&EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-80-038
March 1980
Research and Development
Rock Filters for
Removal of Algae
from Lagoon
Effluents
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-038
March 1980
ROCK FILTERS FOR REMOVAL OF ALGAE FROM LAGOON EFFLUENTS
by
Gregory R. Swanson
Kenneth J. Williamson
Oregon State University
Corvallis, Oregon 97331
Grant No. R805416
Project Officer
Ronald F. Lewis
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommendation for use.
11
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. The complexity of the environment and
the interplay between its components require a concentrated and integrated
attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from munici-
pal and community sources, for the preservation and treatment of public drink-
ing water supplies, and to minimize the adverse economic, social, health, and
aesthetic effects of pollution. This publication is one of the products of
that research, a most vital communications link between the researcher and the
user community.
As part of these activities, this report was prepared to make available
to the sanitary engineering community the results of a study on the upgrading
of wastewater lagoon effluents for the removal of algae by the utilization of
rock filters.
Francis T. Mayo
Director
Municipal Environmental Research
Laboratory
111
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ABSTRACT
Algae are beneficial to lagoon treatment in providing oxygen for aerobic
stabilization; however, their presence in the effluent reduces the water quali-
ty achievable due to their contribution to both total suspended solids (TSS)
and biochemical oxygen demand (BOD). .Rock filtration was shown to be an ef-
fective, low cost unit process for removing algae from lagoon effluents and
correspondingly upgrading lagoon treatment.
Sedimentation is the primary mechanism of algal removal within rock
filters. The settling rates of three species of algae common to lagoons were
measured as varying from 0.02 to 0.3 m/day, depending on species and tempera-
ture. Settling rates of algae from the Veneta, Oregon, lagoon were about
0.05 m/day.
A mathematical model of the sedimentation mechanism was constructed based
on discrete settling theory. This model indicated that TSS removal efficiency
was a function of 6/ds, where 6 is the hydraulic retention time and ds is the
effective settling distance. A linear relationship between TSS removal ef-
ficiency and hydraulic loading rate was demonstrated.
The performance of a full-scale operating rock filter located at Veneta,
Oregon, was evaluated. This horizontal flow rock filter was designed for a
maximum hydraulic loading of 0.28 m^/m^-day (detention time =1.6 days). Week-
ly average effluent BOD^ and TSS did not exceed 20 mg/1.
A pilot-scale rock filter operated at the Veneta lagoon achieved similar
TSS removals at approximately twice the hydraulic loading. This discrepancy
was apparently due to the improved flow distribution in the pilot-scale filter.
The settled algal matter in the rock filter is stored in the voids. With-
in the voids, rates of aerobic decomposition are probably small due to the
limited dissolved oxygen available in the filter influent. However, the
small amount of available dissolved oxygen probably will also limit anaerobic
decomposition. As such, the algal mass in the voids does not degrade rapid-
ly. Clogging does not appear to be a potentially serious problem since suf-
ficient void volume is available to store settled matter for a minimum of 20
years.
This report was submitted in fulfillment of Grant Number R-805415 by
Oregon State University under the sponsorship of the U.S. Environmental Pro-
tection Agency. This report covers a period from June 1, 1977 to March 31,
1978, and was completed as of May, 1978.
IV
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CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables viii
Abbreviations and Symbols ix
Acknowledgement x
1. Introduction 1
2. Conclusions 6
3. Recommendations 7
4. Research Plan 8
5. Characterization of Lagoon Algae 10
6. Mathematical Model of Rock Filter Settling 16
7. Pure Culture Settling Velocities 24
8. Evaluation of Veneta Rock Filter 33
9. Evaluation of Pilot-Scale Rock Filter 48
10. Discussion 55
References 62
Appendices
A. Analytical Data for Veneta Rock Filter 64
B. Algal Speciation for Grab Samples 73
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FIGURES
Number Page
1 Schematic flow diagram of the wastewater treatment sys-
tem at Veneta, OR 4
2 Veneta rock filter plan and profile 5
3 Rock filter settling conceptual model 17
4 Removal efficiency vs. detention time for algal culture
with uniform settling velocity, v 19
5 Hypothetical normal distribution of settling velocities . . 21
6 Removal efficiency vs. detention time for normal distri-
bution of settling velocities 22
7 Schematic diagram of fluorometric apparatus for measuring
algal settling rates 26
8 Typical fluorometric settling curve 27
9 Algal settling rates vs. temperature for two cultures ... 29
10 Settling rates for three cultures under aerobic and
anoxic dark incubation at 21 °C 31
11 Settling rates for mixtures of pure cultures 32
12 Weekly average effluent flow and sampling periods for
Veneta rock filter 36
13 Weekly average TSS and BOD5 for Veneta rock filter .... 41
14 Weekly averages of influent and effluent nitrogen for
Veneta rock filter 42
15 Correlation of chlorophyll to TSS removal for Veneta
rock filter 43
16 TSS removal vs. hydraulic loading rate for Veneta rock
filter 46
VI
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FIGURES (continued)
Number
17 Correlation of BODr to TSS removal for Veneta rock
18
19
20
21
TSS removal vs. hydraulic loading for pilot-scale
Algal settling curve for August 4, 1978 .
Comparison of pilot-scale and full-scale rock filter . . .
49
SI
52
57
Vll
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TABLES
Number Page
1 Algae Most Abundant and Widespread in Wastewater
Ponds (After Palmer (11)) 12
2 Mean Sinking Rates of Diatoms (After Smayda and
Boleyn (13)) 14
3 Comparison of Mean Sinking Rates for Four Species
of Freshwater Phytoplankton (After Titman and
Kilham (15)) 14
4 Sinking Rates of Marine Flagellates (After Eppley,
et al. (9)) 15
5 Summary of Analytical Techniques 35
6 Effluent Limitations During Summer, 1978 37
7 Data Summary for Veneta Rock Filter 38
8 Mean Settling Velocities for Lagoon Samples 44
9 TSS, TVSS, and BOD5 Removal Obtained by the Pilot-Scale
Rock Filter 50
10 Calculation of Effective Settling Depth in Pilot-Scale
Rock -Filter 54
Vlll
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
EPA
BOD5
TSS
mg/1
gal/ ft3- day
cm
ha
MGD
y
m/day
mm
m
kg/ha-day
Ib/acre-day
ft
ml
m-Yday
SYMBOLS
vh
L
W
vc
R
Q
P
PC
V
r
V
Environmental Protection Agency
5-day biochemical oxygen demand
total suspended solids
milligrams per liter
cubic meters per cubic meter per day
gallons per cubic foot per day
centimeter
hectare
million gallons per day
micrometers (10~° meters)
meters per day
millimeters
meters
kilograms per hectare per day
pounds per acre per day
feet
milliliters
cubic meters per day
terminal settling velocity or sinking rate
settling distance
horizontal flow velocity
basin length
basin width
critical settling velocity
algal or suspended solids removal efficiency
flow rate
porosity
depth of void space
depth of rock section
detention time
settling velocity of ith fraction of particles
fraction of particles with settling velocity v^
fraction of particles with settling velocity greater than vc
mean settling velocity
coefficient of correlation
basin volume
IX
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ACKNOWLEDGEMENTS
The cooperation of the City of Veneta Public Works Department, Will
Johnston, Superintendent, and Jim Goebel, former Superintendent, is gratefully
acknowledged. Mr. Ray V. Walter, of Schaudt, Stemm, and Walter, Inc.., pro-
vided design, construction, and cost information for the Veneta wastewater
treatment system and was helpful in originating this project. Mr. Mike
Amspoeker, Botany Department, Oregon State University, performed the algal
speciation analyses. Ms. Susan E. Stutz-McDonald and Mr. Bruce J. Duffe,
former graduate students in the Department of Civil Engineering, Oregon State
University, performed the pure culture settling rate tests and the pilot-scale
rock filte"r testing, respectively. The generous use of their data is ac-
knowledged.
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SECTION 1
INTRODUCTION
BACKGROUND
Aerobic stabilization lagoons are commonly employed by small municipali-
ties and isolated industrial plants for wastewater treatment. Their populari-
ty in these applications is due to relatively low construction costs and mini-
mal operation and maintenance requirements. A major limitation of lagoons,
however, is the presence of occasionally large quantities of algae in the
effluent.
The Clean Water Act of 1972 (Public Law 92-500) required that all munici-
pal effluents meet secondary treatment standards, defined by the EPA as a maxi-
mum of 30 mg/1 of both 5-day biochemical oxygen demand (BODg) and total sus-
pended solids (TSS) on a 30 consecutive day average basis. Compilation of
data on lagoon treatment throughout the U.S., however, soon revealed that most
lagoons could not meet a 30 mg/1 TSS standard year-round because of the algal
content of their effluents.
Consequently, a significant research effort was directed at finding effec-
tive methods for upgrading lagoon treatment, especially through the removal of
algae. Middlebrooks, et al. (1) summarized the results of this research effort
and compiled a list of fourteen possible techniques for upgrading lagoon
treatment:
1. Centrifugation; 7. In-pond chemical precipitation of
2. Microstraining; of suspended materials;
3. Coagulation-Flocculation; 8. Autoflocculation;
4. In-pond removal of 9. Biological harvesting;
particulate matter; 10. Oxidation ditches;
5. Complete containment; 11. Land application;
6. Biological disks, baffles 12. Dissolved air flotation;
and raceways; 13. Granular media filtration; and
14. Intermittent sand filtration.
Successful application of lag-oon upgrading techniques up to the mid-1970's,
however, was generally limited to large scale treatment systems (7000 - 60,000
m-Vd) where technologically complex techniques such as chemical coagulation
and flocculation, dissolved air flotation, and conversion to mechanical aera-
tion could be successfully operated and maintained (2). Techniques that met
the requirements of small communities for ease of operation, minimum mainte-
nance and cost, and dependability of operation were generally not available or
not fully developed.
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As a result of this lack of adequate technology, the secondary treatment
information regulations (40 CFR 133) were amended in 1977 to allow a case-by-
case adjustment in the TSS standard for municipal wastewater treatment lagoons
if the lagoon has a design capacity of 7570 m3/day (2 MGD) or less, and
lagoons are the sole process for secondary treatment. On November 15, 1978
the EPA published in the Federal Register (43 FR 53161) a list of suspended
solids limitations for each state. These concentrations were based upon sta-
tistical analysis to reflect the concentration achievable 90 percent of the
time and vary from 40 to 110 mg/1. This list serves as a guideline for the
states in establishing TSS limitations for municipal lagoons.
Despite the recently relaxed Federal standards, the upgrading of lagoon
treatment may be required in some areas where receiving water quality standards
are stringent. Furthermore, national emphasis on continual improvements in
water quality will eventually require reduced pollutional discharges from all
sources, including municipal lagoons. The above considerations necessitate
the development of dependable lagoon effluent polishing techniques which are
compatible with lagoon treatment technology.
ROCK FILTERS
An additional, promising alternative for the removal of algae from lagoon
effluents is the rock filter. Very simply, a rock filter consists of a sub-
merged bed of rocks (5 to 20 cm diameter) through which the lagoon effluent is
passed vertically or horizontally, allowing the algae to become attached to the
rock surface and thereby be removed. The basic simplicity of operation and
maintenance are the key advantages of this process and make it especially
attractive for application to small lagoons. The effluent quality achievable
and the dependability of long-term operation, however, have not yet been
proven.
Previous Studies
Beginning in 1970, initial research into the rock filter was undertaken
at the University of Kansas (3-7) from which O'Brien (8) concluded that:
1. Peak efficiency for submerged rock filters is achieved during
the summer and early fall; this efficiency can produce an effluent
meeting 30 mg/1 BOD5 - 30 mg/1 TSS discharge standards at a design
of 1.6 nrVm^-day (12 gal/ft -day). During the winter and spring,
the efficiency of suspended solids removal decreases significantly,
and a maximum design loading of 1.2 m3/m3-day (9 gal/ft^-day) is
recommended.
2. The filters may be anaerobic during the summer and early fall and
will produce hydrogen sulfide if sulfates are present.
3. The rate of solids accumulation in submerged rock filters should
allow an effective filter life of greater than 20 years.
4. Rock sizes should be greater than 2.5 cm and less than 12.7 cm.
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O'Brien did not elaborate on the basic mechanism of algal removal in rock
filters, nor the possible effluent quality which could be achieved at lower
loading rates than those used of 0.40 to 2.7 m3/m3-day (3-20 gpd/ft3).
The Veneta Rock Filter
A full-scale rock filter was designed and constructed as part of a
lagoon expansion and upgrading project at Veneta, Oregon in 1975. The rock
filter was conservatively designed to meet 30 mg/1 BOD5 and TSS limitations in
accordance with Federal regulations applicable at that time.
A schematic flow diagram of the Veneta, Oregon wastewater treatment sys-
tem is shown in Figure 1. The system treats wastewater from a sewered popula-
tion of approximately 1400 with no industrial wastewater contribution. Raw
wastewater is pumped into the larger first cell and flows through the smaller
second cell by gravity. Both lagoons are designed for a minimum water eleva-
tion of 0.76 m (2.5 ft) and a maximum of 1.83 m (6.0 ft). The present organic
loading on the lagoon is 17.8 kg BOD5/ha-day (15.9 Ib/acre-day) and the average
detention time is 51 days. Lagoon effluent is pumped to the rock filter by a
submersible pump with a maximum capacity of approximately 2180 m3/day (400 gpm).
A plan and cross-section of the rock filter is shown in Figure 2. The
pressure pipe carrying lagoon effluent discharges into a 0.3 m (1.0 ft) square
influent channel upon entering the rock filter. The lagoon effluent rises
from the influent channel and moves horizontally towards the discharge weirs
where it flows into a covered effluent channel. Finally, the flow from.each
side of the rock filter is combined, chlorinated, and discharged to the nearby
Long Tom River.
The rock surface is approximately 0.30 m (1.0 ft) above the water eleva-
tion to prevent growth of algae on the rock filter. The effective surface
area of the rock filter is 5400 m2 (58,000 ft2) and the effective volume is
8,200 m3 (290,000 ft3). The in situ average porosity of the 7.6 to 15.2 cm
(3 to 6 in) rock bed was measured as 42 percent.
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Raw
Wastewater
Lagoon Cell Number 1
4.51 ha (11.14 acres)
i
Lagoon Cell Number 2
1.47 ha (3.63 acres)
Lagoon
Effluent
Samples
Rock
Filter
0.57 ha
(1.40 acres)
Rock
Filter
Effluent
Samples
Discharge
to
Long Tom
River
Cl,
Figure 1. Schematic flow diagram of the wastewater treatment system at
Veneta, OR.
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_ Affluent
"^ Effluent
Influent f
Effluent
\
r
I
\
\
V
c
36.9m
^-7. 3m
Influent-^
Channel
V
36.9m
«»^
7.3m ^X
N
A
7.3 m^
52.4m
r J
^**
'/
y
\
/
-Effluent
Weir
7.3m
73.8m
Influent Channel
PROFILE
7.3m
Figure 2. Veneta rock filter plan and profile.
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SECTION 2
CONCLUSIONS
The rock filter acts primarily as a clarifier for lagoon algal suspended
solids. Most algae, including green algae, diatoms, and flagellates settle
to the nearest rock surface. Blue green algae, which can form gas vacuoles,
rise and are trapped on the bottom surface of the rocks. The fate of most
settled solids in the rock filter is long-term storage. Lagoon effluent
typically contains only enough dissolved oxygen to aerobically stabilize a
fraction of the settled organic matter. Environmental conditions in the
rock filter are not conductive to extensive anaerobic decomposition.
Plugging of the rock filter bed by settled matter does not appear to be
a problem for the design rock size and loadings of the Veneta rock filter.
Sufficient void volume is available for storage of settled organic matter for
a minimum of 20 years.
BOD removal in the rock filter results from settling of particulate BOD
with little net removal or generation of soluble BOD.
The Veneta rock filter can consistently meet a daily maximum effluent
limitation of 20 mg/1 TSS and 20 mg/1 BOD for hydraulic loadings of less than
0.30 m3/m-day.
Rock filters require a minimum of operation and maintenance and appear
to be compatible with the capabilities of typical lagoon operations.
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SECTION 3
RECOMMENDATIONS
The settling rate of algae is the controlling factor in the design of
rock filters. As such, settling rates should be determined for typical
species and combinations of species found in wastewater lagoons. This in-
formation would allow rational engineering design based on known algal combi-
nations at a particular location.
The operational characteristics of rock filters under severe cold cli-
matic conditions is unknown. A monitoring study should be undertaken for a
facility where icing of the lagoon and rock filter would be encountered during
winter months.
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SECTION 4
RESEARCH PLAN
REMOVAL MECHANISM
Previous research on rock filters did not fully identify the basic algal
removal mechanism or the significant environmental factors affecting
performance of rock filters. Consequently, the first step in this project
was to hypothesize what the basic algal removal mechanism was and to develop
a mathematical model of this mechanism so that field results could be compared
to a theoretical framework. A review of the literature suggested that sedi-
mentation was the primary algal removal mechanism for several reasons. First,
it was considered unlikely that true filtration could be a major mechanism due
to the large pore size within rock filters. Second, it did not appear that
simple impingement of algal particles upon the rocks and subsequent attachment
could be an effective removal mechanism without a difference in specific gravi-
ty between the liquid and particle due to the very slow, laminar horizontal
flow velocities (less than 1.0 m/day). Third, the importance of algal settling
to both marine and freshwater phytoplankton ecology has been recently recog-
nized (8,9,10) and would appear to be important for the physical dimensions of
the rock filter.
OBJECTIVES
The objectives of this project were:
1. To confirm that sedimentation is the primary removal mechanism
operating within rock filters,
2. To determine typical settling velocities of various algal species
that are common in lagoon effluents,
3. To evaluate the present operating characteristics of the rock filter
at Veneta, Oregon for removing algae from lagoon effluents,
4. To estimate the long-term operating characteristics of a rock filter
after considerable algal material has collected in the porous media,
and
5. To determine engineering design criteria for rock filters that would
optimize algal removal rates.
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APPROACH
To fulfill the objectives described above, the project was divided into
three focuses of effort:
1. Measurement of settling velocities for pure cultures of typical
lagoon algal species,
2. Sampling and analysis of influent and effluent samples and physical
observation of the Veneta rock filter to determine its operational
performance, and
3. Construction, operation, and evaluation of a pilot-scale rock filter
and comparison of its performance with that of the full-scale system
at Veneta, Oregon.
A description of the experimental methods and results associated with
each of the above-described three phases are contained in Sections 7, 8 and 9,
respectively.
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SECTION 5
CHARACTERIZATION OF LAGOON ALGAE
Algae perform two primary functions within aerobic stabilization lagoons.
First, they release oxygen into the water due to their photosynthetic activity,
and this oxygen then is available to aerobic bacteria for decomposition of
organic material. Second, they absorb and assimilate nutrients (ammonia, ni-
trates, phosphates) produced by aerobic decomposition of organic wastes.
While generally advantageous within the lagoon treatment process, these algae
constitute the major source of BODg and suspended solids in-^lagoon effluents
and limit the effluent quality achievable by lagoon treatment.
Lagoons typically contain a very heterogeneous, mixed culture of algae.
Additionally, this culture may change rapidly in response to changing environ-
mental conditions. An understanding of the principles behind the successful
removal of algae from lagoon effluents by upgrading techniques can only be
achieved with a basic knowledge of the physical, chemical, and biological
properties of the species occurring within lagoons. To this end, this section
presents a generalized classification scheme for algae. This classification
scheme emphasizes settling behavior and/or motility factors that are directly
related to the proposed sedimentation mechanism for rock filters. Also, the
abundance of the various classes of algae within lagoons is discussed.
CLASSIFICATION
Practically all the algae common to lagoons tend to be planktonic, that
is, they remain dispersed in the water column rather than attached to a surface.
A simplified classification of planktonic algae into four basic groups is often
used in the field of environmental biology and is appropriate for lagoon
studies (11). These groups are blue-green algae, diatoms, flagellate algae,
and green algae. Typical examples of genera representative of the four groups
found in stabilization ponds are: green algae Chlorella, Scenedesmus,
Ankistrodesmus; blue-green algae - Anacystis (Microcystis), Oscillatoria;
flagellate algae - Chlamydomonas, Euglena; and diatoms - Nitzschia, Cyclotella.
It should be noted that some algae within the taxonomic classification for
green algae (Chlorophycae) have flagella for motility (for exapple, Chlamydomonas)
Within the above classification scheme, however, these algae would be classi-
fied as flagellates, not green algae.
ABUNDANCE
A large scale study of algae from lagoons in the U.S. and nearby terri-
tories was conducted by Palmer (11). This study covered 74 ponds, involved
929 sampling sites and lasted more than six years. Of the 125 genera identi-
10
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fied in lagoons, approximately 50 percent were green algae, 25 percent were
pigmented flagellates, 15 percent were blue-green algae, and 10 percent were
diatoms. An index of abundance was developed by Palmer to describe the fre-
quency and extent of occurrence of each algal genera (Table 1). This table
shows that the majority of the 25 most abundant algal genera are green algae,
flagellates are second most abundant, while blue-green and diatoms are much
smaller in abundance.
MOTILITY AND SETTLING BEHAVIOR
Planktonic algae must have some special mechanism or adaptation to remain
in the photic zone, for protoplasm, skeletons, and cell walls are heavier than
water. This mechanism may be active, allowing direct control of depth, or it
may be passive, involving a reduction in sinking rate so that turbulence will
cause resuspension. Active mechanisms include use of flagella for motility
(flagellate algae) and formation of gas vacuoles (most blue-green algae) for
buoyancy. Identified passive mechanisms include:
1. Maintenance of a young culture through rapid growth (e.g., as
organisms age or become nutrient deficient their sinking rate
increases),
2. Reduction in cell size through division (e.g., increased surface
area to volume ratio increases frictional resistance to settling),
and
3. Incorporation of spines or bristles in cell walls which increase
fluid drag.
Additionally, the existence of other passive mechanisms has been hypothesized
including:
1. Accumulation of fatty food reserves or oil,
2. Accumulation of abundant mucilage, and
3. Selective accumulation of ions.
Algae within the four classes described previously utilize different mechanisms
for regulating sinking rate.
Green Algae and Diatoms
Green algae and diatoms are nonmotile, depend upon wind-induced or thermal
circulation to maintain themselves in the photic zone, and will settle under
quiescent conditions (12). Passive mechanisms are utilized to retard settling,
however, and a significant amount of research has been conducted to describe
their settling behavior as a function of physiological and environmental con-
ditions. Most of this research has been conducted on diatoms, particularly
marine diatoms.
Using a settling chamber and an inverted microscope, Smayda and Boleyn
11
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TABLE 1. ALGAE MOST ABUNDANT AND WIDESPREAD IN WASTEWATER
PONDS (AFTER PALMER (11))
Rank
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Genus
Chlorella
Ankistrodesmus
Scenedesmus
Euglena
Chi amydomonas
Oscillatoria
Micractinium
Golenkinia
Anacystis
Oocystis
Chodatella
Nitzchia
Nannochloris
Planktosphaeria
Pandorina
Pteromonas
Closteridium
Crypt omonas
Chlorococcum
Schizothrix
Cyclotella
Phacus
Schroederia
Trachelomonas
Actinastrum
Classification
G
G
G
F
F
BG
G
G
BG
G
G
D
G
G
F
F
G
F
G
BG
D
F
G
F
G
Score
51
49
49
48
47
46
39
37
34
34
33
32
31
29
29
28
28
28
28
28
27
27
25
25
25
1G =
F =
BG =
D =
Green algae
Flagellate
Blue- green
Diatom
2
Score was determined by adding together the highest sampling date abundance
figure, the number of states for the genus, and the number of states where
abundance figure was 5 or above. Genera with similar scores were ranked
according to the first item.
12
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(13) measured the sinking rate of several marine diatoms as summarized in
Table 2. Smayda (14) reported that the mean settling rate of the freshwater
diatom Asterionella formosa progressively increased from 0.26 to 0.62 m/day
over a 22-day growth period. Heat or algicide-killed cells settled more
rapidly than live cells, indicating a loss of phytoplankton suspension mecha-
nisms with cell death.
Titman and Kilham (15) developed a sensitive fluorometric technique (16)
to investigate the sinking rate of freshwater phytoplankton, including one
green species (Scenesdesmus quadricauda) and three diatoms, in both exponential
and stationary growth phases (Table 3). Settling rate for cells in nutrient
limited and nutrient unlimited conditions were also investigated. It was
concluded that the higher settling rates of stationary phase cells were an
apparent response to limited nutrient conditions.
Based on sinking rate measurements with marine diatoms, Eppley, et al.
(9) concluded that sinking rate is not a species specific constant, but is
dependent on cell nutrient status, cell diameter, cell aggregation, and compos-
tion of the culture media. It was hypothesized that cells of a given species
may exist in up to three distinct physiological states with respect to
buoyancy: neutral buoyancy, a moderate sinking rate state, and a high sinking
rate state, with rapid transition to higher sinking rate states occurring
with declining physiological activity.
Flagellate Algae
Little research has been conducted concerning the use of flagellate motili-
ty by algae as a function of environmental or physiological conditions. Veloci-
ties achievable through the use of flagella (up to 2 meters per hour is re-
ported (12)) are more than sufficient to overcome settling tendencies. Ap-
parently, however, these algae depend on light for directional orientation
in the use of their motility and will settle in darkness (9).
Eppley, et al. (9) measured the sinking rates of cultures of marine
flagellates in the absence of light (Table 4). Sinking rates appeared to in-
crease with cell diameter and were similar to those for unicellular diatoms
of similar size. The increased sinking rate of non-growing cultures of
Gonyaular polyedra was interpreted as resulting from cessation of growth
rather than loss of motility.
Blue-Green Algae
Blue-green algae generally contain gas vacuoles for buoyancy regulation.
Although the mechanism regulating formation of gas vacuoles is not entirely
clear, gas vacuoles develop most abundantly (causing flotation) in dim light
and collapse as a result of photosynthesis, often resulting in a diurnal
up-and-down movement. Also, higher concentrations of nutrients favor gas
vacuole formation (17). The formation of mats by certain blue-green algae,
such as Anacystis (Microcystis), is evidence of effective use of gas vacuoles.
Titman and Kilham (15) measured the mean rising rate of the blue-green
algae Spirulina platensis as 0.45 m/day using a modified fluorometric tech-
nique.
13
-------
TABLE 2. MEAN SINKING RATES OF DIATOMS (AFTER SMAYDA AND BOLEYN (13))
Sinking Rate
Cm/day)
Genus
Thalassiosira
Nitzschia
Skeletonema
Rhizosolenia
Minimum
0.05
0.2
0.2
0.05
Young Cells
0.1
0.26
0.3
0.2
Old Cells
0.28
0.5
1.4
1.8
Maximum
45
3
7
23
TABLE 3. COMPARISON OF MEAN SINKING RATES FOR FOUR SPECIES OF FRESHWATER
PHYTOPLANKTON (AFTER TITMAN AND KILHAM (15))
Species
Cell
Radius
(v)
Sinking Rate
(m/day)
Exponential
Growth Phase
Stationary
Growth Phase
Ratio
(Stationary/
Exponential)
Cyclotella meneghiniana 1.0
Scenesdesmus quadricauda 4.2
Asterionella formosa 12.5
Melosira agassizii 27.4
0.08
0.27
0.20
0.67
0.24
0.89
1.48
1.87
3.0
3.3
7.4
2.8
14
-------
TABLE 4. SINKING RATES OF MARINE FLAGELLATES (AFTER EPPLEY. ET AL. (9))
Species
Monochrysis lutheril
Dunaliella tertiolecta
Cricosphaera elongata
Gonyaulax polyedra
Coccolithus huxleyi
with coccoliths
coccoliths removed
Cricosphaera carterae
Cell
Dimensions*
00
5.9
6x9.5
14x16
47
6.5
5.2
17
Sinking Rate
(m/day)
Growing
Cultures
0.18
0.39
0.25
2.8
1.3
0.28
Non- Growing
Cultures
6.0
1.7
*Given as average diameter for spherical cells or average diameter and average
cell length for other cells.
15
-------
SECTION 6
MATHEMATICAL MODEL OF ROCK FILTER SETTLING
INTRODUCTION
As described in the previous section, species of the green algae class
are most commonly dominant within lagoons. While these algae utilize passive
mechanisms to retard settling, they will settle under quiescent conditions.
Diatoms, while infrequently dominant in lagoons, will settle at similar rates.
Flagellate algae, the second most abundant class, will settle under dark con-
ditions and, due to their generally larger size, at rates equal to or greater
than those of green algae. The settling rates involved are often very small
(less than one meter per day), but are measurable by sensitive fluorometric
techniques.
Rock filters provide the dark, quiescent environment and small settling
distance necessary to remove these algae by sedimentation. This section pre-
sents a mathematical model of this settling mechanism based on discrete set-
tling theory.
THE MODEL
Settleable algal particles obviously will follow a tortuous path through
the heterogeneous rock filter bed before finally settling upon a rock surface.
Developing an effective and usable model of rock filter sedimentation charac-
teristics requires that the major variables such as rock size, void volume,
and configuration be considered, while assuming a more homogeneous basin and a
more straightforward settling path. For a horizontal flow rock filter, such
as the one located at Veneta, Oregon, a simplified settling model can be ap-
plied as shown in Figure 3. Within this model, the rocks are envisioned as
comprising a series of horizontal plates, of thickness d , that divide the
total depth of the rock filter into alternate sections of rock and void space.
The depth of each void section is ds and corresponds to the maximum distance
that an individual algal particle must settle to be removed from the flow
stream and become part of the attached slime layer on the rocks. The length
L and width W of the horizontal plates in the conceptual model, corresponds to
the actual length and width of the rock filter basin.
In accordance with simple discrete settling theory, an algal particle
entering the rock filter at the top of one of the settling chambers must set-
tle a distance ds before it travels L horizontally in order to be removed.
That is, for complete removal:
16
-------
Influent
Rock Filter Bed
Effluent
Direction
of Flow
V
* : ' ' ..
. ' . ' * -.
. * * *
n' ":'* .'"
__ i . *
''. *
. \ -
'. ' ' .'
.
" t
^
^.'. ': f .'..'
in/// //it
Algal
Particles
Top of Rock Bed\
"''^i"/''..'-..!' '.' 'v-'./i^:-'-* .^.Vi-.'::;«x
: s-. ^:./J\': v.>.V.r ::..:.> '*>' I ::'?:-
'*.''< '..'
-------
where v, is the horizontal flow velocity, and v is the settling velocity of
the particle. This relationship can also be expressed in terms of a critical
settling velocity vc (the smallest algal settling velocity at which complete
removal occurs) :
The removal efficiency for a uniform vertical distribution of particles
approaching the settling plates, all with the same settling velocity v, can be
expressed as:
R = -7^ . (v * v ) (3)
d v, cj * J
s h
If plug flow is assumed, then:
vh '
where p is the porosity of the rock filter and can be expressed within the
conceptual model as:
dr
s r
Since Vp/Q represents the theoretical detention time 6, Equation (1) can
be rewritten as :
vc=, (6)
and Equation (3) as :
R = . (v * vc) m
Equation (1} , which assumes a constant settling velocity for all algal
particles, is represented graphically in Figure 4. Figure 4a shows removal
efficiency (R) as a function of detention time (9) . For a value of 8 greater
than or equal to ds/v, the removal efficiency is 100 percent as suggested by
Equation (7} . Another useful method for plotting removal efficiency is shown
18
-------
a) 100
R
50
e
b) 100
Figure 4.
1 2345
1/0 or H/p
Removal efficiency vs. detention time for algal culture with
uniform settling velocity, v.
19
-------
in Figure 4b, where R is shown as a function of 1/e. If the hydraulic loading
rate (H) is expressed in terms of cubic meters of flow per cubic meter of
total rock filter volume per day, then 1/9 is equivalent to the hydraulic load-
ing rate divided by the porosity (H/p). Dividing by p converts the hydraulic
loading rate into a loading rate per unit volume of settling volume or void
space.
In an actual lagoon system, of course, the planktonic algae in the ef-
fluent will have a distribution of settling velocities rather than a single,
uniform settling velocity. In this case, Equation (7) can be expanded to:
v
ft c
R = P + 5- Z v. P(v.), (8)
c d n i * ij' * J
s v.=0
where P is the fraction of algal particles with settling velocity greater
than v and P(v^) is the fraction of algal particles with settling velocity v^.
The relationship between removal efficiency and detention time resulting
from Equation (8) will depend upon the distribution of algal settling velocities
in the rock filter influent. This relationship can be illustrated by con-
sidering a hypothetical normal distribution of algal settling velocities as
shown in Figure 5. It is not known whether lagoon algal cultures typically
exhibit a normal distribution of settling velocity, but the available data on
algal settling rates indicates that this may be a reasonable assumption (16).
Figure 6 shows removal efficiency (R) as a function of the detention time
(6) and its inverse (1/6) for the normal distribution of settling velocities
in Figure 5. R is typically a linear function of 6 for values of R up to or
greater than 50 percent where it becomes nonlinear. The plot of R versus
1/9 in Figure 6b shows an approximate linear relationship for R values be-
tween about 60 and 90 percent. Therefore, plots of R versus 1/9 (or versus
the hydraulic loading rate) may be the most useful in that an approximate linear
relationship may be established for the higher removal efficiencies usually
desired for a rock filter.
MODEL APPLICATION
As with any model, the discrepancies between the model and the real physi-
cal system must be examined in applying the model. In the rock filter set-
tling model, it must be recognized that the settling distance ds does not
directly correspond to any physically measurable property of the rock bed,
but is an effective settling distance. The void spaces within the rock filter
are of varying depth and settling of algae takes place not by a gradual
approach towards a horizontal surface, but by statistical chance that the
algal particle comes close enough to a rock surface to settle on it before
the horizontal flow velocity carries it beyond the rock's edge. Due to
reduced flow velocities near rock surfaces, algae may be removed more effect-
ively than an average settling distance would suggest. With this understanding
of an effective settling distance, however, the model can be applied to describe
rock filter performance.
20
-------
N)
0
Settling Velocity, */,
Figure 5. Hypothetical normal distribution of settling velocities.
-------
a) 100
R
75
50
25
b) 100
R
75
50
25
1 2
9 (x 27/dJ
I
I
I
234
1/0 (x cL/27)
Figure 6. Removal efficiency vs. detention time for normal distribution
of settling velocities.
22
-------
An additional problem not heretofore mentioned is the ability of blue-
green algae to rise by gas vacuole formation. Evidence discussed in the pre-
vious section indicates that in the dark, nutrient-rich environment of the
rock filter, gas vacuole formation would occur, resulting in flotation of
blue-green species. In both the model and the rock filter system, however, it
is easy to demonstrate that flotation is only a minor variation of the pro-
posed settling mechanism. Within the model, the same equations apply except
that v corresponds to a rising velocity rather than a sinking velocity.
Within the rock filter, rising blue-green algae would be trapped on the bot-
tom surface of the rocks rather than the top. The behavior of blue-green algae
in the Veneta rock filter and in settling tests during this study is discussed
in subsequent sections.
23
-------
SECTION 7
PURE CULTURE SETTLING VELOCITIES
Research on algal settling rates to date has attempted to describe set-
tling velocity at a given temperature as a species-specific constant or as a
range of values for a given species, depending upon physiological conditions
of the algal cells such as cell nutrient status. This research effort was
undertaken to determine the factors that control the settling rates of various
algal species common to lagoons. The factors considered included temperature,
light availability, algal species, and the presence or absence of dissolved
oxygen.
Specifically, this research was to evaluate whether the temperature de-
pendency of algal settling rates could be predicted by discrete settling theory
as incorporated in Stokes Law:
g(P -Od
v = 5i (9)
18y
2
where: g = the acceleration of gravity (m/s ),
y = the absolute viscosity of the liquid, (pa - s),
p = particle density (kg/m^),
p, = liquid density (kg/m^), and
d = particle diameter (m ).
Also, this research was to evaluate whether the dark, anoxic environment
within the rock filter would cause an alteration of algal settling velocity.
EXPERIMENTAL METHODS
Three species of algae which are common to lagoons were selected for uni-
algal culture settling tests. Two of these pure cultures, Scenesdesmus
acuminatus Lager and Chlorella vulgaris Beij, both green algae, were obtained
from the Starr culture collection (University of Texas7, Austin, Texas). The
third culture, Microcystis aeruginosa Kutz. emend Elenkin, a blue-green alga,
was obtained from the Environmental Protection Agency, National Environmental
Research Center, Corvallis, Oregon. The cultures were maintained in a syn-
thetic algal nutrient medium (18) and transferred weekly to insure a continuous
supply of viable cells. A 14-hr light and 10-hr dark cycle was maintained
using "cool-white" fluorescent bulbs at approximately 4000 lux. To insure the
availability of C02, the pH was maintained below 8.5 by constant agitation
with a wrist shaker with optimum surface to volume ratios (50 m£ of sample in
a 250 mJl flask).
24
-------
The fluorometric method used by Eppley, Holmes and Strickland (9) and
modified for freshwater species by Titman (16) was used to determine the set-
tling rates of the algae. This method measures the fluorescence of algal
cells as they settle past an optical window under quiescent conditions. This
technique allows rapid determination of sinking rate, usually requiring less
than two hours.
The Turner Model III fluorometer was modified to measure the fluorescence
of chlorophyll a, b and c (23) and a circulating water bath door was installed.
A 10 mm inside diameter by 100 mm long cuvette was used as the sample chamber.
The bottom 25 mm and the top 65 mm of the cuvette were covered with black tape.
A sample aliquot of completely-mixed diluted algal culture was placed in the
sample chamber so that the bottom of the meniscus was level with the upper edge
of the clear 10 mm region. The cuvette containing the sample was placed in the
fluorometer door, allowed to reach thermal equilibrium, removed and inverted
several times to remix the sample, and then replaced in the door. Fluorescence
was recorded until at least half the initial reading was reached. A schematic
diagram of this fluorometric apparatus is given in Figure 7.
Several assumptions are involved in the use of this technique to measure
average settling velocities. First, the physiological condition of the cells
is assumed to not be adversely affected during the measurements. The time
period involved in the test (usually less than two hours) helps minimize any
physiological changes. Second, fluorescence readings are assumed to be direct-
ly proportional to cell concentration. For dilute samples (less than one mg/1),
this appears to be valid (19) . Third, settling in the sample chamber is as-
sumed to be free from the effects of convection currents. Maintenance of a
constant temperature in the sample chamber by use of a circulating water bath
minimizes this effect. Finally, reductions in fluorescence are assumed to
result from sinking of cells past the optical window and not from rising of
cells toward the meniscus. This may not always be true, but it is easily
checked.
A typical settling fluorometric settling curve is shown in Figure 8. The
curve is typically linear to below 50 percent fluorescence and this portion of
the curve defines the mean settling velocity v as the average distance cells
sink divided by the time to reach 50 percent fluorescence. Since the algal cells
are uniformly suspended initially, the average settling distance for a 50
percent reduction in fluorescence is one-half the height of the clear por-
tion of the sample chamber, or five millimeters for our apparatus (20).
The non-linear region of the settling curve can be utilized to define the
distribution of settling velocities within the sample tested (16). A small de-
parture from linearity in this range indicates a narrow distribution of set-
tling velocities, while a large departure indicates a broad distribution of set-
tling velocities. Reasonable definition of the settling velocity distri-
bution, however, requires that the settling test be continued to near 0 per-
cent fluorescence. For slow settling samples, this may involve too long a
time period for reasonable accuracy. Consequently, only mean settling veloci-
ties were determined for the pure culture settling tests conducted within this
study.
25
-------
Strip Chart
Recorder
o\
Photomultiplier
(Fluorometer)
Temperature
Control
fok
Sample
Chamber
Ultraviolet
Source
(Fluorometer)
Figure 7. Schematic diagram of fluorometric apparatus for measuring algal settling rates.
-------
100
CD
O
c
0>
O
(/>
o>
k_
O
13
75
50
£ 25
^
0)
Linear
Region
Non-
linear
Region
Time
Figure 8. Typical fluorometric settling curve.
27
-------
Settling rates were measured under varying environmental conditions typi-
cal of rock filters. First, settling rates were determined for cultures grown
and maintained at four different temperatures: 5°C, 10°C, 15°C and 21°C.
Settling rates were measured with the cuvette cooled to the same temperature
as growth. Second, measurements were conducted for cultures that were placed
in the dark at 21°C for a period of two weeks under either aerobic or anoxic
conditions. The anoxic conditions were produced by purging the cultures with
nitrogen in Erlenmeyer flasks with ground-glass stoppers.
Settling rates of mixed cultures were also measured in an attempt to ass-
ess the influence of one species on another. Equal fluorescent aliquots of
S. acuminatus were mixed with C. vulgaris and S. acuminatus with M. aeruginosa
at volume ratios of 2:1, 1:1, and 1:2 at 21°C.
RESULTS
All three cultures tested appeared to exhibit a. specific gravity greater
than 1.0 in that no buoyant cells were observed following the settling tests.
Settling Rates at Constant Temperature
The settling rates of S. acuminatus, C. vulgaris and M. aeruginosa at
21°C were 0.26 ± 0.13, 0.14 ± 0.05 and 0.13 ± 0.11 m/day, respectively
(settling rate ± one standard deviation). The difference in settling rates for
these species were found to be large enough that the performance of the rock
filter could be altered depending on the dominant species present. These
values were similar to the lowest values reported for various marine and fresh-
water algae. Large settling rates (> 1 m/day) observed for some seawater
species were not observed for these species common to wastewater lagoons. Be-
cause these measurements were near the reasonable detection limit for the
fluorometric techniques (approximately 0.1 m/day) (15), the standard deviations
were large in comparison to the measured settling rates.
Settling Rates at Variable Temperatures
The effects of temperature on settling rates of two pure cultures are
shown in Figure 9. S. acuminatus sank about 3 times slower and M. aeruginosa,
about 8 times slower at 5°C as compared to 21°C. Part, but not all, of the
decrease in settling rates is attributable to the effect of decreasing vis-
cosity as expressed by Stokes Law (see Eq. (9)). Over the typical temperature
ranges of lagoons (5° to 20°C), settling rates would be affected by a factor
of about 2.5 to 7.5. This is much larger than the differences measured for
the three different species at 21°C.
The large influence of temperature above the predicted Contribution of
viscosity changes strongly suggests that the physiological condition of the
algal cells are altered at lower temperatures. This may be due to smaller
cell sizes or changes in mechanisms used to retard settling.
Settling Rates After Dark Incubation
Algae settle in rock filters under dark conditions that may alter their
28
-------
100
CD
o
o
C\J
80
60
2> 40
o
tr
o>
c
= 20
Q
Predicted Relationship
Based on Stoke's Law
0
'. acuminatus
aerugnosa
5 10 15 20 25
Temperature (°C)
Figure 9. Algal settling rates vs. temperature for two cultures.
29
-------
physiological condition and, subsequently, their settling rate. The conditions
in rock filters may be either aerobic or anoxic. To determine the influence
of these conditions, the three pure cultures were incubated in the dark at
21°C under either aerobic or anoxic conditions and their settling rates meas-
ured every 3 days for a period of 15 days. Figure 10 shows the results for
S. acuminatus, C. vulgaris, and M. aeruginosa. No large change in settling
rates occurred from the incubation under either aerobic or anoxic conditions.
The settling rates under anoxic incubation for S. acuminatus were lower than
the aerobic values; however, the difference was not large.
Influence of Mixing Cultures
Figure 11 shows the results of the pure cultures when the ratio of algae a
to algae b based on fluorescence was 1:0, 0.67:0.33, 0.50:0.50, 0.33:0.67 and
0:1.0. Theoretically, a linear relationship exists for the settling rate ver-
sus algal fraction if all assumptions for the measurement technique are valid
(i.e., discrete settling and fluorescence equal to concentration). The results
for the S. acuminatus - C. vulgaris mixture are slightly lower than the theor-
etical results. The settling rates for the S. acuminatus - M. aeruginosa
mixtures, however, seemed to be dominated by the slower settling M. aeruginosa.
The reason for this is unknown.
CONCLUSIONS
Based on the results of this phase of the study, it is concluded that:
1) Settling rates for pure cultures of common lagoon algae vary sig-
nificantly between species and are strongly influenced by tempera-
ture,
2) Decreased temperatures result in reduced algal settling rates in
excess of viscosity effects, and
3) Aerobic or anaerobic incubation does not significantly alter algal
settling rates.
30
-------
0.40
0.30
0.20
E
O
cr
0.10
0.20
0.10
5. acuminatus
i i
CD
0.20
0.10
0
I I
Anoxic
M. aerug/nosa
i i
J I
0
3 6 9 12
Incubation Time (days)
15
Figure 10. Settling rates for three cultures under aerobic and anoxic dark
incubation at 21°C.
31
-------
Fraction of 5. acuminatus
0.75 0.50 0.25 0
en
V-
a>
CD
2.0
o
o:
co
1
'-
T
5. acuminatus'> C. vulgar is
_ S. acuminatus:M. aeruginosa
0
1
I
0 0.25 0.50 0.75
Fraction of C. vulgaris or
. aeruginosa
1.0
Figure 11. Settling rates for mixtures of pure cultures.
32
-------
SECTION 8
EVALUATION OF VENETA ROCK FILTER
SAMPLING AND ANALYTICAL METHODS
Samples were collected and analyzed for seven consecutive days out of
each month over a one-year period. This schedule was chosen to provide data
on both long-term and day-to-day performance without overextending available
resources. Samples were collected from the rock filter influent (lagoon
effluent) and the rock filter effluent as shown in Figure 1. Rock filter
effluents were collected prior to chlorination so that interferences from
chlorine would be eliminated in subsequent analysis.
A portable automatic sampler (ISCO model 1580) was installed at each
sample point to collect 24-hour composite samples. The sample bottle in each
sampler was packed in ice to bring sample temperature to 4°C or below. Com-
posite samples were transferred to 1-liter plastic bottles and transported
to the Corvallis laboratory for analysis of major wastewater constituents.
Additionally, grab samples were collected for analysis of constituents whose
concentration could be affected by automatic sampling or by storage for 24
hours. Grab samples were generally taken in the mid-afternoon. Temperature,
dissolved oxygen, pH and sulfide analyses were performed on-site using grab
samples.
Analysis of other constituents, including ammonia, nitrate, suspended
solids (total and volatile), organic nitrogen, phosphorus, chlorophyll,
BOD5, COD, and TOC were performed on 24-hour composite samples. Composite
samples were preserved by refrigeration at 0°C and the addition of 0.8 ml
H-SO. per liter.
An algal speciation and count was performed on one lagoon effluent and
one rock filter effluent grab sample during each sampling period except the
first two.
Additionally, algal settling rates were measured on lagoon effluent grab
samples during several of the sampling periods. The settling tests were per-
formed on undiluted samples by the fluorometric technique described previously.
It should be noted that algal concentration may not be directly proportional to
fluorescence at the high concentrations present in lagoon samples, tilit the use
of undiluted samples was judged as most closely modeling conditions in the rock
filter. The fluorometer was zeroed on sample filtrate so that the measured
fluorescence was attributable only to algal cells. Samples in the settling
chamber were maintained at the temperature of the lagoon effluent.
A summary of parameters analyzed, sample storage and analysis methods,
33
-------
and equipment used is given in Table 5.
RESULTS
Operational History
The City of Veneta is required to meet secondary treatment standards
from Nov. 1 to May 31 and is not allowed to discharge during the summer dry
weather period (June 1 through October 31). Each spring the lagoon is drawn
down to 0.76 m (2.5 ft) to allow for storage of all flows during the dry
weather period. The rock filter is drained after discontinuance of discharge
and left dry until the fall.
Discharge during the Fall 1977 was begun on November 11. The rock filter
was filled with lagoon effluent and drained twice prior to startup. The samp-
ling schedule used is shown in relation to weekly average flow rates in Figure
12. On approximately December 1, the discharge rate was set at the maximum
pumping capacity (~ 2180 m^/day) due to the extremely high lagoon levels
(> 2 m) and maintained at that rate through March, 1978.
Beginning in the February sampling period, a sulfide odor was apparent in
the vicinity of the rock filter effluent manhole. However, sulfides could not
be measured above the minimum detectable concentration of the sulfide probe
(0.1 mg/1). The sulfide odor was reduced, but still noticeable in March, April
and May.
By mid-March, the high effluent pumping rate finally began to reduce the
lagoon level. By April 10, the lagoon was drawn down to the minimum water
level of 0.76 m (2.5 ft) and the discharge rate was reduced.
In the May sampling period, the lagoon level was at 0.9 m (3.0 ft) and an
algal bloom was in progress. Lagoon effluent suspended solids were 33 mg/1 at
the beginning of the week and rose to 61 mg/1 by the end of the week. At the
end of the May sampling period, the rock filter was drained and left dry until
the following sampling period.
A variance of the City's discharge permit was requested of the Oregon De-
partment of Environmental Quality (DEQ) to allow discharge during the summer
dry weather period. This request was made so that data on summer operation of
the rock filter could be obtained. This request was granted by the DEQ based
on the more stringent effluent limitations in Table 6.
34
-------
TABLE 5. SUMMARY OF ANALYTICAL TECHNIQUES
Ul
Parameter
Temperature
DO
pH
H2S
NHj
N0~ > 1 mg/1
Type of Sample
Grab
Grab
Grab
Grab
24-hour composite
24-hour composite
Sample Storage
and/or Preservation
On-site
On-site
On-site
On-site
On-site
On-site
Method of Analysis
Thermistor
Membrane electrode
Electronic pH meter
Ion-specific electrode
Ion-specific electrode
Known addition method
lon-speicific electrode
Known addition method
Reference
(21), p.
(21), p.
(22)
(23)
(27)
450
460
381
< 1 mg/1
TSS rand TVSS
Organic N
Phosphorus (total
and soluble)
Chlorophyll
BOD5
COD
TOC
24-hour composite Refrigeration @ 0°C Brucine method
+ 0.8 ml H2SO /I
24-hour composite On-site filtration
24-hour composite Refrigeration at 0°C
'24-hour composite
24-hour composite
24-hour composite
24-hour composite
24-hour composite
On-site filtration.
Refrigeration @ 0°C
+0.8 ml H2S04/1
Immediate filtration
Immediate preparation
Refrigeration at 0°C
+ 0.8 ml H2S04/1
Refrigeration at 0°C
+ 0.8 ml H2S04/1
Gravimetric
Kjeldahl digestion
Acidimetric finish
Digestion with H2S04 &
HN03- Vanadomolybdophos-
phoric acid method.
Spectrophotometric
BOD bottle
Dichromate digestion
TOC analyzer
(21),p. 94,
96
(21),p. 437
(21),p. 466
(21),p. 1029
(21),p. 543
(21),p, 550
(25),p. 532
-------
2500 r
Sampling Weeks
tri
O\
I T 1
23306 13 20273
Jul Aug
£1000
0
I I I I
6 1320274
Nov
I I I I
11 1825 1
Dec
8 15 22 29 5 12 19 26
Jan Feb
1977
i i i i i i r
9 16 23 30 7 14 21 28
Apr May
1978
Figure 12. Weekly average effluent flow and sampling periods for Veneta rock filter.
-------
TABLE 6. EFFLUENT LIMITATIONS DURING SUMMER, 1978
Parameter Monthly Average Weekly Average Daily Average
BOD 10 mg/1 15 mg/1 20 mg/1
TSS 10 mg/1 15 mg/1 20 mg/1
Fecal
Co 11 form
pH
BOD/DF*
200/100 ml
6-9
1
400/100 ml
6-9
1
-
6-9
1
*BOD in mg/1, DF = dilution factor in receiving stream
Due to the low lagoon level and the high evaporative rate during the sum-
mer, however, discharge was not begun until July 21 when the lagoon level
reached 1.2 m (4.0 ft). By this time, a bloom of blue-green algae had caused
the lagoon effluent TSS to exceed 100 mg/1.
The blue-green algal species that bloomed in July formed a thick (up to
10 cm), odorous mat in the first lagoon cell in August and caused the lagoon
operators severe problems. The operators attempted to skim off the mat manu-
ally and to disperse it using an outboard motorboat with little success. Ad-
ditionally, the low lagoon level required that discharge be discontinued in
late August. Except for a one-week discharge period in September (for sampling
purposes), no flow was discharged until November 10. The blue-green algal mat
was still a problem during the September sampling period and lagoon effluent
suspended solids were near 100 mg/1. Application of lime to the algal mat and
colder temperatures in October eventually broke up the mat. By the November
sampling period, the blue-green algae were absent from the lagoon and wet
weather necessitated a higher discharge flow rate. A sulfide odor was again
present at the rock filter effluent manhole during November.
Analytical Results
A summary of rock filter influent and effluent data is shown in Table 7.
These data are averages of the seven consecutive days of sampling during each
sampling period. Daily data are tabulated in Appendix A. The hydraulic load-
ing indicated is the pumped flow rate divided by the total rock filter volume
(m* water/m-* rock filter-day). The pH of the lagoon effluent approached 10
during summer periods of heavy photosynthesis, but the rock filter provided suf-
ficient detention time without photosynthetic activity so that dissolved carbon
dioxide was replenished and the pH lowered to between 7 and 8.
As mentioned previously, dissolved oxygen (DO) readings were taken on
grab samples, generally in the mid-afternoon. Therefore, the influent (lagoon
effluent) DO values do not represent the large diurnal variations in DO which
37
-------
TABLE 7. DATA SUMMARY FOR VENETA ROCK FILTER
00
11/13/77
Parameter
Hydraulic Loading
(m-Vm^-day)
Temperature (°C)
PH
DO (rag/1)
TSS (mg/1)
TVSS (% of TSS)
BOD5(mg/l)
Soluble COD (mg/1)
Total COD (mg/1)
TOC (mg/1)
NH*-N (mg/1)
Org-N (mg/1)
NO~-N (mg/1)
Soluble P (mg/1)
Total P (mg/1)
Chlorophyll a Jug/1)
Chlorophyll b (yg/1)
Chlorophyll c (yg/1)
Sulfides (mg/1)
I*
0.
8.4
9.0
10.1
42
83
20
-
121
37
0.8
4.1
1.5
4.8
5.2
-
-
-
ND
E*
13
8.6
7.7
4.5
9
89
9
-
77
28
1.7
3.4
2.1
3.9
4.1
-
-
-
ND
Weekly Averages for Week of
1/01/78
I
0.
6.0
8.6
15.4
29
90
27
-
67
24
3.5
5.8
1.0
1.7
2.1
340
39
23
ND
E
28
6.3
7.1
6.2
14
89
14
-
45
16
2.9
5.4
1.6
1.6
1.6
160
23
15
ND
2/05/78
I
0.28
9.3
7.6
11.2
28
90
20
-
51
19
15.5
5.7
0.8
2.5
3.2
260
59
29
ND
E
9.0
7.1
3.2
10
86
10
-
36
13
12.4
1.4
1.1
2.6
3.4
72
15
6
<0.1
3/05/78
I
0.27
10.6
7.9
10.8
22
89
21
-
61
23
15.9
3.9
1.1
2.1
2.7
160
17
19
ND
E
9.8
7.1
3.0
9
86
15
-
44
16
14.3
3.3
0.8
3.5
3.1
32
3
5
<0.1
4/16/78
I
0.
12.3
8.7
10.8
44
92
39
-
147
-
3.8
8.8
1.7
6.0
6.8
690
348
148
ND
E
07
11.
7.
3.
7
66
19
-
80
-
5.
4.
1.
4.
5.
13
12
22
ND
5
2
2
5
5
5
6
0
(continued)
-------
TABLE 7. (Continued)
VO
5/14/78
Parameter
Hydraulic Loading
(m3/m3-day)
Temperature (°C)
pH
DO (mg/1)
TSS (rag/1)
TVSS (% of TSS)
BOD5 (mg/1)
Soluble COD (mg/1)
Total COD (mg/1)
TOC (mg/1)
NH*-N (mg/1)
Org-N (mg/1)
NO~-N (mg/1)
Soluble P (mg/1)
Total P (mg/1)
Chlorophyll a (yg/1)
Chlorophyll b (pg/1)
Chlorophyll c (yg/1)
Sulfides (mg/1)
I
0.17
18.2
9.9
17.4
43
84
42
-
159
56
2.6
8.4
2.3
3.7
5.6
210
240
380
ND
E
17.0
7.3
2.9
9
85
18
-
104
39
7.2
5.2
1.0
4.9
5.4
34
38
80
ND
Weekly
Averages for Week of
7/30/78
I
0.07
22.0
9.6
6.9
105
88
43
72
183
69
0.2
15.3
1.9
-
-
234
156
165
ND
E
20.8
7.6
1.8
10
67
11
76
90
38
3.5
6.5
1.2
-
-
15
18
39
ND
9/17/78
I
0.06
14.5
9.2
6.4
84
87
56
98
201
64
1.8
15.8
1.2
-
-
427
133
34
ND
E
13.8
7.5
2.6
10
66
15
82
85
32
4.6
5.9
0.9
-
-
39
17
12
ND
11/19/78
I
5.6
7.6
10.2
48
87
42
70
189
-
6.6
-
2.6
-
-
179
230
474
_
E
0.13
5.4
7.2
4.5
11
80
19
69
91
-
4.6
-
2.6
-
-
44
63
168
_
* I - rock filter influent, E = rock filter effluent
ND = not detectable (« 0.1 mg/1)
-------
occur in lagoons. The rock filter dampens these diurnal variations to a large
extent so that the effluent DO values shown are probably representative of
average conditions. However, DO measurements taken below the effluent weirs
during the summer months indicated that rock filter effluent DO values were
consistently below 1.0 mg/1, while values at the sampling manhole were up to
3 mg/1 higher. Apparently reaeration was resulting from drops over the weirs
and into the effluent manhole.
Weekly average rock filter influent and effluent BODs and TSS values over
the study period are shown in Figure 13. Good suspended solids removals
(> 70%) were obtained beginning with the first sampling on November 13, 1977,
2 days after the Fall startup.
Percentage removals of suspended solids were lower during the winter
months of January, February, and March due to the high effluent pumping rate,
but the lower algal concentrations in the lagoon effluent during these months
resulted in final effluent BOD5 and TSS values of less than 20 mg/1 on all
days sampled.
Weekly average effluent TSS did not exceed 15 mg/1 during the study period
and were generally not greater than 10 mg/1. Daily effluent TSS values ex-
ceeded 15 mg/1 on only one occasion. Weekly average effluent 8005 values did
not exceed 20 mg/1 and daily values did not exceed 30 mg/1 during the study
period.
Ammonia-nitrogen was observed to increase noticeably in passing through
the rock filter during the warmer months (Figure 14). This is apparently
the result of increased biological degradation rates at these times. Due to
the existence of both aerobic and anoxic zones in the rock filter at most
times, both nitrification and denitrification are possible, even simultaneously.
A small amount of nitrification appeared to occur during the colder months,
changing to an apparent predominance of denitrification in the summer. The
amount of biological nitrification and denitrification that can be supported
in a rock filter, however, is definitely limited: nitrification by the limited
oxygen content in the influent stream and denitrification by variations in the
extent of the anoxic zone.
Chlorophyll concentrations are indicative of viable algae and removal can
be directly correlated with suspended solids removal as shown in Figure 15.
Variations in the proportion of chlorophylls a, b and c between sampling
periods are indicative of changes in dominant algal species.
Beginning in March, 1978, grab samples were analyzed once each sampling
period for algal species and counts. Results of this analysis are contained
in Appendix B. These results indicate that green algae and flagellate algae
were the most common classes present. The massive bloom of blue-green algae
CAnacystis cyenea) that occurred during the summer of 1978 was unprecedented
at the Veneta lagoon.
Results of the fluorometer settling rate analyses are shown in Table 8.
The sample taken August 4 was completely dominated by the blue-green alga
Anacystis cyanea. Observation of the settling chamber in the fluorometer
40
-------
^
1
C/)
C/)
1-
^
ID
Q
O
GO
I£U
100
80
^
60
50
40
30
20
10
0
60
50
40
30
20
10
n
iiit
o Influent
A Effluent
^
_ o
o o
o
A
-A A A
I I i i
o Influent
A Effluent
-
o
- o o o
A A
-A *
1/1/78 3/5/78
I I i i
i I i i i
O
0
^
O ~
o o
-
-
A A A A A-
1 1 1 1 1
0
o o o
o
A A A A"
A
5/14/78 9/17/78
i i i I i
Week of
Figure 13. Weekly average TSS and BOD for Veneta rock filter.
41
-------
25
20
I ro
O
15
o
c 10
o
t_
O
r*
- * 5
0
N03-N
Org. N
NH+-N
I = Influent
E = Effluent
I
IE IE IE IE IE IE IE IE
11/13/77 1/1/78 2/5/78 3/5/78 4/16/78 5/14/78 7/30/78 9/17/78
Week of
Figure 14. Weekly averages of influent and effluent nitrogen for Veneta rock filter.
-------
100
S« 80
o
E
CD
(T
O
^_
_o
o
60
40
20
Q
0
O Chlorophyll a
A Chlorophyll b
Chlorophyll c
I
20 40 60 80
TSS Removal (%)
100
Figure 15. Correlation of chlorophyll to TSS removal for Veneta rock filter.
43
-------
TABLE 8. MEAN SETTLING VELOCITIES FOR LAGOON SAMPLES
Sample
Date
1/3/78
2/4/78
2/5/78
8/4/78
9/21/78
11/25/78
Lagoon
Temperature
C°C)
5
9
10
22
15
5
Dominant*
Class
-
-
green and flagellate
blue -green
i
blue-green and green
green and flagellate
Mean
Settling
Velocity
(m/day)
0.02
0.06
0.08
0.05+
0.03-0.21f
0.07
*See Appendix B for complete speciation and relative abundance
Approximate rising rate
Calculation based on settling versus rising distance
44
-------
indicated that these algae were leaving the optical window by rising into the
meniscus above and not by sinking. The mean velocity was therefore calcu-
lated as a rising velocity using the approximate rising distance in the set-
tling chamber. It should be noted that our apparatus was not set up to meas-
ure rising rates, and this value is therefore approximate. Rising was still
dominant during the September 21 analysis, but settling was also occurring.
Because the rising and settling distances are different in the sample chamber-
it was not possible to calculate a single velocity.
The mean settling velocities measured did not show a distinct correlation
with temperature, /but seemed to be clustered around about 0.05 m/day. Rising
velocities for samples dominated by blue-green algae were similar to settling
velocities of other samples.
It should be noted that the measured settling velocities in Table 8 are
at or below the reported reasonable detection limit of the fluorometric tech-
nique (0.1 m/day). Values in this range have been reported in the literature
(14), but the possible inaccuracy of such measurements are noted.
Physical Observations
During May, 1978, the rock filter was excavated at one point to below
the water line and the rocks were examined. The rocks were covered with a
brown and green slime layer with a thickness of 1-3 mm. A distinct feature
of the slime layer was that it was located primarily on the top surface of the
rocks and the bottom surfaces were relatively clean, indicating that settling
had been the algal removal mechanism. During September, 1978, (after the
blue-green algal bloom) the rocks were again examined. It was then observed
that the bottom surfaces of the rocks were also covered with a slime layer.
Additionally, a brown scum layer of 1-2 cm thickness had formed at the water
surface of the rock filter. These observations indicate that flotation was
a primary mechanism operating in the removal of blue-green algae.
Evaluation of Results
The mathematical model based on discrete settling theory presented in
Section 6 suggested that for a given settling rate distribution, the removal
efficiency of suspended solids should most directly be a function of hydraulic
loading rate in the range of removals observed at the Veneta rock filter. A
plot of weekly average TSS removal efficiency versus hydraulic loading rate
for the nine sampling periods of this study is shown in Figure 16. The good
correlation obtained for a linear relationship suggests that the algal settling
(or rising) velocities did not vary greatly for the different sampling periods,
despite the differences in dominant algal species, temperature, and other
environmental conditions.
In accordance with settling theory, BOD removal in the rock filter should
be through the removal of particulate BOD, primarily algae. This relationship
is shown in Figure 17 as a correlation of BODs and TSS removal. The straight
line suggests a removal of about 0.5 rag BODs per mg of suspended solids.
45
-------
100
80
§ 60
o
E
0)
co 40
CO
I
ct:
20
0
0
R= 99.21-146.1 H
r = -0.953
90% Prediction Limits
0.1
0.2
0.3
3, 3
H-Hydraulic Loading Rate (m /m -day)
0.4
Figure 16. TSS removal vs. hydraulic loading rate for Veneta rock filter.
-------
50
40
_ 30
o
>
o
E
O)
cr 20
in
Q
O
QQ
10
Q
0
I
I
20 40 60 80
TSS Removal (mg/l)
100
Figure 17. Correlation of BOD5 to TSS removal for Veneta rock filter.
47
-------
SECTION 9
EVALUATION OF PILOT-SCALE ROCK FILTER
PURPOSE
The purpose of constructing and operating the pilot-scale rock filter was
to allow a greater range in applied loading rates and to provide a compari-
son to the full-scale system. The full-scale rock filter at Veneta was sus-
pected of having a poor flow distribution from the influent channel, limiting
its performance.
,_ Additionally, it was desired to operate the pilot-scale rock filter at
varying hydraulic loadings over a short time period when the influent algal
composition was relatively constant. Comparison of the removal efficiencies
thereby obtained with the algal settling curve obtained from the fluorometric
settling test would allow estimation of the effective settling distance in
the pilot-scale filter.
The pilot-scale rock filter consisted of a Plexiglas channel, 7.6 m (25
ft) long and 38 cm (15 in) deep. Total volume of the model was 1.33 m3 (46.7
ft3) (Figure 18). Clean rocks were obtained from the Veneta filter and hand-
placed in the channel. The porosity of the pilot-scale filter was measured
as 40 percent compared to 42 percent for the Veneta filter. Thus, the pilot-
scale filter was judged to be fairly close to the operational filter in media
construction.
Lagoon effluent was pumped from the second cell of the Veneta lagoon to
the pilot-scale filter. The flow entered the channel at the base of the media
at one end and flowed through the media to an effluent weir at the opposite
end. This flow pattern closely approximates that designed for the operational
rock filter at Veneta.
The experimental rock filter was initially filled at a high rate from the
Veneta lagoon system during July, 1978. Once filled, the hydraulic loading
rate was reduced to a nominal value for about 8 days to allow stabilization
of the unit. At this time, flow monitoring and sampling was begun. Hydraulic
loading rates through the unit were varied from 0.119 to 0.332 m3/m3-day,
representing anticipated values for rock filters.
Samples of both influent and effluent were taken after a steady-state
situation was reached for each change in loading rate. The effluent samples
were obtained from the wet well located below the effluent weir and the influent
samples were obtained from the influent line (Figure 18). Analysis of each was
made to determine BQD$, TSS and TVSS in accordance with previously described
methods.
48
-------
From
Lagoon
i
-Constant Head Tank
7.6m
Stone Media
Overflow Tube
O.45m
END VIEW
O.38m
LL_ Effluent
Wet
Well
Figure 18. Pilot-scale rock filter.
-------
An estimate of the resuspension rate of algae trapped within the inter-
stices of the rocks and on the rocks was made by running tap water through
the filter and measuring TSS and TVSS of the effluent.
RESULTS
A tracer study indicated that the flow scheme in the pilot-scale filter
resulted in a nearly plug-flow condition. The pilot-scale rock filter was
operated during July and August, 1978 when lagoon suspended solids levels were
exceeding 100 mg/1 due to the bloom of blue-green algae. A summary of removal
efficiencies at the various hydraulic loading rates (based on total volume)
tested is shown in Table 9 and a plot of TSS removal versus hydraulic loading
is shown in Figure 19.
TABLE 9. TSS, TVSS, AND BODs REMOVAL OBTAINED BY THE PILOT-SCALE ROCK FILTER
Hydraulic Loading Rate
(m^/m^-day)
0.119
0.250
0.391
0.532
TSS
87
86
74
70
Percent Removal
TVSS
88
87
75
71
BOD5
84
74
50
47
Removal efficiencies for BODs were similar to those for TSS at lower load-
ing rates, but were smaller at the higher loading rates. This indicates the
presence of a large fraction of soluble BODs in the effluent at the higher
loadings, but it is not known whether this was released from the filter or
was initially present in the lagoon effluent.
Estimating the Effective Settling Distance
The algal settling curve for August 4, 1978 is shown in Figure 20. This
settling curve was obtained as part of the analyses conducted in evaluating
the full-scale filter. Because the testing date was approximately in the mid-
dle of the pilot-scale filter operational period and the same blue-green alga
was dominant during this entire period, it was assumed that this settling
curve was applicable to the entire operational period for purposes of esti-
mating the effective settling distance.
It should be noted again that the blue-green algae dominating the lagoon
during this time period were observed to rise in the fluorometer settling
chamber. Figure 20, therefore, describes the reduction in fluorescence due to
the algae rising past the optical window.
If fluorescence is assumed to be proportional to algal concentration and
50
-------
100
90
g 80
o
E
0)
o:
co 70
CO
h-
60
50
O
I
I
I
0 0.1 0.2 0.3 0.4 0.5 0.6
H-Hydraulic Loading Rate (m /m -day)
0.5
1.0
1.5
(days'1)
Figure 19. TSS removal vs. hydraulic loading for pilot-scale rock filter.
51
-------
100
S 75
C
0)
o
0)
0)
0)
or
50
o 25
T
T
T
Mean Rising Velocity =
0.05 m/day
Temperature = 22°C
August 4,1978
5 10 15 20
Detention Time, 9 (hrs)
25
Figure 20. Algal settling curve for August 4, 1978.
52
-------
therefore, to suspended solids concentration, then the algal removal curve
shown in Figure 20 for the fluorometric settling 'test can be correlated with
the removal efficiencies observed for the rock filter (Figure 19). Using these
assumptions, the settling curve in Figure 20 was converted into a plot of sus-
pended solids removed versus the inverse of the detention time. Figure 19 shows
the same function for the pilot-scale filter operation (1/6 = H/p).
According to the mathematical model of settling presented in Section 6,
equivalent removals can be obtained for two different settling depths only
when 6/ds is the same. Using this requirement, the effective settling depth
in this pilot-scale rock filter can be calculated as shown in Table 10. These
calculations indicate an effective settling depth of about 4 cm in the rock
filter.
An attempt to see if the resuspension of previously settled algae by hy-
draulic shear contributed significantly to the solids content of the effluent
was initiated using clean water as the influent to the experimental filter.
A hydraulic loading of 0.545 m^/m^-day was used, corresponding to the maximum
loading previously applied. The average TSS of the effluent was found to be
less than 3 mg/1; TVSS, less than 2 mg/1. It was concluded that resuspension
of algae will not contribute significantly to effluent solids.
53
-------
TABLE 10. CALCULATION OF EFFECTIVE SETTLING DEPTH IN PILOT-SCALE
ROCK FILTER
Pilot Filter Performance
H
1/6
(days'1)
1/6 in
fluorometer
for same
(days'1)
Effective
Settling
Depth
ds
(cm)
0.119
0.250
0.391
0.532
0.298
0.625
0.978
1.33
87
86
74
70
0.600
0.647
1.20
1.39
7.0
3.6
4.3
3.7
54
-------
SECTION 10
DISCUSSION
REMOVAL MECHANISM
As discussed in Section 5, the classes of algae typically dominant in
lagoons (green algae and flagellates) will settle under the dark, quiescent
conditions present in rock filters. The pure culture settling tests, settling
tests on lagoon samples and the performance analysis and physical observations
of the Veneta rock filter all confirmed that sedimentation was the primary
mechanism by which these algae were removed within the rock filter.
The massive bloom of blue-green algae that occurred during this study at
the Veneta lagoon offered a unique opportunity to observe the behavior of this
class of algae in settling tests and in a rock filter. The literature reviewed
in Section 5 suggested that, under the dark, nutrient-rich conditions present
in rock filters, gas vacuole formation would occur and these algae would rise
in response to an absence of light. The lagoon effluent settling tests indi-
cated that these algae were rising at measurable velocities and physical ob-
servations of the Veneta rock filter indicated that flotation was a major re-
moval mechanism during the time period of blue-green algal dominance.
The apparent settling behavior of the blue-green alga Microcystis
aeruginosa during the pure culture settling tests must be attributed to the
culturing or settling rate measurement techniques and not regarded as typical
behavior for blue-green algae in rock filters. A probable explanation for the
apparent settling behavior is that the light from the fluorometer lamp affected
the algae and, therefore, the behavior observed was not representative of dark
conditions. The dilute algal concentrations utilized in the pure culture set-
tling tests required a larger opening for the fluorometer lamp than the un-
diluted lagoon samples subsequently tested. This may explain the contrary
observation of rising behavior during lagoon sample settling tests.
One topic not extensively discussed to this point is that of the rock-
algae attachment mechanism. Once the algae have settled upon the rock sur-
faces, some form of attachment must occur in order for resuspension to be
prevented at higher hydraulic loadings. Tests on the pilot-scale filter in-
dicated that resuspension of settled algae was not significant. Natural
biological attachment mechanisms may be functioning initially to prevent re-
suspension. Observation of rocks within the Veneta rock filter showed that,
after sufficient time, a gelatinous slime layer of organic material formed on
the rocks that could not be easily removed. These observations imply that
cleaning of rocks within rock filter beds by flushing may not be practical.
55
-------
COMPARISON OF FULL-SCALE AND PILOT-SCALE FILTERS
The TSS removal efficiency as a function of hydraulic loading for the
pilot-scale and the full-scale rock filter at Veneta are compared in Figure
21. This figure shows that the pilot-scale filter was capable of achieving
similar removals to the full-scale filter at approximately twice the hydraulic
loading. This difference is attributed to the poor flow distribution and hy-
draulic short-circuiting that apparently occurs in the Veneta rock filter.
Normal removal efficiencies were obtained upon initial startup for both
the pilot-scale filter and the full-scale filter. The lack of a lag period
for obtaining normal removal efficiencies shows that the sedimentation
removal mechanism does not depend upon development of the slime layer on the
rocks or growth of any biological organisms. Therefore, rock filters probably
can be used intermittently without any loss in performance.
LONG TERM OPERATING CHARACTERISTICS
The analysis of influent and effluent characteristics for the Veneta rock
filter demonstrates that the theoretical oxygen demand of particulate organic
material removed from the lagoon effluent is much larger than the dissolved
oxygen present for aerobic decomposition (see Table 7). The reduction in COD
across the rock filter ranged from 15 to 116 mg/1 (weekly averages) with a
flow-weighted average of about 45 mg/1. Consequently, a major fraction of
the organic material removed within the rock filter has to be simply stored.
Since much of the organic material removed is stored and not aerobically
stabilized, a number of questions have been raised regarding the long-term
operating characteristics of rock filters. These concerns include:
1. The potential plugging of rock filters with organic matter
after a limited number of years in operation,
2. A progessive loss in efficiency due to buildup of organic matter
on the rocks and subsequent short circuiting,
3. The development of odorous conditions or large soluble organic
materials in the effluent due to anaerobic decomposition of the
stored organic material, and
4. The possible practicality of cleaning the rock bed if undesira-
ble conditions develop.
These concerns are addressed in order below.
It is possible to estimate the rate at which solids accumulate in a rock
filter and, subsequently, its useful life. This rate will depend primarily
on lagoon effluent quality, removal efficiency, and DO in the lagoon effluent.
For the Veneta rock filter, the suspended solids removed in the rock filter
are 7900 kg/yr based upon average flows of 264,000 m3/yr and average removals
of 30 mg/1 of TSS. If the oxygen equivalent of the cells is 1.5 mg COD/mg
TSS, the potential aerobic decomposition based on an influent DO of 10 mg/1 is
56
-------
100
80
o 60
o
E
0)
40
c/)
c/)
20
Q
A Pilot-scale Rock Filter
o Operational Rock Filter
0 0.2 0.4 0.6 0.8 1.0
Hydraulic Loading Rate (m3/m3-day)
Figure 21. Comparison of pilot-scale and full-scale rock filter.
57
-------
1760 kg/yr which means 6160 kg/yr of TSS are stored. If this material is 20
percent solids (26), then the volume occupied by this stored organic material
is 31 m3/yr. The Veneta rock filter void volume is 3400 m3 and thus the use-
ful life of the rock filter, assuming 50 percent of the voids can be filled
with TSS, is 55 years. Consequently, possible plugging of the Veneta rock
filter bed does not appear to be a problem. It should be recognized that the
above estimate applies only to the Veneta filter with the assumptions listed.
Design of rock filters for other locations requires that an estimate be made
based upon local conditions.
The second concern listed involves a possible loss in efficiency due to
the buildup of stored organic matter. The mathematical model presented in
Section 6 indicates that removal efficiency is a function of 9/ds where 9 is
the hydraulic detention time and ds, the effective settling distance. While
filling in of the void volume with organic material will reduce the detention
time, it will also result in a proportionate reduction in the settling distance.
Therefore, according to the model, the buildup of stored organic material
should not significantly affect the removal efficiency.
The third concern was that anaerobic decomposition of stored organic ma-
terial would result in objectionable sulfide odors or release of soluble or-
ganic material. During this study, the Veneta rock filter did not exhibit any
tendency to release soluble organic material. Effluent BOD values indicated
a proportionate reduction to TSS removals (Figure 17) and soluble COD analyses
indicated no significant change occurred in passing through the rock filter
(Table 7). Sulfide generation occurred to a limited extent (detectable by
odor only) during the winter months (February through May, 1977 and beginning
November, 1978). The sulfide odor was detectable only in the immediate
vicinity of the rock filter effluent manhole during those months, however, and
was not a significant problem.
It is postulated that several factors functioned to limit the firm estab-
lishment of an anaerobic zone in the Veneta rock filter. First, anaerobic
degradation rates for algal residues are very slow and the refractory organic
fraction is high (20-40 percent) (27). Second, large, diurnal variations in
DO and pH are typical in the lagoon effluent during summer months. These
variations in DO tend to send pulses of oxygen through the filter which prob-
ably inhibits the anaerobic bacteria. The wide variations in pH would also
tend to reduce anaerobic growth. Lastly, the cold temperatures during the
winter months would limit the growth rate and survival of anaerobic bacteria.
Based on these factors, it appears that the conditions that would favor
anaerobic degradation are warm temperatures and low DO due to lack of photo-
synthetic activity. These two factors generally do not occur simultaneously,
but the possibility of developing anaerobic conditions within a rock filter
should be evaluated based on local conditions.
The final concern listed regards whether cleaning of the rock bed is
practical or necessary. Based on the discussion above, it does not appear
that cleaning will be necessary, at least, for the Veneta rock filter. Fur-
thermore, it does not appear that any mechanical methods for cleaning methods
for cleaning, short of physical removal of the rocks and washing, will be
practical. As discussed earlier, the gelatinous organic slime layer that
58
-------
forms on the rocks is not easily removed. Flushing has been suggested, but
the flow rates required to produce turbulent hydraulic shear capable of re-
moving the slime layer would be orders of magnitude greater than normal hy-
draulic loadings. Air injection into the bottom of the rock filter to create
shearing turbulence has also been suggested, but effective removal would re-
quire installation of an extensive network of air piping and diffusers and
provisions for a blower.
A practice at Veneta that apparently serves to limit the extent of the
slime layer and delay any anaerobic activity is draining of the rock filter
during the summer, dry weather period when effluent is not being discharged.
Excavation and investigation of the rocks in the filter bed during the
summer of 1978, approximately one month after it had been drained for the
summer dry-weather period, revealed that the slime layer on the rocks had
completely dried out, leaving a thin layer of dried organic material. Conse-
quently, the draining and drying of rock filters may offer a means to reduce
the volume occupied by the stored organic material and limit the permanent
establishment of an anaerobic zone.
An additional alternative where continuous discharge may not be permitted
or required is to establish a closed loop reaeration scheme. During periods
of non-discharge, liquid from the rock filter could be recycled through an
aerator and pumped back to the rock filter. With a relatively large flow
rate, sufficient oxygen could be supplied to the rock filter to aerobically
stabilize the stored organic material in a relatively short time (typical
non-discharge time is 2 to 3 months out of the year).
ROCK FILTER DESIGN
A primary objective of this study was to establish engineering design
criteria for utilization of rock filters in other locations.
Design Basis
Within this report, a mathematical model of rock filter settling was
developed based on algal settling rates, wastewater constituent data for the
Veneta rock filter was empirically analyzed, and a pilot-scale rock filter was
evaluated and compared to a full-scale operating facility. These results
suggest three possible methods for determining appropriate design loadings:
1. Pilot Testing. Pilot-scale experiments at Veneta indicated that
removal efficiencies could be predicted based on pilot-scale
results. A comparison of the theoretical versus actual hydraulic
detention time in the pilot-scale filter should be evaluated
against the probable effectiveness of flow distribution in the full-
scale system. The pilot-scale system should be monitored (at best
intermittently) for a minimum of one year to analyze performance
when different algal species are dominant in the lagoon:
2. Algal Settling Rates. At the beginning of this study, it was en-
visioned that fluoremetric measurement of algal settling rates
coupled with the theoretical framework of the mathematical model
59
-------
would provide a low-cost, rational method for the design of. rock
filters. During this study, however, several limitations of the
fluorometric settling test were discovered:
a) Accurate measurements of settling velocities in the range of
those found for lagoon effluents (less than 0.1 m/day) is very
difficult. Furthermore, definition of the distribution of
settling velocities requires following the fluorometric trace
to near zero reading, which may take several days. The error
associated with such long-term measurements could be very large.
b) Blue-green algae rise; consequently, the fluorometer must be
designed to measure rising rates as well as sinking rates, even
simultaneously. This presents complications to rapid measure-
ment and the interpretation of results.
c) Flagellates and blue-green algae will respond to light. The
extent to which the light from the fluorometer lamp affects
the settling or rising behavior of these algae is not known.
Consequently, the use of fluorometrically measured algal settling
rates cannot be recommended as a basis for design at present. Future
improvements in the method, however, may allow its use as a design
basis.
3. Empirical Design. Pilot testing may not be practical for all loca-
tions, particularly for very small lagoons. An empirical design is
possible based on the results of this study. The TSS removal curve
shown in Figure 16, with the 90% prediction limits, could be used
as an empirical design curve. The EPA has generally accepted 90%
prediction limits as corresponding to a 30 consecutive day average
(40 CFR 133 and 43 FR 53161).
In utilizing such an empirical approach, it is assumed that the
settling rates of algae in the lagoon to be upgraded are similar to
those observed during this study. The validity of this assumption
is unknown at present. The measurement of algal settling rates,
though they may be only approximate, could serve to test the validity
of this assumption. Also, identification of dominant algal species
during various seasons of the year could be useful.
Application of a safety factor to empirical designs would be
appropriate to account for the possibility of slower settling algal
cultures. It should also be recognized that improvements in flow
distribution over the Veneta rock filter design could result in a
smaller required volume.
Other Design Criteria
The results of this study have suggested several other important design
considerations for rock filters. First, the water surface in the rock filter
should not be exposed to sunlight to prevent regrowth of algae. The use of
60
-------
covered effluent weirs and an extra 0.3 m (1.0 ft) of rock media above the
water surface at Veneta was effective in preventing regrowth of algae. Second,
rock filter systems should be designed for possible generation of sulfides.
Effluent piping should be completely enclosed and reaeration minimized prior
to chlorination. Chlorination facility design should allow for larger doses
to oxidize sulfides. Third, provisions for draining and drying of the rock
filter should be included. Fourth, rock filter effluent will generally be
depleted in DO. Where required, the effluent stream should be aerated prior
to discharge. Fifth, designs should allow for control of flow rates to the
rock filter. Together with proper use of lagoon storage capacities, adjustment
of the hydraulic loading applied to the rock filter is the most effective oper-
ating tool available to meet strict discharge standards.
Flow Scheme
Rock filters should be designed as horizontal, plug-flow basins. The use
of a vertical flow scheme would allow rising algae (blue-greens) to escape in
the effluent. As indicated by the pilot test results, hydraulic short circuit-
ing or poor flow distribution will have a direct negative impact on removal
efficiency.
Rock Size
Definition of rock size to be used in the rock filter is a compromise be-
tween volumetric removal efficiency and potential plugging problems. The use
of smaller rocks will reduce the settling distance within the rock filter and
therefore, increase volumetric efficiency. However, the dangers of plugging
are increased.
The rock size used at Veneta (7.6 to 15.2 cm or 3 to 6 in) appears to be
a reasonable compromise. The use of smaller rocks is not recommended without
extensive pilot testing to evaluate the potential for plugging.
Cost
The Veneta rock filter was constructed in 1975 for a total cost of approxi-
mately $66,000. This cost included all site work and excavation, the rock media,
influent and effluent piping and weirs, and the feed pump. This cost was ap-
proximately 25 percent of the total lagoon expansion cost, which included
essentially all the present facilities except the smaller, second lagoon.
61
-------
REFERENCES
1. Middlebrooks, E.J., D.B. Porcella, R.A. Gearheart, G.R. Marshall,
J.H. Reynolds, and W.J. Grenney. "Techniques for Algae Removal from
Wastewater Stabilization Ponds," J. Water Pollution Control Fed., 46,
2676 (1974).
2. Caldwell, D.H., D.S. Parker, and W.R. Uhte. Upgrading Lagoons, EPA
Technology Transfer Publication, U.S. Environmental .Protection Agency,
Cincinnatti, OH, (1973). 43pp.
3. Martin, D.M. "Several Methods of Algae Removal in Municipal Oxidation
Ponds," M.S. Thesis, University of Kansas, Lawrence, (1970).
4. O'Brien, W.J., R.E. McKinney, M.D. Turvey, and D.M. Martin, "Two
Methods for Algae Removal from Oxidation Pond Effluents," Water and
Sewage Works, 120, No. 3, 66 (1973).
5. Martin, J.L. and R. Weller, "Removal of Algae from Oxidation Pond
Effluent by Upflow Rock Filtration," M.S. Thesis, University of
Kansas, Lawrence (1973).
6. O'Brien, W.J. "Polishing Lagoon Effluents with Submerged Rock Filters,"
Upgrading Wastewater Stabilization Ponds to Meet New Discharge Standards,
Report PRWG 159-1, Utah Water Research Laboratory, Utah State University,
Logan (1974).
7. Hirsekorn, R.A., "A Field Study of Rock Filtration for Algae Removal,"
M.S. Thesis, University of Kansas, Lawrence (1974).
8. O'Brien, W.J. "Algae Removal by Rock Filtration," In: Trans 25th
Annual Conference on Sanitary Engineering, University of Kansas,
Lawrence, (1975).
9. Titman, D. and P. Kilham. "Sinking in Freshwater Phytoplankton: Some
Ecological Implications of Cell Nutrient Status and Physical Mining
Processes," Limn, and Ocean., 21, 409-418, (1976).
10. Eppley, R.W., R.W. Holmes and J.D.H. Strickland. "Sinkjng Rates of
Marine Phytoplankton Measured with a Fluorometer," J. Exp. Marine
Biology and Ecology, 1, 191-208, (1967).
11. Bella, D.A. "Simulating the Effect of Sinking and Vertical Mixing on
Algal Population Dynamics," J. Water Pollution Control Fed., 42, 5,
Part 2, R 140, C1970).
62
-------
12. Palmer, C.M. Algae and Water Pollution, EPA-600/9-77-036, U.S. En-
vironmental Agency, Cincinnati, OH (1977). pp. 46-51.
13. Fogg, G.E. Algal Cultures and Phytoplankton Ecology, University of
Wisconsin Press, Madison, (1975).
14. Smayda, T.J., and B.J. Boleyn. "Experimental Observations on the Flota-
tion of Marine Diatoms," Limn, and Ocean., 10, 499-509, (1965).
15. Smayda, T.J. "Some Experiments on the Sinking Characteristics of 2
Freshwater Diatoms," Limn, and Ocean., 19, 628, (1974).
16. Titman, D., and P. Kilham. "Sinking in Freshwater Phytoplankton: Some
Ecological Implications of Cell Nutrient Status and Physical Mixing Pro-
cesses," Limn, and Ocean., 21, 409-418, (1976).
17. Titman, D. "A FluorometricTechnique for Measuring Sinking Rates of
Freshwater Phytoplankton," Limn, and Ocean., 20, 869-875, (1975).
18. Fogg, G.E., W.D.P. Stewart, P. Fay, and A.E. Walsby. The Blue-Green
Algae, Academic Press, London, (1973).
19. EPA. Algal Assay Procedure Bottle Test, National Eutrophication
Research Program, Environmental Protection Agency, August (1971).
20. Operating and Service Manual, Model III Fluorometer, G.K. Turner Associ-
ates, Palo Alto, CA, (1970).
21. Stutz-McDonald, S.E., and K.J. Williamson. "Settling Rates of Algae
from Wastewater Lagoons," J. Environmental Engineering Div., ASCE,
105, 273-282, (1979).
22. Standard Methods for the Examination of Water and Wastewater, (14th ed.),
American Public Health Association, New York, NY,(1975).
23. Applications Bulletin No. 12: Determination of Total Sulfide Content
in Water, Orion Research, Inc., Cambridge, MA (1969).
24. Instruction Manual: Nitrate Ion Electrode Model 92-07, Orion Research
Inc., Cambridge, MA (1970).
25. Yu, K.Y. and P.M. Berthouex. "Evaluation of a Nitrate-Specific Ion
Electrode," J. Water Pollution Control Fed., 49, 1896 (1977).
26. The Total Carbon System: Operating Procedures, Model 0524B, Oceanography
International Corporation, College Station, TX.
27. Myers, J. "Physiology of the Algae," Annual Review of Microbiology,
5, 172, (1951).
28. Force, E.G. and P.L. McCarty. "Rate and Extent of Algal Decomposition
in Anaerobic Waters," Proc. 24th Industrial Waste Conf., Purdue Univ.,
pp. 13-36, (1969).
63
-------
APPENDIX A
TABLE A-l. ANALYTICAL DATA FOR VENETA ROCK FILTER
O\
Date
11/13/77
11/14/77
11/15/77
11/16/77
11/17/77
11/18/77
11/19/77
wk. avg.
1/01/78
1/02/78
1/03/78
1/04/78
1/05/78
1/06/78
1/07/78
wk. avg.
2/05/78
2/06/78
2/07/78
2/08/78
2/09/78
2/10/78
2/11/78
wk. avg.
Flow
Rate
Cm3/ day)
1100
1100
1100
1100
1060
1140
1210
1120
2270
2310
2310
2310
2270
2270
570
2040
2270
2270
2230
-
2270
2270
2230
2260
Temperature
(°C)
I*
9
10
11
9
8
7
5
8.4
4
2
6
6
8
9
7
6.0
10
10
10
-
9
8
9
9.3
E**
10
10
11
9
8
7
5
'8.6
4
3
5
6
8
9
9
6.3
10
10
10
-
8
8
8
9.0
pH***
I
9.1
8.9
8.9
9.0
9.1
9.0
9.0
9.0
9.0
8.8
-
-
8.5
8.6
7.9
8.6
7.4
7.3
8.2
-
7.5
7.5
, -
7.6
E
7.4
7.5
7.8
7.7
7.8
7.8
7.7
7.7
7.1
7.1
-
-
7.2
7.0
7.3
7.1
7.2
7.0
7.2
-
7.2
7.0
-
7.1
DO*** TSS
(mg/1) (mg/1)
I
. 11.0
8.5
7.6
9.1
11.9
10.4
12.2
10.1
16.5
15.3
16.0
16.2
14.0
15.1
14.3
15.4
8.2
9.1
12.2
-
10.3
14.0
13.2
11.2
E
5.0
4.8
4.3
4.2
4.4
4.7
4.3
4.5
5.3
5.1
6.3
5.2
5.3
6.4
10.1
6.2
3.7
3.2
3.0
-
3.1
3.4
2.8
3.2
I
44
43
38
39
49
42
37
42
30
28
30
32
30
30
26
29
21
24
26
-
33
31
35
28
TVSS BOD5
(% of TSS) (mg/1)
E
13
9
9
8
10
8
10
9
13
15
14
15
14
-
14
14
10
6
10
10
12
12
10
I
61
75
76
94
88
91
98
83
92
93
90
89
90
89
89
90
95
92
82
-
88
90
91
90
E
72
98
77
90
85
100
98
89
89
92
92
90
82
-
86
89
95
84
78
-
84
84
88
86
I
18
17
17
18
19
28
25
20
30
27
28
27
26
28
26
27
18
21
18
-
21
21
22
20
E
10
9
7
7
8
9
10
9
17
16
14
13
12
12
14
14
12
8
9
-
10
10
10
10
**E = Rock filter effluent
***Grab sample measurement, diurnal variations may cause apparent day to day fluctuations.
(continued)
-------
TABLE A-l. (Continued)
tn
Soluble COD Total COD
(mg/1) (mg/1)
Date I E I
11/13/77 122
11/14/77 125
11/15/77 125
11/16/77 122
11/17/77 118
11/18/77 118
11/19/77 115
wk. avg. 121
1/01/78 68
1/02/78 70
1/03/78 65
1/04/78 64
1/05/78 64
1/06/78 67
1/07/78 72
wk. avg. 67
2/05/78 26 27 44
2/06/78 26 28 51
2/07/78 21 23 48
2/08/78 -
2/09/78 23 34 56
2/10/78 21 22 51
2/11/78 20 25 54
wk. avg. 23 26 51
TOC
(mg/1)
E
88
80
78
74
73
73
71
77
56
39
44
44
48
42
42
45
47
32
33
-
34
37
33
36
I
34
38
37
38
38
38
35
37
25
23
22
22
20
26
28
24
18
20
17
-
20
-
20
19
E
29
30
30
28
27
27
27
28
16
18
15
15
17
14
-
16
_
13
13
-
13
14
13
13
NH+-N
(mg/1)
I
1.0
1.3
1.4
0.7
0.5
0.5
0.3
0.8
2.1
2.7
3.1
3.1
3.2
5.3
5.3
3.5
14.9
17.6
14.9
-
14.7
-
-
15.5
E
1.8
2.4
2.4
1.5
1.5
1.6
0.8
1.7
2.4
3.3
2.9
2.6
2.6
3.6
3.2
2.9
11.8
14.9
11.0
-
11.7
-
-
12.4
Org-N
(mg/1)
I
3.2
3.4
8.3
4.1
3.0
4.6
1.9
4.1
N0~-r
4
(mg/1)
E
2.2
5.4
4.1
2.2
3.7
2.7
3.2
3.4
10.5 11.7
4.7
6.7
2.7
-
3.9
6.3
5.8
3.2
6.5
6.3
-
5.6
7.3
5.3
5.7
8.1
3.6
5.8
-
1.5
1.4
5.4
1.7
3.1
1.4
-
0.0
1.4
0.8
1.4
I
1.5
1.2
1.7
2.0
0.8
1.3
1.8
1.5
0.7
1.2
0.8
1.3
1.2
0.9
1.1
1.0
1.1
0.9
0.8
-
0.7
-
0.7
0.8
E
3.7
2.8
2.0
2.3
1.5
1.2
1.5
2.1
1.4
1.7
1.8
1.7
1.8
1.5
1.3
1.6
1.1
1.2
1.1
-
1.1
-
0.8
1.1
Soluble
(mg/1)
I
4.9
5.4
5.5
4.2
5.4
4.2
4.0
4.8
1.9
-
-
-
1.7
1.6
1.5
1.7
2.7 .
2.2
2.1
-
2.3
3.7
2.2
2.5
P
E
3.8
4.6
4.4
3.8
4.6
3.3
2.7
3.9
1.6
-
-
-
2.3
1.3
1.2
1.6
2.1
-
2.7
-
2.8
3.1
2.3
2.6
(continued)
-------
TABLE A-l. (Continued)
Total P
(mg/1)
Date
11/13/77
11/14/77
11/15/77
11/16/77
11/17/77
11/18/77
11/19/77
wk. avg.
1/01/78
1/02/78
1/03/78
1/04/78
1/05/78
1/06/78
1/07/78
wk . avg .
2/05/78
2/06/78
2/07/78
2/08/78
2/09/78
2/10/78
2/11/78
wk . avg .
I
5.6
5.4
5.5
5.0
5.9
4.9
4.4
5.2
2.3
-
-
-
2.7
1.8
1.6
2.1
5.0
3.0
2.7
-
3.6
* 2.7
2.2
3.2
E
4.0
4.4
4.4
4.4
4.4
4.0
2.9
4.1
1.9
-
-
-
1.7
1.5
1.4
1.6
5.7
2.6
3.7
-
3.7
2.6
2.3
3.4
Chlorophyll a
(mg/1)
I
421
356
318
338
294
289
336
336
128
252
198
-
298
323
367
261
E
170
210
124
-
195
104
156
160
30
36
52
-
84
109
118
72
Chlorophyll b
(mg/1)
I
65
46
33
45
33
28
26
39
15
44
43
-
75
83
93
59
E
32
34
10
-
34
8
19
23
7
5
8
-
22
16
31
15
Chlorophyll c
(mg/1)
I
36
32
21
37
12
7
13
23
0
0
40
'-
40
43
49
29
E
24
36
0
-
28
0
0
15
0
4
0
-
10
0
20
6
Sulfides
(mg/1)
I
ND3
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
-
ND
ND
ND
ND
E
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
<0. 1
<0. 1
-
<0. 1
<0. 1
<0. 1
<0. 1
ND = Not detectable («0.1 mg/1)
-------
TABLE A-2. ANALYTICAL DATA FOR VENETA ROCK FILTER
Date
3/05/78
3/06/78
3/07/78
3/08/78
3/09/78
3/10/78
3/11/78
wk . avg .
4/16/78
4/17/78
4/18/78
4/19/78
4/20/78
4/21/78
4/22/78
wk. avg.
5/14/78
5/15/78
5/16/78
5/17/78
5/18/78
5/19/78
5/20/78
wk. avg.
Flow
Rate
(m3/day)
2270
2270
-
2230
2230
-
2230
2250
490
1140
1140
-
1100
1100
1100
1020
1100
1100
-
1100
1590
1590
1590
1360
Temperature
(°C
I
10
11
-
11
11
-
10
10.6
13
12
14
-
12
12
11
12.3
15
15
-
16
19
21
23
18.2
)
E
10
10
-
10
10
-
9
9.8
13
11
13
-
11
11
10
11.5
14
14
-
16
17
19
22
17.0
PH
I
8.5
8.0
-
-
7.3
-
-
7.9
7.4
9.1
8.8
-
9.0
9.0
9.0
8.7
_
9.6
-
9.4
10.0
10.3
10.3
9.9
DO
TSS
(mg/1)
E
7.1
7.1
-
-
7.0
-
-
7.1
7.2
7.2
7.3
-
7.2
7.2
7.3
7.2
_
7.2
-
7.2
7.3
7.4
7.5
7.3
I
15.3
13.7
-
8.4
7.1
-
9.4
10.8
8.5
13.3
10.2
-
11.2
11.9
10.0
10.8
14.2
16.6
-
13.0
19.6
22.0
18.8
17.4
E
3.3
2.8
-
3.0
3.0
-
2.9
3.0
3.8
3.8
2.6
-
2.9
2.9
3.2
3.2
3.0
4.3
-
3.0
2.9
2.4
2.0
2.9
(mg/1)
I
28
22
-
19
23
-
20
22
44
40
47
-
46
50
38
44
33
36
-
38
42
50
61
43
E
9
14
-
8
8
-
8
9
7
5
6
-
9
7
8
7
8
9
-
7
8
9
11
9
TVSS
(% of
I
88
88
-
87
93
-
90
89
91
92
91
-
90
93
95
92
90
87
-
83
83
80
83
84
TSS)
E
85
84
-
83
92
-
85
86
66
73
95
-
30
71
60
66
92
89
-
79
83
84
83
85
BOD5
(mg/1)
I
26
19
-
20
21
-
21
21
40
42
31
-
38
37
44
39
49
42
-
30
38
44
51
42
E
15
14
-
13
14
-
18
15
20
29
15
-
17
19
13
19
22
20
-
16
16
18
19
18
(continued)
-------
TABLE A-2. (Continued)
oo
Soluble COD Total
COD
(mg/1) (mg/1)
Date
3/05/78
3/06/78
3/07/78
3/08/78
3/09/78
3/10/78
3/11/78
wk. avg.
4/16/78
4/17/78
4/18/78
4/19/78
4/20/78
4/21/78
4/22/78
wk. avg.
5/14/78
5/15/78
5/16/78
5/17/78
5/18/78
5/19/78
5/20/78
wk. avg.
I E I
62
66
-
56
68
-
55
61
134
143
143
_
150
163
150
147
146
151
-
174
156
154
173
159
E
39
50
-
39
48
-
46
44
79
77
73
_
82
85
-
80
101
101
-
99
105
105
111
104
TOC
(mg/1)
I
25
26
-
20
21
-
21
23
55
51
-
50
52
66
61
56
E
15
17
-
-
15
-
16
16
41
40
-
-
39
40
37
39
NH
-N
(mg/1)
I
12.7
16.3
-
17.9
17.7
-
15.0
15.9
4.0
3.1
5.1
-
4.5
2.6
3.6
3.8
6.3
3.5
-
3.5
2.6
1.7
1.6
2.6
E
12.7
13.8
-
15.0
15.0
-
15.0
14.3
4.9
6.4
5.2
-
5.7
5.2
5.4
5/5
7.5
6.6
-
7.6
8.8
7.2
6.0
7.2
Org-N
(mg/1)
I
5.0
5.0
-
1.8
4.5
-
3.2
3.9
8.2
6.6
10.1
-
9.0
9.6
9.5
8.8
8.8
9.5
-
8.1
7.3
7.7
9.1
8.4
E
4.5
2.5
-
3.8
3.0
-
2.5
3.3
3.9
3.2
6.0
_
4.9
4.4
4.5
4.5
5.7
6.0
-
6.3
4.4
4.3
4.3
5.2
NO~-N
(mg/1)
I
1.2
1.2
-
0.8
1.0
-
1.4
1.1
2.1
1.5
1.6
_
1.7
1.6
1.9
1.7
.
1.3
-
1.7
2.0
2.6
3.7
2.3
E
1.3
0.9
-
0.6
0.6
-
0.7
0.8
1.7
1.7
1.4
-
1.6
1.3
1.6
1.5
_
0.8
-
1.0
1.1
1.3
1.0
1.0
Soluble P
(mg/1)
I
2.6
1.6
-
2.1
2.9
-
1.5
2.1
5.0
7.4
5.6
_
9.7
4.1
4.3
6.0
4.8
6.1
-
3.5
3.1
2.6
2.7
3.7
E
2.7
1.7
-
2.6
5.0
-
5.8
3.5
_
5.9
5.0
-
5.3
3.9
3.1
4.6
_
5.2
-
5.4
5.0
5.0
3.7
4.9
(continued)
-------
TABLE A-2. (Continued)
to
Total P
(rag/1)
Date
3/05/78
3/06/78
3/07/78
3/08/78
3/09/78
3/10/78
3/11/78
wk. avg.
4/16/78
4/17/78
4/18/78
4/19/78
4/20/78
4/21/78
4/22/78
wk. avg.
5/14/78
5/15/78
5/16/78
5/17/78
5/18/78
5/19/78
5/20/78
wk. avg.
I
2.9
3.0
-
2.3
3.0
-
2.4
2.7
5.7
8.1
8.2
-
7.9
5.4
5.2
6.8
5.0
5.9
-
5.7
5.9
5.0
5.9
5.6
E
2.7
2.9
-
2.3
2.0
-
5.5
3.1
5.7
5.0
7.1
-
3.2
4.1
5.0
5.0
5.2
5.9
-
5.6
5.9
4.4
5.2
5.4
Chlorophyll a
(mg/1)
I
242
177
-
132
119
-
137
161
454
821
587
-
602
935
727
688
152
195
-
197
231
262
-
207
E
46
44
-
28
24
-
18
32
20
9
6
-
13
14
16
13
18
23
-
22
33
48
63
34
Chlorophyll b
(mg/1)
I
35
28
-
17
0
-
7
17
230
422
320
-
307
514
292
348
175
222
-
233
272
277
-
236
E
2
3
-
7
5
-
0
3
20
9
6
-
8
14
15
12
26
28
-
29
40
44
63
38
Chlorophyll c
(mg/1)
I
34
35
-
24
0
-
0
19
119
203
126
-
117
132
192
148
254
351
-
351
410
509
-
375
E
2
9
-
16
0
-
0
5
27
18
9
-
9
31
35
22
57
60
-
62
77
99
93
75
Sulfides
(mg/1)
I
ND
ND
-
ND
ND
-
ND
ND
ND
ND
ND
-
ND
ND
ND
ND
ND
ND
-
ND
ND
ND
ND
ND
E
<0.1
<0.1
-
<0.1
<0.1
-
<0.1
<0.1
ND
ND
ND
-
ND
ND
ND
ND
-ND
ND
-
ND
ND
ND
ND
ND
-------
TABLE A-3. ANALYTICAL DATA FOR VENETA ROCK FILTER
VI
O
Date
7/30/78
7/31/78
8/01/78
8/02/78
8/03/78
8/04/78
8/05/78
wk. avg.
9/17/78
9/18/78
9/19/78
9/20/78
9/21/78
9/22/78
9/23/78
wk. avg.
11/19/78
11/20/78
11/21/78
11/22/78
11/23/78
11/24/78
11/25/78
wk. avg.
Flow
Rate
(m3/day)
530
530
570
570
570
610
610
570
530
530
530
570
530
530
530
530
1100
1100
1100
1140
* 1100
1140
1100
1110
Temperature
(°C) pH
I
22
-
21
22
22
22
23
22.0
16
15
14
12
15
15
-
14.5
8
7
5
5
5
4
5
5.6
E
21
-
21
22
20
20
21
20.8
14
17
14
11
15
11
-
13.7
7
6
5
5
4
5
6
5.4
I
9.4
9.4
9.7
10.0
9.7
9.8
9.4
9.6
9.2
9.2
-
-
-
-
-
9.2
8.0
-
7.5
7.7
7.1
7.7
7.4
7.6
E
7.4
7.4
7.6
7.8
7.7
7.6
7.5
7.6
7.5
7.4
-
-
-
-
-
7.5
7.5
-
7.1
7.1
7.2
7.1
7.1
7.2
DO
(mg/1)
I
_
-
6.8
7.6
7.3
7.2
5.5
6.9
7.3
7.8
4.5
7.5
3.7
7.6
-
6.4
10.3
10.0
-
10.8
9.0
10.5
10.8
10.2
E
_
-
3.2
1.9
1.0
2.3
0.5*
1.8
1.0
2.8
3.5
5.4
2.5
0.2*
-
2.6
4.0
4.6
4.8
-
4.4
-
-
4.5
TSS
(mg/1)
I
105
119
105
97
107
101
100
105
73
88
71
93
108
77
76
84
_
-
50
41
41
43
63
48
E
10
10
11
11
8
13
7
10
4
12
3
18
10
12
18
10
_
-
15
12
10
8
8
11
TVSS
(% of TSS)
I
87
90
89
89
85
86
93
88
79
-
93
89
87
90
86
87
_
-
72
93
84
92
93
87
E
70
76
67
70
62
61
65
67
38
60
70
-
77
80
70
66
_
-
70
79
74
88
88
80
BOD5
(mg/1)
I
49
-.
50
43
49
44
55
48
52
57
58
58
58
55
52
56
42
46
40
45
46
31
-
42
E
16
13
10
9
15
9
8
11
10
17
17
-
14
16
-
15
27
22
21
17
16
13
15
19
*DO measurement taken below effluent weirs.
(continued)
-------
TABLE A-3. (Continued)
Soluble COD
(mg/1)
Date
7/30/78
7/31/78
8/01/78
8/02/78
8/03/78
8/04/78
8/05/78
wk. avg.
9/17/78
9/18/78
9/19/78
9/20/78
9/21/78
9/22/78
9/23/78
wk. avg.
11/19/78
11/20/78
11/21/78
11/22/78
11/23/78
11/24/78
11/25/78
wk . avg .
I
82
53
70
79
62
73
84
72
85
96
99
114
104
94
95
98
_
68
59
80
70
83
59
70
E
68
71
65
73
86
83
84
76
106
80
88
71
93
78
51
82
_
65
66
68
71
76
69
69
Total
COD
(mg/1)
I
188
190
190
188
175
164
188
183
196
212
201
211
204
196
184
201
205
264
175
203
178
166
133
189
E
85
86
79
93
108
96
84
90
80
87
95
93
82
79
82
85
85
71
87
85
106
99
107
91
TOC NH*
(mg/1)
I
86
76
-
69
55
55
72
69
92
72
-
59
50
64
47
64
E
40
30
37
44
37
42
37
38
21
27
28
24
32
51
44
32
-N
(mg/1)
I
_
0.1
0.3
0.3
0.1
0.1
0.1
0.2
2.1
1.4
2.1
1.7
2.0
1.8
1.6
1.8
5.9
6.3
6.2
6.6
6.6
7.6
7.1
6.6
E
_
2.9
3.1
3.6
3.8
3.9
3.6 (
3.5
4.4
4.9
4.2
4.6
4.7
5.0
4.7
4.6
4.9
4.9
4.2
4.2
4.5
5.1
4.5
4.6
Org-N
(mg/1)
I
16.0
15.6
14.2
-
-
-
-
15.3
15.4
24.4
12.1
-
15.6
13.8
13.8
15.8
E
9.8
6.3
6.1
4.0
5.1
5.8
8.2
6.5
4.4
7.3
5.4
5.0
5.2
6.8
7.5
5.9
NO"
-N Soluble P
(mg/1) (mg/1)
I
2.0
2.0
2.5
2.2
2.4
1.3
1.2
1.9
1.1
1.6
1.7
1.5
1.0
0.9
0.8
1.2
1.7
2.2
2.1
2.8
3.5
3.3
2.9
2.6
E I E
1.2
1.4
1.0
1.4
1.3
0.8
1.0
1.2
0.9
1.4
1.3
0.8
0.5
0.6
1.0
0.9
1.5
2.0
1.9
3.9
3.1
3.1
2.4
2.6
(continued)
-------
TABLE A-3. (Continued)
NJ
Total P
(mg/1)
Date I E
7/30/78
7/31/78
8/01/78
8/02/78
8/03/78
8/04/78
8/05/78
wk. avg.
9/17/78
9/18/78
9/19/78
9/20/78
9/21/78
9/22/78
9/23/78
wk. avg.
11/19/78
11/20/78
11/21/78
11/22/78
11/23/78
11/24/78
11/25/78
wk. avg.
Chlorophyll a
(mg/1)
I
218
358
213
202
189
194
265
234
139
219
536
506
732
485
373
427
_
-
-
-
180
187
169
179
E
20
18
20
20
9
9
7
15
16
-
32
39
50
57
40
39
_
-
-
-
49
32
50
44
Chlorophyll b
(mg/1)
I
146
217
110
162
148
127
180
156
106
174
154
264
128
55
45
133
_
-
-
-
229
241
220
230
E
19
22
25
28
13
11
10
18
23
-
12
21
16
27
3
17
_
-
-
-
71
44
75
63
Chlorophyll c
(mg/1)
I
83
326
251
124
117
102
155
165
75
161
0
0
0
0
0
34
_
-
-
-
479
465
474
474
E
34
44
53
72
32
16
22
39
57
-
1
14
0
0
0
12
_
-
-
-
197
112
195
168
Sulfides
(mg/1)
I
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
E
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
-------
Taxon
Chlorophyta
CM
Cyanophyta
Euglenophyta
APPENDIX B
TABLE B-l. ALGAL SPECIATION FOR GRAB SAMPLES - March 9, 1978
Rock Filter Influent
Rel.
genus-species Abundance
Scenedesmus acuminatus 0.195
Chlorella vulgaris 0.189
Pyramimonas cf. tetrarhynchus 0.166
Ankistrodesmus Falcatus 0.112
Chiamydomonas c£. globosa 0.047
Coccomonas sp. 0.30
Scenedesmus quadricauda 0.006
Actinastrum hantzschii 0.006
Schizothrix calcicola 0.018
Astasia sp. 0.016
Euglena sp. 1 0.059
Euglena sp. 2 0.024
Trachelomonas volvocina 0.012
Rock Filter Effluent
genus-species
Scenedesmus acuminatus
Chlorella vulgaris
Ankistrodemus Falcatus
Chlamydomonas cf. globosa
Actinastrum hantzschii
Euglena sp. 1
Trachelomonas volvocina
Trachelomonas sp. 1
Rel.
Abundance
0.139
0.458
0.125
0.028
0.014
0.042
0.180
0.014
Pyrrhophyta Glenodinium sp.
0.030
-------
TABLE B-2. ALGAL SPECIATION FOR GRAB SAMPLES - April 20, 1978
Rock Filter Influent Rock Filter Effluent
Rel. Rel.
Taxon genus-species Abundance genus-species Abundance
Chlorophyta Chlamydomonas cf. globosa 0.99 Chlamydomonas cf. globosa 0.44
Scenedesmus acuminatus 0.01 Scenedesmus acuminatus 0.11
Chlorella vulgaris 0.11
Actinastrum hantzschii 0.06
Euglenophyta Euglena sp. 1 0.17
Trachelomonas sp. 1 0.11
-------
TABLE B-3. ALGAL SPECIATION FOR GRAB SAMPLES - May 18, 1978
in
Taxon
Chlorophyta
Chrysophyta
Pyrrhophyta
Rock Filter Influent
genus -species
Tetraedron regulare
Chlorella vulgaris
Scenedesmus acuminatus
Scenedesmus quadricauda
Ankistrodesmus Falcatus
Nitzschia sp.
Rel.
Abundance
0.75
0.05
0.12
0.04
0.01
0.02
Rock Filter Effluent
Rel.
genus-species Abundance
Tetraedron regulare 0.83
Scenedesmus acuminatus 0.07
Ankistrodesmus Falcatis 0.07
Gilenodinium sp. 0.02
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TABLE B-4. ALGAL SPECIATION FOR GRAB SAMPLES - August 2, 1978
Rock Filter Influent
Taxon genus-species
Cyanophyta Anacystis cyanea
(Microcystis Flos-aquae)
Rel.
Abundance
1.00
Rock Filter Effluent
genus-species
Anacystis cyanea
Schizothrix calcicola
Rel.
Abundance
0.999
0.001
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TABLE B-5. ALGAL SPECIATION FOR GRAB SAMPLES - September 21, 1978
Rock Filter Influent Rock Filter Effluent
Rel. Rel.
Phylum genus-species Abundance genus-species Abundance
Chlorophyta Actinastrum hantzschii 0.15 Ankistrodesmus Falcatus 0.01
Scenedesmus acuminatus 0.10
Scenedesmus quadricauda 0.02
Ankistrodesmus Falcatus 0.02
Cyanophyta Anacystis cyanea 0.70 Anacystis cyanea 0.99
Schizothrix calciola 0.01
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TABLE B-6. ALGAL SPECIATION FOR GRAB SAMPLES - November 24, 1978
Phylum
Chlorophyta
oo
Rock Filter Influent
Rel.
genus-species Abundance
Chlamydomonas cf. pertusa 0.35
Chlorella vulgaris 0.24
Actinastrum hantzschii 0.20
Scenedesmus acuminatus 0.16
Ankistrodesmus Falcatus 0.02
Scenedesmus quadricauda 0.01
Rock Filter Effluent
Rel.
genus-species Abundance
Chlamydomonas cf. pertusa 0.26
Chlorella vulgaris 0.09
Actinastrum hantzschii 0.22
Scenedesmus acuminatus 0.26
Ankistrodesmus Falcatus 0.06
Trachelomonas volvocina 0.02
Cyanophyta
Anacystis cyanea
0.09
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-80-038
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
ROCK FILTERS FOR REMOVAL OF ALGAE FROM LAGOON EFFLUENTS
5. REPORT DATE
March 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Gregory R. Swanson and Kenneth J. Williamson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Civil Engineering
Oregon State University
Corvallis, Oregon 97331
10. PROGRAM ELEMENT NO.
35B1C, D.U. B-124, TaskD71-35
11. CONTRACT/GRANT NO.
Grant #R-805416
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final - 1977-1978
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Ronald F. Lewis (513) 684-7644
16. ABSTRACT
The objective of this project was to show that rock filtration was an effective,
low cost unit process for removing algae from lagoon effluents and correspondingly
upgrading lagoon treatment.
Sedimentation is the primary mechanism of algal removal within rock filter. The
settling rates of three species of algae common to lagoons were measured as varying
from 0.02 to 0.3 m/day, depending on species and temperature. Settling rates of algae
from the Veneta, Oregon lagoon were about 0.05 m/day. A mathematical model of the
sedimentation mechanism was constructed based on discrete settling theory. A linear
relationship between TSS removal efficiency and hydraulic loading rate was demonstrated.
A full-scale horizontal flow operating rock filter designed for a maximum hydraulic
loading of 0.28 m^/m^-day (1.6 days detention time) located at Veneta, Oregon was
evaluated. Weekly average BODs and TSS did not exceed 20 mg/1. With improved flow
characteristics a pilot scale rock filter achieved similar results for a short period
of testing at twice the hydraulic flow. During the one year of testing on the full-
scale rock filter, the rate of decomposition of the sediment accumulating in the filtei
appeared to be low due to the limited D.O. available. Clogging, however, does not
seem to be a major problem since sufficient void volume is available to store settled
matter for a minimum of 20 years.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Filtration
Effluents
*Lagoons (ponds)
*Algae
Rock filter
Algae separation
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
89
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
79
» U S GOVERNMENT PRINTING OFFICE. 1980 -657-146/56ZO
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