The primary objective of this project was to increase the applicability of membrane processes for a variety of feed waters and therefore to develop methods to improve design and operation of membrane water-treatment facilities. This project has included the development of advanced feed pre-treatment facilities based on the use of an ultra-filtration membrane pre-treatment process, the development of improved fouling control techniques based on dosage of anti-scalants and, the development of criteria for selecting optimal pre-treatment facilities depending on the types of raw water and membrane.
Detection of a scaling threshold limit by flux decline measurements and evaluation of anti-scalant effectiveness by a recycle technique require precipitation of sufficient material on the membrane. An overriding consideration is, therefore, the inventory of scaling material in the recycling solution.
Three different laboratory techniques have being developed for characterisation of scaling propensity and anti-scalant effectiveness: a batch recycle technique, a once through technique and an intermittent recycle technique. The simplest technique is the batch recycle technique in which the onset of scaling is simply determined by detecting the water recovery level at which there is a sharp permeability decline. This technique is suitable for a relatively more soluble scaling species such as CaSO4. With very sparingly soluble salts such as CaCO3, the very low inventory of CaCO3 forming species in the recycling solution cannot provide sufficient material to clog the membrane and enable convenient detection of scaling threshold limits in a practical laboratory system.
Two procedures were developed for overcoming the inventory limitation of sparingly soluble salts. In the once-through technique the membrane is continuously fed with water having a specified scaling potential. There are no inventory limitations at all and scale propensity at different levels of the scaling potential, with and without anti-scalants, can be readily evaluated by both the rate of permeability decline and the rate of scale precipitation. In the less demanding intermittent recycle technique, scaling propensity is characterised by evaluating scale deposition rates from changes in the composition of a recycling solution. The solution is periodically replenished to increasing super-saturation levels by adding fresh feed and bleeding permeate.
The above techniques determine an upper limit of the water recovery at which scale precipitation will occur immediately. A technique was also developed for determining the lower water recovery limit at which scaling is prevented or at least delayed for a long period of time. The technique is based on a fundamentally based method for correlating induction time measurements with the water recovery level.