Treatment of high-strength nitrate wastewater by biological methods - operational characteristics study

8

Pergamon

War. Sci. Tech. Vol. 34, No. 1-2. pp. 269-276, 1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 0'00

PH: S0273-l223(96)0054l-0

TREATMENT OF HIGH-STRENGTH NITRATE WASTEWATER BY BIOLOGICAL METHODS • OPERATIONAL CHARACTERISTICS STUDY Shing-Der Chen, Chiu-Yang Chen, Yuan-Chung Shen, Chun-Mao Chiu and Hsiang-Ju Cheng Department of Environmental Engineering, National Chung Hsing University, 250 Kuokuang Road, Taichung, Taiwan 402, Republic of China

ABSTRACT Recently. the research in biological denitrification applied to low nitrate concentration municipal wastewater and polluted groundwater has gained considerable attention. There is, however, much less research on high• strength nitrate industrial wastewater treatment by biological denitrification. The present investigation hence aims towards studying biological treatment of high nitrate concentration wastewater for removal of nitrogen components and understanding its operational characteristics. Three different types of bioreactors, viz.: an activated sludge reactor (ASR), a biologically mediated activated carbon fluidized bed reactor (BAFBR), and an upflow immobilized cell reactor (VICR) were used. Results revealed that the start-up period of VICR was the shortest, followed by BAFBR, while it was longest for ASR. Thus higher volumetric load appears to lead to a shorter start-up period. From long periods of operation, it was observed that the volumetric load of BAFBR was the highest (30 kg N/m 3-d), followed by VICR (8.9 kg N/m 3-d) and the lowest (1.0 kg N/m 3-d) was ASR. The influence of power outage or system shutdown was found to be the most significant for BAFBR, the second most affected was VICR, and ASR was the least affected. The ability to acclimate to shock loading was observed to be best for VICR, the next was BAFBR, while the worst was ASR. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS Activated sludge; denitrification; fluidized bed reactor; immobilized cell; nitrate; shock load. INTRODUCTION The nitrogen compounds existing in wastewater include organic N, ammonia N, nitrate N and nitrite N (Narkis et ai., 1979). The methods applied for removing nitrogen compounds from the water body can be divided into physical, chemical and biological processes (Metcalf & Eddy, 1991). The biological denitrification is the most important and widely used method because it enables the transformation of nitrogen compound into harmless nitrogen gas (N 2 ) (EPA, 1993). Recently, a lot of research has been carried out on biological denitrification of municipal wastewater and contaminated groundwater (Metcalf & Eddy, 1991; Lazarova et aI., 1992). As a result of the complicated characteristics and higher nitrate concentration 269

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270

of industrial wastewater, the research on municipal wastewater treatment can not be applied directly for the treatment of industrial wastewater. It is hence worth while to study the treatability of wastewater containing high levels of nitrogen compounds by biological denitrification. The purpose of the present study is to assess the feasibility and performance of three different bioreactors for biological denitrification of high-strength nitrate wastewater under anoxic conditions. The bioreactors used in the present study are: an activated sludge reactor (ASR), a biologically mediated activated carbon fluidized bed reactor (BAFBR) and an upflow immobilized cell reactor (VICR). EXPERIMENTS The synthetic feed consisted of a mixture of mixed liquor and nutrients which were mainly nitrate (from potassium nitrate) and organic material (from methanol). The substrates used for each of the three systems are listed in Table 1. Table 1. Composition of synthetic wastewater feed Composition N03-N SCOD KH ZP0 4 CaCh MgS04 '7H zO

Concentration (mg/L) 100-1500 440-6000 100 10 70

FeS04 '7HzO

14

ZnS04 '7H zO

0.02

CUS04 '5H zO NazMo0 4 ·2HzO

0.02 0.02

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wastewater tank

reactor

waste sludge

Figure 1. Flow-sheet of the ASR.

influent

-

acid tank

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wastewater

Figure 2. Flow-sheet of the BAFBR.

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c:

Figure 3. Flow-sheet of the mCR.

Treatment of high-strength nitrate wastewater

271

The three different types of laboratory-scale denitrification reactors used in the present investigation are illustrated in Figures 1, 2 and 3, which were employed to obtain the required experimental data. Their effective volumes were 10 1, 2.75 1 and 2.26 1 respectively, and the volumes of filled particles (activated carbon or immobilized cell) were 25% of the effective volumes. Type F400 activated carbons were used as the media in BAFBR, obtained from the Calgon Company of USA, while the immobilized cell beads used in UICR were realized by entrapping the suspended denitrifying organisms by polyvinyl alcohol (PVA) phosphate esterification (Lin and Chen, 1993). The PVA material used was of type BF-20, obtained from Chang-Chuen Petrochemical Company of Taiwan. RESULTS AND DISCUSSION Start-Up of systems The ASR used fresh sludge, obtained from nitrifying effluent of a nearby wastewater treatment plant, as seeding. The system required 80-100 days for reaching a steady state after the acclimation of fresh sludge. When the ASR used acclimated sludge as seeding, the start-up period could be reduced to 25-30 days. Activated carbon was first immersed in the concentrated sludge for one day and then transferred into BAFBR before its start-up. The BAFBR was further seeded with 2 1 of acclimatory sludge every two days, until the distinguished biofilm was grown. The influent NOrN concentration and volumetric load were 1000 mg/l and 5.92 kg N/m 3-d respectively. The system reached a steady state after 21 days. When the volumetric load was increased to 17.4 kg N/m 3-d, it took 15 days for the system to reach a steady state. This indicates that the higher the volumetric load, the shorter is the start-up period. The UICR used fresh immobilized cell beads as bioparticles and operated with an influent N0 3-N concentration and volumetric load of 1130 mg/l and 5.42 kg N/m 3-d respectively. It required 6 days for the system to reach a steady state. Based on the results of the start-up experiment, these three bioreactors are compared below: 1. The start-up period was observed to be shortest when acclimated sludge was used for seeding. 2. Higher volumetric load resulted in a shorter start-up period. 3. The start-up period of UICR was the shortest, followed by BAFBR, while it was longest for ASR. System stability during long period operation Tweleve runs were operated on the ASR. The operational parameters are listed in Table 2. Figure 4 presents the relationship between the volumetric loads and removal efficiency for TN and COD in the ASR. The denitrifying organisms were discovered to be very viscous during the operation. A similar result has been reported by Payne (1981). The gases produced from denitrification adhere to them. When denitrification was not complete, the denitrifying sludge did not settle easily. Hence, a thin layer of sludge rose to the top of the settled zone and resulted in increasing the fraction of SS in the effluent. However, when the operational conditions were changed, the pH of the system did not change noticeably whereas the alkalinity did. This parameter could hence be used as an index of efficiency of the system. Also, it is important to note that although the system achieved almost complete denitrification, the effluent contained significant amounts of soluble COD. Further treatment was hence needed for the control of effluent COD.

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Table 2. Operational parameters of the ASR

Run HRT 1 2 3 4 5 6 7 8 9 10 11 12

(day) 1.04 2.52 0.48 0.44 1.06 1.06 1.03 1.68 1.72 1.06 1.07 0.48

Influent (mglL) COD N03-N 1270 4630 1230 4740 1200 4670 794 3120 1260 4740 1200 4720 494 1890 784 3060 486 1850 494 1900 487 1830 493 1880

Effluent (mgIL) COD N0 3-N NOrN 1160 0.63 315 996 0 72 15 1630 356 10 882 76 0.33 1470 333 3.9 1060 7.8 534 0 46 0 25 667 0.2 414 0 43 0 558 2.5 0 491 11 0 574

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SRT (day) 6.12 15.8 10.7 11.0 11.8 27.3 12.3 13.9 15.2 15.2 25.3 12.2

100

100

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Removal (%) TN COD 75.2 75.0 94.2 78.9 69.1 65.0 89.2 71.7 73.1 68.9 99.0 77.5 90.6 71.7 96.8 78.2 99.9 77.6 91.4 70.6 99.5 73.1 97.8 69.4

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Figure 4. Steady slale operalion of lhe ASR.

Thirteen runs were operated on the BAFBR. The operational parameters are listed in Table 3. The volumetric load was 5.2-39.5 kg N/m 3-d. The results showed that the higher the volumetric load, the lower is the nitrogen removal efficiency. The average TN and COD removal efficiencies were higher than 89.5 and 85.5% respectively, as shown in Figure 5. The specific substrate utilization rate increased correspondingly with the increase in the applied nitrate volumetric load. When the applied nitrate volumetric load was increased to about 30 kg N/m 3-d, it was observed that the effluent nitrate concentration increased drastically, while the specific substrate utilization rate increased gradually to about 1.4 g NOTN/g VSS-d, as shown ir Figure 6. Moreover, the removal efficiencies of the system were found to be quite steady under the constanl volumetric load irrespective of the variations in influent substrate concentration or hydraulic retention time When the volumetric load exceeded 30 kg N/m 3-d, the bioparticles rose to the top of the system and henet made the operation very difficult. On the other hand, the increasing trend of specific substrate utilization rat( became slower and the quality of the effluent deteriorated. The volumetric load of 30 kg N/m 3-d shoul, hence be the operational upper limit for the system under this investigation. Thirteen runs were operated on the UICR. The operational parameters are listed in Table 4. Thl accumulation of nitrogen gases was not significant at the beginning of the operation, however, the quantit~ of gases increased with the increasing volumetric load. This could result in the "channeling" problem sino the gases would mostly remain accumulated in the system even though some could run out of the system du to the recycle flow. Soares et al. (1991) have reported that the accumulation of gases in the system could b reduced by the addition of an intermittent carbon source. It should also be noted that the biofilm grew on th outer layer of the immobilized cell and hence increased the fraction of SS in the effluent.

Treatment of high-strength nitrate wastewater

273

Table 3. Operational parameters of the BAFBR Run

HRT (hr) 1.38 0.92 0.61 0.41 5.08 2.07 1.38 0.92 0.61 3.09 2.06 1.38 0.92

I

2 3 4 5 6 7 8 9 10 11 12 13

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Influent (mg/L) N0 3-N COD 676 2670 677 2640 679 2750 665 2750 1020 3960 3750 1020 1010 3850 3860 1010 3920 1010 5780 1530 5790 1520 5790 1510 1500 5870

Effluent (mg/L) COD N0 3-N N02-N 205 0 0 195 2.2 1.9 11.9 305 8.8 9.5 11.6 359 1.1 0.5 280 156 0.4 0.2 0.1 2.7 300 271 5.0 0.2 26.7 60.4 510 0.9 0.3 280 3.8 0 261 21.6 1.0 556 60.7 96.5 853

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Removal (%) N0 3-N Loading COD TN (kg/m 3-d) 92.4 99.9 11.8 17.7 92.6 99.4 26.7 88.9 97.1 39.2 86.9 96.8 5.2 94.7 99.4 11.8 95.8 99.9 17.7 93.7 99.0 26.3 93.5 99.5 39.5 87.0 91.3 11.8 95.2 99.9 17.7 99.7 95.5 26.3 90.4 98.5 39.5 85.5 89.5

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Figure 5. Steady state operation of the BAFBR.

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S.-D. CHEN et a/.

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Table 4. Operational parameters of the VICR Run 1 2 3 4 5 6 7 8 9 10 11 12

13

HRT (hr) 7.28 6.83 3.08 4.97 4.08 2.13 5.18 4.91 3.09 7.00 2.10 7.03 4.99

Influent (mg/L) COD N03-N 107 442 515 2060 510 2040 501 2060 623 2370 505 1980 100 444 765 3030 757 3000 747 2860 765 2960 1120 4450 1130 4390

Effluent (mgIL) COD NOx-N SS 32.0 0 30.2 54.9 1.49 24.6 76.1 1.38 63.1 87.0 0.15 52.9 1.62 50.0 70.1 2.12 96.4 68.2 33.3 0 35.5 1.22 80.6 42.7 0 54.0 129 0.30 63.0 54.7 1.22 57.8 190 2.69 190 96.4 123 1.58 128

Removal (%) N0 3-N Loading TN (kg/m 3-d) COD 99.9 92.8 0.35 99.7 97.3 1.81 99.7 96.3 3.97 99.9 95.8 2.42 99.7 97.9 3.67 95.1 99.6 5.69 99.9 0.46 92.5 3.74 95.3 99.8 5.88 95.7 99.9 2.56 92.8 99.9 8.75 99.8 93.6 3.83 99.8 93.1 5.42 99.9 97.1

From the system stability experiment, the three bioreactors are compared below: 1. The volumetric load of BAFBR was the highest, followed by VICR, since its ability depends on the material of its immobilized cells as well as on the type of reactor. The lowest was that for ASR since its sludge concentration was the lowest. 2. According to the operation of these reactors, it may be inferred that the influence of power interruption or system shutdown was most insignificant for ASR, whereas it was noteworthy for BAFBR and VICR, because it reduced the removal of nitrate from the system and increased the fraction of SS in the effluent. Influence of shock loading on the system About 30-70 days were required for ASR to achieve a steady state when the volumetric load was changed. For example, the system required 70 days to reach a steady state when the influent NOTN concentration was 1200 mg/l and the volumetric load was changed to 2.5 kg N/m 3-d from 0.5 kg N/m 3-d. Moreover, the time required for the system to reach a steady state while increasing the volumetric load was more as compared to the time required while decreasing the load. The three different conditions employed for operating the BAFBR to detect the shock loading effects were: a doubled influent flow rate, and two and four times increase in the influent nitrate concentration. The results indicate that the system's effluent NOx-N concentration increases slightly and remains in a steady state under the first two conditions (Figures 7 and 8), whereas under the third condition, the system produces large quantities of nitrogen gas from the biological denitrification, thus resulting in an unsteady state (Figure 9). Some of the nitrogen gas was accumulated inside the biofilm and resulted in raising the bioparticles to the top of the bioreactor. While the remaining increased the disturbance inside the bioreactor and resulted in the collision of bioparticles with each other. As a result, the biofilm was sloughed off from the bioparticles. This resulted in reducing the removal efficiency for nitrogen from the system, since the amount of biofilms is an important parameter affecting the efficiency of BAFBR (Mulcahy and Shieh, 1987). In the process of continuous-flow operation, a maximum of 7 days was required for the system to reach a steady state when the volumetric load of the system was increased. The VICR achieved the steady state immediately when the volumetric load was increased gradually from 0.6 kg N/m 3-d to 8.9 kg N/m3-d.

Treatment of high-strength nitrate wastewater

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M -.....- ..._ .

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o

5

10

operation time (hr)

15

20

25

30

35

operation time (hr)

Figure 7. System response to two times increase in influent flow rate of the BAFBR. (.&) effluent N0 -N; 3 (0) effluent N0 2-N; (e) effluent TCOD; (*) effluent SCOD.

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12

0

0

influent flow rate

::J

2

;;::

10

5

15

20

0 25

.S

operation time (hr)

operation time (hr)

Figure 8. System response to two times increase in influent N0 3-N/COD of the BAFBR. (A) effluent N0 3-N; (0) effluent N0 2-N; (e) effluent TCOD; (*) effluent SCOD.

:::J

en

E ......

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40

=3 Uhr

influent flow rate

=3 Uhr

(")

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..-

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~ 10 !E Q.l

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Q.l

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30

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5

10

15

20

25

operation time (hr)

Figure 9. System response to four times increase in influent N0 3-N/COD of the BAFBR. (A) effluent N0 3-N; (0) effluent N0 2-N; (e) effluent TCOD; (*) effluent SCOD.

On the basis of results obtained from the shock loading experiment, the three bioreactors are compared below: 1. The capability of the ASR to respond quickly to shock loading was lowest, followed by BAFBR, and VIeR had the highest capability.

S.-D. CHEN et al.

276

2. While operating under the highest volumetric load, the system produced large quantities of nitrogen gas. This gas accumulated inside the biofilm and resulted in raising the bioparticles to the top of the bioreactor. This caused a washout of sludge from the system. 3. The UICR had the highest capacity for shock loading when the volumetric load was below 8.9 kg N/m 3-d. The main reason for this was that the system contained a large quantity of sludge because of its immobilized cells and had a buffer capacity from its recycle flow. CONCLUSIONS The operation of ASR was simple and the influence of variations in parameters was negligible. However, the ASR was limited by the suspended solids concentration (2000-7500 mg/l). The maximum volumetric load of this system was 1.0 kg N/m 3-d at the steady state operation. The BAFBR could be operated at higher volumetric loads (30 kg N/m 3-d) and its effluent was of good quality. However, when the conditions were changed suddenly, the gases could not be released from the system thus resulting in raising the bioparticles to the top of the system. This resulted in lowering the removal of nitrogen by the system and increasing the SS content in the effluent. The PVA immobilized cells overcame the disturbances arising due to the flow and friction of the bioparticles, however, its permeability to gases was low. When the volumetric load was too high or the conditions were suddenly changed. the bioparticles would rise and result in choking the line. REFERENCES EPA (1993). Nitrogen co//(rol. US EPN62511 R/93101O. Lazarova, V. Z., Capdeville, B. and Nikolov, L. (1992). Biofilm performance of a fluidized bed biofilm reactor for drinking water denitrification. War. Sci. Tech., 26(3/4), 555-566. Lin, Y. F. and Chen, K. C. (1993). Denitrification by immobilized sludge with polyvinyl alcohol gels. Wat. Sci. Tech., 28(7), 159• 164. Metcalf & Eddy (1991). Wastewater engineering: treatment. disposal and reuse. 3rd edn, McGraw-Hili, Inc., New York. Mulcahy, L. T. and Shieh, W. K. (1987). Fluidization and reactor biomass characteristics of the denitrification fluidized bed biofilm reactor. Wat. Res., 21, 451-458. Narkis, N., Rebhun, M. and Sheindorf, C. H. (1979). Denitrification at various carbon to nitrogen ratios. Wat. Res., 13,93-98. Payne, W. 1. (1981). Denitrification, John Wiley & Sons, Inc., New York. Soares, M. I., Braester, c., Belkin, S. and Abeliovich, A. (1991). Denitrification in laboratory sand columns: carbon regime, gas accumulation and hydraulic properties. Wat. Res., 25, 325-332.