Rehabilitation of soil air
Today, there are several dozen methods and principles dealing with removing volatile organic substances from soil air / waste gas. For these purposes, DEKONTA uses sorption filters, biofilters and catalytic incinerators. Within rehabilitation of contaminated localities, it concerns cleaning of soil air by a venting (or bio-venting) system, in industry these methods are applied during elimination of contaminants contained in waste gases from manufacturing processes.
The principle of this method is sorption of organic substances by means of filled filters. Active coal of various types is used as the filter medium. Forms of filters and their construction can be different. In general, there are two types: bed-type filters (e.g. active coal is poured in barrels known from rehabilitation) and cartridge filters (the thickness of the wall is 20-100 mm). The first alternative represents filters with a relatively long service life of the filter medium. Cartridge filters have minimal pressure loss but the service life of their medium is, e.g. several hours (used mainly industrially). Pressure loss varies in the case of direct filtration from upward of 500 Pa (for common barrel filters about 1,000 Pa). First of all, water and mechanical dirt is removed from air in so-called separators of separating demisters. Then, it flows from the exhaustion blower or air-pump into the tank with sorbent. During the flowing of air through the filter medium there is sorption of volatile oil hydrocarbons, including BTEX, CLH, PAH, phenols, and partially PCB (with the use of special sorbents also ketones, ethanol and methanol; ineffective for removing other polar organic substances, inorganic oxides, etc.). After attaining the sorption capacity (e.g. 25% mass), it is necessary to replace or regenerate this medium. Venting sorption-regenerating stations are technically modified for the purpose of operation so that sorption of volatile oil products can be performed at the same time in several filters arranged in parallel (increased capacity) or series-connected filters (increased efficiency). Units have more filters as a rule with active coal with the volume of 200 l each. Filters can work alternately in sorption – regeneration cycles. Regeneration is performed by water steam. Steam is condensed in a cooler, the liquid substance is separated in the separator. The level of automation can be different – from fully automated units, where analysers on the output of the equipment control the sorption-regeneration cycle, to manually operated units. Stations are located in transport containers and can also be delivered in the non-explosive variant.
The principle of this method is filtering of cleaned air through a biologically active filter medium on which, first of all, vapours of organic substances are absorbed and then these absorbed substances are biologically degraded by the effect of specific micro-organisms. Volatile pollutants are diluted together with oxygen in the dampened biofilm which covers the particles of materials used as the filter medium and then the micro-organisms are metabolised under aerobic conditions. Such metabolites (CO2) diffuse by a biological layer, are converted into the gas phase and are collected from the device. The basic principle of biofiltration is a combination of absorption of the contaminant on a suitable carrier and biochemical dissolution by suitable bacterial cultures. Another important aspect is the application of the mentioned technology in suitable equipment ensuring the necessary technological parameters of the process. Sorption and subsequent biotechnical oxidation in the environment of biofilm with a high concentration of biomass under almost optimal conditions (pH, nutrients, oxygen, moisture) causes the removal of volatile organic substances and some inorganic substances from flowing air (not effective for PAH, PCB, multiCLH). Biofilters are constructed from plastic (hardened PP), and as a rule are prismatic, or with several sections or circular. As a filter medium (carrier) which is placed on a special grid, in the case of continually watered biofilters, suitable inorganic carriers are used, i.e. porous materials with high permeability and large specific surface (perlite, active coal, keramzite, zeolite, plastic crushed materials, etc.), in the case of the filter bed, natural organic materials are used (bog coal, soil, wood coat, chips, etc.). Inside the filter it is necessary to keep optimal conditions, in particular, moisture, pH, temperature and concentration of nutrients. Before its activation the filter medium is inoculated by suitable microbial cultures and, at the same time, the necessary inorganic nutrients are added. During the operation, sprinkling of the active medium is performed by water, nutrients, bacterial preparate or buffering agents, and is automated, including continual analysing. Contaminated air supplied to the biofilter can be moistened, if necessary, and pre-heated or cooled. Values of pressure losses of this equipment in real conditions vary widely, however, in general they are higher than in the case of sorption filters. In the case of the mentioned sorption filters, parallel or series sets of biofilters are used more frequently in practice. Sometimes a combination of principally different devices is used, i.e. series connection of the biofilter and sorption filter.
The catalytic-oxidation incinerator is a thermal exchanger for cleaning of soil air containing oil products which are combusted without flame on the catalytic converter. The technology includes a heat exchanger, which ensures maintaining optimal catalytic temperature by the use of the originated thermal energy. The basic series variants of incinerators are from 250 to 5,000 m3 of air per hour. The minimum input concentration may be zero, but autothermal or economical operation of the incinerator is at concentrations between 500 – 1,000 mg/m3 of organic substances. The maximum input concentration varies again according to the kind of combusted substance within the range 8,000 – 15,000 mg/m3. In the case of higher concentrations, it is possible to dilute input air. The catalytic converter can be, e.g. a Pt/Pd type on a corundum carrier, while Pt catalytic converters from iron oxide or copper are also used. The exact type of catalytic converter is specified by the manufacturer according to the kind of contaminant. Working temperatures vary from about 300 C, when the catalytic converter starts to work with sufficient efficiency, to 650 C. In the case of exceeding of this upper limit, it is destroyed.
Polluted air flowing into the incinerator plant flows into the thermal exchanger where it is heated by heat produced by combustion of harmful products on the catalytic converter of the incinerator. The air then flows from the thermal exchanger to the combustion chamber with the catalytic converter where there is flameless combustion of organic substances. Cleaned air flows out from the equipment through the above-mentioned heat exchanger. The technology is completed by auxiliary piping with valves which optimise the allocation of temperatures and flowing in the combustion chamber. The whole combustion process is controlled by a built-in computer connected to the continual analysers of the level of contamination of the input and output air, a device for dilution of input air and other equipment. The incinerator can work in any conditions because it is delivered as a complete self-contained technological unit. Equipment used for rehabilitation of soil air includes the venting station (an air-pump for exhaustion of air, flow meters) and so-called demisters (pre-stage serving for removing mechanical dirt in exhausted air or for condensation of moisture or contaminant). The efficiency of the incinerator is at such a level that the cleanliness of burnt gases fulfils both current and expected emission limits. At present, it concerns the most up-to-date technology available for cleaning of air. A suitably designed catalytic incinerator does not require special servicing, in many cases it provides further utilisable thermal energy in the form of hot air. The concentrator to the catalytic incinerator is a device serving for increased concentration of organic substances in the cleaned air so that the catalytic incinerator connected to this equipment is able to work in an economical regime. The basic module is always the catalytic incinerator with symmetrical pairs of concentrators connected to it. The system is completed with auxiliary elements, measuring and control circuits. This allows for lower operating costs and high reliability.