heterogeneous substrate with a complex chemical composition which comprises
various organic and inorganic compounds. The absence of unified characterization
criteria for food waste makes it impossible to compare results from different studies
or even to reproduce the results of the same study. The quality of COD or the VS is
quite variable and depends on whether the solids are carbohydrate, protein, or fat.
Even for carbohydrates, the quality of the organics depends on whether they are
soluble or particulate, hydrolyzed, or in lignocellulosic structural form. For example,
the COD value of a gram of volatile solids of carbohydrate, protein, or fat is 1.06,
1.42, and 2.9 g/g VS, respectively, assuming that the theoretical molecular formulae
for the three types of organics are C 6 H 10 O 5 , C 5 H 7 O 2 , and C 57 H 104 O 6 , respectively.
Notice that H 2 yield from carbohydrate, protein, and fat-rich waste was 50, 2.5, and
3.3 mL/g VS, respectively [113]. The H 2 production rate of protein-rich material is
slower than that obtained from carbohydrate-rich substrates [69]; thus, protein
requires more time to biodegrade compared to carbohydrate-rich food waste. For
example, the carbohydrate components of bean curd manufacturing waste, rice bran,
and wheat bran waste degraded rapidly, whereas soluble protein degradation
proceeded at very slow rate [25]. Similarly, H 2 yield from carbohydrate-rich high
solid organic (rice and potato-based) waste was 20 times larger than those of fat and
protein-rich waste (meat and chicken skin, egg and lean meat) [113].
A challenge in converting food waste to H 2 is that its carbohydrates – and
protein – content vary in types and concentrations. The degradation of carbohydrates
requires different environmental and operating conditions than that required for
protein degradation during dark fermentative H 2 production; thus, the relative
composition of the waste must be considered when designing the operational
parameters for H 2 dark fermentation. For the fermentation of food waste
(Table 8.3), the H 2 yield obtained at thermophilic conditions (overall average of
119 mL/g VS) is higher than that obtained at mesophilic conditions (overall average
of 98 mL/g VS). The composition and the type of organics in the food waste affect
the HRT required to produce H 2 in continuous reactors. Generally, the yields of
0.9–1.8 mol H 2 /mol hexose that were obtained in batch and continuous reactor
operated at mesophilic and thermophilic conditions are promising. The optimal
conditions for biohydrogen production from food waste via dark fermentation
using mixed culture were reported by Wongthanate and Chinnacotpong as follows:
initial pH 8.0, initial food to microorganisms (F/M) ratio 4.0, initial iron concentration 100 mg FeSO 4 /L, and thermophilic condition (55 Æ 1
C) [114]. They reported
a maximum hydrogen yield of 46.19 mL H 2 /g COD add with 66% COD removal
efficiency.
336
A. Hajizadeh et al.
various organic and inorganic compounds. The absence of unified characterization
criteria for food waste makes it impossible to compare results from different studies
or even to reproduce the results of the same study. The quality of COD or the VS is
quite variable and depends on whether the solids are carbohydrate, protein, or fat.
Even for carbohydrates, the quality of the organics depends on whether they are
soluble or particulate, hydrolyzed, or in lignocellulosic structural form. For example,
the COD value of a gram of volatile solids of carbohydrate, protein, or fat is 1.06,
1.42, and 2.9 g/g VS, respectively, assuming that the theoretical molecular formulae
for the three types of organics are C 6 H 10 O 5 , C 5 H 7 O 2 , and C 57 H 104 O 6 , respectively.
Notice that H 2 yield from carbohydrate, protein, and fat-rich waste was 50, 2.5, and
3.3 mL/g VS, respectively [113]. The H 2 production rate of protein-rich material is
slower than that obtained from carbohydrate-rich substrates [69]; thus, protein
requires more time to biodegrade compared to carbohydrate-rich food waste. For
example, the carbohydrate components of bean curd manufacturing waste, rice bran,
and wheat bran waste degraded rapidly, whereas soluble protein degradation
proceeded at very slow rate [25]. Similarly, H 2 yield from carbohydrate-rich high
solid organic (rice and potato-based) waste was 20 times larger than those of fat and
protein-rich waste (meat and chicken skin, egg and lean meat) [113].
A challenge in converting food waste to H 2 is that its carbohydrates – and
protein – content vary in types and concentrations. The degradation of carbohydrates
requires different environmental and operating conditions than that required for
protein degradation during dark fermentative H 2 production; thus, the relative
composition of the waste must be considered when designing the operational
parameters for H 2 dark fermentation. For the fermentation of food waste
(Table 8.3), the H 2 yield obtained at thermophilic conditions (overall average of
119 mL/g VS) is higher than that obtained at mesophilic conditions (overall average
of 98 mL/g VS). The composition and the type of organics in the food waste affect
the HRT required to produce H 2 in continuous reactors. Generally, the yields of
0.9–1.8 mol H 2 /mol hexose that were obtained in batch and continuous reactor
operated at mesophilic and thermophilic conditions are promising. The optimal
conditions for biohydrogen production from food waste via dark fermentation
using mixed culture were reported by Wongthanate and Chinnacotpong as follows:
initial pH 8.0, initial food to microorganisms (F/M) ratio 4.0, initial iron concentration 100 mg FeSO 4 /L, and thermophilic condition (55 Æ 1
C) [114]. They reported
a maximum hydrogen yield of 46.19 mL H 2 /g COD add with 66% COD removal
efficiency.
336
A. Hajizadeh et al.