Page 433 - Water and wastewater engineering

11-6 WATER AND WASTEWATER ENGINEERING

establishes the following relations between the effective size, uniformity coefficient, geometric
mean size, and geometric standard deviation:

P ( g
S g
E 10 X )( ) 1.28 (11-1)
.
P
U 60 / P ( Sg) 1 535 (11-2)
10
Sand excavated from a natural deposit is called run-of-bank sand. Run-of-bank sand may
be too coarse, too fine, or too nonuniform for use in filters. Within economical limits, proper
sizing and uniformity are obtained by screening out coarse components and washing out fine
components. In rapid sand filters, the removal of “fines” may be accomplished by stratifying
the bed through backwashing and then scraping off the layer that includes the unwanted sand.
In the United States, filter sand is purchased from commercial suppliers based on specifications
provided by the engineer rather than processing the sand at the point of use.

Physical Properties
Standard requirements for filter media are described in ANSI/AWWA standard No. B100-01
(AWWA, 2001). This discussion is to highlight some of the properties.

Hardness. Hardness of the filter material is important as it is an indicator of the resistance to
abrasion and wear that occurs during filter backwashing. Hardness is ranked on the Moh scale.
Talc has a Moh hardness number of 1. Diamond has a Moh hardness number of 10. Sand, garnet
and ilmenite * are hard enough to resist abrasion. Anthracite coal and granular activated car-
bon (GAC) are friable and design specifications must specify the minimum acceptable hardness
value. A minimum Moh hardness of 2.7 is often specified for anthracite (MWH, 2005). GAC
hardness is evaluated using ANSI/AWWA standard B604-96 (AWWA, 1996).

Porosity. The porosity of the in-place media (not of the individual grains) affects the headloss
and filtration efficiency of the media. It is defined as

V v V T V M
(11-3)
V T V T
where porosity, dimensionless
3
V v volume of voids, m
3
V T total volume of media bed, m
3
volume of media, m
V M
Specific Gravity. The specific gravity of the media plays a role in both the arrangement of the
filter media in multimedia filters and in the backwash flow requirements that are required to flu-
idize the bed. The selection of the media used in the design of dual- and tri-media filters is based
on the specific gravity of the media.

* Garnet is a generic term referring to several minerals that are silicates of iron, aluminum, and calcium mixtures. Ilmenite is an
iron-titanium ore associated with hematite and magnetite, which are iron oxides.