A town has 60000 people. Each person adds 110 L of sewage to the drains in a day…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A town has people. Each person adds L of sewage to the drains in a day. The primary settling tank holds the sewage of hours.
- How many cubic metres of sewage reach the plant in a day?
- What is the volume of the settling tank?
- What is the average flow, in litres per second? There are seconds in a day.
Need a reminder? The key ideaShow
Sewage is the waste water of a town. Primary treatment is physical: filtration removes floating debris, sedimentation removes grit, and the settled solids are the primary sludge. The liquid above is the effluent. One m³ is L, and mg per litre is g per m³. The mass of a solid in a flow is the flow times its concentration.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. people, L each in a day, h in the tank.
Plan. Find the flow, then the tank, then the flow each second.
Step 1. The flow. L in a day, which is m³.
Step 2. The tank. m³.
Step 3. Litres each second. L per second.
Answer: m³ in a day; m³; L per second.
Check. million litres, as before.
Watch out. The time in the tank is the volume divided by the flow. A tank that is too small gives the solids too little time to settle.
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