Anoxic Process: An

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Ind. Eng. Chem. Res. 2009, 48, 3526–3532

Full-Scale Application of the Alternating Oxic/Anoxic Process: An Overview Paolo Nardelli,† Giovanni Gatti,† Anna Laura Eusebi,‡ Paolo Battistoni,*,‡ and Franco Cecchi§ SOIS Autonomous ProVince of Trento, Piazza Dante 15, 38100 Trento, Italy, Institute of Hydraulics and Transportation Infrastructures, Engineering Faculty, Marche Polytechnical UniVersity, Via Brecce Bianche, 60131 Ancona, Italy, and Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15, Ca` Vignal 37134 Verona, Italy

This article describes the results obtained in terms of nitrogen removal, energy savings, and waste sludge production upon application of an alternate cycle process and a remote control system in real wastewater treatment plants (WWTPs). The experimentation, which lasted for eight months, was performed in three extended oxidation plants (denoted WWTP1, WWTP2, and WWTP3) characterized by different structural designs and sewerage influent macropollutant concentrations. The alternate cycle (AC) process provided good total nitrogen biological removal. The results obtained were due to the excellent reduction of nitrates. In fact, comparison with the pre-AC period showed an average decrease of NO3-N of about 64% in WWTP1, 62% in WWTP2, and 33% in WWTP3. The behavior of all WWTPs can be rationalized according to a nitrogen loading rate (NLR) approach, showing that its increment determined a reduction of the length of the anoxic phase to allow for ammonia oxidation. All of these conditions assured the satisfaction of the limit on total nitrogen in effluent established in directive EC 91/271. The energy savings (13-26%) was always observed to be related to the time spent in the anoxic phase during which the blowers were switched off and the mixers were used for mixed liquor suspension and nitrates denitrification. Moreover, it was demonstrated, as a new aspect, that application of the AC process was able to reduce the waste activated sludge by a biomass stressing during the anoxic conditions. In addition, in WWTP3 where the influent mass loading and effluent suspended solid removal mainly were controlled, a consistent specific sludge reduction (up to 47%) was observed. Introduction A recent survey by the Italian Environmental Protection Agency1 described the state of municipal wastewater treatment plants (WWTPs), in terms of characteristics, number, and location. It showed that there are about 15 000 WWTPs and most of them have a design treatment capacity lower than 10 000 population equivalents (PEs). Moreover, they generally perform only biological carbon and nitrogen removal. Because of the typical Italian territorial morphology, most WWTPs are located in hilly or mountainous areas. This situation is typical also for the Autonomous Province of Trento (500 000 residential inhabitants), a tourist area located in northern Italy. This area is characterized by the presence of 74 WWTPs, 43 of which have treatment capacities lower than 10 000 PEs, that are spread on a territory of some 6200 km2. Today, the national standard for effluent quality requires a limit of 15 mg of total nitrogen (TN) per liter for WWTPs with sizes between 10 000 and 100 000 PEs, and in Trento Province, a maximum concentration of 3 mg of ammonia (NH4-N) per liter is required in the effluent for temperatures higher than 15 °C. To achieve such a result, an alternate cycle (AC) process application was implemented on several WWTPs. This is a continuous process performed through the automatic alternation of oxic and anoxic phases in the same stirred reactor. The process is characterized by an aeration phase, automatically controlled on the basis of dissolved oxygen and the oxidation reduction potential to perform ammonia nitrification, and an anoxic phase for nitrate reduction,2,3 managed on the basis of the oxidation/reduction potential (ORP) signal. The AC process * To whom correspondence should be addressed. E-mail: [email protected]. † SOIS Autonomous Province of Trento. ‡ Marche Polytechnical University. § University of Verona.

was first validated a small WWTP (700 PE), and a simplified mathematical model was proposed for its design.4 Then, the possibility of completely reclaiming the existing structures and the feasibility of automatic control applications were demonstrated.5 Recently, a performance comparison between the AC process and conventional processes verified the energy savings of the former.6 Moreover, the possibility of upgrading a 17 000 PE WWTP with minor costs compared to anaerobic-anoxic-oxic (A2O) and University of Cape Town (UCT) processes was demonstrated.7 Finally, laboratory experimental results showed that an energy decoupling phenomenon in the AC process can also reduce the production of waste activated sludge.7 This article describes the results of three full-scale WWTPs in which the AC process was applied. A comparison in terms of nutrients removal, energy savings, and sludge production is reported. Materials and Methods Full-Scale Plants. The study was carried out in three fullscale municipal wastewater treatment plants denoted as WWTP1, WWTP2, and WWTP3. The design capacity and flow rate for each plant are summarized in Table 1. Existing Treatment Units and Installed Power. The three plants considered in this study were originally designed to oxidize carbon and nitrogen.8 Square, rectangular, and ringlike Table 1. Design Treatment Capacity and Influent Flow Ratea

WWTP1 WWTP2 WWTP3

design capacity (PE)

average influent flow rate (m3 day-1)

4500 5200 10000

1440 1456 2800

a Calculated using a COD specific mass loading of 120 mg of COD PE-1 day-1.

10.1021/ie8014796 CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3527 Table 2. Biological Reactors and Dimensional Parameters parameter

units

WWTP1

WWTP2

WWTP3

2a 780 rectangular two CSTRs in series 173 354 0.150

2 980 rectangular two CSTRs in series 98 153 0.260

3.8

3.5

0.42

0.40

Biological Unit bioreactor number total volume geometry type Vsp (design) Vsp (actual) F/M (actual) MLSS

2 1000 ringlike three CSTRs in series L per capita 222 L per capita 189 kg of COD 0.270 MLVSS-1 day-1 kg m-3 5.7 m3

Secondary Clarifier overflow loading m3 m-2 h-1

0.28

a

One tank used for aerobic sludge stabilization in the nontourist season. Table 3. Electromechanics and Probes Installed WWTP1 total installed power volumetric compressor type 1 type 2 mixer OD probes ORP probes

kW number kWindividual number kWindividual number kWindividual W m-3 number number

150 2 30 3 15 6 2.5 15 2 2

WWTP2 100 2 12.5 1 18.5 4 2.5 13 2 2

WWTP3 150 2 30 1 15 4 2.5 10 2 2

geometries were used for the bioreactors. The reactor characteristics were maintained for the AC application (Table 2). The retrofitting of WWTPs utilized all of the existing units and required only limited electromechanical modifications. Upgrading of the plants was achieved by installing online dissolved oxygen (DO) and ORP probes for process control and using submerged mixers (Table 3). Basically, a small number of submerged mixers and DO-ORP probes were put in each biological reactor. As a general rule, when the WWTP had two parallel biological tanks, two DO and ORP probes were installed in the middle of the first tank, and two were installed at the end of the second tank. The total and installed power for the electromechanics of each biological reactor are reported in Table 3. Influent Characterization. The main influent characteristics, seasonal variability, and extraordinary incoming peaks during AC application are listed in Tables 4 and 5 (as average values). WWTP2 and WWTP3 were fed with wastewater from a separate sewer systems, whereas WWTP1 was fed with mixed wastewater. For the conditions of the partially separate sewer system, in WWTP1, the influent characterization (Table 4) was performed both before and during AC application. A specific reactor volume (Vsp) of about 200 L per population equivalent (PE) for biological oxygen demand (BOD) oxidation and ammonia nitrification was employed in most of the plants for small communities (treatment capacities of less than 10 000 PE). It was calculated that the plants evaluated were provided with 200 L PE-1. The secondary clarifier overflow loading was 0.42 m3 m-2 h-1 for WWTP2 and 0.40 m3 m-2 h-1 for WWTP3. Process Monitoring and Control: Online and LaboratoryAnalyzed Parameters and Control System Architecture. According to the AC process, the durations of the alternating oxic and anoxic phases were automatically determined by a

patented control device9 that performed the online processing of DO and ORP signals. The percentages of nitrogen removal for the different forms of nitrogen are related to the durations of the aerobic and anoxic phases. The ORP and DO profiles could be not reliably measured when the plant was overloaded or overaerated. In these situations, in fact, the optimal conditions could be not determined. In these cases, two other controllers entered into operation to determine the phase length: the setpoint basis and the time-length (duration) basis. Also, these two control methods determined the flexibility of the process. The data-processing software was purposely engineered to evaluate the reliability of the control system. The software processed the database of analog and digital online signals and gave basically two types of information: (1) the sduration of the aerobic and anoxic phases and (2) the end reason that led the automatic control device to switch from the aerobic to the anoxic phase and vice versa (set-point maximum time, maximum DO and/or ORP, optimal conditions) (Figure 1). The chemical-physical characteristics of the main streams were determined from daily average samples according to standard methods.10 Nitrogen Mass Balance. The nitrogen mass balance was used to objectively evaluate the treatment potential of the AC process in terms of nitrogen removal from wastewater. The balance was calculated according to eq 1, and the nitrification and denitrification performances were studied according to four parameters: the nitrifying efficiency with respect to the total incoming nitrogen (En, in eq 2); the nitrifying efficiency with respect to the amount of the only form of nitrogen that can be nitrified (Enn, in eq 3); the nitrogen removal efficiency with respect to the total incoming nitrogen (Ed, in eq 4); and the nitrogen removal efficiency with respect to the denitrified nitrogen, NOx-N (Edd, in eq 5). LTNden)LTNin - LTNqw - LTNout

(1)

En (%) ) (LTNnit ⁄ LTNin) × 100 ) [(LTNden + LNOx-Nout LNOx-Nin) ⁄ LTNin] × 100 (2) Enn (%) ) [LTNnit ⁄ (LTKNin + LTKNras - LTNqw + LNnb org out)] × 100 (3) Ed (%) ) (LTNden ⁄ LTNin) × 100

(4)

Edd (%) ) [LTNden ⁄ (LTNden+LNOx-Nout)] × 100

(5)

See the Nomenclature section for the definitions of the variables used in these equations. Results and Discussion Influent/Effluent Characteristics and Performances of the Treatment. Operations of the three plants were started in May 2007. Generally, in addition to the loading fluctuations typical of domestic discharges, heavy variations of the incoming nitrogen often depend on the following factors: (1) relevant tourist flows, in both winter and summer seasons; (2) frequent extraordinary discharges in the sewer system from industries, which involves intense and random peaks; and (3) both cases 1 and 2 occurring in the same catchment area. After eight months of experimental work, remarkable seasonal flow rate variations for WWTP2 and WWTP3 were observed: the influent mass load doubled in the tourist season, especially in August (Figure 2, Table 4). In WWTP1, characterized by a partially separated sewer system, a notable increase of the incoming flow rate related to new sewer connections for domestic and industrial wastewater was observed. The industry discharges resulted in

3528 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 4. Average Influent Characteristics of the Selected Plants WWTP1 season: actual treatment capacitya Qin COD TN TP TSS LCOD LTN LTP COD/TN TKN/NH4-N T a

WWTP2

process: PE

pre-AC 2360

AC 4670

m3 day-1 mg L-1 mg L-1 mg L-1 mg L-1 kg day-1 Kg day-1 Kg day-1

360 769 72 8 302 283 30 2 11 1 9-20.5

690 810 125 6 280 560 89 4.4 6.7 2.2 10-22

°C

WWTP3

nontourist AC 1890

tourist AC 1970

nontourist AC 5000

tourist AC 6420

340 580 50 5 290 227 19 1 13 1 15-18

560 500 48 6 220 236 24 2 11 2 10-20

1450 410 25 4 326 600 37 6

1600 455 30 4 337 770 47 7 16 2

11

16

Calculated using a COD-specific mass loading of 120 mg of COD PE-1 day-1.

Table 5. Main Influent Features during Application of the AC Process

WWTP1 WWTP2 WWTP3

influent TN: gross range (mg L-1)

seasonal fluctuations

random irregular peaks from industries

72–200 14–164 13–41

no yes yes

yes yes no

a strong variation in the ratio of organic nitrogen to total nitrogen (from 20% to 55%; Figure 2), whereas the real potentiality increased from 2000 PEs to a constant value of 6000 PEs (Table 6). Considering the actual population equivalents (Table 4), the specific volume of the biological reactor was higher than the design volume for WWTP2 and WWTP3 (354 and 153 L per capita, respectively), whereas it was 189 L per capita for WWTP1. Moreover, further problems could come from the temperature of the liquor, which can remain in the range 8-10 °C for a large part of the winter season, often corresponding to

the period of major skiing tourist arrivals. In addition, some possible critical conditions for sludge and sedimentation of filamentous bacteria occurred during the winter season, because elevated values of overflow loading in the secondary clarifier were calculated (0.42 m3 m-2 h-1 for WWTP2 and 0.40 m3 m-2 h-1 for WWTP3), and also, all three WWTPs were equipped with a secondary clarifier with a traveling bridge. It was observed, in all three plants, that the chemical oxygen demand (COD)/TN ratio was always higher than 6.7, a value sufficient to ensure the denitrification process,11 regardless of seasonal fluctuations. Despite the marked increase in treatment capacity for the three plants, a clear decrease in effluent nitrogen was observed after the retrofitting (Table 7). In particular, application of the AC process resulted in an increase in the reduction of nitrates compared to the conventional process: the removal increase was 64% in WWTP1, 62% in WWTP2, and 33% in WWTP3. The effluent total nitrogen was lower than 10 mg of TN L-1 in all plants (Figure 3). The ammonia concentration in the WWTP1 effluent, during total oxidation and AC process, was dependent on the influent characteristics. Indeed, before retrofitting of the plant, a typical concentration of 72 mg of TN L-1 was observed (Table 5), while after the AC installation, a constant value of 125 mg of TN L-1 and random peaks up to 200 mg of TN L-1 were measured (Figure 4). To better understand the WWTP1 performance, the nitrogen loading rate (kg m-3 day-1) was calculated (eq 6) NLR )

Figure 1. Level AC control automation.

Figure 2. Incoming flow rates in WWTP2 and WWTP3.

LTNin TNu ) V Vsp

(6)

where NLR is the nitrogen loading rate, LTNin is the load of total nitrogen in the influent, TNu is the total nitrogen unit loading factor, V is the reaction volume of the biological tank, and Vsp is the specific per capita volume of the activated sludge tank. The results showed that, during application of the AC process, the NLR increased from 0.030 to 0.240 kg m-3 day-1 because of the TN overload (Figure 5). This effect, added to the low temperature in the winter season (T < 15 °C), determined a critical performance of the nitrification phase: the effluent ammonia reached concentration values up to 6 mg of NH4-N L-1 (Figure 5). The percentile analysis of the ammonia effluent, neglecting the period with temperatures below 15 °C and the startup phase (Figure 5), highlighted values up to the 95th percentile below 3 mg of N L-1 (Figure 6).

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3529 Table 6. Real Treatment Capacity (PE) of Studied Plants Referred to the Incoming COD and TN WWTP1

WWTP2

WWTP3

period

month

COD basis

TN basis

COD basis

TN basis

COD basis

TN basis

pre-AC

01/2007 02/2007 03/2007 04/2007

1680 1740 3280 3130

1750 1170 3360 3840

3050 2050 2480 1200

2230 2070 1980 1140

3290 3260 5550 4570

730 3040 2590 5850

AC

05/2007 06/2007 07/2007 08/2007 09/2007 10/2007 11/2007 12/2007

4070 5110 4370 5650 7891 5356 4554 6256

10150 10150 7350 7520 6695 6022 6725 7182

2510 2170 3550 3360 2636 1436 941 1220

2020 1560 2200 4670 1648 1170 795 969

5310 4910 7690 9850 6110 4640 4382 8241

3380 2850 4120 6180 3177 2361 2673 3715

Table 7. Effluent Nitrogen Concentrations (mg L-1) before and during Application of the AC Process WWTP1

WWTP2

WWTP3

average values

pre-AC

AC

pre-AC

AC

pre-AC

AC

NH4-N NO3-N NO2-N TN

4.3 11.0 0.4 17.0

2.0 3.9 0.1 7.3

1.1 19.0 0.4 22.0

0.9 7.0 0.1 10.0

1.0 6.0 0.1 8.4

1.9 4.0 0.1 7.1

Also, for WWTP3, the AC process allowed for successful denitrification but resulted in an increase in effluent ammonia nitrogen (from 1.1 mg NH4-N L-1 pre-AC to 1.9 mg NH4-N L-1, on average; Table 7). The problems for nitrification were mainly verified during the tourist season (Table 4); this behavior was related to a low concentration of biomass in the biological unit, always lower than 3.5 kg m-3. The operative value of mixed liquor suspended solids (MLSS) was limited by the overflow loading of the secondary clarifier (0.40 m3 m-2 h-1 at the average influent flow rate; Table 2).

Nitrogen Mass Balance and Process Control Automation. The nitrogen mass balances for WWTP1 and WWTP2 highlighted optimal removal efficiencies for the AC process compared to the conventional process (Table 8). The elevated removal performance for all forms of nitrogen was as high as 97% in WWTP1 for Edd and Enn. In WWTP2, ammonia nitrification remained at constant levels (En 84% and 81% for pre-AC and AC, respectively), whereas denitrification showed a meaningful augmentation (Edd changed from 61% to 81%). The best results could be obtained only by

Figure 5. NLR and NH4-N effluent of WWTP1.

Figure 3. Incoming flow rate in WWTP1 and ratio of Norg to TN.

Figure 6. NH4-N Effluent Percentage of WWTP1. Table 8. Nitrogen Mass Balance before and during Application of the AC Process WWTP1

Figure 4. Influent TN of WWTP1.

Ed (%) En (%) Edd (%) Enn (%)

WWTP2

WWTP3

pre-AC

AC

pre-AC

AC

pre-AC

AC

62 71 87 81

91 94 97 97

52 84 61 95

67 81 81 94

62 77 78 92

60 74 79 85

3530 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009

Figure 7. Anoxic and oxic times for WWTP1, WWTP2, and WWTP3.

Figure 8. Successful detection of anoxic time, optimal conditions and lengths in WWTP1, WWTP2, and WWTP3.

increasing the yields of the denitrification process because excellent performance was achieved for the nitrification considering the nitrogen removed with waste activated sludge (Enn 94%). For WWTP3, no improvements in nitrogen removal were observed because the calculations for the AC period included the TN seasonal overloading (Table 8). The nitrogen removal performances of the three plants depended on the durations of the aerobic and anoxic phases. The durations of nitrification and denitrification related to the nitrogen loading rate clearly defined two different conditions (Figure 7): a low NLR zone and a high NLR zone. Indeed, for WWTP2 and WWTP3, the aerobic duration increased from 350 to 860 min day-1 according to the NLR variation from 0.010 to 0.080 kg of TN m-3 day-1. Of course, the previous behavior produced a decrement of the anoxic-phase duration from 1090 to 580 min day-1. In this zone, the lowest NLR caused an overaeration condition influencing the denitrification time, as was demonstrated in previous studies.12 Even though the reduction of the duration of the anoxic phase and the NLR rised 0.080 kg of TN m-3 day-1, an increment of the anoxic optimal conditions (OCs) from 47% to 96% made the optimization of the process possible (Figure 8).

On the other hand, the high-NLR zone (from 0.130 to 0.240 kg of TN m-3 day-1) was characteristic of WWTP1. Under these operating conditions (Figure 7), similar durations (from about 500 to about 800 min day-1) for the oxic and anoxic phases were observed. Obviously, the NLR increase caused a net decrement of the anoxic OCs to 7% (Figure 8), but the other level control automation assured a satisfactory denitrification performance. Electric Energy Consumption. In the AC process, the alternation of aerobic and anoxic phases in the same continuously fed reactor allows for the best exploitation of nitrogenbound oxygen and, as a consequence, enables the elimination of an internal recycle, to achieve an appreciable decrement of energy consumption. The actual power requirements of the three WWTPs were calculated by processing the total adsorbed power. A comparison of the total energy requirements during the pre-AC and AC periods (Table 9) showed that the energy savings derived from application of the AC process was 4200 kWh per month for WWTP2 and WWTP3, substantially the same in the two plants. Indeed, the reduction in energy consumption is strictly linked to the total energy requirements or the design treatment capacity of the WWTP. For these reasons, energy savings of 26% for WWTP2 and

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3531 -1

Table 9. Total Absorbed Power (kWh month ) during the Pre-AC and AC Periods

WWTP1 WWTP2 WWTP3

pre-AC

AC

reduction (%)

20 758 16 176 34 059

19 195 11 945 29 798

8 26 13

Table 10. Specific Energy Consumption (kWh day-1 PE-1) during the Pre-AC and AC Periods

WWTP1 WWTP2 WWTP3

pre-AC

AC

0.370 0.322 0.303

0.120 0.211 0.167

Table 11. Sludge Reduction in WWTP3 sludge production (t) pre-AC (MayDec 2004) pre-AC (MayDec 2005) pre-AC (MayDec 2006) AC (MayDec 2007)

(t of TS) LCOD (t)

specific sludge production (t of TS) (t of COD)-1

225

34.5

132

0.261

239

34.4

122

0.283

278

42.9

114

0.377

214

32.1

161

0.199

13% for WWTP3 can be observed, in agreement with literature values.6 For WWTP1, the energy saving was almost insignificant (only 8%) because of the elevated increase (74%) in the influent mass loading. Finally, the energy reduction in the three plants is clearer in terms of the specific total energy consumption (Table 10). Indeed, from the average value for the three plants of about 0.300 kWh day-1 PE-1 during the pre-AC process, upon AC application, the specific consumption became equal to 0.120 kWh day-1 PE-1 for WWTP1, to 0.211 kWh day-1 PE-1 for WWTP2, and 0.167 kWh day-1 PE-1 for WWTP3. Sludge Reduction. WWTP3 was used to verify the effect of the AC process on the production of waste activated sludge. Here, only civil wastewater (no liquid wastes or landfill leachate) was treated; moreover, the presence of a final filtration step prevented the escape of large amounts of suspended solids in the effluent. Therefore, complete characterizations of the influent and effluent loadings were available. In fact, every week, two daily averaged samples of the influent and effluent streams were taken and analyzed. Also, the total solids content of biosolids was known. All of these conditions made it possible to develop an accurate global mass balance. The results (Table 11) showed the dry and wet production of sludge, the influent COD mass loading, and the specific sludge production during the period of AC application compared with the same months of the three preceding years. The sludge production [as tons of total solids (TS)] during application of the AC process was reduced by about 7%, compared to the same period of 2004 and 2005 and by up to 33.5% compared to the same period of 2006. The decrease observed was similar considering both the dry and wet weights for the sludge. In addition, the decrease of the amount of waste sludge was greater considering the specific sludge production (tons of TSproduced vs tons of CODinfluent) (Table 11). Indeed, from an average value during all of the pre-AC periods of about 0.307 t of TS (t of COD)-1, the specific sludge production became equal to 0.199 t of TS (t of COD)-1 during the AC process with specific reductions of 24% compared to

the same period of 2004, 30% compared to 2005, and 47% compared to 2006. The alternating oxic and anoxic phases could create the ideal conditions for the energy decoupling13 of the biomass. Conclusion This article has investigated the upgrading of three WWTPs in which operation was changed from the conventional process to the alternate cycle process. The startup phase and eight months of different operating steady-state conditions were studied. The main conclusions of the study are as follows: (1) The AC process was able to optimize the biological nitrogen removal and to handle intense variations of influent nitrogen loadings, allowing for a stable quality of the effluent with an average total nitrogen concentration of less than 10 mg of N L-1. (2) A NLR of up to 0.240 kg of TN m-3 day-1 was tolerated by the elevated control level of the AC control device to ensure the optimum reduction of nitrate. (3) Energy savings in the range of 13-26% were observed comparing AC with pre-AC conditions at a constant mass loading of nitrogen. (4) Energy decoupling in the water line related to alternating oxic and anoxic phases resulted in a decrease of waste sludge production. A specific sludge reduction of up to 47% was observed. Nomenclature A2O ) anaerobic-anoxic-oxic (process) AC ) alternate cycle (process) BOD ) biological oxygen demand COD ) chemical oxygen demand CSTR ) continuously stirred tank reactors DO ) dissolved oxygen DOmax ) maximum DO set point DOmin ) minimum DO set point EC ) European Community F/M ) food-to-microorganism ratio LCOD ) chemical oxygen demand mass loading LNnb org out ) nonbiodegradable organic nitrogen mass loading in the effluent LNOx-Nin ) NOx-N mass loading in the plant influent LNOx-Nout ) NOx-N mass loading in the plant effluent LTKNin ) total Kjeldahl nitrogen mass loading in the influent LTKNras ) total Kjeldahl nitrogen mass loading in the return activated sludge LTN ) total nitrogen mass loading LTNden ) total denitrified nitrogen mass loading LTNin ) load of total nitrogen in the influent LTNin ) total nitrogen mass loading in the influent LTNnit ) total nitrified nitrogen mass loading LTNout ) total nitrogen mass loading in the effluent LTNqw ) total nitrogen mass loading in the waste activated sludge LTP ) total phosphorus mass loading MLSS ) mixed liquor suspended solids MLVSS ) mixed liquor volatile suspended solids NH4-N ) ammonia NLR ) nitrogen loading rate NO2-N ) nitrites NO3-N ) nitrates Norg ) organic nitrogen NOx-N ) nitrites and nitrates OCs ) optimal conditions

3532 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 ORP ) oxidation/reduction potential ORPmax ) maximum ORP set point ORPmin ) minimum ORP set point PE ) population equivalent Qin ) influent flow rate TKN ) total Kjeldahl nitrogen Tmax ) maximum duration of oxic or anoxic phase Tmin ) minimum duration of oxic or anoxic phase TN ) total nitrogen TNu ) total nitrogen unit loading factor TP ) total phosphorus TS ) total solids TSS ) total suspended solids UCT ) University of Cape Town (process) V ) reaction volume of the biological tank Vsp ) specific per capita volume of the activated sludge tank WWTP ) wastewater treatment plant

Literature Cited (1) Guida per l’adeguamento, miglioramento e razionalizzazione del serVizio di depurazione delle acque di scarico urbane (Guide to adapt, improVe and rationalize waste water urban serVice); Agenzia per la Protezione dell’Ambiente e per i servizi Tecnici (APAT; Italian Environment Protection and Technical Services Agency): Rome, Italy, 2005. (2) Hao, O.; Huang, J. Alternating aerobic-anoxic process for nitrogen removal: Process evaluation. Water EnViron. Res. 1996, 68, 83–9. (3) Charpentier, J.; Florenz, M.; David, G. Oxidation reduction potential (ORP) regulation: A way to optimize pollution removal and energy savings in low load activated sludge process. Water Sci. Technol. 1987, 19, 645– 655.

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ReceiVed for reView October 1, 2008 ReVised manuscript receiVed December 10, 2008 Accepted January 14, 2009 IE8014796