- Email: [email protected]
Physical and chemical properties of activated sludge floc
War. Res. Vol. 27, No.
12, pp. 1707-1714,1993
0043-1354/93 $6.00+0.00
Printed in Great Britain. All fightsreserved
Copyright© 1993PergamonPress Ltd
PHYSICAL AND CHEMICAL PROPERTIES OF ACTIVATED SLUDGE FLOC ANDREAS D. ANDREADAKIS ~) Water Resources Division, Department of Civil Engineering, National Technical University of Athens, Greece
(First received January 1992; accepted in revisedform April 1993) Abstract--Physical and chemical characteristics of activated sludge such as floc size, density, specific surface, carbohydrate content, dehydrogenase activity and settleabifity were investigated by seven parallel bench scale activated sludge units operated under different sludge ages (1.1-17.4 days). The analytical methods used included a dye adsorption technique for specific surface area determinations, the Coulter Counter method for floc size measurements and interference microscopy for floc density determinations. The typical floc sizes were found to be in the range lO-70/~m with floc densities in the range 1.015-1.034 gem -3. A strong correlation between floc density and size was obtained. The specific surface areas measured (typically 100-200 m2g-t dry sludge) were found to be one to two orders of magnitude higher than the corresponding geometric floc surface areas, indicating a porous floc structure. Sludge settleability, for non-filamentous sludges, was well correlated to both lloc size, density and specific surface area, but not to the sludge carbohydrate content, which was found to vary between 6 and 18%. Key words--activated sludge, floc size, floc density, floc specific surface area, non-filamentous bulking, exocellular polymers
INTRODUCTION
A simple microscopic inspection shows that activated sludge primarily consists of floes which are formed through a process of complex organization of heterogeneous materials such as bacteria, detritus and mucilage of macromolecules. Despite this early recognition, the undisputed key role of the biological action of sludge bacteria placed for many years the emphasis on the study of this particular action and led to the development of several kinetic models based on microbiological concepts. The underlying analogy between activated sludge and pure bacterial cultures in this approach, although simplistic in the case of activated sludge, has led to a very satisfactory description of the organic carbon removal process under quasi-steady state conditions. Under such conditions usually it is the biomass activity which is the rate limiting reaction and the omission of other factors related to the complex nature of the activated sludge, is legitimate. The few aspects of the process, under steady state conditions, which can not be addressed properly, e.g. the accurate determination of the minimum oxygen concentration required in the aeration tank, do not alter the general picture of a satisfactory, particularly for design purposes, approach. The increasing concern for stricter effiuent quality control under transient conditions, created the need for an effective deterministic description of the dynamic performance of the activated sludge process and revealed the limitations of the purely micro-
biological approach. Under transient conditions, phenomena related to the structure of the activated sludge tic,:, such as substrate adsorption and/or intracellular storage, become prominent and require special consideration. Furthermore, during advanced treatment, involving biological nitrogen and phosphorus removal, the fate of the organic carbon during the various phases of the process is important for a better understanding of the treatment schemes involved. But it is in the field of final sedimentation that the knowledge of the floc structure becomes of even greater importance. The settlement of activated sludge is crucial to the operation of the activated sludge system since the overall effectiveness of the process depends, in large measure, on the efficiency of the solids separation step. The bulk of research on sludge settlement has been predominantly based on the quantitative characteristics of the mixed liquor (e.g. flows and solids concentration), and has been mostly empirical. However, settleability is a very variable characteristic, strongly related to the structure and nature of the activated sludge floc which in turn depend on the conditions prevailing in the aeration basin. A number of floc characteristics could be expected to exert some direct or indirect influence on sludge settlement. There are several types of settlement problems which are more or less directly related to the morphological and surface characteristics of the floe. These problems include poor clarification and turbit effiuents, either due to the inability of small clumps of bacteria to flocculate (dispersed growth) or
1707
1708
ANDREAS D. ANDREADAKIS
1. Substrate feed container due to break up of larger floes and formation of small 2. Aeration compartment compact floes which do not settle well (pinpoint floc). 3. Sedimentation tank Furthermore, poor sludge settleabifity, not associated 4. Effluent collector 5. Rotameter (air supply) with excessive amounts of filamentous organisms, 6. Peristaltic pump (feeding and recycle) often referred to as non-filamentous or viscous 7. Sludge wastage bulking (Jenkins, 1992), is most probably related to the morphological characteristics of the floc and the 6 | | presence of large amounts of exocellular slime. In the case of filamentous bulking, often accompanied by filamentous-organism derived foaming (usually due to the presence of Norcadia sp. or Microthrix parvicella), the connection between floc characteristics and the settlement problem is more complex. It has been observed that the presence of filamentous organisms has an effect on the structure of the floc, and according to one theory it is probable that the filamentous organisms' network provides a "backbone" for the buildup of the floc which is subsequently formed with the additional assistance of Fig. I. Schematic representationof one of the parallelbench scale activatedsludge units. various polymer bridges between primary particles and smaller floes (Sergin et al., 1978). On the other hand, it is well established that the extent of filamen- composition shown in Table 1, was prepared daily and tous organisms growth is a reflection of the compara- pumped after appropriate dilution, to the units from several tive physiological properties of filamentous and floc feed containers equipped with low speed rotating paddles forming organisms. However, the conditions that which kept the substrate in suspension. Compressed air was introduced to each aeration tank affect the physiology of these organisms (sludge by means of sintered stone diffusers. The air flow was age, dissolved oxygen level, reactor configuration, controlled with the aid of precalibrated rotameters to ensure nutrient concentration, nature of organic substrate, that a constant level of mixing, resulting in a velocity sulfide concentration etc.) are usually considered at a gradient (G) of 135 s -I in each unit, was maintained. In macroscopic level in the bulk of the mixed liquor and consequence of this requirement, the DO levels in the units were different, but always greater than 2.5 mg 1-t. not in the immediate vicinity of the microorganisms Sludge was recycled from the settlers by means of periinside or at the surface of the floes. The often staring pumps at a recirculation ratio of 1.5. The high reported contradictory results with respect to the recircuiation ratio employed reduced the amount of the effect of such macroscopic environmental conditions sludge at the bottom of the settling tanks and prevented on the growth of filamentous organisms (Tomlinson solids losses with the effluent due to denitrification and high SVI values. and Chambers, 1984) may reflect the variability of these conditions in the vicinity of the bacteria, Analytical methods The units, after an acclimatization and stabilization imposed by variations in the floc structure. A more careful consideration of critical floc characteristics period of 3 weeks, operated under steady state conditions for a period of 4 months. During this period the temperature may therefore lead to a more satisfactory and consist- varied between 12 and 16°C. Regular influent, effluent and ent cause-effect relationship and combined with mixed liquor sampling and analysis for BODs and SS growth kinetic studies, may form the basis of as specified by "Standard Methods" (APHA, 1980), was an effective activated sludge population dynamics performed in order to assess the performance of the system. SVI determinations were made by sludge settlement for theory. 30rain in 1 litre graduated cylinders. Periodically, well mixed samples of activated sludge were removed from each unit and subjected to analysis aiming to determine MATERIALS AND M E T H O D S
i5
Apparatus The experimental system comprised seven parallel bench scale activated sludge units. Five of these units consisted of a 21. aeration tank connected to a 0.31. perspex cylinder for sludge settlement, while in the other two units the volumes of the aeration tanks and settlers were 6.2 and 2.51. respectively. A schematic illustration of a unit is shown in Fig. 1. The units operated in parallel at varying sludge ages (I.1-17.4d) and at a constant hydraulic retention time of 10.5 h. The MLSS concentration was kept practically constant at approx. 2000mgl -t, except in the unit of 1.1 d sludge age, in which the MISS concentration was around 1500 mgl -~. The various sludge ages were achieved by controlled sludge wasting, taking into account the solids lost with the effluent. Synthetic substrate solution, with the
Table I. Composition of synthetic substrate stock solutionfed to the activatedsludge units (BODs = 750mg I-I, SS ffi640mg I-s ) Component Concentration Starch 666.7 Soap 333.3 NH2CONH2 200.0 NaHCO~ 666.7 AI2(SO4)16H20 33.3 I~HPO4 166.5 NaCI 200.0 KCI 46.6 CaCI2 46.6 MgSO47H20 33.3 Nutrient Broth 333.3
Activated sludge floc
1709
Fioc size
120 v e~
=.
80 40 0
I I I I I I 0.02 0.04 0.06 0.08 0.10 0.12 Equil. concentration (retool i -1)
Fig. 2. Typical activated sludge dye adsorption curve.
morphological and chemical characteristics of the floc such as specific surface area, size, density, carbohydrate content and dehydrogenase activity. Spec~ic surface area determination
The specific surface area of the fiocs was determined by a dye adsorption technique. Dyes have been used by many investigators for measuring specific surface of various solids by solution adsorption. The use of dyes is particularly attractive because it is simple and allows for a rapid colorimetric analysis of the solutions. From previous studies (Longmuir, 1975; Andreadakis, 1985), it was found that a naphthalene based dye, known as Lissamine Scarlet 4R, is suitable for dye adsorption measurements on activated sludge because it is stable, its solubility in water is neither too high (competition with solvent) nor too low (micelle formation), it can be efficiently purified and staining of glass tubes is negligible. Before use, the dye was purified by repeated re-crystallization from 60/40 mixtures of ethanol and water as described by Giles and Greczek (1962). The dye adsorption experiments were carried out in 200mi flasks. Each flask contained 100mi of mixture, consisting of 20 ml of activated sludge of known mixed liquor suspended solids concentration, varying combinations of dye stock solution (0.29mmoll -I) and make-up water, totalling 80 ml, and acidified to pH of 2.5 with addition of dilute HCI. The flasks were shaken vigorously for 90 rain in a water bath at a constant temperature of 20°C, to attain equilibrium conditions. Then the contents of the flasks were centrifuged at 3000 rpm for 15rain and the supernatants were collected and diluted where necessary, before spectrophotometric determination of the dye remaining in solution (optimum wavelength at 505 nm). For each activated sludge sample, an adsorption isotherm in the form of a plot of the mmol dye adsorbed kg -I suspended solids versus the mmol dye remaining in solution, was obtained, and the maximum dye adsorption capacity was determined. A typical dye adsorption isotherm is shown in Fig. 2. Dye adsorption capacities of the same sludge sample at various sludge concentrations (1952, 1500, 1300, 970, 640 and 320mgl -m) indicated little variability around a mean value of 131 mmol kg -m (Fig. 3). Once the amount of dye adsorbed as a monolayer is established, the specific surface can be deduced if the area covered by each dye molecule (A) is known. S = Y,N,A
Floc sizes and fioc size distributions in samples harvested from the activated sludge units were determined by the Coulter Counter technique which offers a dynamic method of measuring floc sizes. The Coulter Counter determines the number and size of particles suspended in an electrically conductive liquid. Changes in the resistance between two electrodes immersed in the liquid are caused by particles passing through a small apperture lying between the two electrodes. Resistance changes are monitored as voltage pulses of short duration having a magnitude proportional to particle size, but not related to the shape of the particles. A 200/~m aperture which allows measurement of particles in the range 2-75/~m was used. The range of floc sizes of activated sludge, as reported by various workers, varies between 0.5 and 1000/~m (Knudson et al., 1982), but most of the floes are smaller than 100#m. Data presented by Knocke and Zentkouich (1986) and Barber and Veenstra (1986), show that more than 90% of the floes are smaller than 75/~m, while the corresponding figure obtained from data presented by Sadalgekar et al. (1988) is 87%. Da-Hong Li and Ganczarczyk (1988) reported 80% of the flocs smaller than 100~m, a percentage which is probably higher if the difficulty in detecting flocs <25/Jm with their photographic technique is taken into consideration. FIoc density
Obtaining the bulk density of activated sludge floc is difficult due to the hydration water associated with its surface. The principles of sedimentation (Magara et al., 1976; Da-Hong Li and Ganczarczyk, 1987) and centrifugation (SmyUie, 1969) have been used for floc density determinations, but both methods rely on the assumption of spherical floes. An indirect method involves the determination of the bound water content of the sludge by the dilatometric technique (Heukelekian and Weisberg, 1956; Forster and Lewin, 1972; Barber and Vecnstra, 1986). The method employed in this study makes use of interference microscopy which has been successfully used for the determination of the dry mass of living cells (Barer and Joseph, 1955), as well as activated sludge floes (Coackley and Kiiger, 1964; Andreadakis, 1978). The microscope used was a M41 Vickers interference microscope, fitted with a tungsten Halogen lamp which in conjunction with a precision interference line filter, gives a source of monochromatic transmitted light of 5450 A units wavelength. The x 40 sheafing objective and its corresponding condensing system was used, so that floes up to 160 #m could be examined. A birefringent compensator was incorporated into the microscope enabling phase changes to be read off from a goniometer analyser. Phase change determinations were assisted by use of a half-shade eyepiece. The phase difference (pd) is given by 20 pd = - (2)
360
160 .~ ~
140 0
0
1'3 0
(1)
0
0
where S = specific area (m 2 g- t ) Y = adsorbed dye (tool g- i ) N =Avogadro's number (6.023 × 1023 molecule mol -t) A = area covered by each dye molecule (m2 per molecule).
~, r~
100
I
I
I
I
I
400 800 1200 1600 2000 Solids concentration (mg 1-1)
Fig. 3. Dye adsorption capacity vs MLSS concentration.
ANDRF.ASD. ANDREADAKIS
1710
Table 2. Performanceof the units under steady state conditions Sludge age
lnfluent BODs
(days) (ms I-') I. I 3.1 4.2 5.4 8.8 12.0 17.4
750 580 420 340 240 180 135
Soluble effluent BOD$
(ragI-l)
Effluent SS (mgl -a )
SVI
TTC activity (ragTF MLVSS-l)
76.0 14.6 3.2 3.6 4.4 4.3 5.2
695 116 32 38 51 37 49
32 228 334 249 122 128 241
17.7 10.8 9.3 8.7 7.5 6.0 3.6
where 0 is the angle through which the goniometer analyser is rotated from the position at which matching to the reference area (liquid) is observed to the position at which matching to the floc is observed. The solids concentration for a given floc thickness (d) is: pd c = --
(3)
a*d where a = 0.00185 (Barer and Joseph, 1955). The thickness of the floc was estimated by measuring the optical path difference (pd l-pd2) produced by the same floc in two immersion media (natural suspension medium and a 2% peptone solution) of different refractive indices F 1, F2 which were measured with a sugar refractometer. With C known (in g cm -3) the bulk density (Pr in gcm -3) of the floc can be calculated as follows: (p,- l)*C
tOf----1 -~ -
(4) Ps where p, is the density of dry sludge (g cm-3). The value of p, adopted is the density of protein, 1.34gem -3, which is very close to the typical density of dried activated sludge (Smith and Coackley, 1984). Therefore: pf -~- l + 0 . 2 5 " C
(5)
Carbohydrate content and dehydrogenase activity The carbohydrate content of the sludge samples was determined by the anthrone method (Gaudy, 1962). Dehydrogenase activity was measured by the Triphenyi Tetrazolium Chloride ('I'rC) method (Coackley and O'Neill, 1975). RESULTS AND DISCUSSION
The results from the seven units operated in parallel under steady state conditions, are shown in Table 2. With respect to effluent characteristics it was found that for sludge ages greater than 4.2 days the soluble effluent BODs was less than 5 mg l - ~ and the effluent SS less than 50 mg l-i. Considerable effluent quality deterioration was observed at lower sludge ages, particularly at the sludge age of l . l days where dispersed growth occurred. The physiological condition of the biomass was assessed by the TTC activity method, which gave activities varying from 3.6 to 17.7Fg T F m g MLVSS -I h -l. Weddle and Jenkins (1971) reported a value of TTC activity per viable cell equal to 1.9 × 10-SFg T F cell -~ h -m. Assuming that the average volatile mass per bacterium is 10-12 g it follows that the viable bacteria account for as much as 93%
Sludge Dye ads. carbohydrate capacity (%) (mmolkg-I ) 10.76 8.44 8.56 9.01 10.15 10.98 9.91
342 201 229 150 102 114 188
Median floc size (pro)
Sludge floc density ( g c m -3)
20 -45 -36 36 40
1.034 -1.0150 -1.0204 1.0207 1.0190
of the MLVSS for the dispersed sludge at an age of 1.1 days, and between 19 and 56% for sludge ages in the range of 17.4-3.1 days. These values are higher than the 15-25% viable biomass reported by other studies based on oxygen uptake rate measurements (Kristensen et ai., 1992). Whether the discrepancy can be attributed to the different methods and the inherent assumptions made for each method, or to the possible effect of the wastewater composition (synthetic versus domestic sewage) is not yet clear. The observed reduction of the viable biomass at increased sludge ages is in agreement with the results of other studies (Kristensen et al., 1992; Huang et al., 1985). With respect to sludge settleability very low SVI values were observed during dispersed growth at low sludge ages (1.1 days); increased SVI values were found at sludge ages in the range 3-5.5 days which decreased at higher sludge ages (5.5-12 days), and increased again at even higher sludge ages (17.4 days). These results, in conjunction with a number of contradictory reports in the literature (Ford and Eckenfelder, 1967; Eckenfelder, 1967; Ganczarczyk, 1970; Bisogni and Lawrence, 1971; Rensink, 1974) indicate that the establishment of a valid, for general use, relationship between sludge age and sludge settleability is not possible. This is not surprising in view o f the multitude of factors, in addition to sludge age, that can affect both population dynamics and floc structure in an activated sludge system (wastewater composition, DO concentration, pH, waste feeding regime, presence of unaerated zones, mixing intensity). Microscopic examination of sludge samples from the units showed that filamentous micro-organisms were not noticeable in any of the units. This indicates that the high SVI values observed reflect a "nonfilamentous bulking". In low SVI sludge samples, under microscopic observation, the floc appeared to be fairly dense and more regular-edged, while more irregular-shaped floes characterized the high SVI sludge samples. Application of a polysaccharide staining technique revealed accumulation of abundant polysaccharide material around the floes and dispersed bacteria in the unit operating under a sludge age of 1.1 days. In the other units, the appearance of the floc was different as bacteria seemed to be enmeshed in a network of polysaccharide fibrils.
1711
Activated sludge floc A detailed presentation and discussion of the results with respect to specific critical floe characteristic and their relation to settleability is the subject of the following sections.
Specific surface area of flocs A typical dye adsorption curve for activated sludge is shown in Fig. 2. The curve implies that there is high affinity between the dye molecules and the surface of the floe. Up to a certain limit, all the dye (typically between 40 and 60 mmol dye k g - ' solids) is chemically bound to the floc surface leaving none in solution. A second layer on top of the chemisorbed dye is being formed due to physical adsorption of the dye molecules, up to a point where a monolayer is established (plateau of the isotherm). The determination of the surface area of the floes from equation (1) is based on the amount of the absorbed dye (Y) and the area covered by each dye molecule (A). For the sludge sample corresponding to the isotherm shown in Fig. 2, the total amount of the adsorbed dye is 160 mmol kg -t, and the amount remaining, after deduction of the chemisorbed 50mmolkg -1, is l l 0 m m o l g - L Depending on the orientation of the adsorbed dye molecules, flat or perpendicular, the area occupied by each molecule is, in the case of the dye used, 196 or 90 A 2 respectively (Longmuir, 1975). Therefore, depending on the assumptions with respect to the amount of the adsorbed dye (total or physically adsorbed) and the orientation of the dye molecules, the specific area given by equation (1) varies between 60 and 189 m 2 g-t. These figures are much higher than the geometric area of the surface of activated sludge floes measured by Finstein and Heukelekian (1967), assuming nonporous floes. They found values in the range 20-100cm 2 of surfacecm -3 of mixed liquor. The MLSS values of their samples ranged from 1300 to 2320mgl -~, therefore the specific surface would be 0.862-7.7m2g -1 dry sludge. These findings are comparable to the value of the specific surface area of a non-porous graphite sample (2 m 2 g- ~) reported by Giles et aL (1970). The hypothesis of the porous nature of the activated sludge floe is also supported by consideration of the specific area measured in relation to the floe size. For a typical solids content c = 0.08 g c m -3 (corresponding to a sludge floe density af= 1.020gcm -3, Table 2) the diameter of a sphencai non-porous floe with a surface area of 60--189 m 2 g-~ is 0.50-1.25 #m, which is much smaller than the typical range of floe sizes found in this study (10-70/am) and reported in the literature. On the other hand, for non-porous floes 10-70/zm in size, the corresponding area is between 1.1 and 7.5 m 2 g-J, which is close to the geometric areas reported by Finstein and Heukelekian (1967), but much smaller than the areas determined by the dye adsorption technique. The mean value of the total dye adsorbed by sludge samples harvested from the vari-
800 --
"~ 600
O0
--
i"~ 400
"~ 200
0
~
5..~ o
100
r = 0.86
I
I
I
200
300
400
Dye absorbed (retool kg -t) Fig. 4. Correlation between SVI and activated sludge dye adsorption capacity. ous units, shown in Table 1, vary between 102 and 220 mmol kg -] with the exception of the dispersed sludge which is characterized by the higher value of 342 mmol kg- 1. These mean values are related to the mean SVI values with higher dye adsorption capacifies observed at higher SVI values. This is more clearly demonstrated in Fig. 4 which shows a good correlation between SVI values and corresponding dye adsorption capacities for several sludge samples harvested from the activated sludge units.
Floc size and density Figure 5 shows the floe size distribution obtained by the Coulter Counter technique for samples harvested from the activated sludge units operated at 1.1, 4.2, 8.8, 12 and 17.4 days. With the exception of the unit at 1.1 days, more than 85% of the floes were found in the range 10-70tim, with median values between 35 and 45/zm. The floes from the unit at 1.1 days were significantly smaller with a median value of around 20/am; however again around 80% of the floes had sizes in the range 10-70/am. A strong correlation between floe density (Pr) and size (d) was observed as shown in Fig. 6, and a relationship between density and size was obtained in the form. pf
1 + 0.30d -°'s2
=
(6)
This relationship applies to all sludge samples irrespective of sludge age with the exception of the
100 8O
40 20
o
o t3 - • •
SRT SRT SRT SRT
= = = =
4.2 da 17.4 days s~ax, 8 . 8 a n d 12 d a 1.1 d a y s
I 10
•
I I I 20 30 40 50 6070 Fioc size, d (~tm) Fig. 5. Floc size distribution at different sludge ages.
Ah'D~gS D. ANDREADAKIS
1712 1110 ~oo t~O
811 -6
•~
60
~ L "-;~
35 p f - l +-
-~e
0.3d-0.S2
O~
o SRT ffi 4.2,8.8,12,17.4 days • S R T = 1.1 days
~o 4o
~
25
'~
20
•
•
too.
15 --
20
lo 10
20
30
40
50
60
I
I
20
711
I
1
311 411 Floe size, d (tim)
511
Floc size, d (ttm) Fig. 6. Relationship between flec density and flcc size.
Fig. 7. Relationship between mean fl~ and density and median floc size.
dispersed sludge where for a given floc size (especially for floes smaller than 30#m) the corresponding density is smaller when compared to the density of a floc of the same size from the other units (Fig. 6). This is probably due to the excess polysaccharide material (mostly adsorbed organic material) surrounding the flocs of dispersed bacteria as indicated by the microscopic polysaccharide staining technique. Given that the porosity (~) of the floc is:
1.1-17.4 days the average carbohydrate content was between 8.5 and 11%. Accumulation of carbohydrate material was observed at very low sludge ages (1.1 days) and in the range 8-14 days. The high sludge carbohydrate content at high sludge ages (8-14 days) reflects the accumulation of extracellular polymeric material. At sludge age of 1.1 days the high carbohydrate content was probably caused by accumulation of stored material which resulted in sludge dispersion. To test this hypothesis samples from the two units operating at sludge ages of 1.1 and 12 days respectively were harvested and aerated without feeding for 6 h. At the end of the aeration period the carbohydrate content of the sludges was measured again. It was found that the sample taken from the unit operating at 12 days sludge age had practically the same carbohydrate content as initially (10.7-11.2%); the carbohydrate content of the sludge sample harvested from the unit operating at a sludge age of 1.1 days was reduced at the end of the 6 h aeration period from 10.8 to 9.5%, due to the assimilation of the stored material. A change in the appearance of the sludge was also noticed in the latter case as the colour of the sludge turned from pale yellow (almost white) to darker yellow. The decline in the sludge carbohydrate content observed at a sludge age 17.4 days is striking in view of the expected enhanced exocellular polymer production under starvation or semi-starvation conditions. An explanation can be given by considering
Ps -- Pf
ffi ~
p~ -- p ,
(7)
for p, ffi 1.34 g c m -3, Pw ----1 g c m -3 (density of water) and pf from equation (6), it follows that -- 1 -- 0.88d
-0"s2
(8)
For d in the range 10--70# m the porosities ~ vary from 85 to 97%. These values follow the same trend (with respect to floc size) but are generally higher than the values reported for similar floc sizes by Hong et al. (1987) obtained by settling velocity and photographic techniques. The weighted average floc densities (pf) of the sludge samples from each unit, calculated from: Pf= ~, PiPri
(9)
i-I
where Pt ffi average floc density of sample Pt ffi percentage of particles of size i Pfl ffi density of particles of size i n ffi number of floc sizes examined
250
are shown in Table 1. These average values are correlated to the median floc size (Fig. 7) and the mean SVI values (Fig. 8). A correlation between SVI and median floc size was also obtained (Fig. 9). These correlations of SVI versus floc size and density are in agreement with similar results reported in the literature for non filamentous sludges (Zergin, 1982; Barber and Veenstra, 1986).
Sludge carbohydrate content Sludge carbohydrate content data, presented in Table 2, show that over the sludge age range of
-
200
>
•
150 100 5O
15
20
25
30
I
I
I
35
40
45
(pf-1)xl000
Fig. 8. Relationship between mean SVI and mean floc density.
Activated sludge floc 250 -
.
>
200 -
e / / , ~
150 -
/
r~ 100 -50
--
/
3.
~
.S, 20
I
I
30 40 Floc size, d (Ixm)
50
4.
Fig. 9. Relationship between mean SVI and median floc size.
the ability of the bacteria to acclimatize and produce enzymes which can hydrolyse and assimilate the exocellular polymers surrounding and bonding their surface under conditions of depletion of external substrate (Postgate and Hunter, 1962; Pavoni et al., 1972; Gaudy et al., 1971; Yang and Gaudy, 1974; Yang and Chen, 1977). SVI was not correlated to the sludge carbohydrate content (Fig. 10). Forster (1971) had established a good correlation for samples from a pilot scale treatment plant, but with 30% of the samples having carbohydrate concentrations over 37%. Such high concentrations would be considered atypical in a municipal facility and would probably be experienced in systems treating certain industrial wastes, generally with high C/N and C/P ratios. On the other hand Barber and Veenstra (1986) did not observe any correlation for typical activated sludges with similar to our study carbohydrate contents (in the range 5-16%). It seems therefore that it is not the amount of the exocellular polymers which is a major factor in sludge bulking, but more likely the properties of these polymers and the effect they have on sludge floc morphology. CONCLUSIONS
The results of the study can be summarized as follows: 1. For well mixed activated sludge systems the typical range of floc sizes was found to be between 10 and 70/~m. 2. A strong correlation between floc density and size was observed with density dropping as size 500 400
o o o
300
o
200
o 0
Oo °o° o
100
o
oo
o
~
o
8
o
I
I
I
t
5
10
15
20
Sludge carbohydrate content (%) Fig. 10. SVI vs sludge carbohydrate content. WR
27112--B
5.
6.
1713
increases. An average density of 1.020 g cm -3 was found to be typical for moderately and low loaded activated sludge systems. The specific surface area of the floes as determined by a dye adsorption technique was high (typically in the range 100-200 m 2 g-~ dry sludge) and one to two orders of magnitude higher than the geometric surface area of the floes. Considerations of the specific surface area and density in relation to floc size indicate a porous floc structure, with porosities in the range 85-97%. Non-filamentous sludge settleability, expressed in terms of SVI, was related to floc size and density. A strong correlation between SVI and floc specific surface area was also observed. Exocellular polymers in activated sludge originate either from substrate adsorption and storage (low sludge ages) or as a product of biomass decay (long sludge ages). The carbohydrate content of the activated sludge samples varied between 6 and 18% and was not correlated to sludge settleability. In view of these results it is probable that it is the properties rather than the quantity of the accumulated exocellular polymers that affect sludge settleability. REFERENCES
Andreadakis A. D. (1978) Physicochemical aspects of activated sludge in relation to purification and settle,ability. PhD thesis, Univ. of Strathclyde, Glasgow. Andreadakis A. D. (1985) Surface characteristics of biological solids. In Appropriate Waste Management for Developing Countries (Edited by Curl K.), pp. 139-151. Plenum Press, New York. APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th edition. American Public Health Association, Washington, D.C. Barber J. B. and Veenstra K. N. (1986) Evaluation of biological sludge properties influencing volume reduction. J. Wat. Pollut. Control Fed. 58, 149-156. Barer R. and Joseph S. (1955) Refractometry of living cells. Q. J! microsc. Sci. 96, 423. Bisogni J. J. and Lawrence A. W. (1971) Relationship between biological solids retention time and settling characteristics of activated sludge. War. Res. 5, 753-769. Coackley P. and Kliger B. (1964) Some interferometric observations of the water]solids relation of activated sludge floes using a compression technique. Nature 203, 77-78. Coacldey P. and O'Neill J. (1975) Sludge activity and full-scale plant control. War. Pollut. Control 74, 404-414. Eckenfelder W. W. J. Jr (1967) Comparative biological waste treatment design. J. sanit. Eagng Div. ASCE 93, 157-170. Finstein M. S. and Heukelekian H. (1967) Gross dimensions of activated sludge floes with reference to bulking. J. War. Pollut. Control Fed. 39, 33-40. Ford D. L. and Eckenfelder W. W. J. Jr (1967) Effect of process variables on sludge formation and settling characteristics. J. Feat. Pollut. Control Fed. 39, 1850-1859. Forster C. F. (1971) Activated sludge surfaces in relation to the sludge volume index. War. Res. 5, 861-869.
1714
ANDRF~ D. ANDI~.ADAKIS
Forster C. F. and Lewin D. C. (1972) Polymer interactions at activated sludge surfaces. Eft. War. Treat. J. 12, 520-525. Ganc.zarczyk J. J. (1970) Variation in the activated sludge volume index. War. Res. 4, 69-78. Gaudy A. F. Jr (1962) Colorimetric determination of protein and carbohydrate. Ind. War. Wastes 7, 17-22. Gaudy A. F. Jr, Yan8 P. Y. and Obayashi A. W. (1971) Studies on the total oxidation of activated sludge with and without hydrolytic pretreatment. J. Wat. Pollut. Control Fed. 43, 40-54. Giles C. H. and Grecrek J. J. (1962) Review of methods of purifying and analysing water soluble dyes. Test Res. J. 32, 506-515. Giles C. H., D'Silva and Trivedi A. S. (1970) Use of p.nitrophenol for specific surface measurement of granular solids and fibres. J. Appl. Chem. 20, 37-41. Heukelekian H. and Weisberg E. (1956) Bound water and activated sludge bulking. J. Wat. Pollut. Control Fed. 28, 558-567. Huang J. Y., Cheng M. D. and Nueller J. T. (1985) Oxygen uptake rates for determining microbial activity and application. Wat. ICes. 19, 373-381. Jenkins D. (1992) Towards a comprehensive model of activated sludge bulking and foaming. War. Sci. Technol. 25, 215-230. Knocke W. R. and Zentkovich T. L. (1986) Effects of mean cell residence time and particle size distribution on activated sludge vacuum dewatering characteristics. J. Wat. Pollut. Control. Fed. 58, 1118-1123. Knudson M. K., Wiiliamson K. J. and Nelson P. O. (1982) Influence of dissolved oxygen on substrate utilisation kinetics of activated sludge. J. War. Pollut. Control Fed. 54, 52-60. Kristensen G. H., Jorgensen P. E. and Henze M. (1992). Characterization of functional biomass groups and substrate in activated sludge and wastewater by AUR, NUR and OUR. ;Vat. Sci. Technol. ~ 43-57. Li D. H. and Ganczarczyk J. J. (1987) Stroboscopic determination of settling velocity, size and porocity of activated sludge flocs. Wat. Res. 21, 257-262. Li D. H. and Ganczarczyk J. J. (1988) Flow through activated sludge llocs. Wat. Res. 22, 789-792.
Longmuir G. (1975) Studies on the nature of activated sludge. PhD thesis, Univ. of Strathclyde, Glasgow. Magara Y., Nambu S. and Utozawa K. (1976) Biochemical and physical properties of an activated sludge on settling characteristics. Wat. ICes. 10, 71-77. Pavoni J. L., Tenney M. W. and Echelberger W. F. J. (1972) Bacterial exocelinlar polymers and biological flocculation. J. Wat. Pollut. Control Fed. 44, 414-431. Postgate J. R. and Hunter J. R. (1962) The survival of starved bacteria. J. gen. Microbiol 29, 233-263. Rensink J. H. (1974) New approach to preventing bulking sludge. J. War. PoUut. Control Fed. 46, 1888-1894. Sadalgekar V. V., Mahajan B. A. and Shaligram A. M. (1988) Evaluation of sludge settleability by floc characteristics. J. War. Pollut. Control Fed. 60, 1862-1863.
Sergin M. (1982) Variation of sludge volume index with activated sludge characteristics. War. Res. 16, 83-88. Sergin M., Jenkins D. and Parker D. S. (1978) A unified theory of filamentous activated sludge bulking. J. War. Pollut. Control Fed. $0, 362-380. Smith P. G. (1984) A model of localised oxygen sinks around bacterial colonies within activated sludge. War. Res. 18, 1045-1051. Smith P. G. and Coackiey P. (1984) Diffnsivity, tortuosity and pore structure in activated sludge. Wat. Res. 18, 117-122. Smyllie R. T. (1969) Cake filtration. Phi) thesis, Univ. of Strathclyde, Glasgow. Tomlinson G. J. and Chambers B. (1984) Control strategies for bulking sludge. Wat. Sci. Technol. 16, 15-34. Water Pollution Research Laboratory (1965) Physical Properties o f Sludge. H.M.S.O. Weddle C. L. and Jenkins D. (1971) Viability and activity of activated sludge. Wat. Res. 5, 621--648. Yang P. Y. and Gaudy A. F. Jr (1974) Nitrogen metabolism in extended aeration process operated with and without hydrolytic pretreatment of portions of the sludge. Biotechnol. Bioengng 16, 1-20. Yang P. Y. and Chen Y. L. (1977) Operational characteristics and biological kinetic constants of extended aeration process. J. Wat. Poilut. Control Fed. 49, 678-688.
12, pp. 1707-1714,1993
0043-1354/93 $6.00+0.00
Printed in Great Britain. All fightsreserved
Copyright© 1993PergamonPress Ltd
PHYSICAL AND CHEMICAL PROPERTIES OF ACTIVATED SLUDGE FLOC ANDREAS D. ANDREADAKIS ~) Water Resources Division, Department of Civil Engineering, National Technical University of Athens, Greece
(First received January 1992; accepted in revisedform April 1993) Abstract--Physical and chemical characteristics of activated sludge such as floc size, density, specific surface, carbohydrate content, dehydrogenase activity and settleabifity were investigated by seven parallel bench scale activated sludge units operated under different sludge ages (1.1-17.4 days). The analytical methods used included a dye adsorption technique for specific surface area determinations, the Coulter Counter method for floc size measurements and interference microscopy for floc density determinations. The typical floc sizes were found to be in the range lO-70/~m with floc densities in the range 1.015-1.034 gem -3. A strong correlation between floc density and size was obtained. The specific surface areas measured (typically 100-200 m2g-t dry sludge) were found to be one to two orders of magnitude higher than the corresponding geometric floc surface areas, indicating a porous floc structure. Sludge settleability, for non-filamentous sludges, was well correlated to both lloc size, density and specific surface area, but not to the sludge carbohydrate content, which was found to vary between 6 and 18%. Key words--activated sludge, floc size, floc density, floc specific surface area, non-filamentous bulking, exocellular polymers
INTRODUCTION
A simple microscopic inspection shows that activated sludge primarily consists of floes which are formed through a process of complex organization of heterogeneous materials such as bacteria, detritus and mucilage of macromolecules. Despite this early recognition, the undisputed key role of the biological action of sludge bacteria placed for many years the emphasis on the study of this particular action and led to the development of several kinetic models based on microbiological concepts. The underlying analogy between activated sludge and pure bacterial cultures in this approach, although simplistic in the case of activated sludge, has led to a very satisfactory description of the organic carbon removal process under quasi-steady state conditions. Under such conditions usually it is the biomass activity which is the rate limiting reaction and the omission of other factors related to the complex nature of the activated sludge, is legitimate. The few aspects of the process, under steady state conditions, which can not be addressed properly, e.g. the accurate determination of the minimum oxygen concentration required in the aeration tank, do not alter the general picture of a satisfactory, particularly for design purposes, approach. The increasing concern for stricter effiuent quality control under transient conditions, created the need for an effective deterministic description of the dynamic performance of the activated sludge process and revealed the limitations of the purely micro-
biological approach. Under transient conditions, phenomena related to the structure of the activated sludge tic,:, such as substrate adsorption and/or intracellular storage, become prominent and require special consideration. Furthermore, during advanced treatment, involving biological nitrogen and phosphorus removal, the fate of the organic carbon during the various phases of the process is important for a better understanding of the treatment schemes involved. But it is in the field of final sedimentation that the knowledge of the floc structure becomes of even greater importance. The settlement of activated sludge is crucial to the operation of the activated sludge system since the overall effectiveness of the process depends, in large measure, on the efficiency of the solids separation step. The bulk of research on sludge settlement has been predominantly based on the quantitative characteristics of the mixed liquor (e.g. flows and solids concentration), and has been mostly empirical. However, settleability is a very variable characteristic, strongly related to the structure and nature of the activated sludge floc which in turn depend on the conditions prevailing in the aeration basin. A number of floc characteristics could be expected to exert some direct or indirect influence on sludge settlement. There are several types of settlement problems which are more or less directly related to the morphological and surface characteristics of the floe. These problems include poor clarification and turbit effiuents, either due to the inability of small clumps of bacteria to flocculate (dispersed growth) or
1707
1708
ANDREAS D. ANDREADAKIS
1. Substrate feed container due to break up of larger floes and formation of small 2. Aeration compartment compact floes which do not settle well (pinpoint floc). 3. Sedimentation tank Furthermore, poor sludge settleabifity, not associated 4. Effluent collector 5. Rotameter (air supply) with excessive amounts of filamentous organisms, 6. Peristaltic pump (feeding and recycle) often referred to as non-filamentous or viscous 7. Sludge wastage bulking (Jenkins, 1992), is most probably related to the morphological characteristics of the floc and the 6 | | presence of large amounts of exocellular slime. In the case of filamentous bulking, often accompanied by filamentous-organism derived foaming (usually due to the presence of Norcadia sp. or Microthrix parvicella), the connection between floc characteristics and the settlement problem is more complex. It has been observed that the presence of filamentous organisms has an effect on the structure of the floc, and according to one theory it is probable that the filamentous organisms' network provides a "backbone" for the buildup of the floc which is subsequently formed with the additional assistance of Fig. I. Schematic representationof one of the parallelbench scale activatedsludge units. various polymer bridges between primary particles and smaller floes (Sergin et al., 1978). On the other hand, it is well established that the extent of filamen- composition shown in Table 1, was prepared daily and tous organisms growth is a reflection of the compara- pumped after appropriate dilution, to the units from several tive physiological properties of filamentous and floc feed containers equipped with low speed rotating paddles forming organisms. However, the conditions that which kept the substrate in suspension. Compressed air was introduced to each aeration tank affect the physiology of these organisms (sludge by means of sintered stone diffusers. The air flow was age, dissolved oxygen level, reactor configuration, controlled with the aid of precalibrated rotameters to ensure nutrient concentration, nature of organic substrate, that a constant level of mixing, resulting in a velocity sulfide concentration etc.) are usually considered at a gradient (G) of 135 s -I in each unit, was maintained. In macroscopic level in the bulk of the mixed liquor and consequence of this requirement, the DO levels in the units were different, but always greater than 2.5 mg 1-t. not in the immediate vicinity of the microorganisms Sludge was recycled from the settlers by means of periinside or at the surface of the floes. The often staring pumps at a recirculation ratio of 1.5. The high reported contradictory results with respect to the recircuiation ratio employed reduced the amount of the effect of such macroscopic environmental conditions sludge at the bottom of the settling tanks and prevented on the growth of filamentous organisms (Tomlinson solids losses with the effluent due to denitrification and high SVI values. and Chambers, 1984) may reflect the variability of these conditions in the vicinity of the bacteria, Analytical methods The units, after an acclimatization and stabilization imposed by variations in the floc structure. A more careful consideration of critical floc characteristics period of 3 weeks, operated under steady state conditions for a period of 4 months. During this period the temperature may therefore lead to a more satisfactory and consist- varied between 12 and 16°C. Regular influent, effluent and ent cause-effect relationship and combined with mixed liquor sampling and analysis for BODs and SS growth kinetic studies, may form the basis of as specified by "Standard Methods" (APHA, 1980), was an effective activated sludge population dynamics performed in order to assess the performance of the system. SVI determinations were made by sludge settlement for theory. 30rain in 1 litre graduated cylinders. Periodically, well mixed samples of activated sludge were removed from each unit and subjected to analysis aiming to determine MATERIALS AND M E T H O D S
i5
Apparatus The experimental system comprised seven parallel bench scale activated sludge units. Five of these units consisted of a 21. aeration tank connected to a 0.31. perspex cylinder for sludge settlement, while in the other two units the volumes of the aeration tanks and settlers were 6.2 and 2.51. respectively. A schematic illustration of a unit is shown in Fig. 1. The units operated in parallel at varying sludge ages (I.1-17.4d) and at a constant hydraulic retention time of 10.5 h. The MLSS concentration was kept practically constant at approx. 2000mgl -t, except in the unit of 1.1 d sludge age, in which the MISS concentration was around 1500 mgl -~. The various sludge ages were achieved by controlled sludge wasting, taking into account the solids lost with the effluent. Synthetic substrate solution, with the
Table I. Composition of synthetic substrate stock solutionfed to the activatedsludge units (BODs = 750mg I-I, SS ffi640mg I-s ) Component Concentration Starch 666.7 Soap 333.3 NH2CONH2 200.0 NaHCO~ 666.7 AI2(SO4)16H20 33.3 I~HPO4 166.5 NaCI 200.0 KCI 46.6 CaCI2 46.6 MgSO47H20 33.3 Nutrient Broth 333.3
Activated sludge floc
1709
Fioc size
120 v e~
=.
80 40 0
I I I I I I 0.02 0.04 0.06 0.08 0.10 0.12 Equil. concentration (retool i -1)
Fig. 2. Typical activated sludge dye adsorption curve.
morphological and chemical characteristics of the floc such as specific surface area, size, density, carbohydrate content and dehydrogenase activity. Spec~ic surface area determination
The specific surface area of the fiocs was determined by a dye adsorption technique. Dyes have been used by many investigators for measuring specific surface of various solids by solution adsorption. The use of dyes is particularly attractive because it is simple and allows for a rapid colorimetric analysis of the solutions. From previous studies (Longmuir, 1975; Andreadakis, 1985), it was found that a naphthalene based dye, known as Lissamine Scarlet 4R, is suitable for dye adsorption measurements on activated sludge because it is stable, its solubility in water is neither too high (competition with solvent) nor too low (micelle formation), it can be efficiently purified and staining of glass tubes is negligible. Before use, the dye was purified by repeated re-crystallization from 60/40 mixtures of ethanol and water as described by Giles and Greczek (1962). The dye adsorption experiments were carried out in 200mi flasks. Each flask contained 100mi of mixture, consisting of 20 ml of activated sludge of known mixed liquor suspended solids concentration, varying combinations of dye stock solution (0.29mmoll -I) and make-up water, totalling 80 ml, and acidified to pH of 2.5 with addition of dilute HCI. The flasks were shaken vigorously for 90 rain in a water bath at a constant temperature of 20°C, to attain equilibrium conditions. Then the contents of the flasks were centrifuged at 3000 rpm for 15rain and the supernatants were collected and diluted where necessary, before spectrophotometric determination of the dye remaining in solution (optimum wavelength at 505 nm). For each activated sludge sample, an adsorption isotherm in the form of a plot of the mmol dye adsorbed kg -I suspended solids versus the mmol dye remaining in solution, was obtained, and the maximum dye adsorption capacity was determined. A typical dye adsorption isotherm is shown in Fig. 2. Dye adsorption capacities of the same sludge sample at various sludge concentrations (1952, 1500, 1300, 970, 640 and 320mgl -m) indicated little variability around a mean value of 131 mmol kg -m (Fig. 3). Once the amount of dye adsorbed as a monolayer is established, the specific surface can be deduced if the area covered by each dye molecule (A) is known. S = Y,N,A
Floc sizes and fioc size distributions in samples harvested from the activated sludge units were determined by the Coulter Counter technique which offers a dynamic method of measuring floc sizes. The Coulter Counter determines the number and size of particles suspended in an electrically conductive liquid. Changes in the resistance between two electrodes immersed in the liquid are caused by particles passing through a small apperture lying between the two electrodes. Resistance changes are monitored as voltage pulses of short duration having a magnitude proportional to particle size, but not related to the shape of the particles. A 200/~m aperture which allows measurement of particles in the range 2-75/~m was used. The range of floc sizes of activated sludge, as reported by various workers, varies between 0.5 and 1000/~m (Knudson et al., 1982), but most of the floes are smaller than 100#m. Data presented by Knocke and Zentkouich (1986) and Barber and Veenstra (1986), show that more than 90% of the floes are smaller than 75/~m, while the corresponding figure obtained from data presented by Sadalgekar et al. (1988) is 87%. Da-Hong Li and Ganczarczyk (1988) reported 80% of the flocs smaller than 100~m, a percentage which is probably higher if the difficulty in detecting flocs <25/Jm with their photographic technique is taken into consideration. FIoc density
Obtaining the bulk density of activated sludge floc is difficult due to the hydration water associated with its surface. The principles of sedimentation (Magara et al., 1976; Da-Hong Li and Ganczarczyk, 1987) and centrifugation (SmyUie, 1969) have been used for floc density determinations, but both methods rely on the assumption of spherical floes. An indirect method involves the determination of the bound water content of the sludge by the dilatometric technique (Heukelekian and Weisberg, 1956; Forster and Lewin, 1972; Barber and Vecnstra, 1986). The method employed in this study makes use of interference microscopy which has been successfully used for the determination of the dry mass of living cells (Barer and Joseph, 1955), as well as activated sludge floes (Coackley and Kiiger, 1964; Andreadakis, 1978). The microscope used was a M41 Vickers interference microscope, fitted with a tungsten Halogen lamp which in conjunction with a precision interference line filter, gives a source of monochromatic transmitted light of 5450 A units wavelength. The x 40 sheafing objective and its corresponding condensing system was used, so that floes up to 160 #m could be examined. A birefringent compensator was incorporated into the microscope enabling phase changes to be read off from a goniometer analyser. Phase change determinations were assisted by use of a half-shade eyepiece. The phase difference (pd) is given by 20 pd = - (2)
360
160 .~ ~
140 0
0
1'3 0
(1)
0
0
where S = specific area (m 2 g- t ) Y = adsorbed dye (tool g- i ) N =Avogadro's number (6.023 × 1023 molecule mol -t) A = area covered by each dye molecule (m2 per molecule).
~, r~
100
I
I
I
I
I
400 800 1200 1600 2000 Solids concentration (mg 1-1)
Fig. 3. Dye adsorption capacity vs MLSS concentration.
ANDRF.ASD. ANDREADAKIS
1710
Table 2. Performanceof the units under steady state conditions Sludge age
lnfluent BODs
(days) (ms I-') I. I 3.1 4.2 5.4 8.8 12.0 17.4
750 580 420 340 240 180 135
Soluble effluent BOD$
(ragI-l)
Effluent SS (mgl -a )
SVI
TTC activity (ragTF MLVSS-l)
76.0 14.6 3.2 3.6 4.4 4.3 5.2
695 116 32 38 51 37 49
32 228 334 249 122 128 241
17.7 10.8 9.3 8.7 7.5 6.0 3.6
where 0 is the angle through which the goniometer analyser is rotated from the position at which matching to the reference area (liquid) is observed to the position at which matching to the floc is observed. The solids concentration for a given floc thickness (d) is: pd c = --
(3)
a*d where a = 0.00185 (Barer and Joseph, 1955). The thickness of the floc was estimated by measuring the optical path difference (pd l-pd2) produced by the same floc in two immersion media (natural suspension medium and a 2% peptone solution) of different refractive indices F 1, F2 which were measured with a sugar refractometer. With C known (in g cm -3) the bulk density (Pr in gcm -3) of the floc can be calculated as follows: (p,- l)*C
tOf----1 -~ -
(4) Ps where p, is the density of dry sludge (g cm-3). The value of p, adopted is the density of protein, 1.34gem -3, which is very close to the typical density of dried activated sludge (Smith and Coackley, 1984). Therefore: pf -~- l + 0 . 2 5 " C
(5)
Carbohydrate content and dehydrogenase activity The carbohydrate content of the sludge samples was determined by the anthrone method (Gaudy, 1962). Dehydrogenase activity was measured by the Triphenyi Tetrazolium Chloride ('I'rC) method (Coackley and O'Neill, 1975). RESULTS AND DISCUSSION
The results from the seven units operated in parallel under steady state conditions, are shown in Table 2. With respect to effluent characteristics it was found that for sludge ages greater than 4.2 days the soluble effluent BODs was less than 5 mg l - ~ and the effluent SS less than 50 mg l-i. Considerable effluent quality deterioration was observed at lower sludge ages, particularly at the sludge age of l . l days where dispersed growth occurred. The physiological condition of the biomass was assessed by the TTC activity method, which gave activities varying from 3.6 to 17.7Fg T F m g MLVSS -I h -l. Weddle and Jenkins (1971) reported a value of TTC activity per viable cell equal to 1.9 × 10-SFg T F cell -~ h -m. Assuming that the average volatile mass per bacterium is 10-12 g it follows that the viable bacteria account for as much as 93%
Sludge Dye ads. carbohydrate capacity (%) (mmolkg-I ) 10.76 8.44 8.56 9.01 10.15 10.98 9.91
342 201 229 150 102 114 188
Median floc size (pro)
Sludge floc density ( g c m -3)
20 -45 -36 36 40
1.034 -1.0150 -1.0204 1.0207 1.0190
of the MLVSS for the dispersed sludge at an age of 1.1 days, and between 19 and 56% for sludge ages in the range of 17.4-3.1 days. These values are higher than the 15-25% viable biomass reported by other studies based on oxygen uptake rate measurements (Kristensen et ai., 1992). Whether the discrepancy can be attributed to the different methods and the inherent assumptions made for each method, or to the possible effect of the wastewater composition (synthetic versus domestic sewage) is not yet clear. The observed reduction of the viable biomass at increased sludge ages is in agreement with the results of other studies (Kristensen et al., 1992; Huang et al., 1985). With respect to sludge settleability very low SVI values were observed during dispersed growth at low sludge ages (1.1 days); increased SVI values were found at sludge ages in the range 3-5.5 days which decreased at higher sludge ages (5.5-12 days), and increased again at even higher sludge ages (17.4 days). These results, in conjunction with a number of contradictory reports in the literature (Ford and Eckenfelder, 1967; Eckenfelder, 1967; Ganczarczyk, 1970; Bisogni and Lawrence, 1971; Rensink, 1974) indicate that the establishment of a valid, for general use, relationship between sludge age and sludge settleability is not possible. This is not surprising in view o f the multitude of factors, in addition to sludge age, that can affect both population dynamics and floc structure in an activated sludge system (wastewater composition, DO concentration, pH, waste feeding regime, presence of unaerated zones, mixing intensity). Microscopic examination of sludge samples from the units showed that filamentous micro-organisms were not noticeable in any of the units. This indicates that the high SVI values observed reflect a "nonfilamentous bulking". In low SVI sludge samples, under microscopic observation, the floc appeared to be fairly dense and more regular-edged, while more irregular-shaped floes characterized the high SVI sludge samples. Application of a polysaccharide staining technique revealed accumulation of abundant polysaccharide material around the floes and dispersed bacteria in the unit operating under a sludge age of 1.1 days. In the other units, the appearance of the floc was different as bacteria seemed to be enmeshed in a network of polysaccharide fibrils.
1711
Activated sludge floc A detailed presentation and discussion of the results with respect to specific critical floe characteristic and their relation to settleability is the subject of the following sections.
Specific surface area of flocs A typical dye adsorption curve for activated sludge is shown in Fig. 2. The curve implies that there is high affinity between the dye molecules and the surface of the floe. Up to a certain limit, all the dye (typically between 40 and 60 mmol dye k g - ' solids) is chemically bound to the floc surface leaving none in solution. A second layer on top of the chemisorbed dye is being formed due to physical adsorption of the dye molecules, up to a point where a monolayer is established (plateau of the isotherm). The determination of the surface area of the floes from equation (1) is based on the amount of the absorbed dye (Y) and the area covered by each dye molecule (A). For the sludge sample corresponding to the isotherm shown in Fig. 2, the total amount of the adsorbed dye is 160 mmol kg -t, and the amount remaining, after deduction of the chemisorbed 50mmolkg -1, is l l 0 m m o l g - L Depending on the orientation of the adsorbed dye molecules, flat or perpendicular, the area occupied by each molecule is, in the case of the dye used, 196 or 90 A 2 respectively (Longmuir, 1975). Therefore, depending on the assumptions with respect to the amount of the adsorbed dye (total or physically adsorbed) and the orientation of the dye molecules, the specific area given by equation (1) varies between 60 and 189 m 2 g-t. These figures are much higher than the geometric area of the surface of activated sludge floes measured by Finstein and Heukelekian (1967), assuming nonporous floes. They found values in the range 20-100cm 2 of surfacecm -3 of mixed liquor. The MLSS values of their samples ranged from 1300 to 2320mgl -~, therefore the specific surface would be 0.862-7.7m2g -1 dry sludge. These findings are comparable to the value of the specific surface area of a non-porous graphite sample (2 m 2 g- ~) reported by Giles et aL (1970). The hypothesis of the porous nature of the activated sludge floe is also supported by consideration of the specific area measured in relation to the floe size. For a typical solids content c = 0.08 g c m -3 (corresponding to a sludge floe density af= 1.020gcm -3, Table 2) the diameter of a sphencai non-porous floe with a surface area of 60--189 m 2 g-~ is 0.50-1.25 #m, which is much smaller than the typical range of floe sizes found in this study (10-70/am) and reported in the literature. On the other hand, for non-porous floes 10-70/zm in size, the corresponding area is between 1.1 and 7.5 m 2 g-J, which is close to the geometric areas reported by Finstein and Heukelekian (1967), but much smaller than the areas determined by the dye adsorption technique. The mean value of the total dye adsorbed by sludge samples harvested from the vari-
800 --
"~ 600
O0
--
i"~ 400
"~ 200
0
~
5..~ o
100
r = 0.86
I
I
I
200
300
400
Dye absorbed (retool kg -t) Fig. 4. Correlation between SVI and activated sludge dye adsorption capacity. ous units, shown in Table 1, vary between 102 and 220 mmol kg -] with the exception of the dispersed sludge which is characterized by the higher value of 342 mmol kg- 1. These mean values are related to the mean SVI values with higher dye adsorption capacifies observed at higher SVI values. This is more clearly demonstrated in Fig. 4 which shows a good correlation between SVI values and corresponding dye adsorption capacities for several sludge samples harvested from the activated sludge units.
Floc size and density Figure 5 shows the floe size distribution obtained by the Coulter Counter technique for samples harvested from the activated sludge units operated at 1.1, 4.2, 8.8, 12 and 17.4 days. With the exception of the unit at 1.1 days, more than 85% of the floes were found in the range 10-70tim, with median values between 35 and 45/zm. The floes from the unit at 1.1 days were significantly smaller with a median value of around 20/am; however again around 80% of the floes had sizes in the range 10-70/am. A strong correlation between floe density (Pr) and size (d) was observed as shown in Fig. 6, and a relationship between density and size was obtained in the form. pf
1 + 0.30d -°'s2
=
(6)
This relationship applies to all sludge samples irrespective of sludge age with the exception of the
100 8O
40 20
o
o t3 - • •
SRT SRT SRT SRT
= = = =
4.2 da 17.4 days s~ax, 8 . 8 a n d 12 d a 1.1 d a y s
I 10
•
I I I 20 30 40 50 6070 Fioc size, d (~tm) Fig. 5. Floc size distribution at different sludge ages.
Ah'D~gS D. ANDREADAKIS
1712 1110 ~oo t~O
811 -6
•~
60
~ L "-;~
35 p f - l +-
-~e
0.3d-0.S2
O~
o SRT ffi 4.2,8.8,12,17.4 days • S R T = 1.1 days
~o 4o
~
25
'~
20
•
•
too.
15 --
20
lo 10
20
30
40
50
60
I
I
20
711
I
1
311 411 Floe size, d (tim)
511
Floc size, d (ttm) Fig. 6. Relationship between flec density and flcc size.
Fig. 7. Relationship between mean fl~ and density and median floc size.
dispersed sludge where for a given floc size (especially for floes smaller than 30#m) the corresponding density is smaller when compared to the density of a floc of the same size from the other units (Fig. 6). This is probably due to the excess polysaccharide material (mostly adsorbed organic material) surrounding the flocs of dispersed bacteria as indicated by the microscopic polysaccharide staining technique. Given that the porosity (~) of the floc is:
1.1-17.4 days the average carbohydrate content was between 8.5 and 11%. Accumulation of carbohydrate material was observed at very low sludge ages (1.1 days) and in the range 8-14 days. The high sludge carbohydrate content at high sludge ages (8-14 days) reflects the accumulation of extracellular polymeric material. At sludge age of 1.1 days the high carbohydrate content was probably caused by accumulation of stored material which resulted in sludge dispersion. To test this hypothesis samples from the two units operating at sludge ages of 1.1 and 12 days respectively were harvested and aerated without feeding for 6 h. At the end of the aeration period the carbohydrate content of the sludges was measured again. It was found that the sample taken from the unit operating at 12 days sludge age had practically the same carbohydrate content as initially (10.7-11.2%); the carbohydrate content of the sludge sample harvested from the unit operating at a sludge age of 1.1 days was reduced at the end of the 6 h aeration period from 10.8 to 9.5%, due to the assimilation of the stored material. A change in the appearance of the sludge was also noticed in the latter case as the colour of the sludge turned from pale yellow (almost white) to darker yellow. The decline in the sludge carbohydrate content observed at a sludge age 17.4 days is striking in view of the expected enhanced exocellular polymer production under starvation or semi-starvation conditions. An explanation can be given by considering
Ps -- Pf
ffi ~
p~ -- p ,
(7)
for p, ffi 1.34 g c m -3, Pw ----1 g c m -3 (density of water) and pf from equation (6), it follows that -- 1 -- 0.88d
-0"s2
(8)
For d in the range 10--70# m the porosities ~ vary from 85 to 97%. These values follow the same trend (with respect to floc size) but are generally higher than the values reported for similar floc sizes by Hong et al. (1987) obtained by settling velocity and photographic techniques. The weighted average floc densities (pf) of the sludge samples from each unit, calculated from: Pf= ~, PiPri
(9)
i-I
where Pt ffi average floc density of sample Pt ffi percentage of particles of size i Pfl ffi density of particles of size i n ffi number of floc sizes examined
250
are shown in Table 1. These average values are correlated to the median floc size (Fig. 7) and the mean SVI values (Fig. 8). A correlation between SVI and median floc size was also obtained (Fig. 9). These correlations of SVI versus floc size and density are in agreement with similar results reported in the literature for non filamentous sludges (Zergin, 1982; Barber and Veenstra, 1986).
Sludge carbohydrate content Sludge carbohydrate content data, presented in Table 2, show that over the sludge age range of
-
200
>
•
150 100 5O
15
20
25
30
I
I
I
35
40
45
(pf-1)xl000
Fig. 8. Relationship between mean SVI and mean floc density.
Activated sludge floc 250 -
.
>
200 -
e / / , ~
150 -
/
r~ 100 -50
--
/
3.
~
.S, 20
I
I
30 40 Floc size, d (Ixm)
50
4.
Fig. 9. Relationship between mean SVI and median floc size.
the ability of the bacteria to acclimatize and produce enzymes which can hydrolyse and assimilate the exocellular polymers surrounding and bonding their surface under conditions of depletion of external substrate (Postgate and Hunter, 1962; Pavoni et al., 1972; Gaudy et al., 1971; Yang and Gaudy, 1974; Yang and Chen, 1977). SVI was not correlated to the sludge carbohydrate content (Fig. 10). Forster (1971) had established a good correlation for samples from a pilot scale treatment plant, but with 30% of the samples having carbohydrate concentrations over 37%. Such high concentrations would be considered atypical in a municipal facility and would probably be experienced in systems treating certain industrial wastes, generally with high C/N and C/P ratios. On the other hand Barber and Veenstra (1986) did not observe any correlation for typical activated sludges with similar to our study carbohydrate contents (in the range 5-16%). It seems therefore that it is not the amount of the exocellular polymers which is a major factor in sludge bulking, but more likely the properties of these polymers and the effect they have on sludge floc morphology. CONCLUSIONS
The results of the study can be summarized as follows: 1. For well mixed activated sludge systems the typical range of floc sizes was found to be between 10 and 70/~m. 2. A strong correlation between floc density and size was observed with density dropping as size 500 400
o o o
300
o
200
o 0
Oo °o° o
100
o
oo
o
~
o
8
o
I
I
I
t
5
10
15
20
Sludge carbohydrate content (%) Fig. 10. SVI vs sludge carbohydrate content. WR
27112--B
5.
6.
1713
increases. An average density of 1.020 g cm -3 was found to be typical for moderately and low loaded activated sludge systems. The specific surface area of the floes as determined by a dye adsorption technique was high (typically in the range 100-200 m 2 g-~ dry sludge) and one to two orders of magnitude higher than the geometric surface area of the floes. Considerations of the specific surface area and density in relation to floc size indicate a porous floc structure, with porosities in the range 85-97%. Non-filamentous sludge settleability, expressed in terms of SVI, was related to floc size and density. A strong correlation between SVI and floc specific surface area was also observed. Exocellular polymers in activated sludge originate either from substrate adsorption and storage (low sludge ages) or as a product of biomass decay (long sludge ages). The carbohydrate content of the activated sludge samples varied between 6 and 18% and was not correlated to sludge settleability. In view of these results it is probable that it is the properties rather than the quantity of the accumulated exocellular polymers that affect sludge settleability. REFERENCES
Andreadakis A. D. (1978) Physicochemical aspects of activated sludge in relation to purification and settle,ability. PhD thesis, Univ. of Strathclyde, Glasgow. Andreadakis A. D. (1985) Surface characteristics of biological solids. In Appropriate Waste Management for Developing Countries (Edited by Curl K.), pp. 139-151. Plenum Press, New York. APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th edition. American Public Health Association, Washington, D.C. Barber J. B. and Veenstra K. N. (1986) Evaluation of biological sludge properties influencing volume reduction. J. Wat. Pollut. Control Fed. 58, 149-156. Barer R. and Joseph S. (1955) Refractometry of living cells. Q. J! microsc. Sci. 96, 423. Bisogni J. J. and Lawrence A. W. (1971) Relationship between biological solids retention time and settling characteristics of activated sludge. War. Res. 5, 753-769. Coackley P. and Kliger B. (1964) Some interferometric observations of the water]solids relation of activated sludge floes using a compression technique. Nature 203, 77-78. Coacldey P. and O'Neill J. (1975) Sludge activity and full-scale plant control. War. Pollut. Control 74, 404-414. Eckenfelder W. W. J. Jr (1967) Comparative biological waste treatment design. J. sanit. Eagng Div. ASCE 93, 157-170. Finstein M. S. and Heukelekian H. (1967) Gross dimensions of activated sludge floes with reference to bulking. J. War. Pollut. Control Fed. 39, 33-40. Ford D. L. and Eckenfelder W. W. J. Jr (1967) Effect of process variables on sludge formation and settling characteristics. J. Feat. Pollut. Control Fed. 39, 1850-1859. Forster C. F. (1971) Activated sludge surfaces in relation to the sludge volume index. War. Res. 5, 861-869.
1714
ANDRF~ D. ANDI~.ADAKIS
Forster C. F. and Lewin D. C. (1972) Polymer interactions at activated sludge surfaces. Eft. War. Treat. J. 12, 520-525. Ganc.zarczyk J. J. (1970) Variation in the activated sludge volume index. War. Res. 4, 69-78. Gaudy A. F. Jr (1962) Colorimetric determination of protein and carbohydrate. Ind. War. Wastes 7, 17-22. Gaudy A. F. Jr, Yan8 P. Y. and Obayashi A. W. (1971) Studies on the total oxidation of activated sludge with and without hydrolytic pretreatment. J. Wat. Pollut. Control Fed. 43, 40-54. Giles C. H. and Grecrek J. J. (1962) Review of methods of purifying and analysing water soluble dyes. Test Res. J. 32, 506-515. Giles C. H., D'Silva and Trivedi A. S. (1970) Use of p.nitrophenol for specific surface measurement of granular solids and fibres. J. Appl. Chem. 20, 37-41. Heukelekian H. and Weisberg E. (1956) Bound water and activated sludge bulking. J. Wat. Pollut. Control Fed. 28, 558-567. Huang J. Y., Cheng M. D. and Nueller J. T. (1985) Oxygen uptake rates for determining microbial activity and application. Wat. ICes. 19, 373-381. Jenkins D. (1992) Towards a comprehensive model of activated sludge bulking and foaming. War. Sci. Technol. 25, 215-230. Knocke W. R. and Zentkovich T. L. (1986) Effects of mean cell residence time and particle size distribution on activated sludge vacuum dewatering characteristics. J. Wat. Pollut. Control. Fed. 58, 1118-1123. Knudson M. K., Wiiliamson K. J. and Nelson P. O. (1982) Influence of dissolved oxygen on substrate utilisation kinetics of activated sludge. J. War. Pollut. Control Fed. 54, 52-60. Kristensen G. H., Jorgensen P. E. and Henze M. (1992). Characterization of functional biomass groups and substrate in activated sludge and wastewater by AUR, NUR and OUR. ;Vat. Sci. Technol. ~ 43-57. Li D. H. and Ganczarczyk J. J. (1987) Stroboscopic determination of settling velocity, size and porocity of activated sludge flocs. Wat. Res. 21, 257-262. Li D. H. and Ganczarczyk J. J. (1988) Flow through activated sludge llocs. Wat. Res. 22, 789-792.
Longmuir G. (1975) Studies on the nature of activated sludge. PhD thesis, Univ. of Strathclyde, Glasgow. Magara Y., Nambu S. and Utozawa K. (1976) Biochemical and physical properties of an activated sludge on settling characteristics. Wat. ICes. 10, 71-77. Pavoni J. L., Tenney M. W. and Echelberger W. F. J. (1972) Bacterial exocelinlar polymers and biological flocculation. J. Wat. Pollut. Control Fed. 44, 414-431. Postgate J. R. and Hunter J. R. (1962) The survival of starved bacteria. J. gen. Microbiol 29, 233-263. Rensink J. H. (1974) New approach to preventing bulking sludge. J. War. PoUut. Control Fed. 46, 1888-1894. Sadalgekar V. V., Mahajan B. A. and Shaligram A. M. (1988) Evaluation of sludge settleability by floc characteristics. J. War. Pollut. Control Fed. 60, 1862-1863.
Sergin M. (1982) Variation of sludge volume index with activated sludge characteristics. War. Res. 16, 83-88. Sergin M., Jenkins D. and Parker D. S. (1978) A unified theory of filamentous activated sludge bulking. J. War. Pollut. Control Fed. $0, 362-380. Smith P. G. (1984) A model of localised oxygen sinks around bacterial colonies within activated sludge. War. Res. 18, 1045-1051. Smith P. G. and Coackiey P. (1984) Diffnsivity, tortuosity and pore structure in activated sludge. Wat. Res. 18, 117-122. Smyllie R. T. (1969) Cake filtration. Phi) thesis, Univ. of Strathclyde, Glasgow. Tomlinson G. J. and Chambers B. (1984) Control strategies for bulking sludge. Wat. Sci. Technol. 16, 15-34. Water Pollution Research Laboratory (1965) Physical Properties o f Sludge. H.M.S.O. Weddle C. L. and Jenkins D. (1971) Viability and activity of activated sludge. Wat. Res. 5, 621--648. Yang P. Y. and Gaudy A. F. Jr (1974) Nitrogen metabolism in extended aeration process operated with and without hydrolytic pretreatment of portions of the sludge. Biotechnol. Bioengng 16, 1-20. Yang P. Y. and Chen Y. L. (1977) Operational characteristics and biological kinetic constants of extended aeration process. J. Wat. Poilut. Control Fed. 49, 678-688.