It was the aim of the EU funded research project TECHNEAU to investigate the relevance and feasibility of bank filtration (BF) plus post-treatment for newly industrialised and developing countries. Field studies at BF sites in Delhi (India) were supplemented by literature studies and modelling in order to investigate if this natural drinking water (pre-) treatment is a sustainable option to provide safe drinking water for countries like India. The results showed that especially for those substances that are of relevance in newly industrialised and developing countries subsurface passage can represent an efficient barrier. However, certain limiting factors for BF application also need to be considered: high ammonium levels in surface water, usually associated with high shares of poorly or un-treated sewage, will not be mitigated during subsurface passage and require extensive post-treatment. In order to support decision makers in the difficult task of assessing the feasibility of BF systems at a certain site a simple decision support system was developed. This simple tool enables to assess a range of abstraction rates and well locations for a specific field site that could fit with their needs (e.g. minimum required travel time or share of BF).

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.

Managed aquifer recharge provides efficient removal for many organic water constituents but it is a difficult task to quantify removal under field conditions: Observed concentrations often scatter and may be biased by subsurface mixing of different waters. Removal efficiency is affected by different environmental parameters, such as redox potential, travel times, threshold values, and also field site specifics. In addition, it is crucial to know the corresponding surface water concentration for all samples. We developed a method, which overcomes these difficulties, quantifies the efficiency and removal kinetics and is applicable to extensive databases. It combines both, statistical and graphical evaluation which allows the determination of precise values and also interpretation based on expert knowledge. The database of this study was collected within the NASRI project between 2002 and 2005 at two bank filtration sites (Tegel BF, Wannsee BF) and one basin aquifer recharge site (Tegel AR) in Berlin. In total, 38 organic constituents were analysed (Table 1).

Bank filtration (BF) is a well established and proven natural water treatment technology, where surface water is infiltrated to an aquifer through river or lake banks. Improvement of water quality is achieved by a series of chemical, biological and physical processes during subsurface passage. This paper aims at identifying climate sensitive factors affecting bank filtration performance and assesses their relevance based on hypothetical 'drought' and 'flood' climate scenarios. The climate sensitive factors influencing water quantity and quality also have influence on substance removal parameters such as redox conditions and travel time. Droughts are found to promote anaerobic conditions during bank filtration passage, while flood events can drastically shorten travel time and cause breakthrough of pathogens, metals, suspended solids, DOC and organic micropollutants. The study revealed that only BF systems comprising an oxic to anoxic redox sequence ensure maximum removal efficiency. The storage capacity of the banks and availability of two source waters renders BF for drinking water supply less vulnerable than surface water or groundwater abstraction alone. Overall, BF is vulnerable to climate change although anthropogenic impacts are at least as important.

The use of bank filtration for drinking water treatment in Europe dates back to the days of beginning industrialization in the 19th century. With regard to improved source water quality in Europe, the millennium development goals and global climate change, aquifer recharge (AR) and bank filtration (BF) need to be reassessed in terms of sustainability and their role within an integrated water resource management. Based on the IC-NASRI study comprising 194 drinking water facilities worldwide integrating aquifer recharge techniques in their treatment system, an average AR/BF site would be located in Central Europe alongside a river and is characterized by: a sandy gravel aquifer with a hydraulic conductivity of 2x10-3 m/s, a maximum aquifer thickness of 30 m, 175 m travel distance from bank to well, a travel time of 70 days and by vertical well operation with a daily capacity of 55.000 m³. A literature survey conducted within the TECHNEAU project demonstrated that for substances highly relevant to newly-industrialized or developing countries (e.g. pathogens) the removal efficiency is good. Hydro-chemical analyses from three study sites in Delhi support these results. However, it was also shown that poor surface water quality, saline groundwater or subsurface conditions leading to mobilization of trace metals like iron, manganese or arsenic may limit the applicability of AR / BF without further post-treatment. Climate change might affect the performance of AR / BF worldwide, impairing source water quality and influencing removal efficiency. However, other factors like changes in demography or land-use can impact the systems by far more severely.

Riverbank filtration (RBF) denotes the process whereby river water is induced to infiltrate into a groundwater system by well operation adjacent to banks. In Central Europe, RBF has been common practice for 100 years to produce drinking water. Due to the easy implementation and little maintenance necessary, BF has been suggested to be a useful drinking water treatment for developing and newly-industrialised countries. Experience from Europe has demonstrated that RBF is suitable to remove a range of organic and inorganic contaminants while an exhaustion of cleaning capacity has not been observed. RBF systems can mitigate shock loads and are particularly known for the efficient removal of pathogens, suspended solids and algal toxins from surface water, all being water quality parameters of high relevance in developing and newly-industrialised countries. Another benefit of RBF operation is the storage capacity which may help to balance freshwater availability in areas experiencing high variations of precipitation and run-off. This report aims at evaluating the relevance and opportunities of RBF systems to provide safe water to these countries. In order to evaluate the relevance and opportunities of RBF systems to developing and newly-industrialised countries, the report is structured to address key considerations and (i) identify prerequisites for successful RBF operation based on the experience in Central Europe and the United States, (ii) assess the removal potential of RBF for various water contaminants based on available literature, the TECHNEAU investigations in India and NASRI data from Berlin and (iii) evaluate the sustainability and relevance of RBF operation with regard to the particular needs in developing and newly-industrialised countries.

While climate change is an emerging hazard to water supply, literature on the vulnerability of bank filtration (BF), a proven technique of drinking water production in Central Europe and North America, is yet scarce. The Intergovernmental Panel of Climate Change (2007) has projected a global temperature increase between 1.1 and 6.4 °C by 2100. This will affect vital factors for water supply such as precipitation regime, groundwater recharge, run-off, river discharge and raw water quality. Projections on climate change and the implications are difficult because of the uncertainties associated with climate scenarios and modelling. However, in Europe and North America where BF is in operation, the projected increase in seasonal floods and droughts has already been experienced. In addition, site-specific considerations (e.g. land use, demographic trends) are to be taken into account to evaluate the potential impacts on water supply. To fill the current gap in literature, this report provides a first overview on how changing environmental conditions may affect BF operation.