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Bioprocess Biosyst Eng (2011) 34:57–65 DOI 10.1007/s00449-010-0446-2

ORIGINAL PAPER

Reaction rate constants and mean population percentage for nitrifiers in an alternating oxidation ditch system I. D. Mantziaras • A. Katsiri

Received: 18 March 2010 / Accepted: 4 June 2010 / Published online: 7 July 2010 Ó Springer-Verlag 2010

Abstract This paper presents a methodology for the determination of reaction rate constants for nitrifying bacteria and their mean population percentage in biomass in an alternating oxidation ditch system. The method used is based on the growth rate equations of the ASM1 model (IWA) (Henze et al. in Activated sludge models ASM1, ASM2, ASM2d, and ASM3. IWA Scientific and Technical Report no. 9, IWA Publishing, London, UK, 2000) and the application of mass balance equations for nitrifiers and ammonium nitrogen in an operational cycle of the ditch system. The system consists of two ditches operating in four phases. Data from a large-scale oxidation ditch pilot plant with a total volume of 120 m3 within an experimental period of 8 months was used. Maximum specific growth rate for autotrophs (lA) and the half-saturation constant for ammonium nitrogen (KNH) were found to be 0.36 day-1 and 0.65 mgNH4–N/l, respectively. Additionally, the average population percentage of the nitrifiers in the biomass was estimated to be around 3%.

hc h Qin Qeff Qw tc tn td tc a rgn rn Rgn Rn bA YA lA KNH

Keywords Alternating oxidation ditch system Nitrogen mass balance Nitrifying bacteria Reaction rate constants Wastewater treatment

KOA Mn

List of symbols V Volume of each ditch, (m3)

Mgn SMin

I. D. Mantziaras (&) A. Katsiri (&) Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, Iroon Polytechniou 5, Zografou, 15773 Athens, Greece e-mail: [email protected] A. Katsiri e-mail: [email protected]

SMout SJM SNHin SNHout SNoin

Sludge retention time, (day) V/Q = hydraulic retention time, (day) Influent wastewater flow rate, (m3/day) Effluent wastewater flow rate, (m3/day) Waste sludge flow rate, (m3/day) Total time length of one cycle, (min or day) Nitrification time per tank, (min or day) Denitrification time per tank, (min or day) 2tn ? 2td tn/tc nitrification time fraction Growth rate of nitrifiers, (mg/l min) Nitrification rate, (mg/l min) Average growth rate of nitrifiers, (mg/l min) Average nitrification rate, (mg/l min) Autotrophic decay rate, (day-1) Yield constant for autotrophic organisms (nitrifiers), (gr cell COD formed/gr N oxidized) Maximum specific growth rate of autotrophs, (day-1) Half-saturation constant for NH4–N, (mg NH4–N/l Saturation constant for oxygen during aerobic growth of autotrophs, (mg O2/l) Mass of nitrified nitrogen (NH4–N converted to NOx–N), (mg/cycle) Mass of produced nitrifiers in the system, (mg/cycle) Total influent nitrogen (SMin = SNHin ? SNoin ? SNOxin), (mg/l) Total effluent nitrogen, (mg/l) Total Kjeldahl nitrogen (TKN = SNH ? SNo), (mg/l) NH4–N in the influent, (mg/l) NH4–N in the outlet, (mg/l) Organic nitrogen in the influent, (mg/l)

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58

SNoout Ms SNOxin SNOxout So Nc ixB VA Vout A Xvss k BODin YH kd Yobs

C/N

Bioprocess Biosyst Eng (2011) 34:57–65

Organic nitrogen in the effluent, (mg/l) Nitrogen used for synthesis, (mg/l) NOx-N in the influent, (mg/l) NOx-N in the effluent, (mg/l) Concentration of dissolved oxygen in the aeration ditch, (mg/l) Nitrification capacity, (mg/l) Nitrogen content of the biomass (grN/gMLVSS or grN/grCOD) Concentration of nitrifiers in each ditch, (mg/l) Concentration of nitrifiers in the effluent, (mg/l) Concentration of biomass, (mg/l) VA/Vvss = fraction of nitrifiers in the biomass Influent BOD concentration, (mg/l) Yield constant for heterotrophic organisms, (gr cell COD formed/gr COD oxidized) Decay rate constant for heterotrophic organisms (day-1) Observed yield constant for heterotrophic organisms including decay rate, (gr cell COD formed/gr COD oxidized) Ratio of carbon to nitrogen influent, (gr BOD/gr TKN)

Introduction In the last decade, biokinetic models such as the activated sludge models (ASM) [11] have been developed to simulate and better understand the processes that take place in a wastewater treatment system (WTS). Consequently, the sanitary engineer can better evaluate the system’s capabilities during its operation and make more accurate predictions of its future behaviour, which can subsequently lead to new approaches in design. To achieve this goal, kinetic and reaction rate parameters must be used in the models. Using values for these parameters from the literature is an option, but since every WTS has its own behaviour due to its mode of operation and environmental conditions, together with the different incoming wastewater characteristics, it is (most of the time) a necessity that these values must be determined. The oxidation ditch is a well-known variation of the extended aeration activated sludge system. It has been widely used, mainly in small and medium-sized communities, for secondary treatment of municipal wastewater. Nearly 40% of municipal wastewater treatment plants in the Netherlands are of the oxidation ditch type and more than 9,000 oxidation ditch facilities have also been constructed for municipal wastewater treatment in the USA [21]. In Greece, 75% of the wastewater treatment plants are

123

extended aeration activated sludge systems and the majority of them are oxidation ditches [23, 24]. A popular variation of the oxidation ditch system is the alternating ditch system, or phased isolation ditch system, originally developed in Denmark. The system is characterized by the use of two or three ditches, with or without a separate secondary settlement tank. Alternating ditches have many sub-types, i.e. the D-Ditch (two ditches without separate settlement tank), the DE-Ditch (two ditches with separate settlement tank), the VR-Ditch (two ditches, one inside the other, without separate settlement tank), the T-Ditch (three ditches without separate settlement tank) and the Bio-Denitro and Bio-Denipho process modifications [1–3, 5, 25]. In recent years, there has been a renewed interest in the use of oxidation ditches. Many researchers have investigated their potential efficiency in treating wastewater. New approaches on the design and operation of this system have been made to optimize the removal capabilities concerning carbon or nutrients, employing field strategies such as simultaneous nitrification–denitrification [8, 16, 19], new scheme configurations such as oxidation ditches with vertical circulation [27] or scheme modifications regarding aeration or sedimentation [4, 7, 14], modelling [15, 28] and microbiological analysis [18]. In spite of the renewed interest in oxidation ditch systems, there is lack of information on oxidation, nitrification and nitrogen removal kinetics in phased isolation systems. Experimentally derived data for kinetic constants are scarce and thus most of the values used in biokinetic models are assumed or borrowed from other types of activated sludge systems. This paper presents a methodology to estimate nitrification kinetics using performance results from the operation of a large pilot-scale alternating oxidation ditch with a simple configuration (type D-ditch). The D-ditch is a very stable system, achieving high effluent quality. Under controlled operation, it can easily perform nitrification and denitrification and thus achieve a high total nitrogen removal. Therefore, a better understanding of its behaviour can lead to optimization of operational parameters and to an important reduction in the operating costs. Methods The alternating oxidation ditch system The pilot plant is located in the Sanitary Engineering Research and Development Center (SERDC) and is part of the facilities of the Water and Wastewater Company of Athens (EYDAP) in Greece, treating municipal wastewater mainly from the northern suburbs of Athens. The pilot


system consists of two oxidation ditches, interconnected with a pipe of 10 cm in diameter (see Fig. 1). Each ditch has a water depth of 1.0 m, a volume of approximately 60.0 m3 and a typical oval ring configuration with a trapezoidal cross section. The ditches were designed and operated in such a way as to perform carbon oxidation, nitrification, settling and denitrification. They operate in four phases, functioning successively as aeration–nitrification basins, clarifiers and anoxic basins in 3-h cycles (180 min) as shown in Fig. 1. In the first phase A (60 min), sewage flows into the first ditch (OD1) where aeration is applied and carbon oxidation and nitrification occur. Wastewater circulates in the ditch with an average flow velocity of u = 0.30–0.35 m s-1 and then enters into the second ditch (OD2) through the connecting pipe. In OD2, settling occurs and treated effluent exits via the outlet weir. In the second phase B (30 min), sewage enters into the settled sludge layer of OD2 (occupying the lower 0.30–0.40 m of the ditch), forms a bottom, very slow moving current, (where settling occurs and denitrification is also expected) and exits via the outlet weir. During this phase, in OD1 there is no inlet or outlet flow and the mixed liquor is allowed to tranquilize and settle, which is complete after about 15 min. Phases C and D are mirrors of A and B, with the sewage entering into OD2 and OD1, respectively. The system does not involve sludge recirculation. Sludge wasting (as mixed liquor) is done directly from the interconnecting pipe downstream of the aerated ditch, by means of a discharge valve. Experimental procedures The experimental period lasted for 15 months and was divided into two main periods, characterized by dissolved oxygen levels in the aeration ditch. In the first period, low dissolved oxygen levels in the order of 1–1.5 mg O2/l were maintained in the tank, whilst in the second period, high dissolved oxygen levels were maintained in the order of 4.5–6 mg O2/l. The present work refers to the experimental period with high oxygen levels in the aeration ditch. During this period,

59

biomass is saturated with oxygen and thus no problems for the process of nitrification can appear due to the lack of oxygen. The duration of this experimental period was 8 months. Two horizontal brush aerators (rotors) having different oxygenation capacities are installed in each tank. The first rotor has a two-speed motor, whilst the second has a singlespeed motor. The actual oxygenation capacities of the two rotors were measured in situ and aeration curves were derived. The standard oxygen transfer rates (SOTR) were calculated as 8.32 and 6.34 kg O2 h-1 for the high and low speed of the first rotor, respectively, and 9.51 kg O2 h-1 for the second rotor [17]. Process performance characteristics, including temperature, pH, BOD5, COD, MLSS, MLVSS, SS, DSVI, TKN, NH4–N, NOx–N and alkalinity were measured daily. Analytical details on procedures and methods are given in [17]. Reaction rate constants for autotrophs and their mean population percentage in the system’s biomass were determined using mass balance equations for nitrifiers and ammonium nitrogen. Mass balance equations have been applied assuming a steady-state operational cycle of the alternating system. Process analysis Nitrification capacity Before applying the mass balance equations for this analysis, it is necessary to take a closer look at the fractions of total influent nitrogen in the wastewater that can be available for nitrification, i.e. some of it is required for synthesis of new biomass, some escapes with the effluent and the rest is available for nitrification. For a recirculation type of activated sludge process, van Haandel et al. [26] and Ekama et al. [6] introduced a concept regarding ammonia removal called nitrification capacity, which is defined as the concentration of influent nitrogen that is converted to nitrates. It is actually the difference between the ammonium nitrogen available for nitrification and the ammonium nitrogen concentration, which leaves the system in the effluent:

Fig. 1 Operational cycle of the oxidation ditch system: S settling; N nitrification; TR tranquilization, settling of mixed liquor and DN denitrification

123

60 out Nc ¼ TNin Ns SNout ; ðmg/lÞ O SNH


ð1Þ

where SMin is the total influent nitrogen, Ns is the nitrogen used for synthesis, and SNoout and SNHout are the organic and ammonium nitrogen in the effluent, respectively. This concept can also be applied to the alternating oxidation ditches, considering the whole process from inlet to outlet as a single system and having in mind that nitrification and denitrification take place sequentially. The application of the nitrification capacity concept to this alternating operation can be done by examining the dynamic mass balance over the course of one operational cycle. To simplify the analysis, the following assumptions and approximations to Eq. 1 have been made: (a)

The nitrate concentration in the influent, which is usually very low for domestic wastewaters [12, 20], is neglected and thus SMin = TKNin, where TKN is the total Kjeldahl nitrogen. (b) The concentration of the nitrifiers entering the system is assumed to be zero. (c) The degradable organic fraction of TKN nitrogen in the influent (SNoin) is finally converted to ammonium nitrogen (SNHin). This means, that all the degradable TKNin is assumed to be converted to ammonia (SNHin) and then used for synthesis and nitrification [9]. (d) Ns and SNoout are assumed to be subtracted from TKNin directly. SNoout is the inert organic nitrogen plus the degradable organic nitrogen that has not been assimilated into the biomass or participated in nitrification. If SNoout is not measured, an assumption of an average value of 2–2.5 mg/l can be made, as seen in Table 2 (later in this work), which presents the measured monthly average values during the experimental period. (e) If the outlet ammonia concentration (SNHout) remains below 1 mg/l, which is far less than the concentration of the incoming ammonia (40–50 mg/l), it is neglected; otherwise, it is taken into account. According to the above, Eq. 1 can be rewritten as follows: Nc ¼ TKNin NS SNout SNHout ; ðmg=lÞ ð2Þ O 0 ¼ TKNin , then and if TKNin NS SNout O 0 ð2aÞ Nc ¼ TKNin SNHout ; ðmg=lÞ where all the (TKNin)0 is in the form of ammonium nitrogen (SNH). To apply the nitrification capacity concept as expressed by Eqs. 1 through 2a, the nitrogen (Ns) used for synthesis must be determined as the effluent organic (SNoout) and the

123

ammonia nitrogen (SNHout). In this work, SNoout and SNHout were measured daily, as described in the following paragraph. To determine Ns, instead of assuming arbitrarily a percentage of the TKNin used for synthesis or using mass balance equations for nitrification and denitrification, respectively, which again is not preferable for systems where aerobic and anaerobic conditions alternate with time [13], another method expressed with Eq. 3 is used herein, based on the parameters on which Ns depends [13], [17]. These parameters are: (a) the nitrogen content of the biomass (ixB); (b) the observed yield constant (Yobs = TH/(1 ? kdHc)) for heterotrophic organisms representing the net rate of growth [22]; (c) the ratio of carbon utilized for influent nitrogen (C/N), where C = (BODin–BODout) and N = TKNin, C NS ¼ Yobs ixB N

ð3Þ

The relationship [Yobs(BODin–BODout)] represents the net production of the microorganisms. Multiplying with ixB and dividing by TKNin give the percentage of the influent nitrogen used for synthesis. Determination of the autotrophic decay rate bA and the fraction k of nitrifiers in the biomass Mass balance for the nitrifiers In a steady situation under a constant flow and load conditions, a mass balance for the nitrifiers around the whole alternating process, with cycle time length (tc), can be written as follows:

dX A 2V = QinXAtc – (QwXA – QeffXAout)tc + Mgn dt dX A 2V = QinXAtc – (QwXA – QeffXAout)tc + Mgn dt (QwXA – QeffXAout)tc = Mgn

steady state

(4)

She mass of produced nitrifiers in the system during one operational cycle (tc) is: Mgn ¼ 2V

Ztn

rgn dt ¼ 2VRgn tn

ð5Þ

0

where, rgn is the growth rate of nitrifiers per unit volume, Rgn is the average growth rate of nitrifiers, tn is the nitrifying time in each tank per cycle, XA is the concentration of


61

nitrifiers in each ditch, Xout A is the concentration of nitrifiers in the outlet, Mgn is the mass of produced nitrifiers in the system, Qin, Qeff and Qw is the influent wastewater, effluent wastewater and waste sludge flow rate, respectively, and V is the volume of each aeration ditch. From Eqs. 4 and 5, one can get: Qw XA Qeff Xout A =2V ¼ Rgn ðtn =tc Þ , ð6Þ Qw XA Qeff Xout A =2V ¼ Rgn a which is an expression for the average growth rate of the nitrifiers and a = tn/tc = nitrification time fraction (see ‘‘List of symbols’’). (For example, in this pilot plant tn = 60 min and tc = 2 9 60 ? 2 9 30 = 180 min, as will be mentioned later in this text). But another expression for the average growth rate of the nitrifiers is also: Rgn ¼ YA Rn bA XA

ð7Þ

Combination of Eqs. 6 and 7, gives Eq. 8: 1 Qw XA þ Qeff XAout Rn ¼ YA bA , a 2VXA X A 1 1 YA Rn ¼ bA a 2hc k Xvss

ð8Þ

where YA is the yield constant for autotrophic organisms (nitrifiers), bA is the autotrophic decay rate, Rn is the average nitrification rate in each aeration ditch, hc is the sludge retention time, Xvss is the concentration of biomass, k is the fraction of nitrifiers in the biomass, and a = tn/tc is the nitrification time fraction. Mass balance for nitrification nitrogen In a similar manner, the mass balance for nitrogen to be nitrified with the assumptions described above, in the process analysis paragraph, gives: dN 2V = Qin(TKN)'tc – (QwSNHout + QeffSNHout)tc – Mn dt dN 2V = Qin(TKN)'tc – (QwSNHout + QeffSNHout)tc – Mn dt Qin((TKN)' – SNHout)tc = Mn

steady state

(9)

where, (TKN)0 is the influent nitrogen available for nitrification (in the form of NH4–N), SNHout is the NH4–N in the outlet, Mn is the mass of nitrified nitrogen (NH4–N converted to NOx–N), and tc is the time length of one operational cycle. The mass of nitrified nitrogen in the system during one operational cycle is given by:

Mn ¼ 2V

Ztn

rn dt ¼ 2VRn tn

ð10Þ

0

where, rn is the nitrification rate per unit volume, Rn is the average nitrification rate in the ditch, and tn is the nitrifying time in each tank per cycle. Equations 2a, 9 and 10 together give: Rn ¼

Qin tc Nc Nc 1 ¼ 2Vtn 2h a

ð11Þ

Finally, combining Eqs. 8 and 11, we can get: 1 YA Nc ¼ bA ð2hc aÞ k ð2htn Xvss Þ

ð12Þ

which is an equation of the form c = ax ? b and from which the ratio TA/k and the autotrophic decay coefficient bA can be derived. Additionally, if the yield constant is assumed to be steady and has the theoretical value of 0.24 gr COD/gr NH4–N [10, 12], then the average population percentage of the nitrifiers in the biomass can be estimated. Determination of the maximum specific growth rate for autotrophs (lA) and the half-saturation constant for ammonium nitrogen (KNH) The growth rate of autotrophs, according to ASM1 [10], is given by Eq. 13: SNH SO l ¼ lA ð13Þ XA KNH þ SNH KOA þ SO Also, the average nitrification rate is given by: 1 SNH SO Rn ¼ lA XA YA KNH þ SNH KOA þ SO

ð14Þ

As shown previously (Eq. 11) for the alternating ditch system the nitrification rate is also given by: Rn ¼ N2hc 1a Nc 1 Thus, from Eqs. 11 and 14 ¼) ¼ lA YA 2ha SO SO SNH Nc SNH k , ¼ l X A YA A KNH þSNH KNH þSNH KOA þSO 2ha KOA þSO XVSS , (with the appropriate arithmetic executions and the maintenance of equality for the opposites) , SO 2 KOA þSO XVSS ha k KNH 1 1 þ ð15Þ ¼ YA lA SNH lA Nc which is again (as previously) an equation of the form c = ax ? b and from which the half-saturation constant KNH and the autotrophic maximum specific growth rate lA can be determined.

123

62


Table 1 Monthly average organic loading and influent nitrogen characteristics of the alternating oxidation ditch pilot plant Date

F/M (JgBOD/JgMLVSS day)

SJMin (mgN/l)

SNHin (mgN/l)

SNoin (mgN/l)

SNOxin (mgN/l)

SNHin/TKNin

BODin/TKNin

Jan

0.1

76.4

43.9

32.5

\1

0.57

5.05

Feb

0.08

63.3

48.8

14.5

\1

0.77

5.31

Aug

0.05

52.4

35.6

16.8

\1

0.68

5.25

Sep

0.04

56.4

39.7

16.7

\1

0.7

5.01

Oct

0.08

56.2

39.2

17.1

2.9

0.7

4.88

Nov

0.12

52

37.6

14.4

2.9

0.72

6.01

Dec Jan

0.15 0.21

49.5 50

38.7 43.2

10.8 6.8

2.7 3.3

0.78 0.86

5.98 5.83

123

Total Nitrogen

O2

6.5

100

3

Jan

3.5

30

Dec

4

40 Nov

4.5

50

Oct

5

60

Sep

5.5

70

Aug

6

80

Feb

90

Jan

N removal efficiency, (%)

As mentioned before, the pilot plant received domestic sewage from the northern suburbs of Athens. Raw sewage entered the ditch system after screening and removal of grit, oil and grease. In the influent, the concentrations of total COD (CODt) and BOD (BODt) varied from 603 to 862 mg/l and from 274 to 506 mg/l, respectively. BODt/CODt ratio ranged from 0.4 to 0.6, with a mean value of 0.5. CODs/CODt ratio ranged from 0.33 to 0.49, having the same mean value with the BODs/BODt ratio, which was 0.38. Finally, the BODs/CODs ratio was in the range of 0.37–0.59, with an average of 0.49. The influent suspended solids (SS) varied from 237 to 351 mg/l, having an average of 288 mg/l, whilst the volatile suspended solids (VSS) were in the range of 196– 301 mg/l, with a mean value of 239 mg/l. The VSS were in the range of 73–88% of the SS, with a mean value of 83%. The monthly average organic loading, together with the monthly average nitrogen characteristics of the influent, for the 8 months of the experimental period with high oxygen levels in the ditch, are presented in Table 1. The pH of the influent ranged between 7 and 8.6, whilst the alkalinity ranged from 391 to 614 mg/l as CaCO3. These values of alkalinity ensured complete nitrification without unfavourable repercussions due to the decrease in the pH. The temperature of the incoming sewage varied from 15 °C (lowest) in winter to 33 °C (highest) in summer, with the majority of values ranging between 17 and 27 °C. In Fig. 2, the removal efficiency for NH4–N and Ntot during the 8-month experimental period with high DO in the aeration ditch is presented. It can be seen that nitrogen removal efficiency is high, as the monthly average removal for ammonia is more than 90% for most of the period and for total nitrogen above 80%. The decrease in removal appearing in the last month of the period (January) is due to

Ammonium nitrogen

O2, (mg/l)

Results and discussion

Fig. 2 Ammonium nitrogen and total nitrogen removal in the alternating oxidation ditch pilot plant

the increased F/M ratio (0.21 JgBOD/JgMLVSS day), which is high for such systems of extended aeration. Figure 2 shows that average monthly removal efficiencies for ammonium nitrogen NH4–N was higher than 90% and for total nitrogen Ntot higher than 80%, during the 8-month experimental period where high DO concentrations were maintained in the aeration ditch. Only in the last month, removal efficiencies dropped to\80% for ammonia nitrogen and 65% for total nitrogen, but this was attributed to the higher organic loading (0.21 JgBOD/JgMLVSS day) applied during this period. As described in the paragraph on nitrification capacity, to apply this concept, as expressed in Eq. 1, the nitrogen (Ns) used for synthesis, and the effluent organic (SNoout) and the effluent ammonia nitrogen (SNHout) must be known. In this work, effluent organic (SNoout) and ammonia nitrogen (SNHout) were measured daily through the whole experimental period and their monthly average values are presented in Table 2. Also in Table 2, the measured average BOD (soluble) in the effluent is presented. The nitrogen content (ixB) in the biomass was measured repeatedly and found equal to 8–9% grN/gr VSS or 6% grN/gr COD [17].


63

Table 2 Monthly average effluent nitrogen characteristics of the alternating oxidation ditch pilot plant Date

SJMout (mgN/l)

SNHout (mgN/l)

SNoout (mgN/l)

BODout (mgN/l)

Jan

14.9

1.0

1.0

Feb

16.7

2.6

1.0

7.7

Aug Sept

4.4 5.6

0.4 0.3

2.7 2.9

11.5 4.9

Oct

4.2

1.0

1.8

6.0

Nov

3.5

1.4

0.5

5.0

Dec

11.3

1.5

2.6

4.0

Jan

17.8

4.7

5.8

10.0

3.8

The yield constant for heterotrophic organisms (TH) and the decay rate constant kd were also determined experimentally and found to be equal to TH = 0.67 grCOD/ grCOD and kd = 0.05 day-1 [17]. The average percentage of nitrogen (Ns) used for synthesis for each month, as calculated using Eq. 3, is presented in Table 3. Taking into consideration that nitrification phase time tn = 60 min (phase A or C), denitrification phase time td = 30 min (phase B or D) and the total cycle time tc = 180 min (phases A ? B?C ? D), the application of the field data from the pilot plant to Eq. 12 for the 8 months (assuming almost steady state conditions for each month) of experimental period is also shown in Table 3 and Fig. 3. From Fig. 3, the ratio TA/k was calculated to have an average value of 7.97 and the autotrophic decay coefficient bA, an average value of 0.061 day-1. Additionally, if the yield constant is assumed to be 0.24 grCOD/grNH4–N, as explained earlier in this text, then the average population percentage of the nitrifiers in the biomass can be estimated to be around 0.03 or 3%, which is a common percentage in the activated sludge systems [22]. As with most statistics, the slope (a) and intercept (b) of the regression line c = ax ? b are estimates based on a finite sample, and so there is some uncertainty in them. To

Fig. 3 Determination of the ratio TA/k and of the decay coefficient bA, for the nitrifiers

quantify this uncertainty, confidence limits and other statistics, such as standard error and p or F values, have been used. Applying linear regression analysis on the data in Table 3, the correlation coefficient (R) was found to be R = 0.925, whilst the critical correlation coefficient (Rc) is Rc = 2/(n1/2) = 0.707, where n = 8 (the number of data pairs). This confirms the strength of linear dependency between values in the X and the Y axes of Fig. 3. As one also can see, the coefficient of determination (R2), which estimates the fraction of the variance in Y that is explained by a linear function of X, is R2 = 0.856. The residual standard deviation (or residual standard error, RSE) is found to be RSE = 0.022. The value of the slope (a) and intercept (b), for 98% confidence intervals were in the range of: 3.78 \ a \ 12.15 and -0.147 \ b \ 0.026, respectively. Furthermore, the 98% confidence interval and the 98% prediction interval for the regression line are presented in Fig. 3. It must be reminded that 98% confidence interval is the area that has a 98% chance of containing the true regression line, whilst 98% prediction interval is the area in which one can expect 98% of all data points to fall. Finally, the F value was found to be F = 35.77, which is a quite large value indicating the

Table 3 Application of the field data to Eq. 12 for the determination of ratio TA/k and the decay coefficient bA for nitrifiers Date

F/M (JgBOD/JgMLVSS day)

(Ns/TKNin) (%)

Hc (day)

H (day)

Xvss (mg/l)

Nc/(2htnXvss)

1/(2hca)

Jan

0.10

17

13.7

1.35

2,902

0.0228

0.1098

Feb

0.08

19

22

1.37

3,341

0.0153

0.0682

Aug

0.04

7

35.1

2.23

2,487

0.0123

0.0427

Sep

0.08

7

34.2

2.21

2,883

0.0115

0.0439

Oct

0.12

10

19.8

1.21

2,983

0.0196

0.0758

Nov

0.15

19

16.9

0.75

3,394

0.0231

0.0888

Dec

0.21

22

9.9

0.68

2,896

0.0251

0.1506

Jan

0.22

23

7.6

0.51

2,730

0.0289

0.1984

123

64

Fig. 4 Determination of maximum specific growth rate lA and halfsaturation constant JMH

validity of the model. The accompanying p values for a and b were 0.00098 and 0.0697, respectively, which are quite small and show that this statistic application is reliable. In a similar manner, from the application of Eq. 15 (setting KOA = 1.5 grO2/m3, which is the typical value given in the ASM1 model [10], to the field data, the values of lA and JMH can be estimated as shown in Fig. 4. The value of lA is 0.36 day-1, whereas the value of JMH is found to be 0.65 mgNH4–N/l. The value of lA is in full agreement with the value of other researchers [13] for the same municipal wastewater of Athens. The value of JMH is within the range of values reported in literature, with the typical value given in the ASM1 being KNH = 1 [10]. Applying in Eq. 15 the same statistic analysis, the following characteristics were found: (a) Correlation coefficient R = 0.985, whilst the critical correlation coefficient Rc = 0.707; (b) coefficient of determination R2 = 0.97; (c) residual standard error RSE = 0.381; (d) 98% confidence intervals for the slope (a) and intercept (b) were in the range of: 1.41 \ a \ 2.24 and 2.13 \ b \ 3.45, respectively; (e) 98% confidence interval and 98% prediction interval for the regression line are presented in Fig. 4 and (f) F value = 194.94 and p values for a and b were 8.41 9 10-6 and 1.1 9 10-5, respectively.

Conclusions Using mass balance equations for nitrifiers and ammonium nitrogen, applied to a steady-state operational cycle of an alternating oxidation ditch pilot system, reaction rate constants lA, bA and KNH were determined. The large-scale pilot plant was cited in SERDC-EYDAP, treating

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municipal wastewater of Athens, Greece. The duration of the experimental period was 8 months, during which high oxygen concentrations (4–6 mg/l) in the aeration ditch were maintained. The values calculated for the kinetic parameters were: lA = 0.36 day-1, bA = 0.064 day-1 and JMH = 0.65 mgNH4–N/l. Using the same methodology, an estimate of the mean population percentage of nitrifiers in the biomass of the ditch system was derived. The percentage of nitrifiers was found to be *3% of the whole biomass. This study adds to the existing knowledge regarding the simple alternating oxidation ditch system, with two basins operating in four phases. Also, it helps to better understand and evaluate the nitrification capacity and thus the total nitrogen removal capacity of such a system, leading to operation optimization. At the same time, it provides the design engineer with more field data that can be used for better plant design and performance. Acknowledgments The authors would like to thank the personnel of the Sanitary Engineering Research and Development Center (SERDC) of the Athens Water Company (E.Y.D.A.P) for their help and support during the elaboration of this study.

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