nitrate removal from drinking water in a packed-bed bioreactor coupled by a methanol-based...

Nitrate Removal from DrinkingWater in a Packed-BedBioreactor Coupled by aMethanol-BasedElectrochemical Gas GeneratorRamazan Vagheei,a Hossein Ganjidoust,a Ali-Akbar Azimi,b and Bita Ayatiaa Civil Engineering Department, Environmental Engineering Division, Tarbiat Modares University,Tehran, Iran; [email protected] (for correspondence)b Faculty of Environmental Engineering, Environmental Engineering Division, University of Tehran, Tehran, Iran

Published online 29 October 2009 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10404

Removal of nitrate from drinking water by a cou-pling of methanol-based electrochemical hydrogenand carbon dioxide generator and a packed-bed bio-reactor was investigated. The major goal was to studythe performance of this integrated system for simulta-neous, economical, in situ, and on-demand electrondonor and carbon source production for hydrogeno-trophic denitrification of drinking water in actualscales. The ability of system has been evaluated fortreatment of natural underground water up to 120 mgNO3

2/L (threefold of the drinking water limit) nitrateconcentration. Removal efficiencies above 95% wereachieved for more than 150 days of operation at a 2–5h retention time. Furthermore, the system was operatedonly by injection of two harmless gases (produced byelectrolysis of methanol) and without any chemicaladdition (solid or liquid). In the electrolysis process,H2 and CO2 (H2 as an electron donor and CO2 as acarbon source for denitrifier bacteria) was producedcheaply and simultaneously by applying a very low DCvoltage (4–6 V) to a solution that includes an easilyoxidizable organic substance (methanol) and electro-lyte solution comprised water and a base. In situ and

under control generation of H2 and CO2 by an effi-cient, cost-effective way showed that this process canbe an acceptable and reliable system for treatment ofnitrate contaminated drinking water in actual scales.Additionally, convenient process operation withoutneed of high pressure and explosive gas cylinders andonly by voltage adjustment is other advantage of theprocess. � 2009 American Institute of Chemical EngineersEnviron Prog, 29: 278–285, 2010Keywords: nitrate removal, drinking water, auto-

trophic denitrification, methanol electrolysis

INTRODUCTION

Groundwater is a common and, in some cases,exclusive drinking water source for both human andlivestock in urban and suburban areas. Nitrate is apriority pollutant of groundwater in many countriesdue to its toxicity related to methemoglobinemia andto the possible formation in the gastric system ofN-nitrose compounds, which are known to be carci-nogens in the digestion system and its widespreadpresence [1, 2]. A significant fraction of groundwatercurrently used as municipal water supplies exceedsthe US Environmental Protection Agency (USEPA)and World Health Organization (WHO) standards� 2009 American Institute of Chemical Engineers

278 October 2010 Environmental Progress & Sustainable Energy (Vol.29, No.3) DOI 10.1002/ep

(10 mg NO32-N/L and 1 mg NO2

2-N/L). Nitrate ingroundwater is increasing worldwide that is not easilyremoved by conventional drinking water treatment.Therefore, it has forced abandonment of some commu-nity water supplies. It has been estimated that in someaffected areas, 4% of the underground drinking watersupply is lost each year because of nitrate contamina-tion, which is much more significant than the 0.5% lossattributed to organic chemical contaminants [3].

In reality, anthropogenic sources of nitrate seem tobe the most problematic and often cause the nitratelevels exceed from standard. These sources arerelated to human activity including inorganic fertilizer,livestock waste, industrial discharges, domestic sew-age, and atmospheric deposition.

At present time, there are relatively few optionsavailable for treatment of nitrate polluted ground-water in practical scales and the need for additionaltechnology is obvious. Most often, nitrate is removedusing strong base anion (SBA) exchange resin onwhich nitrate ions are exchanged for chloride or bi-carbonate ions until the resin’s exchange capacity isexhausted. However, this can be costly as it replacesone salts for another and at times is ineffective,depending on the total ionic composition of water [4,5]. Reverse osmosis and electrodialysis process alsodo not have any selectivity for nitrate and are costlyin reduce total dissolved solids (TDS) of treatedwater. Furthermore, disposal a large volume of wastebrine is a critical issue for all of this process [6, 7].

Biological denitrification with heterotrophic micro-organisms has been widely and successfully applied towastewater treatment because of its high efficiencyand low cost, but the residual of carbon sources andbyproducts of disinfection from this process causemany problems in drinking water treatment. Theseproblems make autotrophic denitrification by hydro-gen oxidizing bacteria an excellent alternative. Thesemicroorganisms are naturally present in groundwater[8]. Additionally, hydrogen gas is harmless to humansand clean as an energy source. The bacteria also drawon inorganic carbon, which removes any problems, re-sidual organic carbon may cause. Hydrogen oxidizingbacteria removes nitrate and nitrites by autotrophicdenitrification using hydrogen as an energy sourceand carbon dioxide or bicarbonate as the carbonsource. The major pathway of denitrification is sup-posed to be: NO3

2 ? NO22 ? NO ? N2O ? N2. The

stoichiometry for the reaction of denitrification usingH2 as the electron donor is shown in Table 1.

The Eq. 5 shows that the pH will increase after thereaction because 1 mol of H1 is used when 1 mol ofNO2

2 is converted to nitrogen gas. Autotrophic deni-trification by hydrogen oxidizing bacteria is the mostsuitable process for drinking water treatment becauseit has several advantages: (1) this process has a highlyselectivity for nitrate removal and the reactionbyproducts are harmless for human health; (2) hydro-gen and carbon dioxide used as electron donor andcarbon source are harmless and thereby removes anyproblem that caused by heterotrophic denitrificaton;(3) the process has a small biomass yields and biosol-ids processing costs [11, 12]. In spite of these consid-

erable advantages risk of pressurized gas cylindersand economical issues is the major limitations ofautotrophic denitrification development in mediumand large scales.

To overcome these limitations in this research acoupling of an efficient, economically feasible wayfor adjustable in situ hydrogen and carbon dioxidegeneration by a packed bed bioreactor is designedand tested for the ability to remove nitrate from thewater supply. Hydrogen and carbon dioxide werecheaply and sufficiently generated by applying a verylow voltage (4–7 V) to a solution that includes metha-nol and an electrolyte comprised water and a base.Gas delivered to a biofilter packed with a lightexpanded clay aggregates (LECA) medium that inocu-lated by a mixed culture of natural autotrophic micro-organisms extracted from nitrate polluted well sedi-ments. The goal was to develop a simple, safe, andcost-effective system to facilitate the removal of ni-trate from drinking water.

MATERIALS AND METHODS

Enrichment CultureThe hydrogen-oxidizing denitrifying culture was

enriched from sediments slurry isolated from a highnitrate polluted well (120 mg NO3

2/L) in Tehran aqui-fer. The aquifer has been contaminated by the contin-uous disposal of domestic wastewater and agricul-tural runoff. A total of 200 mL of slurry was added toa 2 L flask containing nitrate contaminated ground-water, KNO3 (2.0 g/L), NaHCO3 (2 g/L), buffer (1.74g/L KH2PO4, 2.14 g/L K2HPO4 per 0.1 g NO3

2-N/Lfeed [13]) and trace element solution (1 mL/L).The trace element solution was composed ofMgSO4�7H2O (10.0 g/L), ZnSO4�7H2O (2.2 g/L),CaCl2�2H2O (7.3 g/L), MnCl2�4H2O (2.5 g/L),CoCl2�6H2O (0.5 g/L), (NH4)6Mo7O24�4H2O (0.5 g/L),FeSO4�7H2O (5.0 g/L), and CuSO4.5H2O (0.2 g/L) and

Table 1. Sequential energy reactions of denitrificationusing H2 (reactions 1–7: after [9] and stoichiometricrelationship between H2/NO3

2 and biomass expressedas C5H7O2N according to [10]).

1. Nitrate reductionNO3

2 1 H2 ? NO22 1 H2O

2. Nitrite reductionNO2

2 1 0.5 H2 1 H1 ? NO(g)1 H2O3. Nitric oxide reduction

2NO(g) 1 H2 ? N2O(g) 1 H2O4. Nitrous oxide reduction

N2O(g) 1 H2 ? N2(g) 1 H2O5. Overall nitrite conversion to N2(g)

2NO22 1 3H2 1 2H1 ? N2 1 4H2O

6. Overall denitrification reaction from NO32 to N2(g)

2NO32 1 5H2 1 2H1 ? N2(g) 1 6 H2O

7. Stoichiometric reaction among e2 donor, e2

acceptor, and biomassNO3

2 1 3.03H2 1 H1 1 0.229CO2 ?0.0458C5H7O2N 1 0.477N2 1 3.37H2O

Environmental Progress & Sustainable Energy (Vol.29, No.3) DOI 10.1002/ep October 2010 279

adjusted to pH 5 7.0 [14, 15]. The flask was com-pletely shacked for 1 h. Hydrogen and carbon diox-ide were fed to the flask by a fine bubble air stone at20 mL/min and 4 mL/min, respectively, for mixingand biochemical reaction and vented out at the top.This starter mixed culture of denitrifying bacteriawere anoxically grown for 7 days at 358C and nextused for bioreactor inoculation.

Denitrification BioreactorThe denitrification set-up is schematically repre-

sented in Figure 1. The pilot-scale plant used in thisstudy consisted of a Plexiglas cylindrical column (120cm high and 11.7 cm internal diameter). The bottomand top 10 cm of the bioreactor was used as inlet gasinjection and outlet zone, respectively. The columnwas equipped with sampling ports at inlet, outlet, 30

cm and 60 cm from the bottom and was packed with3–5 mm in diameter light expanded clay aggregates(LECA) with dry density and effective porosity of0.474 g/cm3 and 0.48, respectively. The starter liquidculture was then mixed by 7.5 L natural groundwaterthat synthetically polluted by 2 g/L KNO3 and trans-ferred to the bioreactor for biofilm formation. Thebioreactor coupled with the electrochemical gas gen-erator after inoculation.

In this way, hydrogen and carbon dioxide injectedat the bioreactor bottom via fine bubble air stone at200 mL/min and 40 mL/min, respectively, and ventedout at the top in batch status for 7 days at room tem-perature (18–238C). Then groundwater containing120 mg NO3

2/L (27 mg NO32-N/L) continuously fed

from the bottom by a peristaltic pump with differentresidence time (based on pore volume) from 25 to 2h (flow rate of 5 to 63.3 mL/min), respectively. The

Figure 1. Diagram of the constructed electrochemical gas generator and denitrification bioreactor. Numericalitems: (1) variable AC transformer, (2) rectifier, (3) capacitor, (4) digital voltmeter and ammeter, (5) electrolytesolution (methanol/water and KOH), (6) graphite plate (EK20), (7) carbon dioxide collector, (8) hydrogen col-lector, (9) gas flow meter, (10) gas injection isolated compressor, (11) methanol vapor trap comprised of gran-ules activated carbon and silica gel, (12) storage tank (nitrate containing water well), (13) peristaltic pump, (14)fine bubble air stone, (15) packed-bed bioreactor, (16) vent out gases, (17) sampling ports, and (18) effluentwater storage tank.


groundwater contains 0.19 mg NO22-N/L, 0.65 mg

PO432-P/L, 102.48 mg/L SO4

22, 79.04 mg/L Cl2 andpH: 7.8.

Electrochemical Gas GeneratorIn recent years, various chemical and electrochem-

ical methods have been proposed for hydrogen pro-duction [4, 16–19]. In this research, a very simple andcost-effective method for simultaneous and on-demand production of hydrogen and carbon dioxidefor drinking water treatment designed and coupledwith the denitrification bioreactor. This gas generatorcomprised four-compartment electrolytic cell, con-structed using a Plexiglas cube (height-width-lengthof 35–20–20 cm with 10 mm in thickness) and di-vided by four cells (10–10–35 cm). Anode and cath-ode electrodes were made of a high-density carbongraphite plate known as EK 20 (SGL group Inc.,Wiesbaden, Germany).

The four pair electrodes were identical with heightto width of 25 to10 cm and 10 mm in thickness bythe geometric area of 570 cm2 (immersed geometricarea was 527 cm2). The cathode and anode in eachcell were separated by 4 cm. The electrolyte solutionconsisted of methanol (Merck Inc., assay: 99.9%) anda conductivity enhancing agent including distilledwater and KOH (Merck Inc., assay: 99%) by themethanol/water volume ratio of 3:1. The concentra-tion of KOH in the electrolyte was 8 M. The gas gen-erator was powered with a variable DC power supply(0–35 amps and 0–60 V), which was equipped by adigital voltmeter and ammeter. The temperature andpH of the electrolyte was measured by a digital pHmeter. For collecting and injecting of gases to bio-reactor a special water trap designed (4 L volume).The gas continuously entered (by voltage and waterlevel adjusting) and delivered to bioreactor by a com-pressor that was equipped by a flow control valve.The pressure of gas holder was always adjusted tothe atmospheric pressure by controlling water level.A column gas drying system consisted of silica gelgranules, activated carbon, and absorbent cottonwool, accompanied by a glass condenser wasdesigned and connected to the gas generator toentrapment of any likelihood methanol vapor andcompletion of drying the gases.

Analytical MethodsThe nitrate, nitrite, and phosphate concentrations

were precisely and simply determined by DR/4000Hatch spectrophotometer. Nitrate was measuredaccording to methods 8171 and 8039 for medium andhigh ranges of 0 to 5.0 and 0 to 30.0 mg/L NO3

2-N,respectively. Nitrite was measured according tomethod 8507 for low ranges of 0 to 0.300 mg/LNO2

2-N. Phosphate was measured according tomethod 8048 for ranges of 0 to 2.500 mg/L PO4

32.The hydrogen concentration in liquid phase wasmeasured by a gas chromatograph (GC) equippedwith a molecular sieves of 5 A. Bacteria concentrationwas analyzed by the spread plate method [20]. Volt-

age and ampere also were measured by a digital volt-meter and ammeter.

RESULTS AND DISCUSSION

Gas Generator SystemIn an extensive review of the most researches in the

field of hydrogenotrophic denitrification of drinkingwater, it has been shown that H2 and CO2 have beenseparately supplied with different methods such aspressurized cylinders which are not economical inactual scale [4, 14, 15, 21, 22]. At present, there are fourhydrogen production categories: biological, chemical,electrochemical (water electrolysis; photoelectrochem-ical; halide electrolysis; H2S electrolysis), and thermaltechnologies [23]. The focus of these processes is onlyon the generation of hydrogen as an energy source.Electrochemical generation of hydrogen is suitable forapplication in the processes that need high purityhydrogen production at room temperature and requirea quick start up and shut down. This process hasfocused on water electrolysis that split water intohydrogen and oxygen gases. In denitrification, the oxy-gen gas is detrimental for the process and it has to beseparated from the products. As a result using organicsubstances as the electrolyser feedstock is very suitablebecause the products consist of pure hydrogen andcarbon dioxide [23, 24]. Methanol electrolysis is thebest choice for simultaneous generation of hydrogenand carbon dioxide for application in hydrogenotro-phic denitrification. Using methanol as the electrolyserfeedstock can tremendously reduce the electricity con-sumption because the standard potential is only 0.02 Vversus NHE for methanol oxidation compared with1.23 V for water. Even when the cost of methanol isincluded, the cost of hydrogen production using elec-trolysis of methanol is still only about half of waterelectrolysis [17, 24]. Moreover, pure hydrogen and suf-ficient carbon dioxide suited it for application in simul-taneous supplying of these harmless gases in denitrifi-cation of drinking water.

The results have shown that methanol cannot beelectrolyzed into H2 and CO2 alone using DC voltage.In that case, a conductivity enhancing agents such aspotassium hydroxide (KOH) solution is required. Tooptimize the process, influence of several factors,including methanol–water ratio, KOH molarity, H2

and CO2 production rate, energy consumption, vari-ous conductivity enhancing agents, and temperature,on the process were investigated. The optimum con-ditions were determined to be 3:1 and 8 M as metha-nol–water ratio and KOH molarity, respectively.

The theoretical electrolytic reaction of methanol(CH3OH(l) 1 H2O(l) ? CO2(g) 1 3H2(g)) indicate that1 mole of methanol generate 3 mol H2 and 1 moleCO2, whereas in the presence of KOH the amount ofCO2 is lower than its theoretical value due to its con-version to CO3

22 and HCO32. Nevertheless, the

amount of CO2 transferred into the bioreactor pro-vided a ratio of 5:1 for H2:CO2 which was sufficientfor the denitrification reaction.

Figure 2 shows the dependence of flow rate of H2

and CO2 in the electrolytic cell on current density.


The theoretical hydrogen and carbon dioxide produc-tion rate at each current density is shown as a straightline (Faraday’s law). The hydrogen flow rateincreased in proportion to the current density andhas a good agreement with the theoretical hydrogenproduction rate. This indicated that hydrogen wasproduced effectively. The flow rate of carbon dioxidewas less than the theoretical production rate (H2:CO2

5 3:1) due to partial conversion to carbonate and bi-carbonate. The experimental mole ratio of hydrogento carbon dioxide production rate was about 5:1.

The lower required external energy, sufficient andon-demand providing of required gases in denitrifica-tion bioreactors without any danger and safety hazardmeans that methanol electrolysis is a cost effective,easy to use, and safe procedure for application todrinking water treatment in the practical scales.

Denitrification BioreactorTo operate the bioreactor, a gas flow rate (H2 1

CO2) of 93–186 mL/min (at STP condition) was pro-vided in 4–6 V, 10–20 A in which the dissolvedhydrogen concentration was held in the range of 0.2to 0.3 mg/L [14]. The gas generator was operatedcontinuously for more than 160 day. The bioreactorhydraulic regime was changed from batch to continu-ous mode (HRT 5 25 h) after 7 days operation whenbiofilm was being formed. Effluent nitrate was notdetectable after 25 days. There was no detectablechange in sulfate and chloride level while phosphatewas decreased from 0.65 to 0.21 mg PO4

32-P/l, result-ing from biomass growth in the bioreactor.

The system was then continuously operated withdifferent retention time from 25 to 2 h in a period of160 days. At the beginning of the operation (about 25days), nitrate removal changed from 85% to 95% dueto denitrifiers acclimatization and formation in the start

up period (see Fig. 3). Whereas from 25th day high re-moval efficiency (95–100%) was achieved for the influ-ent containing 120 mg NO3

2/L. The pH fluctuation inthe effluent was very low (from 7.5 to 8), which indi-cated carbonate alkalinity supplement by carbon diox-ide dissolved in the bioreactor. The denitrificationreaction is known to be differentially sensitive to pH[25, 26]. In such a process, as pH increases, nitriteincreases as well and subsequently the accumulationof nitrite causes an inhibition to denitrification reac-tion. However, in this process owing to buffering envi-ronment provided by CO2, the nitrite production isceased. Nitrogen gas produced from the process isstripped by blowing hydrogen and carbon dioxidegases. Figure 4 indicated dissolved hydrogen concen-tration profile along the bioreactor length in steadystate conditions (e.g., over days 145–150).

The maximum denitrification rate was 338.7 g/m3dNO3

2-N which was achieved for an influent nitrateconcentration of 27 mg/L. It was 250 g NO3

2-N/m3.dfor Denitropur process in commercial scale of denitri-fication plant utilizing hydrogen and the rate is withinthe range as reported by the other investigators(Table 2) [31].

Figure 2. Dependence of hydrogen and carbon diox-ide production rate in electrolytic cell on current den-sity. The solid and dash lines in the figure mean thetheoretical hydrogen and carbon dioxide productionrate, respectively. The experimental production ratesare (*) hydrogen and (h) carbon dioxide.

Figure 3. Nitrate and nitrite concentration history inthe coupled system operated at room temperaturewith different residence time.

Figure 4. Hydrogen concentration profile along thebioreactor length.


System Design and Operation ModelingTo develop a model governing the coupled electro-

chemical gas generator and packed bed bioreactormass balances for hydrogen, nitrate, and nitrite in thebioreactor and hydrogen supply rate from gas genera-tor were done. It was assumed that biomass is notincluded in the mass balance as justified by the lowbiomass yield (only 0.0458 mol C5H7O2N per moleNO3

2 reduced as shown in Table 1). The stoichiometricconsumption ratio of hydrogen to nitrate during nitratereduction and nitrite during nitrite reduction to N2 gasis 1/7 and 3/14 (mg H2/mg N), respectively.

Equation 1 determine hydrogen utilization rate(HUR) based on mass balance:

HUR ¼ Q½5ðN3;i � N3;oÞ � 3N2;o�=14 (1)

In this equation, HUR is the rate of hydrogen utili-zation by the biofilm (mg H2/d), Q is the influentflow rate (L/day), N3,i is the influent nitrate concen-tration (mg N/l), N3,o is the effluent nitrate (mg N/L),and N2,o is the effluent nitrite concentration.

In this process, N2,o is very low and can be elimi-nated from the equation then we have Eq. 2:

HUR ¼ 5QðN3;i � N3;oÞ=14 (2)

On the other hand, H2 and CO2 generation in elec-trochemical reactor is a faradaic process and theamount of gases can easily calculated from Faraday’slaw [32, 33]:

C ¼ nFP (3)

In this equation, C is quantity of electricity orcharge in coulombs (C), n is the number of electronstransferred per mole of product, F (faraday constant)is the quantity of electricity carried by 1 mole of elec-trons (Avogadro’s number3 charge on electron incoulombs 5 6.022 3 1023 mol21 3 1.602192 3 10219

C 5 96,484 C/mol), and P is moles of product.The anode, cathode, and overall reactions that

occur in electrochemical cell are:

CH3OHð1Þ þ H2Oð1Þ ! CO2ðgÞ þ 6HþðaqÞ þ 6e� ðanode;oxidationÞ

6HþðaqÞ þ 6e� ! 3H2ðgÞ ðcathode; reductionÞ

CH3OHð1Þ þH2Oð1Þ ! CO2ðgÞ þ 3H2ðgÞ ðoverall reactionÞ

Based on these equations hydrogen generationrate [HGR (mg)], can be calculated based on electricalcurrent in amps (A) and time (s):

HGR ðmoleÞ ¼ ðI 3 tÞ=ð2 3 96; 484Þ (4)

HGR ðmg=dayÞ¼ ðI 3 86; 4003 10003 2Þ=ð23 96; 484Þ ¼ 895:51 ð5Þ

The amount of hydrogen supplied to the bioreac-tor was utilized by biofilm. Hydrogen transfer fromgaseous phase to biofilm is a function of many varia-bles such as characteristics of water, temperature, re-actor geometry, pressure, and diffuser efficiency thatcan be simplified in one parameter named hydrogentransfer efficiency (HTE). HTE is a key parameter ofthe systems that should be determined in a batch testunder local conditions. Because of low solubility ofH2 in water (H2 saturation concentration in water is

Table 2. Comparison of hydrogenotrophic denitrification systems.

Microorganisms CarriersReactortype

Removalrate

(g NO32 2

N/m3 d )

Influentconcentration

(mg NO32

2 N/l)Temperature

(8C) Reference

Mixed Plastic Fixed bed 400 17 10.5 27Mixed Sand Fluidized

bed130 25 30 28

A. eutrophus PU spongecubes

Fixed bed 200 50 12–20 29

Seeded PU spongestrips

Fixed bed 500 15 12–20 29

Mixed Charcoal Fixed bed 310–340 18–20 15 30Mixed Polyacrylamide Fluidized

bed600–700 22–25 30 14

Rhodocyclus sp. Pea gravel Fixed bed 343 28 20 4Mixed Hollow-fiber HFMB 552 25 20 31Mixed LECA Fixed bed 339 27 18–20 Current

study


1.6 mg/L at 208C and 1 atm) many studies hasfocused on H2 provision scheme such as HFMB reac-tors [31] or separated hydrogen saturation tanks. Thehigher HTE and subsequent lower H2 generation canresult in a significant decreasing in expenditure ofenergy and operation costs.

By combining Eqs. 2 and 5, we can control thesystem operation based on a very easy measurableand adjustable parameter such as electrical current(HUR 5 HGR 3 HTE).

I ¼ 5QðN3;i � N3;oÞ=ð12; 537 3 HTEÞ (6)

Electrical energy as kilowatt-hour per day can becalculated based on current I (amps) and voltage V(volts):

EðKWH=dÞ ¼ ðI 3 V 3 86; 400Þ=ð3:6 3 106Þ (7)

In this study, the HTE was 22% and system oper-ated at 10 A, 4 V that H2 generation rate was 373.12mg H2/h (4179 mL H2/h at STP). Electrical energyneeded for production of 1 kg H2 was 107 kilowatt-hours that is very low and cost effective. For ground-water in current study (27 mg NO3

2-N/l) complete re-moval of nitrate (HTE 5 22%) was achieved by a1.87 KWH energy per cubic meter of water. Further-more, methanol and KOH consumption in the systemis about 7 L and 4 kg KOH per 1 kg H2 produced,respectively. The amount of nitrate removed per 1 kgH2 produced is 508 g NO3

2-N (based on Table 1 andHTE 5 22%) that is sufficient for complete removalof nitrate from 19 m3 of water. In other words, treat-ment of 30 m3 of this polluted water (to standardlimit in drinking water) requires 1 kg H2 that pro-duced by consumption of 7 L methanol, 4 kg KOH,and 107 kilowatt-hours electrical energy. The life timeof electrodes is very high because of low currentdensity and negligible corrosion. Therefore, the costof electrodes should be considered in constructioncosts.

It should be noted in the practical scales therequired energy can be provided from sustainableresources such as wind and solar energy. These logi-cal developments are easily applicable in the systembecause of low-energy demand of the system. Theflexibility of energy provision scheme of the systemand considerable decreasing of operation costmade it suitable for undeveloped areas with energylimitations.

CONCLUSIONS

Autotrophic denitrification in spite of many advan-tages in the field of drinking water treatment couldnot be applied in full scale with the exception of fewcases and there is still a need for further studies. Theintegrated methanol electrolysis/bioreactor systemconstructed for this study can prevail over these limi-tations and have the applicability for design in the

actual scales. Treatment is accomplished using bacte-ria that are readily found in drinking water aquifersand naturally present in nitrate contaminated ground-water. Electrochemical reactor can generates harmlessand on-demand electron donor and inorganic carbonsource for denitrifying bacteria. Evaluation of the sys-tem over 160 days showed, for nitrate contaminateddrinking water, removal efficiencies above 95% byHRT of 2 h can be achieved. The injected carbondioxide eliminates pH fluctuation in the system andnitrite accumulation ceased in stable hydraulic and bi-ological conditions. The maximum denitrification rateachieved for influent nitrate concentration of 27 mgNO3

2-N/L was 338.7 g NO32-N/m3�d. This rate is

adequate for field application.The lower required external energy, sufficiently

providing required gases without any potential ofsafety hazards, easy system operation, and lower gen-eration of biosolids, made it suitable in effectivewater treatment used for drinking water supplieswithout any harmful chemical addition. Accomplish-ment of the feasibility study under local conditions,disinfection testing and long-term monitoring of mi-crobial community structures are the logical nextsteps.

ACKNOWLEDGMENTS

Authors would like to thank Iran Water ResourcesManagement Co. (Ministry of Energy) for funding thisproject (Project No. ENV1-85052). The great coopera-tion of all environmental engineering division ofTarbiat Modares University is appreciated.

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