American industry continues to search for innovative, more cost effective methods of dealing with volatile organic compound (VOC) emissions. Currently, VOC emissions are controlled using nondestructive concentration technologies, such as carbon absorption, or destructive oxidation processes. A preliminary economic comparison of biofiltration and catalytic oxidation suggests that biofiltration can be an effective and economical alternative to catalytic oxidation.

Biofiltration has been an accepted form of VOC and odor control in Europe for a number of years. In the past five years, a number of different industries in the US have accepted biofiltration as an alternative to thermal and sorptive VOC control technologies. These industries include the wood products industry, the flavor and fragrance industry, and a number of different solvent-using industries, such as the film processing and screen printing industries.

Biofiltration is an air pollution control (APC) technology in which VOC's are oxidized into carbon dioxide and water using micro-organisms. Process air is passed through a pretreatment humidifier to saturate the gas stream before it enters the biofilter. The humidified air then flows through the biofilter where the VOC's are absorbed into an aqueous layer surrounding the filter material. The micro-organisms contained in the filter material use the VOC's as their primary carbon source and convert the VOCs to CO₂ and water.

The most attractive feature of biofiltration is the low operating and maintenance costs relative to other APC technologies. The recirculating pump for the humidifier and the fan that moves the air through the system are the only two energy sinks in the system. Coupled with the fact that typical system pressure drops are less than 6" water column (w.c.), this explains why biofilters are inexpensive to operate relative to other control technologies.

An additional benefit of biofiltration over other oxidation technologies is the lack of secondary contaminants. Biofilters produce none of the NOx compounds found in the effluent from thermal or catalytic oxidizers. In ozone non-attainment areas, the reduction or elimination of these compounds is important and makes the biofilter an even more attractive alternative. A third benefit of biofiltration over thermal oxidation technologies is the intrinsic safety of the system . Unlike thermal systems that operate at elevated temperatures, biofilters operate at ambient temperatures. This, along with the fact that the biofilter media is always wet during operation, all but eliminates the possibility of fire or explosion.

Economic Analysis

A biofilter in its simplest form, consists of a hole in the ground filled with filter media, ranging from dirt, wood chips, bark, peat or compost. These types of biofilter designs are inexpensive and easy to construct, but they are not adequate to meet the standards required for regulatory compliance for a number of reasons. These types of systems have a hard time demonstrating compliance due to the difficulty in measuring system performance. These systems also are affected by ambient conditions and make it difficult to maintain consistent and reliable performance. An economic analysis of the biofiltration system was conducted for two case studies (see Table 1). Case I was a press with a higher flow rate and eight VOCs in the emissions that needed to be controlled. Case II had a lower flow rate with fewer, but higher, concentrations of VOCs.

The economic comparisons were based on a VOC control efficiency of 95%. The operating costs were based on a 6000 hour/year operation. The operating costs for the catalytic oxidizers did not include any maintenance costs other than catalyst replacement.

It was conclusively found that biofiltration appears to be the most economical alternative for treating these gas streams. The results of the economic analyses clearly show the benefits of biofiltration (see Table 2). The capital costs are similar between both systems, ranging from $246,000 to $342,000. The biofilter media replacement cost ($31,250 to $50,000 over a five year period) also compares with the catalyst replacement cost. However, the low operating cost of the biofilter reduces the five-year cost by almost 35% for Case I, and almost 40% for Case II. Often, the operating costs of biofilters can be one-tenth that of a comparable oxidizer system.

Associated Cost	Case 1		Case 2
	Biofilter	RCO	Biofilter	RCO
Installed Capital Costs($)	342,000	350,000	246,000	300,000
Annual Utility Costs($)	4,300	43,400	3,000	29,000
Media or Catalyst Replacement Costs Over 5 Years($)	50,000	61,200	31,250	21,400
Total Cost over 5 Years($)	413,500	628,200	292,250	466,400

The biofilter cost estimates used in the above comparisons are based on a relatively sophisticated design. The biofilters compared here are a fully instrumented design, including a totally enclosed biological reactor, a prehumidification system and a PLC moisture control system with data logging capabilities. This type of system makes demonstrating compliance a straightforward process. Compliance can be demonstrated by simply measuring inlet and outlet concentrations, or by relating historical logs of system parameters to the current performance of the system.

Biofiltration has been accepted as the VOC control of choice by many companies. Weyerhaeuser, Kodak, Bush Boake Allen, Mercedes-Benz, Serigraph, and Coca-Cola all are using biofiltration systems. Biofiltration also has successfully handled VOC emissions in pilot tests for companies such as 3M and Dow.

Biofiltration will not be applicable to all gas streams, but in situations where it does apply, the economics indicate that it should be seriously considered as an alternative to current control technologies.

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