Behaviour of Aluminium During Water Treatment
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report No UWRAA 146
Recently there has been increased concern regarding the effect of aluminium on human health, particularly the effect of low levels of aluminium on renal dialysis patients. The long term effect on humans has also been linked with Alzheimer’s disease.(10)
In a survey of aluminium levels in Melbourne tap water, high levels of aluminium above the WHO guideline level of 200 mg/L were observed in the alum treated water from Bunyip and Tarago reservoirs. Particular features of these supplies were the low alkalinities of the natural waters and pre-dosing with fluoride before the alum treatment. A study was undertaken to determine the behaviour of aluminium during water treatment and to obtain a set of operating parameters to minimise aluminium levels in the treated water. It was found that alkalinity and fluoride levels greatly affect aluminium levels in the alum treated water. The presence of about 0.8 mg/L of fluoride, in particular, could markedly cause elevated levels of residual aluminium.
Al speciation studies were carried out on both the raw and treated water, using a chemical separation scheme to isolate various fractions. Graphite furnace atomic absorption spectrometery with detection limits of 2 mg/L was then used to determine aluminium in each one. a significant finding of this work was that aluminium was dominated by organically bound aluminium before alum treatment but was mostly in inorganic forms after the treatment. In the presence of fluoride, the elevated level of aluminium in the treated water was largely in the form of soluble cationic fluoro and hydroxy complexes.
Both the Ion Electrode and Ion Chromatography were used in determining fluoride in the raw and treated water. It was found that loss of fluoride happened during alum treatment which was due to absorption of fluoride by the flocs and significant amounts of fluoride were found in the flocs.