Development of Two Biomass Control Strategies for Extended, Stable

Environ. Sci. Technol. 1996, 30, 1744-1751

Development of Two Biomass Control Strategies for Extended, Stable Operation of Highly Efficient Biofilters with High Toluene Loadings FRANCIS L. SMITH,† GEORGE A. SORIAL,† M A K R A M T . S U I D A N , * ,† ALEXANDER W. BREEN,‡ AND PRATIM BISWAS† Departmentof Civil and Environmental Engineering and Department of Molecular Genetics, University of Cincinnati, Cincinnati, Ohio 45221-0071

RICHARD C. BRENNER National Risk Management Research Laboratory, U.S. EPA, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268

For stable long-term continuous operation of highly loaded trickle bed air biofilters, the prevention of plugging due to accumulating biomass is essential for avoiding biofilter failure. Two biomass control strategies were evaluated to maintain high VOC removal efficiencies at high toluene loadings for over 200 days. A sustained toluene removal efficiency of over 99% was achieved. Backwashing with medium fluidization was found to be very effective in preventing accumulation of excess biomass. A specific flow rate of 190 m/h (for a bed expansion of 40%) for 1 h twice per week was found to be adequate. The use of nitrate (NO3-N) instead of ammonia (NH3-N) as the sole source of nutrient-nitrogen (N) was very effective in reducing the observed biomass yield. A pilot-scale evaluation was made of the performance of two similarly operated, backwashed biofilters receiving influent air with 250 ppmv toluene and varying only the VOC loadings and the form of nutrient-N. The biofilter receiving NO3-N performed significantly better overall. A comparative bacterial enumeration of the two biofilter microbial consortia was performed. This enumeration revealed that the populations of toluene degraders in both biofilters were about the same; however, the populations of total heterotrophs were starkly different.

Introduction For more than 20 years, biofiltration has been recognized as a cost-effective technology for the purification of air * Author to whom correspondence should be addressed; telephone: (513)556-3648; fax: (513)556-2599. † Department of Civil and Environmental Engineering. ‡ Department of Molecular Genetics.

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contaminated with low concentrations of biologically degradable volatile organic compounds (VOCs). Most commercial applications before 1990, however, were for the control of odors (1). The implications of the 1990 Amendments to the Clean Air Act (2) have generated increased interest in the application of biofiltration for the reliable, efficient, and economical control of biodegradable VOCs and airborne toxics (3-5). The emphasis of our research has been to develop well-engineered biofiltration systems that could fill this expanded and more demanding role. Our biofiltration research has focused on expanding the range of application of biofiltration technology to the treatment of high concentrations of VOCs at consistently high removal efficiencies under high loadings. Several different biological attachment media and media geometries were studied by our group (6-9) to establish the feasibility of developing such an engineered system. These studies were performed at pilot scale for the comparison of the long-term operating performance of three media: a patented peat organic mixture and two synthetic inorganic media, one channelized and the other pelletized. The biofilters containing the latter two media were operated as trickle bed air biofilters (TBABs), so called because the medium receives a steady application of water. After 18 months of testing, the inorganic pelletized medium (R-635) was demonstrated to be significantly better than either the peat organic mixture or the inorganic channelized medium for achieving high removal efficiencies of high concentrations of VOCs. This superior performance was particularly evident at higher VOC loadings. However, the biofilter containing R-635 eventually failed, due to plugging caused by the accumulation of excess biomass (VSS). Subsequent work was done to evaluate the performance and behavior of a second biofilter containing R-635. Two significant findings were made. First, a moderate increase in the biofilter operating temperature (from ambient to 32.2 °C) permitted a significantly higher practical VOC loading (i.e., a significantly smaller required medium volume) to achieve the same level of removal efficiency at the same influent VOC concentration. Second, biofilter performance decreased substantially, coincident with the buildup of back pressure due to the accumulation of excess VSS within the medium bed (9-10). This paper presents the results of our research to develop a feasible and successful strategy for long-term TBAB operation at consistently high VOC removal efficiencies under continuous high toluene loadings and influent concentrations. To produce comparative results, our preliminary as well as current work has used toluene as the sole VOC and substrate source. The biofiltration of airstreams containing toluene or the related BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) has been studied by others. Biofilters containing peat or other natural media, under constant VOC loading, have been shown to have limited elimination capacities for these compounds. Weber et al. (11) reported a removal efficiency of 25% for a toluene loading of 4220 g of C m-3 day-1, for a removal rate of 1056 g of C m-3 day-1 [3.62 kg of COD m-3 day-1]. Morales et al. (12) reported a maximum removal rate of 25 g of toluene m-3 h-1 [1.88 kg of COD m-3 day-1]

0013-936X/96/0930-1744$12.00/0

 1996 American Chemical Society

FIGURE 1. Schematic for the pilot-scale trickle bed air biofilter.

for several inlet concentrations. TBABs, however, have been shown to be able to achieve high removal efficiencies at higher loadings of these compounds. Severin et al. (13) reported perhaps the most successful applications, reporting removal efficiencies of 84% for loadings of toluene or BTEX of about 55 g of hydrocarbons m-3 h-1 [4.13 kg of COD m-3 day-1, as toluene] for TBABs containing three different proprietary media. Our objectives were to meet or surpass performance at this level with TBABs using R-635 to eliminate toluene. To achieve this goal, two VSS control strategies were developed and evaluated. The first control strategy developed was to prevent plugging of the medium by the accumulation of excess VSS. Others have studied this problem. Weber et al. (14) employed nutrient-N limitation as well as high ionic strength (NaCl) to reduce plugging in a TBAB degrading toluene. Holubar et al. (15) investigated the limitation of nutrient-N and potassium for plugging control in a TBAB degrading a mixture of hydrocarbons, including some toluene. Both were able to control plugging, but with VOC removal efficiencies reported below 50%. Severin et al. stated that they successfully removed the excess VSS from their media by periodic washing with water, but they presented no specific details of the technique. The VSS control strategy for plugging control discussed in this paper is periodic offline in-situ upflow washing with water or backwashing. The role of the backwashing parameters (frequency, duration, and flow rate) on the performance of the biofilter system has been established (9, 10). The second control strategy developed was to reduce the overall observed yield of VSS, not through nutrient limitation, but by changing the form of the nutrient-N. Ammonia and nitrate were used as the two, alternative, sole nutrient-N sources: NH3-N (biofilter A) and NO3-N (biofilter B) at equivalent mass ratios of COD to nutrientN. It has been shown that the cell yield for a given heterotrophic organism grown on a given substrate can be estimated directly from thermodynamic fundamentals (16).

Similarly, the growth yield for an aerobic culture can be shown to be lower when utilizing NO3-N rather than NH3-N as the sole source of nutrient-N. This reduction in yield results from the need to expend reducing equivalents to convert nitrate to ammonia for cell synthesis, i.e., proteins. Our evaluation was to determine the relative effect of these nutrients on the observed VSS yield as well as on the overall performance of the TBABs. To evaluate these two VSS control strategies, the relative performances of two parallel backwashed TBABs receiving different forms of nutrient-N were studied. The TBABs were operated under similar conditions, varying only the VOC loading. At the end of the experimental run, samples of the R-635 medium were taken from several positions within the bed, and a comparative bacterial enumeration was performed on the attached biofilm.

Experimental Facilities and Methods Experimental Apparatus. The experimental apparatus consists of two independent, parallel, biofilter trains, designated as biofilters A and B (the experimental setup is shown in Figure 1). Each biofilter has a circular crosssection with an internal diameter of 14.6 cm and contains a 1.14 m deep randomly packed bed of 6-mm pelletized biological attachment medium, above an approximate 8-mm layer of 12-mm glass support beads (common glass marbles). The biological attachment medium was Celite R-635 Bio-Catalyst Carrier (Celite Corp., Lompoc, CA). Each biofilter is constructed of 304 stainless steel and consists of several different sections, from top to bottom. The first section is a heat exchanger for preheating the buffered nutrient feed solution to maintain the operating temperature of the biofilter. This is followed by a head space for the air inlet and for housing the buffered nutrient spray nozzle. The third section is a 1.2-m region containing the randomly packed bed of R-635 and its support beads. The last section is a disengagement space for separating the effluent wastewater and air. In order to maintain a

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constant biofilter operating temperature, the biofilters are insulated and temperature controlled by circulating tempered water around the biofilter wall. Raw utility quality air at 690 kPa (100 psig) is purified to remove all particulates, droplets, water vapor, and carbon dioxide (Balston 75-62 FTIR purge gas generator, Haverhill, MA). A granular activated carbon filter is then used to remove all traces of hydrocarbons. The air pressure is reduced to 70 kPa (10 psig) by a pressure control valve, both for safety and for isolating the biofilters from any pressure fluctuations in the upstream air supply. The air flow to each biofilter is controlled by a mass flow controller (MKS Model 247C four-channel readout mass flow controller, Andover, MA) and indicated by a rotameter (Dwyer Instrument, Michigan City, IN). The airstream is then heated to the biofilter operating temperature, the liquid VOC is injected via a syringe pump (Harvard Apparatus, Model 1140-001, South Natick, MA) into the airstream where it vaporizes, and the air with the vaporized VOC is fed to the biofilter. Each biofilter is equipped with an independent system for feeding 20 L/day (0.050 m/h) of a buffered nutrient solution. This solution is made from deionized and PAC filtered water, according to a formulation that contains all necessary macronutrients, micronutrients, and buffers, as described elsewhere (17). The solution is circulated by a stainless steel gear pump from a 70-L feed tank through a solenoid operated three-way valve and back to the feed tank. A programmable controller (Danaher Controls, Eagle Signal Model MX190, Gurnee, IN) activates the solenoid once per minute to divert the feed to the biofilter so that 20 L is fed per day. This feed is first preheated and then sprayed as a fine mist onto the top of the medium bed. The nutrient formulation for each biofilter is essentially the same, containing the same amount of nutrient-N and phosphorous for a given VOC loading (COD/N ) 50 and N/P ) 4). The amount of buffer is adjusted as needed to maintain the desired biofilter operating pH of 7.2. The singular difference between the nutrient formulations for biofilters A and B is that the nitrogen source for biofilter A is in the reduced form (NH3-N) while for biofilter B it is in the oxidized form (NO3-N). During our earlier work, it was discovered that some nitrification was occurring in the nutrient feed system containing NH3-N prior to being fed to the biofilter. Investigation of this condition further revealed that some photosynthesis was also taking place. Therefore, the entire feed system for each biofilter was chlorinated with a bleach solution prior to preparation of each batch of nutrient, and the nitrification inhibitor TCMP [2-chloro-6-(trichloromethyl)pyridine] was added to the formulation of buffered nutrient at a final concentration of 20 ppm for biofilter A (NH3-N). This inhibitor is recommended for use in the 5-day BOD test, according to Method 5210-B (18). Each biofilter train has sampling ports to allow sampling of each stream entering and leaving the biofilter as well as within the medium bed. Ion concentrations were measured in the influent and effluent liquid streams for ammonia and nitrate. The VOC (toluene) concentration was measured in the influent air, both effluent streams, and gas samples taken from within the medium bed. The pellets of R-635 are made from sintered diatomaceous earth and are therefore principally silica (SiO2). They have a circular cross-section with a nominal diameter of 0.635 cm. The length of the pellets vary; from a sample of

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TABLE 1

Experimental Plan for Biofilter A Using (NH3-N) and 250 ppmv Toluenea backwashing

flushing

loading (kg EBRT of COD T frequency duration rate duration test day (min) m-3 day-1) (°C) (per week) (min) (m/h) (min) 1 2 3 4 5 6 7 8

10 28 38 73 80 132 172 185 a

2.0 2.0 2.0 2.0 2.0 2.0 1.0 1.0

2.27 2.27 2.27 2.27 2.27 2.27 4.54 4.54

18 18 32 32 32 32 32 32

1 2 2 2 2 2 2 2

4.4 4.4 8.8 30 60 60 60 120

81 81 81 190 190 190 190 190

n/a n/a n/a 1.9 1.9 0.94 0.94 0.94

Note: ppmv toluene × 0.0037 ) g/(m3) at 32.2 °C.

TABLE 2

Experimental Plan for Biofilter B Using (NO3-N) and 250 ppmv Toluenea backwashing

flushing

loading EBRT (kg of COD T frequency duration rate duration test day (min) m-3 day-1 (°C) (per week) (min) (m/h) (min) 1 2 3 4 5 6

6 18 77 98 117 130 a

2.0 2.0 2.0 2.0 1.0 1.0

2.27 2.27 2.27 2.27 4.54 4.54

32 32 32 32 32 32

2 2 2 2 2 2

8.8 30 30 60 60 120

81 190 190 190 190 190

n/a 1.9 0.94 0.94 0.94 0.94

Note: ppmv toluene × 0.0037 ) g/(m3) at 32.2 °C.

110 pellets, the mean (0.64 cm), median (0.60 cm), and standard deviation (0.21 cm) were measured. From these data and the pellet diameter, the pellet sphericity (0.84) and specific surface (11.9 cm2/cm3) were estimated. The measured pellet internal and external void fractions were about 0.65 and 0.34, respectively, and the uncompacted bulk density was approximately 0.62 g/cm3. Experimental Plan. The experimental plan was designed to compare the performance of two biofilters receiving influent air with a 250 ppmv (parts per million by volume) concentration of toluene and operated in a similar manner. Operating parameters that were varied, the form of the nutrient-N, the VOC loading, the backwashing temperature, frequency, duration and rate, and the flushing duration are summarized in Tables 1 and 2. Performance parameters to be studied were as follows: (1) recovery of VOC removal efficiency with time after backwashing, (2) removal efficiency with respect to depth within the medium bed at steady state conditions, (3) the COD/N utilization ratio, (4) the VSS/COD yield, and at the end of the experimental run (5) a bacterial enumeration of the microbial consortia within the biofilters. Analytical Methods. Concentrations of toluene were measured by chromatographic separation on a 30-m megabore column (DB 624, J&W Scientific, Folsom, CA) using a gas chromatograph (GC) (HP 5890, Series II, HewlettPackard, Palo Alto, CA) equipped with a liquid sample concentrator (LSC 2000, Tekmar, Cincinnati, OH) and a photoionization detector (PID) (Model 4430, OI Corp., College Station, TX). The liquid sample concentrator was programmed according to U.S. EPA Method 601, and a Tenax trap was used with a helium (He) purge flow of 40 mL/min. The GC oven temperature was programmed from 40 to 120 °C at 5 deg/min with a 4-min hold at 40 °C and

FIGURE 2. Performance of biofilter A (NH3-N) at 250 ppmv influent toluene concentration with media backwashing.

FIGURE 3. Performance of biofilter B (NO3-N) at 250 ppmv influent toluene concentration with media backwashing.

a 6-min hold at 120 °C. The carrier gas (He) flow rate was set at 8 mL/min, and the PID detector was used with He makeup gas at a flow rate of 20 mL/min, a sweep gas flow rate (H2) of 100 mL/min, and a base temperature of 250 °C. Gas phase samples for VOC analysis were taken with gas-tight syringes through low bleed and high puncture tolerance silicone GC septa (replaced every week) installed in the sampling ports at the gas inlet and outlet from the biofilters. Samples from the liquid phase for VOC analysis were removed in a similar way from the liquid outlet from the biofilters. Both gas and liquid phase samples were introduced to the GC through the liquid sample concentrator accessory. The gaseous phase VOC analysis was conducted by introducing 5 mL of purged distilled deionized water into the purge vessel of the liquid sample concentrator prior to the injection of the gas sample. Liquid phase samples were analyzed for ammonia and nitrate concentrations by the electrode method of analysis according to Methods 4500-NH3 D,E and 4500-NO3 D (18), respectively. COD analysis was performed according to Method 5220-D (18). Samples were filtered through 0.45µm nylon filters (Micron Separation, Inc., Westboro, MA) prior to analysis. pH determinations were made using a Fisher Accumet pH meter, Model 50 (Fisher Scientific Co., Inc., Fair Lawn, NJ). The pH meter was calibrated using buffers (pH 4.0 and 7.0) supplied by the manufacturer. Bacterial enumeration was performed on samples of the pelletized medium taken from the biofilter medium beds. The pellet samples were vortexed in 1% sodium pyrophosphate to free the biomass from the pellets, and these solutions were then serially diluted in phosphatebuffered saline and plated. Plate count agar (Difco Laboratories, Detroit, MI) was used to determine total heterotrophic bacteria. A mineral salts medium was used for the enumeration of toluene degrading bacteria (19). Following inoculation, the mineral salts plates were incubated in 23-L desiccators that had toluene supplied to the vapor phase.

washed by hand, batchwise, in warm water until they were clean of obvious biofilm and free flowing. After loading, the pellets were seeded by pouring the decanted pellet wash water over the bed. (The original seed, 2 years previously, had been municipal activated sludge). The biofilter was started up at 50 ppmv toluene, 2 min empty bed residence time (EBRT), and 29 mmol of NH3-N/day. By day 4, the removal efficiency was above 99.9%. On day 8, the inlet concentration was increased to the target value of 250 ppmv toluene with 57 mmol/day NH3-N. The removal efficiency dropped to about 97% and ranged between 92% and 98% until day 25 when it again achieved 99%. Subsequently, the removal efficiency dropped as low as 86% before regaining the 99% level on day 81.

Results The performances of biofilters A and B with respect to toluene removal efficiency (a minimum of 12 h after backwashing) are shown in Figures 2 and 3, respectively. Biofilter A. This biofilter was loaded with a 50/50 mixture of fresh pellets and pellets used in a previous run (where toluene and NH3-N had also been used, under similar conditions). These previously used pellets had been

During this period, the principal elements of our backwashing technique were developed. On day 10 (test 1), backwashing was initiated at a frequency of once per week by using 100 L of fresh, cold, once-through tap water at a specific backwashing rate of 81 m/h (33 gpm/ft2). After day 28 (test 2), the backwashing frequency was increased to twice per week, and after day 38 (test 3), the backwashing water volume was increased to 200 L while maintaining the same backwashing rate. Also on day 38, the backwashing water temperature was raised to the biofilter operating temperature of 32.2 °C, and an increase in volatile suspended solids (VSS) removal was observed. However, these procedures were conducted at rates insufficient to produce medium fluidization, and they proved inadequate to prevent pressure drop buildup between backwashings. Therefore, on day 73 (test 4), the backwashing rate was increased to 190 m/h (78 gpm/ft2) to induce full medium fluidization at about 40% bed expansion. By recycling the backwashing water, the backwashing period was increased to 0.5 h, followed by flushing the fluidized medium with another 100 L of once-through water to remove freed solids from the bed. Although the pressure drop increase was minimal, the removal efficiency did not improve. This suggested that some channeling was occurring due to inadequate removal of accumulated VSS. Therefore, on day 80 (test 5), the backwashing period was doubled by recycling the backwashing water for 1 h. Timed samples were routinely taken of this recycled stream, which confirmed the selection of the 1-h backwashing period. As mentioned above, removal efficiency increased to 99% within a few days. For reasons of economy, on day 132 (test 6), the recycle and flush volumes were reduced to 70 and 50 L, respectively. The 70-L recycle volume was selected as a reasonable minimum for our pilot plant, and the 50-L flush volume


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TABLE 4

Biofilter Steady State Removal Efficiency with Depth biofilter A (NH3-N) EBRT 2 min

1 min

FIGURE 4. Schematic for the biofilter backwashing system.

FIGURE 5. Typical backwashing profile, showing the variation of VSS concentration with time. TABLE 3

Biofilter Effluent Recovery of Removal Efficiency with Time after Backwashing biofilter A (NH3-N) EBRT 2 min

1 min

biofilter B (NO3-N)

seq. date

20 min (%)

250 min (%)

seq. date

20 min (%)

250 min (%)

111 122 129 143 153 181 192 209 223 237

68 64 72 68 71 55 52 59 50 48

99 98 98 99 99 78 78 84 90 90

46 60 81 102 112 130 161 172 186

65 72 76 78 77 59 74 63 69

99 99 94 98 99 79 93 94 96

was selected after taking timed samples of flush and observing that the VSS concentration in the flush was essentially zero after 50 L of flush. (The volume of water in the biofilter during backwashing is about 26 L.) The schematic of this final pilot plant backwashing system is shown in Figure 4. A typical backwashing and flushing profile, showing VSS concentration as a function of time, is shown in Figure 5. The response of the biofilter to backwashing was measured, starting on day 111. The biofilter effluent recovery with time after backwashing and the biofilter removal efficiency with depth are shown in Tables 3 and 4, respectively. The significance of these data is discussed below.

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biofilter B (NO3-N)

seq. date

0.3 m (%)

1.2 m (%)

seq. date

0.3 m (%)

1.2 m (%)

112 123 130 144 145 154 156 182 193 194 224 238 245

73 66 70 66 74 74 80 66 49 56 65 67 64

99 99 99 99 99 99 99 98 96 97 96 99 95

47 92 103 104 110 113 115 131 132 162 173 176 187

86 60 86 88 83 71 86 62 62 71 72 70 68

99 99 99 99 99 99 99 97 96 98 99 99 99

For an EBRT of 2 min after day 73, the performance of biofilter A with respect to toluene removal efficiency stabilized at about 99%. On day 172 (test 7), the EBRT was reduced to 1 min at the same inlet concentration, doubling the VOC loading. The NH3-N feed was also doubled to 114 mmol/day at this time. After this increase in VOC loading, removal efficiency dropped initially to 82%, but by day 181 it had risen to 90%. On day 185 (test 8), due to increasing pressure drop, the backwashing period was extended to 2 h. By day 186, the removal efficiency was over 96%, and it ranged between about 94% and 98% until day 245. For an EBRT of 2 min, the pressure drop remained generally under 1.5 cm of water, with an occasional reading as high as 3.8 cm of water. After the EBRT was reduced to 1 min, the pressure drop continued to behave in a similar manner, even after the backwashing period was extended to 2 h. Biofilter B. This biofilter was loaded with fresh pellets and seeded with backwashing water from biofilter A (where NH3-N was being used). It was started up at 50 ppmv toluene, 2 min EBRT, and 29 mmol of NO3-N/day as the sole nitrogen source. By day 3, the removal efficiency was over 92%, and by day 9 it was over 99%. On day 11, the inlet concentration was raised to the target value of 250 ppmv toluene with 57 mmol/day NO3-N, and the removal efficiency remained over 99%. On day 6 (test 1), backwashing was initiated at a frequency of twice per week using 200 L of fresh, warm (32.2 °C), once-through tap water at a specific backwashing rate of 81 m/h (33 gpm/ft2). Some measurable pressure drop developed between backwashings however, and on day 18 (test 2), the backwashing rate was increased to 190 m/h (78 gpm/ft2) to induce full medium fluidization at about a 40% bed expansion. By recycling the backwashing water, the backwashing period was increased to 0.5 h, followed by flushing the fluidized medium with an additional 100 L of once-through fresh water. This modification eliminated the buildup of pressure drop, and the removal efficiency stabilized at about 99%. For reasons of economy, on day 77 (test 3), the recycle and flush volumes were reduced to 70 and 50 L, respectively. On day 98 (test 4), the backwashing period was increased to 1 h to make it equivalent to that of biofilter A. Timed samples taken during backwashing confirmed the selection of the 1-h backwashing period and the 50-L flush volume (Figure 5). The response of the biofilter to backwashing was measured starting on day 46. The biofilter effluent

TABLE 5

Biofilm Bacterial Population Densitiesa

biofilter A (NH3-N) top middle bottom biofilter B (NO3-N) top middle bottom a

heterotrophs (cfu/g)

toluene degraders (cfu/g)

6.5 × 109 4.0 × 109 5.0 × 109

3.0 × 108 1.3 × 108 6.5 × 107

5.5 × 108 6.0 × 108 4.7 × 108

4.0 × 108 2.3 × 108 6.5 × 107

cfu/g of biofilter pelletized media (wet basis).

recovery with time after backwashing and the biofilter removal efficiency with depth are shown in Tables 3 and 4, respectively. The significance of these data is discussed below. For an EBRT of 2 min, the performance of biofilter B with respect to toluene removal efficiency stabilized at over 99%. On day 117 (test 5), the EBRT was reduced to 1 min at the same inlet concentration, doubling the VOC loading. The NO3-N was also doubled to 114 mmol/day at this time. After this increase in VOC loading, removal efficiency dropped initially to about 89%, but by day 130 it had risen to about 92%. Note that with this increase in loading the performance of this biofilter initially dropped less and ultimately recovered better than the previous biofilter. On day 130 (test 6), due to increasing pressure drop, the backwashing period was extended to 2 h. By day 168, the removal efficiency had steadily increased to over 98% and to over 99% by day 173. For an EBRT of 2 min after day 18, the pressure drop through the bed remained below 0.25 cm of water. After reducing the EBRT to 1 min, the pressure drop normally was under 0.5 cm of water, with a few excursions as high as 2.3 cm of water. Bacterial Enumerization. At the end of the experimental run, the contents of the two biofilters were gently removed. Samples were carefully taken from the top, middle, and bottom of the biofilter medium beds. These samples were analyzed to enumerate the biofilm population densities of total heterotrophs and toluene degraders (TDs). The results of this bacterial enumeration study are given in Table 5, expressed as colony forming units per gram (cfu/g) of sample of biofilter pelletized medium (wet basis). Both the heterotrophic and TD counts showed a general trend of decreasing density from top to bottom of the medium beds in either biofilter. The density of heterotrophs in biofilter A was about an order of magnitude greater than that in biofilter B. Nevertheless, the density of TDs in each biofilter was similar at each level. The principal difference between the two biofilters therefore is that the TDs comprised a considerably smaller fraction of the biofilm in NH3-N fed biofilter A than in NO3-N fed biofilter B.

Discussion The data in Tables 3 and 4 include data for 2 min EBRT and for 1 min EBRT. In Table 3 for 2 min EBRT, biofilter B had recovered an average of 5% higher removal efficiency than biofilter A after 20 min following backwashing, although they had recovered to a similar degree after 250 min. However, for 1 min EBRT, biofilter B was consistently superior to biofilter A at 20 and 250 min after backwashing. Plots showing the typical recovery of effluent removal efficiency with time after backwashing for each biofilter

FIGURE 6. Effect of the form of nutrient-N on the recovery of biofilter effluent removal efficiency after startup. Biofilter B (NO3-N) begins with a better removal efficiency and more quickly achieves the ultimate, steady-state removal efficiency.

are shown in Figure 6. As can be seen, the superior performance of biofilter B after backwashing is shown, with a lower drop and a quicker recovery of VOC removal efficiency. In Table 4, for 2 min EBRT, biofilter B exhibited generally better removal efficiency at 0.3 m depth, indicating that the VSS in biofilter B was performing better throughout the bed. For 1 min EBRT, however, biofilter B showed superior performance at both depths (0.3 and 1.2 m), except immediately after the VOC loading was doubled, on day 117 (note that the data point for biofilter A, 99% removal, on day 238, may be an outlier). These differences in relative biofilter performance suggested that the microbial consortia in the two biofilters were functionally if not structurally different. This difference was reflected in the relative utilization of nutrient-N by each consortia. Weekly analyses were made for both ammonia and nitrate, 1 day following backwashing. For each biofilter, the growth of VSS was monitored by reviewing the net nitrogen consumption obtained from the nitrogen balance. This was calculated by subtracting the molar amounts of the nitrogen species in the effluent (both NH41+ and NO31-) from the nitrogen species in the feed. The COD removal was calculated as the difference between the COD of the influent and effluent streams. The influent COD was calculated as the sum the COD equivalent of the toluene present in the influent airstream and the COD measured in the nutrient feed. Similarly, the effluent COD was calculated as the sum the COD equivalent of the toluene present in the effluent air and wastewater streams and the non-toluene COD measured in the effluent wastewater. The average non-toluene COD in the effluent wastewater was less than 0.5% of the influent COD. No compounds other than toluene were detected in the influent or effluent airstreams, or in the effluent wastewater stream. A linear regression of the COD and nutrient-N utilization data was made to evaluate the relationship between them. This approach assumes that the COD/N utilization ratio was independent of the VOC loading and that aerobic heterotrophs accounted for essentially all of the net utilization of the nutrient-N within the biofilters, i.e., that nitrification was negligible. Plots of these data with the “best fit” straight line through the origin for biofilters A and B are shown in Figure 7. It is evident that significantly more NH3-N was utilized for cell growth for a given amount of COD consumed than was NO3-N. From these data, two estimates of the COD/N and VSS/COD ratios were made.


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FIGURE 7. Effect of the form of nutrient-N on the utilization of nitrogen with respect to COD removal for biofilter A (NH3-N) and biofilter B (NO3-N). TABLE 6

Biofilter COD/N Utilization Ratios and Calculated VSS Yields

g of COD/g of N (by linear reg.) (R2) g of VSS/g of COD (yield) g of COD/g of N (by stat. mean) (SD) g of VSS/g of COD (yield)

biofilter A (NH3-N)

biofilter B (NO3-N)

62.1 0.850 0.115 60.2 7.16 0.119

106 0.711 0.0674 112 35.6 0.0638

FIGURE 8. Effect of the form of nutrient-N on the ratio (g of COD/g of N) with respect to time for biofilter A (NH3-N) and biofilter B (NO3-N).

First, from the inverse of the slopes of the straight lines in Figure 7, the COD/N ratios were calculated. Using this ratio, the yield of VSS from COD (VSS/COD) was estimated by assuming that VSS is 14 wt % nitrogen. Second, ratios of these data were also calculated, and the mean values of the ratios were determined. From these mean values, VSS/ COD was again estimated (Table 6). These estimated ratios indicate that two engineering advantages in using NO3-N are that (1) at least 70% more COD can be degraded for a given mass of nutrient-N and (2) at least 40% less VSS is generated for a given mass of COD. The COD/N ratios were plotted for biofilters A and B against the sequential date of operation to check for any relationship with either time or VOC loading (Figure 8). No significant, long-term dependence of the COD/N utilization ratio on either time or VOC loading is apparent.

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The differences in relative biofilter performance was also reflected by the differences in the biofilm population densities. The levels of TDs were about the same in both biofilters, while the proportion of TDs to total heterotrophs was starkly different. Using the ratios of the average densities for each biofilter, the ratio of TDs to all other heterotrophs was 1/30 in biofilter A (3.2%), while that ratio was 1/1.3 in biofilter B (43%). This much larger ratio suggests that NO3-N fed biofilter B had a consortia more highly acclimated to toluene degradation and, therefore, was functionally if not structurally different. The smaller mass of VSS was reflected in its lower operating pressure drop. Although this consortia performed better with respect to toluene removal efficiency, the absence of the additional heterotrophs may have resulted in a consortia more sensitive to the shock of backwashing, which could be reflected in the otherwise unexplained scatter of the data for biofilter B in Figure 8. The effect of the nitrification inhibitor TCMP on either the performance or the VSS yield of biofilter A is unknown, but we do not believe that this inhibitor was responsible for the magnitude of the differences observed. The observed yield of VSS for an aerobic culture grown on NO3-N is lower than that for a culture grown on NH3-N, since reducing equivalents must be expended to convert nitrate to ammonia for cell synthesis, i.e., proteins. Our hypothesis, therefore, is that there was more net energy available in biofilter A for growth and the production of microbial products. These products, such as exopolysaccharides, provided growth substrates supporting a larger population of non-toluene degrading heterotrophs. The absence of these products, therefore, would provide a selective pressure against non-toluene degrading heterotrophs. It is unlikely that toluene oxidation metabolites would directly support these non-toluene degraders. Toluene degradation has been studied extensively (20). Toluene is rapidly oxidized and has not been reported to generate any persistent products (other than CO2 and H2O). Additionally, if toluene metabolites were responsible for this observation, it should also have been observed in the other biofilter. From an engineering standpoint, these findings suggest several things. First, in an actual biofilter application there would be several cost advantages to using NO3-N (rather than NH3-N): higher design VOC loadings could be used, nutrient-N losses to nitrification could be minimized, the design backwashing frequency and duration for a given application could be reduced, and VSS disposal would be substantially reduced. Secondly, under actual conditions of varying VOC composition and loading, the backwashing frequency and duration should be adjusted as needed in order to control the retained VSS to near the minimum required levels. The level of VSS accumulated within the biofilter could be estimated from the COD and nitrogen balances, as was done in this study. However, in an actual application it would probably be better to make an indirect measurement of the VSS removed by measurement of the turbidity of the recycled backwash water and of the VSS retained by periodic measurement of VSS on samples of the pelletized medium taken at the end of the backwashing operation. These samples of medium could be easily taken from any depth of the bed while it remained fluidized. As a final engineering observation, it should be noted that no measurable attrition of the pellets of R-635 has been observed during the period of this research. A layer

of biofilm developed, which covered the complete surface of each pellet. During fluidization, therefore, it was the surface biofilm layers and not the pellets beneath that came in contact and were abraded.

Acknowledgments This research was supported by Cooperative Agreement CR-821029 with the U.S. Environmental Protection Agency. The findings and conclusions expressed in this publication are solely those of the authors and do not necessarily reflect the views of the Agency. We extend our sincere gratitude to Mr. Francis L. Evans III and his staff at the U.S. EPA Testing and Evaluation Facility, Cincinnati, OH, for their consistent support and assistance during this phase of our research.

Literature Cited (1) Ottengraf, S. P. P. In Biotechnology; Rehn, H. J., Reed, G., Ed.; VCH Verlagsgesellschaft: Weinham, Germany, 1986; Vol. 8, Chapter 12, pp 425-452. (2) Lee, B. J. Air Waste Manage. Assoc. 1991, 41, 16-31. (3) Leson, G.; Winer, A. M. J. Air Waste Manage. Assoc. 1991, 41, 1045-1054. (4) Leson, G.; Tabatabal, F.; Winer, A. M. Presented at the 85th annual meeting of the Air & Waste Management Association, Kansas City, MO, 1992; Paper 92-116.03. (5) Ottengraf, S. S. P. Bioprocess Eng. 1986, 1, 61-69. (6) Smith, F. L.; Sorial, G. A.; Smith, P. J.; Suidan, M. T.; Biswas, P.; Brenner, R. C. Presented at the EPA Symposium on Bioremediation of Hazardous Wastes: Research, Development, and Field Evaluations, Dallas, TX, 1993; pp 111-120. (7) Sorial, G. A.; Smith, F. L.; Smith, P. J.; Suidan, M. T.; Biswas, P.; Brenner, R. C. Presented at the 86th annual meeting of Air & Waste Management Association, Denver, CO, 1993; Paper 93TP-52A.04. (8) Sorial, G. A.; Smith, F. L.; Smith, P. J.; Suidan, M. T.; Biswas, P.; Brenner, R. C. Proceedings of the Water Environment Federation

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66th annual conference, Facility Operations Symposia; Water Environment Federation, Alexandria, VA, 1993; Vol. X, pp 429439. Smith, F. L.; Sorial, G. A.; Suidan, M. T.; Biswas, P.; Brenner, R. C. Presented at the EPA Symposium on Bioremediation of Hazardous Wastes: Research, Development, and Field Evaluations, San Francisco, CA, 1994; pp 89-97. Sorial, G. A.; Smith, F. L.; Suidan, M. T.; Biswas, P.; Brenner, R. C. Presented at the 87th annual meeting of Air & Waste Management Association, Cincinnati, OH, 1994; Paper 94RA115A.05. Weber, F. J.; Hartmans, S. Appl. Microbiol. Biotechnol. 1995, 43, 365-369. Morales, M.; Pe´rez, F.; Auria, R.; Revah, S. Adv. Bioprocess Eng. 1994, 405-411. Severin, B. F.; Shi, J.; Hayes, T. Presented at the IGT 6th International Symposium on Gas, Oil, and Environmental Technology, Colorado Springs, CO, 1993. Weber, F. J.; Hartmans, S. VDI Berichte No. 1104, 1994, 161-168. Holubar, P.; Andorfer, C.; Braun, R. Presented at the USC-TRG Conference on Biofiltration, University of Southern California, October 1995; pp 115-121. McCarty, P. L. In Water Pollution Microbiology; Mitchell, R., Ed.; Wiley-Interscience: New York, 1972; Chapter 5. Sorial, G. A.; Smith, F. L.; Suidan, M. T.; Biswas, P.; Brenner, R. C. J. Air Waste Manage. Assoc. 1995, 45, 801-810. Standard Methods for the Examination of Water and Wastewater, 19th ed.; American Public Health Association: Washington, DC, 1995. Hareland, W.; Crawford, R. L.; Chapman, P. J.; Dagley, S. J. Bacteriol. 1975, 121, 272-285. Zylstra, G. J. In Molecular Environmental Biology; Garte, S. J., Ed.; Lewis Publishers: Ann Arbor, 1994; Chapter 5.

Received for review October 6, 1995. Revised manuscript received January 19, 1996. Accepted January 22, 1996.X ES950743Y X

Abstract published in Advance ACS Abstracts, March 15, 1996.


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Development of Two Biomass Control Strategies for Extended, Stable

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