Civil Engineering Reference
In-Depth Information
CHAPTER 12
Filtration
INTRODUCTION
In waterworks parlance, filtration most frequently refers to the use of a relatively deep
[1
1
⁄
2
to 6 feet (0.46 to 1.82 m)] granular bed to remove particulate impurities from
water. Water filtration is a physical-chemical process for separating suspended and
colloidal impurities from water by passage through a bed of granular material. There
are two separate steps:
Transport, in which suspended particles are transported to the immediate vicinity
of the solid filter media
•
Attachment, in which particles become attached to the filter media surface or to
another particle previously retained in the filter
•
The transport step is primarily a physical process, while the attachment step is very
much influenced by chemical and physical-chemical variables.
THE ROLE OF FILTRATION IN WATER TREATMENT
It is generally acknowledged that most surface water supplies should be filtered; but
a significant number of surface water supplies derived from uninhabited, ''protected''
watersheds still are not filtered. These unfiltered systems are undergoing rigorous re-
view by state and federal agencies to assess the need for filtration to supplement
disinfection provided by most facilities.
Some well waters meet water quality standards and goals with no treatment other
than chlorination. However, many well supplies require removal of iron, manganese,
hardness, color, odor, turbidity, hydrogen sulfide, bacteria, viruses, or other undesirable
impurities. In these cases, and in instances where the ground water quality is influenced
by surface water, filtration is ordinarily a part of the overall treatment process.
In water treatment, the general practice is to reduce the turbidity of water to below
10 NTU before application to filters, with a desired goal of 1-2 NTU to ensure op-
timum filter performance. However, some types of filters can handle applied turbidities
of up to 50 NTU on a continuous basis and will tolerate occasional turbidity peaks
up to 200 NTU. Several public water supplies derived from surface sources having
average turbidities of 20 to 50 NTU and peaks of 100 to 200 NTU have operated quite
successfully without the use of settling basins ahead of the filters (direct filtration).
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