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J. of Advanced Engineering and Technology

Vol. 2, No. 3 (2009) pp. 201-205

Removal of Algae and Organic Compounds by Dissolved Air Flotation Process

Dong-Heui Kwak

Department of Civil and Environmental Engineering, California State University, USA(Received : Aug. 08, 2009, Revised : Aug. 31, 2009, Accepted : Sep. 09, 2009)

Abstract : A new hybrid system consisting of powdered activated carbon adsorption and dissolved air flotation

(DAF) processes were studied for simultaneous removal of algae and the organics produced from algae (anabaena

andmycrocystis). Before studying the hybrid system, adsorption equilibrium and kinetics of organics on powdered

activated carbon (PAC) were investigated. Three types of powdered activated carbons (wood-based, coal-based,

coconut-based) were chosen as an adsorbent. The 2-methylisoboneol (MIB) and geosmin were used as a

representative organic compound produced from algae. PACs were characterized using BET, SEM and particle

size analyzer. It was found that MIB and geosmin can be effectively removed by PAC adsorption when a correct

dose was applied. The homogeneous surface diffusion model was applied to predict PAC doses required to reduce

MIB and geosmin concentrations to below 10 ng/l. On the other hand, the flotation efficiency of algae and organic-

loaded PAC particles in DAF process was evaluated with zeta potential measurements. Our experimental results

revealed that simultaneous removal of algae and organics dissolved in water can be successfully achieved using

the hybrid system of adsorption/DAF processes.

Keywords : zeta potential, DAF, algae, MIB, geosmin, hybrid process.

1. Introduction

Blue-green algae in water often produces the musty-earthy tasteand odor compound of 2-methylisoborneol (MIB) and geosmin.Therefore, removing taste and odor compounds from drinkingwater has been attracting much attention internationally becausethey pollute water sources significantly. It has been generallyknown that MIB and geosmin are detected by consumers as musty-earthy odors at levels as low as 10 ng/l [1]. Thus, the treatment ofthese compounds must be very effective. Among many watertreatment strategies, powdered activated carbon (PAC) has beensuccessfully applied because it is relatively inexpensive and canbe applied only when required. However, removing the spentPAC from water is a drawback.

On the other hand, it has been known that dissolved airflotation (DAF) is an effective solid/liquid separation process forlow density particles such as algae, color, clay flocs producedfrom low turbidity water [2]. However, DAF is limited inremoving the organic compounds dissolved in water. A DAF unitconsists of four steps: (1) coagulation and flocculation prior toflotation, (2) bubble generation, (3) bubble-floc collision andattachment in a mixing zone, (4) rising of bubble-floc agglomeratesin a flotation tank. If a bubble-PAC particles collision results insuccessful attachment, and if the resulting bubble-particle agglomerate

is positively buoyant, the agglomerate can rise to the top of theliquid column and collect in a foam layer which can subsequently

be skimmed off. In other words, when the PAC can be floated byDAF, a combination of DAF and PAC adsorption seems to besuccessfully applied in water and wastewater treatment. Manystudies have been conducted on individual process of PACadsorption of MIB and geosmin, and DAF for algae removal [2].However, little has been known about combining the two processessystematically.

Therefore, this study focuses on the feasibility of the hybridprocesses of adsorption and DAF for simultaneous removal ofalgae (anabaena and mycrocystis) and organic pollutants (MIBand geosmin) produced from algae in drinking water treatment.Prior to the studies on the hybrid system, adsorption equilibrium

and kinetics of organics were investigated using three types ofpowdered activated carbons (wood-based, coal-based, coconutbased). The homogeneous surface diffusion model was used todetermine the internal mass transfer coefficient which contributesto the prediction of PAC doses required to reduce MIB andgeosmin concentrations to below 10 ng/l. On the other hand, theflotation efficiency of algae and PAC adsorbing organics wasevaluated in a DAF process.

2. Experimental

The adsorption experiments were carried out using two organic

compounds (2-MIB and geosmin) purchased from Aldrich Co.

Corresponding AuthorTel : +1-714-699-0900

E-mail : kwak124@hanamail.net

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(USA). Chemical properties of MIB and geosmin are listed inTable 1. The concentration of organics was measured by GC/MS.Three different PACs, namely, wood based (WB), coconut-shellbased (HA), and coal based (CB), were obtained from JamesCumming & Sons PTY Ltd. (Australia). Adsorption experimentswere conducted by adding different amounts of PAC into theflask containing 100 ml of synthetic wastewater. After shaking ina constant temperature incubator at constant temperature (298.15K)for 3 days to give sufficient contacting time for equilibrium,samples were taken from the flask and filtered through 150mmfilter paper (ADVANTEC, Japan). The filtrate was then measuredfor organic concentrations. The results of adsorption experimentswere used to determine the equilibrium isotherms for twoorganics (MIB and geosmin). The adsorption capacity of PACwas determined by material balance. On the other hand, adsorption

kinetic experiments were conducted in a Carberry-type batchadsorber (1.0 ~ 2.0 10-3m3) at 300 rpm to obtain the concentrationdecay curves as a function of time.

Cells of algae, anabaena and mycrocystis, were obtained fromAcademia Sinica. Cultivation was conducted using a medium.350 ml of medium was added to each 500 ml Erlenmeyer flaskequipped with a guaze stopper and autocleave. The flasks wereirradiated with UV light for 2 hr before cells were added into, andplaced on a thermostat shaker. The temperature was controlled at25oC, and the speed of shaker at 80rpm. 350ml of medium wasadded to each 500ml Erlenmeyer flask. Cultivation flasks wereshaken for 20days with illumination of solar light for 15 hr each

day. Cells were dried under vacuum state for further experiments.Figure 1 shows SEM photos ofanabaena andmycrocystis.Prior to sedimentation and DAF operation, coagulation of algae

and PAC was conducted on a Jar-Test apparatus using polyaluminiumchloride as a coagulant. 10~ 50 mg of coagulant and 3 ml of

NaOH (0.1M) were added to the distilled water or raw water (1L)and rapidly mixed (150 rpm) for 1 min followed by slow mixing

(50 rpm) for 20 min. The quality of raw water (Dong-Hwa Dam,Korea). The pH was adjusted by adding HCl or NaOH. Thesupernatant after the treatment was examined for residual algaeand PAC concentration. Zeta potential (Photal Otsuka ELS-8000,Japan) was measured to examine the underlying surface charge toobtain further insight in the mechanism of removal.

The schematic diagram of the DAF apparatus is shown inFigure 2. The diameter of the flotation column made of plexiglasswas 10 cm, and the height was 30cm. Algae particles both in thepresence and absence of PAC were suspended initially in thecolumn, then bubbles were introduced in the column from thebottom side of the column. The dissolved air solution was fed intothe flotation column and the particles in the column wereremoved by the rising bubbles. The mean diameter of the bubblesfed into the column was 25 m. After all the bubbles in the cellreached the top of the column, the solution was sampled to obtainthe flotation efficiency. The turbidity was measured using theturbidity meter (HACH 2100P).

3. Results and Discussion

3.1. Adsorption study

PACs used in this study was commercially available, but itsphysical and chemical properties can not be obtained in detail.

Table 1. Chemical Properties of MIB and Geosmin

2-MIB Geosmin

Name 2-methyl-isoborneol Trans-1,10-dimethyl-trans-9-decalol

MW 168 182

Chemical

fomularC

7

H5

OCl C1 2

H2 2

O

Chemical

structure

OTC

(ng/L) a30 10

Ordor Musty Camphorous Earthy-musty

a

ordor threshold concentration

Figure 1. SEM photos of anabaena and mycrocystis.

Figure 2. The schematic diagram of the DAF apparatus.

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Removal of Algae and Organic Compounds by Dissolved Air Flotation Process 203

The surface area was calculated by using the BET method, andthe pore size distribution was measured by BJH (Barrett, Joynerand Halenda) method using the nitrogen desorption data. Thesurface area was found to be 882 to 1200m2/g, and the averagepore diameter was 24 to 31, which belongs to the mesoporerange. Information on the particle size distribution of the PACsused is very important, especially when it is used in conjunctionwith DAF. The average particle size of three PACs was in therange of 11 to 34 m. It is meaningful to investigate how muchconcentration of PAC (i.e., dose) is required to remove dissolvedorganics by PAC adsorption in water treatment. Removalefficiency of suspended solids by DAF process is highly relatedwith the concentration of solids (i.e., turbidity). Prior to the studieson the hybrid system of adsorption and DAF, adsorption equilibriumand kinetics of two organics (MIB and geosmin) were investigated.

Adsorption isotherms of PACs used in this work were the mostimportant information for estimating carbon dose for the removalof MIB and geosmin [1]. The adsorption capacity mainly dependedon the properties of the adsorbate and adsorbent. From ourprevious experimental results, the adsorption capacity of geosminwas found to be much higher than that of MIB for the same PACsat constant temperature (298.15 K). In addition, MIB andgeosmin were successfully removed using PACs at levels as lowas 10 ng/l. The adsorption capacity for MIB and geosmin was theorder of PAC-HA> PAC-WB > PAC-CB. The greater affinity ofgeosmin compared to MIB can be attributed to its chemicalstructure and solubility (Table 1). This is in agreement with the

results reported by Cook et al. [1]. In general, it has been knownthat the adsorption capacity on activated carbon is enhanced byincreasing the molecular size and aromaticity, and by decreasingsolubility, polarity and carbon chain branching. Geosmin has aslightly lower solubility and molecular weight and has a flatterstructure (Table 1), which may make it easier to adsorption in theslit-shaped pores of the activated carbon.

The experimental adsorption kinetics data for MIB and geosminwere predicted by a homogeneous surface diffusion model [3-5].Results showed that the model predictions were in very goodagreement with the experimental data. The determined externalmass transfer coefficient and internal surface diffusivities of MIB

are 6.26 10-5

m/s and 2.59 10-14

m2

/s, and those of geosminare 2.84 10-5 m/s and 4.71 10-15 m2/s. Unlike the results ofequilibrium, the kinetics of adsorption of MIB was faster thangeosmin. On the basis of equilibrium and kinetics obtained in thiswork, PAC-HA seems to be applied to the hybrid system of PACadsorption and DAF processes because it has higher adsorptioncapacity and kinetics.

3.2. DAF study

Zeta potential is an important parameter of double layerrepulsion for individual particles, and it can be used to interpretthe trend of coagulation efficiency. It has been known that colloidal

particles should have zero net surface charge (isoelectric point,

IEP) for agglomeration. This can result from the adsorption ofhydrogen ions or positive-charged ions (i.e., such as aluminumions), on negatively-charged surfaces. When the zeta potential ofparticles is approaching zero, coagulation efficiency is generallyimproved. The coagulation mechanism of PAC is thought to becomplex and involves several reaction routes, including chargeneutralization (electrostatic interaction) and complex formationbetween PAC and alumino precipitates, adsorption, bridge formationand surface precipitation on alumino hydroxide solid precipitate.In addition, it has been addressed that pH is one of the mostimportant parameters in the coagulation processes. Depending onpH values, the interfacial properties and reaction routes may bedifferent.

The results of zeta potential measurements of PACs are carriedout under various experimental conditions, including the effects

of water properties (distilled water and reservoir water), coagulantdose (5~ 20 mg/L), and organic adsorption on PAC dose (5, 10,20 mg/L). The zeta potential values ranged approximatelybetween +20 to -50 mV for distilled water and always negativevalues (3 ~20 mV) for reservoir water. The zeta potentialdecreased with an increasing PAC dose and pH. However, it wasobserved that the influence of the types and dose of PACs on thevariation of zeta potential was quite low. IEP of PACs dissolvedin reservoir water was not seen while that in distilled water waspresent in the pH range of 4 to 5. The zeta potential of organics(MIB and gesosmin) ranged approximately between +40 to 40mV in terms of solution pH (4~ 9) with PACs dose (5, 10, 20mg/

L). It was observed that IEP moved to the neutral range (pH 6 ~7) after adsorbing organics. This result implies that the organicadsorption affects the zeta potential of PACs.

The optimal coagulant condition is the functions of the types ofalgae, coagulant dose, and other operating conditions such asturbidity, pH, and temperature. Polyaluminum chloride was usedfor altering the surface charge of PAC and algae. The variation ofzeta potential of PACs depending on coagulant dose (10~ 50mg/L)and pH (4 ~ 9) were investigated. As expected, the values increasedwith coagulant dose. Also, the surface charge of the PAC particlesdissolved in water was easily controlled by the adjustment ofcoagulant doses. In general, the particles were negatively charged

in water and bubbles were also negatively charged. In this case,the removal efficiency by DAF was very low without adjustmentof the surface charge of the particles. Thus, the surface charge ofPAC should be changed to neutral or positive to float in the DAFprocess because PAC is negatively charged without coagulantdoses.

The variation of zeta potential of two algae was investigated interms of solution pH of 4 ~ 9 with a coagulant dose of 10~ 50mg/L.It was observed that the zeta potential increased much with acoagulant dose at lower pH (4 ~ 6), while it moderately increasedat higher pH (7 ~ 9). The values of zeta potential ranged betweenapproximately 15 ~ +25 mV, and they were slightly different

according to the types of algae. The values were 15~+23mV

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for anabaena and 3 ~ 25 mV for microcystis. Unlike the resultsof PACs, the zeta potential of algae was quite different dependingon the solution pH and coagulant dose. The turbidity removalefficiencies ofanabaena and microcystis by DAF and CGS interms of coagulant dose (10 ~ 50 mg/L) at pH 6. The removalefficiency of two algae increased with coagulant dose although itstendency was changed when the coagulant dose was over 40mg/L.We believe that DAF is an effective method of removing algae.

On the other hand, the removal efficiency of three types of PACswas compared in the absence and presence of coagulant in orderto verify the DAF methodology to float PACs (Table 2). CGSresults were included for comparison. The removal efficiencywithout coagulation was very low (< 10%), while with increasingcoagulant dose the efficiency increased up to 70% by CGS andup to 95% by DAF. We found that the electrostatic interactions

between positively charged algal particles and negatively chargedbubbles became mutually attractive when the coagulant was used.However, the interactions between algal particles and bubbles are

Table 2. Removal Efficiency of PACs by Sedimentation and DAF

(unit:%)

PAC*without coagulant with coagulant

Sedimentation DAF Sedimentation DAFPAC-CB 1.7 9.7 77.5 94.6

PAC-WB 3.8 5.3 72.9 94.2

PAC-HA 1.5 6.0 69.5 93.6

* Conditions :PAC (10 mg/L), coagulant (polyaluminum chloride, 30mg/L)

Table 3. Removal Efficiency of Algae by DAF with and without PAC

(unit:%)

without PAC with PAC*

Anabana 96 94

Mycrocystis 96 95

*

Conditions : PAC-HA (10 mg/L), coagulant (polyaluminum chloride,35 mg/L)

Figure 3. Variation of zeta potential of algae depending on coagulant dose.

Figure 4. Removal efficiency of algae depending on coagulant dose.

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Removal of Algae and Organic Compounds by Dissolved Air Flotation Process 205

repulsive without the use of coagulant. It was also found that theremoval efficiency was almost independent of the types of PAC.On the other hand, the removal efficiency of algae in the absenceand presence of PAC was examined (Table 3). Results showedthat the removal efficiencies of anabana and mycrosystis werevery high about 94 ~ 96% regardless of the presence of PACwhen a coagulant of 35mg/L was used at solution pH 6. Thisresult implies that DAF is an effective process for the removal ofalgae even in the presence of PAC. On the basis of this finding,therefore, we believed that simultaneous removal of algae andPAC adsorbing organic pollutants produced from algae can besuccessfully achieved by employing the hybrid system of PACadsorption and DAF processes.

4. Conclusions

Experimental and theoretical studies on adsorption equilibriumand kinetics were carried out to investigate the adsorptioncharacteristics of MIB and geosmin. Adsorption capacity ofgeosmin on three PACs was much higher than that of 2-MIB. Theequilibrium data were well fitted by Freundlich isotherm. Inaddition, the adsorption kinetic data of MIB and geosmin werepredicted with a homogeneous surface diffusion model. Thedetermined surface diffusivities of MIB and geosmin are 2.59 10-14m2s-1 and 4.71 10-15m2s-1, respectively. On the other hand,

the zeta potential values of algae and PAC increased with coagulationdose, and the flotation efficiencies were also enhanced. Theremoval efficiency of PACs was very low without coagulation,while the efficiency increased up to 70% by CGS and to 95% byDAF when coagulant (i.e., polyaluminium chloride) was used. Inespecial, it was found that the removal efficiency of PAC wasalmost independent of their types. Unlike the results of PAC, theremoval efficiencies of algae by DAF or CGS were very highboth in the absence and the presence of PAC. Our experimentalresults suggest that the hybrid system consisting of adsorption andDAF processes can be effectively applied for simultaneousremoval of algae and PAC adsorbing organic pollutants producedfrom algae in water treatment..

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