Bank filtration, i.e. the abstraction of groundwater from wells along a river or lake, thus inducing infiltration has a long history as (pre-) treatment step for drinking water production in Europe. The goal of this study is to assess whether groundwater waterworks using BF have a cost advantage compared to SWTPs if both, water abstraction and treatment processes are considered.

Berlin’s drinking water is produced from groundwater replenished by 60 % from surface water from the city’s abundant rivers or lakes using bank fi ltration or artifi cial groundwater recharge. Compared to other bank fi ltration sites world wide, the situation in Berlin is characterized by low hydraulic conductivities but nevertheless high capacities. Interdisciplinary research projects have shown that travel times and redox conditions during subsurface passage are highly transient due to seasonal effects and discontinuous pump operation. Trace organics like pharmaceuticals and x-ray contrast media are attenuated during subsurface passage to a varying degree. Substances that were found to be poorly removed under oxic conditions or even persistent include carbamazepine, primidone, sulfamethoxazole, 1,5 NDSA, MTBE and EDTA. Under anoxic to anaerobic conditions others like phenazone and diclofenac show little removal. However, none of these substances occur at relevant concentrations in the fi nished drinking water due to low initial concentrations in the surface water or additional removal during post-treatment (aeration and fi ltration for iron and manganese removal).

In the course of identifying areas of relevance for further research and development the members of the European Water Supply and Sanitation Technology (WssTP) identified Managed Aquifer Recharge (MAR) as an important cross-cutting topic and area relevant for further research. For this reason a Task Force on MAR was initiated with 36 representatives from European research institutes and industry partners with participation of international experts. These task force members developed the basis for a report documenting the state of the art and research needs in the field of MAR that has now been published by the WssTP.

Artificial Recharge (AR) is a method to replenish groundwater in case of insufficient water availability or poor quality. For drinking water production, AR is often used as water purification step to avoid direct surface water abstraction. Besides physical filtration, purification is achieved through chemical processes like precipitation, sorption and (bio-) degradation. These are usually closely linked to redox conditions. It is the activity of micro-organisms and related chemical reactions that change the redox conditions, which in turn control the presence of substances and therefore the water quality. Typical pollutants in surface water that need to be addressed are organic compounds (e.g. pharmaceutical residues or pesticides), pathogens and heavy metals. The purpose of this report is to introduce the theoretical background on redox zoning in infiltration ponds and to review publications in the search for applicable methods capable of controlling redox conditions. This shall serve as basis for further laboratory and technical scale experiments in the course of the OXIRED project. The “optimal redox zonation” for maximum removal of redox-dependent substances is a concept with the aim of defining optimum residence times based on the degradation kinetics of contaminants in the source water: If substances or substance groups that show enhanced removal under anoxic to anaerobic conditions are not present in the source water at drinking water relevant concentrations, anoxic to anaerobic conditions should be avoided in order not to mobilize iron and other inorganic trace elements. Maximum benefit for aerobic subsurface passage is reached after 30 d, for anoxic / anaerobic subsurface passage after 100 d. However, already 15 d of aerobic and 2 d of anoxic / anaerobic passage lead to substantial removal or redox-sensitive substances or substance groups. The main drivers for redox zonation in AR systems are the availability of oxidizing agents (oxygen, nitrate), of reducing agents (organic matter, reduced mineral phases), of nutrients, the biological activity (in infiltration pond and subsurface), and the residence time. These drivers are in turn controlled by many natural, site-specific (exogenous) and design & operation-related (decision) variables. Exogenous variables are e.g. aquifer geochemistry, temperature or natural groundwater recharge whereas the decision variables comprise factors such as pond geometry, distance between pond and well, well depth, pumping rate etc. Theoretically, a wide range of possibilities could be applied to adjust the infiltration pond, the hyporheic zone and the subsurface passage, but only few seem technically feasible. These are e.g. the control of sunlight and temperature in the infiltration pond and upper sediment, the control of water movement in the pond to avoid excessive algal growth while enriching the water with oxygen. For the same reason nutrients could be added or avoided, influencing biomass production. Specific filter material could be used with defined content and characteristic of organic carbon to serve as electron acceptors. Infiltration rates could be controlled by adjusting the hydraulic head in order to enhance the formation of an unsaturated zone. Further downstream the application of redox controlling substances via injection wells could be possible, as well as controlling the residence times by adjusting pumping rates or creating hydraulic barrier wells at different distances from infiltration pond. For newly constructed AR systems the well field design (pond geometry, distance between pond and well, well depth) could be optimized with respect to redox zonation, as long as the other requirements (mainly sufficient production rates) are met. No examples for redox control in infiltration ponds were identified. Therefore, two examples of redox control measures are described: the first serves an artificial reoxidation of a polluted aquifer “BIOXWAND®” and the second provides injection of treated water to influence the redox conditions in the aquifer “Vyridox” and “Nitridox”.

Natürliche und künstliche Systeme zur Infiltration von Wasser (im Englischen: Managed Aquifer Recharge) werden weltweit genutzt, um Grundwasserressourcen quantitativ oder qualitativ zu verbessern. Dies erfolgt beispielsweise bei der Uferfiltration oder künstlichen Grundwasseranreicherung zur Trinkwassergewinnung, bei der Klarwasserverregnung zur weiteren Abwasserreinigung und -nutzung oder bei der Injektion von Süßwasser als hydraulische Barriere in Salzwasserintrusionsgefährdete Grundwasserleiter. Dabei nutzt man nicht nur den mengenmäßigen Ausgleich von überbeanspruchten Grundwasserressourcen, sondern auch die Reinigungsleistung des Untergrundes für eine naturnahe und meist auch kostengünstige Wasseraufbereitung. In Berlin, wo seit über 150 Jahren Trinkwasser mittels Uferfiltration gewonnen wird, wurden in Zusammenarbeit mit den Berliner Universitäten in der Vergangenheit umfangreiche Untersuchungen zur Stoffelimination bei der Untergrundpassage durchgeführt. Diese zeigten, dass auch die Konzentrationen von organischen Spurenstoffen häufig bei der Infiltration oder weiteren Grundwasserleiterpassage zurückgehen. Eine statistische Auswertung von Beobachtungen an verschiedenen Standorten ergab, dass die Mehrheit der untersuchten Substanzen wie beispielsweise Clofibrinsäure, Diclofenac und Phenazon bevorzugt unter oxischen Bedingungen abgebaut werden oder generell eine gute Entfernung erfahren. Einige wie z.B. Carbamazipin oder Sulfamethoxazol werden vor allem unter anoxisch- bis anaeroben Bedingungen entfernt. Aus diesen Beobachtungen ergab sich die Frage, ob ein optimaler Redoxzustand bzw. eine optimal Redoxabfolge für Systeme wie Infiltrationsbecken definiert werden könnte. Erste theoretische Studien erfolgten auf der Basis verfügbarer Abbaukinetiken und unter Einbeziehung weiterer Redox-sensitiver Wasserinhaltsstoffe wie Nitrat und Eisen. Diese ergaben, dass eine Aufenthaltszeit von 30 Tagen im aeroben Milieu und 100 Tagen im anoxischen Milieu während der Untergrundpassage zu einer optimalen Entfernung Redox-sensitiver Problemstoffe führt. Jedoch können bereits 15 Tage aerobe und 2 Tage anoxische / anaerobe Untergrundaufenthalt zu einem deutlichen Rückgang dieser Stoffe führen. Generell sollte jedoch berücksichtigt werden, dass unter anoxischen bis anaeroben Bedingungen mit einer Mobilisierung geogener Spurenelemente wie Eisen und Mangan zu rechnen ist. Obwohl theoretisch eine Vielzahl an Möglichkeiten existiert, den Infiltrationsbereich, die hyporheische Zone und die Untergrundpassage im Hinblick auf eine optimierte Redoxzonierung zu modifizieren oder gar zu steuern, sind nur wenige technisch tatsächlich machbar. Weitere Untersuchungen sollen nun diejenigen Möglichkeiten identifizieren, die in die Praxis übertragbar sind und zu einer Optimierung der künstlichen und natürlichen Systeme zur Infiltration beitragen könnten.

Work package WP 5.2 “Combination of Managed Aquifer Recharge (MAR) and adjusted conventional treatment processes for an Integrated Water Resources Management“ within the European Project TECHNEAU (“Technology enabled universal access to safe water”) investigates bank filtration (BF) + post-treatment as a MAR technique to provide sustainable and safe drinking water supply to developing and newly industrialised countries. One of the tasks within this work package is to assess the costefficiency of BF systems. For this a comparative cost analysis (CCA) between groundwater waterworks using BF as natural pre-treatment step and surface water treatment plants (SWTPs) is performed. The CCA yielded that, under the assumption of equally low surface water quality, BF systems are more cost-efficient than SWTPs. This result is in line with the general water source priority of water suppliers, which prefer resources with the best water quality and security under the constraint of guaranteeing sufficient water availability. Furthermore the sensitivity analysis confirmed that the natural boundary condition 'pumping rate per production well' has a major impact on the specific total costs of BF systems. Lower pumping rates lead to increasing capital costs for the additional production wells, which are not fully compensated through pumping cost savings and thus lead to increasing total costs. In addition the result of the monitoring scenario clearly confirmed that for this aspect groundwater waterworks have a structural disadvantage compared to surface waterworks. Subsequently, if monitoring costs are taken into account, a higher critical pumping rate per production well is required to exceed the break-even-point. In a nutshell the CCA shall support water supply managers in the complex process of making rational investment decisions. However, since within this analysis only water abstraction and treatment process costs are considered, the CCA does not cover the total cost structure of a waterworks (e.g. costs of building sites). Thus the application of the CCA is only valid if both (i) neglected costs and (ii) benefits are in the same order of magnitude for all alternatives (exception: most cost-efficient alternative provides excess benefits). In case that the above stated prerequisites are not fulfilled, the CCA is only a first step in the economic assessment and more powerful evaluation methods (e.g. cost-benefit analysis) are needed.

Berlin’s drinking water is produced from groundwater replenished by up to 60 % of surface water from the city’s abundant rivers or lakes using bank filtration or artificial groundwater recharge. Currently 700 production wells, located along the banks produce more than 200 Mio m³/a of drinking water, which is treated only for iron and manganese removal before distribution. This is due to the fact that different natural treatment processes (e.g. straining of particles, adsorption or biodegradation) occur during subsurface passage so that post-treatment effort is reduced. Compared to other bank filtration sites world wide, the situation in Berlin is characterized by low hydraulic conductivities but nevertheless high capacities. Interdisciplinary research projects have shown that travel times and redox conditions during subsurface passage are highly transient due to seasonal effects and discontinuous pump operation. Trace organics like pharmaceuticals and x-ray contrast media that occur in Berlin’s surface waters due to relevant shares of treated waste water are attenuated during subsurface passage to varying degree. Substances that were found to be poorly attenuated under oxic conditions or even persistent include carbamazipine, primidone, sulfamethoxazole, 1,5 NDSA, MTBE and EDTA. Under anoxic to anaerobic conditions others like phenazone and diclofenac show little removal. However, none of these substances occur at relevant concentrations in the finished drinking water due to low initial concentrations or additional removal during post-treatment. Research is currently focussing on hybrid systems combining subsurface passage with advanced drinking water treatment in order to be prepared in case higher source concentrations occur.

Bank filtration (BF) and aquifer recharge (AR): aquifer storage recharge (ASR), aquifer storage transport recharge (ASTR); are natural and semi-natural methods for drinking water treatment and constitute a major barrier within water supply system. Recent investigations have shown that about 60 % of Berlin’s drinking water is produced via BF or AR (Zippel & Hannappel 2008). Most drinking water therefore originates from surface waters within the cities limits and is pumped from wells adjacent to it’s many lakes and rivers. Since more than 100 years this system has been supplying safe drinking water so that post-treatment is limited to aeration and subsequent sand filtration. Disinfection is usually not applied (SenStadtUm 2008). The research project NASRI (“Natural and Artificial Systems for Recharge and Infiltration”, KWB 2002 – 2006), funded by the Berliner Wasserbetriebe (BWB) and Veolia (VE) had the aim to characterize the specific hydraulic and hydrochemical conditions at selected BF and AR sites in Berlin and to assess the behaviour of major water constituents, trace organic substances, algal toxins and pathogens during subsurface passage. For this, field investigations at three transsects (Lake Tegel BFsite, Lake Tegel AR-site and Lake Wannsee), laboratory and technical scale experiments were carried out by 7 different working groups. The results of the investigations were documented in 6 extensive research reports and were the basis for nearly 50 scientific publications. In 2007 the IC-NASRI project (Integration & Consolidation of the NASRI outcomes) was initiated by VE and BWB in order to support the practical implementation and optimization of bank filtration and aquifer recharge for drinking water production with the experience gained during the NASRI project. The aim was to derive practical guidelines for design and operation of BF & AR systems by i) further interpretation of the NASRI data and ii) integrating experience from other BF / AR sites world wide. Although subsurface passage is characteristic to many systems of managed aquifer recharge (MAR) the investigations within IC-NASRI concentrated on systems where drinking water is produced by infiltration of surface water either from the banks of a lake / river or from infiltration ponds (or similar systems like ditches or irrigation fields). A transfer of the presented results to other MAR systems, which use different recharge methods (e.g. ASR) or different sources (e.g. treated wastewater) therefore needs to be considered carefully, even though many statements may be true for them as well. This reports aims at providing engineers and scientists involved in drinking water production by BF & AR with up-to-date information on settings of similar systems world wide and on the systems’ performance with regard to drinking water treatment. The aim was to give the reader a condensed overview of the topic whereas further details can be taken from the large number of references given in the bibliography.

Within the European project TECHNEAU (www.techneau.org) the Berlin Center of Competence for Water (KWB) is investigating bank filtration (BF) and adjusted post-treatment as a managed aquifer recharge (MAR) technique to provide sustainable and safe drinking water supply to developing and newly industrialised countries. One of the tasks within the project is the development of a Decision Support System (DSS) to assess the feasibility of BF systems under varying boundary conditions such as: (i) quality of surface and ambient groundwater, (ii) local hydrological and hydrogeological properties (e.g. clogging layer) and (iii) well field design (distance to bank) and operation (pumping rates). Since the successful, cost-effective implementation of BF systems requires the optimization of multiple objectives such as (i) optimizing the BF share in order to maintain a predefined raw water quality, (ii) maintaining a predefined minimum travel time between bank and production well and (iii) achieving cost-efficiency of different well field design and operation schemes, all these objectives need to be addressed within the DSS.