D10: Impact of Contaminants - Hydromod
D10: Impact of Contaminants - Hydromod
D10: Impact of Contaminants - Hydromod
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Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas<br />
EUROLAKES<br />
<strong>D10</strong>: <strong>Impact</strong> <strong>of</strong> <strong>Contaminants</strong><br />
FP5_Contract No.: EVK1-CT1999-00004<br />
Version: 4.0<br />
Date: 25/07/01<br />
File: <strong>D10</strong>-vers.4.0.doc<br />
Page 95 <strong>of</strong> 136<br />
The oxygen and nutrients carried in the water to be treated diffuse throughout the bi<strong>of</strong>ilm<br />
until the deepest cellular accumulations are no longer affected by the oxygen and<br />
nutrients. After some time, stratification occurs with an aerobic layer, where the oxygen<br />
is diffused, on top <strong>of</strong> a deeper anaerobic layer in which there is no oxygen. This shall<br />
generally be avoided by hydraulically designing the system so that the when the bio-film<br />
exceeds a certain thickness the bacteria are washed away. The use <strong>of</strong> bio-film methods<br />
for treating water shows that the bacteria attached to a support usually display higher<br />
specific activity than those observed in suspended growth.<br />
This principle has long been applied in the 60ies and 70ies in the so called trickling filters<br />
which were designed for removal <strong>of</strong> carbonaceous compounds and in some cases<br />
even for nitrification and simultaneous sludge stabilisation. However in order to fulfil the<br />
requirement <strong>of</strong> nutrient removal most if these trickling filters have been replaced by activated<br />
sludge process plants in Germany and Austria the last 20 years.<br />
Trickling filters allow the water to be treated to trickle onto a mass <strong>of</strong> material with a<br />
high specific surface area, supporting a film <strong>of</strong> purifying micro-organisms. These filters<br />
are aerated by natural draught, occasionally by forced countercurrent ventilation. When<br />
the bio-film is charged with only a low waste load also bacteria with low growth rates are<br />
able to grow on the supporting material. The advantage that the bacteria are fixed to a<br />
support and need not pass the clarifier, is being used in processes with submerged<br />
contact structures, which are situated in a tank <strong>of</strong> activated sludge, allowing bacteria<br />
with very low growth rates to grow in the system. Thus with that arrangement it is possible<br />
to improve the performance <strong>of</strong> an activated biological sludge stage without enlarging<br />
the clarifier which may be limited by the solids loading applied.<br />
Processes with submerged contact structure are more and more applied since they allow<br />
very specialised bacteria, which can also reduce substances present in very low<br />
concentrations in the water, to settle on the supports.<br />
Treatment <strong>of</strong> surplus sludge - primary sludge and excess biological sludge<br />
Primary sludge<br />
Primary sludge is usually pre thickened in a pre thickener, (frequently together with the<br />
excess biological sludge) or is directly sent to the anaerobic digester. The supernatant<br />
liquid <strong>of</strong> the pre thickener is usually added to the influent. Frequently primary sludge<br />
and excess biological sludge are thickened together.<br />
Excess biological sludge<br />
The excess biological sludge needs further reduction <strong>of</strong> organic material. This is done<br />
either by aerobic stabilisation or by anaerobic digestion. The ratio organic/inorganic<br />
material in excess sludge typically is 2 /3 to 1 /3, thus stabilisation <strong>of</strong> excess sludge allows<br />
a reduction <strong>of</strong> the sludge dry weight by 2 /3.
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