- Email: [email protected]
Particle size distribution in effluent of trickling filters and in humus tanks
PII: S0043-1354(01)00118-X
Wat. Res. Vol. 35, No. 16, pp. 3993–3997, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE PARTICLE SIZE DISTRIBUTION IN EFFLUENT OF TRICKLING FILTERS AND IN HUMUS TANKS WOLFGANG SCHUBERT and F. WOLFGANG GU¨NTHERT* Institute of Hydroscience, Sanitary Engineering and Waste Management, Faculty of Civil Engineering and Surveying, University of the Federal Armed Forces Munich, Germany (First received 22 May 2000; accepted in revised form 19 February 2001) Abstract}Particles and aggregates from trickling filters must be eliminated from wastewater. Usually this happens through sedimentation in humus tanks. Investigations to characterize these solids by way of particle size measurements, image analysis and particle charge measurements (zeta potential) are made within the scope of Research Center for Science and Technology ‘‘Fundamentals of Aerobic biological wastewater treatment’’ (SFB 411). The particle size measuring results given within this report were obtained at the Ingolstadt wastewater treatment plant, Germany, which served as an example. They have been confirmed by similar results from other facilities. Particles flushed out from trickling filters will be partially destroyed on their way to the humus tank. A large amount of small particles is to be found there. On average 90% of the particles are smaller than 30 mm. Particle size plays a decisive role in the sedimentation behaviour of solids. Small particles need sedimentation times that cannot be provided in settling tanks. As a result they cause turbidity in the final effluent. Therefore quality of sewage discharge suffers, and there are hardly advantages of the fixed film reactor treatment compared to the activated sludge process regarding sedimentation behaviour. # 2001 Elsevier Science Ltd. All rights reserved Key words}trickling filters, humus tanks, particle size measurement, particle size distribution, sedimentation
NOMENCLATURE
a BR ;BOD BR;TKN d Ff g n qA;TF vsed X
INTRODUCTION
number of rotary sprinkler arms, dimensionless volumetric loading of a reactor as biological oxygen demand, kg m3 d1 volumetric loading of a reactor as Kjeldahl nitrogen demand, kg m3 d1 diameter of a sedimentation particle, mm flushing force, mm a1 gravity constant, m s2 revolutions per hour, 1 h1 surface flow rate trickling filter, m h1 sedimentation velocity, m h1 equivalent diameter, mm
Subscripts and superscripts A area L length V volume Greek letters density of solids, g cm3 rS rL density of liquid, g cm3 u kinematic viscosity, m2 s1
*Author to whom all correspondence should be addressed. Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany. Tel.: +49-89-6004-2156; fax: +49-89-6004-3858; e-mail: [email protected]
Wastewater treatment with fixed film reactors is a rather old technique. Trickling filters are filled with materials where biofilm grows. Biofilms are organic material (granula), the different species attached to it, i.e. bacteria, fungi, protozoa, higher organisms, and exopolymeric substances from microorganism. Structure of biofilms varies according to facts like wastewater composition and wastewater load. There are several research projects under way within SFB 411 aimed at investigating biofilm structure and microbiological population (Wagner et al., 1998; Lewandowski et al., 1999). The biofilm grows on filling materials and the organisms are fed with substrate taken from wastewater. With microbiological growth biofilms are getting thicker and partially anaerobic zones develop. In these zones there is a lack of substrate and oxygen so that parts of the population die. This leads to biofilm parts getting flushed off (Steinmann, 1989). The size of trickling filters depends on the BOD load per unit reactor volume i.e. BR;BOD 0:4 kg m3 d1 for carbon removal, BR;BOD 0:4 kg m3 d1 and BR;TKN 0:1 kg m3 d1 for nitrification. During wastewater treatment the biofilm grows and surplus sludge must be removed. Therefore a surface flow rate qA;TF between 0.4 m h1 (mineral filling materi-
3993
3994
Wolfgang Schubert and F. Wolfgang Gu¨nthert
als) and 0.8 m h1 (synthetic filler elements) is required (ATV, 1999). A minimum value is also necessary to get an entire wetting of filling material. For the assessment flushing force Ff ¼ qA;TF ða nÞ1 should reach values between 4 and 8 mm a1 to get a permanent erosion of biofilm material (ATV, 1999). Particles in the effluent of trickling filters consist of solids from wastewater and parts of the biofilm. The investigation results regarding the size distribution of those particles presented in this report were obtained at the Ingolstadt wastewater treatment plant (Germany). Grab samples from inflow and effluent of humus tanks as well as from two trickling filters with different filling materials were taken and analysed.
MATERIALS AND METHODS
The Ingolstadt wastewater treatment plant’s operating mode is an activated sludge process treatment with four successive trickling filters. The trickling filters have a filler height of 3 m and a diameter of 38 m. The treatment plant is designed for a population of 345 000. Particle size analysis is carried out by means of Galai Cis 100 particle sizer (Galai Ltd.). The measuring range is 10–3600 mm, with a resolution of 6 mm. Samples from the outlets of trickling filters are taken by placing a sampling beaker at the bottom of the trickling filter (aeration holes). It’s position and the times of placement influence the amount of solids in the sample. Several different samples should be measured to make sure that the results are representative. However, the main advantage of the direct sampling of particles at the bottom of trickling filters is that changes or destruction of particles are avoided. Samples from humus tanks are taken as grab samples with a beaker. For each location there were taken three samples. All samples were measured three times. The measurements are made representative by measuring a large number, for instance 80 000 particles. The quality of the measurements is documented by confidence intervals. The measurements presented in this report are based on a minimum confidence interval of 95%. Variations of the present particle sizes in a grab sample have minor impact and do not induce significant differences on size distribution results.
RESULTS
Trickling filters Particles, flocs and aggregates in wastewater differ regarding shape and size. The vast majority of particles are of very small size, although some bigger particles exist. Particle shapes are very irregular; all kinds of shapes from longish to filamentous, partly voluminous and spherical exist simultaneously. At the wastewater treatment plant in Ingolstadt, grab samples from effluent of trickling filter were taken at different distances from the outer wall. Distance 0.5 m is near the outer wall of the trickling filter, while distance 5 m is more inside the trickling filters (radius 19 m). Particle size distributions results from samples taken at different distances are shown in Fig. 1. There were more large particles found at distance 5 m, while more small particles were encountered at distance 0.5 m. Further investigations are required to establish whether or not this peculiar distribution is caused by hydraulic loading differences inside the trickling filter. Figure 2 shows that wastewater effluent from trickling filter 3 contains larger particles than effluent from trickling filter 4. The average volumetric BOD loading BR;BOD of trickling filter 3 is 0.15 kg m3 d1 and average surface flow rate qA;TF is 1.07 m h1. The average volumetric BOD loading BR;BOD of trickling filter 4 is 0.21 kg m3 d1 and average surface flow rate qA;TF is 1.47 m h1. A major difference between trickling filter 3 and trickling filter 4 is the support media. Trickling filter 3 is filled with random packed synthetic filter media elements with a support media surface of 320 m2 m3, while trickling filter 4 has big built-in filter media bodies with a support media surface of 190 m2 m3. Although trickling filter 3 has a lower surface loading rate, more large sized particles and less small particles were flushed out, independent of sampling location at the bottom of the trickling filter.
Fig. 1. Area distribution, effluent of trickling filter 3 (0.5 and 5 m distance from outer wall).
Particle size distribution in humus tanks
3995
Fig. 2. Comparison between effluent of trickling filters 3 and 4 (5 m distance from outer wall).
Fig. 3. Comparison between effluent of trickling filter 3 (mean values) and inflow humus tanks.
Humus tanks Wastewater effluent from the four trickling filters in Ingolstadt enters a large gully, from where it flows to the humus tanks. Figure 3 shows the differences between the particle size distribution at the effluent of the trickling filter (mean values of the different sampling locations) and the particle size distribution at the inflow to the humus tanks. Particles flushed out from trickling filters are larger than the particles at the inflow to the humus tanks, since they are destroyed on their way. Turbulent stream regime may be responsible for it. On the other hand turbulent flow conditions could lead to particle collision (perikinetic flocculation), which is a desired effect to get larger flocs with better sedimentation behaviour (Mihopulos, 1995; Lyn et al., 1992). Both effects may occur simultaneously but particle destruction seems to exceed flocculation. Humus tanks are supposed to clear wastewater by separating solids from water by sedimentation.
Stokes law can describe theoretical sedimentation velocity of particles in quiescent liquid: vsed ¼
g ðrS rL Þd 2 rL v 18
ð1Þ
Density differences between solids and liquids and particle size are the most important parameters for sedimentation velocity. In horizontal flow settling tanks there is a certain horizontal velocity, so that sedimentation is determined by the resultant of vertical and horizontal force components. Therefore, the flow rate should be limited and maximum horizontal velocity should be less than 0.5 m s1. Detailed design settings for area, support-mediaspecific surface etc. are reported in ATV (1999), Ekama et al. (1997), Gu¨nthert (1995, 1998) and Krebs et al. (1996, 1998). As equation (1) shows sedimentation velocity is influenced by diameter to the power of 2. Particle size determination is a useful instrument for estimation of
3996
Wolfgang Schubert and F. Wolfgang Gu¨nthert
Fig. 4. Comparison between number distribution of effluent and inflow into humus tanks.
Fig. 5. Comparison between volume distribution of effluent and inflow into humus tanks.
sedimentation behaviour and sedimentation success control. Measurement results show that 98% of the particles are smaller than 100 mm. These results are typical for the inflow of humus tanks, there are a huge number of small particles and a few number of large particles present. Figure 4 shows differences between the number of particles in the effluent and inflow of humus tanks in Ingolstadt. There is almost no change in the number of particles with diameters 520 mm, while there are only a few particles with diameters >200 mm in the effluent of humus tanks. Large particles have been removed through sedimentation, while small particles are still in the sample. Especially particles with diameters 5100 mm need sedimentation times beyond the hydraulic residence time in settling tanks (Herold and Mu¨ller, 1987; Klute, 1984; Steinmann, 1989). Figure 5 represents the volume distribution of particle diameters in the inflow and effluent of humus
tanks. The volume distribution as an assumption for the mass of solids shows that 43% of the mass of solids from the effluent is composed of particles with diameters 5100 mm.
DISCUSSION
Particle measurements with Galai Cis 100 are well reproducible, although systematic errors owing to an assumption of spherical particles are included. Nevertheless area distribution was used to get a representative overview of the particle size spectrum across the entire measuring range. The mass distribution of solid particles can be approximated on the basis of volume distribution. Number distribution results show that more than 90% of the particles have diameters smaller than 30 mm. These small particles need sedimentation times beyond the residence time in secondary settling tanks. Other factors like stormwater flow, wastewater
Particle size distribution in humus tanks
compound changes for example on weekends, temperature changes, have additional influence on the solids content (Gu¨nthert, 1996). Volume distribution results show that on average nearly 40% of the solid mass is composed of particles 5100 mm. The solids content of final effluent could be significantly reduced with an improved sedimentation of small particles. A further aspect is that in the effluent of secondary settling tanks nearly 80% phosphorus exists in the particulate fraction (Thiem et al., 1996), so that additional nutrient removal could only be achieved by improved particle separation. In secondary settling tanks flocculation could be improved through changes of the inlet construction as well as by a higher particle collision rate resulting from a higher solids content (sludge blanket filtration). To achieve the latter, the wastewater return rate has to be raised. These aims shall be examined in further investigations within the scope of this research project. CONCLUSIONS
Particle destruction on the way from trickling filters to humus tanks should be avoided. Therefore, further investigations are necessary to determine the reasons for that destruction and also to improve flocculation through particle collision. The microbiological population structure of biofilms, as investigated in the SFB 411, could be influenced in such a way that it forms more stable aggregates. The filling materials of trickling filters as biofilm growth area seem to have influence on size of flushed particles and aggregates. Acknowledgements}The investigation results in this report could only be achieved thanks to a grant of the Deutsche Forschungsgemeinschaft DFG. Our work has also benefited greatly from the inspiration and comments of the other participating members of the common research project.
REFERENCES
ATV (1999) Principles for the dimensioning of filters and biological contactors (German). ATV-Standard A 135. Ekama G. A., Barnard J. L., Gu¨nthert F. W., McCorquodale J. A., Parker D. S. and E. J.
biological Pre-Print Krebs P., Wahlberg
3997
(1997) Secondary settling tanks: theory, modelling, design and operation. IAWQ Scientific and Technical Report No. 6. ISBN 1 900222 03 5. Gu¨nthert F. W. (1995) Final settling tanks}new aspects and design approaches. Seventh IAWQ International Conference on Design and Operation of large Wastewater treatment plants. Gu¨nthert F. W. (1996) Minimising of solid outlet from final settling tanks of activated sludge processes (German). WAP 3/96, 35–38. Gu¨nthert F. W. (1998) Final settling tanks, operation and dimensioning. Dimensioning of Municipal Wastewater Treatment Plants (German). Expert publishing company, Renningen-Malmsheim, ISBN 3-8169-1406-3. Herold M. and Mu¨ller H. (1987) Floc sedimentation velocity and particle shape (German). Chem. Technol. 39(5), 203–204. Klute R. (1984) Modulation of Floc Properties to Separation Techniques (German). DVGW Series Flocculation in Water Preparation 42, 153–172. Krebs P., Stamou A. I., Garcı´ a-Heras J. L. and Rodi W. (1996) Influence of inlet and outlet configuration on the flow in secondary clarifiers. Water Sci. Technol. 34(5–6), 1–9. Krebs P., Armbruster M. and Rodi W. (1998) Laboratory experiments of buoyancy-affected flow in clarifiers. J. Hydr. Res. 36(5), 831–851. Lewandowski Z., Webb D. and Hamilton M. (1999) Quantifying biofilm structure. Water Sci. Technol. 39(7), 71–76. Lyn D. A., Stamou A. I. and Rodi W. (1992) Density currents and shear-induced flocculation in sedimentation tanks. J. Hydr. Engng. ASCE 118(6), 849–867. Mihopulos J. (1995) Interactions of Floc Formation}Floc Separation Under Consideration of Flow Pattern in Sedimentation and Flotation Tanks (German). Series from ISWW, Vol. 72, University Karlsruhe. Steinmann G. (1989) Sedimentation and coagulation processes in final settling tanks from trickling filters with suggestions for dimensioning (German). Reports from water quality and waste management no. 88, Technical University, Munich. Thiem, A. and Neis, U. (1996) Size distribution and pollutant content of suspended solids in wastewater treatment plants (German). Hamburger Series of Sanitary Engineering 18: 9. Expert congress wastewater treatment for protecting North- and East sea, Travemu¨nde, 18/19 November 1996 (pp. 142–153). Wagner M., Hutzler P. and Amann R. (1998) Threedimensional analysis of complex microbial communities by combining confocal laser scanning microscopy and fluorescence in situ hybridisation. In Digital Image analysis of Microbes: Imaging, Morphometry, Fluorometry and Motility Techniques and Application, eds M. H. Wilkinson and F. Schut, pp. 467–484. Willey, Chichester.
Wat. Res. Vol. 35, No. 16, pp. 3993–3997, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE PARTICLE SIZE DISTRIBUTION IN EFFLUENT OF TRICKLING FILTERS AND IN HUMUS TANKS WOLFGANG SCHUBERT and F. WOLFGANG GU¨NTHERT* Institute of Hydroscience, Sanitary Engineering and Waste Management, Faculty of Civil Engineering and Surveying, University of the Federal Armed Forces Munich, Germany (First received 22 May 2000; accepted in revised form 19 February 2001) Abstract}Particles and aggregates from trickling filters must be eliminated from wastewater. Usually this happens through sedimentation in humus tanks. Investigations to characterize these solids by way of particle size measurements, image analysis and particle charge measurements (zeta potential) are made within the scope of Research Center for Science and Technology ‘‘Fundamentals of Aerobic biological wastewater treatment’’ (SFB 411). The particle size measuring results given within this report were obtained at the Ingolstadt wastewater treatment plant, Germany, which served as an example. They have been confirmed by similar results from other facilities. Particles flushed out from trickling filters will be partially destroyed on their way to the humus tank. A large amount of small particles is to be found there. On average 90% of the particles are smaller than 30 mm. Particle size plays a decisive role in the sedimentation behaviour of solids. Small particles need sedimentation times that cannot be provided in settling tanks. As a result they cause turbidity in the final effluent. Therefore quality of sewage discharge suffers, and there are hardly advantages of the fixed film reactor treatment compared to the activated sludge process regarding sedimentation behaviour. # 2001 Elsevier Science Ltd. All rights reserved Key words}trickling filters, humus tanks, particle size measurement, particle size distribution, sedimentation
NOMENCLATURE
a BR ;BOD BR;TKN d Ff g n qA;TF vsed X
INTRODUCTION
number of rotary sprinkler arms, dimensionless volumetric loading of a reactor as biological oxygen demand, kg m3 d1 volumetric loading of a reactor as Kjeldahl nitrogen demand, kg m3 d1 diameter of a sedimentation particle, mm flushing force, mm a1 gravity constant, m s2 revolutions per hour, 1 h1 surface flow rate trickling filter, m h1 sedimentation velocity, m h1 equivalent diameter, mm
Subscripts and superscripts A area L length V volume Greek letters density of solids, g cm3 rS rL density of liquid, g cm3 u kinematic viscosity, m2 s1
*Author to whom all correspondence should be addressed. Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany. Tel.: +49-89-6004-2156; fax: +49-89-6004-3858; e-mail: [email protected]
Wastewater treatment with fixed film reactors is a rather old technique. Trickling filters are filled with materials where biofilm grows. Biofilms are organic material (granula), the different species attached to it, i.e. bacteria, fungi, protozoa, higher organisms, and exopolymeric substances from microorganism. Structure of biofilms varies according to facts like wastewater composition and wastewater load. There are several research projects under way within SFB 411 aimed at investigating biofilm structure and microbiological population (Wagner et al., 1998; Lewandowski et al., 1999). The biofilm grows on filling materials and the organisms are fed with substrate taken from wastewater. With microbiological growth biofilms are getting thicker and partially anaerobic zones develop. In these zones there is a lack of substrate and oxygen so that parts of the population die. This leads to biofilm parts getting flushed off (Steinmann, 1989). The size of trickling filters depends on the BOD load per unit reactor volume i.e. BR;BOD 0:4 kg m3 d1 for carbon removal, BR;BOD 0:4 kg m3 d1 and BR;TKN 0:1 kg m3 d1 for nitrification. During wastewater treatment the biofilm grows and surplus sludge must be removed. Therefore a surface flow rate qA;TF between 0.4 m h1 (mineral filling materi-
3993
3994
Wolfgang Schubert and F. Wolfgang Gu¨nthert
als) and 0.8 m h1 (synthetic filler elements) is required (ATV, 1999). A minimum value is also necessary to get an entire wetting of filling material. For the assessment flushing force Ff ¼ qA;TF ða nÞ1 should reach values between 4 and 8 mm a1 to get a permanent erosion of biofilm material (ATV, 1999). Particles in the effluent of trickling filters consist of solids from wastewater and parts of the biofilm. The investigation results regarding the size distribution of those particles presented in this report were obtained at the Ingolstadt wastewater treatment plant (Germany). Grab samples from inflow and effluent of humus tanks as well as from two trickling filters with different filling materials were taken and analysed.
MATERIALS AND METHODS
The Ingolstadt wastewater treatment plant’s operating mode is an activated sludge process treatment with four successive trickling filters. The trickling filters have a filler height of 3 m and a diameter of 38 m. The treatment plant is designed for a population of 345 000. Particle size analysis is carried out by means of Galai Cis 100 particle sizer (Galai Ltd.). The measuring range is 10–3600 mm, with a resolution of 6 mm. Samples from the outlets of trickling filters are taken by placing a sampling beaker at the bottom of the trickling filter (aeration holes). It’s position and the times of placement influence the amount of solids in the sample. Several different samples should be measured to make sure that the results are representative. However, the main advantage of the direct sampling of particles at the bottom of trickling filters is that changes or destruction of particles are avoided. Samples from humus tanks are taken as grab samples with a beaker. For each location there were taken three samples. All samples were measured three times. The measurements are made representative by measuring a large number, for instance 80 000 particles. The quality of the measurements is documented by confidence intervals. The measurements presented in this report are based on a minimum confidence interval of 95%. Variations of the present particle sizes in a grab sample have minor impact and do not induce significant differences on size distribution results.
RESULTS
Trickling filters Particles, flocs and aggregates in wastewater differ regarding shape and size. The vast majority of particles are of very small size, although some bigger particles exist. Particle shapes are very irregular; all kinds of shapes from longish to filamentous, partly voluminous and spherical exist simultaneously. At the wastewater treatment plant in Ingolstadt, grab samples from effluent of trickling filter were taken at different distances from the outer wall. Distance 0.5 m is near the outer wall of the trickling filter, while distance 5 m is more inside the trickling filters (radius 19 m). Particle size distributions results from samples taken at different distances are shown in Fig. 1. There were more large particles found at distance 5 m, while more small particles were encountered at distance 0.5 m. Further investigations are required to establish whether or not this peculiar distribution is caused by hydraulic loading differences inside the trickling filter. Figure 2 shows that wastewater effluent from trickling filter 3 contains larger particles than effluent from trickling filter 4. The average volumetric BOD loading BR;BOD of trickling filter 3 is 0.15 kg m3 d1 and average surface flow rate qA;TF is 1.07 m h1. The average volumetric BOD loading BR;BOD of trickling filter 4 is 0.21 kg m3 d1 and average surface flow rate qA;TF is 1.47 m h1. A major difference between trickling filter 3 and trickling filter 4 is the support media. Trickling filter 3 is filled with random packed synthetic filter media elements with a support media surface of 320 m2 m3, while trickling filter 4 has big built-in filter media bodies with a support media surface of 190 m2 m3. Although trickling filter 3 has a lower surface loading rate, more large sized particles and less small particles were flushed out, independent of sampling location at the bottom of the trickling filter.
Fig. 1. Area distribution, effluent of trickling filter 3 (0.5 and 5 m distance from outer wall).
Particle size distribution in humus tanks
3995
Fig. 2. Comparison between effluent of trickling filters 3 and 4 (5 m distance from outer wall).
Fig. 3. Comparison between effluent of trickling filter 3 (mean values) and inflow humus tanks.
Humus tanks Wastewater effluent from the four trickling filters in Ingolstadt enters a large gully, from where it flows to the humus tanks. Figure 3 shows the differences between the particle size distribution at the effluent of the trickling filter (mean values of the different sampling locations) and the particle size distribution at the inflow to the humus tanks. Particles flushed out from trickling filters are larger than the particles at the inflow to the humus tanks, since they are destroyed on their way. Turbulent stream regime may be responsible for it. On the other hand turbulent flow conditions could lead to particle collision (perikinetic flocculation), which is a desired effect to get larger flocs with better sedimentation behaviour (Mihopulos, 1995; Lyn et al., 1992). Both effects may occur simultaneously but particle destruction seems to exceed flocculation. Humus tanks are supposed to clear wastewater by separating solids from water by sedimentation.
Stokes law can describe theoretical sedimentation velocity of particles in quiescent liquid: vsed ¼
g ðrS rL Þd 2 rL v 18
ð1Þ
Density differences between solids and liquids and particle size are the most important parameters for sedimentation velocity. In horizontal flow settling tanks there is a certain horizontal velocity, so that sedimentation is determined by the resultant of vertical and horizontal force components. Therefore, the flow rate should be limited and maximum horizontal velocity should be less than 0.5 m s1. Detailed design settings for area, support-mediaspecific surface etc. are reported in ATV (1999), Ekama et al. (1997), Gu¨nthert (1995, 1998) and Krebs et al. (1996, 1998). As equation (1) shows sedimentation velocity is influenced by diameter to the power of 2. Particle size determination is a useful instrument for estimation of
3996
Wolfgang Schubert and F. Wolfgang Gu¨nthert
Fig. 4. Comparison between number distribution of effluent and inflow into humus tanks.
Fig. 5. Comparison between volume distribution of effluent and inflow into humus tanks.
sedimentation behaviour and sedimentation success control. Measurement results show that 98% of the particles are smaller than 100 mm. These results are typical for the inflow of humus tanks, there are a huge number of small particles and a few number of large particles present. Figure 4 shows differences between the number of particles in the effluent and inflow of humus tanks in Ingolstadt. There is almost no change in the number of particles with diameters 520 mm, while there are only a few particles with diameters >200 mm in the effluent of humus tanks. Large particles have been removed through sedimentation, while small particles are still in the sample. Especially particles with diameters 5100 mm need sedimentation times beyond the hydraulic residence time in settling tanks (Herold and Mu¨ller, 1987; Klute, 1984; Steinmann, 1989). Figure 5 represents the volume distribution of particle diameters in the inflow and effluent of humus
tanks. The volume distribution as an assumption for the mass of solids shows that 43% of the mass of solids from the effluent is composed of particles with diameters 5100 mm.
DISCUSSION
Particle measurements with Galai Cis 100 are well reproducible, although systematic errors owing to an assumption of spherical particles are included. Nevertheless area distribution was used to get a representative overview of the particle size spectrum across the entire measuring range. The mass distribution of solid particles can be approximated on the basis of volume distribution. Number distribution results show that more than 90% of the particles have diameters smaller than 30 mm. These small particles need sedimentation times beyond the residence time in secondary settling tanks. Other factors like stormwater flow, wastewater
Particle size distribution in humus tanks
compound changes for example on weekends, temperature changes, have additional influence on the solids content (Gu¨nthert, 1996). Volume distribution results show that on average nearly 40% of the solid mass is composed of particles 5100 mm. The solids content of final effluent could be significantly reduced with an improved sedimentation of small particles. A further aspect is that in the effluent of secondary settling tanks nearly 80% phosphorus exists in the particulate fraction (Thiem et al., 1996), so that additional nutrient removal could only be achieved by improved particle separation. In secondary settling tanks flocculation could be improved through changes of the inlet construction as well as by a higher particle collision rate resulting from a higher solids content (sludge blanket filtration). To achieve the latter, the wastewater return rate has to be raised. These aims shall be examined in further investigations within the scope of this research project. CONCLUSIONS
Particle destruction on the way from trickling filters to humus tanks should be avoided. Therefore, further investigations are necessary to determine the reasons for that destruction and also to improve flocculation through particle collision. The microbiological population structure of biofilms, as investigated in the SFB 411, could be influenced in such a way that it forms more stable aggregates. The filling materials of trickling filters as biofilm growth area seem to have influence on size of flushed particles and aggregates. Acknowledgements}The investigation results in this report could only be achieved thanks to a grant of the Deutsche Forschungsgemeinschaft DFG. Our work has also benefited greatly from the inspiration and comments of the other participating members of the common research project.
REFERENCES
ATV (1999) Principles for the dimensioning of filters and biological contactors (German). ATV-Standard A 135. Ekama G. A., Barnard J. L., Gu¨nthert F. W., McCorquodale J. A., Parker D. S. and E. J.
biological Pre-Print Krebs P., Wahlberg
3997
(1997) Secondary settling tanks: theory, modelling, design and operation. IAWQ Scientific and Technical Report No. 6. ISBN 1 900222 03 5. Gu¨nthert F. W. (1995) Final settling tanks}new aspects and design approaches. Seventh IAWQ International Conference on Design and Operation of large Wastewater treatment plants. Gu¨nthert F. W. (1996) Minimising of solid outlet from final settling tanks of activated sludge processes (German). WAP 3/96, 35–38. Gu¨nthert F. W. (1998) Final settling tanks, operation and dimensioning. Dimensioning of Municipal Wastewater Treatment Plants (German). Expert publishing company, Renningen-Malmsheim, ISBN 3-8169-1406-3. Herold M. and Mu¨ller H. (1987) Floc sedimentation velocity and particle shape (German). Chem. Technol. 39(5), 203–204. Klute R. (1984) Modulation of Floc Properties to Separation Techniques (German). DVGW Series Flocculation in Water Preparation 42, 153–172. Krebs P., Stamou A. I., Garcı´ a-Heras J. L. and Rodi W. (1996) Influence of inlet and outlet configuration on the flow in secondary clarifiers. Water Sci. Technol. 34(5–6), 1–9. Krebs P., Armbruster M. and Rodi W. (1998) Laboratory experiments of buoyancy-affected flow in clarifiers. J. Hydr. Res. 36(5), 831–851. Lewandowski Z., Webb D. and Hamilton M. (1999) Quantifying biofilm structure. Water Sci. Technol. 39(7), 71–76. Lyn D. A., Stamou A. I. and Rodi W. (1992) Density currents and shear-induced flocculation in sedimentation tanks. J. Hydr. Engng. ASCE 118(6), 849–867. Mihopulos J. (1995) Interactions of Floc Formation}Floc Separation Under Consideration of Flow Pattern in Sedimentation and Flotation Tanks (German). Series from ISWW, Vol. 72, University Karlsruhe. Steinmann G. (1989) Sedimentation and coagulation processes in final settling tanks from trickling filters with suggestions for dimensioning (German). Reports from water quality and waste management no. 88, Technical University, Munich. Thiem, A. and Neis, U. (1996) Size distribution and pollutant content of suspended solids in wastewater treatment plants (German). Hamburger Series of Sanitary Engineering 18: 9. Expert congress wastewater treatment for protecting North- and East sea, Travemu¨nde, 18/19 November 1996 (pp. 142–153). Wagner M., Hutzler P. and Amann R. (1998) Threedimensional analysis of complex microbial communities by combining confocal laser scanning microscopy and fluorescence in situ hybridisation. In Digital Image analysis of Microbes: Imaging, Morphometry, Fluorometry and Motility Techniques and Application, eds M. H. Wilkinson and F. Schut, pp. 467–484. Willey, Chichester.