- Email: [email protected]
Experiences of nitrogen and phosphorus removal in deep-bed filters at Henriksdal sewage works in Stockholm
~ Pergamon
Waf. Sci. T«1L Vol.37, No.9, pp. 193-200, 1998. C 1998\A WQ.Published by ElsevierScienc:e LId
po: 50273-1223(98)00288-1
Printed InGreatBritain. 0273-1223J98 $19'00 + 0-00
EXPERIENCES OF NITROGEN AND PHOSPHORUS REMOVAL IN DEEP-BED FILTERS AT HENRIKSDAL SEWAGE WORKS IN STOCKHOLM L. Jonsson Stockholm Water Co; S-l06 36 Stockholm, Sweden
ABSTRACT Deep-bed down-flow two-media filters were used in pilot plant studies with filtration of secondary settled wastewater. FeSO~ or FeClJ was applied as a precipitation agent, and NaAc·3H~ was chosenas a C8Ib0n sourcewhendenitrification was desired. Theconcentration of PO..P in the filtratefromthe pilotplant study neverexceeded 0.0' mg PO~·PII wheniron salts weredosed. The curves showing the concentration of p.tot and PO~·p in the filtrate as a function of the quotient between the dosage of iron and the concentration of PO~·P in the influent to the filter followed approximalely an exponential relationship. The total nitrogen reduction overthe filter bed increased from an average of 2.3 mg (NOJ+N~)-N11 at the beginning of each experiment to an average of 4.3 mg (NOJ+N~NII towards the end of the test. When only secondary settledwastewater, suspended solids, primary settled wastewater, iron salts,or sodium acetatewas added,at a hydraulic loadof 10 mIh, the timebefore clogging became 100h, 10 h, 10- " h, 20 • 40 h, and 20 - 40 h, respectively. Almost the entire pressure drop was located on the surface of the filter bed and 0.2' metre downin the expanded claylayer. ~ 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Acetate; deep-bed filters; denitrification; filtration; phosphorus precipitation; wastewater INTRODUCTIQN As a result of expected increased demands for nitrogen and phosphorus removal in wastewater treatment plants in the Stockholm area the Henriksdal treatment plant expanded the aeration tank volume from about 6S000 m3 to approximately 210000 m3 byincreasing the depth from 4.9 m to 12 m. In order to accomplish fun nitrification and sufficient denitrification the suspended solids concentration in the aeration tanks was also increased. This increase will result ina higher surface load into the secondary sedimentation tanks. A filtration stepas a final process stepinthe plant wastherefore at thattime planned. This filter would reduce the particle bound phosphorus inthe eftluent.
A study of a pilot plant filter was done before the full-scale filter was built. This article presents the results from the pilot plant investigation and discusses the performance of the phosphate precipitation with ferrous sulphate in the full-scale filter. Down-flow two-media filters have recently been installed at the two largest treatment plants in Stockholm (Henriksdal and Bromma wastewater treatment plants). At Henriksdal 60 and at Bromma 24 full-scale tilters have been installed with a length of 10m and a width of 6 m. The sand layer height is O.S m (grain size at Henrilcsdal1.2 - 1.9mm andat Bromma 1.2 - 1.6mm) and the height of the ceramic layer {grain size 2.5 - 3.S
194
L.JONSSON
nun)is 1.0 m. Filters of a similar typeare planned at another large treatment plant (Kappala) in the Stockholm region. The Heoriksdal treatment plant is now a conventional activated sludge wastewater treatment plant with screens, grit chambers, pre-precipitation, pre-aeration, primary sedimentation tanks, aeration tanks with nitrification and denitrification, secondary sedimentation tanks, new-built filters with a surface area of 3 600 m2, sludge digestion, anddewatering of digested sludge in centrifuges. The demands for the mixed effluent from the three wastewater treatment plants in Stockholm during the period 117 1994 - 30/6 1997 are s 10 mg 800 711 and s 0.4 mg P-totll as quarterly average values, and s 10 mg NJl.-N/1 as an average valuefrom July to October. The demands from 1/7 1997 are ~ 10 mg 800711 and ~ 0.3 mgP-totll as quarterly average values, S 3 mg NJl.-NII as an average value from Julyto October, and s 1S mgN-totllas an yearly average value. In thisinvestigation simultaneous particle separation, denitrification, and phosphorus precipitation weretested in a deep-bed down-flow two-media filter. The extent of denitrification in the filter investigated has been
presented earlier by Jonsson et al. (1996) and the lack of difference between ferrous and ferric iron as precipitation agents has, among otherthings, beendiscussed earlier byJonsson (1997). MATERIALS AND METHODS Pilot plant faJter The studywas performed in a stainless steel tube with a diameter of 0 4 m, a height of J.0 m, and a filter bed height of 1.5 m which correspond in height to the full-scale filter which, at that point, was planned. Eight different filters were investigated whereby filters 1 • 5 did not have a full-scale bed height. Filters 6, 7, and 8 are presented in table 1. Expanded clay is a ceramic material marketed as "Filtra purl" by the German manufacturer. It looks like sharp-cornered small pebbles. The full-scale filter at the Henriksdal wastewater treatment plant is also presented intable 1.Thepilot plant filter is presented in figure 1. Inlet
Overflow
Height: 3.0 m
..
Diameter: 0.4m Filterbed: 1.5m • (Sand: 0.3m Expanded clay. 1.2 m)
Washwater. .
Bed with expanded clay
Sampling and
pressure measurement
Sand bed Outlet
Nozzles
.. ,n-_nr+ Washwater
..
sampling
t
Air
Figure 1. Pilot plant experiments with the down-flow filter at the Henriksdal treatment plant.
Nitrogenand phosphorus removalin deep-bedfilters
195
Table 1. Grainsizeand layerheight of sandandexpanded clayin the pilot plantand full-scale filter. Filter Filter6 Filter 7 Filter8 Henriksdal Bromma
Sand size (mm) 1.2- 1.5 0.8 - 1.2 0.6 - 0.8 1.2- 1.9 1.2- 1.6
height (m) 0.5 0.3 0.3 0.5
0.5
Expanded clay size(mm) 2.5 - 3.5 2.5 - 3.5 2.5 - 3.5 2.5 - 3.5 2.5·3 .5
height(m) 1.0 1.2 1.2 1.0 1.0
Sampling and analytical methods
Suspended solids were analysed by a gravimetric method according to the Swedish standard method SS028112-3 but with Whatman GFIC filters (pore size 1.2 urn). When soluble fractions were analysed the samples were first filtered with Munktell MGC filters (pore size 1.2 um). The concentrations of nitrate, nitrite, phosphate, total phosphorus, ammonium, COD (unfiltered and filtered), and iron (unfiltered and filtered) in the filtrate and in the influent were analysed witha Doctor Lange Digital Spectrophotometer LP 2 W. Composite samples from the filtrate and the influent were taken in all experiments and grab samples were taken at selected times along the filter bed in selected experiments. All parameters were not analysed at the sametime, however. Different substances were more interesting to analyse in certain experiments. The grab samples were taken at -0.15 m, 0.25 m, I.l 5 In, and 1.59 m except in experiment 62 after a major loss of expanded clay in which the 0.25 m sample was taken at 0.70 m instead. The zero level was located in the surface of the filter bed. The filtrate samples werecollected 0.09 m belowthe sand layer. The time to reach an abruptly high pressure drop in the filter was measured by a conductive level indicator. The signal from this indicator was recorded. Pressure levels along the filter were measured manually at six different locations in the filter bed and at one location below the water surface during the whole experiment. Oxygen was occasionally measured in the influent and in the filtrate. Operational conditions
Filtration of secondary settled wastewater was investigated during a period from February 1991 to April 1993. Testingwas mainly conducted witha hydraulic load of 10 m1h, but in eight tests the hydraulic load was 5 mIh and in one test it was 3.3 m1h. FerroussUlfhate or ferric chloride was used to precipitate phosphorus. The ferrous sulphate dosagewas 2.8 - 2.9 g Fetm (set point 2.84 g Fetm\ and the ferric chloride dosage 1.9 - 2.0 g Fetm) (set point 1.89 g Fe/rrr' except in experiment 19 and 51 where it was 1.8 and 1.0 g Fetm), respectively. In experiment 18 the average ferric chloride dosage became 0.7 g Fe/m' for filters 6 and 7 and 0.1 g Fetm3 for filter 8, and the average ferrous sulphate dosage in experiment 21 became 1.1 g Fet~3 for filter 6 both as a result of problems with the pump that dosed the iron solution. To simulate conditions of higher values of effluent phosphorus concentrations in the secondary settled wastewater NaJIP04 was added to the filter inlet. In order to achieve denitrification in the tilter sodium acetate was dosed as a carbon source 3 in filter 7 for selected tests. The dosage was 71 - 72 g NaAc·3H20/m (set point 71.6 g/m') except in experiments 34 - 40 and 42 where it was 75 ~m3 (exp. 34), 77 glm: (exp. 35), 81 g/m' (exp. 36), 143 glm) (exp. 37 and 38), 106glm3 (exp. 39), 108 g/m' (exp. 40), and 70 glm (exp. 42). To simulate conditionswitha high hydraulic load to the plant 19 % of primary settledand 81 % of secondary settled wastewaterwere mixed in the study. Sludge flight was simulated by pumping a small flow of activated sludge from the corresponding aeration tank and mixing it with the secondary settled wastewater. This resulted in about 60 - 70 g SS/m 3 in the influent to the filter. The dosage of ferrous sulphate to the filters in the Henriksdal wastewater treatment plant started November 1, 1995 and has varied between 1 and 4 g Fe/m' during the time from the start to January 1997. No carbon source has been added to the filters yet. The denitrification in the anoxic zones of the aeration tanks was estimated to be sufficient.
L.lONSSON
196
RESULTS AND DISCUSSION Phosphorus removal
Figure 2 shows the concentration of phosphate phosphorus in the filtrate andfigure 3 shows the concentration of total phosphorus in the filtrate both as a function of the dosage of ferrous sulphate or ferric chloride divided by the concentration of phosphate phosphorus in the influent to the filter. In figure 2A and 3A the pilot plant studyis presented and the values from the Henriksdal plant can be seenin figure 2B and 3B. 8
A
, ......"
Composite samples. mll P04-PII 0,05 , . . . - - - - - - -....- - - - - - ,
t
o
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•
•
••
i
•
"•
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•
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o
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•
.a. ••
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o
3
8
9
12 15 18 21 24 27 30 mol Fe/mol P04-P
"!II '"
:
mol Fe/mol PO.-p
,• Fe03 • FeS04!
Figure 2. Concentration of phosphate phosphorus in the filtrate as a function of the quotient between the dosageof ironandthe concentration of phosphate phosphorus in the influent to the filter: A. - in the pilotplant study. B. - at the Henriksdal wastewater treatment plant. The pilotplant investigation corresponded well to the pattern from the Henriksdal plant. The concentration of phosphate in the filtrate from the pilot plant study never exceeded 0.05 mg PO.-PII when iron salts were dosed. This result is in closeagreement withthe lower part of the corresponding diagram from the full-scale plant. In figure 3A and 38 the high concentrations of totalphosphorus in the filtrate mostprobably came from tests or conditions in which primary settled wastewater bypassed the biological treatment. Primary settled wastewater consists partly of liquid fat which was not removed by the grains of the filter. Fat consists mostly of organic compounds probably containing organically bound phosphorus which cannot be precipitated by ferrous or ferric precipitation agents. High quotients between the dosage of iron and the concentration of phosphate in the filtrate were a result of very low concentrations of phosphate phosphorus - not of high dosages of iron. In figures 2B, 3A, and 38 the points form exponential curves as expected. The thought lines of these curves are to some extent diffuse depending upon different disturbances. The wastewater in the influent to the filter haddifferent concentrations of substances and different ionic strength over time. Both the concentrations on the x-axis and the y-axis came from analyses on twenty-four-hour composite samples. One greater shortage was that the primary settled wastewater in the Henriksdal plant was added after the point where the samples of secondary settled wastewater weretaken.
Nitrogenand phosphorus removal in deep-bedfilters
A
mg
B
...
Composite samples.
,-tot"
197
~.
1,l10 , . - - - - - - - - - - - - . . . ,
...
0,90
9
t
r r
0,80
0,70 0,60
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0,50 0,40
0,10 0,00 0
3
6
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9
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=
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•
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1.- .-
-:r
C-".-......•
•
•I
12 15 18 21 24 27 30 molFelmol~
Ie Fe03 • FeS04!
mol Fe/mol P04""P
Figure 3. Concentration of total phosphorus in the filtrate as a function of the quotient between the dosageof iron and the concentration of phosphate phosphorus in the influent to the filter: A. - in the pilot plantstudy. B.• at the Henriksdal wastewater treatment plant. Denitrification Grabsamples weretaken alongthe filter bed in the pilot plant study. Figure 4 showsthe reduction ofthe sum of nitrateand nitritenitrogen at different levels in the filter based on the grab samples. In the upper section of the filter, -0.15 - 0.25 m, 0.15 m of water containing more or less deposited suspended solids was situated over a 0.25 m filter bed of expanded clay. The middle section, 0.25 - 1.15 m, contained entirely expanded clay. The lower section, 1.15- LSO m, contained 0.05 m expanded clay over 0.30 m sand. In figure 4A, the samples were taken at the beginning of the experiment (0.1 ·39 hours operational time after backwashing of the preceding experiment) and in figure 4B the samples were taken some time before the end of the experiment or after about 22 hours (4.7 - 28.0 hours operational time after backwashing of the preceding experiment). As can be seenin figure 4, the highest nitrogen reduction occurred in the middle section. During the operation there was a change from a low removal at the uppersection to a highremoval efficiency at the upper sectiondue to deposition of suspended solids. At the same timethe lower section of the filter became lessimportant. The minorrole of the lowersection at the end of the experiments was probably due to lackof biodegradable organic material necessary for the denitrification process. Therewas a lack of carbonsourcein the lowersection in certain experiments, and a lack of nitrateand nitrite nitrogen in a few experiments. Despite this, therewassomenitrogen reduction in the lowersection in someof these mentioned experiments but not in others. In certain experiments nearly all expanded claywas lost in the upper section of the filter bed while the nitrogen removal there was low in these experiments. It was, however, also low in someother experiments. According to figures 4A and4B the nitrogen reduction was not significantly higherin experiments 53 and S5 as a result of a longer retention time due to a lower hydraulic load, S mIh, in experiments 53, S5, and 64. The concentrations of soluble CODat -0.15 m was comparatively high in six experiments while the dosage of acetate was higher in only two of these experiments. In the
L.lONSSON
198
beginning of the test, four of these six experiments and one other experiment have a highernitrogen removal B
A
Grab samples 31/81992 - 3121993.
Grab samples 31/81992 - 3121993.
iS
5...---------,
~
4
~
3
r
~ 5 ,..-~-----,
o
ir : 2
2
o ~
~
:il :g
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~ ~ ~
:il
experiment No. _ _ -o.15·0.25m _ _ 0.25·1.15m _ _ 1.15·1.50m
:a 2
&l
experiment No. r":::;:--o15. 0.25 m _ _ 0.25·1.15m ~1.15.1.50m
Figure4. Removed nitrogen at different depthsof the filter belowthe expanded claysurface: A - 0.1 - 3.9 hours after backwashing. B. - 4.7 - 28.0 hoursafter backwashing. efficiency, but the expected increase as a result of a higher substrate concentration was not clearly seen in the other two of these six experiments. Towards the end of the test. three of five experiments and two other experiments have a higher nitrogen reduction, but this was not the case in the other two of these five experiments. The concentrations of soluble COD at -0.15 m was instead comparatively low in certain experiments. This might be due to evaporation of acetate as acetic acid before addition to the influent of the filter. The nitrogen reduction was extremely low in someof theseexperiments but not in others. It can be concluded that the onlysignificant parameter was the difference between the nitrogen removal at the beginning and towards the end of the tests. From figure 4 it was calculated that the nitrogen reduction was 2.26 mg (N03+N02)-NII as an average in the biofilm after backwashing and 4.32 mg (N03+N02)-NII as an average towards the end of the tests. Thisgave an additional nitrogen removal of2.06 mg (N03+N02)-NII as an average during the tests due to both deposition of suspended solids on the surface of the filter and increasing biofilm growth after the backwashing on the expanded clayand sandgrains. Time before clogging
The concentrations of the analysed substances in the filtrate did not differbetween filter 6, 7, and 8, but the time before clogging differed significantly. The concentration of suspended solids in the filtrate was often 2 3 mg SSII when no additions or dosages were made to the influent of the filter. The clogging time was approximately 67 % to 75 % shorter in filter 8 and 5 % shorter in filter 6 compared to filter 7. This can be seen in figure S. The result was probably due to a higher expanded clay layer in tilter 7, The deposition of suspended solidsoccurred almost completely on the surface of the filter giving filter 7 a longerclogging time despite of a smaller grain size of the sand compared to filter 6. Filter 7 was therefore chosen for the denitrification tests. The great difference in clogging timefor different experiments according to figure 5 was caused by several different operational parameters. In experiments 23, 33, 44, 49, 52, 34 (filter 6), 36 (filter 6), and 64 (filter6) nothing was dosedor added. Thisgavea clogging timeof around 100 hours at a hydraulic load of 10 mIh, and 175 hours at 5 m/h. As mentioned beforethere was some problems with pumping of the iron solution in experiments 18 and 21 (filter6). This gave clogging times similar to those for tests without addition of iron. The time beforeclogging was highly influenced by the addition of suspended solids. Thiscan bee seen in experiments 25 - 27, 31, 56 - 60, and 63 in figure 5. Experiments 25 and 63 have no other
Nitrogen and phosphorus removal in deep-bed filters
t99
additions than suspended solids. At a hydraulic load of 10 II1I1l the time before clogging became 10 hours and 17 hours at 5 m/h. In other words the clogging time was approximately proportional to both the hydraulic load and the concentrationof suspended solidsin the influent to the filter. A
B
Composite samples 81111991 - 217
Composite samples 7/7 1992 - 2913
1992.
1993.
Hour.
Hour. 180 , . - - - - - - - - - - - . , . 160 140 ~
120 100 80 60 40 20
180 . - - - - - _ - - - . . . . . - , 160 140 120 100 80 60 40 20
o L:-~_~~~~~ ~
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~
~
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+ + ! I ! ~ r + + _ l
~
~
~
S
~
~
~
S
experiment No.
l:-+----::Fi-....-:-7-----Fl-...-S-Q
Figure 5. The timebeforeclogging at different experiments: B. - when mainly acetate was added. settled wastewaterwere added.
A. - when mainly Fe, suspended solids, and primary
In experiments 28 - 32, 57, 58, and 60 - 62 primary settled wastewaterwas added to the tilter inlet. In figure 5 it is possible to find a clogging time of 10 - 15 hours when sludge flight was not simulated. In experiments 18 - 32, 50, 51, 54, 55, 58, 59, 62, 65 and 66 (except 23, 25, 28, and 30) ferrous sulphate or ferric chloride was dosed to the tilter. This gave a time before clogging of 20 - 40 hours when no suspended solids or primary settled wastewater was added. In experiment 34 - 64, except 44, 49, 52, 54, 57, and 63, sodium acetate was dosed to filter 7. This gave, according to figure 5B, a time before cloggingof20 - 40 hours when only sodium acetate was dosed. It was noticed that certain additions or dosages occasional1y had a lasting effect on the clogging time for a few experiments after the experiment with a particular addition or dosage until the filter was thoroughly backwashed. Finally, in experiments 44 and 49, with no addition or dosage but with only one backwashing, the clogging time became 60 - 75 hours as compared to 100 hours in a fully backwashed filter (~3 times).
Pressure drop Figure 6 shows the pressure levelin the filter bed of filter 7 in experiment 22. Ferrous sulphate and Na2HPO. were dosed at the filter inlet in this test. Seven pressure levels (see figure I) were measured at -0.15 m, 0.25 m, 0.70 m, 0.85 m, 1.00 m, 1.15 m, and 1.40 m down in the filter bed. As can be seen from tigure 6 the pressure drop was located on the filter surfaceand 0.25 m down in the expanded clay layer. At the beginning of the study in February 1991 the pressure drop was uniform in the filter bed, but after only one month the pattern in tigure 6 was established and this lasted to the end of the study in April 1993. With no additions or dosages the time before clogging became longer but the curve form was similar. During denitrification the filter bed became clogged with nitrogen gas bubbles. In the beginning the curve form fol1owed the curve with only secondary settled wastewater but after about 20 - 40 hours the denitrifying filter clogged rapidly. It should be noted that no higher pressure drop over the sand layer surface between 1.15 m and 1.40 m was detected. See figure 6. The curves of the pressure levels in filters 6 and 8 were similar in shape to the curves for tilter 7.
200
L.IONSSON
Pressure levels In experiment 22. Melr.. of H2O
3.000 . , . - - - - - - - - - - - - - - . , . . . - - - - - ,
r~.O~15m _ _ +O.25m 2.000
-.-+O.70m 1500
"""*- +0.85 m
1000
-tl-+1.00m __ +1.15m
0.500
-+-+1.4Om
0.000 . I - - - -__- - _ - - - _ - - - - - ~ 40 o 10 30 50 20 Time In houri
Figure 6. Pressurelevel profiles in the filterbed of filter7. CONCLUSIONS • The concentration of phosphate phosphorus in the filtrate from the pilot plant study never exceeded 0.05 mg PO.-PII when iron salts were dosed. • Primary settled wastewaterbypassing the biological treatment gave high concentrations of total phosphorus in the filtrate as a result of organicbounded phosphorus in liquid fat. • Reduction of nitrate and nitritenitrogen at different levels in the filter could not be assigned to a particular problem in the operation of the pilot plant filter. There was a change from a low nitrogen removal efficiency at the upper sectionto a highremoval efficiency at the upper section due to deposition of suspended solidson the filter surface. At the same time, the lower sectionof the filter became less important over time probably due to a lack of biodegradable organic material necessary for the denitrification process. The total nitrogen reduction over the filter bed increased from an average of 2.3 mg (NOJ+NOz)-NII at the beginning of each experiment to an average of 4.3 mg (NOJ+N02)-NII towards the end of the test. • The time before clogging was proportional to both the hydraulic load and the concentration of suspended solidsin the influent to the filter. 2 - 8 mg S5IIin the filter inletgave a clogging time of 100 hours at 10 m/h and 175 hours at 5 m!h, while 60 - 70 mg S5II gave 10 hours and about 17 hours, respectively. When only primary settled wastewater, iron salts, or sodium acetate was added the time before clogging became 10 - 15 hours, 20 - 40 hours, and 20 - 40 hours, respectively. • Almost the entire pressure drop was located on the surface of the filter bed and 0.25 m down in the expanded clay layer. No pressuredrop was detectedon the surface of the sand layer. ACKNOWLEDGEMENTS This work was supported by Stockholm Water Co. and NUTEK (within the STAMP project). Valuable support and help in preparation of the manuscript were provided by Bengt Hultman and Erik Levlin. The author is gratefulto Eva Hagland and Johan StAhl for helpwith parts of the analytical works. REFERENCES Jonsson, L. (1997). Phosphorus removal and denitrification in deep-bed two-media filters. Vat/en,
~(1), IS-
20.
Jonsson, L., Plaza, E. and Hultman, B. (1997). Experiences of nitrogen and phosphorus removal in deep-bed filters in the Stockholm area. Wat. Sci. Tech., 36(1), 183-190.
Waf. Sci. T«1L Vol.37, No.9, pp. 193-200, 1998. C 1998\A WQ.Published by ElsevierScienc:e LId
po: 50273-1223(98)00288-1
Printed InGreatBritain. 0273-1223J98 $19'00 + 0-00
EXPERIENCES OF NITROGEN AND PHOSPHORUS REMOVAL IN DEEP-BED FILTERS AT HENRIKSDAL SEWAGE WORKS IN STOCKHOLM L. Jonsson Stockholm Water Co; S-l06 36 Stockholm, Sweden
ABSTRACT Deep-bed down-flow two-media filters were used in pilot plant studies with filtration of secondary settled wastewater. FeSO~ or FeClJ was applied as a precipitation agent, and NaAc·3H~ was chosenas a C8Ib0n sourcewhendenitrification was desired. Theconcentration of PO..P in the filtratefromthe pilotplant study neverexceeded 0.0' mg PO~·PII wheniron salts weredosed. The curves showing the concentration of p.tot and PO~·p in the filtrate as a function of the quotient between the dosage of iron and the concentration of PO~·P in the influent to the filter followed approximalely an exponential relationship. The total nitrogen reduction overthe filter bed increased from an average of 2.3 mg (NOJ+N~)-N11 at the beginning of each experiment to an average of 4.3 mg (NOJ+N~NII towards the end of the test. When only secondary settledwastewater, suspended solids, primary settled wastewater, iron salts,or sodium acetatewas added,at a hydraulic loadof 10 mIh, the timebefore clogging became 100h, 10 h, 10- " h, 20 • 40 h, and 20 - 40 h, respectively. Almost the entire pressure drop was located on the surface of the filter bed and 0.2' metre downin the expanded claylayer. ~ 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Acetate; deep-bed filters; denitrification; filtration; phosphorus precipitation; wastewater INTRODUCTIQN As a result of expected increased demands for nitrogen and phosphorus removal in wastewater treatment plants in the Stockholm area the Henriksdal treatment plant expanded the aeration tank volume from about 6S000 m3 to approximately 210000 m3 byincreasing the depth from 4.9 m to 12 m. In order to accomplish fun nitrification and sufficient denitrification the suspended solids concentration in the aeration tanks was also increased. This increase will result ina higher surface load into the secondary sedimentation tanks. A filtration stepas a final process stepinthe plant wastherefore at thattime planned. This filter would reduce the particle bound phosphorus inthe eftluent.
A study of a pilot plant filter was done before the full-scale filter was built. This article presents the results from the pilot plant investigation and discusses the performance of the phosphate precipitation with ferrous sulphate in the full-scale filter. Down-flow two-media filters have recently been installed at the two largest treatment plants in Stockholm (Henriksdal and Bromma wastewater treatment plants). At Henriksdal 60 and at Bromma 24 full-scale tilters have been installed with a length of 10m and a width of 6 m. The sand layer height is O.S m (grain size at Henrilcsdal1.2 - 1.9mm andat Bromma 1.2 - 1.6mm) and the height of the ceramic layer {grain size 2.5 - 3.S
194
L.JONSSON
nun)is 1.0 m. Filters of a similar typeare planned at another large treatment plant (Kappala) in the Stockholm region. The Heoriksdal treatment plant is now a conventional activated sludge wastewater treatment plant with screens, grit chambers, pre-precipitation, pre-aeration, primary sedimentation tanks, aeration tanks with nitrification and denitrification, secondary sedimentation tanks, new-built filters with a surface area of 3 600 m2, sludge digestion, anddewatering of digested sludge in centrifuges. The demands for the mixed effluent from the three wastewater treatment plants in Stockholm during the period 117 1994 - 30/6 1997 are s 10 mg 800 711 and s 0.4 mg P-totll as quarterly average values, and s 10 mg NJl.-N/1 as an average valuefrom July to October. The demands from 1/7 1997 are ~ 10 mg 800711 and ~ 0.3 mgP-totll as quarterly average values, S 3 mg NJl.-NII as an average value from Julyto October, and s 1S mgN-totllas an yearly average value. In thisinvestigation simultaneous particle separation, denitrification, and phosphorus precipitation weretested in a deep-bed down-flow two-media filter. The extent of denitrification in the filter investigated has been
presented earlier by Jonsson et al. (1996) and the lack of difference between ferrous and ferric iron as precipitation agents has, among otherthings, beendiscussed earlier byJonsson (1997). MATERIALS AND METHODS Pilot plant faJter The studywas performed in a stainless steel tube with a diameter of 0 4 m, a height of J.0 m, and a filter bed height of 1.5 m which correspond in height to the full-scale filter which, at that point, was planned. Eight different filters were investigated whereby filters 1 • 5 did not have a full-scale bed height. Filters 6, 7, and 8 are presented in table 1. Expanded clay is a ceramic material marketed as "Filtra purl" by the German manufacturer. It looks like sharp-cornered small pebbles. The full-scale filter at the Henriksdal wastewater treatment plant is also presented intable 1.Thepilot plant filter is presented in figure 1. Inlet
Overflow
Height: 3.0 m
..
Diameter: 0.4m Filterbed: 1.5m • (Sand: 0.3m Expanded clay. 1.2 m)
Washwater. .
Bed with expanded clay
Sampling and
pressure measurement
Sand bed Outlet
Nozzles
.. ,n-_nr+ Washwater
..
sampling
t
Air
Figure 1. Pilot plant experiments with the down-flow filter at the Henriksdal treatment plant.
Nitrogenand phosphorus removalin deep-bedfilters
195
Table 1. Grainsizeand layerheight of sandandexpanded clayin the pilot plantand full-scale filter. Filter Filter6 Filter 7 Filter8 Henriksdal Bromma
Sand size (mm) 1.2- 1.5 0.8 - 1.2 0.6 - 0.8 1.2- 1.9 1.2- 1.6
height (m) 0.5 0.3 0.3 0.5
0.5
Expanded clay size(mm) 2.5 - 3.5 2.5 - 3.5 2.5 - 3.5 2.5 - 3.5 2.5·3 .5
height(m) 1.0 1.2 1.2 1.0 1.0
Sampling and analytical methods
Suspended solids were analysed by a gravimetric method according to the Swedish standard method SS028112-3 but with Whatman GFIC filters (pore size 1.2 urn). When soluble fractions were analysed the samples were first filtered with Munktell MGC filters (pore size 1.2 um). The concentrations of nitrate, nitrite, phosphate, total phosphorus, ammonium, COD (unfiltered and filtered), and iron (unfiltered and filtered) in the filtrate and in the influent were analysed witha Doctor Lange Digital Spectrophotometer LP 2 W. Composite samples from the filtrate and the influent were taken in all experiments and grab samples were taken at selected times along the filter bed in selected experiments. All parameters were not analysed at the sametime, however. Different substances were more interesting to analyse in certain experiments. The grab samples were taken at -0.15 m, 0.25 m, I.l 5 In, and 1.59 m except in experiment 62 after a major loss of expanded clay in which the 0.25 m sample was taken at 0.70 m instead. The zero level was located in the surface of the filter bed. The filtrate samples werecollected 0.09 m belowthe sand layer. The time to reach an abruptly high pressure drop in the filter was measured by a conductive level indicator. The signal from this indicator was recorded. Pressure levels along the filter were measured manually at six different locations in the filter bed and at one location below the water surface during the whole experiment. Oxygen was occasionally measured in the influent and in the filtrate. Operational conditions
Filtration of secondary settled wastewater was investigated during a period from February 1991 to April 1993. Testingwas mainly conducted witha hydraulic load of 10 m1h, but in eight tests the hydraulic load was 5 mIh and in one test it was 3.3 m1h. FerroussUlfhate or ferric chloride was used to precipitate phosphorus. The ferrous sulphate dosagewas 2.8 - 2.9 g Fetm (set point 2.84 g Fetm\ and the ferric chloride dosage 1.9 - 2.0 g Fetm) (set point 1.89 g Fe/rrr' except in experiment 19 and 51 where it was 1.8 and 1.0 g Fetm), respectively. In experiment 18 the average ferric chloride dosage became 0.7 g Fe/m' for filters 6 and 7 and 0.1 g Fetm3 for filter 8, and the average ferrous sulphate dosage in experiment 21 became 1.1 g Fet~3 for filter 6 both as a result of problems with the pump that dosed the iron solution. To simulate conditions of higher values of effluent phosphorus concentrations in the secondary settled wastewater NaJIP04 was added to the filter inlet. In order to achieve denitrification in the tilter sodium acetate was dosed as a carbon source 3 in filter 7 for selected tests. The dosage was 71 - 72 g NaAc·3H20/m (set point 71.6 g/m') except in experiments 34 - 40 and 42 where it was 75 ~m3 (exp. 34), 77 glm: (exp. 35), 81 g/m' (exp. 36), 143 glm) (exp. 37 and 38), 106glm3 (exp. 39), 108 g/m' (exp. 40), and 70 glm (exp. 42). To simulate conditionswitha high hydraulic load to the plant 19 % of primary settledand 81 % of secondary settled wastewaterwere mixed in the study. Sludge flight was simulated by pumping a small flow of activated sludge from the corresponding aeration tank and mixing it with the secondary settled wastewater. This resulted in about 60 - 70 g SS/m 3 in the influent to the filter. The dosage of ferrous sulphate to the filters in the Henriksdal wastewater treatment plant started November 1, 1995 and has varied between 1 and 4 g Fe/m' during the time from the start to January 1997. No carbon source has been added to the filters yet. The denitrification in the anoxic zones of the aeration tanks was estimated to be sufficient.
L.lONSSON
196
RESULTS AND DISCUSSION Phosphorus removal
Figure 2 shows the concentration of phosphate phosphorus in the filtrate andfigure 3 shows the concentration of total phosphorus in the filtrate both as a function of the dosage of ferrous sulphate or ferric chloride divided by the concentration of phosphate phosphorus in the influent to the filter. In figure 2A and 3A the pilot plant studyis presented and the values from the Henriksdal plant can be seenin figure 2B and 3B. 8
A
, ......"
Composite samples. mll P04-PII 0,05 , . . . - - - - - - -....- - - - - - ,
t
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3
8
9
12 15 18 21 24 27 30 mol Fe/mol P04-P
"!II '"
:
mol Fe/mol PO.-p
,• Fe03 • FeS04!
Figure 2. Concentration of phosphate phosphorus in the filtrate as a function of the quotient between the dosageof ironandthe concentration of phosphate phosphorus in the influent to the filter: A. - in the pilotplant study. B. - at the Henriksdal wastewater treatment plant. The pilotplant investigation corresponded well to the pattern from the Henriksdal plant. The concentration of phosphate in the filtrate from the pilot plant study never exceeded 0.05 mg PO.-PII when iron salts were dosed. This result is in closeagreement withthe lower part of the corresponding diagram from the full-scale plant. In figure 3A and 38 the high concentrations of totalphosphorus in the filtrate mostprobably came from tests or conditions in which primary settled wastewater bypassed the biological treatment. Primary settled wastewater consists partly of liquid fat which was not removed by the grains of the filter. Fat consists mostly of organic compounds probably containing organically bound phosphorus which cannot be precipitated by ferrous or ferric precipitation agents. High quotients between the dosage of iron and the concentration of phosphate in the filtrate were a result of very low concentrations of phosphate phosphorus - not of high dosages of iron. In figures 2B, 3A, and 38 the points form exponential curves as expected. The thought lines of these curves are to some extent diffuse depending upon different disturbances. The wastewater in the influent to the filter haddifferent concentrations of substances and different ionic strength over time. Both the concentrations on the x-axis and the y-axis came from analyses on twenty-four-hour composite samples. One greater shortage was that the primary settled wastewater in the Henriksdal plant was added after the point where the samples of secondary settled wastewater weretaken.
Nitrogenand phosphorus removal in deep-bedfilters
A
mg
B
...
Composite samples.
,-tot"
197
~.
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9
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12 15 18 21 24 27 30 molFelmol~
Ie Fe03 • FeS04!
mol Fe/mol P04""P
Figure 3. Concentration of total phosphorus in the filtrate as a function of the quotient between the dosageof iron and the concentration of phosphate phosphorus in the influent to the filter: A. - in the pilot plantstudy. B.• at the Henriksdal wastewater treatment plant. Denitrification Grabsamples weretaken alongthe filter bed in the pilot plant study. Figure 4 showsthe reduction ofthe sum of nitrateand nitritenitrogen at different levels in the filter based on the grab samples. In the upper section of the filter, -0.15 - 0.25 m, 0.15 m of water containing more or less deposited suspended solids was situated over a 0.25 m filter bed of expanded clay. The middle section, 0.25 - 1.15 m, contained entirely expanded clay. The lower section, 1.15- LSO m, contained 0.05 m expanded clay over 0.30 m sand. In figure 4A, the samples were taken at the beginning of the experiment (0.1 ·39 hours operational time after backwashing of the preceding experiment) and in figure 4B the samples were taken some time before the end of the experiment or after about 22 hours (4.7 - 28.0 hours operational time after backwashing of the preceding experiment). As can be seenin figure 4, the highest nitrogen reduction occurred in the middle section. During the operation there was a change from a low removal at the uppersection to a highremoval efficiency at the upper sectiondue to deposition of suspended solids. At the same timethe lower section of the filter became lessimportant. The minorrole of the lowersection at the end of the experiments was probably due to lackof biodegradable organic material necessary for the denitrification process. Therewas a lack of carbonsourcein the lowersection in certain experiments, and a lack of nitrateand nitrite nitrogen in a few experiments. Despite this, therewassomenitrogen reduction in the lowersection in someof these mentioned experiments but not in others. In certain experiments nearly all expanded claywas lost in the upper section of the filter bed while the nitrogen removal there was low in these experiments. It was, however, also low in someother experiments. According to figures 4A and4B the nitrogen reduction was not significantly higherin experiments 53 and S5 as a result of a longer retention time due to a lower hydraulic load, S mIh, in experiments 53, S5, and 64. The concentrations of soluble CODat -0.15 m was comparatively high in six experiments while the dosage of acetate was higher in only two of these experiments. In the
L.lONSSON
198
beginning of the test, four of these six experiments and one other experiment have a highernitrogen removal B
A
Grab samples 31/81992 - 3121993.
Grab samples 31/81992 - 3121993.
iS
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experiment No. _ _ -o.15·0.25m _ _ 0.25·1.15m _ _ 1.15·1.50m
:a 2
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experiment No. r":::;:--o15. 0.25 m _ _ 0.25·1.15m ~1.15.1.50m
Figure4. Removed nitrogen at different depthsof the filter belowthe expanded claysurface: A - 0.1 - 3.9 hours after backwashing. B. - 4.7 - 28.0 hoursafter backwashing. efficiency, but the expected increase as a result of a higher substrate concentration was not clearly seen in the other two of these six experiments. Towards the end of the test. three of five experiments and two other experiments have a higher nitrogen reduction, but this was not the case in the other two of these five experiments. The concentrations of soluble COD at -0.15 m was instead comparatively low in certain experiments. This might be due to evaporation of acetate as acetic acid before addition to the influent of the filter. The nitrogen reduction was extremely low in someof theseexperiments but not in others. It can be concluded that the onlysignificant parameter was the difference between the nitrogen removal at the beginning and towards the end of the tests. From figure 4 it was calculated that the nitrogen reduction was 2.26 mg (N03+N02)-NII as an average in the biofilm after backwashing and 4.32 mg (N03+N02)-NII as an average towards the end of the tests. Thisgave an additional nitrogen removal of2.06 mg (N03+N02)-NII as an average during the tests due to both deposition of suspended solids on the surface of the filter and increasing biofilm growth after the backwashing on the expanded clayand sandgrains. Time before clogging
The concentrations of the analysed substances in the filtrate did not differbetween filter 6, 7, and 8, but the time before clogging differed significantly. The concentration of suspended solids in the filtrate was often 2 3 mg SSII when no additions or dosages were made to the influent of the filter. The clogging time was approximately 67 % to 75 % shorter in filter 8 and 5 % shorter in filter 6 compared to filter 7. This can be seen in figure S. The result was probably due to a higher expanded clay layer in tilter 7, The deposition of suspended solidsoccurred almost completely on the surface of the filter giving filter 7 a longerclogging time despite of a smaller grain size of the sand compared to filter 6. Filter 7 was therefore chosen for the denitrification tests. The great difference in clogging timefor different experiments according to figure 5 was caused by several different operational parameters. In experiments 23, 33, 44, 49, 52, 34 (filter 6), 36 (filter 6), and 64 (filter6) nothing was dosedor added. Thisgavea clogging timeof around 100 hours at a hydraulic load of 10 mIh, and 175 hours at 5 m/h. As mentioned beforethere was some problems with pumping of the iron solution in experiments 18 and 21 (filter6). This gave clogging times similar to those for tests without addition of iron. The time beforeclogging was highly influenced by the addition of suspended solids. Thiscan bee seen in experiments 25 - 27, 31, 56 - 60, and 63 in figure 5. Experiments 25 and 63 have no other
Nitrogen and phosphorus removal in deep-bed filters
t99
additions than suspended solids. At a hydraulic load of 10 II1I1l the time before clogging became 10 hours and 17 hours at 5 m/h. In other words the clogging time was approximately proportional to both the hydraulic load and the concentrationof suspended solidsin the influent to the filter. A
B
Composite samples 81111991 - 217
Composite samples 7/7 1992 - 2913
1992.
1993.
Hour.
Hour. 180 , . - - - - - - - - - - - . , . 160 140 ~
120 100 80 60 40 20
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experiment No.
l:-+----::Fi-....-:-7-----Fl-...-S-Q
Figure 5. The timebeforeclogging at different experiments: B. - when mainly acetate was added. settled wastewaterwere added.
A. - when mainly Fe, suspended solids, and primary
In experiments 28 - 32, 57, 58, and 60 - 62 primary settled wastewaterwas added to the tilter inlet. In figure 5 it is possible to find a clogging time of 10 - 15 hours when sludge flight was not simulated. In experiments 18 - 32, 50, 51, 54, 55, 58, 59, 62, 65 and 66 (except 23, 25, 28, and 30) ferrous sulphate or ferric chloride was dosed to the tilter. This gave a time before clogging of 20 - 40 hours when no suspended solids or primary settled wastewater was added. In experiment 34 - 64, except 44, 49, 52, 54, 57, and 63, sodium acetate was dosed to filter 7. This gave, according to figure 5B, a time before cloggingof20 - 40 hours when only sodium acetate was dosed. It was noticed that certain additions or dosages occasional1y had a lasting effect on the clogging time for a few experiments after the experiment with a particular addition or dosage until the filter was thoroughly backwashed. Finally, in experiments 44 and 49, with no addition or dosage but with only one backwashing, the clogging time became 60 - 75 hours as compared to 100 hours in a fully backwashed filter (~3 times).
Pressure drop Figure 6 shows the pressure levelin the filter bed of filter 7 in experiment 22. Ferrous sulphate and Na2HPO. were dosed at the filter inlet in this test. Seven pressure levels (see figure I) were measured at -0.15 m, 0.25 m, 0.70 m, 0.85 m, 1.00 m, 1.15 m, and 1.40 m down in the filter bed. As can be seen from tigure 6 the pressure drop was located on the filter surfaceand 0.25 m down in the expanded clay layer. At the beginning of the study in February 1991 the pressure drop was uniform in the filter bed, but after only one month the pattern in tigure 6 was established and this lasted to the end of the study in April 1993. With no additions or dosages the time before clogging became longer but the curve form was similar. During denitrification the filter bed became clogged with nitrogen gas bubbles. In the beginning the curve form fol1owed the curve with only secondary settled wastewater but after about 20 - 40 hours the denitrifying filter clogged rapidly. It should be noted that no higher pressure drop over the sand layer surface between 1.15 m and 1.40 m was detected. See figure 6. The curves of the pressure levels in filters 6 and 8 were similar in shape to the curves for tilter 7.
200
L.IONSSON
Pressure levels In experiment 22. Melr.. of H2O
3.000 . , . - - - - - - - - - - - - - - . , . . . - - - - - ,
r~.O~15m _ _ +O.25m 2.000
-.-+O.70m 1500
"""*- +0.85 m
1000
-tl-+1.00m __ +1.15m
0.500
-+-+1.4Om
0.000 . I - - - -__- - _ - - - _ - - - - - ~ 40 o 10 30 50 20 Time In houri
Figure 6. Pressurelevel profiles in the filterbed of filter7. CONCLUSIONS • The concentration of phosphate phosphorus in the filtrate from the pilot plant study never exceeded 0.05 mg PO.-PII when iron salts were dosed. • Primary settled wastewaterbypassing the biological treatment gave high concentrations of total phosphorus in the filtrate as a result of organicbounded phosphorus in liquid fat. • Reduction of nitrate and nitritenitrogen at different levels in the filter could not be assigned to a particular problem in the operation of the pilot plant filter. There was a change from a low nitrogen removal efficiency at the upper sectionto a highremoval efficiency at the upper section due to deposition of suspended solidson the filter surface. At the same time, the lower sectionof the filter became less important over time probably due to a lack of biodegradable organic material necessary for the denitrification process. The total nitrogen reduction over the filter bed increased from an average of 2.3 mg (NOJ+NOz)-NII at the beginning of each experiment to an average of 4.3 mg (NOJ+N02)-NII towards the end of the test. • The time before clogging was proportional to both the hydraulic load and the concentration of suspended solidsin the influent to the filter. 2 - 8 mg S5IIin the filter inletgave a clogging time of 100 hours at 10 m/h and 175 hours at 5 m!h, while 60 - 70 mg S5II gave 10 hours and about 17 hours, respectively. When only primary settled wastewater, iron salts, or sodium acetate was added the time before clogging became 10 - 15 hours, 20 - 40 hours, and 20 - 40 hours, respectively. • Almost the entire pressure drop was located on the surface of the filter bed and 0.25 m down in the expanded clay layer. No pressuredrop was detectedon the surface of the sand layer. ACKNOWLEDGEMENTS This work was supported by Stockholm Water Co. and NUTEK (within the STAMP project). Valuable support and help in preparation of the manuscript were provided by Bengt Hultman and Erik Levlin. The author is gratefulto Eva Hagland and Johan StAhl for helpwith parts of the analytical works. REFERENCES Jonsson, L. (1997). Phosphorus removal and denitrification in deep-bed two-media filters. Vat/en,
~(1), IS-
20.
Jonsson, L., Plaza, E. and Hultman, B. (1997). Experiences of nitrogen and phosphorus removal in deep-bed filters in the Stockholm area. Wat. Sci. Tech., 36(1), 183-190.