The domestic sewage in large cities  
1. When treated in STPs does not really require the aeration step as the sewage contains adequate oxygen.
2. Has very high amounts of suspended solids and dissolved salts.
3. Has a high BOD as it contains both aerobic and anaerobic bacteria.
4. Is processed by aerobic and then anaerobic bacteria in the secondary treatment in sewage treatment plants (STPs). 

Select the correct statement from the following: 
1. Biogas commonly called gobar gas is pure methane
2. Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria
3. Biogas is produced by the activity of aerobic bacteria on animal waste
4. Methanobacterium is an aerobic bacterium found in rumen of cattle
 

High value of BOD (Biochemical Oxygen Demand) indicates that:
1. water is highly polluted
2. water is less polluted
3. consumption of organic matter in the water is higher by the microbes
4. water is pure

What gases are produced in anaerobic sludge digesters?  

1. Methane and CO₂.
2. Methane, Hydrogen sulphide and CO₂.
3. Methane, Hydrogen sulphide and CO.
4. Hydrogen sulphide and CO₂.