Material: Recycled
Glass
Issue: Slow rate filtration systems
are an alternative to conventional rapid
sand filters in the pre-treatment of
community drinking water.
These systems are relatively
easy to construct and maintain, but
require large quantities of filter media.
The traditional slow rate filtration
medium is sand, but research has suggested
that crushed recycled glass can be an
effective medium in slow rate filtration
systems. Further testing is needed to determine proper
maintenance techniques and long-term
filter performance. This Best Practice briefly discusses current
knowledge of the preparation and placement
of crushed recycled glass as a slow
rate filter medium.
Many of the recommendations presented
here are common to other types of filter
media.
Best
Practice: Slow
rate filtration systems typically consist
of a layer of filter medium placed on
a layer of support gravel, the thicknesses
of which will vary by design. During filtration, water passes through the
filter medium and out the support gravel.
Typical flow rates for filtration
systems of this type vary between 0.04
and 0.10 gallons per minute per square
foot of filter cross-section (gpm/ft2). Research suggests that crushed recycled glass
may be used effectively as a filter
medium in systems of this type. Tests using pilot column filters indicate that
when prepared and placed properly, the
filtration abilities of crushed recycled
glass are comparable to those of natural
sand. These include the reduction of water turbidity
and removal of bacteriological contaminants. Generally, water is not treated before introduction
to slow rate filtration systems.
As such, these systems are usually
limited to water with no heavy algal
blooms and low turbidity.
Prior to use as a filter medium, recycled glass should
be crushed and sieved to the proper
size and gradation. The U.S. Environmental Protection Agency recommends
that slow rate filtration media have
an effective particle diameter (D10)
between 0.15mm (100 mesh) and 0.35mm
(45 mesh). D10 is the particle diameter at
which 10% of the media weight is finer.
In addition, the uniformity coefficient
(U.C.) of the media should be 2.5 or
less.
The uniformity coefficient is
the ratio of D60
to D10, where D60
is the particle diameter at which 60%
of the media weight is finer.
Experience also suggests that
the crushed glass should contain less
than 0.1% by weight smaller than .074mm
(#200 sieve).
This low level of fines may only
be achievable with washing.
Once properly sized, the crushed glass should be washed
to reduce contents of organic material
or dirt that might leach from the medium
or cause excessive turbidity.
To prevent contamination during
shipment, washing should take place
immediately before placement of the
glass.
For a pilot project, this can
be accomplished in a portable electric
cement mixer or in a cement truck.
For large-scale continuous washing
of glass for full-scale project, sand
screws are a common piece of equipment
that can be adapted to glass. Consult the Thomas Register
for manufacturers.
The support medium (gravel) should be lowered first
into the filter apparatus in measured
lifts and compacted. The crushed glass filter medium can then be
placed and compacted in 6-inch thick
lifts.
The thickness of the support
and filter media layers will vary by
design.
Once the glass is placed, the
filter system should be filled with
source water. A disinfectant solution should be added to the system and allowed
to stand for at least 12 hours before
flow begins. A 50 mg/L sodium hypochloride solution can be used for this purpose.
The filter medium in any slow rate filtration system
requires periodic cleaning.
This can be accomplished by “scraping”
or “harrowing”.
Scraping involves lowering the
water level in the filter below the
surface of the media and removing any
residue that has accumulated on top
of the media.
The top one inch of media is
also scraped off and discarded.
“Harrowing” involves turning
over the top few inches of media while
slowly backwashing the filter.
Designers need to account for
the fact that glass has a lower density
than most other forms of filter media
(i.e. sand), and can be fluidized and
disrupted at lower backflow rates.
Implementation: Several studies have documented
the technical efficacy of recycled glass
in water filtration applications.
The processing and marketing
of adequate volumes, with acceptable
quality, at competitive prices, is the
next step to market viability.
Design engineers should be made
aware of the difference in specific
gravity between glass and the materials
with which they are accustomed to work.
Benefits:
Glass
can be processed to be a clean, relatively
inert, granular material that can be
graded in any way required for filtration
applications. Slow rate filtration is only one example of
a filtration application for which glass
appears to be suitable. Investigations into local potential filtration
uses could result in new markets for
glass processors.
Application
Sites: Glass
suppliers, design engineers, water treatment
facilities.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org. References: AWWA Standard for Filtering
Material,
ANSI/AWWA B100-96, American Water Works Association; Cleasby,
J.L., Source
Water Quality and Pretreatment Options
for Slow Sand Filter: Slow Sand Filtration,
American Society of Civil Engineers, 1991; Evaluation of Recycled Glass as a Filtration
Medium for Municipal Water Treatment,
prepared for Clean Washington Center,
1995; Recycled Glass: Development
of Market Potential, Final Report, submitted to City of San Jose by G. Selvaduray, San Jose
State University, San Jose, CA., 1994; Smoley, C.K., Manual of Small Public
Water Supply Systems, U.S. Environmental
Protection Agency, Office of Drinking
Water, 1992 Issue Date / Update: November 1996 |
