- Email: [email protected]
Possibilities for increased nutrient removal with a final filtration stage
~ Pergamon
Wal. Sci. T~c". Vol. 33. No. 12. pp. 147-153. 1996. Copyright C 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Ontain. All nghls reserved. 0273-1223196 S15·00 + 0-00
PU: S0273-1223(96)00468-4
POSSIBILITIES FOR INCREASED NUTRIENT REMOVAL WITH A FINAL FILTRATION STAGE M. Rothman and 1. Hultgren Stockholm Water Co, S-/06 36 Stockholm, Sweden
ABSTRACf Bromma sewage treatment plant (STP) is one of three planls in Stockholm. To meet more stringent requirements for nutrient removal the plant has been extended with a final filtration stage. Earlier it has not been possible 10 operate the plant with nitrificalion during winter time. Bad settling properties of the activated sludge have led to bulking sludge and high concentrations of BOD and phosphorus in the effluent. With the filter stage it is now possible to reduce the load on the biological stage by by-passing part of the flow directly to the filters. The result has been very promising and it seems that the plant can meet the new demands for nitrogen removal without extension of the aerated volumes. Copyright iC) 1996 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Wastewater treatment; activated sl\ldge; phosphorus removal; nitrogen removal; dual-media down flow filter; settling properties. INTRODUCfION In Sweden the requirements for purification of nitrogen and phosphorus in sewage will be more stringent in the future. The three sewage treatment plants in Stockholm. Henriksdal. Bromma and Loudden. are all situated in rock chambers. To meet future requirements for nitrogen removal large extensions are necessary which will result in very high costs. Since the three plants have the same recipient for the effluent, the authorities have agreed to regard the three effluents as one. Each plant will therefore be extended in different ways to use the investment money as efficiently as possible. In Henriksdal, which is the largest plant, the aeration tanks will be extended and a final filtration stage will be installed. Bromma has already been extended with a filtration stage and the pre-precipitation at Loudden will be improved (Hultgren et al., 1993). The final requirements for the common effluent and the design concentrations for each plant are summarized in table I. One of the largest problems in countries like Sweden, when it comes to nitrogen removal, is the low temperature of the sewage during winter time. The crucial point for nitrogen removal is the nitrification and the growth rate of the nitrifying bacteria. The temperature of the sewage in winter could be as low as 9°C for several weeks which demands a sludge age of at least 12 days (Henze et al., 1990). A high sludge age often gives rise to poor settling of the activated sludge (Jenkins et al., 1993; Carlsson et al. 1994). To maintain a high sludge age the activated sludge needs to be returned to the aeration tanks. If there are problems with the 147
M. ROTHMAN and 1. HULTGREN
148
settling of the sludge, the sludge will accumulate in the clarifiers and eventually discharge with the effluent. This will have a large impact on the purification result for the plant. The lost sludge will also affect the nitrification activity by lowering the sludge age. Table I. Required and designed concentrations for the sewage treatment plants in Stockholm Requirement" Tot-P BOD7 Tot-N
Design Concentrations HenriksdaJ
Bromma
Loudden
(m!Y'l)
0.3
0.3
0.3
0.3
(m!Y'l) (mgJI)
10
10
10
10
15
14
17
20
• The flow proportional average of the designed concentrations of Tot-N in the effluent from the three plants corresponds to 15 mgll at the design flow.
Removal of phosphorus may be achieved with precipitation with iron or aluminium. This could be done as pre-, simultaneous- or post-precipitation. By using a sufficient dose of metal the amount of dissolved phosphorus will not exceed 0.1 mg pn. The rest of the phosphorus in the effluent will be bound in particles. This fraction can become very high in plants with pre-precipitation if secondary sedimentation is the final treatment stage and it performs badly. Filtration of the effluent from the secondary sedimentation tanks may then be a very cost efficient way to decrease the particles in the sewage. Secondary sedimentation has previously been the final treatment stage at the plants in Stockholm. To meet the new and more stringent standards for phosphorus both Henriksdal and Bromma need a final treatment stage after secondary sedimentation. Filters were choosen for both plants. It has also been shown that filters may be used to unload the biological stage when the flow is high (Andersson et al. 1992). By by-passing a small stream of settled and precipitated sewage directly to the filters, the flow through the activated sludge stage could be decreased in order to get sufficient settling. Sufficient sludge age for nitrification can then be reached also when the water is very cold. This paper will describe how Bromma sewage treatment plant has performed after the extension with the new filter stage and how the filter stage has given the plant new possibilities to increase the degree of nutrient removal without any further extensions of the biological stage. 25 22.5
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Fiiure I. Temperature of !he sewage at Bromma STP. 1994.
BACKGROUND Activated sludie sta~e The activated sludge stage at Bromma STP is comparatively small with a total volume of the biological reactors of 23400 m3. 88% of the biological reactors are aerated and 12% are anoxic. The plant is designed
Possibilities for increased nutrient removal
149
for an average flow of 160000 m3/day (1.9 m3/s). The designed surface load ofthe secondary sedimentation tanks is 1.2 m/h. The size of the anoxic zones can be rather small due to the large amount of easily degradable carbon in the wastewater from one of the largest breweries in Sweden, which is connected to the plant. The denitrification rate varies between 5-10 mg N/(kg VSS.h). The temperature of the sewage during 1994 is presented in figure I. Due to different growth rates of the nitrifying bacteria at different temperatures the sludge age of the activated sludge needs to vary over the year. The required sludge age in the biological stage at different temperatures is presented in table 2. At high sludge ages, the settling properties for the activated sludge at Bromma STP are often very poor. Table 2. Required sludge age for nitrification at different temperatures Temperature
(C)
10
15
20
Sludge age
(days)
11
6
4
When the settling properties of the activated sludge are bad and the hydraulic flow increases, there is always a risk sludge escaping from the secondary sedimentation tanks. In these situations, the biological stage can be operated in two different ways. If a certain amount of escaping sludge can be accepted. the plant can be operated as usual. If the increased flow only lasts for a short time, most of the nitrifiers will still be kept in the system and the only problem will be increased concentrations of different pollutants in the effluent for a short time. But if the hydraulic flow is high for a longer period, most of the nitrifiers will be lost with the effluent and nitrification can be lost for months. The activated sludge in the effluent can also cause problems With reaching the standards for BOD and phosphorus. An alternative is to by-pass the biological stage with a certain portion of the flow. The problem with escaping sludge can then be eliminated and nitrification can be saved. But on the other hand the pollutants in the settled sewage may increase the concentration of BOD and phosphorus in the effluent above the required standards. Before the installation of the filter stage a combination of these two possibilities was used but the quality of the effluent was often bad and nitrification was always lost in the end. It seemed that it was impossible to run the biological treatment with nitrogen removal in winter time and at the same time meet the standards for BOD and phosphorus. Since the filters have been taken into operation a lot of new possibilities have appeared for the plant. filtration sta~e The filtration stage was taken into operation in August 1993. The type of filter was chosen from the results of pilot plant experiments where ten types of dual-media downflow filters and one upflow filter were tested (Andersson et al., 1992). The filter stage at Bromma STP consists of 24 dual-media downflow filters with a total area of I 440 m2. The filters consist of 0.5 m sand (1.2-1.6 mm in diameter) as a bottom layer and 1.0 m expanded clay (2.5-3.5 mm in diameter) as a top layer. The filter stage is designed for a maximum flow of 4.0 m3/s and a concentration of 50 mg/l of suspended solids. The capacity of sludge accumulation is 6.5 kg SS/m2• The filters are operated at a constant water level and the head-loss over the filters are compensated by a valve on the effluent pipe from each filter. When the valve is completely opened the filter is backwashed. The backwash can also start after a pre-set time of operation. The backwash is repeated twice with a mixture of air (30 m/h) and water (20 m/h) and then the two different layers of media are sorted by a very high flow of water (90 m/h) which is decreased stepwise. The filters are backwashed with the effluent from the filters.
150
M. ROTIlMAN and J. HULTGREN
A backwashing cycle takes approximately 25 minutes. Only one filter can be backwashed at a time which means that the running time for the filters was to be at least 10 hours. 2
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Figure 2. Concentration during 1994 of total ph05phorus in the influent to and effluent from the filtration stage. 50 ~45
::::: 40 01
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... 20 0
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RESULT Filtration
sta~e
The removal of phosphorus from the filtration stage has been approximately 50% and the concentration of phosphorus in the effluent has not exeeded 0.5 mgll (Figure 2). The concentration of suspended solids in the effluent has mainly been below 5 mgll but has been as high as 50 mgll when large portions of the flow are by-passed. The concentration of suspended solids at different flows is presented in figure 3. In figure 4 the concentration of suspended solids and the by-pass flow during 1994 is described.
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Figure 4. Concentration of suspended solids (55) in the effluent and by-pass flow of sellled sewage during 1994.
The sludge accumulation capacity has varied between 1.5 and 5.0 kg SS/m2 with an average of 2.8 kg SS/m2 (p 1.1. n = 23). The backwashing has mostly been controlled by the pre-set running time, which has varied between 40-80 hours. The flow needed for backwashing has been 1.5% of the filtrated flow. About 20% of
Possibilities for increased nutrient removal
the accumulated sludge in the filters are left after backwashing. The head-loss in the filters after backwashing varies (Figure 5), but there is no upwords trend which would indicate that the function of the filters decreases with time. ~ l-
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Figure 6. Concentration of total and ammonium nitrogen in the effluent from the plan\, 1993-1994. liilro~en remoyal
The biological stage was operated with nitrification during eight months in 1994 compared with six months during 1993 (Figure 6). The differellce between 1993 and 1994 was that the biological stage was operated with nitrification for almost the whole winter during 1994.The sludge age varied between 4 and 9 days during periods with nitrification (Fig. 7). 10 ~9 11I8
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1995-01-01:ססOO
Figure 7. Sludge age for the biological stage during 1994.
The actual flow, the maximum allowed flow through the biological stage and the flow which has been by-passed the biological stage during 1994 are presented in figure 8. Large portions of the flow were by-passed during three periods; in January, in March-April and in September. The maximum by-passed flow during one day was 40% of the actual flow.
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Figure 8. Actual flow, maximum allowed flow through the biological stage and by-passed flow during 1994.
The nitrification was lost twice during 1994. The loss of nitrification in April was due to extremely bad sludge settling properties in combination with a very high flow. The portion of water by-passing the biological stage was then around 40% of the total flow but the sludge still did not settle properly, which resulted in high concentrations of suspended solids in the influent to the filters. The filters clogged faster than it was possible to backwash them which led to an impossible situation. The sludge age had to be decreased to improve the settling properties. The nitrification was also lost in the autumn due to problems with sludge digestion caused by the filamentous bacteria Microthrix parvicella in the activated sludge. The filaments caused bad foaming in the digesters. To remove the filaments from the biological stage the sludge age was decreased very rapidly. This gave a fast reply, but with the price of lost nitrification. DISCUSSION The result of operation for 1994 was on the whole good and has given a lot of hope for the future. The nitrogen removal process at Bromma STP worked well although the winter 1994 was comparatively cold (Figure I) and the hydraulic flow high (Figure 8). The nitrification was maintained during the whole winter period for the first time. The result indicates that it is possible to meet the requirements for nitrogen, phosphorus and BOD without any further extension, at least for the next 10-15 years. The concentration of nitrogen in the effluent was well below the designed 17 mg/l N (14.6 mg /I), although nitrification was lost twice during the year. The concentrations of BOD7 and total phosphorus in the effluent during 1994 was 3 and 0.1 mg/l respectively. The new filter stage has met the expectations concerning the degree of purification. The concentration of suspended solids in the effluent increased when large portions of the flow by-passed the biological stage. The cleaning capacity of the filters during these periods has still been comparatively good. The sludge accumulation capacity of the filters has been lower when compared with the result achieved in the previous pilot plant experiments. To calculate the sludge accumulating capacity the running time of the filters has to be set by the opening angle of the valve on the outlet pipe. Mostly the operation time has been set by the pre-set time, so there is limited experience about the sludge accumulating capacity. The filters are not completely cleaned after a backwash. How the remaining sludge affects the running time of the filters has been very difficult to investigate. The flow through the filters has a large impact on the opening angle of the valve. The determining factor of the running time of the filters is therefore a combination of the head-loss in the filters and the flow through the filters. The nitrification was lost in April because the filters needed to be backwashed more often than every 10th hour. There is not enough water to backwash two filters at a time for the moment but this could be possible by reconstruction of the effluent channels. Problems with foaming in the digesters caused by filamentous bacteria is well known (Pujol et ai., 1991; Jenkins et ai., 1993). This problem has not been reported in Sweden before and there is not yet any known solution to it. There are six digesters at Bromma STP and the excess sludge and the primary sludge are today mixed before digestion. By separating the two different sludges, the foaming problem can be limited to a
Possibilities for increased nutrient removal
153
smaller number of digesters. The digesters for the excess sludge can then be rebuilt to prevent stable foam from being formed. The sludge age in the biological stage during winter 1994 required for nitrification was lower than the sludge age recommended in the literature. Sludge age is a very rough measurement for deciding the demanded concentration of mixed liquor suspended solids (MLSS). This could cause higher sludge ages than actually needed. Since the sludge age affects the settling properties of the sludge it would be desirable to adjust the sludge age after the amount of nitrifying bacteria instead. There are also olher ways to further increase the nitrogen removal. Today there are no pumps for recirculating MLSS with high concentrations of nitrate to the anoxic zones to improve denitrification but they will soon be installed. During periods with low flow the filters have capacity to also denitrify a part of the remaining nitrate if there is enough carbon present. Denitrification in filters has in previous studies (Jepsen and Jansen. 1993) proved to be a promising way to polish nitrogen. One drawback to by-passing a part of the flow at high hydraulic loadings is that not all sewage is treated biologically. The goal is of course to always limit the part of the flow that has to be by-passed. but the bUlking conditions often appear very fast and in order to save the filters the by-pass flow is today set with a certain safety margin. By measuring the level of the sludge blanket in the secondary sedimentation tanks and regulating the by-passing flow after this level, the flow through the biological stage can be optimized. More work will also be carried out to increase the understanding of the settling properties of the activated slUdge. By understanding why the ability to settle changes. the biological stage may hopefully be operated to favour good settling properties for the activated sludge in the future.
REFERENCES Andersson. C.• Tendaj. M. and Rothman. M. (1992). Filtration at Bromma Sewage Treatment Plant. Waf. Sci. rl!ch.• 15(415), 59-66. Carlsson. H.• Lee. N. and Welandcr. T. (1994). A case study on the sludge characteristics in a biological nutrient removal pilot plant at the Sjolunda Wastewater Treatment Plant in Malmo. Valll!n. 50. 39-46. Henze, M.. Harremoes. P.• Jansen. 1. la Cour and Arvin. E. (1990). Spildl!vandsrl!nsning Biologisk og kl!misk. Polyteknisk Forlag, Lyngby. Denmark. Hultgren. 1. and Reinius. L. G. (1993). Extension of the sewage treatment plants in Stockholm: Some technical and economical aspects. Waf. Sci. rl!ch.• 27(5/6). 357-367. Jenkins. D.• Richard. M. G. and Daigger. G. T. (1993). Manual on /hl! Causl!s and Con/rol of Activa/I!d Siudgl! Bulking and Foaming. 2nd ed. Lewis Publishers. Michigan. Jepsen. S. E. and Jansen. J. la Cour. (1993). Biological filters for post-denitrification. Wat Sci rl!ch.• 27(5/6). 369-379. Pujol. R.• Duchene, P. H.• Schetrite. S. and Canler. J. P. (1991). Biological foams in activated sludge plants: Characterization and situation. Waf. Rl!s., 25(11).1399-1404.
Wal. Sci. T~c". Vol. 33. No. 12. pp. 147-153. 1996. Copyright C 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Ontain. All nghls reserved. 0273-1223196 S15·00 + 0-00
PU: S0273-1223(96)00468-4
POSSIBILITIES FOR INCREASED NUTRIENT REMOVAL WITH A FINAL FILTRATION STAGE M. Rothman and 1. Hultgren Stockholm Water Co, S-/06 36 Stockholm, Sweden
ABSTRACf Bromma sewage treatment plant (STP) is one of three planls in Stockholm. To meet more stringent requirements for nutrient removal the plant has been extended with a final filtration stage. Earlier it has not been possible 10 operate the plant with nitrificalion during winter time. Bad settling properties of the activated sludge have led to bulking sludge and high concentrations of BOD and phosphorus in the effluent. With the filter stage it is now possible to reduce the load on the biological stage by by-passing part of the flow directly to the filters. The result has been very promising and it seems that the plant can meet the new demands for nitrogen removal without extension of the aerated volumes. Copyright iC) 1996 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Wastewater treatment; activated sl\ldge; phosphorus removal; nitrogen removal; dual-media down flow filter; settling properties. INTRODUCfION In Sweden the requirements for purification of nitrogen and phosphorus in sewage will be more stringent in the future. The three sewage treatment plants in Stockholm. Henriksdal. Bromma and Loudden. are all situated in rock chambers. To meet future requirements for nitrogen removal large extensions are necessary which will result in very high costs. Since the three plants have the same recipient for the effluent, the authorities have agreed to regard the three effluents as one. Each plant will therefore be extended in different ways to use the investment money as efficiently as possible. In Henriksdal, which is the largest plant, the aeration tanks will be extended and a final filtration stage will be installed. Bromma has already been extended with a filtration stage and the pre-precipitation at Loudden will be improved (Hultgren et al., 1993). The final requirements for the common effluent and the design concentrations for each plant are summarized in table I. One of the largest problems in countries like Sweden, when it comes to nitrogen removal, is the low temperature of the sewage during winter time. The crucial point for nitrogen removal is the nitrification and the growth rate of the nitrifying bacteria. The temperature of the sewage in winter could be as low as 9°C for several weeks which demands a sludge age of at least 12 days (Henze et al., 1990). A high sludge age often gives rise to poor settling of the activated sludge (Jenkins et al., 1993; Carlsson et al. 1994). To maintain a high sludge age the activated sludge needs to be returned to the aeration tanks. If there are problems with the 147
M. ROTHMAN and 1. HULTGREN
148
settling of the sludge, the sludge will accumulate in the clarifiers and eventually discharge with the effluent. This will have a large impact on the purification result for the plant. The lost sludge will also affect the nitrification activity by lowering the sludge age. Table I. Required and designed concentrations for the sewage treatment plants in Stockholm Requirement" Tot-P BOD7 Tot-N
Design Concentrations HenriksdaJ
Bromma
Loudden
(m!Y'l)
0.3
0.3
0.3
0.3
(m!Y'l) (mgJI)
10
10
10
10
15
14
17
20
• The flow proportional average of the designed concentrations of Tot-N in the effluent from the three plants corresponds to 15 mgll at the design flow.
Removal of phosphorus may be achieved with precipitation with iron or aluminium. This could be done as pre-, simultaneous- or post-precipitation. By using a sufficient dose of metal the amount of dissolved phosphorus will not exceed 0.1 mg pn. The rest of the phosphorus in the effluent will be bound in particles. This fraction can become very high in plants with pre-precipitation if secondary sedimentation is the final treatment stage and it performs badly. Filtration of the effluent from the secondary sedimentation tanks may then be a very cost efficient way to decrease the particles in the sewage. Secondary sedimentation has previously been the final treatment stage at the plants in Stockholm. To meet the new and more stringent standards for phosphorus both Henriksdal and Bromma need a final treatment stage after secondary sedimentation. Filters were choosen for both plants. It has also been shown that filters may be used to unload the biological stage when the flow is high (Andersson et al. 1992). By by-passing a small stream of settled and precipitated sewage directly to the filters, the flow through the activated sludge stage could be decreased in order to get sufficient settling. Sufficient sludge age for nitrification can then be reached also when the water is very cold. This paper will describe how Bromma sewage treatment plant has performed after the extension with the new filter stage and how the filter stage has given the plant new possibilities to increase the degree of nutrient removal without any further extensions of the biological stage. 25 22.5
U
20 17.5
--
15 12.5
~
10
~
5
•
f
"T'"---------------------------,
~ 7.5
ID2.5O-+-
~
w02 w05 1994-01-01:0000
..L..I.
wOl
vU
vU
v17
v20
v23
v21
v29
H" 1 _...
v32
v35
v31
vU
v44
~......,
v47 w50 1995-01-01:0000
Fiiure I. Temperature of !he sewage at Bromma STP. 1994.
BACKGROUND Activated sludie sta~e The activated sludge stage at Bromma STP is comparatively small with a total volume of the biological reactors of 23400 m3. 88% of the biological reactors are aerated and 12% are anoxic. The plant is designed
Possibilities for increased nutrient removal
149
for an average flow of 160000 m3/day (1.9 m3/s). The designed surface load ofthe secondary sedimentation tanks is 1.2 m/h. The size of the anoxic zones can be rather small due to the large amount of easily degradable carbon in the wastewater from one of the largest breweries in Sweden, which is connected to the plant. The denitrification rate varies between 5-10 mg N/(kg VSS.h). The temperature of the sewage during 1994 is presented in figure I. Due to different growth rates of the nitrifying bacteria at different temperatures the sludge age of the activated sludge needs to vary over the year. The required sludge age in the biological stage at different temperatures is presented in table 2. At high sludge ages, the settling properties for the activated sludge at Bromma STP are often very poor. Table 2. Required sludge age for nitrification at different temperatures Temperature
(C)
10
15
20
Sludge age
(days)
11
6
4
When the settling properties of the activated sludge are bad and the hydraulic flow increases, there is always a risk sludge escaping from the secondary sedimentation tanks. In these situations, the biological stage can be operated in two different ways. If a certain amount of escaping sludge can be accepted. the plant can be operated as usual. If the increased flow only lasts for a short time, most of the nitrifiers will still be kept in the system and the only problem will be increased concentrations of different pollutants in the effluent for a short time. But if the hydraulic flow is high for a longer period, most of the nitrifiers will be lost with the effluent and nitrification can be lost for months. The activated sludge in the effluent can also cause problems With reaching the standards for BOD and phosphorus. An alternative is to by-pass the biological stage with a certain portion of the flow. The problem with escaping sludge can then be eliminated and nitrification can be saved. But on the other hand the pollutants in the settled sewage may increase the concentration of BOD and phosphorus in the effluent above the required standards. Before the installation of the filter stage a combination of these two possibilities was used but the quality of the effluent was often bad and nitrification was always lost in the end. It seemed that it was impossible to run the biological treatment with nitrogen removal in winter time and at the same time meet the standards for BOD and phosphorus. Since the filters have been taken into operation a lot of new possibilities have appeared for the plant. filtration sta~e The filtration stage was taken into operation in August 1993. The type of filter was chosen from the results of pilot plant experiments where ten types of dual-media downflow filters and one upflow filter were tested (Andersson et al., 1992). The filter stage at Bromma STP consists of 24 dual-media downflow filters with a total area of I 440 m2. The filters consist of 0.5 m sand (1.2-1.6 mm in diameter) as a bottom layer and 1.0 m expanded clay (2.5-3.5 mm in diameter) as a top layer. The filter stage is designed for a maximum flow of 4.0 m3/s and a concentration of 50 mg/l of suspended solids. The capacity of sludge accumulation is 6.5 kg SS/m2• The filters are operated at a constant water level and the head-loss over the filters are compensated by a valve on the effluent pipe from each filter. When the valve is completely opened the filter is backwashed. The backwash can also start after a pre-set time of operation. The backwash is repeated twice with a mixture of air (30 m/h) and water (20 m/h) and then the two different layers of media are sorted by a very high flow of water (90 m/h) which is decreased stepwise. The filters are backwashed with the effluent from the filters.
150
M. ROTIlMAN and J. HULTGREN
A backwashing cycle takes approximately 25 minutes. Only one filter can be backwashed at a time which means that the running time for the filters was to be at least 10 hours. 2
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1.8 1.6 1.6
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Influent
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Figure 2. Concentration during 1994 of total ph05phorus in the influent to and effluent from the filtration stage. 50 ~45
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Flow (m 3/s) Figure 3. Concentration of suspended solids (55) in the effluent from the filters at different flows.
RESULT Filtration
sta~e
The removal of phosphorus from the filtration stage has been approximately 50% and the concentration of phosphorus in the effluent has not exeeded 0.5 mgll (Figure 2). The concentration of suspended solids in the effluent has mainly been below 5 mgll but has been as high as 50 mgll when large portions of the flow are by-passed. The concentration of suspended solids at different flows is presented in figure 3. In figure 4 the concentration of suspended solids and the by-pass flow during 1994 is described.
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Figure 4. Concentration of suspended solids (55) in the effluent and by-pass flow of sellled sewage during 1994.
The sludge accumulation capacity has varied between 1.5 and 5.0 kg SS/m2 with an average of 2.8 kg SS/m2 (p 1.1. n = 23). The backwashing has mostly been controlled by the pre-set running time, which has varied between 40-80 hours. The flow needed for backwashing has been 1.5% of the filtrated flow. About 20% of
Possibilities for increased nutrient removal
the accumulated sludge in the filters are left after backwashing. The head-loss in the filters after backwashing varies (Figure 5), but there is no upwords trend which would indicate that the function of the filters decreases with time. ~ l-
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1995-01-01:ססOO
Figure S. Head-loss in the filter bed after the filten have been backwashed. 25 ~
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Figure 6. Concentration of total and ammonium nitrogen in the effluent from the plan\, 1993-1994. liilro~en remoyal
The biological stage was operated with nitrification during eight months in 1994 compared with six months during 1993 (Figure 6). The differellce between 1993 and 1994 was that the biological stage was operated with nitrification for almost the whole winter during 1994.The sludge age varied between 4 and 9 days during periods with nitrification (Fig. 7). 10 ~9 11I8
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1995-01-01:ססOO
Figure 7. Sludge age for the biological stage during 1994.
The actual flow, the maximum allowed flow through the biological stage and the flow which has been by-passed the biological stage during 1994 are presented in figure 8. Large portions of the flow were by-passed during three periods; in January, in March-April and in September. The maximum by-passed flow during one day was 40% of the actual flow.
IS2
M. ROTHMAN and 1. HULTGREN
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Figure 8. Actual flow, maximum allowed flow through the biological stage and by-passed flow during 1994.
The nitrification was lost twice during 1994. The loss of nitrification in April was due to extremely bad sludge settling properties in combination with a very high flow. The portion of water by-passing the biological stage was then around 40% of the total flow but the sludge still did not settle properly, which resulted in high concentrations of suspended solids in the influent to the filters. The filters clogged faster than it was possible to backwash them which led to an impossible situation. The sludge age had to be decreased to improve the settling properties. The nitrification was also lost in the autumn due to problems with sludge digestion caused by the filamentous bacteria Microthrix parvicella in the activated sludge. The filaments caused bad foaming in the digesters. To remove the filaments from the biological stage the sludge age was decreased very rapidly. This gave a fast reply, but with the price of lost nitrification. DISCUSSION The result of operation for 1994 was on the whole good and has given a lot of hope for the future. The nitrogen removal process at Bromma STP worked well although the winter 1994 was comparatively cold (Figure I) and the hydraulic flow high (Figure 8). The nitrification was maintained during the whole winter period for the first time. The result indicates that it is possible to meet the requirements for nitrogen, phosphorus and BOD without any further extension, at least for the next 10-15 years. The concentration of nitrogen in the effluent was well below the designed 17 mg/l N (14.6 mg /I), although nitrification was lost twice during the year. The concentrations of BOD7 and total phosphorus in the effluent during 1994 was 3 and 0.1 mg/l respectively. The new filter stage has met the expectations concerning the degree of purification. The concentration of suspended solids in the effluent increased when large portions of the flow by-passed the biological stage. The cleaning capacity of the filters during these periods has still been comparatively good. The sludge accumulation capacity of the filters has been lower when compared with the result achieved in the previous pilot plant experiments. To calculate the sludge accumulating capacity the running time of the filters has to be set by the opening angle of the valve on the outlet pipe. Mostly the operation time has been set by the pre-set time, so there is limited experience about the sludge accumulating capacity. The filters are not completely cleaned after a backwash. How the remaining sludge affects the running time of the filters has been very difficult to investigate. The flow through the filters has a large impact on the opening angle of the valve. The determining factor of the running time of the filters is therefore a combination of the head-loss in the filters and the flow through the filters. The nitrification was lost in April because the filters needed to be backwashed more often than every 10th hour. There is not enough water to backwash two filters at a time for the moment but this could be possible by reconstruction of the effluent channels. Problems with foaming in the digesters caused by filamentous bacteria is well known (Pujol et ai., 1991; Jenkins et ai., 1993). This problem has not been reported in Sweden before and there is not yet any known solution to it. There are six digesters at Bromma STP and the excess sludge and the primary sludge are today mixed before digestion. By separating the two different sludges, the foaming problem can be limited to a
Possibilities for increased nutrient removal
153
smaller number of digesters. The digesters for the excess sludge can then be rebuilt to prevent stable foam from being formed. The sludge age in the biological stage during winter 1994 required for nitrification was lower than the sludge age recommended in the literature. Sludge age is a very rough measurement for deciding the demanded concentration of mixed liquor suspended solids (MLSS). This could cause higher sludge ages than actually needed. Since the sludge age affects the settling properties of the sludge it would be desirable to adjust the sludge age after the amount of nitrifying bacteria instead. There are also olher ways to further increase the nitrogen removal. Today there are no pumps for recirculating MLSS with high concentrations of nitrate to the anoxic zones to improve denitrification but they will soon be installed. During periods with low flow the filters have capacity to also denitrify a part of the remaining nitrate if there is enough carbon present. Denitrification in filters has in previous studies (Jepsen and Jansen. 1993) proved to be a promising way to polish nitrogen. One drawback to by-passing a part of the flow at high hydraulic loadings is that not all sewage is treated biologically. The goal is of course to always limit the part of the flow that has to be by-passed. but the bUlking conditions often appear very fast and in order to save the filters the by-pass flow is today set with a certain safety margin. By measuring the level of the sludge blanket in the secondary sedimentation tanks and regulating the by-passing flow after this level, the flow through the biological stage can be optimized. More work will also be carried out to increase the understanding of the settling properties of the activated slUdge. By understanding why the ability to settle changes. the biological stage may hopefully be operated to favour good settling properties for the activated sludge in the future.
REFERENCES Andersson. C.• Tendaj. M. and Rothman. M. (1992). Filtration at Bromma Sewage Treatment Plant. Waf. Sci. rl!ch.• 15(415), 59-66. Carlsson. H.• Lee. N. and Welandcr. T. (1994). A case study on the sludge characteristics in a biological nutrient removal pilot plant at the Sjolunda Wastewater Treatment Plant in Malmo. Valll!n. 50. 39-46. Henze, M.. Harremoes. P.• Jansen. 1. la Cour and Arvin. E. (1990). Spildl!vandsrl!nsning Biologisk og kl!misk. Polyteknisk Forlag, Lyngby. Denmark. Hultgren. 1. and Reinius. L. G. (1993). Extension of the sewage treatment plants in Stockholm: Some technical and economical aspects. Waf. Sci. rl!ch.• 27(5/6). 357-367. Jenkins. D.• Richard. M. G. and Daigger. G. T. (1993). Manual on /hl! Causl!s and Con/rol of Activa/I!d Siudgl! Bulking and Foaming. 2nd ed. Lewis Publishers. Michigan. Jepsen. S. E. and Jansen. J. la Cour. (1993). Biological filters for post-denitrification. Wat Sci rl!ch.• 27(5/6). 369-379. Pujol. R.• Duchene, P. H.• Schetrite. S. and Canler. J. P. (1991). Biological foams in activated sludge plants: Characterization and situation. Waf. Rl!s., 25(11).1399-1404.