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Environmental Protection
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DESCRIPTION
Slow rate (SR) land treatment is the controlled
application of primary or secondary wastewater to a
vegetated land surface. It is the oldest and most
widely used form of land treatment. The nutrients
and the water in partially treated wastewater
contribute to the growth of a wide variety of crops,
the maintenance of parks, pasture lands, and forests.
SR systems can produce a very high quality
percolate but also require the largest land area
compared to the other land treatment concepts. On
a worldwide basis, thousands of systems use
wastewater for irrigation in variations of the SR
process.
In the SR process, wastewater infiltrates and
percolates from the vegetated soil surface and flows
through the plant root zone and soil matrix. Water
may percolate to the native groundwater or to
underdrains or wells for water recovery and reuse of
the effluent. Underdrains serve to prevent
groundwater mounding under the site, to control
groundwater flow, and to minimize movement of
leachate onto adjacent property. Figure 1 illustrates
the principal hydraulic pathways of water applied in
SR systems.
SR systems use standard irrigation methods to
distribute the water to agricultural fields, pastures,
or forest lands. SR systems can be classified as
either slow rate infiltration systems (Type 1) or crop
irrigation systems (Type 2). The design objective of
slow rate infiltration systems is to maximize
wastewater treatment while minimizing land area.
Crop irrigation systems are designed to meet crop
water needs, which typically requires the use of a
larger land area.
APPLICABILITY
The simplicity of land treatment makes it an
attractive technology compared with other
wastewater technologies.
Wastewater Technology Fact Sheet
Slow Rate Land Treatment
A large forested sprinkler, slow rate irrigation
system, constructed in the early 1980s, in Dalton,
Georgia, highlights the applicability of land
treatment systems. Dalton is known as the "carpet
capital of the world" with 87 percent of its total
municipal flow attributed to the carpet industry.
The total site contains 3,640 hectares (9,000 acres)
with about 1,860 hectares (4,605 acres) of forest
being irrigated. The terrain varies from flat to
relatively steep (some areas have up to a 40 percent
grade) with soil depths of 0.5 to 1.2 m (1.5 to 4
feet). The three secondary treatment plants that feed
secondary effluent to the site generate a combined
flow of 33 million gallons per day (MGD) (Nutter,
2000).
Evapotranspiration
Percolation
A) Application pathway
Underdrains
ill
Wells
B) Recovery pathways
C) Subsurface pathway
Source: Crites, et al., 2000.
FIGURE 1 HYDRAULIC PATHWAYS FOR
SLOW RATE LAND TREATMENT
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The choice of application method depends upon site
conditions and wastewater characteristics. In
Dalton, lint caused clogging problems in the
sprinkler system but improved industrial
pretreatment and screening at the pumping stations
remediated this problem. In general, advantages of
sprinkler application over gravity methods include:
• More uniform distribution of water and
greater flexibility in application rates.
Applicable to most crops.
Less susceptible to topographic constraints
and reduced operator skill and experience.
Gravity methods that utilize shallow flooding of
carefully graded fields is generally applicable only
for row crops and pastures on relatively flat,
uniform terrain.
ADVANTAGES AND DISADVANTAGES
Advantages
SR systems, like other land treatment methods, may
be an economical system for wastewater treatment
in locations where sufficient land is available at a
suitable price. Specific advantages of this
technology include:
Significantly reduced operational, labor,
chemical, and energy requirements
compared to conventional wastewater
treatment systems.
• Economic return from the use and re-use of
water and nutrients to provide marketable
crops.
• Little or no disposal or effluent production.
• Recycling and reuse of water reduces water
distribution and treatment costs for crop
irrigation.
Disadvantages
SR systems require a thorough investigation of site
suitability before implementation. Land area
requirements are significantly greater for SR
systems than for conventional wastewater treatment
plants and other land treatment methods, such as
rapid infiltration and overland flow systems. Slow
rate application may not be feasible in most
suburban and urban areas. Land requirements
include the application area, roads, and winter
storage during cold weather if seasonal crops are
grown or if frozen soil conditions develop.
Temporary storage may also be required for
harvesting and maintenance activities.
The removal of pathogens and other pollutants is
very effective in SR systems when properly
designed and managed. The complex removal
mechanisms involved with land treatment processes
make site selection a critical part of the design.
Specific problems associated with poor site
selection include:
Soil structure dispersion resulting from high
dissolved salts concentration.
• Runoff and erosion for sites with steep
slopes or lack of adequate erosion
protection.
• Inadequate soil or groundwater
characterization resulting in operational
hydraulic problems.
DESIGN CRITERIA
Proper soils and an adequate land area are
paramount criteria when considering SR systems.
Table 1 shows the general design parameters for SR
systems. The SR process is most suitable for soils
of low to medium permeability. Land requirements
for this technology are relatively large, but can
decrease as the level of influent water quality or
degree of pre-treatment increases.
Vegetation serves to reduce nutrient concentrations
by uptake, to control erosion, and to maintain or
increase infiltration rates.
Considerations for vegetative selection include:
Suitability of climate and soil conditions.
Consumptive water use and water tolerance.
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TABLE 1 DESIGN CRITERIA
Item
Range
Field Area
Application Rate
BOD Loading
Soil Depth
Soil Permeability
Lower Temperature Limit
Application Method
Pretreatment Required
Particle Size (for sprinkler
applications)
56 to 560 acres/MGD
2 to 20 ft/yr
(0.5to4in/wk)
0.2 to 5 Ib/acre/d
at least 2 to 5 ft
0.06to2.0in/hr
25 deg F
sprinkler or surface
preliminary & secondary
Solids less than 1/3
sprinkler nozzle
Source: Crites, et al., 2000.
• Nutrient uptake and sensitivity to
wastewater constituents.
Economic value and marketability.
• Length of growing season.
Ease of management.
Public health regulations.
Design considerations for the sprinkler system
include:
• Field conditions (shape, slope, vegetation,
soil type).
Climate.
Operating conditions.
• Economics.
Design slopes should be less than 15 percent to
promote infiltration rather than surface runoff.
References 1, 2, and 6 provide detailed design
guidance for SR systems. For planning purposes, a
rough estimate of the total land area required for an
SR system can be developed using the following
equations:
Warm climates and/or 12 month per year operation:
A= 190(Q)
Cold climates and/or 6 month per year operation:
A = 280(Q)
Where: A = total site area, acres
Q = design flow, MGD
These equations are valid up to a design flow of
about 10 MGD, and include an allowance for a
temporary storage pond or access roads.
Pretreatment is not included.
PERFORMANCE
Performance of SR systems in reducing BOD, TSS,
nitrogen, phosphorus, metals, trace organics, and
pathogens is generally very good. Table 2 shows
expected removals for typical pollution parameters
by SR systems. Nitrogen removal occurs through
vegetative uptake, biological reduction through
nitrification/denitrification in soil, and ammonia
volatilization.
Limitations
Land treatment of wastewaters by the SR process is
limited by several factors, including climate, the
slope of the land, and soil conditions. Wastewater
application may need to be reduced during wet
weather periods, creating a need for an adequate
storage volume during such periods. In cold
climates, frozen soil conditions may also slow
application during the winter months. Other
TABLE 2 EFFLUENT QUALITY
Parameter
BOD
TSS
TN
TP
Fecal Coliform
Percent Removal
90 to 99+ percent
90 to 99+ percent
50 to 90 percent
80 to 99 percent
99.99+ percent
Source: Crites, et al., 2000.
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disadvantages include high land requirements and
potential odor and vector problems if adequate
pretreatment is not employed. Other limitations of
the SR process include:
Crop water tolerances.
• Nutrient requirements.
• Sprinkling limitations (wind conditions,
clogging of nozzles).
• May need pretreatment for solids, oil, and
grease.
OPERATION AND MAINTENANCE
Proper operation and maintenance (O&M) is
required for SR land treatment systems to perform
as intended. In general, labor requirements for land
treatment systems will be less than those for
conventional waste water systems. When crop
harvesting is required, there will be a greater
requirement for labor. Monitoring requirements can
include applied wastewater, groundwater, soil, and
vegetation. Vegetation grown on SR systems is
usually harvested on a routine basis. Dikes and
berms for ponds require regular investigation to
check for burrowing animals or decay/destruction of
the structure and liner material. Systems that use
sprinklers should have a regular inspection and
cleaning schedule, including regular draining of
lines and pipes in seasonal operation to avoid
corrosion. Pumps, valves, and other mechanical
elements require routine maintenance, including
lubrication.
COSTS
Capital costs for land treatment systems include
(Crites, Reed, and Bastian, 2000):
• Transmission.
• Pumping.
• Preapplication treatment.
• Storage.
• Field preparation.
• Distribution.
• Recovery.
• Land.
There will be operation and maintenance costs with
all of these areas except land purchase and
preparation. Other O&M costs may include
monitoring, site and crop management, and
harvesting. Other costs may include buildings,
roads, relocation of residents, and purchase of water
rights.
A preliminary estimate of costs for planning
purposes can be obtained using the following
equations.
Slow Rate, Sprinklers, Underdrained
Construction costs ($) O&M Costs ($/yr)
C = (3.187)(Q)a9331 C = (0.1120(Q)0'8176
Slow Rate, Sprinklers, Not Underdrained
Construction Costs ($) O&M Costs ($/yr)
C = (1.71)(Q)a999 C = (0.205)(Q)a5228
Where: C = costs in millions of dollars
Q = design flow, MOD
These costs are valid up to about a flow of 10 MOD.
Increase construction costs by about 5 percent for
solid-set sprinklers; decrease construction costs by
about 5 percent for center pivot sprinklers. Increase
O & M by 5 percent for center pivot sprinklers;
decrease by 5 percent for solid-set. Underdrain
costs assume a six foot deep pipe network. A 75-day
storage pond is included in these cost estimates, but
pretreatment and land costs are not.
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REFERENCES
Other Related Fact Sheets
Rapid Infiltration Land Treatment
EPA-832-F-02-018
September 2002
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owm/mtb/mtbfact/htm
1. Crites, R. W. and G. Tchobanoglous, 1998.
Small and Decentralized Wastewater
Management Systems. McGraw Hill.
2. Crites, R. W., S. C. Reed, and R. K. Bastian,
2000. Land Treatment Systems for
Municipal and Industrial Wastes. McGraw
Hill.
3. Metcalf and Eddy, 1991. Wastewater
Engineering: Treatment, Disposal, Reuse.
McGraw Hill.
4. Nutter, W., 2000. Personal communication
with Parsons, Inc.
5. U. S. EPA, 1980. Innovative and Alternative
Technology Assessment Manual. U. S. EPA
MERL, Cincinnati, Ohio.
6. U. S. EPA, 1981. Process Design Manual:
Land Treatment of Municipal Wastewater.
U. S. EPA CERI, Cincinnati, Ohio.
7. U. S. EPA, 1984. Process Design Manual:
Land Treatment of Municipal Wastewater,
Supplement on Rapid Infiltration and
Overland Flow. U. S. EPA CERI,
Cincinnati, Ohio.
ADDITIONAL INFORMATION
Ronald W. Crites
Brown and Caldwell
P.O. Box 8045
Walnut Creek, CA 94596
Sherwood Reed
Environmental Engineering Consultants
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-02-012
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
ICC Building
1200 Pennsylvania Avenue, NW
7th Floor, Mail Code 4204M
Washington, D.C. 20460
* 2002 *
THE YEAR OF
CTEAN WATER
IMTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY B R A^
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