PRIMARY EFFLUENT FILTRATION
WITH A
PULSED BED FILTER
A TECHNICAL NOTE
Prepared For:
U.S. Environmental Protection Agency
Office of Municipal Pollution Control
Municipal Facilities Division
Washington, DC 20460
September 1986
Prepared By:
Environmental Resources Management, Inc.
999 West Chester Pike
West Chester, Pennsylvania 19382
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LIST OF FIGURES AND TABLES
Page
Figure 1 - Cross-Section Through Pulsed-Bed Pilot
Filter 2
Table 1 - Comparison of Primary Effluent and Secondary
Effluent Characteristics 5
Table 2 - Recommended Design Criteria For
Hydro-Clear/PEF 7
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TABLE OF CONTENTS
Page
I. Description of the Process and Process Options 1
A. Process and Process Options 1
B. Existing and Proposed Installations 3
C. Characteristics of Primary Effluent in
Municipal Systems 3
D. Manufacturer's Claims 4
II. Evaluation of Design Procedure - New Construction
and Retrofits 4
A. Design Procedure Recommended by the
Manufacturer/Supplier 4
B. Comments on the Design Procedure 6
III. Evaluation of System Performance 8
A. Availability and Suitability of Existing
Operations Data 8
B. Comments on Manufacturer's Claims 8
IV. Level of Confidence in the Concept of Using
A Pulsed Bed Filter 9
A. Influent Variability and Impact on System
Performance/Suitability of System Design 9
B. Confidence in Designing a System Without
the Benefit of Field Testing 9
C. Benefits of Strengthening Certain Design
Parameters via a Field Test 10
V. Summary of the Hydro-Clear® Filter/PEF 10
Appendix
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ACKNOWLEDGEMENTS AND DISCLAIMER
The information in this document was prepared for the Office of
Municipal Pollution Control under contract number 68-01-7108.
The information was compiled to assist those involved in the
innovative and alternative technology program. This document has
not been subjected to the agency's peer and administrative review
and therefore does not necessarily reflect the views of the
agency.
ERM would like to thank Gary Garzenetti (formerly of Hydro-Clear
Corporation), and Neil Neymier and James Force of Hydro-Clear
Corporation for their assistance in technical review of the
Hydro-Clear® filter as applied to primary effluent filtration
(PEF) . These individuals provided product literature, case study
evaluations, and technical discussions valuable to the completion
of the review.
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SECTION 1
I. Description of the Process and Process Options
A. Process and Process Options
Primary effluent filtration (PEF) has been attempted before with
various filter systems but has shown recent success only as a new
application of the Hydro-Clear® Pulsed Bed Filter manufactured by
Hydro-Clear Corporation. The manufacturer has extensively tested
this filter for secondary and tertiary filtration and obtained a
patent for both complete, prepackaged or partially assembled
designs. In addition, Hydro-Clear promotes primary effluent
filtration, a process which it claims cannot be efficiently
accomplished by conventional deep-bed filters (Reference:
"Hydro-Clear PEF: A Dramatic Breakthrough in Wastewater
Treatment," Hydro-Clear Corporation). Figure 1 shows a
cross-section of the Pulsed Bed Filter.
The Pulsed Bed Filter has five standard features and one optional
one, important in the filtration of primary effluent (Reference:
"Operation of the Hydro-Clear® Filter," Hydro-Clear Corporation).
First, the filter contains uniform, fine-grained sand in order to
remove solids by surface straining. Second, low-pressure
Air-Mix® diffusers release fine bubbles just above the sand
surface. The diffusers are designed to prolong the filter run by
keeping influent solids suspended above the bed and by scouring
solids from the bed surface. Third, the filter uses a Pulse-Mix®
cycle consisting of air forced up through the filter at preset
intervals, in order to dislodge solids clogging the surface of
the bed and subsequently trap them within the bed. These three
concepts are not unique to the Hydro-Clear® filter.
A fourth Hydro-Clear® filter feature is an underdrain system
designed to allow proper functioning of the Pulse-Mix® cycle.
Its design includes modular spaces in which atmospheric air
accumulates and evenly spaced perforations to allow water and air
into the sand bed during various cleaning cycles. During the
pulsed air cycle, underdrain valves close to stop the drain of
water. Backwash pumps then force water into the air chamber, and
propel the air through the perforations and up through the sand
bed. The underdrain system is also designed to provide.the
Hydro-Scour® backwash system, a fifth filter feature. During
backwashing, pumping continues to force both the accumulated air
and clean wash water up through the underdrain. This cycle is
intended to wash out solids into the backwash trough with minimal
bed fluidization.
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Figure 1
Cross-Section Through Pulsed-Bed Pilot Filter
Backwash Sensor -
Backwash Trough
Pulse Mix Sensor
Air Mix Sensor -
Sand Bed
Underdrain -
Influent
- Splash Plate
Backwash Water
c±=> oacmrasn *»a
to Mudweil
Air Diffuser
-Low Pressure Air
-Wire Mesh
Rttrate to Clearweli
Cl
Backwash Water
From Clearweli
Not to Scale
(Adapted from "Primary Effluent nitration: Feasibility and Applications", Hydro Clear
Corporation)
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Cleaning cycles are staggered among all filter cells installed.
Filtration begins with the flow of wastewater over notched weirs,
onto the splash plates, through the sand bed and underdrain, and
to the clear well. When headloss causes the water to rise to the
lowest water level indicator, the Air-Mix® diffusers are
activated and operated for a preset time period as filtration
continues. When headloss can no longer be lowered with the
Air-Mix®, the water level builds up and touches the next highest
level indicator, which is intended to initiate the Pulse-Mix®
cycle without halting filtration. The Pulse-Mix® cycles on and
off a preset number of times, in order to provide some headloss
recovery each time. Afterwards, headloss is allowed to build up
again until water touches the highest level indicator and
triggers the Hydro-Scour® backwashing. The Hydro-Scour® operates
for a preset length of time and is intended to completely restore
the available head.
The Hydro-Clear® filter can be equipped with an optional,
semi-automated Chem-Clean® system, employing alternating
Hydro-Scour® and soaking cycles to wash accumulated grease and
mudballs out of the bed. Cleaning chemicals, such as
hyprochlorite or hydrogen peroxide, are used.
B. Existing and Proposed Installations
While over 350 Hydro-Clear® filter systems have been installed in
the United States, only one retrofit filter and nine new filters
have been installed for PEF in municipal systems, (Reference:
Personal Communication, Hydro-Clear Corporation). Of these, six
are operating, full-scale systems; three are currently operating
pilot plants; and four were pilot plants that are now dismantled
(Reference: "Primary Effluent Filtrations with the
Hydro-Clear® Filter," Hydro-Clear Corporation). A full-scale PEF
project at the Warminster, Pennsylvania Municipal Wastewater
Treatment Plant was only recently completed after five years of
operation. The EPA's "1985 Progress Report: Innovative and
Alternative Technology Projects" indicates that the agency had
funded primary effluent filtration projects at Warminster,
Pennsylvania, and at three other locations. All of these
projects employed Hydro-Clear® filter systems.
C. Characteristics of Primary Effluent in Municipal
Primary effluent is raw wastewater that has been treated by grit
removal and clarification. It often contains high concentrations
of suspended and colloidal solids, which vary in organic content
and range widely in particle size—anywhere from 0.08 microns to
>35 microns (Reference: "Filtration of Primary Effluent," Journal
of Water Pollution Control, Matsumoto et al). Studies conducted
at the University of California at Davis concluded that
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filterable solids in primary effluent are largely organic
(Reference: "The Significance of Filterable Solids in the
Performance of Wastewater Treatment Processes," University of
California at Davis). Table 1 shows the characteristics of a
primary effluent as reported in two recent studies.
D. Manufacturer's Claims
The manufacturer cites the following main advantages of using
Hydro-Clear® filters for primary effluent filtration
( P E F ) ( Reference : Personal Communication, Hydro-Clear
Corporation):
1. Removes discrete particles of less than one micron, which
are found in high percentages in primary effluent. This
results in decreased solids and organic loading of
biological treatment systems downstream and potentially
decreases the size requirements of these systems.
2. Filters primary effluent with up to six times the run time
of conventional sand filters, due to the Air-Mix® and
Pulse-Mix® features.
3. Requires one-sixth or less of the backwash water
requirement of conventional systems due to the longer run
time. This is attributed to the effects of the
Hydro-Scour® and optional grease-cleaning features.
4. Efficiently filters wastewater having a wide variation in
solids content due to the control of cleaning processes by
water-level indicators.
In addition, the Hydro-Clear® filter reduces the relative expense
of PEF by maintaining lower water levels over the bed and thus
allowing the use of smaller, less expensive filter cells.
Retrofits can be used to avoid some of the costs of upgrading
biological systems.
II. Evaluation of Design Procedure-New Construction and
Retrofits '
A. Design Procedure Recommended by the Manufacturer/
Supplier
Hydro-Clear, which designs and manufactures the Hydro-Clear®
filter, has provided no specific information on the actual design
procedure used. However, it reports using design procedures
similar to those used for conventional, uniform-grain sand
filters (Reference: Personal Communication, Hydro-Clear
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TABLE 1
CHARACTERISTICS OF A PRIMARY EFFLUENT
Parameter Primary Effluent
Suspended
solids, mg/1 42 to 212*
Particle size,
microns >0.1 to 35**
Total BOD5, mg/1 55 to 191*
References: *"Filtration of Primary Effluent," Journal, Water
Pollution Control Federation, Matsumoto et al.,
1982; characteristics of University of California
(at Davis) Wastewater Treatment Plant primary
effluent.
**"The Significance of Filterable Solids in the
Performance of Wastewater Treatment Processes,"
Sixth Symposium on Waste Treatment, Tchobanoglous
et al., 1983. ~~~
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Corporation). Hydro-Clear also reports that the filter was
developed over a fifteen year period based on industrial
applications and their own pilot testing. Each bed-cleaning
feature was added at a different time.
The manufacturer has made available the criteria it typically
will use as values for filter design parameters; these criteria
are presented in Table 2. Other design parameters include
influent oil and grease, influent filterable solids (>0.08
micron), desired effluent quality, and expected average and peak
flows. As Table 2 shows, the most significant differences
between Hydro-Clear® filter and conventional deep sand filter
design criteria are the smaller sand size, the lower hydraulic
loading rate (HLR), and the lower rate of backwashing.
It is presumed that the following considerations are made while
designing Hydro-Clear® filters for a particular wastewater
application: filter cells are sized according to influent flow,
Hydro-Clear design criteria, and headloss through the underdrain.
Bed-cleaning features would be designed according to Table 2
criteria, although adjustments in timing and duration of cleaning
cycles would be made after initial filter runs.
B. Comments on the Design Procedure
Further attempts should be made to verify the similarity between
procedures used in the design of Hydro-Clear® filter and
conventional sand filters. The 0.40-0.50 mm design criteria for
sand size is wel 1-supported by both earlier PEF studies and
Hydro-Clear® filter/PEF system studies undertaken at the
University of California at Davis.
The shallow bed depth is apparently sufficient for surface
straining and for solids storage in localized area of the bed.
In contrast to backwashing in deep sand filters, the Hydro-Scour
uses minimal bed fluidization and would understandably require a
lower design value for backwash rate and backwash water usage.
Standard filter design recommendations (Metcalf and Eddy, 1979)
support the manufacturers' claim that bed fluidization is not
necessary for complete bed-cleaning in systems with combined
air/water backwashing. Yet the duration and frequency of
backwashing are more likely to be a function of influent quality
and performance during initial filter runs, and less likely to
conform to the specific values given as design criteria.
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TABLE 2
TYPICAL DESIGN CRITERIA FOR HYDRO-CLEAR®/PEF1
VERSUS CONVENTIONAL DEEP SAND BED/SEF2 FILTERS
Design Parameters
Hydraulic loading
rate, gpm per sq ft
Sand grain size, mm
Bed depth, inches
Operating head,3
inches
Volume of clearwell
Typical Design Criteria
Hydro-Clear®/PEF Conventional deep sand/SEF
2-4
0.40-0.50
10
40 (typical)
60 (maximum)
2 backwash water
volumes
2-8**
0.8-3.0**
36-78**
variable
2 backwash^water
volumes
Backwash:
Frequency, per day
Duration, minutes
Rate, gpm per sq ft
Volume, per backwash
gal per sq ft
based on duration
and rate
Hydro-Scour® pulses;
and duration,(seconds)
Auxiliary Air Scour:
Frequency
Duration
Footnotes
6
3.5-54
12-16
42-80 (8-10% of
flow)
8 per backwash
20
(Pulse Mix®)
8 per filter run
25-30 seconds
^ ~
4-6*
44-49 for 2.0 mm
sand**
75-200
***
not applicable
1-2 times per
backwash**
3 to 4 minutes**
1 PEF refers to primary effluent filtration
2 SEF refers to secondary effluent filtration
3 Height of water over surface of filter bed
4 Does not include time required for initial Pulse Mix
References:*Product literature and specification manual for
Zimpro's Hydro-Clear® filter
**Wastewater Engineering Treatment/Disposal/Reuse,
Metcalf and Eddy, 1979
***Wastewater Filtration: Design Considerations, EPA-
625/4-74-007a, 1974.
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III. Evaluation of System Performance
A. Availability and Suitability of Existing Operations
Data
Hydro-Clear Corporation has provided hydraulic loading rates,
sand sizes, and influent and effluent BOD and suspended solids
data for nine out of twelve known pilot or full-scale municipal
PEF systems (Reference: "Primary Effluent Filtration with the
Hydro-Clear® Filter, and Attachments," Hydro-Clear Corporation).
Other necessary data for evaluating filter systems, such as
influent oil and grease, filter run time, frequency and duration
of bed-cleaning cycles, and backwash water usage rates were
available for only some case studies. Since information on
actual filter designs is incomplete, the suitability of system
designs cannot be fully evaluated.
B. Comments on Manufacturers' Claims
Performance data show that the Hydro-Clear® can remove between 42
and 72 percent of the suspended solids (SS) and between 22 and 66
percent of the BOD from single-stage municipal PEF systems
(Reference: "Primary Effluent Filtration with the Hydro-Clear®
Filter, and Attachments, Hydro-Clear Corporation). In two cases,
addition of a second stage improved SS and BOD removals by about
20 and 25 percent, respectively. Backwash water usage was
relatively low (7.4 percent of the influent) for one single-stage
Hydro-Clear® filter/PEF system. In a single comparative study of
filters operated at a hydraulic loading rate of 4 gallons per
minute per square foot, a Hydro-Clear® filter removed 60 to 70
percent of influent solids compared to 30 to 57 percent removed
by a multi-media filter containing sand and artificial media.
(Reference: "Primary Effluent Filtration: Feasibility and
Applications," University of California, Davis, 1981).
In addition, several pilot plant studies showed that the addition
of a Hydro-Clear® filter for PEF could allow previously
overloaded biological systems downstream to meet their discharge
requirements after clarification. For instance, a pilot filter
study conducted at the Clear Lake Wastewater Treatment Plant in
Clear Lake, Wisconsin, showed that an almost 30 percent reduction
in soluble BOD was feasible and sufficient to upgrade the
capabilities of a subsequent trickling filter system to meet
state regulations (Reference: "Field Evaluation of the
Hydro-Clear® Pulsed Bed Filter Utilized for Primary Effluent
Filtration (PEF), Hydro-Clear Corporation). A second pilot study
in Avon Lake, Ohio, showed that adding a PEF filter upstream
upgraded the plant's trickling filter effluent to the quality
achievable with an activated sludge system without sedimentation
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(Reference: "Primary Effluent Filtration: Feasibility and
Applications," University of California, Davis, 1981).
The available operations information appears to substantiate
Hydro-Clear• s claim that the Hydro-Clear® filter can accomplish
some solids and BOD removal, but is insufficient to prove all of
its claims.
IV. Level of Confidence in the Concept of PEF Using a Pulsed Bed
Filter
m
A. Influent Variability and Impact on Syste
Performance/Suitability of System Design
In pilot studies conducted at the University of California at
Davis, solids size distribution varied between 0.4 and 11 microns
in the primary effluent, both before and after filtration.
However, 60 to 70 percent solids removal was consistently
achieved, even as influent filterable solids varied by as much as
80 percent. Therefore, it can be concluded that PEF is
technologically feasible with the Hydro-Clear® filter. As with
conventional filters, effluent quality decreased and backwash
water usage increased as hydraulic loading rate increased.
B. Confidence in Designing a System Without the Benefit of
Field Testing
Since successful solids and BOD removal have been demonstrated
with a number of Hydro-Clear® filter installations, there would
be a relatively low technological risk in designing a system
without the benefit of additional field testing. Consequently,
the Hydro-Clear® filter/PEF system would fall within Area C:
Innovative Projects to Decide Fully Proven in the EPA's "Window
of Risk" for characterizing the need for field testing innovative
technologies. This would allow additional full-scale projects to
be funded as innovative in order to.provide an opportunity for
more complete data collection and confirm the apparent
capabilities of the Hydro-Clear® filter/PEF system.
Alternatively, more complete system monitoring programs can be
developed and used to confirm the capabilities of existing,
full-scale systems. Nevertheless, for any particular wastewater,
a pilot plant test is needed prior to final filter design. This
is especially important for applications dissimilar to previous
field tests, or cases in which discharge of filtered effluent is
regulated by restrictive permits.
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C. Benefits of Strengthening Certain Design Parameters Via
a Field Test
In cases where pilot (or field) testing is undertaken, such tests
should indicate the optimal values for sand grain size, bed
depth, hydraulic loading rate, maximum allowable headloss, and
various bed-cleaning parameters, such as flow rate, frequency and
duration. The expected length of filter run and the proper size
of pumps, clearwells, and air diffuser systems could then be
determined. Pilot tests would provide opportunities to modify
the filter design prior to final construction, would help avoid
necessary costs associated with undersizing or oversizing a
full-scale system, and would provide operational data on which to
base the design of filter systems for other similar applications.
V. Summary of the Hydro-Clear* Filter/PEF Investigation
The Hydro-Clear® filter employs a fine-grained, shallow sand bed
and four types of bed-cleaning systems in the successful
filtration of primary municipal effluents. Although nine pilot
or full-scale systems have shown ability for some solids and BOD
removal, the operations data are insufficient to fully evaluate
either its design or degree of success. In 1986, EPA completed
an extensive study of the Warminster, PA installation, which was
funded as an innovative technology project. ERM would assign a
relatively low technological risk to the use of the Hydro-Clear®
filter for PEF. Additional monitoring could be performed on
existing systems in order to obtain data for confirming the
performance of the filter system. In summary, the PEF system
falls within the "window of acceptable technological risk...for
further innovative designation" ("Guidance on Innovative
Designations", EPA, October 1984).
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