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Effects of synthetic polymer on the filamentous bacteria in activated sludge
Bioresource Technology 96 (2005) 31–40
Effects of synthetic polymer on the filamentous bacteria in activated sludge Der-Fong Juang
*
Department of Healthcare Administration, Mei-Ho Institute of Technology, 23, Ping-Kuang Road, Nei Pu, Ping Tong 912, Taiwan, ROC Received 1 November 2001; accepted 5 March 2004 Available online 16 April 2004
Abstract Filamentous bulking is one of the solid–liquid separation problems always seen in activated sludge process. The addition of synthetic polymer is always one of the popular ways for the treatment plant operator to immediately solve the poor sludge settling problem. Therefore, it may be interesting to understand the effects of synthetic polymer on the filamentous bacteria in activated sludge. In this study, synthetic polymer was applied to a lab-scale wastewater treatment system with the filamentous bulking problem. The population structure of filamentous bacteria and sludge characteristics were investigated under different conditions. When synthetic polymer was added into the system, it was found that poor sludge settleability caused by filamentous bulking was temporarily solved and filamentous branches growing outside the flocs were damaged or inhibited. However, filamentous growth was still observed inside the flocs. After the addition of polymer was halted, filamentous branches extended out of the flocs immediately. Very serious filamentous bulking occurred and sludge settleability became much worse than that occurring before the addition of polymer. And, it took several weeks for the system to return to normal operation. 2004 Elsevier Ltd. All rights reserved. Keywords: Synthetic polymer; Activated sludge; Filamentous bulking
1. Introduction According to Sezgin et al. (1978), the filamentous bacteria are believed to form the ‘‘backbones’’ of activated sludge flocs to which floc-forming bacteria attach by means of extracellular polymers and form strong flocs. In the absence of filaments, ‘‘pin-flocs’’ will be formed. These tiny and weak flocs will tend to contribute to secondary effluent turbidity. Filamentous bulking occurs if filamentous bacteria grow overabundantly, and it is a condition in which sludge settling rates decrease and the thickening characteristics of the settled sludge are poor (Lau et al., 1984a,b). Sludge volume index (SVI) values higher than 150 ml/g are often associated with filamentous bulking. Many investigators have studied filamentous bulking control for many years. From review of the literature, it seems that certain specific filamentous bacteria cause most sludge bulking episodes. Since actions that will
*
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0960-8524/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.03.005
control one species may not affect others, one is likely to fail to control activated sludge bulking unless the specific causative filamentous microorganisms can be identified. Many filamentous bacteria that cause sludge bulking have been classified (Jenkins et al., 1986; Lau et al., 1984a,b; Richard, 1989), although newly isolated ones are occasionally reported. Jenkins et al. (1986) have proposed that the presence of certain filaments indicates the specific environmental conditions leading to activated sludge bulking. Examples include low DO, low F/M, low pH, increased concentration of sulfides, nutrient deficiency, and so on. Many methods are used by plant operators to control sludge bulking, including chlorine addition, hydrogen peroxide addition, aeration basin pH control, nutrient addition, aeration control, anaerobic selector application, and so on. Synthetic polymer has also been popularly used to improve sludge settleability in many wastewater treatment plants, especially when the poor settling problem is caused by temporary hydraulic overloading. The polymer is usually added either to the activated sludge as it leaves the aeration basin or to the secondary clarifier center well (for circular clarifier).
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
state operation was achieved. At this point, samples were again taken and analyzed as at the end of Phase I. Polymer addition was then halted. During Phase III the activated sludge system was again operated without polymer addition. In order to investigate the effects of polymer on the filamentous bacteria and observe any possible change on the sludge characteristics, samples were taken more frequently and analyzed for numerous parameters during this phase. For those reasons, Phase III was then separated into two stages, which are called Phase III-A and Phase III-B.
Richard (1989) mentioned that synthetic polymer could overcome the physical effects of filaments on sludge settling. There are, however, no detailed reports available specifying how synthetic polymer affects the filamentous bacteria.
2. Methods 2.1. Experimental design To investigate the effects of synthetic polymer on the filamentous bacteria in activated sludge, the whole research period was divided into three phases shown in Fig. 1 (Juang, 2003). During Phase I, no polymer was used, and the system was allowed to operate until steady-state operation was achieved. Stability tests of the system were routinely performed during this phase to see if stable operation had been attained. When the system was determined to be stable, sludge and effluent samples were taken for floc size measurement, identification of filamentous bacteria, analysis of total suspended solids, and so on. After sludge and effluent samples were taken for analysis, Phase II started immediately. In the beginning of Phase II, a synthetic polymer was added to the influent of clarifier and the system was continuously operated. Again, samples were taken and analyzed over a period of time to determine if steady
After Confirmation of Steady State
After Confirmation of Steady State
(~2 months)
Phase I (Prior to the Addition of Synthetic Polymer)
This study was conducted using one lab-scale continuous flow stirred tank reactor (CFSTR) as shown in Fig. 2 (Juang and Morgan, 2001; Juang and Hwu, 2003). The operating data and the feed composition for this CFSTR is described in Table 1 (Juang and Morgan, 2001). A small cubical mixing tank was located between the aeration basin and the clarifier. During Phase II polymer was introduced into this mixing tank. A magnetic stirrer was used to mix the sludge and the polymer. Concentrated (pure) synthetic polymer was diluted 400fold with distilled water before it was used. Based upon a ratio of 1 ml of the concentrate per 10 l of activated sludge, the diluted synthetic polymer solution was continuously pumped into the mixing tank at a flow rate of
After Confirmation of Steady State
Discontinuation of Polymer Addition
Polymer Addition
Operation Start
2.2. System design and operating data
(~2 months)
(~5 weeks)
Phase II
Phase III-A
(After the Addition of Synthetic Polymer)
(~8 weeks)
Phase III
Phase III-B
(After the Discontinuation of Synthetic Polymer Addition)
Fig. 1. Experimental design.
Air Sludge Recycle Feed
Aeration Basin 30L
30L/day Feed
Synthetic Polymer
Sludge Recycle 30L/day
Pump ~
From Feed Container
Scraper
Effluent (To nearby sink) 30L/day
Clarifier
M
7.35L M: Motor
Fig. 2. Bench scale CFSTR system.
Operation End
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
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Table 1 Operating data and feed composition of CFSTR Operating data of CFSTR
Feed Composition of CFSTR
Reactor
CFSTR
Material
Conc., mg/l
Material
Conc., mg/l
Volume feed rate (Aeration Basin) Aeration basin volume Clarifier volume Residual basin volume Volume wasted Hydraulic retention time (Aeration Basin) Solids retention time Sludge recycling rate
30.0 30.0 7.35 – 1.00 1.00 30.0 30.0
Nutrient Broth KH2 PO4 NaOH CaCl2 Æ 2H2 O MgSO4 Æ 7H2 O Glucose KNO3 NaCl
300.0 44.0 16.7 132.4 100.0 140.0 3.0 100.0
FeCl3 Æ 6H2 O MnSO4 Æ H2 O (NH4 )2 SO4 NaHCO3
5.0 12.8 118.4 467
(l/day) (l) (l) (l/day) (day) (day) (l/day)
2.48 liters per day. The resulting polymer dose was equivalent to 1 ml of pure polymer to 10 l of wastewater, a typical dose used in practice. The synthetic polymer used in this study was manufactured by Keystone Labs, Inc., USA. It was marketed specifically as Keystone KF 4249 ‘‘Mannequin Polymer.’’ The polymer has the molecular weight of (2.5–3.5) · l06 , pH of 9–11, absolute viscosity of (16–19) · l05 cps, and bulk density of 112.3 1b/ft3 . 2.3. Confirmation of equilibrium (stability tests) Routine feed and effluent samples were used to check for stable operation. Many tests were performed following the procedures in Standard Methods (Franson et al., 1985) to ensure that the system had reached equilibrium, such as MLVSS, COD, TKN, NH4 -N, NO3 -N, NO2 -N, pH, and DO. Each test was generally performed on at least three consecutive days and the data were averaged. 2.4. Analysis of effluent and sludge characteristics The sludge settling characteristics, the sludge volume index (SVI), and the suspended solids and turbidity of the effluent was observed or analyzed during each Phase. Each test was conducted for at least four days, and the average value was then obtained. Floc size was measured using a technique, called the microscopic sizing method (Mueller et al., 1967). A few drops of mixed sludge sample were taken directly from aeration basin by a pipet with an inside diameter at the mouth of 0.5 cm, and carefully added into a petri dish. The flocs were then solidified by introducing them into a diluted, cooled agar suspension (3 g agar in 100 ml of distilled water) with gentle mixing provided by smoothly rotating the dish. After being solidified, the length and breadth of the flocs were measured with an ocular micrometer and microscope at 100 times magnification (100·). In general, hundreds of flocs from each petri dish were randomly chosen for measurement, and at least three to four dishes were tested with one dish prepared each day.
The average floc size distribution for each Phase was then obtained. 2.5. Identification of filamentous microorganisms Samples were taken from the aeration basin and dried on slides. After drying, the samples were Gram-stained and Neisser-stained, respectively, following the procedures described in the manual on the Causes And Control of Activated Sludge Bulking and Foaming (Jenkins et al., 1986). Crystal violet sheath stains and sulfur oxidation tests were also performed following the procedures described in that manual. The physiological characteristics of each filamentous bacteria, such as size (filament length and diameter), cell shape, attached growth, branching, cell septa, filament dispersion, motility, etc., were observed through a microscope (Eikelboom, 1975; Jenkins et al., 1986; Nowak and Brown, 1990; Pitt and Jenkins, 1990; Rogovskaya and Lazareva, 1959; Salameh and Malina, 1989; US EPA, 1987; van Veen, 1973; van Veen et al., 1978). All data collected for each organism were compiled and compared to the specification of each filamentous bacterium listed in the manual (Jenkins et al., 1986). In many cases, the identification could be completed in this manner. However, additional testing was sometimes required. The numerical population of the various filamentous organisms was estimated according to the rating scale described by Jenkins et al. (1986). The relative abundance of filamentous organisms was quantitated according to seven categories: none, few, some, common, very common, abundant, and excessive. This was accomplished by observing the filaments using a microscope at 100· magnification.
3. Results The effluent quality, sludge characteristics, floc size, and filamentous bacteria in CFSTR during each Phase
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Phase II
Phase III-A
Phase III-B
Phases
Phase I
Items and operating status
Before filamentous bulking
After filamentous bulking
Polymer addition
1st week after polymer addition was halted
2nd week after polymer addition was halted
3rd week after polymer addition was halted
5th week after polymer addition was halted
12th week after polymer addition was halted
Effluent
TSS (mg/l) Turbidity (NTU) SVI (ml/g) Floc size (lm) Description
2 1.1 150–165 – Regularly normal flocs
7.83 4–8 350–360 254 Flocs with filaments
2 1 99–131 631 Strong flocs
17.5 11 310 261 Broad floc size distribution
8 4–11 300–385 240 Normal flocs with filaments
– – – –
Abundant Abundant Very Common Common
Very common Very common Common Common
Abundant Abundant Few Very common
20 14 158 136 Tiny and fragile flocs with many blank filaments Very common Very common None Few
2 4.5–14.7 133 245 Normal flocs
Sphaerotilus natans Type 1851 Type 0092 Unknown
254 50 410 – Very serious filamentous bulking Excessive Excessive None Very common
Abundant Abundant None Common
Abundant Abundant None Common
Sludge charact.
Filamentous bacteria
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
Table 2 Effluent quality, sludge characteristics, floc size, and filamentous bacteria in CFSTR during each phase
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
were depicted in Table 2 (Juang, 2003), and the results of this study were described in the followings. 3.1. Effluent quality and sludge characteristics 3.1.1. Phase I: Prior to the addition of synthetic polymer During the first month after start-up, the system operated well. The sludge settled readily and clear effluent was obtained. The average values of total suspended solids and turbidity in the effluent were about 2 mg/l and about 1.1 NTU, respectively. The SVI was in the range between 150 and 165 ml/g with the average being about 155 ml/g. However, after one month filamentous bulking occurred. This resulted in an increased sludge volume in the clarifier. The SVI increased rapidly from 250 to 360 ml/g in only a few days, then its value held steady in the 350–360 ml/g range for two more weeks without getting worse. At this time, the clarifier was always about halffilled with sludge. The total suspended solids in the effluent rose to an average value of 7.83 mg/l. The turbidity also increased, ranging from 4.0 to 8.0 NTU. 3.1.2. Phase II: After the addition of synthetic polymer Two days after the CFSTR system began to receive the synthetic polymer the settling characteristics of the activated sludge in the clarifier improved greatly. Apparently the adverse effects of the filamentous organisms were completely overcome. The SVI varied within the range of 99–131 ml/g, with an average value of 114.5 ml/g. The activated sludge flocs were visibly much bigger and firmer. The total suspended solids in the effluent dropped to about 2.0 mg/l, and the turbidity decreased to an average value of about 1.0 NTU. 3.1.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition (1) First week: In a few days after the discontinuation of synthetic polymer, the sludge settleability became worse. Because some polymer was still remaining in the system during the first one or two days, the SVI value thus increased gradually from 193 to
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310 ml/g. The total suspended solids in the effluent rose to 17.5 mg/l in about one week. The turbidity of the effluent increased to a value within the range of 10–12 NTU. (2) Second week: During this week, sludge settleability continued to worsen, and the SVI rose from 310 to 410 ml/g. It is obvious that very serious filamentous bulking occurred in the system, and Fig. 3 illustrates the problem. The clarifier was almost filled up with sludge and the activated sludge in the clarifier hardly settled at all. This situation led to great amounts of solids being washed out with the effluent and the total suspended solids in the effluent increased to 254 mg/l. Similarly, the turbidity of the effluent rose to 50 NTU. (3) Third week: During the third week, the sludge settleability in the clarifier started to improve and the SVI gradually dropped to 158 ml/g. The total suspended solids in the effluents also dropped to about 20 mg/l. In the meantime the turbidity of the effluent ranged between 8 and 16 NTU with an average value of 14 NTU. (4) Fourth and fifth weeks: During the fourth and fifth weeks, the system was operating well and the sludge settleability was normal again. The SVI varied from 129 to 137 ml/g with an average value of 133 ml/g. The total suspended solids in the effluent dropped to only 2.0 mg/l and the turbidity of the effluent decreased to between 4.5 and 14.7 NTU. 3.1.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition During this period, the SVI was always in the range of 300–385 ml/g. The total suspended solid in the effluent was about 8.0 mg/l, and the turbidity was between 4.0 and 11.0 NTU. 3.2. Stability tests The stability tests were conducted for 5 consecutive days at the end of each Phase, and the average values were shown in Table 3. It is obvious that after an
Fig. 3. Very serious bulking occurred at the second week of Phase III-A: (a) normal operation, (b) filamentous bulking at Phase I and (c) very serious bulking at Phase III-A.
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
Table 3 Stability tests Item
pH Alk. BOD COD TKN-N NH3 -N NO3 -N NO2 -N PO4 -P
Phase I
Phase II
Phase III-A
Phase III-B
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
7.80 290.00 316.67 389.67 45.19 28.44 0.71 0.01 6.86
6.41 19.60 1.74 33.10 0.26 0.17 57.77 0.64 8.72
7.85 316.00 342.00 490.80 94.20 32.20 0.20 0.03 9.80
5.90 10.00 2.50 5.30 0.40 0.38 68.00 1.07 9.10
8.00 333.30 395.00 458.00 74.60 24.80 0.20 0.004 5.60
6.12 26.70 2.63 65.50 0.49 0.37 61.00 0.31 7.10
8.00 333.30 299.00 354.84 58.04 24.65 0.65 0.03 8.00
6.40 26.70 1.80 20.69 0.46 0.22 54.80 2.44 6.40
Note: Phase I––Aeration Basin: ¼ 2035 mg/l, DO ¼ 5.6–6.4 mg/l. Phase II––Aeration Basin: MLVSS ¼ 2153 mg/l, DO ¼ 6.2 mg/l. Phase III-A–– Aeration Basin: MLVSS ¼ 2073 mg/l, DO ¼ 6.7 mg/l. Phase III-B––Aeration Basin: MLVSS ¼ 2150 mg/l, DO ¼ 6.4 mg/l.
acclimation period of each Phase the activated sludge system operated normally with respect to substrate removal and nitrification. The pH value in the effluent was always above 5.9, and the dissolved oxygen in the aeration basin was usually between 5.6 and 6.7 mg/l. Therefore, this system was determined to be stable (even though some filamentous bulking was occurring). 3.3. Floc size distribution 3.3.1. Phase I: Prior to the addition of synthetic polymer The average floc size was mostly in the range between 150 and 200 lm with an average value of 254 lm. The general characteristics of activated sludge flocs are evident from Fig. 4(a). The flocs shown are irregularly shaped and display some filamentous growth. Basically, the activated sludge flocs during this phase could be described as healthy and large. 3.3.2. Phase II: After the addition of synthetic polymer In general, the floc size during this phase was much larger than before and was mostly in the range between 500 and 600 lm with an average value of 631 lm. The general characteristics of the activated sludge flocs with the growth of filamentous bacteria during this
phase are presented in Fig. 4(b). The flocs appeared much bigger and firmer than before. Only a few filaments were observed to extend out from the flocs. Therefore, the addition of synthetic polymers during this phase apparently not only enhanced the sludge flocculation and caused the formation of larger and heavier flocs, but it had also inhibited (or at least overcome the effects of) the growth of the filamentous organisms. 3.3.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition Since the sludge settling characteristics changed rapidly after the discontinuation of synthetic polymer, floc size measurements were made every two weeks during this phase of the study. (1) First week: Since some synthetic polymer might still remain in the aeration basin during the first week of this phase, the sludge floc size distribution spread out to a broad extent. Most of the sludge flocs however were still sized between 100 and 200 lm, and the average value was about 261 lm. (2) Third week: After two weeks, the flocs appeared tiny and fragile. Most of the sludge flocs had a size of
Fig. 4. Activated sludge flocs at diiferent phases: (a) flocs at phase I (·200), (b) flocs at phase II (·200) and (c) flocs at the third week of Phase III-A (·200).
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
about 100 lm, more or less. The average floc size was about 136 lm, however many flocs sized less than 100 lm were observed during this week. Typical tiny and fragile flocs taken from the aeration basin are pictured in Fig. 4(c). The filamentous backbones of flocs can be easily seen. However, numerous filaments were washed out with the effluent because they were not bound up in large, heavy sludge flocs. Ultimately, the washout of the filaments resulted in improved sludge settling during the third week. As a result, the SVI dropped rapidly. (3) Fifth week: The dimension of the activated sludge flocs during the fifth week increased and size distribution returned to about the same range as before polymer addition was begun. Most of flocs had a size between 150 and 200 lm with the average value of 245 lm.
3.3.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition Most of the flocs are sized between 100 and 200 lm with the average value of about 240 lm. That is, the
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sludge flocs in this system during this phase were in a normal size range. 3.4. Filamentous bacteria 3.4.1. Phase I: Prior to the addition of synthetic polymer Generally, four kinds of filaments were observed. Sphaerotilus natans and Type 1851 were two of the most predominant filamentous bacteria, and they were found ‘‘abundant’’ in this sludge. False branches, Gram-negative and Neisser-negative reactions, rod cells, and sheath presence are the major characteristics of S. natans. Fig. 5(a) and (b) show the false branches and sheath stain of this bacterium, respectively. S. natans is commonly found in activated sludge treatment plants in the USA. Type 1851 always grows in bundles and away from the flocs. Its characteristics and Gram-stain can be seen from Fig. 6(a) and (b), respectively. The presence of Type 1851 tends to cause diffused activated sludge flocs. When attached growth was noted it was always found perpendicular to the filaments. Weak Gram-positive and Neisser-negative reactions were also observed.
Fig. 5. False branches and sheath of Sphaerotilus natans: (a) false branching (·200) and (b) sheath stain (·1000).
Fig. 6. Characteristics and Gram-stain of Type 1851: (a) growth in bundles (·200) and (b) Gram-positive reaction (·1000).
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
From microscopic observation of the sludge flocs, it is believed that the abundant growth of these two filaments was the reason for the sludge bulking previously noted. Type 0092 was found ‘‘very common’’ in this sludge. It exhibited a Gram-negative reaction and very short filaments extending outward from the flocs. Since most filaments of Type 0092 are usually within the flocs, it is often difficult to observe. No attached growth was found on the surfaces of its filaments. Generally, it is believed that the growth of type 0092 in this system was insignificant with respect to leading to sludge bulking. One other filamentous organism was also found in this system, but at a much lower concentration. This organism had the characteristics of strong Gram-positive reaction, granular deposits on the surface of the filaments, attached growth, and no branching. Its filaments, extending out from the flocs, appeared mostly straight and had a diameter of more than 1.0 lm. Fig. 7(a) and (b) show the characteristic and the Gram-stain reaction of this filament, respectively. Complete identification of this organism was not possible. Because of its low population level, its effect on the sludge bulking was possibly not very significant. 3.4.2. Phase II: After the addition of synthetic polymer All four previously discussed types of filamentous bacteria were still observed in the activated sludge after the addition of synthetic polymer. However, most of filamentous organisms intertwined inside the flocs, and only some of the filaments associated with S. natans and type 1851 extended out from the flocs a short distance. Both S. natans and type 1851 were rated as ‘‘very common’’. The S. natans population bordered on the ‘‘abundant’’ category since many filaments were noticed stretching out from the flocs. The population of type 0092 and the ‘‘unknown’’ organism were rated as ‘‘common’’. In general, filamentous growth appeared to be inhibited by the addition of synthetic polymers.
However, they were still present intertwining inside the flocs. 3.4.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition (1) First week: After the discontinuation of synthetic polymer, filamentous bulking started immediately. Filaments extended outward from the flocs very rapidly, and this resulted in higher SVI values. S. natans and type 1851 developed much faster than the others, and they were again rated as ‘‘abundant’’. Only a few of type 0092 were seen. The population of the unknown filamentous organism was rated between ‘‘common’’ and ‘‘very common’’. (2) Second week: Sludge bulking was very serious during the second week. The sludge flocs appeared diffuse and the activated sludge in the aeration basin turned slightly reddish brown. S. natans and type 1851 were still the two major filaments causing the serious sludge bulking. The scale rates for both filamentous bacteria were ‘‘abundant’’ (or ‘‘overabundant’’). Very serious filamentous growth in the system during this period can be detected from both pictures. Type 0092 was hardly seen and the population of the unknown filament was rated as ’’very common.’’ (3) Third week: Sludge settleability started to improve this week. The filaments of type 1851 appeared much less frequently as the week progressed. S. natans was still abundant in the first few days, and then its population started to decrease. Also, some sheaths of these filaments were found suspended in the liquor. Some of them were ‘‘blank’’; that is, there were no cells inside the filaments. The hanging filaments were very likely to be washed out from the system and result in better sludge settling in the clarifier. As mentioned before, the flocs appeared so tiny and fragile at this moment that they would not allow many filaments to bind in. Thus, most of the filaments suspended in
Fig. 7. Characteristics and Gram-stain of unknown filamentous organism: (a) straight filaments and attached growth (·1000) and (b) strong Grampositive reaction (·1000).
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
the liquor were washed out gradually with the effluent. This situation would certainly favor improved sludge settleability. Therefore, the SVI was much lower during this week even though the sludge flocs were tiny and fragile. In fact, the small flocs here did influence the TSS and turbidity of the effluent because according to the data presented before their values, TSS ¼ 20 mg/l and turbidity ¼ 14 NTU, were a little higher than those observed during normal conditions. However, a lot of S. natans and/or type 1851 were still found inside some of the tiny flocs. The population of both filaments was rated close to ‘‘very common’’ during this phase. A few of the unknown filaments were also observed, but type 0092 was never seen again. (4) Fifth week: Compared with the population of those filamentous organisms in the third week, more of them were observed during this week. The rate of Sphaerotilus became ‘‘abundant’’ again, but its filaments were shorter than before. A lot of type 1851 was observed again with the scale rate of ‘‘abundant’’. Similar to S. natans, the filaments of type 1851 did not cause the flocs to be seriously diffused. Type 0092 was still not observed. However, the population of the unknown filament was rated between ‘‘common’’ and ‘‘very common’’. Therefore, according to the results discussed above, it is believed that a new balanced-growth circumstance between flocs and filamentous bacteria had been established so that the sludge settleability remained very good. 3.4.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition S. natans and type 1851 were abundant again with many filaments extending outward from the flocs. These two species were still believed to be the primary ones causing the diffused flocs and sludge bulking. Again, the unknown filament was rated as ‘‘common’’ or ‘‘very common.’’ Type 0092 was not observed.
4. Conclusion Beyond question, synthetic polymers can be used to solve the sludge bulking problems. If they are used continuously, there will be possibly no problems. However, in the event that they are only used for a short period of time (for example, they are applied to temporarily resolve filamentous bulking or hydraulic overloading difficulties), then a serious problem may occur. That is, at the moment that the polymers are discontinued, the sludge settling will become much worse. According to the results of this study, filamentous bulking can be resolved by the addition of synthetic polymers either by physical/chemical means or by biological means. Sludge flocs with filamentous bulking
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agglomerated with polymer and formed bigger flocs. The filamentous bacteria were mostly found intertwining inside the flocs in the presence of synthetic polymers. Only some of the filamentous bacteria extended out of the flocs and their population was much less than that before the addition of polymer. The synthetic polymer used for this study did appear to inhibit the growth of filamentous bacteria, but might not have actually killed the filaments. It seems that not only were they protected by their sheaths, but they were also shielded by the flocs. Accordingly, as the polymer was halted they immediately extended out from the flocs to compete the substrate with floc-formers. And, this would cause very serious sludge bulking and poor settling problem. However, this bulking problem lasted for about one or two weeks. After the addition of synthetic polymer was halted for about three weeks, the flocs appeared tiny and fragile very soon. It is possible that the synthetic polymer remaining in the flocs might be completely washed out at that moment, and the polymer had seriously damaged the floc-formers. During this short period most of the filaments could not find places to attach, thus they were also washed out with the effluent. A new population balance was, thus, established in the activated sludge system. However, the sludge still displayed good settling during this period. As mentioned before, Sezgin et al. (1978) believed that the filamentous bacteria are the ‘‘backbones’’ of activated sludge flocs and the flocforming bacteria attach on them by means of extracellular polymers and form strong flocs. However, Chudoba et al. (1973) and Wanner and Grau (1989) have observed many well-flocculating and settling activated sludge systems that had no filamentous backbones. The results of this study have confirmed that floc formers and filamentous bacteria in fact rely on each other. Synthetic polymer will affect the growth of floc formers and cause tiny and fragile flocs. This will then adversely influence the growth of filamentous bacteria and cause the filaments to be washed out of the activated sludge system without bigger sludge flocs to support. Nevertheless, the activated sludge still displayed good settleability without bigger flocs. As a matter of fact, the filamentous bacteria are inhibited by synthetic polymer as well. Filamentous bacteria are protected by their sheath and strong flocs, therefore their damage will be less than floc formers. Without strong flocs to support, the filamentous bacteria will be washed out eventually. It normally takes at least one month for the activated sludge system to return to normal operation and for the filamentous bacteria to have similar population structure to the one prior to the addition of synthetic polymer. However, it will be recommended that in the future different synthetic polymers may be applied for the confirmation of the results in this study.
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References Chudoba, J. et al., 1973. Control of activated sludge filamentous bulking, part II––selection of microorganisms by means of a selector. Water Res. 7, 1389. Eikelboom, D.H., 1975. Filamentous organisms observed in activated sludge. Water Res. 9, 365–388. Franson, M.A.H., Greenberg, A.F., Trussell, R.R., Clescerl, L.S., 1985. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC, USA. Jenkins, D., Richard, M.G., Daigger, G.T., 1986. Manual on the causes and control of activated sludge bulking and foaming. Juang, D.F., 2003. Effects of synthetic polymer on activated sludge ecology. Proceeding of 9th IWA Specialised Conference––Design, Operation and Economics of Large Wastewater Treatment Plants, p. 208–211. Juang, D.F., Hwu, C.S., 2003. Agar-plated bacteria found in the activated sludge of lab-scale SBR and CFSTR systems. Bioresource Technology 89, 75–79. Juang, D.F., Morgan, J.M., 2001. The applicability of the API 20E and API Rapid NFT systems for the identification of bacteria from activated sludge. Electronic Journal of Biotechnology 4 (1), 1–7. Lau, A.O., Strom, P.F., Jenkins, D., 1984a. Growth kinetics of Sphaerotilus natans and a floc former in pure and dual continuous culture. J. WPCF 56 (1), 41–51. Lau, A.O., Strom, P.F., Jenkins, D., 1984b. The competitive growth of floc-forming and filamentous bacteria: a model for activated sludge bulking. J. WPCF 56 (1), 52–61.
Mueller, J.A., Morand, J., Boyle, W.C., 1967. Floc sizing techniques. Appl. Microbiol. 15 (1), 125–134. Nowak, G., Brown, G.D., 1990. Characteristics of Nostocoida limicola and its activity in activated sludge suspension. Res. J. WPCF 62 (2), 137–142. Pitt, P., Jenkins, D., 1990. Causes and control of Nocardia in activated sludge. Res. J. WPCF 62 (2), 143–150. Richard, M.G., 1989. Activated sludge microbiology, published by Water Pollution Control Federation. Rogovskaya, Z.I., Lazareva, M.F., 1959. Intensification of biological specification of activated sludge purifying various industrial sewages. Mikrobiologiya 28, 565–573. Salameh, M.F., Malina, J.F., 1989. The effects of sludge age and selector configuration on the control of filamentous bulking in the activated sludge process. Res. J. WPCF 61 (9), 1510– 1522. Sezgin, M., Jenkins, D., Parker, D.S., 1978. A unified theory of filamentous activated sludge bulking. J. WPCF, 362–381. US Environmental Protection Agency. The causes and control of activated sludge bulking and foaming. EPA Summary Report, EPA/625/8-87/012. van Veen, W.L., 1973. Bacteriology of activated sludge, in particular the filamentous bacteria. Anton. Leeuwen. 39, 189–205. van Veen, W.L., Mulder, E.G., Deinema, M.H., 1978. The Sphaerotilus–Leptothrix group of bacteria. Microbiol. Rev. 42 (2), 329– 356. Wanner, J., Grau, P., 1989. Identification of filamentous microorganisms from activated sludge: a compromise between wishes, needs and possibilities. Water Res. 23 (7), 883–891.
Effects of synthetic polymer on the filamentous bacteria in activated sludge Der-Fong Juang
*
Department of Healthcare Administration, Mei-Ho Institute of Technology, 23, Ping-Kuang Road, Nei Pu, Ping Tong 912, Taiwan, ROC Received 1 November 2001; accepted 5 March 2004 Available online 16 April 2004
Abstract Filamentous bulking is one of the solid–liquid separation problems always seen in activated sludge process. The addition of synthetic polymer is always one of the popular ways for the treatment plant operator to immediately solve the poor sludge settling problem. Therefore, it may be interesting to understand the effects of synthetic polymer on the filamentous bacteria in activated sludge. In this study, synthetic polymer was applied to a lab-scale wastewater treatment system with the filamentous bulking problem. The population structure of filamentous bacteria and sludge characteristics were investigated under different conditions. When synthetic polymer was added into the system, it was found that poor sludge settleability caused by filamentous bulking was temporarily solved and filamentous branches growing outside the flocs were damaged or inhibited. However, filamentous growth was still observed inside the flocs. After the addition of polymer was halted, filamentous branches extended out of the flocs immediately. Very serious filamentous bulking occurred and sludge settleability became much worse than that occurring before the addition of polymer. And, it took several weeks for the system to return to normal operation. 2004 Elsevier Ltd. All rights reserved. Keywords: Synthetic polymer; Activated sludge; Filamentous bulking
1. Introduction According to Sezgin et al. (1978), the filamentous bacteria are believed to form the ‘‘backbones’’ of activated sludge flocs to which floc-forming bacteria attach by means of extracellular polymers and form strong flocs. In the absence of filaments, ‘‘pin-flocs’’ will be formed. These tiny and weak flocs will tend to contribute to secondary effluent turbidity. Filamentous bulking occurs if filamentous bacteria grow overabundantly, and it is a condition in which sludge settling rates decrease and the thickening characteristics of the settled sludge are poor (Lau et al., 1984a,b). Sludge volume index (SVI) values higher than 150 ml/g are often associated with filamentous bulking. Many investigators have studied filamentous bulking control for many years. From review of the literature, it seems that certain specific filamentous bacteria cause most sludge bulking episodes. Since actions that will
*
Tel.: +886-8-7799821x331; fax: +886-8-7780657. E-mail address: [email protected] (D.-F. Juang).
0960-8524/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.03.005
control one species may not affect others, one is likely to fail to control activated sludge bulking unless the specific causative filamentous microorganisms can be identified. Many filamentous bacteria that cause sludge bulking have been classified (Jenkins et al., 1986; Lau et al., 1984a,b; Richard, 1989), although newly isolated ones are occasionally reported. Jenkins et al. (1986) have proposed that the presence of certain filaments indicates the specific environmental conditions leading to activated sludge bulking. Examples include low DO, low F/M, low pH, increased concentration of sulfides, nutrient deficiency, and so on. Many methods are used by plant operators to control sludge bulking, including chlorine addition, hydrogen peroxide addition, aeration basin pH control, nutrient addition, aeration control, anaerobic selector application, and so on. Synthetic polymer has also been popularly used to improve sludge settleability in many wastewater treatment plants, especially when the poor settling problem is caused by temporary hydraulic overloading. The polymer is usually added either to the activated sludge as it leaves the aeration basin or to the secondary clarifier center well (for circular clarifier).
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
state operation was achieved. At this point, samples were again taken and analyzed as at the end of Phase I. Polymer addition was then halted. During Phase III the activated sludge system was again operated without polymer addition. In order to investigate the effects of polymer on the filamentous bacteria and observe any possible change on the sludge characteristics, samples were taken more frequently and analyzed for numerous parameters during this phase. For those reasons, Phase III was then separated into two stages, which are called Phase III-A and Phase III-B.
Richard (1989) mentioned that synthetic polymer could overcome the physical effects of filaments on sludge settling. There are, however, no detailed reports available specifying how synthetic polymer affects the filamentous bacteria.
2. Methods 2.1. Experimental design To investigate the effects of synthetic polymer on the filamentous bacteria in activated sludge, the whole research period was divided into three phases shown in Fig. 1 (Juang, 2003). During Phase I, no polymer was used, and the system was allowed to operate until steady-state operation was achieved. Stability tests of the system were routinely performed during this phase to see if stable operation had been attained. When the system was determined to be stable, sludge and effluent samples were taken for floc size measurement, identification of filamentous bacteria, analysis of total suspended solids, and so on. After sludge and effluent samples were taken for analysis, Phase II started immediately. In the beginning of Phase II, a synthetic polymer was added to the influent of clarifier and the system was continuously operated. Again, samples were taken and analyzed over a period of time to determine if steady
After Confirmation of Steady State
After Confirmation of Steady State
(~2 months)
Phase I (Prior to the Addition of Synthetic Polymer)
This study was conducted using one lab-scale continuous flow stirred tank reactor (CFSTR) as shown in Fig. 2 (Juang and Morgan, 2001; Juang and Hwu, 2003). The operating data and the feed composition for this CFSTR is described in Table 1 (Juang and Morgan, 2001). A small cubical mixing tank was located between the aeration basin and the clarifier. During Phase II polymer was introduced into this mixing tank. A magnetic stirrer was used to mix the sludge and the polymer. Concentrated (pure) synthetic polymer was diluted 400fold with distilled water before it was used. Based upon a ratio of 1 ml of the concentrate per 10 l of activated sludge, the diluted synthetic polymer solution was continuously pumped into the mixing tank at a flow rate of
After Confirmation of Steady State
Discontinuation of Polymer Addition
Polymer Addition
Operation Start
2.2. System design and operating data
(~2 months)
(~5 weeks)
Phase II
Phase III-A
(After the Addition of Synthetic Polymer)
(~8 weeks)
Phase III
Phase III-B
(After the Discontinuation of Synthetic Polymer Addition)
Fig. 1. Experimental design.
Air Sludge Recycle Feed
Aeration Basin 30L
30L/day Feed
Synthetic Polymer
Sludge Recycle 30L/day
Pump ~
From Feed Container
Scraper
Effluent (To nearby sink) 30L/day
Clarifier
M
7.35L M: Motor
Fig. 2. Bench scale CFSTR system.
Operation End
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
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Table 1 Operating data and feed composition of CFSTR Operating data of CFSTR
Feed Composition of CFSTR
Reactor
CFSTR
Material
Conc., mg/l
Material
Conc., mg/l
Volume feed rate (Aeration Basin) Aeration basin volume Clarifier volume Residual basin volume Volume wasted Hydraulic retention time (Aeration Basin) Solids retention time Sludge recycling rate
30.0 30.0 7.35 – 1.00 1.00 30.0 30.0
Nutrient Broth KH2 PO4 NaOH CaCl2 Æ 2H2 O MgSO4 Æ 7H2 O Glucose KNO3 NaCl
300.0 44.0 16.7 132.4 100.0 140.0 3.0 100.0
FeCl3 Æ 6H2 O MnSO4 Æ H2 O (NH4 )2 SO4 NaHCO3
5.0 12.8 118.4 467
(l/day) (l) (l) (l/day) (day) (day) (l/day)
2.48 liters per day. The resulting polymer dose was equivalent to 1 ml of pure polymer to 10 l of wastewater, a typical dose used in practice. The synthetic polymer used in this study was manufactured by Keystone Labs, Inc., USA. It was marketed specifically as Keystone KF 4249 ‘‘Mannequin Polymer.’’ The polymer has the molecular weight of (2.5–3.5) · l06 , pH of 9–11, absolute viscosity of (16–19) · l05 cps, and bulk density of 112.3 1b/ft3 . 2.3. Confirmation of equilibrium (stability tests) Routine feed and effluent samples were used to check for stable operation. Many tests were performed following the procedures in Standard Methods (Franson et al., 1985) to ensure that the system had reached equilibrium, such as MLVSS, COD, TKN, NH4 -N, NO3 -N, NO2 -N, pH, and DO. Each test was generally performed on at least three consecutive days and the data were averaged. 2.4. Analysis of effluent and sludge characteristics The sludge settling characteristics, the sludge volume index (SVI), and the suspended solids and turbidity of the effluent was observed or analyzed during each Phase. Each test was conducted for at least four days, and the average value was then obtained. Floc size was measured using a technique, called the microscopic sizing method (Mueller et al., 1967). A few drops of mixed sludge sample were taken directly from aeration basin by a pipet with an inside diameter at the mouth of 0.5 cm, and carefully added into a petri dish. The flocs were then solidified by introducing them into a diluted, cooled agar suspension (3 g agar in 100 ml of distilled water) with gentle mixing provided by smoothly rotating the dish. After being solidified, the length and breadth of the flocs were measured with an ocular micrometer and microscope at 100 times magnification (100·). In general, hundreds of flocs from each petri dish were randomly chosen for measurement, and at least three to four dishes were tested with one dish prepared each day.
The average floc size distribution for each Phase was then obtained. 2.5. Identification of filamentous microorganisms Samples were taken from the aeration basin and dried on slides. After drying, the samples were Gram-stained and Neisser-stained, respectively, following the procedures described in the manual on the Causes And Control of Activated Sludge Bulking and Foaming (Jenkins et al., 1986). Crystal violet sheath stains and sulfur oxidation tests were also performed following the procedures described in that manual. The physiological characteristics of each filamentous bacteria, such as size (filament length and diameter), cell shape, attached growth, branching, cell septa, filament dispersion, motility, etc., were observed through a microscope (Eikelboom, 1975; Jenkins et al., 1986; Nowak and Brown, 1990; Pitt and Jenkins, 1990; Rogovskaya and Lazareva, 1959; Salameh and Malina, 1989; US EPA, 1987; van Veen, 1973; van Veen et al., 1978). All data collected for each organism were compiled and compared to the specification of each filamentous bacterium listed in the manual (Jenkins et al., 1986). In many cases, the identification could be completed in this manner. However, additional testing was sometimes required. The numerical population of the various filamentous organisms was estimated according to the rating scale described by Jenkins et al. (1986). The relative abundance of filamentous organisms was quantitated according to seven categories: none, few, some, common, very common, abundant, and excessive. This was accomplished by observing the filaments using a microscope at 100· magnification.
3. Results The effluent quality, sludge characteristics, floc size, and filamentous bacteria in CFSTR during each Phase
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Phase II
Phase III-A
Phase III-B
Phases
Phase I
Items and operating status
Before filamentous bulking
After filamentous bulking
Polymer addition
1st week after polymer addition was halted
2nd week after polymer addition was halted
3rd week after polymer addition was halted
5th week after polymer addition was halted
12th week after polymer addition was halted
Effluent
TSS (mg/l) Turbidity (NTU) SVI (ml/g) Floc size (lm) Description
2 1.1 150–165 – Regularly normal flocs
7.83 4–8 350–360 254 Flocs with filaments
2 1 99–131 631 Strong flocs
17.5 11 310 261 Broad floc size distribution
8 4–11 300–385 240 Normal flocs with filaments
– – – –
Abundant Abundant Very Common Common
Very common Very common Common Common
Abundant Abundant Few Very common
20 14 158 136 Tiny and fragile flocs with many blank filaments Very common Very common None Few
2 4.5–14.7 133 245 Normal flocs
Sphaerotilus natans Type 1851 Type 0092 Unknown
254 50 410 – Very serious filamentous bulking Excessive Excessive None Very common
Abundant Abundant None Common
Abundant Abundant None Common
Sludge charact.
Filamentous bacteria
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Table 2 Effluent quality, sludge characteristics, floc size, and filamentous bacteria in CFSTR during each phase
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
were depicted in Table 2 (Juang, 2003), and the results of this study were described in the followings. 3.1. Effluent quality and sludge characteristics 3.1.1. Phase I: Prior to the addition of synthetic polymer During the first month after start-up, the system operated well. The sludge settled readily and clear effluent was obtained. The average values of total suspended solids and turbidity in the effluent were about 2 mg/l and about 1.1 NTU, respectively. The SVI was in the range between 150 and 165 ml/g with the average being about 155 ml/g. However, after one month filamentous bulking occurred. This resulted in an increased sludge volume in the clarifier. The SVI increased rapidly from 250 to 360 ml/g in only a few days, then its value held steady in the 350–360 ml/g range for two more weeks without getting worse. At this time, the clarifier was always about halffilled with sludge. The total suspended solids in the effluent rose to an average value of 7.83 mg/l. The turbidity also increased, ranging from 4.0 to 8.0 NTU. 3.1.2. Phase II: After the addition of synthetic polymer Two days after the CFSTR system began to receive the synthetic polymer the settling characteristics of the activated sludge in the clarifier improved greatly. Apparently the adverse effects of the filamentous organisms were completely overcome. The SVI varied within the range of 99–131 ml/g, with an average value of 114.5 ml/g. The activated sludge flocs were visibly much bigger and firmer. The total suspended solids in the effluent dropped to about 2.0 mg/l, and the turbidity decreased to an average value of about 1.0 NTU. 3.1.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition (1) First week: In a few days after the discontinuation of synthetic polymer, the sludge settleability became worse. Because some polymer was still remaining in the system during the first one or two days, the SVI value thus increased gradually from 193 to
35
310 ml/g. The total suspended solids in the effluent rose to 17.5 mg/l in about one week. The turbidity of the effluent increased to a value within the range of 10–12 NTU. (2) Second week: During this week, sludge settleability continued to worsen, and the SVI rose from 310 to 410 ml/g. It is obvious that very serious filamentous bulking occurred in the system, and Fig. 3 illustrates the problem. The clarifier was almost filled up with sludge and the activated sludge in the clarifier hardly settled at all. This situation led to great amounts of solids being washed out with the effluent and the total suspended solids in the effluent increased to 254 mg/l. Similarly, the turbidity of the effluent rose to 50 NTU. (3) Third week: During the third week, the sludge settleability in the clarifier started to improve and the SVI gradually dropped to 158 ml/g. The total suspended solids in the effluents also dropped to about 20 mg/l. In the meantime the turbidity of the effluent ranged between 8 and 16 NTU with an average value of 14 NTU. (4) Fourth and fifth weeks: During the fourth and fifth weeks, the system was operating well and the sludge settleability was normal again. The SVI varied from 129 to 137 ml/g with an average value of 133 ml/g. The total suspended solids in the effluent dropped to only 2.0 mg/l and the turbidity of the effluent decreased to between 4.5 and 14.7 NTU. 3.1.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition During this period, the SVI was always in the range of 300–385 ml/g. The total suspended solid in the effluent was about 8.0 mg/l, and the turbidity was between 4.0 and 11.0 NTU. 3.2. Stability tests The stability tests were conducted for 5 consecutive days at the end of each Phase, and the average values were shown in Table 3. It is obvious that after an
Fig. 3. Very serious bulking occurred at the second week of Phase III-A: (a) normal operation, (b) filamentous bulking at Phase I and (c) very serious bulking at Phase III-A.
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
Table 3 Stability tests Item
pH Alk. BOD COD TKN-N NH3 -N NO3 -N NO2 -N PO4 -P
Phase I
Phase II
Phase III-A
Phase III-B
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
Feed (mg/l)
Effluent (mg/l)
7.80 290.00 316.67 389.67 45.19 28.44 0.71 0.01 6.86
6.41 19.60 1.74 33.10 0.26 0.17 57.77 0.64 8.72
7.85 316.00 342.00 490.80 94.20 32.20 0.20 0.03 9.80
5.90 10.00 2.50 5.30 0.40 0.38 68.00 1.07 9.10
8.00 333.30 395.00 458.00 74.60 24.80 0.20 0.004 5.60
6.12 26.70 2.63 65.50 0.49 0.37 61.00 0.31 7.10
8.00 333.30 299.00 354.84 58.04 24.65 0.65 0.03 8.00
6.40 26.70 1.80 20.69 0.46 0.22 54.80 2.44 6.40
Note: Phase I––Aeration Basin: ¼ 2035 mg/l, DO ¼ 5.6–6.4 mg/l. Phase II––Aeration Basin: MLVSS ¼ 2153 mg/l, DO ¼ 6.2 mg/l. Phase III-A–– Aeration Basin: MLVSS ¼ 2073 mg/l, DO ¼ 6.7 mg/l. Phase III-B––Aeration Basin: MLVSS ¼ 2150 mg/l, DO ¼ 6.4 mg/l.
acclimation period of each Phase the activated sludge system operated normally with respect to substrate removal and nitrification. The pH value in the effluent was always above 5.9, and the dissolved oxygen in the aeration basin was usually between 5.6 and 6.7 mg/l. Therefore, this system was determined to be stable (even though some filamentous bulking was occurring). 3.3. Floc size distribution 3.3.1. Phase I: Prior to the addition of synthetic polymer The average floc size was mostly in the range between 150 and 200 lm with an average value of 254 lm. The general characteristics of activated sludge flocs are evident from Fig. 4(a). The flocs shown are irregularly shaped and display some filamentous growth. Basically, the activated sludge flocs during this phase could be described as healthy and large. 3.3.2. Phase II: After the addition of synthetic polymer In general, the floc size during this phase was much larger than before and was mostly in the range between 500 and 600 lm with an average value of 631 lm. The general characteristics of the activated sludge flocs with the growth of filamentous bacteria during this
phase are presented in Fig. 4(b). The flocs appeared much bigger and firmer than before. Only a few filaments were observed to extend out from the flocs. Therefore, the addition of synthetic polymers during this phase apparently not only enhanced the sludge flocculation and caused the formation of larger and heavier flocs, but it had also inhibited (or at least overcome the effects of) the growth of the filamentous organisms. 3.3.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition Since the sludge settling characteristics changed rapidly after the discontinuation of synthetic polymer, floc size measurements were made every two weeks during this phase of the study. (1) First week: Since some synthetic polymer might still remain in the aeration basin during the first week of this phase, the sludge floc size distribution spread out to a broad extent. Most of the sludge flocs however were still sized between 100 and 200 lm, and the average value was about 261 lm. (2) Third week: After two weeks, the flocs appeared tiny and fragile. Most of the sludge flocs had a size of
Fig. 4. Activated sludge flocs at diiferent phases: (a) flocs at phase I (·200), (b) flocs at phase II (·200) and (c) flocs at the third week of Phase III-A (·200).
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
about 100 lm, more or less. The average floc size was about 136 lm, however many flocs sized less than 100 lm were observed during this week. Typical tiny and fragile flocs taken from the aeration basin are pictured in Fig. 4(c). The filamentous backbones of flocs can be easily seen. However, numerous filaments were washed out with the effluent because they were not bound up in large, heavy sludge flocs. Ultimately, the washout of the filaments resulted in improved sludge settling during the third week. As a result, the SVI dropped rapidly. (3) Fifth week: The dimension of the activated sludge flocs during the fifth week increased and size distribution returned to about the same range as before polymer addition was begun. Most of flocs had a size between 150 and 200 lm with the average value of 245 lm.
3.3.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition Most of the flocs are sized between 100 and 200 lm with the average value of about 240 lm. That is, the
37
sludge flocs in this system during this phase were in a normal size range. 3.4. Filamentous bacteria 3.4.1. Phase I: Prior to the addition of synthetic polymer Generally, four kinds of filaments were observed. Sphaerotilus natans and Type 1851 were two of the most predominant filamentous bacteria, and they were found ‘‘abundant’’ in this sludge. False branches, Gram-negative and Neisser-negative reactions, rod cells, and sheath presence are the major characteristics of S. natans. Fig. 5(a) and (b) show the false branches and sheath stain of this bacterium, respectively. S. natans is commonly found in activated sludge treatment plants in the USA. Type 1851 always grows in bundles and away from the flocs. Its characteristics and Gram-stain can be seen from Fig. 6(a) and (b), respectively. The presence of Type 1851 tends to cause diffused activated sludge flocs. When attached growth was noted it was always found perpendicular to the filaments. Weak Gram-positive and Neisser-negative reactions were also observed.
Fig. 5. False branches and sheath of Sphaerotilus natans: (a) false branching (·200) and (b) sheath stain (·1000).
Fig. 6. Characteristics and Gram-stain of Type 1851: (a) growth in bundles (·200) and (b) Gram-positive reaction (·1000).
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D.-F. Juang / Bioresource Technology 96 (2005) 31–40
From microscopic observation of the sludge flocs, it is believed that the abundant growth of these two filaments was the reason for the sludge bulking previously noted. Type 0092 was found ‘‘very common’’ in this sludge. It exhibited a Gram-negative reaction and very short filaments extending outward from the flocs. Since most filaments of Type 0092 are usually within the flocs, it is often difficult to observe. No attached growth was found on the surfaces of its filaments. Generally, it is believed that the growth of type 0092 in this system was insignificant with respect to leading to sludge bulking. One other filamentous organism was also found in this system, but at a much lower concentration. This organism had the characteristics of strong Gram-positive reaction, granular deposits on the surface of the filaments, attached growth, and no branching. Its filaments, extending out from the flocs, appeared mostly straight and had a diameter of more than 1.0 lm. Fig. 7(a) and (b) show the characteristic and the Gram-stain reaction of this filament, respectively. Complete identification of this organism was not possible. Because of its low population level, its effect on the sludge bulking was possibly not very significant. 3.4.2. Phase II: After the addition of synthetic polymer All four previously discussed types of filamentous bacteria were still observed in the activated sludge after the addition of synthetic polymer. However, most of filamentous organisms intertwined inside the flocs, and only some of the filaments associated with S. natans and type 1851 extended out from the flocs a short distance. Both S. natans and type 1851 were rated as ‘‘very common’’. The S. natans population bordered on the ‘‘abundant’’ category since many filaments were noticed stretching out from the flocs. The population of type 0092 and the ‘‘unknown’’ organism were rated as ‘‘common’’. In general, filamentous growth appeared to be inhibited by the addition of synthetic polymers.
However, they were still present intertwining inside the flocs. 3.4.3. Phase III-A: Five weeks after the discontinuation of synthetic polymer addition (1) First week: After the discontinuation of synthetic polymer, filamentous bulking started immediately. Filaments extended outward from the flocs very rapidly, and this resulted in higher SVI values. S. natans and type 1851 developed much faster than the others, and they were again rated as ‘‘abundant’’. Only a few of type 0092 were seen. The population of the unknown filamentous organism was rated between ‘‘common’’ and ‘‘very common’’. (2) Second week: Sludge bulking was very serious during the second week. The sludge flocs appeared diffuse and the activated sludge in the aeration basin turned slightly reddish brown. S. natans and type 1851 were still the two major filaments causing the serious sludge bulking. The scale rates for both filamentous bacteria were ‘‘abundant’’ (or ‘‘overabundant’’). Very serious filamentous growth in the system during this period can be detected from both pictures. Type 0092 was hardly seen and the population of the unknown filament was rated as ’’very common.’’ (3) Third week: Sludge settleability started to improve this week. The filaments of type 1851 appeared much less frequently as the week progressed. S. natans was still abundant in the first few days, and then its population started to decrease. Also, some sheaths of these filaments were found suspended in the liquor. Some of them were ‘‘blank’’; that is, there were no cells inside the filaments. The hanging filaments were very likely to be washed out from the system and result in better sludge settling in the clarifier. As mentioned before, the flocs appeared so tiny and fragile at this moment that they would not allow many filaments to bind in. Thus, most of the filaments suspended in
Fig. 7. Characteristics and Gram-stain of unknown filamentous organism: (a) straight filaments and attached growth (·1000) and (b) strong Grampositive reaction (·1000).
D.-F. Juang / Bioresource Technology 96 (2005) 31–40
the liquor were washed out gradually with the effluent. This situation would certainly favor improved sludge settleability. Therefore, the SVI was much lower during this week even though the sludge flocs were tiny and fragile. In fact, the small flocs here did influence the TSS and turbidity of the effluent because according to the data presented before their values, TSS ¼ 20 mg/l and turbidity ¼ 14 NTU, were a little higher than those observed during normal conditions. However, a lot of S. natans and/or type 1851 were still found inside some of the tiny flocs. The population of both filaments was rated close to ‘‘very common’’ during this phase. A few of the unknown filaments were also observed, but type 0092 was never seen again. (4) Fifth week: Compared with the population of those filamentous organisms in the third week, more of them were observed during this week. The rate of Sphaerotilus became ‘‘abundant’’ again, but its filaments were shorter than before. A lot of type 1851 was observed again with the scale rate of ‘‘abundant’’. Similar to S. natans, the filaments of type 1851 did not cause the flocs to be seriously diffused. Type 0092 was still not observed. However, the population of the unknown filament was rated between ‘‘common’’ and ‘‘very common’’. Therefore, according to the results discussed above, it is believed that a new balanced-growth circumstance between flocs and filamentous bacteria had been established so that the sludge settleability remained very good. 3.4.4. Phase III-B: Three months after the discontinuation of synthetic polymer addition S. natans and type 1851 were abundant again with many filaments extending outward from the flocs. These two species were still believed to be the primary ones causing the diffused flocs and sludge bulking. Again, the unknown filament was rated as ‘‘common’’ or ‘‘very common.’’ Type 0092 was not observed.
4. Conclusion Beyond question, synthetic polymers can be used to solve the sludge bulking problems. If they are used continuously, there will be possibly no problems. However, in the event that they are only used for a short period of time (for example, they are applied to temporarily resolve filamentous bulking or hydraulic overloading difficulties), then a serious problem may occur. That is, at the moment that the polymers are discontinued, the sludge settling will become much worse. According to the results of this study, filamentous bulking can be resolved by the addition of synthetic polymers either by physical/chemical means or by biological means. Sludge flocs with filamentous bulking
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agglomerated with polymer and formed bigger flocs. The filamentous bacteria were mostly found intertwining inside the flocs in the presence of synthetic polymers. Only some of the filamentous bacteria extended out of the flocs and their population was much less than that before the addition of polymer. The synthetic polymer used for this study did appear to inhibit the growth of filamentous bacteria, but might not have actually killed the filaments. It seems that not only were they protected by their sheaths, but they were also shielded by the flocs. Accordingly, as the polymer was halted they immediately extended out from the flocs to compete the substrate with floc-formers. And, this would cause very serious sludge bulking and poor settling problem. However, this bulking problem lasted for about one or two weeks. After the addition of synthetic polymer was halted for about three weeks, the flocs appeared tiny and fragile very soon. It is possible that the synthetic polymer remaining in the flocs might be completely washed out at that moment, and the polymer had seriously damaged the floc-formers. During this short period most of the filaments could not find places to attach, thus they were also washed out with the effluent. A new population balance was, thus, established in the activated sludge system. However, the sludge still displayed good settling during this period. As mentioned before, Sezgin et al. (1978) believed that the filamentous bacteria are the ‘‘backbones’’ of activated sludge flocs and the flocforming bacteria attach on them by means of extracellular polymers and form strong flocs. However, Chudoba et al. (1973) and Wanner and Grau (1989) have observed many well-flocculating and settling activated sludge systems that had no filamentous backbones. The results of this study have confirmed that floc formers and filamentous bacteria in fact rely on each other. Synthetic polymer will affect the growth of floc formers and cause tiny and fragile flocs. This will then adversely influence the growth of filamentous bacteria and cause the filaments to be washed out of the activated sludge system without bigger sludge flocs to support. Nevertheless, the activated sludge still displayed good settleability without bigger flocs. As a matter of fact, the filamentous bacteria are inhibited by synthetic polymer as well. Filamentous bacteria are protected by their sheath and strong flocs, therefore their damage will be less than floc formers. Without strong flocs to support, the filamentous bacteria will be washed out eventually. It normally takes at least one month for the activated sludge system to return to normal operation and for the filamentous bacteria to have similar population structure to the one prior to the addition of synthetic polymer. However, it will be recommended that in the future different synthetic polymers may be applied for the confirmation of the results in this study.
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