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

2.5.1

Biodegradable Plastics from Wastewater

2.5.1.1

Production of PHA from Wastewater

Number of related co-polymers and Alcaligenes spp. were identified in activated

sludge obtained from wastewater. The particular polymer yield has been increased to

about 0.39 g/g dry cells. The yield can be increased by increasing the C/N ratio from

20 to 140. Once the C/N ratio has been maintained at the nitrogen-deficient level

of about 100, the highest polymer production was achieved. The particular polymer

yield in the isolated Alcaligenes spp. reached as high as 0.7 g/g dry cell mass. This

approach not only reduces the cost of production of biodegradable plastics but also

reduces the amount of the excess sludge which was generated from the wastewater

treatment by around 39% [32].

2.5.1.2

Production of PHB

The fresh activated sludge can be collected from the water treatment plant for the

production of PHB. Bacterial strains enriched were isolated by the spreading of the

sludge on the different nutrient agar plates. Five different types of bacterial strains

were obtained on the basis of colony characteristics. It was found that the PHB gran-

ules are produced in all the five different strains. In the normal conditions, the bacte-

ria will synthesize proteins as they grow. During the limited nutrient conditions, the

bacteria will move their proteins for the synthesis of PHB in order to survive [33].

As per an increase in the C/N ratio (24–168), the accumulation of the PHB in the

cell mass also increased. The maximum PHB (33%) was accumulated at C/N ratio

of 144 after an incubation period of 96 hours. For the optimization of production of

PHB, the various concentrations of the activated sludge (biomass) ranging from 0.5

to 3.5 g/l was also used and the maximum production of the PHB was attained at

3 g/l [33].

2.5.2

Production of Bioenergy

The composition, potential, and efficiency of bioenergy from the sludge of wastewa-

ter mainly depend on high-rate algal pond (HRAP) and combination of HRAP with

intensified oxidation ponds and an algal reactor. During the operation of HRAP,

the hydraulic retention time (HRT) of 48 hours results in highest biomass (54 ± 12 g

ash-free dry weight/m²/d) with good settling properties and algal-bacterial popula-

tion compared to a HRT of 72 hours. The two-stage process was found more efficient

than one-stage treatment in the removal of nitrogen and increase of methane yield

(up to 30%, from 267 to 340 ml CH4/gVS).

The change in the composition of algal–bacterial biomass leads to variation

in total energy output (nearly 40%), net energy ratios (1.5–2.2), and efficiencies

(60–68%). However, energy output of only 15–20% of energy available with biomass

and methane yields of only 40–50% of theoretically available with biomass were

achieved. These values can be improved from algal–bacterial biomass of wastewater

in the coming years [34, 35].