2.6 Integrated Approaches for the Production of Biodegradable Plastics and Bioenergy from Waste

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2.6

Integrated Approaches for the Production

of Biodegradable Plastics and Bioenergy from Waste

Integrated approaches for the production of the biodegradable plastics and bioen-

ergy are flexible and aim (i) to use the mixed cultures or microbial strains which

show better capacity for the accumulation of PHA under the specific feeding condi-

tions, (ii) to produce organic acids from a complex organic solid wastes which are

rich in carbohydrates, and (iii) to produce bioenergy or PHA by microorganisms

from the acidogenic effluents.

For the valorization of the waste, biomass derived from municipal organic waste,

food processing factory wastes, agricultural wastes, etc., can be used for the pro-

duction of both biogas and biohydrogen by microbial processes. The advancement

of high-performing microbial strains and the use of the byproducts and wastes as

the substrates make the production cost of biodegradable polymers lower and can

promote their use. Several bacterial strains can synthesize biopolymers from waste

material and store intracellularly (PHA) and extracellularly (EPS). Large number of

bacteria, such as Bacillus spp., Pseudomonas spp., Rhizobium spp., Methylotrophs,

Nocardia spp., Alcaligenes eutrophus, Azotobacter vinelandii, Azotobacter chroococ-

cum, A. latus, Azotobacter beijerincki, and recombinant E. coli, have been efficiently

used for the production of PHAs at an industrial scale from various types of organic

byproducts [1].

Usually, PHA represents intracellular energy and carbon storage, whereas EPS and

the biosurfactants can be produced as extracellular substances for the protection of

cells from desiccation and predation. Biosurfactants will be produced by several vari-

eties of bacterial strains like Bacillus, Rhodococcus, Arthrobacter, Enterobacter, and

Acinetobacter. Several microbes such as A. beijerincki, A. eutrophus, P. oleovorans,

B. megaterium, Nocardia, and Rhizobium are involved in the integrated systems for

the production of bioenergy from the agricultural and industrial wastes, which also

utilize formic acid, acetic acid, and propionic acid as substrates for the production of

PHA. They show the accumulation of PHAs up to 70% of CDW under the nitrogen-

and phosphorous-limited conditions. However, Pseudomonas spp. and Rhizobium

spp. accumulated (PHAs) approximately 60% of CDW. Several other bacterial strains

have also showed the production of PHAs under various conditions with different

yields. Among these species, purple non-sulfur bacteria have shown the production

of both H2 and PHA under nutrient-limited conditions (for example, species such

as Bacillus spp., Rhodopseudomonas palustris, Rhodopseudomonas sphaeroides, and

Rhodospirillum rubrum) [1, 36].

A study on the metabolic activities of Bacillus strains in the transformation of

glucose into PHB and H2 has been conducted in two different stages [37]. During

the first three days in a batch-mode operation, Bacillus thuringiensis EGU45 and B.

cereus EGU44 have reached 1.67–1.92 mol-H2/mol glucose. In the next two days,

B. thuringiensis EGU45 culture has been added with the residual medium which

contains glucose, residual nutrients, and fatty acids, and it produced a PHB yield of

11.3% of CDW. R. palustris WP3-5 has been studied for the estimation of the compe-

tition between the H2 production and PHB synthesis [38]. They tested six different