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BANK FILTRATION SYSTEMS: RIVERS, LAKES, INFILTRATION PONDS AND SLOW SAND FILTERS

BANK FILTRATION SYSTEMS: RIVERS, LAKES, INFILTRATION PONDS AND SLOW SAND FILTERS

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Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



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accumulation of organic matter, too), and if RBF water abstraction rates are not adapted to the

hydraulic conductivity of the river bed and banks or if the ground water table decreases in a

large-scale due to water abstraction at some places. The majority of RBF areas possess in- as

well as exfiltration conditions, but the groundwater flow beneath the rivers is mostly

neglected (Hiscock & Grischek, 2002).

The infiltration conditions, especially the clogging of the river bed by sedimentation of

suspended clayey material, is regulated by floods: The periodically flooding due to snow

smelt or rainy season causes a bed load and a resuspension of fine deposited material, thus the

interstice is re-opened and the hydraulic conductivity of the river bed increases. Only

periodically floods guarantee a long-time infiltration capacity, but flooding affects RBF in

many other ways such as shock load of contaminants, high hydraulic pressure with reduced

filter contact times, and destruction of the developed filter stratification as a complex

biocoenosis within the pore system.



3.2. Lake Bank Filtration

LBF is scarce and only some sites are known such as Naimital, India, Enns Reservoir,

Austria, Lagoa do Peri, Brasil, Lake Müggelsee and Lake Tegel, both Berlin, Germany,

where most experiences exists (Brugger et al., 2001; Sens et al., 2006, Dash et al., 2008;

Massmann et al., 2007). Lake Müggelsee is a shallow lowland lake of the River Spree and

Lake Tegel a shallow lowland lake of the River Havel, water residence times are 63 days

respectively 70 days. Infiltration conditions in lakes are determined by the colmation layer

formed by lake sediments (calcareous mud in oligotrophic lakes, organic mud in eutrophic

lakes, with an extension of many meters), and the bottom of the lake is clogged by these lake

sediments; strait infiltration zones occur only at the lakes shores (Figure 3). In general the

lake littoral zone serves as an infiltration system to ground water if ground water table is

lower than lake water level; but LBF water is mixed with groundwater and infiltration water

of the opposite lake side. Formation of unsaturated conditions beneath the lake’s littoral zone

occurs if groundwater abstraction rates are not adapted to the hydraulic conductivity of the

littoral zone (= over exploration) or if the hydraulic conductivity of the lake shore is reduced

due to clogging (= accumulation of fine particulate material in the littoral sediments). The

introduction of air beneath the lake bottom by an unsaturated sand layer causes the aeration of

anoxic lake sediments, which can occur due to high oxygen consumption of infiltrated water,

and oxic conditions are re-established from the deeper sediment layers upwards to the lake

bottom. Unsaturated sediments beneath the lake lead to a reduction of the hydraulic potential

and to a decrease of the infiltration rates.

In contrast to RBF intensive water level changes with high flood rates and re-opening of

the interstice does not occur in LBF, but in the shallow littoral zone of a lake, wind induced

waves lead to a resuspension of fine sediment components and a re-opening of the interstice;

a wave with an amplitude of 20 cm still affects the sediment in a water depth of 2 m with a

resting orbital movement of 0.6 cm. The resuspended sediment particles will be distributed in

the whole lake by wind induced currents and will finally settle in deeper lake areas.



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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 abstraction 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).



At Lake Tegel infiltration site, a two years investigation was carried out at different local

positions (in front of Phragmites, erosion and water lily stands) and indicated a small scale

differentiation of the hydraulic permeability (Figure 4), the hydraulic potential kf varied from

3 x 10-5 – 8 x 10-8 m sec-1; the mean infiltration rate were 9 L m-2 h-1 (0.7 – 27.0 L m-2 h-1),

leading to a infiltration velocity of 0.5 m day-1 with a variance of 0.05 – 1.8 m day-1. In

general, summer infiltration rates were about 101 times higher than in winter period. The well

abstraction rate during normal production conditions did not influence significantly the

infiltration rate, a consequence of the subsoil water mixing processes as well as of the very

important clogging factor. Exceeded water abstraction rates led to unsaturated soil beneath



Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



145



the lake and to a decreased hydraulic conductivity as mentioned above (Hoffmann & Gunkel,

2009a).

At Lake Tegel the absolute infiltration area for one well is 5,500 m2, which means about

0.3 m2 inhabitant-1. Detailed sediment investigations at Lake Müggelsee, Berlin, Germany,

confirms, that only a small littoral zone < 5 m depth with kf > 10-5 can support bank filtration

(Massmann et al. 2008a).

A hydrogeological model of the bank filtration site in Lake Tegel is given by Massmann

et al. (2008b) pointing out a vertical differentiation of the groundwater: The shallow

groundwater observation wells capture bank filtration water with a transit time of at least 4 - 5

month, deeper groundwater observation wells deliver water with an age of some years to a

few decades. Groundwater underflow of the lake occurs, even being more then 1 km in width.

Due to mixing of these distinct groundwater flows, the portion of bank filtration water from

the proximate lake shore amounts only 48 %. Field studies on the fate and transport of

pharmaceutical residues in Lake Tegel bank filtration site confirm these water transit times

(Heberer et al., 2004).



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 abstraction 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.



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Figure 4. Annual variation of Lake Tegel, Berlin, Germany, bank infiltration rates at different locations

(in front of Phragmites stands, at a palisade passage with high water flow, and in a water lilies stand,

close together < 20 m), and the variance of well abstraction rate (well no. 12 – 14, 100 m beside the

lake shore, in front of the experimental area).



Figure 5. Schematic infiltration situation of an artificial groundwater recharge pond near Lake Tegel,

Berlin, Germany. Stage 1: Surface water infiltrates at the beginning of the recharge cycle, the

infiltration rate is maximal. Early stage 2: Saturated conditions have been established. Late stage 2: A

clogging layer has formed at the bottom of the ground water recharge pond and infiltration rate is

slightly reduced, still saturated conditions occur. Stage 3: Hydraulic resistance of clogging layer is too

high, and an unsaturated zone develops beneath the pond and infiltration rate decreases rapidly, the

groundwater table declines. From Greskowiak et al. (2005).



Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



147



3.3. Ground Water Recharge Ponds and Slow Sand Filters

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

(DVGW, 2006). But the 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).

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. Greskowiak et al. (2005) verified this cycle for the Tegel recharge pond, Berlin,

Germany (Figure 5). 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.

In GWR ponds infiltration velocities range from 0.1 – 0.2 m h-1, and as maximum 0.5 m

-1

h can be reached. These high infiltration rates are realized by the water pressure of 1 - 2

meters of the infiltration pond.

A similar, small scale construction can be used as SSF with the so called direct filtration,

being used for decentralized water treatment; two different technologies are applied, SSF with

up flow or down flow (Graham & Collins, 1996; Gimbel et al., 2006; Bernardo et al., 2006).



4. 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 microorganisms, algae, especially filamentous algae and particulate organic matter (POM; Figure

6). 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. Too, occurrence of unsaturated sand filter conditions leads to a severe decrease of the

hydraulic permeability.



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Chemical processes for clogging are mainly the precipitation of calcium carbonates, iron

oxyhydroxy compounds, sulphur polymers and iron sulphides. Significant biological

processes for clogging are the development of biomass within the interstice (algae, bacteria)

and the excretion of extra-cellular polymeric substances (EPS), in most cases polysaccharides

and polypeptides (Flemming etal., 1999). The clogging processes are strongly determined by

photoautotrophic and heterotrophic production, but too by conversion and decomposition of

FPOM in the interstice. Different pathways for the entry of FPOM into the sediments have to

be distinguished, namely passive settling on the sediment surface layer, the bioconversion of

POM to FPOM, passive transport by convergent infiltration currents and an episodically

burial after sediment movements (Rinck-Pfeiffer et al., 2000; Langergraber et al., 2003). This

intrusion of sestonic5 matter, suspended in river or lake water, is often considered to be the

most important factor for clogging (Hiscock & Grischek, 2002; Langergraber et al., 2003).

According to Okubo & Matsumoto (1983), the concentration of suspended solids should not

exceed 2 mg L-1 to avoid rapid mechanical clogging. But too, re-opening of the interstice is

observed, in rivers by high flood and occurrence of bed load, and in lakes by the resuspension

of fine particles by wind waves in the littoral zone.

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 epipsammic diatoms, forming the biofilm, fine rhizomes of macrophytes and

detritus6. 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 (Figure 7).



Figure 6. Schutzdecke of a groundwater recharge pond near Lake Tegel, Berlin, Germany, after 4

months service with removal of the surface sand layer.



5

6



Seston = suspended inorganic and organic (dead and alive) matter.

Dead organic matter.



Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



149



Figure 7. Mean mass and volume fractions of the water-sand-boundary layer (0 - 8cm depth) at the

bank filtration site at Lake Tegel, Berlin, Germany (mean of 1 year, n = 16).



Within the interstice a biofilm is developed, consisting of an adapted biocoenosis of

bacteria, algae and small invertebrates, living in the pore system of the sand (Beulker &

Gunkel, 1996). An excretion of EPS occurs, build up of carbohydrates and/or proteins, which

form complex three-dimensional structures within the interstice (Figure 8).



Figure 8. Water-sand-boundary layer at the bank filtration site at Lake Tegel, Berlin, Germany,

accumulation of planktonic algae cells, mostly diatoms, and detritus flocs on the superficial sand layer,

0 - 1 cm depth, Lake Tegel bank filtration site, Berlin, Germany (27.04.2004).



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Figure 9. CT of a sediment core at the bank filtration site at Lake Tegel, Berlin, Germany, blue =

overlying water and water in the interstice, white = Ca-carbonate precipitations, grey = sand grains, a

tunnel built by an invertebrate and filled with water is recognizable. The CT technology was developed

and applied by the BAM. Berlin, Germany.



In both, rivers and lakes, the burrowing activity (bioturbation) of the meiofauna is of very

high significance to maintain a hydraulic permeability (Figure 9). Due to the mobility of the

meiofauna and some migration upwards and downwards interstice is re-opened. Furthermore

parts of the POM, being in the pore system, are consumed by these organisms. The CT7 graph

of Lake Tegel, sediment demonstrates the nearly complete filling of the interstice by POM

and shows a typical tunnel formed by sediment living organisms.

At Lake Tegel infiltration site, complete clogging did not occur during the two years

investigation period, but the in situ hydraulic permeability decreases about 10-2 m sec-1

compared with the inert sand interstice, thus a steady state of clogging processes by the

development of the biocoenosis and the re-opening by bioturbation occurs, that means, a local

high accumulation or development of POM is more attractive for sediment feeders, and they

will migrate into this area, a phenomenon well known as patchiness in river and lake littoral

ecology (Yamamuro & Lamberti, 2007). However, experimental data about this dynamic

interaction, the build-up of EPS and the destruction of POM and EPS by interstitial fauna are

scarce. Most of the available information concerns artificial GWR systems, where the inflow

conditions and the turn over processes in the sediment boundary layer are significantly

different from those of natural littoral zones.



7



Computer tomography.



Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



151



5. SEDIMENT BOUNDARY LAYER

Over the past few years the benthic boundary layer has been studied intensively, as

chemical gradients are most obvious and microbial activities are maximal in this layer

supporting biofilm growth (Beulker & Gunkel, 1996; Hiscock & Gruschek 2002). The

biofilm is a complex community of algae, bacteria, fungi and invertebrates, living in the

interstice system: bacteria and algae produce extra-cellular polymeric substances (EPS),

forming a dense three-dimensional structure (Lawrence et al., 1998, 2002; Paterson, 2001).

The development of a high structured biofilm, consisting of several groups of organisms

occurs only under oxic conditions; the limiting oxygen concentration is not yet knows. Do to

the occurrence of algae, a daily cycle with oxygen enrichment during day and oxygen

depletion during night must be considered, thus critical oxygen concentrations are reached

only at night. Thus, the development of a dense layer of superficial filamentosus algae like

the schmutzdecke in GWR ponds will in general impact the development of a high diversity

biofilm because of light limitation in the interstice and, as a consequence, decrease of the

interstitial algae as well as by an increased probability of anoxic conditions during night

(oxygen depletion due to respiration of the schmutzdecke’s algae). Concerning oxygen

balance, two different states have to be clearly distinguished, anoxic or anaerobic infiltration

water and the formation of a micro-zoning in the pore system with small anoxic and

anaerobic zones. Microscopic analyses point out, that too under oxic conditions a microzoning occurs with some anaerobic areas maybe in dead end pores, verified by the presence

of iron sulphide (pyrite).

Biofilms and the associated algae are of special importance for water purification

processes due to a net oxygen production as well as an adsorption of ions like toxic metals

and of dissolved organic matter (DOM). The bacterial community consume and metabolize

DOC, and the excretion of exoenzymes leads to an elevated mineralization efficiency of

DOM and some inorganic polymers (e. g. polyphosphate, colloids; Decho, 2000; Wingender

& Flemming, 2001). Other well-known properties of biofilms are the high water binding

capacity and the stabilization of the surrounding sediment grains (Yallop et al., 2000). The

biofilm has a three dimensional structure and is the basis for a micro-zoning of the filter area

(Flemming et al., 1999), dead end pores occur and serve in retention of small particles (Auset

& Keller, 2006), but too enable the formation of oxygen mirco-zoning in the interstice; some

degradation processes, e. g. of drugs, are linked to a small scale change of oxidative and

reductive conditions in the filter.

The most important factor for the spatial build up and the function of the biofilm is the

excretion of EPS by bacteria and algae as ‘housing’. Structural determinations of EPS

forming biofilms have shown substantial progresses during the last decades, and neutral and

polyanionic polysaccharides as well as peptids (glycolproteins, lipoproteins) form EPS

(Characklis et al., 1990; Wingender et al., 1999; Flemming & Wingender, 2001; Sutherland,

2001). The EPS is a secretion of algae (e. g. diatoms excrete mucus for movement or cell

sheaths) or part of the cell structure (e. g. lipopolysaccharides of gram negative bacteria). EPS

from hydro-gel to viscose elastic structures like filaments, nets and plaques (Figure 10). A

conversion of EPS, that means a re-construction due to enzymatic degradation by hydrolase

and condensation of the formed oligosaccharides can occur, but in general the stability of the



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EPS is very high, and EPS structure consist over some weeks to months, which mean the EPS

outlive the builders for a long-time.

The biofilm biocoenosis is built up by bacteria, fungi and algae, whereas the vertical

distribution of the algae is limited by the transparency, which means the light penetration into

the interstice. Bacteria and fungi settle too in deeper, aphotic zones; bacteria build up an

adapted community, on one hand related to the DOC input and its degradability and on the

other hand by the micro-zoning in the three dimensional interstice system. It must be assumed

that the bacterial community is to a high degree a local one, but up to now only few

investigations are available (Kolehmainen et al., 2007). The abundance of bacteria at Lake

Tegel, Berlin, Germany, was very high, and they reached up to 2 x 109 cells gram-1 sediment,

determined with DAPI8 fluorescence technique; the vertical distribution showed highest cell

numbers at the surface layer of 0 – 5 cm, while in depth of > 20 cm, still 0.2 x 109 cells gram-1

sediment were found. Further specification of these bacteria can be done using FISH 9 or

PCR10 (Spring et al., 2000, Emtiazi et al., 2004).

The bacterial community in the interstice seems to be a complex system of linked

species, adapted to DOM characteristics, fixed in the three dimensional EPS structure and

being at least partly located in dead end pores, this give a analogous community to the

bacterial flocs in surface water (Zimmermann-Timm, 2002).



Figure 10. Biofilm with some algae and bacteria cells and a dense fibrillose net structure of

extracellular polymeric substances (EPS) in the interstice of the water-sand-boundary layer at the bank

filtration site at Lake Tegel, Berlin, Germany (depth of 3 – 4 cm, 17.05.2004).



8



4'6-diamidino-2-phenylindole-2HCl, a fluorescent dye for DNA.

Fluorescence in situ hybridisation.

10

Polymerase chain reaction.

9



Bank Filtration of Rivers and Lakes to Improve the Raw Water Quality …



153



Beside bacteria cells activity, the bacterial exo-enzyme activity (e.g. aminopeptidase,

glucosidase, phosphatise) is of high significance for degradation processes of DOM, too

(Miettinen et al., 1996, Hendel et al., 2001).

The investigations carried out in the fine sand infiltration site of Lake Tegel, Berlin,

Germany, offered a high portion of POM with a maximum at the surface layer with 15 mg g-1

sediment, stretching down to 50 cm (Figure 11; Gunkel et al. 2008). Epipsammic algae11

occur with a high biomass in the upper interstitial zone of about 0 – 6 cm depth, and it has to

be pointed out that planktonic algae species from the lake water are only transported into the

interstice to a small extent (Beulker & Gunkel, 1996; Gunkel et al., 2008). Thus this watersediment-boundary layer serves as a mechanical filter for these algae cells – a process being

of very high importance for the behaviour and fate of all types of POM in bank filtration. This

leads to

1.

2.

3.

4.



the surficial accumulation of algae cells,

an easy resuspension of surficial deposited algae,

an insignificant penetration of algae cells into the interstice,

a high attraction of the surficial sediment layer for herbivorous interstitial fauna with

its burial activity,



oxygen concentrations of deeper sediment layers are not reduced by POM mineralisation but

only by DOM concentration and mineralisation.

Concerning toxic cyanobacteria, too, an accumulation at the surficial sediment layer must

be expected (see Chapter 6.6).

The occurrence of interstice algae lead to a natural bioproduction in the small photic

surface layer of a few centimetres, and both, POC as well as DOC of the infiltrating water is

influenced by the in situ production of POC and DOC (Hoffmann & Gunkel 2009b). The

vertical distribution of chlorophyll (Chl a) confirmed that interstitial algae biomass forms a

significant part of the total POM in the upper interstitial zone of 0 – 6 cm (Figure 12), and

primary production is assumed to be the most important source of organic carbon in the

interstice. Up to now only biomass and no turnover data of interstitial algae are available, too

a lack of information exists concerning pico-algae12 (Dittrich et al., 2004).

At Lake Tegel the Chl a concentrations in the upper 5 centimetres of sediment were very

high (21 to 28 µg cm-3) and decreased with depth, only traces of Chl a were detected below 5

cm depth. A Chl a concentration of about 25 µg cm-3 must be evaluated as extremely high

compared with the lake water, which even under eutrophic conditions contains only about 20

µg L-1 Chl a. Thus, the total algal biomass in the upper sandy layer of the interstice was about

1000 times higher than in the corresponding water body of Lake Tegel (Gunkel et al., 2008).



11

12



Algae attached to sand grains.

Pico-algae are cells between 0.2 and 2 µm that can be either photoautotrophic of heterotrophic.



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