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On the stability of activated sludge flocs with implications to dewatering
War. Res. Vol. 26. No. 12, pp. 1597-1604.1992 Printed in Great Britain. All rights reserved
0043-1354/92$5.00+ 0.00 Copyright C 1992PergamonPresa Ltd
ON THE STABILITY OF ACTIVATED SLUDGE FLOCS WITH IMPLICATIONS TO DEWATERING JACOB HOYGAARDBRUUS,PER HALKJ'AERNIELSEN~ and KRISTIANKEIDING Environmental Engineering Laboratory. University of Aalborg, Sohngaardsholmsvej 57. DK-9000 Aalborg, Denmark (First receh'ed August 1991: accepted in revised form May 1992)
Abstract--It was shown that Ca2. can be extracted from activated sludge from a plant with biological nitrogen and phosphorus removal either by an ion exchange process, where H +, Na +, K ÷ or Mg2÷ served as counter ions. or by chelation by EGTA. The extraction of Caz÷ led to an increase in the number of small particles and subsequently an increase in the specific resistance to filtration. it was argued that approx, half of the total Ca :~ pool was associated with the exopolymers and thereby formed an alginate-like gel, which constituted the backbone of the tic< structure. This notion was further emphasized by the observation that addition of Cu2. which, along with releasing Ca2÷. improved the dewaterability--probably because of the Cu:* ions ability to maintain the three-dimensional structure of the exopolymers. Key word,v--activated sludge, ion exchange, exopolymcrs, dcwaterability
INTRODUCTION One of the most expensive and least understood processes in wastewatcr treatment is the dewatcring process. The dcwatcring of activated sludge is seldom economically and technically optimal. The absence of a scientific understanding of the dcwatering process is due to the complexibility and the dynamics of the sludge matrix. This makes it difficult to find general characteristics to describe the specific sludge in terms of dcwatering. Many characteristics have been reported important for sludge dewatering, for instance: particle size distribution, floc structure and composition (e.g. exopolymcrs and presence of filamentous bacteria), bound water content, added chemicals, viscosity, etc. Floc size and particle size distribution (PSD) are considered two of the most important physical factors in the dewatering of sludge (Karr and Keinath, 1978; Lawler, 1986; Novak et al., 1988). Also the optimal dosage of flocculant before dewatering has been shown to depend on the PSD (Roberts and OIsson, 1975). Karr and Keinath (1978) showed that the supracolloidal fraction of the particles in activated sludge and anaerobic digested sludge strongly affected the dewater ability. The larger the fraction of supracolloidals the poorer the dewaterability. The supracolloidals were, in this case, defined as particles in the size range of 1-100/~m with a density less than or equal to that of water. Furthermore, there seems to be a general agreement that the appearance of smaller particles decreases the dewaterability. Lawler (1986) showed that the dewaterability decreases with decreasing particle size, measured as specific surface area. Also Novak et al. (1988) showed that smaller particles
in a broad particle size distribution tend to blind the sludge during filtration (sludge panicles migrating into the pores of the sludge cake). The PSD of the activated sludge varies from one treatment plant to another and reflects different types of wastewater, design and operation. If smaller particles appear due to changes in operation or wastcwater composition, a decreased dcwaterability may be expected. The occurrence of small particles has been observed by plant operators but the reasons arc usually not clear, although mechanical disruption and anaerobic storage (Rasmussen et al., 1992) have been reported as possible reasons. The increase in small particles may arise from disintegration of the individual floe. Disintegration is possible if the structure of the activated sludge floes is destroyed either by an active microbial process or by some physical-chemical processes. One of the factors suggested by many authors to have an influence on the sludge structure is the presence of exopolymers (Parker et al., 1972; Pavoni et al., 1972; Steiner et ai., 1976; Kang et al., 1990a, b). Exopolymers are extracellular polymers produced by the sludge bacteria and form the network in which the bacteria are embedded. The exopolymers from wastewater bacteria are to be considered the third major component of the activated sludge floc along with water and microorganisms (Li and Ganczarczyk, 1990). They consist largely of neutral sugars and glucoronic acid (Steiner et al., 1976) and also proteins and humic-type substances (Eriksson and Aim, 1991). Steiner and co-workers (1976) also found that polyvalent ions were important for the floc structure because they formed bonds between the exopolymers in the sludge matrix, probably by binding to carboxyi and hydroxyl groups. Busch and Stumm (1968)
1597
JACOBHt~GAAI~DBRUUSeta[.
1598
suggested that the role o f divalent cations was to form complexes or ion pair formation between the functional groups of the exopolymers and the bacteria. Ca 2+ seems to be the most important cation for the ability o f bridging between exopolymers themselves and exopolymers and bacteria in activated sludge (Forster and Lewin, 1972), in biofiims (Turakhia et al., 1983) and also for the floc formation and flocculation o f bacteria in pure culture (Kakii et al., 1990; Eriksson and Axberg, 1981). It is possible to replace extracellular Ca 2÷ in activated sludge by decreasing the pH or to remove it by adding E D T A (Kakii et al., 1985). Ca -'+ has also been removed from biofilm by a more Ca2÷-specific chelant, E G T A (Turakhia et al., 1983). If Ca 2. is replaced or removed from activated sludge floes a weakening of the sludge structure and an appearance of smaller particles may be expected. This change in particle size distribution has recently been shown by Eriksson and Aim 0991) in experiments with the addition o f a more general complexing agent (EDTA). The purpose o f this study has been to confirm the importance of the Ca 2 ~ ion for the structure o f the activated sludge floes, it was examined whether the removal of Ca 2~ would result in disintegration or deflocculation o f the sludge floes, and if so whether this change in particle size distribution to smaller particles would be reflected in a decreased dewaterability. This has been studied by cation exchange experiments and by addition of E G T A to activated sludge from a nutrient removal plant. Cu 2 + was added to activatcd sludge to obtain an indication if Ca z ~ was exopolymer bound, since several exopolymcrs arc reported to be more selective to the Cu 2 + ion than to the Ca: * ion. The preference for Cu 2' was examined by measurements o f dcwaterability.
(2) addition of the calcium-specific chelant [ethylene glycol-bis (p-aminocthyl ether)-N,N-tetraacetic acid (EGTA)]. After the respective additions the bottles were placed horizontally on a shaking table and shaken moderately for 90 rain--shaking speed was the same in all experiments. Experiments with a variation of shaking time have shown that this time was sufficient to obtain reproducible results and constant conditions. Each addition experiment was performed on separate samples, thus slight differences in initial values occurred. Sludge samples for cation experiments were centrifuged (Sigma 3KI2) for 10min at 4500rpm. After filtering the supernatant through a 0.22/zm Milliporefilter (type GV) the cations were measured by atomic absorption spectrophotometry (AAS) on a Perkin-Elmer model 305 B, acetylene-air flame. The burner head was turned 90°. Ca 2+ and Mg 2+ were determined with I% (w/v) La 3÷ (Welz, 1985). as well as without this radiation buffer. The difference in values obtained with and without the radiation buffer was taken as a measure of the strongly complexed Ca -`+ and Mgz+, respectively. 0.1% (w/v) Cs ÷ was added before measuring K + and Na ÷ (Welz, 1985). The total Ca 2÷ concentration was measured after oxidation by adding HNO~ to the dry sample and heating in an oven at 160'C for 4 h. Before measuring H202 and also ! ml 40% LaCI) per 10 ml sample were added. Dewaterability was characterized by measuring specific resistance to filtration according to Christensen and Dick (1985). Two sludge samples of 50 ml each were taken from each I litre polyethylene bottle and measured. A Whatman 41 filter with a diameter of 48 mm was used and the pressure was set to 0.5 bar. The amount of particles in the supernatant was characterized by measuring the turbidity. Sludge samples were poured into 10 ml polyethylene centrifuge vials and centrifuged for 2 rain at 2200 rpm (400g) in a Sigma 302 centrifuge. The turbidity in the supernatant was measured as absorption at 650 nm on a spectrophotometer (Bausch & Lomb, spectronic 2000). The absorption was calibrated to nephelometric turbidity units (NTU) according to Standard Methods (1985). All initial values of the activated sludge from the wastewater treatment plant at Aalborg East are shown in Table I. RESULTS AND DISCUSSION
METHODS All the experiments were performed on activated sludge taken from the Aalborg East wastewater treatment plant. This plant is designed for treatment of domestic wastewater from the city of Aalborg in an amount of 100,000 PE (person equivalents). The plant has full nitrification and denitrification (outlet concentration for total N is below 8 ms/I). Phosphorus is removed partly by biological phosphorus removal by the bio-denipho process (Harremo~s et al., 1991) and partly by chemical phosphorus removal by FeSO4 addition (0.7 tool Fe/mol P). Sludge age is around 30-35 days. It was found that even short time anaerobic storage, such as during transportation, could affect the sludge composition significantly [specific resistance to filtration (SRF) and conductivity] (Rasmussen et al., 1992), thus the sludge sampling, transportation and treatment were standardized before initialing the experiments. The sludge was taken from the aeration tank and immediately transported to the laboratory (approx. 30 rain from sampling to delivery at the laboratory). 1he sludge was thickened after arrival for 90 rain after which the supernatant was removed. After gentle mixing, the sludge was transferred to I litre polyethylene bottles. Two types of experiments were performed. (I) ion exchange experiments with addition of cations from stock solutions of CaCI 2, CuCI2'2H20, CuSO,.5HzO, MgCI2.6H20, KCI and NaCI;
Addition o f cations Mono- or divalent cations were added to thickened sludge and a release of Ca 2~ from the sludge matrix to the liquid phase was observed. It is seen in Fig. ! that the addition of the divalent i o n - - M g 2 + - - h a d an Table I. Approximate initial values for thickened activated sludge from the Aalborg East treatment plant Parameters Value SS 12-148/I VSS 7-8 ~l Ca2. 2.5 mM Mg2' 0.8 mM K" 0.TmM Na * 5-6 mM Tot-Ca 15.3 mM Tot-Mg 2.1 mM Tot-K 1.2 mM Tot-Fe 17.3 mM Tot-P 13 mM SO.~ I mM PC):0.15 mM CI 9 mM Conductivity 1000-1500/~S pH "/.2
On the stability of activated sludge floes mM
1599
NTU
Ca . + 300
-
--2
Na +
2
"0"0.0 i00 !
~
. * ~a +
1
0
! 50
I I00
Added I 150
meq. I 200
cations I 250
pr liter # 300
Released mM
"501 ~ 0
I 05
I
10
I 15
I
20
I 25
Ca +`..
I 30
Fig. I. The concentration of Ca 2+ in the bulk by adding cations to activated sludge.
Fig. 2. Turbidity as a function of released Caz+ in activated sludge.
immediate and large effect on the release of Ca 2+. The Ca z* concentration was asymptotic approaching a maximum release around 5.7 mmol/I. The addition of Mg z+ did not show any release of K ÷ or Na + (data not shown). The addition of monovalent cations showed a more modified release of Ca 2. compared to the release caused by Mg 2÷. Of the two monovalent cations added K + was the most effective in releasing Ca z+. The Ca 2+ release did not however, reach the maximum of 5.7 mM by addition of any monovalent cation, but the trend of the curves (Fig. I) looks as if a further addition would result in a further release of Ca 2~. The release of Mg 2~ by monovalcnt cations was equal to the release of Ca 2* on a relative basis (two times the initial value) (data not shown). It was not possible to measure if there was an uptake of the added ions because the added concentrations were high compared to the concentrations taken up. Ca 2+ was also added to activated sludge to see whether Ca 2+ was able to replace other cations from the sludge floes, and furthermore to assess the influence of a high ionic strength without the Ca 2~ release effect. This experiment showed an increase in the concentration of Mg 2+ from 0.4 to !.4 mM, whereas the addition of Mg 2+ resulted in an increase of Ca =* from 2.7 to 5.7 mM. The addition of Ca 2+ did not show any effect on the K* or Na + concentrations even at high concentrations of Ca 2" (data not shown). The addition of cations and the liberation of soluble Ca "~+ was found in all cases to be followed by an increase in the content of small particles in the supernatant. This was measured as an increase in turbidity on the supernatant of a centrifuged sludge sample. There was an increase in turbidity with increasing cation addition. When the turbidity of the centrifuged sludge sample was plotted against the amount of Ca z* released by addition of cations apparently two different patterns emerged (Fig. 2). The trend of the data for Mg :+ addition in Fig. 2 could either be described as a soft increasing curve or by two connected straight lines. Both ways ofdeseribing the data reflect an initial release of Ca 2+ without any substantial change in turbidity. In the case of Mg z* addition an initial release of approx, i.5 mM
Ca 2+ was found without any distinct changes in turbidity. Following this initial release there was a significant increase in turbidity. This indicates that Mg 2÷, to a certain degree, was able to substitute Ca 2+ in the sludge matrix. In the case of K + and Na + the turbidity seemed to follow the release of Ca 2' all the way. The difference in turbidity for the same concentrations of released Ca" ' indicated that factors other than just the amount of released Ca :+ influenced the floc structure. The process of releasing small particles seemed to depend on the added cation. A decrease in dewaterability with increasing turbidity was seen in all three experiments and it was possible to establish a linear dependency of SRF as a function of turbidity (see Fig. 3). The experiments formerly described showed a specific effect of, respectively, K +, Na + and Mg: + on the release of Ca: ~ and the formation of turbidity. The fact that SRF depended linearly on turbidity indicated that the processes leading to the turbidity had the same physical-chemical effects on the filterability.
Addition of EGTA The Ca2+-specific chelant, EGTA, was found to extract Ca z* from the activated sludge matrix (Fig. 4). There was a linear increase in the concentration of Ca 2. with the addition of EGTA up to a concentration of 4raM EGTA. From 4 to 5mM EGTA the Trn/kg 6.0
El
55
r~
50 45
E
40 35
NTU 0
0
I
50
100
I
150
I
I
t
200
250
300
Fig. 3. SRF vs turbidity for activated sludge with added cations.
1600
JACOB H~GAAIU~ BRUUS et al.
7'
rnM
~ Tm/kg
Ca ~'"
13
6
12 5
11
4
N
io~
i
N
3 2 1
NTU
Added mmol EGTA pr. liter
0
I
I
I
!
I
I
~
3
4
5
100
150
250
200
Fig. 4. The concentration o f Ca 2+ in the bulk by adding E G T A to activated sludge.
Fig. 6. S R F vs turbidity for activated sludge with added
release reached a maximum around 6.3 m M - - a release of approx. 4 mM Ca :+. Addition of EGTA included a pH decrease which also influenced Ca 2+ release. The EGTA addition only showed a very small effect on the release of Mg :+ (data not shown). The slope of the linear regression curve was 0.97 ( R ' = 0.98) which indicated that approx, one Ca "+ ion was exchanged for each EGTA ion added. The turbidity increased with increasing addition of EGTA. The values for the turbidity were within the range of the values found in the cation exchange experiments. This showcd the effectiveness of EGTA compared to K +, Na ' and Mg" ~ in releasing small particles, since the concentration of added EGTA was approx. I% of the concentrations of cations added. The lag-phase found in the Mg "+ additions (Fig. 2) was also found with EGTA addition (Fig. 5). Figure 5 shows, similar to the Mg ~ addition experiments, that there was only a small effect on turbidity until approx. 1.5 mM Ca 2' was released. The S R F measurements were more than two times higher than the results found in the cation addition experiments (Fig. 6). The slope for the turbidity vs S R F in this experiment was 2.67 x 10 -2 (R = 0.914) while the slope for the cation addition experiments was 0.67 x 10 -2 (R 2 =0.825). So not only was EGTA more effective in releasing small particles and thus decreasing the dewaterability, but also the process in which the particles were
released was more influential on the dewaterability. The different slopes showed that the turbidity alone could not be responsible for the decreased filterability. An explanation for the difference in the processes could be that the added cations (Na +, K +, Mg "+ ), despite the removal of Ca z+, still shield the repulsive electrostatic interactions between these sites and thus have some positive effect on the cohesion in the matrix. EGTA only removes Ca 2+ although the increased levels o f H + by EGTA addition might also reduce the repulsive effects.
EGTA.
Addition of Cu :~ Addition of CuCIz and CuSO4 to activated sludge led to the highest release of Ca 2' seen in any of the experiments conducted (Fig. 7). The release of Ca 2+ reached a maximum level around 10 m M - - a release of around 7.5 mM Ca z +, which was approx, twice the concentration released by EGTA addition. Addition of Cu ~~ as chloride or as sulphate also induced a pH effect. A 200 mM solution of CuCI2 has a pH of 3.6 and CuSO4 a pH of 4.0. To investigate the importance of this pH effect a parallel experiment was performed. HCI was added to activated sludge to approximately the same pH values as for the Cu 2+ experiment. Results are shown in Table 2. There was a significant decrease in the concentration of H + from the time HCI was added until measuring 90rain later-mM
NTU 250
12
200
Ca +~"
!
/
150 6
© ~
100
CuSO 4 CuCI
5O 1 0
o
i I
J ~
Released rnM i I 3 4
Ca ÷ ~"
Fig. 5. Turbidity as a function of released Ca ~+ in activated sludge with added EGTA.
0
Added meq o
5o
1oo
z .5o
:'oo
Cu+~pr
2.5o
liter
3oo
Fig. 7. The concentration o f Ca 2+ in the bulk by adding Cu 2~ to activated sludge.
On the stability of activated sludge flogs Table 2.
Resultsof HCIaddition to activated sludge pH after
pH added
90 rain shaking
Ca :+ (mM)
SRF (Tm/kg)
7.20 6.54 6.04
6.80 6.56 6.30
2.00 231 3.48
163 3.98 3.84
5.42 5.03 4,55 4.05
5.85 5.63 5.30 5.00
4.42 5.04 6.09 7.57
4.22 4.73 4.66 4.76
especially at high additions of HCI. The concentration of H + decreased approx. 10-fold for the lowest pH values. Ca :+ was released to a maximum of approx. 7.5 m M - - a release of approx. 5 raM. This showed that the change in pH could not be responsible for the total release of Ca 2+ in the Cu 2+ addition experiments. The measurements of SRF showed a big difference in the function of the Cu:* ions vs the H ÷ ions. The trends of the SRF curves for the respective additions were totally opposite. SRF decreased with Cu 2. addition while SRF increased with addition of HCI. Because of the problems with changing pH from addition to measurement, the SRF is illustrated vs released Ca 2. for the Cu 2~ and the H* additions (Fig. 8).
Turbidity The above results demonstrate a relationship beween the extraction of calcium from the sludge matrix and the increase of the specific resistance to filtration, through an increase in the number of small particles. The small particles could originate from either the sludge matrix or the liquid phase. In the liquid phase small particles could be induced in various ways. The particles could arise from precipitation of the added cations with inorganic matter or reactions with dissolved organic matter. In order to assess the possible reactions of the added cations with dissolved substance, cations were added to the supernatant of otherwise untreated activated sludge samples. The supernatant samples were obtained by centrifugation
Tm/k g
0 HCl
60
O CuSO~
CuCl a
50
O
~
C2-'
40
30 20
| 0
Released mM 0
1
2
3
4
5
6
7
8
Ca ~ +
9
Fig. 8. S R F plotted against the concentration o f released Ca z* by adding Cu:* and H + to activated sludge.
1601
of the activated sludge samples and part of these samples was filtered through a 0.22/am filter in order to retain macromolecules and free bacteria. In neither the filtered nor the unfiltered supernatant samples was particle formation observed when adding K *, Na+ or Mg 2÷. Addition of Ca z÷ above 20raM resulted in the formation of precipitate. Since the concentration of bicarbonate was approx. 3 mM this precipitate was assumed to be CaCO3 particles. However, as the concentration of extracted Ca 2+ in no case exceeded 10 mM in the above-quoted results, CaCO3 particles cannot explain the observed increase in turbidity. When adding Ca 2÷ to the unfiltered samples a minor extra increase in turbidity (5%) was observed, possibly coming from reactions of Ca :+ with organic macromolecules. The absence of a distinct increase in the turbidity in the supernatant when adding the cations shows that the small particles do not originate from the liquid phase. The turbidity increase must then originate from the sludge matrix. From the sludge matrix, turbidity could arise from the release of bacteria, the release of organic and inorganic colloids/particles and from the release of macromolecular organic matter. Microscopic examinations have shown a release of bacteria with the addition of cations and regression analysis has shown that part of the increase in turbidity may be related to the bacterial release. However, experiments by Rasmussen et al. 0992) have shown that a release of organic matter likewise could explain part of the increase in turbidity, thus so far no clear identification of the rise in turbidity is proposed. The turbidity correlates very well with dewaterability measured as specific resistance to filtration in all experiments. According to Karr and Keinath (1978) this could indicate that it is small particles or colloids that are responsible for the turbidity increase. Another interpretation of the turbidity measurements could be that the turbidity increase is to be considered as a symptom of a weakened sludge structure and it is the collapsing of this weakened structure that induces the decrease in dewaterability.
Ca:* pools in activated sludge The experiments also made it possible to give a rough suggestion of the Ca" * pools in activated sludge. Out of a total concentration of 15.3 mM Ca: * approx. 1.5 mM Ca 2. was found directly by analysing the sludge supernatant prior to the addition of cations (without adding La J* ). When adding I% (w/v) La ~+ to the same supernatant an extra I mM Ca 2. was detected. This indicates that approx. 1.5 mM Ca 2+ was present either as free ions or in weakly bound complexes in the untreated supernatant and that 1 mM Ca 2+ was bound in stronger complexes, such as, for example, calcium phosphates in the supernatant. Further, 3 mM Ca:* was released by addition of K ÷ or Na + to the activated sludge. This could be interpretated as a release of weakly bound Ca z÷ from the sludge matrix (e.g. exopolymers and precipitate).
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JAcoB HOYGAARD BRUt;Set al.
The observed increase in Ca :+ concentration by addition of K" and Na + was independent of La 3+ addition. This fact indicates that the possibility of binding Ca:" in strong complexes in the liquid phase was saturated. When adding Mg:÷ a larger initial release of Ca :+ was observed in supernatant samples with La 3. added compared to samples without La ~ . As the Mg:" concentration was increased the effect of adding La ~+ vanished. This suggests that along with the extraction of Ca:* from the sludge matrix, Mg:+ destroys the Ca: + complexes which were initially in the water phase. The large surplus of added cations compared to the release of Ca :+ in the cation exchange reactions suggests that the affinity of the sludge matrix towards the added cations is relatively small. The lag-phase in turbidity appearing by addition of Mg :+ and not by addition of K + and Na + indicates a difference in the affinity to Mg2+ compared to K + and Na +. A much more effective release of Ca "+ emerges from the EGTA addition. This resulted in a release of approx. 4 mM C a : ' , which implies a release of I mM Ca:" for each mM EGTA added. When employing CuCI: a total of 10 mM Ca" ' was released, which is larger than that found in former experiments. This also reflects the ion exchange reaction of H ~ as the high affinity of C u " to the sludge matrix, as discussed later, in the acccptance that the cffcctivcm,~s of Ca :~ is capable of rclcasing all the cxopolymcrbound Ca :~, then approx, half of the total amount of C a : ' would be bound in the cxopolymers. The remaining 5 mM C a " is uncxtractable by the methods applied here or sited within the bacterial mass. A sludge floc model Attempts to describe the desorption curve by Langmuir isotherms were unsuccessful in all experiments. This suggests that Ca-" is released from a number of different types of sites in the sludge matrix, including exopolymer-bound, inorganic precipitatebound or adsorbed to organic particles and also probably from different types of exopolymers, e.g. polysaccharides, proteins and humic-type substances. Assuming that Mg-"', to a certain degree, is capable of substituting Ca "+ in the sludge matrix, there is still a need to explain the lag phase in turbidity by addition of EGTA. In the experiments by Eriksson and Aim (1991) it was shown that addition of EDTA to activated sludge resulted in a release of exopolymers --without any lag phase. This fact compared to the linearity in the added EGTA vs released Ca 2" plot (Fig. 4), that Mg:* exchanges exopolymer-bound Ca" * before any other complexes indicates that EGTA attacks Ca-"~ in the exopolymers without any change in turbidity. This could be explained by a division of the floes into smaller parts large enough to settle by centrifuging. This was seen by Eriksson and Aim (1991) for one out of seven sludges. Addition of CuCI: and CuSO~ to the sludge showed a pattern different from the cation as well
as the EGTA additions. The decrease in the specific resistance to filtration (Fig. 8) suggests that small particles are not formed under these circumstances. Hence, it is assumed that Ca :* is substituted in the exopolymer structure by Cu "+ and that the effectiveness of Cu :+ results in an even more stable structure of the sludge (turbidity was not measured in these samples due to the interfering blue colour of the Cu e+ solution). The selectivity for Cu :+ compared to Ca :+ is a characteristic property for carboxylate-containing exopolymers (Smidsrod, 1974) and in particular for alginates. Also other results of this investigation match the properties of alginates. In these experiments a release of Mg "+ when adding Ca :+ did not affect the dewaterability, so Ca :+ has probably taken its place in the sludge floc structure. This affinity for Ca 2+ compared to Mg 2÷ is a characteristic of the alginates. Furthermore, alginates form a so-called egg-box structure with Ca 2+ which is fairly resistant to increased conductivity (Smidsrod, 1974). This resembles results from these experiments in which addition of C~ 2+ to activated sludge did not affect dewaterability, implying that the exopolymers are still effective and do not coil in spite of the conductivity increase. Alginates are exopolymers consisting of two substantial monomers: 1,4-1inked a-L-guluronic acid and p-D-mannuronic acid. It has not been possible to find literature indicating that these substances are present in activated sludge, but, on the other hand. only a few investigations of the exopolymers in activated sludge have been conducted. AIginates could be present in activated sludge since they are present in the mucilage of some bacteria (Christcnsen, 1989). From these indications we propose that it could be an alginate-like cross-link of exopolymers that creates the floc structure. These exopolymers being linked together by Ca 2+ ions. Novak and Haugan (1978) had a similar proposal. They proposed that since the polymeric materials stripped from the bacterial floes had similar properties to whole floes it indicated that the floc matrix was also of biological origin. Relations to full-scale sludge dewatering The results presented here are believed to relate to phenomena in full-scale activated sludge dewatering. The relationship between simultaneous changes in conductivity by a release of various ions, turbidity and specific resistance to filtration has been observed when storing activated sludge anaerobically. In the early stages of anaerobic storage a release of Ca 2+ and an increase in the turbidity is observed (Rasmussen et al., 1992). This might be explained by an ion exchange process similar to that demonstrated in this paper and maybe by a microbial degradation of Ca 2+ binding exopolymers. At a later stage in anaerobic storage the effects on turbidity and dewaterability exceed what is explicable solely by an ion exchange process.
On the stability of activated sludge floes Another example is given by Teichgr~ber (1991) who described how activated sludge was destroyed by excess acid production during nitrification in plants w/th low alkalinity. This might be understood by an exchange of Ca "+ with H " similar to our experiments with HCI (Fig. 8). Also as salts are applied to roadways in wintertime, increased levels in conductivity are observed at sewage treatment plants. A n ion exchange process similar to that reported for Na ÷ is likely then to take place. It is recognized that the polymer consumption increases noticeably under these circumstances (Berggren, 1991), indicating the same phenomena as seen in this investigation. In the latter examples the exchanging cation is applied externally to the floes, as is also done in our experiments, while in the former example with anaerobic storage, the exchanging cations are produced by the bacteria, and thus applied internally to the floes. Hence, the weakening of the floc structure, i.e. the change in floc strength, is likely to be different in these two situations. The manner in which the floc strength is changed may in turn lead to different responses to shearing forces. In these experiments a fairly mild agitation has been applied, thus a more pronounced effect on the number of small particles and dewaterability may be observed if shearing forces compared to those in full-scale plants were applied. SUMMARY ANt) CONCLUSIONS
Ca' ~ can be extracted from activated sludge from a wastcwater treatment plant either by ion exchange processes or through removal by a chelating agent. The sludge deteriorates along with the removal of Ca '~ which indicates the importance of Ca '+ as binding agent in the sludge flo¢ structure. Turbidity correlates well with filterability in all experiments indicating that either the small particles or colloids arc partly responsible for the filterability decrease or that the increased turbidity is to be considered as a sign on a weakened sludge structure. Addition of Cu '+ improves the filterability of activated sludge along with a release of Ca '+. This indicates that Ca "+ is substituted in the exopolymer structure by the Cu "~ ions resulting in a more stable floc structure. The sludge floc structure is proposed to be considered as a three-dimensional exopolymer matrix (a gel) kept together by divalent cations with varying selectivity to the matrix (Cu '÷ > Ca" ~ > Mg '~ ). It is proposed that approx, half of the total amount of Ca '~ in the activated sludge is bound in the exopolymers. Acknowledgements--The authors want to thank Dr P. Aarne Vesilind for comments on the manuscript. Also thanks go to M. Schneider and L. Wybrandt for technical assistance in the laboratory. The Danish Technical Research Council (Grant 5.26.09.12) is gratefully acknowledged for economical support of this work.
1603 REFERENCES
APHA (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. American Public Health Association, Washington, D.C. Bcrggren H. C. (1991) Alfred Gad's eftr. Personal communication. Busch P. L. and Stumm W. 0968) Chemical interactions in the aggregation of bacteria. Bioflocculation in waste treatment. Era,Jr. Sci. Technol. 2, 49-53. Christensen B. E. (1989) The role of extracellular polysaccharides in biofilms. J. Biotechnol. I0, 181-202. Christensen G. L, and Dick R. I. (1985) Specific resistance measurements: methods and procedures. J. Enrir. Engng !11, 258-271. Eriksson L. and Aim B. (1991) Study of flocculation mechanisms by observing effects of a complexing agent on activated sludge properties. War. Sci. TechnoL 24, 21-28. Eriksson L. and Axberg C. (1981) Direct influence of wastewater pollutants on flocculation and sedimentation behaviour in biological waste water treatment--I. Model system E. coli B. War. Res. 15, 421-431. Forster C. F. and Lcwin D. C. 0972) Polymer interactions at activated sludge surfaces. Effl. War. Treat. J. 12, 520 -525. Harremo6s P., Bundgaard E. and Henze M (1991) Developments in wastewater treatment for nutrient removal. J. Eur. Wat. Pollut. Control Fed. I, 19-23. Kakii K., Hasumi M., Shirakashi T. and Kuriyama M. (1990) Involvement of Ca 2" in the flocculation of Klu)~'era cryocrescens KA-103. J. Ferm. Bioengng 69, 224 -227. Kakii K.. Kitamura S., Shirakashi T. and Kuriyama M. (1985) Effect of calcium ion on sludge characteristics. J. Ferm. Technol. 63, 263-270. Kang S-M., Kishimoto M., Shioya S., Yoshida T. and Suga K. |. (1990a) Properties of extracellular polymer having an effect on expression of activated sludge. J. Ferm. Bioengng 69, I I I-116. Kang S-M., Kishimoto M., Shioya S., Yoshida T. and Suga K. !. (1990b) Dewatering of extracellular polymer and activated sludges by thermochemical treatment. J. Ferm. Bioengng 69, !17-121. Karr P. R. and Keinath T. (1978) Influence of particle size on sludge dewaterability. J. War. Pollut. Control Fed. 50, 1911- 1930. Lawler D. F. (1986) Removing particles in water and wastewater. Envir. Sci. Technol. 20, 856-861. Li D-H. and Ganczarczyk J. J. (1990) Structure of activated sludge floes. Biotechnol. Bioengng 35, 57-65. Novak J. T. and Haugan B.-E. (1978) Chemical conditioning of waste activated sludge. NIVA (Norsk Instituttfor Vannforskning), Report C3-22. Novak J. T., Goodman G. L., Pariroo A. and Huang J. C. (1988) The blinding of sludges during filtration. J. Wac Pollut. Control Fed. 60, 206-214. Parker D. G., Randall C. W. and King P. H. (1972) Biological conditioning for improved sludge filterability. J. Wat. Pollut. Control Fed. 44, 2066-2077. Pavoni J. L., Tenney M. W. and W. F. Echelberger Jr (1972) Bacterial exocellular polymers and biological flocculation. J. War. Pollut. Control Fed. 44, 414-431. Rasmussen H., Bruus J. H., Keiding K. and Nielsen P. H. (1992) Physical, chemical and microbiological changes in anaerobically stored activated sludge from a nutrient removal plant: effects on dewaterability, in preparation. Roberts K. and Olsson O. 0975) Influence of colloidal particles on dewatering of activated sludge with polyelectrolyte. J. envir. Sci. Technol. 9, 945-948. Smidsrod O. (1974) Molecular basis for some physical properties ofalginates in the Gel state. Farada|' Discuss. Chem. Soc. 57, 263-274.
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JACOB H~YGAAI~.DBRUUSel al.
Stciner A. E., McLarcn D. A. and Forster C. F. (1976) The nature of activated sludge flocs. War. Res. 10, 25-30. Teichgrib~" B. (1991) Nitrification of sewage with low buffering capacity. J. Fur. War. Pollut. Control Fed. !, 7-12.
Tu~khia M. H.. Cooksey K. E. and Characklis W. G. (1983) Influemz of a calcium-specific ch¢lant on biofilm removal. J. appl. em'ir. Microbioi. 46, 1236-1238. Welz B. (1985) Atomic Absorption Spectrometry. VCH. Weinheim, Germany.
0043-1354/92$5.00+ 0.00 Copyright C 1992PergamonPresa Ltd
ON THE STABILITY OF ACTIVATED SLUDGE FLOCS WITH IMPLICATIONS TO DEWATERING JACOB HOYGAARDBRUUS,PER HALKJ'AERNIELSEN~ and KRISTIANKEIDING Environmental Engineering Laboratory. University of Aalborg, Sohngaardsholmsvej 57. DK-9000 Aalborg, Denmark (First receh'ed August 1991: accepted in revised form May 1992)
Abstract--It was shown that Ca2. can be extracted from activated sludge from a plant with biological nitrogen and phosphorus removal either by an ion exchange process, where H +, Na +, K ÷ or Mg2÷ served as counter ions. or by chelation by EGTA. The extraction of Caz÷ led to an increase in the number of small particles and subsequently an increase in the specific resistance to filtration. it was argued that approx, half of the total Ca :~ pool was associated with the exopolymers and thereby formed an alginate-like gel, which constituted the backbone of the tic< structure. This notion was further emphasized by the observation that addition of Cu2. which, along with releasing Ca2÷. improved the dewaterability--probably because of the Cu:* ions ability to maintain the three-dimensional structure of the exopolymers. Key word,v--activated sludge, ion exchange, exopolymcrs, dcwaterability
INTRODUCTION One of the most expensive and least understood processes in wastewatcr treatment is the dewatcring process. The dcwatcring of activated sludge is seldom economically and technically optimal. The absence of a scientific understanding of the dcwatering process is due to the complexibility and the dynamics of the sludge matrix. This makes it difficult to find general characteristics to describe the specific sludge in terms of dcwatering. Many characteristics have been reported important for sludge dewatering, for instance: particle size distribution, floc structure and composition (e.g. exopolymcrs and presence of filamentous bacteria), bound water content, added chemicals, viscosity, etc. Floc size and particle size distribution (PSD) are considered two of the most important physical factors in the dewatering of sludge (Karr and Keinath, 1978; Lawler, 1986; Novak et al., 1988). Also the optimal dosage of flocculant before dewatering has been shown to depend on the PSD (Roberts and OIsson, 1975). Karr and Keinath (1978) showed that the supracolloidal fraction of the particles in activated sludge and anaerobic digested sludge strongly affected the dewater ability. The larger the fraction of supracolloidals the poorer the dewaterability. The supracolloidals were, in this case, defined as particles in the size range of 1-100/~m with a density less than or equal to that of water. Furthermore, there seems to be a general agreement that the appearance of smaller particles decreases the dewaterability. Lawler (1986) showed that the dewaterability decreases with decreasing particle size, measured as specific surface area. Also Novak et al. (1988) showed that smaller particles
in a broad particle size distribution tend to blind the sludge during filtration (sludge panicles migrating into the pores of the sludge cake). The PSD of the activated sludge varies from one treatment plant to another and reflects different types of wastewater, design and operation. If smaller particles appear due to changes in operation or wastcwater composition, a decreased dcwaterability may be expected. The occurrence of small particles has been observed by plant operators but the reasons arc usually not clear, although mechanical disruption and anaerobic storage (Rasmussen et al., 1992) have been reported as possible reasons. The increase in small particles may arise from disintegration of the individual floe. Disintegration is possible if the structure of the activated sludge floes is destroyed either by an active microbial process or by some physical-chemical processes. One of the factors suggested by many authors to have an influence on the sludge structure is the presence of exopolymers (Parker et al., 1972; Pavoni et al., 1972; Steiner et ai., 1976; Kang et al., 1990a, b). Exopolymers are extracellular polymers produced by the sludge bacteria and form the network in which the bacteria are embedded. The exopolymers from wastewater bacteria are to be considered the third major component of the activated sludge floc along with water and microorganisms (Li and Ganczarczyk, 1990). They consist largely of neutral sugars and glucoronic acid (Steiner et al., 1976) and also proteins and humic-type substances (Eriksson and Aim, 1991). Steiner and co-workers (1976) also found that polyvalent ions were important for the floc structure because they formed bonds between the exopolymers in the sludge matrix, probably by binding to carboxyi and hydroxyl groups. Busch and Stumm (1968)
1597
JACOBHt~GAAI~DBRUUSeta[.
1598
suggested that the role o f divalent cations was to form complexes or ion pair formation between the functional groups of the exopolymers and the bacteria. Ca 2+ seems to be the most important cation for the ability o f bridging between exopolymers themselves and exopolymers and bacteria in activated sludge (Forster and Lewin, 1972), in biofiims (Turakhia et al., 1983) and also for the floc formation and flocculation o f bacteria in pure culture (Kakii et al., 1990; Eriksson and Axberg, 1981). It is possible to replace extracellular Ca 2÷ in activated sludge by decreasing the pH or to remove it by adding E D T A (Kakii et al., 1985). Ca -'+ has also been removed from biofilm by a more Ca2÷-specific chelant, E G T A (Turakhia et al., 1983). If Ca 2. is replaced or removed from activated sludge floes a weakening of the sludge structure and an appearance of smaller particles may be expected. This change in particle size distribution has recently been shown by Eriksson and Aim 0991) in experiments with the addition o f a more general complexing agent (EDTA). The purpose o f this study has been to confirm the importance of the Ca 2 ~ ion for the structure o f the activated sludge floes, it was examined whether the removal of Ca 2~ would result in disintegration or deflocculation o f the sludge floes, and if so whether this change in particle size distribution to smaller particles would be reflected in a decreased dewaterability. This has been studied by cation exchange experiments and by addition of E G T A to activated sludge from a nutrient removal plant. Cu 2 + was added to activatcd sludge to obtain an indication if Ca z ~ was exopolymer bound, since several exopolymcrs arc reported to be more selective to the Cu 2 + ion than to the Ca: * ion. The preference for Cu 2' was examined by measurements o f dcwaterability.
(2) addition of the calcium-specific chelant [ethylene glycol-bis (p-aminocthyl ether)-N,N-tetraacetic acid (EGTA)]. After the respective additions the bottles were placed horizontally on a shaking table and shaken moderately for 90 rain--shaking speed was the same in all experiments. Experiments with a variation of shaking time have shown that this time was sufficient to obtain reproducible results and constant conditions. Each addition experiment was performed on separate samples, thus slight differences in initial values occurred. Sludge samples for cation experiments were centrifuged (Sigma 3KI2) for 10min at 4500rpm. After filtering the supernatant through a 0.22/zm Milliporefilter (type GV) the cations were measured by atomic absorption spectrophotometry (AAS) on a Perkin-Elmer model 305 B, acetylene-air flame. The burner head was turned 90°. Ca 2+ and Mg 2+ were determined with I% (w/v) La 3÷ (Welz, 1985). as well as without this radiation buffer. The difference in values obtained with and without the radiation buffer was taken as a measure of the strongly complexed Ca -`+ and Mgz+, respectively. 0.1% (w/v) Cs ÷ was added before measuring K + and Na ÷ (Welz, 1985). The total Ca 2÷ concentration was measured after oxidation by adding HNO~ to the dry sample and heating in an oven at 160'C for 4 h. Before measuring H202 and also ! ml 40% LaCI) per 10 ml sample were added. Dewaterability was characterized by measuring specific resistance to filtration according to Christensen and Dick (1985). Two sludge samples of 50 ml each were taken from each I litre polyethylene bottle and measured. A Whatman 41 filter with a diameter of 48 mm was used and the pressure was set to 0.5 bar. The amount of particles in the supernatant was characterized by measuring the turbidity. Sludge samples were poured into 10 ml polyethylene centrifuge vials and centrifuged for 2 rain at 2200 rpm (400g) in a Sigma 302 centrifuge. The turbidity in the supernatant was measured as absorption at 650 nm on a spectrophotometer (Bausch & Lomb, spectronic 2000). The absorption was calibrated to nephelometric turbidity units (NTU) according to Standard Methods (1985). All initial values of the activated sludge from the wastewater treatment plant at Aalborg East are shown in Table I. RESULTS AND DISCUSSION
METHODS All the experiments were performed on activated sludge taken from the Aalborg East wastewater treatment plant. This plant is designed for treatment of domestic wastewater from the city of Aalborg in an amount of 100,000 PE (person equivalents). The plant has full nitrification and denitrification (outlet concentration for total N is below 8 ms/I). Phosphorus is removed partly by biological phosphorus removal by the bio-denipho process (Harremo~s et al., 1991) and partly by chemical phosphorus removal by FeSO4 addition (0.7 tool Fe/mol P). Sludge age is around 30-35 days. It was found that even short time anaerobic storage, such as during transportation, could affect the sludge composition significantly [specific resistance to filtration (SRF) and conductivity] (Rasmussen et al., 1992), thus the sludge sampling, transportation and treatment were standardized before initialing the experiments. The sludge was taken from the aeration tank and immediately transported to the laboratory (approx. 30 rain from sampling to delivery at the laboratory). 1he sludge was thickened after arrival for 90 rain after which the supernatant was removed. After gentle mixing, the sludge was transferred to I litre polyethylene bottles. Two types of experiments were performed. (I) ion exchange experiments with addition of cations from stock solutions of CaCI 2, CuCI2'2H20, CuSO,.5HzO, MgCI2.6H20, KCI and NaCI;
Addition o f cations Mono- or divalent cations were added to thickened sludge and a release of Ca 2~ from the sludge matrix to the liquid phase was observed. It is seen in Fig. ! that the addition of the divalent i o n - - M g 2 + - - h a d an Table I. Approximate initial values for thickened activated sludge from the Aalborg East treatment plant Parameters Value SS 12-148/I VSS 7-8 ~l Ca2. 2.5 mM Mg2' 0.8 mM K" 0.TmM Na * 5-6 mM Tot-Ca 15.3 mM Tot-Mg 2.1 mM Tot-K 1.2 mM Tot-Fe 17.3 mM Tot-P 13 mM SO.~ I mM PC):0.15 mM CI 9 mM Conductivity 1000-1500/~S pH "/.2
On the stability of activated sludge floes mM
1599
NTU
Ca . + 300
-
--2
Na +
2
"0"0.0 i00 !
~
. * ~a +
1
0
! 50
I I00
Added I 150
meq. I 200
cations I 250
pr liter # 300
Released mM
"501 ~ 0
I 05
I
10
I 15
I
20
I 25
Ca +`..
I 30
Fig. I. The concentration of Ca 2+ in the bulk by adding cations to activated sludge.
Fig. 2. Turbidity as a function of released Caz+ in activated sludge.
immediate and large effect on the release of Ca 2+. The Ca z* concentration was asymptotic approaching a maximum release around 5.7 mmol/I. The addition of Mg z+ did not show any release of K ÷ or Na + (data not shown). The addition of monovalent cations showed a more modified release of Ca 2. compared to the release caused by Mg 2÷. Of the two monovalent cations added K + was the most effective in releasing Ca z+. The Ca 2+ release did not however, reach the maximum of 5.7 mM by addition of any monovalent cation, but the trend of the curves (Fig. I) looks as if a further addition would result in a further release of Ca 2~. The release of Mg 2~ by monovalcnt cations was equal to the release of Ca 2* on a relative basis (two times the initial value) (data not shown). It was not possible to measure if there was an uptake of the added ions because the added concentrations were high compared to the concentrations taken up. Ca 2+ was also added to activated sludge to see whether Ca 2+ was able to replace other cations from the sludge floes, and furthermore to assess the influence of a high ionic strength without the Ca 2~ release effect. This experiment showed an increase in the concentration of Mg 2+ from 0.4 to !.4 mM, whereas the addition of Mg 2+ resulted in an increase of Ca =* from 2.7 to 5.7 mM. The addition of Ca 2+ did not show any effect on the K* or Na + concentrations even at high concentrations of Ca 2" (data not shown). The addition of cations and the liberation of soluble Ca "~+ was found in all cases to be followed by an increase in the content of small particles in the supernatant. This was measured as an increase in turbidity on the supernatant of a centrifuged sludge sample. There was an increase in turbidity with increasing cation addition. When the turbidity of the centrifuged sludge sample was plotted against the amount of Ca z* released by addition of cations apparently two different patterns emerged (Fig. 2). The trend of the data for Mg :+ addition in Fig. 2 could either be described as a soft increasing curve or by two connected straight lines. Both ways ofdeseribing the data reflect an initial release of Ca 2+ without any substantial change in turbidity. In the case of Mg z* addition an initial release of approx, i.5 mM
Ca 2+ was found without any distinct changes in turbidity. Following this initial release there was a significant increase in turbidity. This indicates that Mg 2÷, to a certain degree, was able to substitute Ca 2+ in the sludge matrix. In the case of K + and Na + the turbidity seemed to follow the release of Ca 2' all the way. The difference in turbidity for the same concentrations of released Ca" ' indicated that factors other than just the amount of released Ca :+ influenced the floc structure. The process of releasing small particles seemed to depend on the added cation. A decrease in dewaterability with increasing turbidity was seen in all three experiments and it was possible to establish a linear dependency of SRF as a function of turbidity (see Fig. 3). The experiments formerly described showed a specific effect of, respectively, K +, Na + and Mg: + on the release of Ca: ~ and the formation of turbidity. The fact that SRF depended linearly on turbidity indicated that the processes leading to the turbidity had the same physical-chemical effects on the filterability.
Addition of EGTA The Ca2+-specific chelant, EGTA, was found to extract Ca z* from the activated sludge matrix (Fig. 4). There was a linear increase in the concentration of Ca 2. with the addition of EGTA up to a concentration of 4raM EGTA. From 4 to 5mM EGTA the Trn/kg 6.0
El
55
r~
50 45
E
40 35
NTU 0
0
I
50
100
I
150
I
I
t
200
250
300
Fig. 3. SRF vs turbidity for activated sludge with added cations.
1600
JACOB H~GAAIU~ BRUUS et al.
7'
rnM
~ Tm/kg
Ca ~'"
13
6
12 5
11
4
N
io~
i
N
3 2 1
NTU
Added mmol EGTA pr. liter
0
I
I
I
!
I
I
~
3
4
5
100
150
250
200
Fig. 4. The concentration o f Ca 2+ in the bulk by adding E G T A to activated sludge.
Fig. 6. S R F vs turbidity for activated sludge with added
release reached a maximum around 6.3 m M - - a release of approx. 4 mM Ca :+. Addition of EGTA included a pH decrease which also influenced Ca 2+ release. The EGTA addition only showed a very small effect on the release of Mg :+ (data not shown). The slope of the linear regression curve was 0.97 ( R ' = 0.98) which indicated that approx, one Ca "+ ion was exchanged for each EGTA ion added. The turbidity increased with increasing addition of EGTA. The values for the turbidity were within the range of the values found in the cation exchange experiments. This showcd the effectiveness of EGTA compared to K +, Na ' and Mg" ~ in releasing small particles, since the concentration of added EGTA was approx. I% of the concentrations of cations added. The lag-phase found in the Mg "+ additions (Fig. 2) was also found with EGTA addition (Fig. 5). Figure 5 shows, similar to the Mg ~ addition experiments, that there was only a small effect on turbidity until approx. 1.5 mM Ca 2' was released. The S R F measurements were more than two times higher than the results found in the cation addition experiments (Fig. 6). The slope for the turbidity vs S R F in this experiment was 2.67 x 10 -2 (R = 0.914) while the slope for the cation addition experiments was 0.67 x 10 -2 (R 2 =0.825). So not only was EGTA more effective in releasing small particles and thus decreasing the dewaterability, but also the process in which the particles were
released was more influential on the dewaterability. The different slopes showed that the turbidity alone could not be responsible for the decreased filterability. An explanation for the difference in the processes could be that the added cations (Na +, K +, Mg "+ ), despite the removal of Ca z+, still shield the repulsive electrostatic interactions between these sites and thus have some positive effect on the cohesion in the matrix. EGTA only removes Ca 2+ although the increased levels o f H + by EGTA addition might also reduce the repulsive effects.
EGTA.
Addition of Cu :~ Addition of CuCIz and CuSO4 to activated sludge led to the highest release of Ca 2' seen in any of the experiments conducted (Fig. 7). The release of Ca 2+ reached a maximum level around 10 m M - - a release of around 7.5 mM Ca z +, which was approx, twice the concentration released by EGTA addition. Addition of Cu ~~ as chloride or as sulphate also induced a pH effect. A 200 mM solution of CuCI2 has a pH of 3.6 and CuSO4 a pH of 4.0. To investigate the importance of this pH effect a parallel experiment was performed. HCI was added to activated sludge to approximately the same pH values as for the Cu 2+ experiment. Results are shown in Table 2. There was a significant decrease in the concentration of H + from the time HCI was added until measuring 90rain later-mM
NTU 250
12
200
Ca +~"
!
/
150 6
© ~
100
CuSO 4 CuCI
5O 1 0
o
i I
J ~
Released rnM i I 3 4
Ca ÷ ~"
Fig. 5. Turbidity as a function of released Ca ~+ in activated sludge with added EGTA.
0
Added meq o
5o
1oo
z .5o
:'oo
Cu+~pr
2.5o
liter
3oo
Fig. 7. The concentration o f Ca 2+ in the bulk by adding Cu 2~ to activated sludge.
On the stability of activated sludge flogs Table 2.
Resultsof HCIaddition to activated sludge pH after
pH added
90 rain shaking
Ca :+ (mM)
SRF (Tm/kg)
7.20 6.54 6.04
6.80 6.56 6.30
2.00 231 3.48
163 3.98 3.84
5.42 5.03 4,55 4.05
5.85 5.63 5.30 5.00
4.42 5.04 6.09 7.57
4.22 4.73 4.66 4.76
especially at high additions of HCI. The concentration of H + decreased approx. 10-fold for the lowest pH values. Ca :+ was released to a maximum of approx. 7.5 m M - - a release of approx. 5 raM. This showed that the change in pH could not be responsible for the total release of Ca 2+ in the Cu 2+ addition experiments. The measurements of SRF showed a big difference in the function of the Cu:* ions vs the H ÷ ions. The trends of the SRF curves for the respective additions were totally opposite. SRF decreased with Cu 2. addition while SRF increased with addition of HCI. Because of the problems with changing pH from addition to measurement, the SRF is illustrated vs released Ca 2. for the Cu 2~ and the H* additions (Fig. 8).
Turbidity The above results demonstrate a relationship beween the extraction of calcium from the sludge matrix and the increase of the specific resistance to filtration, through an increase in the number of small particles. The small particles could originate from either the sludge matrix or the liquid phase. In the liquid phase small particles could be induced in various ways. The particles could arise from precipitation of the added cations with inorganic matter or reactions with dissolved organic matter. In order to assess the possible reactions of the added cations with dissolved substance, cations were added to the supernatant of otherwise untreated activated sludge samples. The supernatant samples were obtained by centrifugation
Tm/k g
0 HCl
60
O CuSO~
CuCl a
50
O
~
C2-'
40
30 20
| 0
Released mM 0
1
2
3
4
5
6
7
8
Ca ~ +
9
Fig. 8. S R F plotted against the concentration o f released Ca z* by adding Cu:* and H + to activated sludge.
1601
of the activated sludge samples and part of these samples was filtered through a 0.22/am filter in order to retain macromolecules and free bacteria. In neither the filtered nor the unfiltered supernatant samples was particle formation observed when adding K *, Na+ or Mg 2÷. Addition of Ca z÷ above 20raM resulted in the formation of precipitate. Since the concentration of bicarbonate was approx. 3 mM this precipitate was assumed to be CaCO3 particles. However, as the concentration of extracted Ca 2+ in no case exceeded 10 mM in the above-quoted results, CaCO3 particles cannot explain the observed increase in turbidity. When adding Ca 2÷ to the unfiltered samples a minor extra increase in turbidity (5%) was observed, possibly coming from reactions of Ca :+ with organic macromolecules. The absence of a distinct increase in the turbidity in the supernatant when adding the cations shows that the small particles do not originate from the liquid phase. The turbidity increase must then originate from the sludge matrix. From the sludge matrix, turbidity could arise from the release of bacteria, the release of organic and inorganic colloids/particles and from the release of macromolecular organic matter. Microscopic examinations have shown a release of bacteria with the addition of cations and regression analysis has shown that part of the increase in turbidity may be related to the bacterial release. However, experiments by Rasmussen et al. 0992) have shown that a release of organic matter likewise could explain part of the increase in turbidity, thus so far no clear identification of the rise in turbidity is proposed. The turbidity correlates very well with dewaterability measured as specific resistance to filtration in all experiments. According to Karr and Keinath (1978) this could indicate that it is small particles or colloids that are responsible for the turbidity increase. Another interpretation of the turbidity measurements could be that the turbidity increase is to be considered as a symptom of a weakened sludge structure and it is the collapsing of this weakened structure that induces the decrease in dewaterability.
Ca:* pools in activated sludge The experiments also made it possible to give a rough suggestion of the Ca" * pools in activated sludge. Out of a total concentration of 15.3 mM Ca: * approx. 1.5 mM Ca 2. was found directly by analysing the sludge supernatant prior to the addition of cations (without adding La J* ). When adding I% (w/v) La ~+ to the same supernatant an extra I mM Ca 2. was detected. This indicates that approx. 1.5 mM Ca 2+ was present either as free ions or in weakly bound complexes in the untreated supernatant and that 1 mM Ca 2+ was bound in stronger complexes, such as, for example, calcium phosphates in the supernatant. Further, 3 mM Ca:* was released by addition of K ÷ or Na + to the activated sludge. This could be interpretated as a release of weakly bound Ca z÷ from the sludge matrix (e.g. exopolymers and precipitate).
1602
JAcoB HOYGAARD BRUt;Set al.
The observed increase in Ca :+ concentration by addition of K" and Na + was independent of La 3+ addition. This fact indicates that the possibility of binding Ca:" in strong complexes in the liquid phase was saturated. When adding Mg:÷ a larger initial release of Ca :+ was observed in supernatant samples with La 3. added compared to samples without La ~ . As the Mg:" concentration was increased the effect of adding La ~+ vanished. This suggests that along with the extraction of Ca:* from the sludge matrix, Mg:+ destroys the Ca: + complexes which were initially in the water phase. The large surplus of added cations compared to the release of Ca :+ in the cation exchange reactions suggests that the affinity of the sludge matrix towards the added cations is relatively small. The lag-phase in turbidity appearing by addition of Mg :+ and not by addition of K + and Na + indicates a difference in the affinity to Mg2+ compared to K + and Na +. A much more effective release of Ca "+ emerges from the EGTA addition. This resulted in a release of approx. 4 mM C a : ' , which implies a release of I mM Ca:" for each mM EGTA added. When employing CuCI: a total of 10 mM Ca" ' was released, which is larger than that found in former experiments. This also reflects the ion exchange reaction of H ~ as the high affinity of C u " to the sludge matrix, as discussed later, in the acccptance that the cffcctivcm,~s of Ca :~ is capable of rclcasing all the cxopolymcrbound Ca :~, then approx, half of the total amount of C a : ' would be bound in the cxopolymers. The remaining 5 mM C a " is uncxtractable by the methods applied here or sited within the bacterial mass. A sludge floc model Attempts to describe the desorption curve by Langmuir isotherms were unsuccessful in all experiments. This suggests that Ca-" is released from a number of different types of sites in the sludge matrix, including exopolymer-bound, inorganic precipitatebound or adsorbed to organic particles and also probably from different types of exopolymers, e.g. polysaccharides, proteins and humic-type substances. Assuming that Mg-"', to a certain degree, is capable of substituting Ca "+ in the sludge matrix, there is still a need to explain the lag phase in turbidity by addition of EGTA. In the experiments by Eriksson and Aim (1991) it was shown that addition of EDTA to activated sludge resulted in a release of exopolymers --without any lag phase. This fact compared to the linearity in the added EGTA vs released Ca 2" plot (Fig. 4), that Mg:* exchanges exopolymer-bound Ca" * before any other complexes indicates that EGTA attacks Ca-"~ in the exopolymers without any change in turbidity. This could be explained by a division of the floes into smaller parts large enough to settle by centrifuging. This was seen by Eriksson and Aim (1991) for one out of seven sludges. Addition of CuCI: and CuSO~ to the sludge showed a pattern different from the cation as well
as the EGTA additions. The decrease in the specific resistance to filtration (Fig. 8) suggests that small particles are not formed under these circumstances. Hence, it is assumed that Ca :* is substituted in the exopolymer structure by Cu "+ and that the effectiveness of Cu :+ results in an even more stable structure of the sludge (turbidity was not measured in these samples due to the interfering blue colour of the Cu e+ solution). The selectivity for Cu :+ compared to Ca :+ is a characteristic property for carboxylate-containing exopolymers (Smidsrod, 1974) and in particular for alginates. Also other results of this investigation match the properties of alginates. In these experiments a release of Mg "+ when adding Ca :+ did not affect the dewaterability, so Ca :+ has probably taken its place in the sludge floc structure. This affinity for Ca 2+ compared to Mg 2÷ is a characteristic of the alginates. Furthermore, alginates form a so-called egg-box structure with Ca 2+ which is fairly resistant to increased conductivity (Smidsrod, 1974). This resembles results from these experiments in which addition of C~ 2+ to activated sludge did not affect dewaterability, implying that the exopolymers are still effective and do not coil in spite of the conductivity increase. Alginates are exopolymers consisting of two substantial monomers: 1,4-1inked a-L-guluronic acid and p-D-mannuronic acid. It has not been possible to find literature indicating that these substances are present in activated sludge, but, on the other hand. only a few investigations of the exopolymers in activated sludge have been conducted. AIginates could be present in activated sludge since they are present in the mucilage of some bacteria (Christcnsen, 1989). From these indications we propose that it could be an alginate-like cross-link of exopolymers that creates the floc structure. These exopolymers being linked together by Ca 2+ ions. Novak and Haugan (1978) had a similar proposal. They proposed that since the polymeric materials stripped from the bacterial floes had similar properties to whole floes it indicated that the floc matrix was also of biological origin. Relations to full-scale sludge dewatering The results presented here are believed to relate to phenomena in full-scale activated sludge dewatering. The relationship between simultaneous changes in conductivity by a release of various ions, turbidity and specific resistance to filtration has been observed when storing activated sludge anaerobically. In the early stages of anaerobic storage a release of Ca 2+ and an increase in the turbidity is observed (Rasmussen et al., 1992). This might be explained by an ion exchange process similar to that demonstrated in this paper and maybe by a microbial degradation of Ca 2+ binding exopolymers. At a later stage in anaerobic storage the effects on turbidity and dewaterability exceed what is explicable solely by an ion exchange process.
On the stability of activated sludge floes Another example is given by Teichgr~ber (1991) who described how activated sludge was destroyed by excess acid production during nitrification in plants w/th low alkalinity. This might be understood by an exchange of Ca "+ with H " similar to our experiments with HCI (Fig. 8). Also as salts are applied to roadways in wintertime, increased levels in conductivity are observed at sewage treatment plants. A n ion exchange process similar to that reported for Na ÷ is likely then to take place. It is recognized that the polymer consumption increases noticeably under these circumstances (Berggren, 1991), indicating the same phenomena as seen in this investigation. In the latter examples the exchanging cation is applied externally to the floes, as is also done in our experiments, while in the former example with anaerobic storage, the exchanging cations are produced by the bacteria, and thus applied internally to the floes. Hence, the weakening of the floc structure, i.e. the change in floc strength, is likely to be different in these two situations. The manner in which the floc strength is changed may in turn lead to different responses to shearing forces. In these experiments a fairly mild agitation has been applied, thus a more pronounced effect on the number of small particles and dewaterability may be observed if shearing forces compared to those in full-scale plants were applied. SUMMARY ANt) CONCLUSIONS
Ca' ~ can be extracted from activated sludge from a wastcwater treatment plant either by ion exchange processes or through removal by a chelating agent. The sludge deteriorates along with the removal of Ca '~ which indicates the importance of Ca '+ as binding agent in the sludge flo¢ structure. Turbidity correlates well with filterability in all experiments indicating that either the small particles or colloids arc partly responsible for the filterability decrease or that the increased turbidity is to be considered as a sign on a weakened sludge structure. Addition of Cu '+ improves the filterability of activated sludge along with a release of Ca '+. This indicates that Ca "+ is substituted in the exopolymer structure by the Cu "~ ions resulting in a more stable floc structure. The sludge floc structure is proposed to be considered as a three-dimensional exopolymer matrix (a gel) kept together by divalent cations with varying selectivity to the matrix (Cu '÷ > Ca" ~ > Mg '~ ). It is proposed that approx, half of the total amount of Ca '~ in the activated sludge is bound in the exopolymers. Acknowledgements--The authors want to thank Dr P. Aarne Vesilind for comments on the manuscript. Also thanks go to M. Schneider and L. Wybrandt for technical assistance in the laboratory. The Danish Technical Research Council (Grant 5.26.09.12) is gratefully acknowledged for economical support of this work.
1603 REFERENCES
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