Question

Ideally the production of methane and carbon dioxide under anaerobic conditions can be represented by the following equation:

3 2 4 2 NH CO 8 3 2 4 CH 8 3 2 4 O H 4 3 2 4 N O H C d d c a-b d c- b- a d c a-b- d c b a                    

Assuming a rough estimate of municipality solid waste (MSW) composition is as follows:

Component % weight

Paper (assume all cellulose, C6H10O5) 50

Glass 10

Steel 10

Aluminium 10

Food waste (assume sugar, C6H12O6) 20

3.1 If 10, 000 tonnes of this material is placed in a landfill, how much methane gas would theoretically be produced by anaerobic decomposition? (15)

3.2 How much kg methane is produced/kg of waste, comment on the answer?

Answer 1

The rough estimate of MSW is

Paper (assume all cellulose, C6H10O5) 50

Glass 10

Steel 10

Aluminium 10

Food waste (assume sugar, C6H12O6) 20

The equation of production of methane by paper is

C6H10O5+ H2O   --->3CH4+ 3 CO2

So as clear from above equation, one mole of cellulosic paper will give three moles of methane

Molecular weight of cellulose is 162 g / mole and methane is 16 g / mole

So 162 grams of cellulose will give 16 X 3 = 48 grams of methane

Hence 1 gram of cellulse will give = 48 / 162 grams of methane = 0.296 grams of methane

Paper is 50% of MSW (10,000 tonnes)

Mass of paper = 0.5 X 10,000 tonnes = 5000 tonnes = 5 X 10^9 grams of paper

so methane produced = 5 X 10^9 X 0.296 grams = 1.48 x 10^9 grams of methane from paper

b) the equation of production of methane from food waste is

C6H12O6 ---> 3CH4 + 3CO2

So here one moles of foodwaste will give three moles of methane

Molecular weight of food waste = 180 g / mole and molecular weight of methane = 16 g / mole

184 grams of food waste will give 48 grams of methane

so one gram of food waste will give 48 / 180grams of methane = 0.267 grams

The mass of foodwaste is 20% of 10,000 tonnes = 2000 tonnes = 2 X 10^9 grams

So methane produced from 2 X 10^9 grams of food waste = 0.267 X 2 X 10^9 = 5.34 X 10^8 grams of methane

So the mass of methane produced = 1.48 x 10^9 grams + 5.34 X 10^8 grams = 2.014 X 10^9 grams

[Glass steel and aluminium will not undergo decomposition like paper and food waste)

b) the mass of methane produced per 10,000 tonnes of waste = 2.014 X 10^9 grams

so mass of methane per 10,000 Kg will = 2.014 X 10^9Kg

So mass of methane per Kg will be = 2.014 X 10^5 Kg