Rostfärgat slam i dränagesystem vid vattenkraftsanläggningar
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From embankment dams is something that has long been noticed and given rise to different questions. The most serious question is if the sludge is a result of inner erosion through the dam, and thus consists of material transported from the dam. Concerns have also been raised whether the sludge could interfere with the systems drainage capacity and thereby lead to high pore pressure in the dam, or if the measurement of seepage is disturbed, as the measuring is usually made via Thomson weirs, which could risk being blocked.
The result of this investigation shows that the sludge is a result of the oxidation of iron by iron bacteria, and to the greater part consists of these oxides as well as the bacteria?s own structures. The iron oxidizing bacteria, FeOB, oxides ferrous iron to ferric as a part of their metabolism, and thereby create large quantities of iron(oxy)hydroxides. The bacteria that contributes the most to the building of sludge, Gallionella ferruginea and Leptothrix ochracea, are characterized by their extracellular structures, twinned tails in the case of
Gallionella f. and sheaths in the case of Leptothrix. These structures are easy to observe via microscope and both strains have been observed in samples of the sludge from
dams.
In order to grow and produce sludge these bacteria need two things, solved ferrous iron in the water and a gradient from a non-aerobic to an aerobic environment. The ferrous
iron is supplied by another type of bacteria, FeRB, iron reducing bacteria, which in an oxygen free environment reduces ferric iron with organic compounds as electron source.
No sufficient methods to prevent the forming of sludge have been presented. Water flushing of drainage pipes as well as suction of wells has shown to be an efficient method
to temporarily remove the sludge.
Accordingly, the sludge by itself is not a sign of inner erosion in the dam. In extreme cases and in conjunction with neglected maintenance it?s possible for the sludge to
hazard the draining function. As for the measuring of seepage, the sludge becomes a problem when (partially) blocked Thomson weirs give unreliable values.
Until a working method to prevent the forming of sludge emerges, regular maintenance is required in systems where sludge is present. Supplementary instrumentation for flow
measuring, for example water stand pipes or the gauging of temperature in the dam, as well as video surveillance of wells can give more reliable measurements.