Discolouration in drinking water systems - TU Delft

distribution network is on the smaller sized pipes, 150 mm and smaller, at which the house
connections are made. In fact this is the water that actually enters the properties and is
experienced by the customers. The colour of the lines (red, blue and green) in this thesis are
often used to represent the flow direction with red the upstream location, green the
downstream location and blue somewhere in between.
The first process presented in Fig 2-3 is settling of particles, represented by the decreasing
turbidity in the flow direction. With settling, particles are lost and that leads to a decrease in
turbidity in the flow direction. This phenomenon can often be observed when in the turbidity
trace at the treatment plant irregularities occur. These can be the result of a backwash of filters
or resuspension as result of a pump switch.
The spikes in the turbidity can be used as natural tracers to monitor the residence time in a
network. The peak values at each locations are used to identify the origin of the water and that
‘package’ of water can be followed through the network. This has proven to be a very
powerful tool to actually measure the residence time without adding tracers to the water.
In Fig 2-4 the turbidity trace in the three locations are drawn in separate graphs. The three
stylistic traces represent resuspension of particles manifested by local peaks in turbidity,
independent of turbidity elsewhere in the network. Mostly the peaks in turbidity can be related
to increased velocity in the pipes, for instance as result of a hydraulic incident. In this case the
resuspension is caused by the increased flow during the morning peak demand. The increase
starts at 7:00 AM or even earlier and ends after roughly three hours. Resuspension can be
observed both in the transport network and in the distribution network, though in the transport
network there are fewer and mostly lower spikes in turbidity. First, the variation in velocity in
the transport network is less than in the distribution network (Blokker et al., 2006), allowing
for a more regular settling and resuspension. Second, the sampling point is at the top of the
pipe and usually some stratification of the resuspended sediment can be observed. This means
that part of the resuspended sediment will not reach the measuring point at the top of the pipe.
The peaks in turbidity in the distribution network follow the household demand, with high
peaks in the morning and varying peaks in the late afternoon and early evening. The day
peaks are also related to the weather conditions and the types of connections: in hot, dry
weather in areas with gardens the peak in the evening can be much larger compared to the
morning peaks. That also explains why discolouration incidents are often associated with
extreme weather conditions.
The graphs in Fig 2-5 are again separated for the three locations and present the turbidity trace
that can be measured when active corrosion occurs. Active corrosion changes the water
quality (McNeill and Edwards, 2001). The typical pattern is that the turbidity in distribution
pipes rises during the night hours when the water stagnates because of low night use. In the
morning hours the water consumption increases, flushing out water with an increased
turbidity. The rise in turbidity is caused by the particles that are released through the corrosion
process. The phenomenon has also been observed when a stable corrosion products scale is
damaged by aggressive cleaning action (Vreeburg, 1996). The typical difference between
resuspension and corrosion is that with the resuspension phenomenon, the turbidity rises when
demand increases while with corrosion the turbidity drops with increasing demands. It can,
however, be difficult to distinguish the two processes in the case of a combination of settling
and resuspension in a distribution pipe. The pattern of falling and rising turbidity can look the
same, but the difference is the timing of the peaks in turbidity. In the case of settling and
resuspension the turbidity decreases in the night hours and increases in the day hours, while in
the corrosion case this is just the other way around, with increasing turbidity in the night and a
Measuring methods - 15 -