Biological treatment
General information on biotechnological treatment for H2S removal
Hydrogen sulfide, which is normally present in biogas at concentrations between 200-4000 ppm, is a toxic, corrosive and smelly compound as well as poisonous for fuel cells, damaging for gas motors and microturbines and a not allowed compound in the natural gas grid. That means that it has to be removed from the biogas so that it can be used as fuel in all those energy suppliers. Due to its unpleasant characteristics a large variety of methods have been developed to remove H2S from different gas streams. These methods can be classified according to their principle in two big groups: physical-chemical, which are the traditional, and biotechnological. Among the physical-chemical methods, which are still nowadays the most common, the adsorption with activated carbon and iron oxide, and the iron salt and alkaline absorption are the most used to remove H2S from biogas. Nevertheless, in the last years the biotechnological methods have experienced a great development because of their high efficiency (same or higher than the physical-chemical), their lower investment cost and because they lead to savings on energy and avoid catalysts and formation of secondary contaminants. Among the biotechnological technologies the most important are the biofilters, bioscrubbers and biotrickling filters.
The basis of the biotechnological methods is the use of microorganisms to degrade pollutants, in this case H2S. The most suitable microorganisms to degrade H2S belong to the Thiobacillus genus. Thiobacillus are common bacteria, readily found near sulphur substrates, which obtain energy for growth from oxidising inorganic sulphur substrates producing either sulphur if the oxidation is partial or sulphate if it is total. The carbon source of these bacteria is CO2, most of them are aerobic and they don’t oxidise CH4. |
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The most important biotechnological methods are the biofilters, bioscrubbers and biotrickling filters (figure 2):
- Biofilters: in a biofilter the gas to be treated is first humidified and then forced through a bed packet with an organic carrier material (compost, peat, bark …), on which microorganisms are attached as a biofilm. The pollutants contained in the gas stream are transferred from the gas phase into the liquid phase, where are degraded by the microorganisms, being the rest of the nutrients available in the organic carrier material. Limitations are that such filters are not suitable for H2S removal due to the acidification of the system.
- Bioscrubbers: a bioscrubber consists of two reactors. The first part is an absorption tower, where pollutants are absorbed in a liquid phase. This liquid phase goes to a second reactor, which is a kind of activated sludge unit. In the latter, micro organisms grow in suspended flocks in the water, degrading the pollutants. The effluent of this unit is recirculated over the absorption tower in o co- or counter-current way to the flow of the waste gas. Limitations are that these systems represent very high costs due to the necessity of two reactors.
- Biotrickling filters: a biological trickling filter consists of a reactor column where the waste gas is forced through a packed bed filled with a chemical inert material which serves as carrier for biofilm forming micro organisms. While passing through the column, the pollutants from the gas diffuse into the biofilm and are degraded by microbial activity. Because nutrients are not available in the carrier material they are supplied to the micro organisms by recirculating a liquid phase through the reactor in co- or counter-current flow, which also provides moisture and is a mean to control the pH and other operating parameters. In general, most of the pollutant is biodegraded in the biofilm, but part may also be removed by suspended micro organisms in the recycled liquid. Microbial carriers that are used in biotrickling filters are plastic or ceramic structured packings (rings, saddles, etc), open pore foam, unstructured celite, activated carbon or mixtures of different materials.
Any one of these biotechnological methods is cheaper than the physical-chemical ones having the same or even more efficiency (>99 %) than those. Moreover no chemicals need to be added, the energy requirements are lower and there is not formation of secondary contaminant streams that need to be specifically treated. Among them the biotrickling filters are specially promising to develop a cost-effective H2S biogas cleaning unit because they can treat H2S without problems of medium acidification, thanks to the liquid recirculation, reason by which biofilters are not really suitable, and having similar advantages and disadvantages as the bioscrubbers (referring to life expectancy, efficiency, possibility to control the pH and the liquid medium composition and necessity of inoculation of the system with the micro organisms) they are usually cheaper and more compact.
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Siloxanes removal
Volatile Methyl Siloxanes (VMS) are the result of hydrolysis of Polydimethylsiloxane (PDMS), an organosilicon compound which is used in a wide range of consumer applications. Due to its widespread use, PDMS is deposited in landfill where it degrades in lower molecular weight compounds (VMS). PDMS can also enter the wastewater treatment plants were, because of its insolubility in water, partitions to the sludge. Consequently, when this is fed to anaerobic digester, PDMS can hydrolyse to VMS.
The species detected so far in biogas are hexamethylcyclotrisiloxane (D3), decamethyl-cyclopentasiloxane (D5), octamethylcyclotetrasiloxane (D4), hexamethyldisiloxane (L2), octamethyltrisiloxane (L3). The concentration values depend on the origin of the biogases.
When biogas is used in combustion engines, VMSs can be converted to solid inorganic siliceous deposits (silicon dioxide SiO2). In gas engines they form a coating which can be very dangerous, with chemical and physical properties similar to those of glass, leading to the abrasion of gas motor piston surface. Additionally, the glassy residues are responsible for the inactivity of the surfaces of the catalytic system for the control of waste gas. The manufacturers of gas engines have recently introduced a limit value for silicon measured in the oil of the gas engine in order to prevent premature engine failure due to silicon-induced damages.
Currently, a standard method for the elimination of siloxanes from biogas does not exist and treatment systems reported by industry sources are typically based on physical operations. In the past 3 years Profactor GmbH has initiated investigations on biotechnological methods for siloxane removal (in the frame of two European projects, AMONCO y PROBAT) and it is working in its lab with a biotrickling filter system, where biodegradation has been found.
