Some notes on van der Waals forces

The degree of correlation in dielectric response between interacting 

In some instances the raw water reaching water treatment plants may contain pathogens such as the human infectious protozoon Cryptosporidium parvum. The environmental form of C parvum is a spheroidal oocyst of 4-6 microns diameter. The oocyst is resistant to conventional chemical disinfectants that are commonly used in water treatment such as chlorine or chloramines. It is therefore essential that Cryptosporidium be removed during the coagulation and filtration processes stage in the water 

Water treatment by either direct or contact filtration has become common practice for raw water with low turbidity [ 3NTU] and low colour. Simple metal salts such as alum or ferric chloride are added to plant inlet water. Hydrolysis takes place with the formation of hydroxylated species, which adsorb, reducing or neutralizing the charge on the colloidal particles in the raw water, promoting their collision and the formation of floes that settle or can be filtered out. 

Research is continuing on the main factors that influence oocyst flocculation since variations in chemical dosing, water chemistry or oocyst characteristics could potentially lead to oocyst breakthrough in water treatment plants. In recent years researchers have proposed that the interactions between oocysts and different coagulants may be quite different [Bustamante et al., 2001]. 

Dr Heriberto Bustamante Research Scientist Sydney Water Corporation Sydney, Australia 

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