the optimisation of food waste addition as a co-substrate in anaerobic digestion of sewage sludge

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2003 21: 515Waste Manag ResHyun-Woo Kim, Sun-Kee Han and Hang-Sik Shin

The optimisation of food waste addition as a co-substrate in anaerobic digestion of sewage sludge

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Waste Manage Res 2003: 21: 515–x?xPrinted in UK – all rights reserved

Waste Management & Research

Copyright © ISWA 2002Waste Management & Research

ISSN 0734–242X

515

Waste Manage Res 2003: 21: 515–526Printed in UK – all rights reserved

Copyright © ISWA 2003Waste Management & Research

ISSN 0734–242X

Hyun-Woo KimSun-Kee HanHang-Sik ShinDepartment of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea.

Keywords: Anaerobic, co-digestion, food waste, sewage sludge, methane production rate (MPR), optimal mixing ratio, mesophilic and thermophilic conditions, wmr 694–8.

Corresponding author: Hang-Sik Shin, Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea. Tel: +82-42-869-3613 Fax: +82-42-869-8460Email: [email protected]

Received 23 July 2003, accepted in revised form 03 October 2003.

Food waste has been regarded as the main source of various environmental pollution in Korea due to the high volatile solids (VS) and moisture content caused by the features of dietary habits. The feasibility of food waste as a co-substrate in anaerobic digestion of sewage sludge was investigated in mesophilic and thermophilic condi-tions using batch tests. Cumulative methane production, dissolved organic carbon (DOC) and volatile fatty acids (VFA) were monitored to find the optimal mixing ratios of food waste and sewage sludge for the enhanced per-formance of co-digestion. It was observed that adequately mixed food waste led to the enhanced methane produc-tion both at mesophilic and thermophilic conditions. However, a conventional linear regression conducted for the optimisation of co-substrate mixing ratios was not accurate in describing exact methane production trends of co-digestion because of the different biodegradability of substrates. Therefore, a remodified Gompertz equa-tion showing nonlinear relationship between variables was developed to find exact information with the same experimental data obtained at 2g VS/l generally used in biochemical methane potential (BMP) tests. Based on an influential parameter, methane production rate (MPR), the optimal mixing ratios of food waste were 39.3% and 50.1% in mesophilic and thermophilic con-ditions, respectively. To confirm the application of the remodified Gompertz equation, secondary batch tests were conducted with the substrate concentrations of 1-4g VS/l. In overall range tested, the confident mixing ratios of food waste was adjusted to 30-40% and 40% in mesophilic and thermophilic conditions, respectively. The most significant factor for enhanced performance was the improved organic carbon content provided by additional food waste.


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Hyun-Woo Kim, Sun-Kee Han, Hang-Sik Shin

516 Waste Management & Research

IntroductionAnaerobic digestion is one of the common and cost-effective technologies for the stabilisation of organic frac-tion of municipal solid waste (OFMSW) because of its high energy recovery and limited environmental impact (Mata-Alvarez et al. 2000). However, conventional com-pletely-mixed anaerobic sludge digestion processes in most wastewater treatment plants in Korea still suffer from unre-liable performance, even though various researches have been made for higher treatment efficiency.

Among various OFMSW produced, the generation of food waste reaches 11,000 tons per day in Korea, account-ing for 23.2% of MSW (Korea Ministry of Environment, 2002). Food waste is the major source of decay, odour, and leachate in collection and transportation due to the high VS/TS (0.8-0.9) and moisture content (75-85%). Food waste, consolidated in landfills with other wastes, has resulted in serious environmental problems such as odour emanation, vermin attraction, toxic gas emission and groundwater contamination. However, if food waste is used as a co-substrate in anaerobic digestion of sew-age sludge, there is much room for improvement in the digestion performance. The concept of co-digestion has been tested several times as an alternative because feed characteristics might be one of the major governing factors to sound performance. Addition of co-substrate to sewage sludge leads to enhanced biogas production and VS reduc-tion due to deficient nutrient supply, toxic materials dilu-tion, biodegradability improvement, and microbial activity stimulation (Cecchi et al. 1989, Kiely et al. 1997, Converti et al. 1997, Gallert & Winter 1997, Del Borghi et al. 1999, Stroot et al. 2001). However, the food waste fraction and characteristics vary with geographical location. It is neces-sary to suggest the adequate mixing ratio of Korean food waste for adjusting excess amount of inhibitory materials in co-digestion, such as volatile fatty acids (VFA), ammonia, sodium ions, etc (Mata-Alvarez et al. 2000).

Biochemical methane potential (BMP) tests have been conducted because it is essential to identify the methano-genic activity of biomass and the influence of environ-mental conditions on methane conversion. Much useful information on kinetic parameters and process character-istics could be obtained by applying regression techniques. Until now, a conventional linear regression has been popu-lar in the experiments using specific substrates and micro-organisms. However, in co-digestion of sewage sludge and OFMSW, different biodegradability between two major substrates makes it more difficult to fit the cumulative ten-dency of experimental methane production data correctly. Therefore, to overcome the limitations of the simple linear regression, non-linear approach to co-digestion is needed

to estimate more accurate parameter values in methano-genesis of complex substrates.

The reliability of process mainly depends on the charac-teristics of substrate especially in co-digestion. If a closely influential parameter is found in batch tests for mixing ratios of different substrates, it could be useful information and also save effort to evaluate the best performance of co-digestion more reasonably. This research was, therefore, performed to find the optimal mixing ratio of food waste and sewage sludge in co-digestion using the re-modified Gompertz equation. Also, the effect of food waste addition on anaerobic digestion of sewage sludge was investigated in mesophilic and thermophilic conditions on the basis of feed characteristics.

Materials and methodsSeed

Seed micro-organisms were taken from lab-scale mes-ophilic and thermophilic anaerobic chemostats. The total suspended solids (TSS) and volatile suspended solids (VSS) concentrations of the thermophilic seed sludge were 21.8 and 8.0g/l, respectively, and those of the mesophlic seed sludge were 34.9 and 13.6g/l, respectively.

Substrate

The feed was a mixture of food waste and sewage sludge, representing typical Korean food waste and sewage sludge. Food waste, sampled from a dining hall, was crushed by an electrical blender and diluted to 20% (v/v) with liq-uid from food waste. Sewage sludge was sampled from a local wastewater treatment plant. All the substrates were filtered through a stainless steel sieve (U.S. Mesh No. 10 with corresponding sieve openings of 2.00 mm). The char-acteristics of substrate were summarised in Table 1.

Operating procedure

The BMP tests were conducted using 160 ml serum bot-tles with a working volume of 100 ml. The bottles were operated at mesophilic (35°C) and thermophilic (55°C) temperatures, as shown in Tables 2 and 3. The various mixtures of food waste and sewage sludge were added to each serum bottle. Their initial concentrations were all set to 2g VS/l in Experiment I (Cho et al. 1995, Converti et al. 1997). Consulting the obtained mixing ratios results of Experiment I, substrate concentrations were controlled to 1-4g VS/l in Experiment II for verifying effects of substrate on MPR.

After filling all the bottles to 80 ml with anaerobic medium solution, 20 ml of seed sludge was added to indi-vidual serum bottles. Each litre of anaerobic medium solu-





517Waste Management & Research

tion contained 0.53g of NH4Cl, 0.27g of KH2PO4, 0.35 g of K2HPO4, 1.20 g of NaHCO3, 0.075 g of CaCl2·2H2O, 0.10 g of MgCl2·6H2O, 0.02 g of FeCl2·4H2O, 0.05 g of MnCl2·4H2O, 0.05 g of H3BO3, 0.05 g of ZnCl2, 0.03 g of CuCl2, 0.01 g of Na2MoO4·2H2O, 0.50 g of CoCl2·6H2O, 0.05 g of NiCl2·6H2O and 0.05 g of Na2SeO3. All the bot-tles were purged with N2 gas before sealing. The bottles were incubated in a rotary shaker to provide better contact of substrates, nutrients and microorganisms. The volume of biogas was determined using glass syringes of 5-50ml according to Owen et al. (1979).

Analytical methods

The contents of methane and carbon dioxide in the biogas were analysed by a gas chromatograph (GC, Gow Mac series 580) equipped with a thermal conductivity detector (TCD) and a 2 m x 2 mm stainless-steel column packed

with Porapak Q (80/100 mesh). During the experiments, 1 ml of sample was collected at a proper time with a syringe. The samples were immediately filtered through 0.45 µm cellulose nitrate membrane filters (Whatman) and then stored at 4˚C for analysis. For the analyses of individual volatile fatty acid (VFA) concentrations, HPLC (Spectra Physics P2000) was used with an Aminex HPX-87H (300 x 7.8mm) column and a UV (210 nm) detector. Dorhmann DC-180 TOC analyser was used for total organic carbon (TOC) and the concentrations of dissolved organic car-bon (DOC) were converted to chemical oxygen demand (COD) by assuming the average oxidation number of car-bon as zero. Chemical composition of substrates was ana-lysed by Elemental Analyser (Fisons, EA-1110) equipped with a dynamic flash combustion-oxidation chamber and TCD. The total solids (TS), VS, TSS, VSS were deter-mined according to Standard Methods (APHA, 1998).

Table 1: Characteristics of substrates.

Item Unit Food waste Sewage sludge

Physical characteristics

TS % 4.24 3.04

VS % 4.10 1.49

VS/TS 0.97 0.49

Chemical Characteristics

Carbon (C) % 45.7 24.4

Nitrogen (N) % 2.2 3.4

Hydrogen (H) % 6.7 3.9

Sulfur (S) % - 0.7

C/N 20.8 7.2

Table 2: Batch reactor operating conditions of Experiment I.

Temperature Batch reactor No.

Preparation of substrate

Design substrate(g VS/l)

Sewage sludge(% of VS)

Food waste(% of VS)

Thermophilic (55°C)and Mesophilic (35°C) sets

1 2 100 0

2 2 80 20

3 2 50 50

4 2 20 80






Regression analysis Linear regression

In the batch tests, anaerobic degradation after initial lag time is limited by the terms associated with substrate and kinetics, which generally could be represented by a first order kinetic law with some assumptions. Thus, first order kinetic constants on the methane production in the serum bottles were evaluated according to the approach reported by Llabres-Luengo and Mata-Alvarez (1987)

(P – M)––––––– = e –kt (1)

P

Where, M = cumulative methane production (ml) at incubation time (d), P = methane production potential (ml).

Non-linear regression

Lay et al. (1997) reported that the cumulative methane production data in BMP tests were well fitted to a modified Gompertz equation (Zwietering et al. 1990, Zwietering et al. 1992, Cho et al. 1996) as shown in equation (2).

R• e M=P• exp(––––– ( –t) +1 ) (2)

P

Where, M = cumulative methane production (ml) at incu-bation time, t (d), = lag-phase time (d), P = methane production potential (ml), R = methane production rate (ml/d), e = 2.718. However, the equation was not accurate in describing exact cumulative methane production of co-digestion due to different biodegradability between organic

fractions. In this study, equation (3) was developed to over-come the limitation of equation (2) by adding a secondary term. The first term represents the methane production from readily degradable materials in an early stage and the second term expresses the subsequent methane production from slowly degradable materials.

R1• e R2• eM=P1 • exp(––––––(1–t)+1) +P2• exp (–––––– (2–t)+1) (3)

P1 P2

Where, M = cumulative methane production (ml) at incubation time, t (d), 1 = initial lag-phase time (d), P1 = initial methane production potential (ml), R1 = initial methane production rate (ml/d), 2 = secondary lag-phase time (d), P2 = secondary methane production potential (ml), R2 = secondary methane production rate (ml/d). Accordingly, the equation (3) was fitted as a suit-able model to describe the progress of cumulative methane production in batch co-digestion of food waste and sewage sludge.

Results and discussion

Effect of food waste addition on co-digestion perform-ance and its regression (Experiment I)

Based on 2 g VS/l generally used in BMP, five serum bot-tles were operated at 55˚C with different mixtures of food waste and sewage sludge using thermophilic seed sludge. Another five bottles were operated at 35˚C with the same substrates and media using mesophilic seed sludge.

Table 3: Batch reactor operating conditions of Experiment II.

Thermophilic reactor No. Mesophilic reactor No.Design substrate

(g VS/l)Sewage sludge

(% of VS)Food waste(% of VS)

9 25 1 100 0

10 26 1 80 20

11 27 1 60 40

12 28 1 40 60

13 29 2 100 0

14 30 2 80 20

15 31 2 60 40

16 32 2 40 60

17 33 3 100 0

18 34 3 80 20

19 35 3 60 40

20 36 3 40 60

21 37 4 100 0

22 38 4 80 20

23 39 4 60 40

24 40 4 40 60






As shown in Figs. 1 and 2, accelerated hydrolysis was observed by the addition of the proper amount of food waste in both temperature conditions. Thermophilic digestion showed a higher conversion rate in hydrolysis as well as acidogenesis. In thermophilic conditions, the major portion of DOC was identified as VFA. The levels of DOC and VFA sharply decreased after four days by fast acido-genesis and methanogenesis. However, in mesophilic con-ditions, DOC concentrations were kept high because the hydrolysates were not easily converted to VFA, indicating that the acidogenesis was a rate-limiting step. In anaerobic digestion of organic materials including large molecules or sterically incompatible molecules or highly crystalline mol-

ecules, which cannot be easily transported into the bacte-rial cells, a further step of hydrolysis by exoenzymes should be considered (Del Borghi et al. 1999).

Fig. 3 shows that, as the mixing ratios of food waste increased up to 50%, cumulative methane production in both conditions increased. Similar observation was report-ed by Cecchi et al. (1989) for the digestion of separately-collected (SC) OFMSW and sewage sludge. Therefore, it indicated that the application of co-substrate significantly improved feed characteristics, resulting in higher methane potential. Higher ultimate methane yield of food waste can be found in Table 4, which shows reported data from various OFMSW. Also, it was confirmed that co-substrate,

food waste, gave higher methane yields than previously reported BMP values (Table 4).

The methane yields of all the batch reactors are shown in Table 5. The methane yields of the thermophilic digestion were higher by 19.2-29.3% than those of the mesophilic one. These trends of methane yields could be explained by the trends of VFA and DOC shown in Figs. 1 and 2. Ahn & Forster (2000) reported that thermophilic digestion, intrinsically, would have higher degrading capability and methanogenic activity. Moreover, most of the thermophilic co-digestion studies agreed that thermophilic condi-tion was a more balanced fermentation system in biogas production (Griffin et al. 1997, Gallert & Winter 1997, Del Borghi et al. 1999).

In order to evaluate and optimise the previously discussed effects of food waste, a conventional linear regression was conducted by taking all the cumu-lative methane production data into consideration. Because it takes too much time to reach an ultimate value of BMP in case of a real substrate, the consideration of time becomes influ-ential in optimising the use of food waste. Therefore, methane production rate (MPR) was regarded as the most important parameter.

Table 6 shows the first-order rate constants by using equation (1). In all the cases, higher values were obtained in thermophilic conditions. Fig. 3

Time (days)

Time (days)

DO

C (m

g C

OD

/I)

DO

C (m

g C

OD

/I)

Fig. 1: DOC variations during co-digestion at different mixing ratios of food waste and sewage sludge.






also illustrates the addition of food waste rapidly acti-vated methanogens to produce more methane gas when substrates were properly mixed. When the food waste fraction was 50%, the rate constant k50 was superior to that of the control in thermophilic conditions. Also, in mesophilic conditions, k50 was the highest value. Other values of rate constants were similar to or a little higher than the control. However, equation (1) could not reflect the gradual methane production from slowly degradable materials. The fluctuation between regression lines and the value of –ln((P-M)/P) significantly increased after 4 days. Also, the linear regression could not reflect the lag-phase time of batch reactors properly. It is largely

due to the different biodegradability among organic fractions such as car-bohydrates, proteins and lipids. Those obtained rate constants had very low confidence. Therefore, to describe more exact trends of methane produc-tion from complex substrate mixtures, the remodified Gompertz equation was tried as an alternative.

Table 7 shows the estimated values by using equation (3), and Fig. 4 illus-trates the exact description of regres-sion results. The methane production potential (P1) and the lag phase time (1) increased as food waste addi-tion increased. However, in contrast with methane yields, MPR (R1) did not linearly depend on the increase of food waste addition. It decreased when the mixing ratios of food waste were higher than 50% of total VS. This phenomenon could be explained by the variations of VFA concentration. Since the high soluble organics contained in food waste were rapidly converted to VFA as shown in Fig. 2, a drastic pH drop must have inhibited the activ-ity of methanogens as pointed out by Cho et al. (1995). Also, as the loading rate increased, VFA accumulation led methanogenesis to a rate-limiting step instead of hydrolysis (Shin et al. 2000). Those MPR trends implied that there existed adequate food waste fraction showing the highest MPR. Regression models based on food waste fraction and MPR were developed to clarify the influence of food waste addi-

tion on methane production rates as illustrated in Fig. 5. In the results, 50% addition of food waste on VS basis showed the highest MPR in thermophilic reactor set and 20% addi-tion of food waste gave the best result in mesophilic reac-tor set. The suggested quadratic equations presumed the estimated values with high correlation. Accordingly, pre-dicted maximum MPR based on food waste fraction in VS could be calculated by the simple mathematics. Predicted optimal fractions of food waste at 2 g VS/l were 39.3% in mesophilic and 50.1% in thermophilic conditions, respec-tively. Those values meant allowable maximum mixing ratios which could prevent VFA from accumulating during the co-digestion of food waste and sewage sludge.

Time (days)

Time (days)

Tota

l VFA

(mg

CO

D/L

)To

tal V

FA (m

g C

OD

/I)

Fig. 2: Total VFA variations during co-digestion at different mixing ratios of food waste and sewage sludge.






Table 4: Comparisons of methane yields during co-digestion of various OFMSW and sewage sludge in mesophilic and thermophilic conditions.

Reference OFMSWCH4 yield (l CH4/g VS)

Temperatureconditions

This study

aSS only 0.116 & 0.163 35°C & 55°C

Food waste (50%) + SS (50%) 0.215 & 0.280 35°C & 55°C

Previous BMP tests

Owens & Chynoweth (1993) Grass 0.209 35°C

Office paper waste 0.369 35°C

Cho et al. (1995) Food waste 0.472 37°C

Converti et al. (1997) Wood & corn starch (50%)+ SS(50%) 0.103 35°C

Misi & Foster (2001) bWAS 0.130 35°C

Chicken manure (50%) + WAS (50%) 0.180 35°C

Previous continuous experiment

Cecchi et al. (1989) cSC-OFMSW (50%) + SS (50%) 0.364 35°C

Griffin et al. (1997) Paper & food waste (80%) + SS (20%) 0.157 & 0.254 35°C & 55°C

Del Borghi et al. (1999) House-hold waste (50%) + SS (50%) 0.180 55°C

Stroot et al. (2001) Paper & food waste (80%) + SS (20%) 0.259 37°C

a sewage sludge, b wasted activated sludge, c separately-collected

Table 5: Biochemical methane potentials (BMP) during co-digestion at different mixing ratios of food waste and sewage sludge in mesophilic and thermophilic conditions.

Mixing ratio of food waste(% of VS)

Thermophilic conditions (l CH4/g VS)

Mesophilic conditions(l CH4/g VS)

Mesophilic/Thermophilic(%)

0 0.163 0.116 71.2

20 0.222 0.157 70.7

50 0.280 0.215 76.8

80 0.344 0.257 74.7

Table 6: First-order regression coefficients during co-digestion at different mixing ratios of food waste and sewage sludge in mesophilic and thermophilic conditions.

Unit

First-order regression coefficient at different mixing ratios of food waste addition (% of VS)

0 20 50 80

Thermophilic kinetic constant d-1 0.21 0.22 0.35 0.20

Correlation factor (R2) 0.86 0.92 0.93 0.97

Mesophilic kinetic constant d-1 0.15 0.18 0.22 0.18

Correlation factor (R2) 0.88 0.94 0.94 0.82






Confirmation test of the remodified Gompertz equation on co-digestion performance for various substrate con-centrations (Experiment II)

To confirm the effectiveness of the remodified Gompertz equation, the influence of substrate concentrations (1-4 g VS/l) were tested. The mixing ratios of food waste and sew-age sludge were reset to 0:100, 20:80, 40:60 and 60:40 on VS basis for each set of reactors, of which the correspond-ing C/N ratios were 7.2, 9.9, 12.7 and 15.4, respectively. The specific MPR values, MPR divided by the amount of VS added, were calculated for comparisons.

Fig. 6 illustrates the contours of estimated MPR values in mesophilic and thermophilic conditions by non-linear

regression for each set of reactors. In thermophilic conditions, maximum MPR was consistently maintained at about 40% of food waste addition as the substrate concentration increased from 1 to 4 g VS/l. In mesophilic condi-tions, the maximum MPR values (22.5-27.5 ml CH4/g VS/d) were much lower than those (70-80 ml CH4/g VS/l) of thermophilic conditions. Similarly, as the substrate concentration increased, the mixing ratios of food waste for the optimal MPR were almost stable at 40%. Therefore, it was possible to suggest the optimal range of food waste fraction. When the food waste fraction was 40% in thermophilic conditions, the maximum MPR values were 80.0-45.0 ml CH4/g VS/d. On the other hand, in case of mesophilic conditions, 40% of food waste addition resulted in the highest MPR values of 27.5-17.5 ml CH4/g VS/d, respectively. If the food waste addition is controlled depend-ing on these ranges, the MPR would be maintained to the desirable levels. Such a high value could give enough efficiency of methane recovery and VS destruction to anaerobic co-digestion processes.

The results obtained in this study suggested that co-digestion of food waste and sewage sludge could be a good alternative for improving low per-formance of conventional anaerobic digestion of sewage sludge. Previous studies reported that it was feasible to

digest nutrient-rich MSW with other co-substrate containing a high amount of readily biodegrad-able substances (Stroot et al. 2001). One of the important factors for the enhanced performance of co-digestion was high biodegradability of food waste, which compensated for the lack of readily-biodegradable substances in sew-age sludge. It could be monitored by nutrient balance expressed as a carbon to nitrogen (C/N) ratio. Hawkes (1980) suggested that the optimal C/N ratio for anaero-bic digestion was in the range of 20-30. Hasimoto (1983) reported that carbon addition stimulated the CH4 yield at low C/N ratio. The average C/N ratio of typical Korean sewage sludge was just around seven due to the old com-bined sewer system. Thus, if food waste is added as a

Time (days)

Time (days)

-In(

(P-M

)/P

-In(

(P-M

)/P

Fig. 3: Cumulative methane production and first order rate constants during co-digestion at different mixing ratios of food waste and sewage sludge.






co-substrate, the C/N ratio becomes 12.7 which is more appropriate for anaerobic digestion. It indicates that the addition of food waste plays an important role in providing essential organic carbon to sewage sludge. Moreover, food waste has substantially a high hydrolytic kinetic constant (Vavilin et al. 1999). Although the elemental substances of food waste such as carbohydrates, proteins and lipids have different hydrolytic kinetic constants (Christ et al. 1999), fast acidogenesis and methanogenesis can be possible by the enhancement of rate-limiting hydrolysis. Thus, the bal-anced anaerobic environment led to the increased MPR and methane yield of anaerobic biomass. Another specific opinion was reported by Schmidt and Ahring (1994) that the increase of organic carbon could stimulate the produc-

tion of extracellular polysaccharides, common tools for micro-organisms to communicate with outer environment, resulting in improved bacterial attach-ment to solid surface and the high rate biodegradation.

ConclusionsIt was demonstrated that the co-

digestion of sewage sludge mixed with food waste had a distinct positive effect on MPR and methane yields by keeping the adequate fraction of food waste. In order to overcome the incor-rect estimation of linear regression, the complex remodified Gompertz equation was tested to fit the cumula-tive methane production curve of co-digestion. With the same experimental data obtained at 2 g VS/l generally used in BMP tests, the remodified Gompertz equation gave more exact results with high correlation than the conventional linear regression. The predicted optimal fractions of food waste based on MPR were found to be 39.3% in mesophilic and 50.1% in thermophilic conditions, respectively.

To confirm the application of the remodified Gompertz equation, sec-ondary experiments were conducted based on different substrate concentra-tions (1-4 g VS/l). In all the cases, the complex remodified Gompertz equa-tion described the cumulative meth-

ane production with high correlation. From contours, the confidence fractions of food waste guaranteeing the highest MPR was 30-40% in mesophilic and 40% in thermophilic conditions, respectively. Also, it revealed that MPR values were more sensitive to the food waste fraction of VS than the mixed substrate concentra-tion regardless of temperature conditions. The important factors for the enhanced performance of co-digestion were enhanced biodegradability caused by additional organic carbon, nutrient balance and adequate mixing ratio. Additional carbon source supplied by food waste provided preferable environment for the growth and activity of anaerobes. These optimal mixing strategies could give the guideline for the practical operation of anaerobic co-digestion processes.

Time (days)

Time (days)

Cum

mul

ativ

e C

H4

prod

uctio

n (m

l)C

umm

ulat

ive

CH

4 pr

oduc

tion

(ml)

Fig. 4: Regression results of the remodified Gompertz equation during co-digestion at different mixing ratios of food waste and sewage sludge.






Table 7: Non-linearly estimated regression parameters from the remodified Gompertz equation during co-digestion at different mixing ratios of food waste and sewage sludge in mesophilic and thermophilic conditions.

Temperature Item Unit

Non-linearly estimated parameter value at different mixing ratio of food waste(% of VS)

0 20 50 80

Thermophilic con-ditions

Methane production potential (P1) ml 14.0 25.4 40.0 45.2

Methane production rate (R1) ml/d 9.9 13.6 18.731 14.8

Lag-phase time (1) d 0.1 0.1 0.3 1.0

Correlation factors (R2) 0.996 0.996 0.997 0.997

Mesophilic condi-tions

Methane production potential (P1) ml 13.7 22.2 43.8 26.2

Methane production rate (R1) ml/d 2.5 4.7 4.6 2.7

Lag-phase time (1) d 0.0 0.3 1.3 1.4

Correlation factors (R2) 0.993 0.997 0.998 0.998

Fig. 5: Regression models of methane production rates estimated by the remodified Gompertz equation during co-digestion at different mixing ratios of food waste and sewage sludge.

Mixing ratios of food waste (% at 2 g VS/I)

Met

hane

pro

duct

ion

rate

(R 1,

ml/

d)M

etha

ne p

rodu

ctio

n ra

te (

R 1, m

l/d)

Mixing ratios of food waste (% at 2 g VS/I)






Fig. 6: Variations of estimated methane production rates (MPR, l CH4/gVS/d) during co-digestion at different mixing ratios of food waste and sewage sludge depending on various mixed substrate concentrations.

Mixed substrate concentration (g VS/I)

C/N

rat

io o

f m

ixed

sub

stra

te (

mg/

mg)

Mix

ing

ratio

s of

foo

d w

aste

(%

of

mix

ed s

ubst

rate

VS)

Mixed substrate concentration (g VS/I)

C/N

rat

io o

f m

ixed

sub

stra

te (

mg/

mg)

Mix

ing

ratio

s of

foo

d w

aste

(%

of

mix

ed s

ubst

rate

VS)






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References

AcknowledgementsThis research was supported by a grant (No.M1-0203-00-0063) from the National Research Laboratory Program of Korea Ministry of Science and Technology.




the optimisation of food waste addition as a co-substrate in anaerobic digestion of sewage sludge

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