♻️ From Wastewater to Valuable Products

Instead of viewing wastewater sludge as just waste, we can think of it as a resource. Beyond cleaning water, sludge can be reused to:

• Produce biomass (e.g., microbial growth).
• Recover valuable products like:
  • Phosphate (for fertilizers)
  • Biopolymers (used in plastics, gels, fibers, etc.)

🧫 Biopolymers from Biomass

Biopolymers are large natural molecules made by living organisms. In wastewater, we can extract them from biomass or extracellular polymers (EPS). These include:

• Cellulose 🪵
• Starch 🌾
• Alginates 🧃
• Proteins (like gelatin) 🍮

These can be used in:

• 🧵 Textiles
• 📄 Paper industry
• 🍔 Food
• 🏥 Medical applications
• 🧱 Construction
• 🌾 Agriculture

The key factor is purity — higher purity = more specialized use.

🌍 The Rise of Bioplastics

Plastics are everywhere — useful but non-biodegradable and polluting. To replace them, we turn to bioplastics, which are:

• More sustainable
• Often biodegradable
• Sometimes even biocompatible (safe for medical use)

But there’s a catch:

• 💸 Cost: Bioplastics are still more expensive than petroleum plastics.
• 🧪 However, as production scales up, technology costs go down.

⚗️ Microbial Production of Bioplastics

Microbes don’t make plastic directly — they make precursors to bioplastics.

The main precursors:

Polyhydroxyalkanoates (PHAs) — a family of natural polyesters stored by bacteria as energy reserves.

They’re made from small building blocks (monomers) like:

• Acetate
• Propionate
• Butyrate
• Valerate

🧬 How PHAs Work

• PHAs are linear polyesters, and their exact properties depend on which monomers are used.
• They can be:
  • Thermoplastic 🔥 (moldable)
  • Elastic 🧘
  • Biodegradable ♻️
  • Biocompatible ❤️ (for medical devices)

Two main types:

• PHB (polyhydroxybutyrate) – more rigid
• PHV (polyhydroxyvalerate) – more flexible

By mixing these (and others), we can tune the material’s elasticity and resistance — just like adjusting plastic recipes in industry.

🧪 Pure vs. Mixed Cultures

• Pure cultures = high purity, controlled microbes → ideal for medical applications.
• Mixed cultures = cheaper, more adaptable, can use wastewater as input!

Mixed cultures:

• Don’t need sterilization → save energy and cost.
• Are resilient to changing substrates.
• Can be enriched over time — even if we don’t know all the microbes, we can favor the ones that produce PHAs.

🔬 Metabolism & Pathways

Bacteria can make PHAs from:

• Normal carbon sources (like sugars or fatty acids)
• C1 compounds (like methane or methanol)

➡️ This flexibility means multiple production routes — not limited to one substrate or one type of waste.

🧫 The Two-Stage Reactor System

Industrial or lab setups often use two bioreactors:

1️⃣ Fermentation Reactor

• Performs anaerobic fermentation.
• Complex waste → broken down into simple compounds (acetate, butyrate, propionate, etc.).
• These become the “food” for the next reactor.

2️⃣ PHA Production Reactor

• Operates under feast–famine cycles:
  • Feast: Microbes get lots of food → grow fast.
  • Famine: Food runs out → microbes store PHA inside as energy reserves.
• Repeating these cycles selects bacteria that are best at storing PHAs.

Over time:

• Non-storing bacteria get washed out.
• The population becomes enriched in PHA-producers.
• Then, cells are harvested and PHA is extracted for bioplastic production.

🌟 Summary