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Potential of denitrification and solids removal in the rectangular clarifier
PII: S0043-1354(98)00220-6
Wat. Res. Vol. 33, No. 2, pp. 309±318, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
POTENTIAL OF DENITRIFICATION AND SOLIDS REMOVAL IN THE RECTANGULAR CLARIFIER M M G. KOCH1*, R. PIANTA1, P. KREBS2* and H. SIEGRIST1*
Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 DuÈbendorf, Switzerland and 2Institute for Urban Water Management, Dresden University of Technology, D-01062 Dresden, Germany
1
(First received February 1998; accepted in revised form May 1998) AbstractÐIn full scale experiments, the denitri®cation capacity of secondary clari®ers and measures to increase it were evaluated. Due to a higher activated-sludge concentration and a lower scraper velocity, the sludge mass and thus the denitri®cation capacity in the secondary clari®er at the WWTP ZuÈrich± WerdhoÈlzli was signi®cantly increased. During the investigated period, the denitri®cation in the clari®er represented 37% of the total denitri®cation, corresponding to 19% of the total inlet nitrogen. The main part was denitri®ed in the inlet channel of the clari®er, so that doubling the scraper interval increased the denitri®cation in the clari®er by only about 14%. An approach for modelling the sludge-blanket volume was developed on the basis of measurements of the sludge-blanket height along the rectangular clari®er with transverse ¯ow and vacuum-removal scrapers. The limiting sludge-volume load was calculated from sludge settling tests and compared with observed loads on the full-scale clari®er. Activated sludge model No. 2 was veri®ed with the nitrate and ammonium pro®les in the activated sludge tanks, return sludge and clari®er euent. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐactivated sludge model, denitri®cation, dynamic simulation, full-scale experiments, nitrogen removal, secondary clari®er, sludge blanket, solid-¯ux method
NOMENCLATURE js=solids-loading rate (M lÿ2 Tÿ1) Zden=denitri®cation eciency ZN,el=eciency of nitrogen removal Nsol=soluble inorganic nitrogen (M lÿ3) Ntot=particulate and soluble nitrogen (M lÿ3) Qes=excess sludge (l3 Tÿ1) Qin=in¯uent to WWTP (l3 Tÿ1) Qrs=return sludge (l3 Tÿ1) SM=solid mass (M) SRT=sludge retention time (T) SVI=sludge volume index (l3 Mÿ1) T=temperature of waste water (8C) tacc=average sludge accumulation time (T) tL/3=scraper runtime for 1/3 Lsc (T) uscr=scraper velocity (l Tÿ1) V=volume (l3) X=solids concentration (M lÿ3) sb=sludge blanket sc=secondary clari®er t=activated sludge tank
INTRODUCTION
Due to the hydrolysis of particulate degradable COD and the decay of biomass oxygen reduction and denitri®cation in the sludge blanket of the secondary clari®er occur. Only few case studies about these processes have been presented in the literature (Hamilton et al., 1992; Jain et al., 1992; Fillos et *Author to whom all correspondence should be addressed. [Tel: +41-1-823±53±86; Fax: +41-1-8235389, E-mail: [email protected]]. 309
al., 1996). Based on experimental data and activated-sludge simulations Siegrist et al. (1995) clearly showed, that the eect of clari®er denitri®cation on process performance becomes signi®cant with increasing sludge mass in the clari®er. The aim of this study was to investigate the denitri®cation eciency in the secondary clari®ers at the WWTP of ZuÈrich±WerdhoÈlzli and to improve the denitri®cation capacity by maintaining a greater sludge-blanket volume. For this purpose, full-scale experiments were performed (Fig. 1) during seven weeks (28 October±13 December 1996). The experimental data were then used to verify the activated sludge model No. 2 (Gujer et al., 1995) with regard to the hydrolysis, nitri®cation and denitri®cation processes. The increased sludge blanket level during the investigated period required a more detailed monitoring of the plant behavior to prevent overload of the clari®er and escape of sludge. Therefore, the dynamics of the sludge blanket and the sludge mass in the clari®er under dierent solids-¯ux conditions and scraper intervals were analyzed. From these data a model for the behavior of the sludge blanket was developed. Furthermore a solids-¯ux analysis was performed in order to evaluate the settling capacity of the clari®er. An increased sludge-blanket volume can be obtained by setting lower scraper velocities, thus leading to longer intervals between two scraper passages (Siegrist et al., 1995). To this end, the scraper
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Fig. 1. Sample program of the experimental lane of the ZuÈrich±WerdhoÈlzli activated-sludge plant.
motors were equipped with a frequency controller to reduce the maximal velocity by 50%. In April 1996, the maximum combined water in¯uent to the treatment plant was reduced from 9 to 6 m3 sÿ1. As a result, the suspended-solids load of the secondary clari®ers decreased substantially during storm events. This measure allowed the sludge concentration in the activated-sludge tanks Xt to be increased from 3.0 to 4.5 kg TSS mÿ3, leading to an additional increase of the sludge mass in the clari®er during dry-weather conditions.
15 days and the return sludge Qrs was 6 30,000 m3 dÿ1, i.e. about twice the dry-weather ¯ow. Each clari®er tank is equipped with three vacuum scrapers, normally operated at a velocity of uscr=4.8 cm sÿ1, which results in a period of 2 tL/ 3=34 min, referred to the back-and-forth passage of the scraper in one third of the clari®er. To evaluate the eect of the scraper velocity on the denitri®cation capacity in the sludge blanket, uscr was operated at 4.8 and 2.4 cm sÿ1 (week 3, 5, 7; Fig. 3). MATERIALS AND METHODS
FULL SCALE PLANT AND TEST CONFIGURATION
The waste-water treatment plant at ZuÈrich± WerdhoÈlzli (600,000 p.e.) is split up into two lanes, north and south, each with a dry-weather ¯ow of 80,000 m3 dÿ1. Both lanes have six parallel activated-sludge tanks (6 5,000 m3) and six rectangular clari®ers (6 6,000 m3) with transverse ¯ow and scrapers with vacuum removal (Fig. 2). In the north lane, the activated sludge tanks are equipped with a 28% anoxic volume (Fig. 1). SRT was about
Sludge blanket With the aid of a mobile turbidimeter sensor, the sludge-blanket height was recorded at dierent positions (8.0 and 1.0 m screening in longitudinal and transverse direction, respectively) in relation to the time elapsed since the passage of the vacuum scraper (Fig. 2). The interface between the sludge blanket and the clear water was observed by the Staiger-Mohilo 7120 ASH turbidimeter sensor, the height of the sludge blanket was de®ned at X = 3.0 kg TSS mÿ3. A vacuum pump was used to withdraw sludge from within the sludge blanket to determine the density pro®les.
Fig. 2. Cross section of the secondary clari®er and measurement set-up.
Potential of denitri®cation and solids removal in the rectangular clari®er
311
Fig. 3. Denitri®cation in the secondary clari®er (equation 6) and in total (equation 4) during the investigated period. T = 15.58C. Sludge settling parameters The sludge settling properties and the sludge volume index SVI were measured with a plastic cylinder (diameter 28.5 cm, height 1.50 m, settling time for SVI determination 30 min). The SVI observed with the plastic cylinder corresponds to the DSVI (diluted SVI) in the 1 l measuring cylinder. On-line-measurements After ultra-®ltration, the sum of the nitrate + nitrite was analyzed by the Dr. Lange Nitrax analyzer on the basis of ultraviolet absorption. NO2 and NH4 were analyzed photometrically after a color reaction (Skalar, OPA SA9000, nitrite at 540 nm, ammonia at 660 nm). 24 h composite samples The anorganic nitrogen compounds nitrate, nitrite and ammonia were determined colorimetrically by ¯ow injection analysis (FIA) after 0.7 mm ®ltration. The total nitrogen content was measured in the same way after sample digestion with K2S2O8. Both the dissolved and total COD were measured colorimetrically by the HACH method (DR/2000). The total suspended solids (TSS) were ®ltered with a GF/F ®lter (0.7 mm). The ®ltered material was dried for two hours at 1058C.
RESULTS AND DISCUSSION
The ratio between the growing sludge-blanket height and the speci®c sludge-volume load (extracted from Fig. 4) calculated at every measuring position allowed the dynamics of the sludge blanket to be modelled as a function of the sludgevolume load and scraper velocity (Fig. 5). Model calculations show that the clari®er is fed quite inhomogeneously, leading to maximum heights of 0.8 m at rain-weather peak loads and reduced scraper velocity. The volume of the sludge blanket is approximately equal in each third. The average sludge blanket volume, mass and thickness could be calculated for dierent operating states on the basis of the linearized model (Table 1). This shows that the average thickness is increased only slightly due to longer scraper intervals and/or higher mass ¯ow rates (hsb,max=0.36 m). The actual sludge accumulation time in one third of the clari®er depends on the actual position and the velocity of the scraper. Thomann et al. (1996) showed that the average sludge accumulation time for a scraper passage in one third of the clari®er is tacc=2/3 tL/3 only. Under steady-state conditions, the ratio between the solids mass in the whole
Sludge-blanket dynamics in secondary clari®er To describe the behavior of the sludge blanket, the growing sludge level had to be measured at various positions (about 100 sites) depending on the SVI, the actual mass ¯ow of suspended solids and the local accumulation time (Fig. 4). The local accumulation time is equal to the time elapsed after the scraper passage. Measurements showed that the variation of the sludge blanket height is directly related to these parameters. Due to its good settling properties (SVI = 125 ml gÿ1) and a high returnsludge ratio R = Qrs/Qin=0.8±2.0 (i.e. wet-weather and dry-weather conditions, respectively) the sludge blanket is almost completely removed after the passage of the scraper.
Fig. 4. Growing blanket with dierent sludge-volume loads jsSVI. Low load from Thomann et al. (1996).
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Fig. 5. Modelled sludge-blanket height with uscr=2.4 cm sÿ1, Xt=4.5 kg TSS mÿ3, Qin=1,500 m3 hÿ1, Qrs=1,250 m3 hÿ1, SVI = 125 ml gÿ1 (Pianta and Marti, 1997).
sludge blanket SMsb and that in the activated sludge tank SMt is thus SMsb Qrs Qin Xt tacc Qrs Qin tacc : SMt Vt Xt Vt
1
The reduction of the scraper velocity by 50% increases the ratio of equation 1 from 7 to 13% (Table 1) whereas during maximum peak ¯ow about 21% of the total amount of suspended solids in the activated sludge tank is stored in the sludge blanket. With regard to the denitri®cation capacity in the secondary clari®er, it has to be considered that a substantial amount of biomass (approxi-
mately 14% of SMt) is stored in the inlet channel of the clari®er (see Figs 2 and 6). Compared to the ATV-guidelines A131 (1991) the hydraulic retention time of the inlet channel (15 and 23 min at wet- and dry-weather ¯ow, respectively) is relatively high, but satisfy the ¯occulation criterion suggested from Wahlberg et al. (1994). The mass of solids in the transition zone (about 0.20 m with 1.5 kg TSS mÿ3, Fig. 7) de®ned by the surface of the sludge blanket (3.0 kg TSS mÿ3) and the clear water zone is SMtrans=1350 m20.2 m 1.5 kg TSS mÿ3=400 kg TSS. Knowing SMtrans, SMsb and the modelled volume Vsb, the average concentration of suspended
Table 1. Modelled sludge blanket. Qrs=1,250 m3 hÿ1, SMtrans=400 kg TSS, SVI = 125 ml gÿ1 Xt; Qin (kg TSS mÿ3, m3 hÿ1) 3.0; 3.0; 4.5; 4.5;
550 550 550 1,500
uscr (cm sÿ1)
tacc (min)
SMsb (kg TSS)
SMsb/SMt
Vsb (m3)
hsb (m)
Xsb (kg TSS mÿ3)
4.8 2.4 2.4 2.4
11.4 22.4 22.4 22.4
1,030 2,020 3,020 4,620
0.07 0.13 0.13 0.21
100 200 300 460
0.08 0.16 0.24 0.36
6.5 8.2 8.8 9.2
Fig. 6. Flow scheme and operating conditions. The sludge blanket (reactor R1) was simulated with a constant volume with SMsb=XR1VR11(Qrs+Qin)Xttacc. The inlet channel of the clari®er (reactor R7) was simulated as a subsequent anoxic zone.
Potential of denitri®cation and solids removal in the rectangular clari®er
Fig. 7. Density pro®les from 13 December `96, local accumulation time for each pro®le 45 min.
solids Xsb in the sludge blanket below the transition zone is Xsb
SMsb ÿ SMtrans : Vsb
2
At higher sludge-blanket levels, the solids concentration Xsb according to equation 2 increases to about 8±9 kg TSS mÿ3 (Table 1), which agrees well with the average concentrations from density pro®les (Xsb=8.3 kg TSS mÿ3). Suspended-solids removal and settling capacity A solids-¯ux analysis was performed with regard to evaluate the settling capacity of the clari®er during the investigated periods. The limiting solidsmass ¯ow js,limit was determined according to the ¯ux-theory (Kynch, 1952) on the basis of observed settling velocities of the activated sludge (Fig. 8). The limiting solids-mass ¯ow was then compared with the eective speci®c mass ¯ow in the clari®ers. The theoretical limiting solids-mass ¯ow in this study obtains js,limit=9.0 kg TSS mÿ2 hÿ1, which is 25% higher than that obtained in earlier experiments with lower sludge-settling velocities and a higher SVI (Fig. 8, right). Figure 9 illustrates the concentration of suspended solids in the euent as a function of the sludge-volume load jsSVI. It shows that exceeding the limiting sludge volume load may lead to overload and the escape of sludge, whereof lower loads represents safe operating con-
313
ditions. It has to be considered that 2 h-peak-loads up to 1100 (30-11-'96) and 1700 l mÿ2 hÿ1 (16-12'93) respectively, were obtained. The solids-¯ux method, which is based on the assumption of uniform vertical ¯ow, obviously allows the capacity of this clari®er to be estimated in spite of the inhomogeneous ¯ow conditions (Siegrist et al., 1995). Since April 1996 no overloads have occurred and the concentrations of suspended solids in the euent have always been less than 20 mg TSS lÿ1. The increased sludge-blanket level did not aect the removal eciency of the suspended solids (Fig. 9). It is also evident that the denitri®cation eciency in the sludge blanket did not cause lifting of sludge due to rising N2-gas bubbles. Theoretical considerations proved that shallow clari®ers (depth of the water <3 m), high water temperatures (T>208C) and plants with short sludge retention times (SRT) without predenitri®cation are critical for sludge ¯oating (Siegrist and Krebs, 1997). The settling velocity Vs is given as an exponential function of the solids concentration X in the zone settling phase Vs V0 eÿnX :
3
The extrapolated settling velocity V0 at X = 0 and the exponent n may be expressed as functions of the SVI, leading to a much more convenient method for settling-¯ux analysis. Wahlberg and Keinath (1995) formulated these constants on the basis of the SSVI (stirred SVI) which is approximately 67% of DSVI (diluted SVI) (from Ekama et al., 1997). Later regressions were obtained by HaÈrtel and PoÈpel (1992) as a function of the DSVI on the basis of additional data from the literature. A comparison of the approach of Wahlberg and Keinath with measurements shows good agreement (Fig. 8) whereas the relationship obtained from HaÈrtel and PoÈpel leads to a signi®cant underestimation for both experiments. Denitri®cation eciency The total denitri®cation eciency and nitrogen elimination from the activated sludge tank and the
Fig. 8. Observed settling velocity in the cylinder as a function of solids concentration in the zone settling phase. Measurements from this study (left) and from RoÈck and Rufer (1994) (right).
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secondary clari®er were de®ned as follows (iN=0.050 g N/g COD, iCOD=1.13 g COD/g TSS, Nsol=(NO3+NO2+NH4) ÿ N, dissolved organic N in the euent is neglected): Zden,tot
Qin Ntot,in ÿ Nsol,sc,out ÿ TSSsc,out iN iCOD ÿ Qes TSSes iN iCOD , Qin Ntot,in ZN,el
Qin Ntot,in ÿ Nsol,sc,out ÿ TSSsc,out iN iCOD : Qin Ntot,in
The dierence between the equations above refers to the nitrogen being incorporated into the excess sludge. During the investigated period, about 19% of the total inlet nitrogen was incorporated into the biomass, 52% was denitri®ed and 29% was present in the out¯ow (Fig. 10). Purtschert et al. (1996) measured a total denitri®cation capacity of only 35% at the treatment plant in ZuÈrich±WerdhoÈlzli under normal operating conditions (Xt=3.0 kg TSS mÿ3, uscr=4.8 cm sÿ1) with CODtot,in:Ntot,in= 6.5 g COD/g N. From grab samples and dynamic simulation it can be concluded that mainly nitrate was limiting denitri®cation in the anoxic zone except during the night hours (Fig. 11), weekends and wet-weather ¯ows. The simulation showed, that with an additional internal recirculation denitri®cation eciency in the anoxic zone can only be increased slightly from 33 to 38% of total inlet nitrogen. On the basis of an N balance, the denitri®cation eciency of the secondary clari®er is calculated as follows (nitrogen incorporation into the biomass is neglected) Zden,sc
Qin Qrs Nsol,sc,in ÿ Qin Nsol,sc,out ÿ Qrs Nsol,rs Qin Ntot,in Qrs Nsol,sc,out ÿ Nsol,rs Qin Ntot,in
Qin Qrs Nsol,sc,in ÿ Nsol,sc,out : Qin Ntot,in
6
4
5
The ®rst term of equation 6 represents the denitri®cation eciency in the sludge blanket while the second term describes this same eciency in the inlet channel (Fig. 2). Over the whole period, 19% of the total inlet nitrogen was denitri®ed in the clari®er, corresponding to 37% of the total denitri®cation eciency (Fig. 3). Compared to 7% of the total inlet nitrogen load (Thomann et al., 1996) and 6% (RoÈck and Rufer, 1994) from earlier measurements at the WerdhoÈlzli treatment plant (Xt=3.0 kg TSS mÿ3, uscr=4.8 cm sÿ1) the denitri®cation in the clari®er increased substantially. Because of the large amount of sludge in the inlet channel, only approximately 5±6% of the total inlet nitrogen was denitri®ed in the sludge blanket, the remaining 13±14% being denitri®ed in the inlet channel. Considering the sludge mass in the sludge blanket and the inlet channel, the denitri®cation rate results in 0.8 g N/g 8TS hÿ1. During the weekends, the denitri®cation eciency in the secondary clari®er was lower due to the lower COD inlet load. Relative to the inlet ¯ow, about 0.5±1.5 mg NH4±N lÿ1 was nitri®ed in the inlet channel parallel to the oxygen uptake during transient failure of the nitri®cation eciency in the activated sludge tank. It is evident that doubling the scraper interval did not double the denitri®cation eciency in the secondary clari®er, since the sludge mass increased by only 28%. Surprisingly, doubling the scraper interval improved the denitri®cation eciency in the sec-
Fig. 9. Concentrations of suspended solids in the euent vs. sludge-volume load (daily average) at the WerdhoÈlzli clari®er. The limiting sludge-volume load js,limitSVI = 9.0 kg TSS mÿ2 hÿ1125 l kg TSSÿ136.8 kg TSS mÿ2 hÿ1165 l kg TSSÿ1=1100 l mÿ2 hÿ1.
Potential of denitri®cation and solids removal in the rectangular clari®er
Fig. 10. Eciency of nitrogen removal. Qrs/Qin=2.0. CODtot,in:Ntot,in=8.2 g COD/g N.
ondary clari®er by only about 8%. Considering the generally lower temperature during the weeks with uscr=2.4 cm sÿ1 (wet weather), the corrected eciency increases to 14% (kT,hydrolysis=0.038Cÿ1). A possible explanation for the relatively small increase is that the nitrate partly limits the denitri®cation in deeper layers of the sludge blanket (especially at locations with high sludge levels) during periods of lower scraper velocity. Modelling of denitri®cation in the secondary clari®er For the dry-weather simulation (Appendix B, Table 3 and Fig. 12), activated sludge model No. 2 (Gujer et al., 1995) was implemented in Aquasim (Reichert, 1994) and used for simulating nitrate and ammonium pro®les. All processes connected to biological phosphorus removal were switched o. The maximum growth and decay rates for the nitri®ers were selected from Siegrist and Tschui (1992) (Appendix A, Table 2). The saturation coecients for acetate KA and fermentable substrate KF were estimated to be 1.0 mg COD lÿ1 from the oxygen consumption of 14 batch experiments with waste water as the substrate. The modelling of heterotrophic growth on a soluble substrate with two separate processes requires the saturation coecient KS
315
obtained from single substrate simulation to be reduced by half. From 26 anoxic and aerobic respiration experiments with sludge taken from dierent tanks of a pilot plant, the reduction factor for anoxic hydrolysis resulted in Zh' = 0.63 2 0.12, which corresponds well with Zh' = 0.6 selected from Siegrist et al. (1995). Based on the energy requirement of the related metabolic processes the heterotrophic yield coecient is set signi®cantly lower for anoxic than for aerobic growth (Maurer and Gujer, submitted; Orhon et al., 1996; Yhet,ano and Yhet,aer see Appendix A, Table 2). Considering this the correction factor Zh for anoxic hydrolysis reduces to Zh Zh 0
Yano 1 ÿ Yaer 0:42: Yaer 1 ÿ Yano
7
During hours of low oxygen concentrations in the aeration tanks (insucient aeration), ammonia ®ts best with an oxygen saturation coecient of KO=0.5 mg O2 lÿ1 and an ammonia saturation coecient KNH=1.0 mg N lÿ1 (results not shown), which is equal to the results found by Siegrist et al. (1995). Although the dynamic variations of the calculated nitrate pro®les are less pronounced, good agreement with average experimental nitrate concentrations was found (Fig. 11). The inlet channel of the secondary clari®er has to be considered as an additional anoxic zone (R7 in Fig. 6) which substantially increase the nitrogen removal. CONCLUSIONS
The sludge-blanket levels along the rectangular, secondary clari®er of the ZuÈrich±WerdhoÈlzli treatment plant with transverse ¯ow show a linear correlation with the mass ¯ow of suspended solids, SVI and the accumulation time. Because the sludge is almost completely removed after the passage of the vacuum scraper, the sludge-blanket volume can be
Fig. 11. Comparison of experimental and calculated nitrate pro®les from 06.00, 4-12-'96 to 06.00, 6-12'96 (¯ow scheme). uscr=4.8 cm sÿ1, T = 158C. Characterizations of the waste water and the daily variation are described in Appendix A, Table 2.
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modelled as a function of these three parameters. An average solids concentration of 8±9 kg TSS mÿ3 could be estimated from the calculated sludge volume and the mass balance of the suspended solids over the entire secondary clari®er. This value was con®rmed by measured concentration pro®les. The limiting sludge-volume load was calculated on the basis of sludge sedimentation properties and compared with observed sludge-volume loads. It turned out that the solids-¯ux method is suitable for estimating the capacity of the clari®er. Since the peak in¯ow of the plant was reduced from 9 to 6 m3 sÿ1, the higher solids mass in the sludge blanket due to the higher concentration of activated sludge (3.0 to 4.5 kg TSS mÿ3) as well as the lower scraper velocity did not worsen the removal eciency of the suspended solids. However, the denitri®cation in the clari®er improved considerably due to this measure. Over the whole period, about 19% of the total inlet nitrogen was denitri®ed in the entire clari®er, whereby 13±14% was denitri®ed in the inlet channel and the remaining part in the sludge blanket. Doubling the scraper interval increased the denitri®cation in the entire clari®er by only 14%, which is in disagreement with the 28% higher sludge mass calculated from mass balances over the clari®er. The disparity is thought to be due to nitrate limitations in deeper layers of the risen sludge blanket at periods with lower scraper velocity. Good agreement was found between the nitrogen pro®les simulated with activated sludge model No. 2 and experimental data (dry-weather day, uscr=4.8 cm sÿ1).
REFERENCES
A131 (1991) Bemessung von einstu®gen Belebungsanlagen (The design of single stage activated-sludge plants). Abwassertechnische Vereinigung ATV, Gesellschaft zur FoÈrderung der Abwassertechnik, Hennef. HaÈrtel L. and PoÈpel J. (1992) A dynamic secondary clari®er model including processes of sludge thickening. Wat. Sci. Technol. 25(6), 267±284. Ekama G. A., Barnard J. L., GuÈnthert F. W., Krebs P., McCorquodale J. A., Parker D. S. and Wahlberg E. J. (1997) Theory, modelling, design and operation of secondary settling tanks. International Association on Water Quality, Scienti®c and Technical Report (STR) Series, London. Fillos J., Diyamandoglu V., Carrio L. A. and Robinson L. (1996) Full-scale evaluation of biological nitrogen
removal in the step-feed activated sludge process. Water Environ. Res. 68(2), 132±142. Gujer W., Henze M., Mino T., Matsuo T., Wentzel M. C. and Marais G. v. R. (1995) The activated sludge model No. 2: Biological phosphorus removal. Water Sci. Technol. 31(2), 1±11. Hamilton J., Jain R., Antoniou P., Svoronos S. A., Koopman B. and Lyberatos G. (1992) Modelling pilotscale experimental veri®cation for predenitri®cation process. J. Envir. Eng. 118(1), 38±55. Jain R., Lyberatos G., Svoronos S. A. and Koopman B. (1992) Operational strategies for predenitri®cation process. J. Envir. Eng. 118(1), 56±67. Kynch J. J. (1952) A theory of sedimentation. Trans. Faraday Soc. 148, 166±176. Maurer M. and Gujer W. Dynamic modelling of enhanced biological phosphorus and nitrogen removal in activated sludge systems. Water Sci. Technol., submitted. Orhon D., SoÈzen S. and Artan N. (1996) The eect of heterotrophic yield on the assessment of the correction factor for anoxic growth. Water Sci. Technol. 34(5±6), 67± 74. Pianta R. and Marti Ch. (1997) Denitri®kation im NachklaÈrbecken (Denitri®cation in secondary clari®er). Diploma thesis, ETH-ZuÈrich. Purtschert I., Siegrist H. and Gujer W. (1996) Enhanced denitri®cation with methanol at WWTP Zurich± WerdhoÈlzli. Wat. Sci. Technol. 33(12), 117±126. Reichert P. (1994) AQUASIM ± A tool for simulation and data analysis of aquatic systems. Wat. Sci. Technol. 30(2), 21±30. RoÈck C. and Rufer R. (1994) Denitri®kationsversuche an der ARA WerdhoÈlzli (Denitri®cation experiments at the WWTP Zurich±WerdhoÈlzli). Diploma thesis, ETHZuÈrich, Switzerland. Siegrist H. and Tschui F. (1992) Interpretation of experimental data with regard to the activated sludge model No. 1 and calibration of the model for municipal wastewater treatment plants. Wat. Sci. Technol. 25(6), 167± 183. Siegrist H., Krebs P., BuÈhler R., Purtschert I., RoÈck C. and Rufer R. (1995) Denitri®cation in secondary clari®er. Wat. Sci. Technol. 31(2), 205±214. Siegrist H. and Krebs P. (1997) NachklaÈrbecken: Prozesse und Dimensionierung (Secondary clari®er: processes and design). PEAK A7/97, EAWAG-DuÈbendorf, Switzerland. Thomann M., Siegrist H., Diebold J., Zimmerli T. and Gujer W. (1996) Denitri®kation im NachklaÈrbecken (Denitri®cation in secondary clari®er). Gas Wasser Abwasser 11, 854±858. Wahlberg E. J., Augustus M., Chapman D. T., Chen C.-L., Esler J. K., Keinath T. M., Parker D. S., Tekippe R. J. and Wilson T. E. (1994). Evaluating activated sludge clari®er performance using the CRTC protocol: four case studies. Proc. 1±12, 67 Annual WEF Conf., Chicago, U.S.A. Wahlberg E. J. and Keinath T. M. (1995) Development of settling ¯ux curves using SVI: An addendum. Wat. Environ. Res. 67(5), 872±874.
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APPENDIX A
Kinetic parameters, waste-water characterization and stoichiometric constants for simulations with AQUASIM Table 2. Kinetic parameters, waste-water characterization and stoichiometric constants at T = 208C. Earlier studies: Siegrist et al. (1995) and Gujer et al. (1995) Parameter
Symbol
Earlier studies
This study
50/0.03 10 30/0.03 3.0/0.03 0.2 0.5
50/0.03 10 21/0.03 21/0.03 0.15 0.25
2.5/0.07 2.0/0.07 0.25 0.5 0.1 4.0 4.0 0.1 3.0/0.07 20 0.55/0.07
6.0/0.07 4.8/0.07 0.15 0.25 0.01 1.0 1.0 0.1 3.0/0.07 20 0.4/0.07
0.9/0.10 0.5 1.0/0.10 0.25 0.09/0.10
0.9/0.105 0.5 1.0/0.0 0.5 0.01 0.09/0.105
Hydrolysis Kh,aerob Kx Kh,anox Kh,anaer Ko Kno
Aerobic hydrolysis rate constant (dÿ1) Saturation coecient for Xs/Xhet Anoxic hydrolysis rate constant (dÿ1) Anaerobic hydrolysis rate constant (dÿ1) Saturation/inhibition coecient for O2 (g O2 mÿ3) Saturation/inhibition coecient for NO3 (g N mÿ3)
Heterotrophic organisms mhet,aer Maximum aerobic growth rate (dÿ1) ÿ1 Maximum anoxic growth rate (d ) mhet,ano ÿ3 Ko Saturation/inhibition coecient for O2 (g O2 m ) Saturation/inhibition coecient for NO3 (g N mÿ3) Kno ÿ3 Knh Saturation coecient for NH4 (g N m ) Saturation coecient for Sf (g COD mÿ3) Kf ÿ3 Saturation coecient for Sac (g COD m ) Kac ÿ3 Kalk Saturation coecient for Salk (mol m ) qfe Maximum rate for fermentation (dÿ1) ÿ3 Kfe Saturation coecient for fermentation of Sf (g COD m ) ÿ1 Rate constant for lysis of biomass (d ) bhet Nitrifying organisms (autotrophic organisms) maut Maximum growth rate with Fe2+ addition (dÿ1) Saturation/inhibition coecient for O2 (g O2 mÿ3) Ko ÿ3 Knh Saturation coecient for NH4 (g N m ) ÿ3 Saturation coecient for Salk (mol m ) Kalk Kp Saturation coecient for HPO4 (g P mÿ3) ÿ1 baut Rate constant for lysis of biomass (d ) Heterotrophic organisms aerobic Heterotrophic organisms anoxic Autotrophic organism
Yield coecient Yhet,aer Yhet,ano Yaut
0.64 0.64 0.24
0.63 0.53 0.24
Of Of Of Of Of
Nitrogen content iN,BM iN,Si iN,Sf iN,Xi iN,Xs
0.07 0.01 0.03 0.03 0.04
0.07 0.01 0.03 0.04 0.025
Organic fractions of CODtot (primary euent) Sf Sa Si Xi Xs Xh Xaut
0.05 0.05 0.05 0.25 0.60 0.00 0.0
0.05 0.05 0.07 0.20 0.53 0.10 0.0
Fraction fSi fX i
0 0.08
0 0.1
biomass inert soluble COD soluble fermentable substrate COD inert particulate COD particulate substrate COD
Fermentable substrate Volatile acids (acetate) Inert, non-biodegradable organics Inert, non-biodegradable organics Slowly biodegradable substrate Heterotrophic biomass Autotrophic, nitrifying biomass Si from hydrolysis Xs Xi from biomass lysis
Values behind slashes are temperature coecients (8Cÿ1). Bold characters: values from this study. APPENDIX B
Inlet concentrations Table 3. Mean values of primary-euent concentrations of the required model compounds during the investigated period. Qin=19,840 m3 dÿ1. Soluble components Salk Sf Sa Si SNH4 SNO3 SO 2
Unit
Mean value ÿ1
mmol l mg COD lÿ1 mg COD lÿ1 mg COD lÿ1 mg N lÿ1 mg N lÿ1 mg O2 lÿ1
4.8 10 10 14 16.8 1.1 5
Particulate components Xaut Xh Xi Xs
Unit mg mg mg mg
Mean value ÿ1
COD l COD lÿ1 COD lÿ1 COD lÿ1
0 20 40 107
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Fig. 12. Diurnal variation of inlet data from 06.00, 4 December `96 until 06.00, 6 December `96. T = 158C.
Wat. Res. Vol. 33, No. 2, pp. 309±318, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
POTENTIAL OF DENITRIFICATION AND SOLIDS REMOVAL IN THE RECTANGULAR CLARIFIER M M G. KOCH1*, R. PIANTA1, P. KREBS2* and H. SIEGRIST1*
Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 DuÈbendorf, Switzerland and 2Institute for Urban Water Management, Dresden University of Technology, D-01062 Dresden, Germany
1
(First received February 1998; accepted in revised form May 1998) AbstractÐIn full scale experiments, the denitri®cation capacity of secondary clari®ers and measures to increase it were evaluated. Due to a higher activated-sludge concentration and a lower scraper velocity, the sludge mass and thus the denitri®cation capacity in the secondary clari®er at the WWTP ZuÈrich± WerdhoÈlzli was signi®cantly increased. During the investigated period, the denitri®cation in the clari®er represented 37% of the total denitri®cation, corresponding to 19% of the total inlet nitrogen. The main part was denitri®ed in the inlet channel of the clari®er, so that doubling the scraper interval increased the denitri®cation in the clari®er by only about 14%. An approach for modelling the sludge-blanket volume was developed on the basis of measurements of the sludge-blanket height along the rectangular clari®er with transverse ¯ow and vacuum-removal scrapers. The limiting sludge-volume load was calculated from sludge settling tests and compared with observed loads on the full-scale clari®er. Activated sludge model No. 2 was veri®ed with the nitrate and ammonium pro®les in the activated sludge tanks, return sludge and clari®er euent. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐactivated sludge model, denitri®cation, dynamic simulation, full-scale experiments, nitrogen removal, secondary clari®er, sludge blanket, solid-¯ux method
NOMENCLATURE js=solids-loading rate (M lÿ2 Tÿ1) Zden=denitri®cation eciency ZN,el=eciency of nitrogen removal Nsol=soluble inorganic nitrogen (M lÿ3) Ntot=particulate and soluble nitrogen (M lÿ3) Qes=excess sludge (l3 Tÿ1) Qin=in¯uent to WWTP (l3 Tÿ1) Qrs=return sludge (l3 Tÿ1) SM=solid mass (M) SRT=sludge retention time (T) SVI=sludge volume index (l3 Mÿ1) T=temperature of waste water (8C) tacc=average sludge accumulation time (T) tL/3=scraper runtime for 1/3 Lsc (T) uscr=scraper velocity (l Tÿ1) V=volume (l3) X=solids concentration (M lÿ3) sb=sludge blanket sc=secondary clari®er t=activated sludge tank
INTRODUCTION
Due to the hydrolysis of particulate degradable COD and the decay of biomass oxygen reduction and denitri®cation in the sludge blanket of the secondary clari®er occur. Only few case studies about these processes have been presented in the literature (Hamilton et al., 1992; Jain et al., 1992; Fillos et *Author to whom all correspondence should be addressed. [Tel: +41-1-823±53±86; Fax: +41-1-8235389, E-mail: [email protected]]. 309
al., 1996). Based on experimental data and activated-sludge simulations Siegrist et al. (1995) clearly showed, that the eect of clari®er denitri®cation on process performance becomes signi®cant with increasing sludge mass in the clari®er. The aim of this study was to investigate the denitri®cation eciency in the secondary clari®ers at the WWTP of ZuÈrich±WerdhoÈlzli and to improve the denitri®cation capacity by maintaining a greater sludge-blanket volume. For this purpose, full-scale experiments were performed (Fig. 1) during seven weeks (28 October±13 December 1996). The experimental data were then used to verify the activated sludge model No. 2 (Gujer et al., 1995) with regard to the hydrolysis, nitri®cation and denitri®cation processes. The increased sludge blanket level during the investigated period required a more detailed monitoring of the plant behavior to prevent overload of the clari®er and escape of sludge. Therefore, the dynamics of the sludge blanket and the sludge mass in the clari®er under dierent solids-¯ux conditions and scraper intervals were analyzed. From these data a model for the behavior of the sludge blanket was developed. Furthermore a solids-¯ux analysis was performed in order to evaluate the settling capacity of the clari®er. An increased sludge-blanket volume can be obtained by setting lower scraper velocities, thus leading to longer intervals between two scraper passages (Siegrist et al., 1995). To this end, the scraper
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Fig. 1. Sample program of the experimental lane of the ZuÈrich±WerdhoÈlzli activated-sludge plant.
motors were equipped with a frequency controller to reduce the maximal velocity by 50%. In April 1996, the maximum combined water in¯uent to the treatment plant was reduced from 9 to 6 m3 sÿ1. As a result, the suspended-solids load of the secondary clari®ers decreased substantially during storm events. This measure allowed the sludge concentration in the activated-sludge tanks Xt to be increased from 3.0 to 4.5 kg TSS mÿ3, leading to an additional increase of the sludge mass in the clari®er during dry-weather conditions.
15 days and the return sludge Qrs was 6 30,000 m3 dÿ1, i.e. about twice the dry-weather ¯ow. Each clari®er tank is equipped with three vacuum scrapers, normally operated at a velocity of uscr=4.8 cm sÿ1, which results in a period of 2 tL/ 3=34 min, referred to the back-and-forth passage of the scraper in one third of the clari®er. To evaluate the eect of the scraper velocity on the denitri®cation capacity in the sludge blanket, uscr was operated at 4.8 and 2.4 cm sÿ1 (week 3, 5, 7; Fig. 3). MATERIALS AND METHODS
FULL SCALE PLANT AND TEST CONFIGURATION
The waste-water treatment plant at ZuÈrich± WerdhoÈlzli (600,000 p.e.) is split up into two lanes, north and south, each with a dry-weather ¯ow of 80,000 m3 dÿ1. Both lanes have six parallel activated-sludge tanks (6 5,000 m3) and six rectangular clari®ers (6 6,000 m3) with transverse ¯ow and scrapers with vacuum removal (Fig. 2). In the north lane, the activated sludge tanks are equipped with a 28% anoxic volume (Fig. 1). SRT was about
Sludge blanket With the aid of a mobile turbidimeter sensor, the sludge-blanket height was recorded at dierent positions (8.0 and 1.0 m screening in longitudinal and transverse direction, respectively) in relation to the time elapsed since the passage of the vacuum scraper (Fig. 2). The interface between the sludge blanket and the clear water was observed by the Staiger-Mohilo 7120 ASH turbidimeter sensor, the height of the sludge blanket was de®ned at X = 3.0 kg TSS mÿ3. A vacuum pump was used to withdraw sludge from within the sludge blanket to determine the density pro®les.
Fig. 2. Cross section of the secondary clari®er and measurement set-up.
Potential of denitri®cation and solids removal in the rectangular clari®er
311
Fig. 3. Denitri®cation in the secondary clari®er (equation 6) and in total (equation 4) during the investigated period. T = 15.58C. Sludge settling parameters The sludge settling properties and the sludge volume index SVI were measured with a plastic cylinder (diameter 28.5 cm, height 1.50 m, settling time for SVI determination 30 min). The SVI observed with the plastic cylinder corresponds to the DSVI (diluted SVI) in the 1 l measuring cylinder. On-line-measurements After ultra-®ltration, the sum of the nitrate + nitrite was analyzed by the Dr. Lange Nitrax analyzer on the basis of ultraviolet absorption. NO2 and NH4 were analyzed photometrically after a color reaction (Skalar, OPA SA9000, nitrite at 540 nm, ammonia at 660 nm). 24 h composite samples The anorganic nitrogen compounds nitrate, nitrite and ammonia were determined colorimetrically by ¯ow injection analysis (FIA) after 0.7 mm ®ltration. The total nitrogen content was measured in the same way after sample digestion with K2S2O8. Both the dissolved and total COD were measured colorimetrically by the HACH method (DR/2000). The total suspended solids (TSS) were ®ltered with a GF/F ®lter (0.7 mm). The ®ltered material was dried for two hours at 1058C.
RESULTS AND DISCUSSION
The ratio between the growing sludge-blanket height and the speci®c sludge-volume load (extracted from Fig. 4) calculated at every measuring position allowed the dynamics of the sludge blanket to be modelled as a function of the sludgevolume load and scraper velocity (Fig. 5). Model calculations show that the clari®er is fed quite inhomogeneously, leading to maximum heights of 0.8 m at rain-weather peak loads and reduced scraper velocity. The volume of the sludge blanket is approximately equal in each third. The average sludge blanket volume, mass and thickness could be calculated for dierent operating states on the basis of the linearized model (Table 1). This shows that the average thickness is increased only slightly due to longer scraper intervals and/or higher mass ¯ow rates (hsb,max=0.36 m). The actual sludge accumulation time in one third of the clari®er depends on the actual position and the velocity of the scraper. Thomann et al. (1996) showed that the average sludge accumulation time for a scraper passage in one third of the clari®er is tacc=2/3 tL/3 only. Under steady-state conditions, the ratio between the solids mass in the whole
Sludge-blanket dynamics in secondary clari®er To describe the behavior of the sludge blanket, the growing sludge level had to be measured at various positions (about 100 sites) depending on the SVI, the actual mass ¯ow of suspended solids and the local accumulation time (Fig. 4). The local accumulation time is equal to the time elapsed after the scraper passage. Measurements showed that the variation of the sludge blanket height is directly related to these parameters. Due to its good settling properties (SVI = 125 ml gÿ1) and a high returnsludge ratio R = Qrs/Qin=0.8±2.0 (i.e. wet-weather and dry-weather conditions, respectively) the sludge blanket is almost completely removed after the passage of the scraper.
Fig. 4. Growing blanket with dierent sludge-volume loads jsSVI. Low load from Thomann et al. (1996).
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Fig. 5. Modelled sludge-blanket height with uscr=2.4 cm sÿ1, Xt=4.5 kg TSS mÿ3, Qin=1,500 m3 hÿ1, Qrs=1,250 m3 hÿ1, SVI = 125 ml gÿ1 (Pianta and Marti, 1997).
sludge blanket SMsb and that in the activated sludge tank SMt is thus SMsb Qrs Qin Xt tacc Qrs Qin tacc : SMt Vt Xt Vt
1
The reduction of the scraper velocity by 50% increases the ratio of equation 1 from 7 to 13% (Table 1) whereas during maximum peak ¯ow about 21% of the total amount of suspended solids in the activated sludge tank is stored in the sludge blanket. With regard to the denitri®cation capacity in the secondary clari®er, it has to be considered that a substantial amount of biomass (approxi-
mately 14% of SMt) is stored in the inlet channel of the clari®er (see Figs 2 and 6). Compared to the ATV-guidelines A131 (1991) the hydraulic retention time of the inlet channel (15 and 23 min at wet- and dry-weather ¯ow, respectively) is relatively high, but satisfy the ¯occulation criterion suggested from Wahlberg et al. (1994). The mass of solids in the transition zone (about 0.20 m with 1.5 kg TSS mÿ3, Fig. 7) de®ned by the surface of the sludge blanket (3.0 kg TSS mÿ3) and the clear water zone is SMtrans=1350 m20.2 m 1.5 kg TSS mÿ3=400 kg TSS. Knowing SMtrans, SMsb and the modelled volume Vsb, the average concentration of suspended
Table 1. Modelled sludge blanket. Qrs=1,250 m3 hÿ1, SMtrans=400 kg TSS, SVI = 125 ml gÿ1 Xt; Qin (kg TSS mÿ3, m3 hÿ1) 3.0; 3.0; 4.5; 4.5;
550 550 550 1,500
uscr (cm sÿ1)
tacc (min)
SMsb (kg TSS)
SMsb/SMt
Vsb (m3)
hsb (m)
Xsb (kg TSS mÿ3)
4.8 2.4 2.4 2.4
11.4 22.4 22.4 22.4
1,030 2,020 3,020 4,620
0.07 0.13 0.13 0.21
100 200 300 460
0.08 0.16 0.24 0.36
6.5 8.2 8.8 9.2
Fig. 6. Flow scheme and operating conditions. The sludge blanket (reactor R1) was simulated with a constant volume with SMsb=XR1VR11(Qrs+Qin)Xttacc. The inlet channel of the clari®er (reactor R7) was simulated as a subsequent anoxic zone.
Potential of denitri®cation and solids removal in the rectangular clari®er
Fig. 7. Density pro®les from 13 December `96, local accumulation time for each pro®le 45 min.
solids Xsb in the sludge blanket below the transition zone is Xsb
SMsb ÿ SMtrans : Vsb
2
At higher sludge-blanket levels, the solids concentration Xsb according to equation 2 increases to about 8±9 kg TSS mÿ3 (Table 1), which agrees well with the average concentrations from density pro®les (Xsb=8.3 kg TSS mÿ3). Suspended-solids removal and settling capacity A solids-¯ux analysis was performed with regard to evaluate the settling capacity of the clari®er during the investigated periods. The limiting solidsmass ¯ow js,limit was determined according to the ¯ux-theory (Kynch, 1952) on the basis of observed settling velocities of the activated sludge (Fig. 8). The limiting solids-mass ¯ow was then compared with the eective speci®c mass ¯ow in the clari®ers. The theoretical limiting solids-mass ¯ow in this study obtains js,limit=9.0 kg TSS mÿ2 hÿ1, which is 25% higher than that obtained in earlier experiments with lower sludge-settling velocities and a higher SVI (Fig. 8, right). Figure 9 illustrates the concentration of suspended solids in the euent as a function of the sludge-volume load jsSVI. It shows that exceeding the limiting sludge volume load may lead to overload and the escape of sludge, whereof lower loads represents safe operating con-
313
ditions. It has to be considered that 2 h-peak-loads up to 1100 (30-11-'96) and 1700 l mÿ2 hÿ1 (16-12'93) respectively, were obtained. The solids-¯ux method, which is based on the assumption of uniform vertical ¯ow, obviously allows the capacity of this clari®er to be estimated in spite of the inhomogeneous ¯ow conditions (Siegrist et al., 1995). Since April 1996 no overloads have occurred and the concentrations of suspended solids in the euent have always been less than 20 mg TSS lÿ1. The increased sludge-blanket level did not aect the removal eciency of the suspended solids (Fig. 9). It is also evident that the denitri®cation eciency in the sludge blanket did not cause lifting of sludge due to rising N2-gas bubbles. Theoretical considerations proved that shallow clari®ers (depth of the water <3 m), high water temperatures (T>208C) and plants with short sludge retention times (SRT) without predenitri®cation are critical for sludge ¯oating (Siegrist and Krebs, 1997). The settling velocity Vs is given as an exponential function of the solids concentration X in the zone settling phase Vs V0 eÿnX :
3
The extrapolated settling velocity V0 at X = 0 and the exponent n may be expressed as functions of the SVI, leading to a much more convenient method for settling-¯ux analysis. Wahlberg and Keinath (1995) formulated these constants on the basis of the SSVI (stirred SVI) which is approximately 67% of DSVI (diluted SVI) (from Ekama et al., 1997). Later regressions were obtained by HaÈrtel and PoÈpel (1992) as a function of the DSVI on the basis of additional data from the literature. A comparison of the approach of Wahlberg and Keinath with measurements shows good agreement (Fig. 8) whereas the relationship obtained from HaÈrtel and PoÈpel leads to a signi®cant underestimation for both experiments. Denitri®cation eciency The total denitri®cation eciency and nitrogen elimination from the activated sludge tank and the
Fig. 8. Observed settling velocity in the cylinder as a function of solids concentration in the zone settling phase. Measurements from this study (left) and from RoÈck and Rufer (1994) (right).
314
G. Koch et al.
secondary clari®er were de®ned as follows (iN=0.050 g N/g COD, iCOD=1.13 g COD/g TSS, Nsol=(NO3+NO2+NH4) ÿ N, dissolved organic N in the euent is neglected): Zden,tot
Qin Ntot,in ÿ Nsol,sc,out ÿ TSSsc,out iN iCOD ÿ Qes TSSes iN iCOD , Qin Ntot,in ZN,el
Qin Ntot,in ÿ Nsol,sc,out ÿ TSSsc,out iN iCOD : Qin Ntot,in
The dierence between the equations above refers to the nitrogen being incorporated into the excess sludge. During the investigated period, about 19% of the total inlet nitrogen was incorporated into the biomass, 52% was denitri®ed and 29% was present in the out¯ow (Fig. 10). Purtschert et al. (1996) measured a total denitri®cation capacity of only 35% at the treatment plant in ZuÈrich±WerdhoÈlzli under normal operating conditions (Xt=3.0 kg TSS mÿ3, uscr=4.8 cm sÿ1) with CODtot,in:Ntot,in= 6.5 g COD/g N. From grab samples and dynamic simulation it can be concluded that mainly nitrate was limiting denitri®cation in the anoxic zone except during the night hours (Fig. 11), weekends and wet-weather ¯ows. The simulation showed, that with an additional internal recirculation denitri®cation eciency in the anoxic zone can only be increased slightly from 33 to 38% of total inlet nitrogen. On the basis of an N balance, the denitri®cation eciency of the secondary clari®er is calculated as follows (nitrogen incorporation into the biomass is neglected) Zden,sc
Qin Qrs Nsol,sc,in ÿ Qin Nsol,sc,out ÿ Qrs Nsol,rs Qin Ntot,in Qrs Nsol,sc,out ÿ Nsol,rs Qin Ntot,in
Qin Qrs Nsol,sc,in ÿ Nsol,sc,out : Qin Ntot,in
6
4
5
The ®rst term of equation 6 represents the denitri®cation eciency in the sludge blanket while the second term describes this same eciency in the inlet channel (Fig. 2). Over the whole period, 19% of the total inlet nitrogen was denitri®ed in the clari®er, corresponding to 37% of the total denitri®cation eciency (Fig. 3). Compared to 7% of the total inlet nitrogen load (Thomann et al., 1996) and 6% (RoÈck and Rufer, 1994) from earlier measurements at the WerdhoÈlzli treatment plant (Xt=3.0 kg TSS mÿ3, uscr=4.8 cm sÿ1) the denitri®cation in the clari®er increased substantially. Because of the large amount of sludge in the inlet channel, only approximately 5±6% of the total inlet nitrogen was denitri®ed in the sludge blanket, the remaining 13±14% being denitri®ed in the inlet channel. Considering the sludge mass in the sludge blanket and the inlet channel, the denitri®cation rate results in 0.8 g N/g 8TS hÿ1. During the weekends, the denitri®cation eciency in the secondary clari®er was lower due to the lower COD inlet load. Relative to the inlet ¯ow, about 0.5±1.5 mg NH4±N lÿ1 was nitri®ed in the inlet channel parallel to the oxygen uptake during transient failure of the nitri®cation eciency in the activated sludge tank. It is evident that doubling the scraper interval did not double the denitri®cation eciency in the secondary clari®er, since the sludge mass increased by only 28%. Surprisingly, doubling the scraper interval improved the denitri®cation eciency in the sec-
Fig. 9. Concentrations of suspended solids in the euent vs. sludge-volume load (daily average) at the WerdhoÈlzli clari®er. The limiting sludge-volume load js,limitSVI = 9.0 kg TSS mÿ2 hÿ1125 l kg TSSÿ136.8 kg TSS mÿ2 hÿ1165 l kg TSSÿ1=1100 l mÿ2 hÿ1.
Potential of denitri®cation and solids removal in the rectangular clari®er
Fig. 10. Eciency of nitrogen removal. Qrs/Qin=2.0. CODtot,in:Ntot,in=8.2 g COD/g N.
ondary clari®er by only about 8%. Considering the generally lower temperature during the weeks with uscr=2.4 cm sÿ1 (wet weather), the corrected eciency increases to 14% (kT,hydrolysis=0.038Cÿ1). A possible explanation for the relatively small increase is that the nitrate partly limits the denitri®cation in deeper layers of the sludge blanket (especially at locations with high sludge levels) during periods of lower scraper velocity. Modelling of denitri®cation in the secondary clari®er For the dry-weather simulation (Appendix B, Table 3 and Fig. 12), activated sludge model No. 2 (Gujer et al., 1995) was implemented in Aquasim (Reichert, 1994) and used for simulating nitrate and ammonium pro®les. All processes connected to biological phosphorus removal were switched o. The maximum growth and decay rates for the nitri®ers were selected from Siegrist and Tschui (1992) (Appendix A, Table 2). The saturation coecients for acetate KA and fermentable substrate KF were estimated to be 1.0 mg COD lÿ1 from the oxygen consumption of 14 batch experiments with waste water as the substrate. The modelling of heterotrophic growth on a soluble substrate with two separate processes requires the saturation coecient KS
315
obtained from single substrate simulation to be reduced by half. From 26 anoxic and aerobic respiration experiments with sludge taken from dierent tanks of a pilot plant, the reduction factor for anoxic hydrolysis resulted in Zh' = 0.63 2 0.12, which corresponds well with Zh' = 0.6 selected from Siegrist et al. (1995). Based on the energy requirement of the related metabolic processes the heterotrophic yield coecient is set signi®cantly lower for anoxic than for aerobic growth (Maurer and Gujer, submitted; Orhon et al., 1996; Yhet,ano and Yhet,aer see Appendix A, Table 2). Considering this the correction factor Zh for anoxic hydrolysis reduces to Zh Zh 0
Yano 1 ÿ Yaer 0:42: Yaer 1 ÿ Yano
7
During hours of low oxygen concentrations in the aeration tanks (insucient aeration), ammonia ®ts best with an oxygen saturation coecient of KO=0.5 mg O2 lÿ1 and an ammonia saturation coecient KNH=1.0 mg N lÿ1 (results not shown), which is equal to the results found by Siegrist et al. (1995). Although the dynamic variations of the calculated nitrate pro®les are less pronounced, good agreement with average experimental nitrate concentrations was found (Fig. 11). The inlet channel of the secondary clari®er has to be considered as an additional anoxic zone (R7 in Fig. 6) which substantially increase the nitrogen removal. CONCLUSIONS
The sludge-blanket levels along the rectangular, secondary clari®er of the ZuÈrich±WerdhoÈlzli treatment plant with transverse ¯ow show a linear correlation with the mass ¯ow of suspended solids, SVI and the accumulation time. Because the sludge is almost completely removed after the passage of the vacuum scraper, the sludge-blanket volume can be
Fig. 11. Comparison of experimental and calculated nitrate pro®les from 06.00, 4-12-'96 to 06.00, 6-12'96 (¯ow scheme). uscr=4.8 cm sÿ1, T = 158C. Characterizations of the waste water and the daily variation are described in Appendix A, Table 2.
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modelled as a function of these three parameters. An average solids concentration of 8±9 kg TSS mÿ3 could be estimated from the calculated sludge volume and the mass balance of the suspended solids over the entire secondary clari®er. This value was con®rmed by measured concentration pro®les. The limiting sludge-volume load was calculated on the basis of sludge sedimentation properties and compared with observed sludge-volume loads. It turned out that the solids-¯ux method is suitable for estimating the capacity of the clari®er. Since the peak in¯ow of the plant was reduced from 9 to 6 m3 sÿ1, the higher solids mass in the sludge blanket due to the higher concentration of activated sludge (3.0 to 4.5 kg TSS mÿ3) as well as the lower scraper velocity did not worsen the removal eciency of the suspended solids. However, the denitri®cation in the clari®er improved considerably due to this measure. Over the whole period, about 19% of the total inlet nitrogen was denitri®ed in the entire clari®er, whereby 13±14% was denitri®ed in the inlet channel and the remaining part in the sludge blanket. Doubling the scraper interval increased the denitri®cation in the entire clari®er by only 14%, which is in disagreement with the 28% higher sludge mass calculated from mass balances over the clari®er. The disparity is thought to be due to nitrate limitations in deeper layers of the risen sludge blanket at periods with lower scraper velocity. Good agreement was found between the nitrogen pro®les simulated with activated sludge model No. 2 and experimental data (dry-weather day, uscr=4.8 cm sÿ1).
REFERENCES
A131 (1991) Bemessung von einstu®gen Belebungsanlagen (The design of single stage activated-sludge plants). Abwassertechnische Vereinigung ATV, Gesellschaft zur FoÈrderung der Abwassertechnik, Hennef. HaÈrtel L. and PoÈpel J. (1992) A dynamic secondary clari®er model including processes of sludge thickening. Wat. Sci. Technol. 25(6), 267±284. Ekama G. A., Barnard J. L., GuÈnthert F. W., Krebs P., McCorquodale J. A., Parker D. S. and Wahlberg E. J. (1997) Theory, modelling, design and operation of secondary settling tanks. International Association on Water Quality, Scienti®c and Technical Report (STR) Series, London. Fillos J., Diyamandoglu V., Carrio L. A. and Robinson L. (1996) Full-scale evaluation of biological nitrogen
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Potential of denitri®cation and solids removal in the rectangular clari®er
317
APPENDIX A
Kinetic parameters, waste-water characterization and stoichiometric constants for simulations with AQUASIM Table 2. Kinetic parameters, waste-water characterization and stoichiometric constants at T = 208C. Earlier studies: Siegrist et al. (1995) and Gujer et al. (1995) Parameter
Symbol
Earlier studies
This study
50/0.03 10 30/0.03 3.0/0.03 0.2 0.5
50/0.03 10 21/0.03 21/0.03 0.15 0.25
2.5/0.07 2.0/0.07 0.25 0.5 0.1 4.0 4.0 0.1 3.0/0.07 20 0.55/0.07
6.0/0.07 4.8/0.07 0.15 0.25 0.01 1.0 1.0 0.1 3.0/0.07 20 0.4/0.07
0.9/0.10 0.5 1.0/0.10 0.25 0.09/0.10
0.9/0.105 0.5 1.0/0.0 0.5 0.01 0.09/0.105
Hydrolysis Kh,aerob Kx Kh,anox Kh,anaer Ko Kno
Aerobic hydrolysis rate constant (dÿ1) Saturation coecient for Xs/Xhet Anoxic hydrolysis rate constant (dÿ1) Anaerobic hydrolysis rate constant (dÿ1) Saturation/inhibition coecient for O2 (g O2 mÿ3) Saturation/inhibition coecient for NO3 (g N mÿ3)
Heterotrophic organisms mhet,aer Maximum aerobic growth rate (dÿ1) ÿ1 Maximum anoxic growth rate (d ) mhet,ano ÿ3 Ko Saturation/inhibition coecient for O2 (g O2 m ) Saturation/inhibition coecient for NO3 (g N mÿ3) Kno ÿ3 Knh Saturation coecient for NH4 (g N m ) Saturation coecient for Sf (g COD mÿ3) Kf ÿ3 Saturation coecient for Sac (g COD m ) Kac ÿ3 Kalk Saturation coecient for Salk (mol m ) qfe Maximum rate for fermentation (dÿ1) ÿ3 Kfe Saturation coecient for fermentation of Sf (g COD m ) ÿ1 Rate constant for lysis of biomass (d ) bhet Nitrifying organisms (autotrophic organisms) maut Maximum growth rate with Fe2+ addition (dÿ1) Saturation/inhibition coecient for O2 (g O2 mÿ3) Ko ÿ3 Knh Saturation coecient for NH4 (g N m ) ÿ3 Saturation coecient for Salk (mol m ) Kalk Kp Saturation coecient for HPO4 (g P mÿ3) ÿ1 baut Rate constant for lysis of biomass (d ) Heterotrophic organisms aerobic Heterotrophic organisms anoxic Autotrophic organism
Yield coecient Yhet,aer Yhet,ano Yaut
0.64 0.64 0.24
0.63 0.53 0.24
Of Of Of Of Of
Nitrogen content iN,BM iN,Si iN,Sf iN,Xi iN,Xs
0.07 0.01 0.03 0.03 0.04
0.07 0.01 0.03 0.04 0.025
Organic fractions of CODtot (primary euent) Sf Sa Si Xi Xs Xh Xaut
0.05 0.05 0.05 0.25 0.60 0.00 0.0
0.05 0.05 0.07 0.20 0.53 0.10 0.0
Fraction fSi fX i
0 0.08
0 0.1
biomass inert soluble COD soluble fermentable substrate COD inert particulate COD particulate substrate COD
Fermentable substrate Volatile acids (acetate) Inert, non-biodegradable organics Inert, non-biodegradable organics Slowly biodegradable substrate Heterotrophic biomass Autotrophic, nitrifying biomass Si from hydrolysis Xs Xi from biomass lysis
Values behind slashes are temperature coecients (8Cÿ1). Bold characters: values from this study. APPENDIX B
Inlet concentrations Table 3. Mean values of primary-euent concentrations of the required model compounds during the investigated period. Qin=19,840 m3 dÿ1. Soluble components Salk Sf Sa Si SNH4 SNO3 SO 2
Unit
Mean value ÿ1
mmol l mg COD lÿ1 mg COD lÿ1 mg COD lÿ1 mg N lÿ1 mg N lÿ1 mg O2 lÿ1
4.8 10 10 14 16.8 1.1 5
Particulate components Xaut Xh Xi Xs
Unit mg mg mg mg
Mean value ÿ1
COD l COD lÿ1 COD lÿ1 COD lÿ1
0 20 40 107
318
G. Koch et al.
Fig. 12. Diurnal variation of inlet data from 06.00, 4 December `96 until 06.00, 6 December `96. T = 158C.