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 changes.

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 LT20).

Polymer efficiency
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 utilized.

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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