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Municipal wastewater phosphorus removal bycoagulationKai Yang a , Zhenhua Li a , Huaiyu Zhang b , Jiangfeng Qian a & Gang Chen ca School of Civil Engineering , Wuhan University , Wuhan, 430072 Chinab Central and Southern China Municipal Engineering Design & Research Institute , Wuhan,430010 Chinac Department of Civil and Environmental Engineering , FAMU‐FSU College of Engineering ,2525 Pottsdamer Street, Tallahassee, FL, 32310, USAPublished online: 30 Apr 2010.

To cite this article: Kai Yang , Zhenhua Li , Huaiyu Zhang , Jiangfeng Qian & Gang Chen (2010) Municipal wastewaterphosphorus removal by coagulation, Environmental Technology, 31:6, 601-609, DOI: 10.1080/09593330903573223

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Environmental Technology

Vol. 31, No. 6, May 2010, 601–609

ISSN 0959-3330 print/ISSN 1479-487X onlineThis material is published by permission of Florida A&M University for the US Department of Agriculture under Contract No. 2007-35102-18111. The U.S. Government retainsfor itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies tothe public, and perform publicly and display publicly, by or on behalf of the Government.DOI: 10.1080/09593330903573223http://www.informaworld.com

Municipal wastewater phosphorus removal by coagulation

Kai Yang

a

, Zhenhua Li

a

, Huaiyu Zhang

b

, Jiangfeng Qian

a

and Gang Chen

c

*

a

School of Civil Engineering, Wuhan University, Wuhan, 430072 China;

b

Central and Southern China Municipal Engineering Design & Research Institute, Wuhan, 430010 China;

c

Department of Civil and Environmental Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA

Taylor and Francis

(

Received 9 March 2009; Accepted 20 November 2009

)

10.1080/09593330903573223

This study investigated the chemical removal of phosphorus from wastewater by means of adsorption andprecipitation. Using secondary effluent from municipal wastewater treatment plants as the model wastewater,phosphorus precipitation was tested with four commonly used metal salt coagulants, and phosphorus adsorption wasexplored by column experiments with goethite-coated silica sand as porous media. It was concluded that adsorptionplayed a more important role than precipitation in phosphorus removal. As demonstrated by the column experiments(integration of the breakthrough curve), around 65% of phosphorus can be retained through adsorption by goethite-coated silica sand.

Keywords:

phosphorus; coagulant; adsorption; precipitation

Introduction

Eutrophication rendered by excessive nutrient loadinginto the surface water system is one of the most chal-lenging topics environmental engineers have to face[1,2]. Owing to the increased usage of fertilizers andother industrial and agricultural activities, the nutrientlevel of many lakes and rivers has increased dramati-cally over the past 50 years [3]. Excessive nitrogen,phosphorus and other nutrients are loaded into waterbodies, resulting in the fast growth of algae and otherplankton and in deteriorating water quality [4]. Inparticular, phosphorous is one of the major nutrientscontributing to the increased eutrophication of lakes andother natural water bodies. Its presence causes manywater quality problems including increased purificationcosts, decreased recreational and conservation value ofimpoundments, and the possible lethal effect of algaltoxins on drinking water [5].

The blue-algal bloom outbreak in Southern Chinaduring summer 2007 was believed to be caused by phos-phorus pollution. The major phosphorus pollutionsources are from municipal wastewater treatment plants,which represent about 30 to 50% of the total phosphorusdischarge. Thus, controlling phosphorous dischargedfrom municipal and industrial wastewater treatmentplants is a key factor in preventing eutrophication ofsurface waters. Phosphorus can be found in variouschemical forms in the urban wastewater, including insol-uble or dissolved organic phosphorus, orthophosphates

and condensed inorganic phosphates [6,7]. Currently,the most utilized phosphorus removal methods formunicipal and industrial wastewaters are chemicalprecipitation and biological phosphorus removal.Biological phosphorus removal requires complicatedoperating scheme and plant configurations. In contrast,chemical precipitation is more favourable because of itssimplicity, flexibility and cost-effectiveness. It can beused to assist phosphorus removal in aerated biologicalwastewater treatment units [8].

For chemical precipitation of phosphorus, calcium,aluminium, and iron salts are usually used and pH,temperature, and ionic strength are the major impactfactors [8]. Aluminium and iron salts have advantagesover calcium salts since they are not sensitive to pH andare easy to handle. In addition, they produce much lesssludge [9]. Using ferric salts as a model chemical forphosphorus removal, Fytianos

et al

. [10] developed amodel to describe the single-phase precipitation andtwo-phase co-precipitation of phosphorus. They alsoconcluded that pH was the most important factor forphosphorus removal, with the optimum pH being 4.5.Recently, it was demonstrated that phosphorus wasremoved by adsorption to the aluminium or ironhydroxides rather than by precipitation [11]. However,the most acceptable mechanism assumes that phospho-rus removal is a continuous sequence of adsorption andprecipitation, which consists of two processes: revers-ible adsorption and irreversible precipitation [12]. The

*Corresponding author. Email: gchen@eng.fsu.edu

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adsorption process is relatively fast and the precipita-tion process is relatively slow, and, practically, it is verydifficult to distinguish between these two processes.

The objective of this study was to investigate thechemical removal of phosphorus from wastewater usingcommonly used metal salt coagulants. We hypothesizethat the formation of metal hydroxides is an importantfactor for phosphorus adsorption and precipitation.Using secondary effluent from municipal wastewatertreatment plants as the model wastewater, phosphorusremoval was evaluated using four commercial metalsalt coagulants in this research. Previous studies ofphosphorus chemical removal did not distinguishadsorption from precipitation. In this study, phosphorusremoval by adsorption was investigated separately.Based on the experimental observations, it wasconcluded that adsorption played a more important rolein phosphorus removal, i.e. around 65% of phosphorusremoval was attributed to adsorption.

Materials and methods

Effluent collected from six regional municipal sewagetreatment plants located in Wuhan, China was mixedtogether and used as the model wastewater in thisresearch. The model wastewater had the followingcharacteristics: chemical oxygen demand (COD), 60mg/L; biological oxygen demand (BOD), 30 mg/L;dissolved phosphorus, 4.0 mg/L; suspended solid, 60mg/L; pH 7.2.

The commercial metal salt coagulants used inthis research included aluminium sulphate (Al

2

(SO

4

)

3

·18H

2

O), ferric chloride (FeCl

3

·6H

2

O), polyalu-minium chloride (PAC, 28% Al

2

O

3

), and polyferricsulphate (PFS). They were all obtained from PricomInc. (Jiaozuo, Henan, China).

Jar-testing techniques were adopted in this researchusing a six paddle stirrer. In each of the tests, 700 mLof mixed effluent was poured into a 1000 mL glassbeaker and left to settle for approximately 45 min.From each flask, 20 mL of the supernatant was pipettedand measured for dissolved phosphorous using the4500-P.E Ascorbic Acid Method by means of colorim-etry [13]. Specifically, the samples were filteredthrough a 0.45

µ

m membrane filter and preserved with0.2% H

2

SO

4

and then autoclaved with K

2

S

2

O

8

andH

2

SO

4

for 30 minutes at 121

°

C. The samples werethen mixed with ammonium molybdate, ascorbic acidand antimonyl tartrate to form a molybdenum bluecomplex, which was measured at 660 nm andcompared with identically prepared standard phospho-rus and blank solutions using a spectrophotometer.After coagulants were added, the flasks were rapidlymixed at a speed of 450 rpm for 30 seconds, followedby a slow mix at a rate of 80 rpm for another 15

minutes. After 15 minutes of settling, supernatant waspipetted and measured again for dissolved phosphorus.For each set of tests, a control consisting of mixedeffluent with no coagulation addition was used. Zetapotential was estimated parallel to the jar testing, usingmicroelectrophoresis. Zeta potential was quantifiedbased on electrophoretic mobility by dynamic lightscanning (Zetasizer 3000HAS, Malvern InstrumentsLtd., Malvern, UK). During the measurement, a laserbeam passed through the electrophoresis cell, irradiat-ing the particles dispersed in it. The scattered light wasthen detected by a photo-multiplier after passing theelectrophoresis cell. Zeta potential was related to theelectrophoretic mobility by the following equation:

where U

E

is the electrophoretic mobility [m/(V·sec)],

ε

r

and

ε

0

are the relative dielectric permittivities ofthe dispersion medium and the permittivity of vacuum[C/(V·m)], respectively, and

η

is the viscosity. Each testwas repeated six times and the average value wasreported.

To investigate phosphorus adsorption, the effluentwas introduced to a column filled with goethite-coatedsilica sand. Goethite, a frequent and abundant form ofiron oxide in the soil and sediments, is an importantcomponent influencing phosphate adsorption in naturalaquatic environments or in the soil. Goethite has beenextensively used in phosphorus adsorption studies, inpart because it is stable and can be easily produced inthe laboratory. Goethite was prepared as described bySchwertmann and Cornell [14]. Briefly, 1.0 M ferricnitrate was mixed with 1.0 M KOH (1:9, v/v) and agedfor 21 days at 25

°

C. This suspension was then washedextensively with deionized water via centrifugation. Therinsed solid was resuspended in 0.4 M HCl. After beingwashed and dialyzed against deionized water, it wasfreeze-dried to obtain crystalline goethite. The obtainedgoethite was then coated on silica sand following themethod of Schwertmann

et al

. [15] and Scheidegger

etal

. [16]. Specifically, goethite wase mixed with silicasand (1:5, w/w) in 0.01 M NaNO

3

solution (pH 7.5) andshaken for 48 hours. The coated silica sand was thenwashed with 0.1 M NaNO

3

(pH 7.0) via centrifugation.After rinsing with deionized water, the coated silicasand was oven-dried at 110

°

C. The goethite coatingwas determined by dissolving coated silica sand inHNO

3

(95%) and HF (40%) (2:1, v/v). The specificsurface area of goethite-coated silica sand was alsomeasured by the surface area analyser and was deter-mined to be 318 m

2

/g.Column experiments were conducted using an

acrylic column (2.5 cm

×

15 cm, Kimble-Kontes,

UEr= ε ε ζη

0 ( )1

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Vineland, NJ) to investigate phosphorous adsorption onthe goethite-coated silica sand. The column was orientedvertically and sealed at the bottom with a custom frit topermit the flow of water and retain the media. Goethite-coated silica sand was packed in the column through CO

2

solvation to eliminate air pockets. Prior to starting eachexperiment, approximately 100 pore volumes of nano-pure deionized water was eluted through the column bya peristaltic pump to stabilize the column. Before theintroduction of the model wastewater into the column, aconservative tracer of chloride was introduced to thecolumn to estimate the porous media properties. After thetracer study, two pore volumes of model wastewater waspumped into the column at a flow rate of 0.56 mL/min.The column was then flushed with nano-pure deionizedwater for up to 50 pore volumes until no phosphorouscould be detected in the elution. Elution was collectedby a fraction collector and was measured for dissolvedphosphorus. A breakthrough curve was generated and amass balance analysis was performed. For the columnexperiment, three runs were performed, and the incon-sistency of breakthrough curves was within 5% (95%CI). As a control experiment, transport of phosphorus inuncoated silica sand was also conducted.

Under saturated conditions, phosphorus transport ingoethite-coated silica sand is controlled by both equilib-rium adsorption and kinetic deposition, which can bedescribed by:

where C is the phosphorus concentration in the solution(g/m

3

), t is the elapsed time (sec),

ρ

b

is the sediment

bulk density (g/m

3

), K

d

is the partitioning coefficient ofphosphorus between the aqueous phase and porousmedia (m

3

/g),

θ

is the porosity of the porous media (m

3

/m

3

), D is the longitudinal dispersion coefficient (m

2

/sec), x is the coordinate parallel to the flow (m), v is thepore velocity (m/sec), and

µ

is the first-order phospho-rus deposition coefficient on goethite-coated silica sand

(/sec). is defined as the retardation

factor, R, which is an indicator of the ‘lag’ of phospho-rus transport due to reversible adsorption.

Transport parameters in Equation (1) were obtainedby fitting the experimentally obtained phosphorusbreakthrough data using an implicit, finite-differencescheme. All the parameters were optimized by minimiz-ing the sum of squared differences between observedand fitted concentrations using the nonlinear least-square method [17].

During the experiments, all experimental perfor-mances followed standard procedures, and measure-ments using the instrumentation strictly followed theprocedures provided by the manufacturer. The instru-mentation was calibrated regularly and whenever workconditions changed. Data acceptability was determinedin terms of precision, accuracy, representativeness,completeness and comparability. Relative standarddeviation, variance and confidence interval wereapplied to test the accuracy and eliminate systematicand random errors.

Results and discussion

pH and coagulant doses

With the addition of coagulants, solution pH decreasedaccordingly (Figure 1). The decrease in solution pH wasattributed to the alkalinity consumption during coagu-lant hydration. In the case of Al

3+

and Fe

3+

, there is aprimary hydration shell with six octahedrally coordi-nated water molecules, e.g. Al(H

2

O)

63+

and Fe(H

2

O)

63+

.Hydrolysis of Al(H

2

O)

63+

and Fe(H

2

O)

63+

is a sequentialreplacement of the water molecules with hydroxyl ions,the progression of which involves many competingreactions. These reactions of the deprotonation are func-tions of the reaction equilibrium constants and solutionpH. Among these four commercial metal salt coagu-lants, pH decrease was more pronounced for aluminiumsulphate and ferric chloride and less pronounced forPAC and PFS because the PAC and PFS polymersencountered fewer steps of water deprotonation andconsumed less alkalinity during hydration. Comparedwith Al

3+

, Fe

3+

reacted slowly with the natural alkalinityand, consequently, iron salt coagulants encountered lesspH decrease.

Figure 1. Solution pH as a function of coagulant dosing.

The coagulants and aluminium and ferric saltsformed a series of products including monomers, oligo-mers and polymeric hydroxyl complexes, depending onthe pH of the solution (Figure 2). In addition, the exist-ence of organic matter in the wastewater influenced thealuminium and iron speciation. In the presence of 10.0mM HCOOH (model organic matter), Fe(HCOO)

2+

wasthe dominating species, followed by FeOH

2+

andFe(OH)

2+

until pH reached 3.5. For aluminium sulphate,Al(OH)(HCOO)

+

was the dominating species, followedby Al(HCOO)

2+

, Al(OH)

2+

, Al(HCOO)

2+

and Al(OH)

2+

until pH reached 6.0. At pH above 7.0, Al(OH)

3

was thedominating species.

Figure 2. Speciation as a function of solution pH.

Phosphorous removal

Alum or hydrated aluminium sulphate is widely used asa coagulant and also popularly utilized in precipitatingphosphorous. Similarly, ferric chloride or sulphate and

(( )

)

( )

11

22

2

+− ∂

∂=

∂∂

−∂∂

−

ρ θθ

µ

bdK

C

t

DC

xv

C

xC

11+ −ρ θθ

bdK

( )

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ferrous sulphate are also widely used for phosphorousremoval, although the actual removal mechanisms arenot fully understood. Previous research has demon-strated that coagulation of aluminium and iron coagu-lants is an effective method for phosphorus removal[18,19]. In this research, it was demonstrated that allthese four coagulants were efficient at removing phos-phorous from the model wastewater (Figure 3). Theremoval percentage was in the range from 82% to 96%.Among these four coagulants, alum was the most effi-cient coagulant, followed by ferric chloride, PAC andPFS (Figure 3).

Figure 3. Phosphorus removal as a function of coagulant dosing.

For the pH range of this research, Fe(OH)

+

andFe(OH)

3

co-existed for iron salt coagulants, andAlOH

2+

, Al(OH)

2+

and Al(OH)

3

co-existed for alumin-ium salt coagulants (Figure 2). The amorphous Fe(OH)

3

and Al(OH)

3

were the species that were responsible forphosphorus precipitation. The species responsible forphosphorus adsorption were Fe(OH)

+

, AlOH

2+

andAl(OH)

2+

. For phosphorus to adsorb to Fe(OH)

+

,AlOH

2+

and Al(OH)

2+

, phosphorus replaced singlycoordinated OH

−

groups and then reorganized into avery stable binuclear bridge between the cations. Thischemisorption process was coupled with the release ofOH

−

ions, thus this process was favoured by low pHvalues. Because these species co-existed, adsorptionand precipitation of phosphorus occurred at the sametime. Owing to the fact that AlOH

2+

and Al(OH)

2+

were

the dominant species at the pH range of this researchand aluminium salt coagulants were the most effectiveat removing phosphorus, it seemed that adsorptionshould play an more important role in phosphorusremoval, which is consistent with prior research [11].

To further investigate the mechanism, the zeta poten-tial of the solution was monitored. The zeta potential ofthe solution decreased with the increase in solution pH(Figure 4). There was a general trend that simple metalsalt coagulants decreased the zeta potential more thanthe polymers. Compared with iron salts, the decrease foraluminium salts was more obvious. Phosphorousremoval was found to be closely related to the zetapotential of the solution, i.e. phosphorous removalincreased with the increase in the zeta potential (Figure5). The increase in the zeta potential was the result ofaccumulation of Fe(OH)

+

, AlOH

2+

and Al(OH)

2+

, whichwere the species that were responsible for the adsorptionof phosphorus. For the same zeta potential, aluminiumsalt coagulants had greater phosphorous removal effi-ciency than iron salt coagulants.

Figure 4. Zeta potential as a function of solution pH.Figure 5. Phosphorus removal as a function of zeta potential.

Phosphorous sorption on goethite-coated silica sand

Tracer (Cl

−

) transport was studied before the phosphorustransport experiments in both uncoated silica sand andgoethite-coated silica sand. For both cases, nearly all theinput tracer was eluted from the column. The tracer

Figure 1. Solution pH as a function of coagulant dosing.

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breakthrough curve was characterized by a breakthroughfront and an elution tail (Figure 6). The lasting tail of thetracer breakthrough curve indicated possible retardationof nitrate in the column. The tracer breakthrough curvewas simulated with the Advection-Dispersion Equation

(ADE). During the model simulation, the retardationfactor was set to 1.0, i.e. K

d

= 0. This was based on theconsideration that the tracer should not be retarded in themedia, as the tracer was assumed not to adsorb inthe media. In addition, the deposition coefficient,

µ

, was

Figure 2. Speciation as a function of solution pH.

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set to zero, i.e. no retention of the tracer in the media.This was true since nearly all the inputted tracer waseluted from the column at the end of the transport exper-iments. During the simulation, the velocity was fixed at0.058 cm/min and the initial D was set as 8.00 cm

2

/min

for both uncoated and goethite-coated silica sand. Afterthe simulation, D was determined to be 9.96 cm

2

/min foruncoated silica sand and 11.8 cm

2

/min for goethite-coated silica sand. These D values were then used for thesimulations of phosphorus transport in the corresponding

Figure 3. Phosphorus removal as a function of coagulant dosing.

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Figure 4. Zeta potential as a function of solution pH.

Figure 5. Phosphorus removal as a function of zeta potential.

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media. From the tracer study, the porous media porositywas found to be 0.49 for uncoated silica sand and 0.47for goethite-coated silica sand.

Figure 6. Phosphorus transport breakthrough curves in uncoated and goethite-coated silica sand.Phosphorous breakthrough curves were fitted wellwith the ADE equation. The accuracy of bacterial trans-port modelling was expressed by the sum of the squareddifferences between observed and fitted concentrations.The mean square for error of nitrate transport was 0.976and the mean square for error of bacterial transport wasin the range from 0.890 to 0.990. There was a smallretardation and minimal deposition when phosphoruswas transported in uncoated silica sand (R = 1.38 and µ= 1 × 10−7 per min) (Figure 6). When transported ingoethite-coated silica sand, however, more phosphoruswas retarded, as shown by the delayed breakthroughfront (R = 3.80). There was also phosphorous retentionin the column as the peak value of the breakthroughcurve was much lower than that of the uncoated silicasand. Phosphorus transport breakthrough curve also hada long elution tail. The long-lasting tails of the break-through curve indicated kinetic-controlled phosphorusdeposition in the column. Phosphorus adsorption ingoethite-coated silica sand was thought to occur owingto the replacement of OH− groups with phosphorus onthe goethite surface.

As discussed previously, adsorption should play amore important role than precipitation in phosphorusremoval. As demonstrated by the column experiments

(integration of the breakthrough curve), around 65% ofphosphorus can be retained through adsorption by goet-hite. The retention of phosphorus on goethite-coatedsilica sand was attributed to the adsorption since therewas no precipitation during the transport experiments.This observation was supported by the previous obser-vation that the adsorption of phosphorus to Fe(OH)+,AlOH2+ and Al(OH)2

+ was the dominating phosphorusremoval mechanism.

Conclusions

Alum, ferric chloride, PAC and PFS can all effectivelyremove phosphorous from the wastewater. Amongthese four coagulants, alum was the most efficient,followed by ferric chloride, PAC and PFS. Aneconomic evaluation in terms of chemical cost revealedthat the per million gallon cost using aluminiumsulphate, ferric chloride, PAC, and PFS was $23, $42,$36 and $57, respectively. Among these four coagulantsinvestigated in this research, aluminium sulphate wasfound to be the most efficient and cost-effective coagu-lant. Therefore, aluminium sulphate is recommended tobe used for phosphorus removal. During the phosphorusremoval process, amorphous Fe(OH)3 and Al(OH)3

were the species that were responsible for phosphorusprecipitation, and Fe(OH)+, AlOH2+ and Al(OH)2

+ werethe species that were responsible for the adsorption of

Figure 6. Phosphorus transport breakthrough curves in uncoated and goethite-coated silica sand.

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phosphorus. Since these species co-existed, adsorptionand precipitation of phosphorus occurred at the sametime. In this research, it was concluded that adsorptionplays a more important role in phosphorus removal.This was supported by the observed increase in phos-phorous removal with the increase in the solution zetapotential, which was attributed to the accumulation ofFe(OH)+, AlOH2+ and Al(OH)2

+, the species that wereresponsible for the adsorption of phosphorus. Thisconclusion was further supported by column experi-ments where around 65% of phosphorus was removedby adsorption.

AcknowledgementsThe work was supported by the National Research Initiativeof the USDA Cooperative State Research, Education andExtension Service, Grant No. 2007-35102-18111, to FloridaA&M University.

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