with a relatively low solids concentration, and/or in temperate or rainy climates. This is an important important an is This climates. rainy or temperate in and/or concentration, solids low relatively a with especiallybenefi are anybut in used be climate, can 17).tanks Settling-thickening FS treating when cial or (forbeen fully stabilised combinations dewatered of to Chapter reader technologies is 5 referred and fion their liquid and sludge solids as has streamsthe not are stilland nal the fate, yethigh in pathogens, treatment depending liquid of and FS solid the require fractions Following further settling-thickening, infl supernatant. layers sludge from the scumandthickened ow, the separate andto the at turbulence avoid help fl to turbulent used Baffl be with ows. can occur fles not and will otation outlet. Quiescent the hydraulic flthrough of settling, designed ows are thickening rates required, as the discharged is supernatant liquid the tank, the of bottom the fl the at to grease, collected are oat solids and As oils tank. the fats, of top as such particles, Lighter forces. gravitational of result a as of bottomtank the the at thicken and out settle particles heavier the time, retention the During 6.1). (Figure opposite side, and while scum settled fl bottom solids at tank, are the of retained the oats surface on the discharged into an inlet at top the of one side exits and through an supernatant the outlet situated at the that as separation is FS where tanks, solids-liquid rectangular are treatment forFS tanks for Settling-thickening employedseptictanks. in mechanism same the following is clarifi it cation for and and treatment, treatment, wastewater secondary wastewater primary for developed fi rst were They (FS). are Settling-thickening used tanks to achieve separation of liquidthe and solid fractions of faecal sludge 6.1 INTRODUCTION • Be able to design a settling-thickening tank to achieve the desired treatment goal. treatment desired the achieve to tank designasettling-thickening • Beableto achieve to necessary monitoring and maintenance operations, of level appropriate the Know • settling-thickening a operating of disadvantages and advantages potential of overiew an Have • treatment function. tanks ofhow mechanisms settling-thickening fundamental the • Understand appropriate an are tanks settling-thickening contexts what in Understand • Learning Objectives solids-liquid separation in settling-thickening tanks. insettling-thickening solids-liquid separation tank. technology. Settling-Thickening Tanks Pierre-Henri Dodane andMagalie Bassan Pierre-Henri Chapter 6 123

Technology Technology Figure 6.1 Schematic ofthezones in asettling-thickening tank. operational settling-thickening tanks for FS, the information in this chapter is based on theoretical theoretical on based 100 are is Westin there Kumasi,Ghana, In m experiences Africa. knowledge, operating on and chapter this in information the FS, for actual tanks of lack settling-thickening a operational to Due digestion. anaerobic in result times retention longer more is and sludge remove, diffi the to accumulate, cult can sludge more that are tanks and ponds between differences main recommendations, The treatment. design wastewater in ponds anaerobic to similar are which ponds, settling scale mechanisms, larger have to fundamental the of operational overview conditions and performances of settling-thickening tanks for an FS treatment. It is presents also possible chapter This tank. the from sludge settled the remove to devices mechanical clarifiinclude plants typically treatment ers pumps if the sludge is not too thick to pump, or strong vacuum trucks. On the other hand, in wastewater In most existing implementations in low-income sludge the countries, removal is done backhoes, with time. isonly loaded50%ofthe tank periods loading with operated usually are each in volume.parallel, When operated tank from depending one on weekthe ranging to one month, Tanks removed. are sludge thickened and scum the and is drained supernatant when the desludging period, the or during tank, bottom of at the the to thicken should not ease compaction,be of and loaded operations maintenance, tanks sludge during if is the left and compaction sludge increased For removal. sludge and maintenance loading, of cycle operational When using there should settling-thickening tanks be at least two parallel streams to allow for an entire beds. for drying (footprint) area treatment required the bedsreduces drying with dewatering to prior tanks settling-thickening in separation solids-liquid achieving example, For steps. treatment in consideration urban locations where space is limited, as it can reduce required the area of subsequent in contexts. other to shown readily adaptable are chapter inthis 6.2.Thedesignguidelinespresented as Figure (FSTP), plant treatment FS a at beds drying to prior employed are that tanks thickening 124 FS inlet(afterscreening) Thickened layer Separation layer Supernatant layer Scum layer Thickened sludgeextraction(pump) Supernatant outlet 3 settling-

force that retain the particle in the top layer of the tank. tank. layer top ofthe inthe particle the retain force that velocity the defi by is ned It 6.1. Equation by by attained a overcomesstrength particle settling givenbuoyancy the in as and tank the gravitational the drag is particle a of velocity settling theoretical The that is reality FSinspecifi ofthe characteristics the c conditions. tanks, the on based settling-thickening design the for used and of determined are values design empirical tank, settling a designing the when this understanding Although in tanks. the important settling-thickening on is design Based to theory used densities. be lower theoretically could with respective velocities) their particles (and settling FS in particles than of shapes and faster types of distribution settle the settling of density fundamentals greater a with Particles tank. bottom ofthe accumulation ofsolidsatthe an and supernatant, the in concentration solids the of reduction a in result processes combined These suspended solids is where the concentration higher. bottomaccumulates at tank, sludge of the that the layerof the above occurs settling Hindered fl and Discrete tank. the in rapidly happen settling occulant 1977). (Ramalho, ofparticles highconcentration the settling dueto where isreduced hindered, • fl and occurs; settling aggregation dueto accelerated occulant, where • settle independently other; ofeach particles where discrete, • occurare: Thetypes ofsettling that sedimentation. gravitational through tank bottom ofthe the in out settle water than heavier are that particles (SS) solid suspended the tanks settling-thickening In 6.2.1 Settling and fl following section. isgivenmixing mechanisms inthe ofthese overview Abrief through ofparticles. otation separation solids-liquid the hinder can bubbles resulting the and production, gas in results digestion anaerobic as tanks, settling-thickening of goal treatment a not is this although tanks, and thickening, settling, the in occurs also mechanisms: digestion Anaerobic 3. Chapter in fundamental detail more in described flalso main are which otation, three on rely tanks Settling-thickening FUNDAMENTAL MECHANISMS 6.2 Figure 6.2 Left: the twin settling-thickening Left: ofRufi thetwin tanks sludge squefaecal management plant, inDakar, Senegal. 6.2 Figure (photo: SANDEC). for four times as long, itisnotpossible to remove out itwithiscarried apump,andmanualextraction loading,has (4weeks of8weeks anoperational cycle desludging). 4weeks thesludge As isthickened transferred bedswith apump.Right: to thesettling-thickening drying pondinAchimota, Accra, Ghana, (oneweek weeks The operatingtwo lasts loading, cycle oneweek desludging), andsludge is 125

Technology Technology C (m) diameter d=particle

anaerobic digestion. This layer that accumulates at the top of the tank is referred to as the scum layer. scum the as to referred is tank the of top the at accumulates that layer This digestion. anaerobic surface of the tank by fl otation. Some particles are also raised to the surface by gas bubbles resulting from such particles as oils fats, and and a greases, with particles lower Hydrophobic density are water to than raised top the particle. the on force gravitational the than greater is fl buoyancy that the oat, particles fl the to of density For uid. the due from force upward fl explains strength the is differences Buoyancy density otation. gravitational infl of the uence mechanisms, thickening and settling the to Similarly 6.2.3 Flotation for andschedule removal. sludge andmaintenance, ongoing operations the designing in important also is This settling. it for available accumulates, tank the sludge of depth as the reduces because effectively tanks, of design the in consideration important and an settles is it as accumulates storage sludge for tank the in room Allowing particles. of movement individual the hinder strengths inter-particle and high is SS of concentration the when gravity, of This result a as happens solids. total the of concentration the increasing effectively out, squeezed is water down and pressing them, on particles other of of weight process the to the due through compressed compressedare particles further settled The are thickening. tank the of bottom the at accumulate that Particles 6.2.2 Thickening Schematic ofthefi velocity(Vnal settling Figure 6.3 V Where: Equation 6.1: 126 g = gravitational acceleration (m/s acceleration g=gravitational isdiscussedinSection6.3.2. actual designpurposes to to settle out, and will effl remain suspended in the uent 6.3). (as Howshown V in Figure V with outlet.Particles the of level belowsettlethe out to havetime adequate V has to be designed length with to be particles long the enough to ensure is that lengthwise, tank the fl the As sludge. settled the in removal particle in owdesired of percentage the on based designed be to V than greater velocity hydraulic a with particles the the flall of tank, the in ow shortcutting no is there and turbulent) not (i.e. laminar flis the ow if Theoretically, V velocity, settling critical The ρ =fluid density(g/L) ρ s d c = particle density(g/L) =particle =fi (m/h) particle nal settling velocity ofthe = drag coeffi =drag cient FS inlet V c = [] 3 4g . c i slce bsd n h aon o sld ta ae o e removed. be to are that solids of amount the on based selected is , .. (ρ C s d - 2 . ρ) ) ρ L

d 1/2 V c c0 ) needed for a particle to settle in a tank of length L. inatank oflength ) neededfor to aparticle settle Supernatant outlet V c c1 will be removed. This allows the tank tank the allows This removed. be will c

• A layer of scum at the top of the tank. This consists of the fl the matter,of consists This non-organic and tank. organic oating the of top the at scum layerof A • liquid the of consists This layer. scum the and layer separation the between layer supernatant A • A separation layer between thickened layerthe as transition between the and supernatant, the these • at than bottom the at higher is concentration solid The bottom. the at sludge thickened of layer A • 6.1 al. , 1998; (Heinsset inFigure Metcalf and Eddy,illustrated 2003): The of mechanisms interactions fundamental these result in separation of the FS the into four layers, as Solids-liquid zones 6.2.5 sludge fresh and al.,1998; (Heinsset Vonwiller, stabilised tanks for insettling-thickening treatment 2007). appropriate more are of mixture a is that FS and/or tanks) septic from (e.g. FS stabilised Thus, water. bound more contains also stabilised not is that FS and bubbles, upfl gas increased from an ow to contributes FS fresh of digestion anaerobic because is This well. settle not does emptiedfrequently) are that toilets public from (e.g. stabilised not is that FS fresh that shown has experience Operational gasses. The generates and matter organic the of part a layer.degrades process This tank. the inside time retention thickened the the on and temperature, in the FS, of stabilisation initial mainly the of degree the tanks, on depends digestion of level settling-thickening in occurs also digestion Anaerobic Anaerobic digestion 6.2.4 vacuum trucks, sludge should be sampled from the trucks as this is what will be discharged at the the at discharged be will what is this plant. treatment as trucks the from sampled with be companies should transport sludge and trucks, collection vacuum of network existing an is there if example, For treated. be (Strauss concentration SS and accumulation scum for potential ability, thickening ability, settling especially of treated, terms be in to is that FS the on conducted be should analyses laboratory Preliminary tank. specififor the for designofthe adequate isbeingdesignedfor bedetermined needs to tank the c FSthat FS that quantities can and also seasonally.characteristics vary An empirical estimation of settling ability the local and infrastructure existing management system. The design loading needs to take into account for infl value accurate an determining 2, Chapter in on depending challenging be can FS of loading uent tank the determine to order in and volume the and separation, required surface sludge thickened of scum, layers.the supernatant, As discussed is characteristics specifi FS site c of understanding good A infl sludge characteristics tests andfaecal Laboratory 6.3.1 uencing thedesign designexample. casestudies and the providedexamples are inthe and below, discussed are aspects design These separation. solids-liquid the optimise to considered be design needs to include regular and efficient removal of scum the and sludge, thickened which needs to adequate An layers. fl sludge thickened supernatant and scum resulting of production the ow,and and FS, of volume estimated the on based is design tank The knowledge. available current the on based FS This section provides recommendations for design the of for settling-thickening tanks treatment the of TANKS DESIGNOFSETTLING-THICKENING 6.3 Signifioverlooked. be significannotcant scum layersbe and can be can seen cant, on of the settlingsurface and the settling-thickening tanks ponds in FS 6.2. Figure with associated layer scum The tank. the of volume the reduces effectively also it as process design the in consider to important is layer scum The the fats, the oils, and in contained greases FS, as well as particles havethat been raised up by gas up-flow. donot settle outorfl that particles andthe fraction surface. the oat to layer. thickened inthe particles than supernatant is not completely in separation layerthe thickened. Particles can be more easily washed out the with sludge settled the layer, where separation the in mainly occurs settling Hindered immediate. not is layer. ofthis top the , 2000). It is important to ensure that the FS al., used 2000). the tests forwhich to et It ensure is will that is these that actuallyimportant 127

Technology Technology 128 during the fi rst 20 minutes, after which more thickening occurred and continued for 100 minutes minutes 100 for continued al.,2011).(Badji et and occurred thickening more which after minutes, fi 20 the rst during rapidly settled FS that showed Senegal Dakar, in conducted tests SVI Dodane). of experience personal (Heinss mL/g 30-80 of SVI with ability thickening and ability settling good a had FS that showedin Senegal Dakar, and separation Ghana Accra, solids-liquidin FS with good Measurements achieves tanks. plants, settling-thickening SS) (mL/g treatment 100 than wastewater less of for SVI a tanks with settling-thickening sludge wastewater of design the in experiences on Based a provide al. , 1999). (Heinsset isreduced effect litre wall asthe result representative more one than greater volumes with cones Imhoff tank. settling-thickening a in as loading not do continuous not and tests tests batch are they layer,Imhoff as thickened the The of depth below). the of estimates exact provideSVI of calcualtion the on problem example the (see solids per of sludge settled gram of volume the gives which g/L), (in concentration SS the by FS settled of volume minutes, the dividing 30-60 by calculated then After is SVI The 6.4). mL/L. in recorded is FS Figure settled the by (see occupied volume the settle to left is that sample FS the fi with is graduated lled cone a then Imhoff and determined, is FS of content solids suspended fi the SVI, rst the determine To specifitime. a of amount during ed out settle that solids ability suspended settlingof amount the the on based determine sludge of empirically to method laboratory a is (SVI) index volume sludge The Imhoffcones being usedinanalyses ofsludge volume index(photo: SANDEC). Figure 6.4

activated sludge, it is considered that ideal settling conditions are reached with SVI less than than less SVI with reached limits. adequate assess the studies neededto more are are conditions settling ideal that 100 mL/gSS (Pujol With considered tank. is settling-thickening a it in sludge, treatment for activated appropriate be to considered be would FS This =VolumeThe SVI =198/6.6 ofsettled FS/SSconcentration =30mL/g is198 60minutes settled mL/L. FSafter The volume ofthe of6.6g/L. hasaSSconcentration inBurkinaA sample Faso ofFSfrom aseptic tank Example Problem: Calculation ofsludge volume index (SVI) et al.,1990).et also needs and to origin stability For be FS, into account, taken the but et al.,1998), et the and Q C Q =meandaily influent fl ow Where: S = surface of the tank (m tank ofthe S =surface Where: Equation 6.2: The upflThe (V velocity ow h = number of operating hours of treatment the plant (influent is only received operating hours) during upss mtos n cluain t ln SI n V and SVI link to calculations and methods purposes, Q xeine i atvtd lde ramn (Pujol V FS. for exist yet not does knowledge treatment sludge activated in experiences upfl ow velocity is selected to be equal to the fi nal settling velocity of the lightest particles that will V 6.3, Figure shownin as example, For that tank. the in settle particles lightest the of velocity fisettling the nal to equal be to selected is upflvelocity ow the then and removal, solids suspended of percentage desired the for selected is value A supernatant. settling-thickening tank can be calculated as shown in Equation 2, based on the upfl (V the velocity on ow based 2, Equation in shown as calculated be can tank settling-thickening area is used,haveparticles surface and that entire enough tank time the to area settle.of the The surface that ensure to distribution, suffihydraulic be adequate have to and needs cient tank the of length The Tank 6.3.2 andlength surface • separation zone: 0.5 m. zone: separation • zone: 0.5m;and supernatant • weeks 4 (with m 0.8 to cleaning) and compaction week 1 loading, week 1 (with m 0.4 zone: scum • similarcharacteristics: for FSwith for recommended designingtanks are following values (Heinss Dakar and Accra in tanks settling-thickening fiof on Based observations eld ifunderestimated. supernatant the out with washed solids in result will which sludge, thickened and scum of accumulation the to due occur will that depth in reduction the for plan to necessary is 6.1.It Figure in described layers four the of depth the considering calculated, be can volume the determined, been has tank the of area surface the Once Tank6.3.3 volume eoiy o ete u. atce wt a etig eoiy lwr hn V than slower velocity settling a with Particles out. settle to velocity (Ramalho, period” detention theoretical infl acceptable the the 1977). calculate defi to used is It the with allowforwill particles owthat during settlingned tank the of depth effective the to equal exactly and the infland the uent fl ow (Q V and busiest morning, was calculated to the emptying be during 1.6 periods times average. higher the than their have trucks 11:00because be to observed was period peak the Dakar in example, For FSTP. the at The peak coefficient is calculated byof when volumes observation greatest the are of discharging trucks upfl of removal a to of the givensuspended owcorresponds solids. percentage velocity in surface a tank the longer the tank needs to be, and the more particles that willsettle out. that particles more be,andthe needsto tank the longer the (Heinss fi example selected the lower The 1:5. 1:10 velocity,between For to settling nal ratio length to selected. width a recommend be to needs ratio width length: area the surface calculated, the been Once has Dodane). Pierre-Henri experience, (personal 100 than less SVI a have that FS V p u u p p =peakcoefficient can be estimated based on SVI values. Despite the limits of the theoretical calculation for design for calculation theoretical the of limits the Despite values. SVI on based estimated be can =upflow velocity (m/h) =influent peakfl ow (m = Q loading and 4 weeks compactionloading and4weeks andcleaning); .

C p /h, S = u ) is defi ned as “the settling velocity of a particle that settles through a distance distance a through settles that defiparticle is a ) of velocity settling “the as ned p V Q ) (Metcalf andEddy, 2003). 2 ) u 3 p /h) u = 0.5 m/h could be used for rectangular settling tanks treating treating tanks settling rectangular for used be could m/h 0.5 = , 1990). However, this type of empirical empirical of type this However, 1990). al., et u u =V have been developed based on long-term long-term on based developed been have c0 >V c1 . Thus, for a given FS inflFS given a for Thus, . the uent, u will be washed out with the the with out washed be will , 1998), the the 1998), al., et 1998) al., et 129 u )

Technology Technology load and thickening duration. load andthickening and increase in costs.construction Table 6.1 presents examples of SS givenconcentrations initial the FS being able to settle. An underestimation will lead to design the of an large unnecessarily storage volume without out washed effibe maysettling solids reduced as a ciency, to and insuffivolume storage cient Q =meanFSdaily inlet flow (m Equation 6.3: thickened sludge removal need to be defi ned in the fi rst place. The volume of the thickened sludge sludge zone(V storage thickened the of volume The fiplace. and the defi rst scum in be ned to for need methods removal sludge and thickened removal) sludge and compaction loading, (i.e. duration cycle operating expected avoidthe outfl Therefore, to cycle. operating crucial one is sludge during sludge settled of ow al . Concentration ofsludge inthethickening inAccra andDakar tanks (Heinss et zone al. ofsettling , 1998; Table 6.1 (C sludge thickened the of concentration inflload expected the the given and calculated ow be to needs zone sludge thickened the of depth The 130 C FSloadfor onecycle indays ofthe =duration N settling effie =expected as%) ofsuspendedsolidsseparated, ciency (=proportion Care must be taken to ensure a relatively accurate estimate of C of estimate accurate relatively a ensure to taken be must Care maximumeffi the estimate use80%to to recommended ciency. upfl gas and maintenance (Heinss and ow operation poor to due observed, been have increased 60% about effiin only settling fiof result FS ciencies the average removal, In eld, compaction. sludge and to thickening prior loaded not are tanks the when times storage longer but volume zone of thickening is the based on expected settling the of FS. It is not considered in design, the and surface length design to ensure settling is achieved under all the expected operating conditions. The The mean daily flow is used for sludge the accumulation estimate, but peak the flow is used for tank the V Where: C t i t Accra ,FSTP Dakar, FSTP Accra ,FSTP Accra ,FSTP Accra ,laboratory = suspended solids mean concentration ofFSload(g/L) =suspendedsolidsmeanconcentration = suspended solids mean concentration of thickened sludge after the loading period (g/L) loadingperiod the after sludge ofthickened =suspendedsolidsmeanconcentration = volume of thickened sludge storage zone(m storage sludge =volume ofthickened Place ofmeasurement Badji et al.,2011) t ) can be calculated asshown inEquation) canbecalculated 6.3(Metcalf andEddy, 2003). V t = Q .. C i C

Concentration atinlet e t N . 3 /day). (g SS/L) 15-20 15-20 15-20 40 5 t ). The design of a suffia of design The ). thickened the forvolume storage cient 3

) Thickening duration (day) 10 50 30 9 7 t . An overestimation will lead to an to lead will overestimation An . 1998). However, it is it 1998).However, al., et Concentration in thickened zone (g SS/L) 60-70 60-85 >100 140 100 included to allow the sampling of sludge in these zones, and to ensure that easy repair of pumps is pumps of repair easy that possible. ensure to and zones, these in sludge of sampling the allow to be included also should points access Easy sludge. concentrated remove to the adapted of be extraction must the sludge for thickened pumps The settling. solids the improve to tanks of end deep the near zone the of (Heinss energy tanks. the the in turbulence the disperse infl reduce to help and ow, to recommended baffl are Therefore, es short-circuiting. avoid and fltank whole the of the in distribution ow quiescent and uniform the for allow should zone inlet The Technologies. ofTreatment inChapter 5,Overview further explained maintenance (e.g. removal of to coarse to waste facilitate avoid of pumps).potential degradation This is order in tanks settling-thickening the into FS of loading the before undertaken be must screening Grit confi andoutlet Inlet 6.3.4 guration sludge iseasilysludge removed by pumping, asitisnot heavily compacted 6.1). (CaseStudy solid total thickened the expected and depth, minimum a at loading maintained layeris scum the short the that are of compactionperiods and advantages characteristics, The variations. FS seasonal the the and sludge, given defithickened the ned be of concentration to needs period loading FS The the scum, which have a high solids concentration. Large settling-thickening ponds can therefore be diffimore therefore can operate. cult to ponds settling-thickening Large concentration. solids high a have which scum, the and sludge thickened the both remove to used been havediffi more loaders Front-end remove. to cult sludge the before weeks 3-4 scum layercase, the In removal. overthis is sludge deeper, is thickened more compact and the and therefore loaded not are they where phase compaction additional an with weeks, 4 over operated was phase loading The ponds. settling-thickening as to referred also are tank of types these size, the to Due FS. of quantities greater of storage allow to volume larger a have tanks the where Accra, in tanks settling-thickening the for adopted were weeks 4 of periods loading Long of number the served, systems. typeof onsite andthe systems, frequency area ofcollectionfrom onsite households, the including assessed, be should activities designing transport before and required Collection is tanks. treated be to concentrations and volumes the assess to study liminary pre- a 2, Chapter in discussed as Thus h. 1.7 tanks of HRT settling-thickening effective an the with operated treat, and overloaded were to volumes FS the of underestimation initial to Due hours. of 8.6 HRT nominal a with designed were plant treatment Cambérène the of tanks Settling-thickening regular the ensure to effi tank’s ciency andsustainability.the crucial ensure to thus products solid compacted is most the It remove to means available. mechanical of always availability not are which trucks, vacuum powerful requires scum of removal The trucks. vacuum with removed is scum and sludge compacted most the and pumps, by removed mostly is sludge thickened the where Dakar, in plants treatment FS the for adopted were week one about of periods loading 1980s. late Short the since (Ghana), region Accra the of Settling-thickening tanks different sizes have been in operation in Dakar (Senegal) since 2006 and in al.,1998; al. , 2011) from(Adapted Heinsset Badjiet Case Study inDakar, 6.1:Operation ofsettling-thickening tanks andAccra, Senegal Ghana , 1998) recommend locating the inlet the locating recommend 1998) al., et 131

Technology Technology actual on based conditions. operating times removal sludge and compaction optimise to done be can monitoring detailed Sludge removal typically lasts a few hours to a day following the compaction Once period. in operation, altered. appropriately be should operations and expected, than higher is load solid the that means this for,designed was what than accumulated has sludge volumehigher a thickened of that observed is it If Thisshouldbeavoided. surface. onthe weeds seengrowing are removal sludge inadequate from resulting reduction volume scum case, In practices. layerthe this not was as such removeda a that consequence,long during period the of example an shows 6.5 Figure solids-liquid areduced separation. leadingto particles, of settling the for depth adequate is there properly,that functioning and are tanks settling-thickening The timing of removal the of sludge and scum as planned for in design the is essential to ensure the that Sludge andscum removal6.4.1 andscum. sludge thickened inthe increases SSconcentration andthe occurs, lack the of hydraulic disturbance (Heinss to due scum, and sludge thickened of removal the to prior occurs compaction additional loaded, not are tanks the While operations. thickening and solids-liquidseparation expected allow for the periods These cycle. operating an of phases main the comprise scum and sludge thickened the of removal and compaction the and FS, of loading The time. this during loaded be not should tanks as removal sludge for allow to order in parallel, in alternately operated be should tanks settling-thickening two least At OPERATION TANKS OFSETTLING-THICKENING ANDMAINTENANCE 6.4 layer. thickening settled solidsfrom the ofthe carry-over the reduce tank side ofthe 1998). It an must be at the opposite side of ensure the inlet zone. Outlets that are Topositioned near to the shallower supernatant. (Heinss wall the the of width with the along extended scum be flcan hydraulic channel optimal outlet the the ow, of washout avoid to useful Baffl are layer. es storage sludge thickened the above and layer scum the under located be should zone outlet supernatant The 132 Example ofinadequate operation andmaintenance ofasettling-thickening tank inWest The Africa. Figure 6.5 (photo: SANDEC). and scum accumulated does notallow for proper orfor operation ofthetank, solids-liquid separation scum was not removed during a long period, which allowed toplants grow on it. The volume of sludge et al., 1998).et solids-liquid time this separation further During et al., et hog eaoain ol ices te ois otn o te cm Hg tmeaue my also may temperatures High scum. the of scumlayer. heightofthe content the anaerobic digestion process, andtherefore the increase solids the increase could evaporation through water of loss example, infl For may effitanks. characteristics the FS the uence and of conditions ciency and optimise the compaction time and sludge zones removal frequency. different Seasonal of variations meteorological the of depths the assess for time, load requirement the adjust to special useful however is no It periods. is start-up there processes, physical on mainly rely tanks settling-thickening As Start-up periodandseasonal variations 6.4.2 maintenance (Chapter 11). consideration into costs betaken needto and operating increased fl but management exibility, the increase and operation easier thickened sludge out of and the tanks, remove the scum over zone. the supernatant These devices allow the pump and scrape continuously that devices with tanks settling-thickening design to possible is It design,andalsoby visualobservation. loadingsandtimesgiven inthe basedonthe removal sludge for time is it when knows operator The design. the into integrated be to needs cleaning) pump maintained. As are tanks frequently over 2 m deep, adequate access for sludge removal (and for tank and is pressure a as sludge thickened of pumping the facilitate can layer supernatant the as removed, be to removed. When a pump is used for extracting the thickened sludge, been layerthe supernatant has does not supernatant need the after tank chain. the of out treatment shoveled or pumped the be then in can sludge step thickened The next the to or tank settling-thickening parallel the to pumped be can It layer Next, supernatant the is frequently removed by pumping or by (depending gravity on design). the plant. treatment Cambérène asitisdoneatthe emptying tank the after or suckedtanker by avacuum manually removed be also can Scum pumps.strong with or trucks vacuum access, as such means for mechanical by enough narrow is tank the when tank the of sides both from shovels with thickened removed be the can scum after possible, If removed. remain manually be to can needs it case and which in pumped 6.6), (Figure easily removed is sludge be cannot that concentration solids high a has The fi rst step in sludge and scum removal is typically removal of the scum layer. The scum layer generally Settling-thickening tank ofRufisque showing thescum layer (photo: SANDEC). Figure 6.6

133

Technology Technology 134 day and 700 kg TS/day, plant the received 340 m is composed of a combination of settling-thickening tanks (two tanks of 155m of tanks (two tanks settling-thickening of combination a of composed is It 2006. in operation in put city, was Dakar fi serving the FSTP,scale at Cambérène plant treatment rst al.,2011; from(Adapted Badjiet 7.2) continuedinCaseStudy beds andsludge drying tanks Case Study 6.2:Cambérène FST –settling iue67 Effi andflciency uxes analysissludge ofCambérène faecal treatment plantinreal operational Figure 6.7 . (2011), as summarised in Figure 6.7. Although the FSTP was designed for treating 100 treating for designed m was 6.7. (2011),FSTP al. et Figure in the Although summarised as dry (concentration, characteristics sludge of beds byconducted, and was and drying reported Badji matter tanks content) settling-thickening in the monitoring Continuous stages. treatment two the of From 2007 to 2009, daily measurements of the flpollutant uxes were conducted at the inlet and outlets beds. for area drying required the andinorderreduce to before sludge drying dilute the thicken to selected was beds drying and tanks settling/thickening of combination The tanks.. septic from is sludge of majority the and Dakar in table groundwater high a is There g/L. 5 of TS average an with dilute is Dakar in layer.FS scum The the from cleared and pumpedout is tank other the while FS beds drying are to treatment transferred the wastewater plant. Each week, one is tank used for receiving beds by to drying pumping. and The transferred effltanks from leachate and the the uent tank from the drying beds (10 beds of 130m of (10beds beds drying in the plant in real operational condition is presented in the fi inthe conditionispresented operational plantinreal in the below.gure screening storage storage Area of Area of wastes sludge dried conditions (Badjiet al.,2011). 1,700kg 40% 1% T Density of600kg/m S/day 300 kg Incoming sludge 40% for 50%dryness P 7 unplanted (2 x155m drying beds rimary tanks T 2 S m each). The thickened sludge is extracted from the settling-thickening settling-thickening the from extracted is sludge thickened The each). 2 3% /year 3 ) 340m 3 3 /day 3 FS/day and 1,700 kg TS/day. The analysis pollutant Legend 97% 60% W TS flow Supernatant andpercolatedleachates Sludge ater flow treatment plant W astewater 3 each) and unplanted and each) 3 of FS/ of the liquid and solid fractions. Theeffi liquid andsolidfractions. the discussedhere. are this key achieve to mechanisms ciency ofthe of separation is the tanks of settling-thickening performance in the consideration The most important TANKS OFSETTLING-THICKENING PERFORMANCE 6.5 amounts that are partitioned in the solids will be correlated to SS removal effi sludge. thickened inthe removal (Heinss eggswere partitioned Helminth total ciency. 50%ofthe SS 1998) that observed to correlated the be and will solids out, the settle in eggs partitioned are Helminth that as amounts such pathogens larger Many negligible. also is inactivation or (Heinss, supernatant the in COD 25% infland 10% BOD uent inflof 50% include (Badji supernatant the settledin 50% and sludge, the in COD uent or stabilisation not Examples supernatant. will and remain in suspended the particles matter, Dissolvednutrients, organic separation, solids-liquid is tanks pathogen reduction. treatment settling-thickening Further steps are required for both the thickened of solids and supernatant. objective main The Treatment performance 6.5.2 (C sludge thickened the of concentration The al.,1999). (Heinsset operated the in SS settled volume. This effiof 50-60% ciency about can reach up to is 80% have where tanks the been adequately ponds designed and and tanks operating effisettlingof mean ciency fi the the eld, In Solids-liquid separation 6.5.1 of10 oneweek ofloading. had adepth 20cmafter to layerscum observed the FSTP Dakar the In weeks. 8 of cycles with operated tanks settling-thickening FS the (Heinss process. duration, cycle evaporation operating the the and on mainlycharacteristics depends content SS and thickness layer scum The layer.thickened (Heinss weeks, eight about operating of an cycle with Accra, In estimate. reasonable a seems period load day seven a for zone thickened the in SS/L g 60 6.1. Achieving Table in presented as ability), (thickening characteristics FS initial the the parallel tank (see Figure 6.7). (see Figure tank parallel the in overloading increased an to consequence a as and pumping, sludge thickened in delays to led This cleaned. be to needed and blocked by debris often was axle the pumpingas The attention required also were considered of tanks diffithe cult forby tank operator, the scum the in cleaning. entering particular maintenance and Operation plant. the of signifisave can operation real the for money of amounts cant diffiit the – culty despite conditions, local on based degree) fl thickening inlet and concentration, ow (e.g. deficharacteristics to sludge order ne in studies preliminary conducting of importance great the shows This expectations. designer the from far was removal achieved The design. the in envisioned sludge concentration of 60-70 g was achieved TS/L one after week of thickening while 140 g was TS/L thickened a consequence, a As HRTs. longer on ran Ghana in plant the settling-thickening only,while week one the for tank in sludge the left operator Dakar a the Moreover, with with. start g/L to 150 concentration of higher concentration a inflhad Ghana the in that uent TS time the at a known not was it However, obtain tank. settling to similar possibility the reveiled Accra from Experiences .,1998) observed a total solid content of 150 g TS/L in the the in TS/L g 150 of content solid total a observed al.,1998)et t ) achieved depends on the operating cycle duration and duration cycle operating the on depends achieved ) 19) eot su lyr f 0 m in cm 80 of layer scum a report 1998) al., et , 1998). Total pathogen removal removal pathogen Total 1998). al., et , 2011), al., et and et al., et 135

Technology Technology • the expected settling effi expected the ciency (e)is80%ofSS.; • afinal settling velocity ofV • following assumptions the results, anddesigndecisionscanbemade: preliminary Based onthese anddesignAssumptions decisions 6.7.2 studies asshown inTable 6.2. from preliminary obtained information on based is a and implemented design be specifi to can tanks corresponds c settling-thickening the which calculation in situation design typical for a of allow example This to characteristics. context needed local are the to studies according preliminary sufficient situation, real-life a In Initial situation 6.7.1 hydraulicconfi zonevolumes andthe the basin surface, gurations. involves design tanks calculating of the settling-thickening sections, previous As the mentioned in the TANK FOR ASETTLING-THICKENING DESIGNEXAMPLE 6.7 be cannot therefore tanks settling of endproducts the signifi and not cant, is removal pathogen • and inSSandorganics; highly remains concentrated liquid fraction the • dewatering; further andrequires content settled still sludge hasrelatively highwater • FS, and with and operating lack results lack of on data of empirical which experience to base designs • include: tanks ofsettling-thickening Constraints steps. treatment solids-liquid achieve rapidly separation, they are relatively robust and they resilient, and they reduce the volume of as sludge for subsequent step fi treatment a rst effi as are cient tanks Settling-thickening ADVANTAGES TANKS OFSETTLING-THICKENING ANDCONSTRAINTS 6.6 Results to studies ofpreliminary determine design parameters Table 6.2 136 Daily peakflow coeffi cient: FSTP openingtime: Settling ability (1LImhoffcones) ability Settling Concentration ofthickened sludge(1LImhoffcones) Infl uent fl ow: Total volatile solidspercentage FS origin: Initial rawFS concentration: see Chapter 10). enduse onappropriate (for details more bodiesordirectly usedinagriculture water into discharged on; c = 0.5 m/h based on SVI tests andexperience.; tests =0.5m/hbasedonSVI C C 52 weeks/year 5 days/week 7 h/day Good (SVI=23<<100) 60 gSS/L (peak flow isofteninthemorning, afterthefi rotation) rst trucks Q=140 m < 70% (stabilisedFS) Mainly septictanks C p i(SS) i(TS) =1.6 = 5gSS/L = 7gTS/L 3 /day influent peakfl ow (Q Equation 6.4:Q Equation 6.6:M=Q Equation 6.9:S(l following confiEquation 6.5),the shouldbeadopted: guration the thickening zone(M thickening the M Equation 6.7: The daily SS mass of thickened sludge (M sludge The daily SSmassofthickened Thickening zone volume The (S) surface tank needed to allow for selected the final settling velocity (V Design calculations 6.7.3 Thickening zone:0.75m(basedon47 sludge m • al.,1998); zone:0.5m(Heinsset and Separation • al.,1998); zone:0.5m(Heinsset Supernatant • for besafe a2week assumed to cycle); Scumzone:0.4 m(value • given are for zone: each The following designcharacteristics Zone depth The flow. of m distribution 64 to close the surface a facilitate reach To and 0.2. to narrow 0.1 from and ranges ratio long length be to width should recommended tank the of surface The Tank configuration (C initialFSconcentration from the (M) iscalculated The daily SSquantity ofFSdischarged Equation 6.8:V The volume of the thickening sludge storage zone (V zone storage sludge thickening the volumeof The mean). effi expected maximum the (not the ciency be should e of value the design, safe a For 80%. = e Where Equation 6.5:S=Q perday openinghours 7=numberofFSTP Where pumping sludge thickened the therefore and treatment wastewater in experience has operator the • occurs scum and sludge thickened of removal the Hence, days. 2-3 of period compaction short a • a loading of period one week (N = 5 = number of treatment plant opening days per week) to minimise • other; loadingofthe the cleaningofoneduring allow designedto the are tanks two parallel • and tank cleaning is likely to be carried outcorrectly. cleaningislikely becarried to and tank 10 isstill sludge suffievery days bythickened pumping, asthe ciently liquid; and tank; other outonthe andscumiscarried sludge ofthickened extraction whilethe weeks, anaerobic digestion and gas upflow. This means that each tank is loaded for one week out of every two p t t p =M =M =Q . ) as shown in the following equations. ) asshown inthe

L) =3 t ) and the SS concentration achieved in the thickened sludge (C sludge thickened inthe achieved SSconcentration ) andthe p /V . t

. . C .

C e = 560 kg SS/daye =560kg N /C i(SS) p c .

= 64 m =64 /7=32m 22 =66m = 700 kg SS/day= 700kg t =47 m 2 2 3 t 3 ) is then deduced from the SSsettling effi deducedfrom the ) isthen ciency (e): /h 3 storage ina66m storage t ) is related to the mass of the particles trapped in trapped particles the of mass the to related is ) 2 tank). c ) is estimated based on the t ): i ): 2 (see 137

Technology Technology options to further treat the supernatant, a more realistic settling effi ciency (e) of 60% SS should be should SS 40% of C 60% contains supernatant the of such that considered effi(e) settling ciency realistic more a supernatant, the treat for plan further to to order optionsIn removal. SS 80% an on based designed was zone thickened the example, this In Mass fl6.7.4 ow sludge analysistreatment offaecal Figure 6.8 Schematic oftank configuration described indesign example. 6.8. isshown inFigure zonedepths ofthe diagram A schematic 138 Schematic presentations ofthetreatment andmass flows for thetheoretical example. Figure 6.9 with the estimated SSfl estimated sludge. the thickened with andthe supernatant options treatment for the the to ows FS inlet(afternscreening) 0.9 m 60-80% SSremoval P 145 TSS/y 60 gSS/ 47 m T thickening tank rimary settling/ o furthertreatment 140 m 182 TSS/y 5 gSS/ Thickened layer Seperation layer Supernatant layer Scum layer Thickened sludgeextraction(pump) 3 Inlet /week 3 L /day L 0.75 m 1 m 0.4 m Legend T 130 m 73 TSS/y 2.2 gSS/ o furthertreatment i(SS) . These mass fl 6.9, shownare ows in Figure Thickened sludgeandscum Supernatant Faecal sludge 3 /day L Supernatant outlet 0.7 m 1 m EAWAG, Dübendorf, Dakar. in Cambérène plant treatment sludge faecal the of Monitoring (2007) L. Vonwiller, eds. F.L. Burton, G., Tchobanoglous, reuse. disposal, treatment, Engineering: Wastewater (2003). Eddy and Metcalf and Science Water ponds. in sludges faecal Treating (2000). A. Montangero, U., Heinss, S.A., Larmie, M., Strauss, FNDAE, R.S.(1977). Processes, Academic Press. Ramalho, Wastewater Introduction Treatment to ed. activées, boues des foisonnement le sur technique Guide (1990). G. Martin, A., Vachon, R., Pujol, separation. solids-liquid their and sludges faecal of Characteristics (1999). M. Strauss, S.A., Larmie, U., Heinss, Sludges Faecal of Treatment the for Systems Pond and (1998).Separation M. Solids Strauss, S.A., Larmie, U., Heinss, la affectant éléments vidange: de boues des Traitement (2011), D. Koné, M., Mbéguéré, P.H., Dodane K., Badji 6.8 BIBLIOGRAPHY 4. In the design of settling tanks, why is it important to calculate the basin surface, the zone the surface, basin the calculate to important it is why tanks, settling of design the In 4. settling-thickening design to order in calculated be to need that factors three the are What 3. settling- of operation the with associated disadvantages three and advantages three List 2. achieved? is settling-thickening how explain that mechanisms fundamental three are What 1. End ofChapter Study Questions volumes, and the hydraulicconfivolumes, andthe gurations? tanks? tanks. thickening work. Explain how they Switzerland. BookCompany.McGraw-Hill Technology 42(10), p.283-290. et delaForet. Ministère del’Agriculture EAWAG/SANDEC: Janeiro. No. 05/98. EAWAG/SANDEC, Report Design, Preliminary for Recommendations and Learnt Lessons – Tropics the in 30juin–1erjuillet Dakar, 2009,EAWAG/SANDEC. surlaGestion desBouesdeVidange, international des lits etde performance réelle séchage les en non mécanismes taille plantés de séchage. Actes du symposium 139

Technology Technology