Alexander Szabo and Oscar Engle - Svenskt Vatten

Figure 8.1: BOD 5 and COD in effluent from the northern treatment line on the 5-6 November 2009
(before de-sludging).
Figure 8.2: COD in effluent from the northern treatment line on the 9-10 January 2010 (after desludging).
TSS in effluent
The TSS levels are higher and show a different pattern after the de-sludging was carried out. The
average TSS on the 5-6 November was 37 mg/l (Figure 8.3), but 22 mg/l on the 9-10 January
(Figure 8.4). The reason for lower TSS values after de-sludging may be caused by many reasons.
One reason is the fact that if the ponds had high levels of sludge, the volume of water in the
ponds decreased, hence creating less retention time and higher water flow, which disturbs the
settling process. If the sludge reaches a high level from the bottom of the pond, the distance
between the top sludge layer and outlet channel gets narrower and more solid particles may get
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