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Severe bulking and foaming at the Himmerfjärden WWTP
e:>
Pergamon
Wal. ScL Tech. Vol. 33. No. 12. pp. 127-135.1996. CopyrighllC 1996 IAWQ. Published by Elsevier Science LId Pnnled In Greal Dntain. All rights reserved. 0273-1223/96 $15'00 + ()o()O
PH: S0273-1223(96)00466-0
SEVERE BULKING AND FOAMING AT THE HIMMERFJARDEN WWTP Hans Holmstrom, Jan Bosander, Alf-Goran Dahlberg, Asa Dillner-Westlund, Lena Flyborg and Karri Jokinen Southwestern Stockholm Region Water and Sewage Works Inc.• SYVAB. S-/4792 GrOdinge. Sweden
ABSTRACf Several years of trouble-free operation with pre-denitrification at the Himmerfjilrden WWTP concerning bulking and foaming have been followed by an unstable period. Two bulking incidents with no foaming by Microlhrix parvicella were found to be caused by ethanol addition. Other bulking and foaming occurrences by Microthrix could be cOlTelated to the production of volatile fatty acids in some of the sludge treatment steps. It is possible that a more widespread use in society of washing and cleaning products based on vegetable oils has promoted the ability of Microthrix to compete with noc-forrning organisms. especially when ethanol or volatile fatty acids are present. Copyright
KEYWORDS BUlking; ethanol; fatty acids; foaming; Microthrix. INTRODUCfION Demands to introduce nitrogen removal at larger Swedish coastal WWTPs meant that SYVAB decided on a voluntary basis to change the inlet zones of the existing aeration tanks to anoxic zones for pre-denitrification in 1987. The operational problems during the first five years of full scale operation were mainly loss of nitrification due to small aerobic reactor volumes and toxic wastewater from a pharmaceutical company. A long tunnel system also decreased the readily biodegradable fraction of the wastewater, which made denitrification incomplete. The sludge quality index (SQI) during this period was normally in the range ISO200 mUg and no outbreaks of brown foam ever occurred on the aeration tanks. During 1993, the situation changed and from then to the present there have been periodic high SQIs and sometimes also intense fOaming. These new problems took the plant staff by surprise and no experience on how to handle the new Situation was at hand. PLANT DESCRIPTION !he Himmerfjllrden WWTP serves the southwestern part of the Stockholm area and was put into operation tn 1974. It is a conventional plant originally designed for BOD- and P-removal (by post-precipitation). In Figure I the main treatment units are shown after the change to pre-denitrification. The DWF (dry weather flow) is about 100 000 m3/d. 127
H. HOLMSTR6M et al.
128
Figure I. The Himmerfjarden WWTP with pre-denitrificalion.
The biological step (including the post-precipitation units) consists of eight parallel lines. The return sludge is mixed for lines 1-4 (block A) and lines 5-8 (block B) and then distributed to the corresponding anoxic zone in the eight lines. During periods with high flows and high SQls a considerable amount of activated sludge is lost to the post clarifiers, to which no chemicals are added nowadays (only precipitation with Fe 2+). From the post clarifiers the collected sludge was previously pumped back to the return sludge systems in block A and B through a common pipe. The two blocks did not therefore operate completely independently of one another. This was changed in May 1994, after which date the plant can be regarded as two independent subplants. At DWF. the detention time (excluding return sludge) is about 45 minutes in the anoxic zones and four hours in the aeration tanks. Only the conventional return sludge system is used to recycle nitrate to the anoxic reactors. The aerobic sludge age is kept to between five to ten days depending on the water temperature. To improve the denitrification (aim. 50% N-removal) an eJhanol solution from a pharmaceutical company (70% ethanol) is almost continuously added directly to the return sludge (about 12 mgll COD from ethanol). The lack of readily biodegradable organic matter and small anoxic reactors implies that denitrification (ON) is normally not complete. The shortage is more pronounced during weekends and holidays. Methanol is therefore also added d.Jring such periods. METHODS A modified method to determine the sludge quality index (SQI) was used twice a week to evaluate the settling properties of the activated sludge. The key point of this modification is to obtain sludge volumes in a 1000 mI cylindrical measurement glass in the range 150-600 mVI. To reach this range it is often necessary to dilute the sludge samples collected from the aeration tanks with treated wastewater. The following formula is then used to calculate the SQI-value: 150 < SV S 300 mVl
SQI = n x SV/SS x 1000
=n (SVI3 + 2(0)/SS x 1000 where n = 1 for no dilution, n = 2 for dilution 1 + I and n =3 for dilution 2 + I (2 parts water + I part 300 < SV < 600 mVl
sludge).
SQI
Severe bulking and foaming
129
Measurement of fatty acids was performed with gas chromatography and a flame ionozation detector. RESULTS In Figure 2 the SQI-values for block A are shown from September 1992 to November 1994. Several distinct peaks can be noted in the figure.
Sludge Quality Index, block A
:[: :.: :.';.'.: :~.'.::: ::: ~:.'.: ':.' '.':: :. "~'.: ':'.::: :::.::::: :: : 4OO~
:
: :
; : :.. :
3Se! : : 300!· ..:····;···..···:··..:····· 25e~"'~"''''''/'';
200;"':' .
. 150: / ;
. :
l00! .. : 50:'
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."
.
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"
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'j" 'j''':''
:....... . :".:11' ',Ii':'•• : "i" 'j . '~'"i''' ~ ,
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199209010000
I
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199411090000
Figure 2. SQls in block A from September 1992 to November 1994.
&tiod I (September 1992 - April 1993) Two SQI-peaks appeared during period I. The first one coincided with increased removal of suspended Solids (SS) in the primary clarifiers (Sept 1 - Oct 20) to stimulate nitrification. This was achieved by adding about 2 glm3 of a polymer to the inlet of the aerated sand traps and about 0.2 g/m3 of another polymer to the outlet. The effluent SS-concentration fell to about 80 mgll from about 140 mgll. Microscopic evaluation showed that Microthrix parvicella (M p) was the main filament in the activated sludge. No viscous brown foam was observed. As the SQIs improved after the polymer addition had ceased, it was supposed that this operation mode was responsible in some way for the deteriorated sludge quality. The second peak lasted three months. At the end of 1992, nitrification was excellent inspite of water temperatures around 12°C. During the first week in January the ethanol addition was increased three times. A higher addition than normal was then maintained to the end of February. No addition of ethanol or rnethanol took place during the last weeks of 1992. Microscopic examination of the activated sludge again shOwed a large number ofM p during SQI-peak, but also some type 0041 and Nostocoida limicola. An evaluation of operating data showed that pH-drops down to 5.0 had occurred during the holidays at the turn of the year. Such low pH-values had never been observed before in combination with nitrogen removal. The explanation can be found in increased loads of nitrogen to the plant in combination with excellent nitrification, inefficient denitrification (lack of suitable organic matter) and low buffer capacity. The nitrogen loads can be explained by increased intake of food and beverages dUring the Christmas and New Year festivities. A literature survey showed that low pH-values can result in bulking sludge, normally by grOwth of fungi. The gradual increase of the SQIs up to the beginning of March, however, indicated that the Pli-drop was probably not entirely responsible for the bulking. With aerobic sludge ages of about seven days the maximum SQI caused by low pH-values at the end of 1992 should be expected during January (less than three sludge ages).
H. HOLMSTROM et al.
130
Period II: May 1993 tQ April 1994
The beginning Qf this period shQwed increasing (and scattered) SQIs by grQwth Qf M pup tQ early July, in spite Qf increasing water temperatures. This phase was fQnQwed by a decrease during the summer vacaliQn periQd (July). The period frQm the end Qf OctQber tQ the beginning Qf February is again characterized by increasing SQIs. BrQwn visCQUS fQam was nQled fQr the first time Qn the aeratiQn tanks during June 1993. The amQunt was, hQwever, modest during 1993, (20% covered aeration tank surface). During period II SQme changes in the sludge treatment took place: •
An indirect sludge dryer was started in late April 1993. Due to mechanical prQblems the dryer never operated mQre than six fun days each mQnth during 1993 and 1994. No QperatiQn at all went on during July 1993.
•
Gravity-thickened waste-activated sludge (WAS, OS: 1.5-2.5%) was, after OctQber 8, mixed with anaerQbically digested primary sludge (PS) in a holding tank. Mixed sludge was then dewatered with centrifuges (OS: 25%). The purpose Qf this change was tQ limit the recirculatiQn Qf ammQnia via the centrate frQm the centrifuges.
•
After mid-September 1993 all ethanQI was dosed only to block A.
In January 1994, the amQunt Qf brQwn fQam on the aeratiQn tanks started tQ increase and culminated during March and April with up tQ Qne meter of fQam in the first aerated tank (tWQ in series with a submerged cQnnection): the fQam was trapped in the first tank. Loss of fQam frQm the secQnd aeratiQn tank tQ the clarifiers resulted after SQme mQnths in about a half-meter thick layer of flQating sludge Qn the inlet ZQnes. InfQrmatiQn in the literature stated that M p does nQt grow at pH-values belQW 7.1 and at sludge ages belQW eight days (Jenkins, 1992). A first step was therefQre to decrease the aerQbic sludge age to below five days. The nitrificatiQn was then IQst but the pH-value did nQt exceed 6.8. The normal pH-range with nitrQgen remQval is 6.0-6.5. These steps did nQt at all hamper the M p populatiQn. AnQther hypothesis at the time (late 1993) was that lack of phosphQrus CQuid give the M p pQpulatiQn an advantage. The addition Qf ferrQus sulphate was therefore gradually reduced frQm mid-December until the effluent sQluble phQsphate had increased to about 0.2 mg UJ, which happened in mid February. The average additiQn was kept arQund 3 g Fe 2+/m 3 fQr six weeks (normal dose 16 g Fe 2+/m 3). It was Qbvious that the biQIQgical P-removal was very efficient. The SQIs alSQ started to fall (see Figure 2) but the foam layer on the aeratiQn tanks instead increased dramatically. At the end of March the irQn additiQn was again slowly increased. The SQI started tQ increase and the foaming improved. FrQm period I SQme suspiciQns concerning the influence of ethanQl on the grQwth Qf M P lingered on. Laboratory studies were therefore perfQrmed, where the COD-cQnsumption for aerated samples spiked with ethanQl was studied. The samples were prepared with a pH-buffer and nutrients accQrding tQ SJijkhuis and Deinema, 1988. The COD-value was measured on filtered samples after different aeratiQn times up tQ fQur hQurs. Brown fQam (almQst a pure culture of M p) and activated sludge frQm the aeratiQn tank (diverse population) were used. Foam inhibited with a copper salt served as a cQntrol. The COD-cQnsumptiQn appeared tQ be about the same (around 100 mg COD/g SS.h) for both foam and activated sludge frQm the bulk. The cQpper-spiked samples shQwed no COD-consumption. The conclusiQn was that M p can indeed CQnsume ethanQI. Only Qne reference CQuid be found in the literature which cQnfirmed this observation (Wanner and Grau, 1988). With this informatiQn the two bulking incidents during period I could be explained. The first peak was obtained by enhanced SS-removal in the primary clarifiers, which increased the ethanQI part of the available substrate. The secQnd peak was caused by the high ethanol addition at the beginning Qf 1993.
Severe bulking and foaming
131
In March 1994, WAS was again sent to a digester (normal operation. see Figure I). After only one day the digester foamed to such an extent that foam reached the gas burners. which ceased to work. Oil had to substitute the digester gas. The addition of a foam-reducing oil to the digester from above had no effect. The foam consisted almost entirely of M p but with a somewhat different morphology than in the aeration tanks. After three weeks of intense digester foaming the WAS was again directed to the holding tank. ferjod III: May to Noyember 1994 Period II ended with rapidly increasing SQIs but with gradually decreasing foaming intensity on the aeration tanks. In May the following changes were made in order to - if possible - find a way out of this troublesome situation:
•
Thickened WAS was no longer mixed with digested PS in the holding tank. It was instead directly dewatered by a centrifuge and transported to a sludge lagoon (May 2).
•
Block A and B were completely separated (May 3).
•
During the period May 26 to July 7, ethanol was dosed only to block B instead of block A. After July 7. ethanol was no longer added to either of the two blocks
•
Wastewater was fed downstream of the anoxic zones, which thus only contained return sludge (RAS) from the sedimentation tanks (block A May 19. block B June IS and onwards)
The switch of ethanol to block B was made to obtain a definitive answer concerning its effects on M p. The main reason for adding the wastewater directly to an aerated zone was to hopefully hamper the growth of M P. which needs reduced sulphur compounds. This property may explain the often stated observation that M p does not grow well in aeration tanks with high oxygen concentrations (Slijkhuis and Deinema. 1988). ON-studies had revealed a remarkably high ON-rate (about 1 mg N0 3-N/g VSS. h) at 15°C with only RAS. Which is half the normal rate with primary effluent in the anoxic zones. RAS with normal SS (6000-8000 mgll) can therefore denitrify the same amount of nitrate in the anoxic zones due to a longer anoxic detention time for only RAS. It is, however. necessafy to keep a deep sludge blanket in the clarifiers to obtain high DN-rate for the RAS. The explanation is probably that the sludge gets a long time inthe clarifiers to switch over to ON. This mode of operation was named Oillner ON (D-DN) after the plant microbiologist who noted this effect during some routine tests. With low and stable SQIs and a high RAS-flow and SS. it offers the possibility of achieving about 50% N-removal in plants without suitable organic matter for ON. As a safety measure it is advisable to have some reliable additional solids separation step after the main clarifiers. The changes in early May 1994 resulted in a dramatic decrease in SQIs for block A. which can be seen in Figure 3. Block B did not show the same picture. here the SQIs stabilized at around 350 mUg. The behaviour of block B further strengthened the view that ethanol promotes growth of M p. In early July. however. the SQIs in A started to rise again and approached the level in B. which stayed at a high level in spite of the fact that no more ethanol was added (since July 7) and the D-DN mode (since June IS). In mid-June. it was observed that the overflow from the PS-thickener had a blackish instead of a greyish COlour. The reason was thought to be too much WAS in the primary clarifiers in combination with higher Water temperatures. The solids capture of the centrifuge dewatering WAS was periodically as low as 50% due to internal fouling of the machine. Analysis of volatile fatty acids (VFAs) in the overflow from the thiCkener indicated fermentation (see below). The detention time for the PS in the sludge hoppers was obviously too long. The next steps were then to improve the solids capture of the centrifuge and to increase the pumping rate from the hoppers (2 x 90 instead of 2 x 54 m3/h). A rapid response in the form of decreasing SQIs was then obtained, especially in block B. In late September. a slow increase od SQI again started in block A. while block B stayed on a low level (about 130 mUg). A blackish colour was again observed on the surface of the PS-thickener. One reason was again too much WAS in the primary clarifiers.
H. HOLMSTROM et al.
132
It was not possible to get rid of the blackish colour during period III. In November, the SQI in block B also started to rise.
SLudge 500;·············· 4sa~·
Quality Index, bLock A and 8
.. ..
.
..
;
.
;
400:' . .
358; 300:'"
:
..
..
200:
Hie:.. 100;'
..
B
sa: e:··
······· .. · ·.. ··· .. ·.. ··
· ·· ..···· ..·..·.. ·.. ·.. ·.. ···
199405010000
.
199411090000 Figure 3. SQls for block A and B from May 10 November 1994.
Analytical results It is well-known that M p can use some high fatty acids (HFAs) and/or their esters as sole carbon source (Slijkhuis and Deinema, 1988). VFAs can also be used for growth (Jenkins, 1992). During the first months of 1994 water samples were collected and analysed for VFAs (C2·CS) and free and ester bound higher fatty acids (C6-C24). Very little useful information was gained from the measurements of HFAs. Raw wastewater was dominated by C16:0 and C18:1 (oleic acid) and sludge water streams by C16:0 and C16: I. One clear exception was the condensate from the sludge dryer, in which shon HFAs dominated (C6. C8 and C7 respectively). The raw wastewaters seem to resemble French wastewaters in which C 18: I dominated in all three studied cases (Quemeneur and Many. 1994). The VFAs (C2·CS), on the contrary. showed a pattern which directly opened possibililies for changing Ihe operation of the plant. Some typical results are compiled in Table I. No samples from the tunnel end showed any measurable concentrations of VFAs. The gravity thickening of old PS. the mixing of digested PS with gravity-thickened WAS and sludge drying produce VFAs. which may explain the SQI-peaks during 1994. The first observation of viscous brown foam was in May 1993. soon after the stan of Ihe sludge dryer. The condensate from the dryer has low values of HFAs above C12. which is an indication that M p growth really is stimulated by VFAs.
Severe bulking and foaming
133
Table I. Concentrations of VFAS in wastewaters and sludge waters including calculated loads of acetic acid (C2) TABLE 1. CONCENTRATIONS OF VFAS IN WASTEWATERS AND SLUDGE WATERS INCLUDING CALCULATED LOADS OF ACETIC ACIn> C2) Sample location
C2
VFAs (mg/l) C4 C3
CS
Loads ofC2 (kg/d)
Raw ww, tunnel end
<2
<2
<100
Raw ww, pressure main (partly septic)
10
2
<2
20
Overflow, gravity thickening of WAS
<2
<2
<2
Centrate, centrifuge dewatering of gravity thickened WAS
<2
<2
<0.4
Overflow, gravity thickening of PS (blackish water)
31
3
2
80
Centrate, centrifuge dewatering of digested PS
2
<2
<2
Centrate, centrifuge dew. of mixed dig. PS and WAS
213
80
II
22
200
Condensate from sludge drying (COD> 8000 mg/l)
2400
860
31
60
180
DISCUSSION Operation of the Himmerfjlirden plant during 1994 has shown a high sensitivity to the enhanced growth of ~ p induced by small contributions of VFAs from the sludge treatment units. Duchene (1994) reports that Intake of septic sludge water has such an effect. Growth of M p through the addition of sludge fermentation products apparently also induces foaming. while the addition of ethanol does not seem to have the same effect. On the other hand, no negative effects have been observed from dosing the return sludge with methanol. Severe foaming was especially observed when the addition of iron was kept low during a period with a large POpulation of M p. During the same period nitrification was almost entirely absent. The anoxic zones could therefore boost the VFA-production by fermentation of BOD from the primary effluent. Previous periods in ~arJier years (several months) with no nitrification have, however, not resulted in bulking or foaming. The Intense foaming in March and April 1994 was accompanied by improved SQI-values (see Figure 2). A lar~e quantity of the M p during that time were floated from the bulk to the foam-layer. It is possible that the ability of M p to foam is in some way linked to the observed strong biological phosphorus removal.
H. HOLMSTROM It al.
134
Ethanol is not an efficient substrate in the PHB (polybydroxy butyrate)-synthesis, which could explain the low production of brown foam in spite of high SQIs (see figure 3) an a high ethanol addition to block B during June 1994 (Ghekiere et al., 1991). This idea was tested during November 1994 when the iron addition during one week was increased to 20 glm 3 and then during three days lowered to 12 glm 3. The foaming intensity seemed to confirm this supposition but there were no dramatic changes. The M p population was also much lower than during the spring 1994. The failure to correct the bulking at the Himmerfjiirden plant by a low sludge age and a low pH-value indicates that M p can be heterogeneous group of filaments with different properties. Chacin et al. (1994) note that M p in an anaerobic digester has another morphological form than in an aerobic environment and no phosphate granules were observed in the digester. During the first years of operation with nitrogen removal at the Himmerfjiirden plant, the DN was imcomplete due to low addition of external carbon sources. A higher addition of alcohols has decreased the effluent nitrate concentrations from the anoxic zones about 50% to about 2-4 mg N0 3-NII in average. These long-term operating data do not support the hypothesis of AA-bulking with accummulation of nitric oxide due to limited DN (Casey et al. 1994). Reports from Swedish WWTPs indicate that similar problems with bulking and foaming have increased only during the last year (oral information). This is also valid for activated sludge plants with conventional BODand P-removal. Attempts to correct these normally seasonal problems have so far been unsuccessful. Many blame the new generation of washing powders and cleaning agents. A higher concentration of oleic acid esters may have increased the vitality of existing M p, so a strong growth occurs if a suitable readily biodegradable substrate (such as ethanol or VFAs) is also present. If these assumptions are correct it is reasonable to believe that the WWTPs have to be modified in some way to be able to cope with this new situation. The most direct way at a WWTP would be an attempt to lower the concentration in the primary effluent of oleic acid esters, as they are key products for M p and also main components in the popular rapeoil based products. It is possible that a more efficient precipitation than that achieved with Fe 2+ can improve the separation of at least particle bound esters. VFAs from sludge treatment can be eliminated by some biological pretreatment method if the production cannot be stopped (such as by sludge drying). The first thing to try is of course to evaluate if modified operation of, for example, the PS-thickener can help. VFAs may also be removed by aerobic selectors in the activated sludge step. If such a step is successful, contributions from incoming raw wastewater, sludge water streams and pre-treatment units can be handled. The reported effect of aerobic selectors against M p in the literature is, however, ambiguous. With the D-DN mode of operation it is usually a simple procedure to add an aerobic selector at the front of the flrst oxic zone and evaluate the effects, as many anoxic zones have no, or poor, aeration capacity. CONCLUSIONS The operation of the Himmerfjiirden WWTP with pre-denitriflcation since 1987 has become very unstable during 1994 with several outbreaks of bulking and foaming caused by Microthrix parvicella (M p). It has been shown that the addition of ethanol stimulates the growth of M p, but does not seem to cause foaming. Contributions of small amounts of volatile fatty acids (VFAs) from the sludge treatment units result both in bulking and foaming by M p. Observations from different levels of chemical precipitation of phosphorus with iron indicate that the foaming intensity is related to the range of biological P-removal by M p. The recycling of VFAs from sludge treatment units is nothing new for the plant, but no problems with bulking and foaming were observed in previous years. It is possible that the rapid change in detergents towards products based on vegetable oils is the main reason for the high sensitivity to VFAs, probably primarily caused by an increased vitality of M p through higher levels of oleic acid esters in the wastewater. If these assumptions are correct some modiflcations to the plants will be required, which may be detrimental to biological N- and P-removal.
Severe bulking and foaming
135
REFERENCES Casey, T. G. (1994). A hypothesis for the causes and control of anoxic-aerobic (AA) filament bulking in nutrient removal activated sludge systems. War. Sci. Tech., 29(7), 203-212. Chacin. E. (1994). Foam formation, anaerobiosis and Microrhrix parvicel/a. J. IWEM. 8, 534-537. Duchene. P. (1994). Biological foams: The cause-effect relationship. test results and combat strategy. War. Sci. Tech .• 29(7). 239247. Ghekiere. S. (1991). The effect of nitrates and carbon compounds on enhanced biological phosphorus removal from wastewaters. J. EWPCA, 1(4). 15-24. Jenkins. D. (1992). Towards a comprehensive model of activated sludge bulking and foaming. War. Sci. Tech.• 25(6), 215-230. Qu~m~neur. M. and Many. Y. (1994). Fatty acids and sterols in domestic wastewaters. War. Res.• 28.1217-1226. Slijkhuis, H. and Dcinema. M. H. (1988). Effect of environmental conditions on the occurrence of Microthrix parvicella in activated sludge. War. Res.• 21 825-828. Wanner. J. and Grau. P. (1988). Filamentous bulking in nutrient removal activated sludge systems. War. Sci. Tech .• 20(415). 1-8.
Pergamon
Wal. ScL Tech. Vol. 33. No. 12. pp. 127-135.1996. CopyrighllC 1996 IAWQ. Published by Elsevier Science LId Pnnled In Greal Dntain. All rights reserved. 0273-1223/96 $15'00 + ()o()O
PH: S0273-1223(96)00466-0
SEVERE BULKING AND FOAMING AT THE HIMMERFJARDEN WWTP Hans Holmstrom, Jan Bosander, Alf-Goran Dahlberg, Asa Dillner-Westlund, Lena Flyborg and Karri Jokinen Southwestern Stockholm Region Water and Sewage Works Inc.• SYVAB. S-/4792 GrOdinge. Sweden
ABSTRACf Several years of trouble-free operation with pre-denitrification at the Himmerfjilrden WWTP concerning bulking and foaming have been followed by an unstable period. Two bulking incidents with no foaming by Microlhrix parvicella were found to be caused by ethanol addition. Other bulking and foaming occurrences by Microthrix could be cOlTelated to the production of volatile fatty acids in some of the sludge treatment steps. It is possible that a more widespread use in society of washing and cleaning products based on vegetable oils has promoted the ability of Microthrix to compete with noc-forrning organisms. especially when ethanol or volatile fatty acids are present. Copyright
KEYWORDS BUlking; ethanol; fatty acids; foaming; Microthrix. INTRODUCfION Demands to introduce nitrogen removal at larger Swedish coastal WWTPs meant that SYVAB decided on a voluntary basis to change the inlet zones of the existing aeration tanks to anoxic zones for pre-denitrification in 1987. The operational problems during the first five years of full scale operation were mainly loss of nitrification due to small aerobic reactor volumes and toxic wastewater from a pharmaceutical company. A long tunnel system also decreased the readily biodegradable fraction of the wastewater, which made denitrification incomplete. The sludge quality index (SQI) during this period was normally in the range ISO200 mUg and no outbreaks of brown foam ever occurred on the aeration tanks. During 1993, the situation changed and from then to the present there have been periodic high SQIs and sometimes also intense fOaming. These new problems took the plant staff by surprise and no experience on how to handle the new Situation was at hand. PLANT DESCRIPTION !he Himmerfjllrden WWTP serves the southwestern part of the Stockholm area and was put into operation tn 1974. It is a conventional plant originally designed for BOD- and P-removal (by post-precipitation). In Figure I the main treatment units are shown after the change to pre-denitrification. The DWF (dry weather flow) is about 100 000 m3/d. 127
H. HOLMSTR6M et al.
128
Figure I. The Himmerfjarden WWTP with pre-denitrificalion.
The biological step (including the post-precipitation units) consists of eight parallel lines. The return sludge is mixed for lines 1-4 (block A) and lines 5-8 (block B) and then distributed to the corresponding anoxic zone in the eight lines. During periods with high flows and high SQls a considerable amount of activated sludge is lost to the post clarifiers, to which no chemicals are added nowadays (only precipitation with Fe 2+). From the post clarifiers the collected sludge was previously pumped back to the return sludge systems in block A and B through a common pipe. The two blocks did not therefore operate completely independently of one another. This was changed in May 1994, after which date the plant can be regarded as two independent subplants. At DWF. the detention time (excluding return sludge) is about 45 minutes in the anoxic zones and four hours in the aeration tanks. Only the conventional return sludge system is used to recycle nitrate to the anoxic reactors. The aerobic sludge age is kept to between five to ten days depending on the water temperature. To improve the denitrification (aim. 50% N-removal) an eJhanol solution from a pharmaceutical company (70% ethanol) is almost continuously added directly to the return sludge (about 12 mgll COD from ethanol). The lack of readily biodegradable organic matter and small anoxic reactors implies that denitrification (ON) is normally not complete. The shortage is more pronounced during weekends and holidays. Methanol is therefore also added d.Jring such periods. METHODS A modified method to determine the sludge quality index (SQI) was used twice a week to evaluate the settling properties of the activated sludge. The key point of this modification is to obtain sludge volumes in a 1000 mI cylindrical measurement glass in the range 150-600 mVI. To reach this range it is often necessary to dilute the sludge samples collected from the aeration tanks with treated wastewater. The following formula is then used to calculate the SQI-value: 150 < SV S 300 mVl
SQI = n x SV/SS x 1000
=n (SVI3 + 2(0)/SS x 1000 where n = 1 for no dilution, n = 2 for dilution 1 + I and n =3 for dilution 2 + I (2 parts water + I part 300 < SV < 600 mVl
sludge).
SQI
Severe bulking and foaming
129
Measurement of fatty acids was performed with gas chromatography and a flame ionozation detector. RESULTS In Figure 2 the SQI-values for block A are shown from September 1992 to November 1994. Several distinct peaks can be noted in the figure.
Sludge Quality Index, block A
:[: :.: :.';.'.: :~.'.::: ::: ~:.'.: ':.' '.':: :. "~'.: ':'.::: :::.::::: :: : 4OO~
:
: :
; : :.. :
3Se! : : 300!· ..:····;···..···:··..:····· 25e~"'~"''''''/'';
200;"':' .
. 150: / ;
. :
l00! .. : 50:'
:
.
."
.
.
•....
..
I :.
:'.11,' 'j" e:···:··· ~
:
"
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199411090000
Figure 2. SQls in block A from September 1992 to November 1994.
&tiod I (September 1992 - April 1993) Two SQI-peaks appeared during period I. The first one coincided with increased removal of suspended Solids (SS) in the primary clarifiers (Sept 1 - Oct 20) to stimulate nitrification. This was achieved by adding about 2 glm3 of a polymer to the inlet of the aerated sand traps and about 0.2 g/m3 of another polymer to the outlet. The effluent SS-concentration fell to about 80 mgll from about 140 mgll. Microscopic evaluation showed that Microthrix parvicella (M p) was the main filament in the activated sludge. No viscous brown foam was observed. As the SQIs improved after the polymer addition had ceased, it was supposed that this operation mode was responsible in some way for the deteriorated sludge quality. The second peak lasted three months. At the end of 1992, nitrification was excellent inspite of water temperatures around 12°C. During the first week in January the ethanol addition was increased three times. A higher addition than normal was then maintained to the end of February. No addition of ethanol or rnethanol took place during the last weeks of 1992. Microscopic examination of the activated sludge again shOwed a large number ofM p during SQI-peak, but also some type 0041 and Nostocoida limicola. An evaluation of operating data showed that pH-drops down to 5.0 had occurred during the holidays at the turn of the year. Such low pH-values had never been observed before in combination with nitrogen removal. The explanation can be found in increased loads of nitrogen to the plant in combination with excellent nitrification, inefficient denitrification (lack of suitable organic matter) and low buffer capacity. The nitrogen loads can be explained by increased intake of food and beverages dUring the Christmas and New Year festivities. A literature survey showed that low pH-values can result in bulking sludge, normally by grOwth of fungi. The gradual increase of the SQIs up to the beginning of March, however, indicated that the Pli-drop was probably not entirely responsible for the bulking. With aerobic sludge ages of about seven days the maximum SQI caused by low pH-values at the end of 1992 should be expected during January (less than three sludge ages).
H. HOLMSTROM et al.
130
Period II: May 1993 tQ April 1994
The beginning Qf this period shQwed increasing (and scattered) SQIs by grQwth Qf M pup tQ early July, in spite Qf increasing water temperatures. This phase was fQnQwed by a decrease during the summer vacaliQn periQd (July). The period frQm the end Qf OctQber tQ the beginning Qf February is again characterized by increasing SQIs. BrQwn visCQUS fQam was nQled fQr the first time Qn the aeratiQn tanks during June 1993. The amQunt was, hQwever, modest during 1993, (20% covered aeration tank surface). During period II SQme changes in the sludge treatment took place: •
An indirect sludge dryer was started in late April 1993. Due to mechanical prQblems the dryer never operated mQre than six fun days each mQnth during 1993 and 1994. No QperatiQn at all went on during July 1993.
•
Gravity-thickened waste-activated sludge (WAS, OS: 1.5-2.5%) was, after OctQber 8, mixed with anaerQbically digested primary sludge (PS) in a holding tank. Mixed sludge was then dewatered with centrifuges (OS: 25%). The purpose Qf this change was tQ limit the recirculatiQn Qf ammQnia via the centrate frQm the centrifuges.
•
After mid-September 1993 all ethanQI was dosed only to block A.
In January 1994, the amQunt Qf brQwn fQam on the aeratiQn tanks started tQ increase and culminated during March and April with up tQ Qne meter of fQam in the first aerated tank (tWQ in series with a submerged cQnnection): the fQam was trapped in the first tank. Loss of fQam frQm the secQnd aeratiQn tank tQ the clarifiers resulted after SQme mQnths in about a half-meter thick layer of flQating sludge Qn the inlet ZQnes. InfQrmatiQn in the literature stated that M p does nQt grow at pH-values belQW 7.1 and at sludge ages belQW eight days (Jenkins, 1992). A first step was therefQre to decrease the aerQbic sludge age to below five days. The nitrificatiQn was then IQst but the pH-value did nQt exceed 6.8. The normal pH-range with nitrQgen remQval is 6.0-6.5. These steps did nQt at all hamper the M p populatiQn. AnQther hypothesis at the time (late 1993) was that lack of phosphQrus CQuid give the M p pQpulatiQn an advantage. The addition Qf ferrQus sulphate was therefore gradually reduced frQm mid-December until the effluent sQluble phQsphate had increased to about 0.2 mg UJ, which happened in mid February. The average additiQn was kept arQund 3 g Fe 2+/m 3 fQr six weeks (normal dose 16 g Fe 2+/m 3). It was Qbvious that the biQIQgical P-removal was very efficient. The SQIs alSQ started to fall (see Figure 2) but the foam layer on the aeratiQn tanks instead increased dramatically. At the end of March the irQn additiQn was again slowly increased. The SQI started tQ increase and the foaming improved. FrQm period I SQme suspiciQns concerning the influence of ethanQl on the grQwth Qf M P lingered on. Laboratory studies were therefore perfQrmed, where the COD-cQnsumption for aerated samples spiked with ethanQl was studied. The samples were prepared with a pH-buffer and nutrients accQrding tQ SJijkhuis and Deinema, 1988. The COD-value was measured on filtered samples after different aeratiQn times up tQ fQur hQurs. Brown fQam (almQst a pure culture of M p) and activated sludge frQm the aeratiQn tank (diverse population) were used. Foam inhibited with a copper salt served as a cQntrol. The COD-cQnsumptiQn appeared tQ be about the same (around 100 mg COD/g SS.h) for both foam and activated sludge frQm the bulk. The cQpper-spiked samples shQwed no COD-consumption. The conclusiQn was that M p can indeed CQnsume ethanQI. Only Qne reference CQuid be found in the literature which cQnfirmed this observation (Wanner and Grau, 1988). With this informatiQn the two bulking incidents during period I could be explained. The first peak was obtained by enhanced SS-removal in the primary clarifiers, which increased the ethanQI part of the available substrate. The secQnd peak was caused by the high ethanol addition at the beginning Qf 1993.
Severe bulking and foaming
131
In March 1994, WAS was again sent to a digester (normal operation. see Figure I). After only one day the digester foamed to such an extent that foam reached the gas burners. which ceased to work. Oil had to substitute the digester gas. The addition of a foam-reducing oil to the digester from above had no effect. The foam consisted almost entirely of M p but with a somewhat different morphology than in the aeration tanks. After three weeks of intense digester foaming the WAS was again directed to the holding tank. ferjod III: May to Noyember 1994 Period II ended with rapidly increasing SQIs but with gradually decreasing foaming intensity on the aeration tanks. In May the following changes were made in order to - if possible - find a way out of this troublesome situation:
•
Thickened WAS was no longer mixed with digested PS in the holding tank. It was instead directly dewatered by a centrifuge and transported to a sludge lagoon (May 2).
•
Block A and B were completely separated (May 3).
•
During the period May 26 to July 7, ethanol was dosed only to block B instead of block A. After July 7. ethanol was no longer added to either of the two blocks
•
Wastewater was fed downstream of the anoxic zones, which thus only contained return sludge (RAS) from the sedimentation tanks (block A May 19. block B June IS and onwards)
The switch of ethanol to block B was made to obtain a definitive answer concerning its effects on M p. The main reason for adding the wastewater directly to an aerated zone was to hopefully hamper the growth of M P. which needs reduced sulphur compounds. This property may explain the often stated observation that M p does not grow well in aeration tanks with high oxygen concentrations (Slijkhuis and Deinema. 1988). ON-studies had revealed a remarkably high ON-rate (about 1 mg N0 3-N/g VSS. h) at 15°C with only RAS. Which is half the normal rate with primary effluent in the anoxic zones. RAS with normal SS (6000-8000 mgll) can therefore denitrify the same amount of nitrate in the anoxic zones due to a longer anoxic detention time for only RAS. It is, however. necessafy to keep a deep sludge blanket in the clarifiers to obtain high DN-rate for the RAS. The explanation is probably that the sludge gets a long time inthe clarifiers to switch over to ON. This mode of operation was named Oillner ON (D-DN) after the plant microbiologist who noted this effect during some routine tests. With low and stable SQIs and a high RAS-flow and SS. it offers the possibility of achieving about 50% N-removal in plants without suitable organic matter for ON. As a safety measure it is advisable to have some reliable additional solids separation step after the main clarifiers. The changes in early May 1994 resulted in a dramatic decrease in SQIs for block A. which can be seen in Figure 3. Block B did not show the same picture. here the SQIs stabilized at around 350 mUg. The behaviour of block B further strengthened the view that ethanol promotes growth of M p. In early July. however. the SQIs in A started to rise again and approached the level in B. which stayed at a high level in spite of the fact that no more ethanol was added (since July 7) and the D-DN mode (since June IS). In mid-June. it was observed that the overflow from the PS-thickener had a blackish instead of a greyish COlour. The reason was thought to be too much WAS in the primary clarifiers in combination with higher Water temperatures. The solids capture of the centrifuge dewatering WAS was periodically as low as 50% due to internal fouling of the machine. Analysis of volatile fatty acids (VFAs) in the overflow from the thiCkener indicated fermentation (see below). The detention time for the PS in the sludge hoppers was obviously too long. The next steps were then to improve the solids capture of the centrifuge and to increase the pumping rate from the hoppers (2 x 90 instead of 2 x 54 m3/h). A rapid response in the form of decreasing SQIs was then obtained, especially in block B. In late September. a slow increase od SQI again started in block A. while block B stayed on a low level (about 130 mUg). A blackish colour was again observed on the surface of the PS-thickener. One reason was again too much WAS in the primary clarifiers.
H. HOLMSTROM et al.
132
It was not possible to get rid of the blackish colour during period III. In November, the SQI in block B also started to rise.
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199405010000
.
199411090000 Figure 3. SQls for block A and B from May 10 November 1994.
Analytical results It is well-known that M p can use some high fatty acids (HFAs) and/or their esters as sole carbon source (Slijkhuis and Deinema, 1988). VFAs can also be used for growth (Jenkins, 1992). During the first months of 1994 water samples were collected and analysed for VFAs (C2·CS) and free and ester bound higher fatty acids (C6-C24). Very little useful information was gained from the measurements of HFAs. Raw wastewater was dominated by C16:0 and C18:1 (oleic acid) and sludge water streams by C16:0 and C16: I. One clear exception was the condensate from the sludge dryer, in which shon HFAs dominated (C6. C8 and C7 respectively). The raw wastewaters seem to resemble French wastewaters in which C 18: I dominated in all three studied cases (Quemeneur and Many. 1994). The VFAs (C2·CS), on the contrary. showed a pattern which directly opened possibililies for changing Ihe operation of the plant. Some typical results are compiled in Table I. No samples from the tunnel end showed any measurable concentrations of VFAs. The gravity thickening of old PS. the mixing of digested PS with gravity-thickened WAS and sludge drying produce VFAs. which may explain the SQI-peaks during 1994. The first observation of viscous brown foam was in May 1993. soon after the stan of Ihe sludge dryer. The condensate from the dryer has low values of HFAs above C12. which is an indication that M p growth really is stimulated by VFAs.
Severe bulking and foaming
133
Table I. Concentrations of VFAS in wastewaters and sludge waters including calculated loads of acetic acid (C2) TABLE 1. CONCENTRATIONS OF VFAS IN WASTEWATERS AND SLUDGE WATERS INCLUDING CALCULATED LOADS OF ACETIC ACIn> C2) Sample location
C2
VFAs (mg/l) C4 C3
CS
Loads ofC2 (kg/d)
Raw ww, tunnel end
<2
<2
<100
Raw ww, pressure main (partly septic)
10
2
<2
20
Overflow, gravity thickening of WAS
<2
<2
<2
Centrate, centrifuge dewatering of gravity thickened WAS
<2
<2
<0.4
Overflow, gravity thickening of PS (blackish water)
31
3
2
80
Centrate, centrifuge dewatering of digested PS
2
<2
<2
Centrate, centrifuge dew. of mixed dig. PS and WAS
213
80
II
22
200
Condensate from sludge drying (COD> 8000 mg/l)
2400
860
31
60
180
DISCUSSION Operation of the Himmerfjlirden plant during 1994 has shown a high sensitivity to the enhanced growth of ~ p induced by small contributions of VFAs from the sludge treatment units. Duchene (1994) reports that Intake of septic sludge water has such an effect. Growth of M p through the addition of sludge fermentation products apparently also induces foaming. while the addition of ethanol does not seem to have the same effect. On the other hand, no negative effects have been observed from dosing the return sludge with methanol. Severe foaming was especially observed when the addition of iron was kept low during a period with a large POpulation of M p. During the same period nitrification was almost entirely absent. The anoxic zones could therefore boost the VFA-production by fermentation of BOD from the primary effluent. Previous periods in ~arJier years (several months) with no nitrification have, however, not resulted in bulking or foaming. The Intense foaming in March and April 1994 was accompanied by improved SQI-values (see Figure 2). A lar~e quantity of the M p during that time were floated from the bulk to the foam-layer. It is possible that the ability of M p to foam is in some way linked to the observed strong biological phosphorus removal.
H. HOLMSTROM It al.
134
Ethanol is not an efficient substrate in the PHB (polybydroxy butyrate)-synthesis, which could explain the low production of brown foam in spite of high SQIs (see figure 3) an a high ethanol addition to block B during June 1994 (Ghekiere et al., 1991). This idea was tested during November 1994 when the iron addition during one week was increased to 20 glm 3 and then during three days lowered to 12 glm 3. The foaming intensity seemed to confirm this supposition but there were no dramatic changes. The M p population was also much lower than during the spring 1994. The failure to correct the bulking at the Himmerfjiirden plant by a low sludge age and a low pH-value indicates that M p can be heterogeneous group of filaments with different properties. Chacin et al. (1994) note that M p in an anaerobic digester has another morphological form than in an aerobic environment and no phosphate granules were observed in the digester. During the first years of operation with nitrogen removal at the Himmerfjiirden plant, the DN was imcomplete due to low addition of external carbon sources. A higher addition of alcohols has decreased the effluent nitrate concentrations from the anoxic zones about 50% to about 2-4 mg N0 3-NII in average. These long-term operating data do not support the hypothesis of AA-bulking with accummulation of nitric oxide due to limited DN (Casey et al. 1994). Reports from Swedish WWTPs indicate that similar problems with bulking and foaming have increased only during the last year (oral information). This is also valid for activated sludge plants with conventional BODand P-removal. Attempts to correct these normally seasonal problems have so far been unsuccessful. Many blame the new generation of washing powders and cleaning agents. A higher concentration of oleic acid esters may have increased the vitality of existing M p, so a strong growth occurs if a suitable readily biodegradable substrate (such as ethanol or VFAs) is also present. If these assumptions are correct it is reasonable to believe that the WWTPs have to be modified in some way to be able to cope with this new situation. The most direct way at a WWTP would be an attempt to lower the concentration in the primary effluent of oleic acid esters, as they are key products for M p and also main components in the popular rapeoil based products. It is possible that a more efficient precipitation than that achieved with Fe 2+ can improve the separation of at least particle bound esters. VFAs from sludge treatment can be eliminated by some biological pretreatment method if the production cannot be stopped (such as by sludge drying). The first thing to try is of course to evaluate if modified operation of, for example, the PS-thickener can help. VFAs may also be removed by aerobic selectors in the activated sludge step. If such a step is successful, contributions from incoming raw wastewater, sludge water streams and pre-treatment units can be handled. The reported effect of aerobic selectors against M p in the literature is, however, ambiguous. With the D-DN mode of operation it is usually a simple procedure to add an aerobic selector at the front of the flrst oxic zone and evaluate the effects, as many anoxic zones have no, or poor, aeration capacity. CONCLUSIONS The operation of the Himmerfjiirden WWTP with pre-denitriflcation since 1987 has become very unstable during 1994 with several outbreaks of bulking and foaming caused by Microthrix parvicella (M p). It has been shown that the addition of ethanol stimulates the growth of M p, but does not seem to cause foaming. Contributions of small amounts of volatile fatty acids (VFAs) from the sludge treatment units result both in bulking and foaming by M p. Observations from different levels of chemical precipitation of phosphorus with iron indicate that the foaming intensity is related to the range of biological P-removal by M p. The recycling of VFAs from sludge treatment units is nothing new for the plant, but no problems with bulking and foaming were observed in previous years. It is possible that the rapid change in detergents towards products based on vegetable oils is the main reason for the high sensitivity to VFAs, probably primarily caused by an increased vitality of M p through higher levels of oleic acid esters in the wastewater. If these assumptions are correct some modiflcations to the plants will be required, which may be detrimental to biological N- and P-removal.
Severe bulking and foaming
135
REFERENCES Casey, T. G. (1994). A hypothesis for the causes and control of anoxic-aerobic (AA) filament bulking in nutrient removal activated sludge systems. War. Sci. Tech., 29(7), 203-212. Chacin. E. (1994). Foam formation, anaerobiosis and Microrhrix parvicel/a. J. IWEM. 8, 534-537. Duchene. P. (1994). Biological foams: The cause-effect relationship. test results and combat strategy. War. Sci. Tech .• 29(7). 239247. Ghekiere. S. (1991). The effect of nitrates and carbon compounds on enhanced biological phosphorus removal from wastewaters. J. EWPCA, 1(4). 15-24. Jenkins. D. (1992). Towards a comprehensive model of activated sludge bulking and foaming. War. Sci. Tech.• 25(6), 215-230. Qu~m~neur. M. and Many. Y. (1994). Fatty acids and sterols in domestic wastewaters. War. Res.• 28.1217-1226. Slijkhuis, H. and Dcinema. M. H. (1988). Effect of environmental conditions on the occurrence of Microthrix parvicella in activated sludge. War. Res.• 21 825-828. Wanner. J. and Grau. P. (1988). Filamentous bulking in nutrient removal activated sludge systems. War. Sci. Tech .• 20(415). 1-8.