v>EPA
United States
Environmental Protection
Agency
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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