Course 26.2Lesson 2 of 3

Secondary treatment, flocs and BOD

In secondary treatment, the effluent goes into aeration tanks where air is pumped in and aerobic microbes grow as flocs. They use up the organic matter and lower the BOD. The flocs settle as activated sludge. A small part goes back as inoculum, and the rest goes to the anaerobic sludge digesters.

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Feeding the microbes

After primary treatment, the effluent still holds a lot of dissolved and very fine organic matter. Microbes are now put to work. This is secondary treatment, which is also called biological treatment.

The effluent from the primary settling tank goes into large aeration tanks.
The liquid is constantly agitated by machines, and air is pumped into it.
Useful aerobic microbes grow vigorously. They grow into flocs.
While they grow, they consume the major part of the organic matter of the effluent.

A floc is a mass of bacteria that are held together with fungal filaments. The mass has a mesh like structure. It is large enough to settle in still water.

Two parts. On the left, an aeration tank drawn as a blue rectangle with an air pipe along the bottom, many air bubbles rising and curved arrows showing that the liquid is stirred. Brown oval flocs float in the liquid. On the right, a circle shows one floc magnified: red rods, the bacteria, are joined by a mesh of brown threads, the fungal filaments.

BOD, the measure of pollution

How can we tell how much organic matter is in the water? The microbes tell us. The more organic matter there is, the more oxygen they use up.

BOD is the biochemical oxygen demand. NCERT defines it as the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.

The greater the BOD of waste water, the greater is its power to pollute.
Secondary treatment goes on until the BOD has fallen a great deal.

Water with a high BOD takes oxygen from a river. Fish and other water animals then cannot breathe. This is why treated water must have a low BOD.

How a laboratory measures it. This part is not in the NCERT chapter, but A-Level and IB teach it. A small sample of the water is put in a bottle with oxygen rich water, and the bottle is kept in the dark at 20 °C for 5 days. The fall of oxygen is the BOD, written BOD $_5$ . If the sample was diluted, the fall is scaled up by the dilution.

A graph of the oxygen used by microbes in a dark bottle at 20 degrees Celsius, in milligrams per litre, against time in days, from 0 to 20. A dashed vertical line marks 5 days. The red curve for raw sewage rises steeply and passes 220 mg per litre at 5 days, marked as BOD5 equal to 220, and levels off near 320. The green curve for treated effluent rises slowly and passes 15 mg per litre at 5 days, marked as BOD5 equal to 15, and levels off near 22. Both curves are computed from the same first order law with a rate of 0.23 per day.

The graph is computed from a simple law. The microbes use oxygen quickly at first and more slowly later. After 5 days, a good part of the oxygen demand has been used.

What happens next

Settling. When the BOD has fallen a great deal, the effluent goes into a settling tank. The flocs settle. The sediment is called activated sludge.
Inoculum. A small part of the activated sludge is pumped back into the aeration tank. It starts the growth of the next lot of flocs, and so it is called the inoculum.
Digesters. The remaining major part is pumped into large tanks called anaerobic sludge digesters. There, other bacteria that grow without oxygen digest the bacteria and the fungi of the sludge.
Biogas. During this digestion, the bacteria make a mixture of gases: methane, hydrogen sulphide and carbon dioxide. The mixture is the biogas, and it can be used as a source of energy because it burns.
Release. The effluent of the secondary treatment is generally let into rivers and streams.

India has started the Ganga Action Plan and the Yamuna Action Plan to build sewage treatment plants in towns, so that only treated sewage is let into the rivers.

You should find that primary treatment lowers the BOD only a little, and that the aeration tank does most of the work. A river can bear the final value, but not the raw sewage.

Try it in Python

The program follows the BOD from the raw sewage to the final effluent, and works out a laboratory test.

Practise with new numbers

Common mistakes

Saying that BOD is the oxygen produced by microbes. It is the oxygen that they consume.
Saying that the aeration tank has anaerobic bacteria. It has aerobic microbes. The digester has anaerobic bacteria.
Mixing up flocs and activated sludge. The flocs are the masses of microbes. The activated sludge is the sediment of flocs in the settling tank.
Saying that all the activated sludge goes back to the aeration tank. Only a small part does. The rest goes to the digesters.
Thinking that high BOD means clean water. A high BOD means that the water is more polluted.

Remember these

Secondary treatment: aeration tank, air pumped in, constant agitation, aerobic microbes grow as flocs, BOD falls.
Flocs: masses of bacteria with fungal filaments.
BOD: the oxygen that bacteria would use to oxidise the organic matter of 1 L of water. A greater BOD means a greater power to pollute.
Settling tank gives activated sludge. A small part is the inoculum. The rest goes to anaerobic sludge digesters, which make biogas: methane, hydrogen sulphide, carbon dioxide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What is a floc?

Why are air and constant stirring needed in the aeration tank?

Define BOD. What does a high BOD tell you?

What is activated sludge, and what happens to it?

Which gases form in the anaerobic sludge digesters?

References

NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
Tchobanoglous G., Stensel H. D., Tsuchihashi R. and Burton F. (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th edition, Metcalf and Eddy, McGraw Hill, the chapters on suspended growth biological treatment and on the characteristics of wastewater.
American Public Health Association (2017). Standard Methods for the Examination of Water and Wastewater, 23rd edition, method 5210, Biochemical oxygen demand.