As shown in Figure 4.3, in many respects these represent an intermediate tech-nology between biofilters and bioscrubbers, sharing certain features of each. Once again, an engineered vessel holds a quantity of filter medium, but in this case, it is an inert material, often clinker or slag. Being highly resistant to compaction, this also provides a large number of void spaces between particles and a high surface area relative to the overall volume of the filter. The microbes form an attached growth biofilm on the surfaces of the medium. The odourous air is again forced through the filter, while water simultaneously recirculates through it, trickling down from the top, hence the name. Thus a counter-current flow is established between the rising gas and the falling water, as shown in the diagram, which improves the efficiency of dissolution. The biofilm communities feed on substances in the solution passing over them, biodegrading the constituents of the smell.
Process monitoring can be achieved relatively simply by directly sampling the water recirculating within the filter vessel. Process control is similarly straight-forward, since appropriate additions to the circulating liquid can be made, as required, to ensure an optimum internal environment for bacterial action. Though the efficiency of the biotrickling filter is broadly similar to the previous method, it can deal with higher concentrations of contaminant and has a significantly smaller foot-print than a biofilter of the same throughput capacity. However, as with almost all aspects of environmental biotechnology, these advantages are obtained by means of additional engineering, the corollary of which is, inevitably, higher capital and running costs.