Bottom line: Volcanic stone does not filter water by "blocking" — it works through three synergistic mechanisms — physical interception, chemical adsorption, and biological degradation. The third one is the core and the irreplaceable advantage: volcanic stone is a "honeycomb apartment" for nitrifying bacteria, relying on living microorganisms to truly purify water.
Volcanic stone has a porosity of 40%–70%, filled with pores and micropores of various sizes. Its specific surface area is dozens of times larger than ordinary sand or gravel.
When water flows through:
This is the most basic layer, but it is not enough on its own — trapped debris will eventually clog the pores. That is why the next two mechanisms are essential to "digest" what gets caught.
Volcanic stone surface carries a positive charge, while most harmful substances in water (bacteria, colloids, organic molecules) carry a negative charge. This electrostatic attraction makes adsorption extremely efficient.
Specifically, it adsorbs:
| Pollutant | Adsorption Mechanism |
|---|---|
| Heavy metal ions (Pb²⁺, Cu²⁺, Zn²⁺, Cr⁶⁺, etc.) | Ion exchange + chemical adsorption |
| Ammonia nitrogen (NH₄⁺), nitrite (NO₂⁻) | Surface charge attraction + physical adsorption |
| Organic pigments, pesticide residues | Physical adsorption into micropores |
| Residual chlorine | Surface chemical reaction |
This layer combines "physical + chemical" action. Unlike activated carbon, which saturates and loses effectiveness over time, volcanic stone's ion exchange capacity remains stable for long periods.
Research shows that a volcanic rock–activated carbon combined filter achieves higher removal rates for NH₄⁺-N, NO₃⁻-N, TN, BOD, and COD than activated carbon alone, with longer backwash cycles and lower energy consumption.
This is the core advantage that sets volcanic stone apart from quartz sand, ceramsite, and all other traditional filter media.
The porous structure of volcanic stone provides an ideal environment for nitrifying bacteria to attach and multiply:
Step 1: Ammonia-oxidizing bacteria (Nitrosomonas) attach to the volcanic stone surface and convert ammonia (NH₃) from fish waste and leftover food into nitrite (NO₂⁻).
Step 2: Nitrite-oxidizing bacteria (Nitrobacter) then convert nitrite (NO₂⁻) into nitrate (NO₃⁻).
Nitrate is far less toxic and can be removed through water changes or absorbed by aquatic plants.
This entire process is called the nitrogen cycle — the backbone of stable water quality in aquariums and wastewater treatment systems.
Key facts:
This is why volcanic stone is classified as a "biological filter media" — it is not just a rock, it is a living ecosystem carrier.
While filtering, volcanic stone simultaneously does two more things:
Mineral release: Calcium, magnesium, iron, zinc, and over a dozen trace elements slowly leach into the water, supplementing nutrition, boosting fish immunity, and promoting plant growth.
pH buffering: Volcanic stone is near-neutral (pH 6.0–6.5). It gently lowers overly alkaline water by 0.3–0.5 pH units, acting as a buffer to prevent sharp water parameter swings.
The essence of volcanic stone water filtration is: pores intercept debris → surface adsorbs toxins → bacteria degrade ammonia nitrogen — three mechanisms working in series. The first two are passive purification (physical + chemical). The third is active purification (biological) — and that is the irreplaceable core value of volcanic stone. It is not a dead rock. It is a living water purification factory.