Groundwater Abstraction Through Siphon Wells—Hydraulic Design and Energy Savings

Article Groundwater Abstraction through Siphon Wells—Hydraulic Design and Energy Savings

Rico Bartak and Thomas Grischek *

Faculty of Civil Engineering & Architecture, Dresden University of Applied Sciences, 01069 Dresden, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-351-4623350

Received: 16 March 2018; Accepted: 23 April 2018; Published: 27 April 2018

Abstract: Siphon pipes were used for groundwater abstraction from wells before the development of submersible pumps. Many of the existing and operational systems were built before the 1950s and require rehabilitation. Siphon wells are difficult to design and, therefore, are often equipped with submersible pumps when the system is rehabilitated or renewed. This study presents a novel calculation tool for siphon wells and investigates the energy savings of such system in comparison to an alternative equipment with submersible pumps. A theoretical energy savings of 38% was first estimated compared to individually-operated wells (IOW) for a fictional design example just based on the calculated water levels and abstraction rates. Real energy data from two riverbank filtration (RBF) sites, which operate both siphon and IOW, were investigated in the second part of the study. The analysis of measured data revealed energy savings of 36–69%, confirming the theoretical estimation.

Keywords: siphon wells; energy savings; energy efﬁciency; groundwater abstraction

1. Introduction The pumping rate for vertical groundwater wells usually does not exceed 150 m3/h. Several wells are needed for abstracting larger quantities of water (well group). Nowadays groundwater wells are commonly equipped with submersible pumps. Multiple wells from one well group discharge into a main pressure pipe. Before the development of submersible pumps, common groundwater wells were connected via suction pipes to a main gravitational pipe (siphon pipe). The groundwater is hereby abstracted and piped to a central collector caisson without a permanent energy input using gravitational flow (Figure1). Opposite to the free-flow gravity pipes, the pressure inside the siphon pipe is below atmospheric pressure (vacuum) as the water must be first lifted to the top of the siphon crest. Traditionally multiple well groups were connected via individual siphon pipes to a single collector caisson, which acted as sand collector and open air vessel. Gravitational flow is induced from the abstraction wells to the collector caisson after the siphon pipe has been air-evacuated (primed) using a vacuum pump and the water level inside the collector caisson has been lowered below the static groundwater level. The siphon system only works if (1) the outlet of the drop pipe and the inlets of the suction pipes are permanently submerged, (2) the siphon pipe is continuously air evacuated (degassing of dissolved gases or air entry through small leakages) and (3) the piping is permanently vacuum-tight. At the transition from the siphon to the drop pipe a so called “siphon head” is installed. It collects escaping gas, so that it can be automatically extracted by a vacuum system. The maximum suction head is found at the “siphon head”. It is equal to the suction lift from the lowest pumping level to the inside “siphon head” and should not exceed the vapor pressure of water, which would cause a failure of the gravitational flow. In practice this suction head is limited to 8 m, including

Water 2018, 10, 570; doi:10.3390/w10050570 www.mdpi.com/journal/water Water 2018, 10, 570 2 of 10 Water 2018, 10, x FOR PEER REVIEW 2 of 10 gravitationala safety margin. flow. The In groundwaterpractice this suction is pumped head from is limited the collector to 8 m caisson, including to the a safety waterworks margin. using The groundwaterhigh-ﬂow pumps. is pumped from the collector caisson to the waterworks using high-flow pumps.

Figure 1. Schematic of five siphon wells and one collector caisson. Figure 1. Schematic of ﬁve siphon wells and one collector caisson.

The first siphon wells were built in Europe in the mid- to late-19th century preferably in PleistoceneThe first deposits siphon wellsalong were major built river ins Europe. Numerous in the mid-large toscale late-19th siphon century systems preferably were constructed in Pleistocene in Germany,deposits along Poland, major Czech rivers. Republic Numerous, and large Hungary scale siphon with systemscapacities were ranging constructed from 35,000 in Germany, m3/d 3 (DresdenPoland, Czech-Tolkewitz, Republic, Germany) and Hungary to more with than capacities 100,000 ranging m3/d from (Poznan 35,000-Debina m /d, (Dresden-Tolkewitz, Poland). In recent 3 decadesGermany) the to well more-known than 100,000 siphon m technology/d (Poznan-Debina, has been replaced Poland). by In the recent use decades of submersible the well-known pumps. Thissiphon was technology preceded has in been Germany replaced by by an the increasing use of submersible water demand pumps. in This the was mid preceded-20th century. in Germany The requiredby an increasing pumping water rates demand and the in increased the mid-20th screen century. entry Thelosses required caused pumping greater drawdowns rates and the inside increased the collectorscreen entry caisson losses and caused the increased greater drawdownswater demand inside reduced the collector the static caisson water andlevels the in increased the wells. water As a result,demand some reduced well thegroups static could water no levels longer in be the operated wells. As through a result, siphon some well pipes groups as their could abstraction no longer was be limitedoperated by through the drawdown siphon pipes of the as collector their abstraction caisson wasand limitednot by the by thewell drawdown screens. Th ofese the wells collector have caisson been equippedand not by with the well submersible screens. These pumps wells, because have been those equipped were able with to submersibleachieve the necessarypumps, because drawdown those wereand a able higher to achieve yield. Another the necessary reason drawdown for a decreasing and a higher use of yield. siphon Another wells reasonhas been for the a decreasing lack of precise use of calculationsiphon wells tools has beento design the lack and of preciseverify thecalculation productivity tools to of design such and systems, verify which the productivity were historically of such designedsystems, which and built were by historically expertise designed and experience and built [1] by. expertise The computational and experience difficulties [1]. The lay computational, similar to horizontaldifficulties lateral lay, similar screens to, horizontalin the interconnection lateral screens, between in the well interconnection yield, well interference between well, and yield, frictional well pipeinterference, losses. Thus, and frictionalan Excel pipespreadsheet losses. Thus,called an“SIPHON” Excel spreadsheet [2] has been called recently “SIPHON” developed [2] has for been the hydraulicrecently developed calculation for and the design hydraulic of siphon calculation wells. and design of siphon wells. As ofof today,today, many many of of the the old old siphon siphon well well systems systems in Europe in Europe must must be rehabilitated be rehabilitated or rebuilt. or re Duebuilt to. Duethe difficulties to the difficulties in calculating in calculating siphon well siphon systems, well the systems, alternative the alternative equipment equipment of wells with of submersible wells with submersiblepumps is often pumps preferred. is often However, preferred. siphon However, wells offer siphon significant wells advantages offer significant over individually advantages operated over wellsindividually (IOW): operated high operational wells (IOW) safety: high in floodplains operational (no safety electricity in floodplains supply), lower (no electric maintenanceity supply), costs lowerdue to maintenance a reduced number costs due of pumps, to a reduced easier number accessibility of pumps, and maintenance easier accessibility of the dry and mounted maintenance pumps, of theand dry absence mounted of harmful pumps, pressure and absence surges of (air harmful vessel). pressure Their most surges important (air vessel). advantage Their most is that import they canant advantagecontribute tois that long-term they can reductions contribute in energyto long consumption-term reductions and in should, energy therefore, consumption still be and taken should into, thereforeconsideration, still under be taken certain into circumstances, consideration especially under because certain thecircumstances operation of, esiphonspecially wells because appears the to operationbe feasible of again siphon since wells the water appears demand to be perfeasible capita again has gone sinc downe the water in many demand countries per in capita Europe, has so gone that downthe old in siphon many systemscountries will in Europe, not have so to that be equipped the old siphon with submersible systems will pumps not have after to wellbe equipped rehabilitation. with submersibleIn fact, recent pumps examples after of well siphon rehabilitation. well rehabilitation In fact, in recent Germany, example e.g.,s inof Hamburgsiphon well [3], rehabilitation Leipzig [4], and in GermanStuttgarty [, 5e.g.], prove, in Hamburg that the “old-fashioned” [3], Leipzig [4], siphonand Stuttgart systems [5] are, pro worthwhileve that the to “old be considered.-fashioned” siphon systemsThe are crucial worthwhile reason why to be siphon consider wellsed. work more economically than a series of IOW is that these systemsThe require crucial less reason electrical why energysiphon forwells groundwater work more abstraction. economical Insteadly than of a pumpingseries of IOW from is every that singlethese well,systems only require two to less three electrical high-performance energy for groundwater pumps are usually abstraction. installed Instead to abstract of pumping groundwater from every from singlethe collector well, only caisson. two to Typically, three high dry-mounted-performance centrifugal pumps are pumps usually are installed preferred to because abstract they groundwater are easier from the collector caisson. Typically, dry-mounted centrifugal pumps are preferred because they are easier to maintain and have a higher energy efficiency factor compared to submersible pumps.

Water 2018, 10, 570 3 of 10 to maintain and have a higher energy efficiency factor compared to submersible pumps. Those suffer from their compact design and multiple energy losses due to cables and transformers. The average total efficiency (motor and pump) of submersible pumps investigated in Germany was in the range of only 41% (n = 2567, [6]) to 48% (n = 360, [7]). A study by the Berlin waterworks reported a mean value of 44% (n = 650) for submersible pumps compared to 72% (n = 44) for dry-mounted centrifugal pumps [8]. Siphon wells are still operated at more than 50 sites in Germany and at several sites in Poland, Hungary, and the Czech Republic. Most of them were constructed before the 1950s and must be rehabilitated in the near future. Planners often have difficulties to mathematically or numerically prove the capacity of such systems during the design stage, so the siphon wells are, nowadays, often equipped with submersible pumps. A state of the art investigation of the potential energy savings of siphon wells in comparison to IOW, which could justify higher investment costs for the rehabilitation of the “old-fashioned” systems, has not yet been published to assist planers and engineers in their decision. This study, therefore, presents (1) an example hydraulic calculation of a fictional lake bank filtration site using a novel hydraulic calculation tool “SIPHON”, which can be used for the design and rehabilitation of siphon well systems and (2) investigates the energy savings of siphon wells per cubic meter of abstracted water based on real data from two riverbank filtration (RBF) sites operating both siphon wells and individually-operated vertical or small horizontal wells.

2. Materials and Methods

2.1. Hydraulic Calculation of Siphon Wells The calculation of inclining siphon pipes is analogous to pressure pipes except that the absolute pressure in siphon pipes is below the atmospheric pressure and the vapor pressure of water should never be exceeded when determining the most economic elevation of the siphon pipe (Figure1). The first algorithm for the calculation of siphon wells was developed by Paavel (1948) [9] but not useful for system design at that time because of its computational requirements. The MS Excel tool SIPHON [2] is based on Paavel’s algorithm and assumes that (1) all wells influence each other and create a common cone of depression for the steady state, and (2) the radius of influence of a well group, which is not subject to a surface water boundary, cannot be smaller than the distance between the two outermost wells [10]. SIPHON calculates individual well heads and abstraction rates based on a pre-defined pumping water level in the collector caisson. The gravitational flow inside the suction and siphon pipes is controlled by the available driving head. The well water level HBi (m above the bottom of the aquifer) of the well i of a well group with n (-) wells can be calculated using Equation (1) [1] using a counting direction from outside towards the caisson:

n 2 2 HBi = HS + VBLi·Qi + ∑ VSLj·QSLj (1) j=i with: j QSLj = ∑ QK ; i = 1 . . . n (2) k=1 where HS (m above the bottom of the aquifer) is the pre-defined pumping water level in the collector 2 5 3 caisson, VBLi (s /m ) is the flow dependent friction loss coefficient of the suction pipe, Qi (m /s) is the 2 5 abstraction rate of the well i, VSLj (s /m ) is the flow dependent friction loss coefficient of siphon pipe 3 3 segment j, QSLj (m /s) is the flow rate inside siphon pipe segment j; and Qk (m /s) is the abstraction rate of the well k. The segment j extends from the joint of the well j to before the joint of well j + 1. The flow-dependent friction loss coefficients are calculated as [1]:

L f · + ∑ ζ d d VBLi or VSLj = 8 (3) π2·d4·g Water 2018,, 10,, 570x FOR PEER REVIEW 4 of 10 where fd (-) denotes the Darcy-Weisbach friction factor; L (m) is the length of suction pipe well i or wherethe length fd (-) of denotes segment the j; Darcy-Weisbachd (m) is the inner friction pipe diameter factor; L; Σζ (m) (- is) the the sum length of oflocal suction losses pipe; and well g (m/s i or2 the) is length of segment j; d (m) is the inner pipe diameter; Σζ (-) the sum of local losses; and g (m/s2) is the the gravitational acceleration. VSLj=n includes frictional losses of the drop pipe and the “siphon gravitational acceleration. VSL includes frictional losses of the drop pipe and the “siphon head”. head”. In order to calculate HBj=ni (m), a generalized form of the Dupuit-Thiem equation using the InGIRINSKIJ order to calculatepotential HBallowi (m),s the a generalizedsuperposition form of multiple of the Dupuit-Thiem wells including equation image using (hypothetical) the GIRINSKIJ wells potentialto represent allows boundaries the superposition for unconfined of multiple and confined wells including aquifers image[11]: (hypothetical) wells to represent boundaries for unconfined and confined aquifers [11]: n 1 ∅ = ∅0 − 1 ∑n Qi ∙ φ(ri) (4) = −4πK Q ·ϕ(r ) (4) ∅ ∅0 π i=∑1 i i 4 K i=1 where Ø 0 (m2) is the GIRINSKIJ potential of the static water level; Ø (m2) the GIRINSKIJ potential of 2 2 wherethe pumping Ø0 (m water) is the level GIRINSKIJ inside a potential well; K of(m/s) the is static the hydraulic water level; conductivity Ø (m ) the GIRINSKIJof the aquifer potential; ri (m) ofis the pumpingdistance between water level the insidewell axis a well; of well K (m/s) i and is the the bore hydraulic hole annulus conductivity of the of current the aquifer;ly considered ri (m) is thewell distance; and ϕ (r betweeni) is the thespecific well potential axis of well as isum and of the drawdown bore hole annuluspotentials of caused the currently image considered wells plus well;well andentryφ losses(ri) is. theSubstituting specific potential Equation as ( sum4) into of Equation drawdown (1) potentials results in causeda system image of n non wells-linear plus wellequations entry losses.with n Substitutingunknown well Equation abstraction (4) into rates Equation Qi. The (1)system results is insolved a system in Excel of n using non-linear the SOLVER equations function with n unknownand the integrated well abstraction GRG2-code rates for Qi .non The-linear system systems. is solved The in solution Excel using algorithm the SOLVER requires function a pre-defined and the integratedpumping water GRG2-code level and for a non-linear first rough systems. estimate The for solution the well algorithm abstraction requires rate. The a pre-defined tool was test pumpinged with waterfour benchmarks level and a and first real rough data estimate from a site for using the well 43 wells abstraction connected rate. to The three tool individual was tested siphon with pipes four benchmarks[2]. and real data from a site using 43 wells connected to three individual siphon pipes [2].

2.2.2.2. Site Descriptions

2.2.1. Fictional Lake Bank Filtration Site The sitesite layoutlayout (Figure (Figure2) was2) was adapted adapted from from one ofone the of four the benchmarksfour benchmarks [ 12]. A [12] series. A ofseries 15 vertical of 15 wellsvertical (VW) wells with (VW) a bore with diameter a bore diameter of 1.0 m wasof 1.0 placed m was along placed a lake along shore. a lake The shore. groundwater The groundwater ﬂow from theflow individual from the wellsindividual is collected wells inis acollected common in pipe a common (0.2–0.7 mpipe internal (0.2–0. diameter).7 m internal Two diameter) scenarios. wereTwo calculated:scenarios were groundwater calculated: was groundwater abstracted and was pumped abstracted to the and aeration pumped cascades to the eitheraeration from cascades IOW or either from afrom collector IOW caissonor from (siphon a collector wells). caisson Pipe diameters, (siphon wells)friction. P factors,ipe diameter and totals, f abstractionriction factors were, and assumed total toabstraction be equal forwere both assumed scenarios. to be Water equal levels for both and frictionalscenarios. head Water loss levels were and calculated frictional using head SIPHON loss were by enteringcalculated a pre-deﬁnedusing SIPHON pumping by entering rate (IOW) a pre or-defined a pumping pumping water rate level (IOW for the) or siphona pumping collector water caisson level thatfor the was siphon iterated collector to achieve caisson the that same was total iterated abstraction to achieve as for the the same IOW total scenario. abstraction The pump as for head the IOW was H calculatedscenario. The as sum pump of well head drawdown was calculated and static as sum water of height well drawdownAC above and the undisturbed static water waterheight levelHAC (Figureabove the3). undisturbed water level (Figure 3).

Figure 2. Schematic of the hydraulic calculation parameters. Figure 2. Schematic of the hydraulic calculation parameters.

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Figure 3. Schematic of the fictional lake bank filtration site with 15 wells. FigureFigure 3. 3Schematic. Schematic of of the the fictional fictional lake lake bank bank filtration filtration site site with with 15 15 wells. wells. 2.2.2. Waterworks Dresden-Hosterwitz, Germany 2.2.2.2.2.2 Waterworks. Waterworks Dresden-Hosterwitz, Dresden-Hosterwitz, Germany Germany The RBF and managed aquifer recharge site Dresden-Hosterwitz is located on the floodplain of TheThe RBF RBF and and managed managed aquifer aquifer recharge recharge site site Dresden-Hosterwitz Dresden-Hosterwitz is is located located on on the the floodplain floodplain of of the Elbe River in Dresden. The alluvial aquifer is comprised of coarse sand and gravel with a thethe Elbe Elbe River River in Dresden. in Dresden. The alluvialThe alluvial aquifer aquifer is comprised is comprised of coarse of sand coarse and sand gravel and with gravel a saturated with a saturated thickness of 9 to 14 m. Figure 4 shows the waterworks (WW) and the location of 111 thicknesssaturated of thickness 9 to 14 m. of Figure 9 to 4 14 shows m. Figure the waterworks 4 shows the (WW) waterworks and the location (WW) and of 111 the vertical location siphon of 111 vertical siphon wells and 28 IOW. The siphon wells are placed along three individual siphon pipes. wellsvertical and siph 28 IOW.on wells The siphonand 28 wellsIOW. areThe placed siphon along wells three are placed individual along siphon three pipes.individual The siphonsiphon wellspipes. The siphon wells have a three/four-layer gravel pack and a bore diameter of up to 2.0 m (Table 1). haveThe asiphon three/four-layer wells have gravela three/four pack- andlayer a boregravel diameter pack and of a up bore to 2.0diameter m (Table of1 up). The to 2.0 well m spacing(Table 1). The well spacing varies between 12 and 24 m. The abstracted water is a mix of artificially recharged variesThe well between spacing 12 and varies 24 m.between The abstracted 12 and 24 water m. The is a abstracted mix of artificially water is recharged a mix of artificially water, groundwater recharged water, groundwater and bank filtrate. It is abstracted from one collector caisson and pumped to andwater, bank groundwater filtrate. It is abstracted and bank from filtrate. one collectorIt is abstracted caisson fromand pumped one collector to aeration caisson cascades. and pumped The IOW to aeration cascades. The IOW have a bore diameter of 0.8 m and a single layer gravel pack. They haveaeration a bore cascades. diameter The of 0.8 IOW m and have a single a bore layer diameter gravel ofpack. 0.8 Theym and abstract a single artificially layer gravel recharged pack. water They abstract artificially recharged water and pump it directly to the aeration cascades. The siphon wells andabstract pump artificially it directly rec toharged the aeration water cascades. and pump The it siphondirectly wells to the were aeration constructed cascades. before The siphon 1930, when wells were constructed before 1930, when the available filter screens were not as efficient as modern thewere available constructed filter screens before were 1930, not when as efficient the available as modern filter bridge screens slot orwere wire not wrap as screens. efficient The as siphonmodern bridge slot or wire wrap screens. The siphon wells, therefore, outnumber the IOW due to the early wells,bridge therefore, slot or wire outnumber wrap screens the IOW. The due siphon to the wells early, therefore trial and, erroroutnumber well design the IOW and du note dueto the to early the trial and error well design and not due to the siphon operation. siphontrial and operation. error well design and not due to the siphon operation.

Figure 4. Map of WW Dresden-Hosterwitz with approximate well locations. FigureFigure 4. 4Map. Map of of WW WW Dresden-Hosterwitz Dresden-Hosterwitz with with approximate approximate well well locations. locations.

Table 1. Well design parameters WW Dresden-Hosterwitz, Germany. TableTable 1. 1.Well Well design design parameters parameter WWs WW Dresden-Hosterwitz, Dresden-Hosterwitz, Germany. Germany. Parameter Siphon Wells IOW Parameter Siphon Wells IOW No. Parameterof wells Siphon Wells111 IOW 28 No. of wells 111 28 Year of No.construction of wells 1908, 111 1928/29 28 1980s YearYear of of construction construction 1908,1908, 1928/29 1928/29 1980s 1980s Well design VW VW WellWell design design VWVW VW VW Filter screen material Copper & stoneware, slotted & some stainless steel wire Steel, bridge slottedSteel, Filter screen material Copper & stoneware, slotted & some stainless steel wire Steel, FilterFilter screen screen material length Copper & stoneware, 3–5slotted m & some stainless steel wire bridge 3 m slotted Screen diameter 250–300 mm 350bridge mm slotted Filter screen length 3–5 m 3 m FilterNo. screen of pumps length 3 (in collector3– caisson)5 m 46 3 m ScreenType diameter of pump 250 DM–300 mm SP 350 mm Screen diameter 250–300 mm 350 mm No. of pumpsVFD 3 (in collector Yes caisson) No 46 No. of pumps 3 (in collector caisson) 46 TypeVW: of pump vertical well; DM: dry mounted pump; SP: submersibleDM pump; VFD: variable frequency drive. SP Type of pump DM SP VFD Yes No VFD Yes No VW: vertical well; DM: dry mounted pump; SP: submersible pump; VFD: variable frequency drive. VW: vertical well; DM: dry mounted pump; SP: submersible pump; VFD: variable frequency drive.

Water 2018, 10, 570 6 of 10 Water 2018, 10, x FOR PEER REVIEW 6 of 10

2.2.3.2.2.3. Szentendre Island,Island, Budapest Waterworks,Waterworks, HungaryHungary More than 500 wells are are in in use use on on Szentendre Szentendre Island Island in in the the north north of of Budapest Budapest.. They They penetrate penetrate a agravel gravel aquifer aquifer (5 (5–8–8 m m saturated saturated thickness) thickness) that that is is recharged recharged by by the the Danube Danube River. Three different types of wellswells werewere investigatedinvestigated regardingregarding theirtheir energyenergy consumption:consumption: smallsmall individually-operatedindividually-operated horizontal collector wells (sHCW, well group:group: I, II, III), vertical vertical siphon siphon wells wells (VW, (VW, well well g group:roup: IV, IV, V) V),, and siphonedsiphoned sHCW sHCW (well (well group group VI) (FigureVI) (Figure5). The 5 horizontal). The horizontal laterals of laterals the sHCW of werethe sHCW constructed were usingconstructed a modified using Ranneypressa modified Ranney method.press Some method of the. Some sHCW of the have sHCW laterals have on laterals two levels on two (Table levels2). Those(Table were2). Those projected were radiallyprojected into radially the aquifer into the and aquifer extend and partly extend below partly the riverbed. below the The riverbed VW have. The a two-layerVW have a gravel two-layer pack gravel and a bore pack diameter and a bore of 1.2diameter m (Table of2 1.2). The m (Table IOW are 2). locatedThe IOW in theare northernlocated in part the ofnorthern the island. part Waterof the fromisland these. Water wells from is first these pumped wells is into first a pumped low pressure into pipea low system pressure and pipe from system there connectedand from there to a gravitationalconnected toflow a gravitational pipe system flow (1.7–3.3 pipe msystem internal (1.7 diameter)–3.3 m internal on the diameter southern) parton the of thesouthern island. part Water of the from island the siphon. Water collector from the caissons siphon iscollector pumped caissons directly is into pumped the gravitational directly into pipes the thatgravitational transport pipes the water that transport across the the river water and across downstream the river towardsand downstream Budapest towards city. Each Budapest of the three city. siphonEach of well the groupsthree siphon consists well of twogroups individual consists siphon of two pipes individual extending siphon along pipes the riverbankextending in along opposite the directionsriverbank in with opposite the collector directions caisson with in the the collecto middle.r caisson in the middle.

Figure 5. Map of Szentendre Island with approximate location of selected well groups. Figure 5. Map of Szentendre Island with approximate location of selected well groups. Table 2. Well design parameters Szentendre Island, Hungary. Table 2. Well design parameters Szentendre Island, Hungary. Well Group Parameter I II IIIWell Group IV V VI Parameter No. of wells I9 II20 III8 IV56 70 V VI22 Well design sHCW sHCW sHCW VW VW sHCW No. of wells 9 20 8 56 70 22 WellOperation design sHCWIOW sHCWIOW sHCWIOW Siphon VW Siphon VW Siphon sHCW Filter screenOperation material Steel, IOW slotted Steel, IOW slotted Steel, IOW slotted Stainless, Siphon bridge Siphon slotted Stainless, Siphon slotted TotalFilter screenscreen material length Steel,1465 slotted m Steel,3540 slotted m Steel,1733 slotted m Stainless,n.a. bridge slottedn.a. Stainless,3154 slottedm TotalScreen screen diameter length 219 1465 mm m 3540219 mm m 1733200 mmm 300 n.a. mm 300 n.a. mm 200 3154 mm m No.Screen of lateral diameter levels 2191– mm2 219 mm2 200 mm1–2 300n.a. mm 300n.a. mm 2001 mm No. of lateral levels 1–2 2 1–2 n.a. n.a. 1 No. of pumps 9 20 8 2 3 4 No. of pumps 9 20 8 2 3 4 TypeType of of pump SPSP SPSP SPSP SPSP DM SP VFDVFD Yes YesYes YesYes YesYes Yes Yes n.a: notn.a: available not available;; SP: SP:submersible submersible pump; pump; DM DM:: dry mountedmounted pump; pump; VFD: VFD variable: variable frequency frequency drive. drive.

Water 2018, 10, x FOR PEER REVIEW 7 of 10 Water 2018, 10, 570 7 of 10 2.3. Energy Assessment 2.3. EnergyIn the case Assessment of the fictional lake bank filtration, the calculated pumping head and discharge were enteredIn theinto case the ofGrundfos the fictional quick lake-sizing bank web filtration,-tool (https://product the calculated- pumpingselection.grundfos.com/) head and discharge to derive were theentered energy into consumption the Grundfos of quick-sizing dry-mounted web-tool and submersible (https://product-selection.grundfos.com/ pumps. The pump head was )estimated to derive fromthe energy the elevation consumption of the ofaeration dry-mounted cascades and above submersible static groundwater pumps. The table pump HAC head (m) (Figure was estimated 3) plus thefrom pre the-defined elevation drawdown of the aeration HS inside cascades the collector above static caisson groundwater or the calculated table H IOWAC (m) drawdown. (Figure3) plusNote thatthe pre-definedthe energy consumption drawdown HS doinsidees not theinclude collector, e.g., caisson losses through or the calculated cables or IOWVFD. drawdown.Real energy Notedata wasthat available the energy for consumption wells located does on Szentendre not include, Island e.g., lossesas daily through sum counter cables orreadings VFD. Real for the energy duration data wasof one available year (1 forJanuary wells–31 located December on Szentendre 2015) andIsland for Dresden as daily-Hosterwitz sum counter as actual readings energy for thevalues duration with hourlyof one yearresolution (1 January–31 for a duration December of six 2015) weeks and, when for Dresden-Hosterwitz total abstraction decreas as actualed from energy 100% values to 20% with of itshourly maximum resolution treatment for a duration capacity of six (15 weeks, November when total–31 Dec abstractionember 20 decreased17). Contrary from 100% to th toe 20% fictional of its examplemaximum the treatment real data capacity already (15include November–31d cable losses December but also 2017). pump Contrary “aging” to or the possible fictional damage example to, the or incrustationsreal data already on the included impeller cable. losses but also pump “aging” or possible damage to, or incrustations on theThe impeller. specific energy consumption (SEC, kWh/m3) was calculated as energy consumption (kWh) at theThe pump specific divided energy by consumption the volume (SEC,of water kWh/m abstracted3) was calculated(m3). The aspump energying consumptionheads on Szentendre (kWh) at Islandthe pump ranged divided from by 0.8 the and volume 6.3 m of due water to abstractedthe well locations (m3). The and pumping the different heads onpressure Szentendre zones Island. The dailyranged energy from 0.8consumption and 6.3 m duewas to pressure the well-normalized locations and and the divided different by pressure the mean zones. daily The pump dailying energy head. Noconsumption pumping washead pressure-normalized was available for Dresden and divided-Hosterwitz by the. meanHowever, daily as pumping indicated head. in Figure No pumping 4, both thehead siphon was available wells and for IOW Dresden-Hosterwitz. have the same static However, head above as indicated ground ( inaeration Figure 4cascades, both the) a siphonnd it was wells not necessaryand IOW haveto normalize the same the static data. head above ground (aeration cascades) and it was not necessary to normalize the data. 3. Results and Discussion 3. Results and Discussion 3.1. Hydraulic Calculation of Siphon Wells 3.1. Hydraulic Calculation of Siphon Wells It is generally assumed that in siphon well systems the single well abstraction rate decreases with Itincreasing is generally distance assumed from that the in siphoncollector well caisson systems due the to single friction. well The abstraction controlling rate factor decreases for withwell yieldincreasing is the distance well drawdown. from the collector Since the caisson wells dueinfluence to friction. each Theother controlling due to the factor narrow for wellwell yieldspacing, is the a welldistinction drawdown. must be Since made the between wells influence drawdown each from other neighboring due to the wells narrow (well well interference) spacing, a distinctionand “real” drawdownmust be made (well between yield). drawdownFor every well from the neighboring sum of frictional wells (well head interference) loss, well interference and “real” drawdownand “real” (welldrawdown yield). is For equal every to well the drawdown the sum of frictional inside the head collector loss, wellcaisson interference according and to “real”Equation drawdown (1). The degreeis equal of to well the drawdowninterference insidedecreases the collectorwith increasing caisson eccentricity according to from Equation a maximum (1). The plateau degree value of well in theinterference middle of decreases the well withgallery increasing. If with increasing eccentricity distance from a maximumfrom the collector plateau caisson value in the the increase middle in of frictionthe well betweengallery. If two with adjacent increasing wells distance is less from than the collectorthe decrease caisson in thewell increase interference in friction, the between “real” drawdowntwo adjacent (Equation wells is less(1)) thanand, thethus decrease, the single in wellwell interference,abstraction rate the increases “real” drawdown towards th (Equatione outermost (1)) wellsand, thus, (Figure the 6 single), which well isabstraction in opposite rate to what increases is generally towards expected the outermost by only wells assuming (Figure 6an), whichincrease is inin frictionopposite but to whatunderestimating is generally the expected effect byof onlywell assuminginterference an. increaseThis is even in friction more butpronounced underestimating at sites withthe effect larger of pipe well diameters interference. [2]. This is even more pronounced at sites with larger pipe diameters [2].

Figure 6 6.. CalculatedCalculated well heads and flow ﬂow rates for IOW and siphon wells using SIPHON.SIPHON.

Water 2018, 10, x FOR PEER REVIEW 8 of 10 Water 2018, 10, 570 8 of 10

3.2. Energetic Comparison of Siphon and Individually Operated Wells with Submersible Pumps 3.2. Energetic Comparison of Siphon and Individually Operated Wells with Submersible Pumps 3.2.1. Fictional Lake Bank Filtration Site 3.2.1. Fictional Lake Bank Filtration Site The calculated pump head for a cumulative abstraction of 825 m3/h was 25.2 m for the siphon wellsThe and calculated 24.1–26.2pump m for head the for IOWs. a cumulative The resulting abstraction energy of 825consumption m3/h was of 25.2 one m for dry the-mounted siphon centrifugalwells and 24.1–26.2 pump (similar m for the to IOWs. the ones The resulting used in energy WW Dresden consumption-Hosterwitz, of one dry-mounted total efficiency centrifugal 83.8%) pumpingpump (similar the water to the from ones th usede collector in WW caisson Dresden-Hosterwitz, to the aeration cascades total efficiency was 38% 83.8%) (Table pumping 3) less than the neededwater from by 15 the submersible collector caisson pumps to (IOW). the aeration A design cascades total efficiency was 38% (motor (Table 3and) less pump) than neededof 51.9–54.1% by 15 wassubmersible derived pumps from the (IOW). Grundfos A design quick total-sizing efficiency web (motor tool for and new pump) submersible of 51.9–54.1% pumps was that derived was afrompproximately the Grundfos 12% quick-sizinghigher than webthe mean tool for value new of submersible 41% for submersible pumps that pumps was approximately used in Germany 12% reportedhigher than in [6]. the Hence, mean valuethe calculated of 41% for energy submersible savings pumpsmay even used increase in Germany over time reported assuming in [6 ].a higher Hence, degreethe calculated of aging energy or corrosion savings for may submersible even increase pumps over compared time assuming to dry-mounted a higher pumps. degree of aging or corrosion for submersible pumps compared to dry-mounted pumps. Table 3. SEC of siphon wells versus IOW at the fictional lake bank filtration site. Table 3. SEC of siphon wells versus IOW at the fictional lake bank filtration site. Parameter Unit Siphon Wells IOW % Difference ParameterNo. of pumps Unit[/] Siphon1 Wells15 IOW %- Difference No. of pumpsModel [/][/] NK250-350/342 1 SP46-4, 15A21904 15 - - MotorModel power draw [/][kW] NK250-350/34275 SP46-4,7.5 15A21904- - Motor powerPump draw head [kW][m] 25.2 75 24.1–26.2 7.5 - - PumpPump head discharge [m][m3/h] 825 25.2 24.1–26.255 - Pump discharge [m3/h] 825 55 Total efficiency 1 [%] 83.8 51.9–54.1 29.7–31.9 Total efficiency 1 [%] 83.8 51.9–54.1 29.7–31.9 5 5 EnergyEnergy consumption consumption [kWh/a][kWh/a] 5.915.91 ×× 1010 5 9.509.50 × 10× 10 5 37.837.8 SEC SEC [kWh/m[kWh/m3] 3] 0.0820.082 0.132 0.132 37.9 37.9 1 1MotorMotor and and pump.pump.

3.2.23.2.2.. WW WW Dresden Dresden-Hosterwitz-Hosterwitz TheThe SEC SEC of siphon wells at WW Dresden-HosterwitzDresden-Hosterwitz ranged between 0.0810.081 andand 0.1080.108 kWh/mkWh/m33,, 3636 to 52%52% lessless thanthan measuredmeasured forfor thethe IOWIOW gallerygallery (Figure(Figure7 7).). TheThe offsetoffset inin thethe siphonsiphon wellwell curvecurve betweenbetween 750 750 and and 1000 m 33/h/h was was caused caused by by the the activation activation of of another another VFD VFD pump. pump.

FigureFigure 7. 7. MMeasuredeasured SEC SEC as as function function of of discharge discharge for for siphon siphon wells wells and and IOW IOW at at Dresden Dresden-Hosterwitz.-Hosterwitz.

3.2.3.2.3.3. Szentendre Szentendre Island Island TheThe pressure pressure-normalized-normalized SEC SEC of of the the siphon siphon well well groups groups on Szentendre Island range rangedd from from 3 3 0.0.004004 to to 0. 0.011011 kWh/ kWh/(m(m ·m)3·m) compared compared to to0.0 0.01818 to to0.0 0.02828 kWh/(m kWh/(m·m)3 ·m)for forIOW IOW (Table (Table 4) and4) and was was,, on averageon average,, 69% 69% lower lower than than th thoseose of IOW.IOW. The The siphoned siphoned sHCW sHCW had had the lowest the lowest SEC becauseSEC because their higher their higherscreen intake screen enables intake enables a larger a groundwater larger groundwater inﬂow compared inflow compared to vertical to wells vertical at a similar wells at drawdown. a similar drawdownSiphon group. Siphon V, although group equipped V, although with dry-mounted equipped with pumps, dry-mounted had a higher pumps, SEC comparedhad a higher to siphons SEC comparedIV and VI, to which siphon hads IV submersible and VI, which pumps had installed submersible inside pumps the collector installed caisson. inside Thisthe collector was unexpected caisson. Thisand maywas haveunexpected technical and reasons, may have such technical as pump reasons aging or, such an operation as pump awayaging from or an the operation best operation away frompoint the due best to aoperation different systempoint due characteristic to a different curve. system characteristic curve.

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Table 4. Measured SEC of siphon wells and IOW at the Szentendre Island (range of IOW in brackets).

Parameter Unit I (IOW) II (IOW) III IOW IV V VI Pump head [m] 1.3–6.3 0.8–6.2 1.3–5.4 4.3–5.9 3.5–5.9 2.3–4.5 Energy consumption [kWh/a] 4.27 × 105 7.48 × 105 9.90 × 104 7.77 × 104 1.02 × 105 8.62 × 104 0.028 0.018 0.022 Pressure-normalized SEC [kWh/(m3·m)] 0.006 0.011 0.004 (0.017–0.042) (0.011–0.030) (0.014–0.039) 0.075 0.061 0.053 SEC [kWh/m3] 0.026 0.033 0.030 (0.055–0.114) (0.052–0.120) (0.050–0.058)

4. Conclusions The SEC of the siphon wells is 0.026–0.033 kWh/m3 for Szentendre Island with a pump head of ≤5.9 m. The SEC of the WW Dresden-Hosterwitz with a pump head of around 22 to 25 m is 0.081–0.108 kWh/m3. A previous study [13] reported a SEC of 0.05 kWh/m3 for siphon wells along the Rhine River that increased by 20% or 0.01 kWh/m3 during drought periods. In comparison, the SEC of the IOW ranged from 0.053–0.075 kWh/m3 (Szentendre Island) to 0.170 kWh/m3 (Dresden-Hosterwitz). The SEC spans a wide range even for similar well types, thus, data from different sites cannot be easily compared to estimate energy savings. However, the sites investigated in this study each operate both siphon wells and individually operated wells, which allow a comparison. A 38% reduction in energy consumption was projected for the fictional example calculation by using siphon wells instead of IOW equipped with submersible pumps. Real energy data from two investigated RBF sites indicate savings in the range of 36 to 52% for the WW Dresden-Hosterwitz and on average 69% for Szentendre Island. The real data already include losses for cables and VFDs but also pump aging. It should be noted that these values reflect the current state of the art in pump technology. Future advances in pump technology or different site-specific conditions may increase or decrease the energy savings potential for every individual case. However, the authors are confident that an energy savings potential of 30–50% can be achieved by rehabilitating old siphon wells instead of an alternative equipment with submersible pumps, which justifies the higher investment costs. The construction of new siphon wells or the equipment of existing wells with siphon pipes offers similar advantages. However, the siphon operation is not feasible at every site and their operation is not as flexible and controllable as IOW. The suction head between the crest of the siphon pipe and the lowest pumping water level inside the collector caisson should not exceed 8 m. The economic depth for trenching and bedding siphon pipes found in German literature up to the mid-20th century was 4.0–4.5 m or maximum 5.0 m for a large number of wells. Thus, the maximum depth for the pumping water level is considered to be around 12 m below the surface. The construction of nearly vacuum-tight pipes and robust vacuum systems is, nowadays, considered state of the art, but requires a high level of precision. The design of siphon systems still remains somewhat challenging, but the freeware tool SIPHON can be used to plan the rehabilitation of an existing system or to design a completely new one.

Supplementary Materials: SIPHON, the freeware MS Excel calculation tool, is available online together with the documentation at www.htw-dresden.de/heber (German and English language) and in the tools section of http://dss.aquanes.eu/ (English language). Author Contributions: R.B. reviewed the literature, analyzed the energy data, developed the calculation tool and performed the analysis; and T.G. developed the idea, collected literature and contributed expertise in interpreting results. Funding: All primary data was collected within the “AquaNES” project. This project has received funding from the European Union’s Horizon 2020 Research and Innovation Program under grant no. 689450. Acknowledgments: The authors gratefully acknowledge support from Budapest Waterworks (Z. Nagy-Kovács and G. Till) and DREWAG Netz GmbH (R. Haas and R. Opitz). Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. Water 2018, 10, 570 10 of 10

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