High loading denitrification by biological activated carbon process

Wat. Res. Vol. 29, No. 12, pp. 2776-2779, 1995

~

Pergamon

Copyright (C 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0043-t354/95 $9.50 + 0.00

0043-1354(95)00119-0

RESEARCH NOTE H I G H L O A D I N G D E N I T R I F I C A T I O N BY BIOLOGICAL A C T I V A T E D C A R B O N PROCESS N. F. SISON ~*, K. HANAKI'~'@ and T. MATSUO~@ ~Department of Urban Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113 and :Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153, Japan

(First received December 1993," accepted in revised form April 1995) Abstract High loading denitrification was studied using granular activated carbon (Calgon Filtrasorb 400, size: 0.8 1.4 mm) column with injecting carbon source (sucrose) only once a day. Under the condition of EBCT (empty bed contact time) = 80 min, C : N ratio = 1.88, once per day injection mode of organic supply was able to sustain an average denitrification efficiency of 84 to 89% even with influent NO3-N concentration of 80mgl -~. With an influent N O 3 N of 20mgl -~ and C:N ratio of 1.88, however, reduction of EBCT to 20 min resulted in very poor denitrification. In the latter case, 46% of the added carbon was lost in the effluent immediately after the injection. Short EBCT critically affected the process mainly due to insufficient adsorption rate. Microbial denitrification capability and fermentation might also limit the process. Extended organic injection is a possible option to improve the process efficiency. Occurrence of the sulfate reduction was limited in early phase of the cycle.

Key words~enitrification, adsorption, desorption, external carbon source, granular activated carbon (GAC), adsorption rate

INTRODUCTION The authors (Sison et al., 1996) p r o p o s e d a denitrification process using granular activated carbon (GAC) column and dynamic modes o f external carbon addition. This process is suitable for water or wastewater containing N O 3 but insuffÉcient organic matters. It was d e m o n s t r a t e d that 87% o f N O 3 was removed by adding the organic matters [substrate C : N = 1.88, EBCT (Empty Bed Contact T i m e ) = 80min] once a day with continuous inflow o f 20 mgl ~ o f N O 3 . A l t h o u g h such feeding m o d e do not reduce the total a m o u n t o f organic carbon addition, high flexibility is given in the operation o f denitrification process. This point is a great advantage in small scale plants in which a m o u n t o f organic carbon addition is not changed frequently. Application o f this feeding m o d e with high loading was attempted in this study. Possible problems such as insufficient adsorption or denitrification, and sulfate reduction were examined.

this study at 25"C (Fig. 1). The concentrated organic substrate (sucrose, 95%; peptone, 4.5%; yeast extract; 0.5% on TOC basis) was injected for a 10 min period every 24 h interval, whereas substrate containing constant concentration of NO 3 and other trace nutrients was fed continuously. The average substrate C:N ratio was fixed at 1.88 (COD/NO3-N = 5.0) by adjusting the concentration of injected organic substrate. NO; loading was varied either by changing the influent NO; (Experiment 1) or by changing the EBCT in the columns (Experiment 2). Routine samples were collected 3 h and 23 h after substrate injection and analyzed after filtration (0.45 #m) for NO3 and NO: N utilizing high performance liquid chromatography (HPLC), and TOC. The experimental conditions are summarized in

~ ~J'~-'l ~ r~

1 GAC COLUMN 2 SUCROSE + PHOSPHORUS

: o.1~!

I.

~Ii~[il ~

~!i

I~

3 :gL 'g 'E.'S LU ON I

. ~

~o..,~..o. ~ TIMEw~ SEQUENCE CON~OLLER

:

a,b,~,d~ ~MPLING PORTS x WA~ING PORT

MATERIALSAND METHODS An upflow activated carbon column packed with granular Filtrasorb 400 (Calgon Co., size 0.8-1.4 mm) was used in *Present address: Department of Chemical Engineering, De La Salle University, 2401 Taft Avenue, 1004 Manila, Philippines. tAuthor to whom all correspondence should be addressed. 2776

Fig. 1. Experimental arrangement.

Research Note

2777

Table I. Summaryof performanceof once per day injectionmode under various influent NO3-N and EBCT (C:N = 1.88) EBCT (min)

Experiment

Influent NO3-N (mgl -~)

NO3-N loading (gl -~ d -~)

Organic addition (g TOC 1-~ d ~)

Overall N removal (%)

10 40 60 80

0.18 0.72 1.08 1.44 0.36 0.48 0.72 1.44

0.34 1.35 2.03 2.70 0.68 0.91 1.35 2.70

85 84 89 86 91 90 68 29

I 80 2

80 60 40 20

20

Table I. The column was operated until steady state condition was reached. Only steady state data are shown in this paper. EXPERIMENTAL RESULTS

Nitrogen removal and carbon balance As available amount of carbon source changed, nitrogen removal efficiency (removal of NO2-N + NO3-N ) changed within a cycle of 24 h (Fig. 2). Although nitrogen removal tended to decrease in the latter half of the cycle, reasonably high removal was still maintained in Experiment 1. On the other hand, EBCT of 40 min and 20 min resulted in poor nitrogen removal in Experiment 2. Very little nitrogen removal was achieved in the latter half of cycle when EBCT was 20 min. This indicates that injected sucrose failed to be stored by GAC in the initial stage of cycle. Overall nitrogen removal throughout the cycle in each case are tabulated in Table 1. The nitrogen removal was in the range of 84-89% in all cases under Experiment 1. However, nitrogen removal dropped to only 29% at EBCT of 20 min in Experiment 2 although the nitrogen loading in this case was same to the successful case in Experiment 1. These results indicate that once per day injection mode can sufficiently handle influent NO3-N concentration as high as 80 mgl-~ when EBCT was maintained at 80 min. EBCT is more critical factor which can significantly affect the nitrogen removal. Figure 3 shows the fate of the added organic carbon throughout the cycle in each condition. Organic carbon used for denitrification and sulfate

reduction were calculated based on stoichiometrical equation from the change in NO~- and NO~-, and from the reduced sulfate, respectively. The residual carbon in the effluent is based on the measured TOC of the effluent. The leftover in the carbon balance is accounted as the one used for biosynthesis. Carbon used for denitrification decreased moderately while residual carbon increased with the increase of loading at EBCT = 80min (Experiment 1). On the other hand, shortening EBCT greatly affected the carbon balance in Experiment 2. As high as 48% of the injected organic matter was lost in the effluent (shown as residual) when EBCT was reduced to 20 min which severely affected the denitrification performance of this column.

Mechanisms of carbon loss The leakage of organic matter indicated by the effluent TOC within the cycle in both experiments are shown in Fig. 4. As the average C : N ratio was kept constant in all cases, the concentration of the injected organic matter in Experiment 1 was very high in the columns with influent NO3-N of 60 and 80 mgl-~ The percentage of organic substrate lost in the effluent increased with the increase in concentration of the injected organic substrate. The losses are 6.9, 7.2, 12.9 and 17.0% of the TOC fed when infiuent NO3-N were 10, 40, 60 and 80 mgl-~, respectively. Possible mechanisms of loss of added carbon and insufficient denitrification observed under short EBCT in Experiment 2 are (i) insufficient capacity or rate of adsorption of sucrose by GAC, (ii) biological

(a) Experiment I (EBCT = 80 min)

(b) Experiment 2 ( influent NO3-N = 20 rngl "+ )

t

~. so~

~

O!

_o so.

I~

1~

1~

Time within cycle, h

~ I~l~nt~N=lO~ + I ~ l ~ N~N = ~ ~ I ~ 1 ~ ~ N = ~ ~'~ I ~ 1 ~ ~ N = ~ ~'+

24 ! ~ i

0

6 12 18 Tlme w ~ h l n w c l ~ h

~ X

~mln ~ ~ ~T~

24

min min rain

Fig. 2. Change in N removal efficiency within the cycle at various nitrate loadings (C:N = 1.88). WR 29/12--K

2778

Research Note

(a) Experiment1 (EBCT= 80 min)

100

~100 ~

o,.,° ~ 60

60" "6 40" ~.

(b) Experiment2 ( influentNO~-N= 20 rngl-~ )

o 40

~0"

,,~ ~o 0-

10 40 60 80 Influent NO3-N, mgl -~ •

DENITRIFICATION

[]

SYNTHESIS

•

80

RESIDUAL

60 40 EBCT, min []

20

SULFATE REDUCTION

Fig. 3. Fate of TOC under once per day injection mode at various nitrate loadings (C:N = 1.88). conversion of sucrose to non-adsorbable intermediate such as fatty acids, and (iii) insufficient microbial reaction rate of denitrification. Whole amount of sucrose injected once a day (shown in Table 1) must be once adsorbed by the G A C in this operation mode. Successful adsorption and release in Experiment 1 indicates that the total capacity of adsorption was sufficient also in Experiment 2. However, adsorption rate was the potential limiting factor under short EBCT with relatively low influent TOC. Figure 4(b) shows that organic matter was lost in the effluent immediately after the substrate injection at E B C T of 20rain. It is likely that the injected sucrose was lost without being adsorbed. Lower adsorption rate due to shorter contact time than the case with influent NO~ N of 80mgl ~ at E B C T of 80 min perhaps caused insufficient adsorption of injected sucrose when E B C T was shortened to 20 min. One possible alternative to minimize the loss of organic carbon is to prolong the injection period while reducing injected substrate concentration instead. In this way, longer period of time will be facilitated for the adsorption of sucrose. A separate study was conducted (influent NO~ N = 80 mgl ~, E B C T = 80 min) with injection period of 30 min. Nitrogen removal was improved from 86 to nearly 100%. This option seems effective when adsorption rate limits the denitrification. Some of the supplied organic carbon were also lost as fermentation products such as acetate which have low adsorbability on activated carbon. Determi-

nation of these products in the final effluent by H P L C shows that organic acids contributed more than 50% of the residual organic carbon at the early stage when influent N O 3 N levels were 60 and 80 mgl 1. The denitrification activity was estimated by batch denitrification tests using biologically active G A C taken from several points along the column immediately after the end of the experimental run. A small portion of the media was transferred into small vials and mixed with fixed volume of substrate solution consisting NO3 and organic carbon source. The maximum denitrification activity in a 6 h test at 2 5 C was about 0.2 mg N O 3 N (g dry solids.h) ~in which the term "dry solids" in this case refers to the mixture of G A C and biomass. The highest volumetric NO3 loading in the present study (Table 1) is equivalent to about 0.25mg NO~ N (g G A C . h ) ~. Taking into account the biomass contained in the dry solid in the activity test, actual loading and the maximum denitrification capability was almost same. In summary, insufficient adsorption by G A C and microbial denitrification rate were the main limiting factors in this process under high nitrate loading, although formation of fermentative products may also limit the function. Poor denitrification at short E B C T was mainly due to insufficient adsorption rate of injected sucrose.

Su([ate reduction Sulfate (SO£ 2) reduction produces toxic H~S and reduces the availability of organic carbon for (b) Experiment 2 ( influent NO3-N = 20 mgt-1 )

(a) Experiment 1 (EBCT = 80 min) •r

.lS° 1-[~ 1 ~,

2 so1 -= o"o

O • A +

Influent Influent Influent Influent

NO3-N = NO3*N = NO3-N = NO3-N =

10 mg1-1 40 mgl "1 60 mgl -~ 80 rngi"~

' 1 i '

1000, 1.

800 ~

600

l[

• 0 A X

EBCT=80 EBCT=60 EBCT=40 EBCT=20

rain min min rnin

~ 4°°11

|=oo 6 12 18 Time within cycle, h

24

¢

0 0~mo~ _6 12 18 ~ _"c ~'4 ~ Time within cycle, h

Fig. 4. Loss of organic carbon within the cycle at various nitrate loadings (C:N = 1.88).

Research Note

~100] 8° 1 80t ~ 0/ 0

(a) Experiment 1 (EBCT = 80 min)

I Influent lOrng~ Influent NO3-N= NO~-N= 40 rngF-11 J Influent NO3-N= 60rng~1

~

l'a

1'~

"~,

Time within cycle, h

~1ooT 801 8ot

2779

(b) Experiment2 ( influent NO~-N= 20 mgl-~ )

~.

• •

•

EBCT=80rain EBCT=60rain EBCT=40rain

O Time within cycle, h

Fig. 5. Change in sulfate reduction within the cycle at various nitrate loadings (C:N = 1.88). denitrification. Tap water constituent and nutrients were the sources of sulfate in this study. The sulfate reduction calculated from influent and effluent SO4: are shown in Fig. 5. In Experiment 1, as the amount of nutrients added were increased in proportion to the injected organic substrate, the sulfate concentration in the influent also increased. Sulfate reduction was significant only during the initial stages of the cycle when adsorbed sucrose level was high, while the sulfate reduction became almost negligible towards the end of the cycle. In Experiment 2, the reduction of sulfate was more significant in the columns operated at longer EBCT in spite of same influent concentration of sulfate. At short EBCT, denitrification was poor perhaps due to the unavailability of organic carbon and the presence of NO3 and NO£. Sulfate reduction occurred mostly when NO3 + N O ~ N was reduced to concentrations less than 5mgl ~ and the reduction of sulfate took place only after the 40cm zone from the bottom, below which most of the NO3-N were removed.

CONCLUSIONS

Under the condition of E B C T = 80min, C : N ratio = 1.88, once per day injection mode of organic supply was able to sustain an average denitrification efficiency of 84-89% even with influent NO3-N concentration of 80 mgl-i. With an influent NO3-N of 20 mgl-~ and C : N ratio of 1.88, however, reduction of EBCT to 20 min resulted in very poor denitrification. In the latter case, 46% of the added carbon was lost in the effluent immediately after the injection. Short EBCT critically affected the process mainly due to insufficient adsorption rate. Microbial denitrification activity and fermentation might also limit the process. Extended organic injection is a possible option to improve the process efficiency. Occurrence of the sulfate reduction was limited in early phase of the cycle. REFERENCE

Sison N. F., Hanaki K. and Matsuo T. (1996) Denitriflcation with external carbon source utilizing adsorption and desorption capability of GAC. Wat. Res. 30. In press.