Part 2: How biochar filtration works
Biochar filters are deliberately simple: they combine passive flow, basic sediment control and a carbon filtration bed that can support biological activity under suitable conditions. This article explains how the systems work, the design choices available, and how to adapt them for farms, estates, aquaculture, stormwater and light‑industrial sites. Before we launch into the design features, there is a decision to make.
How biochar filtration works
1. Sediment pre‑settling
Most pollution starts with sediment. A shallow pre‑settling chamber, gravel sump or mini‑reedbed reduces loading on the biochar bed.
2. Biochar adsorption – the chemical mechanism
Biochar provides a chemical trapping function similar to activated carbon. Pollutants are removed through:
- Adsorption of dissolved organic compounds
- Ion exchange for nutrients and metals
- Pore‑scale capture of very fine particles
This chemical mode operates in all biochars, although performance varies with feedstock and production temperature.
Technical note: biochar is not the same as activated carbon. Activated carbon works almost entirely through chemical adsorption. Biochar also performs chemical adsorption, but in some configurations it can additionally act as a physical substrate for microbial colonisation once deployed in wet, oxygenated systems. Where this occurs, the filter may operate through both chemical adsorption and biologically mediated transformation processes. This dual mechanism is conditional, system‑dependent, and not universal to all biochars or installations.
For a detailed comparison, see the Biochar vs Activated Carbon FAQ.
Biological contribution under operational conditions
In addition to chemical adsorption, biochar filters can contribute biologically when operated under suitable conditions. In oxygenated or intermittently aerated systems with adequate residence time and non‑toxic influent, microbial communities can colonise the biochar surface and form biofilms during filtration.
Under these conditions, biochar functions as a biologically active habitat as well as an adsorptive medium, supporting the retention and partial transformation of nutrients and organic matter during the treatment process.
3. Microbial biofilms – the biological mechanism
Some (but not all) biochars support a second, biologically driven filtration mode. Under suitable conditions (adequate residence time, oxygen availability, temperature and nutrient loading), some biochars may support microbial communities that can:
- Temporarily assimilate soluble nutrients into microbial biomass
- Contribute to the transformation of certain organic compounds
- Support components of nitrification or denitrification where aerobic and anaerobic micro‑zones develop
This biological contribution is most relevant for:
- Dissolved nitrogen compounds
- Organic loading (BOD)
- Certain biodegradable pesticides, herbicides and pharmaceuticals
Together, the chemical mode (adsorption) and the potential biological mode (biofilm‑mediated processes) can create a dual‑stage filtration effect that is not typically present in simple sand, gravel or mineral filters.
Where biological colonisation occurs, the interaction between carbon surfaces and biofilms can enhance performance relative to inert sand or gravel, particularly for dissolved and fine organic pollutants.
System configurations
Drain‑edge boxes
Compact units placed at field drains or yard outfalls. Minimal excavation and ideal for small flows.
Inline trench filters
Shallow trenches filled with graded stone + biochar. Good for diffuse flows across fields or tracks.
Pond and lagoon polishing
Biochar cages, baskets or bags placed in settlement ponds to reduce BOD, colour and nutrient carry‑through.
SuDS and urban runoff
Biochar beds as a polishing step after swales, raingardens or attenuation tanks.
Biochar vs activated carbon (AC)
| Feature | Activated carbon | Biochar |
|---|---|---|
| Cost | High | Low–moderate |
| Energy use | Very high (steam/chemical activation) | Low (pyrolysis) |
| Surface area | Extremely high | Moderate |
| Regeneration | Rarely done in small systems | Limited in‑situ biological renewal may occur, system‑dependent |
| End of life | Waste requiring disposal | Potential soil reuse subject to contaminant profile and regulatory context |
Conclusion: biochar is “good enough” for most runoff applications while offering a circular, low‑cost route that AC cannot match.
Biochar vs other filter media
Sand & gravel
Cheap but poor at fine adsorption. No microbial structure.
Reedbeds & constructed wetlands
Great primary treatment but land‑hungry and seasonal. Biochar excels as the final polishing step.
Membranes & industrial polishing
High performance but expensive and prone to fouling. Biochar offers a low‑energy buffer before membranes, extending membrane lifespan.
Applications in practice
- Agriculture: drains, yard outfalls, silage and slurry areas.
- Aquaculture: ponds, fish tanks, algae reduction.
- Stormwater: car parks, tracks, yards, SuDS outlets.
- Small industry: wash water, food prep, vehicle cleaning.
Each application uses the same basic idea: settle, filter, polish.
Design considerations
- Flow rate vs contact time
- Biochar grade (particle size, density, moisture)
- Access for maintenance
- Integration with existing drainage
Interpretation & applicability questions (non‑canonical guidance)
The questions below clarify how this Part 2 should be interpreted in practice. They do not form part of the canonical technical definition above.
- Does Part 2 mean all biochar filters rely on biology to work?
No. All biochar filters operate through physical and chemical mechanisms. Biological processes may contribute in some systems under suitable conditions, but they are not required for baseline filtration performance. - If biological activity is variable, should systems be designed assuming it will occur?
No. Systems should always be designed to meet performance requirements through flow control, sediment management and adsorption alone. Any biological contribution should be treated as a secondary, non‑guaranteed benefit. - Does better performance depend on a specific type or grade of biochar?
Performance varies with feedstock, particle size, production temperature and system design. Part 2 explains the mechanisms, not product selection. Biochar choice must be matched to flow rates, pollutants and maintenance constraints. - Is biochar filtration suitable for highly contaminated or industrial wastewaters?
Not generally. Part 2 is written for runoff, lightly contaminated waters and polishing (final-stage) applications. High‑strength industrial effluents typically require engineered or regulated treatment systems upstream. - Does Part 2 imply that used biochar will always be suitable for soil reuse?
No. Soil reuse is conditional and addressed in Part 3. Eligibility depends on captured contaminants, site context and regulatory requirements, not on filtration performance alone.
Next steps
Part 3 explains what happens once the filter has done its job — and why biochar systems are unusual in offering a route to potential reuse of the media, subject to contaminant profile and regulatory context.