water 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 mtechnology/d (Poznan-Debina, has been replaced Poland). by In the recent use decadesof 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 wellsubmersible screens. Thesepumps wells, because have beenthose equipped were able with to submersibleachieve the necessary pumps, 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 the calculation productivity tools toof design such andsystems, verify which the productivity were historically of such designedsystems, which and built were by historically expertise designed and experience and built [1] by. expertiseThe 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 becausecertain thecircumstances operation of, e siphonspecially 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.