500

32 Microbiology of Biogas Production from Food Waste: Current Status, Challenges, and Future Needs

temperature in India, the concentration of ammonia and VFA can be comfortably

handled in the biogas process [3]. Concentrations of ammonia between 1500 and

3000 mg/l can be inhibitory if the reactor pH is greater than 7.4 [6].

32.7.7

Feedstock Composition

32.7.7.1

Protein-Rich Substrate

The unionized ammonia produced during the digestion of protein is toxic to

the microflora when released in high concentration. Hence, ammonia-tolerant

microflora is suggested for digester that is responsible for methane generation from

acetate via syntrophic acetate oxidation (SAO). Bacteria such as Syntrophaceticus,

Thermacetogenium, Tepidanaerobacter acetatoxydans, Clostridium, and Pseudother-

motoga can bring about SAO. Protein-rich food waste feed to digester is known

to increase the population of bacteria of the families Caldicoprobacteraceae,

Porphyromonadaceae, Lactobacillaceae, and Actinomycetaceae.

32.7.7.2

Lipid-Rich Substrate

Lipid-rich feedstock in the digester results in glycerol and LCFA. It is known to

be one of the microbial inhibitors during AD. Biogas generation for pulse feeding

oleate is recorded by hydrogenotrophic Methanoculleus and Methanobrevibac-

ter. Mesophilic co-digestion of lipid-based feedstock can be met with bacteria,

Syntrophomonas.

32.7.7.3

Carbohydrate-Rich Substrate

Carbohydrate-rich feedstock results in high C/N ratio for microbial physiology,

low hydrolysis of lignocellulosic material, and fast acidogenesis from easily

hydrolyzed carbohydrates. Metagenomic studies have revealed the role CAZymes

(Carbohydrate-Active Enzymes) from microbiome acclimatized to lignocellulosic

feedstock [5].

Co-digestion of food waste with cabbage and cauliflower leaves and stalks at C/N

ratio 45 resulted in high biodegradability, a methane yield of 475 mlSTP CH4/g VS,

and an organic loading rate (OLR) of 0.06 kg of VS/m³ h. An increasing C/N ratio in

AD of dairy manure resulted in decreased methane production. Maximum methane

production per unit loading rate was recorded when the C/N ratio of the feedstock

was 25 [10]. Laboratory-scale studies in co-digestion of mixed fruit and vegetable

waste and cow dung as substrates resulted in enhanced methane yield of 112.9 l in a

semi-continuous stir tank over a period of 40 days [4].

32.7.8

Trace Element Supplementation

Acetogenic and methanogenic bacteria require trace elements such as iron, nickel,

cobalt, tungsten, and molybdenum for methane generation. Low availability of

these trace elements in feedstock from food waste and slaughter house waste may

result in low yield of methane. The sulfide produced during protein digestion may

form complexes with metals and result in decreased bioavailability of essential