Approximately 75% of the needed energy is consumed in the biological aerated reactor. Air or oxygen is introduced to the reactor to provide the inflow wastewater the oxygen to sustain the necessary level of biological activity. The demand of oxygen within a biological reactor can be met in various ways, but most aeration devices currently being used in activated sludge process may be classified as either diffused, dispersed, or surface aeration systems. Figure 8 shows the composition of energy demand of different components of the wastewater treatment process.

The treatment process produces effluents with low level of carbonaceous organic and low level of suspended solids. At the same time, however, excess sludge is being produced. Although excess sludge being a nuisance, bio-gas (CH₄ + CO₂) with a high energetic value might be generated under anaerobic digestion. The bio-gas may be used for heating the digester, to produce steam, to produce electricity, or to run direct drive equipment. By this up to 60 – 75 % of the total electrical energy demand and about 100 % of the thermal energy demand of the treatment plant can be covered.

Figure 8: Composition of energy demand of different process components of wastewater treatment plants [Bohn, 1993]