Simultaneous nitrification-denitrification in a fluidized bed reactor

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Pergamon

War. Sci. Tech. Vol.38. No. I. pp. 247-254. 1998. IAWQ @ 1998Published by ElsevierScienceUd. Printed in Great Britain. All rights reserved

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SIMULTANEOUS NITRIFICATIONDENITRIFICATION IN A FLUIDIZED BED REACTOR Priyali Sen* and Steven K. Dentel** • KCI Technologies. 10 North Park Drive. Hunt Valley. Maryland2/030, USA •• Departmentof Civiland Environmental Engineering. University ofDelaware, Newark. Delaware 19716, USA

ABSTRACf This paper describes simultaneous nitrification-denitrification (SND) in a fluidized bed reactor (FBR) without physical separation of the aerobic and anoxic zones. Continuous reactor SND trials demonstrated successful SND over distinct periods. Control was achieved by varying the reactor dissolved oxygen and the feed COD. although optimal performance could not be consistently maintained. Batch studies. using cultures from different vertical sections. demonstrated that variation of the bulk parameters successfully stratified the reactor bed into zones that predominantly either nitrified or denitrified. Evidence also indicated that both types of activity occurred throughout the FBR. © 1998 Published by Elsevier Science Ltd. All rights reserved

KEYWORDS Nitrification; denitrification; SND; fluidized bed; attached film. INTRODUcnON Biological nitrogen removal involves nitrification of ammonia to nitrates followed by denitrification of nitrate-nitrogen to nitrogen gas which is released into the air. Nitrification is an aerobic process whereas denitrification occurs in the absence of oxygen. For total nitrogen removal, multi-reactor systems are common, employing sequential aerobic nitrification and anoxic denitrification processes. When both environments are provided within one reactor, this may be termed simultaneous nitrification-denitrification (SND) (e.g. Wartchow, 1990; Watanabe et al., 1992). This eliminates the need for two separate reactors or intermittent aeration otherwise necessary for complete nitrogen removal. In an FBR, the media (such as sand) are used to immobilize biomass within the reaction zone. This provides high biomass concentrations and hence reaction capacity per unit reactor volume. Typically, the efficiency of an FBR is 10 times that of an activated sludge reactor and it occupies 10% of the space required by stirred tank reactors of similar capacities (Gasser et al., 1975). Both nitrification and denitrification have been individually successful in an FBR (Gasser et al., 1975; Gauntlett, 1981). SND has been reported by a number of investigators in various kinds of reactors such as oxidation ditches (Barnes et al., 1983), aeration basins (Wartchow, 1990), upflow aerated filters (Rogalla et al., 1992), and rotating biological contactors 247

248

P. SEN and S. K. DENTEL

(Watanabe et al., 1992). One pilot-scale study of SND in an FBR has been reported by Fdez-Polanco et al., (1994), but the reactor configuration and operation were different from the one used in this study. While the primary objective was to establish SND with the given reactor configuration, the secondary goal was to determine the mechanism of SND if achieved. Two hypotheses were considered:

i. SND via Vertical Stratification of the Reactor: Nitrification and denitrification activity occur in different portions of the reactor, depending on the availability of various substrates. ii. SND via Stratification of Biofilm: Nitrifying and denitrifying zones occur on a microscopic scale within the biofilm surrounding the media particles.

EXPERIMENTAL METHODS The Fluidized Bed Reactor. The Envirex (Waukesha, WI) Fluid Bed Reactor used in this study was originally designed solely for pilot-scale FBR denitrification. The main reactor vessel was a 3.83 ft (117 ern) clear PVC column with an inside diameter of 1.6 inches (4cm). It included five sampling valves, designated Port I (lowest) through Port 5 (uppermost), The media used was silica sand with an effective size of 0.2 mm and a uniformity coefficient of 1.4. Fluidization was achieved by a peristaltic pump. For this study, the reactor was retrofitted with an aeration loop to obtain DO saturation at the reactor base (Figure 1). The lower portion of the reactor bed was assigned to nitrification and as the DO was depleted with upward flow, denitrification was expected to take over. Hence the feed was introduced part way up the reactor bed (Port 2, 3, or 4) by means of a peristaltic pump. Above the main reactor vessel, the column broadened to allow for a zone of turbulence created by another pump. Here, biosolids were sheared off the media particles, whereafter the latter fell back into the bed while the biosolids were carried out through the top. While most (99%) of the flow from the main reactor vessel was recycled, a part was discharged as reactor effluent. A portion of the recycle flow was removed by a centrifugal pump into the oxygen dissolution loop. There it was aerated by compressed air in a down-flow bubble column and returned back to the main flow. The reactor temperature was maintained at 35°C 5 by a heat exchanger. The reactor with all associated tubing had an approximate volume of 5.OIitres which included 1.00itres of the oxygen dissolution loop. More details can be found in Sen (1997). Reactor Operatjon. Prior to the SND study the reactor had been used exclusively for denitrification in order to obtain a robust denitrifying population. The reactor was then switched to a feed containing both NH 3-N and NOrN, seeded with nitrifiers procured from an industrial wastewater treatment plant, aerated, and set on recycle for the first several days. The reactor was initially fed the effluent from a pilot-scale denitrifying reactor treating nitrogen-rich industrial wastewater to secondary levels. Later in the investigation the denitrifying reactor was shut down and the SND reactor feed was changed to a synthetic preparation using secondary effluent from a nearby domestic wastewater treatment plant. In both cases concentrated HN0 3 was added to provide the NOrN. The industrial feed contained NH 3-N and some COD, but the secondary effluent was supplemented with NH4CI and sodium acetate. The minimum COD requirement was calculated from the following equation that has been computed using the lowest yield values (0.05 weight of cells/weight of substrate) cited by Sharma and Ahlert (1977):

Depending on the reactor performance, higher C:N ratios were used to reduce the reactor DO to anoxic levels. The final feed composition was 50 mgIL of both N0 3-N and NH 3-N, with 800-1230 mgIL COD. The final feed pH was brought up to 8.0-8.8 with NaOH. No vitamins or trace elements were separately added.

Simultaneous nitrification-dentrification

249

GAS COLLECTION

OXYGEN DISSOLUTION LOOP

AIR

D

POATl

FLOW BUFFER VESSEL

POAT2

4-------i I I

I I

FEED POINT OPTIONS

----------

-- --------------~ POAT3

I I I I

I I I I I

I

~~Cl4-------1I

I I I I I I I I I

FLUIDIZATION PUMP

I

PORT 4

I I I I I I

~-------J HEAT TRANSFER LOOP

BUBBLE CONTACTOR

D

Figure I. FBRconfiguration for SNDoperation, including loopsfor mediaabrasion, oxygen dissolution, and heat transfer.

Analytical Methods. The parameters of interest to SND were the three substrates nitrogen, carbon, oxygen; and the reactor pH and temperature. Denitrification was trackedthrough measurements of the NOrN concentration and nitrification by NH3-N determinations. Two methods were used to measure N0 3-N, namely, ion chromatography (Method 4110 B, Standard Methods, 1992) and DeVarda's alloy reduction followed by

P. SEN and S. K. DENTEL

250

acid titration (Methods 350.2 and 417 D, Standard Methods, 1981). NH3-N was analyzed by the acidimetric method following magnesium oxide distillation into boric acid (Methods 350.2 and 417 0, Standard Methods, 1981). COD was measured by the closed reflux, colorimetric method (5220 D of Standard Methods, 1992). Assuming that the biological pathway is the only one available, the reduction in COD is quantitatively equivalent to the carbon consumed. DO was measured by the Membrane Electrode Method (4500- 0 G of Standard Methods, 1992). Oxygen consumed in one pass of the reactor bed was calculated from the difference in the dissolve oxygen readings taken in the calibration tube, directly after aeration and before entering the bed, and at the top of the reactor, just after the media abrasion zone. Batch Experiments. Once SND was established and deemed to have reached a steady-state, batch studies were conducted to determine the mechanism of SND. Seed sample volumes of 50 ml were withdrawn from sample ports I, 2, 3, and 5, diluted to 500 ml (l: 10), and stirred vigorously to separate the biofilm from the sand particles. Seed from each port was given the following feeds: (a) denitrifying feed of 5.0 mg/L N0 3-N and 82 mg/L of COD; (b) nitrifying feed of 5.0 mg/L NH 3-N; and (c) heterotrophic feed of 82 mg/L COD. These diluted feed concentrations were comparable to those inside the reactor. Each experimental unit consisted of a mixture of 25 ml of the seed preparation and 100 ml of feed, combined in a 150 ml serum bottle. Any DO in the denitrifying feed was removed by purging it with nitrogen gas for 15 minutes before adding to the serum bottles. The final feed concentrations in the experimental units were a I: I00 dilution of the reactor feed, a close approximation of the actual FBR conditions. There were a total of twelve (4 ports x 3 seeds) such combinations. Four sets of each combination were prepared and each set allowed to react for a different period of time. The reaction times chosen were 3.0, 6.0, 12.0 and 24.0 hours. The bottles were shaken periodically, for approximately equal duration of time. The units containing nitrifying and heterotrophic feed were allowed to equilibrate with air between the periods of shaking. At the end of a reaction period, a set was removed and refrigerated, to be analyzed later. Media Growth Characteristics. Prior to shut-down, a solids determination of the reactor biomass was conducted. A 40 ml volume of reactor fluid was withdrawn from each port. The samples were weighed and allowed to settle. The volume of the solids was determined approximately from the settled samples. The bed voidage was calculated as the difference between the total sample volume and the volume of solids. Each sample was then gently suspended in order to obtain a representative fraction without breaking any of the bio-particles. A portion of this suspension was filtered on a 45 um glass fiber filter and weighed. Photographs were taken for visual comparison. The filtered residue was next dried at 105°C for 24 hours, cooled, and weighed to obtain water content. It was further dried at 550°C for an hour, cooled, and reweighed to determine biomass as volatile suspended solids (VSS). RESULTS SNP Trials. The reactor was operated as a continuous process, from November 1995 to July 1996, with maintenance shut-downs which did not exceed three days. Initially the feed was positioned to provide equal bed volume to nitrification and denitrification. The control over reactor DO was found to be imprecise due to the lack of an automatic feed-back control loop. It was therefore decided to deplete the DO to anoxic levels with excess COD in the feed. From an operational standpoint this translated into achieving zero DO at the top of the fluidized bed. The feed inlet was lowered to Port 2 in order to provide a heterotrophic zone for depletion of COD and DO and the reactor loading was increased to 0.25 kg N/m3/day (0.14 kg N/m3/d on the basis of the total reactor volume). This increased the reactor performance to 100% denitrification, 4090% nitrification, and 90-97% COD removal in what has been termed SND Period I (Table I and Figure 1). This success only lasted for two weeks. Thereafter, the denitrification continued but ammonia removals deteriorated. At this point the feed inlet was moved further up to obtain a nitrification-denitrification bed volume ratio of 4: I. This step did not help, and it was also found that the reactor was producing large amounts of detached biosolids, attributed to the increase in heterotrophic activity resulting from the excess feed COD used to control DO. A small roll of filter mesh was introduced into the tubing at the top of the reactor for clarification. The filter had to be washed whenever clogged, but SND was re-established (Period 2, Table 1 and Figure 2).

2S1

Simultaneous nitrification-dentrification

Table 1. Average influent and effluent characteristics for periods of SND. ~Nll D~.;~ I 1

....01T

--

~Nn P.. rln I ?

-

Tnn....n

56.7

10.7

60.8

59.4

31.3

65.2

44.5

12.2

32.3

47.0

1.8

37.0

1224.6

94.3

1130.3

809.4

137.2

672.2

Figure 2. Period 2 SND Performance SND Period 2

SND Period 1

•

i

.

NO,-N

Figure 1. Period 1 SND Performance

~ 80 60 II: 40 if. 20 0 160

-

NH 3-N COD

J! 100

--

Itl

•

165

170

••

17So. y.180

1

••

• 185

190

i

195

..'

100 .I. ... 80 60 11:40 if. 20 0 285 29° Dey.295

•

..

~

•

.N03-N

•

• COO

I

300 i

The reactor operationg conditions are summerized in Table 2. The upflow velocity in the reactor was >0.79 mlmin and the bed expansion was>100%. Table 2. Average reactor operating conditions

Nitrogen Loading Temperature pH r

DO removed In one ass

0.25 kg N/m3/dav

0.4 kg N/m 3/dav

35.4°C 6.8

35.9°C 6.8

2.5 m IL

2.8 m IL

Media ~rowth characteristics. Visual indication was that the biofilm was thickest at upper zones, consistent with the design of gradual media migration upwards during film development until the uppermost fraction is sheared by the media abrasion pump. These particles also had the highest water content (Table 3) and thus low density. Particles with the highest VSS were also in the upper zones. The thinnest biofilms were found at the lowest zone due to the higher resulting media density and possibly to the high fluid velocities near the fluidization pump and the narrow throat of the FBR. The continuous transition in properties with vertical location indicates that the reactor successfully stratified the media and attached biofilm, maintaining plug flow characteristics. Table 3. Results of solids analyses conducted on reactor media. Port

Voldage Ratio

Water Content (%)

VSS (weight % dry basis)

5

0.75

78.3

14.47

4

0.625

70.2

12.39

3 2

0.425

67.0

lLlI

0.325

61.5

7.73

1

0.375

15.9

Ll5

252

P. SEN and S. K. DENTEL

Batch experiments. The batch studies were conducted prior to shutdown (Day 302), when the reactor was achieving 80% NH3-N removals and the NOrN removals were up to 100%. The results are presented in Figure 3. Variability in time series results is attributable to inhomogeneity of replicate inoculum samples from each port. Because trends are consistent among samples from different locations but at the same measurement time, conclusions were derived after grouping incubation times and not on the basis of one data set alone. On this basis, one-way analysis of variance tests showed that variations with port location were statistically significant at the 95% confidence level for nitrification, denitrification, and heterotrophic assays. 2

12

1

3

4

5

t----+--------------+----, Top Bottom

10

E8 CIl

~

6

:I: Z

4

?fl.

2

M

A. Nitrifying activity

O-'-----O''----------------....J 25

t----I----+-------+----+----, Bottom

Top

~ 20

~ z

a:

15

8 10 Z ~

5

B. Denltrlfylng activity O-'-------~-='--------_---....J 100 r::::~\_.:;:::;:;;:::;:;:.~--------_:::::-,

Bottom

~ 80

~

60

§

40

C. Heterotrophic activity

oe. 20

O-l----_----4~:;:;...-_--_---+---l

2

3

4

5

Figure 3. Nitrifying, denitrifying, and heterotrophic activitiesat differentsampling ports. Incubation times given in hours.

It should be noted that the feed entered at Port 4 prior to the inoculum sampling. Thus, the high COD level above Port 4 would be expected to decrease the nitrifier density at Port 5, as confirmed in Figure 3A. Because the upper column region consistently achieved zero DO levels, Port 5 should show a high level of denitrifying capability, and this is indicated in Figure 3B. Denitrification is performed by facultative heterotrophs, so heterotrophic capability is also high at this column location (Figure 3C). (Slight heterotrophic activity may take place between ports 4 and 5 due to some DO contributed by the feed flow.) Port I shows a very active heterotrophic capability due to the introduction of dissolved oxygen at this point. Denitrifying activity in the batch results is attributable to facultative aerobes, because the column's biofilm was relatively thin at lower levels and therefore unlikely to support an anoxic layer. (Some carryover of detached biomass from the upper column or tubing walls is also possible.) Nitrifying activity is relatively

Simultaneous nitrification-dentrification

253

low, increasing progressively at Ports 2 and 3 as the heterotrophic activity has decreased the COD. Thus, ports 2 and 3 also display decreased denitrifying and heterotrophic capability. Some denitrifiers also seemed to co-exist with nitrifiers in the mid-column zones. While carbon limited heterotrophic activity, a denitrifying population was evidently maintained. It may be hypothesized that endogenous respiration provided sufficient COD to support denitrification inside a stratified biofilm, with nitrification taking place in the outer layers of the biofilm that are exposed to the bulk DO, and the anoxic inner layers supporting denitrification. However, carrythrough of some suspended denitrifying biomass from other column levels is equally plausible. It is clear from the results that significant vertical stratification of functioning populations occurred within the reactor. Coupled with the fact that the reactor achieved nearly complete removal of nitrogen and COD during this period, it may be concluded that, in spite of a significant fluidization velocity and recycling flow, the stationary biomass within the vertical zones achieved most removal in the initial cycle of feed flow. Fluidization was thus accomplished while maintaining a reactor with essentially plug flow characteristics and thus capable of maintaining distinct populations. DISCUSSION The reactor start-up itself was without problems except those associated with feed composition and physical operating conditions. Establishing the microbial consortium appeared to require only a few days. Once operating, the reactor showed good resilience to changes in the feed and operating variables. Even after being shut down (bed not expanded) for a day or two, it resumed activity within a day. This was significant considering that the bed was stratified into different zones which did not appear to have been disturbed by the re-fluidization. This endorses the fluidization velocity and media size that were selected for this study. Comparison to Previous Studies. Compared to the SND study of Fdez-Polanco et al. (1994), this FBR had a higher nitrogen loading rate. The earlier study had mentioned that zero DO had never been achieved and this Was suspected to be the cause of their denitrification limitation. But performance data from the latter part of Period 1 do not suggest any such limitation. The DO at the reactor top was consistently 0.1-0.3 mgIL. and the reactor was removing 60 mgIL N0 3-N (as opposed to 30 mgIL of the 1994 study). This may be significant in future SND-related work, although here the importance of the zero DO was interpreted with respect to the effectiveness of the aeration loop. Among the control parameters, COD, DO, pH, and temperature, COD and DO were definitely the critical ones. Optimum pH was not essential to achieving SND although it probably would have improved efficiency. Also, the pH measured was that of the bulk liquid and conditions in the biofilms may have been different. While a direct control of DO would have been the ideal, the next best alternative was to control the SND process with COD. The difficulties of this were illustrated by the lack of longer SND periods. The COD dosing is a function of too many variables (Picard and Faup, 1980), including effluent flow rate, N0 3N concentration, and concentration of biomass in the reactor. Therefore, COD is an impractical choice for a full-scale operation. Instead, a better aeration system with feed-back control of DO is recommended. A number of authors have reported nitrite accumulation (Eggers and Terlouw, 1979; c;e~en and Goenec, 1995). But the 99% removals observed in May, 1996, suggest that this was not a point of concern in this study. Given that there was not much change in feed or reactor operating characteristics at the end of Period 1, it is unlikely that the subsequent low N0 3-N removals were a result of nitrite accumulation. The day (#293) associated with low N03-N removal corresponds to insufficient DO removal which also reinforces the earlier significance of zero DO at the reactor top. The experiments conducted to determine the SND mechanism support the initial hypotheses of vertical stratification of active populations. However, the presence of microenvironments within these zones cannot be ruled out. Nonetheless, it is clear that the FBR as configured in this study is capable of attaining SND

254

P. SEN and S. K. DENTEL

without physical separation of the aerobic and anoxic zones. It is important to note that scaling up such a process may requirespecificmeasures to maintain vertical stratification such as lateralbaffles. ACKNOWLEDGEMENTS The authors thank E.I. Du Pont de Nemours and Co. for supportfunding, and in particularRobert A. Reich of DuPontfor his insightful suggestions duringthis project. REFERENCES Barnes, D. and Bliss P. J. (1983). Biological Controlof Nitrogen in Wastewater Treatment, London: E.and F.N. Span. F., Gllene~ I. E. (1995). Criteria for Nitrification and Denitrification of High-Strength Wastes in Two Upflow Submerged Filters, WaterEnvt. Resch.,67, 132-14t. Eggers, E. and Terlouw T. (1979). Biological Denitrification in a Fluidized Bed with Sand as Carrier Material, WaterResearch, 13,1077-1090. Fdez-Polanco, F., Real F. J. and Garcia P. A. (1994). Behaviour of an Anaerobic/Aerobic Pilot Scale Fluidized Bed for the Simultaneous Removal of Carbon and Nitrogen, WaterScL Technol., 29(10111),339-346. Gasser, R. F., Owens R. W. and Jeris J. S. (1975). Nitrate Removal from Wastewater Using Fluid Bed Technology, Purdue r;e~en,

Industrial Waste Conference, 1202-1207. Gauntlett, R. B. (1981). Removal of Ammonia and Nitrate in the Treatment of Potable Water, in Biological Fluidized Bed Treatment of Waterand Wastewater, P.F. Cooper and B. Atkinson, eds., Ellis Horwood, Chichester, 48-58. Picard, M. A. and Faup G. M. (1980). Removal of Nitrogen from Industrial Wastewaters by Biological NitrificationDenitrification, J. lnst. of WaterPoll. Control,79. Rogalla, F., Badard M. and Hansen F. (1992). Upscaling a Compact Nitrogen Removal Process, WaterSci. Technol., 26, 10671076. Sen, Priyali (1997). A Combined Nitrification-Denitrification Fluidized Bed Process for Industrial Waste Treatment." Masters Thesis, University of Delaware, Newark DE. Sharma, B. and Ablert R. C. (1977). Nitrification and Nitrogen Removal, WaterResearch, 11, 897-925. StandardMethods for the Examination of Waterand Wastewater (1992) A.E. Greenberg et al., eds, APHA, 18th edition. StandardMethods for the Examination of Waterand Wastewater (1981). APHA, 15th edition. Wartchow, D. (1990). Nitrification and Denitrification in Combined Activated Systems, WaterSci. Technol., 22(7/8),199-206. Watanabe, Y., Masuda S. and Ishiguro M. (1992). Simultaneous Nitrification and Denitrification in Micro-Aerobic Biofilms, WaterSci. Technol., 26, 511-522.