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Enhanced rapid gravity filtration and dissolved air flotation for pre-treatment of river thames reservoir water
~
Pergamon
War. Sci. Tee/<. Vol. 31, No.2, pp. 35-42, 1998. C 1998 IAWQ. Published by Elsevier Science Ud Printed in Great Britain.
PH: S0273-1223(98)OOOO7-9
0213-1223198 S19'00 + (H)()
ENHANCED RAPID GRAVITY FILTRATION AND DISSOLVED AIR FLOTATION FOR PRE-TREATMENT OF RIVER THAMES RESERVOIR WATER M. J. Bauer*. R. Bayley**. M. 1. Chipps*. A. Eades*. R. J. Scriven* and A. 1. Rachwal* * Thames Water Utilities Limited, Research and Development, Spencer House, Manor Farm Road, Reading, Berkshire RG2 OIN, UK ** Thames Water Utilities Limited, Operational Science, Walton Advanced Water Treatment Works, Hurst Road, Walton-an-Thames, Surrey KTJ2 2EG, UK
ABSTRACT 1bames Water treats approximately 2800MUd of water originating mainly from the lowland rivers Thames and Lee for supply to over 1.3million customers, principally in the cities of London and Oxford. This paper reviews aspects of Thames Water's research, design and operaling experiences of treating algal rich reservoir stored lowland water. Areas covered include experiences of optimising reservoir managemenl, uprating and upgrading of rapid gravity filtration (RGF), standard co-currenl dissolved air flotation (OAF) and counler· currenl dissolved air flotalion/fillralion (COCO-DAFF®) to counler operational problems caused by seasonal blooms of filter blocking algae such as Melosira spp., Aphanizomenon spp. and Anabaena spp. A major programme of uprating and modernisation (inclusion of Advanced Water Treatment: GAC and ozone) of the major works IS in progress which, together with the Thames Tunnel Ring Main, will meet London's water supply needs into the 21st Cenlury. © 1998 IAWQ. Published by Elsevier Science Ud
KEYWORDS Algae; anthracite; COCO-DAFF®; contact filtration; dissolved air flotation; dual media filtration; hydraulic uprating; rapid gravity filtration; reservoir management; slow sand filtration. INTRODUCTION Thames Water treats approximately 2800Mlld of water originating mainly from the lowland rivers Thames and Lee for supply to over 7.3 million customers, principally in the cities of London and Oxford, Traditionally, for London, water treatment has consisted of intake management (pumping water from the rivers Thames and Lee), storage for 30-80 days in deep reservoirs, two stage filtration through rapid gravity filters (RGFs) and slow sand filters (SSFs), then final disinfection with chlorine, with a chloramine residual maintained in distribution (Toms, 1987) The strategy for water treatment and supply in London included the modernisation, together with hydraulic uprating, of the major river-derived water treatment works to match the demands of the Thames Tunnel Ring Main supplying London with treated water (Glendinning and Mitchell, 1996), In addition a strategy to 35
M. J. BAUER et aI.
36
improve water quality for the purposes of complying with the UK Water Supply (Water Quality) Regulations (No 1147, 1989) principally for pesticides has resulted in the implementation of advanced water treatment (AWT) in the form of ozone and granular activated carbon (GAC) at all the major works. Fundamental to achieving the increased treatment plant capacities required is the overcoming of operational constraints caused by seasonal algal blooms which due to differing algal shapes, sizes and motility mean some taxa pass through the RGFs and result in blockage of the SSFs, while others can rapidly block the surface of the RGFs. In 1986, a full scale study at Ashford Common proved that SSF filtration rates could be increased from an average of O.15m1h to O.3m1h, thereby allowing the total 'peak' work's output to be increased from 480MVd to 690MVd. This paper reviews the range of measures which have been taken to achieve increased RGF productivity and improved filtrate quality from improved management of raw water reservoirs to the implementation of a novel patented form of dissolved air flotation termed counter current dissolved air flotation filtration (COCO-DAFF~).
RESERVOIR MIXING Thames Water operates 19 raw water storage reservoirs which provide the first stage of treatment Le. reducing turbidity and pathogenic bacteria and viruses by various processes such as settlement and biological interactions. A key aspect of achieving uprating of works is the continued success of effective reservoir management (Steel, 1975) where each works has access to more than one reservoir. In the prevailing climate of southern England, water bodies such as London's storage reservoirs, with a surface area >lkm2 and depths >10 metres (m), can be subject to thermal stratification during spring and summer. Management options on the deeper reservoirs (> 10m) consist of jet or air mixing systems to prevent thermal stratification. Such mixing systems also allow the control of algal populations via light energy limitation as the euphotic depth is relatively shallow compared to the total depth of the basins. Some basins, notably the Queen Mary, Knight and Bessborough reservoirs, are too shallow «10m) to adequately utilise mixing strategies and algae can proliferate to form a large biomass with subsequent treatment problems at the works. River Thames and Lee derived water is eutrophic and the concentrations of nitrogen and phosphorus are usually in excess of 5mgNn and Imgpn. At present there is no pragmatic, economic method of controlling algal 'blooms' through reducing the nutrient concentrations in these reservoirs. The turbidity of reservoir stored water is generally low in the range of 1-3 NTU, with a peak of 10 NTU during severe algal "blooms" and more generally during the winter caused by filling reservoirs with high turbidity (mineral) water. For operation of SSFs at filtration rates >O.3m1h, RGF filtered water should have an average turbidity of < 2 NTU and should not peak above 5 NTU. A more accurate measure of particle removal is obtained by the use of particle counters. Chlorophyll a is a more sensitive measure of algal biomass and varies by up to 2 orders of magnitude in the 5-24 metre deep reservoirs supplying London. In winter chlorophyll a is typically
Pre-treatment of River Thames reservoir water
37
Chlorophyll a ugll 200
150
r---------------------------,
---
---
----
----------------
---
-----
---
--
-----
100
c=:::J w,.,....,.
_ _ O.E.lI. Q.M...,.
Figure I. Algal biomass as chlorophyll-a for artificially mixed reservoirs (Wraysbury/Queen Elizabeth II) compared with the non-mixed Q. Mary reservoir (Renton et al.• 1995).
RAPID GRAVITY SAND FILTRATION REFURBISHMENT This section outlines the modifications permissible to existing RGFs where hydraulic constraints, notably total available head (in one case restricted to 1.3m), preclude the provision of either dual media (sand/anthracite) or deep bed (2m) coarse media and pre-treatment with ozone/ferric sulphate. The original media specifications for RGFs in London was (BSS mesh 6/22 0.71-2.8mm) laid in a shallow 0.4-0.7m bed of sand overlaying 0.5m of graded gravel(s). The average performance of these filters at 4-6m/h is a 50% (range 30-80%) removal of particles (inorganic and organic) in the size range 4-80 micron. Research at pilot and full scale has shown that reducing the gravel layer(s) and providing a deeper layer of 0.6-0.9m of finer sand (BSS mesh 14/25 0.6-1.18mm) can achieve the target filtration rate of 6-8m/h and particle removal of 50-60%. Effective size cannot be reduced significantly without compromising head loss. Theoretically experimental work has been largely confined to the backwashing of kaolin and alum floes off laboratory filters in sterile conditions (Regan and Amirtharajah, 1984). There is little information on effective backwashing, particularly involving the removal of biological material from un-chlorinated filters_ Antwerp Water Company has investigated different backwashing regimes for cleaning filters operated in the contact filtration mode (Janssens et ai., 1989). Research has proven that an effective backwash using combined air and water is fundamental in ensuring the media is sufficiently cleaned after each run which is in broad agreement with work done by Amirtharajah (Regan and Amirtharajah, 1984). The build up of silt, and organic matter (algal and detrital) in poorly washed filters ultimately leads to the phenomenon of "mud balling" (localised accumulation of mud) and "jetting" (build up of columns of support gravels through the media) which leads to localised fluidisation, sand loss. short circuiting and poor filtrate quality. These inadequately washed filters also develop very high starting bed headlosses often approaching 1m (normal starting bed headloss being a.25m) and consequently can gel locked into a circle of ever increasing cleaning frequency during periods of high algal loading. The method of determining sand cleanliness is by using the parameters of POC and dry weight of solids extracted "silt". Experience has shown that a maximum concentration of POC and "silt" should be 250g/m 2 (or 400g/m 3) and 1.5kg/m 2 (or 2.4kg/m 3) respectively to achieve good filtration performance and acceptable starting bed headloss. .
38
M. J. BAUER el al.
Due to the size of some Thames Water RGFs (150m 2 surface area) additional launders are often required to achieve the satisfactory removal of backwash water. In addition, to minimise media (sand) Joss, particularly during the combined air/water phase, new designs of launder have been developed and successfully implemented. In summary, for existing RGFs a modest increase in filtration rate to (6-8m1h) and consistent particle removal (50-60%) can be achieved by a combination of improved media specification, more effective backwashing (combined air/water) and modifications to the washout launder(s). DUAL MEDIA FILTRATION Two sites, Ashford Common (to replace overloaded microstrainers) and Walton (to replace ageing RGFs), offered the opportunity for developing higher rate (I0-20mlh) higher quality filtration. Pre-ozonation prior to RGF was investigated as it was required at some works to achieve reductions in pesticides, and has been reported by several authors, (Janssens et ai., 1985; Jekel, 1992; Chipps et ai., 1993b) in Europe and the United States, as assisting with the removal of particles, especially algae across filters or clarifiers. Pilot trials evaluating "contact" or "in-line" filtration (without a separate flocculation stage), using iron (III) sulphate rapidly mixed into the feedwater were carried out initially at Ashford Common using 0.6m diameter columns and later at the 5Mld Kempton Advanced Water Treatment Centre (AWTC) commissioned in 1991 using 204m diameter columns. Contact filtration was possible because of the relatively low loadings of turbidity and particles from the reservoirs and is thought to act by utilising a small dose of coagulant for charge neutralisation, allowing floes to form in the filter and/or reducing electrostatic repulsion forces between particles and filter media (grains). Results of the trials confirm that the effect of iron dosing, compared to no dosing, was to improve turbidity removal and produce some small improvement in the parameters which are indicative of algae, Le. chlorophyll a, POC, Total Particle Number (TPN) and Total Particle Volume (TPV) , Figure 2. FILTER PERFORMANCE WITH DIFFERENT PRE-TREATMENTS
Annual average removals Average Removal %
KEY:
100
80
dual media OZONE & IRON
60
dual media OZONE
40
dual media IRON
20
sand filter UNDOSED
0"--
----' POC TPV CHLOROPHYLL a
TPN
TURBIDITY
Water Quality Parameter
dual medIa
UNDOSED
• • •
Figure 2. Average removal data from a control sand filter without chemical dosing and a dual media filter with no chemical pre-treatment and with pre-treatment using pre-ozonation with iron (III) sulphate, on their own, and in combination.
Pre-treatment of River Thames reservoir water
39
The effect of ozone was to increase removals for these parameters compared with iron or no dosing, but turbidity removals were lower than those achieved with iron. The effect of combining iron and ozone resulted in superior removals to that of either chemical alone. Around 50% removals were achieved without chemicals increasing to approximately 90% with ozone and iron. Pre-ozonation was essential for good removals of Microcystis spp. , present in the reservoirs in summer and autumn, and small (5-15lJ.m) filter penetrating diatoms, such as Stephanodiscus astraea, present in the water in the spring. Ozone and iron prevented turbidity breakthrough when Microcystis spp. were present in the water, whereas use of iron alone resulted in turbidity breakthrough. Contact filtration produced a high quality filtrate at high filtration rates, up to 20m/h, from dual media RGFs. The effect of flow rate on filtrate quality was far less than the effect of chemical treatment and is not illustrated (Chipps et ai., 1993a, 1993b, 1994). Following the pilot trials, contact dual media filtration was implemented at Ashford Common enabling the works output to increase from 480 Mid to 690Mld. CO-CURRENT DISSOLVEO AIR FLOTATION A great deal of work was carried out in the late 1960's and early 1970's in South Africa (Van Vuuren and Van Vuuren, 1965; Cillie et ai., 1966), UK (Packham and Richards, 1972; Zabel and Hyde, 1976) and Scandinavia (Rosen and Morse, 1976) on the development and application of dissolved air flotation (OAF) to drinking water treatment. OAF is widely recognised as an effective technology for removing algae. Standard co-current OAF usually comprises chemical coagulation, 20-30 minutes multi-stage mechanical flocculation, followed by dissolved air injection (via the recycle system) prior to the air retention tank. This is then followed by filtration, which is sometimes built into the air retention tank. A simplified schematic of the standard co-current flotation process is shown in Figure 3.
STANDARD OAF AlRUP DISSOLVED
"""N
DEDICATED FLOCCULATORS TO FLOAT
20 MINUTU IlU\.TI4TAGE llEeHAHlCAI. FlOCCUlAlION
w$
CLARifiED WATER ,
* • •* . ."
OUT
WATER UP
•
MECHANICAL SLUDGE SC-RAPER I
!
GO
INLET " - - - - - - ' - - - - - - - - '
'LOC DAMAGE DUE TO RECYCLE
Figure. 3. Schematic of Standard Co-Current DAF unit.
Standard OAF has recently been installed at two Thames Water works, Farrnoor and Grimsbury in Oxfordshire, to increase capacity and improve water quality particularly in the summer, when algal blooms in the reservoirs previously resulted in taste and odour problems and RGF algal blocking or penetration. At Farmoor a new DAF plant (65Mld averagell09Mld maximum) was installed in 1993 in preference to adding more precipitators (clarifiers). At Grimsbury (18Mld averagel21Mld maximum) OAF was also installed in 1993 to replace ageing sludge recirculation accelators (clarifiers).
M. J. BAUER etal.
40
COUNTER CURRENT DISSOLVED AIR FLOTATION FILTRAnON (COCO-DAFF®) Counter current dissolved air flotation filtration (COCO-DAFF®) was born out of a review of the technology of flotation. The patented COCO-DAFF® process has been developed as a compact water treatment process to overcome operational problems with seasonal blooms of filter blocking algae such as Melosira spp.• Aphanizomenon spp. and Anabaena spp. One of the advantages of COCO-DAFF® is its operational flexibilty as it can be turned off (operated in contact filtration mode) and on to ensure the works can maintain maximum capacity during even the worst algal blooms. COCO-DAFF® differs from standard DAF in that the recycle water is introduced after the flocculated water inlet structure (but above the filter media) which generates an even bubble blanket field in the flotation tank through which all the flocculated water must pass, Figure 4. The advantage of this design of moving the recycle inlet away from the flocculated water inlet is that the potential for floc damage (shear) by the recycle is eliminated. Also since the entire sludge blanket is supported by a deep and even bubble blanket over the entire surface area of the filter, on desludging any fall-out of sludge that occurs near the de-sludging weirs will have to go back through the process, leading to subsequent re-floating, resulting in a reduced potential for turbidity spikes.
FIoe.Culll'd INLET DI .. ol~.d
J..,R.cyclio
INLET
"11'''0 iN.t"
OUTL£T
~~~~~~=-Figure 4.
Schematic of COCO-DAFF® unit.
Concept validation trials were carried out on a small scale pilot plant (O.25m 2 surface area) at Swinford WTW. Following encouraging results a 1.4MUd pilot (4m 2 surface area) plant was designed and installed at Kempton (AWTC). The pilot plant incorporated dual media filters (combined air water backwash) in the same filter shell and was designed to operate in the range of 10-15 m/h linear flotation rate. Hydraulic flocculation was chosen as the preferred method of raw water conditioning as it offered operational cost benefits over conventional mechanical sytems. The design of the unit allowed for the testing of up to 17 intermediate underflow/overflow baffle plates in the flocculator, with a total headloss of 350mm across the system. The studies indicated that for a given residual iron (total) target of O.5mg/1 off the flotation unit, 15 minutes total flocculation was sufficient in all seasonal conditions for effective performance (Eades et al., 1993, 1995).
Pre-treatment of River Thames reservoir water
41
During the summer of 1992 algal blooms in the feed reservoirs to Kempton AWTC resulted in chlorophyll a concentrations in excess of 3011gll, the dominating algal genera being Melosira and Microcystis, Figure 5, shows the performance of dual media RGFs with and without COCO-DAFF protection. Figure 5. The advantages of flotation protection of the dual media filters during this period are clear with COCO-DAFF able to maintain 10m/h filtration rates and long filter runs whilst un-protected RGFs had to be down-rated to 8m/h to maintain 4-6 hour filter runs. Process design data from the pilot trials plus the use of computational fluid dynamics (CFD) have been used to design the 200Mld COCO-DAFF plant at Walton. The Walton COCO-DAFF plant has been operational since 1995. and has consistently produced 0.05-0.2 NTU filtered turbidity. This represents +95% removal of raw turbidity across the process with filter run times averaging 36 hours. COCO-DAFF by incorporating both the flotation and filtration stages into the one shell significantly reduces the "foot print" and capital cost of the flotation/filtration plant. COCO-DAFF could also theoretically open up older RGF plants for retro-fitting DAF given that the filter geometry was suitable.
COCO-DAFF
DUAL MEDIA FILTERS
10 mII1r. 2.4mgJ1Fe. 15min CT 22
715
25 -~-
BmII1r, l.4mgIFe + pre - ozone 28 32 12Oll---j---"'-- --"'ii>O<'3'----j
51
~ ~ Ii ~
~ :J
3
.
I-
l5
2
I
I
I
~ ,I
'i ~
0
I
o
08lO9I92
Averaged COCO-DAFF filter run time
= +24 hours
averaged filtrate turbidity 0.'5 NTU
Average dual media filter run time
= 4 hours
average @rate turbidijy 0.12 NTU
Figure 5. COCO-DAFF® versus Dual Media Filters performance data.
CONCLUSIONS For existing sand RGFs productivity (filtration rate/run length) and filtrate quality can be improved by optimising media (grave l/sand) and the backwash protocol. Where new RGFs are required the development of dual media RGFs operated in contact filtration (no additional flocculation stage) mode with pre-ozone and iron has resulted in significant improvements in filter productivity and filtrate quality. Pre-conditioning with ozone and iron significantly improved filtrate quality particularly with respect to the removal of Microcystis spp. which broke through when iron alone was dosed. Standard DAF has been shown to be more effective in reducing the algal load onto the subsequent RGF stage than the existing precipitator clarifiers.
42
M.l. BAUER ~t al.
The COCO-DAFF~ process successfully removed algae allowing long filter runs to be achieved during periods when algae would otherwise have resulted in shorter filter runs on dual media RGFs operated in contact filtration mode. ACKNO~DGEMENTS
The authors wish to thank Mr. J. Sexton, Director of Environment and Science, Thames Water Utilities Ltd, for permission to present this paper. The views expressed in this paper are those of the authors and not necessarily of Thames Water Utilities Ltd. REFERENCES Chipps, M.l., Bauer, M. J. and Bayley, R. G. (1993a). Achieving enhanced filter backwashing with combined air scour and sub• f1uidising water at pilot and operational scale. Proc~~dings of th~ Filt~ch Confer~nc~, Karlsruhe, 1. 63-81. Chipps. M. I., Bauer, M. I., Delanoue, A. and Rachwal, A. 1. (l993b). The importance of pre-ozonation in achieving high rate, high quality, rapid gravity filtration. Proceedings of th~ Int~rnatiollQl Ozone Association 11th World Congress. San Francisco. 2, S-17-62 to S-17·77. Chipps, M. I., Eades, A., Bauer, M. 1. and Rachwal. A. J. (1994). Control of coagulant dose in contact filtration and in-filter flotation of lowland reservoir water. IWSA-IA WQ Workshop Optimal Dosing of Coagulants and Flocculants. Mulheim an der Ruhr. Germany, January 1994. Cillie, G. G., Van Vuuren, L. R.I., Stander, G. 1. and Kolbe, F. F. (1966). The reclamation of sewage effluent for domestic use. Paper presented at the Third Int~rnationaJ Conf~r~nce on Wat~r Pollution R~s~arch, Munich, 1966. Eades, A., Ockleston, G. A.• Rachwal, A.I. and Stevenson. D. G. (1993). Counter current dissolved air flotation (COCO-DAFF). Experimental verification of a new proeess.ICh~mE R~s~arch Event, University College London. Eades, A. and Brignall, W. J. (1995). Counter-current dissolved air flotation/filtration. Wat. Sci. Tuh., 31(3-4), 173-178. Glendinning, D.l. and Mitchell, 1. (1996). Uprating water-treatment works supplying Thames Water Ring Main. J. CIWEM, 10, February. lanssens. 1. G., Van Dijck, H., Meheus. 1. and Van Hoof, F. (1985). Comparative investigation of pre-ozonation and prechlorination in relation to removal of algae by direct filtration. In: Th~ Rol~ of Ozone in Water and Wastewat~r Tr~atmmt (perry, R. and McIntyre. A. E., eds) , Selper, London. lanssens, 1. G., Mus, I. and Delite, C. (1989). Practice of rapid filtration in relation to: removal of algae-filter backwashing. Wat~r Supply, 7('2J3), SSII, 1-23. lekel, M. R. (1992). Aocculation effects of ozone, in the Use of Ozone in Water and Wastewater Treatment Proce~dings of 2nd South African Internationol Ozone Association Conference, 26-28 October 1992. Packham, R. F. and Richards, W. N. (1972). Water clarification by flotation - T~chnical Paper WRA. Regan, M. M. and Amirtharajah, A. (1984). Optimisation of particle detachment by collapse-pulsing during air scour. A~rican Water Works Association. 1984 Annual Confer~nce Procudings, 769-784.
Renton. P. I., Duncan, A., Kubecka, J. and Sed&, J. (l99S). The Management Implications of Low Fish Stocks in the London Reservoirs. J. Wat. SRT-Aqua, 44, Suppl. I., pp. 72-79. Rosen. B. and Morse, J. 1. (1976). Practical experience with dissolved air flotation 011 various waters in Swedell and Finland. Popen and Proceedings ofthe FIoUJtion for Water and Waste Treatment Conf~rence. WRc. 1976. Steel, J. A. (1975). The management of Thames Valley reservoirs. Proceedings. WRc Symposium. "The EJf~cts of storag~ on Water Quality". Publication. WRc UK. 371-419. Toms, I. P. (1987) Developments in London's water supply system. Arch. Hydrbiol. B~ih. Ergebn. Umnol., 28, 149-167. Van Vuuren, L. R. and Van Duuren, F. A. (1965). Removal of algae from wastewater maturation pond effluent. Journal Wat~r Pollution Control Federation, 39(9), 1256-1262. Woodward, C. A. (1991). Pre-treatment of low land river waters. American Wat~r Workr Association Slow Sand Filtration Workshop, 27-30th Oct. 1991. University of New Hampshire, Durham. New Hampshire, USA. Zabel. T. F. and Hyde, R. A. (1976). Factors influencing dissolved air flotation as applied to water clarification. Pap~rs and Proce~dingsof th~ Flotation for Wat~r and Waste Treatm~nt Conferenc~. WRc.
Pergamon
War. Sci. Tee/<. Vol. 31, No.2, pp. 35-42, 1998. C 1998 IAWQ. Published by Elsevier Science Ud Printed in Great Britain.
PH: S0273-1223(98)OOOO7-9
0213-1223198 S19'00 + (H)()
ENHANCED RAPID GRAVITY FILTRATION AND DISSOLVED AIR FLOTATION FOR PRE-TREATMENT OF RIVER THAMES RESERVOIR WATER M. J. Bauer*. R. Bayley**. M. 1. Chipps*. A. Eades*. R. J. Scriven* and A. 1. Rachwal* * Thames Water Utilities Limited, Research and Development, Spencer House, Manor Farm Road, Reading, Berkshire RG2 OIN, UK ** Thames Water Utilities Limited, Operational Science, Walton Advanced Water Treatment Works, Hurst Road, Walton-an-Thames, Surrey KTJ2 2EG, UK
ABSTRACT 1bames Water treats approximately 2800MUd of water originating mainly from the lowland rivers Thames and Lee for supply to over 1.3million customers, principally in the cities of London and Oxford. This paper reviews aspects of Thames Water's research, design and operaling experiences of treating algal rich reservoir stored lowland water. Areas covered include experiences of optimising reservoir managemenl, uprating and upgrading of rapid gravity filtration (RGF), standard co-currenl dissolved air flotation (OAF) and counler· currenl dissolved air flotalion/fillralion (COCO-DAFF®) to counler operational problems caused by seasonal blooms of filter blocking algae such as Melosira spp., Aphanizomenon spp. and Anabaena spp. A major programme of uprating and modernisation (inclusion of Advanced Water Treatment: GAC and ozone) of the major works IS in progress which, together with the Thames Tunnel Ring Main, will meet London's water supply needs into the 21st Cenlury. © 1998 IAWQ. Published by Elsevier Science Ud
KEYWORDS Algae; anthracite; COCO-DAFF®; contact filtration; dissolved air flotation; dual media filtration; hydraulic uprating; rapid gravity filtration; reservoir management; slow sand filtration. INTRODUCTION Thames Water treats approximately 2800Mlld of water originating mainly from the lowland rivers Thames and Lee for supply to over 7.3 million customers, principally in the cities of London and Oxford, Traditionally, for London, water treatment has consisted of intake management (pumping water from the rivers Thames and Lee), storage for 30-80 days in deep reservoirs, two stage filtration through rapid gravity filters (RGFs) and slow sand filters (SSFs), then final disinfection with chlorine, with a chloramine residual maintained in distribution (Toms, 1987) The strategy for water treatment and supply in London included the modernisation, together with hydraulic uprating, of the major river-derived water treatment works to match the demands of the Thames Tunnel Ring Main supplying London with treated water (Glendinning and Mitchell, 1996), In addition a strategy to 35
M. J. BAUER et aI.
36
improve water quality for the purposes of complying with the UK Water Supply (Water Quality) Regulations (No 1147, 1989) principally for pesticides has resulted in the implementation of advanced water treatment (AWT) in the form of ozone and granular activated carbon (GAC) at all the major works. Fundamental to achieving the increased treatment plant capacities required is the overcoming of operational constraints caused by seasonal algal blooms which due to differing algal shapes, sizes and motility mean some taxa pass through the RGFs and result in blockage of the SSFs, while others can rapidly block the surface of the RGFs. In 1986, a full scale study at Ashford Common proved that SSF filtration rates could be increased from an average of O.15m1h to O.3m1h, thereby allowing the total 'peak' work's output to be increased from 480MVd to 690MVd. This paper reviews the range of measures which have been taken to achieve increased RGF productivity and improved filtrate quality from improved management of raw water reservoirs to the implementation of a novel patented form of dissolved air flotation termed counter current dissolved air flotation filtration (COCO-DAFF~).
RESERVOIR MIXING Thames Water operates 19 raw water storage reservoirs which provide the first stage of treatment Le. reducing turbidity and pathogenic bacteria and viruses by various processes such as settlement and biological interactions. A key aspect of achieving uprating of works is the continued success of effective reservoir management (Steel, 1975) where each works has access to more than one reservoir. In the prevailing climate of southern England, water bodies such as London's storage reservoirs, with a surface area >lkm2 and depths >10 metres (m), can be subject to thermal stratification during spring and summer. Management options on the deeper reservoirs (> 10m) consist of jet or air mixing systems to prevent thermal stratification. Such mixing systems also allow the control of algal populations via light energy limitation as the euphotic depth is relatively shallow compared to the total depth of the basins. Some basins, notably the Queen Mary, Knight and Bessborough reservoirs, are too shallow «10m) to adequately utilise mixing strategies and algae can proliferate to form a large biomass with subsequent treatment problems at the works. River Thames and Lee derived water is eutrophic and the concentrations of nitrogen and phosphorus are usually in excess of 5mgNn and Imgpn. At present there is no pragmatic, economic method of controlling algal 'blooms' through reducing the nutrient concentrations in these reservoirs. The turbidity of reservoir stored water is generally low in the range of 1-3 NTU, with a peak of 10 NTU during severe algal "blooms" and more generally during the winter caused by filling reservoirs with high turbidity (mineral) water. For operation of SSFs at filtration rates >O.3m1h, RGF filtered water should have an average turbidity of < 2 NTU and should not peak above 5 NTU. A more accurate measure of particle removal is obtained by the use of particle counters. Chlorophyll a is a more sensitive measure of algal biomass and varies by up to 2 orders of magnitude in the 5-24 metre deep reservoirs supplying London. In winter chlorophyll a is typically
Pre-treatment of River Thames reservoir water
37
Chlorophyll a ugll 200
150
r---------------------------,
---
---
----
----------------
---
-----
---
--
-----
100
c=:::J w,.,....,.
_ _ O.E.lI. Q.M...,.
Figure I. Algal biomass as chlorophyll-a for artificially mixed reservoirs (Wraysbury/Queen Elizabeth II) compared with the non-mixed Q. Mary reservoir (Renton et al.• 1995).
RAPID GRAVITY SAND FILTRATION REFURBISHMENT This section outlines the modifications permissible to existing RGFs where hydraulic constraints, notably total available head (in one case restricted to 1.3m), preclude the provision of either dual media (sand/anthracite) or deep bed (2m) coarse media and pre-treatment with ozone/ferric sulphate. The original media specifications for RGFs in London was (BSS mesh 6/22 0.71-2.8mm) laid in a shallow 0.4-0.7m bed of sand overlaying 0.5m of graded gravel(s). The average performance of these filters at 4-6m/h is a 50% (range 30-80%) removal of particles (inorganic and organic) in the size range 4-80 micron. Research at pilot and full scale has shown that reducing the gravel layer(s) and providing a deeper layer of 0.6-0.9m of finer sand (BSS mesh 14/25 0.6-1.18mm) can achieve the target filtration rate of 6-8m/h and particle removal of 50-60%. Effective size cannot be reduced significantly without compromising head loss. Theoretically experimental work has been largely confined to the backwashing of kaolin and alum floes off laboratory filters in sterile conditions (Regan and Amirtharajah, 1984). There is little information on effective backwashing, particularly involving the removal of biological material from un-chlorinated filters_ Antwerp Water Company has investigated different backwashing regimes for cleaning filters operated in the contact filtration mode (Janssens et ai., 1989). Research has proven that an effective backwash using combined air and water is fundamental in ensuring the media is sufficiently cleaned after each run which is in broad agreement with work done by Amirtharajah (Regan and Amirtharajah, 1984). The build up of silt, and organic matter (algal and detrital) in poorly washed filters ultimately leads to the phenomenon of "mud balling" (localised accumulation of mud) and "jetting" (build up of columns of support gravels through the media) which leads to localised fluidisation, sand loss. short circuiting and poor filtrate quality. These inadequately washed filters also develop very high starting bed headlosses often approaching 1m (normal starting bed headloss being a.25m) and consequently can gel locked into a circle of ever increasing cleaning frequency during periods of high algal loading. The method of determining sand cleanliness is by using the parameters of POC and dry weight of solids extracted "silt". Experience has shown that a maximum concentration of POC and "silt" should be 250g/m 2 (or 400g/m 3) and 1.5kg/m 2 (or 2.4kg/m 3) respectively to achieve good filtration performance and acceptable starting bed headloss. .
38
M. J. BAUER el al.
Due to the size of some Thames Water RGFs (150m 2 surface area) additional launders are often required to achieve the satisfactory removal of backwash water. In addition, to minimise media (sand) Joss, particularly during the combined air/water phase, new designs of launder have been developed and successfully implemented. In summary, for existing RGFs a modest increase in filtration rate to (6-8m1h) and consistent particle removal (50-60%) can be achieved by a combination of improved media specification, more effective backwashing (combined air/water) and modifications to the washout launder(s). DUAL MEDIA FILTRATION Two sites, Ashford Common (to replace overloaded microstrainers) and Walton (to replace ageing RGFs), offered the opportunity for developing higher rate (I0-20mlh) higher quality filtration. Pre-ozonation prior to RGF was investigated as it was required at some works to achieve reductions in pesticides, and has been reported by several authors, (Janssens et ai., 1985; Jekel, 1992; Chipps et ai., 1993b) in Europe and the United States, as assisting with the removal of particles, especially algae across filters or clarifiers. Pilot trials evaluating "contact" or "in-line" filtration (without a separate flocculation stage), using iron (III) sulphate rapidly mixed into the feedwater were carried out initially at Ashford Common using 0.6m diameter columns and later at the 5Mld Kempton Advanced Water Treatment Centre (AWTC) commissioned in 1991 using 204m diameter columns. Contact filtration was possible because of the relatively low loadings of turbidity and particles from the reservoirs and is thought to act by utilising a small dose of coagulant for charge neutralisation, allowing floes to form in the filter and/or reducing electrostatic repulsion forces between particles and filter media (grains). Results of the trials confirm that the effect of iron dosing, compared to no dosing, was to improve turbidity removal and produce some small improvement in the parameters which are indicative of algae, Le. chlorophyll a, POC, Total Particle Number (TPN) and Total Particle Volume (TPV) , Figure 2. FILTER PERFORMANCE WITH DIFFERENT PRE-TREATMENTS
Annual average removals Average Removal %
KEY:
100
80
dual media OZONE & IRON
60
dual media OZONE
40
dual media IRON
20
sand filter UNDOSED
0"--
----' POC TPV CHLOROPHYLL a
TPN
TURBIDITY
Water Quality Parameter
dual medIa
UNDOSED
• • •
Figure 2. Average removal data from a control sand filter without chemical dosing and a dual media filter with no chemical pre-treatment and with pre-treatment using pre-ozonation with iron (III) sulphate, on their own, and in combination.
Pre-treatment of River Thames reservoir water
39
The effect of ozone was to increase removals for these parameters compared with iron or no dosing, but turbidity removals were lower than those achieved with iron. The effect of combining iron and ozone resulted in superior removals to that of either chemical alone. Around 50% removals were achieved without chemicals increasing to approximately 90% with ozone and iron. Pre-ozonation was essential for good removals of Microcystis spp. , present in the reservoirs in summer and autumn, and small (5-15lJ.m) filter penetrating diatoms, such as Stephanodiscus astraea, present in the water in the spring. Ozone and iron prevented turbidity breakthrough when Microcystis spp. were present in the water, whereas use of iron alone resulted in turbidity breakthrough. Contact filtration produced a high quality filtrate at high filtration rates, up to 20m/h, from dual media RGFs. The effect of flow rate on filtrate quality was far less than the effect of chemical treatment and is not illustrated (Chipps et ai., 1993a, 1993b, 1994). Following the pilot trials, contact dual media filtration was implemented at Ashford Common enabling the works output to increase from 480 Mid to 690Mld. CO-CURRENT DISSOLVEO AIR FLOTATION A great deal of work was carried out in the late 1960's and early 1970's in South Africa (Van Vuuren and Van Vuuren, 1965; Cillie et ai., 1966), UK (Packham and Richards, 1972; Zabel and Hyde, 1976) and Scandinavia (Rosen and Morse, 1976) on the development and application of dissolved air flotation (OAF) to drinking water treatment. OAF is widely recognised as an effective technology for removing algae. Standard co-current OAF usually comprises chemical coagulation, 20-30 minutes multi-stage mechanical flocculation, followed by dissolved air injection (via the recycle system) prior to the air retention tank. This is then followed by filtration, which is sometimes built into the air retention tank. A simplified schematic of the standard co-current flotation process is shown in Figure 3.
STANDARD OAF AlRUP DISSOLVED
"""N
DEDICATED FLOCCULATORS TO FLOAT
20 MINUTU IlU\.TI4TAGE llEeHAHlCAI. FlOCCUlAlION
w$
CLARifiED WATER ,
* • •* . ."
OUT
WATER UP
•
MECHANICAL SLUDGE SC-RAPER I
!
GO
INLET " - - - - - - ' - - - - - - - - '
'LOC DAMAGE DUE TO RECYCLE
Figure. 3. Schematic of Standard Co-Current DAF unit.
Standard OAF has recently been installed at two Thames Water works, Farrnoor and Grimsbury in Oxfordshire, to increase capacity and improve water quality particularly in the summer, when algal blooms in the reservoirs previously resulted in taste and odour problems and RGF algal blocking or penetration. At Farmoor a new DAF plant (65Mld averagell09Mld maximum) was installed in 1993 in preference to adding more precipitators (clarifiers). At Grimsbury (18Mld averagel21Mld maximum) OAF was also installed in 1993 to replace ageing sludge recirculation accelators (clarifiers).
M. J. BAUER etal.
40
COUNTER CURRENT DISSOLVED AIR FLOTATION FILTRAnON (COCO-DAFF®) Counter current dissolved air flotation filtration (COCO-DAFF®) was born out of a review of the technology of flotation. The patented COCO-DAFF® process has been developed as a compact water treatment process to overcome operational problems with seasonal blooms of filter blocking algae such as Melosira spp.• Aphanizomenon spp. and Anabaena spp. One of the advantages of COCO-DAFF® is its operational flexibilty as it can be turned off (operated in contact filtration mode) and on to ensure the works can maintain maximum capacity during even the worst algal blooms. COCO-DAFF® differs from standard DAF in that the recycle water is introduced after the flocculated water inlet structure (but above the filter media) which generates an even bubble blanket field in the flotation tank through which all the flocculated water must pass, Figure 4. The advantage of this design of moving the recycle inlet away from the flocculated water inlet is that the potential for floc damage (shear) by the recycle is eliminated. Also since the entire sludge blanket is supported by a deep and even bubble blanket over the entire surface area of the filter, on desludging any fall-out of sludge that occurs near the de-sludging weirs will have to go back through the process, leading to subsequent re-floating, resulting in a reduced potential for turbidity spikes.
FIoe.Culll'd INLET DI .. ol~.d
J..,R.cyclio
INLET
"11'''0 iN.t"
OUTL£T
~~~~~~=-Figure 4.
Schematic of COCO-DAFF® unit.
Concept validation trials were carried out on a small scale pilot plant (O.25m 2 surface area) at Swinford WTW. Following encouraging results a 1.4MUd pilot (4m 2 surface area) plant was designed and installed at Kempton (AWTC). The pilot plant incorporated dual media filters (combined air water backwash) in the same filter shell and was designed to operate in the range of 10-15 m/h linear flotation rate. Hydraulic flocculation was chosen as the preferred method of raw water conditioning as it offered operational cost benefits over conventional mechanical sytems. The design of the unit allowed for the testing of up to 17 intermediate underflow/overflow baffle plates in the flocculator, with a total headloss of 350mm across the system. The studies indicated that for a given residual iron (total) target of O.5mg/1 off the flotation unit, 15 minutes total flocculation was sufficient in all seasonal conditions for effective performance (Eades et al., 1993, 1995).
Pre-treatment of River Thames reservoir water
41
During the summer of 1992 algal blooms in the feed reservoirs to Kempton AWTC resulted in chlorophyll a concentrations in excess of 3011gll, the dominating algal genera being Melosira and Microcystis, Figure 5, shows the performance of dual media RGFs with and without COCO-DAFF protection. Figure 5. The advantages of flotation protection of the dual media filters during this period are clear with COCO-DAFF able to maintain 10m/h filtration rates and long filter runs whilst un-protected RGFs had to be down-rated to 8m/h to maintain 4-6 hour filter runs. Process design data from the pilot trials plus the use of computational fluid dynamics (CFD) have been used to design the 200Mld COCO-DAFF plant at Walton. The Walton COCO-DAFF plant has been operational since 1995. and has consistently produced 0.05-0.2 NTU filtered turbidity. This represents +95% removal of raw turbidity across the process with filter run times averaging 36 hours. COCO-DAFF by incorporating both the flotation and filtration stages into the one shell significantly reduces the "foot print" and capital cost of the flotation/filtration plant. COCO-DAFF could also theoretically open up older RGF plants for retro-fitting DAF given that the filter geometry was suitable.
COCO-DAFF
DUAL MEDIA FILTERS
10 mII1r. 2.4mgJ1Fe. 15min CT 22
715
25 -~-
BmII1r, l.4mgIFe + pre - ozone 28 32 12Oll---j---"'-- --"'ii>O<'3'----j
51
~ ~ Ii ~
~ :J
3
.
I-
l5
2
I
I
I
~ ,I
'i ~
0
I
o
08lO9I92
Averaged COCO-DAFF filter run time
= +24 hours
averaged filtrate turbidity 0.'5 NTU
Average dual media filter run time
= 4 hours
average @rate turbidijy 0.12 NTU
Figure 5. COCO-DAFF® versus Dual Media Filters performance data.
CONCLUSIONS For existing sand RGFs productivity (filtration rate/run length) and filtrate quality can be improved by optimising media (grave l/sand) and the backwash protocol. Where new RGFs are required the development of dual media RGFs operated in contact filtration (no additional flocculation stage) mode with pre-ozone and iron has resulted in significant improvements in filter productivity and filtrate quality. Pre-conditioning with ozone and iron significantly improved filtrate quality particularly with respect to the removal of Microcystis spp. which broke through when iron alone was dosed. Standard DAF has been shown to be more effective in reducing the algal load onto the subsequent RGF stage than the existing precipitator clarifiers.
42
M.l. BAUER ~t al.
The COCO-DAFF~ process successfully removed algae allowing long filter runs to be achieved during periods when algae would otherwise have resulted in shorter filter runs on dual media RGFs operated in contact filtration mode. ACKNO~DGEMENTS
The authors wish to thank Mr. J. Sexton, Director of Environment and Science, Thames Water Utilities Ltd, for permission to present this paper. The views expressed in this paper are those of the authors and not necessarily of Thames Water Utilities Ltd. REFERENCES Chipps, M.l., Bauer, M. J. and Bayley, R. G. (1993a). Achieving enhanced filter backwashing with combined air scour and sub• f1uidising water at pilot and operational scale. Proc~~dings of th~ Filt~ch Confer~nc~, Karlsruhe, 1. 63-81. Chipps. M. I., Bauer, M. I., Delanoue, A. and Rachwal, A. 1. (l993b). The importance of pre-ozonation in achieving high rate, high quality, rapid gravity filtration. Proceedings of th~ Int~rnatiollQl Ozone Association 11th World Congress. San Francisco. 2, S-17-62 to S-17·77. Chipps, M. I., Eades, A., Bauer, M. 1. and Rachwal. A. J. (1994). Control of coagulant dose in contact filtration and in-filter flotation of lowland reservoir water. IWSA-IA WQ Workshop Optimal Dosing of Coagulants and Flocculants. Mulheim an der Ruhr. Germany, January 1994. Cillie, G. G., Van Vuuren, L. R.I., Stander, G. 1. and Kolbe, F. F. (1966). The reclamation of sewage effluent for domestic use. Paper presented at the Third Int~rnationaJ Conf~r~nce on Wat~r Pollution R~s~arch, Munich, 1966. Eades, A., Ockleston, G. A.• Rachwal, A.I. and Stevenson. D. G. (1993). Counter current dissolved air flotation (COCO-DAFF). Experimental verification of a new proeess.ICh~mE R~s~arch Event, University College London. Eades, A. and Brignall, W. J. (1995). Counter-current dissolved air flotation/filtration. Wat. Sci. Tuh., 31(3-4), 173-178. Glendinning, D.l. and Mitchell, 1. (1996). Uprating water-treatment works supplying Thames Water Ring Main. J. CIWEM, 10, February. lanssens. 1. G., Van Dijck, H., Meheus. 1. and Van Hoof, F. (1985). Comparative investigation of pre-ozonation and prechlorination in relation to removal of algae by direct filtration. In: Th~ Rol~ of Ozone in Water and Wastewat~r Tr~atmmt (perry, R. and McIntyre. A. E., eds) , Selper, London. lanssens, 1. G., Mus, I. and Delite, C. (1989). Practice of rapid filtration in relation to: removal of algae-filter backwashing. Wat~r Supply, 7('2J3), SSII, 1-23. lekel, M. R. (1992). Aocculation effects of ozone, in the Use of Ozone in Water and Wastewater Treatment Proce~dings of 2nd South African Internationol Ozone Association Conference, 26-28 October 1992. Packham, R. F. and Richards, W. N. (1972). Water clarification by flotation - T~chnical Paper WRA. Regan, M. M. and Amirtharajah, A. (1984). Optimisation of particle detachment by collapse-pulsing during air scour. A~rican Water Works Association. 1984 Annual Confer~nce Procudings, 769-784.
Renton. P. I., Duncan, A., Kubecka, J. and Sed&, J. (l99S). The Management Implications of Low Fish Stocks in the London Reservoirs. J. Wat. SRT-Aqua, 44, Suppl. I., pp. 72-79. Rosen. B. and Morse, J. 1. (1976). Practical experience with dissolved air flotation 011 various waters in Swedell and Finland. Popen and Proceedings ofthe FIoUJtion for Water and Waste Treatment Conf~rence. WRc. 1976. Steel, J. A. (1975). The management of Thames Valley reservoirs. Proceedings. WRc Symposium. "The EJf~cts of storag~ on Water Quality". Publication. WRc UK. 371-419. Toms, I. P. (1987) Developments in London's water supply system. Arch. Hydrbiol. B~ih. Ergebn. Umnol., 28, 149-167. Van Vuuren, L. R. and Van Duuren, F. A. (1965). Removal of algae from wastewater maturation pond effluent. Journal Wat~r Pollution Control Federation, 39(9), 1256-1262. Woodward, C. A. (1991). Pre-treatment of low land river waters. American Wat~r Workr Association Slow Sand Filtration Workshop, 27-30th Oct. 1991. University of New Hampshire, Durham. New Hampshire, USA. Zabel. T. F. and Hyde, R. A. (1976). Factors influencing dissolved air flotation as applied to water clarification. Pap~rs and Proce~dingsof th~ Flotation for Wat~r and Waste Treatm~nt Conferenc~. WRc.