WATER POLLUTION CONTROL RESEARCH SERIES
17010—02/70
"AN ELECTROCHEMICAL METHOD
FOR REMOVAL OF PHOSPHATES
FROM WASTE WATERS"
>-*""*•• * * .' •—r"'
•3 • 'la*:'. •. .••>'
U.S. DEPARTMENT OF THE INTERIOR • FEDERAL WATER QUALITY ADMINISTRATION
-------
WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe
the results and progress in the control and abatement
of pollution in our Nation’s waters. They provide a
central source of information on the research, develop-
ment, and demonstration activities in the Federal Water
Quality Administration, in the U. S. Department of the
Interior, through inhouse research and grants and con-
tracts with Federal, State, and local agencies, research
institutions, and industrial organizations.
A triplicate abstract card sheet is included in the
report to facilitate information retrieval. Space is
provided on the card for the user’s accession number and
for additional uniterms.
Inquiries pertaining to Water Pollution Control Research
Reports should be directed to the Head, Project Reports
System, Planning and Resources Office, Office of Research
and Development, Department of the Interior, Federal Water
Quality Administration, Room 1108, Washington, D. C. 20242.
-------
AN ELECTROCHEMICAL METHOD FOR REMOVAL OF
PHOSPHATES FROM WASTE WATERS
by
Shafik E. Sadek
Dynatech Corporation
Cambridge, Massachusetts 02139
for the
FEDERAL WATER QUALITY ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Program #17010
Contract #14-12-405
FWQA Project Officer, C. A. Brunner
Advanced Waste Treatment Research Laboratory
Cincinnati, Ohio
February, 1970
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price 50 cents
-------
WQA Review Notice
This report has been reviewed by the Federal
Water Quality Administration and approved for
publication. Approval does not signify that
the contents necessarily reflect the views
and policies of the Federal Water Quality
Administration, nor does mention of trade
names or commercial products constitute
endorsement or recommendation for use.
-------
TABLE OF CONTENTS
Section Page
INTRODUCTION 1
EXPERIMENTAL 4
TEST PROCEDURE 4
TEST RESULTS 8
Current 8
Electrode Consumption 14
Phosphate Removal 14
Raw Sewage Tests 30
SYSTEM OPTIMIZATION AND COST 40
REFERENCES 47
111
-------
ABSTRACT
Phosphates in waste water may be removed electrochemicafly utilizing sacri-
ficial electrodes. The electrode metal is first dissolved by the flow of current
then precipitates out, removing from solution the phosphate ions. This removal is
either dependent on chemical reaction of the metal cation and the phosphate anions
or, possibly, on the adsorption of the phosphate by the metal hydroxide floc.
Data on the phosphate removal was gathered using both aluminum and iron
electrodes. Essentially complete removal was found to occur on using 300
coulombs/liter of charge flow with normal phosphate concentrations for both types
of electrodes. Aluminum consumption averaged about 007 mass units per single
mass unit of P0 4 removed for essentially complete phosphate removal. This mass
ratio was about 2 for iron electrodes.
Treatment costs (excluding labor and filtration) have been estimated to be
about 2.5 cents/l000 gal. and 8 cents/l000 gal. when using iron and aluminum
electrodes respectively.
Exploratory tests indicated that flotation by means of the hydrogen generated
during the electrolysis may be used to remove suspended solids from raw sewage
while phosphates are being removed.
This report was submitted in fulfillment of Program No. 17010,
Contract No. 14-12-405, between the Federal Water Quality Administration
and Dynatech Corporation.
iv
-------
INTRODUCTION
In recent years, increased use of phosphate-based fertilizers and detergents
have contributed to the large concentration of phosphates in waste water. This
phosphate has been identified as one of the major causes of algae growth in receiv-
ing bodies of water, to the detriment of animal life (1,2). There is increasing need
to treat the effluent from municipal treatment plants in order to reduce the phos-
phate content.
Use of electrochemica.l methods for treatment of waste waters is not new;
several studies utilizing such methods have been described in patents and in the open
literature (3,4,5,6). An experimental study has recently been reported in which
several electrode materials were evaluated under a variety of experimental condi-
tions to determine their effectiveness In, removing various contaminants from
waste water (7). It was demonstrated that the phosphate content of the effluent from
a secondary treatment process could be reduced significantly. These experiments
were not geared towards phosphate removal and were not run in a manner to remove
phosphates most economically.
A study has been undertaken to evaluate an electrochemical method for remov-
ing phosphates. The objective of the program was to determine the effectiveness of
an expendable electrode, direct current method for the removal of phosphates from
the effluent of a secondary treatment process. The evaluation consisted of measur-
ing electrode and power consumption and phosphate removal as a function of voltage,
electrode material (aluminum or iron), electrode spacing, and residence time.
Batch tests were followed by continuous flow (steady-state) tests. A number of tests
were nm on raw sewage to evaluate the use of the system as a combined phosphate
removal/bubble flotation operation.
The economics of a conceptual plant were briefly evaluated and the cost of
treating effluent under optimized conditions determined.
1
-------
The term sacrificial electrode (e.g., aluminum or iron) implies that the
metal composing the electrode dissolves forming positive ions. These ions react
with the constituent anions of the solution (hydroxide, phosphate, etc.) and precipi-
tate out as a floe. The result is removal of phosphates. At the cathode, the usual
hydrogen evolution occurs and keeps the solution electrically neutral.
Tests with a non-sacrificial graphite anode showed that no phosphate was
removed at similar current flows even though longer treatment periods were used
(Table 1). The electrode material and the extent of phosphate removal are therefore
intimately related to each other.
Phosphate removal by sacrificial electrode electrolytic techniques may be
explained by either (or possibly both) of the following mechanisms:
1. Physical adsorption of the phosphates onto the hydroxide floc generated; or
2. Chemical reaction of the phosphate in the solution with the metallic ions
followed by precipitation.
Choice of one or tha other of the above-mentioned mechanisms to explain the
phosphate removal phenomenon cannot presently be made with any degree of certainty
owing to the lack of reliable data in the open literature on the solubiity products of
metallic phosphates. This report therefore does not presume to explain the mecha-
nism of phosphate removal. It offers the results of a systematic set of tests which
allows the evaluation of the sacrificial electrode process for phosphate removal on
a sounder basis than was possible earlier.
2
-------
Table 1
EFFECT OF ELECTRODE MATERIAL ON PHOSPHATE REMOVAL
Electrode Material
Poten-
1
tia
(Volts)
Current
(Amps)
Time
(Hours)
Initial
P0
4
mgm/liter)
Final
P0
4
(mgm/liter)
Carbon
Carbon
Aluminum
5
15
5
0.04
0.13
0.14
1.5
1.3
0.5
30.5
30.5
30.5
27.8
29.3
0,2
3
-------
EXPERIMENTAL
TEST PROCEDURE
Three sets of tests were performed. The- first set consisted of batch tests
performed in a Plexiglas cell 4 in. wide by 6-1/2 in. long and 8 in. high (Figure 1).
The electrode spacing was made variable by supporting the electrodes externally.
The dead space behind the electrodes was reduced by filling it with Plexiglas blocks.
A small laboratory stirrer was used to keep the contents of the cell mixed.
The second set of tests consisted of flow tests in single electrode-pair systems.
Two such cells (Figure 2) were assembled with nominal electrode spacings of 1/2 in.
and 1 in. In these two sets, smooth aluminum and iron electrodes were used; each
electrode measured 4 in. x 4 in. No external stirring was used.
The third and final set of tests were run in a multiple electrode (aluminum)
system (Figure 3). Electrode spacings were 1 in. and each electrode was 6 in. x 6 in.
of aluminum screen. Here, separation of the floc from the solution was accomplished
by bubble flotation. No external stirring was used.
The solutions used in these tests were either “synthetic effluent” or true
secondary treatment effluent. The synthetic effluent was used as a convenience to
determine the trends in the performance as a function of the system parameters.
This was a natural choice as it was simple to reproduce and did not deteriorate with
time. True effluent was used to check the trends determined in the synthetic effluent
tests. The synthetic effluent contained 0.25 gm/liter sodium chloride, 0.41 gm/liter
sodium bicarbonate and dibasic sodium phosphate to give 20, 40, 80, and 300 mgm/
liter P0 4 as required.
Raw sewage was used in the multiple electrode system in order to determine
the effectiveness of bubble flotation in reducing suspended impurity concentrations
in conjunction with phosphate removal.
The phosphate measurement was made according to the procedures recommended
in “Standard Methods for the Examination of Water and Waste Water (8).
4
-------
Figure 1
PHOTOGRAPH OF BATCH CELL
5
-------
Figure 2
PHOTOGRAPH OF FLOW CELL
6
-------
Figure 3
PHOTOGRAPH OF LARGE FLOW CELL
7
-------
In both synthetic and secondary effluents, all results are based on an ortho-
phosphate measurement. Preliminary analysis showed that total and orthophosphate
content of secondary effluent were essentially the same.
The test procedure consisted of starting with a known phosphate content in
the solution, adjusting the cell potential to the desired level, and checking the current
as the run proceeded. At the end of the run, the phosphate content was measured.
In some of the batch tests, the reduction in anode weight was also measured
in order to determine the current efficiency with respect to the anode reaction. In
continuous flow tests, the inlet and outlet phosphate concentrations and current were
measured during steady state conditions.
TEST RESULTS
Test results consist of the current as a function of the system parameters,
electrode consumption rates, and phosphate removals.
Current
The current (1) depends on the following system parameters:
• voltage (V)
• solution conductivity (a)
• electrode spacing (s)
• electrode area (A)
• electrode material
Here, it is assumed that edge effects are negligible. For the liquid between the elec—
trodes, one may relate the above parameters as
I _aA
s
where the subscript “ ‘ denotes absence of electrode and polarization potentials.
Since the latter potentials are not negligible, the observed current will be less than
8
-------
that calculated from the above equation using measured voltage. The ratio of
observed to calculated current,
—I
Lo
is therefore less than unity and represents the current effectiveness at a given
voltage.
Figure 4 shows I/ci, current divided by conductivity, of different effluents
plotted against the operating voltage in the batch cell at an electrode spacing of
1.6 cm, The current remained essentially constant throughout the runs and no change
in conductivity was noticed. Aluminum electrodes were used. One set consisted
of tests with aluminum foil electrodes; the other with perforated aluminum sheets.
It is seen that at a given voltage the current is higher for the perforated sheets
than it is for the smooth electrode. This is probably due to the higher surface
area of the screen type electrode.
The current effectiveness, ii , is shown also plotted against the voltage
(Figure 5). This effectiveness is only slightly dependent on the voltage ar 1 equal to
about 0. 13 in the case of the foil electrode and about 0.2 in the case of the perfora-
ted sheet electrode within the range of interest.
The effect of electrode spacing on the efficiency 77 is shown in Figure 6.
These tests were run with synthetic sewage in the batch cell, using perforated
sheet electrodes. The results indicate that smaller spacings give a slightly higher
value of i than wider ones. This is probably due to the higher current densities
through the system at a given voltage when the spacing is reduced. This increase in
current results in an increase in the stirring of the system, owing to the greater
bubble generation rate. The value of i also increases with an increase in opera-
ting voltages. This too is probably caused by the higher current accompanied by an
increase in stirring of the system.
Replotting the data as 77 against the current (I) instead of the voltage (V)
correlates the data with considerably less scatter (Figure 7). A better correlation
is however to be expected as both ordinate and abscissa are proportional to the current.
9
-------
I I
200 A
0
0
o 8
-c
E
E
a
E
too — Effluent Smooth Screen
Source Electrode Electrode
0 Dover o
Marlboro D
0 HudsOn V V
Maynard A
0 I .1
0 5 10 15
V (volts)
Fig.4 Current-Voltage Relationship Using Aluminum Electrodes (Electrode Spacing :1.6cm)
-------
£
0
8
Effluent
Source
Dover
Marlboro
Hudson
Maynard
0
0
5 10 15
Screen
Electrode
V
£
V (volts)
0
0
C
0
V
0
C-)
0
0
4-
C
a,
0
0
4-
4)
E
a,
0 .
‘C
w
0.20
0.10
0
0
0
0
Smooth
Electrode
0
0
V
Fig. 5. Current Flow Effectiveness Using Aluminum Electrodes (Electrode Spacing:l.6cm)
-------
I I
I . U
0.2—
C S
a)
0
V
a)
0
.
C.) S
0
o as
w
E1.ctrod. Spacing
____________________ .6cm 2.5cm 5.0cm
_______________________ _____ _____ _____
0
40 mgm/Iiter P0 4 £
80 mgrn/Iiter P0 4 0
0 300 mgm/Iiter P0 4 0
80 mgm/Iiter (ONoCI)
80 mgm/Iiter (0.5g/ NaCI)
40 mgm/Iiter (no stirrer) X
0 I I I
0 5 10 15
V (volts)
Fig. 6. Effect of Electrode Spacing on Current Effectiveness Using Aluminum Electrodes
-------
0 .
0
x.
U
x
£
S
I
0
I ’S
C
a,
0
V
0)
0
U
0
0
C
a)
0
0
C
a,
E
0)
0.
‘C
w
0
0.2
0.l
0
0
S
Electrode S7ocing
2.5cm 5.0cm
40 mgm/Htor PD 4
30 m m/l fer P0 1
300 rngrn/Hf r P04
80 mg;n1’ cr 0N Cfl
30 d ; ;. ‘3 : c3)
40 m m/. for (no : r r)
1.3 c;.
S
S
0
0
.3
x
I (amps)
0.5 1.0 .5
Fig.7
Current Flow Effectiveness as a Function of Current (Aluminum Electrodes)
-------
No tests were run to determine the effect of electrode height on the current.
Even though it is probably that such an effect does exist (since height wiU affect
degree of turbulence), it may be expected that this effect is small.
The effect of stirring on the current was found to be small in the case of
runs with aluminum electrodes (Figure 7).
With iron electrodes, the same trends were noted (Figures 8 and 9). In this
case, synthetic effluent showed slightly lower current effectiveness than secondary
treatment effluents from Marlboro or Hudson, Massachusetts.
Electrode Consumption
Electrode dissolution rates were determined experimentally. This was
accomplished by using thin foil in the case of aluminum and 0.. 006 In. thick Iron
sheet as electrodes, and weighing them before and after the tests. On comparing
these weight losses to the current, it was shown that under all of the operating
conditions used, the current efficiency scattered around 100% with respect to
aluminum dissolution (i. e., one Faraday dissolved one equivalent). Side reactions
(oxygen evolution) may therefore be considered negligible.
When iron electrodes are used, the current efficiency appeared to be 100%
with respect to the Ferrous iron. Since no gas was observed to evolve it, it is
probable that the electrode did, in fact, dissolve as Fe+ +.
Data supporting these current efficiencies are listed in Tables 2 and 3.
Phosphate Removal
Phosphate removal is intimately related to the quantity of sacrificial electrode
material introduced into the solution. Three steps are necessary for the phosphate
removal to occur:
1. Metal ions (or floe) must first be generated;
2. The phosphate and the metal ion (or the floe) must be transferred from their
respective high concentration regions to the low concentration “reaction” zone;
14
-------
I I
200—
.
0
-c
E
E
E
100
• Marlboro Effluent
• Hudson Effluent
• 4Omgm/liter P0 4
• 80mgm/literPO 4
U
0
0 5 10 15
V (volts)
Fig.8 Current-Voltage Relationship Using Iron Electrodes (Electrode Spacing:l.6cm)
-------
I I
S Marlboro Effluent
0.4—
I Hudson Effluent
S 4 omgm/literPO 4
4 )
• 80 mgm/liter P0 4
U
V
2
U
U .
C
0.2
U
I
— U I
C
4) U
E
4, 0
0.
0.1
w
0 I
0 5 tO I S
V (volts)
Fig.9. Current Effectiveness Using Iron Electrodes
-------
Table 2
CURRENT EFFICIENCY WITH RESPECT TO
ALUMINUM ELECTRODE DISSOLUTION
Electrode
Measured
Dissolution
Calculated
Anode
Current
Run
Sewage
Origin
Potential
(volts)
Current
(amps)
Time
(miii.)
Anode Wt.
Loss (mgm)
(We)
Wt. Loss
(mgm)
(Wm)
Efficiency
(€)
= Wm/Wc)
4R
Dover, N. H.
10
0.3
5
8.4
18.8
2.19
SR
Dover, N. H.
10
0.35
10
19.6
23.9
1.22
5RR
Dover, N. H.
10
0.35
15
29.4
31.8
1.08
5W
Dover, N. H.
10
0.3
10
16.8
18.9
1.13
6R
Portsmouth,N.H.
5
0.95
15
80
109.9
1.37
7
Portsmouth,N.H.
5
0.90
10
50.3
65.9
1.31
8
Portsmouth,N.H.
5
0.90
5
25.2
32.9
1.31
2-3
Marlboro, Mass.
5
0.15
5
4.2
2.5
0.60
2-7
Hudson, Mass.
5
0.15
5
4.2
3.8
0.90
15
Marlboro, Mass.
5
0.17
10
9.5
9.0
0.95
16
Hudson, Mass.
5
0.16
10
9.0
10.9
1.21
19
Dover, N. H.
10
0.315
10
17.2
19.8
1.15
20
Dover, N. H.
5
0.135
10
7.6
7.6
1.0
21
Dover, N. H.
5
0.130
1
7.3
9.0
1.23
22
Dover, N. H.
10
0.29
1
1.6
1.3
0.81
23
Dover, N. H.
14.8
0.48
1
2.7
3.5
1.30
24
Dover, N. H.
5
1.0
1
5.6
5.0
0.89
17
Dover, N. H.
10
0.35
10
19.6
19.3
0.98
18
Dover, N. H.
10
0.32
10
17.9
19.3
1.08
130
Synthetic
5
0.30
5
8.4
7.7
0.92
17
-------
Table 3
CURRENT EFFICiENCY WITH RESPECT TO
IRON ELECTRODE DISSOLUTION
Run
Sewage
Origin
E’otential
(volts)
Current
(amps)
Time
(mm.)
Calculated
No. of
Equivalents
Dissolved
x 1000 (n)
Measured
Electrodewt.
Dissolved
(mgm)4m)
Valency of
Metal
Dissolved
55. 85xn
m
137
92
94
95
96
107
108
113
120
Marlboro
Synthetic
2
10
2
5
10
5
5
2
2
0.1
0.94
012
0.38
0.9
0.39
0.38
0.11
0.13
5
15
5
5
5
10
25
120
60
0.311
8.77
0.373
1.18
2.80
2.42
5.91
8.21
4.85
13.0
249.2
12.2
30.0
82.9
71.9
69.3
216.7
129.6
1.34
1.97
1.71
2.20
1.89
1.88
4.76
2.12
2.09
18
-------
3. When phosphate and metal ion (or floe) come into contact, they must then
react (chemically or by adsorption). At a given overall concentration of both
constituents this reaction proceeds to an equilibrium point beyond which no
more reaction will occur.
It was suspected at the beginning of the program that phosphate removal may
occur during the electrolytic process by some nucleation mechanism Involving the
precipitation of calcium and magnesium phosphates. This was disproved, however,
when it was shown that the calcium and magnesium contents of the effluent remained
unchanged during the process (Table 4). (Note too that ammonia nitrogen also
remains unchanged but that TOC content is reduced slightly.)
Some tests were performed to determine the extent to which each of the above
steps govern the overall phosphate removal rate. It was shown (see FIgure 6) that
variations in the degree of stirring affected the phosphate removal only to a small
degree, at least in the case of aluminum electrodes. Mass transfer, i.e., step 2,
is therefore not limiting the rate of removal appreciably: it is probable that the
rising hydrogen bubbles at the cathode generated sufficient circulation in the pro-
cess so that the presence of a mechanical stirrer had little additional effect on the
removal rate.
Other tests showed that after the voltage was switched off, no further reduc-
tion in phosphate content could be detected--even on stirring the mixture for
periods of up to one hour (Table 5). Step 3, the reaction (chemical or adsorption)
rate, is therefore not limiting the phosphate removal rate.
The removal rate must therefore be limited by the rate of ion or floe genera-
tion, while the extent to which the removal proceeds must be governed by some
equilibrium relationship within the mixture.
This would indicate that the extent of phosphate removal from a given efflu-
ent will depend only on the mass of metallic ion or floe generated and the phosphate
concentration. Since the mass of metal dissolved is directly proportional to the
charge flow through the system, one may expect that the extent of phosphate removal
19
-------
Table 4
EFFECT OF ELECTROLYSIS ON POLLUTANTS IN
MAYNAItD, MASSACHUSETTS EFFLUENT
Sample
P0 4
mgm/liter
Ca
mgm/liter
Mg
mgm/liter
Alkali-
nity
(Ca0O )
mgm/liter
Ibtal Orga-
nic Carbon
TOC
mgm/liter
3
mgm/liter
Untreated
5 volts, 5 mins.
10 volts, SIllins.
33
14
5
11
11
8
2.8
2.8
2.5
170
162
167
25.0
18.0
16.4
36.0
34.6
36.8
20
-------
Table 5
EFFECT OF TIME ON PHOSPHATE REMOVAL
AFTER VOLTAGE HAS BEEN SWITCHED OFF
.)
P0
4
(mgm/liter)
3
2.9
2.9
3.0
*Time indicates the period starting from the instant the voltage was switched
off.
Voltage = 5 volts, applied for 5 mins.
InItial P0 4 content 40 mgm/liter
21
-------
from a given solution in a certain cell may be correlated with the charge flow.
through the system. Variations in voltage, or residence time will affect the
removal only inasmuch as they affect the total charge flow through the system.
Figure 10 shows the phosphate content of synthetic sewage 8amples, initially
containing about 40 and 75 mgm/liter P04 as a function of the charge flow through
the cell when using aluminum electrodes. Runs with different voltages and residence
times correlate well in this set with an electrode spacing of 1.6 cm.
Going one step further, one may expect to correlate the data gathered in
different cell geometries by plotting the residual concentration of phosphate against
the total mass of electrodes dissolved per unit volume of solution. In other words,
it is possible to plot the residual phosphate concentration of a certain solution
against the charge flow per unit volume of solution, irrespective of the voltage, cell
spacing, or residence time used in the test; these last three variables affect the
process only inasmuch as they affect the total charge flow.
The data represented in Figures 11, 12, and 13 show the concentration of
phosphate as a function of the charge flow per unit volume through the cell; the
curves gather the data for various electrode spacings when utilizing aluminum elec-
trodes and for three different initial phosphate concentrations, 40, 80, and 300
mgm/liter P0 4 . Data for secondary treatment effluents from the Maynard and
Marlboro, Massachusetts, plants are also shown in Figure 11. Their initial ortho
(and total) phosphate concentrations were 33 and 41 mgm/liter P0 4 respectively.
The drop in concentration is appreciably greater at the higher concentrations, but
the relative reduction is lower (i. e., fraction removed based on initial content).
This is in keeping with the concept of an equilibrium between the absorbed and the
dissolved phosphate.
A similar approach may be used to correlate the flow tests. Here, too, one
may plot the effluent phosphate concentration against the charge per unit volume,
which in this case is equal to the current divided by the flow rate through the system.
Figures 14, 15, and 16 show the data generated in the single electrode-pair
system. For comparison, the same figures show the curves describing the data
generated in the batch cells (for ‘ 40 and ‘ 80 mgm/liter initial P0 4 content).
22
-------
Electrode Spacing = 1.6 cm
0 00 200 300 400
4-
E
D l
4-
C
C
0
C-)
0
a
0
100
80
60
40
20
7
0
Charge CouIombs
Fig. 10. Effect of Charge Flow on Phosphate Removal Using Aluminum
Electrodes
-------
50
4-
E
01
4-
C
a)
4-
C
0
C)
.-
0
0 .
U)
0
-c
l0
9t
.
&
0
0
0
•Vo I loge
I volt
2
5
l0
1.6 cm
.
.
0
.
+ Maynard
x Marlboro
x
Spocing
2.5 cm
N
0
N
Effluent
Effluent
5cm
‘
£
200
Charge per Volume (Coulombs / liter)
300
400
Fig. II. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Botch Cell
-------
200
Charge per Volume( Coulombs/ liter)
0 NaC
0.5 glt No C C
300
Fig. 12. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Botch Cell
Voltage
Spacing
4)
E
C
a,
C
0
U
4
a
0
.C
0
1.6cm
2.5cm
100
80
60
40
20
0
0
100
400
-------
300
z2 00
E
E
C
C
0
0
Jloo
0
0 tOO 200 300 400
Charge per Volume( Coutombs / liter)
Fig.13. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Batch Cell
-------
I
Fig. 14. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Flow Cells
a)
4-
E
4-
C
a)
4-
0
C-)
a)
4-
0
U,
0
25—
20.—
15
10
5—
0_...
Voltage
Spacing
3/811
3/411
0.5volt
I
2
5
•
,
.
0
.
.
0
0
I
I
I I I 0 I
0 100 200 30Ō 400
Charge per Volume(CoUIOmbS/ liter)
-------
Curve Determined
from Botch Tests
Voltage
0.5 volt
2
5
Spacing
3/8” 3/4”
.
0
200 300
Charge per Volume(COulOmbS/ liter)
Fig. 15. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
.
.
0
E
C
C
0
C-,
0
a.
U)
0
50
40’
30
20
l0
0
.
.
0
100
no
400
Electrodes in Flow Cells
-------
tOO
a)
8O
E
C
a)
C
0
C)
a)
f D
0
4O
‘I ,
0
-c
a-
20
Curve Determined
from Batch Tests
100 200 300 400
Charge per Volume (Coulombs/liter)
Fig. 16. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Flow Cells
Voltage
Spacing
0
0
-------
The data generated in the multiple electrode system (Figure 17) falls slightly
above the data generated in the other systems. The explanation for this discre-
pancy lies in the facts that some unavoidable channeling exists and that current
flows are not exactly equally distributed between the electrodes owing to some non-
uniformity in their assembly. The overall performance of the system is therefore
poorer than may be expected.
The curves of phosphate content versus charge flow are shown cross-plotted
as the mass ratio of aluminum dissolved to phosphate removed (assuming 100%
current efficiency) against the concentration of phosphate removed for four initial
phosphate concentrations of 20, 40,80, and 300 mgm/llter P0 4 (see Figure 18).
Ninety-five per cent P0 4 removal requires an aluminum consumption of about
0.6 - 0.8 mass units per unit of P0 4 removed, the actual value depending on the
initial phosphate content.
Iron electrodes behaved in a manner similar to aluminum electrodes.
Figure 19 shows the data gathered on synthetic and real effluents containing initially
about 20 mgm/liter P0 4 . Both batch and flow test results are plotted in this figure.
Figures 20 and 21 show the data gathered on synthetic effluents initially containing
40 and 80 mgm/liter P0 4 .
Comparing these results to those obtained with aluminum electrodes, there
appears to be a slightly lower charge flow requirement with iron than with aluminum.
Mass consumption of the Iron electrodes (Figure 22) is, however, considerably
greater than the aluminum electrodes owing to the greater equivalent weight of iron.
Raw Sewage Tests
Three sets of tests using raw sewage were run in the multiple electrode flow
cell. The results of these tests are listed in Table 6.
Raw sewage is introduced at the top of the cell and flows downwards through
the electrode array. Bubbles generated at the cathode (hydrogen) attach themselves
to the suspended matter and float it upwards to the top of the cell where it can be
skimmed off. The effluent is considerably clearer than the sewage introduced. A
30
-------
50
Curve Determined
from Batch Cell
100 200 300 400
Charge per Volume (COulombs / liter)
Fig. 17. Effect of Charge Flow Concentration on Phosphate Removal Using Aluminum
Electrodes in Multiple Electrode Flow Cell
0
4
E
0’
C
4)
4-
C
0
U
C.’,
4-
0
a.
0
30
20
l0
0
-------
1.2
V
0
E
0
a-
V
0
E
C
0
U
E
C
E
4
0
0
0
0
0
0
1.0
0.8
0.6
0.4
0.2
0
0
Phosphate
50 100 150
Removed (mgm/liter
Fig. 18. Aluminum ElectrOde
Consumption as a Function of Phosphate Removal
-------
t I
25
E
C
4 ,
0
0
c ,
0
-C
a
U,
0
-C
0
Voltage
BatchTests
Flow Tests
2
volts
•
5
0
D
10
•
•
X Marlboro
÷ Hudson
Effluent
Effluent
5
0
B
0 100 200
Charge per Volume (Coulombs / liter)
-+ I
300 400
Fig. 19. Effect of Charge Flow Concentration on Phosphate Removal Using Iron Electrodes
-------
Volt age
2 volts
5 volts
10 volts
(Batch Tests)
tOO
200
300
400
Charge per Voiume(Coutombs / liter)
Fig.20. Effect of Charge Flow Concentration on Phosphate RemovalUsing Iron Electrodes
50
40
E
C
U,
4—
C
0
U
0
0
a-
30
20
0
0
-------
I00
a
80
60
a40
‘I ,
0
20
Voltoge (Botch Tests)
2 volts .
5 volts 0
10 volts
100 200 300 400
Charge per Volume(COuIOmbs / liter)
Fig. 21. Effect of Charge Flow Concentration on Phosphate Removal Using Iron Electrodes
0
0
-------
Fig. 22.
L
E
a,
E
0
C ’,
0
C
‘-4
Phosphate Removed (mgm/Iiter)
E
U)
0
0
C
0
a)
>
0
E
a)
0
a-
\
0
2
0
0
4-
0
a:
U)
U)
0
a)
0 ’
0
4)
>
0
20 40 60 80
Iron Electrode Consumption as a Function of Phosphate
Removol
-------
Table 6
RAW SEWAGE FLOTATION
(WARD STREET PUMPING STATION, BOSTON, MASS.)
Voltage
Current
(Amps)
Flow Rate
(cc/mm)
Coulombs
liter
Initial
4
rn_gm/liter
Final
P0 4
mg /liter
Total
Ortho
Total
Ortho
1
2
3*
3
3
4
1.2
1.2
1.4
260
360
360
277
200
233
10.7
10.6
11.0
8.0
8.0
8.0
“2.7
‘ ‘2.6
0
0
0
*Measuremeflts at the Cincinnati Water Research Laboratory on this
set are as follows:
Untreated Raw Treated
Suspended solids, mgm/liter 62.0 7.3 (mean of three measurements)
P0 4 - P, mgm/liter 4.5 0.36
TOC, mgm/liter 48.0 20.0
37
-------
photograph of the cell operated with synthetic sewage shows that essentially all the
precipitate is in fact carried to the top of the cell (Figure 23).
Table 6 indicates that in addition to phosphates, suspended solids are quite
effectively reduced by the flotation. TOC content of the raw sewage is reduced from
48 to 20 mgm/liter.
38
-------
Figure 23
PHOTOGRAPH OF MULTIPLE ELECTRODE FLOW CELL
OPERATED WITH SYNTHETIC SEWAGE
39
-------
SYSTEM OPTIMIZATION AND COST
Based on the data shown and the relationships developed in the previous
section, one may proceed with a simple system optimization.
Such an optimization allows us to determine the dependency of the overall
system design on variations In input parameter specification, and allows an
approximate cost estimation to be made.
Assume that:
F = effluent treatment rate, gpd
A = dissolving electrode area, ft 2
s = electrode spacing, cm
V voltage per cell, volts
a = electrical conductivity of effluent, mho/cm
I = total current, amp (In the case of multiple electrode pairs, I would
be divided by the number of these pairs.)
and that
current effectiveness = 0.2.
Therefore, the system volume = A s/30. 5 ft 3 (1)
and
1= v Ax 930 (2)
The charge flow concentration (Q) in coulombs/liter is then equal to:
= I x24 X 3600 coulombs/llter
(3)
= 22,8001/F
Energy dissipation (E) = QV watt-sec/liter
E= 22,800— - watt-sec/liter (4)
40
-------
Now, the total system costs may be simplified and assumed to be broken down
into three contributing parts:
1. energy cost
2. electrode consumption cost
3. equipment cost
Let
CE = electricity cost, cent/watt-sec
CM electrode material cost, cent/lb
CT “treatment tank cost, dollars/gal, capacity
We = equivalent weight of electrode material (9 for aluminum, 27 for iron)
z electrode thickness, in.
= electrode material density lb/ft 3
Energy cost = Q . V. CE cents/liter (neglecting pumping power) (5)
‘
Electrode consumption cost = 96500 454 cents/liter (6)
Equipment cost (neglecting transformers and rectifiers) is composed of tank cost
and electrode interest costs.
lOOx CTx(AS/4.08)X 0.0614
Tank cost 3. 785F 360 cents/liter
CT (As )
906F cents/liter (7)
and
Az CMX 0. 0281
Electrode cost = 12 x 3. 785F x360
p C A
= 581,900F cents/liter
41
-------
where it is assumed that the equipment is amortized over 30 years at 4.5% interest
and that operation is for 360 days per year.
Electrodes are assumed to have a resale value proportional to their residual
weight.
The parameters I, F, and Q are interrelated by Eq. (3), while Eq. (2) relates
V, A, I, and s. Using these two equations, the total cost may be expressed in
terms of V and a after A is eliminated. This is:
Q(W)•CM
total cost = Q. V. CE + 96500 x454
2
+ QCTS (8)
906 x 22800 x 930 V t a
Qp C sz
+ 581,900 x 22,800 x 930 V g cents/liter
The cost is minimized at the lowest value of a and at an optimum value of the
voltage V 0 given by:
sC p zC
0 — 22800 x 93 OflaCE 906 581,900
It is interesting to note that the optimum voltage is not dependent on the plant capacity
or the extent of the treatment required (phosphate removal).
For a typical effluent with a conductivity of 2 x 10 mhos/cm and a value of
= 0.2 with aluminum electrodes, the above equation reduces to
(l. 3 BCT+ O.3 4 1zCM)
42
-------
Based on a tank cost of $100/yd 3 (9), electrical energy costs of 1 cent/KWH
and aluminum costs of 27 cents/lb (10)
CT = $0.49/gallon
CE = 2.8 x cent/watt-sec
CM = 27 cent/lb.
Therefore,
V 0 = 0.228s 2 + 329sz
Without considering pumping power it is impossible to do more than pick a
reasonable value of s for these calculations. When a spacing of 3 cm and an elec-
trode thickness of 0.5 in. are used, the optimum voltage is equal to 2.64 volts.
Furthermore, basing the calculation on a charge flow concentration of
300 coulombs/liter, the energy consumption is
E= 300x2.64
792 watt-sec/liter
= 0.835 KWH/1000 gallons.
For a 1 million gallon per day plant
• Energy consumption 835 KWH/day
• Electrode area required 40,000 ft 2
• System volume 5,000 ft 3
37,400 gallons
• Electrode consumption rate (with aluminum electrodes)
96500x454 x 3.785x 106 lbs/day
233 lbs/day
43
-------
This is equivalent to 1.4 ft 3 /day. Assuming that the aluminum dissolves uniformly
from the surface of the electrode, dissolution is equivalent to a reduction in thick-
ness of 4.2 x 1O in./day. In one year, the aluminum consumption would be
equivalent to a reduction in thickness of 0.3 In. The 0.5 in. figure assumed earlier
would therefore require replacement about once per year.
Of the two operating costs, electrode consumption costs are significantly
higher than electrical energy costs ($63/day for electrode vs. $8.4 day for power)
when using aluminum.
When using iron, the equivalent electrode consumption is 700 lbs/day. At a
cost of 2 cents/lb. this is equal to $14/day. Tables 7 and 8 summarize these results.
44
-------
Table 7
ELECTROLYTIC PHOSPHATE REMOVAL SYSTEM DESIGN
Basis: 1 million gallon per day system
Iron Electrodes
Al Electrodes
Voltage
1.76 volt
2.64 volt
Power Supply
23.2KW
34.8KW
Energy Required
0.557 KWH/bOO
gallon
0.835 KWh/boo
gallon
Electrode Area
60, 000 ft 2
40,000 ft 2
Tank Capacity
56,000 gallons
37,400 gallons
Electrode Consumption
(mass)
oo lbs/day
233 lb/day
Electrode Consumption
(volun e)
1.5 ft 3 /day
1.4 ft 3 /day
Electrode Consumption
(thickness)
0.30 mils/day
0.42 mils/day
NOTE: Electrode spacing assumed = 3 cm
Effluent conductivity = 2 x iO3 mhos/cm
Phosphate removal > 95%
45
-------
Table 8
ELECTROLYTIC PHOSPHATE REMOVAL CONCEPTUAL SYSTEM COSTS
Iron Electrodes Al Electrodes
Energy (at 1 cent/KWH)
Electrode
Equipment Depreciation
0.56
1.40
(at
0.56
cent/1000 gallon
cent/bOO gallon
$40/ton)
cents/1000 gallon
0.84
6.30
(at
0.84
cent/1000 gallon
cent/1000 gallon
27 cent/lb)
cent/1000 gallon
plus interest
TOTAL
2.52
cent/bOO gallon
7.98
cent/bOO gallon
These costs do not include labor, transformer, and rectifier costs, or
filtration costs.
46
-------
REFERENCES
1. Journal AWWA , Nov, 1965, p. 1431.
2. Journal WPCF , June, 1965, p. 800.
3. U. S. Patent 2,997,430 (Aug 22, 1961).
4. U. S. Patent 3,035,992 (May 22, 1962).
5. “Advances in Water Pollution Research,” Proceedings of the International
Conference held in London, Sept, 1962 (Vol. 2).
6. Chemical Engineer pg , Jan 4, 1965, P. 9.
7. AWTR-13, Public Health Service Publication No. 999-WP-19, March, 1965,
“Electrochemical Treatment of Waste Water.”
8. “Standard Methods for the Examination of Water and Waste Water,” Prepared
and published by APHA, AWWA, and WPCF, Twelfth Edition (APHA, N. Y.).
9. Perry’s Chemical Engineers’ Handbook, Fourth Ed., 1963, P. 26-21.
10. “Oil, Paint and Drug Reporter.”
47
U. S. GOVERNMENT PRNTING OFFICE 070 0 - 410-219
------- |