2.5 Wastewater for the Production of Biodegradable Plastics and Bioenergy

25

2.4.2.1

Biodegradable Plastics and Starch Waste

The isolated strain B. cereus (CFR06) is able to accumulate PHAs in the medium

made from soluble starch and PHB was produced at the concentration of 0.48 g/l.

The observed result was less promising than that found in another study. The saccha-

rified waste potato and starch is used as a carbon source by C. necator NCIMB 11599

which produced PHB at the concentration of 94 g/l [24, 25]. The strain, Cupriavidus

sp. KKU38, accumulated PHAs up to 65.27% (at concentration of 2.8 g/l) in the cas-

sava starch hydrolysate medium. However, this process is not cost-effective since the

hydrolysis of starch into glucose is a two-step process (saccharification and lique-

faction) which makes the feedstock less economically viable [26]. The recombinant

E. coli strain SKB99 sheltering plasmids containing genes for the starch hydrolysis

(from Paenibacillus sp.) and the PHB synthesis (from Ralstonia eutropha) utilizes

starch as an exclusive carbon source, with a maximum production of PHB at 1.24 g/l

(with 40% of PHB content) at 2% (w/v) starch. The production of PHB in engineered

E. coli strain SKB99 is not regulated by stress response in contrast to R. eutropha and

other microbes [27].

2.4.2.2

Bioenergy from Starch Waste

Liquefaction and saccharification are the general processes used for the conversion

of starch into oligosaccharides and glucose. In the first liquefaction stage, dextrin

will be obtained from gelatinized starch by the action of thermophilic α-amylase at

high temperature (95–105 ^∘C) and pH of 6–6.5. In the next saccharification process

stage, cooled liquefied starch slurry will be adjusted to pH 4–4.5 and mixed with glu-

coamylase enzyme to hydrolyze dextrins at relatively lower temperature (60–65 ^∘C)

into glucose. The glucose can be used for the production of bioenergy through the

action of microorganisms [28].

Several

microorganisms

produce

highly

effective

hydrogen

from

starch-

manufacturing waste. Mixed culture of C. butyricum and E. aerogenes HO-39

is used in the starch waste medium prepared from sweet potato starch residue

(carbon source) and corn steep liquor (nitrogen source) for high yield production

of hydrogen (2.7 mol-H2/mol glucose). In a repeated batch culture (pH 7.5) of C.

butyricum, E. aerogenes HO-39, and Rhodobacter sp. M-19, a yield of 4.5 mol-H2/mol

of glucose was obtained [23].

2.5

Wastewater for the Production of Biodegradable

Plastics and Bioenergy

The P(3HB) can be produced from excess sludge obtained in waste-water treatment

plants. Methane can be produced from wastewater activated sludge by the action of

bacteria in two-stage treatment [29–32].