Denitrification in secondary clarifiers

e:>

Pergamon 0273-1223(95)001 93-X

Wal. ScL Tech. Vol. 31, No.2, 1'1'. 2OS-214. 1995. Copyrigbt C 1995 IAWQ PrinlCld in Great Bntaill. AU rigbll reserved. 02"73-1223J9S S9'SO + 0-00

DENITRIFICATION IN SECONDARY CLARIFIERS H. Siegrist, P. Krebs, R. BilWer, I. Purtschert, C. Rock and R. Rufer Swiss Federallnslilule lor Environmenlal Science aJUJ Technology (EA WAG) aJUJ Swiss Federallnslitute olTechnology (ETHJ. CH-8600 DUbendorj, Swilurland

ABSTRACT Denitrification in secondary clarifiers is primarily due 10 bydrolysis of particulate degradable COD and decay of biomass. TIle reduction factor of these processes under anoxic as compared 10 aerobic conditiOllS was investigated in the treatment plants of the City of Zuricb. The Zilrich-Glatt plant bas circular clarifiers with cenual inlet The treatment plant ZUricb·WerdMlzli bas rectangular clarifiers with cross flow and scrapers with vacuum removal of the sludge. At ZUrich Glatt denitrification capacity in the clarifiers was 30% of IOtaI denitrification. wbereas aJ ZUricb·WerdMlzli only 15 % was observed. The sludge mass in the secondary clarifier depends on inlet flow, return sludge flow, activated sludge concentration, sedimentation properties of the sludge and scraper intervals. During dry weather and due to good sedimentation properties the activated sludge blanket in the thickening zone of the rectangular clarifiers aJ ZUricb·WerdhOlzli is almost completely removed after passage of the scraper. Denitrification volume in the secondary clarifier can therefore be modelled in relation 10 the ratio of inlet 10 return sludge flow and scraper interval. With the aid of control of return sludge flow and scraper intervals denitrification capacity in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a variable sludge volume in the clarifier related to scraper interval under the assumption of constant sludge mass in the entire system.

KEYWORDS Activated sludge process; denitrification; heterotrophic growth; hydrolysis; mathematical modelling; nitrification; secondary clarifier; sludge blanket. INTRODUcnON

Denitrification in secondary clarifiers can substantially increase overall denitrification of activated sludge systems if the clarifier volume is big compared with denitrification volume or if a substantial sludge blanket is maintained in the thickening zone of the clarifier. Due to lack of experimental data denitrification in secondary clarifiers is hardly reported in the literature (Hamilton et al., 1992). The Ziirich-Glatt plant treats the wastewater of about 110,000 population equivalents (p.e.) with a dry weather flow of 40,000 - 50,000 m3d· l . The activated sludge system consists of four parallel lanes (total volume 7250 m3) with three longitudinal reactors in series, each with a volume of 604 mJ . The plant is nitrifying with SRTs of 4 to 8 days. One of the four lanes was equipped with 33% anoxic volume (Fig. 1). and SRT increased to 10-12 days. The circular secondary clarifiers have central inlets. The thickened sludge is continuously removed to the centre by rotating scrapers. The sludge mass in the clarifier is about 25% of 205

H. SIEGRIST el al.

206

the total sludge volume, calculated from density profile measurements (Q~Q = 2). N0 3-N-removal in the clarifier was almost 15% of the inlet nitrogen load, corresponding to 30% of total denitrification. In the parallel lanes without anoxic zone, denitrification was partly due to simultaneous denitrification in the inlet zone and to denitrification in the clarifier (Fig. 2). retu rn sludge OR

°

Inlet anoxic --"'C""';a;;;';e";;;ro';;';blicaerobic outlet

Fig. I. Flow scheme of the lane with anoxic volume at the Ziirich-Glatt treaUDent planl IlIorncm

~:~13

Effluent 56'10

Den~rtf~

cof1on

23

zone 3
Fig. 2. Nitrogen removal at ZUrich-Glatt with (left) and without anoxic zone (right) related to total inlet nitrogen load (Purtschert and BUhler. 1993). In the lane without anoxic zone denitrification was from the sludge blanket in the clarifier and the inlet area of the aeration tank with low 02 concentration.

The Zurich-WerdhOlzli plant (450,000 p.e.) consists of two lanes with a dry weather flow of 70,000 . 85,000 m3d- 1 each. Both lanes have six parallel longitudinal nitrifying activated sludge tanks (6x5000 m 3) and 6 rectangular clarifiers (6x6000 m 3) with cross flow and scrapers with vacuum removal. In the north lane two tanks are equipped with 28% anoxic volume. Denitrification in the secondary clarifier was about 6% of the total inlet nitrogen load, corresponding to 15% of total denitrification (Rock and Rufer, 1994). An improvement of denitrification by maintaining a greater sludge blanket will be investigated in the future. Both examples illustrate that denitrification in the secondary clarifier can substantially improve denitrification. In this paper we first calibrate the Activated Sludge Model No. I for denitrification in anoxic volumes and secondary clarifiers. Then we discuss clarifier hydraulics, sludge sedimentation properties and operating conditions on increasing the sludge blanket and denitrification capacity in the clarifier. MODELLING OF DENITRIFICATION IN THE SECONDARY CLARIFIER Calibration of the model for anOXic hydrolysis Model No. I describes hydrolysis based on adsorption kinetics (eqn. I). With first-order kinetics similar or better correspondence with experimental data was obtained (Sollfrank and Gujer, 1991; Siegrist and Tschui, 1992). In Model No. I, a first order hydrolysis rate Ph can be introduced if K x » XSIX H:

Ph = with: kh

=

kb ·

= =

Kx Xs XH = kh' kinetics

Xs/Xij . X H .. kh'·XS Kx + Xs/Xij

(g COD rn-3 dol)

(I)

hydrolysis rate constant for adsorption kinetics (d· l ) saturation constant for adsorption kinetics (.) slowly degradable particulate COD (g COD m· 3) heterotrophic biomass (g COD m o3 ) kh/Kx (d· l ) if KX »XS/X H kh' becomes the hydrolysis rate constant for first-order

Denitrification in secondary clarifiers

=

In Henze et al. (1987) a reduction factor for anoxic hydrolysis of"h 0.4 is proposed. At the Glatt and Werdhl:\lzli plants activated sludge was taken from different parts of the activated sludge tanks and investigated in batch reactors for anoxic and aerobic respiration. Average reduction factors of 0.6 and 0.8 were observed. Recent results from measurements of enzyme activity indicate no anoxic reduction of hydrolysis (Richter tt al.• submitted). In the model which was used for the following simulations at the treatment plants Glatt and Werdhl:\lzli, "h =0.6 was selected and good agreement to experimental data was found. Siudgo ogo lei) : Volume IOtaI (m3):

Flow sheet

150 22120 80000 00 18400.0

Inlll (m3Id):

Roc.lnllrn3lel): Recircul. (m3Id1:

FIlICt.:

Inlot.

1.0

Socond. clorll.

Rl

_:

R2

R3

R4

AS

1.0

Rlactor: Volume (m3): 02 control. (gIm3): KJa'voIUl(II1l):

Rl

R2

R3

R4

R5

Nl

400.0 0.0 0.0

3020 0.0 3.0

3020 0.0 3.0

504.0 2.0 0.0

6040 20 00

2600.0

Fig. 3. How scheme and operating conditions at the treatment plant ZUrich-Glatt (aerobic SRT =7.S d). The sludge level in the clarifier (Fig. I) and therefore the sludge blanket volume remained Pr-lCtically constant during the experiments and was simulated with a constant volume of 400 m3 (Reactor I).

15 0::::.

~

8z OlE

,...--=-.---------------,

.

.

•

10

5

••

o +---+--=:::.:;::!!::=a:..::..-.........~~~t____+"~ o 4 8 12 16 20 24 28 32 HOU's Fig. 4. Comparison of experimental and calculated nitrate profiles in the activated sludge tank effiuent (Reactor S), return sludge (Reactor I) and anoxic zone (Reactor 3) of the ZUrich-Glatt plant from 10.00, 9 Nov. until 16.00, 10 Nov. 1992). Model calibration file F, characterisation of the wastewater and daily variation are described in the appendices Nos I to 3.

The increased nitrate concentrations during the first six hours (Fig. 4) are due to digester supernatant addition in the night before starting the daily variation experiments which is not included in the input data of the simulation.

=

During dry weather and thanks to good sedimentation properties (eqn. 3) and sludge volume indices (SVI 60-150 ml g-I TSS), the activated sludge blanket in the thickening zone of the rectangular clarifiers at Ziirich-Werdhl:\lzli is mostly completely removed after passage of the vacuum scraper. Sludge mass (SM) in

208

H. SIEGRIST tt al.

the secondary clarifier is therefore SM described with reactor 1 in Fig. 5:

VI with:

= (Q+QRHsrXl and

the idealised sludge blanket volume (V I).

= (Q+OR)·tsrXJXR = (Q+ORHsrOR/(Q+OR) = tSI' OR =500 m3

=

=

(2)

=

Q inlet flow (m 3d- l ) QR return sludge flow 25.000 m3d- 1 tSI = scraper interval including time for settling of sludge = 0.02 d (ca. 30 min) Xl total particulate concentration in the activated sludge tank (kg TSS m-3) XR = total particulate concentration in the return sludge (kg TSS m- 3)w X!XR = OR/(Q+OR). neglecting biomass in the effluent and excess sludge.

6ltJdge 101

!dl:

130 55000

Vclumo ..taI m3):

Flow sheet

InI« (m3Id):

1~.0

0.0 2llOOO.0

Roc.lnl. (m3Id): R_.(m3Id):

FriICt:

10

1n1e1:

60c0nd.CIItII. Rl

Roelle.:

R5

R4

R3

R2

1.0

Reactor: VOlume 1m3): 02 """lnlI (glm3l: KII.yaJUI(11d1:

Rl

R2

R3

R4

R5

HI

500.0 0.0 0.0

7000 0.0 2.0

7000 00 2.0

1800.0 20 0.0

1800.0 20 0.0

5400.0

Fig. S. Flow scheme and operating conditions at the treatment plant ZOrich-WerdbOlzlL 15

r-------""'""7-==:--------,

Q.j...---ir---+-_+_-__- _ + _ - _ - - t - - - - l

o

4

8

12

16 Hours

20

24

28

32

Fig. 6. Comparison of experimental and calculated nitrate profile in !be anoxic zone (Reactor 3) and secondary clarifier at ZOricb-WerdbOIzli, from 8.00, 2 Dec. until 16.00, 3 Dec. 1993. Model calibration file F, characterisation of the wastewater and daily variation are described in the appendices Nos I, 2 and 4.

CalibratiQn Qf the AmmQnjum-MQnod CQDstant for njtrification The maximum growth rates selected in the model (Jl.max) are discussed in Siegrist and Tschui (1992). For nitrification the maximum observed growth rates 0.27 exp(O.I(T - WOC» d- I and 0.23 d- I with direct Fe 2+

Denitrification in secondary clarifiers

209

addition at 10·C were selected. These values correspond to a maximum model growth rate of 1.1 and 0.90 d-I at 20·C without and with Fe2+ addition, respectively. The decay rate of nitrifiers is assumed to be reduced in the anoxic volumes therefore an average decay rate of 10 % of the maximum growth rate is selected. At Glatt and WerdhOlzli, Monod constants of I and 2 g NH4-N m- 3 fit best, respectively (Fig. 7). Since similar Monod constants were found for 12 and 20·C in earlier studies (Siegrist and Tschui, 1992) we did not consider any temperature effect in spite of the high temperature effect described in Knowles et at. (1964). 10

•

8

"-

~ :x:

6

~

4

z

2

•

0 0

4

8

12

16 Hours

20

24

28

32

Fig. 7. Calibration of Mono
Clarifier hydrauUcs and slud~e sedimentation properties Clarifier hydraulics for the Werdholzli plant (Fig. 8) were simulated by using the fluid dynamics code PHOENICS (Rosten and Spalding, 1987). The numerical procedure is described by Krebs (1991). Steady two-phase flow was computed on a 20 by 20 grid. The well established k-E-turbulence model (Rodi, 1980) was applied. The cross-sectional shape of the clarifier was approximated with a rectangular form of 8 by 4.3 m. The settling velocities observed were approximated with a logarithmic function: (mm s-I) with: SV SVI X VsO

(3)

SVI·X/lOOO = local sludge volume fraction (-) sludge volume index, 60 to ISO ml g-I TSS activated sludge concentration (g TSS I-I) = 3.8 mm s-I, if VsO ·269-sV ~ than Vs 2 mm s-1

= = =

=

Return sludge is removed with vacuum scrapers in intervals of 25 min, In the model a continuous removal of the activated sludge from the entire tank bottom was assumed, resulting in an average sludge blanket height of about 0.3 m as indicated by the high gradients of the horizontal velocity and the sludge density profile (Fig. 8). The simulated maximum horizontal flow velocity of approximately 30 mm s-I at 0.5 m height was conflI1l1ed' by prototype velocity measurements using a cylindrical plastic body of 0.3 m diameter. The sludge blanket level observed with a turbidity meter under similar operating conditions was 30 to 60 em before and practically nil after passage of the scraper.

210

H. SIEGRIST tl al.

-

0.1 mine

I

K I

J~------- -------~ '.Om

Fig. 8. Simulated fields of velocity (above) and sludge volume fraction (below) in a representative cross section of the secondary clarifier (V =6000 m 3, Q - 15,000, QR - 30,OOOm 3d- l , Xl- 2+.5 kg m· 3) of the WerdhOlzli planL

Since the upward flow velocity below the outlet predominantly exceeds the maximum settling velocity, the sludge has to settle during horizontal flow. An increase of the sludge blanket due to peak flow or longer scraper intervals would shift the main horizontal water flow in an upward direction and cause a strong streamline curvature of the flow at the inlet position. This is in strong disagreement with the mass flult method for one-dimensional clarifier modelling which assumes uniform vertical flow. Operation wjth yariable

slud~e

blanket yolume

Depending on temperature and settler depth denitrification in the secondary clarifier should not exceed 6-9 g N03-N m-3 return sludge (Henze et al., 1993). Above this amount of denitrification supersaturation of nitrogen gas at the bottom of the clarifier will lead to rising sludge due to bubble formation. In Switzerland. with maximum ammonium concentrations of 25-35 mg N 1-1. denitrification in the clarifier due to an increased sludge blanket volume should not exceed 30 % of total denitrification. Extended anoxic sludge blankets hinder the obligate aerobic filamentous micro-organisms in their competition against facultative aerobic floc-fonners (Kappeler and Gujer. 1994) and might therefore improve sludge volume index and settling of sludge.

As seen from equation 2 the greater the return sludge flow the bigger the sludge blanket Yet, this runs parallel with increased oxygen input into the clarifier. An increased sludge blanket volume due to incomplete removal of settled sludge by the scraper might be reached with high activated sludge

Denitrification in secondary clarifiers

211

concentrations, bad settling properties, low return sludge to influent flow ratio and high sludge volume index (SVI). But this would probably also result in poor effluent conditions during peak flow. At Werdhl:\lzli, owing to the low SVI and good settling properties only QRlQ < 0.5 led to an enhanced sludge blanket volume, but also to very unstable operating conditions due to nitrogen gas accumulation at the highest point of the vacuum pipe. Higher return sludge flow prevents accumulation of nitrogen gas in the vacuum pipe. Sludge blanket volume can therefore only be increased with lower horizontal scraper velocities leading to longer intervals between two scraper passages. At the WerdhOlzli plant this can be easily obtained by equipping the scraper motor with a frequency control and this will be investigated in winter 1994. The following operating procedures will then be possible: low interval during dry weather and sufficient nitrate supply to the anoxic zone normal interval during elevated rain-water flow and/or suspended solid concentration in the clarifier effluent. Simulation with Aguasim Flow schemes with variable sludge blanket volume can be simulated with Aquasim (Reichert, 1994a,b). To raise the sludge blanket volume (Rl. Fig. 5), the return sludge flow into the sludge blanket will be increased by AQ during a certain time period. In parallel. the flow into the clarifier will be reduced by AQ which leads to a reduced clear water volume of the clarifier (Fig. 9). A horizontal scraper velocity of 50% increases the sludge blanket volume from about 600 to 1200 m3. This measure reduces nitrate concentration in the activated sludge tank effluent by about 1.5-2 gN m- 3, corresponding to 15-20% improved denitrification (Fig. 10).

Fig. 9. Model flow scheme of WerdMlz1i with variable sludge blanket volume. Sludge blanket volume varies between 600 and 1200 m3 and the clear water zone of clarifier from 5400 to 4800 m 3. All other operating data are the same as in Fig. 5.

15

-r--------------------r 2400 N03-N In Reaktor 5

c::. 10

:!

~

~

1200

~

Gl

5 I-

.L-

~__+

600

!

0+---+----+--+---+---+----11---+---+0

o

4

8

12

16

20

24

28

32

Hours Fig. 10. Nitrate profile due to variation of sludge blanket volume. Denitrification capacity is increased by about 15· 20% with elevated sludge blanket volume compared with normal operation.

212

H. SIEGRIST er al.

CONCLUSIONS With the experimental data of investigations at the treatment plants ZUrich-Glatt and Zilrich-Werdhtllzli the activated sludge model No. I was calibrated for nitrification and denitrification for a typical Swiss municipal wastewater. For the hydrolysis of slowly degradable particulate organics flfSt-order kinetics was considered with k H 5.5 exp(0.03(T - 20·C) d-I. A reduction factor for anoxic hydrolysis of T\b 0.6-0.8 was observed. For nitrification a maximum model growth rate of l.l d-I without and 0.90 d-I with Fe 2+ addition directly to the aerobic tank was selected at 20·C. The Monod constant observed for ammonium at 16·C was 1-2 g NH 4N m- 3. Siegrist and Tschui reported similar values in winter and summer experiments of an earlier campaign at ZUrich-Glatt and WerdhOlzli, therefore no temperature effect was include~.

=

=

At ZUrich-Glatt, density profile measurements indicated a sludge blanket volume of 400 m3, corresponding to 25% of the total sludge mass. Denitrification in the secondary clarifier was 30% of total nitrate elimination, corresponding to 15% of inlet nitrogen load. At Werdholzli the sludge volume in the clarifier is the product of return sludge flow and scraper interval, due to complete removal of the sludge blanket after passage of the scraper. Doubling the scraper interval would increase the sludge blanket volume from 600 to 1200 m3 and denitrification capacity in the clarifier from 6 to 12% of total inlet nitrogen. The scraper interval may be controlled by a motor with frequency control, therefore an immediate change to normal operation during rainy weather is possible. Calculation of clarifier hydraulics using the fluid dynamics code PHOENlCS illustrates a horizontal flow velocity above the sludge blanket of up to 30 mm s-I. The sludge is settled during horizontal flow over the sludge blanket because upward velocity below the outlet position is above the maximum settling velocity. This is in strong disagreement with the mass flux method for onedimensional clarifier modelling which assumes uniform vertical flow.

REFERENCES Hamilton, J~ Jain, R., Antoniou, P., Svoronos, S. A., Koopman. B. and Lyberatos, G. (1992). Modeling and pilot-scale experimenlal verification for predenitrification process, J. Envir. Engrg~ 118, 38-SS. Henze, M., Grady, C. P. L., Gujer. W., Marais, G. V. R. and Matsuo, T. (1987). 'Activated Sludge Model No. I', Scielllific and Technical Report No. 1,IAWPRC. London. Henze, M~ Dupenl, R, Grall, P. and De La SOla, A. (1993) Rising sludge in secondary settlers due to denitrification. War. Res., 27, 231-236. Kappeler, J. and Gujer, W. (1992) Estimation of kinetic parameters of beterotrophic biomass under aerobic conditions and cbaracterization of wastewater for activated sludge modeUing, WaL Sci. Tech.• 25(6), 125-139. Kappeler, J. and Gujer, W. (1994) Verification and application of a mathemaL model for aerobic bulking. War. Res., 28, 311-322. Knowles, G., Downing. A. L., and Barretl, M. J. (1964) Detel'llllnation of kinetic constants for nitrifying bacteria in mixed culture, with the aid of an electronic computer, J. Gen. Microbiol., 38, 263·278. Krebs, P. (1991) Modellierung und Verbesserung der StrOmung in Nachklarbecken. Dissertation No. 9486, Swiss FederallnstilUte of Technology (ETH) ZUrich, Switzerland. Purtsebert, I. and Biihler, R. (1993) Denitrifikationsversuche an der ARA Glatt, Diplomarbeit an der Abt. VIII der ETH-ZUricb. Reicber!, P. (I 994a) AQUASIM - A tool for simulation and data analysis of aquatic systems, War. Sci. Tech., 30(2), 21.30. Reicher!, P. (I994b) Concepts underlying a computer program for the identification and simulation of aquatic systems. Scbriftenreihe der EAWAG Nr. 7, EAWAG, CH-8600 Dubendorf. Richter, H., Jardin N. and POpel H. J. (1994) Hydrolyseaktivillit von Belebtscbllimmen unterscbiedlicber Herkunfi (submiued to gwf-WasseriAbwasser). ROCk, C. and Rufer. R. (1994) Denitrifikationsversucbe an der ARA WerdhOlzli, Diplomarbeit an der Abl VIII der ETH-ZOrich. Rodi, W. (1980) Turbulence models and rheir appUcation in hydrauUcs. InL Ass. on Hydraulic Researcb, IAHR, Delft. The Netherlands. Rosten, H. I. and Spalding, D. B. (1987) The PHOENICS Reference Manual. Report TR2OO. CHAM Ltd. London. Siegrist, H. and Tscbui, F. (1992) Interpretation of experimental data with regard to the activated sludge model No. I and calibration of the model for municipal wastewater treatment plants. War. Sc~ Tech.• 25(6), 167·183. SoUfrank, U. and Gujer, W. (1991) Cbaracterisation of domestic wastewater for mathematical modeUing of the activated sludge process, Wat. Sc~ Tech.• 23(4-6). 1057-1066.

Denilrificalion in secondary clarifiers

213

APPENDICES Apl2'ndjx No I' Calibration file for simulations wtb AS40 (AQua System AG CH-8400 Winterthur) Calibration file of model E (Siegrist and Tschui. 1992) was modified for nitrogen content of inert particulate and inert soluble COD (see appendix 2); readily degradable COD (see appendix 2); reduction factor for anoxic hydrolysis llb and decay rate of nitrifiers (see calibration section). Heterotrophic yield Fraction inert from biomass decay N-content in biomass N-cont. in biomass decay products N-content of inert particulate in feed N-content in inert soluble COD Yield of nitrifying organisms Max. heterotr. growth rate (20 Deg C) COD-saturation constant for heterotrophs 02-saturation constant for heterotrophs NH4-N-saturation constant for heterotr. Heterotrophic decay rate (20 Deg C) HC03-saturation constant for heterotr. Eta denitrification qrowth Eta denitrification hydrolysis N03-N-saturation constant for denitrif. Hydrolysis rate (20 Deq C) Hydrolysis saturation ratio (20 Deg C) Ammonification rate const. (20 Deq C) Max. growth rate of nitrif. (20 Deg C) NH4-N-sat.-const. for nitrit. (20 Deg C) HC03-satur.-constant for nitrifiers 02-satur.-constant for nitrifiers Decay rate of nitrifiers (20 O8g C) Temp.-coefficient heterotrophic growth Temp.-coefficient heterotrophic decay Temp.-coefficient hydrolysis rate Temp.-coefficient hydro satur. ratio Temp.-coefficient ammonification rate Temp.-coefficient growth rate of nitrif. Temp.-coefficient decay rate ot nitrif. Temp.-coeff. NH4-N-satur.-const. nitrif. Fraction readily biodegradable COD Fraction slowly biodegradable COD Hinimum temperature Maximum temperature Ratio NH4-N/TKN soluble in feed

COD/q COD COD/q COD N/q COD N/q COD q N/q COD q N/q COD q COD/q N lid 9 COD/m3

q q q q

q 02/m3 q N/m3 lid mol/m3

q N/m3 lid q COD/q COD m31 (gCOD*d) lid q N/m3 mol/m3 q 02/m3 lid l/08q C l/Deg C l/Deq C l/Deq C

I/Deq l/08q l/Deq I/Deq

Deq C Deq C

C C

C C

Hodel E 0.64 0.08 0.07 0.06 0.04 0.06 0.24 2.50 5.00 0.25 0.10 0.55 0.10 0.80 0.35 0.50 50.00 10.00 0.08 • 0.90/1.10 2.00 0.25 0.50 0.15 0.07 0.07 0.03 0.00 0.07 0.10 0.10 0.00 0.20 0.60 10.00 20.00 0.85

Hodel F 0.64 0.08 0.07 0.04 0.03 0.02 0.24 2.50 5.00 0.25 0.10 0.55 0.10 0.80 0.60 0.50 50.00 10.00 0.08

0.90/1.10 2.00/1.00

0.25 0.50

a) b)

O.Og/O.l1

0.07 0.07 0.03 0.00 0.07 0.10 0.10 0.10

0.15

0.60 10.00 20.00 0.85

a) ~x • 1.10 d- 1 without direct re 2 + addition (Glatt). At ZUrich WerdMlzl1 Fe 2 + was added directly to the aeration section therefore ~x • 0.90 d- l was selected. b) At Glatt and WerdhOlzli Honod terms of 2 and 1 gN m- 3 were observed, respectively.

Appendix No 2: Characterisation of the wastewater Readily degradable COD observed with batch experiments as described in Kappeler and Gujer (1992) was 6 to 15% of total COD. In the simulation 10% was selected. The inert dissolved COD is assumed to be 90 % of the total dissolved COD in the secondary effluent which was 6-12% of total inlet COD. The average nitrogen content of the activated sludge was 4.3% and 5.4% of the particulate COD at Glatt and WerdhOlzli. respectively. This corresponds approximately to the nitrogen contents selected in model F for the different

214

H. SIEGRIST er al.

COD fractions. The nitrogen content of inert soluble COD was reduced due to dissolved organic nitrogen effluent concentration of 0.5 to I g N m·3. The inert particulate COD was about 25% of total COD at both plants as estimated from the activated slUdge production. Table I. Wastewater flow and mean value of the primary effluent concentrations of the required model compounds during the investigated time period (Purtsehert and Biihler, 1993; R&k and Rufer, 1994). Com1Xlllnds How COD tot COD dissolved. inert COD readily deJmld. COD Dart. inert (estimated) Ki-Ntol Ki-N diss. (estimated) NHA-N NO't-N Bicarbonate OXY2en

m.)/d Jr: COD m-3 Jr: COD m'3 R COD m·3 II COD m- 3 II Nm-3 IlNm- J gNm- 3 gNm- J mM ll02m'J

ZQrich-Glalt 9'000 190 IS 19

ZOrich-Werdhlllzli

14'500 250 20 2S

50 23 20 17

60 33 28 24 0.8

O.S S

6.S 6

2

Agpendix No.3: Diurnal variation of inlet data at ZUrich Glatt 9-10 November 1992 Time 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00

Averaqe: Weiqhted mean:

01

o avq

0.99 1.00 0.96 0.91 0.92 0.93 0.89 0.73 0.65 0.61 0.79 0.96 0.95 1.07 1.50 1.50 0.96

COOl

COD avq 0.67 1.55 1.21 1.30 1.15 1.25 0.97 0.82 0.64 0.45 0.58 0.95 0.98 1.88 0.63 0.24 0.95 0.91

TKNI TKN avq 1.17 1.25 1.05 1.15 1.19 1.26 1.29 1.28 1.04 1.02 0.95 1.39 1.18 1.11 0.35 0.24

1.06 0.96

Oxl

Ox avq

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Temp.T avq

1.20 1.50 1.40 1.70 1.60 1.40 1.40 1.20 1.50 1.40 0.90 0.70 1.20 1.40 -0.30 -1.50 16.04

Appendix No.4: Diurnal variation data at the WerdbOlzli plant 2-3 December 1993 Time 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00

Average: Weighted mean:

01

o avq 0.81 0.92 0.98 0.98 0.92 0.92 0.87 0.71 0.78 0.48 0.24 0.48 1.02 1.14 1.32 1.02 0.85

COOl

COD avq 1.03 1.16 1.48 1. 64 1.82 1.67 1.75 1.52 1.41 1.28 1.12 1.08 1.05 1.22 1.55 1.69 1.40 1.22

TKNI TKN avq 1.00 1.05 1.03 1.09 1.41 1.66 1. 76 1.23 1.41 1.38 1.15 1.22 0.97 1.10 0.89 0.97 1.21 1.01

oxl

Ox avq

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Temp.T avq

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16.00