Biofilters vs. Adsorption
In the last three decades biological technologies have emerged as a sustainable and economically-viable alternative compared to traditional techniques like adsorption/absorption for purifying gases. They were well-received initially in the industry and interesting products with good results were carried out for several years. However, owing to a series of reasons that will be detailed in this article, physical-chemical techniques –like adsorption– continue as the most common leading methodologies in purification systems for polluting gaseous effluents.
In the case of adsorption, the operating principle is the toxic transfer during the gas to solid phase. Adsorption is conducted in fixed and fluidised beds created from porous materials, such as zeolite or active carbon. After the porous bed has reached its adsorption limit, the material may or may not be regenerated. Where possible, it is advisable for the pollutant to be a highly volatile compound, as desorption is conducted faster and to a greater degree.
This can be done via different methods: thermal treatment, employing steam, inert gases, in a vacuum, etc. Whatever the method, the most economical alternative often consists of discarding or incinerating the material. In the event that the bed is regenerated with steam, wastewater will be obtained after desorption, which has to be purified again. The main disadvantage seen in this technology is that –similar to absorption– the pollutant is not actually eliminated, but is instead limited to being transferred to a different phase.
On the other hand, the main feature of biofilters compared to adsorption technology is the fact that the pollutant is truly and effectively degraded to products that are generally innocuous compounds or, in all cases, less hazardous than the initial pollutants, which is not always the case in physical-chemical technologies.
However, in order for biodegradation to be carried out adequately, the values of the reactor’s operating parameters must be relatively stable and fall within ranges that permit the action of microorganisms. If these requirements are not met, the treatment of the current can end up being done with low efficiency rates, or not done at all.
The most important operating variables in a standard biofilter are:
- Composition of the gaseous current
- Filling material
- Contribution of nutrients
- Drop in pressure
- Oxygen concentration
By way of example, the response of these two technologies is presented for two common industrial scenarios.
- Peak of pollutant concentration in influent: treatment with bioreactors is generally only applied to currents with low pollutant loads, given that a substance that is, in principle, biodegradable can give rise to toxicity effects if concentrations are too high. It is also important to ensure some stability in discharge concentration, given that brusque variations in toxic contribution negatively affects how the reactor operates. For adsorbent media, a peak in concentration affects the autonomy of the media, but does not decrease its overall capacity. In fact, many biological treatment systems tend to install a pre-treatment stage with adsorbent media to buffer possible spikes in concentration.
- Increase in temperature: as mentioned, bioreactors are normally inoculated with mesophilic microorganisms. An increase in temperature is likely to cause the deactivation of a percentage of the consortium, with the consequent drop in the bioreactor’s efficiency. In adsorbent treatment systems, the temperature range admitted tends to be quite extensive. In cases where the reaction mechanism is chemisorption, removal can even be favoured by temperature increases.
In light of the above, it is obvious that biological systems require an instrumental complexity level that is much greater than that necessary to apply adsorption solutions. This fact implies highly intensive maintenance of operating variables. A change in temperature, a drop in pH due to failure of the pH control system, spikes in pollutant concentration, and so forth, can cause –in the best of cases– a loss of the microbial consortium’s efficiency and even the need to re-inoculate the media due to the death of microorganisms.
This greater complexity has led many sectors to mistrust biological systems compared to adsorption technologies.