Effect of Organic Loading Rate on Anaerobic Digestion of Food Waste

Feb 3, 2017 - ABSTRACT: This study investigated the effect of organic loading rate (OLR) on anaerobic digestion (AD) of food waste under mesophilic an...
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Effect of Organic Loading Rate on Anaerobic Digestion of Food Waste under Mesophilic and Thermophilic Conditions Chao Liu,† Wen Wang,*,† Naveed Anwar,† Zonghu Ma,‡ Guangqing Liu,*,† and Ruihong Zhang§ †

Biomass Energy and Environmental Engineering Research Center, Beijing University of Chemical Technology, Beijing 100029, China ‡ China Huadian Engineering Company limited, Beijing 100160, China § Department of Biological &Agricultural Engineering, University of California, Davis, California 95616, United States ABSTRACT: This study investigated the effect of organic loading rate (OLR) on anaerobic digestion (AD) of food waste under mesophilic and thermophilic conditions. Results showed that the performance of the AD system was distinctly influenced by temperature and OLR in terms of biogas production, intermediate metabolism, and degradation performance. The optimal OLR under thermophilic condition was 2.5 g of volatile solids (VS)/L/day with methane yield (MY) of 541 mL/g of VSadded. In addition, the optimal OLR under mesophilic condition was 1.5 g of VS/L/day with a MY of 371 mL/g of VSadded. At the same OLR, the MY under thermophilic condition was 33−49% higher than that under mesophilic condition. Under thermophilic condition, steady methane production and degradation efficiency were achieved with considerably high OLR of 7.5 g of VS/L/ day. Under mesophilic condition, stability was obtained only when the OLR was controlled below 2.5 g of VS/L/day. Results also revealed that food waste is a highly desirable substrate with a high bearing OLR under the thermophilic condition of biogas production. of 1.2 g of VS/L/day. Tanimu et al.5 carried out the AD of FW at OLRs ranging from 1.0 to 6.1 g of VS/L/day and obtained the optimum OLR of 2.1 g of VS/L/day under thermophilic condition (55 °C). Different optimum OLRs are possibly caused by different operational conditions and substrates. In conventional AD, OLRs of 1−4 g of VS/L/day are commonly used for treatment of organic wastes.1,11−14 Temperature is an important environmental factor that directly affects the dynamic situation of microorganisms.8 Conventional AD is operated under mesophilic condition to maintain stable performance.15 Thermophilic digestion offers several benefits over mesophilic digestion; these benefits include high degree of waste stabilization, thorough destruction of viral and bacterial pathogens, a decreased detention time, a small reactor capacity, a high biogas production, and a strong capacity for metabolism indicated by the increased activity of microorganisms.16 Therefore, the reactor may operate steadily in a higher OLR under thermophilic condition than that under mesophilic one. Previous studies mainly focused on improving the efficiency of FW digestion at a certain operating temperature.17−19 Additional data are required to compare the digestion performances with different temperatures under a wide range of OLRs. Given these considerations, the present study aimed to evaluate and compare the stability, biogas production, intermediate metabolism, and degradation performance of AD under a wide range of OLRs under mesophilic and thermophilic conditions. Semicontinuous reactors were used, and FW was utilized as substrate.

1. INTRODUCTION Food waste (FW) generation continues to increase worldwide; Koreans and Americans annually dispose of approximately 3.8 × 107 and 4.36 × 107 tons of FW, respectively.1 The amount of FW generated in China is 6 × 107 tons annually.2 FW is characterized by high moisture and organic matter contents, and it may cause environmental pollution when inappropriately treated.3 In view of the increasing public concerns about environmental pollution and energy shortage, the conversion of FW to energy becomes an economical practice.4 Anaerobic digestion (AD) is a spontaneous process mediated by enzymatic and bacterial activities that convert organic materials into biogas with low sludge production, which recovers energy and satisfies the energy requirement.5 FW is comprised of high concentrations of carbohydrates, proteins, and lipids and is an ideal substrate for AD. The equilibrium and productivity of AD are criteria for its performance and can be considerably influenced by the organic loading rate (OLR).6 High OLR enriches different bacterial species, reduces the capacity requirements of the reactor, and requires less energy for heating.7 However, a high OLR results in the accumulation of volatile fatty acid (VFA) and ethanol, uneven distribution during stirring, and poor transfer of heat that could eventually lead to an irreversible failure.8 To date, different optimal OLRs are obtained during AD treatment of organic wastes. Agyeman et al.9 conducted the anaerobic codigestion of FW and dairy manure at an OLR range of 0.67− 3 g of VS/L/day, and they obtained the highest methane yield (MY) at the OLR of 2 g of VS/L/day. Marañoń et al.10 codigested a mixture containing 70% manure, 20% FW, and 10% sewage sludge in continuous stirred-tank reactors operated at OLRs ranging from 1.2 to 1.5 g of VS/L/day under mesophilic (36 °C) condition; they obtained the optimal OLR © XXXX American Chemical Society

Received: January 3, 2017 Revised: February 2, 2017 Published: February 3, 2017 A

DOI: 10.1021/acs.energyfuels.7b00018 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

inoculum for thermophilic reactors. This sludge was obtained from a continuously stirred tank reactor in a cassava ethanol plant in Jiangsu province. 2.2. Experimental Procedures. Twelve identical 500 mL laboratory-scale bottles with working volume of 400 mL were used as digesters for AD of FW. The hydraulic retention time (HRT) of all reactors was 20 days during the whole experiment. Approximately 20 mL of digests was removed, and 20 mL of substrate (certain amount of distilled water was added into the pretreated FW to make the initial OLR of 1.5 g of VS/L/day) was fed into reactors every 24 h. Six bottles labeled as R1, R2, R3, R4, R5, and R6 were operated under mesophilic condition. The remaining six bottles labeled as R7, R8, R9, R10, R11, and R12 were operated under thermophilic condition. All reactors were initially operated with OLR of 1.5 g of VS/L/day for about two HRTs after obtaining steady and similar biogas production performances (phase I). On the 35th day, different OLRs of 1, 1.5, 2.5, 5, 7.5, and 10 g of VS/L/day were adjusted by varying the ratio of pretreated FW and distilled water added into R1, R2, R3, R4, R5, and R6, respectively. Similarly, the OLRs in R7, R8, R9, R10, R11, and R12 were also changed to 1, 1.5, 2.5, 5, 7.5, and 10 g of VS/L/day, respectively, on the same day. Afterward, all the reactors were semicontinuously operated for about 23 days (phase II). In the above experiments, the headspace of each bottle was purged with nitrogen gas for 5 min to create an anaerobic environment before connecting the bottle with silicone tubing to a 2 L aluminum foil bag for biogas collection. R1−R6 were controlled at 37 ± 1 °C, and R7−R12 were controlled at 55 ± 1 °C. The bottles were subsequently placed in a reciprocating air bath shaker with a shaking speed of 90 rpm. The biogas volume was measured by using a syringe, and biogas composition was analyzed during daily collection. Liquid samples were also collected and characterized periodically. 2.3. Analytical Methods. Total solids (TS), volatile solids (VS), total chemical oxygen demand (TCOD), and soluble chemical oxygen demand (SCOD) were tested according to standard methods.20 The

2. METHODS AND MATERIALS 2.1. Substrate and Inoculum. FW was collected from a canteen at Beijing University of Chemical Technology. The main components of the mixed FW were organic matter, including meat, rice, noodles, leafy vegetables, fish, eggs, and some impurities that were mainly comprised of plastics, bones, and toothpicks. After sampling, the impurities were initially separated manually from the FW. After being smashed to 2−5 mm with a pulverizer (Waste King, SS3300, USA) and homogenization, the FW was kept frozen at −20 °C to prevent biological decomposition. Table 1 presents the characteristics of the

Table 1. Substrate Characteristics parameter TS VS pH TN TCOD SCOD soluble protein NH3-N element analysis C H S N O

units

food waste

g/L g/L g/L g/L mg/L

24.30 ± 2.11 22.50 ± 1.32 5.02 ± 0.03 5.95 ± 0.13 181.05 ± 0.24 103.53 ± 0.31 33.75 ± 0.22 96.0 ± 3.5

% % % % %

53.39 ± 1.22 6.93 ± 0.71 0.22 ± 0.01 2.31 ± 0.42 29.50 ± 0.25

% %

TS TS TS TS TS

substrate. Sewage sludge from a biogas station in Shunyi District, Beijing, China was used directly as the inoculum for mesophilic reactors. Thermophilic-digested sludge was used directly as the

Figure 1. Methane yield, methane content, and biogas production under mesophilic condition. B

DOI: 10.1021/acs.energyfuels.7b00018 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Summary of Biogas Composition at Mesophilic and Thermophilic Digestions Mesophilic Digestion phase IIa

phase I OLR (g of VS/L/day) bottle H2% CO2% CH4%

1.5 0.0 40.0−48.0 56.0−58.0

1 R1 0.0 45.0 54.0

1.5 R2 0.0 43.0 56.0 Thermophilic Digestion

2.5 R3 19.5 77.0 3.4

5 R4 19.6 78.0 2.3

a

1.5 0.0 41.0−43.0 55.0−57.0

10 R6 34.5 64.5 0.1

7.5 R11 0.0 41.8 57.0

10 R12 0.0 43.0 55.7

phase IIa

phase I OLR (g of VS/L/day) bottle H2% CO2% CH4%

7.5 R5 28.0 68.0 1.8

1 R7 0.0 44.0 55.0

1.5 R8 0.0 42.3 56.7

2.5 R9 0.0 39.0 59.0

5 R10 0.0 40.4 58.6

For R1−R11, biogas composition was the average value at a steady state. For R12, data were collected on the 55th day.

pH value was determined using an le438 pH electrode (Mettler Toledo, USA). Elemental contents (C, H, N, and S) were tested by using an organic elemental analyzer (Vario El Cube, Germany), and the O content was estimated by assuming C + H + O + N = 99.5% on a VS basis.21 The Lowry−Folin method was used to determine soluble protein concentrations using bovine serum albumin as the standard.22 Soluble carbohydrate concentrations were measured by the phenol sulfuric acid method with glucose as the standard.23 Biogas (CH4 and CO2) compositions were measured by a gas chromatograph (GC; Agilent, 7890B) equipped with a thermal conductivity detector and an analytical column of Agilent Hayesep Q. The operational temperatures at the detector and column oven were 220 and 60 °C, respectively. Helium was used as the carrier gas at a constant pressure of 5 psi. The ethanol and VFA (acetate, propionate, isobutyrate, n-butyrate, isovalerate, and n-valerate) concentrations were determined by a gas chromatograph (Agilent, 7890A) with a flame ionization detector and a DB-wax capillary column (30 m × 530 μm × 1.0 μm). The temperatures of the injector and detector were 200 and 250 °C, respectively. Nitrogen was used as the carrier gas at a flow rate of 10 mL/min. The GC oven was programmed to start at 55 °C for 1 min, increase at a rate of 30 °C/ min until 110 °C, hold at 110 °C for 1 min, increase at a rate of 30 °C/ min until 220 °C, and hold at 220 °C for 1 min. The injection volume was 1.0 μL. 2.4. Data Analysis. The methane production efficiency was evaluated using the methane content in the biogas (%), MY (the calculated methane production per gram of VS added, mL/g of VSadded). The gas volume reported in this study was calibrated to standard temperature (273 K) and pressure (1 atm). ANOVA was used to evaluate the significance of the results, and p < 0.05 was considered statistically significant.

at about 250 mL/g of VSadded in the initial 10 days of operation, but eventually decreased to approximately zero on the 50th day. Given that the OLRs of R3, R4, R5, and R6 increased, a large amount of substrate was utilized by the hydrolysis and acidification of bacteria in those reactors. In addition, the accumulation of superfluous VFA resulted in intense inhibition of methane production, which finally led to the failure of R3− R6. By contrast, the MYs in R1 and R2 with the OLRs of 1 and 1.5 g of VS/L/day remained stable at approximately 390 and 370 mL/g of VSadded, respectively. This result indicated that, with FW as the substrate, the OLR in the mesophilic AD reactors should be controlled below 2.5 g of VS/L/day to maintain a stable operation. Notably, the MY of R1 was about 5% higher than that of R2, but no significant difference was observed between the two reactors (p > 0.05). The decrease in MY with increasing OLR was noted in many previous studies. Gou et al.8 found that when the OLR increases from 1 to 6 g of VS/L/day, the MY varies from 0.26 to 0.15 L of CH4/g of VSadded when waste-activated sludge is codigested with FW. Linke et al.24 also evaluated the OLR of AD in the range 0.8− 3.4 g of VS/L/day, with potato as the substrate; the obtained MY decreases from 0.42 to 0.32 L of CH4/g of VSadded. Methane contents are shown in Figure 1b, and the patterns are similar to those of the MYs. Prior to the change in OLR, six reactors presented almost the same methane content of around 50%. After changing the OLR, the methane contents of R4, R5, and R6 decreased to around zero within 5 days of operation. The methane content in R3 decreased, remained at about 40% for 10 days, and subsequently decreased rapidly to zero before the 50th day. The methane contents of R1 and R2 were maintained at approximately 50% during the whole process in phase II and similar to those in phase I. Biogas productions upon changing of different OLRs in the reactors are shown in Figure 1c. The first 35 days served as a stable stage for each reactor at the same OLR of 1.5 g of VS/L/ day. After changing the OLRs, the biogas productions in R1 and R2 remained steady with average values of about 300 and 400 mL/day, respectively. The biogas productions in R3 and R4 drastically decreased and remained at around 30 mL/day after the 50th day. However, the biogas productions in R5 and R6 first decreased to about 50 mL/day, gradually increased, and remained stable with average biogas productions of 150 and

3. RESULTS AND DISCUSSION 3.1. Effect of OLR on Biogas Production under Mesophilic Condition. The MYs during digestion of FW under mesophilic condition are shown in Figure 1a. The MYs in all reactors remained relatively stable for about 35 days, with an average value of around 370 mL/g of VSadded prior to the change in OLR. On the 35th day, the OLRs of R1, R2, R3, R4, R5, and R6 were adjusted to 1, 1.5, 2.5, 5, 7.5, and 10 g of VS/ L/day, respectively. Intense inhibition of methane production was immediately observed in R4, R5, and R6, as reflected by the decreased MYs. The MYs in R5 and R6 decreased to zero on the 37th day, and the MY in R4 decreased to zero on the 40th day. The MY in R3 with OLR of 2.5 g of VS/L/day fluctuated C

DOI: 10.1021/acs.energyfuels.7b00018 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 2. pH during anaerobic digestion of food waste under mesophilic and thermophilic digestions.

Figure 3. Methane yield, methane content, and biogas production under thermophilic condition.

of VFA degradation and thereby cause problems during AD.28 Previous studies found increased molecular hydrogen and hydrogen sulfide concentration, decreased production of methane, and acidification of AD at the end of the experimental period.29,30 The maximum OLR should be controlled under 2.5 g of VS/L/day for stable and efficient methane production under mesophilic condition. 3.2. Effect of OLR on Biogas Production under Thermophilic Condition. Figure 3 presents the plot of MY, methane content, and biogas production with six different OLRs tested (1, 1.5, 2.5, 5, 7.5, and 10 g of VS/L/day) under thermophilic condition. As shown in Figure 3, the reactors were relatively stable for about 35 days with methane contents, MYs, and biogas productions of around 52%, 500 mL/g of VSadded, and 560 mL/day, respectively. The methane content under

100 mL/day, respectively. As shown in Table 2, increased contents of hydrogen and carbon dioxide were detected in R3− R6, and the average hydrogen concentrations were 19.5, 19.6, 28, and 34.5% in reactors R3, R4, R5, and R6, respectively. AD proceeded to acidification in R3−R6, and the environment was suitable for hydrogen production. As shown in Figure 2a, the pH values in R1 and R2 showed no evident changes. Nevertheless, the pH decreased sharply in R3−R6, with a final value of approximately 4.5 in R3 and less than 4.0 in R4−R6. Acetate could be converted into hydrogen and carbon dioxide at pH 4.0−4.5.25 The pH range of 6.8−7.2 is suitable for methane production.26 Therefore, low pH may lead to complete failure of reactors through inhibition of methanogenesis.4,27 The increased hydrogen partial pressure in the biogas reactor might also theoretically lead to the inhibition D

DOI: 10.1021/acs.energyfuels.7b00018 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Comparison of Yields of Mesophilic Methane, Thermophilic Methane, and Theoretical Methane OLR (g of VS/L/day)

mesophilic methane yieldsa (mL/g of VSadded)

mesophilic methane yields/theor methane yield (%)

thermophilic methane yieldsa (mL/g of VSadded)

thermophilic methane yields/theor methane yield (%)

1 1.5 2.5 5 7.5 10

386.73 370.57 − − − −

62.00 59.34 − − − −

512.67 551.40 541.39 443.93 401.00 −

82.10 88.30 86.70 71.09 64.22 −

a

Data obtained at steady state.

Figure 4. Total VFA/ethanol concentration in the effluent of mesophilic and thermophilic reactors.

thermophilic condition was nearly the same as that under mesophilic condition. However, at the same OLR, the MYs and daily biogas productions under thermophilic condition were about 1.5 times higher than those under mesophilic condition. These data indicated that AD performed more effectively under thermophilic condition than under mesophilic condition. This result was also consistent with those of previous studies that reported the superior performance of thermophilic processes over mesophilic processes.31,32 For example, Gou et al.8 found that, at the same OLR, the average gas production rate with codigestion of waste-activated sludge and FW at 55 °C is approximately 1.6 and 1.3 times higher than those at 35 and 45 °C, respectively. The high MY under thermophilic condition might be due to the higher hydrolysis efficiency and activity of methanogens compared with those under mesophilic condition. After 35 days of operation, the OLRs of R7−R12 were changed. Steady states were observed in R7−R9 with MYs of 513, 551, and 541 mL/g of VSadded, respectively. No significant difference was observed in the MYs of R8 and R9 (p > 0.05). The MYs in R10−R12 decreased after the increase in OLRs. Nevertheless, unlike the phenomenon under mesophilic condition, decreased but steady MYs of 440 and 401 mL/g of VSadded were obtained in R10 and R11 with OLRs of 5 and 7.5 g of VS/L/day, respectively. As shown in Figure 2b, the pH of R10 and R11 slightly decreased when the OLR increased, but still remained stable at above 7.1; this pH value is suitable for methane production. When the OLR further increased to 10 g of VS/L/day, the MY in R12 decreased gradually until the end of the experiment, and the pH decreased to below 6.1 in R12, which also indicated the failure of the process (Figure 2b). Biogas productions in the reactors were similar to the patterns of the MYs. On the basis of these results, the optimal OLR for AD of FW was 2.5 g of VS/L/day, and the maximum OLR for stable methane production could reach 7.5 g of VS/L/day

under thermophilic condition. These data implied that, unlike under mesophilic condition, the methanogens could function well and present high activity with high OLR under thermophilic condition. The MYs were further compared with the theoretical MY of FW (623 mL/g of VSadded), which was calculated according to the elemental contents. As shown in Table 3, under mesophilic condition, the AD systems could only be maintained stable with OLRs of 1 and 1.5 g of VS/L/day. The ratio of measured MY/ theoretical MY under mesophilic condition was around 60%, which suggested insufficient degradation performance. Under thermophilic condition, the AD systems remained stable with OLRs of 1, 1.5, 2.5, 5, and 7.5 g of VS/L/day, and the ratio of measured MY/theoretical MY could reach more than 85%. These results indicated that most of the organic compounds were degraded and transferred into methane under thermophilic condition. At the same OLR of 1 g of VS/L/day, the MY under thermophilic condition was 33% higher than that under mesophilic condition. With the same OLR of 1.5 g of VS/L/ day, the MY under thermophilic condition was 49% higher than that under mesophilic condition. These data further revealed that FW could be thoroughly degraded under thermophilic condition. 3.3. Combined Effects of OLR and Temperature on VFA/Ethanol Accumulation and Distribution. Reactor acidification due to a rapid increase of VFA/ethanol is one of the most important and common reasons of inhibitory effect on methanogens, a subsequent decrease of methane production, and the consequent failure of the system.4,27 Periodic monitoring of VFA/ethanol concentration is necessary during AD to evaluate the biological process and prevent collapse.15 The total volatile fatty acid and ethanol (TVFA/ethanol) concentrations in R1−R12 were almost the same (