SEPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600/6-91/001
March 1991
Preliminary Risk
Assessment for
Parasites in Municipal
Sewage Sludge
Applied to Land
-------
-------
EPA/600/6-91/001
March 1991
PRELIMINARY RISK ASSESSMENT FOR PARASITES
IN MUNICIPAL SEWAGE SLUDGE APPLIED TO LAND
Environmental Criteria and Assessment Office
Office of Health and Environmental Assessment
Office of Research and Development ,
U. S. Environmental Protection Agency
Cincinnati, OH 45268
Printed on Recycled Paper
-------
DISCLAIMER
This document has been reviewed in accordance with U.S. Environmental Protection
Agency policy and approved for publication. Mention of trade names, or commercial
products does not constitute endorsement or recommendation for use.
11
-------
PREFACE
. v Section ,405 of the Clean Water Act requires the U.S. Environmental Protection
Agency to develop and issue regulations that identify: (1) uses for sludge, including disposal;
(2) specific factors (including costs) to be taken into account in determining the measures
and practices applicable for each use or disposal; and (3) concentrations of pollutants that
interfere with each use or disposal. To comply with this mandate, the U.S. EPA has
embarked on a program to develop four major technical regulations: land application,
including distribution and marketing; landfilling; incineration and surface disposal. The
development of these technical regulations requires a consideration of pathogens as well
as chemical constituents of sludge. Public concern related to the reuse and disposal of
municipal sludge often focuses on the issue of pathogenic organisms. The purpose of this
report is to use, the pathogens methodology, Pathogen Risk Assessment for Land
Application of Municipal Sludge, to develop a .preliminary assessment of risk to human
health posed by parasites in municipal sewage sludge applied to land as fertilizer or soil
conditioner. The preliminary risk assessment includes a description of the most critical data
gaps that must be filled before development of a definitive risk assessment and recommends
research priorities.
in
-------
DOCUMENT DEVELOPMENT
Cynthia Sonich-Mullin, Project Officer
Norman E. Kowal, Technical Project Manager
Randall J.F. Bruins
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH
Authors
Charles T. Hadden
Marialice Wilson, Project Manager
Science Applications International Corporation
Oak Ridge, TN
Reviewers
Thomas T. Evans
Exposure Applications Branch
Office of Research and Development
U.S. Environmental Protection' Agency
Washington, D.C.
Walter Jakubowski
Frank W. Schaefer, III
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH
Milovan S. Beljin
Groundwater Research Center
The University of Cincinnati
Cincinnati, OH
Judith Olsen, Editorial Review
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH
IV
-------
TABLE OF CONTENTS
1. EXECUTIVE SUMMARY . 1
2. INTRODUCTION , ... 6
3. LITERATURE REVIEW OF PARASITES ...................... . 12
3.1. SIGNIFICANCE OF INTESTINAL PARASITES 12
3.1.1. Reproduction and Life Cycle . . . I 14
3.1.2. Transmission/Exposure Routes . 15
3.1.3. Occurrence of Parasites in Sludge 18
3.1.4. Epidemiology ...'.' 18
3.2. SURVIVAL IN TREATMENT PROCESSES AND DENSITY
IN TREATED SLUDGE 21
3.2.1. Survival in Treatment Processes 21
3.2.2. Density of Parasites in Treated Sludge ! ',',', 27
3.3. VIABILITY AND'SURVIVABILITY IN SOIL AND WATER ".' 28
3.4. TRANSPORT ......... 31
3.4.1. Transport in Soil 31
3.4.2. Transport in Surface Runoff ' ' 34
3.4.2. transport by Wind . ...:......]....... ] ]'.'.'. . 35
4. PARAMETERS FOR MODEL RUNS 37
4.1. RATIONALE FOR PARAMETER SELECTION 37
4.2. PARAMETER VALUES . 41
4.2.1. Main Program Parameters ... 41
4.2.2. Parameters for Subroutine RISK ^
4.2.3. Subroutine GRDWTR 44
4.2.4. Subroutine RAINS '. 44
5. SITES FOR MODEL RUNS 46
5.1. SITE 1: ANDERSON COUNTY, TENNESSEE 46
5.1.1. Description of Soil 45
5.1.2. Narrative Climatological Summary 45
-------
TABLE OF CONTENTS (continued)
5.1.3. Temperature 47
5.1.4. Rainfall ..,..., . . ^, 47
' 5.1.5. Parameters for Subroutine RAINS 48
5.2. SITE 2: CHAVES COUNTY, NEW MEXICO ..'..' : . 49
5.2.1. Description of Soil 49
5.2.2. Narrative Climatological Summary .... . ../...... 49
5.2.3. Temperature . . .-vv. . .= , . ij. ;,-,;: '. > '':'-, -;- ; ^9
5.2.4. Rainfall . ;, .,......,... 50
5.2.5. Parameters for Subroutine RAINS 50
5.3. SITE 3: CLINTON COUNTY,' IOWA 51
5.3.1. Descriptipn of Soil . , . ....... ซ 51
5.3.2. Narrative Climatplogical .Summary . ., . . ... . ,..- 51
5.3.3. Temperature "':' i-V'r' v,......... 52
5.3.4. Rainfall .->.;>;:...;.'' ,,;:- ' ซ'. 52
5.3.5. Parameters for Subroutine RAINS . 52
5.4. SITE 4: HIGHLANDS COUNTY, FLORIDA - 53
5.4.1. Description of Soil ..... .-. . .*..;. ... . .... ._../; :' ....... 53
5.4.2. Narrative Climatological Summary 53
5.4.3. Temperature : :, 54
5.4'.4. Rainfall '. .' 54
5.4.5. Parameters for Subroutine RAINS . .... . . ..- '54
5.5. SITE 5: KERN COUNTY, CALIFORNIA 55
5.5.1. Description of Soil 55
5.5.2. Narrative CUmatological Summary 55
5.5.3. Temperature 55
5.5.4. Rainfall . . ; 56
5.5.5. Parameters for Subroutine RAINS . . : 56
5.6. SITE 6: YAKIMA COUNTY, WASHINGTON 56
5.6.1. Description of Soil 56
5.6.2. Narrative Climatological Summary , . . 57
5.6.3. Temperature ' 57
5.6.4. Rainfall . 57
5.6.5. Parameters for Subroutine RAINS 58
VI
-------
TABLE OF CONTENTS (continued)
6. RESULTS ..............;........: 59
6.1. SENSITIVITY TO VARIABLES . 59
6,2. EXPOSURE COMPARTMENTS ' 64
6.2.1. ONSITE . .... ..... . ....;.... .-. . . . .- . . 68
6.2.2. FOOD CONSUMER (EATER) ................ 71
6.2.3. SWIMMER . . . . . 76
7. CONCLUSIONS . . go
7.1. SENSITIVITY ANALYSIS 80
7.2. ONSITE EXPOSURES . . . . 81
7.3. SEDIMENT TRANSPORT AND SURFACE RUNOFF .. 81
1A. OFFSITE EXPOSURES . 82
7.5. WAITING PERIOD . . 82
8. RESEARCH NEEDS 84
8.1. INFORMATION NEEDS FOR PARASITES 84
8.2. MODEL DEVELOPMENT ........ 86
9. REFERENCES . . . 89
APPENDIX. MODEL OVERVIEW .............. . . . . . . . . .... . ...... A-l
vu
-------
LIST OF TABLES
No. Title
2-1 Compartments Included in the Sludge Management Practices ..........
2-2 Sludge Management Practices and Descriptions in Land Application Model
3-1 Computer Search Strategy 1".
3-2 Pathogens of Concern
3-3 Concentration, Viability and Survivability of Parasites in Sludge ........
4-1 Die-off Rates of Parasites in Sludge .,
6-1 Sensitivity Coefficients of Site-specific Variables
6-2 Sensitivity to Parameters of Subroutine RAINS ,
6-3 Probability of Infection Onsite
6-4 Probability of Infection, Swimmer
6-5 Maximum Probability of Infection, Site 1
6-6 Maximum Probability of Infection, Practice I
6-7 Probability of Infection by Consumption of Contaminated Crops
6-8 Maximum Probability of Infection of Food Consumer (Eater)
i
6-9 Variation of EATER Risk with Time of Harvesting
6-10 Probability of Infection by Exposure to Runoff Water (Swimmer)
Page
. "7"
9
13'
16
22
39
62
65
66
67
69
70
72
73
75
78
Vlll
-------
LIST OF FIGURES
No, Title
6-1 Sensitivity Coefficient of APRATE .......
6-2 SWIMMER Exposure by Practice, Site 1 . . .
6-3 SWIMMER Exposure by Site, Practice I
63
77
79
IX
-------
LIST OF ABBREVIATIONS
D&M Distribution and marketing
dia Diameter
g Gram
ha Hectare ...... .
hr Hour
m Meter
MID Minimum infective dose
min Minute
NOAA National Oceanic and Atmospheric Administration
PSRP Processes to significantly reduce pathogens
sec Second
USD A U.S. Department of Agriculture
-------
1. EXECUTIVE SUMMARY'
This preliminary risk assessment study focuses oh-the probability of -'human infeetioii
from protozoa and helminths, usually referred to as parasites, in municipal sludge applied
to land. It is based on the Pathogen Risk Assessment computer model and methodology
described in Pathogen Risk Assessment for Land Application of Municipal'''Sludge.
This document reports (1) the results of a literature review designed to find the data
on parasites required by the pathogens methodology, and (2) the results, of numerous site-
specific computer simulations, running the Pathogen Risk Assessment Model with a wide
range of values for the parameters required. The parameters required for parasites are (1)
density of viable parasites in treated sludge destined for land application; (2) die-off rates
in soil, dry particulates, liquid aerosols, and water; (3) dispersion in the environment, i.e.,
transport in water, soil and air; and (4) minimum infective dose, which for parasites is
assumed to be MID = 1 since single eggs'of helminth's and single cysts of protozoa havb
produced infections in humans. Of these parameters/''density is site-specific and requires
a standard method for enumerating parasites, die-off rate data are very limited, transport,
data are essentially non-existent, and infective dose has been determined 'to be MID = 1.
Locations selected for site-specific application of the model include Anderson
County, Tennessee; Chaves County, New Mexico; Clinton County, Iowa; Highlands County,
Florida; Kern County, California; and Yakima County, Washington. The sites were chosen
to provide - diversity in geographic location, topography, soil type, rainfall pattern and
temperature.
.Parasites are of health significance in land application practices because they tend
to become concentrated in sludge during sewage treatment processes and because they can
remain viable as environmentally stable protozoan cysts or helminth ova for months or
years under favorable conditions. Although epidemiological studies suggest little risk to
human health from parasites in treated municipal sludge or wastewater applied to land,
their low minimum infective dose and persistence in soil mean that the issue cannot, be
dismissed.
Density and Viability of parasites in sludge are site-specific, based on source of
wastes, species of parasites present, climate, and efficacy of sludge treatment. Densities of
-------
parasites have been reported to be generally higher in sludges from southern than from
northern states. However, accurate risk assessment would require site-specific analysis of
density levels by standard methods for enumerating parasites in sludges and soil. Parasite
densities reported in the literature range from 100-2000 ova/kg dry wt in dried sludge and
0-30,000 cysts or ova/kg in liquid sludge; however, the values are highly dispersed and
geometric means are in the range of 200-2000 ova/kg dry wt. According to EPA
regulations, composted sludge for distribution and marketing (D&M) must have no more
than 1 ovum/g or cyst/g volatile sludge solids. Based on the literature ranges, values
* . . -
suggested for use in the Pathogen Risk Assessment model are 5000 ova or cysts/kg for
liquid sludge, 500 ova or cysts/kg for dried sludge and 1000 ova or cysts/kg for composted
(D&M) sludge. "The value used in the following risk assessment was 5000 ova or cysts/kg
for all practices.
During storage under unfavorable conditions, ova and cysts may become inactive
(non-infective) before they die. Death may be followed by disintegration. Although some
of the studies discussed include information on infectivity, in many cases only viability of
eggs and cysts was reported, and some studies reported only occurrence, not viability.
Inactivation of parasites appears to be most closely tied to temperature during
treatment or storage, with higher temperatures contributing to increased inactivation.
Temperatures in the 45-55 ฐC range are likely to kill resistant parasites within a few hours.
Alternate freezing and thawing reduce viability more rapidly and to a greater extent than
constant above- or below-freezing temperatures.
Field studies of parasite-contaminated sludge applied to agricultural plots, however,
have not produced a direct statistical correlation between viable Ascaris ova concentration
and solar radiation, relative humidity or soil temperatures. In fact, no statistical correlation
was found between" parasite egg concentration and chemical, physical or biological
parameters.
Data on die-off rates are very limited, but a published 90% die-off time of 270 days
implies an exponential rate of 10fr0'oool?4)/hour. Published ranges for die-off are approximately
10WlOOOI)/hour to lO^'^Vhour at ambient temperature. Based on these ranges, suggested
values in the model for die-off of ova and cysts are:
-------
During application/incorporation
0 for Temp < 20ฐC;
" or 0.00041/hour for 20 .<_Temp < 40;
or 0.65/hour for Temp J>_40;
In moist soil
0 for Temp < 20 ฐC or for 8 hours after irrigation;
lO'"0-00023' or 0.000533/hour fpr 20 <.Temp < 40;
10<-o.ซ7> or o.7845/hour for 40 _<_Temp ^.50;
10(4)-12S) or 0.25/hour for Temp > 50;
On crop surfaces
or 0.7845/hour at all temperatures;
In water
Qr o.000533/hour at all temperatures.
Although detailed data on survival and transport of parasites in soil are lacking, the
Pathogen Risk Assessment Model appears to confirm the general observations in the
literature that parasites are persistent, justifying land-use restrictions. Model runs implied
that restrictions on the consumption of below-ground crops may be overly conservative.
Model runs showed that within narrow limits, the probability of human infection by
parasites as a result of exposure to soil contaminated with sewage sludge is proportional to
the concentration of organisms in the sludge, the amount of sludge applied and the amount
of contaminated soil to which the individual is exposed, either by casual contact or ingestion
of food grown in the contaminated soil. Many of the parameters of the model seemed to
have little bearing on the probability of infection, apparently because they had no effect on
the number of organisms to which the human receptor was exposed in each exposure
compartment or they exerted their effect after the time of maximum .exposure. The
probability of infection was sensitive to the rate of inactivation or die-off of the parasite ova
or cysts and to the method of application. According to the model, human exposure via
subsurface application of sewage sludge would be unlikely because it is believed that ova
or cysts cannot move significant distances through soil.
The model predicted that the most significant potential source of infection would be
exposure to runoff water and sediment transported to an onsite pond after rainfall. Model
runs indicated that it would be prudent to limit access to runoff water and sediment from
a sludge-amended field, either by mulching to reduce runoff, ditching and diking to contain
-------
the runoff or restricting access to any onsite ponds receiving runoff.
Various model runs predicted that it was unlikely that a significant number of ova
or cysts would be transported off-site either by droplet aerosols or wind-blown dusts. The
model also predicted that parasites moving through the soil column into groundwatei* was
unlikely. Therefore, one can infer, based on the model parameters used, that there is
relatively little risk to human health from parasitic infection via inhalation of contaminated
fugitive dust emissions or ingestion of contaminated groundwater.
Using a benchmark probability of infection of IxlO"4 as an indicator of sufficient
protection of human health, a waiting period appeared to be unnecessary for consumption
of aboveground crops contaminated with 0.1 g soil/crop unit. A waiting period of 18
months appeared to be adequate for below-ground crops, whose consumption is currently
forbidden for 5 years after sludge application.
The current version of the Pathogen Risk Assessment Model does not address some
of the properties of parasite survival in soil. Mathematical descriptions of the die-off of
parasite ova, cysts and oocysts as a function of temperature and moisture are not yet
adequate to allow construction of algorithms for die-off rates. It may be appropriate to add
a diurnal cycle to the model's temperature algorithm. Other changes may be limited by the
constraint that the model should run on a personal computer.
The following research priorities are recommended to allow development of a
definitive risk assessment for parasites in land-applied sludge:
For Helminths:
Standard quantitative methods for examining helminths in sludge and soil
samples;
Data on transport in water, soil and aerosols;
Die-off rates in water, soil and aerosols;
The relationship of those decay rates to environmental conditions, previous
sludge treatment, method of sludge application and various effects of crop
cover.
For Protozoa:
Quantitative methods for examining protozoa in sludge and soil samples; and
-------
Quantitative data on occurrence and survivability of protozoa in treated
' sludge. j. ; .. .; ,.' . .
If results indicate that protozoa survive in sludge, the following additional research needs
become apriority:
Data on transport in water, soil and aerosols;
Die-off rates in water, soil and aerosols;
The relationship of those decay rates to environmental conditions, previous
sludge treatment, method of sludge application and various effects of crop
cover.
-------
2. INTRODUCTION
This preliminary risk assessment study fqcuses^on the probability of human infection
from intestinal protozoa and helminths in municipal sludge applied to land. These two types
of microorganisms are usually grouped under the heading of "parasites" (Kowal, 1985).
S}udge, a byproduct of sewage treatment, is the mixture of solids and liquids remaining
after settling processes remove solids from municipal or domestic wastewater. Secondary
and tertiary sludges contain biomass resulting from microbial digestion of the sewage.
Being derived from human sanitary wastes, sludge contains microorganisms that colonize
humans and can cause infection and "disease.
This risk assessment is based on the Pathogen Risk Assessment computer model arid
methodology described in Pathogen Risk Assessment for Land Application of Municipal
Sludge (U.S. EPA, 1990b). The purpose of the model is to determine the probability of
infection of a human receptor from pathogens in the land-applied sludge. The model
consists of a series of compartments (Table 2-1). representing discrete points in the
application pathway. The compartments are the various locations, states or activities in
which sludge or sludge-associated pathogens exist; they vary to some extent among practices.
Compartments representing sources of human exposure are designated with an asterisk in
Table 2-1. In each compartment, pathogens increase, decrease or remain the same in
number with time, as specified by "process functions" (growth, die-off or no population
changes) and "transfer functions" (movement between compartments). Infection rather than
disease is used to measure risk in the methodology, since exposures to parasites may lead
to human infection that is asymptomatic (Kowal, 1985). The outputs produced by running
the model are numerical values for the probability of a human receptor receiving an
exposure in 24 hours exceeding the minimum infective dose (MID). The MID is assumed
to be one since single eggs of helminths and single cysts of protozoa have produced
infections in humans (Kowal, 1985). The model will run until the day specified or until the
number of pathogens in each compartment -decreases to < 1 at which point the number is
rounded to zero. .''.
Two categories of land application are emplbyed' in the methodology, (1) agricultural
utilization and (2) distribution and marketing (D&M), and the source of parasites is either
-------
TABLE 2-1
COMPARTMENTS INCLUDED IN THE SLUDGE MANAGEMENT PRACTICES
Compartment
Name and Number
Application
Incorporation
Application/Tilling .
Emissions ,
Soil Surface
Particulates ^
Surface Runoff
Direct Contact
Subsurface Soil
Groundwater '
Irrigation Water
Soil Surface Water
Offsite Well
Aerosols
Crop Surface ,
Harvesting
(Commercial) Crop
Animal Consumption
Meat
Manure
Milk
Hide
Udder
Liquid Sludge
Management Practices
'. ;.-3i ;.".'..;. /.,ซ....' ' .,;'''.; '.-:Y.: .' .
I II
,,1 - ... -.-. -1
..2;-... ;.-. 2_
3* "" 3*
4 . ..( :> 4
5* . . 5*;,
.6* x ..;...-. (^*
7f..'. ;:;-.^7*
8* '-- t ~~- : * l- 1 ' Q'
. . i, , . ,., ' ' Q-
- 9 .-> :'*::-g--
:- ' ','''
'10 ' " " 10
11 " 11
12* 12*
, 13*.'... ,,,,.^*
14 ... 14,
15 -
, -16Vv--:;-,,v-
..->; ......^-'i7
.- .-'r.':.-^- jjjS-1
'19
20*
_ >ih,..."'2lf.
22
III
....",. 1,
.-:.;., - 2,.,.. ., '
3*
4. '
;,::-. :S*.'-
-.- :....-.. 6* ,
. ' ' 7* -. ,
:,;:,;,. r,8
'- o
i .'-'- .=.">'
11
12*
14
15
'>"- ':--n, :
:"': '" 18*
19
. 2Q*
'.'; ' '^i,-'-
22
Dried/Composted
Sludge Management
Practices
IV V
1 1
'*",-,","
3* ' 3* '
4 , , , 4 . -.
5* 5*
7* 7*
8 - - -8
11 11
14 14
15
16*
*Indicates exposure compartment ,f ^ V ;,; ," ^f;:
:7
-------
liquid or dried/composted municipal. sewage sludge. The five municipal sewage-sludge
management practices (Table 2-2) included in the model, are application of liquid treated
sludge (I) for production of commercial crops for human consumption, (II) to grazed
pastures, and (HI) for production of crops processed before animal consumption; and
application of dried or composted sludge (TV) to residential vegetable gardens and (V) to
residential lawns. Practices III and V, while ostensibly limited to hay fields and residential
lawns, respectively, can be modified by selection of appropriate parameters to represent
sludge application on golf courses, parks, roadsides, etc. Although Practice V does not
include an onsite pond, the risk to SWIMMER can be modeled by using appropriate
parameters in Practice III. ,
Risk assessment for pathogens in land-applied municipal sludges requires the
following input data:
Types of pathogens and their concentrations in the sludge, their
survivabilities, and their infective doses (MID = 1 for parasites);
The sludge reuse/disposal option used and the conditions of sludge
application (quantities, frequencies, application method);
The fate of the pathogens in the environment, i.e., the die-off rate
under different conditions including moist soil, dry particulates, droplet
aerosols and water; and
The level of exposure of human receptors to the applied sludge.
This document reports (1) the results of a literature review designed to find the data.
on parasites required by the pathogens methodology, and (2) the results of numerous
computer simulations, running the Pathogen Risk Assessment Model with a wide range of
values for the parameters required. Six sites, chosen to provide diversity in geographic
location, topography, soil type, rainfall pattern and temperature, were selected for site--
specific applications of the model: Anderson County, Tennessee; Chaves County, New
Mexico; Clinton County, Iowa; Highlands County, Florida; Kern County, California; and
Yakima County, Washington. Because of the unlimited number of possible sites, the final
selections were somewhat arbitrary, being based on an attempt to represent different
geographic regions and to ensure a variety of weather patterns.
Exposure pathways, i.e.,migration routes of parasites from or within the application
8
-------
TABLE 2-2
SLUDGE MANAGEMENT PRACTICES AND DESCRIPTIONS IN
LAND APPLICATION MODEL
PRACTICE
DESCRIPTION
Application of Liquid Treated Sludge for Production of Commercial
Crops for Human Consumption
II
III
IV
V
Application of Liquid Treated Sludge to Grazed Pastures
Application of Liquid Treated Sludge for Production of Crops
Processed before Animal Consumption
Application of Dried or Composted Sludge to Residential Vegetable
Gardens
Application of Dried or Composted Sludge to Residential Lawns
Two types of sludge are used in this model - liquid and dried/composted. The extent of
treatment or conditioning prior to application is variable and must be determined for each
case.
-------
site to a receptor, for sludge applied to land include the following:
Inhalation ' and ingestion of emissions from application of sludge or
tilling of sludge/soil;
Inhalation and ingestion of windblown or mechanically generated
particulates;
Swimming in a pond fed by surface water runoff;
Direct contact with sludge-contaminated soil or crops (including grass,
vegetables, or forage crops);
Drinking water from an off site well;
Inhalation and subsequent ingestion of aerosols from irrigation;
Consumption of vegetables grown in sludge-amended soil;
Consumption of meat or milk from cattle grazing on or consuming
forage from sludge-amended fields.
This methodology assumes that exposure to parasites will not result in infection
unless the organisms are actually swallowed. Risks due to inhalation of enteric pathogens
will be considered only because the organisms can be subsequently swallowed. Disease can
result through routes of exposure other than the alimentary tract; calculations of exposure
by direct contact in the Pathogen Risk Assessment methodology do not distinguish among
routes of-infection.
The following human receptors are the exposed individuals whose probability of
infection by parasites is calculated by this model:
Onsite person (ONSITE) who is exposed by ingestion (includes pica
in children) or skin penetration of parasites following direct contact
with soil, vegetables, or forage or by inhalation and subsequent
ingestion of aerosols (particulates or liquid);
Offsite person (OFFSITE) who is exposed to particulate or liquid
aerosols carried by wind;
Food consumer (EATER) who eats vegetable crops, meat or milk
produced on sludge-amended soil;,
Groundwater drinker (DRINKER) who consumes water from a well
near but not on the sludge application site;
10
-------
Pond swimmer (SWIMMER) whose skin is penetrated by parasites or who
ingests, a small amount of water while swimming in the pond that receives the
surface runoff from the application site.
The model conceptualization (U.S. EPA, 1990b) specifies that workers engaged in the
transportation, handling and application of liquid sludge are not included as exposed
individuals because such activity is an occupational exposure.
The U.S. EPA (1985, 1986) has provided extensive information relevant to the
conceptual risk assessment framework for land application of sludge. These key studies
address the pathogens associated with sewage sludge, as well as exposure pathways and the
potential risks to humans from each of the pathways. Most of that information will not be
repeated here. Additional information about the computer model and methodology is
available in Volumes I and II of Pathogen 'Risk Assessment for Land Application of
Municipal Sludge (U.S. EPA, 1990b) and in Wilson et aL (1989); a brief overview of the
model is included as an Appendix.
11
-------
3. LITERATURE' REVIEW OF PARASITES
A literature search was performed to find the most current information available for
the parameters required by the model for simulating land application of sludge. The
parameters required for parasites are (1) density of viable parasites in treated sludge
destined for land application; (2) die-off rates in soil, dry particulates, liquid aerosols, and
water; (3) dispersion in the environment, i.e., transport in air, soil and water; and (4)
infective dose, which for parasites is assumed to be MID = 1.
Appropriate codes and keyword truncation were used to produce the most effective
search strategy for each data base. Table 3-1 lists the computerized data bases queried and
the keywords used. The three columns of keywords were "anded" together to produce a set
in which at least one keyword in each column was a descriptor or was contained in a
retrieved record. ' ,, = .-.. V v
References in review articles and in relevant 'articles retrieved by the computer
search were also evaluated, 'and names of pertinent,'authors were searched to find recent
" -v - *' , " . '' ' ' "- - " ''
papers that may not have been incorporated into online1 databases. ." , .
,'-".' * " ''",*-'; ซ"" ' -
3.1. SIGNIFICANCE OF INTESTINAL PARASITES .' ,
The protozoan pathogens cause a variety of symptoms by colonizing the
gastrointestinal tract. Protozoan diseases may be debilitating but are - rarely fatal -in
developed countries. Protozoa are present in sewage and..sludge as cysts and oocysts,
dormant structures resistant to adverse 'environmental conditions (U.S. EPA, 1990b).
The pathogenic helminths include a variety of .worms, some of which are only
incidental parasites of humans. Among them are pinworms, roundworms, whipworms, and
a variety of tapeworms. The larval stages of helminths often migrate through the body
before maturing in the gut and can cause serious tissue and organ damage. Adult forms
primarily cause malnutrition and anemia while residing in the gut. Helminths are present
in sewage and sludge as ova. More complete data on pathogenic helminths and protozoa
are discussed in Kowal (1985) and U.S. EPA (1985, 1986).
12
-------
TABLE 3-1
COMPUTER SEARCH STRATEGY
DATA BASES
KEYWORD GROUPS
AGRICOLA
AGRIS
BIOSIS
CAB ABSTRACTS
CRIS/USDA
ENVIROLINE
FSTA
NTIS
POLLUTION ABSTRACTS
TOXLINE
WATER RESOURCES ABS
ZOOLOGICAL RECORD
PARASITE. , . :
HELMINTH/HELMINTHES
ROUNDWORM
HOOKWORM
TAPEWORM
CESTODE/CESTODA
NEMATODE/NEMATODA
GIARDIA
CRYPTOSPORIDIUM
PROTOZOA
TAENIA
ASCARIS
ENTAMOEBA/ENTAMEBA
AMEBA/AMOEBA
ACANTHAMOEBA/
ACANTHAMEBA
SURVIVAL
DISPOSAL
TRANSPORT
FATE
VIABILITY/
VIABLE
LIFE CYCLE
MOVEMENT
DIE-OFF
SEWAGE
SOIL-'
AIR
AEROSOL
WATER
SLUDGE
' 13
-------
3.1.1. Reproduction and Life Cycle. Protozoa typically reproduce asexually, by fission, but
many also have sexual cycles in which they 'form zygotes, which mature to cysts or oocysts
(Daly, 1983b). Trophozoites are the active stage of flagellate protozoans in the intestines
of infected individuals, whereas sporozoites are the active stage for coccidians. Following
a period of reproduction, the trophozoites or sporozoites can become precysts, capable .of
secreting a tough membrane .to protect the parasite (Kowal, 1985). It is these thick-walled,
environmentally resistant, dormant structures that are excreted in the feces and are found
in sewage and sludge. These forms are capable of causing human infection.
Helminths are parasitic worms that typically reproduce in the gut: and generally
require more than one host to complete their life cycle. They may have simple life cycles
in which humans become infected by ova or larvae produced by the worms. Or, the life
cycle may be more complex, requiring several hosts before reaching humans. Typically, the
adult tapeworm lives in the gut of the definitive (final) host and sheds fertilized ova, either
free or contained in proglottids,. in the feces. Helminth ova are the resistant stage found
in sludge. The ova are eaten by the intermediate host and develop into larvae, which may
form cysts within the tissues of the intermediate .host. The definitive host species ingests
the infected tissue, and the larva develops into an adult in the gut of this host. An example
of this life cycle is the beef tapeworm, Taenia saginata. found in the small intestine of
humans. It is obtained through eating poorly cooked beef. The adult tapeworm can attain
lengths of ~5 m, containing as many as 2000 proglottids that pass eggs as they move
through the lower gastrointestinal tract. These ova must be eaten by herbivores such as
cattle to allow further development. The larva released from the egg penetrates the
intestine and form a cysticercus in the striated muscle. When ingested by humans, the
cysticercus transforms into a mature tapeworm (Daly, 1983a). Rarely, humans have been
known to be infected by Taenia ova (Beaver et al., 1984).
Other important parasitic helminths are tissue roundworms that can .cause visceral
larval migrans or cutaneous larval migrans in humans. Larvae of the dog and cat ascarids
Toxocara canis and Toxocara cati can produce visceral larval migrans as they migrate
through human tissues, producing inflammatory reactions in organs such as the liver
(hepatomegaly) and lungs (pneumonitis). Serious irreversible damage is possible if critical
areas such as the nervous system are affected. The life cycle of these nematodes is similar
14
-------
to that of human Ascaris lumbricoides. with the source of infection being ingested soil
containing embryonated ova. Children suffering from pica, an abnormal soil-eating
behavior, are more likely than adults to acquire this infection. However, for Toxocara.
humans are abnormal hosts in whom the life cycle cannot be completed (Daly, 1983a).
Cutaneous larval migrans is a skin inflammation most commonly caused by the
hookworm Ancylostoma braziliense. The filariform larvae penetrate the skin and produce
lesions within the skin. As they travel around under the skin, the larvae produce an
irritative pruritis that leads to scratching and can result in secondary bacterial infections.
As is the case with Toxocara. the larvae cannot complete their life cycle in a human host.
Other dog hookworms and some other nematodes can also cause cutaneous larval migrans
(Daly, 1983a). .
3.1.2. Transmission/Exposure Routes. Transmission of parasitic infections is usually by
one of the following routes: ingestion of contaminated food or water, direct contact with
the parasite form in feces, soil or water, or consumption of the undercooked flesh of the
host. The modes of transmission for those parasites of concern in sewage sludge are listed
in Table 3-2. ' "
Toxoplasma infections are transmitted by direct contact with cat feces or by
ingestion of undercooked meat containing the oocyst, but the'most hazardous route is
transplacental. While typically asymptomatic for most infected individuals, toxoplasmosis
during pregnancy can seriously harm the fetus (Hershey and McGregor, 1987).
The transmission potential for Crvptosporidium. a coccidian protozoan, is as yet
unknown, although it has been the cause of several outbreaks of waterborne' illness (Rose,
1988; Crawford and Vermund, 1988; Hayes et al., 1989). Features of the pathogen's
taxonomy and life cycle contribute to the likelihood for waterborne transmission (Current,
1987). Cryptosporidium may be the cause of much of the diarrheal illness in humans and
other mammals worldwide. Infected individuals excrete "an environmentally stable oocyst"
in feces, and evidence of significant cross-transmission among mammals (wild and domestic
animals and humans) suggests the organism is ubiquitous in the environment (Payer and
Ungar, 1986; Current, 1987; Rose, 1988).
Data are as yet insufficient to determine the significance of Crvptosporidium in
municipal sewage sludge destined for land application. However, Kayed and Rose (1987)
15
-------
TABLE 3-2
PATHOGENS OF CONCERN
,
Pathogen
HELMINTHS -
Ancvlostoma
duodenals
Ancyclostoma
braziliense
Ancvlostoma
caninum
Ascaris
lumbricoides
Ascaris
suum
Enterobius
vermicularis
Necator
americanus
Strongyloides
stercoralis
Toxocara canis
Toxocara cat!
Common
Name or
Class
Nematoda
Hookworm
Cat
hookworm
' Dog
hookworm
Roundworm
Swine
roundworm
Pinworm
Hookworm
Threadworm
Dog
roundworm
Cat
roundworm
Trichuris trichiura Whipwonn
HELMINTHS -
Echinococcus
granulosus
Echinococcus
multilocularis
Hvmenolepis
nana
Cestoda
Dog
tapeworm
Tapeworm
Dwarf
tapeworm
Nonhuman
Disease Reservoir
. . ,L , ,,. , ., , . ... .lt , ,
Hookworm disease
Cutaneous cat, dog*
larva migrans
Cutaneous dog*
. larva migrans
Ascariasis
, ~ ^ - -
Ascariasis pig*
.. .. . .. -.
Enterobiasis
Necatoriasis
Strongyloi- dog
diasis
Visceral dog*
larva migrans
Visceral cat* ...
larva migrans
Trichuriasis
Unilocular dog*
hydatid disease ,
Alveolar dog, fox
hydatid disease
Taeniasis rat,
mouse
Human
Infective
Stage
., , . , , , , ,,,,, , ,
free-living
larva
free-living
larva
free-living
larva
embryonated
ovum
embryonated
ovum
ovum
embryonated
ovum
free-living
larva
embryonated
ovum
embryonated
ovum
ovum
ovum
ovum
ovum
(auto-
possible)
Mode of
Transmission.
. ,, = , n , * i
skin penetration,
soil contact
skin penetration,
soil contact
skin penetration,
soil contact
ingestion of
water, food,
soil
ingestion of
water, food,
soil
ingestion of ova
skin penetration,
soil contact
skin penetration,
soil contact
ingestion of
water, food, soil
ingestion of
water, food, soil
ingestion of
food
ingestion of
water, food
ingestion of
water, food
ingestion of
water, food
16
-------
TABLE 3-2
PATHOGENS: OF CONCERN (continued)
Pathogen
Taenia saginata
Taenia solium
PROTOZOA.
Balantidium coir
Crvptosporidium
parvum
Dientamoeba
fragilis
Entamoeba
histolytica -"':
Giardia lamblia
. Isospora, belli
Isospora hominis
Toxonlasma
gondii
Common
Name or-
Glass
Beef1 '
tapeworm
Pork
tapeworm
..-.', ' :
Ciliate
(dysentery)
Sporozoan
(Coccidia)
Amoeba
Amoeba
* ..' r
Flagellate
Sporozoan ,
(Coccidia)
' Sporozoan
(Coccidia)
Sporozoan
(Coccidia)
Nonhuman
Disease Reservoir
Taeniasis .-
Taeniasis,
cysticerosis
Balantidiasis pigs, other
mammals
Cryptosporidiosis cattle
Amebiasis
Amebiasis
(amebic \
dysentery)
Giardiasis mammals
.-..-. - ป --- dog, .
dog
Toxoplasmosis -cat
Human
Infective
Stage
ovum, as well
as larva in or
from inter-
mediate host
ovum,larva
in or from
intermediate
host
cyst
oocyst
unknown
cyst
cyst
oocyst
oocyst
oocyst
Mode of
Transmission
ingestion of
water, food
ingestion of
water,- food
ingestion of
water, food
contact,
ingestion of
water, food
ingestion of
water, food
ingestion of
water, food
contact, '
ingestion of
water, food
ingestion of
water, food
ingestion of
water, food
ingestion of
water, food
Source: Kowal, 1985; U.S. EPA, 1988; Sorber and Moore, 1986
* Definitive host; humans only incidentally infested
.17
-------
reported Cryptosporidium oocyst concentrations in anaerobically digested sludges ranging
from 1250-38,700/g dry wt. Madore efal. (1987) found that Cryptosporidium oocyst
removal efficiency by sewage treatment, using activated sludge, approached 79%. Sewage
treatment utilizing sand filtration in conjunction with activated sludge resulted in lower
levels of oocysts (10/L) in finished effluent than treatment using activated sludge treatment
only (1300 oocysts/L), suggesting that the filtered oocysts would be found in the sludge.
The importance of Cryptosporidium as a waterborne pathogen and its similarity to Giardia
suggests further studies are needed to clarify its survivability in water and wastewater
treatment processes. There is some disagreement over whether Cryptosporidium or
Giardia duodenalis is the' more- common intestinal parasite and which has the greater
potential for causing waterborne disease (Rose, 1988; Sykora et al., 1990).
3.1..3. Occurrence of Parasites in Sludge. Several literature reviews have included surveys
of parasites present in sludge at different stages of treatment, and most discuss the diseases
associated with these pathogen populations (WHO, 1981; Kowal, 1982, 1985; U.S. EPA,
* . .
1986). Several reviews have summarized > information on parasites in sludge applied to
land: Kowal (1985), Sorber and Moore (19'86)}' Pedersen (1981), Reimers et al. (1981,
1986, 1990), Yanko (1988), and U.S. EPA (1985). Most of that information will not be
repeated here, although conclusions derived in these reviews are included in the relevant
sections of this document.
Table 3-2 lists the protozoa and helminths most commonly found in sewage sludge
and wastewater that are significant human pathogens. For a descriptive summary of these
parasites, their life cycles, and the symptoms of infection in humans, see Kowal (1985). His
review provides information on the occurrence and viability of pathogens in sludge and in
the environment following application. Information on their levels, survival, and behavior
in soil, groundwater, surface water, aerosols and animals is discussed; and available data on
infective dose, risk of infection and epidemiology of these agents are summarized.
3.1.4. Epidemiology. Epidemiological studies have suggested little risk to human health
from parasites in municipal sludge or wastewater applied to land. The low. MID for
parasites, however, and the persistence of some parasites, particularly helminths, in soil
suggests that all questions of public health risk have not been answered (Kowal and
Pahren, 1982).
18
-------
Yanko (1988) studied the occurrence of pathogens in 498 samples of municipal
composts, air-dried and heat-treated sludge products, often referred to as distributed and
marketed (D&M) sludge. Products sampled weekly for one year were from a windrow
composting facility and from an aerated static pile composting facility. Final sludge
products from 24 other municipalities were sampled bimonthly for a year. No protozoan
cysts were found in the samples, and helminth ova,' while regularly detected, were not
viable. Trichuris and Ascaris were the helminth ova most commonly found, and evidence
suggests that many of the Trichurus ova were from non-human, sources. Hymenolepis and
Toxocara ova were detected infrequently. No health hazards from parasites were found
associated w'ith the treated sludge products. ,
With respect to wastewater, Clark et al. (1981) reported results of a prospective
seroepidemiological study of municipal wastewater workers and controls in Memphis,
Cincinnati, and Chicago. They concluded that wastewater workers were not subject to "any
detectable risks due to parasites in wastewater."
Amebic infections have been linked epidemiologically with vegetables fertilized with
night soil or irrigated with untreated wastewater (Bryan, 1977; Geldreich and Bordner,
1971). When raw wastewater was used to irrigate fields on Israeli kibbutzim, however, no
excess of enteric diseases was found (Shuval and Fattal, 1980).
The occurrence of giardiasis is common and may be of epidemic proportions due to
infection acquired by ingesting Giardia cysts in public water supplies or in surface water
(Meyer and Radulescu, 1979). The potential for waterborne disease transmission of
Crvptosporidium may equal or exceed that of Giardia (Rose, 1988; Current, 1987). In
addition, its transmission by the fecal-oral route from host to host by means of its
environmentally stable oocyst and its capacity for auto-infection makes its epidemiology of
extreme importance (Rose, 1988; Current, 1987).
Dorn et al. (1985) concluded that health risks to humans and animals on Ohio farms
receiving yearly applications of municipal sewage sludge were not significantly different
from those on control farms. Sludge was applied at relatively low rates of 2-10 dry metric
tons/hectare(ha)/year in accordance with U.S. EPA guidelines. The authors caution that
higher application rates, higher concentrations, of disease organisms in the sludge or
increases in treated acreage/farm may produce different health risks. In fact, Fertig and
19
-------
Dorn (1985) summarized an epidemiological investigation of an outbreak of Taenia
saginata cysticercosis on an Ohio cattle farm, concluding that it is likely that the infection
of the seven slaughtered cattle was associated with application of municipal sewage sludge.
Wallis et al. (1984) concluded that health risks were no greater from sludged fields
than from the control hayfield and pasture receiving no sludge application. They recorded
decreases in numbers of parasite eggs from 11,000 eggs/kg dry wt in the initial sludge
applied to a grass plot to 7000 parasite eggs/kg dry wt after 7 days. No eggs were found
at 43, 84 or 118 days following sludge application. Sludge injected into' the soil showed
reductions from the initial 11,000 eggs/kg dry wt to almost none within 4 weeks. Storey
and Phillips (1985), however, found that Taenia saginata and Ascaris lumbricoides ova
could be washed into the soil where they were afforded protection from radiation and
desiccation, increasing survival with increasing depth in the soil profile.
In Europe, researchers have concluded that the chief hazards of sludge application
for agriculture are from Salmonella spp. and from the ova of Taenia saginata and Ascaris
spp. (Block et al., 1986). Matching the degree of pathogen reduction with restrictions on
use of the sludged land is the method used to eliminate risks. Since the use of disinfected
sludge has the fewest restrictions, pasteurization of sludge at 70 ฐC for 30 minutes has been
developed in Switzerland (where it is now required) and in Germany to protect cattle from
salmoneUosis (Pike et al., 1988).
Burger (1984) reviewed the prevalence of Taenia saginata in humans in Europe; it
ranged from a low of 0.013% in an unselected population in Poland to 8.04% in clinic
patients in Turkey. The prevalence of cysticercosis (the larval stage) in cattle ranged'from
0.13% in Cuba to 4.2% in Sudan. Insufficient data on infectivity of T. saginata eggs allow
only a recommendation to prohibit grazing by cattle for at least 4 months following spring
application of sludge to pastures and longer during cooler seasons in a moderate climate.
20
-------
3.2. SURVIVAL IN TREATMENT PROCESSES AND DENSITY IN TREATED
SLUDGE ,
The density of parasites in municipal sludge (Table 3-3) is site-specific, being related
to the population served by the sewage treatment system, i.e.,source of the wastes, species
, of parasites present, geographic area, and season or climate. Density and^ viability of
parasite species are also dependent on the type of sludge treatment. Primary sludge has
.received primary treatment such as screening and settling; secondary sludge is produced' by
biological waste treatment, or secondary treatment; primary and secondary sludge are
combined to produce mixed sludge (U.S. EPA, 1985). Studies of municipal sludges from
southern states showed that 99% of primary sludges and 89% of final sludges contained
large numbers of viable parasite cysts and ova (Reimers et al., 1981; Leftwich :et ,al.r!981).
3.2.1. Survival in Treatment Processes. Ward et al. (1984) summarized the reductions: of
parasites in various processes to significantly reduce pathogens (PSRP) expressed as Iog10:
, mesophilic anaerobic ,digestion, 0.5; aerobic digestion, 0.5; composting, 2-4; air drying, 0.5-
4; and lime stabilization,; 0.5. ; : , . , , ,.-.
Pedersen (1981) reviewed the literature concerning parasites in municipal
wastewater sludge and concluded that digestion does not .effectively reduce levels of
parasites in sludge, but that sludge lagooning-.could produce a 1-log reduction of parasitic
ova if carried out for > 6 months at temperatures > 20ฐ Cor for 3 years at temperatures
<20ฐC. --., . - , . - ..... :
.... Reimers et al. (1990) found that Ascaris suum eggs in lagoon-stored sludge in
Louisiana and Texas were inactivated after 15 months. , Die-off of Ascaris in municipal
sludge appeared to be a function of temperature except in petroleum-contaminated sludge.
Information on survivability of Giardia and Crvptosporidium in treated sludge is
.lacking, but-waterborne outbreaks of cryptosporidiosis are possible when Crvptosporidium
oocysts, resistant to routinely-used disinfectants, escape filtration (Payer and Ungar, 1986;
Current, 1987). A survey of one drinking water treatment plant found that a large number
of oocysts were recovered off the filter, indicating concentration of oocysts by filtration and
reduction in finished water of as much as 91% (Rose, 1988). An outbreak of
cryptosporidiosis in Carrollton, GA was linked to oocysts in drinking water that had been
through the disinfection process (Hayes et al., 1989).
.'21
-------
TABLE 3-3
CONCENTRATION, VIABILITY, AND SURVIVABILITY
OF PARASITES IN SLUDGE
Pathogen
Density in
Treated
Sludge
(Mean ova/
kg dry wt)
Viability
Survivability
Reference
HELMINTHS -Nematoda
Ancvlostoma duodenale
Ancvlostorna
braziliense
Ancvlostoma
jnur
iscaris
lumbricoides
Ascaris
suum
11,000
(digested,
lagooned);
100-2000
D&M
(Ascaris spp.)
9600 southern
states; 2030
Chicago; 565
northern'
states, in
of final
sludges;
geometric
mean of 1360
in secondary
sludges
69%
64%,
15 years
Wallis, 1984
Yanko, 1988
Reimers et
al., 1981;
Kowal, 1985;
Arther et al.,
1981; Reimers
et al., 1986
Enterobius
vermicularis
Necator
americanus
Strongvloides
stercoralis
22
-------
TABLE 3-3
CONCENTRATION, VIABILITY, AND SURVIVABILITY
OF PARASITES IN SLUDGE (continued)
Pathogen
Density in
Treated
Sludge
(Mean ova/
kg dry wt)
Viability
Survivability
Reference
Toxocara canis
Toxocara cati
Trichuris trichiura
(Toxcocara
spp.) 700
southern
states; 1730
Chicago; 370
northern
states, in
of final
sludges;
geometric
mean of 280
in secondary
sludges
265 northern
states, in 22%
of final
sludges; max
of 7700 and
geometric
mean of < 10
hi secondary
sludges
HELMINTHS - Cestoda
Echinococcus
granulosus
Echinococcus
multilocularis
Hvmenolepis
nana
52%
53%
55%
6 years
(in soil)
Reimers et
al., 1981;
Arther et al.,
1981; Reimers
et al., 1986
Seattle Metro, '
1983;
Reimers et al.,
1981,1986
23
-------
TABLE 3-3
CONCENTRATION, VIABILITY, AND SURVIVABILITY
OF PARASITES IN SLUDGE (continued)
Pathogen
Density in
Treated
Sludge
(Mean cysts/
kg dry wt)
Viability
Survivability
B.eference
PROTOZOA
Bnlantidium
cpli
Crvptosporidium
narvum
Dientamoeba
fragilis
Entamoeba
histolvtica
None detected
in D&M sludge
1250-38,700
oocysts/g dry vyt
in anaerobically
digested sludge;
range of 140-4000
oocysts/L in
treated sewage
effluence (activated
sludge)
70-30,000
cysts/L; max
44 cysts/L in
treatment
plant effluent;
avg concn
387-1723
cysts/L in
liquid sludge
Max: 10- days (on
(on soil);
common max:
2 days (soil)
> 140 days in
water (laboratory
study)
Yanko, 1988;
Kowal, 1985
18-24 hours
(dry soil);
42-72 hours
(moist soil);
8-10 days
(damp loam
, and sand);
153 days
(water)
1 year in
liquefied feces
stored at 4ฐC;
< 24 hours (air-
dried at 4ฐC or
21 ฐC); <24hours
(artificial sea
water at 4 ฐ C)
Kayed and Rose, 1987;
Current, 1987;
Madore et al., 1987
Rudolfs et
al.,, 1951;
Beaver and
Deschamps,
1949;
Mitchell,
1972
Sykora, 1990;
Craft, 1982;
Jarroll et al.,
1984
ara
hominis
Toxoplasma
gondii
(Tbxoplasma
spp.)
334-410 days
Frenkel et
al., 1975
Source: Jakubowski, 1990; Kowal, 1985; U.S. EPA, 1988;Sorber and Moore, 1986
24
-------
Sykora et al. (1990) determined the occurrence of Giardia cysts following wastewater
treatment in eleven cities across the United States. Occurrence in sludges, determined by
direct count (centrifugation), ranged from 70-30,000 cysts/L. Giardia cysts were present in
all raw sewage samples, but only half the wastewater treatment plant effluents contained
cysts, with maximum counts of 44 cysts/L. Higher cyst concentrations in raw wastewater
during the colder months may be due to factors such as higher rates of infection, increased
survival of cysts and lower settling rates at lower temperatures (Jakubowski et al., 1990).
McHarry (1984) detected Giardia in effluent from sewage treatment plants in Illinois to be
~1 cyst/L. Giardia cysts were found to be infective to rats after one year's storage in
liquefied feces at 4ฐC (Craft, 1982).
Schwartzbrod et al. (1989) compared the effects of wastewater treatments on
helminth eggs; activated sludge, lagoon treatment and sand filtration reduced the level of
helminth eggs by 77.7%, 100% and 98.8%, respectively, in the final effluent destined to be
used for crop irrigation. This finding confirms the typical results: concentration of parasites
in sludge and relatively low density or absence in wastewater effluent. However, the
authors note that the contaminated sludge poses a problem. In addition, they point out
that they did not determine the viability and infectivity of the eggs.
Leftwich et al. (1981) evaluated domestic wastewater treatment processes with
respect to their ability to inactivate parasites. They considered removal processes,
stabilization processes, and decontamination or inactivation processes. In general, aerobic
and anaerobic processes are lethal to parasite eggs if carried out at >55ฐC. Likewise,
composting is effective for inactivating eggs if all matter reaches 60 ฐC for at least 2 hours.
Drying beds are most effective if moisture is reduced to <5%.
Pike and associates (1988) determined that complete destruction of viable Ascaris
ova was possible only by digestion at 49ฐC or by heating for 15 minutes at 55ฐC. Recovery
of viable ova from anaerobically digested sludge was significantly affected by temperature
of digestion-63% at 35 ฐC but only 0.6% at 49ฐCปbut not by retention period. Digestion
alone reduced viability of the recovered ova only slightly. Heating alone also had little
effect at temperatures below 51 ฐC for 1 hour, but heating to 55 ฐC for 15 minutes resulted
in no viable ova being recovered. When heat treatment reduced viability of Ascaris suum
incompletely, subsequent digestion at 35,ฐC further reduced viability.
25
-------
The results of Pike et al. (1988) closely parallel those of Arther et al. (1981), who
found that ova of four genera of parasites survived anaerobic sludge digestion and
t
lagooning by a Chicago sanitary district treatment plant: Ascaris spp., Toxocara spp.,
Toxascaris leonina and Trichuris spp. (see Table 3-3). , Viabilities following treatment
ranged from 20-64%.
Black et al. (1982) studied the effects of mesothermic anaerobic or aerobic sludge
digestion on survivability and viability, of eggs from Ascaris suum. Toxocara cam's. Trichuris
yulpis, Trichuris suis and Hymenolepis diminuta. Anaerobic digestion destroyed 23 % of
Ascaris eggs, and the aerobic digestion destruction was 38%. Trichuris 'eggs were not
destroyed by anaerobic digestion, but 11 % of the eggs were destroyed by aerobic digestion.
Toxocara eggs were destroyed by neither method. The viability of those Ascaris and
Toxocara eggs surviving digestion was not affected, but aerobic digestion decreased the
viabilities of Trichuris eggs.
Mbela et al. (1990) assessed temperature effects on Ascaris ova viability following
aerobic and anaerobic digestion of municipal sludges. Both aerobic and anaerobic
digestion processes fail to inactivate Ascaris eggs in the 25-35 ฐC temperature range (Reyes
et al, 1963; Reimers et al., 1987). They concluded that high temperature is a significant
factor in the inactivation of pathogens. Increasing detention times from 10 to 31 days while
maintaining constant temperature increased Ascaris inactivation from 7% to 16% by
anaerobic digestion. Aerobic digestion at 35 ฐC and 45 ฐC, with a 10-day detention period,
decreased viability by 57% and 82%, respectively. Temperatures of 45-55 ฐC achieved
complete inactivation of Ascaris ova within two days. Liming and caustic stabilization
increased inactivation; temperatures > 35 ฐC for sludge digestion coupled with < 1000 mg
lime/g sludge solids completely destroyed Ascaris eggs.
O'Donnell et al. (1984) performed controlled laboratory studies of lagooning of
sludge. Results indicated that viability and infectivity of eggs were related to storage
temperature, with eggs stored at a higher temperature (25 ฐC) becoming nbnviable in 10-
16 months whereas those eggs held at 4ฐC were viable and infective at 25 months.
Storey (1987) also concluded that temperature was the major controlling factor in
the survival of Taenia saginata eggs during. simulated sewage treatment processes. Eggs
survived treatment at 55 ฐC for only a few hours, and eggs treated at 35 ฐC were killed
26
-------
faster than those at 20ฐ C. - ;' .
3.2.2. Density of Parasites in Treated Sludge. Since densities of parasites in sludge vary
by source and treatment method, there is no true representative parasite concentration for
any given PSRP treatment or source of sludge destined for land application. For the most
accurate risk assessment, parasite density should be tested by a standard method for a-given.
source of. sludge, i.e., a given treatment plant and treatment method. Reimers and
associates (1990) and Little et al. (1988) have developed methods for enumerating parasites
in sludges and soil, but widely-accepted standard methods would make comparisons of
results across studies more meaningful. .
Densities of helminth ova in positive samples of distributed and marketed sludge
ranged from 0.1-2 ova/g dry wt (100-2000 ova/kg dry wt) (Yanko, 1988).
Pedersen (1981) critically reviewed the original literature detailing quantitatively the
density levels of pathogenic organisms in municipal wastewater sludge and septage. He
evaluated conventional municipal sludge stabilization and dewatering processes for their
effectiveness in reducing those density levels. U.S. EPA (1985), summarizing the results of
Pedersen (1981), warns of several qualifications relative to data quality: laboratory studies
may not adequately mimic operations at full-scale treatment plants; seeded pathogen
behavior may not represent that of naturally occurring organisms; operating conditions
during data collection and pathogen numbers reported are uncertain; and there is
inconsistency relative to die-off rates during sludge stabilization.
Reimers et al. (1981, 1986, 1990) investigated parasite density in southern sludges,
northern sludges, and lagooned sludges, respectively, and the effectiveness of disinfection
techniques on their inactivation. Levels of viable helminth ova were typically somewhat
lower in northern sludges than in southern sludges for some species, and densities of T.
trichiura varied Inversely with population size served by the treatment facility (Reimers et
al., 1981, 1986). ...
Reimers and associates (1986) conducted a study of parasites in municipal
wastewater sludges from treatment plants in four northern states. In approximately 90%
of the sludge samples, they found resistant stages of twenty types of parasites. The most
common were Ascaris spp..Trichuris trichiura. Trichuris vulpis and Toxocara spp., with one
or more of these parasites detected in 89% of the samples examined. The geometric mean
27
-------
density of eggs was determined for each of these four parasites in sludge samples destined
for disposal: 565, 265, 270 and 370 eggs/kg dry wt, respectively (see Table 3-3). Ascaris
eggs occurred in final sludges at higher densities than the eggs of the other three parasites.
Although the authors gave no confidence limits, they stated that there were no significant
differences between geometric mean densities of total eggs in aerobic and anaerobic
sludges (3000 and 2150 for Ascaris. 460 and 600 for T. trichiura. 345 and 485 for T. yulpjs,
1410 and 1155 for Toxocara. respectively). However, there were fewer viable Ascaris and
Toxocara eggs in anaerobic sludges (1720 and 740, respectively) than in aerobic sludges
(4090 and 1320, respectively). The authors suggested this effect may be a result of the
higher temperature (35-40 ฐC) typical of anaerobic digestion than the typical ambient
temperature of aerobic digestion. Viability of these four parasites was determined in
samples of undigested, digester and postdigestion sludge and also for the total of all three
sludge types. The ratio of geometric means of viable eggs to total eggs (viable arid
nonviable) in the total of all three sludge types is 710/1000 eggs/kg dry wt of sludge for
Ascaris. 400/440 for Trichuris trichiura. 370/400 for Trichuris vulpis. and 670/880 for
Toxocara (see Table 3-3). The ratio of viable to total eggs in all sludge- samples from the
four northern states is 1400/1900 eggs/kg dry wt of sludge for Ascaris. 200/380 for T.
trichiura. 260/290 for T. vulpis. and 1100/1400 for Toxocara (see Table 3-3). A similar
comparison of sludge samples from five southern states showed a significantly higher
density of all parasites except Toxocara. with ratios of geometric means of viable to total
eggs of 2500/2800 eggs/kg dry wt of sludge for Ascaris. 880/910 for T. trichiura. 430/470
for T. vulpis. and 680/790 for Toxocara.
Sykora et al. (1990) determined the density of Giardia cysts following wastewater
treatment in eleven cities and found the highest average concentration (1723 cysts/L, or
~ 1723 cysts/kg wet wt) was from Pennsylvania samples of sludge that were dewatered but
not digested; the lowest average concentration was 387 cysts/L (or -387 cysts/kg wet wt)
in samples from the Illinois plant. Giardia cysts were detected in the wastewater treatment
plant effluents, with maximum counts of 44 cysts/L.
3.3. VIABILITY AND SURVTVAfflLITY IN SOIL AND WATER
Microorganisms are inactivated in soil at rates that vary with the type of organism,
28
-------
the degree of predation by other microorganisms, the amount of sunlight, and the physical
and chemical composition of the soil, including moisture content, pH and temperature'
(Gerba et al., 1975; Kowal, 1985). Moist, cold soils contribute to increased survival time
of protozoan cysts and helminth ova; therefore, soils with a higher percentage of organic
matter, which have a greater water-holding capacity, may be more conducive to pathogen
survival. Protozoan cysts are extremely sensitive to drying. Likewise, helminth eggs and
larvae are susceptible to die-off when exposed to desiccation and sunlight, but in cool,
moist soil they may remain infective for several years (Kowal, 1985).
Storey and Phillips (1985) found that the survival of H saeinata and A. lumbricoides
ova, introduced into the upper 1 cm of soil, increased at increasing depth of the soil profile.
The ova survived longer at the lower levels of the soil profile. At levels below 12 cm, the
number of H saginata eggs surviving at 200 days was only slightly reduced from the initial
number at the beginning of the experiment. . :
Reimers et al. (1986) noted that field studies adequately describing parasite egg .
survival following land application are lacking. These authors indicated that once applied
to land, viable helminth eggs may develop into the infective stage. The limited data
available suggest that the more resistant eggs (e.g., Ascaris) may survive for years.
With helminths having the longest survival times of the microorganisms of concern,
ova of Toxocara and Ascaris have been shown to persist in soil or on pasture for several
years. Ascaris has been called the most hardy and resistant of all excreted pathogens
(Feachem et al., 1983) with survival times in soil of up to 10-12 years (Brudastov et al.,
1970; Oganov et al., 1975). In addition, many sewage treatment methods serve to'
concentrate Ascaris eggs in the sludge. Lack of accurate and consistent quantitative data
on the survival of parasites under land application conditions prompted the U.S. EPA to
investigate Ascaris ova survival following land application of municipal wastewater
treatment plant sludge (Jakubowski, 1988). Despite problems associated with field studies,
results indicated that infective Ascaris eggs survived throughout the 3-year duration of the
study. Sludge applied to the surface grass plots produced inactivation of Ascaris ova more
rapidly than either subsurface or tilled applications.
Leftwich et al. (1988b) studied parasite survival in anaerobically digested sludge,
spiked with Ascarij eggs, applied to agricultural plots in Ohio, Texas and Louisiana. They
29
-------
found that the concentration of Ascans ova decreased >90% on untilled, grassed plots, a
fact possibly attributable to biological activity, and lower, freezing soil temperatures.
Incorporation into soil increased survival of the parasites. Although a laboratory
experiment showed that survival was greatly reduced at extremely low soil moisture
(Leftwich et al., 1988a), under field conditions no statistical correlation was obtained
between soil moisture and survival. The authors were unable to draw a direct statistical
correlation between viable Ascaris ova concentration and solar radiation, relative humidity
or soil temperatures. In fact, no statistical correlation was found between parasite egg
concentration and chemical, physical or biological parameters.
Grenfell et al. (1986) also conclude that quantitative analysis of field data fails to
show that mortality of infective larvae of Ostertagia ostertagi and CoQEejla oncophora,
parasitic gastrointestinal nematddes of cattle, varies with climatic parameters, despite that
generally widespread view in the literature.
Repeated freezing and thawing of soil samples spiked with Ascaris ova reduced egg
viability more rapidly and to a greater extent than did constant room temperature
(controls) or below-freezing temperatures (Leftwich et al., 1988a). Viable Ascaris eggs in
anaerobic digested sludge and in sediment were reduced by 100% within 6 weeks when
spiked in soils of 4%, 10% and 20% moisture and subjected to freeze-thaw conditions. The
lower the soil moisture, the greater was the reduction in percentage viable Ascaris ova,
regardless of temperature. ,
Burger (1984) found that Taenia saginata eggs survived differentially depending on
the season of contamination of the site. The shortest survival time, 4-8 days, occurred in
summer when egg-contaminated soil was exposed to the sun (Soviet Union). The longest
survival time was 365 days (Kenya) when eggs were applied to open pasture (infectivity
unknown). Burger concludes that the time interval between sludge application and
introduction of cattle to the pasture should be at least four months during seasons of
pasture growth and longer during the cooler seasons in the northern European Community.
In a study of the development and survival of Trichuris suis ova on pasture plots in
southern England, Burden and Hammet (1979) found that ova required 62-90 weeks to
develop to the infective stage (embryonation). The rate was dependent on temperature,
assuming adequate moisture and oxygen. There was little if any development during the
30
-------
winter. Most susceptible to adverse environmental conditions were the early developmental
stages. The highest percentage of ova perished in the plots contaminated during the
summer drought of 1975 when maximum air temperatures were above 20ฐC. Following
development to the infective stage, ova survived at least two years.
Burden et al. (1976) studied the closely-related Trichuris trichiura and found a
relatively rapid death rate of ova in soil in southeast England. During an 18-month
observation period, 80% of the eggs died.
Data on survival of parasites in water is very limited. Entamoeba histolytica
survived 153 days in distilled-water at temperatures ranging from 12-22ฐ C and the same
length of time in natural waters, but with a decrease of 30% for each 'lOฐC rise in
temperature (Mitchell, 1972). These data are generally confirmed byGrenfell et al. (1986),
who found that the optimal survival rate for Ostertagia ostertagi in water is in the range of
0-15ฐC with higher die-off rates above and below this range.
There is abundant evidence that Crvptosporidium bocysts and Giardia cysts persist
in water. Crvptosporidium oocysts have been identified in surface water (Madore et al.,
1987; Crawford and Vermund, 1988) and in a filtered, treated public water supply (Hayes
et al., 1989). Giardia cysts survive relatively long periods in water, particularly at
temperatures below 20ฐC; above 20ฐC, cyst inactivation is rather rapid (Jakubowski, 1990).
Evidence suggests that Giardia cysts in water survive best at 4-8 ฐC (Jakubowski, 1990).
Jarroll et al. (1984) reported that cysts did not survive when Giardia were exposed for 24
hours to artificial sea water at 4ฐC or to air-drying at 4ฐC or 21 ฐC.
Extremes in temperature can reduce oocyst viability. Tzipori (1983) reported that
freeze-drying destroyed infectivity of Crvptosporidium oocysts as did 30-minute exposure to
temperatures below freezing and above 65 ฐC. Laboratory studies have shown that
Cryptosporidium oocysts stored in containers that exclude air can remain viable for 8-9
months, but excystation seems to occur soon after exposure to air (Tzipori, 1983).
Quantitative data on die-off rates in water or soil were not found.
3.4. TRANSPORT
3.4.1. Transport in Soil. It is assumed in the model that pathogens are distributed in the
upper soil layer by incorporation and tilling. However, materials in the soil can also be
31
-------
transported by water moving through the soil. This movement occurs through pores, spaces
between the particles or grains of soil. Soil pores are generally classified as macropores
(>62/zm) or micropores; because water is bound tightly to the surface of the grains and
nearly fills micropores, the bulk of water movement is in macropores. Downward bulk
transport of water may occur as a result of gravity.; Bulk water movement occurs only when
the amount of water in the soil exceeds the field capacity. In this case, the path of the
water is tortuous because -pores are not_arranged in straight channels. There may also be
upward and horizontal movement as a result of capillarity.
Particulate matter suspended in soil water can be transported as the water moves.
Particulates entering soil pores may become lodged because they are too large to move
through the pores or because of electrostatic binding to soil particles. Thus bulk transport
of particulates occurs more readily in macropores, cracks and channels than in micropores.
Capillarity is less likely to transport large particulate matter for significant distances
because it occurs mainly in the micropores, which are too small for large particles.
Protozoan cysts (~5-25jum dia) and helminth ova (~ 15-80nm dia) are large enough'
that they exhibit very little migration through soil (U.S. EPA, 1985). They do not move
vertically into groundwater because of the physical barrier provided by the soil, unless there
are vertical cracks or fissures. In the Seattle Metro study (1983), no appreciable downward
movement of Ascaris ova occurred after. 15 days, nor were Ascaris or hookworm eggs or
Entamoeba histolytica cysts able to pass through 24 inches of sand.
Sorber and Moore (1986) conclude in their critical literature review that "there are
essentially no data available in the published literature that would permit estimating
transport rates for pathogens from sludge-amended soils." They go on to indicate that the
size of protozoan cysts and helminth ova appears to prevent the vertical migration of these
parasites from sludge-amended soil, but no studies designed to resolve this issue were
found in their review of the literature. However, Storey and Phillips (1985) measured the
rate of transport of T. saginata ( 30 p,m dia) and A. lumbricoides (~50/.tm dia) ova in
laboratory soil columns. The mean distances traveled in 72 hours by the smaller T\
saginata eggs and the larger A. lumbricoides eggs were 2.21 cm and 1.78 cm, respectively,
with a drip rate of 0.25 mL/minute, and 2.54cm and 2.14cm, respectively, with a drip rate
of 0.5 mL/minute. The soil columns were 1 cm in diameter (0.79 cm2 in area), so the
32
-------
amount of water percolating through them at 0.25 ml/min was 0.25/0.79 = 0.316 cm/min,
or 1367 cm in 72 hr. Therefore the relative rate of migration of T. saginata- was 2.21/1367
= 1,62x10ฐ, and the relative rate of migration of A. lumbricoides was 1.78/1367 =
1.30x10ฐ. From these ratios, a rough calculation could be made of the time required to
transport parasite ova through soil. Even if the relative rate of migration were as high as
0.01 and all rain water were to pass through the soil layer containing the ova, the time
required to move the ova 100 cm would be greater than '65 years if the annual rainfall
averaged 60 inches.
Results from the current literature search confirm the dearth of data with respect
to the pathogens, listed in Table 3-2. However, there have been a few recent reports on
other parasites that may be useful as representatives of the group.
Krecek and Murrell (1988) observed that larvae of Ostertagia ostertagi migrated at
least 15 cm down into the soil, moved laterally around a barrier and subsequently returned
to the surface grass within 5 weeks after the beginning of the experiment. The total
distance covered was >30cm. Bovine fecal pats containing 200,000 eggs were placed on
pasture whose base soil was composed of 72% sand, 20% silt and 8% clay/ This capability
for vertical soil migration and return to surface herbage raises the issue of the
epidemiological significance of soil sequestration during periods of environmental stress as
a reservoir of Ostertagia. The authors made no determinations of total numbers of larvae
re-emerging onto grass. Krecek and Murrell (1988) also reviewed other studies of larval
migration in soil and found results to be inconsistent: free-living third-stage larvae did
migrate but authors differed in their conclusions about the epidemiological importance of
such migration.
Free-living larval forms may move through the surface soil, but these movements are
usually active rather than a result of passive transport. Larval migration in soil tends to
keep the pathogens near or above the surface, where they are more likely to encounter a
suitable host.
Altaif and Yakoob (1987) studied the survival of Haemonchus contortus infective
larvae on soil and pasture in Iraq for a period of.12 months. The authors found little
evidence of migration of infective larvae in the soil; the majority of larvae of this parasitic
gastrointestinal nematode of sheep and goats was discovered in the herbage.
33
-------
Grenfell et al. (1986) analyzed the survival and migration rates of the infective
stages of Ostertagia ostertagi and Cooperia oncophora. parasitic gastrointestinal nematodes
of cattle. They determined, by use of a mathematical model of larval demography,
maximum-likelihood estimates of mortality rates of larvae as 0.0284/day in feces and on
herbage as 0.0087/day. Average migration rate from feces to herbage in the temperate
climate of northern Europe was estimated as 0.00884/day. The life-span of the 3rd-stage
larvae (L3) may be 1-2 years.
Burden and Hammet (1979) recovered Trichuris suis ova from contaminated pasture
plots at all depths sampled-0-10 cm, 10-20 cm, and 20-30 cm~for the 30-month duration
of the test. The samples demonstrated that T. suis ova did not leach rapidly through the
chalky/flinty soil at Compton in southern England but that they remained available to pigs
grazing the plots up to 30 months later. In some of the samples, the majority of ova were
located in the 20-30 cm fraction, presumably because the ova were washed down the cracks
and fissures that were common on the plots following periods of hot, dry weather.
3.4.2. Transport in Surface Runoff. Whenever the amount of water applied to the land
surface is greater than can be absorbed by the soil or soil cover, water will pool on the
surface or run off to a lower point on the surface. Microorganisms, along with other
particulates in the soil, can be suspended in this surface water and transported as surface
runoff. In the Pathogen Risk Assessment Model, it is assumed that runoff occurs only after
rainfall, because it is required that irrigation be limited to prevent runoff 1 In addition, it
is assumed that both runoff from offsite onto the site and runoff from the site to an offsite
location are prevented by ditching, diking or other means. Therefore, runoff is limited to
that occurring in the specified field area.
In the computer model, pathogens in runoff water are considered separately from
those associated with suspended sediments. The concentration of pathogens in runoff water
depends on how readily they are separated from the soil particles and how well they
remain in suspension. Viral particles may be tightly bound to soil particles and thus be
difficult to suspend in runoff water, but suspended viruses do not readily settle out of
suspension because they are so small. In contrast, parasite ova, cysts and oocysts are less
tightly bound by soil particles, but their large size makes them settle out of suspension
rapidly.
34
-------
It is assumed that organisms in the top 1 cm of soil are available for suspension in
runoff water after a rainfall. The fraction of total rainfall occurring as runoff is calculated
by Subroutine RAINS, and the fraction of organisms in the soil surface that are suspended
in runoff water is given in the model by the variable SUSPND [P(45)]. Although there
have been several studies on suspension of soil-associated viruses and bacteria, there are
few data describing the suspension of parasites in runoff water. In the model it is assumed
that the fraction of parasites suspended is 0.01 in Practice I, in which the soil surface has
little cover, and 0 in Practices II and in, in which the soil has a continuous grass surface.
3.4.3. Transport by Wind. The ability of protozoan cysts and Cryptosporidium oocysts to
survive in aerosols has not been determined. However, Lawande and associates (1979)
recovered soil amebae, transmitted by the dust-laden air during the harmattan period, from
the nasal passages of children in Nigeria. Of the 50 children evaluated, 24% had positive
cultures for the soil amebae. The pathogenic strains of Naegleria fowleri recovered from
two children were viable and infective, killing mice in five days.
Rivera et al. (1987) have also isolated airborne, free-living amebae from the
atmosphere of Mexico City. The authors suggest that the ability of the cyst-forming
amebae to exist and remain viable in the atmosphere can, under favorable environmental
conditions, contribute to dispersion and invasion of water supplies, food, or healthy people.
Airborne transport of dry larvae or eggs of nematodes has been modeled by Carroll
and Viglierchio (1981) using simplified Gaussian plume models. They measured the
sedimentation velocities of dry larval, egg and cyst forms of a number of parasites and
found that most fell between 0.1 and 0.6 m/sec, indicating that they would be more
erodible than dry soil particles. Their results indicated that, given the presence of eggs or
larvae on a loose, dry soil surface and with sufficient wind velocity (11 m/sec, particles
lofted to a height of 30 m by tilling), eggs could be carried up to'0.6 km and larval forms
1-3 km as a result of tilling. The authors concluded that nearly all dust suspended by wind
vortices (dust devils) would be deposited within 4 km, but a few individual organisms could
be transported as far as 40 km away.
The authors' calculations indicated that the maximum concentration (pathogens/m 3)
of larval forms at a distance of 80 m during cultivation at a windspeed of 2 m/sec was
approximately 4% of the release rate (in pathogens/sec). The U.S. EPA model for tilling
35
-------
emissions (U.S. EPA, 1983) yields a release rate of approximately 1.0(5 kg soil/hr, or
approximately 0.3g/sec. At a sludge application rate of 10T/ha, a concentration of 5000
pathogens/kg, and dispersal of the pathogens in 2x10* kg surface soil/ha (Naylor and
Loehr, 1982), the concentration of pathogens in soil would be 0.025 pathogens/g. Thus the
concentration of pathogens transported 80 m by wind in the case described above would be
0.04x0.3x0.025=3x10"" pathogens/m 3. This figure implies a low risk of infection by
windblown pathogens as a result of tilling and subsequent air transport.
36
-------
4. PARAMETERS FOR MODEL RUNS .
4.1. RATIONALE FOR PARAMETER SELECTION
The assessment of human health risk from pathogenic parasites as a result of land
application of sewage sludge requires a realistic description of the fate and transport of the
pathogens. Information in the published literature confirms that protozoan cysts and
parasite ova survive during the sewage treatment process, but quantitative data on survival
of protozoa in sludge are scarce. There appear to be few quantitative measures of
movement or die-off rates of ova or cysts in soil or their inclusion in aerosols. Most
researchers assume that, ova and cysts are too large to migrate in soil or into groundwater.
That assumption was maintained in this analysis. Some researchers conclude that ova and
cysts are unlikely to be included in droplet aerosols. In this analysis, however, it was
assumed that ova and cysts would be included in any droplet aerosols formed by spray
application, as well as in any paniculate aerosols formed as a result of disturbance of the
soil by wind or cultivation. Although the droplet aerosol model includes droplets smaller
than ova and cysts, no modifications were made to exclude these particles from the
infectiveness calculation. Therefore, estimates of the infectiveness of very small droplets
may be unrealistically high.
Various estimates have been made for the rates of inactivation of parasite ova and
protozoan cysts. These forms of the pathogens are more resistant to environmental
conditions than the unencysted, larval or adult forms, which do not survive sewage
treatment. Therefore, ova and cysts are considered to be more significant with regard to
exposure risk from sewage sludge. However, the survival properties of ova and cysts
depend on the specific organism in question, as well as on the conditions to which they are
exposed. Therefore, a single mathematical description of die-off will at best only
approximate the behavior of all parasites. Default values in the model for die-off of ova
and cysts are those found in the initial version of the model (U.S. EPA, 1980). They are:
During application/incorporation
0 for Temp < 20ฐC;
1(r.ooo,7s> or o.00041/hour for 20 _< Temp < 40;
10(-ฐ'456) or 0.65/hour for Temp >i40;
37
-------
In moist soil
0 for Temp < 20 ฐC or for 8 hours after irrigation;
10punai) Qr o.000533/hour for 20 _< Temp < 40;
10W667) or 0.7845/hour for 40 ^Temp jC 50;
10WM or 0.25/hour for Temp > 50; ,.
On crop surfaces
KjtMtQ or o.7845/hour at all temperatures;
In water
Of o.000533/hour at all temperatures.
A 90% die-off time of 270 days for Ascaris ova in soil reported by Sorber and
Moore (1986) implies an exponential rate of lO*'"270*24' -or lO^'^/hour (fractional rate
0.00035/hour); this value is near the model's default value. In the model analysis a value
of 10<4>>cool)/hour was used as a lower limit for die-off rate in moist soil.
O'Donnell et al. (1984) reported a decrease in the number of recoverable Ascaris
ova treated in sludge and stored in soil. The decrease occurred at rates of approximately
lO^Ymonth for aerobically treated sludge and approximately 10C"ฐ'3Y month for
anaerobically treated sludge; viability of the ova recovered dropped at a rate of - 10"
Oi003)/month for aerobically treated sludge and lO^'^Vmonth for anaerobically treated sludge,
giving composite rates for Ascaris of lO^'^/month (lO'^^^Vhour) for aerobically treated
sludge and lO'^/month (10(-aooฐ45)/hour) for anaerobically treated sludge. The value for
aerobically treated sludge agrees well with the default value. A value of 10l"ฐ-(1005)/hour was
used as an upper limit for the model analysis. Recovery and viability of Toxocara.
Trichuris and Hymenolepis ova appeared not to be greater than for Ascaris (O'Donnell et
al.,1984).
Published data on die-off rates for parasites in sludge are summarized in Table 4-1.
For maximum utility, rates should be determined for all of the organisms listed. However,
quantitative data are not available for most of them.
38
-------
8
ง
u
oi
w
o
Q
1
.1
V
3
I
CQ
AH
UH
O
I
fcj
9
u
5
g
1
PL,
Q
tu
9
w
>-H
Q
5 q
ง
Ofl
c
I
t/1
c
Ancvlostoma
duodenale
Ancvlostoma
braziliense
Ancvlostoma
caninum
Ascaris
lumbricoid
Ascaris suum
Enterobius
vermiculari
3
Necator ameri
en
Stronevloides
stercoralis
Toxocara cani
Toxocara cati
39
-------
al o
ปa w
w
CD
Q
I
CO
2
o
= ซ
Pi
o
N
o rs
J!
U
Q
a
40
-------
4.2. PARAMETER VALUES
Prior to the site-specific simulations, an initial sensitivity analysis was performed.
In. this analysis, several parameters were systematically varied to simulate a variety of
possible conditions, application methods and agricultural practices. The parameters varied
included those of the main program, Subroutine RISK and Subroutine RAINS. The ranges
of values and rationale for their selection are discussed below.
4.2.1. Main Program Parameters. The main program parameters varied for this analysis
are listed below (default values are in bold-face type):
VARIABLE
# NAME
1 ASCRS
2 APRATE
4 TREG
6 APMETH
13 TRAIN
17 IRMETH
18 DILIRR
VALUES
200
5000
11000
l.OxlO4
2.5xl04
0
-1
0
+ 1
Site-specific
0
1
0
1
DEFINITION
VALUES
AND RATIONALE FOR
Concentration of organisms in sludge (number/kg
dry wt). Values as high as 11,000/kg have been
reported (Wallis et aL, 1984), and values less
than the default have been reported.
Rate of application of sludge (kg/ha).
Demonstrate the effect of increased sludge
concentration in soil.
Waiting period before harvesting (months).
Make parasites available immediately for
exposure as a worst case.
Application method (flag). Demonstrate the
relative effects of spray application, surface
application, and subsurface application.
Flag for rainfall subroutine menu. Rainfall is the
most significant factor in surface runoff/sediment
transport to the onsite pond.
Irrigation method (flag). Compare the effects of
spray irrigation and ditch irrigation.
Fraction of irrigation water that is contaminated.
Demonstrate the effect of irrigation with sludge
as compared to uncontaminated water.
41
-------
VARIABLE
# NAME
30 COVER
31 AEREFF
45 SUSPND
66 CROP
67 TCULT
68 TCROP
69 THARV
VALUES
0
0.9
IxlO'3
2x10 "2
0.01
0.001
DEFINITION
VALUES
AND RATIONALE FOR
59 DTCTMT 0
1
60 DTCTMK 0
1
-1
0
+1
0
_2
240
300
Percent of ground surface covered by vegetation.
Compare surface runoff/sediment transport for
bare soil and soil with vegetation (will be
matched with values in Subroutine RAINS).
Efficiency of aerosol formation. 'Compare default
value to an unrealistically high value to
determine whether the model is sensitive to
off site aerosols.
Fraction of soil surface organisms suspended in
soil surface water. Vary to determine whether
model is sensitive to resuspension from surface
soil into soil surface water. !
Fraction of pathogens ingested by animals that is
transferred to meat. No transfer of parasites to
meat [P(59)=0], or invasion of edible tissue by
every infective ovum or cyst [P(59) = l].
Fraction of pathogens ingested by animals thai: is
transferred to milk. No-transfer of parasites to
milk [P(60)=0], or invasion of milk by every
infective cyst [P(60) = l].
Type of crop (flag). Type of crop is important in
exposure during consumption of crop.
Cultivation time (hr) or flag. Include regular
cultivation to determine whether model is
sensitive to generation of particulate aerosols.
Time crop surface is present (hr). Establish early
appearance of crop surface to facilitate
demonstration of sensitivity of exposure to
presence of crop. ,
Harvest time (hr). Establish early harvesting of
crop to facilitate demonstration of sensitivity ,of
exposure to presence of crop.
42
-------
The variable DTCTMT [P(59)] is used to describe the transfer of parasites from
contaminated soil or feed to meat. This process accounts for ingestion .of infective ova or.
larvae by livestock, with subsequent migration of larvae to edible tissues. Upon ingestion
of meat containing cysts, infection of the food consumer (EATER) would occur. In all
other exposure pathways, it is assumed that the EATER ingests infective ova or larvae.
4.2.2.Parameters for Subroutine RISK. Parameters varied for Subroutine RISK are listed
below (default values are in bold-face type):
PARAMETER
# NAME
5 DRECTC
VALUE
DEFINITION AND
CHOICE OF VALUES
RATIONALE FOR
6 DRECTS
34 - XDIST
0.1
0.02
0.1
0.02
0.2
200
100
Ingestion of crop surface (g/day). Determine
effect of reducing ingestion of contaminated crop
surface during routine daily work in the field.
Ingestion of or direct contact with soil (g/day).
Determine effects of reducing ingestion of
contaminated soil during routine daily work and
of including transdermal invasion from soil on
the skin surface.
Distance (m) downwind of receptor from
particulate source. Determine effects of reducing
distance to receptor of offsite aerosol to Verify
initial indications that offsite exposure is
negligible.
DRECTS (Variable 6) is intended to represent infection by direct contact with
contaminated soil or crop surfaces. This exposure pathway includes both transdermal
invasion by parasites and inadvertent ingestion of infective forms. The default value (0.1
g/day) was chosen to be conservative in comparison to U.S. EPA estimates of ingestion of
dust and soil by adults (0.02 g/day; U.S. EPA, 1983). Typical rates of soil ingestion by
children are higher, with a 95th percentile value of 0.4-0.6g/day (Binder et al., 1986). The
U.S. EPA suggests a daily intake of 0.2 g/day for children under 5 for assessment of risk
from toxic chemicals (U.S. EPA, 1990a). However, children (and adults) with pica, a
behavior characterized by intentional ingestion "of soil or other mineral substances, may
ingest much larger amounts of soil. For example, Calabrese (1988) reports ingestion of 5-
8 g soil/day by one subject. For these persons, the risk of infection from contaminated soil
43
-------
is much greater than for the general population. To model risk of infection to persons
exhibiting pica, any chosen value can be used for DRECTS during the input phase of the
model run. Exposure by ingestion of or direct contact with contaminated soil or crop
surfaces is summed with aerosol exposures to calculate the probability of infection ONSITE.
4.2.3. Subroutine GRDWTR. Because it was assumed that parasite ova and protozoan
cysts are not transported into groundwater, this subroutine was not used in the model test
runs. However, since protozoan cysts are small (5-25 /LOU), their migration through porous
soil may not be completely prevented. If the user wished to model subsurface transport in
groundwater, appropriate values for parasites would be entered during the input phase of
a model run using the bacteria or virus option of the model. ' However, data on such
parameters as retardation coefficient and hydrodynamic dispersion coefficient have not been
found. ........
4.2.4. Subroutine RAINS. The Modified Universal Soil Loss Equation, which is the basis
for Subroutine RAINS, depends on soil type, topography and land-use practices, so
parameters for Subroutine RAINS are influenced strongly by the choice of site. A variety
of locations representing different soil types, topography and climate were selected as test
sites for model 'runs. Values for the parameters in Subroutine RAINS were chosen "to be
appropriate for soil type, topography and meteorological patterns for the chosen locations.
Although the model is limited in its ability to represent the rainfall pattern of any location
because of its restriction to no more than ten rainfall events, these events were included as
early in the model run as possible to ensure that the effects of rainfall on surface
runoff/sediment transport were maximized.
44
-------
Subroutine RAINS is described in more detail in U.S. EPA (1990b). Parameters
used by Subroutine RAINS include the following: . . ,
...-" ' . ' -' " .~-\~ , .. ... , " i
PARAMETER
#
2 .
3
4,
5
6
?'.
8
9
10
11
NAME
PDUR
PTOT
BTLAG
CN
AMC
STAD
USLEK
USLEL
USLES
USLEC
DEFINITION
Event-specific duration of rainfall (hours) ,
Event-specific amount of rainfall (cm)
Basin time lag. Depends on site-specific properties.
Curve number. Depends on site-specific properties.
Antecedent moisture conditions (dry, intermediate1 or wet)
Event-specific storm advancement coefficient
USLE K factor (soilcredibility)
USLE L factor (from length of slope)
USLE S factor (from steepness of slope)
USLE C factor (coyer management)
Values used^for these variables in the model runs are given in Section 5 along with the site
descriptions. , ,,. . ,.,.,
. Since Practices IV and V do not include an onsite pond to receive surface water
runoff, parameters for Subroutine RAINS are given only for Practices I, H and III.
45
-------
5. SITES FOR MODEL RUNS
Six sites were chosen to provide a variety of soil types, topography arid
meteorological patterns. Other than Anderson County, TN, for which more detailed
meteorological data were available to the authors, specific sites were chosen arbitrarily with
the goal of geographic diversity. Data on soil properties were taken from U.S. Soil
Conservation Service soil surveys, which have been developed for each county in the United
States. Meteorological data were taken from the National Oceanic and Atmospheric
Administration Local Climatological Pata Annual Summaries for 1981 (NOAA, 1981). The
sites chosen for the model runs are described below.
5.1. SITE 1: ANDERSON COUNTY, TENNESSEE
Values of site-specific variables were chosen to reflect.conditions at an agricultural
location in the Clinch River Valley of East Tennessee.
5.1.1. Description of Soil. The soil chosen for the model run is the Claiborne series, which
comprises fine-loamy, siliceous, mesic Typic Paleudults. It is further described as follows
(USDA, 1981a):
The Claiborne series consists of deep, well drained soils that formed
in sediment deposited by water or in residuum of dolomite. These soils are
on ridgetops, on hillsides, and at the base of slopes. The slope range is 5 to
45 percent, but in most areas the gradient is 12 to 30 percent.... :
The soliim is more than 60 inches thick. Depth to dolomite bedrock
is more than 72 inches. The soil is strongly acid or very strongly acid
throughout except for the surface layer where limed. The content of coarse
chert fragments ranges form 5 to 25 percent in each horizon. These
fragments commonly increase in size and abundance with increasing depth.
Claiborne soils are of hydrologic group B, characterized by moderately low runoff potential,
moderate infiltration rates and moderate rates of water transmission.
For this analysis, it was assumed that sites with slopes greater than 10% (6ฐ) would
not be used because of the likelihood of excessive runoff.
5.1.2. Narrative Climatological Summary. The following climatological summary for Oak
Ridge, Anderson County, TN, was taken from NOAA (1981):
Oak Ridge is located in a broad valley between the Cumberland Mountains,
which lie to the northwest of the area* and the Great Smoky Mountains, to
the southeast. These mountain ranges are oriented northeast-southwest and
the valley between is corrugated by broken ridges 300 to 500 feet high and
46 :
-------
oriented parallel to the main valley. The local climate is noticeably
influenced by topography. Prevailing winds are usually either up-valley, from
west to southwest, of down-valley, from east to northeast. During periods of
; light winds daytime winds are usually southwesterly, nighttime winds usually
northeasterly. Wind velocities are somewhat decreased by the mountains and
ridges. Tornadoes rarely occur in the valley between the Cumberlands and
the Great Smokies. In winter the Cumberland Mountains have a moderating
influence on the local climate by retarding the flow of cold air from the north
and west.
The coldest month is normally January but differences between the mean
temperatures of the three winter months of December, January, and February
are comparatively small. The lowest mean monthly temperature of the winter
has occurred in each of the months December, January, or February in
different years. The lowest temperature recorded during the' year has
occurred in each of the months November, December, January, or February
in various years. July is usually the hottest month but differences between the
mean temperatures of the summary months of June, July, and August are also
relatively small. The highest mean monthly temperature may occur in either
of the months June, July, or August and the highest temperature of the year
has occurred in the months of June, July, August, and September in different
: years. Mean temperatures of the spring and fall months progress orderly
from cooler to warmer and warmer to^ cooler, respectively, without a
secondary maximum or minimum. Temperatures of 100ฐ [38 ฐC] or Higher
are unusual, having occurred during less than one-half of the years of the
period of record, and temperatures of zero or below are rare. The average
number of days between the last freeze of spring and the first freeze of fall
is approximately 200. The average daily temperature range is about 22ฐ
[ 12 ฐC] with the greatest average range in spring and fall and the smallest in
winter. Summery nights are seldom oppressively hot and humid. Low level
temperature inversions occur during approximately 57 percent of the hourly
observations. Fall is usually the season with the greatest number of hours of
low level inversion with the number decreasing progressively through spring
and winter to a summertime minimum but seasonal differences are small.
5.1.3. Temperature. The monthly average temperatures at this location ranged between
a low of 2.8ฐ C and a high of 24.8ฐ C.
5.1.4. Rainfall. An hourly rainfall record for April and May, 1989, was obtained from the
Atmospheric Turbulence and Diffusion Laboratory, National Oceanic and Atmospheric.
Administration, Oak Ridge, TN. Profiles of the first ten rain events beginning April l,the
time the model run is initiated, were constructed from this record. Profiles consisted of the
duration of the event (PDUR), the total amount of precipitation in the event (PTOT) .and
the storm advancement coefficient (STAD), which was determined by inspection of the
47
-------
hourly precipitation. The resulting parameters were as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
flirt
77
174
726
826
924
1180
1340
1549
1590
1650
PDUR PTOT
flirt
5
8
11
7
4
5
12
2
9
14
STAD
(cm)
1.60
1.52
1.55
3.30
1.50
2.31
4.06
1.63
2.52
3.48
0.65
0.36
0.12
0.27
0.56
0.19
0.52
0.46
0.52
0.45
5.1.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions for Anderson County, TN. Values
were calculated as described in Appendix B of U.S. EPA (1990b). The values used in the
model run were based on a field with dimensions 500 m by 200 m, sloping at an angle of
6ฐ (10.5%). It was assumed for Practice I that before a crop Was present, the cover
management factor was not modified, whereas after the crop was present, a canopy cover
of 30%, a canopy height of 0.5 m and a relative root network factor of 30% were provided;
for Practices n and III, the canopy cover was taken to be 90%, the canopy height was taken
to be less than 0.5 m and a relative root network factor of 90% was iassumed. The
resulting values were:
Parameter
No. Name
4
5
6
8
9
10
11
BTLAG
CN
AMC
USLEK
USLEL
USLES
USLEC
PRACTICE NUMBER
I II ! TIT
0.2
78
3 (TCROP)
0.32
4.76
1.25
0.45 (TCROP)
0.31
64
,
0.32
4.76
1.25
0.02
0.31
64
2
I
0.32
4.76
1.25
0.02
The initial value (0.02) for USLEC in Practices II and III was subsequently shown
to cause errors which halted operation of the program, so in subsequent runs and for all
other sites that parameter value was changed to 0.05.
48
-------
5.2. SITE 2: CHAVES COUNTY, NEW MEXICO
Values for site-specific variables for Site 2 were chosen to represent an agricultural
area near Roswell, a city in southeast New Mexico.
' 5.2.1. Description of Soil. The soil chosen for the model run is the Pecos Series, which
comprises fine, mixed, thermic Torrertic Haplustolls. It is further described as follows
(USDA, 1980):
The Pecos series are deep, moderately well drained, very slowly permeable
soils on flood plains. The soils formed in calcareous, saline, stratified, clayey
alluvium. Slope is 0 to 1 percent.
Typically, the surface layer is reddish brown silty clay loam about 12 inches
thick. The upper 10 inches of the substratum is reddish brown clay, the next
20 inches is reddish brown silty clay and silty clay loam, and the lower part
to a depth of 60 inches or more is brown loam and fine sandy loam. Salinity
is moderate. Available water capacity is high.
Pecos soils are of hydrologic group D, characterized by having a very slow infiltration rate
(high runoff potential) when thoroughly wet. They consist chiefly of clays that have a high
shrink-swell potential, soils that have a permanent high water table, soils that have a
claypan or clay layer at or near the surface, and soils that are shallow over nearly
impervious material. These soils have a very slow rate of water transmission.
5.2.2. Narrative Climatological Summary (NOAA, 1981).
The climate at Roswell conforms to the basic trend of the four seasons, but
shows certain deviations related to geography. A location south and west of
the main part of major weather activity affords a degree of climatic seclusion.
There are also topographic effects that are inclined to alter the course of the
weather in this area. Higher landmasses almost surround the valley location,
with a long, gradual descent from points southwest through west and north'.
The topography acts to modify air masses, especially the cold outbreaks in
wintertime. Downslope warming of air, as well as air interchange within a
tempering environment, often prevents sharp cooling. Moreover, the
elevation of 3,600 feet in common with the geographic situation, discourages
a significant part of the heat and humidity that originates in the south in
summer. In winter, subfreezing at night is tempered by considerable warming
during the day. Zero [ฐF] or lower temperatures occur as a rule a time or
two each winter. Subzero cold spells are of short duration. Winter is the
season of least precipitation. , '
5.2.3. Temperature. The monthly average temperatures at this location ranged between
a low of 4.2ฐ C and a high of 26.2ฐ C.
49
-------
5.2.4. Rainfall. Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 at the location (NOAA, 1981). The record provided the
date and amount of the largest rainfall during a 24-hour period each month, as well as the
total amount of rainfall each month. The largest rainfall (greater than the subroutine's
lower limit of 1 cm) was always used, and the remaining rainfall during the period was
divided into events placed at arbitrary times. The storm advancement coefficient was
chosen to reflect the nature of rainfall in the region; the low number used reflects a
preponderance of thunderstorms' and sudden showers, whereas larger numbers were used
for some other sites to reflect a more gradual buildup of the rainstorm. The resulting
. parameters were as follows: .
Event
No.
1
2
3
4
5
6
7
8
9
10
START
fhf)
328
784
966
1280
1830
2174
2366
2800
3328
3518
PDUR
flirt
5
' 8
6
3
8
10
10
5
10
.6
PTOT
(cml
1.17
4.5
4.55
1.5
3.05
7.75
12.47
3.45
4.19
3.0
5.2.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions. The slope value used in the model
run was 1 degree (1.7%). The resulting values were:
Parameter
No. Name
4
5
7
8
9
10
11
BTLAG
CN
STAD
USLEK
USLEL
USLES
USLEC
PRA<
I
0.32
89
0.25 "
0.32
2.54
0.16
0.45(TCROP)
CTICE NUME
II
0.38
84
0.25
0.32
2.54
0.16
0.05
tER
III
0.38
84
0.25
0.32
2.54
0.16
0.05
50
-------
5.3. SITE 3: CLINTON COUNTY, IOWA
Values for site-specific variables for site 3 were chosen to represent an agricultural
area in eastern Iowa in a county that borders on the Mississippi River.
5.3.1. Description of Soil. The soil chosen for the model run is the Fayette Series, which
comprises fine-silty, mixed, mesic type Hapludalfs. It is further described as follows
(USDA, 1981b):
The Fayette series consists of well drained, moderately permeable soils on
loess-covered uplands. These soils formed in loess that is more than 40
inches thick. Slope ranges from 2 to 40 percent.
The solum ranges from 40 to 60 inches In thickness. There are no carbonates
to a depth of 40 inches to 60 inches.
Fayette soils are of hydrologic group B, characterized by moderately low runoff potential,
moderate infiltration rates, and moderate rates of water transmission.
5.3.2. Narrative Climatological Summary. Because a meteorological report for Clinton
County was not included in NOAA (1981), the Climatological summary and data reported
for nearby Dubuque, IA, (NOAA, 1981) were used:
The principal feature of the climate in Dubuque is its variety. Standing, as
it does, at the crossroads of the various air, masses that cross the continent,
the Dubuque area is subject to weather ranging from that of the cold, dry,
arctic air masses in the winter with readings as low as 32ฐ below [-36ฐC],
when the ground is snow covered, to the hot, dry weather,;of the air masses
from the desert southwest in the summer when the temperatures reach as
high as 110ฐ [43 ฐC]. More often the area is covered by mild Pacific air that
has lost considerable moisture in crossing the mountains far to the west, or
by cool, dry Canadian air, or by warm, moist air from the Gulf regions. Most
of the year the latter three types of air masses dominate Dubuque weather,
with the invasions of Gulf air rarely occurring in the winter.
The seasons- vary widely from year to year at: Dubuque; for example,
successive invasions of cold air from the north may just reach this far one
winter and bring a long, cold winter with snow-covered ground from mid-
November until March, and many days of sub-zero temperatures, while
another season the cold air may not reach quite this far arid the winter can ,
be mild with bare ground most of the season, and only a few sub-zero
readings. The summers, too, may vary from hot and humid with considerable
thunderstorm activity when the Gulf air prevails, to relatively cool, dry
weather when air of northerly origin dominates the season.
51
-------
5.3.3. Temperature. The monthly average temperatures at this location ranged between
a low of-7.5ฐ C and a high of 23.2ฐ C.
5.3.4. Rainfall. Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 (NOAA, 1981). The resulting rainfall parameters were
as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
fhrt
180
231 .
396
636
970
1264
1791
1834
1934
2080
PDUR PTOT
flirt
8
6
8
6
6
-.8
, 10
10
6
4
(cm)
2.84
1,0
1.6
1.2 .
1.0
1.27 .
6.12
4.56
3.0
2.5
5.3.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe soil conditions for Clinton County, IA, and rainfall for Dubuque, IA,
the nearest reporting station. The slope value used in the model run was-4.6ฐ (8%). The
resulting values were:
Parameter
No. Name
4
5
6
7
8
9
10
11
BTLAG
CN
AMC
STAD
USLEK
USLEL
USLES
USLEC
PRACTICE NUMBER
I II III
0.17
78
3 (TCROP)
0.375
0.37
4.76
0.85
0.45(TCROP)
0.26
61
2 ' . .
0.375 .
0.37
' 4.76
0.85
0.05
0.26
61
: 2
0.375
0.37
4.76
0.85
0,05
, -
52
-------
5.4. SITE 4: HIGHLANDS COUNTY, FLORIDA ;
Values for site-specific variables for Site 4 were chosen to represent a sandy soil in
central Florida, These soils can be productive for agriculture but can be improved greatly
by amendment. :'./'..'
5.4.1. Description of Soil. The soil chosen for the model run is the Archbold Series, which
comprises hyperthermic, uncoated Typic Quartzipsomments. It is further described as
follows (USDA, 1989): , .
The Archbold series consists of nearly level to gently sloping, moderately well
drained, droughty soils that formed in marine and eolian deposits. These soils
are on moderately high ridges in the ridge part of the county. The slopes
range from 0 to 5 percent.
Typically, the surface layer is gray sand about 4 inches thick. The underlying
material to a depth of 80 inches or more is white sand.
The soil reaction is slightly acid to extremely acid. The texture is sand or fine
sand. The content of silt plus clay in the 10- to 40rinch control section is less
than 2 percent.
Archbold soils are of hydrologic group A, characterized ,by having a high infiltration rate
(low runoff potential) when thoroughly wet. They consist mainly of deep, well drained to
excessively drained sands or gravelly sands. These soils have a high rate of water
transmission.
5.4.2. Narrative Climatological Summary. Because meteorological information was not
given in NOAA, 1981 for Highlands County, the summary and data for nearby Orlando, FL,
were used. ., , , . , ,, .
Orlando, by virtue of. its location in the central section of the Florida
peninsula (which is abounding with lakes), is almost surrounded by .water and,
therefore, relative humidities remain high here the year round, with values
hovering near 90 percent at night and dipping to 40 to 50 percent in the
afternoon (sometimes, to 20 percent in the winter).
The rainy season extends from June through September (sometimes, through
October when tropical storms are near). During this period, scattered
afternoon thundershowers are an almost daily occurrence, and these bring a
drop in temperature to make the climate. bearable (although, most summers,
temperatures above 95ฐ.[35ฐC] are rather rare). Too, a breeze is usually
present, and this also contributes towards general comfort.
53
-------
5.4.3. Temperature. The monthly average temperatures at this location ranged between
a low of 15.8ฐ C and a high of 28.0ฐ C,
5.4.4.- Rainfall. Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 at Orlando (NOAA, 1981). The resulting parameters
were as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
flirt
1043
1166
1667
1789
1958
2536
2918
3301
3547
4025
PDU
fhtf
7
8
10
9
. 6
10
6
6
7
10
PTOT
(cm)
2.1
3.12
11.17
5.7
3.0
3.17
2,0
1.6
2.44
9.93
In model runs from Practice I, Subroutine RAINS returned a floating-point error
during computations for rainfall event 9. This error did not occur for Practices II and III,
so it was probably related to both the number of organisms and the long time over which
the subroutine operated. In order to complete the model run, it was necessary to delete
rainfall events 9 and 10 for Practice I.
5.4.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe local rainfall reported for Orlando, FL, and soil conditions for
Highlands County, FL. The slope value used in the model run was 1.2ฐ (2%). The
resulting values were:
Parameter
No. Name
4
5
7
8
9
10
11
BTLAG
CN
STAD
USLEK
liSLEL
USLES
USLEC
PRACTICE NUMBER
I II III
0.45
67
0.2
0.1
2.54
0.26
0.45 (
-------
'5.5. SITE 5: KERN COUNTY, CALIFORNIA
Values for site-specific variables for Site 5 were chosen to represent "a'soil near
'Bakersfield, CA, which is located in southern California.' ' '"''"'''"''" "'v'
5.5.1. Description of Soil. The soil chosen for the model run is the Arvin series, which
comprises coarse-loamy, mixed, nonacid, thermic Mollic Xerofluvents.' ' It is further
described as follows (USDA, 198Ic):
The Arvin series consist of very deep, well drkined soils on alluvial fan,
stream flood plains, and stream terraces: - These; soils formed in mixed
. alluvium derived from granitic rock. Slope ranges from 2 to 9 percent.
Clay content ranges from 5 to 18 percent in the control section. Organic
matter content is 0.9 percent or less. Reaction Is,slightly acid to mildly
alkaline throughout. '.;.'.
Arvin soils are of hydrologic group B, characterized by moderately low runoff potential,
moderate infiltration rates and moderate rates of water transmission.
5.5.2. Narrative Climatological Summary (NOAA, 1981).
Bakersfield, situated in the extreme south end of/ the; great, San Joaquin
Valley, is partially surrounded by a horseshoe-shaped rim of mountains with
an open side to the northwest and the crest at an average distance of 40
miles. - ,- .-. - .-,- ... - -,.-. -..;:; ;.: I ;,;..-. '.ป;. .-...:..',: -.-
The Sierra Nevadas to the northeast shut out most of the cold air that flows
southward over the continent during winter. They also catch and store snow,-.; .
. which provides irrigation water for use during the dicy months,, .The..Tehachapi }
Mountains, forming the southern boundary, act as aii obstruction to northwest
wind, causing heavier precipitation on the windward slopes,;high wind velocity .
over the ridges and, at times, prevailing cloudiness in the south end of the
valley after sMes have cleared elsewhere. To the west are the coast ranges,;
and the ocean shore lies at a distance of 75 to 100 miles. , -,,,,1 ;; ;
Because of the nature of the surrounding topography, there are large climatic
variations within relatively short distances. These zones of variation may be
classified as Valley, Mountain, and Desert .areas.. The, .overall, .climate,
however, is warm and semi-arid. There is only one wet season during the
year, as 90 percent of all precipitation falls from October thorough April,
inclusive. Snow in the valley is infrequent, with only a trace: occurring in
about one year out of seven. Thunderstorms also seldom occur in?the valley.
5.5.3. Temperature. The monthly average temperatures at this location ranged between
a low of 8.5ฐ C and a high of 28.8ฐ C. , '.-"-a:: -.
55
-------
5.5.4. Rainfall. Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 at the location (NOAA, 1981). The resulting parameters
were as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
flirt
16
4378
7256
. 7530
8016
8320
8606
8782
13164
16022
PDUR PTOT
fhrt
8
10
9
6
6
5
8
8
10
9
(cm)
1.0
,1.78
1.47
1.07
1.0
1.0
1.63
1.0
1.78
1.47
5.5.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions. The slope value used in the model
run was 1.7ฐ (3%). The resulting values were:
Parameter
No. Name
4
5
6
8
9
10
11
BTLAG
CN
AMC
USLEK
USLEL
USLES
USLEC
PRACTICE NUMBER
I II III
0.3
78
3 (TCROP)
0.32
2.54
0.26
0.45 (TCROP)
0.45
61
2
0.32
2.54
0.26
0.05
0.45
61
; 2
0.32
2.54
0.26
0.05
5.6. SITE 6: YAKIMA COUNTY, WASHINGTON
Values for site-specific variables for Site 6 were chosen to represent a soil near
Yakima, WA, which is located in south-central Washington along the Yakima River. This
is a region of fairly low rainfall, but which is successfully farmed by irrigcition.
5.6.1. Description of Soil. The soil chosen for the model run is the Kittitas Series, which
comprises fine-silty, mixed (calcareous), mesic Fluvaquentic Haplaquolls. It is further
described as follows (USDA, 1985):
56
-------
The Kittitas series consists of very deep, somewhat poorly drained soils on
flood plains. These soils formed in mixed alluvium. Slopes range from 0 to
2 percent. ,
Kittitas soils are of hydrologic group C, characterized by a slow infiltration rate when
thoroughly wet. They consist chiefly of soils having a layer that impedes the downward
movement of water or soils of moderately fine texture or fine texture. These soils have a
slow rate of water transmission.
5.6.2. Narrative Climatological Summary (NQAA, 1981).
Yakima is located in a small east-west valley in the upper (northwestern) part
of the irrigated Yakima Valley. Local topography is complex with a number
of minor valleys and ridges giving a local relief of as much as 500 feet. This
complex topography results in marked variations in air drainage, winds, and
minimum temperatures within short distances.
The climate of the Yakima Valley is relatively mild and dry. It has
characteristics of both maritime and continental climates, modified by the
Cascade and the Rocky Mountains, respectively. Summers are dry and rather
hot, and winters cool with only light snowfall. The maritime influence is
strongest in winter when the prevailing westerlies are the strongest and most
steady. The Selkirk and Rocky Mountains in British Columbia and Idaho
shield the area from most of the very cold air masses that sweep down from
Canada into the Great Plains and eastern United States. Sometimes a strong
polar high pressure area over western Canada will occur at the same time
that a low pressure area covers the southwestern United States. On these
occasions, the cold arctic air will pour through the passes and down the river
valleys of British Columbia, bringing very cold temperatures to Yakima. That
this happens infrequently is shown by the occurrence of temperatures of 0
degrees [F] or below on only 4 days a winter on the average. On about 21
days during the winter the temperature will fail to rise to the freezing point.
In January and February 1950, there were 4 consecutive days colder than -
20ฐ [-29ฐC], including -25ฐ [-32ฐC] on February 1. However, over one-half
of the winters remain above 0 degrees [F (-18 ฐC)].
5.6.3. Temperature. The monthly average temperatures at this location ranged between
a low of-1.5ฐ C and a high of 22.3ฐ C.
5.6.4. Rainfall. Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 at the location (NOAA, 1981). The resulting parameters
were:
57
-------
Event
No.
1 '
2
3
4
5
6
7
8
9
10
START
fhrV
1628
4290
4506
5490
5722
5966
7002
7212
7498
7816
PDUR PTOT
flirt
6
8
10
6
6
6
10
8
8
8
(cm)
1.0
1.25
2.06
1.14
1.0
1.0"
2.65
1.5
1.2
1.0
5.6.5. Parameters for Subroutine RAINS. Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions. The slope value used in the model
run was 0.6ฐ (1%). The resulting values were:
Parameter
No. Name'
PRACTICE -NUMBER
II
III
4
5
8
9
10
11
BTLAG
CN
USLEK
USLEL
USLES
USLEC
0.4
85
0.43
2.54
0.12
0.45(
-------
6. RESULTS
6.1. SENSHTVITY TO VARIABLES
Limitations in mathematical processes and in the computer's operating system result
in calculated probabilities of infection that, strictly speaking, are approximations rather than
accurate evaluations of risk. For example, the probability of infection is calculated as 1.0
minus the probability of not being infected, which is an exponential function of exposure.
The population of a compartment may be zero either because operations in the model have
not yet transferred a population into that compartment or because the program has
rounded off a population of < 1 to zero. If the calculated exposure is zero, the probability
of not being infected is calculated as 1.0, and the resulting risk is reported as 0. It is
unlikely that there are actually zero pathogens in most compartments; the user has the
option of specifying the number of pathogens in each compartment during the input phase
of the model run.
A preliminary assessment was made using site-specific data for Site 1. Seventy-five
model runs were made to assess the effects on Practices I-III of variations in the
parameters chosen (see Section 4.1 above). Several of the parameters were also tested in
Practices IV and V. In all model runs, the probability of infection for OFFSITE and
DRINKER was calculated as zero. For the subsurface injection option, the probability of
infection was calculated as zero in all exposure compartments, because with this application
option the parasites are assumed to be deposited below any zone in which exposure could
occur. ONSITE exposure occurs as a result of inhalation of dust generated by
incorporation of sludge or tilling the soil, or by direct contact, with infected soil (U.S. EPA,
1990b). Because pathogens are transferred gradually to the soil compartment during
incorporation in Practices I through III, the maxiinum exposure by direct contact is
calculated as occurring when all of the sludge has been incorporated. During incorporation,
hourly exposures are less than after incorporation has been completed, so the highest
exposure occurs on the day following incorporation. In every model run for Practice I
(which requires 24 hours for the sludge to dry before the field is tilled), the maximum
probability of infection ONSITE occurred at day 3; whereas for Practices II and III (which
do not require the 24-hour wait), the maximum probability of infection ONSITE occurred
59
-------
on day 2. For Practices IV and V, the maximum probability occurred on day 1; in these
practices, application and incorporation result in more extensive direct contact than in the
agricultural practices. Using default values for the main program variables, the maximum
daily probability of infection ONSITE was 0.0191 in Practice land 0.0027 for Practices II
and III. Using the same value for ASCRS > in Practices IV and V yielded maximum
probabilities of infection of 1.42xlO~3 and 8.67X10"4, respectively. However, the statutory
requirements for D&M sludge allow .a maximum of 1 parasite ovum or cyst per g volatile
sludge solids (U.S. EPA, 1989). Using the similar value of 1000/kg dry weight for ASCRS,
the maximum probability of infection would be 2.85x10"* for Practice IV and 1.73x10"" for
Practice V. On the basis of this result, a probability of infection of IxlO"4 will be used as
a benchmark level for acceptable risk in discussions below.
A preliminary sensitivity analysis of the model was carried out (U.S. EPA, 1990b)
to determine the relative sensitivity of model output to variations in input parameters. In
this analysis, the values .of selected parameters were varied singly, and the calculated
numbers of organisms in various compartments were compared. For many parameters,
there was no effect on the number of pathogens in the direct contact compartment or in
the onsite pond (the model was not sensitive to these parameters). The model was shown
to be sensitive to variations in application rate and size of field, which are related to the
number of organisms applied; method of application, which determines the surface
availability of pathogens; and rate of inactivation of the organisms, which determines the
number of pathogens surviving. In the preliminary risk assessment for parasites, the
sensitivity of probability of infection to variations in input parameters was analyzed as
described in the document- Pathogen Risk Assessment for Land Application of Municipal
Sludge. Volume I: Methodology and Computer Model (U.S. EPA, ;1990b). In this,
methodology, the change in .the input variable of interest is divided by its baseline value
(dB/JJ), and the resulting change in the output of interest is divided by the baseline result
(dC/C). A ratio is then taken of the quotients, S=(dC/C)/(d5/6). For example, when the
value of Variable 1, ASCRS, was changed from 5000 to 200, the maximum probability of
infection ONSITE was reduced from 0.01907 to 0.00077. Thus d&/R was -4800/5000 and
dC/C was -0.0183/0.01907. The sensitivity coefficient S was -0.96/-0.96 = 1.00. In most
cases, the sensitivity coefficient was 0 for one or both relevant exposure compartments; that
60
-------
isy there -was no effect of changing the value of the variable. Table 6-1 summarizes the
results when there was a response to a change in the value of a variable' in Practice I, using
site-specific data for Site 1.
These results indicate-that at low exposure levels, the probability of infection varies
directly (S = 1) with the concentration of organisms in the sludge (ASCRS) and soil
(APRATE) and with amount of soil ingested or contacted directly (DRECTS). However,
further investigation of the relationship between APRATE and probability of infection
showed that as APRATE is changed farther from the default value, S diverges from 1. To
illustrate this divergence, values of S were calculated ' for ONSITE, EATER and
SWIMMER at various values of APRATE and plotted as a function of APRATE (Figure
6-1).
The value of S varied much more for SWIMMER than for ONSITE when the
exponential decay rates were varied. "This difference occurs because of the nature of the
Poisson distribution used to calculate risk of infection: as the average exposure increases,
the fraction of individuals exposed to more than, one infective dose increases, yielding a less
than proportional increase in infection rate. In addition, the highest ONSITE exposure
occurs at day 2 or 3, whereas the highest SWIMMER exposure occurs at day 70; therefore,
varying the die-off rate affects the SWIMMER exposure more than the ONSITE exposure.
Because the onsite-pond pathogen densities vary according to rainfall events, the relative
concentrations of the peaks in concentration change as die-off rate changes. Thus, in some
cases (but not all) changes in die-off rate change the day on which the maximum
probability of infection occurred. '
Overall, the sensitivity analyses of Practices II and III gave results similar > to those
for Practice I. Practices TV and V do not include an onsite pond, so-there are no
SWIMMER exposures in these compartments. As with the other practices,- there were only
small variations in S for ONSITE exposures as input variables were changed.
These results indicated that analysis of input variables other than SUSPND and
DECAY would not be productive. Therefore, in subsequent modeling runs, only SUSPND
and DECAY were changed.
Variables in Subroutine RAINS were also tested. Tn this test, a single rainfall event
was modeled for simplicity. The largest rainfall used for Site 1, 12.47cm total rainfall at
61
-------
TABLE 6-1
SENSITIVITY COEFFICIENTS OF SITE-SPECIFIC VARIABLES
Variable
Baseline
1 ASCRS
1 ASCRS
2 APRATE
45 SUSPND
R6 DRECTS
R6 DRECTS
DECAY
DECAY
Value
200
l.lxlO4
2.5X104
0.001
0.02
0.20
-0.0005
-0.0001
Probability of Infection
ONSITE SWIMMER
0.01907
0.00077
0.0416
0.0470
0.00384
0.0378
0.01819
0.01915
0.1388
0.0060
0.2802
0.3.45
0.1656
0.0868
0.1655
Sensitivity
ONSITE
1.0
0.978
0.976
1.0
0.98,
-0.0717
-0.0096
i
Coefficient
SWIMMER
0.997
0.849
0.941
-0.21
-0.330
-0.439
62
-------
O 8
El
SI
ง
J L
oq
co
o
g
n
o o d
6
AXIAU1SN3S
63
-------
a duration of 10 hr, was arbitrarily placed at Hour 240. Other variables for the base
rainfall were BTLAG = 0.32, CN =89, AMC = 2, STAD = 0.25,USLEK = 0.32,USLEL
= 2.54.USLES = 1.25,USLEC = 0.05 AND USLEP = 1.0. Because a number of the
variables (BTLAG, CN, USLEL and USLES) are interdependent, their minimum and
maximum values were combined into two model runs. Other parameters were varied
independently, as indicated in Table 6-2. None of the changes in parameters for
Subroutine RAINS had an effect on ONSITE exposure, and effects on the food consumer
(EATER) were minimal, except for changes in SUSPND [P(45)]. Effects on the
SWIMMER were variable and somewhat contradictory. Specifying a good soil and a 1%
slope (RAIN4L) slightly increased the EATER exposure and reduced transport to the
onsite pond. Conversely, increasing the erodibility of the soil by specifying; a poor soil and
increasing the slope to 7% (RAIN4H) slightly increased both the EATER and SWIMMER
exposures. Specifying a moist soil (RAIN6H) or poor ground cover, (RAIN 11H) markedly
increased the SWIMMER exposure. i
Both of these conditions would be expected to increase the amount of runoff water relative
to transported sediment. In contrast, increasing the resuspension factor SUSPND [P(45)],
which describes the transfer of pathogens from soil to runoff water, greatly decreased both
the EATER and the SWIMMER exposures. This result implies that sediment transport is
more important than runoff water as a transfer route to the pond. In summary, increasing
the amount of runoff water increased exposure, but increasing the fraction of soil pathogens
suspended in runoff water decreased exposure. The reason for this apparent inconsistency
is not clear.
6.2. EXPOSURE COMPARTMENTS
The results of model runs under the various conditions are summarized in Tables
6-3 and 6-4, which give the probability of infection under these conditions, at all six sites.
Default conditions are compared to changes in SUSPND [P(45)] and die-off rates (LOW
and HIGH). No SWIMMER exposures occurred in Practices IV and V because it is
assumed that there is no onsite pond collecting runoff from the site.
In these model runs, site-specific values were used for Subroutine RAINS (Section
5), and default, values were used for the main program variables, with the exception of
64
-------
TABLE 6-2
SENSITIVITY TO PARAMETERS OF SUBROUTINE RAINS
FILE
NAME
RAINBASE
RAIN4L
RAIN4H
RAIN6L
RAIN6H
RAIN7L
RAIN7H
RAIN8L
RAIN8H
RAINBASE
RAIN11H
RAIN12L
RAINBASE
RAINSPL
RAINSPH
.VARIABLE
NAME VALUE
BTLAG
CN
USLEL
USLES
BTLAG
CN
USLEL
USLES
AMC
AMC
STAD
STAD
USLEK
USLEK
USLEC
USLEC
USLEP
USLEP
SUSPND
SUSPND
1.2
40
2.54
0.12
0.2
90
4.76
0.7
1
3
0.2
0.5
0.1
0.5
0.05
0.5
0.25
1.0
0
0.1
PROBABILITY OF INFECTION
ONSITE EATER SWIMMER
. 0.019
0.019
/
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
0.019
5.73xlO'5
6.07xlO-5
5.84X10'3
,5.80xlO'5 .
5.66xlO'5
5.73xlO'5
5.73xlO'3
5.73xlO-5
5.73xlO'5
5.73xlO'5
5.72xlO'3
5.73xlO'5
5.73xlO'3
6.53xlO'5
L66xlO-5
0.274
0.190
0.288
0.202
0.312
0.275
0.273
0.271
0.278 .
0.274
0.319
0.271
. 0.274
0.287
0.176
65
-------
TABLE 6-3
PROBABILITY OF INFECTION, ONSITE
SITE PRACTICE
1 I
n
m
IV
V
2 I
II
in
IV
V
3 I
II
III
IV
V
4 I
n
in
IV
V
5 I
n
in
IV
' V
6 I "
n
in
IV
V
DEFAULT
1.907xl02
2.665xl03
2.665xl03
1.424xl03
8.667x10"-
1.907xl02 . ,,
2.665xl03
2.665xl03
1.424xl03
8.667x10"
1.907xl02
2.665xl03
2.665xl03
1.424xl03
8.667x10"
1.907xl02
2.616xl03
2.616xl03 [
1.424xl03
8.667x10"
1.907xl02
2.664xl03
2.664xl03 .
1.424xl03
8.667x10"
' 1.907xl02
2.665xl03
2.665xl03
1.424xl03
8.667x10"
'SUSPND
1.907x10*
2.665xl03
,2.665xl03, ,
l^^xlO3
8.667x10"
.. -L907X102
2.665xl03
2.665xl03
1.424xig3
8.667x10"
1.907xig2
2.665xl03
2.665xl03
* i , ".
1.424X1O3
8.667x10"
1.907xl02 ,
2.649xl03
2.649xl03
1.424xl03
8.667x10"
1.907xl02
2.662xl03
2.662xio3
, 1.424X103
. 8.667x10"
M.907xl02
2.665xl03
2.665xl03
1.424xl03
8.667x10"
DIE-OFF LOW
1.915xl02
2.671xl03
2.671xl03 -
6.170X103
6.190xl03
1.915xl02
2.671xl03
2.67 IxlO3
6.170xl03
'6.190xl03
1.915xl02
2.671xl03
2.671xl03
6.170xl03
6.190xl03
1.915xlO2
2.66 IxlO3
2.661xl03
6.170xl03
6.19Qxl03
1.915xlO2
2.671xl03
2.671xl03
6.170xl03
6.190xl03
1.915xl02
2.671xl03
2.671xl03
6.170xl03
6.190xl03
DIE-OFF HIGH
1.819xl02
2.592xlO3
2.592:xig3
5.934xig3
6.030xl03
1.819xl02
2.592xl03
2.592xl03
5.934xl03
6.030xl03
1.819xl02
2.592xl03
2.592xl03
5.934xl03
6.030xl03
1.819xl02
2.592xl03
2.592xl03
5.934xl03
6.030xl03
1.819xlO2
2.592xl03 '
2.592xl03
5.934xl03
6.030xl03
1.819xl02
2.592xl03
2.592xl03
5.934xl03
6.030:id03
66 ,
-------
: TABLE 6^4, , ,
MAXIMUM PROBABILITY 'OF INFECTION, SWIMMER
SITE PRACTICE
1 I
ป
II
III
2 I
II
III
3 I
II
III
4 I
II
III
5 " I '
II
III
6 I
II
III
DEFAULT
1.388x10'
9.013x10"
9.691x10"
2.669x10'
2.433xia2
6.240xia3
1.617x10'
i.o9ixia2
i.isoxia2
7.357xia2
3,383x10"
3.767X1Q4
3.358x10"
2.930xl05
2.932xia5
4.540x10"
2.800xia4
2.803x10"
SUSPND
1.656xiar
9.008xi(T4'
9.686X104
3.572x10''
5.250xlQ3
5.448xia3
1.981x10'
L090xlCr2 ;
1.179xia2
9.279xia2
3.380X104
1.425X104
5.386xia4
2.928xia3
2.931xia5
5.396xiaV
2.797xl(J5 '
2. 80 Ix 10s
DIE-OFF .LOW
1.655x10'
1.102xl03
1.183xl03
3.605x10'
8.397xl03
8.649X103
2.057xi01;-.
1.243'xi02
1.46ixl02
1.077x10''
6.206X104
6.785X104
9.810x10"
2.938xl05
2.940xl05
6.694x10"
4.154X104
4.157xl04
DIE-OFF HIGH '
1.093x10'
5.920xl04
6.247X104
1.416x10'
1.407xl03
1.500xl03
1.383x10'
7.902xl03
8.212xl03
2.413xl02
1.425x10"
1.438x10"
1.743xl05
2.896xl05
2.898xl05
1.497xl03
9.276xl03
9.284xl05
67
-------
FCROP [P(46)]=5xlO-6,TCROP [P(68)]=240 and THARV [P(69)]=300. More detailed
comparisons of the results are made below.
As a comparison of the effects of application practice on potential human exposures
to parasites in land-applied sludge, the results of model runs using site-specific data for Site
1 and practice-specific data for all five practices are presented in Table 6-5. This table
shows that ONSITE -exposures related to the grazing and feed crop applications are lower
than those for field crops, but exposures for residential applications are even lower. Runoff
and sediment transport are significantly lower in the grazing and feed crop applications,
resulting in a much lower exposure for the SWIMMER. The probability of infection of the
ป
food consumer (EATER) is higher for aboveground crops in the residential application
than in the application for commercial production of food crops for human consumption.
The effects of site-specific variables on exposure are demonstrated in Table 6-6,
which shows the maximum probability of infection at all sites, using practice-specific values
for Practice I. The largest effect was on the SWIMMER exposure because the variation
in timing of rainfalls had a major effect on the amount of surface runoff arid sediment
transport moving parasites into the onsite pond. Rainfall times also had an effect on
contamination of the crop surface, resulting in slight variations in both ONSITE and
EATER exposures. .
The effect of timing and amount of rainfall on surface runoff was studied further by
using the times and amounts of rainfall events at Site 6 (with infrequent but,intense
rainfall) along with other site-specific values for Site 1 (with more frequent and more
moderate rainfall), and vice versa. When this was done using Practice I, there were no
significant changes in maximum probability of infection in the ONSITE or EATER
compartments. For SWIMMER, site-specific variables were shown to be significant, but the
rainfall pattern was shown to be more significant than soil properties: the maximum
probability of infection to the SWIMMER at Site 1 changed from 0.14 to 0.008 when the
rainfall pattern of Site 6 was substituted, and the maximum probability at Site 6 changed
from 0.0005 to 0.22 when the rainfall pattern of Site 1 'was substituted.
6.2.1. ONSITE. The maximum probability of infection ONSITE was essentially the same
at all sites, because site-specific variables had little time to act on the number of organisms
in the first three days of the model run. The maximum probability of infection at each site,
68
-------
TABLE 6-5
MAXIMUM PROBABILITY OF INFECTION, SITE 1"
PRACTICE
1
2
3
4
5
ONSITE
1.907xl02
2.665xl03
2.665xl03
1.424xl03
8.667xl04
EATER
5.944xlOs
,
3.057xl03
SWIMMER
1.388x10'
9.013xl04
9.691xl04
.
.
'Site-specific values for Subroutine RAINS are described in Section 5; main program values
were default values, except FCROP [P(46)] = lxlO'6, TCROP [P(68)]=240, THARV
[P(69)]=300 '-
69
-------
': TABLE 6-6
! [
MAXIMUM PROBABILITY OF INFECTION, PRACTICE I
SITE ONSITE
1 . 1.907x10*
2 1.907x10*.
3 1.907xlO'2
4 1.888x10*
5 1.889xlO'2
6 1.907xlO'2
EATER
, 5.944xlO-5
,6.341xlQ-5
6.015x10-*
6.279xlO'5
6.282xlO-5
6.341X10'5
SWIMMER
1. 388x10'! -
2.669x10"'
1.6l7xlO-' .
7.357xia2
3.358X10-4
4.540xlO'4
"Site-specific variables are described in Section 5; main program variables were default
values, except FCROP [P(46)]=5xlO'6, TCROP [P(68)]=240, THARV [P(69)]=300.
70
-------
using default values for SUSPND and die-off rates and a parasite density of ASCRS=5000,
was: Practice I, 0.191; Practices II and III, 0.0027; Practice IV, 0.0014; and Practice V,
0.00087.
6.2.2. Food Consumer (EATER). The calculated probability of infection of the food
consumer (EATER) as a result of consuming the crop was < 10"16 in every model run in
which the default values for CROP [P(66)] (aboveground crop) and FCROP1 [(P46)] were
used. However, when the on-ground crop was modeled [P(66) = 0] and the crop was
harvested at the unrealistically early time of 300 hours, the probability of infection
immediately after harvesting was 0.068. When a,below-ground crop was modeled [P(66)
= -l],the probability of infection immediately after harvesting at 300 hours was 0.146. The
reduced probability of infection via aboveground crops reflects the assumption that the
exposure of aboveground crops to contaminated soil (by blown dust or splashing during a
rain) is much less than the exposure of on-ground or below-ground crops (by direct contact
with soil).
The amount of soil associated with the crop was significant in determining the
probability of infection. The default conditions include a value of IxlO'8 for FCROP1
[P(46)], the fraction of sludge remaining on the aboveground crop and consumed by the
food eater. Because of dilution of applied sludge by surface soil, that fraction represents
20 g of soil on the entire crop surface, or about 2x10^ g/crop unit (tomato). If FCROP1
is increased to represent 0.1 g soil remaining on each crop unit at harvesting, the
probability of infection per serving of the aboveground crop became 6.47xlO'5 if the crop
was harvested at 300 hours and 8.91xlO"6 if the crop was harvested at 150 days after
} . - .,.. flJIr ,..,. ,t ,,,.,........ ...^ .. .- . .,.-,">-
application of sludge. For subsequent model runs, P(46) was, increased to. 5x10'6,to ensure
that there would be a significant exposure in the EATER compartment. -
Harvesting of on-ground and below-ground crops after 3600 hours (150 days) was
also modeled. This time was chosen to represent a crop planted 30 days after sludge
application and harvested 120 days later. In this case, the below-ground crop gave a risk
of infection of 0.018/serving, compared to 0.0083/serving for the on-ground crop. These
results are summarized in Table 6-7.
Table 6-8 gives a comparison of infection probabilities for food consumers under
site-specific conditions for all six sites. The time specified for harvesting in this comparison
71
-------
TABLE 6-7 '
MAXIMUM PROBABILITY OF INFECTION
BY CONSUMPTION OF CONTAMINATED CROPS
Conditions
g soil/
crop unit
Harvest at
300 hours
Harvest at
150 days
Aboveground
Aboveground
On-ground
Below-ground
0.0002
0.1
0.2
0.2
-------
TABLE 6-8
MAXIMUM PROBABILITY OF INFECTION
OF FOOD CONSUMER (EATER)*
Practice
Site
Aboveground
Type of Crop
On-ground
Below-ground
I 1
2
3
4 .."
5
6
IV '-.. 1
2
3
4
. 5
6
5.94X10'5
6.34xlO'5
6.02xlO'5
6.28xlO'5
6.28xlO-5
6.34xlO'5
3.10xlO'3
3.20xlO'3
3.10X10'3
3.16xlO'3
1'l'OxlO*
-3:10x10*
6.80xlO'2
_:; , 7.24xlO'2
6.88xlO'2
/' ; '"''T.iTxia^;. ;";."
7.18xlO'2
- - -7.24x10*
3.06xlO'3
3.16xlO-3
3.06xlO'3
3.13xlO'3
3.06x10*
3.06xlQ-3
1.46x10-'
1.55x10''
1.48x10-'
1.54x10-'
1.54x10-'
1.55x10-'
1:64x10*
1.70xlO-3
1.64xlO-3
1.68xlO'3
1.64x10*
1.64x10* -:
* Site-specific values for Subroutine RAINS are given in Section 5; main program values
were default values, except FCROP1 [P(46)] =5x10 *, TCROP [P(68)]=240 THARV
[P(69)]=300.
73
-------
was 300 hours. Results are shown only for Practices I and IV because the calculated
probabilities of infection in Practices II, III and V were < 10"'6 at all sites. These results
show that site-specific variables had little effect on the calculated risk; the variation in
infection probability within each type of crop and each practice was less than 10%. The
probability of infection from consumption of aboveground crops was calculated to be
approximately 50-fold higher for home crops than for the agricultural application. In
contrast, despite the fact that the modeled application rate for Practice IV was 2.5 times
the rate for Practice I, the calculated probabilities of infection from consuming on-ground
crops were lower for home gardens by 2 to 2.5-fold and lower for consuming; below-ground
crops by approximately 100-fold. These results must reflect differences between the
practices in distribution of sludge pathogens above, on and below the surface.
Land access and crop use restrictions are applied on the basis of the extent of
pathogen reduction effected by the treatment process .(U.S. EPA, 1989). Class A treatment
is assumed to render sludge microbiologically harmless,, requiring no waiting period before
exposure is allowed. Class B and C treatments require a waiting period of 1.8 months after
application before aboveground and on-ground food crops can be grown, and a 60-month
waiting period before beloW'ground crops can be grown (18 months if there are no
parasites in the sludge). The waiting period for grazing on sludge-amended pasture is 1
month for Class B treatment and 2 months for Class C treatment. In the computer model,
the class of treatment is entered in response to a prompt. A waiting period based on the
land use practice is then assigned to the model run. Exposure (time of harvest) is not
allowed before the waiting period has elapsed. Thus the operator is forced to choose a
time of harvest [P(69)] that exceeds the waiting period. The effect of time of harvesting
on the probability of infection by ingestion of aboveground crops was assessed by varying
P(69) in a series of model runs, using the default application rate and 5000/kg as the
parasite density. The results are presented in Table 6-9. The values given for months 15
and 18 for the aboveground crop are extrapolations; the model gives a result of 0 because
at these times the concentration of parasites on the crop is calculated to be < IxlO'Vg,
which is assigned the value of 0. These results show that a waiting period may be
unnecessary for consumption of aboveground crops, and a waiting period of 18 months
should provide adequate protection (probability of infection < IxlOVserving) for ingestion
74
-------
TABLE 6-9 "
VARIATION OF EATER RISK
WITH TIME OF HARVESTING
10NTH
0
1
2
3
.6
9
12
15
18
ABOVEGROUND
7.439xlO'5
' . . 4.628X10'3
2.699xlO-5
1.632xlO-5
4.502xlOr6
L242xlO-6
3.195xlO'7
, <8xlO'8 , .
<2xlO;s ,
BELOW-GROUND
1.729x10'' ,
. . , ,1. 156x10.-', ,
. .. . 6.914X10"2 .....
: , 4.240X10'2 . ' ' '. .
1 188x10'
3.290xlO-3
8.477X10-
2.351xlO;4
; 6,247xlO'5
75
-------
of root crops.
6.2.3. SWIMMER. As indicated by trial runs with bacteria (U.S. EPA, 1990b), the most
significant source of exposure was the surface runoff pond. A peak in probability of
infection for the SWIMMER occurred after each rainfall, which transported significant
amounts of contaminated sediment into the onsite pond (Figure 6-2). The maximum
probability of infection for the SWIMMER in Practice I, 0.139, was observed to occur on
day 70, after the heavy rainfall beginning .at hour 1650. In Practices II and III the
maximum probability of infection for the SWIMMER occurred on day 66, at a level of
0.00090 in Practice II and 0.00097 in Practice HI. Practices II and III limit the amount of
surface runoff and sediment transport because of the extent of ground cover characterizing
these practices. Extensive mulching of soil not covered by crop plants in Practice I would
also be expected to reduce the' amount of transport to the onsite pond by a small amount.
The maximum probability, of infection for the SWIMMER in the onsite pond was
dependent on site-specific variables. The number of organisms transferred to the pond by
surface runoff and sediment transport depended on both the extent of rainfall and the
elapsed time during which the organisms died off in the surface soil before they were
transported. The maximum probability at'each site and for each practice, using default
values for SUSPND arid die-off rates, is given in Table 6-10.
The time course of infection probability for Practice I at each site is displayed
graphically in Figure 6-3. It is clear from these results and the rainfall data presented in
Section 5 that the maximum probability of infection as a result of exposure in the onsite
pond is closely related to the timing and total amount of rainfall and to the farming
practice.
76
-------
CD ฃ
'--- Ou
W fe
...j -W
PC. W
O
cc
UJ
S
o
o
o
o
8
o
T
8
o
CO
o
M
i
o
Q Q q Q
o o . o' o
CL,
O
*
S a
cu
UJ
y
1
n
jo
77
-------
TABLE 6-1.0
MAXIMUM PROBABILITY OF INFECTION
BY EXPOSURE TO RUNOFF WATER (SWIMMER)
Site
1.
2..
3.
4.
5.
6.
Anderson Co.,TN
Chaves Co.,NM :
Clinton Co.,IA
Highlands Co.,FL
Kern Co., CA
Yakima Co., WA
I
'K39xlO-'-
2.67x10-'
, 1.62x10-';,,,
7.36xlO-2 ,; -,-,.
3.36x10^
4.54xiO'4
Practice
II
9.01xlO'4
... 2.43X10'2
1.09X10'2
. 3.83X10'2
2.93xlO-5
2.80xlO'4
III .
9.69X10"1
6.24xlO'3 "'
LlSxlO'2
3.77xlO-2
2.93xiO'5
2.80xlO'4
78
-------
n GO
ป n
LU
b
X
M
4-
o o o o
n
N011D3JN1 JO
79
-------
7. CONCLUSIONS
Although detailed data on survival and transport of parasites in soil are lacking, the
model appears to confirm the general observations in the literature that parasites are
persistent, justifying land-use restrictions. Model runs implied that restrictions on the
consumption of below-ground crops may be overly conservative. Reports of offsite infection
by parasites in sludge-amended soil are rare; model runs confirm the low probability of
offsite infection except by uncontrolled surface runoff.
7.1. SENSITIVITY ANALYSIS
Model runs showed that within narrow limits, the probability of infection -by parasites
as a result of exposure to soil contaminated with sewage sludge is proportional to the
concentration of organisms in the sludge, the amount of sludge applied and the amount of
contaminated soil to which the individual is exposed, either by casual contact or ingestion
of food grown in the contaminated soil. Direct proportionality of response to exposure
level is not maintained over a wide range of concentrations, however, because the
probability of infection is calculated by a Poisson distribution, which is an exponential
function of exposure rather than a proportional one. Variations in the probability of
infection can be extrapolated from variations in parasite concentrations up to a probability
of about 0.1; the departure from linearity is 5% at a probability of 0.093 and 10% at a
probability of 0.176. Many of the parameters of the model seemed to have little bearing
on the probability of infection, apparently because they ultimately had no. effect on the
number of organisms to which the human receptor was exposed in each exposure
compartment, or because they exerted their effect on survival or transport after the
maximum probability of infection had occurred.
The probability of infection was sensitive to the rate of inactivation or die-off of the
parasite ova, cysts and oocysts and to the method of application. Subsurface application
resulted in no exposure to any individual, because the ova, cysts and oocysts are modeled
as being unable to move significant distances in soil. Future revisions of the model should
probably allow for redistribution of pathogens from subsurface soil into surface soil (see
Section 8.2).
80
-------
7.2. ONSITE EXPOSURES
, . Significant onsite .exposures were calculated in all practices when 5000/kg dry weight
:was used as the parasite concentration in the sludge. The greatest risk, approximately: 0.02
per day,,was associated with Practice I, application of sludge for production of commercial
crops. These calculations imply that the field worker, who inadvertently ingests soil during
daily activity on the sludge application site is at significant risk of infection by parasites.
The results imply that there should be a waiting period before routine daily activity en.the
site. The length of the waiting period should depend on the initial application rate and
pathogen concentration as well as the die-off rate of the parasite; in the model runs
reported here, the probability of infection ONSITE in Practice I was >0.01 for more than
30 days and > 10'7 for about 27, months. , ;:.:,. ;
The maximum risks of infection calculated- in domestic applications were lower
(0.0014 for Practice IV and 0.0009 for .Practice V), but -not low enough to be protective.
However, it must be noted again that a concentration of 5000 parasites/kg was used in the
model runs, whereas U.S. EPA regulations for D&M sludge call for a limit of 1 parasite/g
volatile sludge solids (U.S. EPA, 1989), or approximately 1000/kg dry weight. It is assumed J
that- incorporation in these practices is done by hand or with power tools rather than -by
farm machinery; therefore, the calculated exposures/ include direct exposure -to .'non-
incorporated sludge. During incorporation, the parasite concentration is rapidly reduced
by dilution in soil, and by the end of the second day, the extrapolated probabilityvof
infection is below IxlO'7 in both practices. In summary, it appears that * a probability of
infection greater; than the arbitrary benchmark value of 1x10'4 is likely during application
and incorporation of D&M sludge. A person engaged in these activities could .probably
reduce the risk by wearing a protective mask and washing thoroughly before handling food.
7.3. SEDIMENT TRANSPORT AND SURFACE RUNOFF
The most significant potential source of infection was exposure to runoff water and
transported sediment after rainfall. Rainfall events were modeled as being able-to
transport contaminated.-. soil from the field to the onsite pond, where the suspended or
particle-bound parasites accumulated. A swimmer in the pond was therefore exposed to
the parasites, either by ingestion of ova, cysts or oocysts or by infective larvae penetrating
81
-------
the skin. Model runs indicated that it would be prudent to limit access to runoff water and
sediment from a sludge-amended field, either by mulching to reduce runoff, ditching and/or
diking to contain the runoff or restricting access to any onsite ponds receiving runoff.
7.4. OFFSITE EXPOSURES
No health hazard was indicated as a result of offsite transport of parasites by droplet
aerosols or by wind-blown dust. This may be a model limitation caused by over-
simplification of the Gaussian-plume aerosol transport subroutine in the model, or it may
reflect a very low concentration or probability of transport of viable parasites in aerosols.
Because it was assumed that transport of ova, cysts and oocysts to groundwater does not
occur, no risk from consumption ! of groundwater from a drinking water well was
demonstrated.
7.5. WAITING PERIOD
Practice-specific waiting periods are required by the U.S. EPA Pathogen Reduction
Requirements (U.S. EPA, 1989)-before access to sludge-amended land or consumption of
crops grown thereon. For exposure comparisons, a probability of infection of IxlO'4 was
tentatively chosen as a benchmark for sufficient protection of human health (Section 6.1).
Using this benchmark value, the default values for application rate and die-off rate, and a
parasite density of5000/kg, the initial maximum probability of infection for aboveground
crops was
-------
currently true, but also on sludge application rate and pathogen concentration. In
calculating a safe waiting period, conservative assumptions should be made about amounts
of soil ingested with crops.
83
-------
8. RESEARCH NEEDS
8.1. INFORMATION NEEDS FOR PARASITES
As indicated in Tables 3-3 and 4-1, data on the concentration of parasite ova, cysts
and oocysts in sewage sludge after treatment and on survival of the organisms in sludge-
amended soil are sparse. A review of published literature reveals that data are available
for only a few indicator organisms.
Distribution of pathogens in soil or groundwater is poorly understood. This model
assumes random distribution of pathogens in environmental media, a commonly-made
assumption that is probably frequently violated. Another obvious data gap, illustrated by
this methodology and model development, is-the degree of survival and transport of
pathogens in the environment. Information on the fate of parasites in groundwater and
subsurface soil is extremely limited. The concentration and survival rates of pathogens
leaching through soil into groundwater are unavailable for protozoa and helminths (U.S.
EPA, 1986). In addition, the survival of these organisms in soil depends strongly on prior
conditions of treatment. For example, mild heating or digestion alone may have little effect
on viability of Ascaris ova, whereas mild heating of digested sludge rapidly destroys the ova
(Pike et al., 1988).
Characterizing the average or typical concentration of viable ova in treated sludge
may not be of great significance because the probability of infection is directly related to
the concentration of parasites in the particular sample or batch of treated sludge. Each
community's treatment system and source material will be different in some details from
the ideal or average system characterized in research studies. The treated sludge from
individual treatment systems can be characterized more accurately by din;ct counts than by
assumptions that they conform to a typical concentration distribution. However, it is
difficult to characterize the survival of parasites in soil in each individual case, and a well-
characterized range of survival under defined conditions would be useful.
Reimers et al. (1986) concluded that, since Ascaris and Toxocara are typically
present in most municipal sludges applied to land, the following information is needed to
clarify associated health risks: (1) survival of resistant stages of parasites (ova and cysts)
in soil following land application; (2) effects of sewage-treatment method, application
84
-------
method, soil type, climate, and land use on survival; and (3) degree of risk to humans and
animals based on field situations.
Reimers et al. (1990) recommend further work in. the areas of (1) determining the
stabilization conditions needed to produce sludges meeting PFRP criteria; (2) investigating
the feasibility of using combined treatment processes, such as digestion followed by lagoon
and/or drying bed storage, to inactivate pathogens in sludges processed at small Publicly
Owned Treatment Works; (3) determining whether petroleum hydrocarbons would
inactivate pathogens in petroleum-contaminated sludges; and (4) selecting appropriate
controls for studies of Ascaris egg survival in different types of sludges.
Further research on pathogen exposure pathways and infectious doses will facilitate
the predictive accuracy of this model and its successors. 'Especially useful will be:
Data on the relation of die-off rate of different parasites to moisture,
temperature, method of application and previous sludge treatment. No single
organism is really adequate to represent all other parasites. Ascaris can serve
as a "worst case" in most instances, but, for example, some protozoa are much
smaller and thus are more likely to be transported through soil and in
aerosols. Data suggest that moisture and temperature would probably have
significant effect on parasite survival, but the one conclusion reached
following such a field study is that there is no statistically significant
correlation between parasite egg concentration and environmental parameters
(Leftwich et al., 1988b).
More detailed quantitative information on any protection provided by crop
cover. It has been shown that subsurface injection of sludge increases the
survival of parasite ova and cysts, but quantitative data on relative effects of
grass, hay, truck crops or forests on parasite survival are lacking.
Data allowing a mathematical description of die-off as a function of
environmental conditions. Some authors indicate that environmental
conditions play an essential role in determining the die-off rate of parasites,
whereas others (Grenfell et al., 1986; Young, 1983) see no statistically
significant effects. It is likely that a variety of confounding variables have
obscured the effects of easily-measured environmental parameters on die-
85
-------
off rates. With sufficient study, the effects of environmental conditions on
survival may become more obvious.
In summary, the following research priorities are recommended to allow
development of a definitive risk assessment for parasites inland-applied sludge:
ESI Helminths:
Standard quantitative methods for examining helminths in sludge and soil
samples and in liquid droplet and dry particulate aerosols;
Data on transport in water, soil and aerosols;
Die-off rates in water, soil and aerosols;
The relationship of those decay rates to environmental conditions, previous
sludge treatment, method of sludge application and various effects of crop
cover.
gfir Protozoa:
Standard quantitative methods for examining protozoa in sludge and soil
samples and in liquid droplet and dry particulate aerosols; and
. . Quantitative data on occurrence and survivability of protozoa in treated
sludge.
If results indicate that protozoa survive in sludge, the following additional research needs
become a priority:
Data on transport in water, soil and aerosols;
Die-off rates in water, soil and aerosols;
The .relationship of those decay rates to environmental conditions, previous
sludge treatment, method of sludge application and various effects of crop
cover.
8.2. MODEL DEVELOPMENT
The current version of the Sewage Sludge Pathogen Risk Assessment model is not
adequate to address some of the properties of parasite survival in soil. In particular, it has
been noted that cycles of freezing and thawing may kill parasite ova more rapidly than
exposure to a constant temperature at either extreme ofthe cycle (Leftwich et al., 1988a).
86
-------
The model calculates a daily average temperature for each day of the model run, but it
does not address the diurnal temperature cycle which can result in subfreezing temperatures
at night on days with an average temperature above freezing. It should be possible to
program a diurnal cycle into the temperature calculation subroutine and flag for a freeze-
thaw cycle. The flag can in turn result in decreasing the population of viable parasites by
a fraction specified by default or by the user as one of the operating parameters.
Mathematical descriptions of the die-off of parasite ova and cysts as a function of
temperature and moisture are not yet adequate to allow construction of algorithms in the
model for die-off rates. However, it appears that a simple relationship between
temperature and die-off rate of ova may not occur (Ferris et al., 1978; Grenfell et al., 1986;
Young, 1983), and the equation used to calculate die-off rate will probably have to be of
a different form than that currently used for bacteria and viruses.
In the present version, the model does not allow for direct contact with sludge
during incorporation in Practices I, II and HI. Instead, it is assumed that all exposures
during incorporation occur via inhalation of dry aerosols generated by the machinery used
to incorporate the sludge. It might be more reasonable to include inadvertent ingestion of
unincorporated sludge in these practices, as occurs in Practices IV and V.
The model assumes that tilling will not disturb the soil below 15 cm, so parasites
injected into subsurface soil are not transferred to surface soil. However, plowing is deeper
than tilling and would be expected to redistribute subsurface soil. Some transfer factor
could be added to the model to allow for redistribution of pathogens from subsurface to
surface soil when the field is plowed.
As described in Section 3 above, there are significant differences in size and
probably in survival between helminths and protozoa, and it might be prudent to include
the option to model transport of protozoa in groundwater. Future revision of the Pathogen
Risk Assessment Model might include providing separate options for helminths and
protozoa, with separate sets of default values, or a flag to enable or disable Subroutine
GRDWTR, the groundwater transport subroutine.
The present version of the Pathogen Risk Assessment Model does not include an
onsite pond in Practice V, the home lawn application. For a better description of sludge
use on public parks and golf courses, which are more 'likely to have ponds, it might be
87
-------
beneficial to add the option for existence of a pond onsite. For even more flexibility, the
user could be asked to specify which exposure calculations to print in the output table.
The limits of Subroutine RAINS should be further characterizcjd to establish
operating boundaries for input variables. These boundaries could be set by having the
program return a warning message and possibly revise the input data to a value that would
not cause the program to crash.
Limitations in offsite transport subroutines may limit accuracy of the model, but
constraints on the size of a model able to run on a personal computer make it unlikely that
more sophisticated routines can be added. For example, sophisticated air dispersion models
are large and complex and could probably not be added to the current model. Adding the
capacity to model more than one windstorm (the current model limit) would probably be
feasible without making the model too unwieldy. It is likely that further analysis of the
model using other pathogens will reveal additional areas in which the model can be
improved.
-------
9. REFERENCES
Altaif, K.I. and A.Y. Yakoob. 1987. Development and survival of Haemorichus contortus
larvae on pasture in Iraq. Trap. Anim. Hlth. Prod. 19: 88-92.
Arther, R.G.,P.R. Fitzgerald and J.C. Fox. 1981. Parasite ova in anaerobically digested
sludge. J. Water Poll. Contr. Fed. 53(8): 1334-1338.
Beaver, P. and G. Deschamps. 1949. The viability of Entamoeba histolytica in soil Am
J. Trop. Med. Hyg. 29: 189-191. (Cited in U.S. EPA, 1988) , :
Beaver, P.C.,R.C. Jung and E.W. Cupp; 1984. Clinical Parasitology, 9th ed. Lea and
Febiger, Philadelphia, p. 512-522.
Binder, S.,D. Sokal and D. Maughan. 1986. Estimating soil ingestion: The use of tracer
elements in estimating the amount of soil ingested by young children. Arch Environ
Health 41: 341-345.
Black, M.I.,P.V. Scarpino, C.J. O'Donnell, K.B. Meyer, J.V. Jones and E.S. Kaneshiro.
1982. Survival rates of parasite eggs in sludge during aerobic and anaerobic digestion
Appl. Environ. Microbiol. 44(5): 1138-1143.
Block, J.C.,Havelaar, A.H. and L'Hermite, P. 1986. Epidemiological Studies of Risk
Associated with the Agricultural Use of Sewage Sludge: Knowledge and Needs. Elsevier
Applied Science Publishers, London, p. 154-165. (Cited in Pike et al., 1988)
Brudastov, A.N., L.N. Krasnonos, V.R. Lemeler and Sh. Kr. Kholmukhamedor. 1970.
Invasiveness of Ascaris lumbricoides eggs for man and guinea pigs after a 10-year stay in
the soil. Med. Parazitol. Parazit. Bolezni 39(4): 447-451. (Cited in Jakubowski, 1988)
Bryan, F.L. 1977.- Diseases transmitted by foods contaminated by wastewater J Food
Protection 40(1): 45-56. (Cited in Kowal, 1985)
Burden, D.J. and N.C.Hammet. 1979. The development and survival of Trichuris suis ova
on pasture plots in the south of England. Res. Vet. Sci. 26: 66-70.
Burden, D.J.,Whitehead, A., Green, E.A.,McFadzean, J.A. and Beer, RJS 1976 J Hve
77:377. (Cited in Burden and Hammet, 1979)
Burger, H.-J. 1984. Survival of Taenia saginata eggs in sewage and on pasture.' In:
Processing and Use of Sewage Sludge. Proc. of 3rd Int. Symp., September 27-30 1983 P
L'Hermite and H. Ott, Ed. Boston, D. Reidel Publishing Company, p. 191-201.
Calabrese, EJ. 1988. Improving the risk assessment process. In: The 1988 Washington
Conference on Risk Assessment, September 22-23, 1988, Alexandria, VA. Sponsored by
the Center for Energy and Environmental Management.
89
-------
Carroll, J.J. and D.R. Viglierchio. 1981. On the transport of nematodes by the wind., J.
NematoL 13(4): 476-483. ,
Clark, C.S.,C.C.Linnemann, G.L. Van Meer, G.M. Schiff and P.S.Gartside. 1981, Healtit}
Risks of Human Exposure to Wastewater. U.S. Environmental Protection, Agency,
Cincinnati, OH. EPA-600/1-81-002. NTIS PB81-143406. (Cited in Kowal and Pahren,
1982)
Craft, J.C. 1982. Experimental infection with Giardia lambJia in rats. J. Infect. Dis. 145:
495-498. (Cited in Jakubowski, 1990)
Crawford, F.G. and S.H. Vermund. 1988. Human cryptosporidiosis,^ CRC Grit. Rev.
Microbiol. 16(2): 113-159. '
*
Current, W.L. 1987. Cryptosporidium: Its biology and potential for environmental
transmission. Grit. Rev. Environ. Control 17: 21.
Daly, JJ. 1983a. Parasitic Helminths of Humans. In: Microbiology: Basic Principles and
Clinical Applications, N.R. Rose and A.L. Barron, Ed. New York, Maemillan. p. 487-
509.
Daly, JJ. 1983b. Parasitic Protozoa of Humans. In: Microbiology: Basic Principles and
Clinical Applications, N.R. Rose and A.L. Barron, Ed. New York, Macmillan. p. 467-
486.
Dorn, C.R.,Reddy, C.S.,D.N. Lamphere, J.V. Gaeuman and R. Lanese. 1985. Municipal
sewage sludge application on Ohio farms: Health effects. Environ. Res. 38: 332-359.
Payer, R. and B.L.P.Ungar. 1986. Cryptosporidium spp. and cryptosporidiosis. Microbiol.
Rev. 50(4): 458-483. . .
Feachem, R.G., DJ. Bradley, H.. Garelick and D.D. Mara. 1983. Ascaris and ascariasis.
In: Sanitation and Disease, Health Aspects of Excreta and Wastewater Miinagement. John
Wiley & Sons and The World Bank, Washington, D.C. (Cited in Jakubowski, 1988)
Ferris, H.,H.S.DuVernay and R.H. Small. 1978. Development of a soil-temperature data
base on Meloidogyne arenaria fora simulation model. J. NematoL 10(1): 39-42.
Fertig, D.L.,and C.R. Dorn. 1985. T_aenia saginata cysticercosis in an Ohio cattle feeding
operation. Department of Veterinary Preventive Medicine, Ohio State University,
Columbus, OH.
Frenkel, J.K.,A. Ruiz and M. Chinchilla. 1975. Soil survival of Toxoplasma oocysts in
Kansas and Costa Rica. Amer. J. Trop. Med. Hyg. 24(3): 439-443.
90
-------
Geldreich, E.E. and R.H. Bordner. 1971. Fecal contamination of fruits and vegetables
during cultivation and processing for market: A review. J. Milk Food Tech. 34: 184-195.
(Cited in Kowal, 1985)
Gerba, C.P.,G. Wallis, and J.L. Melnick. 1975. Fate of wastewater bacteria and viruses
in soil. Proc; ASCE, J. Irrig. Drain. Div. 101: 157-174.
Grenfell, B.T.,G. Smith and R.M.Anderson. 1986. Maximum-likelihood estimates of the
mortality and migration rates of the infective larvae of Ostertagia ostertagi and Cooperia
oncophora. Parasitology 92: 643-652.
Hayes, E.B., T.D. Matte, T.R. O'Brien, et al. 1989. Large community outbreak of
cryptosporidiosis due to contamination of a filtered public water supply. New Engl. J. Med.
320(21): 1372-1376.
Hershey, D.W. and J.A. McGregor. 1987. Low prevalence of Toxoplasma infection in a
Rocky Mountain prenatal population. Obstet. Gynec. 70(6): 900-902.
Jakubowski, W. 1988. Ascaris Ova Survival in Land Application Conditions.
Administrator's Item, Deliverable #2799. Toxicology and Microbiology Division, Health
Effects Research Laboratory, U.S. EPA, Cincinnati, OH.
Jakubowski, W. 1990. The control of Giardia in water supplies. In: Giardiasis, E.A.
Meyer, ed. Elsevier Science, New York. p. 335-353.
Jakubowski, W., J.L. Sykora, C.A. Sorber, L.W. Casson and P.D. Gavaghan. 1990.
Determining Giardiasis prevalence by examination of sewage. IAWPRC Health-Related
Water Microbiology Symposium, Tuebingen, Germany, 1-6 April 1990. (Cited in Sykora
et al., 1990)
Jarroll, E.L., J.C. Hoff and E.A. Meyer. 1984. Resistance of cysts to disinfection agents.
In: Giardia and Giardiasis, S.L. Erlandsen and E.A. Meyer, ed. Plenum, New York p
311-328. (Cited in Jakubowski, 1990)
Kayed, D. and J.B. Rose. 1987. Development of a method for the detection of
Crvptosporidium in sewage sludge. Abstr. Ann. Meet. Amer. Soc. Microbiol. 87: 296.
Kowal, N.E. 1982. Health Effects of Land Treatment: Microbiological. Health Effects
Research Laboratory, U.S. EPA, Cincinnati, OH. EPA-600/1-82-007.
Kowal, N.E. 1985. Health effects of land application of municipal sludge. Health Effects
Research Laboratory, Office of Research and Development, U.S. EPA, Research Triangle
Park, NC. EPA/600/1-85/015.
Kowal, N.E. and H.R. Pahren. 1982. Health effects associated with wastewater treament
and disposal. J. Water Poll. Contr. Fed. 54(6):677-687.
91
-------
Krecek, R.C. and K.D. Murrell. 1988. Observations on the ability of larval Ostertagia
ostertagi to migrate through pasture soil. Proc. Helminthol. Soc. Wash. 55(1): 24-27.
Lawande, R.V., S.N.Abraham, I.John and L.J. Egler. 1979. Recovery of soil amebas from
the nasal passages of children during the dusty harmattan period in Zaria. Am. J, Clin.
Pathol. 71:201-203. . . ' ' ' '
Leftwich, D.B., R.S. Reimers and AJ. Englande. 1981. Inactivation of parasite-
contaminated domestic wastewater sludges. In: Chemistry in Water Reuse, Vol. 2. Edited
by WJ. Cooper. Ann Arbor, MI: Ann Arbor Science, p. 613-634.
Leftwich, D.B.,D.B. George and C.A. Hetzel. 1988a. Effects of repeated freezing and
thawing on Ascaris eggs. In: W. Jakubowski, Ascaris ova survival in land application
conditions. Administrator's .Item, Deliverable #2799. Toxicology and Microbiology
Division, Health Effects Research Laboratory, U.S. EPA, Cincinnat, OH. ,
Leftwich, D.B.,D.B. George, R.S. Reimers, M.D. Little and N.A.Klein. 1988b. Afield
investigation of Ascaris ova survival in domestic, sludge applied to land. In: W. Jakubowski,
Asearis ova survival in land application conditions. Administrator's Item, Deliverable
#2799. Toxicology and Microbiology Division, Health Effects Research Laboratory, U,S.
EPA, Cincinnat, OH.
Little, M.D,,R. Badeaux, R.S. Reimers and D.B. Leftwich. 1988. Sensitivity of a procedure
for the parasitologic examination of soil. In: W. Jakubowski, Ascaris ova survival in land
application conditions. Administrator's Item, Deliverable #2799. Toxicology and
Microbiology Division, Health Effects Research Laboratory, U.S. EPA, Cincinnat, OH.
Madore, M.S.,J.B. Rose, C.P. Gerba, M.J. Arrowood and C.R. Sterling. 1987. Occurrence
of Cryptosporidium oocysts in sewage effluents, and selected surface waters. J. Parasit.
73(4): 702-705.
Mbela, K.K..D.B. McDonell, D.B. Leftwich, R.S. Reimers, M.D.;Little and T.G. Akers.
1990. Evaluation of temperature effects on inactivation of Ascaris eggs in both aerobic and
anaerobic digestion processes. (Submitted to J. Water Poll. Contr. Fed.)
McHarry, M.J. 1984. Detection of Giardia is sewage effluent. J. Protozool. 31:362-364.
(Cited in Jakubowski, 1990)
Meyer, E.A., and S. Radulescu. 1979. Giardia and Giardiasis. In: Advances in
Parasitology, Vol. 17. W.H.R. Lumsden, R. Muller and J.R. Baker, eds. Academic Press,
New York. p. 1-47.
Mitchell, R. 1972. Water Pollution Microbiology. Wiley-Interscience, New York. p. 207-
241. (Cited in U.S. EPA, 1988) .
Naylor, L.M., and R.C. Loehr. 1982. Priority pollutants in municipal sewage sludge.
Biocycle 23(4): 18-22.
92 .
-------
NOAA (National Oceanic and Atmospheric Administration). 1981. Local Climatological
Data: Annual Summaries for 1981. Environmental Data and Information Service, National
Climatic Center, Asheville, NC.
O'Donnell, C.J.,K.B. Meyer, J.V. Jones, T.Benton, EiS. Kaneshiro, J.S.Nichols and F.W.
Schaefer III. 1984. Survival of parasite eggs upon storage in sludge. Appl. Environ
Microbiol. 48(3): 618-625.
Oganov, L.I..N.P. Aleksandrova and E.A. Baturina. 1975. Time of development and
longevity of Ascarid eggs in the soil. Med. Parazitol Parazit. Bolezn 44(1)- 104-105
(Cited in Jakubowski, 1988)
Pedersen, D.C. 1981. Density levels of pathogenic organisms in municipal wastewater
sludge: A literature review. Prepared by Camp Dresser & McKee Inc., Boston, MA for
the Municipal Environmental Research Laboratory, Office of Research and Development
U.S. EPA, Cincinnati, OH. EPA-600/2-81-170.
Pike, E.BV E.G. Carrington and S.A. Harman. 1988. Destruction of Salmonellas,
enteroviruses and ova of parasites in wastewater sludge by pasteurisation and anaerobic
digestion. Wat. Sci. Tech. 20(11/12): 337-343.
Reimers, R.S., M.D. Little, A.J. Englande, D.B. Leftwich, D.D. Bowman and R.F.
Wilkinson. 1981. Parasites in Southern Sludges and Disinfection by Standard Sludge
Treatment. Prepared by the School of Public Health and Tropical Medicine, Tulane
University, New Orleans, LA in cooperation with the Health Effects Research Laboratory,
U.S. EPA, Cincinnati, OH for the Municipal Environmental Research Laboratory, Office
of Research and Development, U.S. EPA, Cincinnati, OH. EPA 600/2-81-166.
Reimers, R.S., M.D. Little, A.J. Englande/Jr., D.B. McDonell, D.D. Bowma and J.M.
Hughes. 1986. Investigation of Parasites in Sludges and Disinfection Techniques. Prepared
by the School of Public Health and Tropical Medicine, Tulane University, New Orleans, LA
in cooperation with the Municipal Environmental Research Laboratory, U S EPA
Cincinnati, OH for the Health Effects Research Laboratory, Office of Research and
Development, U.S. EPA, Cincinnati, OH. EPA 600/1-85/022.
Reimers, R.S., M.D. Little and T.G. Akers'. 1987. Effectiveness of different municipal
sludge treatment process for attaining 'PFRP' status. Sessions A5 the 60th Annual
Conference, Water Pollution Control Federation, Philadelphia, PA, p. 10, October 1987
(Cited in Mbela, 1990)
Reimers, R.S.,M.D. Little, T.G. Akers, W.D. Henriques, R.C. Badeaux, D.B. McDonnell,
and K.K. Mbela. 1990. Persistence of Pathogens in Lagoon-stored Sludge. Project
Summary. Risk Reduction Engineering Laboratory, U.S. EPA, Cincinnati, OH.
i A/oOO/o2-89-015.
93
-------
Reyes, W.L.,C.W. Kruse and M.C. Batson. 1963. The effect of aerobic and anaerobic
digestion of eggs of Ascaris lumbricoides var suum in night soil. Am. J. Trop. Med. Hyg.
12:46-55. (Cited in Mbela et al., 1990) x :,
Rivera, F.,G,Roy-Ocotla, I. Rosas, E. Ramirez, P, Bonilla and F. Lares. 1987. Amoebae
isolated from the atmosphere of Mexico City and environs. Environ. Res. 42: 149-154.
Rose, J.B. 1988. Occurrence and significance of Cryptosporidium in water. J. Am. Water
Works Assoc. 80(2): 53-58. .
Rudolfs, W.,L.L.Falk and R.A. Ragotzkie. 1951. Contamination of vegetables grown in
polluted soil. H. Field and laboratory studies on Endamoeba cysts. Sewage Ind. Wastes
23:478-485. (Cited in Kowal, 1985) ,
. - . '
' Schwartzbrod, L, J.L. Stien, K. Bouhoum and B. Baleux. 1989. Impact of wastewater
treatment on helminth eggs. Wat. Sci. Tech. 21(3):295-297.
Seattle Metro (Municipality of Metropolitan Seattle). 1983. Health effects of sludge land
application: A risk assessment. Municipality of Metropolitan Seattle Water Quality
Division.
Shuval, H.I. and B. Fattal. 1980. Epidemiological study of wastewater irrigation in
kibbutzim in Israel. In: Wastewater Aerosols and Disease. Proceedings of a Symposium,
September 19-21, 1979, H. Pahren and W. Jakubowski, Eds. Sponsored by the Health
Effects Research Laboratory, Office of Research and Development, Cmcinnat, OH. EPA-
600/9-80-028. p. 228-238.
Sorber, C.A. and B.E. Moore. 1986. Survival and transport of pathogens in sludge-
amended soil: A critical literature review. Prepared by the University of Texas at Austin
for the Water Engineering Research Laboratory, Office of Research and Development, U.S.
EPA, Cincinnati,. OH.
Storey, G.W. 1987. Survival of tapeworm eggs, free and in proglottids, during simulated
sewage treatment processes. Water Res. 21(2): 199-203.
Storey, G.W. and R.A. Phillips.. 1985. The survival of parasite eggs througout the soil
profile. Parasitology 91(3): 585-590.
Sykora, J.L., C.A. Sorber, W. Jakubowski, L.W.Casson, P.D. Gavaghan, M.A.Shapiro and
MJ.Schott. 1990. Distribution of Giardia Cysts in Wastewater. U.S: EPA, Environmental
Monitoring Systems Laboratory, Cincinnati, OH. (Submitted)
Tzipori, S. 1983. Cryptosporidiosis in animals and humans. Microbiol. Rev. 47: 84-96.
94
-------
U.S. EPA. 1980. Sewage.Sludge Pathogen Transport Model Project. Prepared under IAG-
78-D-X0116 by The BDM Corporation, Albuquerque, NM in cooperation with Sandia
National Laboratories, Albuquerque, NM and U.S. Department of Energy, Advanced
Nuclear Systems and Projects Division, Washington, D.C. Health Effects Research
Laboratory, Office of Research and Development, Cincinnati, OH. EPA 60071-81 -049a.
NTIS PB82-109000.
U.S. EPA. 1983. Section 11.12,Fugitive Dust Sources. An AP-42 Update of Open Source
Fugitive Dust Emissions. EPA 450/4-83-010. Office of Air Quality Planning and'Standards,
Research Triangle Park, NC.
U.S. EPA. 1985. Pathogen Risk Assessment Feasibility Study. Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH
EPA/600/6-88/003. NTIS PB88-191440.
U.S. EPA. 1986. Development of a Qualitative Pathogen Risk Assessment Methodology
for Municipal Sludge Landfilling. Office of Health and Environmental Assessment,
Environmental Criteria arid Assessment Office, Cincinnati, OH. EPA/600/6-88/006. NTIS
PB88-198544.
U.S. EPA. 1989. Standards for the Disposal of Sewage Sludge: Proposed Rule. Federal
Register 54(23): 5886-5887,
U.S. EPA. 1990a. Corrective Action for Solid Waste Management Units (SWMUs) at
Hazardous Waste Management Facilities: Proposed Rule. Federal Register 55(123): 30798-
30884.
U.S. EPA. 1990b. Pathogen Risk Assessment for Land Application for Municipal Sludge.
Volume I: Methodology and Computer Model. Volume II: User's Manual. Prepared by
Science Applications International Corporation, Oak Ridge, TN for the Environmental
Criteria and Assessment Office, Office of Health and Environmental Assessment,
Cincinnati, OH. EPA/600/6-90/002a,b. NTIS PB90-171901/AS, PB90-171919/AS.
USD A (U.S. Department of Agriculture), Soil Conservation Service. 1980. Soil Survey of
Chaves County, New Mexico, Northern Part.
USDA (U.S. Department of Agriculture), Soil Conservation Service. 198la. Soil Survey
of Anderson County, Tennessee.
USDA (U.S. Department of Agriculture), Soil Conservation Service. 1981b. Soil Survey
of Clinton County, Iowa.
USDA (U.S. Department of Agriculture), Soil Conservation Service. 1981c. Soil Survey
of Kern County, California, Southeastern Part.
95
-------
USDA (U.S. Department of Agriculture), Soil Conservation Service. 1985. Soil Survey of
Yakima County, Washington.
USDA (U.S. Department of Agriculture), Soil Conservation Service. 1989. Soil Survey of
Highlands County, Florida. .
Wallis, P.M.,D.L. Lehmann; D.A. Macmillan, and J.M. Buchanan-Mappin. 1984. Sludge
application to land compared with a pasture and a hayfield: Reduction of biological health
hazard over time. J. Environ. Qual. 13(4):645-650.
Ward, R.I., G.A. McFeters and J.G. Yeager. 1984. Pathogens in sludge: Occurrence,
inactivation, and potential for regrowth. U.S.D.E. Contract DE-AC04-76DP00789. Sandia
National Labs, Albuquerque, NM. (Cited in U.S. EPA, 1988) ;
WHO (World Health Organization). 1981. The Risk to Health of Microbes in Sewage
Sludge Applied to Land. WHO Regional Office for Europe, Copenhagen. 27 p.
Wilson, M., C.T. Hadden, E.L. Burress, J.M. Webb, R.J.F. Bruins, N.E. Kowal and C.
Sonich-Mullin. 1989. Microbiological risk assessment for sludge reuse and disposal options.
Joint American Water Works Association-Water Pollution Control Federation Conference,
San Diego, CA, August 1989.
Yanko, W.A. 1988. Occurrence of pathogens in distribution and marketing municipal
sludges. Health Effects Research Laboratory, U.S. EPA, Research Triiingle Park, NC.
EPA/600/1-87/014:
Young, R.R. 1983. Populations of free-living stages of Ostertagia ostertagi and O.
circumcincta in a winter rainfall region. Aust. J. Agric. Res. 34: 569-58J..
96
-------
APPENDIX ,
MODEL OVERVIEW
Five sludge management practices, representing land application, and D&M
management options, are included in the present model and are numbered I-V. They are
listed in Table A-l and illustrated in Figures A-1 through A-5. Two of the practices use
heat-dried or composted sludge for residential purposes and three use liquid sludge for
commercial farming operations. Since each of these two types of sludge represents a wide
range of sludge treatment possibilities, the extent of treatment or conditioning prior to land
application must be approximated for each case (i.e., the pathogen concentration in the
applied sludge must be specified). The computer model represents the compartments and
transfers among compartments of the five management practices. The compartments are
the various locations, states or activities in which sludge or sludge-associated pathogens
exist; they vary to some extent among practices. - In each compartment, pathogens either
increase, decrease or remain the same in number with time, as specified by "process
functions" (growth, dieoff or no population changes) and "transfer functions" (movement
between compartments). The population in each compartment, therefore, generally varies
with time and is determined by a combination of initial, pathogen input, "transfer functions"
and "process functions." The populations of pathogens in the cornpartments representing
human exposure locations (designated with an asterisk in Figures A-l through A-5 and in
Table A-2), together with appropriate intake and infective dose data, are used to estimate
human health risk.
Although each practice listed in Table A-l is different, all five practices share
common characteristics. All compartments that appear in one or more of the five sludge
management practices are listed in Table A-2. Those compartments with an asterisk
represent exposure sites for the human receptor:
3* inhalation or ingestion of emissions from application of sludge or
tilling of sludge/soil;
5* inhalation or ingestion of windblown or mechanically generated
particulates; .
6* swimming in a pond fed by surface water runoff;
.A-l
-------
TABLEAU
COMPARTMENTS INCLUDED IN. THE SLUDGE MANAGEMENT PRACTICES
Compartment
Name and Number
-
Application
Incorporation
Application/Tilling
Emissions
Soil Surface
Participates
Surface Runoff
Direct Contact
Subsurface Soil
Groundwater
Irrigation Water
Soil Surface Water
Offsite Well .
Aerosols
Crop Surface
Harvesting
(Commercial) Crop
Animal Consumption
Meat
Manure
Milk
Hide
Udder
Liquid Sludge
Management Practices
;,,!., j
1
2
3* ' TV..;;-
4 "
5* '
6* - -.
' .7* ... '...I '.'"
8 \.
9 >.-.v.-"ป.
10 ,.. ,
' .iju.^, '.:,:._.
ii*":',;"~--'
"134T" /"'
14
15
16* < --.
*
II
1
2
3*
4 .
5*
6*
..7* . ' '
8
9
10
11
12*
13*
14
17
18*
19
20*
21
22
III
V": ' '
2 ' :"
3*
4
5*
6*
7*
8
9
10
11
12*
13*
14
15
17
18*
19
20*
21
22
Dried/Composted
Sludge Management
Practices
IV V
1 1
3* 3*
4 ' 4
.-5* 5*
. 7* . -.. .7*
8 8
11 ' . 11 .
14 * 14
15
16* ,
'''Indicates exposure compartment
A-2
-------
Application/Tilling
Emissions
External
Sourca
FIGURE A-1
Input/Output Diagram for Practice I - Application of Liquid Sludge
for Production of Commercial Crops for Human Consumption
-------
Application
Application/Tilling
Emissions
Soil
Surface
Crop
Surface
Animal
Consumption
Manure
Irrigation
Watar
External
Source
FIGURE A-2
Input/Output Diagram for Practice II - Application of Liquid
Sludge to Grazed Pastures
A-4
-------
Subsurface
Soil
Groundwater
Offslta
Well
Application
Partlculates
Soil
Surface
Crop
Surface
ir 15
Harvesting
Animal
Consumption
Manure
Irrigation
Water
External
Source
Application/Tilling
Emissions
FIGURE A-3
Input/Output Diagram for Practice III - Application of Liquid Sludge
for Production of Crops Processed before Animal Consumption
A-5
-------
Subsurfac*
Soil
3*
Application/Tilling
Emissions
FIGURE A-4
Input/Output Diagram for Practice IV- Application of Dried
or Composted Sludge to Residential Vegetable Gardens
A-6
-------
Subsurface
Soil
Application/Tilling
Emissions
FIGURE A-5
Input/Output Diagram for Practice V - Application of Dried
or Composted Sludge to Residential LaWris
A-7
-------
TABLE A-2
SLUDGE MANAGEMENT PRACTICES AND DESCRIPTIONS ;IN
LAND APPLICATION MODEL
PRACTICE
DESCRIPTION
Application of Liquid Treated Sludge for .Production of Commercial
Crops for Human Consumption
H Application of Liquid Treated Sludge to Grazed Pastures
III Application of Liquid Treated Sludge for Production of Crops
Processed before Animal Consumption
IV Application of Dried or Composted Sludge to Residential Vegetable
Gardens
V Application of Dried or Composted Sludge to Residential Lawns ,
Two types of sludge are used in this model. - liquid and dried/composted. The extent of
treatment or conditioning prior to application is variable and must be determined for each
case.
A-8
-------
7* direct contact with sludge-contaminated soil or crops (including
grass, vegetables, or forage crops);
12* drinking water from an offsite well;
13* inhalation and subsequent ingestion of aerosols from irrigation;
16* consumption of vegetables grown in sludge-amended soil;
18* consumption of meat or
20* consumption of milk from cattle grazing on or consuming forage from
sludge-amended fields. " ,
The first 14 compartments, * most of which are common to all practices,; are described
below. .-'.- .: ;.
APPLICATION (1) represents the application of sludge to a field (default size 10
ha) or to a yard or garden of specified size. Liquid sludge may be applied by spread-flow
techniques, by spray, or by subsurface injection. The application rate and pathogen
concentrations are variables to be entered by the user of the model. During spread-flow
and spray application, sludge will be spread thinly on the soil, where it will be subject to
drying, heating and solar radiation, thus losing the protective benefits provided by bulk
sludge. It is assumed, therefore, that inactivation will occur at a rate characteristic of the
organism in soil at 5ฐC above the ambient temperature (Brady, 1974; USD A, 1975). It is
also assumed that liquid sludge is absorbed by the upper 5 cm of soil surface during this
time. The default time period for transfer from APPLICATION (1) to
INCORPORATION (2) is 24 hours, which allows a field treated with liquid sludge to dry
sufficiently to plow or cultivate. If the injection option is chosen, the liquid sludge goes
directly to SUBSURFACE SOIL (8) at hour 10. During spray application of liquid sludge
or application of dry composted sludge, droplets or loose particulates may become
airborne. Liquid aerosols are modeled by a Gaussian-plume air dispersion model that
calculates the downwind concentration of airborne particulates.. Dry particulate emissions
are calculated using models for generation of dust by tilling or mechanical disturbance of
soil. Both are represented as transfers from APPLICATION (1) to
APPLICATION/TILLING EMISSIONS (3).
INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
and sludge-associated pathogens evenly throughout the upper 15 cm of soil. Process
A-9
-------
functions associated with this compartment are the same as for the relevant pathogen type
in soil. Particulate emissions generated by cultivation are represented by a. transfer from
INCORPORATION (2) to APPLICATION/TILLING EMISSIONS (3) beginning at hour
24, extending for enough time to cultivate the field (at' a rate of 5 ha/hr) or till the garden
or lawn (at a rate of 0.002 ha/hr). At the end of this time, all remaining pathogens are
transferred to SOIL SURFACE (4).
APPLICATION/TILLING EMISSIONS (3*) is an exposure compartment that
receives the dust, or suspended particulates, generated by application or by the tilling of
dried sludge or sludge-soil mixture. It also receives aerosols generated by spray application
ป -
of liquid sludge. All process functions associated with this compartment are incorporated
in the aerosol subroutines. Exposure in this compartment is by inhalation but, as in all
inhalation exposures, model simplification limits the exposure to the pathogens assumed to
be ingested after the inhaled dust or aerosol spray is trapped in the upper respiratory tract,
swept back to the mouth by ciliary action and swallowed.
SOIL SURFACE (4) describes the processes occurring in the upper 15 cm (Practices
I, IV and V) or upper 5 cm (Practices II and III) of the soil layer. Microbes are
inactivated at rates characteristic for moist ' soil at 5ฐC above the chosen ambient
temperature (Crane and Moore, 1986; Kibbey et al., 1978). Transfers from SOIL
SURFACE (4) occur by wind to WIND-GENERATED PARTICULATES (5), at a time
chosen by the user, by surface runoff and sediment transport after rainfall events to
SURFACE RUNOFF (6), by a person walking through the field or contacting soiled
implements or clothing or by other casual contact to DIRECT CONTACT (7), by leaching
after irrigation or rainfall to SUBSURFACE SOIL (8), by resuspensipn during irrigation
or rainfall to SOIL SURFACE WATER (11), or at harvest to CROP SURFACE (14).
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind. Process functions are the same as for the organism in air-dried soil at
the ambient temperature (Crane and Moore, 1986; Kibbey et al., 1978). The exposed
individual is standing in the field or at a user-specified distance downwind from the field
during a windstorm. The wind-generated exposure is calculated from user-specified values
for duration and severity of the windstorm (default values, 6 hr at 18 m/sec (40 mph)).
SURFACE RUNOFF (6*) is an exposure compartment describing an onsite pond
A-10
-------
containing pathogens transferred from SOIL SURFACE (4)'by surface runoff arid sediment
transport after rainfall. These processes are described by a separate subroutine.
Inactivation rates in this compartment are characteristic of microbes in water and are much
lower than rates for soil. Water is removed from the pond by infiltration and, recharge of
the groundwater aquifer, but it is assumed that no microbes are transferred by this process.
The human receptor is an individual who incidentally ingests 0.1 L of contaminated water
while swimming in the pond. This compartment is also an exposure compartment for cattle
drinking 20 L of water daily from the pond (Practice II).
DIRECT CONTACT (7*) is the exposure compartment for a worker or a child less
than 5 years old who plays in or walks through the field, yard or garden, incidentally
! - ' : . *
ingesting O.lgof soil or vegetation at the daily geometric mean concentration of pathogens.
This human receptor represents the worst-case example of an individual contacting
contaminated soil or soiled clothing or implements. No process functions are associated
with this compartment because it is strictly an exposure compartment.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 5 or 15.cm depth and the water table. It also serves as the
incorporation site for subsurface injection of liquid sludge. Process functions in
SUBSURFACE SOIL (8) are the same as for moist soil at the ambient temperature. The
transfer from SOIL SURFACE (4) occurs after each rain or irrigation event as a result of
leaching from the soil surface. The time of transfer is calculated by dividing the depth of
rainfall or irrigation' by the infiltration rate. Transfer to GROUNDWATER (9) is
arbitrarily set at one hour later. At present, the relation between unsaturated water flow
and subsurface transport has not- been Well-established. Thus, this model lacks a
satisfactory subroutine to describe pathogen transport from the subsurface soil to
groundwater. Instead, user-specified variables are used to describe the fraction of
pathogens transferred from SOIL SURFACE (4) to SUBSURFACE SOIL (8) and from
SUBSURFACE SOIL to GROUNDWATER.
GROUNDWATER (9) describes the ' flow of pathogens in the saturated zone.
Process functions are the sarhe as for other water compartments. Transfers occur to
IRRIGATION WATER (10) if the water is needed for irrigation or to OFFSITE WELL
(12*) if the water is used for drinking. The number of pathogens transferred to
A-11
-------
IRRIGATION WATER (10) is based on the concentration of pathogens in the
groundwater compartment and the total depth of irrigation. The transfer to OFFSITE
WELL (12) is described by a modification of the subsurface solute transport model of van
Genuchten- and Alves (van Genuchten and Alves, 1982). Because microbes in suspension
are passively transported by bulk water flow and interact with soil particles by adsorption
and desorption, they behave similarly enough to dissolved chemicals that existing solute
transport models can be used to describe their fate in the saturated zone (Gerba, 1988).
IRRIGATION WATER (10) describes the transfers for pathogen-contaminated
water used for irrigation. No processes are associated with this compartment because it is
intended as a transition compartment. Irrigation of the field, lawn or garden takes place
a user-specified number of times each week. This irrigation water may come from either
an onsite well fed by GROUNDWATER (9) or from an outside source of treated, liquid
sludge. The default conditions vary by practice. In either case, AEROSOLS (13) are
'
generated unless a non-spray option is chosen. Spray irrigation is the default since it would
be most likely to cause a significant exposure to workers or offsite persons,, In addition to
aerosol emissions, irrigation transfers pathogens to CROP SURFACE (14) and to SOIL
SURFACE WATER (11).
SOIL SURFACE WATER (11) represents any irrigation water or rainfall in contact
with the ground prior to infiltration. This compartment describes the temporary suspension
of pathogens in such a water layer and their subsequent transfer to CROP SURFACE (14)
or to SOIL SURFACE (4). Process functions are the same as for other water
compartments. - :
OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day
of contaminated water whose pathogens have been transported through groundwater.
Process functions are the same as for groundwater. The groundwater transport subroutine
supplies the concentration of pathogens in the well at a user-specified distance from the
source. No transfers out of the compartment 'are specified because it is an exposure
compartment only.
AEROSOLS (13*) describes fugitive emissions from spray irrigation, which occurs
at a default rate of 0.5 cm/hr for 5 hr. The source of irrigation water producing
AEROSOLS can be an onsite well-(i.e., GROUND WATER) or liquid sludge. A Gaussian-
A-12
-------
plume model is used to,calculate concentrations of airborne microbes downwind. The
human reqeptor is an onsite worker or a person offsite who is exposed during the time of
irrigation. .- ,
..CROP SURFACE (14) describes. contamination of vegetable or forage crops by
transfer of user-specified amounts to or from SOIL SURFACE (4), from IRRIGATION
WATER (10), or to or from SOIL SURFACE .WATER (11). Process functions are not
well characterized but are assumed to be influenced by drying, thermal inactivation and
solar radiation; they are thus most characteristic of pathogens in surface soil.
*
These preceding 14 compartments are common to most of the, five practices
modeled. The following descriptions of the five management practices help clarify the
differences among the practices. ,
Practice I: Application of Liquid Treated Sludge for Production of Commercial Crops for
Human Consumption. .
Liquid sludge may be applied as fertilizer/soil conditioner for the production of
agricultural crops for human consumption or for animal forage or prepared feed. Both
existing (40 CFR 257.3-6) and proposed (U.S. EPA, 1989) regulations prohibit direct
application of sewage sludge to crop surfaces. Therefore, this .model practice is designed
for a single application of liquid sludge, which is incorporated into the soil before the crop
is planted. Regulations also require various waiting periods before the planting of crops
that will be consumed uncooked by humans. These restrictions, however, are optional in
the model and can be tested. .....-.,. , .
Vegetables can be grown aboveground, on-ground or below-ground. These are
represented by tomatoes, zucchini and carrots, respectively. At HARVESTING (15) time,
all pathogens remaining on CROP SURFACE (14) are transferred to HARVESTING (15),
which represents a .single harvest of all of the crop. The same process functions apply as
in CROP SURFACE (14). The crop is held for 24 hours before being processed. The
number of pathogens is then transferred to COMMERCIAL CROPS (16*), the
compartment in which further processing takes place. The number of pathogens/crop unit
following processing is calculated in this compartment and is the figure used in the
A-13
-------
vegetable-exposure risk calculations. A 24-hour pathogen exposure is computed by
Subroutine VEG. Pathogen concentrations are determined as number/crop unit for each
sludge management practice. Before being consumed, vegetables normally are processed
in some way. Included in the program is a series of user-selectable processing steps. The
user has the option of choosing any or all processing steps and of specifying some
conditions within processing steps. The human receptor is a person who consumes
minimally prepared vegetables (washed, but not peeled or cooked) at a rate of 81 g
tomatoes, 80 g zucchini or 43 g carrots per eating occasion (Pao et al., 1982).
# . , "- , '-
Practice II: Application of Liquid Sludge to Grazed Pastures.
* . . . . ,
In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops for pasture. This model practice is designed for
repeated applications of liquid sludge, initially on a field with a standing forage crop used
for pasture. It is assumed that spray irrigation will be used because this method is effective
for delivering large amounts of sludge to a large area. In this way, the pasture is also used
as a final treatment and disposal system for the treated sludge. The irrigation rate, the
total weekly depth and the number of times per week can be specified by the user. A
sludge solids concentration of 5% is assumed.
The model assumes that each hectare of pasture supports 12 head of cattle, although
both area and herd size may be varied. This may be a higher density than is the common
practice for fields that receive no irrigation, but with adequate irrigation, sufficient forage
is expected to be produced. Current and proposed regulations require various waiting
periods before animals can be grazed. These requirements can also be tested by the
model.
ANIMAL CONSUMPTION' (17) describes the ingestion of CROP SURFACE (14)
by cattle grazing in the pasture. Transfers from ANIMAL CONSUMPTION (17) are to
MEAT (18*), MANURE (19) and MILK (20*).
MEAT (18*) is the compartment describing transfer of pathogens from ANIMAL
CONSUMPTION (17) to meat. The human receptor is assumed to consume 0.256kg of
meat daily (U.S. FDA, 1978). Contamination of meat by gut contents during slaughter or
by systemic infection by sludge-borne pathogens can be modeled. The model allows for
A-14
-------
inactivation of pathogens in meat by cooking, assuming reasonable cooking times and
temperatures.
The production and consumption of milk from cattle pastured on the sludge-
amended field are modeled when the dairy cattle option is chosen. The default condition
is for consumption of raw milk because commercial production' of milk poses an extremely
small hazard of exposure to pathogens. In the model, contamination from dirty utensils
and careless handling are combined as a transfer from the manure-contaminated udder
[MANURE (19)], which occurs at each milking. All three pathogens can enter milk by this
route. MILK (20*) is the compartment describing production and consumption of milk
from cattle pastured on the sludge-amended field when the dairy cattle option .is" chosen.
The default condition models the consumption . of raw milk that has been stored for 24
hours. In exposure calculations, it is assumed that the human receptor consumes 2 kg
milk/day, roughly three times the national average milk consumption (U.S. FDA, 1978).
Practice III: Application of Liquid Treated Sludge for Production of Crops Processed
before Animal Consumption. ,
In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops to be processed and stored for animal feed. This
model practice is designed for repeated applications of liquid sludge, initially on a field
with a standing forage crop. It is assumed that spray irrigation will be used for the
application of liquid sludge, because this method is effective for delivering large amounts
of sludge to a large area. In this way, the field is also used as a final treatment and
disposal system for the treated sludge. The rate, the total weekly depth and the number
of irrigations per week can be changed by the user. A sludge solids concentration of 5%
is assumed. The risks to the human receptor are similar to those for the preceding
practice, i.e.,exposure through meat or milk, in addition to direct contact with the forage
grown in the field.
Practice IV: Application of Dried or Composted Sludge to Residential Vegetable Gardens.
Dried or composted treated sludge may be sold or given away to the public as a
bulk or bagged product for use as fertilizer or soil, conditioner for the production of
A-15
-------
domestic garden crops for human consumption. Although some studies have shown that
composting is highly effective in removing pathogens from sludge (Wiley and Westerberg,
1969), other studies have shown that bacterial pathogens may grow in dried or composted
sludge to concentrations of Ixltf organisms/kg dry weight (U.S. EPA, 1988). Exposure of
individuals to materials used in home gardening would be expected to be more frequent
than exposure in a commercial agricultural setting, Therefore, this practice would be
expected to pose a greater risk of infection. This model practice is designed tp describe
the application of dried or composted treated sludge, which is incorporated into the soil
before the crops are planted.
Vegetables can be grown aboveground, on-ground or below-ground. These are
represented by tomatoes, zucchini and carrots, respectively. The user may specify the
proportions of above-ground, on-ground and below-ground crops in the garden. At
HARVESTMG (15) time, all pathogens remaining on CROP SURFACE (14) are
transferred to HARVESTING (15). The same process functions apply as in CROP
SURFACE (14). The crop is held for 24 hours before being processed. The number of
pathogens is then transferred to CROP (16*), the compartment in which further processing
takes place. The number of pathogens/crop unit following processing is calculated in this
compartment and is the figure used in the vegetable-exposure risk calculations. A 24-hour
pathogen exposure is computed by Subroutine VEG. Pathogen concentrations are
determined as number/crop unit for each sludge management practice. Pathogen
concentrations are determined as number/crop unit.
Before being consumed, vegetables normally are processed in some way. Included
in the program is a series of user-selectable processing steps. The user has the option of
choosing any or all processing steps and of specifying some conditions within processing
steps. In the default condition, the human receptor is a person who consumes minimally
prepared vegetables (washed, but not peeled or cooked) at a rate of 81 g tomatoes, 80 g
zucchini or 43 g carrots per eating occasion (Pao et al., 1982).
^
Practice V: Application of Dried or Composted Sludge to Residential Lawns.
Dried or composted treated sludge may be made available to the public as a bulk
or bagged product to be sold or given away for use as fertilizer or soil conditioner for the
A-16
-------
preparation 'of a seed bed for domestic lawns. Individuals engaged in preparing a seed bed
for a lawn are likely to come into contact with the soil and any additives used to improve
the seed bed. If the soil or the additives contain pathogens, this practice would be
expected to pose a risk of infection. This model practice is-designed to describe the
application of dried or composted treated sludge, which is incorporated into the soil before
the lawn is seeded.
The main exposure in this practice is for the lawn worker or for a child younger
than 5 years old who plays in or walks through the lawn site, incidentally ingesting soil or
crop surface at the daily geometric mean concentration of pathogens. This human receptor
represents the worst-case example of an individual contacting contaminated soil or soiled
clothing or implements. Before all pathogens have been transferred to SOIL SURFACE
(4), exposure is at the pathogen concentration found in undiluted sludge whereas, after the
transfer, the concentration is that calculated for the soil-sludge mixture.
After 840 hours, the time assumed necessary for the. lawn to require mowing, the
lawn is mowed weekly, and a fraction of the pathogens associated with CROP SURFACE
(14) are transferred to DIRECT CONTACT (7). It is assumed that the person mowing the
lawn is exposed by inhalation and/or, ingestion at each mowing.
A-17
-------
REFERENCES
Brady, N.C. 1974. The Nature and Properties of Soil. Macmillan Publishing Co., Inc.,
New York. p. 266-276. ' ';
CFR (Code of Federal Regulations). 1988. Disease, 40 CFR 257.3-6. In: U.S. EPA,
Criteria for classification of solid waste disposal facilities and practices, 4(3 CFR 257.3.
Crane, S.R. and J.A. Moore. 1986. Modeling enteric bacterial die-off: A review. Water
Air Soil Pollut. 27: 411-439.
Gerba, C.P. 1988. Alternative Procedures for Predicting Viral and Bacterial Transport
from the Subsurface into Groundwater. Draft. U.S. Environmental Protection Agency,
Cincinnati, OH.
Kibbey, HJ.,C. Hagedorn and E.L. McCoy. 1978. Use of fecal streptococci as indicators
of pollution in soil. Appl. Environ. Microbiol. 35:711-717.
Pao, E.M.,Fleming, K.H.,Guenther, P.M. and Mickle, SJ. 1982. Foods commonly eaten
by individuals: Amount per day and per eating occasion. U.S. Department of Agriculture,
Economics Report No. 44.
U.S. EPA. 1983. Section 11.2, Fugitive Dust Sources. An AP-42 Update of Open Source
Fugitive Dust Emissions. EPA 450/4-83-010. Office of Air Quality Planning and
Standards, Research Triangle Park, NC.
U.S. EPA. 1988. Occurrence of Pathogens in Distribution and Marketing Municipal
Sludges. Prepared by County Sanitation Districts of Los Angeles County for the Health
Effects Research Laboratory, Office of Research and Development, Research Triangle
Park, NC. EPA/600/1-87/014. NTIS PB88 154273.
U.S. EPA. 1989. Standards for the Disposal of Sewage Sludge; Proposed Rule. Federal
Register 54(23): 5886-5887. ,
U.S. FDA (U.S. Food and Drug Administration). 1978. FY78 Total Diet Studies.
van Genuchten, M.T. and W.J. Alves. 1982. Analytical solutions on the one-dimensional
convective-dispersive solute transport equation. USDA Technical Bulletin No.l66i.
Wiley, B.B. and S.C. Westerberg. 1969. Survival of human pathogens, in composted
sewage. Appl. Microbiol. 18: 994-1001. .
ซU.S. GOVERNMENT PRINTING OFFICE: 19 91 . S18 . 1.8 7,2 0 5 8 1
A-18
-------
-------
3> O
-r 3
m
o>
c ex rs 5" ฐ- 3?
EOJ o T S Q)
O C 3" O O)
2. 3" ex 01 3- O
5.0ฐ ฃ03
ar o o o S 2T
s|s งi:
o-'i 5 m - =
n 3 =r '53
| ป0 i-S
ฎ O CD (!>(/)
si
5"
(0 O
> rn i
QET
o ฐ
5'3
oo
-o w,
S ^c
?ปm^=|
> -n
m
m
C/5
m
en
------- |