You are the city engineer for a town containing 4, 000 residences. Each residenc

Question

You are the city engineer for a town containing 4, 000 residences. Each residence contains approximately 2.5 people. Assume that each person created 4 lb refuse/day. It is your job to figure out how big the refuse trucks needs to be (in yd^3). You can buy 3 trucks and they can compact the refuse to 500 lb/yd^3. The trucks can make 3 trips per day for 5 days a week. Calculate the minimum size of the trucks to meet the needs of the community. (12.4 yd^3) Assume that the waste is 45% biodegradable with a formula of C_65H_110O_45N. Calculate how much methane can be produced per year if the waste is placed in a landfill. (2.2 million lbs of methane/year) If methane has a heat capacity of 0.3 kWh/ft^3 and a density of 0.042 lb/ft^3 and the methane power point is 70% efficient, calculate kWh/year that can be generated from this refuse. (11 million kWh/year)

Explanation / Answer

1.

Total amount of refuse = 4000(Residents)* 2.5 people/residence * 4 lb/day =40000 lb/day

Waste generated per week = 40000*7= 2,80,000 lb/ week

Capacity of truck = 500lb/yd3, number of trucks= 3

Capacity of total of 3 truckes = 500*3=1500 lb/yd3

They can have 3 trips per day hence

Capacity for one week = 1500*5*3=22500 lb/yd3

Minimium size of the tuck = waste generated/ Carrying capacity of truck= 280000/22500= 12.4 yd3

2.

The reaction of biodegradation of   C65H110O45N to produce CH4 can be written as ( balanced for CH4)

C65H110O45N +(65-110/4- 45/2-3*1/4)H2O---à (65/2+110/8-45/4-3/8)CH4 + CO2+NH3’ that if for every moles of C65H110O45N, 34.625 moles of CH4 is generated

Moles of waste = mass/Molecular weight

Molecular weight of waster= 65*12+110+45*16+14 =1624

Moles of waste = 2,80,000/1624=172.4 moles

But only 45% is biodegraded, hence moles biodegraded= 172.4*0.45=78

1 lb mole produces of waste produces 34.625 lb mole of CH4

78 lb moles produces 78*34.625 =2701 lb mole CH4

Mass of CH4 generated= 2701*16 =43216 lbs

There are 52 week in a year, Hence CH4 generated= 43216*52=2.24*106 lb CH4/years

3.

Heat capacity / lb = 0.3 Kwh/ft3/ 0.042 lb/ft3= 7.14 Kwh/lb

Power output= 2.24*106 lb * 7.14 Kwh/lb

But the efficiency is only 75%, hence power output= 2.24*106*7.14*0.75=1.1*107 KwH/year

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