A student draws the steps of secondary treatment in the wrong order: the sediment is called…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student draws the steps of secondary treatment in the wrong order: the sediment is called activated sludge; air is pumped into a tank and flocs grow; the effluent goes into a settling tank; a small part of the sludge goes back to the first tank; the rest goes to a digester where biogas forms.
- Write the steps in the right order.
- Why is the small part of the sludge sent back?
- Which gases form in the digester?
Need a reminder? The key ideaShow
Secondary treatment is biological. Aerobic microbes grow as flocs in the aeration tank and use up the organic matter. BOD is the oxygen consumed in oxidising the organic matter of one litre of water. In a BOD test with a diluted sample, BOD (fall of oxygen) (volume of the bottle) (volume of the sample). The sediment of the flocs is the activated sludge: a small part is the inoculum, and the rest goes to the anaerobic sludge digesters.
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.
Step 1. (a) 1. Air is pumped into a tank and flocs grow. 2. The effluent goes into a settling tank. 3. The sediment is called activated sludge. 4. A small part of the sludge goes back to the first tank. 5. The rest goes to a digester where biogas forms.
Step 2. (b) It carries microbes into the aeration tank, so it acts as the inoculum and starts the next growth of flocs.
Step 3. (c) Methane, hydrogen sulphide and carbon dioxide, which together are the biogas.
Answer: the order is aeration, settling, activated sludge, return of a small part, digester; the small part is the inoculum; the gases are methane, hydrogen sulphide and carbon dioxide.
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