Understanding Water Dynamics In The Biofilter Excessive or insufficient water in a biofilter may cause a variety of operating problems. Neglecting the water content is the most common cause of poor biofilter operation.

Bradley A. Striebig, Hyun-Keun Son and Raymond W. Regan

B IOFILTERS have been success- ful in treating a wide variety of odorants a t wastewater t reat-

ment and composting facilities. Biofilters include any type of reactor packed with natural or synthetic support media on which a film ofwa- ter is attached, rich with odor-re- moving microorganisms. Examples of synthetic media include packing materials made of ceramic, metal or plastic. Woodchips, bark, compost, sawdust, peat moss and soil a re some of the natural materials com- monly used in biofilters. Often biofilter media are composed of a mixture of natural materials. With-

in the media, microorganisms in- habit a thin film of water that forms on the surface of media particles. This "biofilm" also contains dis- solved organic compounds, nutri- ents and gases, including oxygen (see "Biofiltration - Black Box Or Biofilm?," June, 1997).

Exhaust air from a wastewater treatment operation or composting system contains numerous com- pounds that can cause odors - prin- cipally volatile organic compounds (VOCs), ammonia and hydrogen sul- fide. When a contaminated a i r stream passes through a biofilter, the pollutants are transferred to the

Air ducts (left) deliver composting building exhaust to an outdoor biofilter (above). Significant cooling of air and condensation can take place in transfer ducts.

biofilm, where they are biodegraded to simple end products, such as wa- ter and carbon dioxide. Instead of using large amounts of thermal en- ergy to destroy pollutants or remov- ing pollutants via chemical conver- sion (e.g. incinerators and chemical scrubbers), biofilter systems har- ness the natural abilities of microor- ganisms to biochemically degrade odorous chemicals into environmen- tally harmless end products.

The reliance on biology requires that conditions in the biofilter be managed to sustain microbial activ- ity. Of all the critical conditions, biofilter media moisture or water content is the single most important parameter. The importance of the water content in biofilter operation has been demonstrated by numer- ous researchers (see References). It is the operating parameter most rel- evant to performance of the biofil- ters and also the most sensitive to changes. Significant problems re- sult when the moisture content of the biofilter is either excessive or in- sufficient.

Optimum Water Content The optimum water content for

biofilters is not well established be- cause it depends on the media com- position and the physical character- istics of the pollutant. The optimum water content varies significantly for organic, inorganic and mixed media. In practice, biofilters packed with synthetic media are normally

operated as "bioscrubbers" where the pollutants are first absorbed into the liquid and then degraded in solution. Therefore the water con- tent of synthetic inedia bioscrub- bers is extremely high, with liquid flow rates often exceeding the air- flow rates. With typical na tura l biofilter media (e.g. wood chips plus compost) operators have found that wa te r content should be main- tained in the range of 40 to 60 per- cent. It is not a coincidence that this range is s imilar to t h a t recom- mended for composting. The mois- ture goals are the same in both the compost pile and the biofilter - provide enough water to support microbial activity without exces- sively hindering airflow.

Up to a point, higher water content generally leads to better degradation of odor-causing chemicals. However, the media normally should not be saturated to the point where water is free flowing (except when irrigating the biofilter to add water). It is also important that water content is con- sistent within the biofilter, i.e., i t neither fluctuates greatly over time nor varies from spot to spot within the bed. A consistent water content leads to steady removal rates and de- creases system maintenance.

When the water content fluctuates to either extreme, operational prob- lems result. A low water content de- creases microbial activity and there- fore the odor degradation rate in the biofilin layer decreases. Further- more, under dry conditions, cracks develop within the biofilter media. The air is short circuited through these cracks, leaving parts of the biofilter without adequate aeration and limiting its effectiveness.

Excessively high water content can cause a variety of problems. Air pressure drop across the biofilter increases as water displaces air in the spaces between the particles, thereby restricting the flow of air. I n addition, wet biofilter media tends to consolidate, which further restricts airflow. Excessive inois- ture slows the rates of diffusion of nutrients, oxygen and gases to the microorganisins in the biofilin, hin- dering odor degradation. This is es- pecially t rue for cheillical com- pounds with low water solubility. The oxygen transfer rate decreases because the area of airlwater inter- face is lower per unit biofilin vol- ume . Consequent ly , anaerobic zones form that promote odor for- mation and slow degradation rates. If excessive moisture leads to free flowing water , nut r ien ts can be flushed from the biofilter media, also leading to production of high-

strength, low pH leachate that re- quires further treatment.

Mechanisms Of Water Content Change Three factors largely control the

water content in a biofilter: Humid- ity of the air stream; Evaporation due to t h e h e a t genera ted by biooxidation; and Hydraulic conduc- tivity of the biofilter media. The hu- midity and temperature of the in- coming and outgoing air, relative to the humidity and temperature of the biofilter, have an important influ- ence on the water content of the biofilter. These factors can work to either add or remove n~oisture. In addition, the hydraulic conductivity of the media (i.e. its ability to trans- port water) affects how water moves through the biofilter.

Humidity: Relative humidity val- ues above or below equilibrium con- ditions can change the water content of the biofilter media. Moist incom- ing air, warmer than the biofilter, can add water as it cools within the biofilter because the moisture that it carries condenses in the media. How- ever, more often, incoming air is at or below the biofilter temperature and i t tends to steadily remove water from the bed, unless its relative hu- midity is 100 percent (i.e. saturated). For this reason the air entering the biofilter is usually passed through a humidification process.

Building a i r or exhaus t from many wastewater treatment areas are not normally saturated with wa- ter vapor. To avoid excessive drying of the biofilter, these air streams re- quire aggressive humidification. Air vented from coinposting piles and vessels, enclosed aerobic treatment tanks, equalization tanks and other liquid processes are often close to 100 percent humidity and do not re- quire as aggressive humidification.

Condensation of inoisture from the air stream can cause uncontrolled water addition to the biofilter. For example, in upflow operation (air in- let a t the base of the media, exhaust a t the top) of enclosed units, a lower temperature in the headspace above the bed can generate condensate that will eventually saturate the top of the bed material. Similarly, with open biofilters, condensation may oc- cur as the exiting air passes through the cooler outer layers of the biofil- ter. This recaptures some of the wa- ter that had previously been evapo- rated but concentrates the moisture in the outer layers. Over long periods of time, condensate formation can represent a significant input of wa- ter in either upflow or downflow op- eration. Operators should inonitor moisture in areas of the biofilter

prone to condensation. E v a p o r a t i o n f rom H e a t of

Biooxidation: The second inecha- nisin causing water content changes is t he h e a t genera t ion from biooxidation, the microbial oxida- tion of organic matter (e.g. VOCs and organic biofilter media). The ox- idation of an organic coinpound is an exothermic reaction that steadily re- leases heat into the biofilter and causes the temperature to increase. Moisture loss due to biooxidation in- creases a s the incoming load of VOCs increases because there are more organic compounds to decom- pose and thus more heat to be gen- erated. The temperature rise in the biofilter inay be predicted by the concentration of VOCs in the incom- ing air stream plus air flow rate.

The heat generated within the biofilter also can promote water evaporated from the surface of open biofilters. An enclosure and roof will prevent evaporative losses from the exposed surface and are highly rec- ommended in inany applications. An enclosure or roof also will prevent di- rect sunlight from heating the biofil- ter and subsequently increase evap- orative losses of moisture.

Heat is not the only product of biooxidation. The decomposition of organic compounds also produces water (organic mat te r + 0, --> biomass + CO, + H20). The produc- tion of this metabolic water poten- tially can add moisture to the biofil- t e r . However, our models have shown that the amount of metabol- ic water formed through biological degradation is normally insignifi- cant compared to the changes in water content due to evaporation and condensation.

Hydraulic Conductivity: The third mechanism for wa te r content change is the hydraulic conductivity of biofilter media. Water within the biofilter is either bound to the media by surface tension to create t h e biofilm, or i t may move freely through the biofilter and is called "free water." Hydraulic conductivity is the measure of the velocity of wa- ter inovement through the media, and governs the movement of free water. Water penetrates the biofil- t e r inedia rapidly when the hy- draulic conductivity is high as it is in very porous media. When the biofil- ter media are very dense, the hy- draulic conductivity is very low. In this case, movement of the water added to the biofilter is restricted and inay resul t in localized dry spots. Therefore the hydraulic con- ductivity property of the inedia must allow water to evenly penetrate the biofilter in order to maintain the

proper moisture content throughout the system.

Maintaining Water Content Recent research into the distribu-

tion and addition of water to biofil- tration systems has provided insight into management practices for con- trolling water content. Moisture is best maintained by assuring proper humidification of the entering air stream, monitoring water content throughout the biofilter and period- ically adding water in appropriate quantities.

Proper humidification of the en- tering air stream is the most impor- tant step in maintaining water con- tent. A simple spray chamber or packed bed scrubber can be installed prior to the biofilter to condition the entering air stream and maintain the relative humidity. A properly designed humidification system can maintain humidity above 95 per- cent, moderate temperature fluctua- tions, and act as a buffer for un- s teady airflow and pol lu tant concentrations - all of which leads to improved removal efficiencies of the biofilter and lower maintenance costs. In addition, significant reduc- tions in odor and VOCs often occur within the humidifying device due to biodegradation and absorption.

The location of the blower moving air into the biofilter can affect the in- let humidity. After the humidifier, the air stream is very near satura- tion. If the blower is located between the humidifier and biofilter bed, compression of the air by the blower increases temperature and conse- quently decreases relative humidi- ty. Since the air entering the biofil- ter is no longer saturated, i t may remove moisture from the biofilter. This scenario is a greater concern when air pressure loss through the media is high because the blower works harder and increases pres- sure and temperature more.

Water content within the biofilter may be monitored by conventional sampling and oven drying or elec- tronically using time domain reflec- tometry (TDR) technology. TDR de- te rmines t h e volumetric wa te r content of biofilter media by mea- suring changes in the propagation speed of an electromagnetic pulse in the bed material. Probes for the TDR method can be inserted into the biofilter media to provide on-line data without disturbing the biofilter media. TDR probes can show the distribution of water within the bed and can be used to add water only to the areas that need irrigation (see "Monitoring Moisture in Compost- ing Systems," October, 2000).

Recent studies have shown that switching the direction of airflow through the biofilter more evenly distributes water within the media and increases operational efficiency. In most biofilters, airflows in one di- rection. Assuming the incoming air is properly humidified, microorgan- isms tend to concentrate a t the inlet because the oxygen and pollutant concentration is greater there. This leads to more heat generation and a temperature increase a t the inlet. I t subsequently causes moisture to mi- grate from the inlet to other areas of the media. Reversing the direction of the airflow through the biofilter tends to correct this pattern. The outlet becomes the inlet and vice versa. Thus moisture uniformity im- proves and less water needs to be ap- plied to maintain minimum mois- t u re content in al l a r eas of t he biofilter.

Even with proper humidification, moisture monitoring and air distri- bution, water must be added to the system eventually. Adding i t in moderation is the key to successful- ly maintaining operational efficien- cy. The method of water addition may affect how the biofilter per- forms.

An internal irrigation system, us- ing multiple layers of drip irriga- tion piping or hose to deliver water to the biofilter, is optimal. Irriga- tion pipes are placed a t multiple levels and locations that prevent lo- calized drying. The system should have numerous water emitters (i.e. pipe holes) to evenly distribute the water. Suitable drip irrigation sys- tems are available from various suppliers. Care must be taken to avoid over saturating the biofilter with an internal system. In addi- tion, relatively high water pressure may be required to overcome air pressure within the biofilter and clogging due to build up of micro- bial biomass a t the emitters.

Spray irrigation systems are an- other option. Systems that add water i n the direction of the air stream im- prove the distr ibut ion of water throughout the biofilter. The smaller the droplet size, the better the distri- bution of the water . In fact, re- searchers have recently studied the atomization of water droplets using ultrasonic nozzles in order to carry the water deep into the biofilter. For most applications, water will tend to move in the direction of the airflow because the hydraulic conductivity of most media is small compared to the effects of water vapor transport, evaporation and condensation. Adding small amounts of water to maintain consistent water content

will result in better removal efficien- cy and fewer operational problems. H

Bradley Striebig and Hyun-Keun Son are with The Applied Research Laboratory, and Raymond Regan is w i th the Department of Civil and Environmental Engineering, at The Pennsylvania State University i n State College, PA.

Note: This article was adapted from a literature review conducted on the topic by the authors. For copies of the literature review or questions c o n c e r n i n g t h e ar t i c l e , con tac t Bradley Striebig, Applied Research L a b o r a t o r y , T h e P e n n s y l v a n i a State University, P.O. Box 30, State College, PA 16804-0030. e-mai l : basl24@psu.edu).

References Auria, R., Aycaguer, A.C. and Devin-

ny, J. S. 1998. Influence Of Water Content On Degradation Rates For Ethanol In Biofiltration, Journal of the Air & Waste Management As- sociation. Vol. 48, No. 1, Jan. pp. 65-70.

Devinny, J.S. 1998. Clearing The Air, Biologically, Civil Engineering. Vol. 68. No. 9, Sep. pp. 46-49.

Devinny, J.S., Deshusses, M.A. and Webster, T.S. 1999. Biofilteration for Air Pollution Control, CRC Press LLC, 2000 Corporate Blvd., N.W., Boca Raton, Florida 33431.

Gostomski, P.A., Sisson, J.B. and Cherry, R.S. 1997. Water Content Dynamics In Biofiltration: The Role Of Humidity And Microbial Heat Generation, Journal of the Air & Waste Management Association. Vol. 47, No. 9, Sep. pp. 936-944.

Park, S.J., Cho, K.S, Hirai, M. and Sho- da, M., 1993, Removability of Mal- odorous Gases from a Night Soil Treatment Plant by a Pilot-Scale Peat Biofilter Inoculated with Thiobacillus thioparus DW44, Jour- nal of Fermentation and Bioengi- neering. Vol. 76, No. 1, pp. 55-59.

Sorial, G.A., Smith, F.L., Suidan, M.T., Pandit, A., Biswas, P. and Brenner, R.C. 1997. Evaluation Of Trickle Bed Air Biofilter Perfor- mance For BTEX Removal, Jour- nal of Environmental Engineering. Vol. 123, No. 6 Jun. pp. 530-537.

Swanson, W.J. and Loehr, R.C. 1997. Biofiltration: Fundamentals, De- sign And Operations Principles, And Applications, Journal of Envi- ronmental Engineering. Vol. 123, No. 6, Jun. pp. 538-546.

van Lith, C., Leson, G. and Michelsen, R. 1997. Evaluating Design Op- tions for Biofilters, Journal of the Air & Waste Management Associa- tion. Vol. 47, Jan. pp. 37-48.