Alexander Szabo and Oscar Engle - Svenskt Vatten

8. Water flow and quality
Sampling procedure for effluent water
Effluent samples were collected every two hours from the pond outlet at the 5-6 November, 18-19
November and 9-10 January. From the samples made COD, BOD and TSS were analyzed. Since
the ponds have large volumes and the retention time is several days, the effluent quality does not
change drastically during a time span of a few hours. Therefore, for 5-6 November and 9-10
January, composite samples were created from three effluent samples under approximately a 6
hour period. This was done in order to reduce work and costs in the laboratory. The analysis
procedures are described in chapter 7.
De-sludging conditions
The treatment efficiency is dependent on the sludge, since the sludge contains bacteria that
degrade pollutants in the waste water. If the sludge has been removed recently, the system may
operate ineffectively due to the lack of bacteria. If however, the sludge has accumulated for a
long period, the volume of the sludge will reduce the HRT in the pond system, hence making the
treatment less effective. One can assume that the best treatment occurs somewhere in the middle
between the de-sludging operations.
The WSP (north treatment line) was de-sludged after the first measurements were done on the 5-6
November 2009. During de-sludging one treatment line at a time is closed down. The waste water
is pumped out and the accumulated sludge from the bottom of the ponds is removed. The desludging
operation takes a few weeks to be completed and is done with 10 year intervals. On the
9-10 January 2010, the WSP had been in operation, after de-sludging, for about 2 weeks when the
samples were collected. The average effluent COD was 64 mg/l before and 58 mg/l after desludging
(see Figure 8.1 and 8.2). The average BOD 5 for 5-6 November was 39 mg/l BOD 5 on
average (Figure 8.1).
Since the samples were collected just before and after de-sludging, the results presented above
may be slightly higher than during most of the operation time. Zahari & Zain (1999) showed in
their report an average effluent of 40 mg/l COD (see Appendix A) which is approximately 30%
better than the results in this report. This could be due to the sludge conditions discussed above.
BOD 5 and COD in effluent
The quality of the effluent water is relatively stable (see Figure 8.1 and 8.2). Variations may
depend on sunlight that changes the concentrations of algae in the surface layer. Another factor to
consider is the effluent pump that is working discontinuously where the treated waste water is
released to the small stream when the waterlevel in the ponds has reached a certain level (more
information about the pumping effect is discussed on page 36).
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