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Parameter determination and model fitting — Two approaches for modelling processes in wastewater treatment plants
~
Wat. Sci. Tull . Vol. 34. No. 5-6. pp, 27-33. 1996. CopyrightC 1996IAWQ. Published byElsevierScience Ltd Printedin Great Britain. All rightsreserved. 0273-1223196 S15'00 + frOO
Pergamon
PH:S0273-1223(96)00625-7
PARAMETER DETERMINATION AND MODEL FITTING - TWO APPROACHES FOR MODELLING PROCESSES IN WASTEWATER TREATMENT PLANTS M. Liebeskind, D. Schapers, C. Bornemann, E. Brands, M. Freund and T. Rolfs lnstitutfiirSiedlungswasse1wirtschaft. lSA, RWTHAachen, Templergraben 55. 52056Aachen, Germany
ABSTRACf This study shows results from four lab-scale wastewater treatment plants. which were insla!led behind the grit chamber of a municipal wastewater treatment plant The lab-scale plants were fed with original wastewater. diluted with water from the final clarifier to give a constantconcentration of 300 mg n COD. The activated sludges were adapted to different sludge ages from 1.88 to 24.0 days. The steady-state behaviourwas measured along with the decay rate. the yield coefficient, the oxygen demand. the C- and N· balance and the DNA of biomass XBH' The steady-state equations of Marais and Ekama were used to calculate the masses of active biomass. endogenous residue and inert VSS in the reactors. Measured parametervaluesweredifferentfor all sludgeages. They gave similarvaluesfor inen particulate VSS (Xi) of 7 to 9%. Parameter valuesof the IAWQ-Model No.1 togetherwith Xi·fitting could also be used to solve the equations. but in this case different Xi values for the different reactors had to be used for the same wastewater. Copyrighte 1996 IAWQ.Publishedby ElsevierScienceLtd.
KEYWORDS Activated sludge; decay rate; IAWQ-Model No.1; parameters; wastewater; Xi-determination . INTRODUcnON In recent times dynamic simulation of single sludge wastewater treatment processes bas become increasingly important. Various attempts have been made to simulate municipal wastewater treatment plants (for example Dupont, 1994; Londong, 1994; Spiller, 1994; van der Kuij, 1994). Often the kinetic and stoechiometric parameters for modelling are not determined to reduce the simulation costs. As a total mass balance of C and N for an existing wastewater treatment plant is difficult to obtain and even complete sets of bydrograpbs for several weeks are usually not available for full-scale plants. The modeller has to deal with great uncertainty concerning his data or has to install a small on-site pilot or lab-scale plant to get data sets that are SUfficiently complete to fit the model parameter values. The latter approach is quite costly. Compared with this, parameter determination is definitely less expensive and less time consuming. The question arises. Whether determination methods are sufficiently developed to obtain parameter values which fit into the dynamic model. This study shows a comparison of model-fitting techniques and parameter determination 27
28
M. LIEBESKlND et al.
techniques for the steady-state behaviour of four lab-scale wastewater treatment plants which were operated with municipal wastewater over three months . METHODS Lab-scale treatment plants
Three plants (designed for COD removal and nitrification) consisted of a 100 I storage tank (daily filled with fresh municipal wastewater and treated wastewater to give a final concentration of about 300 mgll COD) , a 15-1 aerated reactor for nitrification and COD removal, a 14 I settling tank and a storage tank for treated wastewater. A fourth lab-scale plant, designed for COD-removal, nitrification and denitrification, consisted of a 500 I storage tank (daily filled with fresh municipal wastewater and treated wastewater to give a final concentration of about 300 mgll COD), a 60 I reactor which was periodically aerated to enable nitrification and denitrification, a 481 settling tank and a 130 I storage tank for treated wastewater. The reactors 1-3 were heated to give a temperature in the aeration tank of 20°C. Reactor 4 had a temperature of 16°C. The surplus sludge was taken directly from the aeration tank to give a fixed sludge age of 1.88 d, 5.89 d, 8.50 d and 24.0 d. Small losses of particulate COD with the treated wastewater of about 7 mgll were taken into account and were added to the wasted surplus sludge . Activated sludge from the municipal wastewater treatment plant, which was adapted to the respective wastewater and had a sludge age of about 18 d, was used as inoculum. As the sludge age was fixed, the VSS concentration in the reactor adjusted itself according to the feed and the surplus sludge wasted . Oxy~en
demand
Mixed liquor from the respective aerated reactors (1-3) was filled into a closed batch respirometer and the reduction of oxygen was measured immediately. Measurements showed, that even a delay of one minute (which is the minimum for handling of the mixed liquor) gives a reduction of the measured oxygen demand . Therefore, within the 60 I reactor No.4 (which had a favourable surface to volume ratio), measurements were done directly within the reactor after establishing by Nraeration that the oxygen input from the surface hadlittle effect on the measurement. C-balance The COD of wastewater, treated wastewater, mixed liquor and surplus sludge was measured two to three times a day (3 parallel samples each) by a test kit from Macherey & Nagel. After two to three sludge ages, a week with extended measurements was carried out. Within this measurement week a time segment X of three to five following days was chosen , which gave the best C-balance. Therefore the standard deviations for the balance are usually smaller than for the whole time of the experiment. The measured COD values in Tables 1-4 are given as total COD in [mg) within the time segment X. Oxygen demand for nitrification was calculated with the factor 4.57 mgOimgN COD removal for denitrification )Vas calculated with the factor 2.86 mg COD/mg N (Barker, 1995). N-balance The wastewater, treated wastewater, mixed liquor and surplus sludge were daily examined for NH 4-N, N03-N, NOrN and total N. The NH 4-N, and N03-N were measured according to DIN 38406 ES-l and 091 [old] and the total N was measured according to DIN 38409 (Part 27). The balance was set up for total N. Both, C- and N-balance were established for the steady state after two to three sludge ages. Yield coefficient To determine the yield coefficient for easily degradable substrate (Ss) a closed batch respirometer is used . The activated sludge is diluted with effluent from the final clarifier to give approximately 2 gVSSIl. NH.CI solution is added to give a maximum nitrification rate and thereby a stable basic level of respiration. The
Parameter determination and model fitting
29
respirometer is filled and the respiration rate (endogenous plus nitrification) is recorded until it shows a straight line. Then a known COD amount of sodium acetate is added to the mixture. The subsequent increase of respiration rate is recorded until it drops to the initial level again. The area between the basic level and the measured respiration rate for the additional sodium acetate is integrated to give the amount of oxygen used. The difference between the amount of substrate COD added and the amount of oxygen used (which can be described in terms of negative COD) has been converted into biomass COD (storage COD or cell structure COD) thereby giving the yield coefficient for Ss.
Decay rate Activated sludge, maintained at 20'C, is mixed and aerated in a 2 I batch reactor. DNA is determined as described by Liebeskind (1994). The slope of the half logarithmic plot of colour extinction against time equals the net decay rate. Nomenclature is according to IAWQ Model No. 1 (Henze et al., 1987) or explained separately. Table 1. Cod balance of the four reactors No. I
No.2
No.3
1.88 d 3.7 (7 d)
5.89d
8.50d
No.4 24.,)d
2.0 (12 d)
2.1 (18d)
1.4 (34 d)
+ 89101 mg
+ 75700 mg
+ 60179 mg
+104667mg
(X=3d) 1.89 % 9.98% - 35528 mg
(X=3d) 2.30 % 4.37% -24333 me
(X = 5d) 6.97% 16.4%
Surplus sludee output:
(X=4d) 5.86 % 11.7% - 4 1044 OIl!
-40585ml!
X. in COD:
20524 me
72 1100111
73080 OIl!
1878340111
1.368 g
4.80711
4.872 g
3.131 g
Deviation from a constant 100ai sludge mass (steady stale) within the reactor over time qm. X in mg COD and in %:
+ 3450mg
·1575111g
- 345 mg
+ 697401g
+ 16.8%
Measured oxygen demand :
- 43 275 IlIg (in resp.O)
+ 2.2% . 45 124 mg (in resp .)
-0.5% • 49065 IlIg (in resp.)
Oxygen demand for nitrifi cation (calculated on the basis of NO,·N -production):
+ 18403mg
+ 15145mg
+ 21822 rug
+ 30852mg
Omg
omg
Omg
· 18424 mg
- 1490 1 mg 18.5% + 11734 mg
.7099mg 16.7%
-921201g
7%
- 11996 mg 16.9%
+ 1519mg
·954 rug
+ 18644to
+
+ 2.0%f)
·1.6%f)
Reactor: Sludge a~e: Sludge ages before measurement week: COD Input {sum over a time segment of X days after reaching steady state behav iour (COD used hy denitrification in the storage tank is already subtracted j): standard deviatio n (time segm , X): standard deviation (whole time » :
X. in COD
rsr liter:
COD·removal because of measured loss of N'Ol.'J (denitrific ationj: CODOUlPU1: standard deviation : COD-halance:
+ 3.71 % ·52844 mg (in resp.O ) to · 6821OO1g (in reactor)
+3278 Deviation from O-halance over lime segment X in %:
o f}
13.2 %0
= The respiranon measurement was too slow.
+
17,8%0 to + 3.1%f)
= The minor plus or loss of COD is assumed 10 be due to measurement standard deviations.
30
M. LIEBESKIND et al.
RESULTS AND DISCUSSION With the daily COD-load and the chosen sludge age. COD-concentrations in the reactors of 1.368 gil to 4.872 gil were obtained. Even after 3.7 sludge ages the activated sludge mass in the reactor with the lowest sludge age and the smallest sludgeconcentrationwas unstable.As those variationscould not be avoided.the change in sludge mass was taken into account for the COD-balance. Althoughthere was no true steady state within this reactor (just as there is never a true steady state within a full-scale wastewater treatment plant). the data were nevertheless included in the calculations. The changes within the activated sludge mass of the other three reactors were very small (0.5 to 3.7%). The performance of these reactors could be described as steady-statebehaviour. Tables I and 2 show the C- and N-balancesfor the different reactors. Usually an approach of 2 sludge ages is considered to be sufficient to reach steady-state behaviour. The activated sludge for the initial filling of the different reactors was taken from the municipalwastewatertreatmentplant and had a sludge age of about 18 d. It was already adapted to the municipal wastewater used. Therefore a start-up phase of 34 days was considered to be sufficient for reactor 4. The removal of COD by denitrification. calculated on the basis of the factor published by Barker (1995). gave a very good fit for the C- and N-balance. The significant nitrification in reactor I was not expected. It is assumed to be due to the high wastewater temperature of 20·C in the reactor throughoutthe experiment. Table 2. Ntotal-balance of the four reactors Sludge age: Ntotal input: NO,-N input: NO,-N demand instorage tank (assumed for No. 1-3. measured for No.4): Ntotal outnur with sumlus sludge: (iyl• + i m): Measured NH.-N demand for nitrification: NO,-N removal because of denitrification: :(oxygen concentration in reactor): Ntotal oumut Ntotal balance Deviation from Q-halance in % Comment:
No. I 1.88 d + 10259mg + 1641 OIl!
No.2 5.89d + 6612 mg + 209mg
No.3 8.5d + 8279mg + 2839mg
No.4 24d + 9895 mg + 867 me
"-
1641 me - 2611 me 0.064
- 209 me - 2112 nil! 0.059
-, 996 mn ·1368 OIl! 0.056
-651m2 - 1890012 0.047
4027012 -Omg
3314mg -Omg
4775mg -Omg
6740 me - 6442 mg
17m!! 0./1) 17m\! 0./1) me 0./1) 10 mil 0./1) ·5762mg -4291 OIl! - 5915 me -912m2 + 245 0 0 0 0% 0% 0% + 2.4% TIle loss ofCOD and NO,-N inthe storage tank was not As N~ was not taken into aCCOU1\t and had to be estimated 01\ the basis of measured the balance the results from exreriment No 4 toobtain a O-halance. isO. (7
.
After the set up of balances for the four reactors the steady state equationsof Marais and Ekama (1976) were used with minor modifications to calculate the masses of active biomass. endogenous residue and inert volatile material in the reactors. Calculationof the total mass of biodegradablesubstrate in the influent MSbioJ.,in:
(1)
Parameter detennination and model fitting
31
Calculationof the total mass of active biomassMXB,R in the reactor at steady state:
(2) Calculationof the inert particulatefractionMXp,R generatedby decay of biomassMXB,R at steady state:
(3) Calculationof the inert particulatefractionMXj in the reactor at steady state:
MX,.R
=MX total.R - MX B•R - MX P•R
(4)
: biologically degradablesubstrate in the influent[mg COD] : oxygen used by heterotrophic biomass for COD removal[mg COD] : yield coefficientof heterotrophic biomass [mg COD/mg COD] : net decay rate [lid] : gross decay rate [lid] : active biomass in reactor [mg COD] : inert particularmatter generatedby decay of biomass [mg COD] : inert particularmatterof the influent, accumulated in the reactor [mg COD] : factor for generation of inert particular products of biomass decay [mg COD] (fp
MXtotal,R tTS
=0.08)
: total COD in the reactor [mg COD] : sludge age [d)
The net decay (in terms of the "death-regeneration concept") is here defined as respiration losses of COD plus losses of COD by productionof MXp. The net decay can be measuredby DNA degradation as only the remainingliving biomass is determined. The gross decay rate (Henze, 1987)is defined as:
(5) The calculated total COD of the influent (MSbiol in+MXi + MSi) and total COD of the reactor are compared with the measured total COOs. A consistent solution for the equations can either be found by model fitting or by parameter determination, but a uniform Xi-value could only be found with parameter determination methods. The parameter determination gave the results shown in table 3. With the calculation methods a small set of consistent values for Xi, Xtotal,R • ~otal.in and M(02) is obtained.As the resulting sets were very much alike (for example: Xi (wastewater) of reactor No.3: 8.7% - 9.4%), only one set of consistent values is shown in Table 3. As shown in Tables 3 and 4 the measurements gave consistent Xi values for the municipal wastewaterof 7% to 9%, whereas the parameters of the IAWQ Model No. I gave values for Xi of 6% to 29%. The same wastewater was fed in parallel into reactors 3 and 4 and, one week later, reactor 1. Reactor 2 was fed with wastewaterfrom the same municipal wastewatertreatment plant some time earlier, The discrepancy between Xi values cannot be explained. Of course a search for parameter values (a fitting), which can give a unique Xi value for the different reactors, may result in new parametersfor bH and YH' but for full-scale plants data for only one sludge age are available. Therefore such a fitting is not possible without the set-up of a pilot plant, whereas parameter determination techniques can be used without such experiments. For the steadystate behaviourthe measurement of parametersgave better results than the fitting of data for one sludge age. One major discrepancyis found between the values of Model No. I and the values received from parameter measurements of the decay rates. The latter were much smaller in the measurements than in Model No.1. Also the decay rates and yield coefficients varied with sludge age. The lowest decay rate and highest yield
M. L1EBESKIND et al.
32
coefficient were determined for the longestsludgeage of 24 days. Figure I shows the measured decay rates in relation to sludge age. Table 3. Measured parameter valuesand calculated Xi-fraction Reactor: Sludge age:
No. I
No.2
No. 3
No.4
1.88 d
5.89d
8.50 d
24.0d
b (gross) !lId]: b.. (net) (lId]:
0.297
0.225
0.187
0.157
0.136
0.099
0.0804
0.0588
0.59
0.609
0.62
0.68
8.7 %
15.4 %
9%
0%
8.4 %
0%
9.6 % 0%
Xi municipal wastewater:
8.7%
9.0%
9.6%
Si, in (measured):
13.9 %
11.4 % 79.6 %
11.5 % 78.9 %
YII:
Xi.in: artificially added Xi.in I (measured):
77.4 %
7.0% 7.5 % 85.5 %
MX•• :
15280 me
41607 me
48890mg
59660 ma
MX••:
708 mn
6285 mu
MX,.:
4536 me
4615 mil 2588711lj!
18840 ma 26167 rug
Xs.in- Ss.in:
17905 me
Table 4. IAWQ modelno. I parameter valuesand calculated Xi-fraction Reactor:
No. I
No. 2
No.3
No.4
Sludee ace: b I (gross) IlIdl bill (net) Il/dl
1.88 d 0.62
5.89d 0.62
8.50d 0.62
24.0d 0.62
0.24 0.67
0.24 0.67
0.24 0.67
0.24 0.67
6.5 %
22.45 %
19.3 %
29.1 %
0%
8.4 %
0%
0%
Xi municipal wastewater:
6.5% 13.9 % 79.6 %
14.5 % 7.5 %
19.3%
Si, in (measured):
11.4 %
29.1 % 11.5 %
78 %
69.3 %
59.4 %
YII Xi,in: artificially added XUn I(measured):
Xs.in-s Ss, in:
Net decay rate [1/d] 0,16 0,14 0,12 0,1 0,08
0,06 0,04 0,02
o
o
5
10
15
20
25
30
Figure I. Measured net decay rates for different sludge ages.
35
Sludge age [d)
Parameter determination and modelfitting
33
Duringsteady-state behaviour only very few parameters can be determined and simultaneously checked by modelling. Dynamic states of the reactors will be examined to find out whetherthe results can be confirmed for dynamic processes. In summary very good C- and N-balances for 4 reactors of different sludgeage wereobtained. The measured parameter values for yield coefficients and decay rates were used to calculate the Xi in the feed and gave similarresults of 7% to 9%. Obviously parameter values were obtained by measurements, which fit directly into the model. Defaultvaluesof the IAWQ Model No.1 could also be used to solve the equations, but gave differing Xi valuesfrom 6% to 29%.The measured decay rateswere muchlower(bu=Q.16 - 0.30 [lid)) than the valuespublished by Henze(1987)(bH=O.62 [lId)).
REFERENCES Barker, P. S. and Dold, P. L. (1995). COD and Nitrogen Mass Balances in Activated SludgeSystems. Water Research, 29,633643. Henze, M., Grady,C. P. L. Jr, Gujer,W., Marais, G. v. R. and Matsuo, T. (1981).A GeneralModelfor SingleSludgeWastewater Treatment Systems. Water Researr:h, 21(5),pp. 50.5-515. Liebeskind, M. and Dohmann, M. (1994).lmprovedmethod of activated sludgebiomass determination. Wat. Sci. Tech. 29(1),pp. 1-13. Londong, J. (1994). Konsequenzen aus dem dynamischen Verhalten von Belebtschlammanlagen bei MischwasserzuflUssen. Korrespondenz; Abwasser, 9, pp. 1526-1538. Marais, G. v. R. and Ekama, G. A. (1916). The Activated SludgeProcessPart I - SteadyState Behaviour. Water SA, 2(4), pp.163200. Spiller, K. and Schmitt, 1. (1994). Innovatives Bewirtschaftungs- und Bedienungskonzept fUr die K1liranlage Conbus. gwfWasser Abwasser, 135(4), 213-222. vander Kuij,R. J. and van der Roest,H. F. (1994). Upgrading sewage treatment plantsusingcomputersimulation. Poster,Water QualityInternational,lA WQ17thBlennial Intemational Conference, 24-29.July 1994, Budapest.
Wat. Sci. Tull . Vol. 34. No. 5-6. pp, 27-33. 1996. CopyrightC 1996IAWQ. Published byElsevierScience Ltd Printedin Great Britain. All rightsreserved. 0273-1223196 S15'00 + frOO
Pergamon
PH:S0273-1223(96)00625-7
PARAMETER DETERMINATION AND MODEL FITTING - TWO APPROACHES FOR MODELLING PROCESSES IN WASTEWATER TREATMENT PLANTS M. Liebeskind, D. Schapers, C. Bornemann, E. Brands, M. Freund and T. Rolfs lnstitutfiirSiedlungswasse1wirtschaft. lSA, RWTHAachen, Templergraben 55. 52056Aachen, Germany
ABSTRACf This study shows results from four lab-scale wastewater treatment plants. which were insla!led behind the grit chamber of a municipal wastewater treatment plant The lab-scale plants were fed with original wastewater. diluted with water from the final clarifier to give a constantconcentration of 300 mg n COD. The activated sludges were adapted to different sludge ages from 1.88 to 24.0 days. The steady-state behaviourwas measured along with the decay rate. the yield coefficient, the oxygen demand. the C- and N· balance and the DNA of biomass XBH' The steady-state equations of Marais and Ekama were used to calculate the masses of active biomass. endogenous residue and inert VSS in the reactors. Measured parametervaluesweredifferentfor all sludgeages. They gave similarvaluesfor inen particulate VSS (Xi) of 7 to 9%. Parameter valuesof the IAWQ-Model No.1 togetherwith Xi·fitting could also be used to solve the equations. but in this case different Xi values for the different reactors had to be used for the same wastewater. Copyrighte 1996 IAWQ.Publishedby ElsevierScienceLtd.
KEYWORDS Activated sludge; decay rate; IAWQ-Model No.1; parameters; wastewater; Xi-determination . INTRODUcnON In recent times dynamic simulation of single sludge wastewater treatment processes bas become increasingly important. Various attempts have been made to simulate municipal wastewater treatment plants (for example Dupont, 1994; Londong, 1994; Spiller, 1994; van der Kuij, 1994). Often the kinetic and stoechiometric parameters for modelling are not determined to reduce the simulation costs. As a total mass balance of C and N for an existing wastewater treatment plant is difficult to obtain and even complete sets of bydrograpbs for several weeks are usually not available for full-scale plants. The modeller has to deal with great uncertainty concerning his data or has to install a small on-site pilot or lab-scale plant to get data sets that are SUfficiently complete to fit the model parameter values. The latter approach is quite costly. Compared with this, parameter determination is definitely less expensive and less time consuming. The question arises. Whether determination methods are sufficiently developed to obtain parameter values which fit into the dynamic model. This study shows a comparison of model-fitting techniques and parameter determination 27
28
M. LIEBESKlND et al.
techniques for the steady-state behaviour of four lab-scale wastewater treatment plants which were operated with municipal wastewater over three months . METHODS Lab-scale treatment plants
Three plants (designed for COD removal and nitrification) consisted of a 100 I storage tank (daily filled with fresh municipal wastewater and treated wastewater to give a final concentration of about 300 mgll COD) , a 15-1 aerated reactor for nitrification and COD removal, a 14 I settling tank and a storage tank for treated wastewater. A fourth lab-scale plant, designed for COD-removal, nitrification and denitrification, consisted of a 500 I storage tank (daily filled with fresh municipal wastewater and treated wastewater to give a final concentration of about 300 mgll COD), a 60 I reactor which was periodically aerated to enable nitrification and denitrification, a 481 settling tank and a 130 I storage tank for treated wastewater. The reactors 1-3 were heated to give a temperature in the aeration tank of 20°C. Reactor 4 had a temperature of 16°C. The surplus sludge was taken directly from the aeration tank to give a fixed sludge age of 1.88 d, 5.89 d, 8.50 d and 24.0 d. Small losses of particulate COD with the treated wastewater of about 7 mgll were taken into account and were added to the wasted surplus sludge . Activated sludge from the municipal wastewater treatment plant, which was adapted to the respective wastewater and had a sludge age of about 18 d, was used as inoculum. As the sludge age was fixed, the VSS concentration in the reactor adjusted itself according to the feed and the surplus sludge wasted . Oxy~en
demand
Mixed liquor from the respective aerated reactors (1-3) was filled into a closed batch respirometer and the reduction of oxygen was measured immediately. Measurements showed, that even a delay of one minute (which is the minimum for handling of the mixed liquor) gives a reduction of the measured oxygen demand . Therefore, within the 60 I reactor No.4 (which had a favourable surface to volume ratio), measurements were done directly within the reactor after establishing by Nraeration that the oxygen input from the surface hadlittle effect on the measurement. C-balance The COD of wastewater, treated wastewater, mixed liquor and surplus sludge was measured two to three times a day (3 parallel samples each) by a test kit from Macherey & Nagel. After two to three sludge ages, a week with extended measurements was carried out. Within this measurement week a time segment X of three to five following days was chosen , which gave the best C-balance. Therefore the standard deviations for the balance are usually smaller than for the whole time of the experiment. The measured COD values in Tables 1-4 are given as total COD in [mg) within the time segment X. Oxygen demand for nitrification was calculated with the factor 4.57 mgOimgN COD removal for denitrification )Vas calculated with the factor 2.86 mg COD/mg N (Barker, 1995). N-balance The wastewater, treated wastewater, mixed liquor and surplus sludge were daily examined for NH 4-N, N03-N, NOrN and total N. The NH 4-N, and N03-N were measured according to DIN 38406 ES-l and 091 [old] and the total N was measured according to DIN 38409 (Part 27). The balance was set up for total N. Both, C- and N-balance were established for the steady state after two to three sludge ages. Yield coefficient To determine the yield coefficient for easily degradable substrate (Ss) a closed batch respirometer is used . The activated sludge is diluted with effluent from the final clarifier to give approximately 2 gVSSIl. NH.CI solution is added to give a maximum nitrification rate and thereby a stable basic level of respiration. The
Parameter determination and model fitting
29
respirometer is filled and the respiration rate (endogenous plus nitrification) is recorded until it shows a straight line. Then a known COD amount of sodium acetate is added to the mixture. The subsequent increase of respiration rate is recorded until it drops to the initial level again. The area between the basic level and the measured respiration rate for the additional sodium acetate is integrated to give the amount of oxygen used. The difference between the amount of substrate COD added and the amount of oxygen used (which can be described in terms of negative COD) has been converted into biomass COD (storage COD or cell structure COD) thereby giving the yield coefficient for Ss.
Decay rate Activated sludge, maintained at 20'C, is mixed and aerated in a 2 I batch reactor. DNA is determined as described by Liebeskind (1994). The slope of the half logarithmic plot of colour extinction against time equals the net decay rate. Nomenclature is according to IAWQ Model No. 1 (Henze et al., 1987) or explained separately. Table 1. Cod balance of the four reactors No. I
No.2
No.3
1.88 d 3.7 (7 d)
5.89d
8.50d
No.4 24.,)d
2.0 (12 d)
2.1 (18d)
1.4 (34 d)
+ 89101 mg
+ 75700 mg
+ 60179 mg
+104667mg
(X=3d) 1.89 % 9.98% - 35528 mg
(X=3d) 2.30 % 4.37% -24333 me
(X = 5d) 6.97% 16.4%
Surplus sludee output:
(X=4d) 5.86 % 11.7% - 4 1044 OIl!
-40585ml!
X. in COD:
20524 me
72 1100111
73080 OIl!
1878340111
1.368 g
4.80711
4.872 g
3.131 g
Deviation from a constant 100ai sludge mass (steady stale) within the reactor over time qm. X in mg COD and in %:
+ 3450mg
·1575111g
- 345 mg
+ 697401g
+ 16.8%
Measured oxygen demand :
- 43 275 IlIg (in resp.O)
+ 2.2% . 45 124 mg (in resp .)
-0.5% • 49065 IlIg (in resp.)
Oxygen demand for nitrifi cation (calculated on the basis of NO,·N -production):
+ 18403mg
+ 15145mg
+ 21822 rug
+ 30852mg
Omg
omg
Omg
· 18424 mg
- 1490 1 mg 18.5% + 11734 mg
.7099mg 16.7%
-921201g
7%
- 11996 mg 16.9%
+ 1519mg
·954 rug
+ 18644to
+
+ 2.0%f)
·1.6%f)
Reactor: Sludge a~e: Sludge ages before measurement week: COD Input {sum over a time segment of X days after reaching steady state behav iour (COD used hy denitrification in the storage tank is already subtracted j): standard deviatio n (time segm , X): standard deviation (whole time » :
X. in COD
rsr liter:
COD·removal because of measured loss of N'Ol.'J (denitrific ationj: CODOUlPU1: standard deviation : COD-halance:
+ 3.71 % ·52844 mg (in resp.O ) to · 6821OO1g (in reactor)
+3278 Deviation from O-halance over lime segment X in %:
o f}
13.2 %0
= The respiranon measurement was too slow.
+
17,8%0 to + 3.1%f)
= The minor plus or loss of COD is assumed 10 be due to measurement standard deviations.
30
M. LIEBESKIND et al.
RESULTS AND DISCUSSION With the daily COD-load and the chosen sludge age. COD-concentrations in the reactors of 1.368 gil to 4.872 gil were obtained. Even after 3.7 sludge ages the activated sludge mass in the reactor with the lowest sludge age and the smallest sludgeconcentrationwas unstable.As those variationscould not be avoided.the change in sludge mass was taken into account for the COD-balance. Althoughthere was no true steady state within this reactor (just as there is never a true steady state within a full-scale wastewater treatment plant). the data were nevertheless included in the calculations. The changes within the activated sludge mass of the other three reactors were very small (0.5 to 3.7%). The performance of these reactors could be described as steady-statebehaviour. Tables I and 2 show the C- and N-balancesfor the different reactors. Usually an approach of 2 sludge ages is considered to be sufficient to reach steady-state behaviour. The activated sludge for the initial filling of the different reactors was taken from the municipalwastewatertreatmentplant and had a sludge age of about 18 d. It was already adapted to the municipal wastewater used. Therefore a start-up phase of 34 days was considered to be sufficient for reactor 4. The removal of COD by denitrification. calculated on the basis of the factor published by Barker (1995). gave a very good fit for the C- and N-balance. The significant nitrification in reactor I was not expected. It is assumed to be due to the high wastewater temperature of 20·C in the reactor throughoutthe experiment. Table 2. Ntotal-balance of the four reactors Sludge age: Ntotal input: NO,-N input: NO,-N demand instorage tank (assumed for No. 1-3. measured for No.4): Ntotal outnur with sumlus sludge: (iyl• + i m): Measured NH.-N demand for nitrification: NO,-N removal because of denitrification: :(oxygen concentration in reactor): Ntotal oumut Ntotal balance Deviation from Q-halance in % Comment:
No. I 1.88 d + 10259mg + 1641 OIl!
No.2 5.89d + 6612 mg + 209mg
No.3 8.5d + 8279mg + 2839mg
No.4 24d + 9895 mg + 867 me
"-
1641 me - 2611 me 0.064
- 209 me - 2112 nil! 0.059
-, 996 mn ·1368 OIl! 0.056
-651m2 - 1890012 0.047
4027012 -Omg
3314mg -Omg
4775mg -Omg
6740 me - 6442 mg
17m!! 0./1) 17m\! 0./1) me 0./1) 10 mil 0./1) ·5762mg -4291 OIl! - 5915 me -912m2 + 245 0 0 0 0% 0% 0% + 2.4% TIle loss ofCOD and NO,-N inthe storage tank was not As N~ was not taken into aCCOU1\t and had to be estimated 01\ the basis of measured the balance the results from exreriment No 4 toobtain a O-halance. isO. (7
.
After the set up of balances for the four reactors the steady state equationsof Marais and Ekama (1976) were used with minor modifications to calculate the masses of active biomass. endogenous residue and inert volatile material in the reactors. Calculationof the total mass of biodegradablesubstrate in the influent MSbioJ.,in:
(1)
Parameter detennination and model fitting
31
Calculationof the total mass of active biomassMXB,R in the reactor at steady state:
(2) Calculationof the inert particulatefractionMXp,R generatedby decay of biomassMXB,R at steady state:
(3) Calculationof the inert particulatefractionMXj in the reactor at steady state:
MX,.R
=MX total.R - MX B•R - MX P•R
(4)
: biologically degradablesubstrate in the influent[mg COD] : oxygen used by heterotrophic biomass for COD removal[mg COD] : yield coefficientof heterotrophic biomass [mg COD/mg COD] : net decay rate [lid] : gross decay rate [lid] : active biomass in reactor [mg COD] : inert particularmatter generatedby decay of biomass [mg COD] : inert particularmatterof the influent, accumulated in the reactor [mg COD] : factor for generation of inert particular products of biomass decay [mg COD] (fp
MXtotal,R tTS
=0.08)
: total COD in the reactor [mg COD] : sludge age [d)
The net decay (in terms of the "death-regeneration concept") is here defined as respiration losses of COD plus losses of COD by productionof MXp. The net decay can be measuredby DNA degradation as only the remainingliving biomass is determined. The gross decay rate (Henze, 1987)is defined as:
(5) The calculated total COD of the influent (MSbiol in+MXi + MSi) and total COD of the reactor are compared with the measured total COOs. A consistent solution for the equations can either be found by model fitting or by parameter determination, but a uniform Xi-value could only be found with parameter determination methods. The parameter determination gave the results shown in table 3. With the calculation methods a small set of consistent values for Xi, Xtotal,R • ~otal.in and M(02) is obtained.As the resulting sets were very much alike (for example: Xi (wastewater) of reactor No.3: 8.7% - 9.4%), only one set of consistent values is shown in Table 3. As shown in Tables 3 and 4 the measurements gave consistent Xi values for the municipal wastewaterof 7% to 9%, whereas the parameters of the IAWQ Model No. I gave values for Xi of 6% to 29%. The same wastewater was fed in parallel into reactors 3 and 4 and, one week later, reactor 1. Reactor 2 was fed with wastewaterfrom the same municipal wastewatertreatment plant some time earlier, The discrepancy between Xi values cannot be explained. Of course a search for parameter values (a fitting), which can give a unique Xi value for the different reactors, may result in new parametersfor bH and YH' but for full-scale plants data for only one sludge age are available. Therefore such a fitting is not possible without the set-up of a pilot plant, whereas parameter determination techniques can be used without such experiments. For the steadystate behaviourthe measurement of parametersgave better results than the fitting of data for one sludge age. One major discrepancyis found between the values of Model No. I and the values received from parameter measurements of the decay rates. The latter were much smaller in the measurements than in Model No.1. Also the decay rates and yield coefficients varied with sludge age. The lowest decay rate and highest yield
M. L1EBESKIND et al.
32
coefficient were determined for the longestsludgeage of 24 days. Figure I shows the measured decay rates in relation to sludge age. Table 3. Measured parameter valuesand calculated Xi-fraction Reactor: Sludge age:
No. I
No.2
No. 3
No.4
1.88 d
5.89d
8.50 d
24.0d
b (gross) !lId]: b.. (net) (lId]:
0.297
0.225
0.187
0.157
0.136
0.099
0.0804
0.0588
0.59
0.609
0.62
0.68
8.7 %
15.4 %
9%
0%
8.4 %
0%
9.6 % 0%
Xi municipal wastewater:
8.7%
9.0%
9.6%
Si, in (measured):
13.9 %
11.4 % 79.6 %
11.5 % 78.9 %
YII:
Xi.in: artificially added Xi.in I (measured):
77.4 %
7.0% 7.5 % 85.5 %
MX•• :
15280 me
41607 me
48890mg
59660 ma
MX••:
708 mn
6285 mu
MX,.:
4536 me
4615 mil 2588711lj!
18840 ma 26167 rug
Xs.in- Ss.in:
17905 me
Table 4. IAWQ modelno. I parameter valuesand calculated Xi-fraction Reactor:
No. I
No. 2
No.3
No.4
Sludee ace: b I (gross) IlIdl bill (net) Il/dl
1.88 d 0.62
5.89d 0.62
8.50d 0.62
24.0d 0.62
0.24 0.67
0.24 0.67
0.24 0.67
0.24 0.67
6.5 %
22.45 %
19.3 %
29.1 %
0%
8.4 %
0%
0%
Xi municipal wastewater:
6.5% 13.9 % 79.6 %
14.5 % 7.5 %
19.3%
Si, in (measured):
11.4 %
29.1 % 11.5 %
78 %
69.3 %
59.4 %
YII Xi,in: artificially added XUn I(measured):
Xs.in-s Ss, in:
Net decay rate [1/d] 0,16 0,14 0,12 0,1 0,08
0,06 0,04 0,02
o
o
5
10
15
20
25
30
Figure I. Measured net decay rates for different sludge ages.
35
Sludge age [d)
Parameter determination and modelfitting
33
Duringsteady-state behaviour only very few parameters can be determined and simultaneously checked by modelling. Dynamic states of the reactors will be examined to find out whetherthe results can be confirmed for dynamic processes. In summary very good C- and N-balances for 4 reactors of different sludgeage wereobtained. The measured parameter values for yield coefficients and decay rates were used to calculate the Xi in the feed and gave similarresults of 7% to 9%. Obviously parameter values were obtained by measurements, which fit directly into the model. Defaultvaluesof the IAWQ Model No.1 could also be used to solve the equations, but gave differing Xi valuesfrom 6% to 29%.The measured decay rateswere muchlower(bu=Q.16 - 0.30 [lid)) than the valuespublished by Henze(1987)(bH=O.62 [lId)).
REFERENCES Barker, P. S. and Dold, P. L. (1995). COD and Nitrogen Mass Balances in Activated SludgeSystems. Water Research, 29,633643. Henze, M., Grady,C. P. L. Jr, Gujer,W., Marais, G. v. R. and Matsuo, T. (1981).A GeneralModelfor SingleSludgeWastewater Treatment Systems. Water Researr:h, 21(5),pp. 50.5-515. Liebeskind, M. and Dohmann, M. (1994).lmprovedmethod of activated sludgebiomass determination. Wat. Sci. Tech. 29(1),pp. 1-13. Londong, J. (1994). Konsequenzen aus dem dynamischen Verhalten von Belebtschlammanlagen bei MischwasserzuflUssen. Korrespondenz; Abwasser, 9, pp. 1526-1538. Marais, G. v. R. and Ekama, G. A. (1916). The Activated SludgeProcessPart I - SteadyState Behaviour. Water SA, 2(4), pp.163200. Spiller, K. and Schmitt, 1. (1994). Innovatives Bewirtschaftungs- und Bedienungskonzept fUr die K1liranlage Conbus. gwfWasser Abwasser, 135(4), 213-222. vander Kuij,R. J. and van der Roest,H. F. (1994). Upgrading sewage treatment plantsusingcomputersimulation. Poster,Water QualityInternational,lA WQ17thBlennial Intemational Conference, 24-29.July 1994, Budapest.