Simultaneous Nitrification, Denitrification, and Phosphorus Removal in Single-Tank Low-Dissolved-Oxygen Systems Under Cyclic Aeration

By Ju, Lu-Kwang Huang, Lin; Trivedi, Hiren

ABSTRACT: Simultaneous nitrification and denitrification (SND or SNdN) may occur at low dissolved oxygen concentrations. In this study, bench-scale (approximately 6 L) bioreactors treating a continuous feed of synthetic wastewater were used to evaluate the effects of solids retention time and low dissolved oxygen concentration, under cyclic aeration, on the removal of organics, nitrogen, and phosphorus. The cyclic aeration was carried out with repeated cycles of 1 hour at a higher dissolved oxygen concentration (HDO) and 30 minutes at a lower (or zero) dissolved oxygen concentration (LDO). Compared with aeration at constant dissolved oxygen concentrations, the cyclic aeration, when operated with proper combinations of HDO and LDO, produced better-settling sludge and more complete nitrogen and phosphorus removal. For nitrogen removal, the advantage resulted from the more readily available nitrate and nitrite (generated by nitrification during the HDO period) for denitrification (during the LDO period). For phosphorus removal, the advantage of cyclic aeration came from the development of a higher population of polyphosphate-accumulating organisms, as indicated by the higher phosphorus contents in the sludge solids of the cyclically aerated systems. Nitrite shunt was also observed to occur in the LDO systems. Higher ratios of nitrite to nitrate were found in the systems of lower HDO (and, to less dependency, higher LDO), suggesting that the nitrite shunt took place mainly because of the disrupted nitrification at lower HDO. The study results indicated that the HDO used should be kept reasonably high (approximately 0.8 mg/L) or the HDO period prolonged, to promote adequate nitrification, and the LDO kept low (

KEYWORDS: simultaneous nitrification and denitrification, low dissolved oxygen concentration, solids retention time, cyclic aeration, nitrite shunt, enhanced biological phosphorus removal.

doi:10.2175/106143007X175942

Introduction

Regulations on the nitrogen and phosphorus contents in wastewater discharge are increasingly more stringent for controlling the rate of eutrophication in the aquatic environment. Biological reduction of the nitrogen contents in wastewater relies primarily on two mechanisms-aerobic nitrification and anoxic denitrification (Gee and Kim, 2004; Grady et al., 1999; Schmidt et al., 2003). Generally, the two processes are carried out in physically separated oxic and anoxic treatment zones (Barnard, 1975; Ju et al., 1995; Ludzack and Ettinger, 1962). They can also be achieved in the same reactor, but temporally separated with alternating oxic and anoxic periods, by turning the aeration on and off in repeated cycles (Alleman and Irvine, 1980; Randall et al., 1992; Sedlak, 1991; Silverstein and Schroeder, 1983).

Recent studies have revealed that nitrification and denitrification can occur simultaneously in the same reactor (Trivedi and Heinen, 2000), arising from the following mechanisms (Kaempfer et al., 2000; Satoh et al., 2003; Stensel, 2001):

(1) Presence of microscopie anoxic/oxic zones within sludge floes. Aeration promotes the dissolution of oxygen into the water. The dissolved oxygen subsequently diffuses into the floes and, along the diffusion path, is consumed by the organisms. The diffusion und consumption cause a gradient of dissolved oxygen concentration (DO) in the floes and, at suitably low dissolved oxygen concentrations, create floes that nitrify in the oxic outer layer and denitrify in the anoxic inner core.

(2) Presence of macroscopic anoxic/oxic zones within bioreactors. Such zones are commonly formed as a result of nonhomogeneous mixing and aeration, particularly in bioreactors with surface aerators, where the dissolved oxygen concentration is high near the aerators and low or zero away from the aerators.

(3) Presence of novel microorganisms. For example, Robertson et al. (1988) reported that Thiosphaera pantotropha could simultaneously nitrify and denitrify under aerobic conditions. In addition, several bacteria were found to perform aerobic denitrification (Davies et al, 1989). More recently, Chen et al. (2003) confirmed, with continuous culture of Pseudomonas aeruginosa, that aerobic denitrification functioned as an electron-accepting mechanism supplementary to aerobic respiration. Some nitrifiers, such as Nitrosomonas europea and N. eutropha, could also denitrify at low dissolved oxygen concentrations (Zart et al., 1996).

Single-tank simultaneous nitrification and denitrification (SND) processes have potential advantages in eliminating the need for separate tanks and for recycling the mixed liquor from oxic nitrifying zones to the typically upstream anoxic zones for denitrification. In addition to the simpler process design, SND has been estimated to require smaller total tank sizes (Kaempfer et al., 2000; Stensel, 2001). The SND also helps to maintain a relatively neutral pH in the bioreactor, without the addition of an external acid or base. The alkalinity consumed by nitrification is partially recovered by denitrification (Grady et al., 1999).

While offering the above potential benefits, SND faces several challenges in design, control, and operation. The single-stage, continuously stirred tank reactor configuration and the low dissolved oxygen environments required for SND processes are conventionally considered more susceptible to sludge bulking, primarily because of the excessive growth of filamentous bacteria (Grady et al., 1999; Jenkins et al, 2003; Martins et al., 2004). A study conducted in this laboratory concluded that the cyclic aeration, with repeated cycles of 1 hour at a higher dissolved oxygen concentration (HDO approximately 0.8 mg/L), followed by 30 minutes at a lower dissolved oxygen concentration (LDO

In addition to the potential bulking problem, the performance of SND relies on achieving a dynamic balance between nitrification and denitrification. The objective of this study was to evaluate the treatment performance (i.e., the removal of organic matter and nutrients [nitrogen and phosphorus]) of the cyclically aerated lowdissolved-oxygen systems maintained at different SRT and HDO and LDO. The potential occurrence of “nitrite shunt” and the development of sludge with elevated biophosphorus contents in such systems were also examined. Nitrite shunt referred to the shortcut biochemical pathway with partial nitrification of ammonia (NH^sub 3^) to nitrite (NO^sub 2^^sup -^), followed by the partial denitrification of NO^sub 2^^sup -^ to N^sub 2^, bypassing the formation and consumption of nitrate (Yoo et al., 1998).

Methodology

Experimental Setup and Procedure. Two modified Eckenfelder’s reactors (approximately 6 L) were used simultaneously in this study. The reactor setup is shown schematically in Figure 1. A sliding baffle was used to divide the bioreactor into a mixing zone and a settling/clarifying zone, with a volume ratio of 5:1. A pump was used to slowly return the settled sludge from the settling zone to the mixing zone, to avoid the sludge loss resulting from flotation caused by the denitrification-generated bubbles in the accumulated blanket of sludge. Depending on the studied SRT (5, 10, or 20 days), a calculated volume of mixed liquor was removed once to thrice daily, after pulling up the dividing baffle to allow thorough mixing of the reactor contents.

Mixing in the mixing zone was achieved by mechanical agitation and diffused-air aeration. A marine propeller operated at a constant speed (approximately 300 r/min) provided the mechanical agitation. The aeration, through a diffuser placed at the bottom of the mixing zone, was supplied by two air pumps, which were separately controlled by two timers to allow the study of various aeration schemes. In this study, the aeration was applied in three different modes. One experimental run (run 1, Tables 1 to 3) was carried out under constant aeration, to maintain constant dissolved oxygen levels. Some experiments (runs 2 to 4) were aerated in repeated on/ off cycles, with an air pump being turned on for 1 hour and then off for 30 minutes. Aeration to the third group of experiments (runs 5 to 7) was also done in repeated 1-hour/30-minute cycles. In this group, however, both air pumps were turned on for 1 hour to obtain a certain dissolved oxygen concentration. One of the pumps was then turned off for 30 minutes, to maintain a lower dissolved oxygen concentration in the mixing zone. Different dissolved oxygen levels were studied for each aeration mode, as summarized in Table 1.

The bioreactors were operated at room temperature (22 +- 1[degrees]C). By controlling the wastewater feeding rate, the hydraulic retention time was maintained at 24 hours for all of the experiments. The influent feed was continuous during the experiment. A synthetic wastewater was used in this study, with the following composition modified from that used by Kiso et al. (2000): 500 mg/L skim milk, 80 mg/L NH^sub 4^Cl, 20 mg/L K^sub 2^HPO4, 580 mg/L NaHCO^sub 3^, 100 mg/L MgSO^sub 4^ * 6H^sub 2^O, and 100 mg/L CaCl^sub 2^ * 2H^sub 2^O. The wastewater had approximately 480 mg/L chemical oxygen demand (COD), 50 mg/L total Kjeldahl nitrogen (TKN), and 8 mg/L total phosphorus. Fresh sludge samples were taken from a secondary clarifier of the nearby Water Pollution Control Division Station at Akron, Ohio. The sludge was diluted with tap water to make an initial concentration of total solids (TS) of 1000 to 1500 mg/L. After seeding, the bioreactor was maintained for a period of 3 SRTs. The mixed-liquor samples were then taken every 1 to 2 days to monitor the concentrations of TS and volatile solids (VS). After TS and VS stabilized (with random fluctuations within +- 1 standard deviation), more samples (3 to 6 per day) were taken for the additional analyses of ammonium (NH^sub 4^^sup +^) and nitrate (NO^sub 3^^sup -^) concentrations. The system was considered to have reached (pseudo)steady state when the NH^sub 4^^sup +^ and NO^sub 3^^sup -^ concentrations also only fluctuated randomly within +-1 standard deviation. Afterwards, the bioreactor was maintained for an additional period of 1 to 2 SRTs, during which, more frequent samples were taken and analyzed to evaluate the process characteristics and performance. The pH in the mixing zone tended to increase because of the protein-rich synthetic wastewater used in this study. The pH was therefore maintained at 7.2 +- 0.3 using an assembly of pH probe, meter/controller, and pump for automatic addition of 1 N hydrochloric acid (or, occasionally, 1 N sodium hydroxide). Dissolved oxygen concentration was measured continuously using a YSI 5739 dissolved oxygen probe with a meter (model 58, YSI Inc., Yellow Springs, Ohio).

Analytical Methods. The sample analyses included those for the sludge properties (TS, VS, TKN, and total phosphorus contents) and those for water quality (COD, ammonia-nitrogen, [NH^sub 3^-N], nitrate-nitrogen [NO^sub 3^^sup -^N], nitrite-nitrogen [NO^sub 2^^sup -^ -N], and orthophosphate [P^sub i^]). All tests were conducted in accordance with Standard Methods (APHA et al., 1995), as follows: TSS by method 2540 B, VSS by method 2540 E, COD by the closed reflux titrimetric method (5220 D), NH^sub 3^-N by using an ammonia selective electrode (method 4500-NH^sub 3^ D), combined NO^sub 3^^sup -^-N and NO^sub 2^^sup -^-N by the titanous chloride reduction method (4500-NO^sup 3^^sup -^ G), NO^sub 2^^sup -^-N by the colorimetrie method (4500-NO^sub 2^^sup -^ B), P^sub i^ by method 4500-P E, total phosphorus first digested by the Persulfate Digestion Method then followed by method 4500-P E, and TKN by method 4500-N^sub org^ C.

Results

Overall Treatment Performance. Sludge bulking occurred in some experimental runs, particularly those under constant aeration and/ or with a longer SRT (20 days). Only the results from the nonbulking runs (sludge volume index [SVI] <150 mL/g) are summarized in Tables 1 to 3-Table 1 for the operating conditions, SVI, and solids concentrations; Table 2 for the experimentally measured data on mixed-liquor and effluent properties; and Table 3 for the calculated/ derived results. As shown in Table 3, the COD removal (91 to 96%) was good and did not differ much under various operating conditions. Nitrogen removal, however, ranged from 62 to 86%. Phosphorus removal varied even more, from approximately 3 to 65%. Among all the operating conditions examined, run 4 had the highest nitrogen removal and the second highest phosphorus removal. It produced good overall treatment results for the synthetic wastewater used in mis study.

Variations of Nitrogen Species and Phosphate Concentrations During Aeration Cycles. To monitor die variation of water properties during the alternating HDO and LDO aeration cycle, a 15-mL mixed- liquor sample was taken every 10 minutes, totaling 18 samples over 2 full aeration cycles. The supernatants collected by centrifugation were frozen and later analyzed for concentrations of nitrogen species (ammonium, nitrite, and nitrate) and inorganic phosphate. The above sampling was repeated on multiple days to confirm the reproducible profiles obtained. The profiles obtained for nitrogen species are described first.

As an example, the profiles from run 4 are shown in Figure 2a. Opposite trends were observed for NH^sub 3^-N and NO^sub x^^sup -^- N (i.e., combined NO^sub 3^^sup -^-N and NO^sub 2^^sup -^-N) concentrations. During the LDO period, the NH^sub 3^-N concentration increased and NO^sub x^^sup -^-N concentration decreased, because the low dissolved oxygen concentration (0 mg/L, in this case) prevented nitrifiers from converting the influent NH^sub 4^^sup +^ to NO^sub x^^sup -^, while promoting denitrification. During the HDO period, the higher dissolved oxygen concentration enabled nitrification and inhibited denitrification. The influent NH^sub 4^^sup +^ was converted to NO^sub x^^sup -^, causing NH^sub 3^-N concentration to decrease and the NO^sub x^^sup -^-N concentration to increase.

Similar profiles were seen for other runs (data not shown), although the range of variation differed depending on the operating conditions, particularly SRT (which affected nitrification, because nitrifiers grew much slower at low dissolved oxygen concentrations and required longer SRT to establish in the sludge, as described more in die Discussion section) and the difference between HDO and LDO (i.e., DeltaDO = HDO – LDO).

The variation of the phosphate-phosphorus concentration over two aeration cycles is shown in Figure 2b. The phosphate concentration increased during the LDO period, and then decreased during the HDO period. The phenomenon was similar to that observed in the common biophosphorus removal processes, with sequential anaerobic and aerobic stages, where the phenomenon was attributed to the activity of phosphorus-accumulating organisms (PAOs) (Grady et al., 1999). As shown in Table 3, the solids from die systems under cyclic aeration had higher phosphorus contents (2.7 to 4.3%) than the system under constant aeration did (run 1, 1.5%). This finding strongly supported the feasibility of enriching PAOs in the low-dissolved-oxygen SND process with cyclic aeration.

Discussion

The implications associated with the observed variations in nitrogen and phosphorus removal are discussed in more details in separate sections below.

Balance of Nitrification and Denitrification. Successful nitrogen removal in low-dissolved-oxygen SND depends on the balance between nitrification and denitrification occurring in the process. A three- dimensional bubble plot is shown in Figure 3 for the effects of dissolved oxygen concentration (time averaged for runs with cyclic aeration) and SRT on nitrogen removal, where the bubble size represents the magnitude of nitrogen-removal percentage. Within the investigated ranges, the nitrogen removal tended to increase with increasing dissolved oxygen concentration and SRT. Because high dissolved oxygen concentrations favor nitrification and low dissolved oxygen concentrations favor denitrification, the observed trend indicated that the overall nitrogen removal in the low- dissolved-oxygen SND process is more susceptible to the limitation in nitrification. This susceptibility to nitrification limitation is also consistent with the observed trend of higher nitrogen removal at longer SRTs. The chemoautotrophic nitrifiers are typically slow growers that require a longer SRT to develop properly in the bioreactors (Grady et al., 1999), particularly under the low- dissolved-oxygen environments of SND processes.

However, the time-averaged dissolved oxygen concentration is not a perfect parameter to describe the effects of cyclic aeration. For example, comparing runs 4 and 7 of the same SRT (10 days), the former had a slightly lower time-averaged dissolved oxygen concentration (0.53 mg/L) than the latter (0.6 mg/L), but achieved an appreciably higher nitrogen removal (86 versus 74%). As shown in Table 2, the less ideal nitrogen removal in run 7 resulted from the incomplete denitrification, giving much higher NO^sub 2^^sup -^-N and NO^sub 3^^sup -^-N concentrations in the effluent (4.0 and 5.4 mg/L, respectively). More effective denitrification was obtained in the cyclically aerated systems wiuh larger differences between the higher dissolved oxygen concentration (HDO) used during the 1-hour aeration period and the lower dissolved oxygen concentration (LDO) in the subsequent 30-minute period. This can be explained as follows.

The effectiveness of denitrification in SND is affected by how the nitrification-generated NO^sub x^^sup -^ (NO^sub 2^^sup -^ and NO^sub 3^^sup -^) becomes available to the microorganisms in the anoxic zone of floes. This availability may occur in two mechanisms- (1) diffusion of NO^sub x^^sup -^ from the outer aerobic zone of floes to the inner anoxic zone and (2) change of the aerobic zone to anoxic zone as the dissolved oxygen concentration drops from HDO to LDO under cyclic aeration. The latter mechanism is more effective, because it does not require the slow diffusion of NO^sub x^^sup -^. The latter mechanism can be promoted by larger differences between HDO and LDO, because larger fractions of floe volume are involved in the dynamic swing between aerobic and anoxic conditions.

Nitrite Shunt. Nitrite shunt is known to require lower COD to drive the nitrate-bypassed denitrification. The COD/NO^sub x^^sup – ^-N ratio required for complete denitrification of nitrate was 5.0 to 6.0 (w/w) (Tarn et al., 1992), while complete denitrification in a sequencing batch reactor with nitrite shunt occurring was achieved at a much lower ratio of 2.9 (Yu et al., 2000). Therefore, the low- dissolved-oxygen SND process may be particularly beneficial for wastewater with low carbon-to-nitrogen ratios. More importantly, the nitrite shunt pathway was also reported to require less aeration energy (Yoo et al., 1998). As shown in Figure 2a for run 4, NO^sub 2^^sup -^-N constituted a rather high fraction (approximately 36%) of the total NO^sub x^^sup -^-N. This observation was consistent with the nitrite shunt reported in low-dissolved-oxygen systems (Gee and Kim, 2004; Yoo et al., 1998; Yu et al., 2000; Zeng et al., 2003). The extent of nitrite shunt occurrence was expected to correlate with the ratio of NO^sub 2^^sup -^-N/ NO^sub 3^^sup -^-N, which would be larger at a higher extent of nitrite shunt (because of the bypassed involvement of NO^sub 3^^sup -^). Accordingly, the N0^sub 2^^sup -^-N/NO^sub 3^^sup -^-N ratio was plotted against SRT and time-averaged dissolved oxygen concentration (DO) (Figure 4a) and against HDO and LDO (Figure 4b) in three-dimensional bubble plots, where the bubble size corresponds to the magnitude of the NO^sub 2^^sup -^-N/NO^sub 3^^sup -^-N ratio. There was no clear effect of SRT on nitrite shunt, although the observation was inconclusive because of limited data (only 3 SRTs studied). As for the effects of DO, according to the results from runs 5 to 7 with an SRT of 10 days, LDO of 0.2 mg/L, and different HDO (0.4, 0.6, and 0.8 mg/L), the ratio clearly decreased with increasing DO (or HDO). The trend, however, did not apply to the experiments conducted under different aeration modes; for example, for the two runs with an SRT of 5 days (runs 1 and 3), the ratio was larger under constant aeration, despite the larger DO used (Figure 4a). Therefore, DO is not an ideal parameter for predicting the occurrence of nitrite shunt. Instead, the HDO and LDO used should be considered, as suggested by the more consistent trends in Figure 4b. The ratio appeared to increase with decreasing HDO and increasing LDO, and the dependency on HDO was more pronounced. The observations could be explained as follows.

Nitrite accumulation could result from disrupted nitrification or, if nitrification was complete, unbalanced denitrification. The accumulation resulting from disrupted nitrification would occur when ammonia oxidation (to nitrite) was faster than nitrite oxidation (to nitrate). This might result from the higher sensitivity of nitrite- oxidizing species to low dissolved oxygen concentrations than the ammonia-oxidizing species (Ju and Nallagatla, 2003; Yu et al., 2000). On the other hand, the nitrite accumulation resulting from unbalanced denitrification would appear when nitrite reduction (eventually to N^sub 2^) was slower than nitrate reduction (to nitrite). The unbalanced denitrification might be a result of the higher susceptibility of nitrite reductases to oxygen inhibition/ repression than nitrate reductases (Chayabutra and Ju, 2000; Drury et al., 1991). Accordingly, nitrite accumulation resulting from disrupted nitrification would be more significant at a lower HDO, while that resulting from unbalanced denitrification would be more significant at a higher LDO. These expectations were reflected in the trends observed in Figure 4b, and the more pronounced dependency on HDO suggested that the nitrite shunt took place mainly because of the disrupted nitrification at low-dissolved-oxygen conditions.

Biophosphorus Sludge Establishment. It is generally agreed that, in biological nutrient removal processes, PAOs break down intracellular polyphosphates in the anaerobic stage, to obtain the energy for taking up readily biodegradable organic substrates and storing them as polyhydroxyalkanoates (PHAs). The phosphate produced from the polyphosphate hydrolysis is released to the water, causing an increase in phosphate concentration. In the subsequent aerobic stage, PAOs oxidize the stored PHAs to generate energy for growth, glycogen synthesis, and phosphate uptake. This causes removal of phosphate from the water, while replenishing the intracellular polyphosphate pool depleted during the previous anaerobic stage.

In the low-dissolved-oxygen systems maintained under cyclic aeration in this study, a portion of the population/volume inside each floe also experienced the anaerobic and aerobic cycles. The observed profile of phosphate concentration, shown in Figure 2b, suggested the establishment of PAOs under such conditions. The effects of operating conditions on phosphorus removal in the systems evaluated in this study are shown in Figure 5. Several observations can be made. First, the run with constant aeration (dissolved oxygen concentration = 0.8 mg/L) clearly gave the lowest phosphorus removal, confirming the beneficial effects of cyclic aeration in enhanced biophosphorus removal. Second, for the three runs of SRT = 10 days and LDO = 0.2 mg/L, phosphorus removal increased with increasing DO (or HDO). Thus, larger differences between HDO and LDO in the cyclic aeration had potentially positive effects on phosphorus removal. Third, phosphorus removal was low (approximately 12%) in run 2 (Tables 1 to 3), with SRT = 20 days, LDO = 0 mg/L, and HDO = 2 mg/L. Two factors might have effected the low phosphorus removal. One was the long SRT (20 days) involved, although the effect of SRT was inconclusive, because run 2 was the only nonbulking system attained at this long SRT. The other factor was the high HDO (2 mg/L) involved. Development of PAOs requires the existence of cyclic aerobic and anaerobic states. In run 2, active nitrification during the 1-hour aeration at HDO = 2 mg/L would yield appreciable amounts of nitrate and/or nitrite, which sustained longer periods of anoxic denitrification and, thus, allowed shorter anaerobic periods and smaller anaerobic cores in the floes during the 0.5-hour LDO operation. Such conditions would be less favorable for enhanced biophosphorus removal (but achieving effective nitrogen removal).

Plant Case Studies. The above findings in laboratory low- dissolved-oxygen SND systems with alternating aeration between HDO and LDO phases are largely consistent with the phenomena observed in full-scale wastewater treatment plants that incorporated alternating low-dissolved-oxygen aeration in their operation. Some case studies are presented in the following sections.

Big Bear, California. Big Bear Area Regional Wastewater Agency in California operates an oxidation ditch plant with a design flow of 12 000 m^sup 3^/d (3.2 mgd). Of the three oxidation ditches at the facility, only two are normally operated in parallel. Each ditch has a volume of 6000 m^sup 3^ (1.6 mil. gal) and uses brush aerators (total 134 000 W [180 hp] installed aeration capacity in each ditch) for aeration and mixing. The plant was designed as a conventional nitrification plant based on ammonia removal requirements in the past. As the effluent requirement changed to 10.0 mg/L total inorganic nitrogen (TIN = ammonia-nitrogen + nitrite-nitrogen + nitrate-nitrogen), the plant opted for the cyclic on/off low- dissolvedoxygen aeration to introduce simultaneous denitrification within the ditches. Initially, during a 3-month trial period in the summer of 2000, the brush aerators were controlled in on/off mode, with dissolved oxygen concentrations in the range 0.2 to 0.8 mg/L during the “on” period. The modified operation generated effluent TIN values below 2.0 mg/L, while the ammonia concentrations were maintained below 0.5 mg/L. This clearly indicated that effective nitrification was maintained, while simultaneous denitrification was introduced using the low-dissolved-oxygen SND technology. Over 30% of aeration energy savings were achieved.

In this case, the above aeration control was based on the online monitored fluorescence profile of intracellular NAD(P)H (commercially known as the SymBio technology, Enviroquip, Inc., Austin, Texas). The NAD(P)H (i.e., NADH + NADPH) are the reduced forms of the coenzymes nicotinamide adenine dinucleotides. The NAD(P)H fluorescence intensity changes significantly when the heterotrophic microorganisms in the sludge experience “local” environmental changes that cause them to switch their respiratory mechanisms among aerobic respiration, anoxic denitrification, and anaerobic fermentation. More detailed descriptions are available elsewhere for the effects of aeration on the NAD(P)H profile in wastewater treatment processes (Arunachalam et al., 2005; Ju et al., 1995) and, particularly, in low-dissolved-oxygen SND systems (Huang and Ju, 2007).

Subsequently, in March 2001, variable frequency drives were installed on the brush aerators for better aeration control. The NAD(P)H fluorescence was monitored in the biomass, and the speed of the aerators was regulated to match the oxygen demand and vary the dissolved oxygen concentration between HDO (<0.8 mg/L) and LDO (>0.2 mg/L). Effluent nitrogen concentration results from this automatic operation during 2001 to 2003 are provided in Figure 6. Table 4 provides the average plant effluent results during this period. In addition, the plant showed enhanced biophosphorus removal, as observed in the laboratory systems and explained earlier. Typical influent total phosphorus levels were 7 mg/L, and the effluent total phosphorus averaged approximately 1 mg/L.

Rochelle, Illinois. This 18 400-m^sup 3^/d (4.87 -mgd) plant, operated by Rochelle Municipal Utilities, uses a parallel operation between four plug-flow reactors, each using a two-pass system. Only two reactors are typically used at a given time. Fine-bubble diffusers coupled with centrifugal, multistage blowers are used for aeration. The plant treats a combination of industrial (food processing) and domestic wastewater. Currently, it is required to perform only nitrification and not denitrification. The city, however, decided to install the SymBio system in 2001 at this facility, to maximize the energy savings (and to use the NAD(P)H measurement for monitoring the organic loading fluctuations from a food-processing industry). Further, a denitrification requirement is expected in the future. Because of the industrial contribution, the influent TKN loading is relatively high, in the range 50 to 60 mg/ L, and, correspondingly, the effluent nitrates were high before the introduction of the low-dissolved-oxygen SND process. Simultaneous denitrification (potentially more than 70%) in the plug-flow reactors was achieved after the installation. The effluent TKN results are shown in Figure 7. The overall plant effluent results for 2001 to 2003 are given in Table 5. The plant, before using SND, had settling issues during summer months, as a result of denitrification in the clarifiers. Because of fluctuating loadings, the overaeration caused the pin-floc phenomenon, at times. This also had negative effect on solids separation. One additional benefit with the low-dissolved-oxygen SND operation for this facility has been the improvement in sludge settling. As indicated in Table 5, the SVl, based on 30-minute settling tests, was maintained at approximately 130. No filamentous bulking was observed. A significant increase in phosphorus removal resulting from the low- dissolved-oxygen SND operation was also observed in this plant. Phosphorous removal efficiency improved by 43%, with the influent total phosphorus levels in the range 55 to 60 mg/L, as a result of the food-processing-industry waste contribution (data not shown).

Conclusions

For the laboratory low-dissolved-oxygen SND studies, the systems with a 5-day SRT (runs 1 and 3) exhibited poor nitrification, presumably as a result of the slow growth of nitrifiers at low dissolved oxygen concentrations. Compared with constant low- dissolved-oxygen aeration, cyclic aeration, when operated with proper combinations of HDO and LDO, achieved more effective nitrogen and phosphorus removal. For nitrogen removal, the advantage of cyclic aeration presumably resulted from the more readily available nitrate and nitrite (generated by nitrification during the HDO period) for denitrification (during the LDO period). Under constant low-dissolved-oxygen aeration, denitrification would rely on the slow diffusion of nitrate and nitrite from the outer nitrifying zone of the floes into the inner denitrifying zone. Nonetheless, nitrogen removal in the systems investigated was more susceptible to nitrification limitation than denitrification limitation. Therefore, the HDO used should be kept reasonably high (0.8 mg/L or higher) or the HDO period prolonged, to promote adequate nitrification. Nitrite shunt was also observed in low-dissolved-oxygen SND systems, with higher ratios of NO^sub 2^^sup -^/NO^sub 3^^sup -^in systems of lower HDO (and, to less dependency, higher LDO). The results suggested that the nitrite shunt took place mainly because of the disrupted nitrification at low dissolved oxygen concentrations. For phosphorus removal, the advantage of cyclic aeration came from the development of higher polyphosphate-accumulating populations, as indicated by the higher phosphorus contents in the sludge solids established in the systems under cyclic aeration. The above findings in the laboratory systems were largely consistent with the observations in full-scale wastewater treatment plants, as indicated in the two case studies. The feasibility of simultaneous nitrification, denitrification, and enhanced phosphorus removal in single-tank, low-dissolved-oxygen, cyclically aerated systems was clearly demonstrated in both laboratory-scale systems and full- scale plants.

Credits

This research was supported by Enviroquip, Inc. (Austin, Texas). The authors are grateful for the technical assistance of Shuyan Qiu, Nicholas J. Hamilton, Andrew S. Lash, Kristen M. Dudak, and Elizabeth A. Amaddio.

Submitted for publication April 6, 2006; revised manuscript submitted November 13, 2006; accepted for publication November 28, 2006.

The deadline to submit Discussions of this paper is November 15, 2007.

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Lu-Kwang Ju1*, Lin Huang1, Hiren Trivedi2 1 Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio.

2 Enviroquip, Inc., Austin, Texas.

* Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325-3906; e-mail: Ju@uakron.edu.

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