Bioresource Technology 97 (2006) 1360–1364

Electrochemical oxidation of the effluent fromanaerobic digestion of dairy manure

Ikko Ihara a, Kazutaka Umetsu a,*, Kiyoshi Kanamura b, Tsuneo Watanabe b

a Department of Agro-Environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro 080-8555, Japanb Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji 192-0397, Japan

Received 16 February 2005; received in revised form 7 July 2005; accepted 11 July 2005Available online 25 August 2005

Abstract

The electrochemical oxidation of the digested effluent from anaerobic digestion of dairy manure was investigated in this study.The digested effluent sample containing with suspended solids was pretreated by filtration for the electrochemical experiment. Theinfluence of direct anodic oxidation and indirect oxidation was evaluated through the use of dimensionally stable anode (DSA) andTi/PbO2 as anode. The decreasing rate of chemical oxygen demand (COD) was higher at lead dioxide coated titanium (Ti/PbO2)electrode than at DSA, however the DSA was preferred anode for the decrease of ammonium nitrogen (NH4-N) due to the controlof ammonium nitrate (NO3-N) accumulation. The results showed that the filtration of suspended solids as a pretreatment and addi-tion of NaCl could improve the whole removing efficiency of NH4-N in the digested effluent on electrochemical oxidation.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Ammonium nitrogen; COD; DSA; Effluent from anaerobic digestion; Electrochemical oxidation

1. Introduction

Waste management has been widely recognized as aserious problem for livestock production. Anaerobicdigestion has become an option for sustainable treat-ment of livestock manure, converting it to biogas andeffluent. Digested effluent from anaerobic digestion oflivestock manure usually contains high strength ofammonium nitrogen (NH4-N) and persistent organicsubstances. The components in digested effluent hadbeen applied as fertilizer for recycling of nutrients backto agricultural field (Salminen et al., 2001; Umetsu et al.,2002). The excessive spreading of livestock manure onthe field should be attributable to nitrogen pollution infarming areas (Woli et al., 2004). A simple and effectiveprocess for removing nitrogen and residue organic sub-

0960-8524/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2005.07.007

* Corresponding author. Tel.: +81 155 49 5515; fax: +81 155 495519.

E-mail address: umetsu@obihiro.ac.jp (K. Umetsu).

stances is required as a posttreatment of the effluentfrom anaerobic digestion. The electrochemical oxidationtreatment of various wastewaters has been investigatedin recent years. Both organic pollutants and NH4-N inwastewater containing chloride can be destroyed electro-chemically (Chiang et al., 1995). In this work the effluentfrom anaerobic digestion of dairy manure was treatedby the application of electrochemical oxidation. Thepurpose of this study was to identify the main para-meters influencing the performance of an electrochemi-cal oxidation process.

2. Methods

2.1. Anaerobic digestion

The digested effluent used in this study was collectedfrom a full-scale anaerobic digester of ObihiroUniversity of agriculture and veterinary medicine

I. Ihara et al. / Bioresource Technology 97 (2006) 1360–1364 1361

(Hokkaido, Japan). A 60 m3 anaerobic digester was in-stalled next to free stall barn and was operated withdairy manure slurry at a digester temperature of 55 �C.In the coldest season, average biogas production was150 m3/day, consisting of 56% methane gas with an aver-age loading rate of 6.75 kg/m3/day which established ahydraulic retention time of 13 days at an average ambi-ent temperature of �15 �C and slurry temperature of2 �C.

2.2. Pretreatment for digested effluent

To remove suspended solids, the effluent sample waspretreated with membrane filters. After the filtrationwith a nylon membrane filter (pore size: 37 lm), thesample was filtered with a hollow fiber membrane (poresize: 5.0 or 0.5 lm). All filtrated samples were diluted 1:2with distilled water before electrochemical oxidationtreatment.

3.0

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0.0

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12.0

pH

E

E (

V)

pH (

-)

2.3. Experiments of electrochemical oxidation

The electrochemical oxidation experiment was con-ducted in a glass beaker, equipped with a 100 · 50 mmmesh anode and a plate cathode. The anodes were adimensionally stable anode (DSA) based on mixed oxi-des of RuO2 + IrO2 and lead dioxide coated titanium(Ti/PbO2) electrode. The stainless steel was used as cath-ode. They were placed vertically and parallel to eachother with an electrode gap of 10 mm in a beaker. Theelectrochemical oxidation was carried out at a constantcurrent of 1.5 A using a DC power supply. The samplesolution was agitated by a magnetic stirrer. The surfacebubbles were recycled by a peristaltic pump forantifoam.

0

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400

0 2 4 6 8 10

0

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time (h)

Cl- (

mg/

L)

CH

3CO

OH

(m

g/L

)

COD CH3COOH

NH4 -N NO3 -N ClO -

Cl -

CO

D (

mg

/ L)

NH

4-N

, NO

3-N

, ClO

- (m

g/L

)

Fig. 1. Electrochemical oxidation at DSA with addition of 0.5 g NaCl.

2.4. Analytical method

Chemical oxygen demand (COD) was determined bydichromate method. Ammonium nitrogen (NH4-N)was determined using salicylate reaction. The concen-trations of these analytical parameters were measuredby a HACH DR4000 spectrophotometer. Ammoniumnitrate (NO3-N), chloride ion, hypochloride ion andacetic acid were analyzed by capillary electrophoresis(CE) system (Agilent Technologies, G1600A). Thebasic anion buffer and a fused silica capillary with104 cm in length and 50 lm internal diameter wereobtained from Agilent technologies. The temperaturecontrolled cartridge for fused silica capillary was setat 30 �C and the applied dc voltage was �30 kV. Thewavelength of diode array detector was set at 350 nm(signals)/275 nm (reference). Before the CE analysis,the effluent sample was pretreated by 0.45 lm pore sizemembrane filter.

3. Results and discussion

3.1. Variation of parameters during electrochemical

oxidation at DSA

The diluted sample with 0.5 g of NaCl, pretreated by0.5 lm membrane filter was tested for the electrochemi-cal oxidation using a DSA (Fig. 1). The concentration ofCOD was reduced by 32% in 9 h. In a previous work,the electrochemical oxidation applying for wastewatertreatment was explained by a direct anodic oxidationor an indirect oxidation (Chiang et al., 1995). In thedirect anodic oxidation, the organic pollutants weredestroyed on oxide anode by electrochemical conversionor combustion (Comninellis, 1994). In the indirectoxidation, the electrogenerated oxidant such as hypo-chlorite (Comninellis and Nerini, 1995) or peroxodisul-phates (Canizares et al., 2003) destroyed the pollutantsin the bulk solution. The concentration of acetatewas increased consistently whereas the COD wasdecreased from the beginning. The result showed thatthe electrochemical oxidation at DSA for the wastewatercontained chloride had low degradability to acetic acid.The NH4-N was decreased rapidly with time. Thedecrease of NH4-N could be explained by indirect oxida-tion with hypochlorite.

0

2000

4000

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8000

0 2 4 6 8 10

0

100

200

300

400

COD (DSA)

COD (Ti/PbO2)

NH4-N (DSA)

NH4-N (T i/PbO2)

NO3-N (DSA)

NO3-N (T i/PbO2)

conc

. (m

g / L

) co

nc. (

mg

/ L)

time (h)

Fig. 3. Effect of anode material for the decrease of NH4-N and CODduring electrochemical oxidation with addition of 0.5 g NaCl.

1362 I. Ihara et al. / Bioresource Technology 97 (2006) 1360–1364

Hypochlorite is generated as the product of hydro-lysis of chloride. The decreasing process of NH4-Nmight be regarded as similar to the chemistry of the Sel-leck-Saunier breakpoint phenomenon (Chiang et al.,1995; White, 1998). The electrochemically generatedhypochlorite was consumed to produce nitrogen fromammonium ion. Since the sample contained 1496 mg/Lof chloride ion at the start of the experiment and DSAhad high catalytic properties for chlorine evolution(Trasatti, 2000), hypochlorite was effectively producedand responsible for NH4-N decrease during the electro-chemical oxidation. The data showed that NH4-N wasvanished in 8 h, and then hypochlorite ion was detected.

3.2. Influence of NaCl addition

Fig. 2 showed the influence of the chloride ions onelectrochemical oxidation using DSA. The diluted sam-ple used in the experiments contained approximately700 mg/L of chloride ion. When 0.5 or 2.0 g of NaClwas added, the decreasing rate of NH4-N was increaseddramatically. The increased concentration of chlorideion had positive influence for the enhancement in therate of NH4-N decrease. An increase in the concentra-tion of chloride ion produced a high generation rate ofhypochlorite. The result also indicated that the dilutedsample of digested effluent had only a low concentrationof chloride ion for the decrease of NH4-N on electro-chemical oxidation. To achieve an economical opera-tion, the addition of adequate amount of chloride ionwas required for the digested effluent.

In contrast, the addition of NaCl had less effective forthe decrease of COD during electrochemical oxidation.The generated amount of hypochlorite increases withhigh initial concentration of chloride ion on constantcurrent electrolysis. The result indicated that the indirectoxidation caused by electrogenerated hypochlorite was

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8

no addition

0.5 g NaCl added

2.0 g NaCl added

time (h)

NH

4 -N

(-)

Fig. 2. Effect of NaCl addition for the decrease of NH4-N duringelectrochemical oxidation at DSA.

not predominant in the degradation of organics in thedigested effluent.

3.3. Comparison of anode materials between

DSA and Ti/PbO2

The influence of anode material on electrochemicaloxidation was examined with DSA and Ti/PbO2 anodefor the decrease of NH4-N and COD (Fig. 3). Thedecreasing rate of NH4-N was higher at DSA than atTi/PbO2. It was indicated that the DSA had higher cata-lytic property for chlorine evolution. In contrast, theaccumulation rate of NO3-N was higher at Ti/PbO2

than at DSA. The data showed that NO3-N was theintermediate product in the electrochemical oxidationof NH4-N. It was significant that the electrochemicaloxidation with DSA can prevent the accumulation ofNO3-N. White (1998) noted the side reaction fromammonium ion to nitrogen gas in the breakpoint chlori-nation, which was affected by factors such as the initialratio of chloride to NH4-N, pH and alkalinity.

Fig. 3 also illustrated the degradation of COD incomparison with DSA and Ti/PbO2. It was clear thatthe decreasing rate of COD was higher at Ti/PbO2

I. Ihara et al. / Bioresource Technology 97 (2006) 1360–1364 1363

anode than at DSA. The indirect oxidation with electro-generated hypochlorite might have little influence of thededuction rate of COD. Thus, it was considered thatthe direct anodic oxidation was allowed to enhance thedecreasing rate of organic pollutants contained in thedigested effluent. The differential decrease of CODbetween DSA and Ti/PbO2 could be explained by differ-ent two states for ‘‘active oxygen’’ at anode surface ondirect anodic oxidation (Comninellis, 1994). The physi-cally adsorbed ‘‘active oxygen’’ (OH) can cause thecombustion of organic compounds at the surfaceof the inactive electrode such as Ti/SnO2 and Ti/PbO2

(Simond et al., 1997). In contrast, the chemicallyadsorbed ‘‘active oxygen’’ can favor selective oxidationof organic compounds with active electrode such as Pt,Ti/IrO2 (Comninellis, 1994) and DSA (Polcaro et al.,2000). The result showed that the anode material wasessential parameter for the elimination of both organicpollutants and NH4-N for wastewater treatment byelectrochemical oxidation.

3.4. Effect of pretreatment by membrane filtration

To achieve more effective degradation, filtration bymembranes was performed as a pretreatment to evaluatethe efficacy for the electrochemical oxidation treatment.Fig. 4 illustrated the effect on different pore sizes of amembrane filter for the pretreatment of electrochemicaloxidation. The decreasing rate of NH4-N for the pre-treated sample with 0.5 lm membrane filter was higherthan with 5.0 lm. The enhancement of decreasing rateof NH4-N can be explained by taking into account thatsuspended solids was separated by membrane filter. Itwas noted that suspended solids contained in waste-water would impede the electrochemical oxidation(Kim et al., 2003). In general, the effluent from anaero-

0.0

0.2

0.4

0.6

0.8

1.0

6

5.0 μm

0.5 μm

time (h) 80 2 4

NH

4-N

(-)

Fig. 4. Effect of pretreatment with membrane filtration for thedecrease of NH4-N during electrochemical oxidation at DSA (NaCl0.5 g added).

bic digestion of livestock manure contained high con-centration of suspended solids. The result indicatedthat the pretreatment such as membrane filtration wasimportant for the economical operation on electrochem-ical oxidation of high concentration of wastewater.

4. Conclusions

The electrochemical oxidation could be feasible forthe treatment of the effluent from anaerobic digestionof dairy manure. Both NH4-N and COD were decreasedin proportion to the electric charge. The high chlorideconcentration was possible to accelerate the indirect oxi-dation for the decrease of NH4-N. The electrochemicaloxidation combined with pretreatment of membrane fil-tration allowed also more effective decrease of NH4-N.DSA showed the advantage of decreasing NH4-N buthad lower efficiency of organic pollutants. The resultsconcluded that the DSA was more suitable anode thanTi/PbO2 to achieve the control of nitrate accumulation.

Acknowledgement

This work was supported by Japan Society Promo-tion of Science (JSPS) for the future program.

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