Cleaning up our waste - naturally

Following the primary treatment of sewage by filtering and allowing heavier particles to settle, the effluent is either discharged into a waterway or passed on for secondary treatment. This is where microbes come in!



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Questions
A town's waste water following primary treatment contains 450mg per litre of organic matter (CH2O). It is treated in a secondary treatment plant that can process 200,000 litres per day. During its passage through the plant, 50% of the organic matter is oxidised:

CH2O + O2 ----------> CO2 + H2O

1. If the percentage of oxygen in the air is 20% by volume, what volume of air is required to oxidise the organic matter in the waste water?


to find out if your correct

Answer

If you answered '180'. litres x 103 you were correct. And this is how to work it out:

450mg x 200,000 litres = 90kg organic matter;
Mr(relative molecular mass) CH2O = 30g per mol, so 90kg = 90,000/30 = 3000 mol;
50% oxidised = 1500 mol, so (acc to eqn) 1500 mol oxygen needed;
If 20% air is oxygen, mass of air = 1500 x 100/20 = 7500 mol;
Volume of air with this mass = 7500 x 24 litres = 180,000 litres (or 180 litres x 103)

 

2. What volume of methane would be produced each day if all the organic matter were converted into methane?

2 (CH2O) ----------> CH4 + CO2



to find out if your correct

 

 

 

 

 

Answer

If you answered '36'. litres x 103 then you were correct

Mass of organic matter oxidised = 3000 mol;
Acc to eqn, 3000 mol CH2O = 1500 mol CH4;
Volume of methane with this mass = 1500 x 24 litres = 36,000 litres (or 36 litres x 103)