Biological processes of bank filtration,
In the capital city of Germany the entire water supply for 3.5 billion inhabitants is derived from intra-urban water resources. Thus, in Berlin artificial recharge techniques such as bank filtration play an important role for groundwater recharge and drinking water abstraction. Focus of the investigations was the analysis of the lake/low land river infiltration zone, the interstitial, as a small boundary layer of about 30 cm depth with high biological activity. The significance of the biotope for the self purification processes in natural and induced groundwater recharge was evaluated.
Clogging phenomenon occurs, but up to now the significance of the processes with respect to mechanical (e.g. POM input, gas bubbles), chemical (e.g. precipitation of calcium carbonates) and biological effects (e. g. the formation of extracellular polymeric substances, EPS) is not sufficiently known. In the upper 10 cm, nearly half of the interstitial volume is filled with particulate organic material (POC) to which epipsammic diatoms contribute 10 % in the upper 5 cm, and their extracellular polymeric substances form biofilms that account for approximately 10 vol.-%.
The colmation leads to an effective particle retention. No significant transport of inert microparticles (2,44 mm) or natural FPOM (< 1 mm) as acting as fluorescent tracers was observed beneath 1 cm during 3 to 14 days or even longer periods.
Interstitial systems are a habitat with a highly activated biocenosis of bacteria, algae, protozoa and metazoan (Meiofauna), which ensure biological degradation and regeneration in a river/lake bed as well as effectiveness and sustainability of bank filtration.
The meiofauna counteracts clogging processes by detritivorous activity and surficial tube dwellings but also reduces pore volume when abundance is high. Population maxima up to 17,008 Ind. dm-3 respectively 4,400 Ind. dm-3 as annual mean occur.
The turnover of POC (particulate organic carbon) exceeds that of DOC (dissolved organic carbon) during summer approximately tenfold. POC and probably also biofilms being prone to degradation due to high temperatures and enhanced microbial activity.
Climate change effects on bank filtration
During induced bank filtration, the quality of the infiltration water is improved by temperature changes due to climate change. Mean water temperatures in the Berlin urban area have increased significantly, e. g. the mean surface water temperature has already been raised about 2.4 °C at Lake Tegel within 28 years (1980 – 2008). With respect to climate change, the effects of increasing temperatures on artificial bank filtration are a matter of special importance, especially the risk of sulfide formation due to its toxic effects to the interstitial biocenosis.
It had been pointed out that the water temperatures occurring in the Berlin urban area had an effect on redox conditions during bank filtration process. A significant correlation of the inorganic nitrogen concentration by denitrification as well as manganese and iron reduction occurred. Up to now, the sulphate reduction is without significance, but with increasing water temperature this process will take place.
The purpose of this research was in particular the monitoring of changes on hydraulic properties of the aquifer as well as the clogging effect caused by disturbances (e.g. effects of floods on the river bed or waves on lake shores and drought periods with high particle sedimentation); as well as temporal and spatial variations of physical-chemical gradients under natural instabilities triggered by floods, droughts or shore erosion.
Implementation of bank filtration in tropical countries
The results of this investigation form the basis for the implementation of bank filtration in semi-arid areas, too. The knowledge about physical, chemical and biological processes during bank filtration is still scarce for regions with climates different from that in Central Europe. Considerable experience exists in Germany that validates river bank filtration in an array of environmentally different settings and under conditions that have rigorously tested the technology. The same cannot be transferred to tropical countries as surface waters have higher temperature and more algae. River bank filtration is required to be tested for Brazil raw water quality e.g. Lago Peri, Santa Catalina and Rio Beberibe, Pernambuco.