United States
Environmental Protection
                         Wastewater Technology Fact Sheet
                         Rock Media Polishing Filter for Lagoons
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,

                       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.


                       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.

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.


Some advantages and disadvantages of rock filters
are listed below:


       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.


      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
      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

      No provisions exist for cleaning rock filters.


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.

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.


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.


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

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.


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.


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.


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.


Other EPA  Fact  Sheets  can be  found  at the
following web address:


1.     Middlebrooks,  E.J.,  September   1988.
       Review  of Rock Filters for Upgrade of
       Lagoon Effluents. Journal WPCF,  60(9),

2.     Middlebrooks,  E.  I,  December   1995.
       Upgrading Pond Effluents: An Overview.
       Water Research, 31 (12), 353-368.

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.


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
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
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
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