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Tube settling of high-rate pond algae
e:>
Pergamon
Wal. Sci. Tech. Vol. 33. No.7. pp. 229-241.1996. Copyright Q 1996 IAWQ. Published by Elsevier Science Ltd Printed in Greal Bntam. All nghts reserved. 0273-1223/96 $1 S'OO + 0'00
PH: S0273-1223(96)00358-7
TUBE SETTLING OF HIGH-RATE POND ALGAE Yakup Nurdogan* and William J. Oswald** • Bechtel National. Inc.• 50 Beale St.• 45-28-C33. San Francisco, CA 94105, USA •• University o/California. 1301 South 46th Street, Buildingg ll2, Richmond, CA 94804. USA
ABSTRACT The effluent of a hlBh-rate pond (HRP) may contain 200 to 400 mgIL of algal suspended solids (55). Microal&ae must be separated before the effluent is discharged. Gravity sedimentation is usually the first method considered in algal wastewater treatment systems. However. required overflow rates (OFRs) to remove algae in the conventional clarifiers are too low. Therefore, the shallow-depth sedimentation concept was studied for the separation of Micractinium from HRP effluent in order to increase the OFRs of gravity settling equipment. Using the orthogonal squares experimental plan. the design parameters, such as OFR. tube diameter. tube length, and inclination. were evaluated for algal 55 removal. A circular upflow clarifier was run as a control. At an OFR of 8.1 S Um 2 . min (0.2 gpmlft 2), SS removal efficiencies of tube settling and upflow clarifier were 80% and 18%. respectively. When the OFR was Increased to 16.3 Um 2 • min (0.4 gpmlft2). SS removal efficiencies decreased to 61% in tube settler and to 11% in upflow c1anfier. SS removal efficiencies of tube settlers were 40% at 30.6 Um 2 . min (0.75 gpmlft2) and 20% at 71.3 Um 2 • min (1.75 gpmlft 2). A linear relationship ellists between removal efficiency and the diameter and length of tubes. The relationship is parabolic for the inclination of tubes and the OFR. The overall algae removal efficiency increased with the tube length but it was an inverse function of OFR. tube diameter. and inclination. AI an OFR of 12.2 Um 2 . min (0.3 gpmlft2). alilae removal efficiencies were 7 to 8 times better than that of upflow clarifier. It was demonstrated that OFR in a gravity settler could be increased 4 to 5 times using tube settlers. Copyright@ 1996IAWQ. Published by Elsevier Science Ltd
KEYWORDS Algae; gravity settling; high-rate pond; Micractinium; orthogonal squares; sedimentation; separation; tube settler.
INTRODUCTION GraYity sedimentation of al~ae The preference of the algae separation method depends on the species of algae, chemistry of growth medium, ultimate use of algal biomass, and most economically available separation technology. Algae cell size is an important factor to consider while selecting the technique for separation. Colonial algae (Micractinium, Scenedesmus) and filamentous algae (Spirulina) could be harvested by a low-cost separation technology such as sedimentation, flotation, and filtration. Smaller microalgae. like most single-eell (Chlorella) and motile algae (Euglena, Chlorognium), have to be flocculated before their gravity separation. Motile algae can also be separated utilizing their phototactic characteristics (e.g., swimming to the light 229
230
Y. NURDOGAN and W. J. OSWALD
source). Miller (1972) reported the phototactic separation of motile algae Pandorina species, which were cultivated in llib-scale growtb units in order to investigate the removal of strontium from low-level radioactive wastes. Because of the high volumetric flow rates associated with wastewater discharge, tbere are economic constraints on the capital and operating costs that can be incurred in such systems. Therefore, gravity sedimentation as a low-cost technology is usually considered first to clarify the dilute algal suspensions in algal wastewater treatment systems. Algae separation by gravity sedimentation is inexpensive but its reliability is low without some kind of flocculation. Colonial algae growth and the conditions for natural flocculation (auto- and bio-) should be maintained in the pond for reliable gravity settling without chemical flocculation. Costly separation techniques such as centrifugation may be justified if algae or their byproducts are used for animal or human consumption (e.g., protein, valuable chemicals). In order to improve the efficiencies of algae separation from HRP effluents, Nurdogan and Oswald (1993) investigated the enhancement of naturally occurring autoflocculation in a paddle-wheel-mixed HRP receiving primary sewage. Calcium- and magnesium-deficient autoflocculation was perfected by adding 60 mgIL of lime (CaD) into the pond. Although the primary objective of the study was to remove phosphorus, this simple procedure improved the removal efficiencies of algal solids from 72% to 95% in bench-scale batch gravity settlers with a I-day detention time. Eisenberg (1982) tried to separate algal solids from HRP effluents in a 30 m 3 batch settling unit operated in a fill-and-draw fashion with a 2-day detention time. The sedimentation basin, which was converted from a backyard swimming pool, had poor characteristics and produced higher than desirable concentrations of SS in the processed effluent. Some were as high as 150 mgIL but most varied between 40 to 100 mglL. These poor removal efficiencies may also have been due to unsettleable algal genera growing in the pond or to an adverse physiological state of the algae during his experiments. Koopman (1981) studied a process known as pond isolation for algae separation. Isolation of facultative pond (FP) from inflow promoted the clarification of pond effluent. Fill-and-draw operation of a secondary pond allowed significant removal of algae from FP effluent but a cycle of 2 to 3 weeks was required for desirable algae removal. The time required for pond isolation may be reduced if the conditions in the pond lead to bioflocculation and/or autoflocculation of algae. Secondary ponds were also used for algae settling from paddle-wheel-mixed HRP effluent (Benemann et al., 1980). Tubes of different diameters and cross sections (flat, corrugated, and other type plates) are also used to upgrade perfonnance in existing sedimentation basins. Mohn (1980) reported that the pond algae were concentrated to 1.6% as total suspended solids (TSS) using chemical flocculation and a lamella settler. Golueke and Oswald (1965) extensively studied the chemical flocculation of waste-grown algae preceding gravity clarification. They reported that alum flocculation followed by gravity settling was reliable method of algae separation. Eighty-five percent of suspended biomass from HRP effluent was separated with a resultant algae slurry of 1.5% TSS. Shallow-depth sedimentation concept Sometimes the term high-rate sedimentation is used in referring to a shallow-depth gravity sedimentation. The hydraulic residence time of a high-rate settler may be much less than 30 minutes to achieve comparable or better settling efficiencies than those normally obtained in a conventional settling basin having residence times around 2 hours. The main feature of a high-rate settler is its shallow depth, which is usually not more than a few inches. In 1904, Hazen presented the idea that settling basin efficiency is dependent primarily upon the OFR and maximum particle settling distance, and independent of detention time in the basin (Culp et al., 1969). Since then it has been recognized tbat a settling basin should be as shallow as possible and that a detention time of only a few minutes can be used in very shallow basins. For example, a particle settling at a rate of I inch per
Tube sellling of high-rale pond algae
231
minute requires 120 minutes to fall to the bottom of a- IO-foot conventional clarifier. If the basin was 2 inches deep, this particle would fall to the bottom in only 2 minutes. By inserting parallel plates in an ordinary clarifier, the flow could be increased by a factor directly proportional to the number of plates used without adverse impact on the treated water quality. Yao (1970) studied the theory of shallow-depth sedimentation and provided some infonnation on designing tube and parallel plate (lamella) settlers. Using the known hydrodynamic principals and assuming a single discrete particle moving in laminar one-dimensional flow, he developed a general equation for the trajectories of suspended particles in a laminar flow through a circular tube as follows:
y] - - ) - ~ y Sin 8 2 3 v.
y2
8 (-
V
+~ v.
(X - L) COS 8
=0
(I)
=~ . X =~, x, y =Coordinates L =Relative settler length =.!... I =Length of tube, d =Diameter of tube d v, =Fall velocity of a suspended particle
where Y
v.
= Average velocity of flow through tube
8 = Angle of tube inclination Among this family of trajectories, there is a limiting trajectory that defines the critical particle fall velocity 0 for a given system. Any suspended particle with its fall velocity greater than or equal to this critical fall velocity would be completely removed in the tube settler. The perfonnance of a tube setller can be characterized by a parameter S as follows: S
=~ (Sin 8 + L Cos 8) v.
(2)
The critical S-value, Sc ' is dependent only on the shape or geometry of a settler. It is 4/3 for a circular tube settler and I for a plate or lamella settler. Any suspended particle in a circular tube settler with its -value greater than or equal to 4/3 would be completely removed. The basic assumption here is that a particle is removed once it strikes the bottom wall of tube. A low Sc indicates a small vSC' which in tum means settlers having smaller values will remove more of the lighter or smaller particles. The design of settling tanks for wastewater treatment is usually based on the parameter OFR, expressed as gpm/ft 2. The concept was originated from the fact that, for an ideal unifonn flow in a horizontal-flow rectangular settling tank, the OFR represents the vsc' The design OFR should be equal to or smaller than the vsc to be removed. The same concept is readily adaptable to high-rate settlers since the vse can be estimated easily from Sc' The OFR or vsc can be expressed by the following equation: (3)
K = S. L (Sin 8 + L Cos 8)
(4)
where C is a constant and its magnitude depends on the units used in Equation 3. For example, if v 0 is in fps and OFR in gpm/ft2 , C =6.54 x lOs. The performance of a settler theoretically deteriorates when the degree of inclination is increased due to an increase in the settling depth. However, some inclination is desired to remove settled solids continuously and this may compensate for the reduction in particle removal efficiency. As solids are deposited onto the tubes or plates, the hydraulic flow path becomes partially restricted. Clearly, as the channels fill with settled
Y. NURDOOAN and W. J. OSWALD
232
solids, they must be removed periodically to prevent their reentrainment in horizontal settlers. To effect this cleaning requirement, the tubes or plates are inclined at an angle of 45' to 60', which is in excess of the slide-away angle of separated particles. The settled solids flow downwards in the reverse direction of liquid flow. The flow path through a separator, the hydraulic diameter, and the flow velocity should all be minimized to achieve a lower Reynolds number 0 and better settling conditions. Longitudinal flow through tubes with a diameter of a few inches theoretically offers optimum hydraulic conditions for sedimentation. Such tubes would have a large wetted perimeter relative to the wetted area and thereby would provide laminar flow, as evidenced by the very low for such systems as shown in Table I. Table 1. Reynolds numbers of tube settlers at various flow rates
OFR (gpmlrr)
Diameter (in.)
Reynolds Number·
0.5 0.5 0.5 0.5
1 2 3 4
6.6 13.2 19.8 26.4
1.0 1.0 1.0 1.0
1 2 3 4
13.2 26.4 39.6 52.8
2.0 2.0 2.0 2.0
1 2 3 4
26.4 52.8 79.2 105.6
• at a water temperature of 10°C.
'"
Effluent
-Influent d (In.) I (ft.) e F) 1 2 30 2 4 45 3
4
6
8
60
75
Imhoff Cone
Sludge Sludge
Figure I. Diagram of model tube sellier used in shallow·depth sedimenlation experiments.
Tube settling of high-rate pond algae
233
MATERIALS AND METHODS Two paddle-wheel-mixed HRPs were used to grow algae, which were mostly colonial green algae
Micractinium. The area of each pond was approximately 0.1 hectare and the wastewater source was primary sewage from a predominantly residential section of the city of Richmond, California. The ponding system, wastewater characteristics, and environmental conditions were described previously by Nurdogan (1988). The shallow-depth sedimentation concept was studied for the separation of colonial green algae Micractinium from HRP effluent in order to increase the OFRs of gravity settling equipment. Various modifications of model tube settlers of the type shown in Figure I were constructed from schedule-40 PVC pipes and mounted on a wooden structure. The design parameters, such as OFR, tube diameter, tube length, and inclination, were evaluated for algal 55 removal. A circular upflow clarifier with a conical bottom was run as a control in parallel with tube settlers. This unit had a diameter of 2.5 feet, total depth of 3.2 feet, and total volume of 260 liters. The pond water was pumped to the central well. The solids were collected from the conical bottom and clarified water overflowed to the collection weir. A scraper at 1.4 rpm was operated to help the downtlow movement of settled solids.
A typical procedure used in research studies related to multivariable analysis is to vary one factor at a time while holding all others constant. When the number of variables and length of time required for individual measurements is small, this method works well. However, as these increase, the conventional method becomes excessively time consuming and expensive. By carefully selecting each combination of independent variables, it is possible to greatly reduce the number of individual measurements required in a multivariate analysis. A special experimental procedure called "orthogonal squares" (OS) was used for the tube settling experiments (Harris et ai., 1952). Two sets of OS experiments (4x4 and 3x3) were carried out according to the preplanned experimental procedure. According to the OS test program, if there are four independent variables, a fifth factor enters the picture in the data analysis. This is experimental error and represents the portion of the relationship not accounted for by four independent variables. The specific combinations of variables required in the experimental design are most easily determined by the matrix shown in Fig. 2. Outlines of the experiments are shown in Table 2.
313 Orthogonal Squares
414 Orthogonal Squares
A A A
A B 1 B A B
A C 2 C B C
A 0 3 D C D
A 4 D
A
A
B B 5 D D A
B A 6 A C 0
B D
A
B C
B 8 A
C B 9 C B D
C C 10 B A A
C 0 11 A 0 B
C 12 C
A
D B 13 A C C
D C 14 D 0 B
D D 15 C A A
D 16 B
C B A D
Key
C
DIA LEN
A B
ERR
# INC OFR
D
7 B
A
A B
B A
6
B
A A
C
C C
C B
8
C B
B C 5
C C 7
C
B C
B B
B
3
2
4
A
A C
A B 1
9
A B
A
Key CIA LEN # ERR OFR
Figure 2. Experimental design matrix.
SS and TSS determinations were made according to Standard Methods (1985). Chlorophyll-a (Chi-a) was measured using a methanol extraction method modified from VoUenweider (1969).
234
Y. NURDOGAN and W. J. OSWALD
Table 2. Experimental design outlines
4 x 4 Orthogonal Squares
Code #
Diameter (in.)
Length (ft.)
(gpmlft 2)
OFR
Inclination (degree)
I 2
2 4 6 8
0.25 0.75 1.25 1.75
30 45 60 75
A B C D
3 4
Error Level
I II
III IV
3 x 3 Orthogonal Squares
Code #
Diameter (in.)
Length (ft.)
(gpmlft2)
Error Level
A B C
I
2 3
2 4 6
0.15 0.30 0.45
I II III
OFR
RESULTS Effects of design parameters of a tube settler on the algal removal efficiencies of were studied using the OS experimental methodology. The time savings achieved by using the as test program made it possible to complete all of the tube settling runs before the settling characteristics of the algal cultures changed due to seasonal changes in growth conditions. Micraclinium spp. were the predominant algae and they were readily settling. Figure 3 shows the photomicrograph of Micraclinium spp. and the settled volume in an Imhoff cone after 2 hours.
B
A .
. •
~b
·fiti ..
Figure 3. Photomicrograph of Micractinium predominant in HRP during the tube senling experiments (AI and volume of algae biomass senled in Imhoff cones (8).
Tube settling of high-rate pond algae
235
4x4 orlhof:onal square results Four parameters (tube diameter, tube length, tube inclination and OFR) were varied four times. Pipes having 1-,2-, 3-, and 4-inch diameters and 2-, 4-, 6-, and 8-foot lengths were used. The tube inclinations of 30°, 45°, 60°, 75° were used together with a 90° control. The OFRs were varied through four levels: 0.25, 0.75, 1.25, and 1.75 gpmlft 2. The first set of these 16 experiments was run to determine the most important parameters of tube settler for HRP algae. In order to determine the algae removal efficiency, 55 and Chl-a determinations were made on influent and effluent samples of tube settler. Table 3 shows the study range, the optimum operational conditions for the best algal removal, and the optima for practical operation.
100
I
• 55
80 ~ 0
60
'"
40
E
a:
'if.
~
20 0
LEN 5.0 ft. OFR 1.0 gpmlft 2 INC 52.5°
o Chl-a
---v ----0---2
0
3
0
----
4
5
Diameter, in. Figure 4. Effect of tube diameter on algal removal efficiency.
100...----.------,-----r---r---,
~E
DIA 2.5 In. OFR 1.0 gpmlft 2
• 55
80
o Chl-a
INC 52.5°
60
... ..
'" 40 t-:.:-:-~ ~-:-::-~ ~-::-;-~;:.:--O--J ~ 20
o o
L-_-I.-_---l'--_-I-_--I._.---J
2
4 6 Length, ft.
8
10
Figure 5. Effect of tube length on algal removal efficiency.
A linear relationship exists between removal efficiency and the diameter and length of tubes. The effect of tube diameter on algae removal is shown in Figure 4. The smaller the tube diameter the better the algae separation. A I-inch-diameter tube resulted in an average of 49% 55 and 46% Chl-a removal efficiencies. Average 55 and Chi-a removal efficiencies with 4-inch-diameter tube were 39% and 32%, respectively. Figure 5 indicates that algal removal efficiencies increase with increasing tube length. The 8-foot long tube produced 47% 55 and 45% Chl-a removal efficiencies on the average. The 2-foot long tube produced average 55 and Chl-a removal efficiencies of 40% and 33%, respectively. The effect of inclination on algae removal is parabolic and presented in Figure 6. The 30° inclination produced a better algae removing condition because when the inclination gets closer to horizontal, ideal conditions were achieved for shallow depth sedimentation, which is the principal of tube settling. However, inclinations steeper then 45° facilitate the settled sludge removal by gravity (self-cleaning action). This will in turn contributes the better algae removal efficiencies. Therefore, a 60° inclination gave better removal efficiencies than a 45° angle. The effect levels off after 60° and sharply declines after 75° angle because the benefit of shallow-depth sedimentation is completely lost at the 90° angle. In order to obtain a simple
Y. NURDOGAN and W. J. OSWALD
236
relationship between inclination and algae removal efficiencies, the overall effect of inclination from zero to 90° can be approximated to a declining linear relationship.
100 80 ~ 0
E CI>
60
a: 40
.55
LEN 5.0 ft. CIA 2.5 in. OFR 1.0 gpm/ft 2
o Chi-a
e:-:-:....... -~ .. e---~ ......
20 0 15
30
45 60 Inclination, degree
75
90
Figure 6. Effect of tube inclination on algal removal efficiency.
There is a parabolic relationship between removal efficiency and the OFR. The most important parameter of all was the OFR as shown in Figure 7. Increased OFRs substantially decreased the algae removal efficiencies. This is expected in all solid-liquid separation systems and for all types of particles. Removal efficiencies were 75% Chl-a and 70% SS at 0.25 gpmlft 2 and 24% SS and 16% Chl-a at 1.75 gpmlft 2. Because of very high OFR and averaging used in OS data analysis, algae separation efficiencies came out low. Therefore, a second set of OS experiments were designed with lower OFR varying from 0.15 to 0.45 gpmlft2. In this way, the optimum tube operating conditions were refined using a 3x3 OS experiment.
100 LEN 5.0 ft. CIA 2.5 in. INC 52.5°
80
~E
....
60
a: 40 CI>
20 0 0.0
e55 o Chi-a
lSl..
..... ......
-O' ...... __ 'E)
1.0 1.5 0.5 Ovenlow Rate, gpm/ft 2
2.0
Figure 7. Effect of overflow rate on algal removal efficiency.
Using the least squares method of curve fitting, correlation equations can be written to predict the removal efficiencies for SS and Chi-a in the study range of variables. Based on a maximum error level of 6.3% SS and 7.9% Chi-a, the correlation equations for 4x4 OS test results are as follows: % 5S Removal = 109 - 3.1 DIA + 1.2 LEN - 0.72 INC + 0.007 (INC)2 - 60 OFR + 15 (OFR)2 % Chl-a Removal
=128 - 4.7 DIA + 1.9 LEN - 0.83 INC + 0.008 (INC)2 - 93 OFR + 27 (OFR)2
(5) (6)
3x3 ortho~onal square results After examining the results obtained in 4x4 OS experiments, an OS of 3x3 was conducted. This time the range of variables was decreased, but inclination was constant at 60'. One-, 2-, and 3-inch diameters and 2-, 4-, and 6-foot lengths were used for 0.15, 0.30, 0.45 gpmlft 2 OFR. Nine more experiments were run to refine the results of the previous 16 experiments.
Tube settling of high-rate pond algae
237
Table 3. Optimization of tube settler for algae separation 4 x 4 Orthogonal Squares Study Range
Best Removal
Practical
Diameter. in.
1-4
1
2
Length. ft.
2-8
8
6
30-7S
30
60
0.25-1.75
0.25
0.50
Parameters
Inclination. degrees 2
OFR, gpmlft
3 x 3 Orthogonal Squares Study Range
Parameters
Best Removal
Practical
Diameter, in.
1-3
1
2
Length. ft.
2-6
6
4
0.15-0.45
0.15
0.45
2
OFR. gpmlft
Results are tabulated in Table 3 showing the study range, the optimum operational conditions for the best algae removal, and the optima for practical operation. Correlation equations between algae removal efficiencies and operational parameters of tube settler are written using the least squares method of curve fitting. A linear relationship exists between removal efficiency, the diameter and the length of tubes, and the OFR. These equations can predict the removal efficiencies for SS and Chl-a in the study range of variables. The calculated maximum percent errors in predicting removal efficiencies were minimal. Figure 8 shows the effect of the tube diameter on algae removal. Similar to the previous experiment. when the tube diameter increases the algae removal efficiencies decrease. The best algae removal was obtained with l-inch-diameter tube and the least removal with 3-inch-diarneter tube. A l-inch-diameter tube produced 83% Chl-a and 80% SS removal efficiencies while a 3-inch-diameter tube produced 73% Chl-a and 63% SS removal efficiencies.
100 r---r---~--...---.,
~E '"
a: ";!.
40 20
• 55
LEN 4.0 ft.
o Chl-a OFR 0.23 Opmlft 2 o~--;-----:---~----J.
o
2
Diameter, in.
3
4
Figure 8. Effect of tube diameter on algal removal effiCiency.
JWST 33:7-1
238
Y. NURDOGAN and W. 1. OSWALD
100r-------------, 80
~E 60
.----~----.
..
~----
Q)
ex: 40 ~
.55
20
D1A 2.0 in. OFR 0.23 gpmlft 2
o Chl·a
O':-_ _~--.L---.L--~
o
2
4 Length. ft.
6
8
Figure 9. Effecl of lube lenglh on algal removal efficiency.
Figure 9 shows the effect of tube length on algae removal efficiency. Increasing the length of tubes increased the algae sedimentation efficiency. The 6-foot length produced average efficiencies of 81 % Chl·a and 74% SS. and the 2-foot long tube produced 76% Chl-a and 68% S5 removal efficiencies. The OFR was again the single most important parameter effecting algae separation in the tube settlers. An OFR of 0.15 gpmlft2 produced the best algae removal efficiencies (89% Chl-a and 86% SS) and 0.45 gpmlft 2 OFR rate produced the least algae removal efficiencies (68% Chl-a and 59% SS) as seen in Fig. 10. t5-1lltlUOI
100 80
.
!E
60
a::
40
~
20
.55 o
CIA 2 In. LEN 4 II.
Ch~a
o0.00 ~-...,..J-:-----,~-~L.:----,~ 0.15 0.30 0.45 0.60 Overftow Rate, Opmlh 2
Figure 10. Effect of overflow rale on algal removal efficiency.
Based on a maximum error level of 1.7% SS and 0.6% ChI-a. the correlation equations for 3x3 OS tests are as follows:
=108 - 8 DIA + 1.7 LEN - 92 OFR % Chl-a Removal =105 - 5 DlA + 1.2 LEN - 72 OFR % SS Removal
(7) (8)
DISCUSSION Microalgae. in general. have slow settling rates and it is difficult to separate them from liquid medium by a simple gravity sedimentation. Required OFRs to remove algae from oxidation pond effluents are too low and hydraulic residence times are out of the conventional range of practicality. Earlier attempts by Eisenberg (1982) and Koopman (1981) on separation of microalgae using phase isolation ponds or clarifiers showed some success at very low OFRs (less than 0.1 gpmlft 2) and long detention times (days to weeks). Such low OFRs are not practical for HRP or for FP. Therefore. where effluent 55 are controlled. microalgae are normally flocculated using chemical agents followed by gravity sedimentation.
Tube settling of high-rate pond algae
239
If the colonial algae Micractinium can be maintained in a HRP, algae separation by gravity sedimentation is possible without using any chemical flocculants. Batch settling tests with Imhoff cones indicated SS removal efficiencies approaching to 95% in a few hours when Micractinium were predominant in HRP. The Richmond HRP system fosters growth of colonial Micractinium. These algae have been maintained almost as a monoculture for several years and were also successfully sustained in the pond during the tube settling study. The tube settling of algal solids in this study was carried out using no chemical flocculation. High-rate settlers can also be used to separate autoflocculated algae. Based on the experience in Richmond, autoflocculation of algae could be relied on for at least 6 to 8 months in a year in places climatically comparable to San Francisco Bay Area. During last 15 years, high-rate settlers have been used in water and industrial wastewater treatment where chemical coagulants and flocculants are always used to form settelable flocs. Needless to say, a FP effluent could not be clarified by tube settlers unless the algae were flocculated. Therefore, no attempt has been reported, until now, to separate the waste-grown algae in tube settlers without some kind of chemical flocculation. Chemicals used in separation of algae may be detrimental if the biomass is to be used for protein or methane generation.
100 80 ;; > 0
E
.
.55 o Chl-a
60
a::
Q)
:>!!
20 0 0.0
0.6 0.9 1.2 1.5 Overflow Rate, gpmlft 2
1.B
Figure II. Comparison of tube settlers with conventional upllow clarifier in tenns of algal removal efficiency.
100 ~ 0
E Q) a:: tr.l tr.l
'#-
80 60 030° 45° • 60° A 75°
40
lJ.
20 0
1
10
NR
100
Figure 12. Effect of Reynolds number in tubes on algal removal efficiency.
The pilot-scale conventional upflow clarifier was run parallel to the model tube settlers at varying OFRs. Figure 11 shows the SS removal efficiencies achieved in an upflow clarifier and in the tube settlers. At 0.2 gpmlft 2 OFR, 55 removal efficiencies of tube settling and upflow clarifier were 80% and 18%, respectively. At 0.4 gpmlft 2 OFR, 55 removal efficiencies decreased to 61 % in tube settler and to II % in upflow clarifier. It was apparent that the most important parameter governing the algae removal efficiency was the OFR. 70% 55 removal efficiency was obtained at 0.3 gpmlft 2 and 86% S5 removal efficiency at 0.15 gpmlft 2. When the OFRs were increased the removal efficiencies were rapidly decreased. 55 removal efficiencies were 40% at 0.75 gpmlft 2 and 20% at 1.75 gpmlft 2.
Y. NURDOGAN and W. J. OSWALD
240
100
~ 0
E Q)
a:
(/) (/)
80 60 030° 45° .60° .75°
40
A
20 0
1
100
10 d
v2 - *102 ° I
Figure 13. Effect of flow velocity term on algal removal efficiency.
SS removal efficiency seems decreasing with the angle of inclination from 30' to 45'. The efficiency improves between 45' and 60', probably due to the benefit of self-cleaning action. SS removal efficiency was better at 60' than at 45'. However, after 60' the benefits of shallow-depth sedimentation start to diminish due to increased settling depths. In order to extract a physical meaning from the tube settling results, %SS and %Chl-a removal efficiencies were correlated with the NR and the ratio of tube length to tube diameter. The SS removal efficiency decreases with increasing NR and a velocity term (21) as seen in Figures 12 and 13. Two equations to predict the performance of a similar tube settler for algae sedimentation without the help of chemical agents are given as follows:
(9) = 115 (NR ) -fJ.27 (1..) 0.029 d (10) % Chl-aRemovai 115 (Nil) -fJ.l8 (1.. )0.006 d The error levels in the OS experiments were checked using an additional parameter that was not varied but presumably could vary due to errors. Figure 14 indicates that error levels were minimal. %SSRemovai
=
4x4 Orthogonal Squares
.55
80 ]! 0
E .,
o Chl-a
..
60
a: .,e 40
3x3 Orthogonal Squares
100
100 LEN 5.0 ft D1A 2.5 In. OFR 1.0 gpnVft 2 INC 52.5 degree
~E .,
..
60
a: 40
---~--~---~--~---
.,e
.55
o Chl-a
20
20 0 0
3 2 Error Level
0 4
5
0
CIA 2.0 In. LEN 4.0ft OFR 0.23 gpmllt 2 2 3 Error Level
4
5
Figure 14. Error levels in orthogonal square tube selliing experiments.
SUMMARY AND CONCLUSIONS The experimental tube settlers separated the HRP algae much better than a conventional upflow clarifier. At an OFR of 0.3 gpmlft2, SS and Chl-a removal efficiencies were 7 to 8 times better than that of an upflow clarifier. It was demonstrated that OFR in a gravity settler could be increased 4 to 5 times, or settling time could be shortened 4 to 5 times using tube settlers.
Tube seltling of high-rate pond algae
241
The OS experiments demonstrated that the overall algae removal efficiency increases with the tube length but it is an inverse function of OFR, tube diameter, and tube inclination. Smaller OFRs and tube diameters result in smaller Reynolds numbers, which is better for particle settling in general. Tube settling cannot dependably clarify pond effluent to required standards throughout 4 seasons without chemical or other types of flocculation. Because algae genera and biomass settling rates vary (some times sharply) due to changes in wastewater characteristics and environmental conditions.
It is possible to harvest the algal biomass by gravity separation techniques when colonial algae such as Micractinium spp. predominate and the natural flocculation processes such as autoflocculation, bioflocculation are attained in the HRP. This study demonstrated that Micractinium spp. can settle in a few hours without using any chemical agents. REFERENCES APHA (1985). Standard Methods for the Examination of Water and Wastewater, 16th cd., Washington. D.C. Benemann. lR., B.L Koopman. J.C. Weissman. D.M. Eisenberg and R. Goebel (1980). Development of microalgae harvesting and high-rate pond technologies in Califorma. In: Algae Biomass. Shelef G.and C.J. Soeder (eds), Elsevier Biomedical Press. Amsterdam. Culp. G.L. K. Hsiung and W.R. Conley (1969). Tube clarification process. operating experiences. J. San. Eng. Div., Proc. of ASCE. 95. SA5. p. 829. Eisenberg. D.M. (1982) Large·Scale Freshwater Microalgae Biomass Production for Fuel and Fenilizer. PhD. Dissertation, University of California. Berkeley. Golueke. J.C.• W.J. Oswald and H.K. Gee (1965). Harvesting and processing sewage-grown planktonic algae. Journal WPCF. 37. 4. Harris. D.N.• F.R. Watson and T.F. Thomson (1952). Magic squares: preplaned tests exemplified by flight research. Shell Aviation News. June and July. Koopman. B. (1981). Factors Affecting Algal Separation in Batch-Operated Secondary Ponds. PhD. Dissertation. University of California, Berkeley. Miller, S.F. (1972). Utilization of Algae for Wastewater Purification and Protein Production. PhD. Dissertation. University of CalifornIa, Berkeley. Mohn. H.F. (1980). Experiences and strategies in the recovery of biomass from mass cultures of microalgae. In: Algae Biomass, Shelef G. and C.J. Soeder (eds). Elsevier Biomedical Press. Amsterdam. Nurdogan. Y. (1988). Microalgal Separation From High-Rate Ponds. Ph.D. Dissertation. University of California. Berkeley. Nurdogan. Y and W.J. Oswald (1993). Enhanced nutrient removal in high-rate ponds. In: Proceedings of the 2nd IAWQ Conference on Waste StabilizatIOn Ponds. Oakland. California. Vollenweider. R.A. (1969). A Manual on Methodsfor Measuring Primary Production in Aquatic Environment. IBP Handbook 12. Oxford: Blackwell. p. 213. Yao, K.M. (1970). Theoretical Study of high-rate sedimentation. Journal WPCF. 42. p. 218-223.
Pergamon
Wal. Sci. Tech. Vol. 33. No.7. pp. 229-241.1996. Copyright Q 1996 IAWQ. Published by Elsevier Science Ltd Printed in Greal Bntam. All nghts reserved. 0273-1223/96 $1 S'OO + 0'00
PH: S0273-1223(96)00358-7
TUBE SETTLING OF HIGH-RATE POND ALGAE Yakup Nurdogan* and William J. Oswald** • Bechtel National. Inc.• 50 Beale St.• 45-28-C33. San Francisco, CA 94105, USA •• University o/California. 1301 South 46th Street, Buildingg ll2, Richmond, CA 94804. USA
ABSTRACT The effluent of a hlBh-rate pond (HRP) may contain 200 to 400 mgIL of algal suspended solids (55). Microal&ae must be separated before the effluent is discharged. Gravity sedimentation is usually the first method considered in algal wastewater treatment systems. However. required overflow rates (OFRs) to remove algae in the conventional clarifiers are too low. Therefore, the shallow-depth sedimentation concept was studied for the separation of Micractinium from HRP effluent in order to increase the OFRs of gravity settling equipment. Using the orthogonal squares experimental plan. the design parameters, such as OFR. tube diameter. tube length, and inclination. were evaluated for algal 55 removal. A circular upflow clarifier was run as a control. At an OFR of 8.1 S Um 2 . min (0.2 gpmlft 2), SS removal efficiencies of tube settling and upflow clarifier were 80% and 18%. respectively. When the OFR was Increased to 16.3 Um 2 • min (0.4 gpmlft2). SS removal efficiencies decreased to 61% in tube settler and to 11% in upflow c1anfier. SS removal efficiencies of tube settlers were 40% at 30.6 Um 2 . min (0.75 gpmlft2) and 20% at 71.3 Um 2 • min (1.75 gpmlft 2). A linear relationship ellists between removal efficiency and the diameter and length of tubes. The relationship is parabolic for the inclination of tubes and the OFR. The overall algae removal efficiency increased with the tube length but it was an inverse function of OFR. tube diameter. and inclination. AI an OFR of 12.2 Um 2 . min (0.3 gpmlft2). alilae removal efficiencies were 7 to 8 times better than that of upflow clarifier. It was demonstrated that OFR in a gravity settler could be increased 4 to 5 times using tube settlers. Copyright@ 1996IAWQ. Published by Elsevier Science Ltd
KEYWORDS Algae; gravity settling; high-rate pond; Micractinium; orthogonal squares; sedimentation; separation; tube settler.
INTRODUCTION GraYity sedimentation of al~ae The preference of the algae separation method depends on the species of algae, chemistry of growth medium, ultimate use of algal biomass, and most economically available separation technology. Algae cell size is an important factor to consider while selecting the technique for separation. Colonial algae (Micractinium, Scenedesmus) and filamentous algae (Spirulina) could be harvested by a low-cost separation technology such as sedimentation, flotation, and filtration. Smaller microalgae. like most single-eell (Chlorella) and motile algae (Euglena, Chlorognium), have to be flocculated before their gravity separation. Motile algae can also be separated utilizing their phototactic characteristics (e.g., swimming to the light 229
230
Y. NURDOGAN and W. J. OSWALD
source). Miller (1972) reported the phototactic separation of motile algae Pandorina species, which were cultivated in llib-scale growtb units in order to investigate the removal of strontium from low-level radioactive wastes. Because of the high volumetric flow rates associated with wastewater discharge, tbere are economic constraints on the capital and operating costs that can be incurred in such systems. Therefore, gravity sedimentation as a low-cost technology is usually considered first to clarify the dilute algal suspensions in algal wastewater treatment systems. Algae separation by gravity sedimentation is inexpensive but its reliability is low without some kind of flocculation. Colonial algae growth and the conditions for natural flocculation (auto- and bio-) should be maintained in the pond for reliable gravity settling without chemical flocculation. Costly separation techniques such as centrifugation may be justified if algae or their byproducts are used for animal or human consumption (e.g., protein, valuable chemicals). In order to improve the efficiencies of algae separation from HRP effluents, Nurdogan and Oswald (1993) investigated the enhancement of naturally occurring autoflocculation in a paddle-wheel-mixed HRP receiving primary sewage. Calcium- and magnesium-deficient autoflocculation was perfected by adding 60 mgIL of lime (CaD) into the pond. Although the primary objective of the study was to remove phosphorus, this simple procedure improved the removal efficiencies of algal solids from 72% to 95% in bench-scale batch gravity settlers with a I-day detention time. Eisenberg (1982) tried to separate algal solids from HRP effluents in a 30 m 3 batch settling unit operated in a fill-and-draw fashion with a 2-day detention time. The sedimentation basin, which was converted from a backyard swimming pool, had poor characteristics and produced higher than desirable concentrations of SS in the processed effluent. Some were as high as 150 mgIL but most varied between 40 to 100 mglL. These poor removal efficiencies may also have been due to unsettleable algal genera growing in the pond or to an adverse physiological state of the algae during his experiments. Koopman (1981) studied a process known as pond isolation for algae separation. Isolation of facultative pond (FP) from inflow promoted the clarification of pond effluent. Fill-and-draw operation of a secondary pond allowed significant removal of algae from FP effluent but a cycle of 2 to 3 weeks was required for desirable algae removal. The time required for pond isolation may be reduced if the conditions in the pond lead to bioflocculation and/or autoflocculation of algae. Secondary ponds were also used for algae settling from paddle-wheel-mixed HRP effluent (Benemann et al., 1980). Tubes of different diameters and cross sections (flat, corrugated, and other type plates) are also used to upgrade perfonnance in existing sedimentation basins. Mohn (1980) reported that the pond algae were concentrated to 1.6% as total suspended solids (TSS) using chemical flocculation and a lamella settler. Golueke and Oswald (1965) extensively studied the chemical flocculation of waste-grown algae preceding gravity clarification. They reported that alum flocculation followed by gravity settling was reliable method of algae separation. Eighty-five percent of suspended biomass from HRP effluent was separated with a resultant algae slurry of 1.5% TSS. Shallow-depth sedimentation concept Sometimes the term high-rate sedimentation is used in referring to a shallow-depth gravity sedimentation. The hydraulic residence time of a high-rate settler may be much less than 30 minutes to achieve comparable or better settling efficiencies than those normally obtained in a conventional settling basin having residence times around 2 hours. The main feature of a high-rate settler is its shallow depth, which is usually not more than a few inches. In 1904, Hazen presented the idea that settling basin efficiency is dependent primarily upon the OFR and maximum particle settling distance, and independent of detention time in the basin (Culp et al., 1969). Since then it has been recognized tbat a settling basin should be as shallow as possible and that a detention time of only a few minutes can be used in very shallow basins. For example, a particle settling at a rate of I inch per
Tube sellling of high-rale pond algae
231
minute requires 120 minutes to fall to the bottom of a- IO-foot conventional clarifier. If the basin was 2 inches deep, this particle would fall to the bottom in only 2 minutes. By inserting parallel plates in an ordinary clarifier, the flow could be increased by a factor directly proportional to the number of plates used without adverse impact on the treated water quality. Yao (1970) studied the theory of shallow-depth sedimentation and provided some infonnation on designing tube and parallel plate (lamella) settlers. Using the known hydrodynamic principals and assuming a single discrete particle moving in laminar one-dimensional flow, he developed a general equation for the trajectories of suspended particles in a laminar flow through a circular tube as follows:
y] - - ) - ~ y Sin 8 2 3 v.
y2
8 (-
V
+~ v.
(X - L) COS 8
=0
(I)
=~ . X =~, x, y =Coordinates L =Relative settler length =.!... I =Length of tube, d =Diameter of tube d v, =Fall velocity of a suspended particle
where Y
v.
= Average velocity of flow through tube
8 = Angle of tube inclination Among this family of trajectories, there is a limiting trajectory that defines the critical particle fall velocity 0 for a given system. Any suspended particle with its fall velocity greater than or equal to this critical fall velocity would be completely removed in the tube settler. The perfonnance of a tube setller can be characterized by a parameter S as follows: S
=~ (Sin 8 + L Cos 8) v.
(2)
The critical S-value, Sc ' is dependent only on the shape or geometry of a settler. It is 4/3 for a circular tube settler and I for a plate or lamella settler. Any suspended particle in a circular tube settler with its -value greater than or equal to 4/3 would be completely removed. The basic assumption here is that a particle is removed once it strikes the bottom wall of tube. A low Sc indicates a small vSC' which in tum means settlers having smaller values will remove more of the lighter or smaller particles. The design of settling tanks for wastewater treatment is usually based on the parameter OFR, expressed as gpm/ft 2. The concept was originated from the fact that, for an ideal unifonn flow in a horizontal-flow rectangular settling tank, the OFR represents the vsc' The design OFR should be equal to or smaller than the vsc to be removed. The same concept is readily adaptable to high-rate settlers since the vse can be estimated easily from Sc' The OFR or vsc can be expressed by the following equation: (3)
K = S. L (Sin 8 + L Cos 8)
(4)
where C is a constant and its magnitude depends on the units used in Equation 3. For example, if v 0 is in fps and OFR in gpm/ft2 , C =6.54 x lOs. The performance of a settler theoretically deteriorates when the degree of inclination is increased due to an increase in the settling depth. However, some inclination is desired to remove settled solids continuously and this may compensate for the reduction in particle removal efficiency. As solids are deposited onto the tubes or plates, the hydraulic flow path becomes partially restricted. Clearly, as the channels fill with settled
Y. NURDOOAN and W. J. OSWALD
232
solids, they must be removed periodically to prevent their reentrainment in horizontal settlers. To effect this cleaning requirement, the tubes or plates are inclined at an angle of 45' to 60', which is in excess of the slide-away angle of separated particles. The settled solids flow downwards in the reverse direction of liquid flow. The flow path through a separator, the hydraulic diameter, and the flow velocity should all be minimized to achieve a lower Reynolds number 0 and better settling conditions. Longitudinal flow through tubes with a diameter of a few inches theoretically offers optimum hydraulic conditions for sedimentation. Such tubes would have a large wetted perimeter relative to the wetted area and thereby would provide laminar flow, as evidenced by the very low for such systems as shown in Table I. Table 1. Reynolds numbers of tube settlers at various flow rates
OFR (gpmlrr)
Diameter (in.)
Reynolds Number·
0.5 0.5 0.5 0.5
1 2 3 4
6.6 13.2 19.8 26.4
1.0 1.0 1.0 1.0
1 2 3 4
13.2 26.4 39.6 52.8
2.0 2.0 2.0 2.0
1 2 3 4
26.4 52.8 79.2 105.6
• at a water temperature of 10°C.
'"
Effluent
-Influent d (In.) I (ft.) e F) 1 2 30 2 4 45 3
4
6
8
60
75
Imhoff Cone
Sludge Sludge
Figure I. Diagram of model tube sellier used in shallow·depth sedimenlation experiments.
Tube settling of high-rate pond algae
233
MATERIALS AND METHODS Two paddle-wheel-mixed HRPs were used to grow algae, which were mostly colonial green algae
Micractinium. The area of each pond was approximately 0.1 hectare and the wastewater source was primary sewage from a predominantly residential section of the city of Richmond, California. The ponding system, wastewater characteristics, and environmental conditions were described previously by Nurdogan (1988). The shallow-depth sedimentation concept was studied for the separation of colonial green algae Micractinium from HRP effluent in order to increase the OFRs of gravity settling equipment. Various modifications of model tube settlers of the type shown in Figure I were constructed from schedule-40 PVC pipes and mounted on a wooden structure. The design parameters, such as OFR, tube diameter, tube length, and inclination, were evaluated for algal 55 removal. A circular upflow clarifier with a conical bottom was run as a control in parallel with tube settlers. This unit had a diameter of 2.5 feet, total depth of 3.2 feet, and total volume of 260 liters. The pond water was pumped to the central well. The solids were collected from the conical bottom and clarified water overflowed to the collection weir. A scraper at 1.4 rpm was operated to help the downtlow movement of settled solids.
A typical procedure used in research studies related to multivariable analysis is to vary one factor at a time while holding all others constant. When the number of variables and length of time required for individual measurements is small, this method works well. However, as these increase, the conventional method becomes excessively time consuming and expensive. By carefully selecting each combination of independent variables, it is possible to greatly reduce the number of individual measurements required in a multivariate analysis. A special experimental procedure called "orthogonal squares" (OS) was used for the tube settling experiments (Harris et ai., 1952). Two sets of OS experiments (4x4 and 3x3) were carried out according to the preplanned experimental procedure. According to the OS test program, if there are four independent variables, a fifth factor enters the picture in the data analysis. This is experimental error and represents the portion of the relationship not accounted for by four independent variables. The specific combinations of variables required in the experimental design are most easily determined by the matrix shown in Fig. 2. Outlines of the experiments are shown in Table 2.
313 Orthogonal Squares
414 Orthogonal Squares
A A A
A B 1 B A B
A C 2 C B C
A 0 3 D C D
A 4 D
A
A
B B 5 D D A
B A 6 A C 0
B D
A
B C
B 8 A
C B 9 C B D
C C 10 B A A
C 0 11 A 0 B
C 12 C
A
D B 13 A C C
D C 14 D 0 B
D D 15 C A A
D 16 B
C B A D
Key
C
DIA LEN
A B
ERR
# INC OFR
D
7 B
A
A B
B A
6
B
A A
C
C C
C B
8
C B
B C 5
C C 7
C
B C
B B
B
3
2
4
A
A C
A B 1
9
A B
A
Key CIA LEN # ERR OFR
Figure 2. Experimental design matrix.
SS and TSS determinations were made according to Standard Methods (1985). Chlorophyll-a (Chi-a) was measured using a methanol extraction method modified from VoUenweider (1969).
234
Y. NURDOGAN and W. J. OSWALD
Table 2. Experimental design outlines
4 x 4 Orthogonal Squares
Code #
Diameter (in.)
Length (ft.)
(gpmlft 2)
OFR
Inclination (degree)
I 2
2 4 6 8
0.25 0.75 1.25 1.75
30 45 60 75
A B C D
3 4
Error Level
I II
III IV
3 x 3 Orthogonal Squares
Code #
Diameter (in.)
Length (ft.)
(gpmlft2)
Error Level
A B C
I
2 3
2 4 6
0.15 0.30 0.45
I II III
OFR
RESULTS Effects of design parameters of a tube settler on the algal removal efficiencies of were studied using the OS experimental methodology. The time savings achieved by using the as test program made it possible to complete all of the tube settling runs before the settling characteristics of the algal cultures changed due to seasonal changes in growth conditions. Micraclinium spp. were the predominant algae and they were readily settling. Figure 3 shows the photomicrograph of Micraclinium spp. and the settled volume in an Imhoff cone after 2 hours.
B
A .
. •
~b
·fiti ..
Figure 3. Photomicrograph of Micractinium predominant in HRP during the tube senling experiments (AI and volume of algae biomass senled in Imhoff cones (8).
Tube settling of high-rate pond algae
235
4x4 orlhof:onal square results Four parameters (tube diameter, tube length, tube inclination and OFR) were varied four times. Pipes having 1-,2-, 3-, and 4-inch diameters and 2-, 4-, 6-, and 8-foot lengths were used. The tube inclinations of 30°, 45°, 60°, 75° were used together with a 90° control. The OFRs were varied through four levels: 0.25, 0.75, 1.25, and 1.75 gpmlft 2. The first set of these 16 experiments was run to determine the most important parameters of tube settler for HRP algae. In order to determine the algae removal efficiency, 55 and Chl-a determinations were made on influent and effluent samples of tube settler. Table 3 shows the study range, the optimum operational conditions for the best algal removal, and the optima for practical operation.
100
I
• 55
80 ~ 0
60
'"
40
E
a:
'if.
~
20 0
LEN 5.0 ft. OFR 1.0 gpmlft 2 INC 52.5°
o Chl-a
---v ----0---2
0
3
0
----
4
5
Diameter, in. Figure 4. Effect of tube diameter on algal removal efficiency.
100...----.------,-----r---r---,
~E
DIA 2.5 In. OFR 1.0 gpmlft 2
• 55
80
o Chl-a
INC 52.5°
60
... ..
'" 40 t-:.:-:-~ ~-:-::-~ ~-::-;-~;:.:--O--J ~ 20
o o
L-_-I.-_---l'--_-I-_--I._.---J
2
4 6 Length, ft.
8
10
Figure 5. Effect of tube length on algal removal efficiency.
A linear relationship exists between removal efficiency and the diameter and length of tubes. The effect of tube diameter on algae removal is shown in Figure 4. The smaller the tube diameter the better the algae separation. A I-inch-diameter tube resulted in an average of 49% 55 and 46% Chl-a removal efficiencies. Average 55 and Chi-a removal efficiencies with 4-inch-diameter tube were 39% and 32%, respectively. Figure 5 indicates that algal removal efficiencies increase with increasing tube length. The 8-foot long tube produced 47% 55 and 45% Chl-a removal efficiencies on the average. The 2-foot long tube produced average 55 and Chl-a removal efficiencies of 40% and 33%, respectively. The effect of inclination on algae removal is parabolic and presented in Figure 6. The 30° inclination produced a better algae removing condition because when the inclination gets closer to horizontal, ideal conditions were achieved for shallow depth sedimentation, which is the principal of tube settling. However, inclinations steeper then 45° facilitate the settled sludge removal by gravity (self-cleaning action). This will in turn contributes the better algae removal efficiencies. Therefore, a 60° inclination gave better removal efficiencies than a 45° angle. The effect levels off after 60° and sharply declines after 75° angle because the benefit of shallow-depth sedimentation is completely lost at the 90° angle. In order to obtain a simple
Y. NURDOGAN and W. J. OSWALD
236
relationship between inclination and algae removal efficiencies, the overall effect of inclination from zero to 90° can be approximated to a declining linear relationship.
100 80 ~ 0
E CI>
60
a: 40
.55
LEN 5.0 ft. CIA 2.5 in. OFR 1.0 gpm/ft 2
o Chi-a
e:-:-:....... -~ .. e---~ ......
20 0 15
30
45 60 Inclination, degree
75
90
Figure 6. Effect of tube inclination on algal removal efficiency.
There is a parabolic relationship between removal efficiency and the OFR. The most important parameter of all was the OFR as shown in Figure 7. Increased OFRs substantially decreased the algae removal efficiencies. This is expected in all solid-liquid separation systems and for all types of particles. Removal efficiencies were 75% Chl-a and 70% SS at 0.25 gpmlft 2 and 24% SS and 16% Chl-a at 1.75 gpmlft 2. Because of very high OFR and averaging used in OS data analysis, algae separation efficiencies came out low. Therefore, a second set of OS experiments were designed with lower OFR varying from 0.15 to 0.45 gpmlft2. In this way, the optimum tube operating conditions were refined using a 3x3 OS experiment.
100 LEN 5.0 ft. CIA 2.5 in. INC 52.5°
80
~E
....
60
a: 40 CI>
20 0 0.0
e55 o Chi-a
lSl..
..... ......
-O' ...... __ 'E)
1.0 1.5 0.5 Ovenlow Rate, gpm/ft 2
2.0
Figure 7. Effect of overflow rate on algal removal efficiency.
Using the least squares method of curve fitting, correlation equations can be written to predict the removal efficiencies for SS and Chi-a in the study range of variables. Based on a maximum error level of 6.3% SS and 7.9% Chi-a, the correlation equations for 4x4 OS test results are as follows: % 5S Removal = 109 - 3.1 DIA + 1.2 LEN - 0.72 INC + 0.007 (INC)2 - 60 OFR + 15 (OFR)2 % Chl-a Removal
=128 - 4.7 DIA + 1.9 LEN - 0.83 INC + 0.008 (INC)2 - 93 OFR + 27 (OFR)2
(5) (6)
3x3 ortho~onal square results After examining the results obtained in 4x4 OS experiments, an OS of 3x3 was conducted. This time the range of variables was decreased, but inclination was constant at 60'. One-, 2-, and 3-inch diameters and 2-, 4-, and 6-foot lengths were used for 0.15, 0.30, 0.45 gpmlft 2 OFR. Nine more experiments were run to refine the results of the previous 16 experiments.
Tube settling of high-rate pond algae
237
Table 3. Optimization of tube settler for algae separation 4 x 4 Orthogonal Squares Study Range
Best Removal
Practical
Diameter. in.
1-4
1
2
Length. ft.
2-8
8
6
30-7S
30
60
0.25-1.75
0.25
0.50
Parameters
Inclination. degrees 2
OFR, gpmlft
3 x 3 Orthogonal Squares Study Range
Parameters
Best Removal
Practical
Diameter, in.
1-3
1
2
Length. ft.
2-6
6
4
0.15-0.45
0.15
0.45
2
OFR. gpmlft
Results are tabulated in Table 3 showing the study range, the optimum operational conditions for the best algae removal, and the optima for practical operation. Correlation equations between algae removal efficiencies and operational parameters of tube settler are written using the least squares method of curve fitting. A linear relationship exists between removal efficiency, the diameter and the length of tubes, and the OFR. These equations can predict the removal efficiencies for SS and Chl-a in the study range of variables. The calculated maximum percent errors in predicting removal efficiencies were minimal. Figure 8 shows the effect of the tube diameter on algae removal. Similar to the previous experiment. when the tube diameter increases the algae removal efficiencies decrease. The best algae removal was obtained with l-inch-diameter tube and the least removal with 3-inch-diarneter tube. A l-inch-diameter tube produced 83% Chl-a and 80% SS removal efficiencies while a 3-inch-diameter tube produced 73% Chl-a and 63% SS removal efficiencies.
100 r---r---~--...---.,
~E '"
a: ";!.
40 20
• 55
LEN 4.0 ft.
o Chl-a OFR 0.23 Opmlft 2 o~--;-----:---~----J.
o
2
Diameter, in.
3
4
Figure 8. Effect of tube diameter on algal removal effiCiency.
JWST 33:7-1
238
Y. NURDOGAN and W. 1. OSWALD
100r-------------, 80
~E 60
.----~----.
..
~----
Q)
ex: 40 ~
.55
20
D1A 2.0 in. OFR 0.23 gpmlft 2
o Chl·a
O':-_ _~--.L---.L--~
o
2
4 Length. ft.
6
8
Figure 9. Effecl of lube lenglh on algal removal efficiency.
Figure 9 shows the effect of tube length on algae removal efficiency. Increasing the length of tubes increased the algae sedimentation efficiency. The 6-foot length produced average efficiencies of 81 % Chl·a and 74% SS. and the 2-foot long tube produced 76% Chl-a and 68% S5 removal efficiencies. The OFR was again the single most important parameter effecting algae separation in the tube settlers. An OFR of 0.15 gpmlft2 produced the best algae removal efficiencies (89% Chl-a and 86% SS) and 0.45 gpmlft 2 OFR rate produced the least algae removal efficiencies (68% Chl-a and 59% SS) as seen in Fig. 10. t5-1lltlUOI
100 80
.
!E
60
a::
40
~
20
.55 o
CIA 2 In. LEN 4 II.
Ch~a
o0.00 ~-...,..J-:-----,~-~L.:----,~ 0.15 0.30 0.45 0.60 Overftow Rate, Opmlh 2
Figure 10. Effect of overflow rale on algal removal efficiency.
Based on a maximum error level of 1.7% SS and 0.6% ChI-a. the correlation equations for 3x3 OS tests are as follows:
=108 - 8 DIA + 1.7 LEN - 92 OFR % Chl-a Removal =105 - 5 DlA + 1.2 LEN - 72 OFR % SS Removal
(7) (8)
DISCUSSION Microalgae. in general. have slow settling rates and it is difficult to separate them from liquid medium by a simple gravity sedimentation. Required OFRs to remove algae from oxidation pond effluents are too low and hydraulic residence times are out of the conventional range of practicality. Earlier attempts by Eisenberg (1982) and Koopman (1981) on separation of microalgae using phase isolation ponds or clarifiers showed some success at very low OFRs (less than 0.1 gpmlft 2) and long detention times (days to weeks). Such low OFRs are not practical for HRP or for FP. Therefore. where effluent 55 are controlled. microalgae are normally flocculated using chemical agents followed by gravity sedimentation.
Tube settling of high-rate pond algae
239
If the colonial algae Micractinium can be maintained in a HRP, algae separation by gravity sedimentation is possible without using any chemical flocculants. Batch settling tests with Imhoff cones indicated SS removal efficiencies approaching to 95% in a few hours when Micractinium were predominant in HRP. The Richmond HRP system fosters growth of colonial Micractinium. These algae have been maintained almost as a monoculture for several years and were also successfully sustained in the pond during the tube settling study. The tube settling of algal solids in this study was carried out using no chemical flocculation. High-rate settlers can also be used to separate autoflocculated algae. Based on the experience in Richmond, autoflocculation of algae could be relied on for at least 6 to 8 months in a year in places climatically comparable to San Francisco Bay Area. During last 15 years, high-rate settlers have been used in water and industrial wastewater treatment where chemical coagulants and flocculants are always used to form settelable flocs. Needless to say, a FP effluent could not be clarified by tube settlers unless the algae were flocculated. Therefore, no attempt has been reported, until now, to separate the waste-grown algae in tube settlers without some kind of chemical flocculation. Chemicals used in separation of algae may be detrimental if the biomass is to be used for protein or methane generation.
100 80 ;; > 0
E
.
.55 o Chl-a
60
a::
Q)
:>!!
20 0 0.0
0.6 0.9 1.2 1.5 Overflow Rate, gpmlft 2
1.B
Figure II. Comparison of tube settlers with conventional upllow clarifier in tenns of algal removal efficiency.
100 ~ 0
E Q) a:: tr.l tr.l
'#-
80 60 030° 45° • 60° A 75°
40
lJ.
20 0
1
10
NR
100
Figure 12. Effect of Reynolds number in tubes on algal removal efficiency.
The pilot-scale conventional upflow clarifier was run parallel to the model tube settlers at varying OFRs. Figure 11 shows the SS removal efficiencies achieved in an upflow clarifier and in the tube settlers. At 0.2 gpmlft 2 OFR, 55 removal efficiencies of tube settling and upflow clarifier were 80% and 18%, respectively. At 0.4 gpmlft 2 OFR, 55 removal efficiencies decreased to 61 % in tube settler and to II % in upflow clarifier. It was apparent that the most important parameter governing the algae removal efficiency was the OFR. 70% 55 removal efficiency was obtained at 0.3 gpmlft 2 and 86% S5 removal efficiency at 0.15 gpmlft 2. When the OFRs were increased the removal efficiencies were rapidly decreased. 55 removal efficiencies were 40% at 0.75 gpmlft 2 and 20% at 1.75 gpmlft 2.
Y. NURDOGAN and W. J. OSWALD
240
100
~ 0
E Q)
a:
(/) (/)
80 60 030° 45° .60° .75°
40
A
20 0
1
100
10 d
v2 - *102 ° I
Figure 13. Effect of flow velocity term on algal removal efficiency.
SS removal efficiency seems decreasing with the angle of inclination from 30' to 45'. The efficiency improves between 45' and 60', probably due to the benefit of self-cleaning action. SS removal efficiency was better at 60' than at 45'. However, after 60' the benefits of shallow-depth sedimentation start to diminish due to increased settling depths. In order to extract a physical meaning from the tube settling results, %SS and %Chl-a removal efficiencies were correlated with the NR and the ratio of tube length to tube diameter. The SS removal efficiency decreases with increasing NR and a velocity term (21) as seen in Figures 12 and 13. Two equations to predict the performance of a similar tube settler for algae sedimentation without the help of chemical agents are given as follows:
(9) = 115 (NR ) -fJ.27 (1..) 0.029 d (10) % Chl-aRemovai 115 (Nil) -fJ.l8 (1.. )0.006 d The error levels in the OS experiments were checked using an additional parameter that was not varied but presumably could vary due to errors. Figure 14 indicates that error levels were minimal. %SSRemovai
=
4x4 Orthogonal Squares
.55
80 ]! 0
E .,
o Chl-a
..
60
a: .,e 40
3x3 Orthogonal Squares
100
100 LEN 5.0 ft D1A 2.5 In. OFR 1.0 gpnVft 2 INC 52.5 degree
~E .,
..
60
a: 40
---~--~---~--~---
.,e
.55
o Chl-a
20
20 0 0
3 2 Error Level
0 4
5
0
CIA 2.0 In. LEN 4.0ft OFR 0.23 gpmllt 2 2 3 Error Level
4
5
Figure 14. Error levels in orthogonal square tube selliing experiments.
SUMMARY AND CONCLUSIONS The experimental tube settlers separated the HRP algae much better than a conventional upflow clarifier. At an OFR of 0.3 gpmlft2, SS and Chl-a removal efficiencies were 7 to 8 times better than that of an upflow clarifier. It was demonstrated that OFR in a gravity settler could be increased 4 to 5 times, or settling time could be shortened 4 to 5 times using tube settlers.
Tube seltling of high-rate pond algae
241
The OS experiments demonstrated that the overall algae removal efficiency increases with the tube length but it is an inverse function of OFR, tube diameter, and tube inclination. Smaller OFRs and tube diameters result in smaller Reynolds numbers, which is better for particle settling in general. Tube settling cannot dependably clarify pond effluent to required standards throughout 4 seasons without chemical or other types of flocculation. Because algae genera and biomass settling rates vary (some times sharply) due to changes in wastewater characteristics and environmental conditions.
It is possible to harvest the algal biomass by gravity separation techniques when colonial algae such as Micractinium spp. predominate and the natural flocculation processes such as autoflocculation, bioflocculation are attained in the HRP. This study demonstrated that Micractinium spp. can settle in a few hours without using any chemical agents. REFERENCES APHA (1985). Standard Methods for the Examination of Water and Wastewater, 16th cd., Washington. D.C. Benemann. lR., B.L Koopman. J.C. Weissman. D.M. Eisenberg and R. Goebel (1980). Development of microalgae harvesting and high-rate pond technologies in Califorma. In: Algae Biomass. Shelef G.and C.J. Soeder (eds), Elsevier Biomedical Press. Amsterdam. Culp. G.L. K. Hsiung and W.R. Conley (1969). Tube clarification process. operating experiences. J. San. Eng. Div., Proc. of ASCE. 95. SA5. p. 829. Eisenberg. D.M. (1982) Large·Scale Freshwater Microalgae Biomass Production for Fuel and Fenilizer. PhD. Dissertation, University of California. Berkeley. Golueke. J.C.• W.J. Oswald and H.K. Gee (1965). Harvesting and processing sewage-grown planktonic algae. Journal WPCF. 37. 4. Harris. D.N.• F.R. Watson and T.F. Thomson (1952). Magic squares: preplaned tests exemplified by flight research. Shell Aviation News. June and July. Koopman. B. (1981). Factors Affecting Algal Separation in Batch-Operated Secondary Ponds. PhD. Dissertation. University of California, Berkeley. Miller, S.F. (1972). Utilization of Algae for Wastewater Purification and Protein Production. PhD. Dissertation. University of CalifornIa, Berkeley. Mohn. H.F. (1980). Experiences and strategies in the recovery of biomass from mass cultures of microalgae. In: Algae Biomass, Shelef G. and C.J. Soeder (eds). Elsevier Biomedical Press. Amsterdam. Nurdogan. Y. (1988). Microalgal Separation From High-Rate Ponds. Ph.D. Dissertation. University of California. Berkeley. Nurdogan. Y and W.J. Oswald (1993). Enhanced nutrient removal in high-rate ponds. In: Proceedings of the 2nd IAWQ Conference on Waste StabilizatIOn Ponds. Oakland. California. Vollenweider. R.A. (1969). A Manual on Methodsfor Measuring Primary Production in Aquatic Environment. IBP Handbook 12. Oxford: Blackwell. p. 213. Yao, K.M. (1970). Theoretical Study of high-rate sedimentation. Journal WPCF. 42. p. 218-223.