D10: Impact of Contaminants - Hydromod

Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas
EUROLAKES
D10: Impact of Contaminants
FP5_Contract No.: EVK1-CT1999-00004
Version: 4.0
Date: 25/07/01
File: D10-vers.4.0.doc
Page 108 of 136
4. Filtration (mostly rapid filtration, formerly slow sand filtration)
5. Polishing processes (ozonisation, activated carbon adsorption of odours and
taste)
6. Disinfection
These processes are combined according to requirement and have to be completed
according to other substances contained in the raw water.
In developing countries the processes described above are still being applied as the
only treatment for raw water. In Europe modern sea and river water plants are more
and more applying ozone and activated carbon as completing and substituting treatment
stages to the traditional ones.
Ozone
• can support coagulation and flocculation by converting large monopolar molecules
into smaller polar compounds
• can crack organic molecules like phenoles
• can oxidise soluble gases as sulphides and mercaptanes, in order to reduce odour
and taste
• can de-colourise by destroying colloids and colours of humic acids
• can rise the BOD/COD ratio what increases the biodegradability of organic substances
Ozone is usually used as pre-ozonisation prior to flocculation and filtration and as main
ozonisation after flocculation and filtration in order to reduce odour colour and taste and
for disinfection. Ozonisation is mostly applied in combination with activated carbon in
order to reduce residual ozone in the treated water. [LEITZKE, 1999, a presentation of
applied ozone processing from the company Philaqua, Germany]
Activated carbon
Activated carbon has a broad spectrum of adsorptive activity, as most organic molecules
are retained on its surface. The hardest to retain are the molecules which are the
most polar and the linear ones with a very low molecular weight.. Molecules which are
slightly polar, generating taste and smell, and molecules with a relatively high molecular
weight are for various reasons well adsorbed on carbon.
Beyond these adsorbent properties activated carbon is also a bacteria support that is
capable of breaking down a fraction of adsorbed phase. Thus, a part of the support is
continuously being regenerated and capable of freeing sites, allowing new molecules to
be retained. (DEGREMONT, Water Treatment Handbook, 1991)
The fact that activated carbon is also a bacteria support lead to the development of
biological activated carbon, which operates similar to trickling filters, allowing the growth
of bacteria which can specialise in metabolising difficult biodegradable substances.
Activated carbon is applied in two forms:
Powdered activated carbon (PAC) and granular activated carbon.(GAC). PAC takes the
form of grains between 10 and 50µm and its use is generally combined with clarification