Waste Gas Biofiltration: 25 Years of Practical Experience
Efficiency, Capital and Operating Costs
For a period of time in the early 1990’s the treatment of organically laden industrial process gases via biofiltration application received significant attention in the technical journals, academia, and elsewhere, and was being practiced somewhat substantially in Europe at the time.
The principals and process dynamics and kinetics were being discussed, evaluated and tested in real world applications and on bench and pilot scale. While interest in this technique has not evaporated, it appears not to receive the attention it once did. The failure of many systems due to poor design, inadequate maintenance or inappropriate application did not help to elevate biofiltration as a premier technology.
Having designed, installed, and evaluated numerous individual systems since 1994, we have gained a wealth of practical operating and design experience in treating a variety of substances via vapor phase biofiltration. Initially, the primary constituent(s) of a client gas stream were evaluated in bench scale units, and performance compared to pre-existing models for any particular component for which they existed. After decades of application, this has become more predictive without the need for preliminary evaluation or scaling.
Design Considerations:
The design parameters for candidate biofilter systems, principally EBRT, are a function of the refractory nature or biodegradeability, of the compounds to be treated, and their concentrations. High concentrations of terpenes, for example, which are functionally strict hydrocarbons, would have less efficiency and require more extended bed retention times than say primary alcohols, which are metabolized at a much more rapid rate, even at concentrations of several thousand ppmv. Conducting a simple BOD test is an excellent predictive indicator for the effective treatment in a biofilter, and can be used for design (size) considerations. Indeed, BOD values for a broad array of organic compounds are well known and have been established and published, making use of this technique even easier. Generally speaking, those components with oxygen substituents, unsaturated carbon bonds, or a combination of the two, such as ethers, alcohols, ketones, acetates, esters, organic acids, etc. are eminently biodegradeable and excellent candidates for biofiltration, even at high concentrations. Biogenic materials, into which category many of these classes of compounds fall, are almost universally readily biodegraded. Terpenes are a special category. Even though they are biogenic, such as constituents of citrus fruits, they are not as readily amenable as most naturally produced organics. Methane is another example, being almost totally refractory. However, there has been very effective success in treating terpenes in typical concentrations. Typical EBRT’s range from 25-30 sec., for more difiicult materials, to as low as 15-20 sec. for easily metabolized components. Both, however are contingent on actual concentration as well. Comparatively, a carbon bed operates in retention time windows of 1-2 seconds or less.
Aside from dimensional considerations, one of the most significant design parameters is the means by which the bed material is maintained in a humid or moistened state. Rather than strictly a gas-solid interface and contact, successful biofiltration more resembles the models for gas-liquid phase chromatography. The biomass is the stationary phase and may be said to be coated with a thin, even mono-molecular layer of water, the liquid phase, which facilitates adsorption, particularly of condensable or water soluble components, in addition to being a critical phase for the survival and metabolism of a viable microbiological community, which after all is what is doing all the “work”. Media design factors in as necessary to facilitate diffusion and efficient contact, captive nutrient supply, and minimal pressure drop. Therefore moisture control is of paramount consideration in the design and operation of any system. The ideal situation is for the feed gas to be 100% saturated RH. This is rarely accomplished, and so auxiliary moisture must be imparted to the vapor phase feed gas. This may be done in a variety of ways. The most efficient, but which entails additional capital cost, is to pass the gas through a humidification or conditioning chamber which is essentlially a counter-current packed bed scrubber. This may achieve saturation as high as 96-98%. In cold weather conditions, the water feed to the humidifier may be heated to facilitate evaporation/saturation. Lack of moisture is detrimental to the microbiological community, overly wetted material may impart increased pressure drop and potentially lead to pre-mature breakdown of the organic phase in the biomass. However, this latter situation has not been encountered, except in other designs utilizing 100% organic materials such as various types of compost, admixtures, or bark or wood chips. Moisture control is easily maintained through the use of the humidification chamber, and auxiliary sprays.
The use of a packed irrigated section also serves to facilitate the removal of dust or particulate, and, depending on the blowdown rate, remove condensable or water soluble organics as well, mitigating the load on the bio beds. In any event, significant particulate must be controlled to prevent occlusion of the beds. There is a certain amount of moisture loss in the beds due to metabolic activity, particularly in the surface layer, and this may be accommodated by periodic spraying of internally mounted misting spray headers. Alternatively, 100% of moisture control may be accomplished by internal spraying, however these sprays would have to be operated 30-50% of the time to match the performance of a packed bed conditioning unit.
Thus the two major considerations in design and operation of a successful biofilter are retention time and moisture control. Moisture control is automated through a simple control mechanism, and the entire system is virtually maintenance free, other than periodic inspection and a few simple tests on the biomass on an infrequent basis.
A third design consideration, and no less significant, is selection of the biomass used in the beds. Over the decades, a variety of materials have been tried and are still used. Such materials include peat, bark chips, and other organic matrices or combinations thereof. Material considerations are the provision of adequate captive nutrients to sustain vigorous microbiological growth and metabolism, pressure drop, diffusion, and adsorption. Many purveyors of biofiltration systems rely on replacement of the biomass when it becomes compacted, or otherwise deteriorates, as a source of downstream revenue. Indeed, most purely organic materials will breakdown and consolidate, diminish in volume, and become occluded with fines over time, rendering them ineffective and contributing to poor drainage and increases in pressure drop across the system, increasing power consumption and reducing efficiency.
Our systems use an artificial support material as an admixture with a select organic substrate which has proven to last indefinitely. This results in pressure drop design and operation of less than 12″ wg, while optimizing diffusion and adsorption. Certain nutrient parameters such as organic nitrogen, phosphorous, and certain cations have been shown to remain at sustainable levels within the biomass system for over 20 years and counting. This “engineered” material is resistant to compaction, comparatively light in weight, imparting advantageous structural consideration, and highly resistant to compaction, fissuring, and deterioration. Initially, the biomass was guaranteed for 1-2 years, on a performance basis, and clients could opt to purchase additional guarantees at the time of installation. However, certain systems that have been installed and in service for 23 years show no signs of deterioration or nutrient depletion. These situations exist in those units that are at least regularly monitored through the PLC/PC control system and properly attended to in terms of replacement or repair of periodic solenoid failures or other infrequent, low impact repairs. Even with gross neglect, where the moisture control systems were virtually unattended, causing the biomass to become desiccated, the bed material was re-hydrated, still contained adequate nutrient levels, and after a suitable re-aclimation time, was put back into service.
Capital and Operating Costs:
A properly designed and operated biofilter is capital cost competitive with alternative controls such as RTO’s, virtually maintenance free when compared to competitive systems, generates no green house or combustion gases, requires no fossil fuel consumption and is easier to permit. A usual complaint is the foot print. However, a 15,000-20,000 cfm system would take up no more room than approximately typical 16 parking places, exclusive of ancillary equipment which may be sited externally or adjacent to the unit. A comparable RTO may require an equivalent amount of space as well as have a higher capital cost, and certainly a higher operating cost. Biofilter capital costs may be estimated at about $28-$30/cubic foot, depending on the size of the unit.
From an operating standpoint, a biofilter is energy efficient, has inherent design flexibilty, and minimal water consumption. A recently installed 45,000 cfm unit requires approximately 3600 GPD of evaporated water, assuming incoming gas at 50% saturation, year round. That is comparable to the design criteria of personal water consumption for 100 employees. It is eminently suitable and competitive for food and beverage, flavor and fragrance, and countless other production processes. A wide spectrum of HAPS compounds may also be efficiently treated. With biomass longevity running in decades, rather than months or years, it is hugely advantageous to carbon beds, for which humid or saturated streams decrease efficiency, and for which fire or explosive hazards exist. Biofiltration should be getting more recognition and consideration as cost effective and efficient waste gas treatment technology, particularly in an era of concern over greenhouse gas accumulations.