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Biological treatment of waste water in the compact reactor
229
Biological Treatment of Waste Water in the Compact Reactor Die biologische E. A. NAUNDORF.
Reinigung
D. SUBRAMANIAN,
Institut fur Thermische Zellerfeld (F. R. G. ) (Received
January
von AbwGsern N. RABIGER
Verfahrenstechnik,
im Kompaktreaktor and A. VOCELPOHL
Technische
Universitat
Claus&a?, 3392
Clausthal-
2, 198.5)
Abstract A newly developed loop reactor has been shown to treat various domestic and industrial waste waters highly efficiently. In pilot studies BOD5 capacities per unit volume as high as 28 kg m p3 dd’ have been achieved for domestic waste water. In treating the effluents from a paper factory the compact reactor gives the same degree of conversion for only l/6 of the residence time of the factory’s conventional effluent treatment plant. Kurzfassung Ein neu entwickelter Schlaufenreaktor hat bei der biologischen Reinigung kommunaler als such industrieller Abwasser hervorragende Ergebnisse gezeigt. In Pilotversuchen wurden fur kommunales Abwasser Raumbelastungen bis zu 28 kg m-’ d-r erreicht. Die AbwPsser einer Papierfabrik werden bei gleichem Abbaugrad in l/6 der Verweilzeit konventioneller Anlagen gereinigt. Synopse Bei der Reinigung van Abwassern zeichnet sich der 7rend ah, die konventionellen offenen Belebungsbecken durch Reaktoren zu ersetzen mir dem Ziel der Kostenersparnis sowie eines umweltjieundlichen Betriebs. Die bishengen Entwicklungen wie die Behrilterbiologie und das Tiefschachtverfahren zeichnen sich zwar durch einen vem.ngerten Grundjkichenbedarj; eine verbesserte Sauerstoffausnutzung sowie einen umweltfratndlichen Betrieb aus, eine Verbesserung der biologischen Abbauleistung wurde indessen nicht erreicht. In dieser Arbeit wird uber Ergebnisse beider Reinigung synthetischer, kommunaler und industrieller Abwasser mit einem neu entwickelten Verfahren (Abb. 3) berichtet, das sich zusatzlich durch eine hohe volumenbezogene Abbauleistung auszeichnet. Kernstuck des Verfahrens ist ein Schlaufenreaktor (Abb. I) mit einem Umlaufrohr und einer oben angeordneten Zweistoffduse (Abb. 2). Der Lltissigkeitsstrahl dieser Zweistoffdtise erzeugt in dem Umlaujrohr ein hohes Scherfeld, in dem die zugefihrte Luft zu kleinsten Btischen zerteilt wird sowie die Bakten’enflocken zu kleineren Agglomeraten dispergiert *Extended version of a paper given by E. A. Naundorf at the meeting of the working party ‘Bioverfahrenstechnik’ of the GVC/VDI Society of Chemical Engineering and Processing, May 2X-30, 1984. in Lindau Bodcnsee, F.R.G. 025.52701/85/$3.30
(Ihun.
Ens. F’rnc~ss., 19 (1985)
werden. Die dadurch erreichten grossen Stoffaustauschmchen gas-fltissig sowie fliissig-fest begunstigen den Sauerstoff- und Substrattransport und sind neben einer definierten Strtimungsfiihrung die Ursache fur die hohen in Abb. 5, 9 und 10 dargestellten Abbauleistungen. Wie die in Abb. 5 und 10 gezeigten Vergieiche mit konventionellen Anlagen beweisen, betragen die Abbauleistungen des neaen Verfahrens sowohl fir kommunales Abwasser als such das untersuchte Abwasser einer Papierfabrik ein Vielfaches der Abbauleistung konventioneller Anktgen. 1. Introduction In the biological stage of waste water treatment plants the organic pollutants are converted to sludge by bacteria under addition of oxygen. Recently, there has been a shift from conventional treatment basins with a water depth of 3-4 m to large-size tower reactors with a height between 15 and 30 m like the ‘Turmbiologie’ of Bayer AC, the ‘Biohoch-Reaktor’ of Hoechst AC, or the ‘deep shaft process’ of ICI, with water depths between 50 and 200 m [I]. These new developments have greatly reduced the ground surface required and the emission of- airborne pollutants as well as the air intake owing to better oxygen usage. The space-time yield, however, has not improved significantly, and the separation of the sludge from the treated water still requires huge classificatron or sedimentation tanks. 229-233
0 Elsevier Sequoia/Printed
in The Netherlands
230 The ‘Hubstrahlreaktor’ proposed by Brauer and Sucker [2] and the ‘compact reactor’ developed at the Technical University of Clausthal [3] demonstrate, on the other hand, a high space-time yield and improved sludge handling properties, and thus may be regarded as high performance reactors with respect to the biological treatment of waste water. Design of the ‘compact reactor’ and operating data from the treatment of different waste waters are described in the following chapters.
2. Design C-R)
and operation
of the compact
reactor
The CPR, as shown in Fig. 1, is a loop reactor made up from a cylindrical vessel (1) with an inner draft tube (2) and a height:diarneter ratio of about 7: 1. A twofluid nozzle (3) is located at the top of the vessel and reaches down into the draft tube. According to Fig. 2 the air is introduced through the centre of the nozzle and the liquid through the surrounding annular space. In this way a liquid jet is formed which has the shape of a hollow cylinder and provides two rnomentum transfer areas. In the interior of the hollow jet the air is sucked in and dispersed while liquid circulation and redispersion of the circulating two-phase fluid are effected at the outer surface of the jet. This particular kind of operation allows both dispersion of the air sucked in (primary dispersion) and efficient recirculation of the reactor fluid and redispersion of the circulating gas (secondary dis-
persion) to be independently optimized. The zone of high turbulence due to the liquid jet produces extremely small gas bubbles, resulting in efficient oxygen transfer. It also reduces the clusters of bacteria to small agglomerates and thus enhances the uptake of cjxygen and substrate because of the increased outer surface of the bacteria. In addition. the defined flow of the circulating fluid prevents stagnant areas in the reactor and provides for a largely homogeneous distribution of the bacteria and thus improved local reaction conditions. Whether submission of the bacteria to mechanical stresses in the CPR accelerates the metabolism as claimed in the literature and thus improves their pcrformance is stilt open to question. Studies of the population of baclelia generated in the CPR, however, show a higher ratio of aerobic bactcrla over anaerobjc bacteria compared with conventional treatment systems. The better performance of the CPR is partially explained by the higher activity of the aerobic bacteria. Finally, by proper adjustment of the geometry and operating conditions, the mixing characteristics of the CPR may be changed from complete backmixing to almost plug flow performance.
3. Experimental
test unit
Figure 3 shows the test unit used in the experiments. The treated water plus biomass leaves the reactor (1) at the top and is either recycled to the reactor along with the water to be treated or sent to the settling tank (2). The concentration of biomass in the reactor is measured at position (7) and controlled by recycling part of the sludge produced in the settling tank. The loop (13) serves to measure the oxygen concentration in the reactor. The BODS, COD and TOC values of both the feed waste water as well as the purified water are determined at position (8). The flow rates of the waste water and the liquid passing through the nozrle are obtained via the flowmeters (5). The air flow rate through the intake (10) is measured by an anemometer. The heat exchanger (4) serves to control lhe temperatill-e in the
Fig. 1. Compact reactor (CPR): 1, cylindrical vessel; 2, inner draft tube; 3, two-fluid nozzle.
Prlmory
Secondary
Fig. 2. ‘l’wo-phase jet.
dlsperswn
Fig. 3. Test unit: 1, compact reactor; 2, settling tank; 3, circulating pump; 4, heat exchanger; 5, flowmeter; 6, oxygen measurement; 7, concentration measurement; 8, TOC measurement; 9, waste water feed; IO, air feed; 11, treated effluent; 12, excess sludge removal.
231 reactor. In addition, the pH value within the reactor is measured. From the measured data the following characteristic quantites are calculated. TOC or BODS removed Capacity
= (ci - co)/Ci
(1)
per unit volume:
BR = CiVw/ V, Capacity
(2)
per unit dry mass:
BTS = BR/TS Capacity
(3)
per unit total dry mass:
BFS = B,/FS Energy dissipation
(4) rate:
E = rnUT2/2VR
(5)
Sludge volume index: SVI = V,,/TS
4. Experimental
(6)
results
The experiments were carried out with synthetic and domestic waste water as well as the effluents from a paper plant processing recycled paper. The composition of the synthetic waste water is shown in Table 1 and the organic loadings of the waste water are summarized in Table 2. In the experiments two different reactors were used, namely, one of diameter 0.14 m and height 1 m and one of diameter 0.3 m and height 2.5 m with a volume of 14 and 150 1, respectively. In the experiments the energy dissipation rate of the liquid jet was changed within the range 0.5-2.5 kW m-‘, the dry mass in the reactor between 1 and 6 g 1-l and the residence time of the waste water within the range 15-45 min.
TABLE 1. Composition
4.1. Synthetic waste water Figure 4 gives a comparison between the percentage TOC removed in the CPR and a conventional treatment basin of similar size as a function of the dry mass content (concentration of bacteria) and the energy dissipation rate under otherwise similar conditions. Whereas the percentage removed is above 80% for the CPR, and reaches almost 100% at higher biomass concentrations, the conversion in the conventional plant drops to less than 40% at high capacities per unit volume. The Figure also shows that under the given conditions the CPR allows operation at three times the capacity per unit volume of the conventional treatment basin. Similar results are shown in Fig. 5 which gives the capacity per unit dry mass as a function of the TOC value remaining in the treated water, again for the CPR and a conventional treatment basin under otherwise identical conditions. In particular, at low TOC values the CPR allows for an up to eightfold increase in capacity. By increasing the jet velocity an even higher efficiency can be obtained from the CPR. Figure 6 shows the effect of the liquid jet velocity on the percentage TOC removed at constant dry mass content. The increase is explained by the rise in the turbulence causing an increase in the bacterial surface area resulting in more efficient oxygen and substrate transfer to the bacteria.
(mg 1-l) of synthetic waste water 1000 221 100 0.5 521 1070 64
GlUCOX
Urea Magnesium sulphate Iron(lll) chloride Primary potassium phosphate Secondary potassium phosphate Calcium chloride
00 0.0
1.0
2.0
Fig. 4. Percentage content.
3.0 4.0 ORY MASS
6. 0 7. 0 9. 0 5.0 CONTENT TS/kg m-3
TOC removed as a function
9.0
10.0
of the dry mass
TABLE 2. Organic loadings of the waste waters
BOBS (mg 1-l) COD (mg I-‘) TOC (mg 1-1) Dry matter Q 1-l) (non-biodegradable)
Domestic
Synthetic waste water
water
890 1000 445 -
180-360 270-540 120-240 -
waste
Paper factory effluent
200-420 650-1200 1 SO-220 <1.5
50 TOC *
100 lB’/kS
m-’
150 in the
200 250 EXIT STREAM
300
Fig. 5. Capacity per unit dry mass as a function of the TOC value.
232
00.
Fig. 6. Percentage
TOC removed
as a function
of the liquid jet
velocity.
Figure 7 shows the effect of the geometry on the CPR efficiency. The percentage TOC removed is plotted against the shear rate with the ratio of the reactor to liquid jet diameter as the parameter. The shear rate is a function of the liquid jet velocity and the diameter of the draft tube. The results show the existence of an optimum shear rate at constant reactor to jet diameter. l:urther, the percentage TOC r-emoved is seen to increase with decreasing jet diameter. The reason for this behaviour is seen to he in the change of the residence time distribution of the reactor and the decrease of the liquid jet velocity at a constant energy dissipation rate. This is because an increase in the jet or in the draft tube diameter increases the recirculation rate and thus the backmixing outside or inside the reactor, respectively. The result is, in both cases, an increasing tendency toward the residence time distribution of an ideally stirred reactor, resulting in the decrease in the TOC removal efficiency. On the other hand, however, the possibility of changing the residence time distribution of the reactor over a wide spectrum can be of considerable benefit. For example, in the case ofstrong variations in the capacity per unit volume of the feed water the percentage TOC removed can be kept virtually constant by choosing a corresponding residence time distribution for the CPR. The CPR shows, in addition to the considerable increase in the TOC removal rate, a marked decrease in
50 0. 00
•1 . .I 0 I
0. 50 SHEAR
1. 00
RATE
Fi *
o/q
-
[l/d,_ D/d7
-
J. 50
XT
volume
iO.O “ELOCIT”
index
20.
15. 0 s-1
“,/rn
as a function
of the liquid
0
jet
the sludge volume index SVI with respect to conventional treatment basins. The cause of this is the fact that the relatively small bacterial agglomerates formed m the CPR flock together to form denser particles in the settling tank than is the case in conventional plants. Figure 8 shows that a large reactor to jet diameter is beneficial with respect to the sedimentation properties of the sludge produced in the CPR reactor. The excellent settling properties allow the settling tank to be operated at surface loadings of up to 8 m3 me2 h--l. III comparison to the CPR, the settling tanks of conventional treatment basins are designed for an average SVI of 1 SO ml g-’ and a surface loading of 1 m3 n-’ II-‘. Preliminary studies also indicate that the separation of the sludge from the treated water in a cyclone or flotation cell should be possible with the benefit of a large reduction in the volume required for this separation step. 4.2. Domestic waste water Typical data from expel-iments with waste water from the city of C:lausthal-Zellerfeld, as shown in Fig. 9, confirm the high efficiency of the CPR in sewage treatment. Pilot studies in a larger reactor of diameter 0.8 m and a height of 6 m have actually given an 8tY%reduction of the organic load of seepage water from a depository with a BOD, value of more than 2000 mg l--’ at capacities per unit volume as high as 80 kg nY3 d--l.
00; 0. 0
2. 00
4. 0
8. 0
12.
CAPAC,;Y/\nI I3g.
of the shear rate.
5. e LiDbID
Fig. 8. Sludge velocity.
28.0 15.5 ii. I
103/e“
Fig. 7. Perccntagc TOC removed as a function
I /
I
0. 0
9. Biodegradation
volume.
0 i,MF
16. B,,‘Iq
as a function
0
2a. m-3
m
24.
a
28.
d -:
of the capacity
per unit
0
233 possibility of treating waste waters directly at their place of formation and thus incorporating the waste treatment in the original process.
Nomenclature
BR BTS
BFS BODs 50 BOO,
/mg
100 1-l I-
the
150 EXIT
200 STREAM
ci CO
Fig. 10. Capacity per unit dry mass as a function of the BODs
COD
value.
‘f ,
4.3. Industrial effluents
DE d
Figure 10 gives a comparison of the sludge loadings for the CPR and the newly installed treatment plant of a paper factory as a function of the BOD5 loading of the effluent. The treated water originates from the processing of recycled paper and has a total dry mass content (concentration of bacteria plus non-biodegradable material) between 1 and 5 g I-’ from which about 30% are non-biodegradable fibres and minerals. Because of large variations of the organic as well as the solid loading of the waste water, the CPR was operated at a high recycle rate so that the CPR performed almost like an ideally mixed reactor in these experiments. Again, the CPR allows for a much higher capacity per unit total dry mass compared with the conventional treatment plant. For a BODs value of 20 mg I-‘, for example, the CPR needs only about l/6 of the volume of a conventional plant.
6
D
FS m SVI TOC TS UT
vR
VSL VW
capacity per unit volume, kg rnp3 d-’ capacity per unit dry mass, 1 capacity per unit total dry mass, 1 biochemical oxygen demand, mg 1-l BODs of organic matter in waste water, mg 1-l BOD5 of organic matter in treated water, mg 1-l chemical oxygen demand, mg 1-l diameter of liquid jet, m diameter of reactor, m diameter of draft tube, m shear rate, s-l energy dissipation rate, kW me3 total dry mass content (concentration of bacteria plus non-biodegradable material), g I-’ mass flow rate of liquid jet, kg h-’ sludge volume index, ml g-’ total organic carbon, mg 1-l dry mass content (concentration of bacteria), g 1-l velocity of liquid jet, m s-r volume of reactor, m percentage volume of settled sludge (after 30 min) feed rate of waste water per unit dry mass, m3
References M. Zlokarnik, Verfahrenstechnik der aeroben Wasserreinigung, Chem.-lng.-Tech..
5. Summary The systematic application of chemical engineering principles to the biological treatment of waste water has led to a new process which is highly efficient, compact and flexible. The new process offers the
54 (1982)
939-952.
H. Brauer and D. Sucker, Biological waste water treatment in a high efficiency reactor, Ger. Chem En&, 2 (1979) 77-86. U. Wachsmann, N. RLbiger and A. Vogelpohl, The compact reactor--a newly developed loop reactor with a high mass transfer performance, Ger. Chem. Eng., 7 (1984) 39-44. ATV-Regelwerk, Gesellschaft zur FGrderung der Abwassertechnik c.V. (GFA), D-5205 St. Augustin, Nov. 1981.
Biological Treatment of Waste Water in the Compact Reactor Die biologische E. A. NAUNDORF.
Reinigung
D. SUBRAMANIAN,
Institut fur Thermische Zellerfeld (F. R. G. ) (Received
January
von AbwGsern N. RABIGER
Verfahrenstechnik,
im Kompaktreaktor and A. VOCELPOHL
Technische
Universitat
Claus&a?, 3392
Clausthal-
2, 198.5)
Abstract A newly developed loop reactor has been shown to treat various domestic and industrial waste waters highly efficiently. In pilot studies BOD5 capacities per unit volume as high as 28 kg m p3 dd’ have been achieved for domestic waste water. In treating the effluents from a paper factory the compact reactor gives the same degree of conversion for only l/6 of the residence time of the factory’s conventional effluent treatment plant. Kurzfassung Ein neu entwickelter Schlaufenreaktor hat bei der biologischen Reinigung kommunaler als such industrieller Abwasser hervorragende Ergebnisse gezeigt. In Pilotversuchen wurden fur kommunales Abwasser Raumbelastungen bis zu 28 kg m-’ d-r erreicht. Die AbwPsser einer Papierfabrik werden bei gleichem Abbaugrad in l/6 der Verweilzeit konventioneller Anlagen gereinigt. Synopse Bei der Reinigung van Abwassern zeichnet sich der 7rend ah, die konventionellen offenen Belebungsbecken durch Reaktoren zu ersetzen mir dem Ziel der Kostenersparnis sowie eines umweltjieundlichen Betriebs. Die bishengen Entwicklungen wie die Behrilterbiologie und das Tiefschachtverfahren zeichnen sich zwar durch einen vem.ngerten Grundjkichenbedarj; eine verbesserte Sauerstoffausnutzung sowie einen umweltfratndlichen Betrieb aus, eine Verbesserung der biologischen Abbauleistung wurde indessen nicht erreicht. In dieser Arbeit wird uber Ergebnisse beider Reinigung synthetischer, kommunaler und industrieller Abwasser mit einem neu entwickelten Verfahren (Abb. 3) berichtet, das sich zusatzlich durch eine hohe volumenbezogene Abbauleistung auszeichnet. Kernstuck des Verfahrens ist ein Schlaufenreaktor (Abb. I) mit einem Umlaufrohr und einer oben angeordneten Zweistoffduse (Abb. 2). Der Lltissigkeitsstrahl dieser Zweistoffdtise erzeugt in dem Umlaujrohr ein hohes Scherfeld, in dem die zugefihrte Luft zu kleinsten Btischen zerteilt wird sowie die Bakten’enflocken zu kleineren Agglomeraten dispergiert *Extended version of a paper given by E. A. Naundorf at the meeting of the working party ‘Bioverfahrenstechnik’ of the GVC/VDI Society of Chemical Engineering and Processing, May 2X-30, 1984. in Lindau Bodcnsee, F.R.G. 025.52701/85/$3.30
(Ihun.
Ens. F’rnc~ss., 19 (1985)
werden. Die dadurch erreichten grossen Stoffaustauschmchen gas-fltissig sowie fliissig-fest begunstigen den Sauerstoff- und Substrattransport und sind neben einer definierten Strtimungsfiihrung die Ursache fur die hohen in Abb. 5, 9 und 10 dargestellten Abbauleistungen. Wie die in Abb. 5 und 10 gezeigten Vergieiche mit konventionellen Anlagen beweisen, betragen die Abbauleistungen des neaen Verfahrens sowohl fir kommunales Abwasser als such das untersuchte Abwasser einer Papierfabrik ein Vielfaches der Abbauleistung konventioneller Anktgen. 1. Introduction In the biological stage of waste water treatment plants the organic pollutants are converted to sludge by bacteria under addition of oxygen. Recently, there has been a shift from conventional treatment basins with a water depth of 3-4 m to large-size tower reactors with a height between 15 and 30 m like the ‘Turmbiologie’ of Bayer AC, the ‘Biohoch-Reaktor’ of Hoechst AC, or the ‘deep shaft process’ of ICI, with water depths between 50 and 200 m [I]. These new developments have greatly reduced the ground surface required and the emission of- airborne pollutants as well as the air intake owing to better oxygen usage. The space-time yield, however, has not improved significantly, and the separation of the sludge from the treated water still requires huge classificatron or sedimentation tanks. 229-233
0 Elsevier Sequoia/Printed
in The Netherlands
230 The ‘Hubstrahlreaktor’ proposed by Brauer and Sucker [2] and the ‘compact reactor’ developed at the Technical University of Clausthal [3] demonstrate, on the other hand, a high space-time yield and improved sludge handling properties, and thus may be regarded as high performance reactors with respect to the biological treatment of waste water. Design of the ‘compact reactor’ and operating data from the treatment of different waste waters are described in the following chapters.
2. Design C-R)
and operation
of the compact
reactor
The CPR, as shown in Fig. 1, is a loop reactor made up from a cylindrical vessel (1) with an inner draft tube (2) and a height:diarneter ratio of about 7: 1. A twofluid nozzle (3) is located at the top of the vessel and reaches down into the draft tube. According to Fig. 2 the air is introduced through the centre of the nozzle and the liquid through the surrounding annular space. In this way a liquid jet is formed which has the shape of a hollow cylinder and provides two rnomentum transfer areas. In the interior of the hollow jet the air is sucked in and dispersed while liquid circulation and redispersion of the circulating two-phase fluid are effected at the outer surface of the jet. This particular kind of operation allows both dispersion of the air sucked in (primary dispersion) and efficient recirculation of the reactor fluid and redispersion of the circulating gas (secondary dis-
persion) to be independently optimized. The zone of high turbulence due to the liquid jet produces extremely small gas bubbles, resulting in efficient oxygen transfer. It also reduces the clusters of bacteria to small agglomerates and thus enhances the uptake of cjxygen and substrate because of the increased outer surface of the bacteria. In addition. the defined flow of the circulating fluid prevents stagnant areas in the reactor and provides for a largely homogeneous distribution of the bacteria and thus improved local reaction conditions. Whether submission of the bacteria to mechanical stresses in the CPR accelerates the metabolism as claimed in the literature and thus improves their pcrformance is stilt open to question. Studies of the population of baclelia generated in the CPR, however, show a higher ratio of aerobic bactcrla over anaerobjc bacteria compared with conventional treatment systems. The better performance of the CPR is partially explained by the higher activity of the aerobic bacteria. Finally, by proper adjustment of the geometry and operating conditions, the mixing characteristics of the CPR may be changed from complete backmixing to almost plug flow performance.
3. Experimental
test unit
Figure 3 shows the test unit used in the experiments. The treated water plus biomass leaves the reactor (1) at the top and is either recycled to the reactor along with the water to be treated or sent to the settling tank (2). The concentration of biomass in the reactor is measured at position (7) and controlled by recycling part of the sludge produced in the settling tank. The loop (13) serves to measure the oxygen concentration in the reactor. The BODS, COD and TOC values of both the feed waste water as well as the purified water are determined at position (8). The flow rates of the waste water and the liquid passing through the nozrle are obtained via the flowmeters (5). The air flow rate through the intake (10) is measured by an anemometer. The heat exchanger (4) serves to control lhe temperatill-e in the
Fig. 1. Compact reactor (CPR): 1, cylindrical vessel; 2, inner draft tube; 3, two-fluid nozzle.
Prlmory
Secondary
Fig. 2. ‘l’wo-phase jet.
dlsperswn
Fig. 3. Test unit: 1, compact reactor; 2, settling tank; 3, circulating pump; 4, heat exchanger; 5, flowmeter; 6, oxygen measurement; 7, concentration measurement; 8, TOC measurement; 9, waste water feed; IO, air feed; 11, treated effluent; 12, excess sludge removal.
231 reactor. In addition, the pH value within the reactor is measured. From the measured data the following characteristic quantites are calculated. TOC or BODS removed Capacity
= (ci - co)/Ci
(1)
per unit volume:
BR = CiVw/ V, Capacity
(2)
per unit dry mass:
BTS = BR/TS Capacity
(3)
per unit total dry mass:
BFS = B,/FS Energy dissipation
(4) rate:
E = rnUT2/2VR
(5)
Sludge volume index: SVI = V,,/TS
4. Experimental
(6)
results
The experiments were carried out with synthetic and domestic waste water as well as the effluents from a paper plant processing recycled paper. The composition of the synthetic waste water is shown in Table 1 and the organic loadings of the waste water are summarized in Table 2. In the experiments two different reactors were used, namely, one of diameter 0.14 m and height 1 m and one of diameter 0.3 m and height 2.5 m with a volume of 14 and 150 1, respectively. In the experiments the energy dissipation rate of the liquid jet was changed within the range 0.5-2.5 kW m-‘, the dry mass in the reactor between 1 and 6 g 1-l and the residence time of the waste water within the range 15-45 min.
TABLE 1. Composition
4.1. Synthetic waste water Figure 4 gives a comparison between the percentage TOC removed in the CPR and a conventional treatment basin of similar size as a function of the dry mass content (concentration of bacteria) and the energy dissipation rate under otherwise similar conditions. Whereas the percentage removed is above 80% for the CPR, and reaches almost 100% at higher biomass concentrations, the conversion in the conventional plant drops to less than 40% at high capacities per unit volume. The Figure also shows that under the given conditions the CPR allows operation at three times the capacity per unit volume of the conventional treatment basin. Similar results are shown in Fig. 5 which gives the capacity per unit dry mass as a function of the TOC value remaining in the treated water, again for the CPR and a conventional treatment basin under otherwise identical conditions. In particular, at low TOC values the CPR allows for an up to eightfold increase in capacity. By increasing the jet velocity an even higher efficiency can be obtained from the CPR. Figure 6 shows the effect of the liquid jet velocity on the percentage TOC removed at constant dry mass content. The increase is explained by the rise in the turbulence causing an increase in the bacterial surface area resulting in more efficient oxygen and substrate transfer to the bacteria.
(mg 1-l) of synthetic waste water 1000 221 100 0.5 521 1070 64
GlUCOX
Urea Magnesium sulphate Iron(lll) chloride Primary potassium phosphate Secondary potassium phosphate Calcium chloride
00 0.0
1.0
2.0
Fig. 4. Percentage content.
3.0 4.0 ORY MASS
6. 0 7. 0 9. 0 5.0 CONTENT TS/kg m-3
TOC removed as a function
9.0
10.0
of the dry mass
TABLE 2. Organic loadings of the waste waters
BOBS (mg 1-l) COD (mg I-‘) TOC (mg 1-1) Dry matter Q 1-l) (non-biodegradable)
Domestic
Synthetic waste water
water
890 1000 445 -
180-360 270-540 120-240 -
waste
Paper factory effluent
200-420 650-1200 1 SO-220 <1.5
50 TOC *
100 lB’/kS
m-’
150 in the
200 250 EXIT STREAM
300
Fig. 5. Capacity per unit dry mass as a function of the TOC value.
232
00.
Fig. 6. Percentage
TOC removed
as a function
of the liquid jet
velocity.
Figure 7 shows the effect of the geometry on the CPR efficiency. The percentage TOC removed is plotted against the shear rate with the ratio of the reactor to liquid jet diameter as the parameter. The shear rate is a function of the liquid jet velocity and the diameter of the draft tube. The results show the existence of an optimum shear rate at constant reactor to jet diameter. l:urther, the percentage TOC r-emoved is seen to increase with decreasing jet diameter. The reason for this behaviour is seen to he in the change of the residence time distribution of the reactor and the decrease of the liquid jet velocity at a constant energy dissipation rate. This is because an increase in the jet or in the draft tube diameter increases the recirculation rate and thus the backmixing outside or inside the reactor, respectively. The result is, in both cases, an increasing tendency toward the residence time distribution of an ideally stirred reactor, resulting in the decrease in the TOC removal efficiency. On the other hand, however, the possibility of changing the residence time distribution of the reactor over a wide spectrum can be of considerable benefit. For example, in the case ofstrong variations in the capacity per unit volume of the feed water the percentage TOC removed can be kept virtually constant by choosing a corresponding residence time distribution for the CPR. The CPR shows, in addition to the considerable increase in the TOC removal rate, a marked decrease in
50 0. 00
•1 . .I 0 I
0. 50 SHEAR
1. 00
RATE
Fi *
o/q
-
[l/d,_ D/d7
-
J. 50
XT
volume
iO.O “ELOCIT”
index
20.
15. 0 s-1
“,/rn
as a function
of the liquid
0
jet
the sludge volume index SVI with respect to conventional treatment basins. The cause of this is the fact that the relatively small bacterial agglomerates formed m the CPR flock together to form denser particles in the settling tank than is the case in conventional plants. Figure 8 shows that a large reactor to jet diameter is beneficial with respect to the sedimentation properties of the sludge produced in the CPR reactor. The excellent settling properties allow the settling tank to be operated at surface loadings of up to 8 m3 me2 h--l. III comparison to the CPR, the settling tanks of conventional treatment basins are designed for an average SVI of 1 SO ml g-’ and a surface loading of 1 m3 n-’ II-‘. Preliminary studies also indicate that the separation of the sludge from the treated water in a cyclone or flotation cell should be possible with the benefit of a large reduction in the volume required for this separation step. 4.2. Domestic waste water Typical data from expel-iments with waste water from the city of C:lausthal-Zellerfeld, as shown in Fig. 9, confirm the high efficiency of the CPR in sewage treatment. Pilot studies in a larger reactor of diameter 0.8 m and a height of 6 m have actually given an 8tY%reduction of the organic load of seepage water from a depository with a BOD, value of more than 2000 mg l--’ at capacities per unit volume as high as 80 kg nY3 d--l.
00; 0. 0
2. 00
4. 0
8. 0
12.
CAPAC,;Y/\nI I3g.
of the shear rate.
5. e LiDbID
Fig. 8. Sludge velocity.
28.0 15.5 ii. I
103/e“
Fig. 7. Perccntagc TOC removed as a function
I /
I
0. 0
9. Biodegradation
volume.
0 i,MF
16. B,,‘Iq
as a function
0
2a. m-3
m
24.
a
28.
d -:
of the capacity
per unit
0
233 possibility of treating waste waters directly at their place of formation and thus incorporating the waste treatment in the original process.
Nomenclature
BR BTS
BFS BODs 50 BOO,
/mg
100 1-l I-
the
150 EXIT
200 STREAM
ci CO
Fig. 10. Capacity per unit dry mass as a function of the BODs
COD
value.
‘f ,
4.3. Industrial effluents
DE d
Figure 10 gives a comparison of the sludge loadings for the CPR and the newly installed treatment plant of a paper factory as a function of the BOD5 loading of the effluent. The treated water originates from the processing of recycled paper and has a total dry mass content (concentration of bacteria plus non-biodegradable material) between 1 and 5 g I-’ from which about 30% are non-biodegradable fibres and minerals. Because of large variations of the organic as well as the solid loading of the waste water, the CPR was operated at a high recycle rate so that the CPR performed almost like an ideally mixed reactor in these experiments. Again, the CPR allows for a much higher capacity per unit total dry mass compared with the conventional treatment plant. For a BODs value of 20 mg I-‘, for example, the CPR needs only about l/6 of the volume of a conventional plant.
6
D
FS m SVI TOC TS UT
vR
VSL VW
capacity per unit volume, kg rnp3 d-’ capacity per unit dry mass, 1 capacity per unit total dry mass, 1 biochemical oxygen demand, mg 1-l BODs of organic matter in waste water, mg 1-l BOD5 of organic matter in treated water, mg 1-l chemical oxygen demand, mg 1-l diameter of liquid jet, m diameter of reactor, m diameter of draft tube, m shear rate, s-l energy dissipation rate, kW me3 total dry mass content (concentration of bacteria plus non-biodegradable material), g I-’ mass flow rate of liquid jet, kg h-’ sludge volume index, ml g-’ total organic carbon, mg 1-l dry mass content (concentration of bacteria), g 1-l velocity of liquid jet, m s-r volume of reactor, m percentage volume of settled sludge (after 30 min) feed rate of waste water per unit dry mass, m3
References M. Zlokarnik, Verfahrenstechnik der aeroben Wasserreinigung, Chem.-lng.-Tech..
5. Summary The systematic application of chemical engineering principles to the biological treatment of waste water has led to a new process which is highly efficient, compact and flexible. The new process offers the
54 (1982)
939-952.
H. Brauer and D. Sucker, Biological waste water treatment in a high efficiency reactor, Ger. Chem En&, 2 (1979) 77-86. U. Wachsmann, N. RLbiger and A. Vogelpohl, The compact reactor--a newly developed loop reactor with a high mass transfer performance, Ger. Chem. Eng., 7 (1984) 39-44. ATV-Regelwerk, Gesellschaft zur FGrderung der Abwassertechnik c.V. (GFA), D-5205 St. Augustin, Nov. 1981.