Control of activated sludge filamentous bulking—VII. Effect of anoxic conditions

Wat. Res. Vol. 21, No. 12, pp. 1447-1451, 1987 Printed in Great Britain. All rights reserved

0043-1354/87 $3.00+0.00 Copyright © 1987 Pergamon Journals Ltd

CONTROL OF ACTIVATED SLUDGE FILAMENTOUS BULKING--VII. EFFECT OF ANOXIC CONDITIONS J. WANNER, J. CHUDOBA*, K. KUCMAN a n d L. PROSKE Department of Water Technology and Environmental Engineering, Prague Institute of Chemical Technology, Suchb~itarova 5, 166 28 Prague 6, Czechoslovakia

(Received February 1986) Abstract--The effect of anoxic conditions on the occurrence of filamentous organisms in mixed cultures was studied in laboratory activated sludge systems. It was repeatedly demonstrated that anoxic conditions are able to suppress the growth of some undesirable filamentous organisms, for instance, type 021N and Sphaerotilus natans. It was also found that severely filamentous mixed cultures had maximum rates of denitrification or nitrate respiration one order of magnitude lower than non-filamentous mixed cultures. On the basis of these findings it is concluded that some filamentous organisms cannot use nitrate nitrogen as an electron acceptor.

Key words--activated sludge, filamentous bulking, anoxic conditions, nitrate respiration, kinetic constants

INTRODUCTION The first report of a positive effect o f anaerobiosis on the suppression of activated sludge bulking was presented by W e s t g a r t h et al. 0 9 6 4 ) . Their results, however, were n o t sufficiently convincing. Recently, C h a m b e r s (1982) a n d W a g n e r (1982) reported on the results o b t a i n e d in pilot plants clearly showing a

means of a persistaltic pump in such volumes so that the hydraulic retention time in all systems was constant, namely 24 h. Also the volumetric loading of I kg m - 3 d - i (as COD) was kept constant in all systems. Analytical methods All analyses were carried out according to the Czechoslovak Standard Methods (1965). Ammonia was determined by means of a direct Nesslerization method, nitrites,

positive effect o f anoxic zones o n the activated sludge settleability. Price (1982) d e m o n s t r a t e d this effect in the Thigley Sewage T r e a t m e n t Works. N o n e o f these authors, however, gave a n explanation o f this p h e n o m e n o n . The aim of the experiments described in this p a p e r was to give a n acceptable explanation o f this p h e n o m e n o n .

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Laboratory models System 1. This system consisted of an anoxic selector (AS, 0.25 1.) followed by an anoxic completely mixed tank (CMT, 1.51.) and an oxic completely mixed tank (CMT, 3.01.) (Fig. la). System 2. This system was similar to the previous one with the exception that the anoxic selector was removed from the technological line (Fig. lb). The first completely mixed tank was periodically oxic and anoxic as shown below. The influent was decreased accordingly so that the total hydraulic retention time (HRT) remained the same as with the system 1. System 3. This system consisted only of an anoxic completely mixed tank (2.91.) followed by a small post-aeration tank (PA, 0.1 1.) (Fig. Ic). System 4. This system was an oxic completely mixed tank and served as a control (Fig. ld). Further technological parameters for all the above systems are given in Table 1. The composition of influents into the individual systems is summarized in Table 2. The influents were dosed by *Author to whom correspondence should be addressed,

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(d| Fig. 1. Schematic outline of the performed activated sludge systems, l - - l s t CMT; 2--2nd CMT; 3--anoxic CMT; 4--oxic CMT; AS--anoxic selector; PA--post-aeration unit; S---settler.

1447

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J. WAr~NER et al.

Table I. Technological parameters of activated sludge systems System Parameter Volume of AS (1) Volume of Ist CMT (I) Volume of 2nd CMT (I) Volume of PA unit (1) Volume of settler (1) Recirculation ratio-HRT in system (h) SRT in system (d) Volumetric loading on COD basis (kgm 3d-~) Temperature (°C) *Only in periods with low SVI.

1

2

3

4

0.25 1.5 3.0 -1.0 1.0 24 7

-1.5 3.0 -1.0 1.0 24 7*

--2.9 0.1 1.0 1.0 24 7

--3.0 -1.0 1.0 24 7*

I 1 1 20-25 20-25 20-5

1 20-25

Table 2. Composition of influents (mgl -t) dosed into the individual systems System Component 1 2 3 4 Glutamic acid (as COD) 200 ---Glucose (as COD) 300 500 500 500 Ethanol (as COD) 500 500 500 500 Total COD 1000 1000 1000 1000 NH~--N [as (NH4)2CO3] 130 130 50 130 PO~ -P (as KHzPO,) 15 15 15 15 NO3~) (as NaNO 3) 706 706* 706 -NaHCO 3 200 200 -400 *Dosed only during periods when 1st CMT was anoxic. with sulphanilic acid and N-(1-naphtyl)-ethylenediamined~hydrochloride, nitrates, by a salicylate method, and COD by a dichromate method. The concentration of biomass was determined by means of membrane filters. SVI was determined in unstirred, 100-ml cylinders. SVI values obtained in this way are usually higher than those obtained from stirred, 1000-ml cylinders, Besides the chemical analyses, a microscopic investigation according to the procedure developed by Eikelboom (Eikelboom and Buijsen, 1981) was carried out at least once a week. RESULTS AND DISCUSSION Experiments with systems 1 a n d 2 can be divided into six periods as indicated in Table 3. D u r i n g the first period, the system with the anoxic selector was operated. Organic loading of the anoxic selector was 19 kg m - 3 d-~ o n the C O D basis. As s h o w n in Table 4, most substrate was removed in the anoxic selector a n d m o s t oxygen c o n s u m e d originated from nitrates. As the selector was not covered, a precise oxygen balance c a n n o t be carried out because some oxygen could diffuse into the liquid from the atmosphere. A mixed culture developed in

Table 3. Indication of various periods and their duration during the performance of systems 1 and 2 Performance ofCMT Period Days of No. experiments System • 1st 2nd 1 1-102 1 Anoxic Oxic 2 103-138 2 Anoxic Oxic 3 139-195 2 Oxic Oxic 4 196--267 2 Anoxic Oxic 5 268-302 2 Oxic Oxic 6 303-330 2 Anoxic Oxic

Table 4. Average values of soluble COD and NO{-N along system 1 during period 1 Concentration (mg 1-1)

Component

lnfluent

Effluent from AS

E~uent from 1st CMT

COD 1040 200 110 NO{-N 252* 55 3 *Dosed 247 mg 1 t plus 5 mg 1- t from tap water. tNitrification in oxic conditions.

Effluent from 2nd CMT 70 33t

this system h a d very good settling properties as can be seen in Fig. 2. F i l a m e n t o u s organisms appeared sporadically a n d formed short filaments p r o t r u d i n g from the flocs. Higher organisms, including attached a n d free swimming ciliates a n d rotifers, were also present. In order to elucidate whether the suppression of growth of the filamentous organisms was due to the anoxic selector or not, the selector h a d been removed from the technological line before the second period was started. A t the same time, glutamic acid was replaced by glucose in the influent. Both glucose a n d ethylalcohol are k n o w n to support the excessive growth of filamentous organisms in completely mixed systems ( C h u d o b a , 1985; C h u d o b a et al., 1985). In this way, the potential for filamentous bulking was increased considerably. Both changes resulted in a transient increase of SVI values as d e m o n s t r a t e d in Fig. 2. A t the end of the second period the settleability of the mixed culture was again very good. The second period d e m o n s t r a t e d that not the selector but the anoxic conditions were responsible for the suppression of growth of the filamentous organisms. To o b t a i n more evidence o f the positive effect of anoxic conditions on filamentous bulking suppression, oxic a n d anoxic conditions were repeatedly changed during further periods. The results are summarized in Fig. 2. It can be seen in this figure that oxic conditions in the first C M T always resulted in severe sludge bulking as measured a n d expressed by the SVI. As soon as filamentous bulking appeared, it was n o t possible to keep the S R T o f 7 days. It d r o p p e d dramatically due to the escaping of the filaments from the settler. Severe bulking took place in spite o f the fact that the mixed culture h a d a sufficiently long regeneration period (8 h) in the second, oxic C M T . As shown in Table 5, most substrate was removed in the first C M T , a n d the second C M T served for sludge regeneration a n d nitrification. O n the other h a n d , anoxic conditions in the first C M T resulted always in an effective suppression of growth of the filamentous organisms, of which the type 021N was p r e d o m i n a n t . D u r i n g all anoxic periods, sodium nitrate was dosed into the first, anoxic C M T so t h a t the source of nitrate oxygen would be i n d e p e n d e n t o f the rate of nitrification in the second, oxic C M T . The filamentous bulking suppression in anoxic conditions was very fast a n d usually finished within 3 weeks. D u r i n g the transient period, the length o f the

Activated sludge bulking--VII

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Fig. 2. A course of SV! din-Jagthe experimentswith systems l and 2. The figures indicate the individual periods given in Table 3. filaments got shortened and the rest got covered with bacteria. These covered filaments formed a basis for newly formed flocs (Fig. 3). A characteristic feature of the transient period was also a change in the contents of higher organisms. While in the oxic conditions higher organisms were present sporadi-

summarized in Fig. 4. This figure clearly shows that the anoxic system itself, without an oxic part can suppressfilamentous hulking. Values of SV! varied from 30 to 70 m] g-~. The anoxic system contained a small post-aeration unit with the mixed liquor retention time of only 0.8 h to remove nitrogen bubbles

cally in the f i l a m e n t o u s m i x e d culture, u n d e r the a n o x i c c o n d i t i o n s their n u m b e r increased in the n o n - f i l a m e n t o u s m i x e d culture. A t t a c h e d a n d free s w i m m i n g ciliates a n d rotifers were p r e d o m i n a n t , Although the above experiments showed that a n o x i c c o n d i t i o n s were able to s u p p r e s s f i l a m e n t o u s bulking, the role o f the oxic p a r t ( s e c o n d C M T ) in this s u p p r e s s i o n r e m a i n e d unclear. In o r d e r to elucid a t e this, e x p e r i m e n t s with systems 3 (anoxic C M T ) a n d 4 (oxic C M T ) were started. T h e results are

w h i c h c o u l d interfere with settling in t h e settler. T h u s t h e d o s e d nitrates were a m a i n source o f o x y g e n in this system. T h e oxic c o m p l e t e l y m i x e d s y s t e m o p e r a t e d in parallel c o n t a i n e d a severely f i l a m e n t o u s m i x e d culture with values o f SVI v a r y i n g f r o m 300 to 1100 ml g-~. T h e p r e d o m i n a n t f i l a m e n t o u s o r g a n i s m s were again o f type 0 2 I N . It is i m p o r t a n t to p o i n t o u t t h a t t h e effluent f r o m the s y s t e m 3 was o f a p o o r quality, c o n t a i n i n g as high as 1 0 0 m g l -~ s u s p e n d e d solids c o m p o s e d m a i n l y o f

Table 5. Average values of soluble COD in effluents from the first and the second completely mixed tank in system 2 during periods 2--6 (mg I-~ )

Table 7. Changes in the maximum denitrification rates of the mixed cultures developed in systems 3 and 4 after their transfer into systems 4 and 3, respectively

Effluent from Period No. 2 3 4 5 6

1st CMT

2nd CMT

160 (anoxic) 120 (oxic) 130 (anoxic) 140 (oxic) 150 (anoxic)

r~.m of denitrification 0NO{-N) (h -] )

65 80 75 70 80

Days after replacement

System 3

System 4

7 14 21

0.0129 0.0191 0.0245

0.0050 0.0024 0.0028

Table 6. Kinetic constants of mixed cultures developed in systems 2, 3 and 4 and the maximum rates of denitrification. The measurement was carried out with ethanol, and r~.m, Ks and Y are expressed in oxygen units

Rate of denitrification

Oxic conditions System 2 3 4

Conditions Anoxic 1st CMT Anoxic CMT with PA Oxic CMT

rx.,~ (NO~--N)

rxm

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Y

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0.132 0.132 0.104

0.11 0.52 0.05

0.65 0.64 0.67

0.026 0.027 0.0023

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J. WANNERet al.

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Fig. 3. A microscopic photograph of the mixed culture developed in system 2 with the first CMT anoxic.

was too short to enable the growth of higher organisms. In practice, an optimum proportion between anoxic and oxic zones should always be found. The last experiment was started on the 67th day of operation of both systems. The non-filamentous mixed culture from the anoxic system 3 was transferred into the oxic system 4, while the severely filamentous culture from the oxic system 4 was transferred into the anoxic system 3. The results are also shown in Fig. 4. During 3 weeks, the originally filamentous culture with the SVI of 1000ml g-~ became non-filamentous in the anoxic conditions with the SVI of 6 0 m l g -] and the originally nonfilamentous culture with the SVI of 60 ml g - ' became in the oxic conditions severely filamentous with the SVI of 1000ml g-l. It is believed that all these results are a sufficient proof of the capability of anoxic conditions to suppress the growth of some filamentous organisms, for instance, type 021N and Sphaerotilus natans. In the end, one question should be answered, namely, what is the explanation of this phenomenon. The measurement carried out during the experiments enables to explain this phenomenon. Some filamentous organisms, especially type 021N and Sphaerotilus natans do not have an enzymatic system

enabling them to use oxygen from nitrates, i.e the nitrate respiration. This is well documented in Table minute flocs and many dispersed bacteria. Although 6. While the kinetic constants (r. . . . Ks, Y), deterthe mixed culture developed under exclusively anoxic mined by a respirometric method developed by (~ech conditions was not filamentous, it contained very et al. (1984) and expressed in oxygen units, are with minute flocs, considerable amounts of dispersed bac- non-filamentous (system 2 and 3) and filamentous teria, and no higher organisms. Only Vorticella m i (system 4) mixed cultures nearly the same, the maxicrostoma appeared sporadically. The absence of cili- mum rate of denitrification or nitrate respiration is ates and rotifers resulted in higher contents of one order of magnitude lower with the severely dispersed bacteria. Thus it is evident that the mixed filamentous culture than with the non-filamentous liquor retention time in the post-aeration unit of 0.8 h one. This suggests that some filamentous organisms are unable to use nitrate nitrogen as an electron acceptor. 12 ~ A further proof of this fact was obtained after a 11 /\ mutual exchange of both mixed cultures developed in 1o I ) systems 3 and 4. The originally severely filamentous mixed culture developed in the oxic system 4 with the 9 maximum rate of dentrification (NO3-N) 0.0023 h - l ~o 8 (Table 6), after being transferred into the anoxic 4 ,~ 7 system, gradually raised its denitrification capability 6 during 3 weeks up to a maximum rate of 0.0245 h-J (Table 7), depending on decreasing contents of the ~, 5 filamentous microorganisms in the culture. On the 4 other hand, the originally non-filamentous mixed 3 culture developed in the anoxic system 3 and having the maximum rate of denitrification (NO~--N) 2 0.027 h -~ (Table 6), after being transferred into the 1 3 3 oxic system, gradually lowered its denitrification I J capability during the same period up to a maximum o 2o 40 6o 8o 1oo rate of 0.0028 h-~ (Table 7), depending on increasing Days contents of the filamentous organisms in the culture. Fig. 4. A course of SVI in the anoxic (3) and the oxic (4) These findings are of great practical importance. It completely mixed tank. The arrow shows the day on which is evident now that the anoxic selector is more the mixed culture from both systems were replaced, efficient in the suppression of filamentous bulking

Activated sludge bulking--VII and more reliable than the oxic one. In connection with the present findings it would be interesting to know whether filamentous microorganisms can accumulate polyphosphates and use them as a source of energy in anaerobic conditions. Experiments leading to the elucidation of this question are under preparation in our laboratory. SUMMARY Effect of anoxic conditions on the occurrence of filamentous organisms in mixed cultures was studied in laboratory activated sludge systems. It was repeatedly demonstrated that anoxic conditions are able to suppress the growth of some undesirable filamentous organisms, for instance, type 02IN and Sphaerotilus natans. It was also found that severely filamentous mixed cultures had maximum rates of denitrification or nitrate respiration one order of magnitude lower than non-filamentous mixed cultures cultivated under similar conditions. On the basis of these findings it is concluded that some filamentous organisms cannot use nitrate nitrogen as an electron acceptor. REFERENCES t~ech J. S., Chudoba J. and Grau P. (1984) Determination

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of kinetic constants of activated sludge microorganisms. Wat. Sci. Technol. 17, 259-272. Chambers B. (1982) Effect of longitudinal mixing and anoxic zones on settleability of activated sludge. In Bulking of Activated Sludge: Preventive and Remedial Methods (Edited by Chambers B. and Tomlinson E. J.). Ellis Horwood, Chichester. Chudoba J. (1985) Control of activated sludge filamentous bulking--VI. Formulation of basic principles. Wat. Res. 19, 1017-I022. Chudoba J., (~ech J. S. and Chudoba P. (1985) The effect of aeration tank configuration on nitrification kinetics. J. War. Pollut. Control Fed. 57, 1078-1083. Czechoslovak Standard Methods for Water Analysis (1965) SNTL, Praha (in Czech). Eikelboom D. H. and Van Buijsen H. J. (1981) Microscopic Sludge Investigation Manual. IMG-TN0 Report A 94 A, Delft. Price G. J. (1982) Use of an anoxic zone to improve activated sludge settleability. In Bulking of Activated Sludge: Preventive and Remedial Methods (Edited by Chambers B. and Tomlinson E. J.). Ellis Horwood, Chichester. Wagner F, (1982) Study of the cause and prevention of sludge bulking in Germany. In Bulking of Activated Sludge: Preventive and Remedial Methods (Edited by Chambers B. and Tomlinson E. J.). Ellis Horwood, Chichester. Westgarth W. C., Sulzer D. A. and Okun D. A. (1964) Anaerobiosis in the activated sludge process. Paper presented at the 2nd IAWPR Conference, Tokyo.