Case studies
Biochar has now been used in many different forms: from tiny polishing bags inside domestic composters to large suspended mats in industrial waterways. Although these systems operate under very different loads and conditions, looking across them provides valuable clues about how biochar behaves over time, where it excels, and what design pitfalls need attention when moving from gas-phase odour polishing to water filtration.
This article highlights three contrasting examples and summarises what they do and do not tell us about practical water-filtration design.
Case 1 — The HOTBIN lid biochar bag (12 years in service)
Inside the HOTBIN insulated composter, a small quantity of biochar sits in a breathable pouch in the lid, acting as a polishing step for warm, humid exhaust air. Some users have had the same bag in place for more than a decade without feeling the need to replace it.
What the system actually does
- Treats low-concentration odours (ammonia, amines, VOCs)
- Experiences intermittent, low airflow
- Operates at warm, moist conditions ideal for biofilm formation
- Acts as a secondary polishing stage—the main odour control comes from aerobic composting inside the bin
Why such long service life is plausible
- The pollutant load is relatively low
- Biochar provides chemical adsorption for peaks in odour
- A thin biofilm develops over time and can biologically oxidise captured volatiles
- Variations in temperature and airflow allow partial natural regeneration
- The original fill mass may be over-spec for the duty required
What this example does not imply
A 12-year life in a compost lid does not translate directly to agricultural water filtration.
Water systems have:
- far higher mass loads
- constant or event-driven hydraulic flow
- suspended solids and biofilm thickening
- nutrients, pesticides, sediments and dissolved organics in much greater quantity
However, the HOTBIN bag is still useful: it shows that biochar can operate as a durable, dual-mode chemical + biological polishing medium under gentle loading.
Case 2 — Large suspended biochar mats (e.g., German watercourse systems)
In several European projects, large geotextile “mats” or hanging bags filled with biochar have been suspended in channels or small waterways to polish surface water. These units are often several metres long and must be installed by machinery.
What they demonstrate
- Biochar can be used in flow-through water environments at larger scales
- Regulators recognise biochar as a valid polishing medium
- Passive systems can treat diffuse pollution without pumps or power
- Modular, removable designs are possible in real watercourses
Limitations for smallholder / farm design
- Units can become extremely heavy when waterlogged
- They are prone to surface clogging if flows carry a high sediment load
- Installation and removal typically require mechanical lifting
- Hydraulic resistance must be managed to avoid unwanted backwater effects
These systems show what is possible under industrial conditions, but are not directly suited to farms, estates or small SuDS features.
Case 3 — Designing practical biochar “pillows” for ponds and ditch edges
Between the small HOTBIN bag and the giant German mats, there is a highly practical middle ground: biochar pillows in the 20–60 L range. These can be deployed in ponds, settlement bays, ditch edges or low-flow channels without machinery.
A key design decision is the fabric or netting used to contain the biochar.
Option A — Fine non-woven geotextile (e.g., 50 g/m² weed membrane)
Pros:
- Retains all char fines
- Easy to sew or weld
- Gives a strong “clarity boost” initially
Cons:
- Acts like a microfilter skin
- Suspended solids accumulate on the outside
- Rapid risk of blinding and flow bypass
- Most of the biochar volume becomes under-used
Option B — Coarser PE/PP mesh or gauze (0.5–1 mm apertures)
Pros:
- High permeability → water passes through the entire char bed
- Biofilm forms inside the media (not just on the shell)
- Much slower clogging
- Encourages volumetric filtration
Cons:
- Minor escape of char fines unless the char is screened
- Must be strong, UV-stable mesh (not textile organza)
Design principle that emerges
The containment fabric should hold the char, not act as the primary filter.
The filtration should occur inside the biochar bed, where both chemical adsorption and biological processes can operate.
This principle echoes both earlier cases:
- The HOTBIN bag works because most load is inside the bed, not on a clogged shell
- German mats work by having large, permeable surfaces where flow passes through, not around, the media
Case 4 — Woodtek’s farm-scale biochar filter (UK example)
A useful point of reference within the UK is the passive biochar filtration system installed by Woodtek on a working farm. Although every site is different, this installation shows how a practical, box-based system can be integrated into existing farm drainage without pumps or complex infrastructure.
What the system consists of
- A compact settlement or interception chamber.
- A downstream biochar-filled compartment in a simple box housing.
- Flow directed through the media before entering a ditch or watercourse.
- No power requirement and relatively low maintenance.
- Designed around the real drainage volumes of a farm environment.
Note: description based on publicly shared material; detailed design choices belong to Woodtek.
What this example demonstrates
- Biochar filtration can be successfully embedded into working farm drainage.
- A box-based design is practical, serviceable, and familiar to land managers.
- Flow can be managed effectively with simple baffling and pre-settlement.
- It builds confidence that UK-relevant deployments already exist, even if each farm requires adaptation.
Why this is relevant to design reasoning
Woodtek’s system sits between the small HOTBIN odour bag and the large European biochar mats:
| System | Size | Flow | Role | Key insight |
| HOTBIN bag | Very small | Gas-phase | Polishing | Biochar’s biological mode can be long-lived |
| German mats | Very large | High-volume water | Polishing | Biochar works as a passive catchment-scale media |
| Woodtek farm box | Farm-scale | Moderate flows | Treatment + polishing | A maintainable box is practical and realistic in UK farms |
| Biochar pillows / trench beds | Modular | Local flows | Polishing | Ease of handling and reduced clogging are design priorities |
Woodtek’s example underlines an important message:
Farm-scale biochar filtration does not need to be complicated — but it must be designed around flow, load, and maintenance access.
Where the analogy stops
The Woodtek system is still site-specific, shaped by their feedstock, char grade and farm layout. It is not a universal template. But, as with the other cases, it provides design signals rather than a specification:
- Simple box geometries work.
- Staged flow (settlement → char bed) is effective.
- Periodic char replacement is manageable.
- The concept aligns with UK farm drainage patterns.
Case 5 — ARTi’s pond-scale BioFilter (USA example)
A useful international reference comes from ARTi in the United States, who operate a pond-side biochar filtration unit using several cubic metres of biochar and a solar-powered recirculation pump.
Although the hydraulic and nutrient loads are very different from farm drains, this system demonstrates how biochar can function at larger water volumes. ARTi report that the biochar becomes “charged” with nitrogen and organics over a period of months, after which the spent media is removed and reused as a soil amendment.
The key lesson from this example is that, under higher loading, biochar filters can require more frequent media turnover, and active recirculation may be necessary to achieve meaningful water turnover. This reinforces the idea that biochar performs best as a polishing medium and that filter life depends on flow, load and contact time rather than a fixed calendar interval.
6. What these three cases teach us when designing water filters
Looking across these very different systems reveals several consistent lessons:
1. Duty matters more than material
Biochar behaves differently under:
- gas-phase polishing
- low-flow pond polishing
- through-flow channels
- high-load drainage events
Recognising load differences prevents over-claiming and supports credible design choices.
2. Geometry and hydraulics are critical
A system can succeed or fail based on:
- whether flow goes through the media or around it
- how quickly external skins blind
- whether the media remains submerged, oxygenated, or intermittently dry
3. Biochar performs best as a polishing medium
In all three examples, biochar is not asked to capture everything.
Pre-settlement, sediment traps, reedbeds or initial aerobic treatment do most of the heavy lifting.
4. Biofilm development is real and useful
Warm, moist, oxygenated environments support microbial communities that:
- transform nutrients
- degrade many organics
- extend media life
But only when hydraulics and fabric choice prevent premature clogging.
5. Material selection is not trivial
A seemingly simple decision—membrane vs mesh—can determine whether:
- the pillow clogs in weeks
- or runs effectively for months or years
This is where practical experience converges with engineering principles.
7. Where this insight leads in future design
The next generation of biochar filtration for farms and estates will not look like domestic odour bags or industrial mats. It will likely emphasise:
- modular drain-edge boxes
- inline trench beds
- biochar polishing pillows
- floating baskets for ponds/lagoons
- and, in advanced settings,
nanobubble-enabled biochar reactors that support deeper biological activity.
These systems all follow the same logic: allow water to flow through the media, encourage microbial colonisation, avoid clogging skins, and ensure the biochar’s second life in soil remains viable.
Closing reflection
Biochar has now been used successfully in very low-load odour control and in large-scale watercourse polishing. Studying these extremes helps guide the design of practical, farm-scale filtration systems that are maintainable, modular and biologically active.
Each example has limits—and acknowledging those limits is part of responsible stewardship.
But taken together, they provide a strong foundation for designing effective, real-world biochar filters that support both water quality and soil improvement.