United States
Environmental Protection
Agency
Water Engineering Research
• Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/037 June 1985
Project Summary
Risk Assessment of Wastewater
Disinfection
David Hubly, Willard Chappell, John Lanning, Martin Maltempo, Daniel Chiras,
and John Morris
A risk assessment data base is pre-
sented for several wastewater disinfec-
tion alternatives, including chlorination,
ozonation, chlorination/dechlorination,
and ultraviolet radiation. The data base
covers hazards and consequences of
onsite use and transportation of the
disinfectants and ultimate disposal of
disinfected effluents. A major segment
of the data base deals with the effects of
chlorination products in aquatic eco-
systems. Energy consumption and cost
analyses are also presented for chlori-
nation and ozonation. Example risk
calculations are presented for two hypo-
thetical wastewater treatment plants.
The usefulness of the data base for risk
assessment is also discussed.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory. Cincinnati. OH, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Risks in today's world have been the
center of growing attention. Increased
risk awareness in modern technological
societies is an outgrowth of technologic
development and economic achievement.
Technologies with inherent risks (air,
water, and soil pollution, for example)
cannot continue to develop without con-
sidering the net impact of those inherent
risks on humans, other living organisms,
and the environment. Effective risk man-
agement is based on a qualitative and
quantitative understanding of the risks
associated with public policy decisions.
Risk assessment, the primary focus of
this study, provides that understanding
and is the first step in risk management.
The assessment in this study focused
on chlorination and those disinfection
processes that appeared most likely to
replace chlorination. The alternatives
selected for the risk assessment were as
follows:
1. Chlorination,
2. Chlorination followed by dechlori-
nation,
3. Ozonation,
4. Ultraviolet radiation, and
5. No disinfection.
The specific products expected from
this risk assessment are designed for
local public policy-setting applications.
The two primary products are:
• The collection and evaluation of a data
base, and
• The development of a method for using
that data base in a wastewater disin-
fection risk assessment.
Methodology
For each disinfection alternative, the
study first focused on identifying hazards
associated with its onsite use, its trans-
portation to the site, and the residuals left
in the wastewater after its use. Data were
also collected that might be used to
estimate the probability of each hazard
occurring. Then the consequences as-
sociated with each hazard were identified,
frequently in the form of a dose-response
model. Probability-estimating data were
also collected for the hazard-consequence
relationship. All of this work relied solely
on published literature and personal
communications with selected users and
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Table 1. Example Comparisons of Energy \
Requirements for Alternative
Disinfectants
experts. The literature dealing with the
aquatic toxicology of chlorine and chlori-
nation products proved to be massive and
is the dominant feature of the collected
data base.
Discussion of Results
Though wastewater treatment plants
have a poor overall safety record, with an
accident rate similar to that of metal
mining, exposure to toxic substances
(primarily chlorine) accounts for only 4%
of these accidents. However, the possibil-
ity of a low-probability, high-consequence
event (i.e., massive exposure) cannot be
discounted and is impossible to quantify
without a more detailed analysis (i.e., a
fault tree).
Most chlorine is shipped by rail (85%).
The bulk of the rest is shipped by tank
truck (9.9%) or by common carrier in
cylinders (114 kg or 0.9 metric ton).
Though railroads generally have a much
better safety record than truck shipments
(particularly in 114-kg cylinders), a Feb-
ruary 1978 accident in Youngstown,
Florida, illustrates the possibility of low-
frequency, high-consequence events for
railroad shipment. This case involved 8
fatalities, 260 injuries, more than $1
million in property damage, and a release
of 45,400 kg of chlorine gas.
A review of the literature on the aquatic
toxicology of chlorine indicates that for all
chlorination residuals (e.g., chloroform)
except total residual chlorine (TRC), the
observed effluent levels are below those
known to be acutely toxic to aquatic
organisms. Thus only TRC will lead to an
acute response, and then only if the
dilution in the receiving stream is inade-
quate to lower the resulting level suffic-
iently. The acute response can range
from avoidance to death of aquatic organ-
isms. Further studies are needed to
determine whether TRC and various
byproducts could lead to chronic effects
in aquatic organisms at the levels en-
countered.
The compounds found in effluents are
well below the acute toxicity levels for
human ingestion, and human risks'
through body contact with water con-
taining these compounds were not found
in the literature. In general, the contribu-
tion that wastewater disinfection makes
to finished drinking water will be much
smaller than the contribution made by
drinking water treatment. Though chloro-
form and trichloroethylene are carcino-
gens and (assuming no threshold) will
therefore contribute some additional risk
of cancer to humans, this contribution is
relatively small (less than one excess
case of cancer for every 50,000 persons
exposed to 5 yug/L of chloroform over a
lifetime). This figure assumes no dilution
of the effluent or subsequent loss. Since
drinking water chlorination is the major
cause of human exposure to byproducts
of chlorination, the risks associated with
wastewater disinfection are not expected
to be an important consideration.
Though it is possible to identify hazards
associated with the disinfection alter-
natives of chlorination-dechlorination,
ozonation, and UV irradiation, the lack of
data made quantitation impossible. Ac-
cidental releases of SOs (used in dechlo-
rination) can pose hazards to humans and
terrestrial and aquatic organisms. Ozone
poses a risk to workers in the plant and to
vegetation in the vicinity of the facility.
Though Europe has had considerable
experience with ozone disinfection of
drinking water, no data on human risks
are readily available. Although ozone is
toxic to aquatic life, its lack of stability in
water makes that risk minimal. Even less
information is available on the risk of
ultraviolet disinfection. The primary haz-
ards are from human exposure to the
radiation itself (burns), exposure to ozone
produced by the radiation, and exposure
to high electrical voltages (a hazard also
present with ozonation).
The risks of not disinfecting wastewater
were also analyzed. No discussion existed
in the scientific literature on the effects of
pathogens on aquatic organisms. The
human hazards, of course, are related to
exposure to pathogenic organisms, and
the most common consequence would be
gastrointestinal illness, although there is
also some risk of exposure to life-threat-
ening or disabling organisms as well.
Recent work relating disease incidence to
disinfection measures in body contact
marine waters is reviewed and applied in
the risk assessment examples.
The energy requirements for chlorina-
tion, ultraviolet irradiation, and ozonation
were compared using data reported in the
literature. An example calculation for a
3790-mVday plant and a set of given
assumptions appear in Table 1. The
energy use of chlorination is somewhat
understated because offsite energy use is
not included, but the chlorination process
has an obvious advantage over ozonation
in this example.
The costs associated with chlorination
and ozonation were also compared, and
the results of those analyses are shown
in Tables 2 and 3. The capital costs are
about twice as great for ozone as for
Disinfection Alternative
Energy
Requirement
fkWh/dayj
Chlorination 71
Ozonation
Air-fed 332
Oxygen-fed 586
Ultraviolet
Without photoreactivation 121
With photoreactivation 242
chlorine, and operating and maintenance
costs are, in the best case, 35% higher for
ozone than for chlorine. However, wide
variation exists in the operation and
maintenance costs for ozone, depending
on the efficiency of ozone generation and
absorption and on the cost of energy.
Note, however, that the cost of disinfec-
tion is only a few percent of the total cost
of wastewater treatment, resulting in a
maximum difference of 10% for the total
cost.
A risk assessment data base for the
wastewater disinfection alternatives of
chlorination, ozonation, ultraviolet radia- m
tion, chlorination/dechlorination, and no
disinfection has been collected and re-
ported in this study. Portions of the
chlorination data base are listed in the
reference section of this report. The data
base is heavily skewed toward the chlori-
nation alternative. This imbalance focus-
es an excessive amount of attention on
the hazards of chlorination and may
create the illusion that the other alter-
natives involve less risk. In addition, the
nature of the data base is not well suited
to quantitative risk assessment because
many of the data do not support the
development of dose-response relation-
ships for many acute responses and for
essentially all chronic responses.
Finally, the utility of the data base is
demonstrated by performing two example
risk assessments. The results of one
example are shown in Table 4. The
examples illustrate that even though the
data base does not provide quantification
of all risks, it will provide a limited
assessment that can be used for guidance
in setting policy and selecting disinfection
alternatives.
The full report was submitted in ful-
fillment of Grant No. R-806586 by the
University of Colorado at Denver under
the sponsorship of the U.S. Environmen-^
tal Protection Agency. ™
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Table 2. Chlorination Cost Summary ($/1,OOO m3)*
Plant
Size
(m3/day)
% Utilization
60%
8O%
10O%
3790
37900
379000
$19.19 - $20.79
9.81 - 11.41
5.27 - 6.47
$15.23 - $16.43
7.68 - 8.88
4.08 - 4.98
$12.84 - $13.80
6.41 - 7.37
3.37 - 4.08
'Minimum unit cost correspondes to 6 mg/L Cli dosage;
Maximum unit cost corresponds to 10 mg/L Cli dosage;
Interest rate = 9%;
Amortization period = 20 years.
Table 3. Ozonation Cost Summary ($/1,000 m3)*
Plant
Size
(m3/day)
3790
37900
379000
% Utilization
60%
$41.47
21.58
13.88
- $75.50
- 39.85
- 26.57
80%
$35.15
18.42
12.40
- $64.06
- 34.58
- 24.30
10O%
$31.35
16.52
11.51
- $57.19
- 31.42
- 22.94
'Absorbed ozone dosage = 5 mg/L;
Minimum unit cost corresponds to 90% transfer efficiency;
Maximum unit cost corresponds to 4O% transfer efficiency;
Interest rate =. 9%;
Amortization period = 2O years.
Table 4. Risk Summary—Example A
Description
Chlorination
Ozonation
Transportation
Case 1 - truck only
114 kg cylinders
Deaths/yr
Injuries/yr
Property damage—$/yr
Releases—kg/yr
Case 2 - rail + truck
0.91 metric ton cylinders
Deaths/yr
Injuries/yr
Property damage—$/yr
Releases—kg/yr
On-site accidents—lost work days/yr
Energy Use—kWh/hr
Cost—$/yr
Human health risk-
cancer cases/lifetime
Ecosystem effects
0.
0.014
$1.92
0.64
0
0.000075
$0.31
0.17
0.1
31,000
21,000
0 - 2x10~*
trout population
stress near the
outfall
Not
Applic-
able
Insuff.
Data
240,000
51.000
Insuff.
Data
None
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David Hubty, Willard Chappell. John Lanning, Martin Maltempo, Daniel Chiras,
and John Morris are wittt the University of Colorado. Denver, CO 80202.
Albert D. Venosa is the EPA Project Officer (see below).
The complete report, entitled "Risk Assessment of Wastewater Disinfection,"
(Order No. PB 85-188 845/AS; Cost: $17.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
* U.S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27085
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAII
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