Electrochemical oxidation of the effluent from anaerobic digestion of dairy manure

Electrochemical oxidation of the effluent from anaerobic digestion of dairy manure

Bioresource Technology 97 (2006) 1360–1364 Electrochemical oxidation of the effluent from anaerobic digestion of dairy manure Ikko Ihara a, Kazutaka Um...

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Bioresource Technology 97 (2006) 1360–1364

Electrochemical oxidation of the effluent from anaerobic digestion of dairy manure Ikko Ihara a, Kazutaka Umetsu a

a,*

, Kiyoshi Kanamura b, Tsuneo Watanabe

b

Department of Agro-Environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro 080-8555, Japan b 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 2005 Available 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. The influence of direct anodic oxidation and indirect oxidation was evaluated through the use of dimensionally stable anode (DSA) and Ti/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 control of ammonium nitrate (NO3-N) accumulation. The results showed that the filtration of suspended solids as a pretreatment and addition 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 a serious problem for livestock production. Anaerobic digestion has become an option for sustainable treatment of livestock manure, converting it to biogas and effluent. Digested effluent from anaerobic digestion of livestock manure usually contains high strength of ammonium nitrogen (NH4-N) and persistent organic substances. The components in digested effluent had been applied as fertilizer for recycling of nutrients back to agricultural field (Salminen et al., 2001; Umetsu et al., 2002). The excessive spreading of livestock manure on the field should be attributable to nitrogen pollution in farming areas (Woli et al., 2004). A simple and effective process for removing nitrogen and residue organic sub*

Corresponding author. Tel.: +81 155 49 5515; fax: +81 155 49 5519. E-mail address: [email protected] (K. Umetsu). 0960-8524/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.07.007

stances is required as a posttreatment of the effluent from anaerobic digestion. The electrochemical oxidation treatment of various wastewaters has been investigated in recent years. Both organic pollutants and NH4-N in wastewater containing chloride can be destroyed electrochemically (Chiang et al., 1995). In this work the effluent from anaerobic digestion of dairy manure was treated by the application of electrochemical oxidation. The purpose of this study was to identify the main parameters influencing the performance of an electrochemical oxidation process.

2. Methods 2.1. Anaerobic digestion The digested effluent used in this study was collected from a full-scale anaerobic digester of Obihiro University of agriculture and veterinary medicine

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The electrochemical oxidation experiment was conducted in a glass beaker, equipped with a 100 · 50 mm mesh anode and a plate cathode. The anodes were a dimensionally stable anode (DSA) based on mixed oxides of RuO2 + IrO2 and lead dioxide coated titanium (Ti/PbO2) electrode. The stainless steel was used as cathode. They were placed vertically and parallel to each other with an electrode gap of 10 mm in a beaker. The electrochemical oxidation was carried out at a constant current of 1.5 A using a DC power supply. The sample solution was agitated by a magnetic stirrer. The surface bubbles were recycled by a peristaltic pump for antifoam.

9.0

12.0

7.0

8.0 pH

5.0

4.0

E

0.0

3.0 6000

200

4000

100

COD CH3COOH

2000

0

0

NH4 -N, NO3 -N, ClO- (mg/L)

Chemical oxygen demand (COD) was determined by dichromate method. Ammonium nitrogen (NH4-N) was determined using salicylate reaction. The concentrations of these analytical parameters were measured by a HACH DR4000 spectrophotometer. Ammonium nitrate (NO3-N), chloride ion, hypochloride ion and acetic acid were analyzed by capillary electrophoresis (CE) system (Agilent Technologies, G1600A). The basic anion buffer and a fused silica capillary with 104 cm in length and 50 lm internal diameter were obtained from Agilent technologies. The temperature controlled cartridge for fused silica capillary was set at 30 C and the applied dc voltage was 30 kV. The wavelength 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 size membrane filter.

COD (mg / L)

300

2.4. Analytical method

E (V)

2.3. Experiments of electrochemical oxidation

The diluted sample with 0.5 g of NaCl, pretreated by 0.5 lm membrane filter was tested for the electrochemical oxidation using a DSA (Fig. 1). The concentration of COD was reduced by 32% in 9 h. In a previous work, the electrochemical oxidation applying for wastewater treatment was explained by a direct anodic oxidation or an indirect oxidation (Chiang et al., 1995). In the direct anodic oxidation, the organic pollutants were destroyed on oxide anode by electrochemical conversion or combustion (Comninellis, 1994). In the indirect oxidation, the electrogenerated oxidant such as hypochlorite (Comninellis and Nerini, 1995) or peroxodisulphates (Can˜izares et al., 2003) destroyed the pollutants in the bulk solution. The concentration of acetate was increased consistently whereas the COD was decreased from the beginning. The result showed that the electrochemical oxidation at DSA for the wastewater contained chloride had low degradability to acetic acid. The NH4-N was decreased rapidly with time. The decrease of NH4-N could be explained by indirect oxidation with hypochlorite.

CH3COOH (mg/L)

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

3.1. Variation of parameters during electrochemical oxidation at DSA

400

NH4 -N NO3 -N ClOCl-

300

1500

1000

200

Cl- (mg/L)

2.2. Pretreatment for digested effluent

3. Results and discussion

pH (-)

(Hokkaido, Japan). A 60 m3 anaerobic digester was installed next to free stall barn and was operated with dairy manure slurry at a digester temperature of 55 C. In the coldest season, average biogas production was 150 m3/day, consisting of 56% methane gas with an average loading rate of 6.75 kg/m3/day which established a hydraulic retention time of 13 days at an average ambient temperature of 15 C and slurry temperature of 2 C.

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500

100

0

0 0

2

4

6

8

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time (h) Fig. 1. Electrochemical oxidation at DSA with addition of 0.5 g NaCl.

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Hypochlorite is generated as the product of hydrolysis of chloride. The decreasing process of NH4-N might be regarded as similar to the chemistry of the Selleck-Saunier breakpoint phenomenon (Chiang et al., 1995; White, 1998). The electrochemically generated hypochlorite was consumed to produce nitrogen from ammonium ion. Since the sample contained 1496 mg/L of chloride ion at the start of the experiment and DSA had high catalytic properties for chlorine evolution (Trasatti, 2000), hypochlorite was effectively produced and responsible for NH4-N decrease during the electrochemical oxidation. The data showed that NH4-N was vanished in 8 h, and then hypochlorite ion was detected.

400 NH4-N (DSA) NH4-N (T i/PbO2)

300

conc. (mg / L)

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no addition

1.0

0.5 g NaCl added 2.0 g NaCl added

0.8

NH4 -N (-)

200

0 8000

0.6 0.4 0.2 0.0 4

6

8

time (h) Fig. 2. Effect of NaCl addition for the decrease of NH4-N during electrochemical oxidation at DSA.

COD (DSA) COD (Ti/PbO2)

6000

conc. (mg / L)

Fig. 2 showed the influence of the chloride ions on electrochemical oxidation using DSA. The diluted sample used in the experiments contained approximately 700 mg/L of chloride ion. When 0.5 or 2.0 g of NaCl was added, the decreasing rate of NH4-N was increased dramatically. The increased concentration of chloride ion had positive influence for the enhancement in the rate of NH4-N decrease. An increase in the concentration of chloride ion produced a high generation rate of hypochlorite. The result also indicated that the diluted sample of digested effluent had only a low concentration of chloride ion for the decrease of NH4-N on electrochemical oxidation. To achieve an economical operation, the addition of adequate amount of chloride ion was required for the digested effluent. In contrast, the addition of NaCl had less effective for the decrease of COD during electrochemical oxidation. The generated amount of hypochlorite increases with high initial concentration of chloride ion on constant current electrolysis. The result indicated that the indirect oxidation caused by electrogenerated hypochlorite was

2

NO3-N (T i/PbO2)

100

3.2. Influence of NaCl addition

0

NO3-N (DSA)

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2000

0 0

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4

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time (h) Fig. 3. Effect of anode material for the decrease of NH4-N and COD during electrochemical oxidation with addition of 0.5 g NaCl.

not predominant in the degradation of organics in the digested effluent. 3.3. Comparison of anode materials between DSA and Ti/PbO2 The influence of anode material on electrochemical oxidation was examined with DSA and Ti/PbO2 anode for the decrease of NH4-N and COD (Fig. 3). The decreasing rate of NH4-N was higher at DSA than at Ti/PbO2. It was indicated that the DSA had higher catalytic property for chlorine evolution. In contrast, the accumulation rate of NO3-N was higher at Ti/PbO2 than at DSA. The data showed that NO3-N was the intermediate product in the electrochemical oxidation of NH4-N. It was significant that the electrochemical oxidation with DSA can prevent the accumulation of NO3-N. White (1998) noted the side reaction from ammonium ion to nitrogen gas in the breakpoint chlorination, which was affected by factors such as the initial ratio of chloride to NH4-N, pH and alkalinity. Fig. 3 also illustrated the degradation of COD in comparison with DSA and Ti/PbO2. It was clear that the decreasing rate of COD was higher at Ti/PbO2

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anode than at DSA. The indirect oxidation with electrogenerated hypochlorite might have little influence of the deduction rate of COD. Thus, it was considered that the direct anodic oxidation was allowed to enhance the decreasing rate of organic pollutants contained in the digested effluent. The differential decrease of COD between DSA and Ti/PbO2 could be explained by different two states for ‘‘active oxygen’’ at anode surface on direct anodic oxidation (Comninellis, 1994). The physically adsorbed ‘‘active oxygen’’ (OH) can cause the combustion of organic compounds at the surface of the inactive electrode such as Ti/SnO2 and Ti/PbO2 (Simond et al., 1997). In contrast, the chemically adsorbed ‘‘active oxygen’’ can favor selective oxidation of 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 was essential parameter for the elimination of both organic pollutants and NH4-N for wastewater treatment by electrochemical oxidation.

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bic digestion of livestock manure contained high concentration of suspended solids. The result indicated that the pretreatment such as membrane filtration was important for the economical operation on electrochemical oxidation of high concentration of wastewater.

4. Conclusions The electrochemical oxidation could be feasible for the treatment of the effluent from anaerobic digestion of dairy manure. Both NH4-N and COD were decreased in proportion to the electric charge. The high chloride concentration was possible to accelerate the indirect oxidation for the decrease of NH4-N. The electrochemical oxidation combined with pretreatment of membrane filtration allowed also more effective decrease of NH4-N. DSA showed the advantage of decreasing NH4-N but had lower efficiency of organic pollutants. The results concluded that the DSA was more suitable anode than Ti/PbO2 to achieve the control of nitrate accumulation.

3.4. Effect of pretreatment by membrane filtration To achieve more effective degradation, filtration by membranes was performed as a pretreatment to evaluate the efficacy for the electrochemical oxidation treatment. Fig. 4 illustrated the effect on different pore sizes of a membrane filter for the pretreatment of electrochemical oxidation. The decreasing rate of NH4-N for the pretreated sample with 0.5 lm membrane filter was higher than with 5.0 lm. The enhancement of decreasing rate of NH4-N can be explained by taking into account that suspended solids was separated by membrane filter. It was noted that suspended solids contained in wastewater would impede the electrochemical oxidation (Kim et al., 2003). In general, the effluent from anaero-

1.0 5.0 μm 0.5 μm

NH4-N (-)

0.8

0.6 0.4 0.2

0.0

0

2

4 time (h)

6

8

Fig. 4. Effect of pretreatment with membrane filtration for the decrease of NH4-N during electrochemical oxidation at DSA (NaCl 0.5 g added).

Acknowledgement This work was supported by Japan Society Promotion of Science (JSPS) for the future program.

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