What substance do bacteria in activated sludge primarily feed on?

In activated sludge systems used for wastewater treatment, bacteria primarily feed on organic material. This organic matter typically consists of nutrients such as biodegradable waste, sugars, proteins, and fats, which are present in the wastewater. The process of aeration in activated sludge systems promotes the growth of beneficial bacteria that utilize this organic material as an energy source, effectively breaking it down through metabolic processes.

Bacteria play a crucial role in reducing the biochemical oxygen demand (BOD) of the wastewater, facilitating the removal of contaminants. These organisms thrive on organic substances, converting them into microbial biomass, carbon dioxide, and water, thus purifying the water for release or further treatment. The other options—aluminum sulphate, salts, and polyelectrolytes—do not serve as primary food sources for the bacteria in this context, as they are either coagulants or chemical agents that aid in the treatment process rather than providing nutrition to the microorganisms.