
3.9
gravel
8-12
10
4.0
9-70
37
2-7
3.7
53
4.6
Pilot scale "Boby" upflow
gravel
2-3
10
5.0
12-128
55
1-17
7.1
42
5.6
filter
sand
1-2
60
6.0
5-97
37
2-22
9.9
35
4.7
Letchworth, England
7
(1968)
gravel
20-30
4
3-4
81
10-26
22
1-12
5.5
gravel
10-15
4
4-6
66
10-28
16
2-15
6.7
Pilot scale "Boby" upflow
gravel
2-3
4
6-8
65
7-24
14
6-14
8.8
filter
sand
1-2
60
Los Angeles, Calif. (1961)
8
sand
0.95 ES
11
2
5
19-34
27
7-21
15
6-15
10
2.8
4
Preliminary tests
1.6 UC
Philowith, Ore. (1967)
9
mixed
—
30
5
30-2180
59
1-20
4.6
17-36
26
1-4
2.5
Extended aeration AS)
media
Peoria, III. (1964)
10
sand
1.1
35
8
45
17
(High rate AS)
Cleveland, Ohio
11
dual media
coal
1.78 ES
60
8
1
20
5
19
14
1.63 UC
16
1
27
8
9
5
sand
0.95 ES
24
24
2
22-23
9-11
9-10
4-6
1.41 UC
32
1
29
14
9
7
Declining rate filters at
dual media
Cleveland. (Avg filter
coal
4.0 ES
60
8
1
13
4
7
5
rate presented)
1.5 UC
16
1
13
4
7
5
sand
2.0 ES
24
24
1
13
6
7
5
1.32 UC
Location Source Media Filter JhF Suspended solids (mg/l)
~~ Rate
Type Sizeb Depth gpm/sq ft Influent Effluent
• (mm) ('"¦) (U.S.) Range Avg Range Avg
Prepared by Gary A. Rice and John L. Cleasby, Iowa State University, March, 1974.
aBlank spaces in table due to data missing or not presented in manner needed for table, e.g., for averaging. All mg/l values rounded to nearest 1 mg/l.
bRange in size given, British practice, or ES (effective size) and UC (uniformity coefficient), U.S. practice.
CN = number of values reported in the range and average presented. N generally represents individual filter runs unless followed by the letter m which indicates the number of
average monthly values presented, m without numeral means average of several months data (unspecified duration).
-------
SOURCES
1 A. E. J. Pettet, W. F. Collett, and T. H. Summers "Mechanical Filtration of Sewage
Effluents. I. Removal of Humus," J. Proc., Inst. Sewage Purif., Part 4:399-411, 1949.
2 S. C. Evans, "Ten Years of Operation and Development at Luton Sewage Treatment Works,"
Water and Sewage Works, 104:214-219, 1957.
3 A. A. Naylor, S. C. Evans, and K. M. Dunscombe, "Recent Developments on the Rapid
Sand Filters at Luton," Water Pollution Control (British), 66:309-315, 1967.
4 J. R. Joslin and G. Greene, "Sand Filter Experiments at Derby," Water Pollution Control
(British), 69:611-622,1970.
5 K. J. Merry, "Tertiary Treatment of Domestic Wastewater by Rapid Sand Filtration,"
unpublished master's thesis, Iowa State University Library, Ames, la., 1965.
6 R. Wood, W. S. Smith, and J. K. Murray, "An Investigation into Upward Flow Filtration,"
Water Pollution Control (British), 67:421-426, 1968.
7 G. A. Truesdale and A. E. Birkbeck, "Tertiary Treatment of Activated Sludge Effluents,"
Water Pollution Control (British), 67:483-492, 1968.
8 F. B. Laverty, L. A. Meyerson, and R. Stone, "Reclaiming Hyperion Effluent," J. Sanit.
Engr. Div., Amer. Soc. Civil Engr., 87:1-40, 1961.
9 G. L. Culp and S. P. Hansen, "Extended Aeration Effluent Polishing by Mixing Media
Filtration," Water and Sewage Works, 114:46-51, 1967.
10 E. B. Fall, Jr. and L. S. Kraus, "Tertiary Treatment for High Rate Activated Sludge
Effluent," unpublished paper presented at the 37th Central States Water Pollution Control Associa-
tion meeting, June 1964.
11R. Nebolsine, I. Pouschine, and C. Y. Fan, "Ultra-High Rate Filtration of Activated Sludge
Plant Effluent," U.S. Environmental Protection Agency, Office of Research and Monitoring, EPA-
R2-73-222, Apr. 1973.
48 -&U.S. GOVERNMENT PRINTING OFFICE-. 1977-757- 1W6569 Res I on No. 5-11
-------
METRIC CONVERSION TABLES
Recommended Units
Recommended Units
Description
Length
Mats
Force
Moment or
torque
Symbol
Customary
Equivalents"
meter
kilometer
millimeter
micrometer or
micron
square meter
square kilometer
square millimeter
hectare
cubic meter
litre
kilogram
gram
milligram
tonne
newton meter
Flow (volumetric) cubic meter
per second
liter per second
m
km
mm
|im or n
m2
km2
mm2
ha
kg
9
mg
t
m3/s
Basic SJ unit
Thi hectare (10,000
m2l is a recognized
multiple unit and will
remain in interna-
tional use.
Basic SJ unit
1 tonne = 1,000 kg
The newton is that
force that produces
an acceleration of
1 m/s2 in a mass
of 1 kg.
The meter is mea-
sured perpendicular
to the line of action
of the force N.
Not a joule.
39.37 m = 3.281 ft =
1.094 yd
0.6214 mi
0.03937 in
3.937 X 10 s in = 1 X 104 A
10.76 sq ft = 1.196 iq yd
0.3861 sq mi - 247.1 acres
0.001550 sq in
2.471 acres
35.31 cu ft 3 1,308 cu yd
1.057 qt = 0.2642 gal =
0,8107 X 10"4 acre ft
2.205 lb
0.03527 oz- 15.43 gr
0.01 543 gr
0.9842 ton (long) s
1.102 ton (short)
0.2248 lb
* 7.233 poundals
0.7375 lb ft
23.73 poundal-ft
15.850 gpm =
2,119cfm
15.85 gpm
Description
Unit
Symbol
Comments
Customary
Equivalents*
Velocity
linear
meter per
second
millimeter
per second
kilometers
per second
m/s
mm/s
km/s
3.281 fpi
0.003281 fps
2,237 mph
angular
radians per
second
rad/s
9.549 rpm
Viscosity
pascal second
Pa-s
0.6722 poundal{i)/sq ft
centipoise
Z
1.450 X 10 7 Reyn (#i)
Pressure or
stress
newton per
square meter
or pascal
N/m2
or
Pa
0.0001450 Ib/sq in
kilonewton per
square meter
or kilopascel
kN/m2
or
kPa
0.14507 Ib/sq in
bar
bar
14.50 Ib/sq in
Temperature
Celsius (centigrade)
°C
(°F-32|/1.8
Kelvin (abs.)
°K
°C+ 273.2
Work, energy,
quantity of heat
joule
J
1 joule ¦ 1 N-m
where meters are
meesured along
the line of Ktion
of force N.
2.778 X 10 7
kw-hr =
3.725 X 10"7
hp-hrs 0.7376
fft-lb = 9.478 X
10-4 Btu
kilojoule
kj
2.77BX 10"4 kw-hr
Power
watt
kilowatt
joule per second
W
kW
J/s
1 watt * 1 J/s
44.25 ft-lbs/min
1.34 Up
3.412 Blu/hr
Application of Units
Application of Units
Description
Precipitation,
runoff,
evaporation
Flow
Discharges or
abstractions,
yields
Usage of water
Unit
Symbol
Comments
Customary
Equivalents*
millimeter
cubic meter
per second
liter per second
cubic meter
per day
cubic meter
per year
liter per person
per day
m3/s
m3/d
n3/yei
l/person/
day
For meteorological
purposes, it may be
convenient to meas-
sure precipitetion in
terms of mast/unit
area (kg/m2).
1 mm of rain s
1 kg/m2
1 l/t * 86.4 m3/d
35.31 cfs
15.85 gpm
0.1835 gpm
264.2 gal/year
0.2642 gcpd
Description
Density
BOO loading
Hydreulic toed
per unit area,
e.g., filtration
rates
Air supply
Optical units
Symbol
Comments
Customary
Equivalents*
kilogram per
cubic meter
milligram per
liter (water)
kilogram per
cubic meter
per dey
cubic meter or
liter of free air
per second
kg/m3
mg/i
kg/m3/d
m3/s
The density of water 0.06242 Ib/cu ft
under standard
conditions is 1,000
kg/m3 or 1,000 g/l
or 1 g/ml.
1 ppm
0.06242 Ib/cu ft/day
cubic meter m3/m2/d
per square metir
per day
If this is converted
to a velocity, it
should be expressed
in mm/s (Imm/ss
86.4 m3/m2/day>.
3.281 cu ft/sq ft/day
lumen per lumen/m2
square meter
0.09294 ft candle/sq ft
•Miles ire U.S. statute, qt and gal are U.S. liquid, and oz and lb are avoirdupois.
------- |