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
There are a huge number of pollutants, both organic and inorganic, that can be treated using biofilters. The main condition that must be met so that purification takes place is that the substance or substances must be biodegradable and not have toxic effects on microorganisms. Conversely, substances that are quite insoluble in water, with high molecular weight and/or that have complex links have lower biodegradability rates. For adsorbent media, there is a wide range of materials that, when mixed together, can encompass a large number of pollutants.
Filling material is one of the vital parameters in working with conventional biofilters and percolating biofilters. Depending on the application, the material is sought that meets attributes such as having a large biofilm-media contact area, lightness, durability, high chemical and mechanical resistance, must not cause large drops in pressure, economical, must permit good biomass adhesion and must not have toxic effects on the microorganisms. The majority of adsorbent media tend to have mechanical properties that can take on the most aggressive environments while preserving their initial geometry.
Contribution of nutrients
To maintain biofilters’ high-level and stable efficiency, it is essential that the microorganisms have all the nutrients they need available to them to perform their activity, and that the nutrients are present in suitable proportions. Microorganisms’ nutritional requirements include sources of carbon, nitrogen, phosphorous, potassium, sulphur, magnesium, iron and other elements. This additional contribution generates extra expense, which in non-existent with adsorbents.
Biofiltration systems generally employ mesophylls, which are organisms with maximum metabolic activity between 20 and 45º C. Outside of these ranges, high and low temperatures can have negative effects on the microbial consortium, with the consequent loss of efficacy. The temperature ranges at which adsorbent media can operate tend to be much broader and even, in many of them, have increased efficiency with higher temperatures.
While water availability does not entail a problem with bioscrubbers and percolating biofilters, it is conversely a vital operating parameter in conventional biofilters. In these filters, the air is saturated with steam prior to its entry into the reactor. Indeed, the periodic addition of liquid media to the reactor is recommended. In general, microorganisms have high water requirements, due to which the desiccation of the filling causes a reduction in biological activity. Insufficient water can also cause the bed to become dry at the air input area, causing channelling of the current, which contributes to decreasing the efficiency of the biofilter. For adsorbent media, there is no limitation due to the percentage moisture in the influent, provided that condensation on it is prevented.
One of the most common problems in biofiltration systems is the drop in pH that takes place, due to the production of acidic degradation intermediaries, common in the treatment of halogenated hydrocarbons, nitrogenous or sulphurous compounds, some VOC, etc. Due to this phenomenon, microbial activity can decrease considerably, causing a drop in treatment efficacy. The importance of this phenomenon is higher in conventional biofilters than in other bioreactors, given that the absence of a moving liquid phase makes the removal of acidic compounds difficult.
Drop in pressure
The growth of the biomass inside the reactor can, after a certain operating time, give rise to problems obstructing and channelling the gaseous current. Bed compaction significantly increases operating costs, which ends up making it necessary to replace the media, with the associated costs this entails.
The microorganisms most commonly employed in biofiltration are strict aerobes, requiring minimum oxygen content between 5 and 15% in the incoming air current to survive. Values below this level will favour the development of anaerobic areas in the reactor, with the ensuring negative effects. Atmospheric air contains around 20% oxygen, so that under normal conditions this does not tend to be a limitation. Depending on the application, there are different types of adsorbent media that can operate efficiently with and without oxygen.
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.