Studies of modified pumice and titanomagnetite filtration media

Abstract

There is a continuing interest in filtration technologies capable of more efficient filtration, particularly of small microbial particles, but which place less reliance on the use of water treatment chemicals. In order to develop new options for granular filtration systems, two indigenous New Zealand media, titanomagnetite (TM) and pumice were investigated. TM was of interest because its particle size is finer than most filtration media and its magnetic properties allow the possibility of magnetic conditioning of the bed. A modified pumice, marketed as Silicon Sponge (SS), is already finding application in filtration systems where its low effective density and porous structure make it ideally suited for use as the upper layer of dual media filters.

Particle size and surface charge are key parameters influencing the effectiveness of a given filtration medium. Finer medium particle size improves filtration efficiency but at the expense of lower hydraulic conductivity. Magnetic conditioning of fine TM filtration beds was used to improve hydraulic conductivity. Hydraulic conductivity was increased by 140% when a vertically aligned magnetic field 0.018 T was applied. However filtration efficiency was decreased by about 9% in the conditioned (expanded) bed. Adjustment of pH to enhance electrostatic interactions more than compensated for this decrease.

The particulate matter of most waters requiring filtration is negatively charged. Surface treatment to make a filtration medium more electropositive is known to improve the medium’s filtration effectiveness. The isoelectric point (IEP) of natural TM, even after rigorous chemical cleaning, was found to be 3.64 ± 0.06 This value was confirmed by the preparation of a synthetic TM and the measurement of its IEP. Attempts to raise the IEP of natural TM by treatment with aluminium and iron polycation solutions were unsuccessful.

Greater success was achieved in the modification of the properties of SS by treatment with aluminium and iron polycation solutions and mixtures of these solutions. IEPs ranging from the 4.10 ± 0.06 of acid rinsed SS to the 8.52 ± 0.06 of aluminium treated SS were achieved. In addition, it was found possible to impart magnetic properties to fine SS by the use of treatment solutions containing a mixture of ferric and ferrous ions. The presence of aluminium in the treatment solutions allowed higher IEPs to be obtained but the magnetic effect was reduced. In the absence of aluminium, the ferric/ferrous mixture gave a product with a magnetic effect that was 7% of that observed for pure magnetite.

Filtration studies using 1.7 μm negatively charged kaolin as a model negative colloid showed that significant increases in filtration efficiency were achieved by medium pre-treatments with the aluminium and iron solutions. For example, turbidity removal for 710-1000 μm SS was increased from 22% to 94% by aluminium pre-treatment.

The information on the performance of magnetically conditioned beds and the ability to enhance filtration efficiency by surface modification using iron and aluminium has potential applications in the design of novel systems for water and wastewater treatment. A small-scale system based upon the use of a permanent magnet has been proposed and work in this area is continuing.