vvEPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Rock Media Polishing Filter for Lagoons
DESCRIPTION
Rock filters are often used to remove algae from
lagoon effluents. These systems consist of
submerged beds of rocks, 75 to 200 mm (3 to 8 in)
in size, through which lagoon effluent is passed
horizontally or vertically. Vertical flow rock filters
generally provide the highest level of performance.
In rock filters, algal solids are expected to settle on
or become attached to the rock where biologically
active surfaces induce decomposition. Well
designed systems can usually produce a final
effluent with 5-day biochemical oxygen demand
(BOD5) and total suspended solids (TSS)
concentrations of less than 30 mg/L. Rock filters
are neither as successful nor as reliable in the
removal of ammonia (NH3-N) as other methods of
filtration. Nonetheless, their low cost and simple
operation make them attractive for small
communities that are not subj ect to ammonia limits.
The concept of the rock filter was developed in
Kansas in the early 1970s. There are about 20
operating systems in the United States with most
constructed between 1970 and 1985. The design
flow of these operational systems ranges from 150
to 19,000 m3/d (0.04 to 5.0 MOD). New
applications of the rock filters have diminished in
recent years based on the problems with ammonia
removal and the emergence of constructed wetlands
to upgrade lagoon performance.
Most rock filter operating systems were designed
for horizontal flow with the rock bed placed at or
near the effluent end of the final cell in the lagoon
system. In general, vertical flow systems, such as
ones located in Veneta, Oregon and West Monroe,
Louisiana, perform better than horizontal flow
systems. In some systems, effluent is collected by
a manifold buried in the rock bed while in other
systems effluent is discharged from an open water
area on the downstream side of the filter bed.
Significant after growth of algae has been observed
when open water downstream of the discharge is
used. It is better to discharge the effluent without
exposure to sunlight, but this may be difficult where
reoxygenation is required before discharge. Filter
beds typically extend about 0.3 m (1 foot) above the
maximum water level. Hydraulic loadings range
from 250 to 1,200 L/m3 d (2 to 9 gal/ft3 d).
Hydraulic loading rates that exceed 250 L/m3 d of
media (1.9 gal/ft3 d) do not appear to provide the
consistent effluent quality observed at Veneta,
Oregon.
Common Configurations
The most common configuration for rock media
polishing is the horizontal flow system. If
constructed within an existing lagoon, the
configuration depends in part on the location of the
effluent pipe. Vertical flow systems as well as
some horizontal flow systems have been
constructed in a separate basin. For example, a
configuration used in Illinois provides open water
zones in the horizontal flow bed. Aerators are then
placed in these open water zones to increase
dissolved oxygen levels in the water flowing
through the bed. The Illinois designs produced
wide variations in effluent quality.
APPLICABILITY
The rock filter may find continued use for low-cost,
low-maintenance polishing of wastewater treatment
lagoon effluents. The Veneta, Oregon system
produced an effluent of 30 mg/L (or less) BOD and
TSS for more than 20 years, but the process will not
reliably remove ammonia. In many cases, ammonia
concentrations in the final effluent exceed that in
the influent to the rock filter. This is due to the
anaerobic decomposition of the algae trapped in the
bed. This is a seasonal response, with the highest
loss of ammonia occurring during the warmest
summer months.
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The systems in Oregon and West Monroe,
Louisiana, are designed as upflow vertical filter
beds. Influent is delivered to a buried perforated
pipe along the center line of the filter basin and
effluent is collected in weirs on the side of the bed,
near the top of the rock surface. The Louisiana
system has a filter bed 1.8 m (6 feet) deep
composed of rocks 50 to 120 mm (2 to 5 in) in size.
Pumps deliver the lagoon effluent to the rock filter
at an average rate of 400 L/m3 d (3.0 ga./ft3 d) of
media. The hydraulic loading rate at the Oregon
system was 250 L/m3 d of media which is much
lower than the 400 L/m3 d of media hydraulic
loading rate used at the Louisiana system. This
difference in loading rate may be related to the
more consistent effluent quality observed at the
Oregon system.
ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of rock filters
are listed below:
Advantages
Provides a method to improve some aspects
of facultative lagoon effluent at the lowest
possible cost.
Simple operation and low costs are
attractive to small communities that do not
face ammonia limits.
• Provides low maintenance polishing of
wastewater treatment lagoon effluents.
Disadvantages
• Significant ammonia (NH3-N) removal
should not be expected. In some cases,
ammonia content may be increased.
• The process is impractical for communities
with strict ammonia limits.
• The final effluent ammonia concentration
can exceed that in the influent to the rock
filter.
• Many designs have not been able to
consistently or reliably meet a 30 mg/L
discharge standard for BOD and TSS.
• Rock filters may accumulate a heavy
concentration of slime and Psychoda fly
larvae.
• No provisions exist for cleaning rock filters.
DESIGN CRITERIA
While systems have been in operation for about 20
years, there is still no consensus on design
procedures. There are no equipment requirements
for operation of a typical horizontal flow rock filter
bed. Vertical flow beds may require a pump for
influent delivery to the filter bed. A 25 L/s (400
gpm) pump was used at the 760 m3/d (200,000
gallon per day) system in Oregon.
State of Illinois
The State of Illinois Department of Environmental
Protection published a set of guidelines for the
design of horizontal flow rock filters, but
performance has varied widely, perhaps because
allowable hydraulic loading rates are too high.
Based on the successful operation of rock filters
with hydraulic loading rates of less than 250
L/m3 d of media (2 gal/ft3 d), it appears prudent
to design systems accordingly.
Hydraulic loading rate: 800 L/m3 d (6 gal/ft3 d).
It should be noted that hydraulic loading rates of
less than 250 L/m3 d of media provide better and
more consistent effluent quality.
Rock Characteristics: 75 to 150 mm (3 to 6 in)
diameter. Rock should be free of fines, soft-
weathering stone, and flat rock.
Bed Depth: Top of bed must extend 0.3 m (one
foot) above maximum water surface.
Post Aeration: Typically required to meet dissolved
oxygen discharge limits.
Disinfection: If required by discharge permit.
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Veneta, Oregon
Design data from the 760 m3/d (200,000 gallons per
day) vertical flow rock filter in operation in Veneta,
Oregon include:
Hydraulic loading rate: 0.29 cubic meters per day
per cubic meter (2.2 gallons per day per cubic foot).
Pump Capacity: 25 L/s (400 gpm).
Rock Size: 75 to 200 mm (3 to 8 in).
Bed Porosity: 42 percent.
Operations
With horizontal flow through the filter media,
pumps and their related energy requirements may
not be required. Pumps are usually necessary for
vertical up-flow type filter beds. Energy
requirements depend on site-specific factors.
PERFORMANCE
In general, rock filter systems perform adequately;
however, effluent from these systems occasionally
exceeds 30 mg/L BOD and TSS. The Veneta,
Oregon, system has performed consistently better
than the Louisiana system with respect to these
parameters. The only difference between these
systems is the hydraulic loading rate. The Oregon
system received 200 L/m3 d (1.9 gal/ft3 d) while
the Louisiana system received 290 L/m3 d (2.7
gal/ft3 d). Thus, the better performance in Oregon
may be attributable to the lower hydraulic loading
rate.
Overall, rock filters can provide effective BOD and
TSS removal most of the time. The low cost and
ease of operation also make them attractive for non-
critical applications.
Limitations
A major limitation of rock media polishing filters is
their capability to meet a consistent 30 mg/L
discharge standard for BOD and TSS. After an
extensive study of rock filters in Illinois, the states
concluded that such systems could meet an effluent
limitation of 30 mg/L BOD and 37 mg/L TSS. The
Illinois study also found that rock characteristics
were very important. Flat rocks, excess fines, soft-
friable rocks, and rock sizes of less than three
inches in diameter should all be avoided to prevent
plugging problems. If stringent ammonia limits
prevail, presently designed rock filters may not
produce an acceptable effluent.
OPERATION AND MAINTENANCE
Residuals generated
Inorganic solids, biological slime, and non-
degradable residues of biological activity will
accumulate in void spaces in the filter bed. The rate
of accumulation depends on remaining biological
activity and transport of inert materials. The
frequency of plugging varies from every few years
to never. Provisions for cleaning do not exist in
present systems. In the worst case, it might be
necessary to remove the rock media, dredge out
accumulated detritus, and replace the rock.
COSTS
Construction costs include excavation when
necessary, rock placement, inlet and outlet piping,
land and pumps, for vertical flow beds. Operation
and maintenance (O & M) costs are minimal for
both horizontal and vertical flow beds. Power and
pump maintenance costs are additional expenses for
vertical flow type beds.
REFERENCES
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
1. Middlebrooks, E.J., September 1988.
Review of Rock Filters for Upgrade of
Lagoon Effluents. Journal WPCF, 60(9),
1657-1662.
2. Middlebrooks, E. I, December 1995.
Upgrading Pond Effluents: An Overview.
Water Research, 31 (12), 353-368.
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3. Reed, S.C., et al., 1995 2nd Ed. Natural
Systems for Waste Management and
Treatment. McGraw Hill Book Co., New
York, NY.
4. U.S. EPA, 1983. Design Manual -
Municipal Wastewater Stabilization Ponds.
EPA - 625/1-83-015, US EPA CERI,
Cincinnati, OH.
5. WPCF, 1990. MOP FD-16, Natural
Systems for Wastewater Treatment. WPCF,
Alexandria, VA.
ADDITIONAL INFORMATION
Richard H. Bowman, P.E.
West Slope Supervisor
Colorado Dept of Public Health and Environment
Water Quality Control Division
222 South 6th Street, Room 232
Grand Junction, CO 81502
E. Joe Middlebrooks, Ph.D., P.E., DEE
Environmental Engineering Consultant
360 Blackhawk Lane
Lafayette, CO 80026-9392
Sherwood Reed
Principal
Environmental Engineering Consultants (EEC)
50 Butternut Road
Norwich, VT 05055
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S. Environmental
Protection Agency.
Office of Water
EPA 832-F-02-023
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY B R A
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