Bank Filtration

Bank Filtration

During subsurface passage in biologically active soil layers (with aerobic, anaerobic and anoxic milieus), water quality of surface water can be improved before being mixed with groundwater and extracted for use. BF systems involve several physical, chemical and biochemical processes and are particularly known for the efficient reduction/removal (or even elimination) of suspended solids, organic pollutants, microorganisms, heavy metals, nitrogen, toxic algae as well as organic trace compounds (e.g.: pharmaceutical products), salinity or taste- and odour causing compounds (RAKESH et al. 2010; SPRENGER et al. 2006; HUELSHOFF et al. 2009b).

Relying on natural processes, design and treatment capacity (and efficiency) of BF systems strongly depends on local circumstances such as quality and quantity of available river- and ground water, hydraulic residence times of the water in the soil, the porosity of the soil, the hydraulic potential of the aquifer, temperature, pH values and oxygen concentrations as well as underlying redox processes (SCHMIDT et al. 2003; ZIEGLER 2001). Depending on the bank filtrate quality, disinfection or even supplementary treatment steps are necessary to achieve drinking water quality. Besides its polishing function, BF also provides huge fresh water storage capacity for buffering extreme climatic conditions and shock loads (HUELSHOFF et al. 2009a&b; SHARMA & AMY 2009; SCHMIDT et al. 2003), but also represents an artificial groundwater recharge technique preventing the overuse of aquifers, saltwater intrusion and land subsidence (NRMMC 2009).

Basic requirements for the operation of a BF system are the availability of surface water as primary water source and a detailed consideration of the groundwater level in the surroundings of the abstraction well. Water abstraction should not result in adverse effects on the aquifer or the river downstream of the site. Depending on the BF site’s characteristics and purpose of the output water, operation of a BF system is easy and only little maintenance is needed (HUELSHOFF et al. 2009a; HISCOCK & GRISCHEK 2002). Compared to high-end technologies, requirements for skilled labour and energy & chemical use are very low (RAY et al. 2002; ZIEGLER 2001). However, more requirements may arise in relation to design, operation and maintenance of the water abstraction well. One challenge in relation to well operation is the prevention/handling of colmation of the infiltration path.

Costs for establishing riverbank filtration systems depend on many factors, including aquifer characteristics, type of well-screen installation, facility design, and distance to the population served. However, costs can be classified as moderate. Using natural treatment processes, BF system can be considered as cost-effective system, which ideally can reduce costs for subsequent treatment steps (SCHMIDT et al. 2003). Additional costs can arise in dependency of raw-water quality and continuative treatment steps for diverging intended purpose (e.g. drinking water use).

Investment costs are costs for the abstraction well (construction, pump, main, control system etc.) as a minimum, as well as costs for groundwater monitoring of BF processes and water quality. Operational costs are primarily costs for pumping electricity for abstraction well operation. For abstraction (and treatment) facilities skilled personal is required.

For many decades Bank Filtration has been used in Europe and the United States for drinking water supply in communities located on river banks, applying both large-scale schemes involving high-tech extraction methods (providing water for central water supply entities) as well as low-tech household-based schemes. In Germany, more than 300 water works use BF producing about 16% of the German drinking water. Along the Danube River large-scale BF-systems exist in all major cities like Vienna (Austria), Bratislava und Gabicikova (Slovakia), Budapest (Hungary) and Belgrad (Serbia). About 50% of the public drinking water supplies in Slovakia and about 45% in Hungary are provided through BF (HISCOCK & GRISCHEK 2002). Production capacities are varying significantly not only depending on the productivity of the aquifer, but also on the type and number of extraction wells used. In Vienna, for example, up to 2.000 L/s are extracted with BF from the Danube using a total of 13 radial collector wells and 11 vertical filter wells.

In Central Europe, increasing chemical pollution, high concentrations of ammonia, organic compounds and micro-pollutants in the river water called for the introduction of supplementary pre- and post-treatment steps to build up a multi-barrier system (e.g. granular activated carbon filters, often combined with isolation and filtration) (SCHMIDT et al. 2003). SANDHU et al. (2011) emphasised on the necessity to be aware of the severe differences in the geological, hydrogeochemical, hydrological and river morphological characteristics of river systems, when transferring experiences from Europe to India. Thus, design and operation of BF systems should not only use adapted well designs, but also take into account prevalent cultural and operational constraints.

SANDHU et al. (2011) investigated the potential of riverbank filtration for drinking water supply in India based on experiences with operating large-scale riverbed filtration schemes in the cities of Ahmedabad, Delhi, Haridwar, Mathura, Medinipur and Kharagpur, Nainital, Patna and Srinagar. Large diameter caisson wells, vertical filter wells, radial collector wells and small-scale radial collector wells are used for water abstraction with production capacities ranging from 29 m³/day to up to 110.000 m³/day. Reported travel times of the bank filtrate range from min. 2 to max. 100 days. BF proved to be advantageous as pre-treatment in situations with high concentrations of organic compounds. Also the removal of pathogenic microorganisms, colour and dissolved organic carbon, UV absorbance, turbidity, total and faecal coliform counts during monsoon season could be observed. All investigated examples proved to sustain their treatment efficiency. They conclude that BF has a great potential for improving both quality and quantity of many water supplies throughout the country.

In cities where BF is already applied, the BF process is accepted as purification treatment step in combination with – compared to other supply systems – much more limited post-treatment chlorination or ozonation (SANDHU et al. 2011; SINGH et al. 2010). SANDHU et al. (2011) identified a number of feasibility issues such as sufficient flow in rivers, protection of landside groundwater from contamination, high arsenic concentrations at shallow depths, insufficient scouring of riverbed and removal of the clogging layer due to heavily regulated surface flows and discontinuous well operation due to lack of continuous electricity supply or electricity saving measures.

Facing increasing quantities of insufficiently treated wastewater being discharged into rivers, the authors propose to preferably locate BF systems upstream of big cities to minimise the risk of landside groundwater contamination. Moreover, attention should be paid to private production wells and unmonitored pumping resulting in decline of the groundwater level (e.g. as been observed in Delhi). LORENZEN et al. (2010) propose to accompany planning and operation of BF sites by field studies on a local scale to optimise the systems in accordance with local conditions (e.g.: observing content of undesired substances, obtain the desired shares from each source and attenuate contaminant concentrations).

Hydrogeological investigations are pointed out to be essential, not only to assess hydraulic parameters but also to evaluate the risk of contamination by different substances and to understand the contributing processes. Water levels and quality in surface water should be monitored throughout the first year at least, to identify temporal and spatial variations in the interaction zone of surface water and groundwater. Temperature logging is proposed as relatively easy and cheap method that may reveal valuable information on the flow regime.

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