From Sludge to Solution, Eco-Friendly Biochar from Wastewater and Microalgae

Wastewater, especially from industries and households, is one of the greatest environmental challenges of our time. Every day, millions of liters of dirty water flow into treatment plants. During purification, solid and semi-solid sludge is also produced. If not managed properly, this sludge can become a new source of pollution.

But hidden within this challenge lies a remarkable opportunity: what if this sludge could be transformed into an environmentally friendly, useful material? This is exactly what a new approach seeks to answer—by converting sludge from wastewater treatment into biochar, a pollutant-absorbing material that also supports plant growth.

Transforming Microbes, Aquatic Plants, and Bark into Biochar

Researchers applied pyrolysis (heating without oxygen) to biological residues from wastewater treatment. The feedstock included:

• Biomass of fungi and bacteria attached to natural fibers from Furcraea andina (commonly known as fique plants)

• Microalgal sludge (Chlorella sp.) used to clean dye-polluted wastewater

• Pine bark sawdust

The process produced two types of biochar:

• BC300, pyrolyzed at 300°C with a pH of 5.08

• BC500, pyrolyzed at 500°C with a pH of 6.78

The higher the pyrolysis temperature, the more stable and active the surface of the biochar becomes.

Microalgae: More Than Just Water Cleaners

In wastewater treatment containing synthetic dyes such as malachite green, Chlorella sp. microalgae act as “living adsorbers.” After treatment, the microalgae are separated using a cationic coagulant. Remarkably, more than 90% of the algae can be recovered within 50 minutes at pH 6.5.

Instead of being discarded, these recovered microalgae are reused as raw material for biochar production. This reflects the principle of a circular economy—where waste from one process becomes a valuable input for another.

Biochar + Bacteria: A Smart Combo for Plants

Once biochar is produced, the next step is to turn it into a biologically active growing medium. This is done by adding beneficial bacteria, including:

• Nitrogen-fixing bacteria

• Phosphate-solubilizing bacteria

• Heterotrophic bacteria

Tests showed that these microbes thrived within the biochar, with densities reaching 7.5 × 10⁸ CFU per gram. This makes biochar not just a pollutant absorber but also a natural biofertilizer.

Safe for Plants: Growth Tests with Grass Seeds

To ensure biochar and its feedstocks are safe for plants, germination tests were conducted using Lolium sp. grass seeds. The results were impressive:

• Water from the coagulation process: 100% germination

• Biochar without bacteria: 98% germination

• Biochar with bacteria: 99% germination

This shows that biochar is safe to use as a growing medium and even promotes healthy plant development.

Biochar’s Ability to Capture Dyes

One major challenge in industrial wastewater is synthetic dyes like malachite green, which are hard to break down and harmful to ecosystems. In trials, BC500 demonstrated the best performance in dye adsorption, particularly at pH 4.0.

The maximum adsorption capacity reached 0.525 mg per gram of biochar, following a second-order kinetic model that indicates high efficiency.

A Promising Environmental Solution

From wastewater sludge and microalgae to pollutant-absorbing, microbe-carrying biochar—this process represents an intelligent and sustainable environmental management system.

Key benefits include:

• Reducing sludge volume from wastewater treatment

• Producing multifunctional biochar: pollutant absorber, plant growth medium, and microbial carrier

• Turning waste into valuable products that support sustainable agriculture

• Reducing reliance on chemical fertilizers and hazardous adsorbents

Innovations like this reshape how we view waste—not as a problem, but as a new resource. With relatively simple yet impactful technology such as co-pyrolysis, we can pave the way toward healthier agriculture, cleaner water, and a more sustainable environment.

Discover more about SAWA’s Innovative Biochar production and its impact on sustainable farming.

Explore SAWA Biochar