1.5 Bank Filtration for Water Treatment
Bank filtration (BF) process occurs when the hydraulic head in surface water is higher than in underlying groundwater (Figure 1.3).
Figure 1.3 The basic principle of bank filtration.
During the BF, the four types of filters act together and their interactions are very strong to remove particles, biodegradable organic compounds, trace organics, microorganisms, as well as ammonia and nitrates. It is a reliable, natural, and multi-objective treatment process applied to rivers or lakes (Gillefalk et al., 2018). BF has the potential to replace or support other treatment process by providing a robust barrier within a multi-barrier system and also reducing the costs of water treatment in general (Sharma and Amy, 2009). It is known that BF is an efficient and well accepted treatment method of surface water in Europe. For more than 100 years, riverbank filtration has been used as a treatment method in European countries for public and industrial water supply along Rhine, Elbe, and Danube rivers (Grischek et al., 2003). There exist several mechanisms for improvement of water quality during BF. During infiltration, the surface water moves through the soil and aquifer sediments. During the latter process, surface water is subjected to a combination of physical, chemical, and biological processes such as 1) filtration, 2) solution-precipitation, 3) ion-exchange, 4) sorption-desorption, 5) complexation, 6) redox reactions, 7) microbial biodegradation, and 8) dilution that significantly improve water quality. The main advantages and limitations of BF systems are listed below (Ray et al., 2002; Gale, 2005).
Advantages:
– BF is a natural treatment process, which avoids or reduces the use of chemicals and produces biologically stable water as end-product;
– BF improves water quality by removing particles (suspended solids), organic pollutants, microorganisms, metals, and nitrogen from the surface water;
– BF dampens concentration peaks associated with spills (in river/lake) as well as dampens temperature peaks;
– BF replaces or supports other surface water treatment processes by providing a robust barrier and reduces the overall cost of surface water treatment.
Limitations:
– BF is site specific (rivers or lakes) and is feasible only when the local hydrogeological conditions are favorable and known;
– Leaching of the aquifer materials can occur under reducing conditions, sometimes leading to increased concentration of iron and manganese in extracted water;
– Another problem is clogging of the aquifer due to accumulation of suspended matter that is filtered out when river/lake water enters the aquifer;
– BF and groundwater recharge may be only a limited barrier for certain contaminants that pass through the system;
– Influence of surface water and operation on quality is poorly known.
1.6 Constructed Wetlands (CWs)
In terms of wastewater treatment, natural self-purification processes involving vegetation, sediments and their associated biocenosis and microbiocenosis (phytobiome) are applied in CWs (Vymazal, 2007). These latter, together with iron oxidation bioreactors and composting reactors belong to the so-called passive biological treatments. The advanced technologies include acid mine drainage (AMD) treatment in bioreactors with sulfate-reducing bacteria cocktails (Neculita et al., 2007) and CWs (Johnson and Hallberg, 2005). The cost factor involved in the passive treatment of water may be a large start-up cost but is ideal in the natural rehabilitation of water over time.
Natural wetlands have been used for a long time for disposal of waste-water, without really considering the negative effect that pollution can have on these aquatic ecosystems. CWs aim to reproduce (and intensify) the processes observed in natural wetlands, swamps, or marshes. They can be classified according to: 1) hydrology (surface flow versus subsurface flow); 2) macrophyte types (emergent, free-floating, or submerged plants); and 3) flow path (horizontal or vertical). The major flow types are as follows:
– Free Surface Flow CWs (FSF-CWs), as shown in Figure 1.4a, are ponds with emergent macrophytes. A free-water CW reproduce natural processes of a natural wetland, where water flows slowly through the wetland, promoting the deposition of particles, the removal of pathogens and the degradation and uptake of OM and nutrients by microbes and macrophytes;
– Floating macrophyte mats on CW surface, or Floating Treatment CWs (FT-CWs), as shown in Figure 1.4b, use natural water macrophytes on buoyant mats drifting on the surface of the water. The floating macrophyte mat promotes the hydraulic flow of water below and through the macrophytes, with a free root system that grows deeper into the water column and function as a natural filter;
– In subsurface flow CWs (SSF-CWs), there is no superficial water, and the water level is kept below in the filter material, as shown in Figure 1.4 (c and d): their advantages and limits are presented in Table 1.1.
Figure 1.4 (a) Simplified scheme of a Free Surface Flow CW (SFS-CW); (b) Schematic representation of a floating macrophytes surface flow CW (FT-CW); (c) Principles of Horizontal subsurface flow CW (HSSF-CW); and (d) Principles of