Biofilm Air-lift Suspension reactors can be used to treat waste water at a high volumetric loading rate combined with a low sludge loading. The biofilms are ...
Wal. Sci. Tech. Vol. 26, No. 34, pp. 647-654, 1992. Printed in Great Britain. All rigbts reserved.
0273-1223192 $15.00 Copyright@ 1992 IAWPRC
FORMATION OF BIOFILMS IN A BIOFILM AIR-LIFT SUSPENSION REACTOR J. J. Heijnen*, M. C. M. van Loosdrecht*, A. Mulder** and L. Tijhuis* *Department Biochemical Engineering, Delft University of Technology, lulianalaan
67, 2628 Be Delft, The Netherlands **TNO, Department of Biology, P. O. Box 217, 2600 AE Delft, The Netherlands
ABSTRACT Biofilm Air-lift Suspension reactors can be used to treat waste water at a high volumetric loading rate combined with a low sludge loading. The biofilms are formed on small suspended particles (r 0.1 mm). We have studied the effect of particle characteristics and hydraulic retention time on the biofilm formation. It is shown that small, rough particles show the best biofilm formation. Low hydraulic retention times benefit the formation of biofilms. This results from the fact that suspension growth is minimal under these conditions. The effect of biofilm detachment became distinct from the observation that most of the bacterial growth in the biofilm is transferred to the liquid. The biofilm formation process is concluded to be a three stage process: (i) the outgrowth of single cells to micro-colonies, this process is positively influenced by the carrier surface roughness; (ii) the outgrowth of micro-colonies to small biofilms, this process is negatively influenced by the concentration of carrier material; (iii) the outgrowth of biofilms, this occurs when the majority of particles are covered with a biofilm. At that time the influence of shear due to particle-particle interactions diminishes. =
KEYWORDS Biofilm; carriers; air-lift reactor; shear; aerobic waste water treatment; hydraulic retention time.
INTRODUCTION For the aerobic treatment of waste water an airlift reactor containing biofilms on small suspended particles has been developed (Heijnen 1990) (Figure 1). The main advantage of this type of treatment process is the possibility to maintain high active biomass concentrations in the reactor. This is achieved by growing biofilms on small suspended carriers (radius 0.1 mm) which can be separated easily from the treated waste water (Figure I). Due to the large carrier surface area in the reactor (2000-4000 m2/m3) thin biofilms are formed (0.1 mm), so there are no problems relating to oxygen diffusion in these reactors. These characteristics result in: (i) a high volumetric loading ( 10 kgCOD/m3.day) combined with a low sludge loading (0.3 kgCODlkgVSS.day) (ii) good nitrification even at lower temperatures (iii) low (or no) sludge production (iv) a small area requirement (small and tall reactor with settler on top). The results of a full
647
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648
et
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scale application of a biofilm air-lift suspension reactor has been described earlier (Heijnen� 1990).
... ===; WASTE ·w....
Biofilm development can be described as a three phase process of (i) cell attachment, (ii) microcolony formation, and (iii) biofilm formation. Attachment of cells has been described before, in turbulent systems with high shear rates this process is only of minor importance (van Loosdrecht ct....al... 1991). Also biofilm formation has been reviewed extensively (Characklis a..Jl... 1990). The effects of shear are however still poorly understood. An important aspect of biofilm formation in air-lift suspension reactors is the difference with conventional biofilm systems. These are among others (i) relatively high specific surface area, (ii) turbulent flow conditions, (iii) spherical biofilm geometry (iv) change in biofilm surface area during growth of biofilms.
4U:t·UFT AUCTOR
PAlM
Fig.
W4StE .. WATER
1 Geometry of air-lift suspension reactor
The phenomenon of biofilm formation on small suspended particles in a turbulent air-lift reactor is of obvious importance for the process performance. Especially biofilm detachment due to shear can have a dominating effect. Therefore we have studied the (i) effect of carrier characteristics upon steady state biofilms and (ii) effect of hydraulic retention time upon steady state biofilms, and (iii) dynamics of the biofilm formation process.
MATERIALS AND METHODS The experiments where conducted in concentric tube air-lift reactors of 2.0 I (HID 20, ratio of surface area innerlouter tube 0.8). The influence of carrier characteristics was studied under the following conditions: superficial gas velocity 3.8 cm/s, 35 C, carrier concentration 125 gIl, hydraulic retention time 1 or 2 hours, effluent of an anaerobic wa�1e water treatment reactor (NH. 100 - 200 mgNII, COD 150 - 200 mg COD/I). The influence of retention time was studied under the following conditions: superficial gas velocity 7.5 cmls, · 30 C, carrier concentration (basalt) 175 gIl, hydraulic retention time 0.5 to 4 hours, artificial waste water containing nutrients and acetate at the desired COD concentration. The different phases in biofilm formation were determined by microscopic observations. Biomass concentrations were determined by TOC measurements. Conversions were assayed by ammonium, COD or acetate measurements. =
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+
=
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RESULTS AND DISCUSSION
Influence of carrier characteristics on steaay state biofilm formation. In total twenty one carriers of different composition were tested, some of them in different diameter fractions. The carriers were classified in three different classes (figure 2, table I): Class I: Carriers with a low potential for biofilm formation. The biofilm coverage is poor and very irregular. Only a partial ammonium and no nitrite oxidation occurs. Class 2: Carriers with a moderate potential for biofilm formation. The biofilm on individual particles is inhomogeneous. Only conversion of ammonia to nitrite occurs. Class 3: Carriers with a good potential for biofilm development. The biofilm coverage of the individual particles is homogeneous. Complete nitrification within 10 weeks.
Biofilm air-lift suspension reactor
649
:>
Fig.
2
class 1
class 2
class 3
phase 1
phase 2
phase 3
Classification of biofilms in the experiments with different carrier types. Phases refer to experiments on the growth dynamics of biofilms.
TABLE 1 Classitication criteria for biofilm carriers Quality class carrier Biofilm properties
Bad (1)
Moderate (2)
Good (3)
Coverage: Qualitative
irregular
not homogeneous
homogeneous
10
20- 50
80- 180
0- 40
60
0.1
0.6
0.7
0
0.1
0.9
Quantitative (mgVSS/gCarrier) Biofilm thickness (J.lm) NH.' -oxidizing capacity (gNH4 -N/gVSS.day) +
N02-oxidizing capacity (gNOz-N/gVSS.day)
The results of the classification are summarized in table 2. The results show that there are two main carrier characteristics determining the biofilm development: the carrier diameter and the surface roughness. The effect of carrier diameter is demonstrated best in the experiments with lava. With the two fine fractions the biofilm development is very good, while the results with the coarser fraction are moderate. The observed effects might originate from the effect that with larger particles the impact of particle-particle collisions is greater. This results in a larger detachment rate of the biofilm. The effect of surface I;"oughness is clearly demonstrated by the fact that all the carriers of class 3 have a rough surface, while the carriers with a smooth surface belong to classes 1 and 2. This might result from the fact that initially the biofilm is formed in the surface cavities of the particles, i.e. protected from shear effects. When such cavities are absent, the microcolonies are subject to the full liquid shear and a strong detachment will occur. The effect of biofilm detachment rates on reactor performance can be illustrated by the different nitrification characteristics of the biofilms formed on the different carrier types. With carriers of class 3 biofilm specific detachment is less than with carriers of class 1. This results in a lower biofilm wash-out in the case of rough particles, and therefore a lower specific sludge growth rate. The resulting difference in sludge age provokes the different nitrification behavior of the biofilms on the three different carrier classes. A low growth rate of the biomass results in a good nitrification. In the case of particles of class 1 or 2 the higher wash-out rate results in a higher growth rate and therefore only partial or no nitrification. For application in full scale treatment plants other selection criteria besides biofilm formation characteristics have to be evaluated. Such criteria are settling velocity, cost and availability. Based on these criteria basalt has turned out to be the best choice. Basalt is therefore used in the experiments below.
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TABLE 2 Effect of carrier type on biofilm formation. Carrier
Density
Surface
Diameter
Alumina
3600
0.2 - 0.4
Anthrite
1450
0.2 - 0.3
Class
3
0.4 - 1.0 Basalt
2900
Hydro-Filt BS
1970
rough
0.2 - 0.3
3
0.2 - 0.3
3
0.3 - 0.5
2
1.0 Lava
2600
rough
0.1 - 0.2
3
0.2 - 0.3
3
0.3 - 0.5
2
Lava red
2830
rough
0.2 - 0.3
2
Limestone
2670
rough
0.2 - 0.3
3
Manganese dioxide
3750
0.3 - 0.5 0.3 - 0.8
Effect
1.4 - 5.9
Norit supra
850
Olivine sand
2900
smooth
0.1 - 0.2
2
Plastorit
2290
rough
0.2 - 0.3
3
Phonolite
2470
rough
0.2 - 0.3
3
Pouzzolane
2580
rough
0.3 - 0.6
2
Pumice (Bims 2B)
1600
v.rough
0.2 - 0.4
3
Pumice (Bims 02S)
2000
v.rough
0.3 - 0.4
3
Quartz porphyry
2700
smooth
0.2 - 0.3
2
Silica beads
2600
smooth
0.8 - 1.3
Sillimanite
3500
smooth
0.1 - 0.2
2
Silver sand
2600
smooth
0.1 - 0.2
2
Sodium feldspar
2200
rough
0.1 - 0.5
2
Vasil Z
2300
angular
0.2 - 0.3
of hydraulic retention time on hiofilm formation.
In biofilm reactors there will always exist a competition between organisms growing in suspension and organisms growing in the hiofilm. Biofilm formation will only occur under conditions where suspended cel\s are quickly washed-out (Heijnen 1984). The effect of hydraulic retention time (at a constant volumetric COD loading rate), on the formation of biofilms is demonstrated in figures 3 and 4. As expected relatively more biomass grows in biofilms at lower hydraulic retention times. At higher retention times the suspended
651
Biofilm air-lift suspension reactor 120 c 0
•
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•
96
� c
'" '" '" E
72
•
0
§ Q; ."
48
� c 0
'" '" '" E
24
0
iii
Hydraulic retention time (h)
Fig.
3 Influence of hydraulic retention times on the relative contribution of
biofilm and suspended biomass in an air-lift reactor.
(COD-load 5 kg/m3.day)
biomass is washed out slower. The suspended biomass has therefore the ability to consume more substrate as compared to lower retention times. This results in a higher accumulation of biofilm biomass at lower retention times. In figure 4 the effect of competition between biofilm and suspension organisms is shown. At the longer hydraulic retention times only an intermediate type of biofilms is formed, whereas at the lower retention times complete biofilms are formed. This results, for these reactor types, in an increasing sludge age with decreasing hydraulic retention times.
Dynamics of biofilm formation During start-up of the reactor three growth phases can be distinguished (figure 2):
(l) bare carriers, without any microscopically visible biofilm,
(2) intermediately covered particles: particles only partially covered with biofilm, (3) biofilm particles: particles which are fully covered by biofilms. These three stages can easily be distinguished by microscopic examination. Note that the three phases correspond with the three carrier classes defined above. 100
HRT
=
0.5 h
80 0'2 Q) Cl ctl
C Q) u
Phase 1 ..
60
•
40
Phase 2
iii
0..
20
Phase 3 10
15
20
Time (days)
25
30
35 0
10
15
20
25
30
Time (days)
Fig. 4 Effect of hydraulic retention time on the biofilm development in air-lift suspension reactors (COD-load 5kg/m3.day).
35
J. J. HEIJNEN et al.
652
4
Cumulative total
'" '" '" E .S?
..
Biomass in biofilms
CD
..
10
Fig.
15
20
..
25
30
35
40
45
50
Time (days) 5 Cumulative biomass production and biofilm accumulation in an air-lift reactor. (HRT � 0.6 h; COD-load � 5 gjl.day)
During start-up (without any specific inoculation) COD conversion was 100 % within 2 days. The experiments were generalIy stopped after 16 - 31 days, when there was (visually) reached a steady state biofilm i.e. no change in biofilm coverage or biomass concentration in the reactor system. The microscopic observations of biofilm development during a representative experiment are displayed in figure 4. The general trend is that bacteria colonize the carrier surface at certain sites in the form of microcolonies. These gradually grow out to a full biofilm. Sometimes this process stops at an intermediary phase where the particles are only partially covered (compared to class 2 carriers from above). There might be two reasons for this behavior. (i) Biofilm formation is balanced by biofilm detachment. This detachment is mainly caused by the shear in a turbulent air-lift reactor. The main factor in this context is presumably particle-particle collisions (see below). This implies that the carrier concentration besides the carrier characteristics (see above) should have a great influence on biofilm development. (ii) Competition for substrate by suspended and biofilm bacteria. If the hydraulic retention time is high, growth of biofilm bacteria is far less, resulting in a bigger influence of detachment due to shear. TABLE 3 Biofilm accumulation in a biofilm air-lift reactor (HRT 0.6 h, COD-load 5 kg/ml.day) =
Time
Biomass in reactor
Biomass on carrier
(day)
(g/ml)
12
=
Feed per biofilm area
Bare carriers
Detachment rate
(g/m2)
Biofilm surface area (m2/ml)
(gCOD/m2.d)
(%)
(gVSS/m2.d)
600
3.6
167
30
90
. II
19
1100
4. 5
244
20
84
7
26
1700
4.4
386
13
70
5
30
2400
4.3
558
9
60
3
45
4900
5. 1
960
5
40
2
One remarkable point is the fact that even when most of the biomass (99 %) in the reactor is present in the form of biotilms, the majority of the produced biomass is transferred to the liquid phase and washed-out of the system. This results in the very low biofilm accumulation rate observed in the experiments. Figure 5 shows that only 10 % of the produced biomass remains in the biofilm. Table 3 summarizes some data on the biofilm accumulation in an air-lift reactor. It follows that the amount of biomass per square metr e does
Biofllm air-lift suspension reactor
653
not increase with an increase of biofilm in the reactor. This implies that when on a particle a biofilm develops this biofilm grows rapidly to its "steady state" thickness. The increase of biomass in the reactor is thus caused by an increased number of particles covered by biofilms and not by a gradual increase in biofilm thickness. The observed characteristics of biofilm formation might result from the fact that going from microcolonies to complete biofilms, the surface area per volume of biofilm decreases. This leads to a decreased detachment rate per volume of biofilm. In other words when a microcolony develops on a particle it will quickly grow out to a complete biofilm. The growth of the biofilm stops when the complete particle is covered and the shear equals the biomass production in the biofilm. The detachment rate during this experiment drops from 11 to 2 g/m2.day. This detachment rate is initially higher as the detachment rate observed by Trulear (1982) for a 100 I'm biofilm in a laminar flow system (1.2 g/m2.day). At this moment it is impossible to state which factor determines the decreasing detachment during the experiment. One factor might be the decreasing amount of bare carriers in the system, resulting in less particle shear. However at the same time the sludge loading of the system drops. This result in a lower specific biomass production rate, which might also lead to a decrease in the detachment rate. The overall yield of biomass on substrate (Y ) lies initially between 0.4 and 0.5 C-moI/C-mol. This value is in the same range as for conventional suspended cell cultures. With increasing biomass content of the reactor system the yield (and thus sludge production) drops (see figure 5). ..
CONCLUSION The maximum amount of biomass accumulated in a biofilm air-lift suspension reactor depends on: (i) . the carrier characteristics, small and rough particles give the best biofilm development; (ii) the hydraulic retention time, at higher retention times the competition by suspended biomass leads to decreased biofilm formation; (iii) the detachment rate.
ATTACHED CELLS
+
- :1
MICRO
liqUid
COLONIES
shear
I
�I particle
surface roughness
BIOFILM
shear
particle concentration: particle roughness:
Fig.
?
6 Schematic representation of processes involved in biofilm formation in air-lift suspension reactors (-:
negative effect,
+:positive effect on biofilm formation)
a
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J. J. HEUNEN
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Shear plays a dominant role in the biofilm formation process (figure 6). The formation of biofilms can be divided in the following stages. (i) Attachment of single cells to a carrier surface. (ii) Growth of attached cells to microcolonies. This process as well as the first one is negatively influenced by liquid shear, and positively influenced by surface roughness. (iii) Outgrowth of microcolonies to complete biofilms. This proces is negatively influenced by particle particle shear. This shear is influenced by the concentration of particles in the reactor and probably also by the geometry of the particle.
REFERENCES
Characklis, W.G., Marshall, K.C. (1990) . Biofilms. John Wiley & Sons, New York. Heijnen, J.J. (1984). Biological industrial waste water treatment minimiting biomass production and maximizing biomass concentration. PhD-thesis, Delft University of Technology. Heijnen, J.J., Mulder, A., Weltevrede, R., Hols, J., van Leeuwen, H.L.J.M. (1990). Large scale anaerobic/aerobic treatment of complex industrial waste water using immobilized biomass in fluidized bed and air-lift suspension reactors. Chem. Eng. Tech. a 145-220. Trulear, M.G., Characklis, W.G., (1982). Dynamics of biofilm processes. J. Wat. Poll. Cont. Fed. � 1288-1300. Van Loosdrecht, M.C.M., Lyklema, J., Norde, W., Zehnder, AJ.B., (1988) Bacterial adhesion: a physicochemical approach. Microb. Ecol. 11.. 1-15.
