nitrogen removal from synthetic wastewater by ...

PII: S0043-1354(98)00159-6

Wat. Res. Vol. 33, No. 1, pp. 145±154, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $ - see front matter

NITROGEN REMOVAL FROM SYNTHETIC WASTEWATER BY SIMULTANEOUS NITRIFICATION AND DENITRIFICATION (SND) VIA NITRITE IN AN INTERMITTENTLY-AERATED REACTOR M HYUNGSEOK YOO1*, KYU-HONG AHN1* , HYUNG-JIB LEE1, 2 2 KWANG-HWAN LEE , YOUN-JUNG KWAK and KYUNG-GUEN SONG1 1 Environment Research Center, Korea Institute of Science and Technology (KIST), P.O. Box 131 Cheongryang, Seoul, South Korea and 2Department of Environmental Engineering, Seoul National Polytechnic University, Gongneung-Dong, Nowon-Ku, Seoul, South Korea

(First received August 1997; accepted in revised form March 1998) AbstractÐAn intermittently aerated and decanted single-reactor process was proposed and some key control parameters investigated for nitrogen removal from wastewater by simultaneous nitri®cation and denitri®cation (SND) via nitrite. Two types of synthetic wastewater with acetate as the main carbon source, and chemical oxygen demand (COD):nitrogen (N) ratios of approximately 5:1 and 10:1 were used. For both types of wastewater the average COD removal eciency reached above 95%, and under optimal conditions nitrogen removal eciency also reached above 90%. This process consisted of 72 min aeration, 48 min settling and 24 min euent decanting. In¯uent wastewater was fed from the bottom of the reactor, and did not require a separate mixing phase. In this process, nitritation (1st step of nitri®cation) was induced but nitratation (2nd step of nitri®cation) was eectively suppressed and denitri®cation was carried out using nitrite. Most important parameters for eective SND via nitrite in the proposed process were: (1) the increase rate and the minimum and maximum dissolved oxygen (DO) concentration during the aeration period, (2) length of one cycle, and of aeration per cycle, taking advantage of the lag-time of nitrite-oxidizers behind ammonia-oxidizers when adjusting from anoxic/anaerobic to aerobic condition, and (3) close contact between the mixed liquor suspended solids (MLSS) in the reactor and the in¯uent wastewater under anoxic/anaerobic condition. During the aeration period, DO concentration dropped initially reaching the minimum value after several minutes, and then rose in 2nd-order fashion. The optimal maximum DO concentration (at the end of the aeration period) for nitrogen removal was determined to be around 2.0±2.5 mg/l in this proposed process. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐnitrogen removal, simultaneous nitri®cation and denitri®cation (SND), nitrate accumulation, nitritation, nitratation, intermittently decanted extended aeration (IDEA)

INTRODUCTION

Simultaneous nitri®cation and denitri®cation (SND) which has advantages over the separated nitri®cation and denitri®cation processes, means that nitri®cation and denitri®cation occur concurrently in the same reaction vessel under identical operating condition. In continuously operated plants, SND oers the potential to save cost for a second (anoxic) tank, or at least reduce its size, if it can be ensured that a considerable amount of denitri®cation takes place together with nitri®cation in the aerated tank. If SND is accompanied by the inhibition of the second step of nitri®cation (oxidation of nitrite to nitrate), theoretically a saving of organic energy of *Author to whom all correspondence should be addressed. Hyung Yoo, Dept of Civil and Environmental Engineering, Stanford University, Stanford, CA 943054220, USA. [Tel: +1(650)723-0315; Fax: +1(650)7253162; E-mail: [email protected]]. 145

up to 40% could result (Turk and Mavinic, 1986, 1987; Abeling and Seyfried, 1992). This is of particular interest when biologically removing nitrogen from wastewater with low COD:nitrogen ratio. Research by Sutherson and Ganczarczyk (1986) and Turk and Mavinic (1989) has shown the interest of mastering nitritation and achieving the direct coupling of nitritation and denitri®cation by performing a nitrate shunt, which was achieved by reducing the activity of Nitrobacter and giving Nitrosomonas species growth advantages. Nitri®cation and denitri®cation via nitrite has been investigated in detail (Abeling and Seyfried, 1992; Yang and Alleman, 1992; Akunna et al., 1993; Turk and Mavinic, 1986; Prakasam and Loehr, 1972; Murray et al., 1975; Laudelout et al., 1976; Sauter and Alleman, 1980; Blaszczyk et al., 1981). Investigation of Turk and Mavinic (1986, 1987) of the shortened nitrogen removal pathway via nitrite revealed: (1) 40% reduction of COD demand

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during denitri®cation; (2) 63% higher rate of denitri®cation; (3) 300% lower biomass yield during anaerobic growth; and (4) no apparent nitrite toxicity eects for the microorganisms in the reactor. Abeling and Seyfried (1992) discovered in their half-technical experiences, that saving cost was also possible with the reduced oxygen demand. For nitritation only 75% of the oxygen was required compared to that of the complete oxidation to nitrate. SND is also eective in maintaining neutral pH level in the reactor, without the addition of external acid/base source. This is important since a narrow optimal range between pH 7.5±8.6 is known to exist for the nitrifying bacteria. During nitri®cation alkalinity is consumed, but alkalinity is produced during denitri®cation. The optimal pH lies between 7 and 8 for denitri®cation with dierent optimums for dierent bacterial populations. However, high nitrite build-up may, in many cases, be harmful to the operation of the wastewater treatment plant, and therefore its concentration in the euent must be kept low due to its toxicity to some species of ®sh; therefore, SND via nitrite must accompany far-reaching denitri®cation. This investigation proposes a simple, single-reactor nitrogen-removal process utilizing SND via nitrite, without a separate mixing period, evaluates its performance, and suggests some key control parameters for eective operation. The design of the proposed process is based on the following literature review; key control parameters and their optimal values were determined in the laboratory. FACTORS FOR NITRITE ACCUMULATION DURING NITRIFICATION

Some of the best works investigating nitrite build-up during nitri®cation were performed by Anthonisen (1976), Sutherson and Ganczarczyk (1986), Jayamohan and Ohgaki (1988) and Hanaki and Wantawin (1990), which focused on a number of factors, such as the free ammonia (FA) concentration, the free hydroxylamine (FH) concentration, the pH, the temperature, and the dissolved oxygen (DO) concentration, on the transient build-up of the nitrite ion. High free ammonia (NH3) concentration According to the research by Anthonisen (1976), the non-ionized forms of the ammonium and nitrite have Ð as ammonia (NH3) and nitrous acid (HNO2) Ð an inhibition eect on both the Nitrosomonas and the Nitrobacter. The Nitrobacter react more sensitively so that concentration of NH3 in relatively low range is sucient for inhibition. Abeling and Seyfried (1992) reported that concentration of 1±5 mg NH3/l inhibited nitratation but not the nitritation. In the research of Turk and Mavinic (1986, 1987) nitrite build-up was achieved

with intermittent contact to high FA levels (5 mg NH3±N/l) in the ®rst cell of a four-cell system. However, nitrite build-up could not be sustained inde®nitely, due to the acclimation of the Nitrobacter to FA. Wong-Chong and Loehr (1978) observed that the Nitrobacter acclimated to FA could tolerate concentrations as high as 40 mg NH3±N/l, while unacclimated ones were inhibited at concentrations of 3.5 mg NH3±N/l. Many other researchers found out similar results. Ford et al. (1980) reported total inhibition of nitri®cation activity at FA levels of 24 mg NH3±N/l, but noted that system recovery was possible, even at levels as high as 56 mg NH3±N/l. Cecen and Gonenc (1994) noted, in the batch start-up phase, the combined eect of high ammonia and high pH (8.5) inhibited Nitrobacter and led to accumulation of nitrite. Mauret et al. (1996) showed that high FA concentration inhibits Nitrobacter, in the range of 6.6 and 8.9 mg NH3±N/l. Balmelle et al. (1992) reported that the inhibition eect of nitratation could be observed at concentrations as low as 1 mg NH3±N/l. Anthonisen (1976) found out values of 0.1±1.0 mg NH3±N/l. Abeling and Seyfried (1992) stated that in order to attain the highest nitritation rate, it was decisive to prevent the inhibition of the Nitrosomonas caused by FA. The FA concentration necessary for the inhibition of Nitrobacter must be kept low enough to ensure that the inhibition of nitritation does not also take place. At pH = 8.5 and T = 208C, the optimal FA concentration for maximum nitritation and minimum nitratation was found to be around 5 mg NH3/l (Abeling and Seyfried, 1992). On and after approximately 7 mg NH3/l an inhibition on nitritation could be noticed. At concentrations of around 20 mg NH3/l there was very little nitri®cation activity. Limits to prevent nitritation inhibition had been found out in batch tests which ranged between 10 and 150 mg NH3/l (Anthonisen, 1976). Also Neufeld et al. (1980) con®rmed the beginning of nitritation inhibition at 10 mg NH3/l. Temperature Balmelle et al. (1992) showed that in spite of concentration of FA normally inhibiting for the Nitrobacter of between 2 and 5 mg NH3±N/l, the Nitrobacter was active over a range of temperature between about 10 and 208C. Under these conditions, nitrite build-up remained low, since the eect of the Nitrobacter activation by temperature prevailed over its inhibition by FA. On the other hand, beyond a temperature of 20±258C, a slowing of the nitratating activity was observed together with an activation of the nitritating activity, which passes through a maximum at 258C. These results were in agreement with the data of Quinlan (1986), that showed high Nitrobacter activity for temperature below 158C. With respect to Nitrosomonas

Nitri®cation and dentri®cation via nitrite

147

under these conditions, the inhibiting eect of FA was preponderant for temperature higher than 258C. This result con®rmed that of Anthonisen (1976), but fairly disparate observations had been found in the literature, notably those of Ford et al. (1980) which suggested the optimal temperature range between 30 and 368C.

the microorganisms in direct contact with the in¯uent wastewater in oxygen-de®cient condition to induce contact with high concentration of FA and/ or FH (factors1 and 5); (4) raise the pH (factors 1 and 5); (5) add hydroxylamine to the reactor (factor 5); and (6) maintain the reactor temperature near 258C (factor 2).

Lag-time when changing from anoxic to aerobic condition

PROPOSED PROCESS

Turk and Mavinic (1986) observed a lag time in nitratation. Even with a bacterial population containing Nitrobacter acclimated to FA, nitritation rates lagged nitratation rates following the exit from the anoxic cell and entering the aerobic cell, resulting in a temporary, but signi®cant, accumulation of nitrite which lasted for several hours in the aerobic cell. Senanyake (1982), and Turk and Mavinic (1986) have reported that the extension of aeration time alleviates stress on the nitrite oxidizers and results in the loss of nitrite build-up. Duration of aeration time was found to be inversely related to the degree of nitrite build-up. Low DO concentration during aeration Cecen and Gonenc (1994) reported that nitrite accumulation reached a considerable degree at DO to FA concentration ratios lower than 5 during nitri®cation, and the formation of nitrate was inhibited. No nitrite occurrence was encountered when this ratio exceeded 5, which implied that oxygen limitation lead to nitrite accumulation. Therefore, eective nitritation might be achieved by supplying less air during nitri®cation. Free-hydroxylamine (FH) concentration Yang and Alleman (1992) concluded that the DO level alone did not appear to be the dominant factor behind nitrite build-up, and its correlation with free ammonia (FA) concentration alone was also erratic. On the other hand, hydroxylamine (NH2OH/NH3OH+), an intermediate in nitri®cation by Nitrosomonas, was found to inhibit nitratation by the Nitrobacter. Indeed, hydroxylamine appeared likely to accumulate in a nitrifying system with high NH3/NH+ 4 concentration, de®cient oxygen, and high pH. This research showed that the presence of unionized or the so called free hydroxylamine (FH) fraction (as opposed to ``total'') appeared to have a consistent correlation with low nitratation activity. FH was believed to be a major, if not principal, factor behind nitrite build-up in the batch nitrifying systems. Process guidelines for suppressed nitratation Therefore, to achieve eective SND via nitrite, it is necessary to (1) use simultaneous and/or alternating nitri®cation/denitri®cation process in the same reactor (factors 1, 3 and 4 above); (2) maintain low DO during aeration (factors 1, 4, and 5); (3) keep

The following cyclic activated sludge process consisting of three dierent periods (Fig. 1) was proposed. It is a modi®ed form of an intermittently decanted extended aeration (IDEA) process (®rst developed by the Department of Public Works and Services, NWS, Australia in 1965). `Ànoxic'' is de®ned, in this process, as the state or condition where denitrifying bacteria carry out denitri®cation utilizing nitrite and/or nitrate as electron acceptors, independent of the DO concentration in the surrounding environment. Such condition may take place in the entire water body of the reactor during the initial aeration phase, and in the settling/settled sludge layer during the settling phase. `Ànaerobic'' condition in this process takes place inside the settled sludge layer after all the nitrite and nitrite are used up as electron acceptors inside the layer. `Àerobic'' condition takes place in the reactor after some time has elapsed into the aeration period-denitri®cation is suppressed due to the rising DO concentration and nitri®cation takes place. As shown in Fig. 1, the in¯uent wastewater is fed constantly from the bottom throughout the cycle. During the settling and decanting periods oxygen is de®cient, and the Nitrobacter are in direct contact with the FA in the in¯uent wastewater, which theoretically has a high probability to be converted to FH by the Nitrosomonas species. The settling/ settled sludge acts as an anoxic reactor where denitri®cation takes place. When the aeration period of the subsequent cycle starts, the condition in the reactor is anoxic even with the added air; it takes some time for the DO level to rise high enough to suppress denitri®cation. Denitrifying activity is not completely suppressed, and the converted nitrite and nitrate from nitri®cation with the added oxygen will be denitri®ed-thus the simultaneous nitri®cation and denitri®cation. Therefore, this modi®ed IDEA process takes advantage of the non-instantaneous transition from anoxic/anaerobic to aerobic condition of the microorganisms. Such alternating between anoxic/anaerobic and aerobic condition is necessary to take advantage of the lag-time of the nitrite-oxidizers behind the ammonium-oxidizers. Such lag time may be dierent for each species of denitrifying bacteria, and certain species may carry out denitri®cation throughout the entire cycle. The length of the aeration period should be long enough for eective COD removal, nitri®cation and denitri-

148


Fig. 1. Modi®ed IDEA process description and operation strategy: (a) three periods of the process; and (b) length ratios of the three periods, corresponding conditions and the ideal DO pattern (determined in this experiment).

®cation, but short enough to prevent the adaptation of nitrite-oxidizers to the aerobic environment. Ideal dissolved oxygen (DO) level curve for one cycle is shown in Fig. 1(b). Minimum DO concentration of the aeration period is achieved several minutes into the period. Intuitively, optimal average and maximum DO concentration levels exist for nitrogen removal in this process. DO levels above these values will yield too much nitrite/nitrate (NOx) in the euent, whereas levels too low will induce a high level of NH3/NH+ 4 . It was one of the purposes of this experiment to determine such optimal DO levels. This proposed process does not require a separate mixing period, since the settling and decanting periods, and the initial portion of the aeration period are anoxic/anaerobic treatment phases. The amount of air supplied is such that the DO level does not rise too quickly into the aeration period.

MATERIALS AND METHODS

Composition of the synthetic wastewater Two types of synthetic wastewater, made according to Table 1, were fed to reactors 1 and 2. The dierence between in¯uent wastewater 1 and 2 was the COD/NH4±N ratio Ð approximately 5:1 and 10:1 Ð and COD/phosphate ratio-approximately 10:1 and 20:1, respectively.

Experimental set-up and reactor operation The two modi®ed IDEA reactors were variable-volume, and seeded with a grab sample of mixed liquor from the aerobic basin of the Kwangdongli Sewage Treatment Plant (Kyung-Gi-Do, Korea). They were operated for 122 days. Sample analysis commenced after approximately 22 days of operation and continued for the next 100 days. The reactors had circular bottom with diameter of 20 cm, and their maximum working volume was 17 l. Discharge and sampling ports were located at dierent levels. The top of the reactors was open to air. The reactors were operated in parallel, with varying hydraulic retention times (HRT's), and sludge retention times (SRT's). The in¯uent wastewater was kept in a refrigerator (at 48C). The reactor temperature was maintained at 22±278C. Cyclic operation as described in Fig. 1 was controlled by a programmable logic controller (PLC). Aeration period was set at 72 min, settling period at 48 min, and decanting period at 24 min for the reactors; the duration of each cycle was 144 min (2.4 h) i.e. 10 cycles per day. 10 l of wastewater was fed to each reactor everyday, and Table 1. Composition of the two types of in¯uent wastewater Added compounds (per l in¯uent) CH3COONa CH3COONH3 KH2PO4 NaHCO3 CaCl2 FeCl2 MnSO4 ZnSO4 MgSO4 Yeast extract

In¯uent wastewater 1 In¯uent wastewater 2 (mg) (mg) ÿ

240.88 43.94 125.00 10.00 0.375 0.038 0.035 25.00 50.00

256.41


149

Table 2. In¯uent wastewater characteristics In¯uent 1

In¯uent 2

Parameter

average std. dev. average

COD (TCOD = SCOD) (mg/l) NH4±N (mg/l) HPO4±P (mg/l) NO3 ±N (mg/l) COD/NH4±N ratio

226.2

10.0

401.1

21.3

41.3 13.5 1.1

1.8 7.5 0.3

41.4 11.5 1.1

0.8 6.5 0.6

5.5

std. dev.

9.7

all in¯uent, euent, and waste activated sludge (WAS) streams were controlled using variable-speed Master¯ex pumps. When changing the HRT, the amount of in¯uent feed was kept constant and the reactor volume was changed. All water and WAS tubes were cleaned once a week. To control the SRT, WAS was wasted during the last minute of the aeration period (not collected). Stirrers, operating at 30 rpm, provided mixing during the aeration period in addition to the compressed air. Air was diused by aquarium-type diuser-stone, and the supply rate was controlled using ¯owmeter. Supplied air was in the range of 0.3±1.0 l/min. DO level control system was purposely not used to observe nitrogen removal at various DO concentration and pattern, and to obtain the optimal DO level. DO concentration was measured 5 min into (minimum DO) and at the end of (maximum DO) the aeration period, using Series 5905 DO probe and Model 58 DO meter (YSI, OH); the membrane of the DO electrode was changed every 2 weeks. The DO probe was inserted into the water surface about 5 cm for measurement.

Sampling methods Grab in¯uent wastewater samples were analyzed once a week. Euent samples were collected as the entire decanted euent from the same cycle of everyday, two to four times a week and analyzed. Cyclic studies were performed several times throughout the operation with sampling intervals between 1 and 15 min. Samples for the determination of soluble components were immediately ®ltered using 0.45 mm ®lter paper and cooled (48C) in order to prevent further reaction after sampling. Sample analysis was performed according to the standard methods (APHA, 1992).

General reactor operation Table 2 shows the characteristics of the two types of in¯uent wastewater. Since this process combined anoxic, aerobic and settling phases in a single reactor, and aimed for high performance, initial HRT's were set at relatively high values. HRT's were to be reduced if the reactors showed high performance with continued decrease in the MLSS concentration. Table 3 describes change in the HRT's and SRT's. (All calculation was based on the reactors' volume at the bottom water level (BWL) at the end of the decanting period.) HRT's for both reactors were lowered nearly by half on day 61. (``Day'' will be referred according to the sample analysis dates.) The sludge settleability for both reactors was good, with the sludge volume index (SVI60) between 78 and 165 ml/g. SVI60 was not aected by changes in HRT and SRT. pH was measured at the end of aeration, settling and decanting periods; pH values for both reactors were in the neutral range between 6.9 and 7.8. Average pH values were 7.2 and 7.5 for reactors 1 and 2 respectively. Although pH buer solution was added to the in¯uent wastewater, SND aided in maintaining the neutral pH.

Fig. 2. MLSS concentration for reactor 1 (W) and reactor 2 (w).

RESULTS AND DISCUSSION

Mixed liquor suspended solids (MLSS) and COD removal As shown in Fig. 2, MLSS concentration for both reactors dropped relatively quickly from day 1 until day 30, and continued to drop until day 60; however such drop did not aect the two reactors' COD removal which was very high (Figs 3 and 4). Therefore, the initial HRT's were believed to be too high, and on day 61, the HRT's were nearly reduced by half (Table 3). The HRT reduction did not aect the reactors' COD removal, and resulted in stable MLSS concentration in reactor 1 from day 61, but in reactor 2 MLSS concentration increased slightly. SRT for both reactors was increased to 45 days on day 26 to slow down the dropping rate of MLSS concentration. The SRT increase had no eect on the reactors' performance (Figs 3 and 4). COD removal rate for both reactors was very high throughout the operation, unaected by the changes in the MLSS concentration, HRT and SRT. Based on the average incoming soluble COD (SCOD) concentration of 226.2 mg/l (reactor 1) and 401.1 mg/l (reactor 2), the removal eciency was between 89% and 98% with the average of 95% (Std. Dev. = 2.0) for reactor 1, and between 85% and 99% with the average of 97% (Std.

Fig. 3. Euent SS concentration of reactor 1 (W) and reactor 2 (r), and SCOD concentration of reactor 1 (.) and reactor 2 (w).

150


Fig. 4. Euent nitrogen compounds and DO concentration of reactor 1 (a) and reactor 2 (b). (``NH3 ± N'' in this Figure is technically the sum of NH3±N and NH+ 4 ±N.)

Dev. = 2.4) for reactor 2, respectively. Figure 3 shows the euent suspended solids (SS) and SCOD concentration. Scum ¯oated on the water surface of both reactors; an attempt was not made to remove the scum. In full-scale operation, decanting system that separates the scum from the euent should be installed. In this experiment, euent samples contained the scum and exhibited relatively high SS concentration. Nitrogen removal, and changes in DO, HRT and SRT Figure 4 shows the euent nitrogen (N) compounds' concentration with the minimum and maximum DO concentration for reactors 1 and 2. Surprisingly, nitrite accumulation was very successful in both reactors from day 1. For many days, high concentration of nitrite was observed in the euent, but low nitrate concentration. A correlation between the euent nitrite concentration and DO levels was observed.

According to Fig. 4, high concentration of nitrite resulted when the maximum and minimum DO concentration in the reactors was relatively high. High NH3/NH+ 4 concentration was observed when the maximum and minimum DO concentration was relatively low. Generally, the minimum DO concentration was below 0.4 mg/l. From Fig. 4, SND via nitrite seemed very successful. Also, the initial guess of the existence of optimal DO levels for N-removal proved to be correct. pH was never intentionally raised or the addition of hydroxylamine made to suppress nitratation; initially it was thought that these steps would be necessary. To con®rm the ÿ SND via nitrite, NOx (sum of NOÿ 2 and NO3 ) con-

Table 3. HRT's and SRT's of the two reactors Reactor 1 Day 1±25 26±60 61±100

HRT (h) 25 25 13

SRT (days) 39 45 45

Reactor 2 HRT (h) 31 31 17

SRT (days) 25 45 45

Fig. 5. Nitrite concentration for reactor 1 (W) and reactor 2 (.), and nitrate concentration for reactor 1 (r) and reactor 2 (w), at the end of aeration period.


151

Fig. 6. Cyclic study of reactor 1 on day 87. (``NH3±N'' is the sum of NH3±N and NH+ 4 ±N.)

centration was measured at the end of the aeration period. Figure 5 shows high concentration of nitrite at the end of aeration period in both reactors. Nitrite oxidation was eectively suppressed in this proposed process. Cyclic study Figure 6 shows data from one of the cyclic studies, which showed ideal DO pattern described in Fig. 1. All samples for the cyclic studies were collected by immersing 50 ml beaker slightly into the water surface and extracting about 20 ml. The samples were then immediately ®ltered using 0.45 mm ®lter paper. DO level changed sensitively in all cyclic studies. In Fig. 6, DO concentration rose in 2nd-order fashion after about 5 min into the aeration period, reaching 2.5 mg/l by the end. The median DO level, de®ned as the mid-value between the minimum and maximum DO values in a given aeration period, was 1.3 mg/l in this cyclic study. To obtain the true average DO concentration, integration must be performed for the DO curve during the aeration period and then division by the time; however, such integration was not performed in this experiment. Using the average DO concentration together with the minimum and maximum concentration would

be the accurate way to describe the pattern of DO level. During settling and decanting periods, DO level in the upper region of the reactor dropped quickly reaching 0.4 mg/l by the end. Figure 6 showed the ideal pattern of DO concentration; it dropped and then started to rise several minutes into the aeration period, and passed 1.0 mg/l at minute 38, about half-way into the period. Thus, during the initial phase, mostly denitri®cation took place indicated by the drop and low concentration of nitrite, and slight increase in pH. NH3/NH+ 4 ±N concentration stayed constant at around 6.5 mg/l during the initial phase, but then dropped as the DO concentration rose, and disappeared when the DO concentration was between 1.5±2.0 mg/l. On the contrary, nitrite concentration rose with the DO level, indicating nitri®cation was active in the later portion of the aeration period. Nitrite appeared as the DO level passed through 1.0 mg/l and its concentration stayed high until the end. Due to nitri®cation, pH dropped slightly. Nitrite concentration rose steadily until the end of aeration. After several minutes into the settling period, nitrite concentration started to drop. During the decanting period, NH3/NH+ 4 appeared again due to diusion of the in¯uent wastewater's NH3/NH+ 4 . About 1.3 mg/l of nitrite and 0.4 mg/l of NH3/NH+ 4 was

Fig. 7. Euent NO3±N (*), NOx±N (R), and (NH3+NOx)±N (w) concentrations of reactors 1 (a) and 2 (b) versus median DO levels (data from entire operation). (``NH3±N'' in this Figure is the sum of NH3±N and NH+ 4 ±N. ``NOx ±N'' is the sum of NO2 ±N and NO3 ±N.)

152


Fig. 8. The eect of DO level on speci®c nitri®cation and denitri®cation rates for reactor 1 during days 61±100. (Dotted lines show the intuitive pattern.)

observed in the euent of this speci®c cycle indicating eective N-removal. From the cyclic studies, the importance of DO level was realized. DO level should be controlled two-dimensionally: (1) the maximum level of DO during the aeration period, and (2) the increase rate of the DO level-aerobic condition should not be created too quickly or slowly. Therefore, aeration system capable of such DO control must be implemented in the full-scale plant of this proposed process. The aeration system should be easy to install since there are four control parameters of air supply: (1) DO level sensor, (2) ¯owmeter installed at the air pump, (3) air-supply valve, and (4) ¯exibility in the length of the aeration period. Determining the optimal DO concentration for nitrogen-removal The eect of maximum and minimum DO concentration on the euent N compounds is shown in Fig. 7; the median DO concentration is the x-axis. According to Fig. 7, when the median DO level was high, NOx concentration was high and NH3/ NH+ 4 was mostly removed from the euent, and

the opposite trend occurred when the DO value was low. The median DO value of 1.0±1.5 mg/l seemed to be optimal, implying that if the minimum DO level was in the range of 0.0±0.4 mg/l, the maximum DO level should be in the range of 1.6 and 3.0 mg/l. It is generally known that DO concentration above 1 mg/l is essential for nitri®cation; if the DO level is lower, oxygen becomes the limiting factor and nitri®cation slows or ceases. On the contrary in denitri®cation, high DO level will suppress the necessary enzyme system. Thus, when SND via nitrite becomes the focus, controlling the DO level is critical to balance the degrees of nitri®cation and denitri®cation, and the resulting levels of N compounds in the euent. With certain portion of the data, speci®c nitri®cation, denitri®cation and N-removal rates were calculated to study the eect of dierent DO levels on the relative degrees of nitri®cation and denitri®cation. For such study to be valid, MLSS concentration must be constant. Therefore, data from reactor 1 during days 61±100 (Fig. 2) were used. Figure 8 shows that nitri®cation rate increased with the DO concentration, whereas the denitri®cation rate decreased. Figure 9 shows the speci®c Nremoval rate at various DO levels. The optimal median DO level for nitrogen removal was 1.3 mg/l. Review of factors for nitratation suppression

Fig. 9. Speci®c N-removal rates for reactor 1 during day 61±100. (Dotted line shows the intuitive pattern).

Concentration of FA (NH3) was calculated throughout the operation. For reactor 1, the maximum NH3/NH+ 4 ±N in the euent was 23.1 mg/l obtained on day 71; pH was 6.9 during aeration and 7.2 during decanting which corresponded to 0.10 and 0.20 mg/l NH3±N at 258C. For reactor 2, the maximum NH3/NH+ 4 ±N in the euent was 27.6 mg/l obtained on day 64; pH was 7.1 during aeration and 7.5 during decanting which corresponded to 0.19 and 0.48 mg/l NH3±N at 258C. If no NH3/NH+ 4 ±N removal is assumed for reactor 1,


153

Fig. 10. Extension of the aeration period in reactor 1 on day 96.

the in¯uent 41.3 mg/l NH3/NH+ 4 ±N at 258C should yield 0.07 mg/l NH3±N at pH = 6.5, 0.23 mg/l at pH = 7.0, and 0.71 mg/l at pH = 7.5; so in reactor 1, NH3±N concentration should not exceed 1.0 mg/l at any time unless the pH reaches 7.65; such pH was never recorded. For reactor 2, the in¯uent 41.4 mg/l NH3/NH+ 4 ±N at 258C should yield 0.71 mg/l NH3±N at pH = 7.5, and 1.40 mg/l at pH = 7.8 which was the maximum pH recorded. Such result implies that the reported values by Abeling and Seyfried (1992), Turk and Mavinic (1986, 1987), Wong-Chong and Loehr (1978), Ford et al. (1980), and Mauret et al. (1996) for the FA level inhibiting nitratation was not reached in either of the reactors. Only the values reported by Balmelle et al. (1992) at 1 mg NH3±N/l, and Anthonisen (1976) at 0.1±1.0 mg NH3/l were close to the levels observed in our experiment. Inhibition eect of FA was maximum in the region of in¯uent feed at the bottom of the reactor, especially during settling and decanting periods. Hydroxylamine was not measured in this experiment. Thus, correlation between hydroxylamine concentration and nitritation/nitratation activity cannot be stated in numerical terms. However, hydroxylamine was never added to the in¯uent or to the reactors, as initially thought necessary to achieve SND via nitrite. Theoretically, hydroxylamine had high probability of build-up (by the Nitrosomonas) in the settled sludge during settling and decanting periods due to de®cient oxygen, and could have suppressed nitratation irreversibly. Temperature was in the range of 22±278C during the entire operation, inducing maximum nitritation activity. Maximum nitratation activity is expected in the temperature range below 158C. DO to FA ratio was calculated. Cecen and Gonenc (1994) suggested that below the ratio of 5, nitrite accumulation was high and the formation of nitrate inhibited. For reactor 1, maximum FA concentration in the euent was 0.20 mg/l NH3±N (calculated previously), which corresponded to 0.24 mg/l NH3. For the DO to FA (NH3) ratio to be below 5, the DO level must be lower than

1.2 mg/l. For reactor 2, maximum FA concentration was 0.58 mg/l NH3 (0.48 mg/l NH3±N) in the euent, and the DO level must be lower than 2.9 mg/l for the ratio to be below 5. DO level in both reactors changed throughout the cycle as well as from day to day. The calculated DO levels (as reference points) suggest that DO:FA ratio was below 5 more than half of the time in both reactors. The most in¯uential factor was believed to be the lag-time of nitrite-oxidizers behind ammonia-oxidizers during the transition from anoxic/anaerobic to aerobic condition. Turk and Mavinic (1986) stated such lag-time to be ``several hours'' but did not provide a speci®c numerical value. To study the eect of the lag-time, on day 96, aeration period in reactor 1 was extended for 330 min (the normal length had been 72). DO level was controlled at approximately 2.5 mg/l by adjusting the air ¯owmeter. In¯uent wastewater was fed continuously, but the euent was not decanted; the water surface level continued to rise. All other experimental settings stayed the same. 72 min, the normal length of aeration, was adequate for eective COD and N-removal (Figs 3 and 4). In Fig. 10, normally the aeration period should have ended at minute 77. After minute 77, pH continued to drop but never fell below 6.5; ammonia did not appear and nitrite concentration increased indicating on-going nitri®cation. Nitrate concentration rose as the aeration continued indicating that (1) the nitrite-oxidizers were present in the reactor, and (2) they were adjusting to the aerobic environment. Substantial portion of nitrite-oxidizers could not adapt to the aerobic environment in the 72 min of normal aeration. It was concluded that the active nitritation and suppressed nitratation was caused by the combined eect of all the factors reviewed, and the most important one was the lag-time of nitrite-oxidizers when transitioning from the anoxic/anaerobic to aerobic environment. The investigation to determine exactly to what degree each of the factors was in¯uential would be the subject of future research.

154

Hyungseok Yoo et al. CONCLUSION

This investigation showed that N-removal from synthetic wastewater utilizing SND via nitrite was successful in this proposed intermittently-aerated cyclic activated-sludge single-reactor process consisting of 72 min aeration, 48 min settling and 24 min decanting, which should be advantageous in treating wastewater with low COD:N ratio. SND via nitrite was sustained for 122 days. However, for eective SND via nitrite to take place in this proposed process, careful control of parameters such as the DO level, pH, FA and FH concentration, temperature and aeration length is necessary. Also, the continuous feeding of the in¯uent wastewater from the bottom of the reactor was critical for eective SND via nitrite. Aeration time per cycle should not be too long or short. Aeration of 72 min per cycle, and 12 h per day was adequate and eective for this proposed process. However, the data showed that longer aeration time per cycle may be possible once the sludge in the reactor is acclimated to nitratationsuppressed condition (Fig. 10). DO concentration during aeration should be controlled two-dimensionally to eectively suppress nitratation: control of the maximum DO concentration reached at the end of aeration period, and the DO concentration increase rate. The minimum DO level was generally below 0.4 mg/l. Ideal maximum DO concentration should be 2.0±2.5 mg/l. Ideal median DO level was 1.3 mg/l. Active denitri®cation took place in the initial phase of the aeration period when the DO concentration was below 1.0 mg/l. However, during the later portion of the period when the DO concentration exceeded 1.0 mg/l, nitri®cation was more active. Active denitri®cation commenced again once the settling period started and continued until the end of the cycle. It is important to create the environment where oxygen is de®cient and the sludge is in close contact with the high level of FA in the in¯uent wastewater. Under such condition, build-up of hydroxylamine by the Nitrosomona has high probability to take place. In this proposed process, continuous feeding of the in¯uent from the bottom of the reactor during settling and decanting periods was eective in creating such environment. AcknowledgementsÐThis research was funded jointly by the Ministry of Science and Technology (MOST), and the Kumho Construction Company, Ltd., of the Republic of Korea. REFERENCES

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