Solid and Hazardous Waste Management: Collection to Landfill
This domain covers waste generation and characterisation, recycling and composting, thermal treatment, landfilling and hazardous-waste regulation. Numerical items often involve stoichiometry, energy content or landfill gas yield.
Core rules and concepts
- Characterisation: waste composition by wet weight; moisture and volatile solids; calorific value on dry or wet basis; C/N ratio 25-30 and moisture 50-60% for composting.
- Incineration: the three T's (temperature around 850-1100 °C, residence time, turbulence), excess air typically 6-12% for municipal waste; energy recovery depends on lower heating value.
- Stoichiometry: CH4 + 2O2 -> CO2 + 2H2O; 1 kg CH4 needs 4 kg O2, and with air at 23.2% O2 by mass, about 17.2 kg air.
- Landfill: composite liner, leachate collection and head control, daily cover, gas extraction; landfill gas is roughly 50-60% CH4 and 40-45% CO2, with generation peaking years after closure.
- Hazardous waste: characteristics are ignitability, corrosivity, reactivity and toxicity; TCLP estimates leachability, not total content; stabilisation/solidification with cement or pozzolans immobilises metals.
Worked example
Calculate stoichiometric air for complete combustion of 1 kg methane. Moles CH4 = 1000/16 = 62.5 mol; O2 required = 125 mol = 4 kg. Air = 4/0.232 = 17.2 kg per kg CH4. With 10% excess air, actual air is 18.9 kg/kg. The reasoning step is converting to moles before applying the stoichiometric ratio; treating 2O2 as 2 kg of oxygen per kg of methane is the common error.
Traps that cost marks
Mixing higher and lower heating values; reporting TCLP results as total concentration; forgetting that wet-basis calorific value falls as moisture rises; and assuming landfill gas production stops at closure rather than peaking afterwards.