- Email: [email protected]
Sludge viability in a biological reactor
Water Research Vol. 15. pp. 953 to 959. 1981 Printed in Great Britain. All rights reserved
0043-13M/81/080953-07$02,00/0 Copyright © 1981 Pergamon Press Ltd
SLUDGE VIABILITY IN A BIOLOGICAL REACTOR M. GREEN and G. StmLEF Department of Environmental and Water Resources Engineering, Technion, Israel Institute of Technology, Haifa, Israel
(Received January 1980) Abstract--The volatile suspended solids (VSS) concentration cannot be used as a measure for the active biomass in a reactor which operates under a wide range of operating conditions since the viable organism content of the VSS is not constant. Using substrate saturation conditions the kinetic parameters maximum substrate removal rate (~) and oxygen uptake rate (J)--both per mass of VSS--were determined in an experimental pulse fed batch biological reactor, It was found that ~ and J both doubled during the experimental period (6 h). It was concluded that the increases in ~ and J values were due to the increase in the sludge viability which are here defined as the percentage of VSS which is active biomass. Using the variations in ~ and J values during each experiment, it was possible to calculate sludge viability. During a 6 h experimental period at substrate saturation level the sludge viability increased on average from 8.9 to 23.30o. In a loop type sewage conduits system operated as a plug flow reactor and enriched with biomass and air, it is possible to achieve high specific substrate removal rates when step feeding creates saturation conditions. This is further attenuated by a marked increase in the sludge viability.
INTRODUCTION
therefore the VSS concentration cannot be used as an indicator of active biomass concentration. The low viability in conventional systems stems from the limiting substrate concentration in the reactor, which is sufficient only just for maintenance requirements. This results in a very low net growth rate. As the effective life time of the bacteria is shorter than the nominal sludge age the majority of the bacteria cells are dead. An increase in the food to microorganisms ratio (F/M) (# > 0.3 day - t ) is followed by higher net growth rates and this results in steadily increasing sludge viability. Experiments were conducted to determine variations in sludge viability under substrate saturation conditions. The sludge viability was estimated using the kinetic parameters, maximum substrate removal rate (~) and oxygen uptake rate (J)---calculated per unit mass VSS.
Sludge viability, defined as the percentage of the volatile suspended solids (VSS) which is viable biomass, is less than 20% in conventional activated sludge systems. The 10w value is typical for units operating under severe substrate limitations, very common to continuously stirred sewage treatment reactor (CSTR). The standard index for biomass concentration is the VSS concentration which includes active and inactive cells, inert organic suspended solids and biodegradable matter adsorbed on biomass floes. The VSS concentration can be used as a measure of the active biomass concentration only when the active biomass is a constant fraction of the VSS. It has been reported (Weddle & Jenkins, 1971; Postgate & Hunter, 1962) that the sludge viability was found to be 15-20% and generally does not vary under the operational conditions of a conventional biological reactor for sewage treatment (# = 0-0.3 day- ~, where EXPERIMENTAL is the specificgrowth rate). The experimental work was carried out in a batch reacHowever, Prakasam & Dondero (1967), Austin & tor with pulse feeding, under substrate saturation conForster (1969) and Wooldridge & Standfast (1933) ditions, The reactor was aerated through porous diffusers. reported that the sludge viability is lower and varies The dissolved oxygen concentration was maintained above 2 mg I- i and the temperature was kept constant, between I and I0°/0under similar conditions. The reactor was fed with filtered sewage from the Haifa Under a narrow range of operational conditions, suburb, Neve Shaanan, The filtered sewage was strengthsuch as those found in activated sludge systems, the ened to 1200mgl -I COD by addition of a glucose solactive biomass is a constant fraction of the VSS con- ution and yeast extract and was diluted as required. centration and therefore the VSS concentration is a The biomass originated in an activated sludge pilot plant reliableindicator of the active biomass concentration. situated at the Technion, Haifa. The biomass was acclimaO n the other hand, when operating under a wider tized by feeding it with the strengthened sewage in a continuous laboratory system, for a period equivalent to range of operational conditions, as in the batch sys- several detention times such that steady state conditions tems case, or plug flow systems, the fraction of the were achieved (organic loading of about I g,COD per VSS which is active biomass is not constant and g. VSS' day and temperature range of 23--25°C!. This accli-
953
954
M. GREENand G. SHELEF
matized biomass was used for the experiments. In each experiment (which extended for about 6 h) the sewage was added three times at constant time intervals. The ~ and J coefficients were determined for each stage, where "stage" is defined as the time interval between 2 successive sewage additions. The 3 additions were conducted so that the instantaneous feed to microorganism ratio g,COD per g'COD'day immediately after the feeding, was the same in all 3 stages. The VSS concentration in the innoculum was 5000 mg 1- ~ and its volume was I I. The volume of each of the 3 sewage additions was about 21. Mixed liquor samples were taken every 3-10min and were analyzed for COD and VSS. At the termination of each stage (which extended 1~ h) the mixed liquor was allowed to settle and the supernatant was decanted to prevent any possible effect of substrate residuals enzymes and metabolic by-products which were produced during the stage. Because of the decantation it was possible to maintain similar substrate concentrations (i.e. preventing substrate dilution and maintaining substrate saturation conditions) in all the stages, while keeping the same organic loading, about 7 g. COD per g. VSS'day. The effect of removing the supernatant was determined in a number of experiments by comparing the kinetic behavior of the biomass with and without decantation: no difference was found. It is important to note that in all the experiments sewage from the same source was used, and that the glucose solution + yeast extract were a constant fraction of the strengthened sewage.
KINETICS Direct determination of sludge viability which is based on bacterial counting using microscopic methods or plate counts could not be used in this study because of the lack of sensitivity of these techniques. These techniques may be used for estimating order of magnitude changes in the number of microorganisms. However. they cannot be used for determining the fractional changes which were anticipated. Therefore the sludge viability was determined using an indirect technique. The kinetic parameters ~ (maximum substrate removal rate per unit mass VSS) and d (oxygen uptake rate per unit mass VSS) were used for determining changes in sludge viability. The experiments were performed under substrate saturation conditions. These conditions enable rapid increase in the sludge viability. Likewise, the determination of ~ and J is simple and more accurate under zero order reaction conditions as is explained in the following section.
Cl Determination The Monod equation is:
ds q's q = x.dt - K~+ S when q = S= x = K~ =
specific substrate removal rate substfate concentration biomass concentration saturation coefficient
= maximum specific substrate removal rate coefficient. However at substrate saturation condition S ~- K,, and the equation becomes: I ds q = ~ ~- =
Zr
I21
For each experiment it is possible to assume a constant value for the VSS concentration throughout the stage, because the variations in the VSS concentration are small relative to the VSS concentration in the reactor (Ax/x = 0.05-0.1 for each stage), Hence the coefficient can be determined from the gradient of the argumental results graphed as S/x vs t. Because the changes in VSS are very small relative to the VSS concentration the derivation of kinetic constants based on these changes, i.e. deriving #, are difficult and less accurate than techniques based on changes in substrate concentrations. Hence the determination of ~ was based on changes in substrate concentrations and not changes in VSS concentrations. Chemical oxygen demand (COD) was chosen as the analytical technique used for the determination of the substrate concentration. C O D was preferred over BOD method because of the time and work saving especially because of the large number of samples involved in batch experiments. Another reason for favoring the C O D method is that it has been shown that the conventional BOD test often underestimates the amount of organic substrate available for the aerobic bilogicat degradation of wastewater (Arthur, 1974: Ballinger & Lishka, 1962), In general it is necessary when using the C O D analysis to determine the fraction of the C O D which is not biodegradable. However, because of the technique chosen for determining ~, using the gradient of the data graphed as ,s/x vs t, this problem does not arise. This is because the gradient of the line (~) is the same whether s is the total C O D or only the biodegradable COD.
The determination of the specific oxygen uptake rate The oxygen uptake rate was determined by measuring the decrease in the dissolved oxygen concentration with a YSI Dissolved Oxygen Analyzer in I 1. of mixed liquor sample mixed by a magnetic stirrer. Under substrate saturation conditions, the value of the specific oxygen uptake rate is constant and is independent of substrate concentration.
The determination qf sludye viability (1)
When the sludge viability (i.e. percentage of active biomass per unit mass VSS) is not constant, the values of the kinetic parameters ~ and J, which are calculated per unit mass of VSS, will also change. It is possible to calculate the sludge viability from the changes in these parameters. The determination of sludge viability was based o n two assumptions: firstly that the change in ~ and J
Sludge viability in a biological reactor
values is the result of changes in the sludge viability only. Secondly, the increase in the VSS concentration is due to an increase in viable cell concentration only. The first assumption was applied t o the first two stages in order to determine the initial sludge viability. This assumption was then checked for the second and the third stages. Based on the first assumption it is possible to write that; qB ~A
--
=
average sludge viability in stage B (SVB) average sludge viability in stage A (SV,)
as:
x~
/~ j
\ Iv" 100|, and the average sludge viability in each stage
SVA =
(XoA + XrA)/2
(5)
(v + AXA + - ~ ) " 100 (3)
when SV, and SVa are the sludge viability in stage A and stage B respectively. By defining v as the initial concentration of active biomass (an unknown value), XoA, XfA,Xoa, XrB, are initial and final VSS concentrations in stage A and stage B respectively, and AX is the increment in VSS concentration in the whole stage (i.e. x r - Xo), it is possible to calculate the initial sludge viability (v) from equation (3) expressed
+
955
SVs =
(xos + xr,)/2
(6)
(c+AxA+Axa+~)'lO0 SVc =
(Xoc + Xrc)/2
(7)
If the sludge viability ratio in the stages B and C is equal to the ~ ratio in these stages, then the first assumption has been verified. A detailed description of the sludge viability determination is given elsewhere (Green, 1979).
IOO RESULTS AND DISCUSSION
\
Xoa
The kinetic parameters maximum specific substrate removal rate (9) and specific oxygen uptake rate (J)
XfB
,4, Determination of changes in VSS concentration (x) can be based on direct measurement of VSS concentration (Standard Methods, 1978) or on the removed substrate concentration multiplied by the yield coefficient (under substrate saturation conditions cell decay can be neglected). Calculating the VSS concentration changes using the reduction in the substrate concentration is preferred when the change in VSS concentration in each stage is small relative to its concentration in the reactor. The net yield coefficient was derived from the equation: dx -y. dt
-ds dt
when Y, is the net yield coefficient, dx is the net VSS addition and - d s is the COD reduction. Y net is very similar to Y gross (VSS produced per COD removed) under substrate saturation conditions, when the cell decay is negligible in comparison with the total growth. The yield coefficient is independent of the active cell concentration in the sludge and therefore the change in sludge viability should not affect the yield coefficient value. The yield coefficient was found to have a similar value during all the stages of an experiment: 0.5 From equation (4) it is possible to calculate the initial active biomass concentration (v). Having calculated v it is possible to calculate the initial sludge viability
The ~ coefficient g. COD per g. VSS.day was determined for every stage in l l experiments. Three values were determined for every experiment: qA for the time interval between the first and second feedings, stage A; tta, for stage B; and qc for stage C. values are given in Table 1 and an example of ~ determination is shown in Fig. I. The results in Table 1 show variations in the ~ values of the same stage between the experiments. The differences between the values of the same stage are due to daily and hourly changes in the sewage characteristics and changes in the biomass composition and sludge age of the activated sludge taken from a pilot plant. The qA values are lower than published values of ~. The reason for this could be the low organic loadings and the high sludge age in the source pilot plant (sludge age > 10 days), conditions which caused low sludge viability. Nevertheless, the ~ values in these experiments are similar to those obtained in other studies at the Technion (Rom, 1971, ~ = 0.9; Scheindorf, 1976, ~ = 2.29). The results in Table 1 show a significant increase in values from stage to stage in the same experiment. coefficient values increased from an average value of 1.3 day-~ in stage A to 1.88 day-1 in stage B (an average increase of 45~) and to 2.7 day-1 in stage C (an average increase of 449/0). The total average increase in ~ from stage A to stage C was 108~o (from 1.3 to 2.7 day-1). The specific oxygen demand rate g - O 2 per g. VSS.day was determined for each stage in 3 experiments. Since the reactor operated under substrate saturation conditions, the substrate removal rate was independent of the substrate concentration. Similarly
M. GREEN and G. SHELEF
956
Table 1. Values of the specific maximum substrate removal rate (~) in the different stages Experiment no.
i-stage A*
i-stage B*
i-stage C*
~,,.~.,
qc,~).
I 2 3 4 5 6 7 8 9 10 11
1.93 1.30 1.57 -1.37 0.67 1.53 1.26 1.05 0.85 1.51
2.78 1.9 2.t2 2.13 1.98 1.29
4.16 2.36 3.37 3.09 2.58 2.04 2.38
1.61 1.56 1.29 2.09
2.25 1.95 2.81
1.45 1.46 1.35 -1.45 1.9 -1.28 1.5 1.52 1.38
1,5 2.15 1.24 1.82 1.59 2.15 1.45 -1.3 1.88 1.58 3.04 ~ 1.56 . . . . . . . 1.44 2.14 1.51 2.29 1.34 2.86
Average
1.30
1.88
2.7
1.48
1.44
~)c,'~)A
2.21
* g ' C O D per g. VSS.day. stage. The results in Table 2 show an increase of t 13% in the average value of J, from stage A to stage C. This increase is equal to the increase in d in the 3 experiments. Since ~ and J values were determined using different techniques, the excellent agreement between the increase of these t w o parameters is a reflection of the accuracy and reliability of the techniques.
the small changes in VSS concentration during each stage can be neglected, thus, the oxygen uptake rate was approximately constant for each stage. The results are given in Table 2 and in Figs 2 and 3. (The decrease in VSS concentration from stage to stage as shown in Fig. 2, is due to the mixed liquor samples which were taken frequently.) For comparative purposes the ¢~ values were also determined here for each
0.,~
o
0~.~
Stage
t.~.__."2~
A
~.I~ ®y'~
0.287~ o.z~ o~o'~
o.'oio4
oi&e4 T,
0.o204
o~
-
days
Fig. 1. Specific maximum substrate removal rate (~) determination (Experiment No. 14).
Table 2. Comparison of the maximum substrate removal rate Iq) and the oxygen uptake rate {J) in the different stages
Stage
Oxygen uptake rate per mass VSS J (day - i)
Maximum substrate removal rate per mass VSS ¢~(day - ~)
12
A B C
0.82 1.34 1.89
1.64 3.12 3.96
13
A B C
1.01 1.51 2.12
1.5 2.42 3.24
14
A B C
0.90 1.46 1.74
1.53 1.93 3.0
Experiment no.
JcJA
qc/qA
2.30
2.41
2.10
2.00
1.93
1.99
Sludge viability in a biological reactor
957
The sludge viability Stage C VSS- I I09 rng I"l J - 1,74 day -I
0 0 0 0 0
C
i
I
I
l
I
I
I
i
l
Stage B VSS • 1300 mg I"l J - 1.46 day-I
T 0 0 0
C
m
=
=
t
t
l
l
StoOe A 6
V S S . 1600 rng I~ J • 0.90 day
4-0 0 0 0
,
,
,
;
=
Time, rain
Fig. 2. Oxygen uptake rate (J) determination (Experiment No. 14).
As the sludge was not changed during each 6 h experiment and a constant composition substrata was added to each stage, it was expected that kinetic parameters would be constant except if there was a change in sludge viability. The sludge was acclimatized before each experiment hence acclimatization effects are negligible. Changes in the dominant species within the micro organism population would not be significant over such a short time (6 h). Therefore the marked increase in the values of ~ and J (which were calculated per mass of VSS and not per mass of active biomass) was due to the increase of sludge viability and hence these parameters should be used for the determination of the sludge viability. The sludge viability values, which were calculated from ~ values in the different stages, are given in Tables 3 and 4. As was noted, the q^/qa ratio was used for the determination of the sludge viability ratio in stages A and B. The correlation between the sludge viability ratio in stages B and C and ~ ratio in these stages was used to verify the second assumption that the increase in ~ value per mass VSS was caused by the sludge viability increase. The results in Table 4 show a good correlation between cls/~lc ratio and the sludge viability ratio in these two stages. The differences between these two ratios vary from 0 to + 7%, with an average value of + 3%. The reason for these small differences can be the sludge viability determination method or because of laboratory errors. These differences are considered negligible when dealing with biological reactors data. From the results in Tables 3 and 4 it can be concluded that the increase in ~ values stems only from the increase in sludge viability and therefore ~ values can be used to determine sludge viability. If the ~ coefficient is calculated per unit mass of active biomass (and not per unit mass of VSS) then
A B C
A B
C
A
B
C
~eae
A
A
C
A
B
C
SmO,
B
C
Exp 12
B
Exp 13
Exp 14
Fig. 3. Comparison of maximum substrate removal rate (~) and' oxygen uptake rate (J), per mass VSS, in the different stages.
w Jr. ! 5/8 - - .
958
M. GREEN and G. SHELEF Table 3. The sludge viability Experiment no.
Original sludge
1 2 3 5 6 8 10 11 Average
11.2 7.7 10.5 8.7 3+9 8.3 7.7 13.5 8.9
Sludge viability (9o) Stage A Stage B
no differences should have been found in ~ between the different stages of each experiment. The maximum substrate removal rate per mass of active biomass, qv, was calculated from the following equation: 3'100 SV when SV is the average sludge viability during the stage.
13.7 9.3 12.4 11.2 6.2 10.2 10.0 15.3 11.0
19.8 13.5 16.8 16.2 11.9 15.2 15.2 21.2 16.2
Stage C 27.0 19.5 23.6 23.0 19.3 21.3 23.1 29.2 23.3
The results shown in Table 4 indicate that qv is constant during each experiment (an average van, ation of +39"0 between the stages). The results m Table 3 show that the initial sludge viability in stage A varies between 3.9 and 13.59/o with an average value of 8.9~o. This viability ~s in agreement with the other published results (Blok, 1976: Prakasam & Dondero, 1967). F r o m Table 3 it can be seen that the sludge viability in stage C is 2.6 times higher than that of the original sludge. The active biomass increased from 445 mg in the original sludge (8.990 of 5000rngVSS)
Table 4. The variation of the specific maximum substrate removal rate with the sludge viability
Exp. no.
Max. substrate removal rate per mass of active biomass c~,,(day- t)
Stage (st.)
Viability (o~,}
Max. substrate removal rate per mass VSS ~ (day- l)
A B C
13.73 19.79 27.00
1.93 2.78 4.16
14.1 14.1 15.4
A
9.25
14.1 14. I 12.1
B
13.50
C
19.50
1.30 1.90 2.36
A B C
12.38 16.75 23.60
1.57 2.12 3.37
12.7 12.7 14.3
A B C
11.18 6.20 23.00
1.37 1.98 2.58
12.3 12.3 11.2
A
6.15
10.9 10.9 10.6
B
11.87
C
19.30
0.67 1.29 2.04
A
10.24
1.05
10.3
B C
15.20 21.30
1.56 2.25
10.3 10.6
10
A B C
9.99 15.20 23.05
0.85 1.29 1.95
8.5 8.5 8.5
tl
A B C
15.30 21.20 29.15
1.51 2.09 2.81
9.9 9.9 9.6
Viability (°,oFst. C Viability (°o)-st. A
qc/q,~
1.97
2.16
2.11
1.82
1.91
2.15
2.06
1.88
3.14
3.04
2.08
-.'~14
2.31
2.29
1.91
1.86
Sludge viability in a biological reactor to i 384 mg in stage C (23.3% of 5939 mg VSS, including VSS which was taken out with the samples). The nominal increase is the same (939 rag) but this an increase of 211~o in the active biomass and only 18.8% in the VSS. The initial sludge used in stage A of each experiment originated in a CSTR type system which was severely substrate limited, hence there was a very slow sludge viability. This is because the substrate concentration was only sufficient for maintenance requirements and minimal growth. However, on transferal to experimental substrate saturation conditions a high nett growth was achieved which caused the increase in the sludge viability during each experiment. Preliminary experiments which extended over 16 h have indicated that it is possible to increase the sludge viability to 60% and perhaps more. Operation of sewage treatment facilities at hioh substrate concentrations
Conventional CSTR sewage treatment plants operate under severe substrate limiting conditions because the final etttuent quality requirements dictate the substrate concentration in the reactor. This is not the case in a two or more stage reactor or in a plug flow reactor. A plug flow reactor with step feeding for biological treatment of wastewater can operate along most of its length under saturation levels of substrate concentrations (i.e. under conditions close to ~) and therefore with steadily rising sludge viability. Such conditions can be achieved using a main sewer which is used as a plug flow reactor with entering sewage flows from branch mains being analogous to step feedings (Green, 1979: Green & Shelef, 1980). The system should be highly loaded along most of its length to maximize removal efficiency. Only the final section of the system should not be fed to ensure that a high quality effluent (low in dissolved substrate) is produced.
CONCLUSIONS Under a wide range of operation conditions the VSS concentration cannot be used as a criterion for active biomass concentration since the viable organism content of the VSS is not constant. The sludge viability was determined from changes in maximum specific substrate removal rate coefficient (9) and the specific oxygen uptake rate (J) when these two parameters were calculated per mass VSS. In experiments with sludge which was taken from an activated sludge plant (operating under limiting substrate concentration), acclimatized and transferred to
959
substrate saturation conditions in a batch reactor, a gradual and steady increase in the value of 9 was found. The values ranged from an average value of i . 3 g . C O D removed per g.VSS'day to 2.7g.COD removed per g. VSS. day, an increase of over 100%. A similar increase was found in the specific oxygen uptake rate (from 0.91 to 1.92g.O 2 per g,VSS.day). The main reason for the increase in these kinetic parameters is the increase in the sludge viability as a result of the higher net growth rate under substrate saturation conditions. When the maximum substrate removal rate was calcrlated per mass of active biomass (and not per mass of VSS) no change in the substrate removal rate was found. Contrary to the substrate limiting conditions and the low viability which are typical of standard CSTR sewage treatment plants, it is possible to obtain a marked increase in sludge viability under substrate saturation conditions. Such a system which operated under high substrate concentrations has a double advantage over standard biological wastewater treatment plants, namely, the possibility of working at high substrate removal rate (9) levels and the increase in the sludge viability which results from a higher net growth rate of the biomass. The previously mentioned advantages can be applied in a two or more stage reactor or in a sewerage system operating as a step fed plug flow reactor for sewage purification. REFERENCES
Arthur R. M. (1974) Let's upgrade the BOD test. War. Sewage Wks 111, 100-102. Austin B. L. & Forster C. F. (1969) The microbiology of Lubeck activated sludge plant, Wat. Waste Treat. J. 208. Ballinger D. G. & Lishka R. J. (1962) Reliability and precision of BOD and COD determinations. J. Wat. Pollut. Control Fed. 34, 470--479. Blok J. (1976) Measurements of viable biomass concentration in the activated sludge by respirometric techniques. Water Res. 10, 919-925. Green M. (1979) Ph.D. Thesis. Technion, Israel Institute of Technology, Haifa, Israel. Green M. & Shelef G. (1980) Sewer trunk lines can be treatment facilties. War. Sewage Wks August. Postgate J. R. & Hunter J. R. (1962) The survival of starved bacteria. J. gen. Microbiol. 29, 233-263. Prakasam T. B. S. & Dondero N. C. (1967) Aerobic heterotrophic bacteria population of sewage and activated sludge. Appl. Microbiol. 15, 461-467. Rom D. (1971) Ph.D. Thesis. Technion, Israel Institute of Technology. Haifa, Israel. Scheindorf C. (1976) M.Sc. Thesis. Technion, Israel Institute of Technology, Haifa, Israel. Standard Methods for the Examination of Water and Sewage
(1978) American Public Health Association. Weddle C. L. & Jenkins D. (1971) The viability and activity of activated sludge. Water Res. 5, 621-640. Wooldridge W. R. & Standfast A. F. B. (1933) The biochemical oxygen demand of sewage. Biochem. J. 27, 183.
0043-13M/81/080953-07$02,00/0 Copyright © 1981 Pergamon Press Ltd
SLUDGE VIABILITY IN A BIOLOGICAL REACTOR M. GREEN and G. StmLEF Department of Environmental and Water Resources Engineering, Technion, Israel Institute of Technology, Haifa, Israel
(Received January 1980) Abstract--The volatile suspended solids (VSS) concentration cannot be used as a measure for the active biomass in a reactor which operates under a wide range of operating conditions since the viable organism content of the VSS is not constant. Using substrate saturation conditions the kinetic parameters maximum substrate removal rate (~) and oxygen uptake rate (J)--both per mass of VSS--were determined in an experimental pulse fed batch biological reactor, It was found that ~ and J both doubled during the experimental period (6 h). It was concluded that the increases in ~ and J values were due to the increase in the sludge viability which are here defined as the percentage of VSS which is active biomass. Using the variations in ~ and J values during each experiment, it was possible to calculate sludge viability. During a 6 h experimental period at substrate saturation level the sludge viability increased on average from 8.9 to 23.30o. In a loop type sewage conduits system operated as a plug flow reactor and enriched with biomass and air, it is possible to achieve high specific substrate removal rates when step feeding creates saturation conditions. This is further attenuated by a marked increase in the sludge viability.
INTRODUCTION
therefore the VSS concentration cannot be used as an indicator of active biomass concentration. The low viability in conventional systems stems from the limiting substrate concentration in the reactor, which is sufficient only just for maintenance requirements. This results in a very low net growth rate. As the effective life time of the bacteria is shorter than the nominal sludge age the majority of the bacteria cells are dead. An increase in the food to microorganisms ratio (F/M) (# > 0.3 day - t ) is followed by higher net growth rates and this results in steadily increasing sludge viability. Experiments were conducted to determine variations in sludge viability under substrate saturation conditions. The sludge viability was estimated using the kinetic parameters, maximum substrate removal rate (~) and oxygen uptake rate (J)---calculated per unit mass VSS.
Sludge viability, defined as the percentage of the volatile suspended solids (VSS) which is viable biomass, is less than 20% in conventional activated sludge systems. The 10w value is typical for units operating under severe substrate limitations, very common to continuously stirred sewage treatment reactor (CSTR). The standard index for biomass concentration is the VSS concentration which includes active and inactive cells, inert organic suspended solids and biodegradable matter adsorbed on biomass floes. The VSS concentration can be used as a measure of the active biomass concentration only when the active biomass is a constant fraction of the VSS. It has been reported (Weddle & Jenkins, 1971; Postgate & Hunter, 1962) that the sludge viability was found to be 15-20% and generally does not vary under the operational conditions of a conventional biological reactor for sewage treatment (# = 0-0.3 day- ~, where EXPERIMENTAL is the specificgrowth rate). The experimental work was carried out in a batch reacHowever, Prakasam & Dondero (1967), Austin & tor with pulse feeding, under substrate saturation conForster (1969) and Wooldridge & Standfast (1933) ditions, The reactor was aerated through porous diffusers. reported that the sludge viability is lower and varies The dissolved oxygen concentration was maintained above 2 mg I- i and the temperature was kept constant, between I and I0°/0under similar conditions. The reactor was fed with filtered sewage from the Haifa Under a narrow range of operational conditions, suburb, Neve Shaanan, The filtered sewage was strengthsuch as those found in activated sludge systems, the ened to 1200mgl -I COD by addition of a glucose solactive biomass is a constant fraction of the VSS con- ution and yeast extract and was diluted as required. centration and therefore the VSS concentration is a The biomass originated in an activated sludge pilot plant reliableindicator of the active biomass concentration. situated at the Technion, Haifa. The biomass was acclimaO n the other hand, when operating under a wider tized by feeding it with the strengthened sewage in a continuous laboratory system, for a period equivalent to range of operational conditions, as in the batch sys- several detention times such that steady state conditions tems case, or plug flow systems, the fraction of the were achieved (organic loading of about I g,COD per VSS which is active biomass is not constant and g. VSS' day and temperature range of 23--25°C!. This accli-
953
954
M. GREENand G. SHELEF
matized biomass was used for the experiments. In each experiment (which extended for about 6 h) the sewage was added three times at constant time intervals. The ~ and J coefficients were determined for each stage, where "stage" is defined as the time interval between 2 successive sewage additions. The 3 additions were conducted so that the instantaneous feed to microorganism ratio g,COD per g'COD'day immediately after the feeding, was the same in all 3 stages. The VSS concentration in the innoculum was 5000 mg 1- ~ and its volume was I I. The volume of each of the 3 sewage additions was about 21. Mixed liquor samples were taken every 3-10min and were analyzed for COD and VSS. At the termination of each stage (which extended 1~ h) the mixed liquor was allowed to settle and the supernatant was decanted to prevent any possible effect of substrate residuals enzymes and metabolic by-products which were produced during the stage. Because of the decantation it was possible to maintain similar substrate concentrations (i.e. preventing substrate dilution and maintaining substrate saturation conditions) in all the stages, while keeping the same organic loading, about 7 g. COD per g. VSS'day. The effect of removing the supernatant was determined in a number of experiments by comparing the kinetic behavior of the biomass with and without decantation: no difference was found. It is important to note that in all the experiments sewage from the same source was used, and that the glucose solution + yeast extract were a constant fraction of the strengthened sewage.
KINETICS Direct determination of sludge viability which is based on bacterial counting using microscopic methods or plate counts could not be used in this study because of the lack of sensitivity of these techniques. These techniques may be used for estimating order of magnitude changes in the number of microorganisms. However. they cannot be used for determining the fractional changes which were anticipated. Therefore the sludge viability was determined using an indirect technique. The kinetic parameters ~ (maximum substrate removal rate per unit mass VSS) and d (oxygen uptake rate per unit mass VSS) were used for determining changes in sludge viability. The experiments were performed under substrate saturation conditions. These conditions enable rapid increase in the sludge viability. Likewise, the determination of ~ and J is simple and more accurate under zero order reaction conditions as is explained in the following section.
Cl Determination The Monod equation is:
ds q's q = x.dt - K~+ S when q = S= x = K~ =
specific substrate removal rate substfate concentration biomass concentration saturation coefficient
= maximum specific substrate removal rate coefficient. However at substrate saturation condition S ~- K,, and the equation becomes: I ds q = ~ ~- =
Zr
I21
For each experiment it is possible to assume a constant value for the VSS concentration throughout the stage, because the variations in the VSS concentration are small relative to the VSS concentration in the reactor (Ax/x = 0.05-0.1 for each stage), Hence the coefficient can be determined from the gradient of the argumental results graphed as S/x vs t. Because the changes in VSS are very small relative to the VSS concentration the derivation of kinetic constants based on these changes, i.e. deriving #, are difficult and less accurate than techniques based on changes in substrate concentrations. Hence the determination of ~ was based on changes in substrate concentrations and not changes in VSS concentrations. Chemical oxygen demand (COD) was chosen as the analytical technique used for the determination of the substrate concentration. C O D was preferred over BOD method because of the time and work saving especially because of the large number of samples involved in batch experiments. Another reason for favoring the C O D method is that it has been shown that the conventional BOD test often underestimates the amount of organic substrate available for the aerobic bilogicat degradation of wastewater (Arthur, 1974: Ballinger & Lishka, 1962), In general it is necessary when using the C O D analysis to determine the fraction of the C O D which is not biodegradable. However, because of the technique chosen for determining ~, using the gradient of the data graphed as ,s/x vs t, this problem does not arise. This is because the gradient of the line (~) is the same whether s is the total C O D or only the biodegradable COD.
The determination of the specific oxygen uptake rate The oxygen uptake rate was determined by measuring the decrease in the dissolved oxygen concentration with a YSI Dissolved Oxygen Analyzer in I 1. of mixed liquor sample mixed by a magnetic stirrer. Under substrate saturation conditions, the value of the specific oxygen uptake rate is constant and is independent of substrate concentration.
The determination qf sludye viability (1)
When the sludge viability (i.e. percentage of active biomass per unit mass VSS) is not constant, the values of the kinetic parameters ~ and J, which are calculated per unit mass of VSS, will also change. It is possible to calculate the sludge viability from the changes in these parameters. The determination of sludge viability was based o n two assumptions: firstly that the change in ~ and J
Sludge viability in a biological reactor
values is the result of changes in the sludge viability only. Secondly, the increase in the VSS concentration is due to an increase in viable cell concentration only. The first assumption was applied t o the first two stages in order to determine the initial sludge viability. This assumption was then checked for the second and the third stages. Based on the first assumption it is possible to write that; qB ~A
--
=
average sludge viability in stage B (SVB) average sludge viability in stage A (SV,)
as:
x~
/~ j
\ Iv" 100|, and the average sludge viability in each stage
SVA =
(XoA + XrA)/2
(5)
(v + AXA + - ~ ) " 100 (3)
when SV, and SVa are the sludge viability in stage A and stage B respectively. By defining v as the initial concentration of active biomass (an unknown value), XoA, XfA,Xoa, XrB, are initial and final VSS concentrations in stage A and stage B respectively, and AX is the increment in VSS concentration in the whole stage (i.e. x r - Xo), it is possible to calculate the initial sludge viability (v) from equation (3) expressed
+
955
SVs =
(xos + xr,)/2
(6)
(c+AxA+Axa+~)'lO0 SVc =
(Xoc + Xrc)/2
(7)
If the sludge viability ratio in the stages B and C is equal to the ~ ratio in these stages, then the first assumption has been verified. A detailed description of the sludge viability determination is given elsewhere (Green, 1979).
IOO RESULTS AND DISCUSSION
\
Xoa
The kinetic parameters maximum specific substrate removal rate (9) and specific oxygen uptake rate (J)
XfB
,4, Determination of changes in VSS concentration (x) can be based on direct measurement of VSS concentration (Standard Methods, 1978) or on the removed substrate concentration multiplied by the yield coefficient (under substrate saturation conditions cell decay can be neglected). Calculating the VSS concentration changes using the reduction in the substrate concentration is preferred when the change in VSS concentration in each stage is small relative to its concentration in the reactor. The net yield coefficient was derived from the equation: dx -y. dt
-ds dt
when Y, is the net yield coefficient, dx is the net VSS addition and - d s is the COD reduction. Y net is very similar to Y gross (VSS produced per COD removed) under substrate saturation conditions, when the cell decay is negligible in comparison with the total growth. The yield coefficient is independent of the active cell concentration in the sludge and therefore the change in sludge viability should not affect the yield coefficient value. The yield coefficient was found to have a similar value during all the stages of an experiment: 0.5 From equation (4) it is possible to calculate the initial active biomass concentration (v). Having calculated v it is possible to calculate the initial sludge viability
The ~ coefficient g. COD per g. VSS.day was determined for every stage in l l experiments. Three values were determined for every experiment: qA for the time interval between the first and second feedings, stage A; tta, for stage B; and qc for stage C. values are given in Table 1 and an example of ~ determination is shown in Fig. I. The results in Table 1 show variations in the ~ values of the same stage between the experiments. The differences between the values of the same stage are due to daily and hourly changes in the sewage characteristics and changes in the biomass composition and sludge age of the activated sludge taken from a pilot plant. The qA values are lower than published values of ~. The reason for this could be the low organic loadings and the high sludge age in the source pilot plant (sludge age > 10 days), conditions which caused low sludge viability. Nevertheless, the ~ values in these experiments are similar to those obtained in other studies at the Technion (Rom, 1971, ~ = 0.9; Scheindorf, 1976, ~ = 2.29). The results in Table 1 show a significant increase in values from stage to stage in the same experiment. coefficient values increased from an average value of 1.3 day-~ in stage A to 1.88 day-1 in stage B (an average increase of 45~) and to 2.7 day-1 in stage C (an average increase of 449/0). The total average increase in ~ from stage A to stage C was 108~o (from 1.3 to 2.7 day-1). The specific oxygen demand rate g - O 2 per g. VSS.day was determined for each stage in 3 experiments. Since the reactor operated under substrate saturation conditions, the substrate removal rate was independent of the substrate concentration. Similarly
M. GREEN and G. SHELEF
956
Table 1. Values of the specific maximum substrate removal rate (~) in the different stages Experiment no.
i-stage A*
i-stage B*
i-stage C*
~,,.~.,
qc,~).
I 2 3 4 5 6 7 8 9 10 11
1.93 1.30 1.57 -1.37 0.67 1.53 1.26 1.05 0.85 1.51
2.78 1.9 2.t2 2.13 1.98 1.29
4.16 2.36 3.37 3.09 2.58 2.04 2.38
1.61 1.56 1.29 2.09
2.25 1.95 2.81
1.45 1.46 1.35 -1.45 1.9 -1.28 1.5 1.52 1.38
1,5 2.15 1.24 1.82 1.59 2.15 1.45 -1.3 1.88 1.58 3.04 ~ 1.56 . . . . . . . 1.44 2.14 1.51 2.29 1.34 2.86
Average
1.30
1.88
2.7
1.48
1.44
~)c,'~)A
2.21
* g ' C O D per g. VSS.day. stage. The results in Table 2 show an increase of t 13% in the average value of J, from stage A to stage C. This increase is equal to the increase in d in the 3 experiments. Since ~ and J values were determined using different techniques, the excellent agreement between the increase of these t w o parameters is a reflection of the accuracy and reliability of the techniques.
the small changes in VSS concentration during each stage can be neglected, thus, the oxygen uptake rate was approximately constant for each stage. The results are given in Table 2 and in Figs 2 and 3. (The decrease in VSS concentration from stage to stage as shown in Fig. 2, is due to the mixed liquor samples which were taken frequently.) For comparative purposes the ¢~ values were also determined here for each
0.,~
o
0~.~
Stage
t.~.__."2~
A
~.I~ ®y'~
0.287~ o.z~ o~o'~
o.'oio4
oi&e4 T,
0.o204
o~
-
days
Fig. 1. Specific maximum substrate removal rate (~) determination (Experiment No. 14).
Table 2. Comparison of the maximum substrate removal rate Iq) and the oxygen uptake rate {J) in the different stages
Stage
Oxygen uptake rate per mass VSS J (day - i)
Maximum substrate removal rate per mass VSS ¢~(day - ~)
12
A B C
0.82 1.34 1.89
1.64 3.12 3.96
13
A B C
1.01 1.51 2.12
1.5 2.42 3.24
14
A B C
0.90 1.46 1.74
1.53 1.93 3.0
Experiment no.
JcJA
qc/qA
2.30
2.41
2.10
2.00
1.93
1.99
Sludge viability in a biological reactor
957
The sludge viability Stage C VSS- I I09 rng I"l J - 1,74 day -I
0 0 0 0 0
C
i
I
I
l
I
I
I
i
l
Stage B VSS • 1300 mg I"l J - 1.46 day-I
T 0 0 0
C
m
=
=
t
t
l
l
StoOe A 6
V S S . 1600 rng I~ J • 0.90 day
4-0 0 0 0
,
,
,
;
=
Time, rain
Fig. 2. Oxygen uptake rate (J) determination (Experiment No. 14).
As the sludge was not changed during each 6 h experiment and a constant composition substrata was added to each stage, it was expected that kinetic parameters would be constant except if there was a change in sludge viability. The sludge was acclimatized before each experiment hence acclimatization effects are negligible. Changes in the dominant species within the micro organism population would not be significant over such a short time (6 h). Therefore the marked increase in the values of ~ and J (which were calculated per mass of VSS and not per mass of active biomass) was due to the increase of sludge viability and hence these parameters should be used for the determination of the sludge viability. The sludge viability values, which were calculated from ~ values in the different stages, are given in Tables 3 and 4. As was noted, the q^/qa ratio was used for the determination of the sludge viability ratio in stages A and B. The correlation between the sludge viability ratio in stages B and C and ~ ratio in these stages was used to verify the second assumption that the increase in ~ value per mass VSS was caused by the sludge viability increase. The results in Table 4 show a good correlation between cls/~lc ratio and the sludge viability ratio in these two stages. The differences between these two ratios vary from 0 to + 7%, with an average value of + 3%. The reason for these small differences can be the sludge viability determination method or because of laboratory errors. These differences are considered negligible when dealing with biological reactors data. From the results in Tables 3 and 4 it can be concluded that the increase in ~ values stems only from the increase in sludge viability and therefore ~ values can be used to determine sludge viability. If the ~ coefficient is calculated per unit mass of active biomass (and not per unit mass of VSS) then
A B C
A B
C
A
B
C
~eae
A
A
C
A
B
C
SmO,
B
C
Exp 12
B
Exp 13
Exp 14
Fig. 3. Comparison of maximum substrate removal rate (~) and' oxygen uptake rate (J), per mass VSS, in the different stages.
w Jr. ! 5/8 - - .
958
M. GREEN and G. SHELEF Table 3. The sludge viability Experiment no.
Original sludge
1 2 3 5 6 8 10 11 Average
11.2 7.7 10.5 8.7 3+9 8.3 7.7 13.5 8.9
Sludge viability (9o) Stage A Stage B
no differences should have been found in ~ between the different stages of each experiment. The maximum substrate removal rate per mass of active biomass, qv, was calculated from the following equation: 3'100 SV when SV is the average sludge viability during the stage.
13.7 9.3 12.4 11.2 6.2 10.2 10.0 15.3 11.0
19.8 13.5 16.8 16.2 11.9 15.2 15.2 21.2 16.2
Stage C 27.0 19.5 23.6 23.0 19.3 21.3 23.1 29.2 23.3
The results shown in Table 4 indicate that qv is constant during each experiment (an average van, ation of +39"0 between the stages). The results m Table 3 show that the initial sludge viability in stage A varies between 3.9 and 13.59/o with an average value of 8.9~o. This viability ~s in agreement with the other published results (Blok, 1976: Prakasam & Dondero, 1967). F r o m Table 3 it can be seen that the sludge viability in stage C is 2.6 times higher than that of the original sludge. The active biomass increased from 445 mg in the original sludge (8.990 of 5000rngVSS)
Table 4. The variation of the specific maximum substrate removal rate with the sludge viability
Exp. no.
Max. substrate removal rate per mass of active biomass c~,,(day- t)
Stage (st.)
Viability (o~,}
Max. substrate removal rate per mass VSS ~ (day- l)
A B C
13.73 19.79 27.00
1.93 2.78 4.16
14.1 14.1 15.4
A
9.25
14.1 14. I 12.1
B
13.50
C
19.50
1.30 1.90 2.36
A B C
12.38 16.75 23.60
1.57 2.12 3.37
12.7 12.7 14.3
A B C
11.18 6.20 23.00
1.37 1.98 2.58
12.3 12.3 11.2
A
6.15
10.9 10.9 10.6
B
11.87
C
19.30
0.67 1.29 2.04
A
10.24
1.05
10.3
B C
15.20 21.30
1.56 2.25
10.3 10.6
10
A B C
9.99 15.20 23.05
0.85 1.29 1.95
8.5 8.5 8.5
tl
A B C
15.30 21.20 29.15
1.51 2.09 2.81
9.9 9.9 9.6
Viability (°,oFst. C Viability (°o)-st. A
qc/q,~
1.97
2.16
2.11
1.82
1.91
2.15
2.06
1.88
3.14
3.04
2.08
-.'~14
2.31
2.29
1.91
1.86
Sludge viability in a biological reactor to i 384 mg in stage C (23.3% of 5939 mg VSS, including VSS which was taken out with the samples). The nominal increase is the same (939 rag) but this an increase of 211~o in the active biomass and only 18.8% in the VSS. The initial sludge used in stage A of each experiment originated in a CSTR type system which was severely substrate limited, hence there was a very slow sludge viability. This is because the substrate concentration was only sufficient for maintenance requirements and minimal growth. However, on transferal to experimental substrate saturation conditions a high nett growth was achieved which caused the increase in the sludge viability during each experiment. Preliminary experiments which extended over 16 h have indicated that it is possible to increase the sludge viability to 60% and perhaps more. Operation of sewage treatment facilities at hioh substrate concentrations
Conventional CSTR sewage treatment plants operate under severe substrate limiting conditions because the final etttuent quality requirements dictate the substrate concentration in the reactor. This is not the case in a two or more stage reactor or in a plug flow reactor. A plug flow reactor with step feeding for biological treatment of wastewater can operate along most of its length under saturation levels of substrate concentrations (i.e. under conditions close to ~) and therefore with steadily rising sludge viability. Such conditions can be achieved using a main sewer which is used as a plug flow reactor with entering sewage flows from branch mains being analogous to step feedings (Green, 1979: Green & Shelef, 1980). The system should be highly loaded along most of its length to maximize removal efficiency. Only the final section of the system should not be fed to ensure that a high quality effluent (low in dissolved substrate) is produced.
CONCLUSIONS Under a wide range of operation conditions the VSS concentration cannot be used as a criterion for active biomass concentration since the viable organism content of the VSS is not constant. The sludge viability was determined from changes in maximum specific substrate removal rate coefficient (9) and the specific oxygen uptake rate (J) when these two parameters were calculated per mass VSS. In experiments with sludge which was taken from an activated sludge plant (operating under limiting substrate concentration), acclimatized and transferred to
959
substrate saturation conditions in a batch reactor, a gradual and steady increase in the value of 9 was found. The values ranged from an average value of i . 3 g . C O D removed per g.VSS'day to 2.7g.COD removed per g. VSS. day, an increase of over 100%. A similar increase was found in the specific oxygen uptake rate (from 0.91 to 1.92g.O 2 per g,VSS.day). The main reason for the increase in these kinetic parameters is the increase in the sludge viability as a result of the higher net growth rate under substrate saturation conditions. When the maximum substrate removal rate was calcrlated per mass of active biomass (and not per mass of VSS) no change in the substrate removal rate was found. Contrary to the substrate limiting conditions and the low viability which are typical of standard CSTR sewage treatment plants, it is possible to obtain a marked increase in sludge viability under substrate saturation conditions. Such a system which operated under high substrate concentrations has a double advantage over standard biological wastewater treatment plants, namely, the possibility of working at high substrate removal rate (9) levels and the increase in the sludge viability which results from a higher net growth rate of the biomass. The previously mentioned advantages can be applied in a two or more stage reactor or in a sewerage system operating as a step fed plug flow reactor for sewage purification. REFERENCES
Arthur R. M. (1974) Let's upgrade the BOD test. War. Sewage Wks 111, 100-102. Austin B. L. & Forster C. F. (1969) The microbiology of Lubeck activated sludge plant, Wat. Waste Treat. J. 208. Ballinger D. G. & Lishka R. J. (1962) Reliability and precision of BOD and COD determinations. J. Wat. Pollut. Control Fed. 34, 470--479. Blok J. (1976) Measurements of viable biomass concentration in the activated sludge by respirometric techniques. Water Res. 10, 919-925. Green M. (1979) Ph.D. Thesis. Technion, Israel Institute of Technology, Haifa, Israel. Green M. & Shelef G. (1980) Sewer trunk lines can be treatment facilties. War. Sewage Wks August. Postgate J. R. & Hunter J. R. (1962) The survival of starved bacteria. J. gen. Microbiol. 29, 233-263. Prakasam T. B. S. & Dondero N. C. (1967) Aerobic heterotrophic bacteria population of sewage and activated sludge. Appl. Microbiol. 15, 461-467. Rom D. (1971) Ph.D. Thesis. Technion, Israel Institute of Technology. Haifa, Israel. Scheindorf C. (1976) M.Sc. Thesis. Technion, Israel Institute of Technology, Haifa, Israel. Standard Methods for the Examination of Water and Sewage
(1978) American Public Health Association. Weddle C. L. & Jenkins D. (1971) The viability and activity of activated sludge. Water Res. 5, 621-640. Wooldridge W. R. & Standfast A. F. B. (1933) The biochemical oxygen demand of sewage. Biochem. J. 27, 183.