Abstract:
The performance of three different types of PAC was analysed for the
efficiency at which they remove natural organic matter (NOM) from untreated
(raw) water, with the objective of ultimately reducing disinfection bi-product
(DBP) formation. The treatment of raw water by PAC was also examined in
conjunction with the addition of iron salts in the form of ferric sulphate, for
coagulation and flocculation of NOM.
The optimum dosing of PAC was achieved at pH 3 and at a dose of
around 20mgl-
1
which gave 32.7% removal. Coagulation could be seen to give
88% removal and when used together PAC and coagulant gave between 93.6%
and 97% removal of NOM.
The dosing sequence of PAC and coagulant was investigated whereby
it was found that optimum removal was achieved by dosing PAC prior to dosing
coagulant, although if the delay between dosing PAC and coagulant was more
than 30 seconds it was found to impair NOM removal and increase turbidity.
Scale up trials of the dosing strategy were implemented at both Ewden
water treatment works, in Yorkshire, and at the Pilot Hall in Cranfield University.
The greatest reduction in DBP formation could be seen using Norit SA
Super and coagulation. Dosing this activated carbon at 20mgl-
1
resulted in a
decrease of DBP formation by 86% in raw water and by 78.7% using
coagulation alone. Dosing PAC before coagulant produced the lowest levels of
DBPs, which were 27.2μgl-1. This is much lower than the 80μgl-1 limit to which
water treatment works must adhere.
