Bank Filtration

[Audio] Natural Bank Filtration. Bank Filtration.

[Audio] Bank filtration ( BF) is a drinking water pre-treatment step, where river water is induced to percolate in subsurface passage through a river bed and mix with ambient (or natural) groundwater, before being extracted through a pumping well adjacent to the river bed. It can be applied as first step within a multi-barrier approach in an overall treatment chain where groundwater quantity is insufficient or of poor quality (e.g. geogenic pollution)..

[Audio] BANK FILTRATION SYSTEMS river bank filtration ( RBF) in riverine zone, • lake bank filtration ( LBF) at lake shore, • ground water recharge ( GWR) with water charged artificial ponds, •slow sand filtration ( SSF) in constructed filters..

[Audio] Some relevant processes for water purification during bank filtration are mechanical retention of POM (particulate organic matter)at the surficial sand layer of the infiltration zone, • adsorption of NOM (natural organic matter) and anthropogenic contaminants on soil compounds during underground water passage to the ion well, •biological degradation of NOM and anthropogenic contaminants during sand filter passage, • decrease of bacteria and encysted bacteria (e. g. E. coli) due to limited survival time during the groundwater passage of about 50 days..

[Audio] Riverbank filtration ( RBF) is the main water ion method used in many European regions. The long-time use of bank filtration led to a good practical knowledge in construction of wells and management of infiltration ponds. Many investigations of the RBF describe: - hydrogeological processes, - water purification during infiltration - occurrence of an interstitial flora and fauna, and verify a biological self- purification process in the sandy infiltration layer.

[Audio] RBF at the riverine is the most common system and is characterized by: - the dynamic of the flow regime of the river - coarse sands - or even fine gravels in bank filtration sites. In general an infiltration occurs at the river bed too..

[Audio] RBF is a natural process, occurring if the water level of the river is above the ground water table. That means : i. infiltration conditions exists; ii. exfiltration conditions occur during low discharge of the river, - means water level of the river is beneath the ground water table. Thus, river infiltration conditions are dynamic and determined: (a) surface water and ground levels, (b) low and high flood conditions of the river.

[Audio] Figure 2. Schematic representation of different types of flow conditions at river bank filtration sites. Type 1: River serve as drainage system with infiltration conditions, but at the filtration site, exfiltration occurs. Type. 2: River with exfiltration conditions. Type 3: River with exfiltration and infiltration and a strong groundwater flow beneath the river. Type 4: Formation of unsaturated conditions beneath the river occurs if groundwater ion rates are not adapted to the hydraulic conductivity of the river bed or if the hydraulic conductivity of the river bed material becomes clogged due to surface water pollution inputs. Type 5: The river bed cuts into the confining layer and the filtration site is completely isolated from the groundwater flow. Type 6: Bank filtration with lateral wells. From Hiscock & Grischek ( 2002)..

[Audio] Design and construction principles. Diagram Description automatically generated.

[Audio] 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 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 freshwater storage capacity for buffering extreme climatic conditions and shock loads. Also represents an artificial groundwater recharge technique preventing the overuse of aquifers, saltwater intrusion and land subsidence ( NRMMC 2009)..

[Audio] Lake Banking Filtration -2,600,000 habitants are served with LBF water as main water treatment since about 70 years - additionally only oxygenation and iron precipitation by water sprinkling and fast sand filtration for iron elimination are done..

[Audio] LBF can be done next to lake shore consisting of fine to middle sands, but in lakes sedimentation of organic and inorganic seston4 occurs, leading to an accumulation of fine material at the lake bottom, and infiltration conditions vary with lake's water depth..

[Audio] Figure 3. Schematic representation of two different types of flow conditions at lake shore filtration sites. Type 1: The lake littoral zone serve as a drainage system and bank filtration water is mixed with groundwater and infiltration water of the opposite lake side. Type 2. Formation of unsaturated conditions beneath the lake littoral zone occurs if groundwater ion rates are not adapted to the hydraulic conductivity of the littoral zone or if the hydraulic conductivity of the lake shore becomes clogged, an aeration of the lake sediment from beneath occurs..

[Audio] Process during bank filtration. Process during bank filtration.

[Audio] Ground Water Recharge Ponds and Slow Sand Filters Artificial ponds for GWR can be constructed at any sites with middle to coarse sand and good infiltration conditions, they are filled with surface water, and water level determines the hydraulic pressure and thus the infiltration. Worldwide ground water recharge ponds are used to stabilize and increase the ground water level as well as to guarantee sufficient water purification during infiltration process..

[Audio] Ground Water Recharge Ponds and Slow Sand Filters Nutrient content of the raw water (mainly N and P) lead to a high primary production in infiltration ponds and more eutrophic conditions will be established in the ponds. Due to low water depth of 1 to 2 m and light penetration to the bottom, dominantly benthic algae develop and form a dense layer at the sand surface, the so called schmutzdecke. This leads to clogging of the water-sand-interface, 1) mechanical by the dense growth of algae, and 2) by the precipitation of calcium carbonate due to increase of the pH during photosynthesis (= biological Ca-precipitation)..

[Audio] The infiltration conditions of artificial recharge ponds are characterized by the time period after start to work, and three phases have to be distinguish: -An open sand filter without bioactivity, the development of an interstitial biocoenosis with rapid clogging and reduced infiltration capacity, and a complete clogging, that makes necessary a removal of the upper sand layer. -At the beginning of the recharge cycle the infiltration rate is maximal and saturated conditions have been established beneath the pond. -During GWR working, a clogging layer has been formed at the bottom of the GWR pond and infiltration rate is reduced. - With an increasing hydraulic resistance of the clogging layer, an unsaturated zone develops beneath the pond and infiltration rate decreases rapidly, a removal of a few centimetres of the surficial sand layer becomes necessary. -This surficial sand is washed and re-deposited in the pond several times per year..

[Audio] CLOGGING OF THE INFILTRATION ZONE Clogging is a well known process in slow sand filters, where the hydraulic permeability decreases during working period and lead to an extreme reduction of the infiltration capacity; visually this process leads to the formation of the schmutzdecke, a surface layer of micro-organisms, algae, especially filamentous algae and particulate organic matter ( POM; Figure 6)..

[Audio] Clogging can be caused by some mechanical, chemical and biological processes, and is the limiting factor for infiltration capacity in many bank filtration sites. Well-known mechanical factors for clogging are the input of fine sand particles ( silt, clay), fine particulate organic matter ( FPOM), and the oversaturation and development of gas bubbles ( oxygen by primary production or methane by methanogenese) in the sand pore system. Occurrence of unsaturated sand filter conditions leads to a severe decrease of the hydraulic permeability..

[Audio] Chemical processes for clogging are mainly the precipitation of : - calcium carbonates, - iron oxyhydroxy compounds, -sulphur polymers - iron sulphides. biological processes for clogging : - development of biomass within the interstice ( algae, bacteria) - the excretion of extra-cellular polymeric substances ( EPS),.

[Audio] Clogging of the interstice under natural, but induced infiltration conditions at Lake Tegel, Berlin, Germany, is caused to a large extent by POM, which is composed of living biomass such as : 1.epipsammic diatoms, 2.forming the biofilm, 3. fine rhizomes of macrophytes 4. detritus. This biological clogging reaches down to a sediment depth of least 10 cm. The interstice of the sandy littoral zone is filled up to about 50 % with POM such as detritus, living bacteria and algae cells..

[Audio] Water purification processes during bank filtration are influenced mainly by the infiltration rate and the extension of the active sandy filter zone, that together determine the effective contact time of contaminants with the microorganisms, •geological and hydrogeological conditions (e.g. pH, redox potential, sand grain surface characteristics), • characteristics of the interstice, especially the filling of the interstice with inorganic and organic matter as well as the formation of dead-end pores, •the interstitial flora and fauna as a highly adapted community of bacteria, fungi, algae and small invertebrates..

[Audio] Thus many different contaminants are of significance for drinking water quality and have to be analysed and evaluated for the behaviour in bank filtration processes, especially physical effects like surface water heating, inorganic contaminants, especially metal ions, NH4+, NO3- and SO42-, • DOC as a summary parameter for natural and anthropogenic compounds • POM such as leaves and other plant rests, • pathogens like viruses, bacteria ( coliform bacteria) and new protozoa like Giardiaintestinalis and Cryptosporidium parvum, •toxic cyanobacteria and extracellular cyanotoxins..

[Audio] Advantages Can dampen pollution peaks and buffer extreme climatic conditions ( quality and quantity). Huge freshwater storage capacity. Can reduce costs of supplementary treatment steps. Low requirements for skilled labour, chemicals and energy use (depending on purpose of output water)..

[Audio] Disadvantages Prone to clogging/ colmation at high levels of suspended solids. Permeability can be influenced by high (seasonal) temperature amplitude. High organic pollution and high mean temperature.

Reference. 1. https://sswm.info/step-nawatech/module-1-nawatech-basics/appropriate-technologies-0/bank-filtration 2. Gertsen , N and Sonderby , L. 2009. Water Purification. 2009 by Nova Science Publishers, Inc..