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 subject 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 MGD). 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.
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, softweathering 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.

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 noncritical 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, softfriable 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 nondegradable 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. J., December 1995.

Upgrading Pond Effluents: An Overview.

Water Research, 31 (12), 353-368.
For more information contact:

Municipal Technology Branch

U.S. EPA

1200 Pennsylvania Avenue, NW

Mail Code 4204M

Washington, D.C. 20460

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