f/EPA
United States
Environmental Protection
Agency
Office of Environmental
Engineering and Technology
Washington DC 20460
Research and Development
EPA-600/S7-81-152 Oct. 1981
Project Summary
Long-Term Effects of
Slow-Rate Land Application of
Municipal Wastewater
Alex Hershaft and J. Bruce Truett
Long-term effects of applying par-
tially treated municipal wastewater to
croplands were examined at six
locations in the western United States.
All locations had received wastewater
for at least 10 years. The effects on
soil, groundwater, and crop tissues
were measured and compared with
measurements made at nearby control
sites where crops were irrigated with
water from wells or other conventional
sources.
Data on some 50 pollutants and
other parameters measured at the six
locations are summarized in the full
report. The data revealed that the soil
and vegetation effectively reduce the
concentrations of most pollutants in
the wastewater. although certain
pollutants accumulate in the soil.
Some pollutants pass through the soil
to the groundwater, but usually in
concentrations that do not exceed
Federal standards for drinking water
supply. An important exception is
nitrate-nitrogen, which exceeded the
drinking water standard at three
project locations. At no site was there
any reported evidence of adverse
health effects on farm workers or
nearby residents.
Values of two parameters—hydraulic
loading and nitrogen concentration in
the leachate—measured at the sites
were compared with calculated values
obtained using estimation procedures
from the Process Design Manual for
Land Treatment of Municipal Waste-
water, published in 1976 jointly by the
Environmental Protection Agency,
the Army Corps of Engineers, and the
Department of Agriculture. This
publication presents the two above-
named parameters as limiting criteria
for the design of slow-rate land
application systems, and gives pro-
cedures for estimating their values
from climate data, soil conditions,
evapotranspiration data, and other
site parameters. These procedures
were retrospectively applied to site
data at the six locations, and the
results were compared with actual
measurements. Measured values of
hydraulic loading at the project loca-
tions fell within the allowable limits
estimated by the Design Manual pro-
cedure, but measured values of leachate
nitrogen concentration did not cor-
respond closely to estimated allowable
limits. Recommendations are made in
the report concerning possible modi-
fications to the Design Manual esti-
mating procedures.
This Project Summary was devel-
oped by EPA's Office of Environ-
mental Engineering and Technology,
Washington, DC, to announce key
findings of the research work that is
fully documented in a separate report
of the same title. (See Project Report
ordering information at back.)
-------
Background
Disposal of domestic or municipal
wastewater by application to the land
was widely practiced in Europe and the
United States during the 1800s for
irrigation, for utilizing the plant nutrient
content of the wastewater, and for
reducing the amount of pollutants
discharged to rivers and other surface
waters. During the first half of the 20th
century, many land application systems
in the United States were replaced by
waste treatment plants using mechan-
ical, chemical, and biological processes.
The treated effluent from such plants
has generally been discharged to rivers
or other surface waters. In some
instances, particularly in western
states, the effluent has been used for
irrigation and groundwater recharge.
As late as the mid-1970s, treatment
plant effluent was applied to the land
primarily for irrigating crops. The
effectiveness of land application in
removing pollutants has been widely
recognized, however, and its use as a
step in total wastewater treatment
systems became the subject of sub-
stantial research efforts by the U.S.
Environmental Protection Agency (EPA)
and other agencies. This work led to the
consideration of land application as a
wastewater treatment process, to be
evaluated as a supplement, or possibly
an alternative, to the more conventional
processes.
The number of wastewater treatment
systems that use land application of
effluent has increased over the past 40
years from about 300 facilities, serving
an aggregate population of 0.9 million in
1940, to 940 systems, serving an
estimated 9 million in 1978. This is still
only a small part of the total municipal
wastewater treatment facilities, esti-
mated at approximately 22,700 plants
serving about 164 million people in
1979.
The growth of land treatment has
been encouraged by Federal environ-
mental legislation and policies during
the 1970s. Regulations developed
pursuant to the Federal Water Pollution
Control Act Amendments of 1972
(Public Law 92-500) require that land
treatment be considered, together with
other alternatives, for federally funded
municipal wastewater treatment pro-
jects. Continued emphasis was placed
on the land treatment alternative by the
Water Pollution Control Act Amend-
ments of 1977. There was widespread
interest in the land treatment provision
of the regulations related to these two
Acts, raising questions as to what
criteria would be used to evaluate land
treatment systems and to compare
them with conventional systems. In
response to these questions, the
Environmental Protection Agency, in
cooperation with the Department of
Agriculture and the Army Corps of
Engineers, issued the Process Design
Manual for Land Treatment of Municipal
Wastewaters in 1977.
There has been a continuing concern
about the environmental and public
health impacts of land treatment of
municipal wastewater, especially where
the practice extends over several
decades. Potential problems relate to
the accumulation of heavy metals and
other toxic substances in the soil, the
transport of these pollutants to the
groundwater, and the dispersal of
pathogens (bacteria, viruses, protozoa,
and nematodes) during spray irrigation.
In the early 1970s, EPA initiated a
program of research, development, and
demonstration to address the technical
aspects of land treatment and offered
the prospect of producing the sound
information base needed for a better
understanding of land treatment. The
program included a series of site studies
that examined 10 land application
systems that had been in operation for
at least 10 years. While the studies
cover only a short interval (12 to 18
months) in the total period of wastewater
application at each site, the resulting
data permit inferences to be drawn
about the long-term effects of this
practice. In addition, the data are
expected to be useful in evaluating
design criteria in the Design Manual.
Six of the 10 studies examined slow-
rate systems, where vegetation plays a
critical role in the absorption of water
and nutrients. The other four dealt with
rapid infiltration systems, which rely
principally on physical and chemical
action of the soil to purify the applied
wastewater.
The results of the six slow-rate
systems studies, performed during
1976-1978 are summarized in this
report. A parallel summary, issued in
1980 by the EPA's Robert S. Kerr
Environmental Research Laboratory,
Ada, OK, covers the four rapid infiltration
land treatment systems (Report No.
EPA-600/2-80-165).
The slow-rate land treatment project
sites cover a broad range of climatic
conditions, soils, and topographic and
hydrogeological configurations. All six
systems investigated had received
municipal wastewater for at least 10
years and one for more than 30 years.
The projects were located in the vicinity
of:
Camarillo, Roswell,
California New Mexico
Dickinson,
North Dakota
Mesa,
Arizona
San Angelo,
Texas
Tooele,
Utah
System Design and Operation
The general design of the projects
using slow-rate land treatment systems
is shown in Figure 1. While this design
does not correspond specifically to any
one of the six projects, it represents all
major components typically found in a
slow-rate treatment system. All of the
components shown in Figure 1 are not
required for slow-rate land treatment,
and are not included in every treatment
system studied.
Effluent from the primary or secondary
treatment plant is.transferred to one or
more facultative lagoons where itfl
undergoes additional oxidation and*
settling. Effluent from the lagoon(s) is
transported to a holding pond, then to
the test site where it is applied to the
land by any of several methods. A
nearby control site with characteristics
similar to the test site is irrigated by
water from a conventional source such
as a well or a river. One or more
principal crops are grown on the major
portion of the area at both the test and
control sites. Some of the projects also
have experimental garden plots where a
variety of crops is grown.
Samples of applied water, leachate,
groundwater, soil, and crops were
collected at the test and control sites
and analyzed. Crop yields were reported
for three of the projects.
Site Characteristics,
Operations, and Selection
Criteria
All six projects were in western
states, and all but one were in arid or
semiarid climates. Test sites and control
sites at five of the six projects were on
privately-owned farms. The test site for
the sixth project was on a municipally-
operated farm.
-------
Municipal Treatment Facility
J_. __ ,
Municipal
'Collection'
System
.1 ^ "'
1
1
_!
1
1
1
y
_^ Primary and
Secondary Plant
X
Facultative Lagoons
"\
r
Experimental Farm
Effluent
for
Land
Application
Ho/ding
Pond
I
Effluent
Discharged to
Surface Waters
r
Control Farm
1
Irrigation Water
1
1
^ I
Control
Site
Well
or River
Figure 1. Schematic of general project design.
Specific test sites for each project
were selected after examination and
evaluation of several alternatives in
each of six geographic regions specified
by EPA. Criteria for selection of the test
sites included the following:
• The site had been irrigated with
effluent for at least 10 years, and
historical records were available
for that period.
• The applied wastewater should be
an effluent from either primary or
secondary treatment facilities.
• Flow rates should be at least 4*38
liters/second (l/s) [0.1 million
gallons/day (mgd)].
Crops should be representative of
common usage.
A "good control site" should be
available within a reasonable
distance and have the same general
type of soil and hydrogeological
conditions.
Results at the Six Locations
The results of measurements of
approximately 50 pollutants and other
water quality parameters in the three
media (soil, vegetation, and ground-
water, including leachate) at the six
locations are presented in a series of 20
tables in the full report. One of these
(Table 1) is included in this summary to
indicate the arrangement of data. The
tabular data ~. e grouped first by media
and then by pollutant category within
each medium. The pollutant categories
are: nitrogen and phosphorus, organics,
suspended and dissolved soilds, alkali
and alkaline earth metals, heavy metals,
nonmetallic elements, and bacterio-
logical indicators of pathogenic organ-
isms.
Each table reports the concentrations
of several pollutants in the applied
water and in selected locations within a
given medium for each of the six
projects (for example, at different
depths in the groundwater or different
parts of a crop plant). The media-based
approach is used on the assumptions
that most readers will prefer to access
the data by medium rather than by
pollutant or location.
The entries in the columns for each
pollutant show four values: the concen-
trations at the test and control sites, the
percent difference with respect to the
control site value, and the significance
level of the difference. In cases where
the values are not large enough to be
detected by the analytical techniques
used, the detection limits are reported
but difference and significance levels
are not, as they have no meaning. In
some instances, control site data were
not available and comparisons were
omitted or were drawn between con-
centration measured upgradient and
downgradient of the test site or at
different times in the effluent application
period.
Transport and Fate of Pollutants
Nitrogen
Total nitrogen and ammonium nitro-
gen concentrations were much higher
in the sewage effluent than in the
irrigation water at the four reporting
projects (Camarillo and Mesa for total N,
Dickinson and Roswell for NH/). Nitrate
levels did not differ as markedly.
Considerable equalization of concen-
trations occurred with depth as the
nitrogen compounds were taken up by
plants. However, nitrate levels rose with
depth, in part through nitrification and
in part because of low affinity for soil
particles. In fact, nitrate levels in
groundwater i n both the test and control
sites consistently exceeded drinking
water criteria in at least one location
(Camarillo).
-------
Table 1. Concentration of Nitrogen and Phosphorus in Leachate and Groundwater (mg/l)
Camarillo
• Applied water
• Leachate - 50cm
• Leachate - 100 cm
• Groundwater-top"
• Groundwater-bottom*
Dickinson
• Applied water
• Groundwater
Mesa
• Applied water
• Leachate - 50 cm
• Leachate -100cm
• Groundwater • top*
• Groundwater-bottom"
Roswett
• Applied water
• Groundwater<6m
• Groundwater>6m
San Angelo
• Lagoon effluent
• Groundwater<10m
*Groundwater<10m
» River water"
Test""
160
36.6
47.5
539
45.6
249
1S2
11.9
41.3
07
0.8
352
05) 0.21
21 6 95
OO6 O04
0.03 002 +50 0.02
0.08 006 +330005)0024
P-POS
Cont %A
0.5 +2,260
38 +26
3.0 -3
0.4 +25
028 +114
(soluble)
<0t +37,900
005 -40
0.5 +1 740
1 0 +400
36 +17
10 +70
12 +75
(soluble)
0.05 +12.840
0.11 +91
(dissolved)
O01
0017 +41
(SL)
(001-004)
O01)
O01)
00.1)
O0.1)
OO05)
(0.014.04)
O01)
OO 11
O01)
O0.1t
/<05I
O05)
•
O005)
"Concentrations at the test site
"Upgradient/downgradient concentrations at test site
""Test = concentration at test site
Cont = concentration at test site
%A = /concentration at test site) - (concentration at control site)
concentration at control site
(SL) = significance level
""The value for total organic nitrogen in applied water appears, on review of analytical procedures in this reference, to represent total K/eldahl-N
It is used as TKN in the sample calculation
Concentrations in the soil as a
function of depth were reported for total
nitrogen and nitrate (Camarillo, Mesa,
and Tooele), for organic and inorganic
nitrogen (Dickinson and Roswell), and
for ammonium (Tooele). The values at
both the control and test sites generally
decreased with depth, but the dif-
ferences between the control and test
sites did not exhibit any obvious trends.
Phosphorus
Phosphorus concentrations in leach-
ate and groundwater decreased with
depth. In the soil, phosphorus concen-
tration decreased rather consistently
with depth for both the control and test
sites, although the decrease at the test
sites appeared to be more rapid.
Phosphorus levels in vegetation were
•reported in the form of total phosphorus
(Dickinson, Roswell, San Angelo,
Tooele) and phosphate (Camarillo and
Mesa). In those investigations that
afforded a comparison between the
same plant species at the control and
test sites, the test site plants exhibited
considerably higher phosphorus levels.
Organics
The quantity of organic matter found
at some of the facilities was affected by
changes in the algal growth in the
effluent storage and treatment lagoons
(Dickinson and San Angelo), by drop-
pings from grazing animals, and by
plowing under crops grown on the sites
(Camarillo). Interpretation of the data to
determine the effect of land treatment
in removing organic matter in applied
wastewater was greatly complicated by
the other sources of these materials. In
most cases, the levels appeared to
decrease with depth, but the number
and uniformity of results were insuf-
ficient to establish definite patterns.
Solids
The term "total solids" encompasses
suspended solids and dissolved solids.
The first two terms are of small con-
sequence in land treatment because
suspended matter is usually filtered out
by the first few centimeters of the soil.
Concentrations of both total and in-
dividual dissolved solids were generally
higher in the sewage effluent than in
the irrigation water. The notable ex-
ception was at the Roswell facility,
which used groundwater for irrigation.
Other exceptions were the concentra-
tions of magnesium and sulfate at most
of the four reporting facilities.
The inequality in dissolved solids
concentrations at test and control sites
persisted in water sampled at increasing
depths, with the exception of deep
groundwater samples. Furthermore, 4
concentrations of total and individual^
dissolved solids in leachate and
groundwater generally increased with
depth at both the test and control sites.
The key exception was potassium which
was taken up by plants as an important
nutrient in relatively large amounts.
Heavy Metals
Heavy metals are of special concern
in land treatment because of their
toxicity, their persistence in the en-
vironment, and the ability of living
organisms to bioaccumulate them.
These substances may be taken up by
crops grown on land treatment sites and
ingested by humans or animals, or they
may percolate to groundwater or
surface water supplies. Concentrations
of cadmium, chromium, copper, lead,
molybdenum, nickel, and zinc were
reported as a function of depth for
leachate and groundwater at the
Camarillo and Mesa projects. More
limited results were provided for most of
these metals as well as for aluminum,
cobalt, iron, manganese, and mercury at
Dickinson, Roswell, and San Angelo. In
general, metal levels were low at both
the control and test sites and did not
-------
change appreciably with depth. In
several cases the concentrations were
higher at the control sites.
Heavy metals levels in the soil
showed no appreciable change at
different depths. In a number of cases,
higher concentrations were noted at the
control sites. Heavy metals levels in
various portions of crops grown on the
control and test sites were reported for
all six projects. At those projects that
afforded a comparison between the
same crops, all metals exhibited higher
or equivalent concentrations in plants
grown on the control site. None of the
concentrations approached hazardous
limits.
Biological Indicators of
Pathogenic Organisms
Although indicator organisms levels
were substantially higher in the sewage
effluent than in the irrigation water, all
but total coliforms were reduced to
below detectable limits by passage
through 300 cm of soil. Some elevated
levels on crops were attributed to
droppings from grazing animals.
Comparison of Leachate
Parameters with Drinking
Water Standards
Although the Design Manual does not
recommend that drinking water stan-
dards other than nitrate-nitrogen con-
centration be considered as limiting
criteria in the design of slow-rate land
treatment systems, it is nevertheless
interesting to compare the allowable
levels of criteria parameters specified in
EPA standards for drinking water supply
with the measured concentration of the
same substances in the groundwater at
test sites. An overall summary compar-
ison is shown in Table 2.
Data in Table 2 indicate that the
concentrations of most drinking water
criteria parameters are well within the
acceptable maximum levels at most of
the treated sites. However, the measured
levels at nitrate-N, selenium, and total
coliform equal or exceed the criteria
levels at all sites reported. This observa-
tion does not, in itself, indicate that the
higher-than-acceptable levels of these
constituents are a result of land
treatment, since the levels of selenium
and total coliform in groundwater at
some of the control sites also exceed
criteria levels for drinking water supply.
Comparison of System Design
Factors with Criteria in Process
Design Manual
The report examines possible design
and predicted performance changes
that might have resulted if the six slow-
rate systems had been designed accord-
ing to criteria in the Design Manual.
Such comparisons between the features
of existing systems and the design
criteria in the Design Manual are
entirely hypothetical, since all of the
systems were designed and in operation
long before the Design Manual was
published.
The Design Manual considers two
principal limiting criteria for the design
of slow-rate systems: hydraulic loading,
and concentration of nitrogen in the
leachate.
Given minimal information on soil
characteristics at any location in the
Table 2. Comparison of Drinking Water Criteria and Groundwater Parameters at Treatment Sites
Constituent
Chemical fmg/l)
Drinking
Water
Standard
(Value) Camarillo Dickinson Mesa Roswell San Angelo
Concentrations in Groundwater or Leachate at Treated Sites
Tooele
Arsenic
Barium
Cadmium
Chromium
Fluoride
Lead
Mercury
Nitrates (as N)
Selenium
Silver
0.05
1.0
0.01
0.05
1.4 to 2.4*
0.05
0.002
10.
0.01
0.05
0.01
0.16
0.02
0.03
1.1
0.12
—
52.8
O.01
—
0.074
—
<0.01
<0.02
—
<0.1
<0.0001
1.8
O.Q2
—
0.02
0.19
0.01
0.03
0.79
0.19
—
9.9
O.O1
—
<0.02
—
<0.02
<0.02
—
<0.10
<0.0005
6.5
O.O3
—
—
<0.004
<0.005
—
0.005
0.01
0.01
*»»
— -0.00001 —
*#*
***
— -0.000008 —
— -0.00003 —
***
0.00007
**»
***
0.00001
0.00003
—
—
—
—
—
—
— = Not reported.
*= Dependent on temperature; higher limits for lower temperatures.
**= TNTC - Too numerous to count.
*** = Not observed at detectable limit.
-------
Table 3. Acceptable Wastewater Hydraulic Loading (I.*,) as Calculated by Design Manual Procedure
Location
Camanllo
Dickinson
Mesa
Roswell
San Angelo
Tooele
Textural
Classification
of Soil
Loam {mocho
loam with
25% clay)
Fine sandy
loam
OVinity loam
fine sand
GGilman Loam
Silty clay,
loam
Silty clay
Sand
Grave/
Loam
Sandy loam
Clay loam
Permeability Upper Limit of
Range Application
Value Used Rate Excluding
For Computation Evapotranspiration
In /hr In /week
Moderate to
Moderately Slow
106)
(0.031
Moderately Rapid
1301
Moderate
12.0)
(1 04)
Moderately Slow
104)
Moderately Slow
104)
Slow to
Moderately Slow
104)
to
0.5
45
38
20
70
70
70
Wastewater Loading (L*J
Percolation Evapotranspiration
Growing Rate Less Precipitation
Season IWf) (ET-Pr)
Weeks In /yr Ft/yr Ft/yr
52 520
26
30 1350
52 1976
1040
22 154
52 364
30 210
433 17
2.2
1125 1 7
165 46
87
128 46
30.3 4.2
17 B 2.5
Max. Allowable
per
Design Manual
Ft/yr
45.0
39
1142
170
92
174
345
20.0
Actual
Measurements
Ft/yr In /wk
50 (12)
2 6 10.6)
165 (3.8)
26 (06)
87 (20)
20 (0.5)
continental United States, a rough
estimate of the allowable hydraulic load
(Lw) can be made from information in the
Design Manual. Estimates of Lw have
been made for each project location in
this study and are compared with the
actual wastewater loading applied.
Actual wastewater application rates
and estimated acceptable rates obtained
by means of the Design Manual pro-
cedure for all stx slow-rate projects
(together with data used in determining
the estimated rates) are presented in
Table 3. Actual application rates for
each project are plotted against perm-
eability rates for the six. projects in
Figure 2. The format of this figure shows
the line indicating maximum acceptable
wastewater application. It is clear from
both Figure 2 and Table 3 that actual
application rates at the sites fall below
the acceptable maximum as estimated
from the Design Manual in all cases,
except the Camarillo estimate, based on
field measurements of soil permeability.
For this one exception, actual rate
exceeds the estimated acceptable rate
by about a factor of two.
The concentration of total nitrogen in
the percolate from the treatment site is
one of the limiting criteria in the Design
Manual. The allowable concentration
depends on the use classification of the
groundwater. In this example, the most
stringent use classification—drinking
water source—will be assumed. The
allowable concentration for this use is
10mg/l.
Input data for all projects are presented
in Table 4, and computed values of CNP
are compared with measured values at
each project. There is relatively low
7000
x. 400 -
.c
| 200 -
C -g.
2 .5
§. 240 -
(0 >y
lii "5.
0,^-20 -
c ^
•S o
-§ 2 70 -
|j j) 4.0-
| I 2°-
\ 1.0-
^
9)
| 0.4-
1 0.2-
0.1.
• Bas€
>d on soil
permeab
procedures. ]
• Base
Camarill
m
/'
d on field
j
3 /
measurn
ility as es
timated f
rom desig
] T 1
nents of s
oil perme
10 /
<$>y
rt\* .
$s
t&jT
r^^
Y$S
<$*/
f
•Si
Mesa
1 i
n Angelo
t Camarill
|
• Roswell
T
• Tc
ole
Mesa
t
0
ability.
• Dicker son
n manua
1
Permeability Rates of Most Restrictive Layer in Soil Profile, in./hr
Permeability*, Soil Conservation Service Descriptive Terms
Very Slow
<0.08
Slow
0.06-0.20
Moderately
Slow
0.20-0.60
Moderate
0.60-2.0
Moderately
Rapid
2.0-8.0
Rapid
6.0-20.0
Very Rapid
>2O.O
* Measured with clear water. 1 in./wk = 2.54 cm/wk
Figure 2. Measured values of application rate versus soil permeability at six slow-
rate project sites.
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Table 4. Nitrogen Concentration in Percolate
Con of Hydraulic Crop Uptake Percolation Nitrogen Net Cone, of- Nitrogen Applied Total
Total N in Loading of of Nitrogen Rate (from Loading from Nitrogen in Percolate as Fertilizer Nitrogen
Wastewater Wastewater (Un) Site Data) Wastewater Loading Cw= (Lm> Loading
(CwJ Actual WJ «.7CN»iJ O.SL^-Un 087N»t/N <.NT=
LN*} - UN
mg/l ft/yr Ib/a-yr ft/yr Ib/a-yr Ib/a-yr mg/l Ib/a-yr Ib/a-yr
CAMARILLO 15 5.1 194 2.9 207 1-281 Calculated
Negative
242 165
Nitrogen Concentrations
in Percolate, CND
Calculated Values
tNt/2.7Wp
mg/l
21.1
Measured
Values
at Sites
mg/l
64
Tomatoes
Broccoli
Combined
Uptake
DICKINSON
MESA
Barley
Sorghum
Corn
ROSWELL
Corn
SANANGELO
TOOELC
Grasses
(50%)
Alfalfa
(S0%)
11.8
(Total organic-N
24.9
(Total N)
42.9
34 5 avg
13.2
4.6 160 08
16.S 262 1 1.6
Combined
Uptake
26 160 1.1
8.7 550 7.5
2.0 310 avg. -03
147 1-13) Calculated
Negative
1109 625 20
-300 80 26.9
832 115 57
71 3 (-2531 Calculated
Negative
14 (-18)
None
None
62.5 165
(Does not include
manure from live-
stock grazing)
51 -202
Calculated 2.4
Negative
20 464
269 13.7(<6m)
64(>6m)
8.1 5.8
/Groundwater)
6.4
IRiveri
-
correlation between the observed
average values of percolate nitrogen
I concentration and the estimated values
obtained from the procedures in the
Design Manual. The differences between
observed and estimated values of this
parameter suggest that the estimating
procedures should be carefully reviewed
and their limitations explicitly identified.
Conclusions
• All six slow-rate land treatment
systems generally reduced the
levels of major pollutants in the
applied wastewater well below the
corresponding levels typically
found in effluent from secondary
treatment.
• None of the six investigations
reported any significant threat to
the public health through airborne
dispersion of pathogens or con-
tamination of drinking water sup-
plies or vegetation.
• Large amounts of nitrogen and
phosphorus were taken up by the
crops, but nitrate levels in the
leachate increased with depth and
in some cases exceeded the EPA
standard for drinking water supply.
• The organic matter content, as
indicated by BOD and COD levels,
was effectively attenuated by the
soil, although precise measure-
ment of the attenuation was not
always possible when the amount
of applied wastewater varied rapidly
with time.
1 Since suspended solids were fil-
tered out within the first few
centimeters of the soil bed, their
concentration in the leachate was
not measured.
Concentrations of both total dis-
solved solids and individual dis-
solved solids generally increased
with depth.
None of the six facilities received
much industrial wastewater with
its typically high concentration of
heavy metals. The low concentra-
tions of heavy metals found in the
soil did not change appreciably
with depth. Heavy metal levels in
vegetation were within normal
ranges, and levels in groundwater
were within the limits set by
drinking water standards.
Water-borne pathogen levels, as
indicated by measurements of
fecal coliform bacteria, protozoa,
and nematodes, appear to have
been reduced below limits of
detection by passage through the
soil; some indicators of pathogenic
organisms detected on vegetation
were attributed to droppings by
grazing animals.
All six test sites investigated would
have met the Design Manual's land
area criteria based on hydraulic
conductivity of the soil.
Three of the six test sites would not
have met the Design Manual's land
area criteria based on nitrogen
content in the percolate.
On the basis of experience during
10 or more years of wastewater
application at each of the six sites,
the effective life of land treatment
systems is estimated to be several
decades.
The economic benefits of waste-
water application extend beyond
the lower cost of the water and
include, in some cases, higher crop
yields than at sites where conven-
tional irrigation water was used.
One project (Camarillo) reported a
12 percent higher yield at the test
site.
Recommendations
• Slow-rate land treatment of muni-
cipal wastewater is demonstrated
to be an effective means of reducing
the concentration of most cate-
gories of pollutants. On the basis of
the results of. the six studies
examined here, this method of
treatment is recommended for
GOVERNMENT PRINTING OFFICE:1981--559-092/3353
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consideration as a technically and
economically viable supplement to
conventional treatment processes
and as an alternative to certain of
the processing steps typically
included in a treatment plant that
discharges its effluent to surface
waters.
Although levels of many pathogens
(fecal coliform, protozoa, and
nematodes) were reduced below
limits of detection by soil treat-
ment, the degree of reduction of
viruses is not clearly established.
Information on viruses in the
source reports suggests that further
investigations be undertaken to
obtain a better understanding of
the prevalance and transport of
viruses under various conditions of
land treatment of wastewater.
However, the subject of viruses
was not the primary emphasis in
the source documents, and was
not addressed in depth. It is there-
fore recommended that the need
for additional studies of viral
prevalance and transport be re-
viewed in the light of more recent
investigations on these subjects.
• The Design Manual procedure for
estimating land requirements for
slow-rate wastewater treatment
systems should be reviewed for
adequacy. In particular, the follow-
ing modifications should be con-
sidered for any future edition of the
Design Manual:
• The procedure for estimating
maximum allowable application
rates on the basis of the hydraulic
conductivity of the most restrictive
layer should be more thoroughly
discussed.
The procedure for estimating
percolate nitrogen concentration
could easily be modified to ac-
count for sources of nitrogen other
than the applied wastewater (for
example, fertilizer or animal drop-
pings) Also, the discussion should
point out that the procedure is
based on the assumption of equi-
librium (or steady-state) condi-
tions, which could yield estimates
that differ markedly from meas-
urements made in the field, where
conditions may be highly dynamic
and may not approach equilibrium
during a short-term period of
measurement
Alex Hershaft and J, Bruce Truett are with The MITRE Corporation, Metrek
Division, McLean, VA 22101.
H. R. Thacker is the EPA Project Officer (see below).
The complete report, entitled "Long- Term Effects of Slow-Rate Land Application
of Municipal Wastewater," (Order No. PB 82-105 610; Cost: $9.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Office of Environmental Engineering and Technology (RD-681)
U.S. Environmental Protection Agency
Washington, DC 20460
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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