🧫 Microbes in Anaerobic Digesters — Who Lives There?

Anaerobic digesters are like giant stomachs where microbes break down organic waste without oxygen to produce methane (CH₄) — a renewable energy source.

🧩 Microbial Signatures & Diversity

• Every treatment plant has a unique microbial fingerprint — its microbial community composition reflects its operational conditions, especially temperature and substrate type.
• These microbes can be classified into two main temperature groups:
  • Mesophilic (moderate temps, ~35–40°C)
  • Thermophilic (hot temps, ~55°C)

Temperature strongly affects which microbes dominate and how efficiently they convert waste into methane.

🔥 Thermophilic Hydrolysis Pretreatment (THP)

Also called the “pressure cooker” treatment:

• Sludge is heated to 120–140°C at 1.5–2 bars of pressure.
• This breaks open microbial cells, making organic material more accessible to digestion.
• It effectively “cooks” the sludge, killing existing microbes and destroying most foreign DNA.

🧠 Why it matters: THP sludge has a cleaner microbial background, so the microbes you find afterward are truly those growing in the digester, not just dead leftovers from the input material.

🌱 Indigenous vs. Immigrant Microbes

Not all microbes in a digester actually live there:

• Some come from the incoming waste (like activated sludge).
• Others are true residents — the core anaerobic community that thrives inside.

For example:

• Phosphorybacter and Microthrix (Microtreats) are abundant in activated sludge, but not native to the digester.
• They get carried in with the feed but don’t grow there — their DNA still appears in 16S sequencing because that technique detects all DNA, living or dead.

🧬 The NIDAS Project — Tracking the Microbial Community

Over years, scientists have tracked how microbial communities shift with:

• 🌡 Temperature
• ⚗️ Organic loading rate
• 💧 Substrate composition and pretreatment
• 💦 Mineral water quality

By combining these data, they can correlate changes in microbes with operational performance, helping operators maintain healthy, methane-producing digesters.

🧠 Practical Use: Microbial Fingerprinting as a Diagnostic Tool

Each month, the lab sends microbial reports to treatment plants — like a “microbial health check.”

Example: A plant claimed it was running thermophilic digestion, but its microbial profile matched mesophilic conditions. ➡️ Turns out, their temperature sensor was broken! Microbial data revealed the truth.

💡 Microbial signatures can act as “biological thermometers” and early-warning systems for operational issues.

☁️ The Foaming Problem

One of the main operational challenges in digesters is foaming, which can:

• Disrupt gas collection,
• Overflow tanks,
• Cause expensive clean-ups.

🧪 Causes:

• Filamentous bacteria with hydrophobic surfaces trap gas bubbles.
• These filaments can come from activated sludge, even if they don’t grow in the digester itself.

Two main types of foaming microbes:

1. Native anaerobic foaming filaments — grow directly in the digester (e.g., Brevitalea).
2. Imported foaming filaments — come from sludge but don’t grow in anaerobic conditions (e.g., Candidatus Microthrix).

🧫 Spotlight: Candidatus Microthrix vs. Brevitalea

Key Insight:

• Microthrix → Stop it from entering the digester.
• Brevitalea → Control its growth, not eliminate it — it helps fermentation and methanogenesis when kept in check.

⚡ Energy and Efficiency (Intro to Next Section)

At the end, the instructor connects the biological lessons to energy conversion efficiency:

• When methane is burned for power, turbines are only ~30–34% efficient.
• So understanding and optimizing microbial methane production has direct energy implications — it ties biology to sustainability.

🧾 Summary Recap