Dissolved ozone flotation (DOF) — a promising technology in municipal wastewater treatment

*Corresponding author.

Byoung Ho Lee*, Won Chul Song, Biswaranjan Manna, Jong Kyu HaDepartment of Civil and Environmental Engineering, University of Ulsan, Korea 680-749

Tel. +82 (52) 259-2279; Fax +82 (52) 259-2629; email: [email protected]

Received 22 January 2007; accepted 8 July 2007

Abstract

In this study, dissolved ozone flotation and conventional mechanical diffuser ozonation systems have beenapplied to treat the downstream municipal wastewater. The process efficiency has been evaluated in the lab benchscale on the basis of the removal rate of some wastewater quality parameters, like turbidity, SS, color, UV254,COD, BOD, T-N, T-P and bacteria. DOF is found to be an efficient ozonation technique in comparison to themechanical diffuser system under identical conditions, though the removal efficiencies of coliform bacteria arefound to be the same in both cases. The optimum ozone dose of 6.1 mg/L is found in the DOF system. As for theresult of this investigation, a DOF pilot plant with the capacity of 200 m3/d was designed and set up to treat theeffluence of secondary wastewater, and detailed assessment of performance was carried out during one monthoperation. The results indicate a good removal rate of 86.9% turbidity, 81% TSS, 72.6% color, 82.4% BOD and92% total phosphorus. However, lower removal efficiencies of 42.9% and 33.4% were obtained for COD and T-Nrespectively. Almost 100% disinfection efficiency was achieved by removing heterotrophic and coliform bacteria.DOF technology is very effective and economically viable for municipal wastewater treatment in the present daycontext.

Keywords: Dissolved ozone flotation; Mechanical diffuser; Wastewater; Micro bubble; Mass transfer; Disinfection

1. Introduction

The rapid development of our society is con-tinuously creating huge demand of water in vari-ous sectors including domestic, agricultural, in-

low rainfall areas. With a view to meet our re-

quirements, wastewater effluents can be used asan alternative potential source. But disposal oftreated wastewater often poses a risk to the envi-ronment and ecology due to the presence of harm-ful organic compounds and pathogenic microor-ganisms [1]. Conventional treatment does notmeet the criteria of water quality parameters forreuse and safe disposal to the environment. Ad-

municipal wastewater treatmentDissolved ozone flotation (DOF) — a promising technology in

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.

dustrial uses and aquifer recharge particularly in

doi:10.1016/j.desal.2007.07.011

D esalination 225 (2008) 260–273

B.H. Lee et al. / Desalination 225 (2008) 260–273 261

vanced treatment technology is, therefore, ur-gently needed for further decreasing the residualconstituents in downstream of municipal waste-water treatment plants. As a result, constant ad-vancement is being made by wastewater treatmentindustries to improve the methods of removal. Inthis context, the use of ozone as an oxidant anddisinfectant for the treatment of wastewater isgaining popularity.

Various advance treatment technologies havebeen adopted to obtain effluents with high qual-ity. Among the common processes used for ad-vanced treatment are membrane filtration, acti-vated carbon adsorption and chemical oxidationfollowed by disinfection using chlorine or ozone[2]. Membrane technologies including microfil-tration and ultrafiltration have proved to be effi-cient with regard to disinfection, but they are con-sidered unsuitable owing to the high operationalcost and the requirement of continual backwash-ing to avoid system clogging [3]. The residualdissolved organic matters of secondary wastewa-ter may effectively be removed by the applica-tion of activated carbon adsorption; however, theregeneration of spent carbon and disposal ofregenerant wastes create further problems [4].Chemical oxidation by chlorine also creates prob-lems due to the formation of toxic disinfectionby-products such as trihalomethanes and otherhalogenated organic compounds, by reacting withthe dissolved organic matters [5]. Municipal wa-ter systems around the world are being re-engi-neered to incorporate the use of ozone, to restrictthe formation of such carcinogenic disinfectionby-products, for providing safe and potable wa-ter.

Ozone is an unstable gas with half life in theorder of minutes and is sparingly soluble in wa-ter. It has been found that the main limitation inozonation systems comes from the low mass trans-fer rate of ozone from the gas phase to the liquidphase [6–9]. Mass transfer efficiency from the gasphase to the liquid phase depends on mixing char-acteristics of the gas liquid contractor used, the

kinetics of ozone decay in water, number and sizeof bubbles produced [9,10]. The effectiveness ofozone as either an oxidant or a disinfectant canbe increased by creating higher surface area tovolume ratio, for contact of ozone with a solutionthrough the generation of smaller bubbles in DOFtechniques [11].

A number of different bubble generators arecurrently used in water and wastewater systems,including submersible orifices such as nozzlessintered metal or glass plates, sprinklers and rub-ber membranes [12]. In combination with thesebubble generation techniques, devices providingmechanical agitation are sometimes used to in-crease contact between the phases. Such devicesinclude impellers and other types of mixers thatprovide kinetic energy to the bulk fluid, wastinga significant amount of energy through dissipa-tion into the liquid and creating undesirable backmixing conditions. As a result of the shortcom-ings of bubble generation devices currently uti-lized, dissolved ozone floatation techniques havebeen developed with the view of enhancing masstransfer, increasing energy efficiency and creat-ing more suitable mixing conditions.

The objectives of the present study are to in-vestigate i) the process efficiency of a DOF sys-tem with a conventional mechanical diffuser sys-tem of ozonation, ii) optimization of the DOFsystem, and iii) implementation of the effectiveDOF ozonation techniques in the municipal waste-water treatment plant in Korea and assessment ofperformance on the basis of removal efficienciesof some water quality parameters like SS, turbid-ity, color, UV254, COD, BOD, T-N, T-P, and bac-teria to meet the reuse and safe effluent dischargecriteria.

2. Materials and methods

The generation of ozone is accomplished bycorona-discharge systems in which pure oxygen(99.7%) is subjected to an electric discharge.Power is supplied in the form of high alternating

262 B.H. Lee et al. / Desalination 225 (2008) 260–273

current voltage, creating ionized gas in the spacebetween two electrodes. Ozone generation ismaintained by adjusting the oxygen flow rate, andoutput voltage into the ozone generator.

The lab bench scale mechanical diffuserozonation system consists of an ozone generator,an ozone pump, a reactor, a raw water tank and afeed pump. The reactor is a cylindrical containerof steel with the height and inner diameter of 50cm and 7 cm2 respectively. Wastewater is injectedin down flow direction into the reactor throughthe feed pump and ozone is supplied through aspherical porous diffuser fitted at the bottom ofthe reactor. In the DOF system, the reactor is arectangular flotation tank attached to the ozonedissolving tank, which is fitted with the ozonegenerator. All experiments were performed by thesemi batch method using microbiologically pro-cessed municipal wastewater at 20°C and origi-nal pH of wastewater. The input and off gas ozoneconcentrations were measured by KI method [13].

The water quality parameters (pH, SS, turbid-ity, color, UV254, COD, BOD, T-N, T-P and totalcoliform bacteria) were measured according tostandard methods [13]. Color was measured spec-trophotometrically at the visible absorbance wave-length of 400 nm and UV254, representing thatthe aromatic content of wastewater was also mea-sured spectrophotometrically. COD was estimatedby the standard dichromate method. For BOD

measurement, samples were incubated for 5 daysat 20°C, and BOD5 was determined based on thedifference between the initial DO and final DO.T-N was estimated by the alkali potassiumpersulphate digestion method and T-P was mea-sured calorimetrically by the ascorbic acidmethod. Total and fecal coliform counts were gen-erally used as indicators to determine the degreeof disinfection, and were measured by Petri filmmethod.

A DOF pilot plant, with the capacity of 200 m3/dhas been designed and set up in a municipal waste-water treatment plant (32,000 m3/d) in Korea.Samples were taken for analysis at regular inter-vals during a two-month period. The schematicdiagram and operating conditions of the plant arerepresented in Fig. 1 and Table 1 respectively.

Fig. 1. Schematic diagram of the DOF system.

Table 1Operating conditions of DOF pilot plant

Flow rate, L/min 10 Rapid mixing, rpm 200 HRT of rapid mixing zone, min 1 Flocculation tank 1st, rpm 9 2nd, rpm 6 3rd, rpm 3 HRT of flocculation tank 20 Recycle rate, % 20 Flocculant dose, mg/L 15


3. Chemistry of ozone in water

Several phenomena occur in an ozonation re-action with water containing organics. These phe-nomenon include i) direct chemical reaction ofozone with organics in water as an electron ac-ceptor ii) indirect pathway through the genera-tion of a very strong non-selective hydroxyl radi-cal oxidant by the hydroxyl ion catalyzed decom-position of ozone in the solution. Once ozone en-ters water, it becomes highly unstable and rap-idly decomposes through a series of reactions in-cluding ignition (1), propagation (2)–(6) and ter-mination (7), (8) as follows [14,15]:

3 2 2OH O H O O− −+ → + (1)

+2 2H O H O−→ + (2)

2 3 2 3O O O O− −+ → + (3)

+3 3O H HO− + → (4)

3 2HO O OH→ + i (5)

2 2 2O OH HO O+ → +i (6)

3 4O OH HO+ → (7)

4 3 2 2 3 2HO HO H O O O+ → + + (8)

However, it has been demonstrated that thechemical composition of water can greatly influ-ence these reactions [14]. The chain of reaction isinitiated by the hydroxyl ions present in water withthe generation of HO2 (1) which can initiate fur-ther reactions. The decomposition of ozone to itssecondary oxidants, particularly hydroxyl radical,is very rapid in high pH level in water solution(5). It is reported that oxidation of ozone is highlyselective whereas oxidation by hydroxyl is non-selective and hence more effective oxidant thanozone [16].

4. Results and discussion

Dissolved ozone flotation and conventionalmechanical diffuser ozonation systems have beenapplied to treat the downstream of the municipalwastewater treatment plant, using ozone dose of6.1 mg/L at 20°C for reclaiming water for reuseand safe effluent discharge requirements. The re-sults (Table 2) were analyzed on the basis of re-moval of some water quality parameters, includ-

Table 2Comparative performance of DOF and mechanical diffuser ozonation systems

Treated water Parameters Raw water Mechanical diffuser DOF

pH 6.55 6.52 6.43 Turbidity, NTU 6.2 4.6 1.3 SS, mg/L 8.25 5.60 2.40 Color, CU 12 5 2 UV254 absorbance, cm–1 0.131 0.094 0.076 CODCr, mg/L 35 27 20 CODMn, mg/L 10.4 8.3 3.2 BOD, mg/L 4.1 2.8 1.1 T-N, mg/L 9.082 9.379 7.684 T-P, mg/L 0.36 0.34 0.018 Coliform bacteria, MPN/ml 960 0 0 Total aerobic bacteria, CFC/ml Uncountable 37 21


ing turbidity, SS, color, UV254, COD, BOD, T-N,T-P and bacteria.

The treatment capacity and efficiency of DOFtechniques is found to be better than the conven-tional mechanical diffuser ozonation process. Thehigher removal efficiency achieved in DOF is dueto the increased surface area to volume ratio offine micro bubbles, leading to enhanced masstransfer rates of ozone from the gas phase to thesolution phase. Moreover, low density particles(algae, alum flocs) are easily separated in DOFwhile the removal of them by normal ozonationis more difficult. DOF provides the needed en-ergy for effective flotation in the form of ex-tremely fine bubbles, which become attached tothe suspended materials to be removed. Particlesand globules in water act as nucleation sites, andair seeks out these sites to begin micro bubbleformation. The attraction between the air bubblesand particles is a result of adsorption forces thatare a function of the characteristics of the particlesurface or physical entrapment in the particle. Thisattachment of bubbles to the particle reduces thedensity of the particle, resulting in increased buoy-ancy, thus effecting flotation.

The effect of the ozone dose for mineraliza-

Table 3Ozone dose optimization in treating municipal wastewater (downstream) by the DOF process

Removal efficiency of parameters (%) Ozone dose (mg/L)

Parameters Raw water

2.0 4.6 6.1 7.6 Turbidity, NTU 12.5 91.0 90.9 94.4 94.0 SS, mg/L 18.3 82.1 90.7 93.4 93.7 Color, CU 27 52.4 66.5 88.8 90.0 UV254 absorbance, cm–1 0.0176 47.0 58.4 75.4 75.6 CODCr, mg/L 35.0 34.5 44.8 58.6 60.5 CODMn, mg/L 10.2 30.6 42.5 55.1 55.9 BOD, mg/L 4.4 27.9 36.4 73.2 75.6 T-N, mg/L 8.23 31.0 28.8 42.9 42.1 T-P, mg/L 0.84 94.9 95.7 89.3 92.1 Coliform bacteria, MPN/ml 860 99 100 100 100 Total aerobic bacteria, CFC/ml 550 90 97 99 100

tion is carried out with the variation from 2.0 to7.6 mg/L and the results are represented in Table 3.The optimum ozone dose obtained is 6.1 mg/Land further increase in the dose concentration doesnot improve the removal efficiency greatly. Morethan 90% removal efficiency is found for turbid-ity, SS and T-P, whereas lower removal efficiencyis observed for COD and T-N. Ozone selectivelyoxidizes higher molecular weight aromatic com-pounds to lower molecular weight non-aromat-ics, which is reflected by a 75% decrease inUV254. However, the ozone dose requirement inorder to achieve definite removal efficiency un-der identical conditions solely depends on thenature and concentration of the wastewater con-stituents.

A DOF pilot plant of 200 m3/d capacity hasbeen designed and installed to treat the second-ary waste effluents of a municipal wastewatertreatment plant situated at P city in Korea. Removalcharacteristics of some water quality parameterswere investigated using the optimum ozone doseof 6.1 mg/L, and the plant performance was evalu-ated with some reported works. Removal of tur-bidity about 87% was observed by reducing theaverage raw water turbidity from 10.1 to


0.91 NTU (Fig. 2). The effluent turbidity meetsthe EPA water reuse guideline (1 NTU) satisfac-torily [18].

In contrast, very poor turbidity removal effi-ciency of 10% was found by M. Petala and co-workers [19] using ozone doses of 7.1 mg/L tothe secondary waste effluents with the initial tur-bidity of 23.8 NTU. Approximately 25% turbid-ity removal was also observed by P. Xu et al. [20]from municipal wastewater with the applicationof the transfer ozone dose of 10 mg/L. M.P. delPino et al. [21] applied a dual membrane process(microfiltration followed by reverse osmosis) totreat secondary effluents and obtained a good re-duction of turbidity from 15 NTU to less than1 NTU. Advanced treatment and reverse osmosismembranes also effectively remove turbidity by90% from municipal wastewater as reported byM. Abdel-Jawad et al. [22]. The removal of tur-bidity is accompanied with the removal of SS. Inthe DOF system, a good SS removal of 81% isachieved by reducing the values in an averagefrom 15.4 to 2.9 mg/L. The attachment of par-ticles with bubbles due to strong adsorption forcesreduces the density of the particles resulting inincreased buoyancy and consequently removes by

Fig. 2. Removal of turbidity in DOF pilot plant at pH 6.7±0.2 and temperature 20±5°C.

0

2

4

6

8

10

12

0 5 10 15 20 25 30

Operation time (day )

Turb

idity

(NTU

)

0

20

40

60

80

100

Rem

ova

l ra

te (

%)

Raw water Treated water Removal rate

flotation effectively. In agreement with this, K.Neilson et al. [23] reported that 50–60% SS wasremoved from municipal secondary waste efflu-ent by ozone enhanced electro coagulation. A simi-lar result (70% TSS removal) was also achievedby M.F. Hamada et al. [24] in treating municipalwastewater by pre-chlorination-granular media(sand) filtration techniques.

High color removal efficiency was alsoachieved by the DOF system (Fig. 3). The resultindicates that almost 73% color was removed inthis case. The DOF technique can efficiently re-move color from wastewater, containing high sus-pended solids, by flotation. The reduction of coloroccurs due to the adsorption of some color im-parting organic molecules by suspended solidparticles in addition to the direct breakdown ofthe chromophoric bonds in dyes by ozone. Theaverage color of the treated water was found as7 CU by applying ozone dose of 6.1 mg/L, butfurther increase in the ozone dose failed to im-prove the reduction in color significantly whichwas attributed to the presence of some color gen-erating organic molecules without neucleophilicsites that are reluctant to ozone oxidation. P. Xuet al. [20] have also investigated the effect of


Fig. 3. Removal of color in DOF pilot plant at pH 6.7±0.2 and temperature 20±5°C.

0

5

10

15

20

25

30

35

0 5 10 15 20 25

Number of day s

Co

lor

(CU

)

0

10

20

30

40

50

60

70

80

90

100

Rem

ova

l ra

te (

%)


ozonation on color abatement and found similarresults. With the increase in the transfer ozonedose from 2 to 13 mg/L at HRT 4 min, 60–85%reduction in color from wastewater effluents wasachieved by them. Moreover, an excellent decol-orization efficiency of 90% from synthetic waste-water zqs obtained by X.J. Wang et al. [25] usingO3/dye ratio of 3.6.

The UV254 absorbance values may be con-sidered as a rough indication of organic com-pounds in the reclaimed wastewater, mostly in theform of humic-like substances [26]. The averagereduction of 65.5% in UV254 absorbance wasobserved by the DOF system (Fig. 4). The resultsindicate that the ozone reacts and oxidizes theorganic matter, in particular, the compounds hav-ing double bonds or aromatic structures that de-termine the value of the absorbance of UV254. P.Xu and his co-worker found that with the increasein the transfer ozone dose from 2 to 13 mg/L atHRT 4 min, the variation of UV254 absorbancein secondary effluent increased from 28% to 55%.In contrast, very poor UV254 absorbance reduc-tion around 20% was observed by M. Petala and

his co-workers on ozone treatment at the dose of7.1 mg/L.

The COD removal efficiency by the DOF pro-cess is represented in Fig. 5. The result shows only43% reduction from the average initial COD valueof 35 mg/L. The higher molecular weight recalci-trant organics are selectively oxidized to lowermolecular weight organics, without a significantchange of total organic carbon, and thereby com-paratively less COD reduction is observed. More-over, 38% removal of COD is reported by M.F.Hamoda et al. [24] for tertiary treatment of mu-nicipal wastewater, using sand filtration with pre-and post-chlorination (chlorine dose — 10 mg/L).Again, only 10% total COD removal was obtainedby P. Xu et al. [20] from wastewater using thetransfer ozone dose of 9.2 mg/L and HRT of 4 min,whereas a change of dissolved COD was observedby them. The phenomenon is due to the conver-sion of particulated organic compounds to solublefragments by ozonation.

BOD is an important water quality parameterthat determines the portability of effluent water,and possesses the EPA guideline value not exceed-


ing 5 mg/L. In this study, the average BOD valueof 9.13 mg/L for secondary effluent was reducedto 2.38 mg/L which satisfactorily meets the EPAguideline value (Fig. 6). In contrast, less BODremoval rate (50–60%) is reported by K. Neilson

Fig. 4. Removal of UV254 in DOF pilot plant at pH 6.7±0.2 and temperature 20±5° C.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 5 10 15 20 25

Number of days

Abso

rbance

0

10

20

30

40

50

60

70

80

90

100

Rem

ova

l ra

te (

%)


Fig. 5. Removal of CODCr in DOF pilot plant at pH 6.7±0.2 and temperature 20±5° C.

et al. in O3, which enhances electroflocculationtechnique. Moreover, P. Xu et al. [20] also foundvery poor BOD reduction of 11% using the trans-fer O3 dose of 9.2 mg/L.

The removal of T-P and T-N in the DOF pilot

0

10

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30

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50

60

0 5 10 15 20 25Number of days

CO

D (

mg/L

)

0

10

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30

40

50

60

Rem

ova

l eff

icie

ncy

(% )

Raw water Treated water Removal efficiency


Fig. 6. Removal of BOD in DOF pilot plant at pH 6.7±0.2 and temperature 20±5°C.

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30 35

Number of day s

BO

D (

mg

/L)

0

10

20

30

40

50

60

70

80

90

100

Rem

ova

l eff

icie

ncy

(%)


plant has been investigated. A good removal effi-ciency of TP was achieved by 92% (Fig. 7) whileT-N was removed by 33.5% (not shown) only.Ozone acts as a coagulant aid and oxidizes or-ganic phosphorus to soluble phosphate, which issubsequently removed by adsorption with flocparticles in this process. The treatment of munici-pal wastewater by ozone-enhanced electrofloc-culation techniques using the ozone dose of 6–7 mg/L, and the current density of 300 Å also ex-hibits good removal of TP by more than 85% asreported by K. Nielson and D.W. Smith [23].Moreover, the removal capacity of phosphorousdid not exceed 20% during the addition of polyaluminum coagulant, followed by sand filtration[19]. The DOF techniques cannot remove the in-organic nitrogen and hence the removal efficiencyis poor. A similar result in TN reduction was foundusing aluminum electrode with ozone addition[23]. Liberti and Notarnicola [5] also did not ob-serve a distinctive ammonium nitrogen removalduring coagulation with poly aluminum chloride,followed by sand filtration of secondary effluents.

The experimental results (Figs. 8 and 9) ob-tained at pilot scale confirm the efficiency of DOFfor wastewater disinfection. All the coliform bac-teria were completely destroyed and removed byusing a very low ozone dose of 2 mg/L. The sameozone dose also removes the heterotrophic bacte-ria, reducing their count to 5 CFU/ml and satis-factorily meeting the EPA wastewater reuse dis-infection criteria.

Ozone kills microorganisms with a processknown as cellular lysis. During oxidation, ozoneruptures the cellular membrane of microorganismsand disperses the bacterial cytoplasm into the so-lution, thus making reactivation impossible. Thisprocess takes place in about 2 s with a relativelylow ozone dose for fecal coliform inactivation,because of the fast kinetics between ozone andcoliform bacteria. It is reported [20] that in orderto achieve inactivation of microorganism trans-fer, the ozone dose must have to meet the imme-diate ozone demand of wastewater, and optimumdisinfection can be achieved at ozone dosagesbetween 2–15 mg/L, depending upon the quality


of the secondary effluents. However, adequateremoval of total coliform was not found by otherresearchers [5] even using the ozone dose of15 mg/L. Similar results were also observed byM. Petala and co-workers [19] indicating the re-quirement of the optimum ozone dose of 26.7 mg/L

Fig.7. Removal of TP in DOF pilot plant at pH 6.7±0.2 and temperature 20±5° C.

0

0.5

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1.5

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Number of days

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(m

g/L

)

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40

60

80

100

120

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ova

l (%


Fig. 8. Removal of E-coli bacteria in DOF pilot plant.

for complete removal of total and fecal coliforms.The assessment of performances of the DOF tech-nique with some other reported works on the ba-sis of the removal efficiency of some water qual-ity parameters from municipal wastewater is rep-resented in Table 4. It is apparent from the results

0

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)


Fig. 9. Removal of E-coli bacteria in DOF pilot plant treatment.

a) Raw water: 860 MPN/ml b) Treated water: Not detected

that DOF in an efficient treatment option in thisregard.

5. Economical viability

Treatment cost of a plant plays a significantrole for its acceptability and implementation inthe real field. It is the deciding factor for practicalapplications of a plant. The cost of treatment inthe DOF system is evaluated as US$ 0.031/m3

which include the costs of chemicals, ozone gen-eration and power consumption. The treatmentcost of municipal wastewater by granular mediafiltration and chlorination technique is reported[24] as US$ 0.5/m3. Similarly, the high treatmentcost (CAN$ 0.57/m3) is observed in ozone-en-hanced electroflocculation [23]. Moreover, thecost of huge energy consumption, conductivityenhancing chemicals and ozone generation in-creases the overall cost of this system in com-parison to other competing technologies. In ad-vance treatment (RO) for municipal wastewaterpurification, the variable costs (including chemi-

cals, energy and maintenance) obtained from treat-ment plants with the capacity of 9 m3/d and 1150m3/d were US$ 0.26/m3 and US$ 0.246/m3 respec-tively [22]. The production cost of tertiary treat-ment by a system consisting of a moving bed sandfilter, a granular activated carbon adsorption bedand ozone disinfection was also estimated to beUS$ 0.31/m3 [19]. The treatment cost per m3 wa-ter in DOF technology is estimated with someother adopted treatment options and representedin Table 5. It is obvious that DOF is an economi-cally viable technology in comparison to otherrelevant technologies for municipal wastewatertreatment in the present day context.

6. Conclusion

In view of the experimental results, it can beconcluded that the DOF technique is more effi-cient than the conventional mechanical diffusersystem to achieve discharge limits by reducingpollutants from municipal wastewater. The re-moval efficiencies of water quality parameters like


Tabl

e 4

Com

para

tive

perf

orm

ance

of D

OF

tech

niqu

es w

ith so

me

othe

r rep

orte

d w

orks

in re

mov

ing

wat

er q

ualit

y pa

ram

eter

s fro

m m

unic

ipal

was

tew

ater

Rem

oval

eff

icie

ncy

(%)

Tech

niqu

es

Con

ditio

ns

Turb

idity

(N

TU)

SS

(mg/

L)

Col

or

(CU

) U

V25

4A

bs

COD

(m

g/L)

BO

D

(mg/

L)

TP

(mg/

L)

TN

(mg/

L)

Colif

orm

(M

PN/m

l)

Ref

. No.

DO

F O

3 dos

e =

6.1

mg/

L 87

71

72

.6

65.5

43

70

92

33

.5

100

Pres

ent

wor

k O

3 enh

ance

d el

ectro

flocc

ulat

ion

O3 d

ose

= 6–

7 m

g/L

Cur

rent

= 3

00 Å

—

45

—

—

35

50

–60

>85

20

100

[23]

Sand

filtr

atio

n an

d ch

lorin

atio

n C

hlor

ine

dose

=

10 m

g/L

—

55

—

—

38

61

—

—

100

[24]

Ozo

natio

n O

3 dos

e =

7.1

mg/

L 10

—

—

20

—

—

–9

14

<1

00

[19]

O

zona

tion

O3 d

ose

= 9.

2 m

g/L

HR

T 4

min

20

—

80

20

10

11

—

—

10

0 [2

0]

Elec

trodi

alys

is —

—

74

—

—

40

83

59

79

25

[2

1]


Table 5Assessment of performance of DOF with some other technologies on the basis of operational costs

Treatment technology Wastewater Plant capacity (m3/d) Cost (US$) Ref. No. DOF Municipal 200 0.031 Present work. Sand filtration and chlorination Municipal 42,000 0.50 [24] O3 enhanced electroflocculation Municipal — 0.57 (CAN$) [23] Sand filtration-AC adsorption-ozonation

Municipal — 0.31 [19]

Dual-membrane Municipal 12,000 0.21 [21] Reverse osmosis Municipal 9

1150 0.433 0.306

[22]

SS, turbidity, color, UV254, COD, BOD, TP, TNand total coliform are enhanced with increasingthe ozone dose, and the optimum ozone dose wasfound as 6.1 mg/L in most cases. The DOF pilotplant successfully removes turbidity, SS, color,BOD and TP by 89.6%, 81%, 72.6%, 82.4% and92% respectively from municipal wastewater us-ing the optimum ozone dose. Comparatively poorremoval of COD by 43% and TN by 33.5% wasobserved in this study. DOF provides an outstand-ing performance for disinfection, and completeremoval of total coliform bacteria was found at alow ozone dose of 2.4 mg/L. It also improves thetaste and odor of the treated water. In DOF, truedestruction of pollutants occurs instead of dis-placement without generation of secondary pol-lutants. It is cost-effective compared to other ad-vance treatments as well. It can meet satisfacto-rily the water quality standards of treated waterused in various sectors including domestic, agri-cultural, industrial, and aquifer recharging. In thisstudy, the DOF technology has been proven as agood option for municipal wastewater treatment.

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Dissolved ozone flotation (DOF) — a promising technology in municipal wastewater treatment

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