Short note: Bacteria-algal symbiosis

Bacteria-algal symbiosis is the mutually beneficial relationship between aerobic bacteria and algae that makes natural purification possible in oxidation ponds (waste stabilisation ponds).

Process

1. Bacteria oxidise the organic matter of sewage using dissolved oxygen:

$$\text{organics}+O_2\xrightarrow{\text{bacteria}}CO_2+H_2O+NH_3+PO_4^{3-}+\text{new cells}$$

2. Algae use the CO₂, ammonia and phosphates released by the bacteria as nutrients and, by photosynthesis in sunlight, produce oxygen:

$$CO_2+H_2O+\text{sunlight}\rightarrow\text{algal cells}+O_2$$

3. This oxygen is used by the bacteria to oxidise more organic matter, and the cycle repeats.

         sunlight
            |
        +---v----+   O2   +----------+
        | ALGAE  |------->| BACTERIA |<-- organic matter
        |        |<-------|          |
        +--------+ CO2,   +----------+
                   NH3,PO4

Importance

• Each group supplies what the other needs, so oxygen is supplied free of cost and mechanical aeration is not needed.
• Organic matter is converted into algal and bacterial cells and stable products, so the BOD of the sewage is reduced by 70-90%.
• Oxygen is produced only in the daytime with enough sunlight; at night the algae also respire and the DO falls, and in cloudy weather the oxygen supply may be insufficient. So a shallow depth and a proper loading rate are used.