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Effect of sludge age and substrate composition on the settling and dewatering characteristics of activated sludge
f4ater Rc~. Vol 17. N o II, pp 1511 1515. 1983 Print~:d in Great Britain All rights reser',ed
0043-1354835300---0.00 Copyright ~j 1983 Pergamon Press ktd
EFFECT OF SLUDGE AGE AND SUBSTRATE COMPOSITION ON THE SETTLING AND DEWATERING CHARACTERISTICS OF ACTIVATED SLUDGE D. A. LOVETT, B. V. KAVANAGHand L. S. HERBERT CSIRO Division of Food Research. Meat Research Laboratory. P.O. Box 12. Cannon Hill. Queensland, Australia 4170
(Receiced May 1982) Abstract--Substrate composition and sludge age are known to influence the biokinetics of the activated sludge process and recent work has shown that substrate composition also influences the settling and dewatcring characteristics of the sludge. This paper presents results from an experimental study of an activated sludge reactor ted with a solution of a soluble meat-extract. Settling and dewatering data from this work compared with published data, demonstrate a considerable effect of substrate compositions on the relationships between sludge characteristics and sludge age. The effect on sludges of low age were particularly marked. The maximum rate of biodegradability appears to be the substrate property which most strongly inlluences sludge characteristics.
INTRODUCTION In the activated sludge process, biomass solids are separated from water in two stages. In [he first, suspended solids in the mixed liquor from the aeration pond are allowed to settle, and yield activated sludge: and in the second, some ol" the activated sludge is dewatered by filtration to produce waste activated sludge. A knowledge of sludge settling and dewatering characteristics is essential for proper design and operation of activated sludge treatment plants, and these aspects have been studied by Ford and Eckenfelder (1967), Bisogni and Lawrence (1971), Pitman (1975), Heddle (1977), Kiff(1978) and Chao and Keinath (t 979). The information however, is limited, and particularly for sludge settling, is sometimes contradictory. For example, Bisogni and Lawrence (1971) report that, at low sludge ages, solids settle more readily as sludge age decreases. The opposite conclusion, that solids settle less readily as sludee a_e decreases, hits been drawn by Ford and Eckenfelder (1967). It is possible that such differences rektte to different substrates used by these authors. In this paper, we report solids settling and dewatering data obtained with laboratory-scale, activated sludge reactors fed with a meat concentrate solution. These data are compared with published data to illustrate the effect of substrate composition in determining the settling and dewatering characteristics of activated sludge.
MICROBIM. GrO~VTH KINETICS
Theoretical descriptions of bacterial growth and substrate utilisation in completely mixed, activated sludge systems have been presented by workers such as Lawrence and McCarty (1970), Sherrard (1977),
and Metcalf and Eddy (1979). Using the notation of Metcalf and Eddy, the net specific growth rate of microorganisms in such systems is given by
Fl'= Y U - k a
(I)
where IL'= net specific growth rate (time -~) Y = m a x i m u m yield coefficient during logarithmic growth; defined as the mass of cells formed per unit mass of substrate chemical oxygen demand (COD) consumed U = specific substrate utilisation rate (time -~) rsu
X kj = endogenous decay coefficient (time -~) r~u = rate of substrate utilisation (mass vol -t time-~); mass of COD removed per unit volume of reactor per unit time X = c o n c e n t r a t i o n of viable microorganisms (mass vol-L); approximated by the concentration of mixed liquor suspended solids. Using a Monod-type growth relationship, U can be expressed as
kS K,+S
U= - -
(21
where k = maximum rate of substrate utilisation per unit mass of microorganisms (time-') K~= half velocity constant, substrate concentration at one-half the maximum growth rate (mass voL-~) S =substrate concentration surrounding the microorganisms (mass vol-~). For a completely mixed system, S is the substrate concentration in the effluent. 1511
1512
D . A . LO~,ETT c't d./. Table 1. Operating parameters for acti,,ated ~ludge reactors
Sludge age Ida2, I
F M {g C O D g MLSS -~ day -')
C O D of influent ~mgl-'l
Hydraulic residence time (day)
pH
dissolved ox?gen ling i-~l
MLSS irng I - ' )
..: "~ 8 II 13 lS 20
0.93 084 0.46 036 030 025 0 24
2400 2460 2510 [ 220 1780 1640 1280
1.4 [ .4 I6 12 28 27 19
79 7S 77 6.5 6.3 7.0 63
2.0 2.0 1.3 2.0 15 1.5 30
I851) 2100 340~) 2800 2150 2400 285/)
It follows from e q u a t i o n s (1) a n d (2) that vahtes o f Y, k ~. k and K can be calculated by d e t e r m i n i n g values o f / l ' , U a n d S at several different o p e r a t i n g conditions. In line with m a n y o t h e r w o r k e r s in this area (Bisogni a n d L a w r e n c e , 1971: P i t m a n , 1975: S h e r r a r d a n d S h r o e d e r , 1973), we have c h o s e n "'Sludge A g e " (0,) in p r e f e r e n c e to "'loading f a c t o r " or "'food to m i c r o o r g a n i s m mass r a t i o " ( F / M ratio) to define the o p e r a t i o n a l state o f an a c t i v a t e d sludge reactor. F o r a c o m p l e t e l y mixed s y s t e m at s t e a d y state, sludge age equals the ratio o f total b i o m a s s in the s y s t e m to the daily w a s t a g e or p r o d u c t i o n rate o f b i o m a s s i.e. 0 = l i d . In general, low sludge ages are a s s o c i a t e d with rapid rates o f m i c r o b i o l o g i c a l g r o w t h and high rates o f sludge p r o d u c t i o n a n d wastage. High sludge ages are a s s o c i a t e d with slower g r o w t h a n d low rates o f sludge p r o d u c t i o n . APPARATUS. MATERIALS AND PROCEDURES Two activated sludge reactors were constructed from polycarbonate sheet. Each consisted of a stirred, aerated compartment of 9 I.. separated by a vertical baffle from a I 1. settling chamber. Operating conditions were set to allow the reactors to operate at sludge ages of 4, 5, 8. II, 13, 18 and 20 da>s by removing a volume ot" mixed liquor each day sulficient to provide a total daily solids loss equzd to 10, of the total biomass in the reactor. The mixed liquor removed was replaced by water, which was taken into account when calculating hydraulic residence times. Temperature of reactor contents was maintained between 19 and 23 C. A soluble meat concentrate--"Bonox"--manufacturcd by Kraft Foods Lid was used as the substrate because wc wished to simuhtte the treatment of wastewater from abattoir operations. Bonox has a crude protein content of 37",.. fat I",, and total phosphorus 0.4'!,,, dry solids basis. Its COD content is 0.678 C O D g -I dry solid and it has a C O D : N : P ratio of 156: 14:1, compared to 170:9: I for an untreated abattoir effluent, reported by Heddle (1977). Bonox was pumped to the reactors by peristaltic pumps controlled by timers at feed rates corresponding to 4.5-27 g COD day -~. Other pumps fed water to the reactors to maintain the required hydraulic residence times. [n Table I the values of the operatiomd parameters used at the different steady state conditions are shown. Low values of COD removal etficiency were obtained at low sludge ages (68". at 4 days, 73,, at 5 days} rising to higher values at the higher sludge ages (91, 88, 91. 91 and 94", at 8. 11. 13. 18 and 20 days respectively). Similar variation of COD removal efficiency with sludge age have been widely reported by other workers. The reactors were seeded with sludge from an activated sludge plant treating abattoir wastewater. Microscopic examination shinned the sludge initially contained filamentous organisms, which persisted throughout the
tests. On several occasions, one or more types of filamentous organism became domimmt in the mixed liquor. seriously reducing settling rates. The only effective remedy was to discard the affected mixed liquor and start the test again. To determine when stable conditions had been attained at a given sludge age. selected parameters were monitored, usually dail), until constant values were obtained. In the mixed liquor, the concentration of mixed liquor suspended solids (MLSS), sludge capillary suction time and sludge volume index were monitored and. in the effluent, soluble COD. Considerable time was required for reactor conditions to stabilise, up to 3 months in tests in which biomass had to acclimatise to the new substrate. Large variations in sludge and mixed liquor characteristics were observed during the stabilisation period. At various times, the biomass became almost completely dispersed and non-settling for several weeks. A thick foam would sometimes develop on the liquid surface. At other times the liquid viscosity increased, and greatly increased aeration rates were required to maintain adquate dissolved oxygen levels. Similar changes have been reported to occur in full-scale plants (Schwartz et aL, 1980: Dhaliwal, 1979). Sludge Volume Index (SVI) measurements were made in accordance with Standard Method~ (APHA, 1971), except that all samples were diluted with tap water to give MLSS concentrations of 0.3 times original concentration. The resulting samples had solids concentrations of around 1000 m g l - t . Dick and Veselind (t969) have shown that for non-bulking sludges, SVI values are independent of solids concentration lit around 1000mgI-~, and this was contirmed in our experiments. At MLSS concentrations o[" 2000rag[ -~ or more, SVI values varied markedly with solids concentration for samples from the same source (curves I and II of Fig. l) and even for consecuti,,e measurements on the same sample (curves Ill and IV of Fig. 1). Similar values of SVI were obtained for samples
250
~"~, 2 0 0
-~
150
~00
@o • O
Sample from full scale p~ant freahng oboTloir wastes
•
Samole from laborotory scale plant
50
0
l 2000
I 4000
MLSS
I 6000
f 8000
(rag t-')
Fig. 1. Variation of SVI with suspended solids concentrations.
Sludge age and substrate composition on characteristics of activated sludge Table 2. Characteri:itics of polyelectrolytes u.~d in conditionin~
studies
Charge density Pol',mer t,,pe
Charge type
(%)
Non-ionic Non-ionic Non-ionic Cationic Cationic Cationic Anionic Anionic Anionic
Nil Nil Nil 20 25 211 35 30 30
-\cc, lamide
~cr~lamide quaternary amine copol?mer ~,cr.'.lamide act'.kite topoi? mer ND =
not
1513
Mol'm:ular',.,eight x
I0~'
ND
>9 II >2 4 4 ND >9 II
determined
diluted ~ith mixed liquor supernatant, and those diluted with tap water. Dispersed solids were measured by the method of Bisogni and Lawrence (1971). Samples used for SVI tests were allowed to settle for a further 30 rain, and the suspended solids in the supernatant were measured and expressed as a percentage of the total suspended solids in the SVI sample. Dispersed solids values were independent of M LSS concentration. Specilic resistance to filtration and capillary suction time (SRF and CST) of mixed liquor samples were measured according to the methods described by Kavanagh (1980). SRF values are expressed in units of m kg -~ and CST values in seconds. The physico-chemical properties of the polyelectrolytes used in conditioning tests are listed in Table 2. An appropriate volume of an approx. 1000mg I-~ solution of polyelectrolyte was added to 300 ml of mixed liquor, and the mixture was stirred vigorously for 10. Stircing was reduced to low speed for a further 1-2 rain before SRF or CST measurements were made.
RESUI.TS
AND
DISCUSSION
In Fig. 2, SVI values are plotted against sludge age for our experiments with a meat concentrate sub-
strate, and for work using different substrates reported by others. The results of Heddle (1977), from work on abattoir wastewater, are similar to our own. The results of Bisogni and Lawrence (1971), and those of Chao and Keinath (1979) for activated sludge plants operating on glucose-based substrates are similar. The results of Pitman (1975) for domestic sewage substrate show quite different behaviour of SVI with sludge age. The high MLSS concentrations used by Bisogni and Lawrence and Pitman at sludge ages greater than 8 days may have increased their SVI values, but this would not appreciably alter the comparisons in Fig. 2. Ford and Eckenfelder (1967) and Kilt (1978), also report SVI data for domestic sewage substrate, Their results agree qualitatively with Pitman's, showing a steady increase in SVI with increasing loading factor (decreasing sludge age). It appears that there is reasonably good agreement between plots of SVI/sludge age results reported for activated sludge plants treating similar substrates, and very wide differences between results for plants treating different substrates. However, other factors, such as the dominance of filamentous micro-
\
800
3isogni and Lawrence (1971) (glucoie substrale)
~
tic sewage)
600
T
-~ (.~
400
200
/
O k l
•
0
Present
study
I
I
5
I0
(meat
t
concentrate) l 15
O: (days) Fig. 2. Variation of SVI with sludge age for different substrates.
I
20
1514
D.A. LOVETT et 50
-~
4o
/
3o
I Present Study
~ ~o
=
Bisoglni and Lawrence (1971)
c5
I
0
4
8
12 0c (days)
I
16
20
Fig. 3. Variation of dispersed solids with sludge age. organisms in the biomass, could overshadow the effects of either substrate composition or sludge age on SVI data obtained in a particular treatment plant at a given time. In Fig. 3, dispersed solids are shown as a function of sludge age. As sludge age increases-above 8 days, dispersion decreases slowly from about 37~, towards zero. At sludge ages below 8 days, there is a dramatic increase in dispersion to 35'}0 at 4 days. The results of Chao and Keinath (1978) and Bisogni and Lawrence (1971), working with glucose-based substrates are also presented in Fig. 3 and show a similar trend, although the increase in dispersion occurs at lower sludge ages. Similar trends are noted by Lawrence and McCarty (1970) and Pipes (1979), working with industrial wastewaters. The increase in dispersion was accompanied by a decrease in substrate removal efficiency, indicating that the specific growth and substrate removal rates of the microorganisms were at or near their maximum values. In our results, the rapid increase in dispersion occurs at 8 days, implying that maximum specific growth and substrate removal rates are low for meat concentrate substrates, compared to glucose substrates where the rapid increase occurs at a sludge age of less than 1 day. High levels of dispersed solids and low substrate removal efficiences were not observed by Heddle (1977) in abattoir wastewater treatment, even at sludge ages approaching 1 day. The explanation may be that fat and suspended solids which make up approx. 60'~.;;of the COD of abattoir wastewater, have been observed to be efl'ectively removed from activated sludge systems by adsorption onto microbial flocs (Mulligan and Sheridan, 1975: and Lordi and Lue-Hing, 1976). Such a mechanism increases the removal rate of substrate, particularly at low sludge ages where a greater fraction of the biomass is wasted each day. Further. Benedict e t al. (1979) have shown that the presence of svspended solids in the substrate reduces
al.
the microbial growth rate at a given sludge age. and is therefore likely to reduce solids dispersion. Filtration results are presented as values of SRF and CST plotted against sludge age in Fig. 4. The CST data have been adjusted to represent equivalent times for samples of 1"i, suspended solids concentration. Kavanagh (1980) has shown from theoretical considerations that there is a direct relationship between SRF and CST values measured on the same sample, and the similarity in the shapes of the CST vs 0, curves confirms this relationship. Both curves show improved filterability with sludge age, with approx. 200-fold increases as sludge age increases from 4 to 20 days. Also shown in Fig. 4 are SRF data for 2 samples obtained from a plant treating abattoir wastewater (Kavanagh, personal communication) and from a domestic sewage treatment plant (Pitman, 1975). Kavanagh's results agree well with our own. as do those of Pitman at sludge ages greater than 8 days. Pitman demonstrated that filtration resistance is directly related to the concentration of dispersed solids in the sludge, and our SRF results are also consistent with the increase in dispersion at sludge ages below about 8 days. Conditioning tests on samples having sludge ages of 5, 7 and 20 days, showed that non-ionic and anionic polyelectrolytes were ineffective in improving the filterability of our activated sludge, as assessed by CST and SRF measurements. Effectiveness of cationic polyelectrolytes varied considerably, depending on sludge age (Fig. 5). At a sludge age of 5 days, there was no improvement in SRF. whilst at sludge ages of 7 and 20 days, progressively larger reductions in SRF values were obtained. These results are in agreement
7 J5 E
u. OE co 14
13
L2
4 0" 0
1--
o~
(.9 O_ g
3
-
\
2
I
0
I
I
4
8
°lx~_._o J2
o_~ 16
2o
0 c (days}
Fig. 4. Variation of SRF and CST with sludge age.
Studge age and substrate composition on characteristics of activated sludge
8 c - 5 days
&. q5 7
E
14 ~
0c - 7days
C~ 03
relatively constant for substrates as different as glucose and yeast (Bisogni and Lawrence, 1971) and abattoir wastewater (Heddle, 1977). This result is suprising in view of the variety of factors which ma,, influence the value of Y. These factors include the oxidation state of the carbonaceous substrate, the degree of substrate polymerisation and the activity of specific metabolic pathways (Sherrard and Shroeder, 1973). REFERENCES
2 o~
1515
f3
A P H A (1971) Standard Methods /or the Examination o/" Water and Wastewater. A m e r i c a n Public Health Associa t i o n , New York.
Oc - 2 0 days ~2
I 2
I 4
I 6
I 8
I IO
Polyelectrolyte dose (g kg -~) Fig. 5. Variation o f SRF
with dose o f cationic poly-
electrolyte at different sludge ages.
with Pitman's observation that "'the greater the degree of bioflocculation of sludge, the greater the effect of chemical conditioners and the lower the amount of chemicals required to produce good dewatermg". In sludges of age 5 days, polyelectrolyte doses as high as 100 g kg -~ sludge solids proved ineffective in reducing filtration resistance. At this sludge age, about 20% of the solids are dispersed, i.e. only about 80')o are bio-flocculated, so that a poor effectiveness may be expected. However, the absence of any improvement, even at very high polyelectrolyte dose rates, may indicate that other factors, such as changes in the surface characteristics of the flocs, may also play a role in determining conditioning requirements. Values of microbial growth coefficients and substrate utilisation rate coefficients were: Y =0.41 mg MLSS mg -~ C O D ; k~= 0.04 day-~: k = 0.67 day-~: K~= 1 5 0 m g l -~ COD. The valtte of the maximum specific substrate utilisation rate, k, is substantially lower than for most other substrates (Lawrence and McCarty 1970; Nelson and Lawrence, 1980; Metcalf and Eddy, 1979), indicating a low rate of biodegradability. A similar conclusion has already been drawn from the dispersed solids and filtration data presented above. The reasons for the low rate of biodegradability of meat concentrate are not known. The values of the other coefficients, on the other hand, are within the range of values obtained by other workers for synthetic substrates and domestic sewage. The report values of Y, in particular, are
Benedict A. H.. Merrill M. S, and Mauseth G. A. (19791 Sludge production, waste composition, and BOD loading effects for activated sludge systems. J. Wat. Pollut. Control Fed. 51, 2898-2915. Bisogni J. J. and Lawrence A. W. (1971) Relationships between biological solids retention time and settling characteristics of activated sludge. Water Res. 5, 753-763. Chao A. C. and Keinath T. M. (1979) Influence of process loading intensity on sludge clarification and thickening characteristics. Water Res. 13, 1213-I 223. Dhaliwal B. S. (I 979) Nocardia amarae and activated sludge foaming. J. Wat. pollut. Control Fed. 51, 344-350. Dick R. I. and Vesclind P. A. (19691 The sludge volume index--what is it'? J. 142tt. Poll,a. C¢mtrol Fed. 41, 1285-1291. Ford D. L. and Eckenfelder W. W. (1967) Effect of process variables on sludge fioc [brmation and settling characteristics. J. Wat. Pollut. Control Fed. 39, 1850-1859, Heddle J. H. (1977) Activated sludge treatment of meat wastes with protein recovery. Meat Industry Research Institute of New Zealand Technical Report No. 616. Kavanagh B. V. (I 980) The dewatering of activated sludge: measurement of specilic resistance to t]ltration and capillary suction time. |Vat. Pollut. Control 79, 388-398. Kiff R. J. (1978) A study of the factors affecting bioflocculation in the activated sludge process. Ifat. Pollut. Control 77, 464-470. Lawrence A. W. and McCarty P. L. (1970) Unilicd basis for biological treatment design and operation. J. sanit. Em.zng Dir. Am. Sac'. cir. Engrs 96, 757-778. Lordi D. T. and Leu-Hing C. (1976) Fats oils and greases-effluent regulations and municipal treatment plant operations. Proc. 31st htd. tt2tste Con]~ Pur~hw Unit'. Metcalf and Eddy Inc. (I979) Wastewater L)tghwerhtg: Treatment Disposal and Reuse. T M H Edition, Chap. 9.
McGraw-Hill, New York. Mulligan T. J. and Sheridan R. P. (1975)Treatment of high strength fatty acid derivative waste~atcrs. Proc. 30 Ind. Waste Conf Purdue Unit'.
Nelson P. O. and Lawrence A. W. (1980) Microbial ',lability measurements and activated sludge kinetics. Water Rex. 14, 217-225. Pipes W. O. (1979) Bulking. defiocculation and pinpoint floe. J. Wat. Pollut. Control Fed. 51, 62-70. Pitman A. R. (1975) Bioflocculation as a means of improving the dewatering characteristics of activated sludges. Wat. Pollut. Control 74, 688-700. Schwartz H. G.. Popowchak T. and Becket K. (1980) Control of sludge bulking in the bre~ing industr 3. J. l['at. Pollut. Control Fed. 52, 2977-2993. Sherrard J. H. (1977) Kinetics and stoichiometry of completely mixed activated sludge. J. fg'at. Po[[ut. Control Fed. 49, 1968-1975. Sherrard J, H. and Schroeder E. D. (1973) Yield and growth rate in activated sludge. J. Wat. Pollut. Control Fed. 45, 1889-1896.
0043-1354835300---0.00 Copyright ~j 1983 Pergamon Press ktd
EFFECT OF SLUDGE AGE AND SUBSTRATE COMPOSITION ON THE SETTLING AND DEWATERING CHARACTERISTICS OF ACTIVATED SLUDGE D. A. LOVETT, B. V. KAVANAGHand L. S. HERBERT CSIRO Division of Food Research. Meat Research Laboratory. P.O. Box 12. Cannon Hill. Queensland, Australia 4170
(Receiced May 1982) Abstract--Substrate composition and sludge age are known to influence the biokinetics of the activated sludge process and recent work has shown that substrate composition also influences the settling and dewatcring characteristics of the sludge. This paper presents results from an experimental study of an activated sludge reactor ted with a solution of a soluble meat-extract. Settling and dewatering data from this work compared with published data, demonstrate a considerable effect of substrate compositions on the relationships between sludge characteristics and sludge age. The effect on sludges of low age were particularly marked. The maximum rate of biodegradability appears to be the substrate property which most strongly inlluences sludge characteristics.
INTRODUCTION In the activated sludge process, biomass solids are separated from water in two stages. In [he first, suspended solids in the mixed liquor from the aeration pond are allowed to settle, and yield activated sludge: and in the second, some ol" the activated sludge is dewatered by filtration to produce waste activated sludge. A knowledge of sludge settling and dewatering characteristics is essential for proper design and operation of activated sludge treatment plants, and these aspects have been studied by Ford and Eckenfelder (1967), Bisogni and Lawrence (1971), Pitman (1975), Heddle (1977), Kiff(1978) and Chao and Keinath (t 979). The information however, is limited, and particularly for sludge settling, is sometimes contradictory. For example, Bisogni and Lawrence (1971) report that, at low sludge ages, solids settle more readily as sludge age decreases. The opposite conclusion, that solids settle less readily as sludee a_e decreases, hits been drawn by Ford and Eckenfelder (1967). It is possible that such differences rektte to different substrates used by these authors. In this paper, we report solids settling and dewatering data obtained with laboratory-scale, activated sludge reactors fed with a meat concentrate solution. These data are compared with published data to illustrate the effect of substrate composition in determining the settling and dewatering characteristics of activated sludge.
MICROBIM. GrO~VTH KINETICS
Theoretical descriptions of bacterial growth and substrate utilisation in completely mixed, activated sludge systems have been presented by workers such as Lawrence and McCarty (1970), Sherrard (1977),
and Metcalf and Eddy (1979). Using the notation of Metcalf and Eddy, the net specific growth rate of microorganisms in such systems is given by
Fl'= Y U - k a
(I)
where IL'= net specific growth rate (time -~) Y = m a x i m u m yield coefficient during logarithmic growth; defined as the mass of cells formed per unit mass of substrate chemical oxygen demand (COD) consumed U = specific substrate utilisation rate (time -~) rsu
X kj = endogenous decay coefficient (time -~) r~u = rate of substrate utilisation (mass vol -t time-~); mass of COD removed per unit volume of reactor per unit time X = c o n c e n t r a t i o n of viable microorganisms (mass vol-L); approximated by the concentration of mixed liquor suspended solids. Using a Monod-type growth relationship, U can be expressed as
kS K,+S
U= - -
(21
where k = maximum rate of substrate utilisation per unit mass of microorganisms (time-') K~= half velocity constant, substrate concentration at one-half the maximum growth rate (mass voL-~) S =substrate concentration surrounding the microorganisms (mass vol-~). For a completely mixed system, S is the substrate concentration in the effluent. 1511
1512
D . A . LO~,ETT c't d./. Table 1. Operating parameters for acti,,ated ~ludge reactors
Sludge age Ida2, I
F M {g C O D g MLSS -~ day -')
C O D of influent ~mgl-'l
Hydraulic residence time (day)
pH
dissolved ox?gen ling i-~l
MLSS irng I - ' )
..: "~ 8 II 13 lS 20
0.93 084 0.46 036 030 025 0 24
2400 2460 2510 [ 220 1780 1640 1280
1.4 [ .4 I6 12 28 27 19
79 7S 77 6.5 6.3 7.0 63
2.0 2.0 1.3 2.0 15 1.5 30
I851) 2100 340~) 2800 2150 2400 285/)
It follows from e q u a t i o n s (1) a n d (2) that vahtes o f Y, k ~. k and K can be calculated by d e t e r m i n i n g values o f / l ' , U a n d S at several different o p e r a t i n g conditions. In line with m a n y o t h e r w o r k e r s in this area (Bisogni a n d L a w r e n c e , 1971: P i t m a n , 1975: S h e r r a r d a n d S h r o e d e r , 1973), we have c h o s e n "'Sludge A g e " (0,) in p r e f e r e n c e to "'loading f a c t o r " or "'food to m i c r o o r g a n i s m mass r a t i o " ( F / M ratio) to define the o p e r a t i o n a l state o f an a c t i v a t e d sludge reactor. F o r a c o m p l e t e l y mixed s y s t e m at s t e a d y state, sludge age equals the ratio o f total b i o m a s s in the s y s t e m to the daily w a s t a g e or p r o d u c t i o n rate o f b i o m a s s i.e. 0 = l i d . In general, low sludge ages are a s s o c i a t e d with rapid rates o f m i c r o b i o l o g i c a l g r o w t h and high rates o f sludge p r o d u c t i o n a n d wastage. High sludge ages are a s s o c i a t e d with slower g r o w t h a n d low rates o f sludge p r o d u c t i o n . APPARATUS. MATERIALS AND PROCEDURES Two activated sludge reactors were constructed from polycarbonate sheet. Each consisted of a stirred, aerated compartment of 9 I.. separated by a vertical baffle from a I 1. settling chamber. Operating conditions were set to allow the reactors to operate at sludge ages of 4, 5, 8. II, 13, 18 and 20 da>s by removing a volume ot" mixed liquor each day sulficient to provide a total daily solids loss equzd to 10, of the total biomass in the reactor. The mixed liquor removed was replaced by water, which was taken into account when calculating hydraulic residence times. Temperature of reactor contents was maintained between 19 and 23 C. A soluble meat concentrate--"Bonox"--manufacturcd by Kraft Foods Lid was used as the substrate because wc wished to simuhtte the treatment of wastewater from abattoir operations. Bonox has a crude protein content of 37",.. fat I",, and total phosphorus 0.4'!,,, dry solids basis. Its COD content is 0.678 C O D g -I dry solid and it has a C O D : N : P ratio of 156: 14:1, compared to 170:9: I for an untreated abattoir effluent, reported by Heddle (1977). Bonox was pumped to the reactors by peristaltic pumps controlled by timers at feed rates corresponding to 4.5-27 g COD day -~. Other pumps fed water to the reactors to maintain the required hydraulic residence times. [n Table I the values of the operatiomd parameters used at the different steady state conditions are shown. Low values of COD removal etficiency were obtained at low sludge ages (68". at 4 days, 73,, at 5 days} rising to higher values at the higher sludge ages (91, 88, 91. 91 and 94", at 8. 11. 13. 18 and 20 days respectively). Similar variation of COD removal efficiency with sludge age have been widely reported by other workers. The reactors were seeded with sludge from an activated sludge plant treating abattoir wastewater. Microscopic examination shinned the sludge initially contained filamentous organisms, which persisted throughout the
tests. On several occasions, one or more types of filamentous organism became domimmt in the mixed liquor. seriously reducing settling rates. The only effective remedy was to discard the affected mixed liquor and start the test again. To determine when stable conditions had been attained at a given sludge age. selected parameters were monitored, usually dail), until constant values were obtained. In the mixed liquor, the concentration of mixed liquor suspended solids (MLSS), sludge capillary suction time and sludge volume index were monitored and. in the effluent, soluble COD. Considerable time was required for reactor conditions to stabilise, up to 3 months in tests in which biomass had to acclimatise to the new substrate. Large variations in sludge and mixed liquor characteristics were observed during the stabilisation period. At various times, the biomass became almost completely dispersed and non-settling for several weeks. A thick foam would sometimes develop on the liquid surface. At other times the liquid viscosity increased, and greatly increased aeration rates were required to maintain adquate dissolved oxygen levels. Similar changes have been reported to occur in full-scale plants (Schwartz et aL, 1980: Dhaliwal, 1979). Sludge Volume Index (SVI) measurements were made in accordance with Standard Method~ (APHA, 1971), except that all samples were diluted with tap water to give MLSS concentrations of 0.3 times original concentration. The resulting samples had solids concentrations of around 1000 m g l - t . Dick and Veselind (t969) have shown that for non-bulking sludges, SVI values are independent of solids concentration lit around 1000mgI-~, and this was contirmed in our experiments. At MLSS concentrations o[" 2000rag[ -~ or more, SVI values varied markedly with solids concentration for samples from the same source (curves I and II of Fig. l) and even for consecuti,,e measurements on the same sample (curves Ill and IV of Fig. 1). Similar values of SVI were obtained for samples
250
~"~, 2 0 0
-~
150
~00
@o • O
Sample from full scale p~ant freahng oboTloir wastes
•
Samole from laborotory scale plant
50
0
l 2000
I 4000
MLSS
I 6000
f 8000
(rag t-')
Fig. 1. Variation of SVI with suspended solids concentrations.
Sludge age and substrate composition on characteristics of activated sludge Table 2. Characteri:itics of polyelectrolytes u.~d in conditionin~
studies
Charge density Pol',mer t,,pe
Charge type
(%)
Non-ionic Non-ionic Non-ionic Cationic Cationic Cationic Anionic Anionic Anionic
Nil Nil Nil 20 25 211 35 30 30
-\cc, lamide
~cr~lamide quaternary amine copol?mer ~,cr.'.lamide act'.kite topoi? mer ND =
not
1513
Mol'm:ular',.,eight x
I0~'
ND
>9 II >2 4 4 ND >9 II
determined
diluted ~ith mixed liquor supernatant, and those diluted with tap water. Dispersed solids were measured by the method of Bisogni and Lawrence (1971). Samples used for SVI tests were allowed to settle for a further 30 rain, and the suspended solids in the supernatant were measured and expressed as a percentage of the total suspended solids in the SVI sample. Dispersed solids values were independent of M LSS concentration. Specilic resistance to filtration and capillary suction time (SRF and CST) of mixed liquor samples were measured according to the methods described by Kavanagh (1980). SRF values are expressed in units of m kg -~ and CST values in seconds. The physico-chemical properties of the polyelectrolytes used in conditioning tests are listed in Table 2. An appropriate volume of an approx. 1000mg I-~ solution of polyelectrolyte was added to 300 ml of mixed liquor, and the mixture was stirred vigorously for 10. Stircing was reduced to low speed for a further 1-2 rain before SRF or CST measurements were made.
RESUI.TS
AND
DISCUSSION
In Fig. 2, SVI values are plotted against sludge age for our experiments with a meat concentrate sub-
strate, and for work using different substrates reported by others. The results of Heddle (1977), from work on abattoir wastewater, are similar to our own. The results of Bisogni and Lawrence (1971), and those of Chao and Keinath (1979) for activated sludge plants operating on glucose-based substrates are similar. The results of Pitman (1975) for domestic sewage substrate show quite different behaviour of SVI with sludge age. The high MLSS concentrations used by Bisogni and Lawrence and Pitman at sludge ages greater than 8 days may have increased their SVI values, but this would not appreciably alter the comparisons in Fig. 2. Ford and Eckenfelder (1967) and Kilt (1978), also report SVI data for domestic sewage substrate, Their results agree qualitatively with Pitman's, showing a steady increase in SVI with increasing loading factor (decreasing sludge age). It appears that there is reasonably good agreement between plots of SVI/sludge age results reported for activated sludge plants treating similar substrates, and very wide differences between results for plants treating different substrates. However, other factors, such as the dominance of filamentous micro-
\
800
3isogni and Lawrence (1971) (glucoie substrale)
~
tic sewage)
600
T
-~ (.~
400
200
/
O k l
•
0
Present
study
I
I
5
I0
(meat
t
concentrate) l 15
O: (days) Fig. 2. Variation of SVI with sludge age for different substrates.
I
20
1514
D.A. LOVETT et 50
-~
4o
/
3o
I Present Study
~ ~o
=
Bisoglni and Lawrence (1971)
c5
I
0
4
8
12 0c (days)
I
16
20
Fig. 3. Variation of dispersed solids with sludge age. organisms in the biomass, could overshadow the effects of either substrate composition or sludge age on SVI data obtained in a particular treatment plant at a given time. In Fig. 3, dispersed solids are shown as a function of sludge age. As sludge age increases-above 8 days, dispersion decreases slowly from about 37~, towards zero. At sludge ages below 8 days, there is a dramatic increase in dispersion to 35'}0 at 4 days. The results of Chao and Keinath (1978) and Bisogni and Lawrence (1971), working with glucose-based substrates are also presented in Fig. 3 and show a similar trend, although the increase in dispersion occurs at lower sludge ages. Similar trends are noted by Lawrence and McCarty (1970) and Pipes (1979), working with industrial wastewaters. The increase in dispersion was accompanied by a decrease in substrate removal efficiency, indicating that the specific growth and substrate removal rates of the microorganisms were at or near their maximum values. In our results, the rapid increase in dispersion occurs at 8 days, implying that maximum specific growth and substrate removal rates are low for meat concentrate substrates, compared to glucose substrates where the rapid increase occurs at a sludge age of less than 1 day. High levels of dispersed solids and low substrate removal efficiences were not observed by Heddle (1977) in abattoir wastewater treatment, even at sludge ages approaching 1 day. The explanation may be that fat and suspended solids which make up approx. 60'~.;;of the COD of abattoir wastewater, have been observed to be efl'ectively removed from activated sludge systems by adsorption onto microbial flocs (Mulligan and Sheridan, 1975: and Lordi and Lue-Hing, 1976). Such a mechanism increases the removal rate of substrate, particularly at low sludge ages where a greater fraction of the biomass is wasted each day. Further. Benedict e t al. (1979) have shown that the presence of svspended solids in the substrate reduces
al.
the microbial growth rate at a given sludge age. and is therefore likely to reduce solids dispersion. Filtration results are presented as values of SRF and CST plotted against sludge age in Fig. 4. The CST data have been adjusted to represent equivalent times for samples of 1"i, suspended solids concentration. Kavanagh (1980) has shown from theoretical considerations that there is a direct relationship between SRF and CST values measured on the same sample, and the similarity in the shapes of the CST vs 0, curves confirms this relationship. Both curves show improved filterability with sludge age, with approx. 200-fold increases as sludge age increases from 4 to 20 days. Also shown in Fig. 4 are SRF data for 2 samples obtained from a plant treating abattoir wastewater (Kavanagh, personal communication) and from a domestic sewage treatment plant (Pitman, 1975). Kavanagh's results agree well with our own. as do those of Pitman at sludge ages greater than 8 days. Pitman demonstrated that filtration resistance is directly related to the concentration of dispersed solids in the sludge, and our SRF results are also consistent with the increase in dispersion at sludge ages below about 8 days. Conditioning tests on samples having sludge ages of 5, 7 and 20 days, showed that non-ionic and anionic polyelectrolytes were ineffective in improving the filterability of our activated sludge, as assessed by CST and SRF measurements. Effectiveness of cationic polyelectrolytes varied considerably, depending on sludge age (Fig. 5). At a sludge age of 5 days, there was no improvement in SRF. whilst at sludge ages of 7 and 20 days, progressively larger reductions in SRF values were obtained. These results are in agreement
7 J5 E
u. OE co 14
13
L2
4 0" 0
1--
o~
(.9 O_ g
3
-
\
2
I
0
I
I
4
8
°lx~_._o J2
o_~ 16
2o
0 c (days}
Fig. 4. Variation of SRF and CST with sludge age.
Studge age and substrate composition on characteristics of activated sludge
8 c - 5 days
&. q5 7
E
14 ~
0c - 7days
C~ 03
relatively constant for substrates as different as glucose and yeast (Bisogni and Lawrence, 1971) and abattoir wastewater (Heddle, 1977). This result is suprising in view of the variety of factors which ma,, influence the value of Y. These factors include the oxidation state of the carbonaceous substrate, the degree of substrate polymerisation and the activity of specific metabolic pathways (Sherrard and Shroeder, 1973). REFERENCES
2 o~
1515
f3
A P H A (1971) Standard Methods /or the Examination o/" Water and Wastewater. A m e r i c a n Public Health Associa t i o n , New York.
Oc - 2 0 days ~2
I 2
I 4
I 6
I 8
I IO
Polyelectrolyte dose (g kg -~) Fig. 5. Variation o f SRF
with dose o f cationic poly-
electrolyte at different sludge ages.
with Pitman's observation that "'the greater the degree of bioflocculation of sludge, the greater the effect of chemical conditioners and the lower the amount of chemicals required to produce good dewatermg". In sludges of age 5 days, polyelectrolyte doses as high as 100 g kg -~ sludge solids proved ineffective in reducing filtration resistance. At this sludge age, about 20% of the solids are dispersed, i.e. only about 80')o are bio-flocculated, so that a poor effectiveness may be expected. However, the absence of any improvement, even at very high polyelectrolyte dose rates, may indicate that other factors, such as changes in the surface characteristics of the flocs, may also play a role in determining conditioning requirements. Values of microbial growth coefficients and substrate utilisation rate coefficients were: Y =0.41 mg MLSS mg -~ C O D ; k~= 0.04 day-~: k = 0.67 day-~: K~= 1 5 0 m g l -~ COD. The valtte of the maximum specific substrate utilisation rate, k, is substantially lower than for most other substrates (Lawrence and McCarty 1970; Nelson and Lawrence, 1980; Metcalf and Eddy, 1979), indicating a low rate of biodegradability. A similar conclusion has already been drawn from the dispersed solids and filtration data presented above. The reasons for the low rate of biodegradability of meat concentrate are not known. The values of the other coefficients, on the other hand, are within the range of values obtained by other workers for synthetic substrates and domestic sewage. The report values of Y, in particular, are
Benedict A. H.. Merrill M. S, and Mauseth G. A. (19791 Sludge production, waste composition, and BOD loading effects for activated sludge systems. J. Wat. Pollut. Control Fed. 51, 2898-2915. Bisogni J. J. and Lawrence A. W. (1971) Relationships between biological solids retention time and settling characteristics of activated sludge. Water Res. 5, 753-763. Chao A. C. and Keinath T. M. (1979) Influence of process loading intensity on sludge clarification and thickening characteristics. Water Res. 13, 1213-I 223. Dhaliwal B. S. (I 979) Nocardia amarae and activated sludge foaming. J. Wat. pollut. Control Fed. 51, 344-350. Dick R. I. and Vesclind P. A. (19691 The sludge volume index--what is it'? J. 142tt. Poll,a. C¢mtrol Fed. 41, 1285-1291. Ford D. L. and Eckenfelder W. W. (1967) Effect of process variables on sludge fioc [brmation and settling characteristics. J. Wat. Pollut. Control Fed. 39, 1850-1859, Heddle J. H. (1977) Activated sludge treatment of meat wastes with protein recovery. Meat Industry Research Institute of New Zealand Technical Report No. 616. Kavanagh B. V. (I 980) The dewatering of activated sludge: measurement of specilic resistance to t]ltration and capillary suction time. |Vat. Pollut. Control 79, 388-398. Kiff R. J. (1978) A study of the factors affecting bioflocculation in the activated sludge process. Ifat. Pollut. Control 77, 464-470. Lawrence A. W. and McCarty P. L. (1970) Unilicd basis for biological treatment design and operation. J. sanit. Em.zng Dir. Am. Sac'. cir. Engrs 96, 757-778. Lordi D. T. and Leu-Hing C. (1976) Fats oils and greases-effluent regulations and municipal treatment plant operations. Proc. 31st htd. tt2tste Con]~ Pur~hw Unit'. Metcalf and Eddy Inc. (I979) Wastewater L)tghwerhtg: Treatment Disposal and Reuse. T M H Edition, Chap. 9.
McGraw-Hill, New York. Mulligan T. J. and Sheridan R. P. (1975)Treatment of high strength fatty acid derivative waste~atcrs. Proc. 30 Ind. Waste Conf Purdue Unit'.
Nelson P. O. and Lawrence A. W. (1980) Microbial ',lability measurements and activated sludge kinetics. Water Rex. 14, 217-225. Pipes W. O. (1979) Bulking. defiocculation and pinpoint floe. J. Wat. Pollut. Control Fed. 51, 62-70. Pitman A. R. (1975) Bioflocculation as a means of improving the dewatering characteristics of activated sludges. Wat. Pollut. Control 74, 688-700. Schwartz H. G.. Popowchak T. and Becket K. (1980) Control of sludge bulking in the bre~ing industr 3. J. l['at. Pollut. Control Fed. 52, 2977-2993. Sherrard J. H. (1977) Kinetics and stoichiometry of completely mixed activated sludge. J. fg'at. Po[[ut. Control Fed. 49, 1968-1975. Sherrard J, H. and Schroeder E. D. (1973) Yield and growth rate in activated sludge. J. Wat. Pollut. Control Fed. 45, 1889-1896.