6

1 Anaerobic Co-digestion as a Smart Approach for Enhanced Biogas Production

(C/N ratio) content of the feedstocks, as few feedstocks are either rich in carbon

(agricultural) or found to be rich in nitrogen (animal waste). High C/N ratio of

feedstock will ultimately lead to reduction in microbial load due to overall nitrogen

deficiency while lower C/N can result in ammonia poisoning that could particularly

affect methanogens leading to lower biogas production. Excess of carbohydrates

in feedstocks needs shorter retention time (RT) in digesters attributed by its quick

oxidation, while excess protein content leads to lesser biogas production ascribed to

accumulation of toxic levels of ammonia; on the other hand, excess lipids though

results in higher biogas production but RT nearly doubles [1] further characterized

by high concentrations of volatile fatty acids (VFAs) and low pH, thus leading to

a consensus that excess of any nutrient cannot be beneficial for biogas production

[2]. The anaerobic co-digestion (AcD) thus offers an opportunity to modify the

composition of the waste to our need that suits our microbial consortium very well,

and in this regard, C/N ratio can be altered to the optimum range. WWTPs around

the world have increasingly opted for co-digestion to increase biogas output, and a

WWTP in Mesa, USA, has successfully evaluated co-digestion of commercial solid

food waste with sewage sludge in pilot-scale anaerobic digesters [3]. Lipid-rich

restaurant waste has been co-digested with sewage sludge [4].

1.2.1

Zero Waste to Zero Carbon Emission Technology

The biogas as renewable energy can contribute in a big way to meet an overzeal-

ous future goal of zero emission economy by supplying fuel to major contributors

of greenhouse gas emissions such as transportation and heavy industries (power

plants, steel and cement industry, to name a few). Presently the biogas, which is

rich in methane, burns clean and helps in the cutdown of carbon emissions at a

domestic level. It is evident now as many countries have taken initiatives in setting

goals for tapping the renewal energy resources, the Australian water industry is said

to have generated 187 GW/year of electricity from biogas via WWTPs and an addi-

tional 5.5 GW/year through AcD [5]. Channeling of organic wastes from land fill,

restaurants, other urban wastes toward existing and time-tested WWTPs is advo-

cated by many countries and has envisioned zero carbon emission by the year 2040.

Figure 1.1 summarizes the scope of AD.

1.2.2

Alternative Feedstocks

Feedstock refers to the particular form of organic waste available for AD but if

left unattended can lead to environmental pollution. United State Environmental

Protection Agency (USEPA) has assigned each feedstock a unique RIN (renewable

identification number) that helps to rate how much of greenhouse gas it can emit

in comparison to fossil fuel [3]. Cattle dung has been traditionally preferred as

the typical substrate for AD; however, in terms of substrate quality it represents

the semi-digested material excreted by ruminants. However, the advantage of

cattle dung as a substrate is that it has inherent microbes catered from intestines

of ruminants specialized in AD and biogas production. Any substrate for AD is