Letter

pubs.acs.org/journal/ascecg

Improved Electrocoagulation Reactor for Rapid Removal of

Phosphate from Wastewater
Yushi Tian,† Weihua He,† Xiuping Zhu,‡ Wulin Yang,‡ Nanqi Ren,*,† and Bruce E. Logan*,†,‡
†
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang
District, Harbin 150090, People’s Republic of China
‡
Department of Civil & Environmental Engineering, Penn State University, 231Q Sackett Building, University Park, Pennsylvania
16802, United States

ABSTRACT: A new three-electrode electrocoagulation reac-

tor was investigated to increase the rate of removal of
phosphate from domestic wastewater. Initially, two electrodes
(graphite plate and air cathode) were connected with 0.5 V of
voltage applied for a short charging time (∼10 s). The direction
of the electric field was then reversed, by switching the power
supply lead from the anode to the cathode, and connecting the
other lead to a sacrificial aluminum mesh anode for removal of
phosphate by electrocoagulation. The performance of this
process, called a reverse-electric field, air cathode electro-
coagulation (REAEC) reactor, was tested using domestic
wastewater as a function of charging time and electro-
coagulation time. REAEC wastewater treatment removed up
to 98% of phosphate in 15 min (inert electrode working time of 10 s, current density of 1 mA/cm2, and 15 min total
electrocoagulation time), which was 6% higher than that of the control (no inert electrode). The energy demand varied from
0.05 kWh/m3 for 85% removal in 5 min, to 0.14 kwh/m3 for 98% removal in 15 min. These results indicate that the REAEC can
reduce the energy demands and treatment times compared to conventional electrocoagulation processes for phosphate removal
from wastewater.
KEYWORDS: Energy efficiency, Wastewater Treatment, Electrocoagulation, Air cathode, Aluminum electrode, Ion mobility

■ INTRODUCTION
Anaerobic bioreactors, such as anaerobic fluidized bed reactors,
Anode: Al(s) − 3e− = Al3 + (1)

anaerobic membrane bioreactors, and microbial fuel cells Cathode: 2H 2O + 2e− = H 2(g) + 2OH− (2)
(MFCs), are being increasingly investigated for removal of
the organic matter from wastewater.1−4 However, effluents
from these reactors still have high phosphorus concentrations Solution: Al3 + + 3H 2O = Al(OH)3 + 3H+ (3)
in the form of orthophosphate, polyphosphate and organic
phosphate.5 The wastewater must be further treated to remove Al3 + + PO4 3 − = AlPO4 (4)
the phosphorus to low levels to avoid stimulating eutrophica-
tion of receiving water bodies.6,7 These anaerobic systems will The high energy requirements of EC is one of the main
therefore require additional treatment processes in order to disadvantages of this process.15,16,29,30 Using an air cathode
achieve nutrient removal.8 instead of a metal electrode has been shown to reduce the
Electrocoagulation (EC) has been successfully applied to energy requirements for EC, as the reaction at the cathode is
removal of nutrients and other pollutants from different types favorable due to oxygen reduction rather than hydrogen
of wastewaters,9−16 such as orthophosphate and boron from evolution. The use of a passive air cathode also uses less energy
synthetic wastewater,17−19 degradation of disperse red 167 in than that needed for oxygen reduction based on electrodes
textile industry wastewater,20 and remediation of hydrofluoric using dissolved oxygen due to the high energy demands needed
wastewater.21 EC is a process where metallic hydroxide ion for gas sparging.27 Inexpensive air cathodes have been
flocs are produced by electrocoagulation of sacrificial anodes developed for MFC applications due to the use of activated
typically made of iron or aluminum11,22−27 because these carbon as the catalyst.30,31
materials are cheap, readily available, and effective coagulants.
When aluminum ions are released into the water, they form Received: July 12, 2016
hydroxides that can then react with phosphate to form Revised: October 23, 2016
precipitates of AlPO4, according to the following:28 Published: November 1, 2016

© 2016 American Chemical Society 67 DOI: 10.1021/acssuschemeng.6b01613

ACS Sustainable Chem. Eng. 2017, 5, 67−71
ACS Sustainable Chemistry & Engineering Letter

Figure 1. (A) Photo and (B) schematic diagram of the electrocoagulation reactor with an air cathode.

A new type of EC process, called a reverse-electric field, air distance between the aluminum mesh anode and the air cathode was
cathode electrocoagulation (REAEC) process was developed to set at 1.5 cm based on previous optimization tests.27 The aluminum
improve removal and energy efficiencies of EC nutrient mesh was inserted into a u-shaped polycarbonate plate with a reaction
removal. The REAC process is different from a conventional area (6 cm × 3 cm) and a sedimentation zone (1 cm × 1 cm) on the
bottom. The inert electrode was inserted in a u-shape polycarbonate
EC process as it contains a graphite plate electrode that is
plate (7 cm × 3 cm) 2 cm from the air cathode. The plates were
charged prior to treatment, so that less time and energy is connected using plastic screws.
subsequently needed for EC treatment. The REAC reactor also Reactor Operating Conditions. Wastewater was collected from
contained an aluminum mesh electrode as the sacrificial anode, the primary clarifier at the Penn State Wastewater Treatment Plant
and an air cathode that has been shown to reduce energy and stored at 4 °C. The wastewater had a pH of 7 ± 2, conductivity of
demands compared to that needed for hydrogen evolution at 1.2 ± 0.1 mS/cm, and a dissolved phosphate concentration of 20 ± 1
the cathode. The reactor was operated by applying a voltage of mg/L. All reactors were operated in fed-batch model.
0.5 V across the graphite plate anode and air cathode for a short Treatment of domestic wastewater was examined as a function of
time to polarize the electrodes and draw positively charged ions different inert electrode working times (inert electrode connected to
to the graphite electrode, and negatively charged ions to the the air cathode) and electrocoagulation times (aluminum anode
cathode (charging phase). The direction of the electric field was connected to the air cathode), with voltages or current set using a
potentiostat (VMP3; BioLogic, Claix, France). Voltage was applied
then reversed, by switching the anode lead to the air cathode,
during to the inert electrode circuit with the inert electrode connected
and then connecting the other end of the circuit to the to the negative lead of the power supply. The direction of the current
sacrificial aluminum mesh anode (EC treatment phase). During was then switched during the electrocoagulation period with the
this subsequent EC treatment time, aluminum was oxidized to positive lead from the power supply connected to the cathode. Inert
produce aluminum hydroxides and aluminum phosphate electrode working times were set at 10, 30, 60, and 120 s with a
particles that subsequently settled out and were removed. constant applied voltage of 0.5 V, followed by a fixed electro-
The impact of inert electrode working time and subsequent coagulation time of 10 min at 1 A/cm2. Control tests (no inert
electrocoagulation was examined relative phosphate removal, electrode operation) were conducted using wastewater with only
and compared to a control lacking the inert electrode. To electrocoagulation times of 5, 10, 15, 20, and 30 min to determine the
reduce the impact of particles on the electrodes, the reactor was time needed to achieve similar removals compared to inert electrode
designed with a sedimentation zone to remove particles tests.
produced during the electrocoagulation process. The impact of electrocoagulation on phosphate removal and energy

■
utilization was compared with a set inert electrode time of 10 s, with
electrocoagulation times of 5, 10, and 15 min. Following each
MATERIALS AND METHODS electrocoagulation period, the particles were allowed to settle in the
Electrode Materials and Reactor Construction. REAEC reactor for 1 h (no applied current). Cathodes were cleaned after 20
reactors were constructed based on a previous EC system design batch cycles by soaking the cathode in a diluted (10%) hydrochloric
that29 consisted of a 40 mL electrolyte chamber and an air cathode, acid solution (BDH ARISTAR PLUS, VWR) for 2 min to remove salt
but no charging (graphite) electrode. The sacrificial anode was a single deposits. All tests were conducted using duplicate reactors at 30 °C,
piece of aluminum mesh (mesh size 200 per 2.54 cm, wire diameter with averages reported ± a standard error.
0.053 mm, opening 0.074 mm; TWP Corporation). Cathodes Solution Measurements and Calculations. All samples were
contained an activated carbon catalyst, and were made by a continuous filtered through 0.45 μm pore diameter syringe filters (polyvinylide-
rolling and press process, using a polytetrafluoroethylene (PTFE) nedifluoride, PVDF, 25 mm diameter; Restek Corporation) and
binder, and a PTFE/carbon black diffusion layer to avoid water analyzed for phosphate using standard methods (method 8190,
leakage, as previously described.30 The surface areas of the electrodes HACH Company, Loveland, CO).32 Removals were calculated based
were: cathode, 18 cm2 (6 cm × 3 cm); aluminum mesh anode, 15 cm2 on initial and final concentrations. A probe (SevenMulti, Mettler-
(5 cm × 3 cm), and graphite plate, 21 cm2 (7 cm × 3 cm). The Toledo International Inc.) and meter (SevenMulti, Mettler-Toledo,
charging electrode was a graphite sheet (>99% carbon, <1000 ppm OH) were used to measure solution pH and conductivities.
sulfur, <10 ppm leachable chloride, 0.127 mm thick; Beyond Materials, Voltage and the counter electrode potentials were recorded at 10 s
Inc.). intervals, and analyzed using the potentiostat software (EC-Lab
REAEC reactors were built from polycarbonate plates (0.5 cm × 5 V10.02). Energy consumption, W (kWh/m3), was calculated for the
cm × 8 cm) cut to produce an inner chamber (6 cm × 3 cm), with the electrocoagulation period as W = UIt/v, where U is the voltage (V), I
plates separated by rubber gaskets 0.1 cm wide (Figure 1). The the current (A), t the electrocoagulation time (h), and v the volume

68 DOI: 10.1021/acssuschemeng.6b01613
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ACS Sustainable Chemistry & Engineering Letter

Figure 2. (A) Phosphate removal using the REAEC system, as a function of the inert electrode working time for a fixed 10 min electrocoagulation
time (current density 1 mA/cm2). (B) Derivative of the current as a function of the inert electrode working time at a fixed applied voltage (0.5 V).
(C) Phosphate removal in the control lacking an inert electrode, at different electrocoagulation times at a fixed current density of 1 mA/cm2.
(Phosphate concentrations shown are after 1 h of settling time.)

Figure 3. (A) Phosphate concentration and (B) energy requirements for REAEC system (10 s fixed inert electrode working time) versus the control
(no inert electrode) as a function of electrocoagulation time (fixed current density of 1 mA/cm2; phosphate concentrations shown are after 1 h of
settling time).

(m3). The power (P = UI) to polarize the inert electrode was not approximately twice the treatment time (20 min) would be
included in this calculation, as there was only a minor current.

■
needed to achieve the same removal as the REAEC reactor in
10 min.
RESULTS AND DISCUSSION These results indicate that a short period of only ∼10 s was
Polarization of the inert electrode for 5 s, followed by a fixed 10 needed to polarize the inert circuit electrodes, and greatly
min electrocoagulation time and settling, resulted in removal of improved performance of the REAEC system compared to a
90 ± 1% of the phosphate in the wastewater, and 95 ± 2% for typical electrocoagulation process. By switching the direction of
10 s. Phosphate removal was not further improved by using the ionic current flow in the electrolyte, the subsequent
longer inert electrode polarization times of up to 120 s (Figure electrocoagulation process was more effective, likely due to the
2A). The lack of an impact of longer inert electrode operation migration of the negatively charged phosphate ions toward the
times was likely due the nearly constant circuit potentials after aluminum electrode.
∼10 s, as shown by a plot of the first derivative of the current Treatment Times and Energy Requirements. Treat-
(Figure 2B). The energy used to polarize the inert electrode ment of the REAEC process was further examined at two other
was very low, as the current in the inert electrode circuit after total electrocoagulation times, with a fixed inert electrode time
120 s was only 0.01 mA/cm2. of 10 s. The phosphate removal was 85 ± 2% for a shorter
The performance of the REAEC system was compared to a treatment time of 5 min, and 95 ± 2% for a 10 min time
control reactor lacking an inert electrode, at the same fixed (Figure 3A). Phosphate removal further increased to 98 ± 1%
current density of 1 mA/cm2. Phosphate removals increased when for a 15 min treatment time (Figure 3A), which was
from 67 ± 2% with a 5 min treatment time to 87 ± 1% for the about half the time needed by the control (Figure 2C) for the
same 10 min period used in the REAEC tests (Figure 2C). same percentage of phosphate removal.
Longer treatment times increased removals, with 98 ± 1% The energy used for treating domestic wastewater using the
phosphate removal achieved after 30 min of electrocoagulation. REAEC ranged from 0.05 kWh/m3 for 85% phosphate removal
Based on the percentage removal line shown in Figure 2C, at a 5 min electrocoagulation time, to 0.14 kWh/m3 for 98%
69 DOI: 10.1021/acssuschemeng.6b01613
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Table 1. Energy Consumption of Electrocoagulation and Removals of Phosphorus, Nitrogen, COD, and TSS Reported in
Different Studies Using Fe or Al Electrodes
electrode type wastewater type energy (kWh/m3) current density (mA/cm2) phosphate removals (%) reference
Al and Fe synthetic solution 22 1 97 33
Al and Al urban wastewater 4.5 1 99 34
Al and Fe synthetic solution 0.76 16 98 35
Al and air cathode domestic wastewater 0.4−9.9 0.6−1.2 97−99 27
graphite sheet, Al and, air cathode domestic wastewater 0.05−0.14 1 85−98 this study
0.07−0.18 (control) 1 67−92 this study

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■ AUTHOR INFORMATION
Corresponding Authors
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