Air biofiltration

A biofilter is a packed bed carrying a wet microbial biofilm through which a waste gas is pushed slowly. The organisms oxidise the pollutant to carbon dioxide, water and mineral salts at ambient temperature, which is why the technique is used where thermal oxidation makes no thermodynamic sense: very large flows carrying very little contaminant, where the fuel would be spent heating air.

The transfer step, not the reaction step#

Bacteria consume dissolved substrate. The pollutant is in the gas phase. Between them sits a partition equilibrium described by Henry’s law, and it is this step, not the enzymology, that sets performance for most compounds. A dimensionless Henry coefficient near or below 0.01 — alcohols, aldehydes, organic acids, hydrogen sulfide, ammonia — means the gas gives its load up readily to the water film, and removal can approach completeness at an empty-bed residence time of a few tens of seconds. A coefficient one to two orders of magnitude higher — alkanes such as hexane, terpenes such as α-pinene, and most chlorinated solvents — means the compound stays in the gas. The biofilm then runs starved, its degradation capacity is not the constraint, and the only remedies are longer residence time, more specific interfacial area, or a carrier phase such as silicone oil that dissolves the compound and shuttles it to the cells.

This shows in the standard performance curve. Below a critical inlet load, removal is near-complete and the bed is reaction-limited. Above it, elimination capacity plateaus at a maximum set by transfer, and additional load passes straight through. Reported maxima for readily soluble compounds sit in the range of tens to low hundreds of grams per cubic metre of bed per hour; for poorly soluble ones they are several times lower.

The bed degrades itself#

Water content is the operating variable that fails first. Compost, peat or wood-chip media perform in a narrow band, roughly 40–60 % by mass. The incoming gas is usually drier than the bed, so the bed loses water continuously and must be humidified upstream. Drying is not fully reversible: a dried organic medium shrinks, cracks and channels, and gas thereafter short-circuits through the fissures while the mass of the bed sits unused. Overwatering is the opposite failure, producing anaerobic pockets, odour breakthrough and rising pressure drop.

Chemistry attacks the bed too. Oxidising hydrogen sulfide yields sulfuric acid, and oxidising chlorinated compounds yields hydrochloric acid. An organic bed has finite buffering capacity; once it is spent, pH falls, and the heterotrophs that degrade the organic load are lost while the acid-tolerant sulfur oxidisers continue. That is the reason sulfide and VOC loads are often treated in separate stages, and the reason biotrickling filters exist at all: recirculating liquid over inert packing lets alkalinity and nutrients be dosed and acid products be bled off, at the cost of needing nitrogen supplied deliberately rather than mineralised from the medium.

Two limits remain hard. Excess biomass clogs packing and must be removed. And intermittent operation is poorly tolerated: the biofilm needs a continuous substrate supply, so plants with long shutdowns lose activity that takes days to weeks to rebuild.