498

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

found to be 6.7–7.4. Fruit industry waste and certain urban solid waste feedstocks

are known to initiate high VFA flux with drop in pH which has an inhibitory effect

on methanogenic bacteria [3]. The methane workers are inhibited when the pH falls

below 4.2 in reactor operation. The amount of volatile acids produced and the alka-

linity in the reactor control the pH of the liquid undergoing anaerobic digestion.

If a large amount of readily digestible organic matter were added suddenly, excess

amounts of acids would be produced and will lower the pH. When this occurs, the

methane formers slow down; they cannot keep up with the acid formers, and volatile

acids accumulate in the reactor. The pH values in anaerobic reactors are mainly the

result of the presence of weak acids (carbon dioxide, VFA, hydrogen sulfide) and

weak bases (ammonia), which buffer the wastewater [6].

32.7.3

VFA

Acetic, propionic, and butyric acids are the common VFA produced in the initial

stages of biogas process in an AD. VFA levels above 6 g/l of acetic acid equivalent are

known to decrease the functioning of methanogenic microflora. Beyond this level,

the long-chain VFA was found to be detrimental to several bacteria and thereby

limiting essential process in the anaerobic digester. Application of pretreatment

methods was found to control VFA accumulation [3]. Methanosarcina was more

efficient at higher concentration of VFA than Methanothrix which was important

for granulation in upflow anaerobic sludge blanket (UASB) system. In a healthy

reactor, the volatile acids of the digesting reactor content usually run in the range

of 2500–3500 mg/l expressed as acetic acid [6, 7].

32.7.4

Microbial Consortia in AD

The structure of the microbial community varies in different biogas digesters,

depending on parameters such as type of substrate, operational conditions, etc.

However, abundance microorganism is considered to improve stability and to

decrease the risk of disturbances of biogas process. Generally, the bacteria dominate

the microbial community, whereas the methanogenic bacteria only represent a few

percentages of the total microbial flora as shown in Figure 32.2 [8].

Generally, hydrolysis of the agro-residue and feedstock with minimal processing

is the rate-limiting step in the AD process. The growth of acidogens was faster than

methanogens. The methanogens dominating during short solid retention time (SRT)

and long SRT were Methanosarcina and Methanothrix, respectively. The dry biomass

feedstock has low key microbial inoculum as compared to the animal-based feed-

stock essential for the generation of biogas [3].

The microbial consortia in chicken droppings inoculum included Cam-

phylobacter spp., Bacillus cereus, Staphylococcus epidermidis, Staphylococcus

aureus, Rhizopus spp., and Aspergillus spp., Yersinia enterocolitica, Clostridium,

Methanosarcina, Methanobacterium, Aspergillus, and Penicillium species. AD

in fabricated laboratory-scale biodigester with feedstock of chicken dropping,

vegetable waste, animal waste, and fruit waste in the ratio of 1 : 1 : 1 : 1 generated