Material: Recycled
Glass
Issue: Sand filters are a common adjunct to conventional on-site septic treatment
systems.
When the drain field for a septic
treatment system does not meet percolation
standards, a sand filter creates an area
with the correct permeability for effluent
from the septic treatment system.
Sand filters are part of a non-proprietary
anaerobic septic treatment system in which
the sand acts as a mechanical and biological
filter. Research suggests that using crushed recycled glass in place of
sand may lower construction costs and
minimize the potential of system failures. Best Practice: This best practice details the benefits of using crushed
recycled glass as an alternative to C-33
sand as a medium in septic treatment systems.
For methods of reducing the recycled
glass to the desired size, refer to the
Small Scale Glass Processing Best Practice.
For details on the use of crushed
glass as a medium in potable water filtration
systems, refer to the Crushed Recycled Glass as Medium in
Slow Rate Filtration Systems
Best Practice. Sand filter septic treatment
systems typically consist of a septic
tank, a sand media filter, and a drain
field.
The septic tank collects the influx,
allows sedimentation to occur, and transfers
the effluent through the sand filter to
a drainfield, where the effluent infiltrates
into the ground.
The filter slows the rate of effluent
flow and provides a place for microorganisms
to break down organic matter and pathogens,
and to convert ammonia to nitrate. In the state of Washington, the specification
for the filter medium has been ASTM
C33-01 Standard Specification for Concrete
Aggregates, also known as cement sand,
in which up to 10% by weight of the material
can be finer than a No. 100 sieve.
The flow of effluent through the
sand can cause the fines to migrate and
form low-permeability lenses.
These lenses reduce the rate of
flow and encourage accumulations of biological
material, called “biomats.” Biomats can cause clogging of the filter, resulting
in system failure.
Repair requires replacement of
the filter sand.
Minimizing the fines content inhibits
the formation of biomats, but also increases
the cost of the sand. Tests conducted with in-situ residential septic treatment systems
indicate that when crushed to C-33 specifications,
the fines in glass wash out with effluent
flow easier than the fines in sand, reducing
the potential for biomat formation and
the associated filter clogging. Experience in field tests indicates that crushed
glass used as a replacement for sand starts
with much higher permeability than C-33
sand.
The higher permeability appears
to reduce the potential of filter clogging. Furthermore, the increased filter permeability
may allow greater hydraulic loading of
the septic system, therefore reducing
the required size of the filter, and thus
the installation cost of the filter.
In addition, research indicates
that recycled crushed glass filters appear
to perform as well as C-33 sand in the
treatment of sewage effluent.
These treatment parameters include
the reduction of five-day biochemical
oxygen demand (BOD5), fecal
coliform count (FC), total suspended solids
(TSS), oil and grease (O&G), and nitrates. Reducing the BOD5 and TSS may increase the permeability
of drainfield soils by attracting worms
and other, higher, life forms, which can
reduce the area of leachfield required
for a given volume of effluent. Based
on test data accumulated over two years,
in October 1996, the Washington State
Department of Health amended its filter
media specifications to include crushed
glass along with mineral sand in intermittent
sand filter and sand-lined drainfield
trench filter media specifications. The Department of Health cited the higher permeability
of glass along with data indicating that
the BOD, TSS, and fecal coliforms were
kept within acceptable ranges for the
duration of the research project.
The Department of Health also noted
that the glass filter took longer to reach
maturity than a sand filter run in parallel.
This may have been due to the higher
permeability requiring more water volume
to establish the biological activity. Implementation: Recycled
glass is collected in excess of traditional
market needs in many areas of the country.
If some of the cost savings from
not needing to landfill the excess glass
can be invested in processing equipment,
recycled glass can be crushed to meet
ASTM C-33 sand gradation specifications.
Any local agency or group interested
in investigating the possibilities for
this application needs to determine the
local sand filter material and gradation
specifications and to investigate the
necessary approvals at the state, county,
and local levels.
It is not correct to assume that
simply because there is ready availability
of inexpensive natural sand from local
resources, that the sand is of the correct
quality and gradation for sand filters.
Although sand is readily available
in most Washington counties, some counties
will not allow local sand to be used in
sand filters because of problems with
fines or clay. Initial approvals should probably
be sought at the state level.
State approval may make county
and local approval easier to achieve.
In addition, more data needs to
be accumulated from research around the
country to more closely quantify the effects
of using glass in septic treatment systems. Benefits: The attributes described in
this best practice will help promote the
use of recycled crushed glass as a filter
medium in residential sewage treatment
systems. The use of glass as an alternative to conventional
filter sand may lower construction costs
and improve the overall performance of
the treatment system. Application Sites: Glass suppliers, residential septic system designers.
In particular, at the time this
was written, all of the glass collected
in San Juan County, Washington, was being
processed and used in sand filters. Contact: for more information about
this Best Practice, contact CWC mailto:info@cwc.org. References: Intermittent Sand Filter Systems,
Washington State Department of Health,
2000, page 5. Crushed Glass as Filter Medium for Onsite
Treatment of Wastewater, 1997, Final
Report, prepared for the Clean Washington
Center by Aqua Test, Inc. and Stuth Co.,
Inc., Maple Valley, WA, 98109. Issue Date / Update: May, 2001 |
