Characteristics of hydrogen production from food waste and waste activated sludge

This study was conducted for microbial hydrogen production from food waste and sewage sludge. Thirty three batch tests with different VS concentration (from 0.5 to 5.0 %, w/v) and mixing ratio of food waste to sewage sludge (from 0:100 to 100:0) were performed at 35°C. Heat-treated anaerobic sludge was used to seed the serum bottles. In all the tests, cumulative hydrogen production reached the maximum values within 2.5 days. n-Butyrate was produced simultaneously with hydrogen production, of which the amount was proportional to that of nbutyrate. Clostridium sp. are, therefore, considered to be the dominant microorganisms in this study because these microorganisms are responsible for n-butyrate fermentation. The hydrogen production potential of food waste was found over 34.0 mL/g VS at all the VS concentration. The maximum potential of 59.2 mL/g VS was found at 3.0 % of VS concentration. The potential decreased as sewage sludge composition increased due to the methanogens contained in sewage sludge and low carbohydrate concentration; however, the addition of sewage sludge to food waste enhanced hydrogen yield because of sufficient protein. The maximum hydrogen yield of 1.01 mole H2/mole hexoseadded was achieved at the food waste to sewage sludge ratio of 80:20 at the VS concentration of 3.0 %. The specific hydrogen production rate increased up to 22.6 mL H2/g VSS/h as both food waste composition and VS concentration increased

Characteristics of hydrogen production from food waste and waste activated sludge

*Hang-Sik Shin1, Sang-Hyoun Kim1 and Byung-Chun Paik2

and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea

San 96-1, Dundeok-dong, Yeosu-si, Jeollanam-do, 550-749, Korea

ABSTRACT

This study was conducted for microbial hydrogen production from food waste and sewage sludge Thirty three batch tests with different VS concentration (from 0.5 to 5.0 %, w/v) and mixing ratio of food waste to sewage sludge (from 0:100 to 100:0) were performed at 35°C Heat-treated anaerobic sludge was used to seed the serum bottles In all the tests, cumulative hydrogen production reached the maximum values within 2.5 days n-Butyrate was produced simultaneously with hydrogen production, of which the amount was proportional to that of

n-butyrate Clostridium sp are, therefore, considered to be the dominant microorganisms in this

study because these microorganisms are responsible for n-butyrate fermentation The hydrogen production potential of food waste was found over 34.0 mL/g VS at all the VS concentration The maximum potential of 59.2 mL/g VS was found at 3.0 % of VS concentration The potential decreased as sewage sludge composition increased due to the methanogens contained in sewage sludge and low carbohydrate concentration; however, the addition of sewage sludge to food waste enhanced hydrogen yield because of sufficient protein The maximum hydrogen yield of 1.01 mole H2/mole hexoseadded was achieved at the food waste to sewage sludge ratio of 80:20 at the VS concentration of 3.0 % The specific hydrogen production rate increased up to 22.6 mL H2/g VSS/h as both food waste composition and VS concentration increased

Key Words: Food waste, hydrogen, mixing ratio, sewage sludge, VS concentration

INTRODUCTION

Due to the finite quantities and pollutants emission (CO2, CO, CnHm, Sox, NOx, ashes, etc.), fossil fuels should be alternated by renewable and non-polluting energy sources in recent

future (Momirlan and Veziroğlu, 1999) As a sustainable energy source with minimal or zero use of hydrocarbons, hydrogen is a promising alternative to fossil fuels With high energy yield (122 kJ/g), hydrogen is clean and renewable In addition, hydrogen can be directly used

to produce electricity through fuel cells (Rifkin, 2002) Since conventional physico-chemical production methods (e.g water electrolysis or chemical cracking of hydrocarbons) require electricity derived from fossil fuel combustion, interest in biohydrogen production has

increased significantly (BenneHawkes et al., 2002) Between two biological processes,

fermentative process that uses refuse or organic wastes seems technically simpler than photosynthetic process

Clostridium species (sp.) are the representative anaerobic fermentative hydrogen producing

bacteria (Hawkes et al., 2002) Due to the ability to produce endospore, they can be easily selected from natural environments such as anaerobic sludge, compost and soil by inhibiting

other bacteria using heat, acid/base, ultrasound, chemicals, freezing/thawing, etc (Sparling et

al., 1997; Van Ginkel et al., 2001; Chen et al., 2002; Wang et al., 2003) Clostridium sp are

also able to use wide range of biopolymers with various extracellular enzymes or enzyme complexes (Mitchell, 2001) Carbohydrates are the preferred organic carbon source for

hydrogen producing fermentation Stoichiometrically, Clostridium sp can produce 2 moles of

hydrogen with 1 mole of n-butyrate or 4 moles of hydrogen with 1 mole of acetate from 1 mole of hexose In most cases using soluble defined substrates, hydrogen production yield and major byproduct were 0.7-2.1 mole/hexoseconsumed and n-butyrate, respectively (Mizuno et

al., 2000; Fang and Liu, 2002) However, hydrogen was hardly produced from protein and

lipids (Okamoto et al., 2000; Noike and Mizuno, 2000)

Up to now, fermentative hydrogen production was studied using organic wastes such as

high-strength wastewater (Ueno et al., 1996), lignocellulosic waste (Sparling et al., 1997), municipal solid waste (Lay et al., 1999; Okamoto et al., 2000; Lay et al., 2003), food manufacturing waste (Noike et al., 2000; Noike et al., 2002) and waste acitivated sludge (Wang et al., 2003a; Wang et al., 2003b) The maximum hydrogen production potentials were

in the range of 10-70 mL H2/g VS However, systematic studies of the anaerobic fermentation

of solid wastes are still lacking, although hydrogen yield and hydrogen production rate may significantly depend on the characteristics of organic wastes, such as water content, carbohydrate composition, carbohydrate/nutrient balance, etc

Food waste and sewage sludge are the most abundant and problematic organic solid wastes

in Korea The generation of food waste reaches 11,237 tons per day in Korea, accounting for 23.2 % of municipal solid wastes (Ministry of environment, 2002) Food waste is the major source of decay, odor, and leachate in collection and transportation due to the high volatile solids (VS; 80~90%) and moisture content (75~85%) Food waste, consolidated in landfills with other wastes, has resulted in serious environmental problems such as odor emanation, vermin attraction, toxic gas emission and groundwater contamination However, food waste

might be suitable for anaerobic hydrogen production, because it is the carbohydrate-rich, and easily hydrolysable waste (Han and Shin, 2002) 5,689 tons of digested and dewatered sewage sludge cakes are generated per day (Ministry of environment, 2002) Now, 72 % of them are disposed by ocean dumping, however it will be prohibited according to London Convention in recent future The enhancement of anaerobic digester is, therefore, urgent to reduce the amounts of the sludge cakes and improve the quality for reuse Phase separation and/or co-digestion with carbon-rich waste is known as an economic and feasible approach to retrofit the conventional digester (Lafitte-Trouquē and Forster, 2000; Schafer and Farrell, 2000) If hydrogen can be produced in acidogenesis of sewage sludge or sewage sludge/food waste co-digestion, sewage sludge will be the important source for hydrogen production due to its amounts Thus, in this work, food waste and sewage sludge were used for fermentative production of hydrogen Effects of VS concentrations and mixing ratio of two substrates were investigated by serum bottle tests

MATERIALS AND METHODS

Seed

The seed sludge was taken from an anaerobic digester in a local wastewater treatment plant and heat-shocked at 90°C for 10 min to inhibit the bioactivity of hydrogen consumers and to

harvest spore-forming anaerobic bacteria (Hawkes et al., 2002) The pH value, alkalinity, and

volatile suspended solids (VSS) concentration of the sludge were 7.6, 2.83 g CaCO3/L, and 5.5 g/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 under anaerobic condition 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 opening of 2.00 mm) The characteristics of the substrate were summarized in Table 1

Operating procedure

The experiments were conducted using 415 mL Wheaton media lab bottles A total of 33 bottles with different volatile solids (VS) concentrations and mixing ratios of food waste and sewage sludge were simultaneously operated Total VS concentrations were controlled to 0.5, 1.0, 1.5, 2.0, 3.0, and 5.0 % The mixing ratios of food waste to sewage sludge were designed 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100 on VS basis; however, the experiments at 20:80

and 0:100 for 3.0 and 5.0 % of total VS concentrations could not be conducted because of low

VS concentration of sewage sludge 20 mL of seed sludge and appropriate amounts of food

waste and sewage sludge were added in individual bottles Each bottle was supplemented with

200 mg of KH2PO4, 14 mg of MgCl2•4H2O, 2 mg of Na2MoO4•4H2O, 2 mg of CaCl2•2H2O,

2.5 mg of MnCl2•6H2O, and 10 mg of FeCl2•4H2O, which was modified from Lay et al

(1999) NaHCO3 was also added to adjust total carbohydrate : alkalinity ratio to 1.0±0.1 Each

bottle was then filled to 200 mL with distilled water and pH value was adjusted to 6.0 using

either 1 M HCl or 1 M KOH Subsequently, the headspaces of the bottles were flushed with

N2 gas for 1 min and the bottles were tightly sealed using open-top screw caps with rubber

septum The bottles were then placed in a reciprocating shaker at 35oC and 100 rpm The

biogas production was determined using a glass syringe of 20-200 mL (Owen et al., 1979)

At the same time, gas composition was measured and the sample from the supernatant was

taken to analyze pH and organic concentrations If the pH value was out of the range from 5.0

to 6.0, it was re-adjusted using injection of either 1 M HCl or 1 M KOH by syringes

Table 1 Characteristics of substrate

Analytical methods

Hydrogen content in biogas was measured by a gas chromatography (GC, Gow Mac series

580) using a thermal conductivity detector and a 1.8 m × 3.2 mm stainless-steel column

packed with molecular sieve 5A with N2 as carrier gas The contents of CH4, N2, and CO2

were measured using a GC of the same model noted previously with a 1.8 m × 3.2 mm

stainless-steel column packed with porapak Q (80/100 mesh) using helium as a carrier gas

The temperatures of injector, detector, and column were kept at 80, 90, and 50°C, respectively,

in both GCs VFA (C2-C6), and lactate were analyzed by a high performance liquid

chromatograph (Spectrasystem P2000) with an ultraviolet (210 nm) detector and an 300 mm ×

7.8 mm Aminex HPX-97H column using H2SO4 of 0.005 M as mobile phase Aliphatic alcohol was determined using another high performance liquid chromatograph (DX-600, Dionex) with an electrochemical detector (ED50A) and an 250 mm × 4 mm Dionex CarboPac PA10 column using NaOH of 0.01 M as mobile phase The liquid samples were pretreated with 0.45 µm membrane filter before injection to both HPLCs Chemical oxygen demands (COD), Suspended solids (SS), VSS, TKN, ammonia, and pH were determined according to Standard Methods (APHA, 1998) Carbohydrate was determined by the colorimetric method

of Dubois et al (1956) with UV wavelength at 480, 484 and 490 nm using glucose as

standard Soluble protein was also measured by the colorimetric method at a wavelength of

562 nm with bovine serum albumin as standard (Smith et al., 1985) Total protein was calculated from organic nitrogen (9.375 g COD/g organic nitrogen) (Miron et al., 2000)

RESULTS AND DISCUSSION

Fermentation characteristics

In all the tests, cumulative hydrogen production reached the maximum values within 2.5 days as shown in Fig 1 The hydrogen production curve was fitted to a modified Gompertz equation (1), which has been used as a suitable model for describing the hydrogen production

in batch tests (Lay, 2001; Lee et al., 2001; Chen et al., 2002)

}]

1 ) ( exp{

∗

P

R P

where H was cumulative hydrogen production (mL), P was hydrogen production potential (mL), R m was hydrogen production rate (mL/day), λ was lag-phase time (days), and e was

exponential 1

All the correlation coefficients, R2, were larger than 0.98 Additionally, all the t-values for

parameters were larger than t0.975, 5 = 2.571 (table value) The specific hydrogen production

potential (mL/g VS) was obtained by dividing P by the substrate weight (g VS), while the specific hydrogen production rate was calculated by dividing R m by the inoculum weight (g

VSS) Hydrogen production yield (mole/mole hexose) was determined by dividing P by

22,400 (mL/mole) and by either carbohydrate added (mole hexose) or carbohydrate consumed

(mole hexose) Lag-phase times (λ) for hydrogen production were not longer than 0.8 day,

which was shorter than reported values (2-4 days) in batch tests with heat-treated inocula (Lay

et al., 1999; Lay 2001) It was seemed that environmental conditions such as substrate, amino

acids, inorganic nutrients, and pH were sufficient for spore germination in this study (Hawkes

et al., 2002)

0

2000

4000

6000

8000

10000

12000

0

50

100

150

200

250

Food waste composition 1.00 Food waste composition 0.60 Food waste composition 0.20

0

5

10

15

20

25

30

0

2000

4000

6000

8000

10000

12000

0

1000

2000

3000

4000

5000

Time (days)

0

1000

2000

3000

4000

5000

Fig 1 Development of biogas (H2 and CH4),

carbohydrate and soluble products

with time at 2.0 % of VS

concentration

Methane was observed in all the bottles where sewage sludge was added, due to methanogenic bacteria in sewage sludge

(Wang et al., 2003) However, the amount of

methane was less than 8.1 mL/g VS, which was much lower than reported values (17.5 L/g VS) in which the methanogenic bacteria

was externally dosed (Chu et al., 2002)

Carbohydrate degradation and organic acids production almost ceased as hydrogen production ended up In most cases, n-butyrate was produced simultaneously with hydrogen production Simultaneous n-butyrate production with hydrogen was also reported in anaerobic hydrogen fermentation

(Lay et al., 1999; Noike et al., 2000), meaning that Clostridium sp were related

with hydrogen production in this study

(Payot et al., 1998; Yokoi et al., 1998)

H2/VFA production was followed by alcohols production In normal batch culture,

exponential growth phase, and alcohols in

the late growth phase (Lay et al., 1999; Ueno et al., 2001) Alcohols represent

hydrogen that has not been liberated as a gas

(Hawkes et al., 2002) Ethanol was the most

abundant alcohol, and small amounts of 2-propanol, butanol and 2-pentanol were also detected

Effects of VS concentration and mixing ratios on hydrogen fermentation

The hydrogen production potential of food waste was found over 34.0 mL/g VS at all the

VS concentrations as shown in Fig 2 It is known that hydrogen production using concentrated substrates higher than 1% TS is needed for suitable energy production system

(Hawkes et al., 2002) In this study, hydrogen production potential increased as VS

concentration increased up to 3.0 % (3.15 % as TS) The maximum potential was 59.2 mL/g

VS, which was in the range of reported maximum potential of carbohydrate-rich biomass such

as rice bran, carrot, cabbage (Okamoto et al., 2000; Noike et al., 2000) The hydrogen

production potential decreased as VS concentration increased further Product inhibition by

H2 and VFAs might cause the decrease in the hydrogen production potential at 5.0 % of VS

concentration (Van Ginkel et al., 2001; Lay, 2001) The hydrogen production potential

decreased as sewage sludge composition increased Hydrogen production over 2.2 mL/g VS could be achieved only when food waste composition was higher than 20% The reasons of insignificant hydrogen production from sewage sludge might be the methanogens contained

in sludge and low carbohydrate concentration (Wang et al., 2003a)

Food waste composition (%, VS basis)

1

2

3

4

5

5

5

5

5

20

15

15

15

15

15

10

10

10

10

30

30

30

30

30

25

25

25

25

25

20

20

20

35

35

35

35

35 20

45 45 45

40 40

40 40

55 55

50 50 50

Fig 2 Constant hydrogen production

potential (mL H2/g VS) contour lines

against food waste composition and

VS concentration

However, the maximum hydrogen yield

of 1.01 mole H2/mole hexoseadded (1.12 mole

H2/mole hexoseconsumed) was found at the mixing ratio of 80:20 (food waste:sewage sludge) and at the VS concentration of 3.0 %

as shown in Fig 3 Addition of sewage sludge to 20 % of total VS enhanced hydrogen yield (based on carbohydrate) at

VS concentrations ranging 1.0 to 5.0 % It was reported that adequate control of inorganic nutrient can enhance the hydrogen

production (Hawkes et al., 2002) In this

study, however, the concentrations of nutrients such as phosphorus and iron were

sufficiently supplemented (Lee et al., 2001; Fang et al., 2002) Nitrogen was not externally dosed, but carbohydrate to nitrogen ratio was

less than 19.3 g carbohydrate-COD/g TKN-N, which meant nitrogen was also sufficient

(Mizuno et al., 2000; Van Ginkel et al., 2001; Lin and Chang, 2003)

Protein would be a better explanation for the synergic effect It was well known that protein such as peptone or yeast extract was better a nitrogen source than ammonium salts or urea for

activation and growth of Clostridium sp (Mitchell, 2001) Addition of protein was helpful or

even indispensable, sometimes, for the hydrogen production in both pure and mixed culture

(Taguchi et al., 1996; Ueno et al., 2001; Yokoi et al., 2001) Food waste is a carbohydrate-rich

waste (0.56 g carbohydrate-COD/g VS and 0.25 g protein-COD/g VS), while sewage sludge

is a protein-rich waste (0.20 g carbohydrate-COD/g VS and 0.73 g protein-COD/g VS) Addition of sewage sludge from 0 to 20 % of total VS decreased carbohydrate to protein ratio

from 2.24 to 1.85 g carbohydrate-COD/g protein-COD

The specific hydrogen production rate increased as both food waste composition and VS concentration increased as shown in Fig 4 It was reported that the production rate was less inhibited than the production yield (Lay, 2001; Van Ginkel et al., 2001) The maximum hydrogen production rate was 22.6 mL H2/g VSS/h, which was in the range of reported values

in serum bottle tests using organic wastes (Lay et al., 1999; Lay, 2001).

Food waste composition (%, VS basis)

1

2

3

4

5

0.20 0.20

0.20 0.20

0.20 0.10

0.10

0.10

0.10

0.40 0.30 0.30

0.30 0.30 0.30

0.60 0.60 0.60 0.60

0.60 0.50

0.50

0.50

0.50 0.50

0.40

0.40

0.40

0.70 0.70

0.70

0.70

0.80 0.80

0.80

0.80

0.90 0.90

0.90

Fig 3 Constant hydrogen yield (mole

H2/mole hexoseadded) contour lines

against food waste composition and

VS concentration

Food waste composition (%, VS basis)

1 2 3 4

5

2

2

2

2

2

4 4

4 4 4

8 8

8 8

6 6

6 6 6

10 10

10 10

14 14 14

12 12

12 12

18 18

16 16

20

Fig 4 Constant specific hydrogen production rate (mL H2/g VSS./h) contour lines against food waste composition and

VS concentration

CONCLUSIONS

Food waste and sewage sludge at various VS concentration (from 0.5 to 5.0 %, w/v) and mixing ratio of food waste to sewage sludge (from 0:100 to 100:0) were used for fermentative production of hydrogen After lag-phase shorter than 0.8 day, hydrogen was produced rapidly The metabolic results indicated that the characteristics of the heat-shocked digester sludge converting the organic wastes were similar to those of anaerobic spore-forming bacteria,

Clostridium sp The hydrogen production potential of food waste was found over 34.0 mL/g

VS at all the VS concentrations The maximum potential of 59.2 mL/g VS was found at 3.0 %

of VS concentration The potential decreased as sewage sludge composition increased The maximum hydrogen yield of 1.01 mole H2/mole hexoseadded was, however, achieved at the sewage sludge composition of 20 % and at the VS concentration of 3.0 % Increase of protein concentration by adding sewage sludge might cause the synergic effect The specific

hydrogen production rate increased up to 22.6 mL H2/g VSS/h as both food waste composition and VS concentration increased Use of food waste and sewage sludge as the main and the auxiliary substrates seems feasible way to produce hydrogen

ACKNOWEDGEMENT

This work was supported by grant No M1-0203-00-0063 from the National Research Laboratory Program of the Korean Ministry of Science and Technology

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