A large underground tank where solid matter or sewage is disintegrated by bacteria

• A. Septic tank
• B. Tank
• C. Cistern
• D. Pit

Answer: (A)

This describes a septic tank: an underground chamber where wastewater is held so that solids can settle and be broken down by bacteria in anaerobic conditions. The bacteria digest much of the organic matter, solids form a sludge layer at the bottom, oils and grease form a scum layer at the top, and the clarified liquid then moves on to the drainage system. The idea is on-site sewage treatment, using microbial action to reduce waste. The other options don’t fit as well: a tank is a generic container and doesn’t specify treatment by bacteria; a cistern stores water, not sewage; a pit is just a hole and doesn’t imply the biological digestion process.