The process used depends on whether the source is groundwater or
surface water. Surface water requires considerably more treatment to
make it fit for public distribution. Groundwater typically has
minimal bacteriological contamination, but tends to have more
dissolved solids.
Here is a description of a typical process:
"Drinking water production from surface water
On this page you will find an explanation of a drinking water
preparation process. All process steps are numbered and the numbers
correspond with the numbers in the schematic representation of the
drinking water process found below. This is a summing up of the
process steps:
a: Prefiltration
1) The uptake of water from surface waters or groundwater and storage
in reservoirs. Aeration of groundwater and natural treatment of
surface water usually take place in the reservoirs. Often softening
and pH-adjustments already happen during these natural processes.
2) Rapid sand filtration or in some cases microfiltration in drum filters.
b: Addition of chemicals
3) pH adjustment through addition of calcium oxide and sodium hydroxide.
4) FeCl3 addition to induce flocculation for the removal of humic
acids and suspended particulate matter, if necessary with the addition
of an extra flocculation aid. Flocs are than settled and removed
through lamellae separators. After that the flocs are concentrated in
sludge and pumped to the exterior for safe removal of the particulates
and sludge dewatering.
5) Softening in a reservoir, through natural aeration or with sodium
hydroxide, on to 8,5 oD. This is not always necessary. For instance,
in case natural filtration will be applied, softening takes place
naturally.
c: Natural filtration
6) Drinking water preparation step that is specific for the
Netherlands: Infiltration of the water in sand dunes for natural
purification. This is not applied on all locations The water will
enter the saturated zone where the groundwater is located and it will
undergo further biological purification. As soon as it is needed for
drinking water preparation, it will be extracted through drains.
d: Disinfection
7) Disinfection with sodium hypochlorite or ozone. Usually ozonation
would be preferred, because ozone not only kills bacteria and viruses;
it also improves taste and odour properties and breaks down micro
pollutants. Ozone diffuses through the water as small bubbles and
enters microrganisms cells by diffusion through cell walls. It
destroys microrganisms either by disturbance of growth or by
disturbance of respiratory functions and energy transfers of their
cells. During these processes ozone is lost according to the reaction
O3 -> O2 +(O).
e: Fine filtration
8) Slow sand (media) filtration for the removal of the residual
turbidity and harmful bacteria. Sand filters are backwashed with water
and air every day.
9) Active carbon filtration for further removal of matter affecting
taste and odour and remaining micro pollutants. This takes place when
water streams through a granular activated carbon layer in a filter.
Backwash is required regularly due to silting up and reactivation of
an active carbon filter should be done once a year.
f: Preservation and storage
10) Addition of 0.3 mg/L sodium hypochlorite to guarantee the
preservation of the obtained quality. Not all companies chlorinate
drinking water. The water will eventually be distributed to users
through pipelines and distribution pumps.
11) Aeration for recovery oxygen supply of the water prior to storage.
This is not always applied.
12) Remaining water can be stored in drinking water reservoirs."
"Drinking water production from surface water" Lenntech Water
treatment & air purification Holding B.V. (2005)
http://www.lenntech.com/drinking-water-preparation.htm
Here is another description of typical utility water treatment:
"MUNICIPAL OR UTILITY WATER TREATMENT
Most municipal water found in a city or community today has been
treated extensively. Specific water treatment methods and steps taken
by municipalities to meet local, state, national, or international
standards vary but are categorized below.
Screen prefiltration
A coarse screen, usually 50 to 100 mesh (305 to 140 microns), at the
intake point of a surface water supply, removes large particulate
matter to protect downstream equipment from clogging, fouling, or
being damaged.
Clarification
Clarification is generally a multi-step process to reduce turbidity
and suspended matter. Steps include the addition of chemical
coagulants or pH-adjustment chemicals that react to form floc. The
floc settles by gravity in settling tanks or is removed as the water
percolates through a gravity filter. The clarification process
effectively removes particles larger than 25 microns. The
clarification process is not 100% efficient; therefore, water treated
through clarification may still contain some suspended materials.
LIME TREATMENT
The addition of lime (Ca) and soda ash (Na2CO3) reduces the level of
calcium and magnesium and is referred to as "lime softening." The
purpose of lime softening is to precipitate calcium and magnesium
hydroxides (hardness) and then clarify the water. The process is
inexpensive but only marginally effective, usually producing water of
50 to 120 ppm (3 to 7 gpg) hardness.
Disinfection
Disinfection is one of the most important steps to municipal water
treatment. Usually, chlorine gas is fed into the supply after the
water has been clarified and/or softened. The chlorine kills bacteria.
In order to maintain the "kill potential", an excess of chlorine is
fed into the supply to maintain a residual. The chlorine level must be
constantly monitored to assure that no harmful levels of chloramines
or chlorinated hydrocarbons develop.
pH adjustment
Municipal waters may be pH adjusted to a pH of approximately 7.5 to
8.0 to prevent corrosion of water pipes, particularly to prevent
dissolution of lead into the water supply. In the case of excessive
alkalinity, the pH may be reduced by the addition of CO2."
"Methods of Water Purification" from the "Pure Water Handbook"
published by Osmonics, Inc.
http://www.gewater.com/library/tp/716_Methods_of.jsp
Another article explaining the process in detail and the
characteristics of different water sources can be found here:
"Water purification" Wikipedia (March 28, 2005)
http://en.wikipedia.org/wiki/Water_purification
Sincerely,
Wonko |
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