**4. Waste management processes**

There are different methods of solid waste management which are well known. Among the various methods, bio-process-based are the most suitable for managing the waste in a safe way and they generate the nontoxic degraded foams.

1.**Landfills:** Landfill is the most primitive and commonly used waste disposal method (**Figure 3**). In this method, waste organic materials are simply collected and thrown in depth or pits. There are two main processes involved in an efficient landfilling method: 1. Collection of waste in a very small and confine place. 2. Compaction of collected waste by compactors or bulldozers.

There are five phases involved in this method:

i. Phase I: Adjustment of waste with O2 supply

The waste compacted in this phase with void spacing for oxygen exchange (O2). Initially, O2 level is high due to activity of microbes for aerobic decomposition process but O2 level gradually decreased due to high microbial density.

ii. Phase II: Transition from aerobic to anaerobic

Due to high microbial density, O2 level decreases and CO2 level increases. The anaerobic condition occurs in the layers and primary electron acceptors are sulfates and nitrates instead of O2.

iii. Phase III: Generation of organic acids by acidification

This phase is also known as acidification phase, and in this, volatile fatty acids (VFA) are decomposed to acetic acid (CH3COOH), CO2, and H2. The H2 formation phase occurred in the last and activates the H2 oxidizing bacteria. The acidogenic bacteria increase the decomposition rate of waste.

**Figure 3.** *Landfill mechanism of waste decomposition.*

iv. Phase IV: Methonogenesis

The organic acids products, viz., acetic, propionic, and butyric acids are converted into CH4 and CO2 as intermediate products by methoanogenic microorganisms. In this phase, pH changes from acidic to neutral. This phase is the longest phase of decomposition.

v. Phase V: Humification

All the microbial activity slows down in this phase and CH4 production also completely disappears. All the remaining organic materials are converted into gas phase by the oxidative processes and converted into humic acid compounds.

2.**Anaerobic decomposition:** Anaerobic decomposition is gaining attention in various energy sources to fulfill the demand of increasing population in a safer and economical way [17]. There are around 267 plants that have been distributed all over the world. In anaerobic decomposition, microorganisms decompose the solid waste into biodegradable and nontoxic forms in the absence of oxygen (1). There are two types of bacteria which play a significant role in anaerobic decomposition reactions, which include hydrolytic bacteria and the acidogenic bacteria. These two bacteria belong to two kingdoms, the archea and the bacteria [18].

There are four successive stages of anaerobic decomposition which are: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These four stages are carrying out by the interaction of various microbes [19].

#### a. Hydrolysis

The decomposition pathways start with the hydrolysis of the waste agricultural biomass, which is rich in complex polymeric constituents. The hydrolytical cleavage broke the polymeric interconnecting bonds by the hydrolysis process. Agricultural waste is rich in organic polymers such as cellulose, hemicelluloses, lignin, etc. [20]. Hydrolysis breakdown is an electrochemical process driven by the hydrolytic bacteria by cellulolytic enzymes, lipase enzymes, etc. Hydrolysis cleavage converts these insoluble long chain polymers into soluble simpler derivatives like ammonium and other organic constituents [21].

Kinetically, hydrolysis reactions are the first-order reactions in which complex biodegradable waste material is hydrolyzed at constant temperature and pH [22].

$$(\text{C6H10O5})\text{n} + \text{n H2O} \rightarrow \text{n C6H12O6} + \text{n H2}$$

$$\frac{\text{dXdegr}}{\text{dt}} = -\text{Kb.Xdegr} \tag{1}$$

where Xdegr = Amt. of Decomposed material (kg/m<sup>3</sup> )

t = time interval (days)

kh = hydrolysis constant in first order reaction.

Hydrolysis reactions in anaerobic decomposition can be considered as ratedetermining steps, which depend upon the ration of hydrolytic product to microorganisms, pH 5–7, and 30–35°C [23]. The hydrolysis reactions make the agricultural postharvesting biodegradable waste residue into e simpler forms and make it accessible for acidogenic bacteria for further breakup.
