Good day to you vivian maxine
"The prefix dia- occurs in loanwords from Greek ( diabetes; dialect) and used, in the formation of compound words, to mean “passing through” ( diathermy), “thoroughly,” “completely” ( diagnosis), “going apart” ( dialysis), and “opposed in moment” ( diamagnetism)."
In the word ‘dia-filtration’ it is used almost redundantly because filtration is a passing-through process itself.
Nevertheless it has become a well-known process in milk product preparations. The process is used to concentrate skim milk and to isolate certain protein and carbohydrate fractions in order to improve and standardise the taste and texture of certain cheeses and yoghurts.
This paper will probably gives you a rough idea - http://onlinelibrary.wiley.com/doi/10.1002/(SICI
)1097-0010(199801)76:1%3C10::AID-JSFA928%3E3.0.CO;2-%23/abstract Abstract"Defatted milk was ultrafiltered–diafiltered in a 20 kDa polysulphone flat membranes UF unit to obtain concentrated milk with different protein/lactose ratios. Different samples of concentrated–diafiltrated milks with protein concentration between 4•7% and 8•9% and lactose concentration between 2•7 and 0•8% (protein/lactose ratios between 1•7 and 9•5) were fermented (Lactobacillus bulgaricus and Streptococcus thermophilus) to obtain yoghurt-like products. Firmness of the coagulum as well as pH, colony count and organoleptic test were considered in the final products. Concentrated-diafiltrated milk with lactose and protein contents lower than 2% and 6•5%, respectively, lead to a very liquid product. Enriched protein low-lactose fermented milks can be obtained by this technology."
Note: ‘Defatted milk’ is skim milk.