Continuous co-digestion of aquatic weeds with food waste and the availability of digested effluent

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1 Continuous co-digestion of aquatic weeds with food waste and the availability of digested effluent M. Fujiwara*, M. Koyama **, S. Akizuki* K. Watanabe*, K. Ishikawa***, S. Ban**** and T. Toda* * Graduate School of Engineering, Soka University, Tangi-cho, Hachioji, Tokyo, Japan (Corresponding author: fujimasa523@soka.gr.jp) ** School of Environment and Society, Tokyo Institute of Technology, Ookayama, Meguroku, Tokyo, Japan *** Lake Biwa Environmental Research Institute, 5-34 Yanagasaki, Ostu, Shiga, Japan **** School of Environmental Science, University of Shiga Prefecture, 25 Hassaka-cho, Hikone, Shiga, Japan Abstract Anaerobic digestion is expected as an effective way to treat excess growing aquatic weeds because it can recover methane as an energy resource from high moisture content wastes. Aquatic weeds often contain high lignin and nitrogen in their body, leading to low anaerobic biodegradability. In this study, alkaline thermophilic pre-treatment and co-digestion with food waste were applied to improve methane productivity by removing lignin from weeds and adjusting C/N ratio in the reactor. Continuous co-digestion of harvested aquatic weeds from Lake Biwa in Japan with food waste was conducted for 12 days. During the experimental period, ph showed stable value ranging from 7. to 7.8. Average methane production rate was 542±45 ml L -1 day -1 which corresponds with methane conversion rate of 76.4±6.4% based on COD (Chemical Oxygen Demands). Nutrient concentration of digested effluent was analysed to clarify the usefulness of the effluent as a source for microalgae cultivation. As a result, the chemical composition of the effluent such as nitrogen as a form of NH 4, Mg, Ca and P showed more suitable to cultivate microalgae compared to other common digested effluent. Keywords Anaerobic co-digestion; Food waste; Aquatic weeds; Anaerobic digestion effluent INTRODUCTION In recent years, overgrowing aquatic weeds have led to serious social problems. In Lake Biwa of Japan, overgrowth of aquatic weeds cause serious economic and environmental problems such as sail disturbance, environmental pollution, decreasing of the fishery catches [1]. Currently, management of excessive growing aquatic weed has been treated as a national scaled problem. Therefore, an appropriate management of aquatic weeds must be established urgently. Anaerobic digestion is expected as the suitable treatment methods for aquatic weeds due to possibility to recover methane gas as an energy resource from wastes with high moisture content. In addition, anaerobic digestion effluent can utilize for the nutrient resource such as the medium for the micro algal cultivation. To achieve the stable AD performance and nutrient concentration in long term operation, increasing of biodegradability and optimum substrate supply are required. The difficulty of treating aquatic weeds by anaerobic digestion is commonly due to the high lignin contents. Lignin is undegradable compounds under anaerobic condition, and interferes the degradation of other lignocellulosic components such as hemicellulose and cellulose [2]. Alkaline thermophilic pretreatment is an effective way to remove lignin, leading to significant increase of methane potential from aquatic weed [3]. In addition, aquatic weeds have the lower C/N ratio, possibly leading to low microbial activities due to free ammonia inhibition [4]. Anaerobic co-digestion with other easily degradable carbon rich substrate can adjust the both of ammonia concentration and C/N ratio in the reactor, increasing methane productivity compare to the mono-digestion of aquatic weeds [5]. In this study, continuous co-digestion of harvested aquatic weed from Lake Biwa with food waste was conducted by using bench-scale digester. In addition, the nutrient characteristic of the effluent was evaluated to clarify the usefulness of the effluent.

2 MATERIALS AND METHODS 1. Substrate and seed sludge Aquatic weeds from Lake Biwa located in Shiga Prefecture, Japan, were harvested in May, 216. Alkaline thermophilic pretreatment was carried out according to Koyama et al., (215) [3]. In details, 1% w/v NaOH was added to aquatic weeds leading to.1 g g-ts-substrate -1 and heated for 3 h at 8 C. After the pre-treatment, 4.2 M HCl was added to neutralize ph. Food waste was made by mixing of vegetable waste and rice in the ratio of 2:1. The mesophilic anaerobic sludge was collected from Hokubu Sludge Treatment Center, Kanagawa Prefecture, Japan. 2. BMP test To evaluate the methane potential from harvested aquatic weed and food waste, BMP test was conducted. Identical medium bottles which have an effective volume of 5 ml were used as reactors. The substrate and seed sludge were added to medium bottle, leading to the ratio of 1 to 2 on VS basis. Batch reactors were purged with N2 gas to make anaerobic condition in the reactor. The BMP tests were performed at 37±1 C for 1 days. 3. Continuous anaerobic co-digestion of pretreated aquatic weeds and food waste Bench-scale continuous stirred tank reactor (CSTR) which has an effective volume of 16 L was used. 8. kg of substrate, which is consisted by the aquatic weed, food waste and tap water in the wet weight ratio of 3:2:3, were fed into the reactor once in 2 days, leading to organic loading rate (OLR) and hydraulic retention time (HRT) of 1.89 g-vs -1 L -1 day -1 and 4 days respectively. The experiment was conducted at 37±1 o C for 12 days. 4. Analytical parameter Total solids (TS), volatile solids (VS), chemical oxygen demand (COD) were analysed following methods in the APHA standard (1998). In the BMP test, methane production was measured by using Automatic Biomethane Potential Test SystemⅡ (Bioprocess Control CTD., Sweden). In the continuous anaerobic digestion, biogas (CH4, CO2) composition was monitored using a gas chromatograph (SHIMADZU, GC-14B, Japan) equipped with a packed column (Shincarbon ST) and a thermal conductivity detector. The temperature of injector and detector were maintained at 12 C and 26 C, respectively. The column temperature was gradually increased from 4 C to 25 C. Helium was used as carrier gas with the flow rate of 4mL min 1. RESULTS AND DISCUSSION 1. BMP test Fig.1 shows the methane yield from harvested aquatic weed and food waste. Methane yield of untreated aquatic weed was 279 ml g-vs -1, and by alkaline thermal pretreatment, methane yield was increased to 321 ml g-vs -1 This result confirms that alkaline thermal pretreatment is an effective way to increase the methane yield from aquatic weeds. Methane yield of food waste was 374 ml g-vs -1, and in co-digestion of pretreated aquatic weed and food waste, 368 ml g-vs -1 was obtained. These results indicates that methane yield of aquatic weeds could be increased by treating with food waste. 2. Continuous anaerobic co-digestion During the experimental period, ph showed stable value ranging from 7. to 7.8. Methane production rate and methane conversion rate during the experimental period are shown in Fig. 2.

3 Methane production rate showed stable values of 542±45 ml L -1 day -1 which corresponds with high methane conversion rate of 76.4±6.4% based on substrate COD. From these result, anaerobic co-digestion of aquatic weeds with food waste was performed effectively through the long-term operation. Soluble COD (SCOD) concentration and its component (i.e. acid precipitation and other COD) were shown in Fig. 3. Although high methane conversion rate was achieved during the experiment, significant amount of SCOD around 6, mg L -1 was observed. Almost of SCOD was regarded as anaerobically undegradable substances such as soluble lignin and corrosion materials. To remove these substances, aerobic treatment or chemical treatment is required. Thus, posttreatment for anaerobic digestion by aerobic process is probably required to improve the effluent quality. 3. Characteristics of anaerobic digestion effluent Table 1 shows the nutrient concentrations of digested effluent treating different substrates. In previous study, when NH4 concentration was more than 4 ppm, low microorganisms activity was occurred due to free ammonia inhibition in mesophilic conditions [4]. Digested effluent of this study showed lower NH4 concentration as 385 mg L -1, suggests that NH4 concentration was decreased by the effect of co-digestion, and free ammonia inhibition would not be occurred. As a result, preferable treatment performance may be achieved. Even NH4 concentration was low, metal elements such as Ca, Mg shows higher compared to common digested effluent. Table 2 shows the chemical composition of microalgae. Compared with other effluent, the effluent treating aquatic weeds and food waste shows similar ratio of nutrient concentration to that of microalgae. It indicates that the effluent treating aquatic weeds and food waste has the possibilities applying to the medium for microalgae cultivation. CONCLUSION During the experimental period, steady treatment efficiency was revealed from ph, methane production rate and methane conversion rate. To remove the significant amount of SCOD contained in the effluent, aerobic post-treatment is probably required. The ratio of nutrient concentration shows similar ratio to that of microalgae, indicating that the effluent treating aquatic weed and food waste has the possibilities applying to the medium for microalgae cultivation. ACKNOWLEDGEMENT This study was funded by "The Environment Research and Technology Development Fund from the Ministry of the Environment, Japan (4-146, ). We gratefully acknowledge the Hokubu Sludge Treatment Center, in Yokohama, Japan, for providing digested sludge and the Lake Biwa Policy Division and the Ohmi Environment Conservation Foundation for the donation of the aquatic weed. REFERENCES [1] Maruno, S. and Hamabata, E. (216). Effects of macrophyte reaping on macrophyte community at southern part of Lake Biwa. Izunuma-Uchinuma Wetland Researches, 1, [2] Triolo, J., Sommer, S., Møller, H., Weisbjerg, M. and Jiang, X. (211). A new algonism to characterize biodegradability of biomass during anaerobic digestion: Influence of lignin concentration on methane production potential. Bioresource Technology, 12, [3] Koyama, M., Yamamoto, S., Ishikawa, K., Ban, S. and Toda, T. (215). Enhancing anaerobic digestibility of lignin-rich submerged macrophyte using thermochemical pre-treatment. Biochemical Engneering Journal, 99,

4 [4] Rajagopal, R., Massé, D. I. and Singh, G. (213). A critical review on inhibition of anaerobic digestion process by excess ammonia. Bioresource Technology, 143, [5] Zhou, Q., Shen, F., Yuan, H., Zou, D., Liu, Y., Zhu, B., Jaffu, M., Chufo, A., and Li, X. (214). Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover. Bioresourse Technology, 166, [6] Racharaks, R., Ge, X., Li, Y. (215). Cultivation of marine microalgae using shale gas flowback water and anaerobic digestion effluent as the cultivation medium. Bioresource Technology, 191, [7] Cheng, J., Ye, Q., Xu, J., Yang, Z., Zhou, J., Cen, K. (216). Improving pollutants removal by microalgae Chlorella PY-ZU1 with 15% CO2 from undiluted anaerobic digestion effluent of food wastes with ozonation pretreatment. Bioresource Technology, 216, [8] Richmond, A. (24).Algal nutrition, Handbook of Microalgal Culture,

5 Methane production rate (ml L -1 day -1 ) Untreated Aquatic weed pretreated Aquatic weed Food waste Co-digestion Figure.1. Methane yield from aquatic weeds and food waste Methane production rate Methane conversion rate Methane conversion rate (%-COD) Operation time (day) Figure 1. Methane production rate and methane conversion rate during the experimental period Figure.2. Methane production rate and methane conversion rate during the experimental period.

6 Dissolved COD concentration (mg L -1 ) 1 Total soluble COD Acid precipitations COD (soluble lignin, corrosion materials) Others COD (volatile fatty acid sugar) Operation time (day) Figure Methane Concentration production of dissolved rate and methane COD during conversion the experimental rate during the period. experimental period Table 1. Nutrient concentration of digested effluent treating different substrates. Substrates NH 4+ -N P Na Ca Mg References Municipal wastewater 1447 {1} 16 {.7} 13 {.9} 83 {.6} 2 {.1} [6] [4] Food waste 212 {1} 44 {.2} 16 {.5} 1 {.5} 27 {.1} [5] [7] Aquatic weeds Food waste 385 {1} 52 {.14} 189 {.49} 15 {.36} 59 {.15} This study {} means the ratio of the each nutrient concentration to NH 4 concentration. Table 2. Chemical composition of micro algae modified from Romand [6]. [8] N P Na Ca Mg 1-14{1}.5-33 {.24}.4-47 {.34}.-8 {.57}.5-75 {.54} {} means the ratio of the each nutrient content to N content.