Sludge is decomposed in a separate tank by the _________.

Sludge is decomposed in a separate tank by anaerobic bacteria.

This process, known as anaerobic digestion, breaks down the organic matter in the sludge in the absence of oxygen, producing biogas (a mixture of methane and carbon dioxide) and reducing the volume of sludge. The treated sludge can then be used as fertilizer or disposed of safely.