Discolouration in drinking water systems - TU Delft

the order of 2 can be expected, making it difficult to draw conclusions from these
measurements. The measurements are reported here because the methodology can still be
applied if the filter itself is also analysed for suspended solids. In this case, however, that
could not be done because the filters were already too tampered with and partly destroyed.
3.5 Discussion
Not surprisingly all measurements show that the sediment load at the Research Area was
significantly less than at the Reference Area. Theoretically the sediment load to the Research
Area should be negligible, but in practise it turns out that even in the outflow of the UFreservoir
some calculated particle volume was present. These particles were probably picked
up in the UF-reservoir. Though the reservoir had been cleaned before being taken into service,
it obviously contained some particles that settled in the first pipe of the network. The
Hemoflow measurement that had been done on the outflow of the UF-installation shows that
the actual amount of TSS was virtually negligible. The total amount of loose deposits
removed from the Research Area at the end of the measuring period was less than 8% of the
amount removed from the Reference Area. The main difference between the two systems was
the particle volume in the water that was supplied, since materials that were used and the
diameters that were applied are the same. The UF-treatment only removed the TSS from the
water, but left other components as AOC and other soluble substances unchanged. The
chemical properties of the water were identical.
The average VSS level of the samples flushed from the Research area was 60,6%, while the
average VSS level of the samples flushed from the Reference Area was 42,5%. In the
Research Area 89,9 mg/m of loose particles were flushed out and in the Reference Area
1071,2 mg/m. Those percentages applied to the deposits would indicate that the
corresponding organics are 54,0 mg/m and 450,0 mg/m respectively.
In the Reference Area 27% of the incoming TSS was VSS (Table 3-15). If this percentage is
applied to the sediments found that would account for 121 mg/m of the total 450 mg/m found
in the Reference Area. If the VSS is considered to be the product of biological regrowth then
the net growth in the Reference Area results in 329 mg/m organic matter and in the Research
Area in 54,0 mg/m organic matter, a significant difference of a factor of more than 6. The UFmembrane
treatment doesn’t take out the AOC, as this is soluble. Other biological properties
of the water such as colony counts are not considered in this study, but with these results it is
plausible that the amount of particles in the water and those accumulated in the network
promotes biological regrowth, because they offer a service to grow on (van der Kooij, 2000;
Huck and Gagnon, 2004).
In the Research Area a sediment amount has been removed that is equal to a sediment rate of
89,9 mg/m while in the Reference Area a sediment rate of 1071,2 mg/m was found. (Carriere
et al., 2005) report sediment rates varying from 30 to 24500 mg/m and (Barbeau et al., 2005)
gives values of 260 to 410 mg/m. These sediment rates, however, were based on samples
taken under different and non-uniform circumstances that can lead to both over- and
underestimating of the ‘real’ sediment rate.
In earlier research sediment rates in small diameter pipes are found in the order of 3,9 to 247,3
mg/m. In a cast iron network that is supplied with water treated with slow sand filtration as
the finishing treatment step, sediment rates were found ranging from 9,1 to 63,3 mg/m
(Vreeburg et al., 2007).
The Surf-90% and Surf+90% values of the particle volume of the Reference and Research
Areas show that the major part of the particle loading was done in a relatively short time. The
actual particle load to the system is also dependent on the volume flow. In the Reference Area
Particle free water - 60 -