Bioresource Technology 98 (2007) 3521–3525

Electrochemical treatment of anaerobic digestion effluentusing a Ti/Pt–IrO2 electrode

Xiaohui Lei a,*, Takaaki Maekawa b

a Doctor’s Program in Life and Environmental Science, University of Tsukuba, 31-207 Ichinoya, 2-1 Tennodai, Tsukuba, Ibaraki 3050006, Japanb Graduate School of Life and Environmental Science, University of Tsukuba, Japan

Received 26 July 2006; received in revised form 9 November 2006; accepted 11 November 2006Available online 4 January 2007

Abstract

Electrochemical treatment of the anaerobic digestion effluents using a Ti/Pt–IrO2 electrode was evaluated in this study. The effects ofelectric current, NaCl dosage, and initial pH on ammonia, nitrate, total organic carbon (TOC), inorganic carbon (IC), final pH, andturbidity variations were studied in a series of batch experiments. It was found that the electric current and NaCl dosage had a consid-erably larger effect on the oxidization of ammonia; this was less for the effect of the initial pH. In addition, electroflotation was the mainmechanism for turbidity, TOC, and IC removals. Further, the IC removal was mainly affected by the pH of wastewater. The electrochem-ical treatment using Ti/Pt–IrO2 electrode without pretreatment was feasible for the anaerobic digestion effluent.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical; Electroflotation; Ammonia removal; pH effect

1. Introduction

The anaerobic digested effluents typically contain highamounts of ammonia, phosphate, total suspended solids(TSS), and persistent organic substrate—1000–3000 mg/Lof ammonia, 100–500 mg/L of ortho-phosphorus, and15000–25000 mg/L of TSS. Digested effluents have beenused as fertilizers for recycling nutrients in agriculturalfields (Salminen et al., 2001). However, the excessive appli-cation of digested effluents is the probable cause of nitro-gen pollution in farming areas (Woli et al., 2004). Inaddition, this method is unsuitable for urban areas becauseof the unpleasant odor of the effluent and the limited agri-cultural area. A simple and effective process for removingnitrogen and residual organic substances is required forthe posttreatment of the anaerobic digestion effluents.

The high ammonia, phosphate, and TSS contents aregenerally difficult of access to conventional biological

0960-8524/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2006.11.018

* Corresponding author. Tel.: +81 805 412 1741; fax: +81 29 853 7496.E-mail address: rain_fields@hotmail.com (X. Lei).

treatment processes such as activated sludge process (Bat-tistoni et al., 1997; Li and Zhao, 1999) soil trench system,etc. (Bouwer and Chaney, 1974). In addition, a relativelylow COD/TN ratio (1–3) is insufficient to facilitate effi-cient TN removal (Meinhold et al., 1998). Physicochemi-cal methods such as ammonia stripping (Liao et al.,1995;Cheung et al., 1997; Bonmati and Flotats, 2003)magnesium ammonium phosphate hexahydrate method(MAP) (Maekawa et al., 1995; Suzuki et al., 2002; Leiet al., 2006) were generally used to pretreat wastewaterwith similar characteristics. However, these methods,ammonia stripping in particular, are generally not costeffective and are inadequate. Ammonia stripping methodrequires higher infrastructure investment due to theammonia stripping tower and higher running cost due tothe high temperature steam. In the MAP method, theexpensive reagents and the incomplete treatment contrib-ute to the costs. Generally, a subsequent purification suchas biological treatment is required. In our previousresearch, a soil trench system (Lei et al., in press) was usedto treat an anaerobic digestion effluent that was pretreatedby ammonia stripping.

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Fig. 1. Effect of electric current on pollutants removal (unadjusted initialpH, 1% NaCl): solid line for 1.0 A, dotted line for 0.5 A; n for ammonia,s for Nitrate, j for TOC and r for IC.

3522 X. Lei, T. Maekawa / Bioresource Technology 98 (2007) 3521–3525

In this study, electrochemical treatment was used totreat the anaerobic digestion effluent using a Ti/Pt–IrO2

electrode. The feasibility and main parameters influencingthe performance of this process were evaluated.

2. Methods

2.1. Characteristics of wastewater

In 2005, an anaerobic digestion energy plant using pigexcreta and kitchen garbage as the substrate was built inKoga city, Japan. The plant produced about 200 m3 of bio-gas and 5 m3 of digested effluent daily from a two-phaseplug flow anaerobic digestion reactor. The digested effluentobtained from the plant was stored at 4 �C prior to theexperiments. The characteristics of the effluent were ana-lyzed before the experiments were conducted. The pH ofthe wastewater was 7.7, and the concentration of TotalNitrogen (TN), ammonia, nitrite, nitrate, Total OrganicCarbon (TOC), Inorganic Carbon (IC), TSS and Turbiditywere (mg/L) 1770, 1124, 0, 81, 2549, 1122, and 25000 and3249 FAU, respectively.

2.2. Electrochemical experiments

Based on the preliminary experiment using 500-mLbeakers, a cylinder electrochemical cell (height: 140 mm;radius: 70 mm) with cap was used for treating the anaero-bic digestion effluent. There was a gap in just below the capin the electrochemical cell, which was used to dischargefoam produced in the electrochemical reactions. The Ti/Pt–IrO2 electrode (100 mm · 43 mm) was used as theanode, and a stainless steel electrode of the same size wasused as the cathode. After 300 mL of wastewater waspoured into the electrochemical cell, reagents were addedand pH was adjusted before the experiments. The effectivearea of electrode was 33 cm2, and the distance between thetwo electrodes was 1 cm. The effect of the electric currents,pH, and NaCl dosages on electrochemical treatment effi-ciency was studied using batch experiments at about 25 �C.

2.3. Analytical method

The analytical procedures for the determination of pH,TN, ammonia, nitrite, nitrate and TSS of wastewater wereconducted according to Standard Methods (APHA, 1998).The turbidity was assayed at 860 nm by using AttenuatedRadiation Method (HACH, 2003) with a Hach DR4000spectrophotometer. The TOC and IC contents of thewastewater were measured using a Shimadzu TOC5000A.

3. Results and discussion

3.1. Effect of electric current

In the experiments for evaluating the effect of electriccurrent, 1% NaCl was added into wastewater before each

experiment and electric currents of 0.5 A and 1 A wereapplied respectively. The variations of residual nitrogenin the reactions are shown in Fig. 1. It was shown thatammonia could be completely removed in 5 h using an elec-tric current of 1 A; however, 620 mg/L of ammonia stillremained when an electric current of 0.5 A was used. Dur-ing the removal of ammonia, although some of the ammo-nia was transformed into nitrate (nitrite was undetectablein all samples), a major portion of it was transformed intonitrogen gas. Ihara et al. (2006) and Vlyssides et al. (2002)also indicated that some nitrate was produced with theoxidization of ammonia.

The ammonia removal was mainly accomplished by theoxidization by electrogenerated hypochlorous acid (HClO)(Comninellis and Nerini, 1995). Besides ammonia, organiccarbon can also oxidized into inorganic carbon by hypo-chlorous acid (Feng et al., 2004). The variations of residualcarbon in the reactions are also shown in Fig. 1. It wasreported that N2 and CO2 were produced during the oxidi-zation of ammonia and TOC by hypochlorous acid (Fenget al., 2003). And, the CO2 in the inorganic carbon wasthen stripped out by electrogenerated gas (H2, Cl2, O2,etc.).

Electroflotation is a separation treatment process thatuses small bubbles to remove low density particulates frompotable water and wastewaters (Ahmed and Jameson,1985; Ketkar et al., 1991; Khelifa et al., 2005; Santoset al., 2006). In this method, small gas bubbles with diam-eters ranging from 22 to 50 lm (Ketkar et al., 1991) aregenerated at the surface of electrodes. These bubbles thenrise to the surface of the liquid and act as collectors of fineparticles in the solution. The electroflotation process isused as an alternative to sedimentation for removing lowdensity materials like clays or algae (Burns et al., 1997;Murugananthan et al., 2004).

These results showed that the electroflotation removednot only turbidity but also the TOC contained in thedischarged foams. However, it was unclear whether the

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TOC removal was mainly achieved by either of these reac-tions (oxidization, electroflotation) or both. The effect ofelectric current on turbidity removal was also evaluated(data not shown). It was observed that in the electrochem-ical treatment process, black foam was produced anddischarged from the gap, the wastewater color lightenedgradually, and the treated wastewater turned almost trans-parent finally for the 1 A electric current. The reactions atthe anode and cathode were both electric current depen-dent, and it was observed that relatively higher nitrogen,carbon, and turbidity removal efficiencies were obtainedusing an electric current of 1 A.

It has been reported that suspended solids containedin wastewater impede the electrochemical oxidation (Kimet al., 2003). Ihara et al. (2006) employed filtration anddilution to remove the TSS before ammonia oxidizationby the electrochemical treatment using the Ti/RuO2 + IrO2

and Ti/PbO2 electrodes. However, dilution and filtration inthe pretreatment was not used in this work and a lowerelectric current was used, and nearly twice of the ammoniawas removed. In addition, a higher turbidity removal wasalso achieved. Hence, it was affirmed that the direct treat-ment of the anaerobic digestion effluent with the Ti/Pt–IrO2 electrode not only was possible but also could achievegood treatment efficiencies.

3.2. Effect of NaCl dosages

The indirect oxidization of ammonia and organic car-bon was mainly achieved using electrogenerated hypochlo-rous acid; therefore, the treatment efficiency was highlyrelated to the NaCl dosage. The relationship between theNaCl dosages and the ammonia removal efficiency isshown in Fig. 2. The ammonia removal efficiency wasgreatly improved with the addition of NaCl but the differ-ence between the different dosages was relatively small.These results indicated that a 1% NaCl dosage was almostsaturated. Therefore, this dosage could be regarded suffi-

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Fig. 2. Effect of NaCl dosages on pollutants removal (unadjusted initialpH, 1 A): · for 0% NaCl, s for 1% NaCl, n for 2% NaCl and h for 3%NaCl; dashed line for ammonia, solid line for TOC and dotted line for IC.

cient for ammonia removal. The result was comparableto that obtained by Ihara et al. (2006).

The NaCl dosage effect on TOC and the IC removal areshown in Fig. 2. An interesting phenomenon was thatunlike ammonia oxidization, the TOC removal maintainedalmost the same level for all dosages of NaCl, including noNaCl dosage. The independence of the TOC removal onNaCl dosages showed that most of the TOC removalsmight not be achieved by the oxidization of the electrogen-erated hypochlorous acid. In addition, the continuousdecrease of IC also indicated that the TOC might not betransformed into IC by the oxidization, although IC wassimultaneously discharged from the gap by the electrogen-erated gas. Therefore, the TOC removal might be mainlyachieved by electroflotation that is dependent on the elec-trogenerated gas production rate, which was determinedby the electric current. Therefore, the TOC and the IC rem-ovals were determined by the electric current (Fig. 1). Therelatively low IC removal in the absence of NaCl dosage(Fig. 2) was caused by the higher pH of the wastewater pro-duced. Because the form of the IC is determined by the pHaccording to the following reaction, a lower pH wouldincrease the percentage of CO2 in IC. Therefore, a greaterquantity of IC could be discharged from the acidic waste-water, vise versa.

CO2 þH2O() HCO�3 þHþ

Fig. 3 shows the effect of NaCl dosage on the pH varia-tions of wastewater. In the absence of the NaCl dosage, thepH increased gradually. This indicated that the cathodereaction was predominant, which produced more OH�,thereby increasing the pH. However, with the addition ofNaCl, the pH increased gradually first and finallydecreased to almost the same level as that of the originalwastewater. This indicated that the anode reaction pre-dominated the second half reaction period, which producedmore H+ thereby decreasing the pH. For all NaCl dosages,higher turbidity removals were achieved entirely, although

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a relatively low turbidity removal was observed in theabsence of a NaCl dosage (data not shown). Based on theseresults, a 1% NaCl dosage was regarded as sufficient forammonia, TOC, and turbidity removals. This will be usedin the following experiments.

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Fig. 5. Effect of initial pH on pH variations (1% NaCl, 1 A): s for initialpH of 2.6, d for initial pH of 4.6, n for initial pH of 6.6, m for initial pHof 7.7, h for initial pH of 9.0 and j for initial pH of 11.6.

3.3. Effect of initial pH

Free chlorine is defined as the concentration of residualchlorine in water present as dissolved gas (Cl2), HClO, and/or hypochlorous acid ions (OCl�). Three forms of freechlorine exist together in equilibrium. Their relative pro-portions are determined by the pH value and temperature;free chlorine is the most effective at pH of 5–7 where HClOis the predominant form (Cole, 1987). Fig. 4 shows theeffect of initial pH on ammonia removal. Although betterammonia removal efficiency was achieved below the pHs6.6 and 7.7, which was consistent with the previous theory,those below other pHs did not have a bad ammoniaremoval efficiency. In addition, ammonia was also removedby ammonia stripping below pH 9.0 and 11.6. Althoughhigher effect of initial pH on ammonia removal wasobtained by Vlyssides et al. (2002), present results indicatedthe negligible effect of initial pH on the ammonia removal,which might be caused by different anode material, waste-water etc.

Fig. 4 also shows the effect of initial pH on TOC and ICremovals. Based on the previous analysis that TOC wasmainly removed by electroflotation, the higher TOCremoval efficiencies for the pH 2.6 and 4.6 were causedby the increased Cl2 that was produced as electrogeneratedgas. This was because more Cl2 existed in the form offree chlorine below this pH range. Similarly, lower TOCremoval efficiencies below the pH of 9 and 11.6 were causedby the low Cl2 in the free chlorine in this pH range. Fur-ther, as analyzed previously, the IC removal was alsomainly determined by the pH of the wastewater. It was

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Fig. 4. Effect of initial pH on pollutants removal (1% NaCl, 1 A): s forinitial pH of 2.6, d for initial pH of 4.6, n for initial pH of 6.6, m forinitial pH of 7.7, h for initial pH of 9.0 and j for initial pH of 11.6;dashed line for ammonia, solid line for TOC and dotted line for IC.

observed that the IC below an initial pH of 2.6 and 4.6decreased to about 60 mg/L in 1 h and maintained a lowlevel. The IC below the initial pH of 11.6 maintained analmost identical level during the electrochemical treatmentprocess.

The pH variations for different initial pH are shown inFig. 5. Below the pH of 9.0 and 11.6, the pH of the waste-water decreased gradually. The waste water under lowerpH exhibited similar pattern of an increase and thendecrease, although their amplitudes differed. Below thepH of 4.6 and 6.6, the amplitudes were much larger andthe final pH was about 2.2 for an initial pH of 4.6. Forall pH, higher turbidity removals were achieved com-pletely, although a relatively low turbidity removal wasattained for the initial pH of 11.6 (data not shown).

Based on these results, an unadjusted pH presented aconsiderably suitable engineering application. In addition,because of the less effect of pH on ammonia removal, sub-sequent or simultaneous treatments could be applied toachieve a significantly better treatment. These treatmentsincluded that the initial pH of the wastewater was adjustedto the acidic range (4–5), then the final pH of the treatedwastewater decreased to about 2–3, which just met the opti-mal pH for Fenton’s oxidization (Kang and Hwang, 2000).Therefore, further TOC removal could be expected byapplying Fenton’s oxidization subsequently. Further, afterthe Fenton’s oxidization, the pH of the treated wastewaterwould increase close to neutral. In addition, if the initialpH of wastewater was adjusted to around 11–12, and aer-ation was also applied, a higher ammonia removal could beexpected by a combination of ammonia oxidization andammonia stripping.

4. Conclusions

Electrochemical treatment of anaerobic digestion efflu-ents using a Ti/Pt–IrO2 electrode was feasible withoutany pretreatment. Using a 1 A electric current and a 1%

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NaCl dosage, ammonia could be completely removed inabout 5 h, although some nitrate was produced. TOC,IC, and turbidity removals reached 51.4, 73.8, and 95.5,respectively.

Acknowledgement

This research was supported by the Strategic Interna-tional Cooperative Program, JST.

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