Optimising pre-treatment and filtration
Torgeir Saltnes and Rene' Kristensen of Optiroc AS report on Norwegian research into the benefits of using expanded clay aggregates in granular filter beds for the treatment of drinking water.
Surface water sources around the world are experiencing an increase in colour
caused by humic substances, which many experts say is a result of global climate
A granular filter bed for drinking water treatment with expanded clay aggregates
(Filtralite) as filter media is extensively tested in pilot scale experiments
at the Norwegian University of Science and Technology (NTNU ). The objective
of the work was to improve the direct filtration process for removal of humic
substances from water. The filter bed used consisted of a upper layer of 118cm
Filtralite NC (Normal density, Crushed) and a bottom layer of 60cm Filtralite
HC (High density, Crushed) with grain sizes of 1.6-2.5 mm and 0.8-1.6 mm respectively.
This filter bed is designed to give long filter runs (high storage capacity)
and good effluent water quality. For the results shown in the figure the raw
water had a colour level of 50 mg Pt./l and a turbidity of about 0.2 NTU. The
raw water was pH adjusted before coagulant addition and flocculation, with variable
detention times and a G value of 110 s-1 in the tube flocculator. In the experiments
were polymer was used, it was added just after flocculation prior to filtration.
The production period for a filter is confined by a maximum allowed effluent
turbidity and/or a maximum head loss through the filter. In these three experiments
the predetermined maximum effluent turbidity was set to be 0,3 NTU, and all
the runs were limited by breakthrough (effluent turbidity exceeding maximum
value). The polymer used was a non-ionic high molecular weight polymer (Magnafloc
As can be seen in the figure the colour removal is high for all three runs.
There is a slight increase in colour removal for the two runs where polymer
is added, from 90 to 93% without and with polymer respectively.
When flocculating and adding polymer as filter aid, a major lengthening of
the run length and an improvement in effluent turbidity can be observed. This
is because the polymer makes the flocs bigger and stronger. The adding of long
chained polymers increase the efficiency of the adhesion in the filter, and
the storage capacity in the relatively coarse Filtralite filter is much better
The run length increase significantly with an increasing detention time in
the flocculator. Longer detention times enhance the growth of flocs. Visual
observations during the experiments support this theory. For the experiment
with 5 minutes detention time in the flocculator and the addition of polymer,
large flocs could easily be observed above the filter media, for the shorter
detention time the flocs were smaller, and for the experiment with alum alone
(without flocculation), visible flocs were not observed.
The combination of a metal salt and polymer as filter aid with the Filtralite
filter have shown that it is possible to obtain long filter runs and good effluent
water quality when treating humic waters in a direct filtration plant.
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