Alternative Filter Media for Potable Water Treatment M.E. Steele*, M.J. Chipps*, R. Bayley*, A. Mikol** & C.S.B. Fitzpatrick**
* Thames Water Research & Development ** Department of Civil & Environmental Engineering University College London
Acknowledgments Research & Development, Thames Water Utilities Ltd University College London Engineering Doctorate Supported by UK EPSRC Eng.D. award
Contents Need for research The ideal filter medium Alternative media â€˜candidatesâ€™ Hydraulic performance testing (Phase 1) Quality performance testing for roughing filter application (Phase 2: pilot scale trials) Scanning Electron Microscopy Analysis Full scale trials in roughing filters (Phase 3) Conventional filtration application (Phase 4)
Need for Research Roughing filters at London works:
Low backwash rates for filters at older works Inability to keep sand clean (rising normalised starting head loss) Vulnerability to algal laden water Loss of production at critical times during the year Objective: identify mono-medium & dual media alternatives to â€“ BSS 14/25 sand â€“ BSS 14/25 sand / grade 2 coarse anthracite Which demonstrate hydraulic advantage & produce acceptable filtrate quality
Ideal Filter Media (emphasis: Roughing Filters) 1. Ability to remove particles: acceptable filtrate quality 2. DWI approved material (or scope for approval) 3. Hydraulic advantage – Low normalised (clean bed) starting head loss (NSHL) – Low rate of head loss development – Ability of low backwash rates to keep it clean (i.e. constant NSHL)
4. Commercial availability of appropriate grade(s) (grain size, uniformity co-efficient, specific gravity etc) 5. Durable (does it turn to dust within weeks of installation?)
Other considerations 1. Launder design (will it stay in the filter?) 2. Dual media: do media layers separate during fluidising rinse? 3. Conventional filters: how does media perform when fed with coagulant-dosed, clarified water? (Phase 4)
Phase 1 Hydraulic testing of Media â€˜Candidatesâ€™
Phase 1 Testing Hydraulic Suitability Velocity of minimum fluidisation % Bed expansion Settling rate Size grading Attrition testing Ability of dual media layers to separate
Media selected for pilot trials
Crushed recycled glass Expanded clay Sand (‘control’) Anthracite / Sand (‘control’) (Various grades & combinations)
Phase 2 Quality Performance Testing Roughing Filter Pilot Trials 15 pilot filters operated since 2003 Not all results reported here: Focus on : 4 pilot filter rig
Phase 2 Pilot Plant Trials
4 x 150mm diameter PVC columns (pilot filters) Fed with reservoir (stored) water & operated as roughing filters ‘Collapse-pulse’ backwashing (rates identified by Amit for sand) Monitoring: included head loss, flow rate, turbidity & particle counts +12 months operation Glass vs. Sand •
1m Grade 1 (0.5-1.0mm) crushed recycled Glass
1m BSS 14/25 (0.6-1.18mm) Sand
Expanded clay dual media vs. Anthracite / Sand •
500mm HC 1.5-2.5mm Expanded Clay / 500mm NC 0.81.6mm Expanded Clay
500mm (1.7-2.5mm) coarse Anthracite / 500mm BSS 14/25 Sand
Hydraulic Wins: Longer Run Times / Less Backwashing 20-40% longer for glass compared to sand 100-800% longer for dual expanded clay compared to anthracite / sand 20
Sand Mono-medium Glass Mono-medium Anthracite / Sand Dual Media Expanded Clay Dual Media
No. of Runs / Month
Acceptable Filtrate Quality Comparable filtrate turbidity 7.0
6.0 5.0 4.0 3.0 2.0 1.0 0.0 Dec-05
Sand Mono-medium Anthracite/Sand Dual Media Expanded Clay Dual Media
Glass Mono-medium Stored Water
Performance under Algal Challenge Head Loss across Alternative & Conventional Media during February (2006) Melosira varians Challenge.
Flow and Temperature Normalised Head Loss (m)
Glass mono medium Sand mono medium Expanded clay dual media Anthracite / Sand dual media
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Date in February 2006
Comparable filtrate turbidity when under algal challenge. 7
Individual spikes relate to post backwash ripening
Source water change
5 4 3 2 1 0 1
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Sand Mono Medium Anthracite / Sand Dual Media Stored Water
Glass Mono Medium Expanded Clay Dual Media
Date in February 2006
Vulnerability to Algal Blinding? Algal Spiking Trials Cultivation of algal bloom of Mycrocystis Sand vs. Glass (pilot RGFs operated at 5m/h) Anthracite/Sand vs. Expanded Clay dual media (pilot RGFs operated at 10m/h) 18 16
Run Time (mins) .
14 12 10 8 6 4 2 0 Sand
Anthracite / Sand
Influent water Chlorophyll-a concentration = 331.8 Âľg/l Suspended solids = 632mg/l
Expanded Clay Dual Media
95% removal achieved Glass just as vulnerable as sand Expanded clay least vulnerable
Phase 3 Full Scale Trials
Phase 3 Full Scale Trial: Expanded Clay Roughing Filter •
Mono-medium sand in roughing filters
Low rate backwashing: causing rising starting head losses & loss of production capacity
Installed 700mm of coarse (1.5-2.5mm) expanded clay in 1 RGF (December 2006)
Compared to sand-only RGFs (both recently refurbished & old)
Example results (next slides): over the Easter weekend expanded clay RGF was backwashed twice (runs of just over 24 hours). The sand-only RGFs backwashed 9-10 times each (runs of 6 to 15 hours).
Improved Hydraulic Performance at Full Scale Extended run times: Easter weekend: 27-30 hours for expanded clay, compared to 6-15 hours for sand Consistently low normalised starting head loss for the expanded clay RGF: 0.15m for expanded clay compared to 0.4m for clean sand
Rate of Head Loss Development (m/day) .
0.0 Non Refurbished Sand
Acceptable quality performance Filtrate Suspended Solids, chlorophyll-a & turbidity values similar Backwashing returning filter to clean bed head loss (despite low backwash rates) Bed depth measurements indicating media loss is not significant 10
Sand (not refurbished) 8
Sand (recently refurbished) Expanded clay
7 6 5 4 3 2 1 0 Jan-07
NB spikes = post backwash ripening
Phase 4 Alternative Media Performance in Conventional Filters
Alternative Media for Conventional Filters Does expanded clay or pumice out-perform conventional sand media when treating coagulated & DAF clarified water?
Media being tested:
Expanded clay (mono & dual media) Pumice (mono media) Sand control Anthracite / Sand control Floated Water
Preliminary results: Longer run times appear to be reproducible under flocculation conditions
Scanning Electron Microscopy How is enhanced performance achieved? Biological? Physical: media packing, particle capture? Both?
14/25 SAND RGF Media from Pilot plant 10 months operation Collapse-pulse backwashing
Relatively clean â€“ some biofilm
Glass RGF Media from Pilot plant 10 months operation Collapse-pulse backwashing
Some biofilm Similar to sand
Anthracite (Grade 2 Coarse) RGF Media from Pilot plant 10 months operation Collapse-pulse backwashing
Relatively clean Pockets of biofilm in protected areas
Expanded Clay RGF media from pilot plant 10 months operation Collapse-pulse backwashing
Teeming with biological activity Concentrated in cracks & crevices Porous media provides greater surface area for colonisation
Alternative Media Cost Saving? Assume – backwashing once/3 days rather than once/day – saving = electrical cost + cost to treat & discharge wash water Cost of backwash = works dependant (i.e. filter size, backwash design) Potential saving: £1k to £10k per filter per year Payback time for media purchase & installation is: 3-4 years? This does not account for cost associated with lost production during backwashing – which could be considerable if more expensive treatment units need to be brought into operation in order to achieve the necessary output, – only a true cost at some older works & during periods of very poor stored water quality (i.e. when algal blooms cause production to drop).
Summary 1. Glass media provided some advantage over sand media, but not significant to operations: â€“ Biological fouling of glass media was evident despite collapse-pulse backwashing. â€“ Just as vulnerable to blinding by algae as sand. â€“ Run lengths 20-40% longer than sand-only media.
2. Dual media expanded clay pilot filters demonstrated consistently longer run times (100-800% longer) than conventional dual media pilot filters (anthracite / sand), whilst producing acceptable filtrate quality.
3. SEM analysis indicated scope of expanded clay for biological colonisation, however this was thought to benefit treatment rather than hinder it.
4. Full scale results are promising: replacement of sand by a bed of expanded clay (in a operational RGF) has lead to longer run times, whilst still meeting quality requirements.
5. Expanded clay media appears less vulnerable to blinding by algae (pilot scale & full scale results)
What Next? The full scale trial will continue for 1 year. If results continue to be positive it is likely that sand will be replaced in additional operational RGFs.
Further full scale alternative media trials are planned for later this year at 3 of London’s key water treatment plants:
– two will use the dual media expanded clay in place of anthracite / sand, – the third will test a mono medium (expanded clay or pumice) in place of sand.
The TW research programme has recently expanded to include assessment of alternative media performance in conventional filters treating coagulated, clarified stored water. – Pilot trials will run until October 2007 – First full scale trial in TW conventional RGF is planned for January 2008.