CELEBRATING: “-Thames Water has strongtraditions for being in the forefront of water

treatment. Our long term research into differentfiltering materials has been highly fruitful.“

Dr. Michael Chipps, head of reserach atKempton Water Laboratories.

How Thames Water reinvented its fast sand filters

RenovatingAfter years of research with trialsand full scale testing, ThamesWater have today a deeper under-standing of rapid filtering and pre-cise data on the performance ofcurrent filter materials. “The research has given us uniquebasis for building better filters. Sofar we have renovated many of ourfilters with success, and the workcontinues”, tells Dr. MichaelChipps, lead scientist of the waterresearch centre of Thames Waterat Kempton Park Water Work inMiddlesex.

Way backThe first water works of theThames region were established in1845. According to regulations,water should only be producedabove the maximum tidal level, toavoid risk of re-contamination fromsewage. All drinking water wasproduced by “slow filtering” inlarge outdoor filters, with the sizeof half a football pitch. The waterpasses through a deep layer ofsand that filters the natural way.The slow sand filters are main-tained by occasionally scraping off2-3 centimeters of the top layer.The slow sand filters of today havebeen modernized with a layer ofactivated carbon, to eliminate odorand taste.

A change of the climate However as the slow sand filterssome decades ago started strug-gling to keep up with the demand,

new rapid sand filters wereinstalled to pre-treat the water. Theslow sand filters are still oftenused in some water works ofThames Water, as a finishing stepwith a barrier against parasitesetc. Changes in the climate and morenutrients in the water source (TheThames River) most likely causedconstraints on the fast sand filters.The primary reason is algal bloomsin the water reservoirs. This led toan increasing need of back wash-ing of the primary sand filters, andthe production capacity hasdecreased due to this. The slow sand filters have a limit-ed scope for improvement, sincethey cannot be back flushed andwork only by natural processes.

“Our focus was therefore toimprove the rapid sand filters andour ambition was to find new filter-

When algal blooms put the sand filtersunder pressure, Thames Water started itsown in-house research into new filter materials. Not only did the effort cure theconstraint, the use of new filter materialshas saved millions in investments.

THAMES WATER

TESTING: Research staff with great interest in improving filtering.

A pioneer in filtration reserach

ADVANCED: The research facilities at Kempton Park.

THE USE OF SAND for water filtra-tion has for more than a centurybeen the backbone of water treat-ment in the Thames region. This simple method still brings safe waterto the millions of inhabitants in theregion, including the city of London.However, some years ago the pri-mary sand filters of some waterworks of Thames Water had difficul-ties dealing with the increasingamount of organic load in the rawwater, which was caused by algalblooms in the reservoirs. ThamesWater had to search for other filtermaterials.

ing materials for these, furthermoreto rebuild and re-furnish them witha new filtering material, and theright support systems”, explainsDr. Chipps.

Water, energy and timeThe problem of the rapid sand fil-ters was actually not the ability tofilter itself, although the capacity ofa single filter was reduced. The keyproblem was a large need for backflushing.“Back flushing is our biggestexpense. It requires energy, cleanwater and reduces productiontime. Water, energy and time aremoney. Ultimately, the need forback flushing is a limit for the totalcapacity of a plant”, explains Dr.Chipps. The research has produced valu-able results. Dr. Chipps and histeam has managed to improve onthe output of the fast sand filtersgreatlly. First by introducing dual-media, and secondly by usingexpanded clay (Filtralite).The need for backwashing of the

filters has been reduced, and theproduction is today far less vulner-able to algal blooms than it usedto be. “With new filter materials, andespecially Filtralite Mono Multi, wehave increased the capacity ofwater works without having toincrease the actual footprint. Ourresearch has been profitable”, saysDr. Chipps. Today Filtralite Mono-Multi®(Filtralite HC 0.8-1.6 in bottom andFiltralite NC 1.5-2.5 on top) is thepreferred choice of filter material,and 37 filters have so far been ren-ovated with this. Thames Water is on a long termprogram to replace all filters withFiltralite. It has given the filters bet-ter performance and lower operat-ing costs.

Broad approachTesting new filtering materials firststarted on a broad basis. All com-mercial available filter materialsWorld Wide were tested. A widerange of materials were first tested

for hydraulic performance andcrude filtering performance. Tosupport the observations alsoscanning electron microscopy(SEM) was taken of the filteringparticles. The SEM revealed physi-cal characteristics, as well as arevealing the amount of biologicalgrowth on the material.“We have studied all aspects, likethe filtering ability itself of differentsizes and types of particles, thehydraulic resistance, the amount ofparticles a filtering material is ableto hold, and how often they needbackwashing. As algae are a partof the problem, we have of coursestudied how the filtering materialbehaves during high loads ofalgae, as in algal blooms”, explainsDr. Chipps.More than a dozen filter materialswere studied, further research wasdone with the top four filteringmaterials. These were sand,crushed glass, sand/ anthracite andFiltralite Mono-Multi®.

CANDIDATES: Both anthracite, sand and expanded clay has been thoroghly tested and compared for hydraulic propertiesand filtering performance.

MONO-MULTI: Filtalite at work, noteits porous nature and distinct layers

LAB: A separate monitoring room provides control and data recording.

ADVANCED: Parallell trials with differentfiltering materials give relevant data.

Further studiesThese four top candidates werethen compared and characterizedin all respects; Minute studies ofthe development of the pressureduring filtering cycles were done.Filtering performance and dynam-ics were characterized. The con-tinuous monitoring included col-lecting data on head loss, flowrate, turbidity & particle counts ofthe filtrate. The research went onfor several months.This work was carried out in coop-eration with the Department ofCivil & Environmental Engineeringat the University College ofLondon. “We also looked at the degree ofbiological growth on the differentmaterials, and discovered that thismay affect the filtering perform-ance. Material that graduallybecome “fouled” with microbialgrowth is becomes increasinglydifficult to backwash”, says Dr.Chipps. The durability and thecommercial availability of thematerial were also looked upon.

The duo filterThe best results were achievedwith so-called dual-filters. The two

combinations sand/anthracite andFiltralite Mono-Multi® (two sizes)performed far better than anyother material. The ability to catchparticles of different sizes in thefilter bed seem to be of high rele-vance, and this ability seem togive less head loss and longer fil-tering cycles, i.e. a reduced needof backflushing. “If we take all considerations intothe picture, 2 layers of Filtralite fil-ters are highly effective, so wehave also studied different frac-tions of these. It seems clear thatthe Filtralite Mono-Multi® possessan extra filtering dimension, whichmay be explained by its openstructure. This is the reason whywe have already renovated 37rapid sand filters with FiltraliteMono Multi”, states Dr. Chipps.The discovery of Filtralite Mono-Multi® has already saved ThamesWater great investments, as olderfast filters can be renovated andgiven increased capacity withoutincreasing the footprint.“Our new precise knowledge of fil-tering material has saved the com-pany millions of GBP in invest-ments into new systems”, statesDr. Chipps.

The Filtralite functionFiltralite is made from expandedclay and developed especially forwater filtering purposes. Filtraliteworks by a different principle thansand. The two kinds of theFiltralite used at Thames Watertypically have the fractures ofFiltralite HC 0,8-1,6 mm andFiltralite NC 1,5-2,5 mm respec-tively. Filtralite can be character-ized as small hard pieces of opensponge, with internal pores of vari-ous sizes. As the water passesthrough the filtering bed, the parti-cles are trapped in the pores ofthe Filtralite bodies. The water ishence free to pass right throughthe Filtralite itself, making cloggingdifficult. Thanks to its porousstructure Filtralite is a more openfiltering medium than sand, withextra large room for storing parti-cles. As the pores of Filtralite aresmaller than the spaces betweensand grains Filtralite also catchcyanobacteria, and reduce thetotal amount of small particlesescaping the filters as well.The wash water flow needed foreach backwashing is alsoreduced, due to the lower specificweight of Filtralite than sand.

THAMES WATER The giant steam engines at Kempton Park are a proof of advancedengineering, they were built to pump fresh water all the way to the cityof London and are today a listed historic place. “ Local knowledge and engineering skills has been a trademark ofThames Water”, tells Michael Chipps, lead scientist at the researchcentre at Kempton Park.Today Thames Water has started a technological revolution in filtering. “ We have actually re-invented our sand filters, today we save millionsof GBP thanks tonew filtering mate-rials, and amongstthese, Filtralite isthe most outstand-ing.” The ThamesRegion includingthe capital ofLondon has asteadily increasingneed for water. Theraw water availableis however notincreasing in the same degree. “ Our major concern are periodical algal blooms, that clog the filtersor reduce their filtering capacity by half and demand more frequentback flushing. In some cases we have been on the verge of watershortage”, explains Dr. Michael Chipps, chief scientist at The ThamesRegion’s research laboratories at Kempton Park Water works.

EXACT: Studying the pressure dynamics of filtering cycles provides valuable data

FILTRALITE FACTS: Filtralite is a lightweight ceramic particle aggregate made from expanded clay. Filtralite isdesigned, and proven to be an excellent material, for water and wastewater purification. The low density granuleshave large pore volumes with large surface areas, which are ideal characteristics for conventional filtration.Filtralite is also an ideal medium for biofilm growth. See more at www.filtralite.com

Fact summary1. Glass media provided some advan-tage over sand media, but not signifi-cant to operations: – Biological fouling of glass media wasevident despite collapse-pulse back-washing. – Just as vulnerable to being cloggedby algae as sand. – Run lengths 20-40% longer thansand-only media.2. Dual media - Filtralite - pilot filtersdemonstrated consistently longer runtimes (100-800% longer) than conven-tional dual media pilot filters (sand/ -anthracite), whilst producing accept-able filtrate quality.3. SEM analysis indicated scope ofexpanded clay for biological colonisa-tion,however this was thought to bene-fit treatment rather than hinder it.4. Full scale results are promising:replacement of sand by a bed ofFiltralite (in a operational RGF) has leadto longer run times, whilst still meetingquality requirements.5. Filtralite appears less vulnerable toblinding by algae (pilot scale & fullscale results)

Engine house at Kempton water park. Photo: Ron Early