In environmental engineering and waste management, what is an anaerobic digester? (Choose the option that correctly describes its purpose and role in converting organic/agricultural residues into useful energy and by-products.)

Introduction:
An anaerobic digester is a sealed system that decomposes organic matter without oxygen to produce biogas (primarily methane and carbon dioxide) and a nutrient-rich digestate. This question tests your basic understanding of what an anaerobic digester is and is not, with emphasis on its role in circular bioeconomy and on-farm waste-to-energy applications.

Given Data / Assumptions:

• The process is oxygen-free (anaerobic) and relies on a microbial consortium.
• Feedstocks include manure, crop residues, food waste, and sewage sludge.
• Outputs are biogas (fuel) and digestate (fertilizer/soil amendment).
• Engineering designs include continuously stirred tank reactors and plug-flow systems.

Concept / Approach:
Anaerobic digestion proceeds in stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Methanogenic archaea convert acetate and hydrogen/carbon dioxide to methane. The technology is used to manage waste, reduce odors and pathogens, capture renewable energy, and recycle nutrients back to land with lower greenhouse gas emissions compared to raw manure storage.

Step-by-Step Solution:
Identify the domain: waste-to-energy and wastewater/solid waste management. Connect process conditions: absence of oxygen enables specific microbial pathways. Link outputs to uses: biogas for heat/electricity; digestate for soil fertility. Evaluate options: only one correctly defines the technology rather than unrelated items. Select the description that matches the engineered process converting waste into biogas.

Verification / Alternative check:
Operational digesters on farms, food processing plants, and wastewater facilities consistently produce 50–70% methane biogas and stabilized digestate, confirming the technology's definition and purpose.

Why Other Options Are Wrong:

• New diet drink: Irrelevant to waste treatment or energy.
• Microbe that eats hazardous waste: A digester is a system; microbes work inside it, but the digester itself is not a single organism.
• All of the above: Contains incorrect statements; cannot be correct.

Common Pitfalls:
Confusing the digester (a reactor) with the microbial community (its biological “engine”). Also, assuming only animal manure is suitable feedstock—many organic wastes can be co-digested for better gas yields.

Final Answer:
Method to convert agricultural waste into a biogas correctly describes an anaerobic digester.