Sustainable Waste

Unit 1

Composting

Composting is a biological stabilization process in which the organic fraction of municipal solid waste

(MSW) is decomposed under controlled environmental conditions to produce a stable, humus-like

material called compost. The objective of composting is to reduce the volume and weight of waste, destroy pathogens and weed

seeds, eliminate odour, and convert unstable organic matter into stable humic substances.

Two types of Composting

Aerobic And Anaerobic Composting

Aerobic Composting

Aerobic composting is the controlled biological oxidation of organic waste materials in the presence of

oxygen. In this process, microorganisms utilize oxygen to metabolize organic matter, producing carbon

dioxide, water vapour, heat and stabilized compost.
General Reaction:

Organic matter + O₂ → CO₂ + H₂ O + Heat + Stabilized compost
The process is exothermic, and the heat generated raises the temperature of the composting mass to

thermophilic levels (55–65°C), which ensures rapid decomposition and pathogen destruction.

Process Desciption of Aerobic Composting

(1) Segregation and Pre-treatment

(2) Formation of Composting Piles

(3) Aeration and Turning

(4) Biological Decomposition Phases:

i) Mesophilic Phase (20–40°C):

ii)Thermophilic Phase (45–65°C):

iii) Cooling Phase:

iv) Maturation Phase:

Types of Aerobic Composting

Aerobic composting systems are classified based on the method of aeration, level of process control,

and reactor configuration. The major types of aerobic composting are:

1. Windrow Composting

2. Aerated Static Pile Composting

3. In-Vessel Composting (including Rotary Drum Systems)

Factors Affecting Aerobic Composting

Factors Affecting Aerobic Composting

Efficient composting requires control of the following parameters:

 Moisture content: 50–60%

 Carbon to Nitrogen (C/N) ratio: 25:1 to 30:1

 Temperature: 55–65°C during thermophilic phase

 pH: 6.5–8.0

 Adequate oxygen supply

What is Anaerobic composting?

Anaerobic composting, more commonly referred to as anaerobic digestion, is a biological stabilization

process in which organic matter is decomposed in the absence of molecular oxygen by a consortium of

anaerobic microorganisms. The process results in the production of a combustible gas known as biogas,

primarily composed of methane (CH₄ ) and carbon dioxide (CO₂ ), along with a partially stabilized

residue termed digestate.

Process Description of Anaerobic Composting

Anaerobic digestion is carried out in an airtight reactor known as a digester, designed to exclude

oxygen completely. The process may be operated in batch or continuous mode, and digestion may occur

under mesophilic (30–38°C) or thermophilic (50–55°C) temperature conditions.
a) Waste Preparation

Before digestion, the organic waste is screened to remove non-biodegradable materials. The

biodegradable fraction is mixed with water to form a slurry of suitable consistency. The slurry ensures

adequate contact between microorganisms and substrate and facilitates pumping into the digester. In

some systems, mechanical mixing is provided to maintain uniformity.

b) Digestion in the Reactor

The slurry is introduced into the sealed digester through an inlet chamber. Inside the reactor, a complex

series of biochemical reactions occurs in the absence of oxygen. Microbial degradation converts organic

solids into biogas and soluble products. As digestion proceeds, gas accumulates in the upper portion of

the reactor, while the stabilized slurry gradually moves toward the outlet chamber.

The retention time generally varies from 20 to 40 days, depending on operating temperature, feed

composition, and design of the digester.

c) Biogas Production and Collection

The gas produced during digestion contains:

 Methane (50–70%)

 Carbon dioxide (30–50%)

 Small quantities of hydrogen sulfide, ammonia, and water vapor

d) Rem oval of Digestate

The effluent slurry, known as digestate, is discharged from the outlet chamber. Although partially

stabilized, it may require further aerobic curing. The digestate is rich in nutrients such as nitrogen and

phosphorus and can be used as organic fertilizer.

Overall Reaction:

Organic matter → CH₄ + CO₂ + Stabilized residue