Algae contamination of surface water and drinking water supplies is a significant problem particularly in rural areas. A decentralized inexpensive technology that would effectively remove algae from water would be beneficial. Electrocoagulation (EC) combined with electroflotation (EF) as a single process (ECF) is a promising algae harvesting technique with no moving parts that may be powered using a modest array of photovoltaic panels if a low power system can be developed. Here, an ECF system was constructed to study the energy required to remove algae from a simulated drinking water supply. Results from 18 ECF experiments indicated a >95% improvement of water clarity measured by optical density (OD750) could be achieved with as little energy as 1.25 kWh m−3. The key was to find the ideal combination of gas bubbles produced by EF (G) and coagulant from the EC relative to the concentration of suspended solids (S). The ideal gas to solids (G/S) ratio for the ECF system ranged from 0.09 to 0.17. In solutions containing chloride (Cl) ions ECF produced chlorine gas which is known to contribute to disinfection. Results suggest that ECF can efficiently remove algae and simultaneously contribute to disinfecting contaminated drinking water supplies.

INTRODUCTION

The World Health Organization estimates that more than 80% of the nearly 1 billion people lacking access to clean drinking water are located in rural areas (WHO & UNICEF 2008). In many of these areas, the problem stems from inadequate wastewater treatment and other sources of surface water pollution responsible for water-borne illnesses, which cause millions of deaths annually (Grant et al. 2012). While these problems can be solved by centralized water treatment and distribution systems, this approach is not practical for most rural areas, which lack both the infrastructure and capital to build and support such systems (Mollah et al. 2004). In rural areas, algal growth due to eutrophication is a major contributor to water contamination (Shen et al. 2011; El-Dars et al. 2015) and an efficient way to remove algae would be an important step in the direction of improving water quality.

It is well established that algae can be removed from suspension using flotation techniques that typically combine coagulating agents with bubbles (Green et al. 1996; Wiley et al. 2009,). The coagulants form algae aggregates that adhere to rising bubbles, causing them to float. While effective, many flotation techniques require a constant supply of expensive chemical coagulants and energy-intensive compressed gas systems to operate, making them poorly suited for rural areas (Wiley et al. 2011).

Electrocoagulation (EC) combined with electroflotation (EF) as a single process (ECF) is a relatively inexpensive technique shown to be effective in removing algae and a wide range of pollutants from water (Feng et al. 2003; Azarian et al. 2007; Emamjomeh & Sivakumar 2009; Gao et al. 2010; Sirés & Brillas 2012). The EC process generates coagulants and metal hydroxides that destabilize and aggregate suspended particles in situ, through the electrochemical dissolution of soluble electrodes (Jiang et al. 2002). Previous research has shown that EC requires lower coagulant dose, reduces sludge production, is less pH sensitive, and removes turbidity with higher efficiency than conventional chemical coagulation (Abuzaid et al. 2002; Han et al. 2002; Gao et al. 2010). The EF portion of the system generates hydrogen and oxygen gas bubbles by the electrolysis of water. As these gas bubbles rise through the water column, they attach to aggregated particles and transport them to the surface, producing clear subnatant. A combined ECF system is a promising approach to remove algae from contaminated water in rural areas because it has no moving parts, requires only electricity as an input, and can be powered completely with photovoltaic modules (Mollah et al. 2001; Rodriguez et al. 2007).

It is also known that if chloride (Cl) is present in the bulk solution the ECF process can produce chlorine gas, which acts as a disinfectant (Hernlem & Tsai 2000; Abuzaid et al. 2002). This combination of clarifying and disinfecting contaminated water suggests ECF would be beneficial in rural areas, particularly because it requires little maintenance. The key will be developing effective ECF systems that require as little electricity as possible to operate (Mollah et al. 2001; Rodriguez et al. 2007).

Here we evaluated the energy requirements of ECF for 95% algae removal, determined the gas to solids (G/S) ratio and monitored the pH, temperature and conductivity. We compared calculated coagulant dose using Faraday's law with the actual experimental data and determined conditions for the greatest treatment efficiency with minimal power consumption.

METHODS

Algae cultivation

An algae cultivation system was constructed from a 1,000-liter plastic container cut in half to improve solar exposure. Two diffusers at the bottom of the container were aerated continuously to control oxygen and provide mixing. The system was filled with tertiary wastewater effluent from the Santa Paula Water Recycling Facility (Santa Paula, CA, USA). No attempt was made to maintain axenic conditions, but the system was inoculated with Chlorella vulgaris (UTEX 2714; University Texas Culture Collection; Austin, TX, USA). The dissolved oxygen, which remained near saturation level, and pH were measured using Sensorex submersible electrodes (S8000 and DO6442-T; Garden Grove, CA, USA). A DL-06 programmable logic controller (PLC) (Automation Direct, Cumming, GA, USA) activated a mass flow controller (Aalborg, Orangeburg, NY, USA) that injected CO2 at a rate of 5 l min−1 to maintain the culture pH between 7.5 and 7.7. Ammonia-nitrogen (NH3-N) and nitrate-nitrogen (NO3-N) were monitored using a Hach (Loveland, CO, USA) DR2800 Spectrophotometer. Approximately 30 grams of urea-nitrogen fertilizer (46–0–0) were periodically added to the 400 l culture when the combined concentration of NH3-N and NO3-N was <5 mg l−1.

Electrocoagulation and electroflotation (ECF) system

The combined ECF system used aluminum electrodes for EC and non-soluble iridium oxide coated (20 g m−2 coating density) titanium (Ti/IrO2) electrodes for EF, operated independently to increase the longevity of the soluble aluminum electrodes and optimize bubble production for flotation. The ECF system was tested in 18 experimental trials, using different current settings to determine the most efficient coagulant dose and gas production for flotation based total power consumption and improvements in water clarity. The ECF vessel was constructed from 9.5 mm thick sheets of clear acrylic plastic (30 cm wide × 40 cm long × 28 cm deep). The interior of the vessel was separated into three zones: (1) the coagulation zone with two aluminum electrodes (20.3 cm × 2.5 cm × 0.32 cm) held together with non-conductive 5 mm plastic spacers, (2) the flotation zone with four Ti/IrO2 electrodes (25.4 cm × 1.3 cm × 0.20 cm) wired in parallel with 5 mm plastic spacers, and (3) the underflow zone, which permitted clear subnatant to recirculate to the coagulation zone using a pump (model 2088-343-170 12 VDC diaphragm pump, SHURflo, Costa Mesa, CA, USA) (Figure 1). The speed of the pump was adjusted by varying the voltage of a HY1520EX DC power supply (VOLTEQ, San Jose, CA, USA). Two independently controlled Y030LX2400 DC power supplies (Acopian, Eastman, PA, USA), operated in constant current mode, delivered the power to each of the electrode assemblies.
Figure 1

The ECF vessel with instrumentation installed during leak testing. The coagulation zone (a), flotation zone (b) and underflow zone (c) are separated by baffles. The tubing on the exit of the underflow zone is connected to a recirculation pump that feeds back into the bottom of the coagulation zone. The valves in each of the zones are for sample collection.

Figure 1

The ECF vessel with instrumentation installed during leak testing. The coagulation zone (a), flotation zone (b) and underflow zone (c) are separated by baffles. The tubing on the exit of the underflow zone is connected to a recirculation pump that feeds back into the bottom of the coagulation zone. The valves in each of the zones are for sample collection.

A model 2537 paddlewheel flow meter (GF Signet, Schaffhausen, Switzerland) measured the recirculation rate through the system. Temperature and pH electrodes (GF Signet, model 2750) were mounted in the coagulation zone and a conductivity sensor (GF Signet, model 2850) was mounted in the flotation zone. Data from the sensors were transmitted to a custom human-machine interface (HMI) designed using Lookout Direct Software (Automation Direct, Cumming, GA, USA) with a DL06 PLC. The HMI was used to start and stop ECF power supplies at user-specified current set-points and to log data.

The sacrificial aluminum EC electrodes released ions into solution when direct current (DC) was applied to the anode (Equation (1)). Several monomeric species with different coagulation properties are produced when the aluminum generated by EC is released into solution (Equations (2)–(5)). 
formula
1
 
formula
2
 
formula
3
 
formula
4
 
formula
5
The total amount of aluminum dissolved into solution (coagulant dose) is related to the current applied to the electrodes by Faraday's Law (Equation (6)). 
formula
6
where, w = mass of aluminum dissolved into solution, g; I = current applied to the electrodes, amperes; t = time, seconds; M = molecular weight of aluminum, g mol−1; z = number of electrons involved in the redox reaction; F = Faraday's constant, 96,485 C mol−1.
Hydrogen and oxygen bubbles produced during this process at the anode (Equation (7)) and cathode (Equation (8)), respectively, comprise the EF component of the harvesting system. 
formula
7
 
formula
8
Four 20-minute trials using different amperage settings (0.75, 1.75, 2.75 and 3.75) were performed to determine the dissolution rates of the aluminum EC electrodes. The electrodes were submerged in a 1,000 ml graduated cylinder containing distilled water mixed with NaCl to increase the conductivity to approximately 2,000 μs cm−1. Every 5 minutes electrodes were removed, dried in an oven at 104 °C for 10 minutes, placed in a desiccator to cool for 10 minutes, and weighed with a TE214S analytical balance (Sartorius, Bohemia, NY, USA). The measured weight loss was compared to predictions based on Faraday's Law.

ECF operation

In the 18 experiments a variety of ECF current combinations were tested to achieve a 95% reduction in algae OD750 while minimizing power consumption (Table 1). Each trial was conducted using 30 liters of algae culture transferred to the ECF vessel. The solution was continuously circulated through the vessel at approximately 3.5 l min−1 for each 30-minute trial. The initial total suspended solids (TSS) concentration, as determined using method 2540D of Standard Methods (American Public Health Association 1998), and OD750 of the algae solution were determined prior to initiating the ECF processes. The OD750 was measured on samples collected from the underflow zone at 5-minute intervals. Improvements in water clarity over time relative to initial conditions were then quantified using Equation (9). 
formula
9
where, %red = % reduction of OD750; OD750i = initial OD750; OD750n = OD750 after n minutes of treatment.
Table 1

The current setpoints for ECF during each of the 18 experimental trials

TrialEC current, amperesEF current, amperesTrialEC current, amperesEF current, amperes
3.75 5.75 10 4.50 3.00 
6.25 15.3 11 4.50 3.00 
2.25 3.25 12 6.00 6.00 
3.75 5.75 13 6.00 9.25 
5.25 3.00 14 1.00 1.75 
1.50 12.2 15 2.80 4.25 
4.50 12.2 16 4.25 6.00 
1.60 12.2 17 2.25 8.00 
4.80 12.2 18 4.25 8.25 
TrialEC current, amperesEF current, amperesTrialEC current, amperesEF current, amperes
3.75 5.75 10 4.50 3.00 
6.25 15.3 11 4.50 3.00 
2.25 3.25 12 6.00 6.00 
3.75 5.75 13 6.00 9.25 
5.25 3.00 14 1.00 1.75 
1.50 12.2 15 2.80 4.25 
4.50 12.2 16 4.25 6.00 
1.60 12.2 17 2.25 8.00 
4.80 12.2 18 4.25 8.25 
The mass of gas required to remove a mass of suspended solids, referred to as the gas to solids (G/S) ratio is an important process control metric for any flotation technology. Decreasing G/S ratios indicate more efficient operation, as less power is necessary to remove the solids. As the volume of hydrogen and oxygen generated used for flotation is a function of current applied to the system, the total volume of gas produced by ECF was calculated with Equation (10). 
formula
10
where, Qt = total gas production rate, l sec−1; I = current applied to the electrodes, amperes; V = molar volume of gas, 22.4 l mol−1; F = Faraday's constant, 96,485 C mol−l; t = time, seconds; ηH = mole of electrons transferred per mole of H2 produced; ηO = mole of electrons transferred per mole of O2 produced.
The result of Equation (10) can then be converted to mass (Equations (11) and (12)) and used to determine the G/S ratio (Equation (13)). 
formula
11
where, n = mol; P = pressure, atm; V = volume, l; R = ideal gas constant, 0.0821 L-atm mol-K−1; T = temperature, K. 
formula
12
where, MT = mass of total gas produced, g; = mol of H2; = molecular weight of H2, g; = mol of O2; = molecular weight of O2. 
formula
13
where, G/S = gas to solids ratio; MT = mass of total gas produced, g; TSSi = initial [TSS], g l−1; Svol = system volume, l.
The power consumed for each experimental trial was then calculated using Equation (14): 
formula
14
where, E = energy consumed, kWh m−3; kW = power consumption, kW; t = time, hours; Svol = system volume, m−3.
An additional benefit of ECF is that potential exists to generate chlorine gas from Cl ions in solution for in situ disinfection. As current is applied to the submerged electrodes, chlorine gas is generated at the anode, which dissolves in the surrounding solution to form hypochlorite ions (Equations (15)–(17)). 
formula
15
 
formula
16
 
formula
17
The concentration of Cl present in the Santa Paula effluent averages 140 ± 10.7 mg l−1 (n = 65), suggesting that disinfection by-products are likely to be generated during the ECF process. The total chlorine residual of the subnatant following ECF treatment was quantified using Hach method 8167 and a DR890 spectrophotometer.

RESULTS AND DISCUSSION

The primary goal was to find ECF conditions in which algae were effectively removed from simulated drinking water in a reasonable time with the lowest power consumption. To this end we measured the changes in optical density (OD750) of the system and the amount of current applied to the electrodes submerged in the experimental chamber. The current influenced the quantity of aluminum ions released as coagulant from the aluminum electrode, and the amount of gas produced for flotation which is produced at both the aluminum and Ti/IrO2 electrodes. The challenge in finding the most efficient operation was to determine the current to each electrode that produces sufficient aluminum coagulant and gas for flotation without applying too much current to the aluminum electrode, which erodes. This balance in the current was determined using the gas to solids (G/S) ratio, which compares the mass of the gas produced (G) to the total mass of suspended solids (S) (see Equations (12) and (13)).

Algae removal and power

To determine the ECF conditions that effectively removed suspended algae with minimal power, the OD750 was compared to the G/S ratio, with different current settings to the EC and EF electrodes (Figure 2). In 18 experiments the OD750 reduction ranged from 29.9 to 99.5%, the G/S ratio ranged from 0.01 to 0.52 and power consumption ranged from 0.3–7.4 kWh m−3. Nine of the experiments decreased OD750 by 75 to < 95% (Figure 2, Δ), had G/S ratios of 0.08–0.35, and power requirements ranging from 0.9–7.4 kWh m−3. Nine of the ten experiments consuming the most power had the highest G/S ratios (0.27–0.54), suggesting too much current was applied to the EF electrodes. The experiment in this group with the lowest G/S ratio (0.15) used an EC current setting of 6.0 amperes, which accounted for the elevated power consumption. In the seven experiments with low G/S ratios (0.08–0.17) and low power consumption (0.9–2.2 kWh m−3) (Figure 2, circled lower left), three had a 95% reduction in OD750 indicating that a G/S ratio in this range is adequate for algae removal. The other four experiments had OD750 reductions of 80–90%, suggesting the need for a higher coagulant dose.
Figure 2

The relationship between G/S ratio, power consumption and treatment efficiency as determined by OD750 reduction (□ = ≥95%, Δ = 75– < 95%, ○ = 50– < 75%, ◊ = <50% reduction in OD750). The wide spread of the data is indicative of a mismatch between ideal power for coagulation and optimum bubble production with EF. High removal efficiencies paired with lower power requirements and G/S ratios indicate efficient operation of both ECF systems. The large circle containing a cluster of points signifies experimental trials (n = 7) with optimum G/S ratios. While four of these trials did not meet the target treatment objective of 95% OD750 reduction, these data provide insight regarding current setpoints providing most efficient EF operation.

Figure 2

The relationship between G/S ratio, power consumption and treatment efficiency as determined by OD750 reduction (□ = ≥95%, Δ = 75– < 95%, ○ = 50– < 75%, ◊ = <50% reduction in OD750). The wide spread of the data is indicative of a mismatch between ideal power for coagulation and optimum bubble production with EF. High removal efficiencies paired with lower power requirements and G/S ratios indicate efficient operation of both ECF systems. The large circle containing a cluster of points signifies experimental trials (n = 7) with optimum G/S ratios. While four of these trials did not meet the target treatment objective of 95% OD750 reduction, these data provide insight regarding current setpoints providing most efficient EF operation.

Experiments 7 and 8 improved the clarity by 58 and 54%, had higher G/S ratios (0.34 and 0.35) and required large amounts of electrical power (7.1 and 5.7 kWh m−3) due to EF overdose (Figure 2, ○). The poor removal may be due to inadequate coagulant dose given that the G/S ratio was high. Experiments 6 and 14 had the least reduction in OD750 (29.9 and 31.8%) (Figure 2, ◊) due to insufficient coagulant and gas excess in Experiment 6, indicated by the high G/S ratio (0.36) and large power requirement (5.5 kWh m−3), and insufficient coagulant and gas in Experiment 14, indicated by the low power used 0.25 kWh m−3.

In five of the 18 experiments algae removal was >95% (Figure 2, □) within 30 minutes and in four of the five more than 50% of the algae were removed in the first 10 minutes (Table 2). In two trials (Figure 3: X and Δ) the rate of removal was initially slow, but rapidly increased after five or ten minutes, presumably when the aluminum coagulant and bubbles had accumulated sufficiently.
Table 2

The change in OD750 measurements during ECF operation and the corresponding percent improvement (italicized) for the experiments achieving the treatment objective of 95%

TrialOD750τ0OD750τ5OD750τ10OD750τ15OD750τ20OD750τ25OD750τ30
0.413 0.294 (28.8) 0.109 (73.6) 0.049 (88.1) 0.014 (96.6) 0.012 (97.1) 0.002 (99.5) 
0.899 0.617 (31.4) 0.358 (60.2) 0.140 (84.4) 0.045 (95.0) 0.034 (96.2) 0.025 (97.2) 
0.731 0.595 (18.6) 0.233 (68.2) 0.115 (84.2) 0.066 (91.0) 0.036 (95.0) 0.023 (96.9) 
15 0.414 0.381 (8.0) 0.342 (17.4) 0.152 (63.3) 0.085 (79.5) 0.044 (89.4) 0.014 (96.6) 
18 0.398 0.384 (3.5) 0.176 (55.8) 0.063 (84.2) 0.026 (93.5) 0.018 (95.5) 0.009 (97.7) 
TrialOD750τ0OD750τ5OD750τ10OD750τ15OD750τ20OD750τ25OD750τ30
0.413 0.294 (28.8) 0.109 (73.6) 0.049 (88.1) 0.014 (96.6) 0.012 (97.1) 0.002 (99.5) 
0.899 0.617 (31.4) 0.358 (60.2) 0.140 (84.4) 0.045 (95.0) 0.034 (96.2) 0.025 (97.2) 
0.731 0.595 (18.6) 0.233 (68.2) 0.115 (84.2) 0.066 (91.0) 0.036 (95.0) 0.023 (96.9) 
15 0.414 0.381 (8.0) 0.342 (17.4) 0.152 (63.3) 0.085 (79.5) 0.044 (89.4) 0.014 (96.6) 
18 0.398 0.384 (3.5) 0.176 (55.8) 0.063 (84.2) 0.026 (93.5) 0.018 (95.5) 0.009 (97.7) 
Figure 3

The percent OD750 reduction over time for each experiment meeting the treatment objective. The horizontal bar represents the target reduction of 95% (◊ = Experiment 2; ○ = Experiment 4; □ = Experiment 5; X = Experiment 15; Δ = Experiment 18).

Figure 3

The percent OD750 reduction over time for each experiment meeting the treatment objective. The horizontal bar represents the target reduction of 95% (◊ = Experiment 2; ○ = Experiment 4; □ = Experiment 5; X = Experiment 15; Δ = Experiment 18).

In Experiment 2 (Figure 3: ◊) the OD750 was reduced by 96.6% after 20 minutes and 99.5% after 30 minutes. However, this trial used the most power and had the highest G/S ratio (0.52), indicating there was an excess of bubbles generated than required for effective separation (Table 3).

Table 3

The power consumption, G/S ratio and applied current for the experiments with ≥95% reduction in OD750

TrialPower used, kWh m−3Total gas produced, mgTSS, mg l−1G/S ratioEC current, amperes (SD ±)EF current amperes (SD ±)
5.75 2,414 154 0.52 6.28 (0.03) 15.31 (0.04) 
1.25 1,069 415 0.09 3.80 (0.03) 5.76 (0.03) 
2.07 1,146 362 0.11 5.23 (0.03) 2.97 (0.03) 
15 1.49 1,086 214 0.17 2.80 (0.03) 4.21 (0.03) 
18 3.41 1,615 128 0.42 4.28 (0.03) 8.26 (0.03) 
TrialPower used, kWh m−3Total gas produced, mgTSS, mg l−1G/S ratioEC current, amperes (SD ±)EF current amperes (SD ±)
5.75 2,414 154 0.52 6.28 (0.03) 15.31 (0.04) 
1.25 1,069 415 0.09 3.80 (0.03) 5.76 (0.03) 
2.07 1,146 362 0.11 5.23 (0.03) 2.97 (0.03) 
15 1.49 1,086 214 0.17 2.80 (0.03) 4.21 (0.03) 
18 3.41 1,615 128 0.42 4.28 (0.03) 8.26 (0.03) 

Additionally, an aluminum precipitate was observed in the floating algal mat, which suggests an excess of coagulant. Excess coagulant dosing should be avoided, as it shortens electrode life and can increase the amount of residual aluminum in solution, which may have adverse health effects (Frisbie et al. 2015). Similarly in Experiment 18 (Figure 3: Δ) the 95% OD750 reduction goal was reached in 25 minutes, but resulted in a high G/S ratio (0.42) due to an excess of EF current and lower solids loading (Table 3).

In contrast, Experiment 4 (Figure 3: ○), which reduced the OD750 by 95.0% after 20 minutes and 97.2% after 30 minutes, used the least amount of power (1.25 kWh m−3), and had the most favorable G/S ratio (0.09), indicating efficient operation of both EC and EF components of the system (Table 3). Trial 5 had only a slightly higher G/S ratio (0.11), suggesting efficient operation of the EF system, but the overall power consumption was much higher (2.07 kWh m−3) due to the higher current applied to the EC system (Table 3).

Experiment 15 (Figure 3: X) required the entire 30 minutes to meet the OD750 reduction goal of 95%, but required only 1.49 kWh m−3 of power and had a G/S ratio of 0.17. While this trial was efficient, the additional time needed to achieve the desired separation implies that the coagulation dosing may have been slightly low (Table 3).

ECF power consumption and the factors that affect it

The power consumption of the three most efficient experiments ranged from 1.25 to 2.07 kWh m−3 and had G/S ratios between 0.09 and 0.17. Experiments consuming the most electrical power had the highest G/S ratios, which demonstrates the importance of this metric when optimizing the efficiency of ECF. The ideal G/S ratio identified here is much lower than the 0.26–0.88 previously reported by Wiley et al. (2009) for the removal of algae grown in a wastewater oxidation pond using dissolved air flotation (DAF). The reduced G/S ratio observed with ECF is attributed to smaller bubble diameters that yield higher removal efficiencies (Burns et al. 1997; Dimoglo et al. 2004). Gao et al. (2010) reported that ECF power requirements for effective algae removal at the laboratory scale were 0.3 kWh m−3. However, additional research is required to determine the scalability of this result and to assess whether the performance of ECF is influenced by the composition of the algal consortium and size of the algal cells.

Identifying the optimum coagulant dose with EC is also needed to optimize the energy efficiency of the ECF system. However, the predictability of coagulant dose using Faraday's Law is highly dependent on the configuration of the EC electrodes. We observed that coagulant dose exceeded theoretical predictions by 109–215% when aluminum dissolution rates were measured using electrodes submerged in a 1,000 ml graduated cylinder; values consistent with those reported elsewhere (Donini et al. 1994; Chen et al. 2000; Calvo et al. 2003). Mouedhen et al. (2008) attributed this deviation from Faraday's Law to chemical dissolution at the cathode (not the anode, which followed the law), and suggested it was due to high pH during hydrogen evolution creating more basic conditions. This discrepancy between actual and theoretical dosing can be alleviated by altering the flow around the electrodes or by using insoluble cathodes, although the formation of an anode passivation layer would increase resistance and therefore increase power requirements.

Increasing temperature and pH shifts towards neutral were observed during ECF operation (Table 4). The shift toward neutral pH is advantageous because it precludes the need for chemical pH adjustment (Ghernaout et al. 2011), but has an impact on coagulation by changing the solubility of aluminum. For example, aluminum solubility is minimal near neutral pH, and precipitating aluminum hydroxides provide coagulation in a process known as ‘sweep flocculation’, where suspended particles bind to precipitates and form larger aggregates (Duan & Gregory 2003). In contrast, the higher temperatures increase solubility, which could inhibit the formation of aluminum hydroxides. This contributes to the difficulties associated with predicting coagulation performance, as these variables could differ greatly between water sources.

Table 4

The pH, temperature, and conductivity before and after ECF for the experiments with ≥95% reduction in OD750

TrialInitial pHFinal pHInitial temp. °CFinal temp. °CInitial conductivity, μS cm−1Final conductivity, μS cm−1
8.48 8.18 24.2 28.1 2,413 2,259 
5.65 5.78 23.5 24.5 2,750 2,589 
8.03 7.98 20.4 22.0 2,000 1,915 
15 8.54 8.21 21.7 23.0 2,234 2,132 
18 8.26 8.04 19.0 21.7 2,310 2,022 
TrialInitial pHFinal pHInitial temp. °CFinal temp. °CInitial conductivity, μS cm−1Final conductivity, μS cm−1
8.48 8.18 24.2 28.1 2,413 2,259 
5.65 5.78 23.5 24.5 2,750 2,589 
8.03 7.98 20.4 22.0 2,000 1,915 
15 8.54 8.21 21.7 23.0 2,234 2,132 
18 8.26 8.04 19.0 21.7 2,310 2,022 

The power requirements of the ECF system are strongly influenced by the conductivity of the bulk solution. Higher conductivity is advantageous as it provides less electrical resistance between the electrodes, lessening the voltage required to deliver the desired current. However, it was noted that conductivity of the solution decreased during operation of the ECF system (Table 4). This can be explained through the oxidation of ionic compounds to gases that are stripped from solution by hydrogen and oxygen bubbles or to adsorption to aluminum hydroxide. Thus prolonged operation may require increased voltage to overcome increased resistivity, but this can be controlled if small amounts of NaCl are added. The addition of NaCl has the added benefit of forming chlorine gas, which is a disinfectant.

ECF production of chlorine gas as a disinfectant

In water containing Cl ions from natural sources or from additions used in water treatment, the ECF system generates chlorine gas that acts as a strong disinfectant (Hernlem & Tsai 2000). To determine the disinfecting capacity of the system, we used a spectrophotometer to determine the total chlorine residual of the subnatant following treatment. The total amount of chlorine generated electrochemically can theoretically be calculated based on the Cl concentration and current. However, this does not accurately describe how well the solution is disinfected. For example, if all chlorine generated reacts with dissolved solids and solids in solution there may not be enough to provide disinfection. The residual is more telling because it indicates that the chlorine has reacted with the solution with excess remaining.

Predicting the current required to produce enough chlorine for adequate disinfection is challenging because it depends on the Cl concentration of the source water and the chlorine demand exerted by the solution. For the system described here, there was a poor correlation (r2 = 0.19) between the total chlorine residual and the total current applied (Figure 4). However, a much stronger correlation coefficient (r2 = 0.68) was observed between the chlorine residual and the change in OD750 with time. Less total chlorine residual was detected in experiments with the greatest change in OD750, which suggests that the chlorine produced is reacting with suspended solids. If additional power is required beyond that needed for effective removal of suspended solids, it could be generated using the non-soluble EF electrodes to prevent shortening the utility of the sacrificial aluminum electrodes. A similar approach was adopted by Hernlem & Tsai (2000), who successfully utilized EF for the clarification and disinfection of food process water, reporting strong reductions in the presence of E. coli. However, a more in-depth analysis is needed to assess the disinfecting efficiency of the ECF system presented here.
Figure 4

The weak correlation (r2 = 0.19) between chlorine residual and current (left) and the stronger correlation (r2 = 0.68) between chlorine residual and OD750 (right), support the hypothesis that chlorine is reacting with the suspended solids in solution.

Figure 4

The weak correlation (r2 = 0.19) between chlorine residual and current (left) and the stronger correlation (r2 = 0.68) between chlorine residual and OD750 (right), support the hypothesis that chlorine is reacting with the suspended solids in solution.

CONCLUSIONS

The ECF system effectively removed algae from water with low power requirements under the right combination of conditions. The most successful experiment reduced the OD750 by 95% after 20 minutes of treatment and required 1.25 kWh m−3 of electricity. The G/S ratio providing effective removal of coagulated algae ranged between 0.09 and 0.17, lower than values reported for DAF. This results from the smaller diameter and therefore greater surface area of electrochemically generated H2 and O2 bubbles. While the power consumption obtained during these experiments is reasonably low, advancements in electrode configuration and containment vessel design could further reduce energy requirements and allow for more accurate predication of coagulant dose. The ECF process has the potential to provide some disinfection through the electrochemical production of chlorine gas. However, the extent of disinfection is dependent on the chlorine demand exerted by the source water and the concentration of Cl ions in solution. The low power requirements, effective removal of suspended algae and potential to provide disinfection suggest that development of ECF technology can improve water quality for rural areas with limited resources and problems with contaminating algae.

ACKNOWLEDGEMENTS

The authors acknowledge Professor Elliott Campbell from the University of California, Merced, Brian Cullen and Nate Owen from PERC Water, and the NASA Ames Research Center GSRP for providing financial support.

REFERENCES

REFERENCES
Abuzaid
N. S.
Bukhari
A. A.
Al-Hamouz
Z. M.
2002
Ground water coagulation using soluble stainless steel electrodes
.
Advances in Environmental Research
6
,
325
333
.
Azarian
G. H.
Mesdaghinia
A. R.
Vaezi
F.
Nabizadeh
R.
Nematollahi
D.
2007
Algae removal by electro-coagulation process, application for treatment of the effluent from an industrial wastewater treatment plant
.
Iranian Journal of Public Health
36
(
4
),
57
64
.
Burns
S. E.
Yiacoumi
S.
Tsouris
C.
1997
Microbubble generation for environmental and industrial separations
.
Separation and Purification Technology
11
,
221
232
.
Calvo
L. S.
Leclerc
J. P.
Tanguy
G.
Cames
M. C.
Paternotte
G.
Valentin
G.
Rostan
A.
Lapicque
F.
2003
An electrocoagulation unit for the purification of soluble oil wastes of high COD
.
Environmental Progress
22
(
1
),
57
65
.
Chen
X.
Chen
G.
Yue
P. L.
2000
Separation of pollutants from restaurant wastewater by electrocoagulation
.
Separation and Purification Technology
19
,
65
76
.
Dimoglo
A.
Akbulut
H. Y.
Cihan
F.
Karpuzcu
M.
2004
Petrochemical wastewater treatment by means of clean electrochemical technologies
.
Clean Technologies and Environmental Policy
6
,
288
295
.
Donini
J. C.
Kan
J.
Szynkarczuk
J.
Hassan
T. A.
Kar
K. L.
1994
The operating cost of electrocoagulation
.
The Canadian Journal of Chemical Engineering
72
,
1007
1012
.
Duan
J.
Gregory
J.
2003
Coagulation by hydrolysing metal salts
.
Advances in Colloid and Interface Science
100–102
,
475
502
.
El-Dars
F. M. S. E.
Rahman
M. A. M. A.
Salem
O. M. A.
2015
Algal control and enhanced removal in drinking waters in Cairo, Egypt
.
Environmental Science an Indian Journal
10
(
8
),
295
307
.
Emamjomeh
M. M.
Sivakumar
M.
2009
Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes
.
Journal of Environmental Management
90
,
1663
1679
.
Feng
C.
Sugiura
N.
Shimada
S.
Maekawa
T.
2003
Development of a high performance electrochemical wastewater treatment system
.
Journal of Hazardous Materials
103
,
65
78
.
Gao
S.
Yang
J.
Tian
J.
Ma
F.
Tu
G.
Du
M.
2010
Electro-coagulation-flotation process for algae removal
.
Journal of Hazardous Materials
177
,
336
343
.
Grant
S. B.
Saphores
J. D.
Feldman
D. L.
Hamilton
A. J.
Fletcher
T. D.
Cook
P. L. M.
Stewardson
M.
Sanders
B. F.
Levin
L. A.
Ambrose
R. F.
Deletic
A.
Brown
R.
Jiang
S. C.
Rosso
D.
Cooper
W. J.
Marusic
I.
2012
Taking the ‘waste’ out of ‘wastewater’ for human water security and ecosystem sustainability
.
Science
337
,
681
686
.
Green
F. B.
Bernstone
L.
Lundquist
T.
Oswald
W.
1996
Advanced integrated wastewater pond systems for nitrogen removal
.
Water Science & Technology
33
,
207
217
.
Han
M.
Song
J.
Kwon
A.
2002
Preliminary investigation of electrocoagulation as a substitute for chemical coagulation
.
Water Science & Technology: Water Supply
2
(
5–6
),
73
76
.
Hernlem
B. J.
Tsai
L. S.
2000
Chlorine generation and disinfection by electroflotation
.
Journal of Food Science
65
(
5
),
834
837
.
Jiang
J. Q.
Graham
N.
Andre
C.
Kelsall
G. H.
Brandon
N.
2002
Laboratory study of electro-coagulation-flotation for water treatment
.
Water Research
36
,
4064
4078
.
Mollah
M. Y. A.
Schennach
R.
Parga
J. R.
Cocke
D. L.
2001
Electrocoagulation (EC) – science and applications
.
Journal of Hazardous Materials
84
,
29
41
.
Mollah
M. Y. A.
Morkovsky
P.
Gomes
J. A. G.
Kesmez
M.
Parga
J.
Cocke
D. L.
2004
Fundamentals, present and future perspectives of electrocoagulation
.
Journal of Hazardous Materials
114
,
199
210
.
Mouedhen
G.
Feki
M.
Wery
M. D. P.
Ayedi
H. F.
2008
Behavior of aluminum electrodes in electrocoagulation process
.
Journal of Hazardous Materials
150
,
124
135
.
Rodriguez
J.
Stopić
S.
Krause
G.
Friedrich
B.
2007
Feasibility assessment of electrocoagulation towards a new sustainable wastewater treatment
.
Environmental Science and Pollution Research
14
(
7
),
477
483
.
Shen
Q.
Zhu
J.
Cheng
L.
Zhang
J.
Zhange
Z.
Xu
X.
2011
Enhanced algae removal by drinking water treatment of chlorination coupled with coagulation
.
Desalination
271
,
236
240
.
Standard Methods for the Examination of Water and Wastewater
1998
20th edn,
American Public Health Association/American Water Works Association/Water Environment Federation
,
Washington, DC
,
USA
.
WHO, UNICEF
2008
Progress on Drinking Water and Sanitation: Special Focus on Sanitation
.
World Health Organization (WHO) and United Nations Children's Fund Joint Monitoring Programme for Water Supply and Sanitation, UNICEF
,
New York
,
USA
,
WHO, Geneva, Switzerland
.
Wiley
P. E.
Brenneman
K. J.
Jacobson
A. E.
2009
Improved algal harvesting using suspended air flotation
.
Water Environment Research
81
(
7
),
702
708
.
Wiley
P. E.
Campbell
J. E.
McKuin
B.
2011
Production of biodiesel and biogas from algae: a review of process train options
.
Water Environment Research
83
(
4
),
326
338
.