United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86-060 Sept. 1986
&EPA Project Summary
Forecasting Onsite Soil
Absorption System Failure
Rates
RECEIVED
James Hudson
NOV181986
ENVIRONMENTAL PROTECTION AGENCY
LIBRARY, REGION V
Reviews were made of four statistical
studies of onsite soil absorption system
failure rates in four different areas of
the United States. From this data base,
a series of forecasting techniques were
developed with varying levels of accu-
racy. The techniques are intended for
use by facility planners in evaluating
the costs of alternative plans for pollu-
tion abatement in small communities.
The required input data and level of ac-
curacy expected for each forecasting
method are also provided along with
the assumptions made in development
and testing of the models against avail-
able data.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce 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
This project was undertaken to pro-
vide small community wastewater facil-
ity planners with appropriate tools for
assessing the total life-cycle costs of on-
site wastewater disposal options so that
the latter can be compared with other
alternatives for abating water pollution
or public health hazards. No guidance of
this type has previously been available
to facility planners. As a result, tech-
niques of cost-effective analysis have
been variable and often inadequate for
small communities where the status
quo and partial sewering options are
evaluated against total community sew-
erage and centralized treatment and
disposal.
Procedures
A comprehensive literature search
was conducted for analyses of failures
of onsite soil absorption systems. This
search yielded four excellent analyses
performed in Acton, Massachusetts,
Fairfax County, Virginia, Glastonbury,
Connecticut, and Malibu and Topanga,
California. The methods used in these
studies were analyzed for their utility in
developing a useful array of forecasting
methods.
A rational approach was developed
by first using linear regression on 14 in-
dividual cohorts. Some statistical at-
tempts were then made to determine
whether significant differences could be
found between specific-year failure
rates in an individual cohort. An attempt
was also made to determine why indi-
vidual cohorts exhibited significant dif-
ferences. Community or study area fail-
ure rates were also compared.
The models were developed from
past failure rate information and used to
estimate future failure rates. The use of
the models in the facility planning proc-
ess was then delineated in a step-by-
step manner, and an example was pre-
sented.
Results
Evaluation of previous failure analy-
ses indicated that except for potentially
higher failure rates during the initial
years, the rate was generally the same
in each subsequent year up to 30.
Newer codes generally reduced failure
-------
rates, and system longevity was primar-
ily a function of how well soil types and
code requirements were matched.
Larger systems in a given soil tended to
last longer.
The simple model that best matched
the available failure data is:
F = b, + bz A = b3 E
where: F = failure rate (number/
year)
A = age of system (years)
E= early failures (use
value of 1 for ages <1-
5 years)
bn = coefficients estimated
from data
Conclusions
1. The best compromise between accu-
racy and usefulness in forecasting
onsite soil system failure rates is the
use of the cohort raw failure rate ap-
proach described and demonstrated
here. The reason is that it allows the
forecaster to evaluate code changes,
soil differences, and management
method shifts.
2. The next best approach is the use of
raw failure rates as an annual aver-
age during the planning period.
However, this method fails to ac-
count for major categories of
changes or differences that can sig-
nificantly influence future failure
rates.
3. More complex methods may be fea-
sible if sufficient data are available,
but the cost of gathering these data
and the complexity of the analyses
are not generally justified for rural
communities.
Recommendations
1. Rational failure forecasting methods
should be applied to all small com-
munity facility planning alternatives
that involve either continued use of
existing onsite wastewater disposal
systems or installation of new ones.
2. Application of the developed failure
forecasting method (or more com-
plex methods) is recommended as a
prerequisite for any community con-
templating a major revision in their
onsite system codes. The forecasting
method provides a basis for that de-
cision and for a review of the revision
at a later date.
The full report was submitted in par-
tial fulfillment of Contract No. 68-03-
3057 by Urban Systems Research and
Engineering, Inc., under the sponsor-
ship of the U.S. Environmental Protec-
tion Agency.
4
James Hudson is with Urban Systems Research and Engineering, Inc., Cambridge.
MA 02138.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Forecasting Onsite Soil Absorption System Failure
Rates," (Order No. PB86-216744/AS;Cost:$11.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES I
EPA
PERMIT No G-3
Official Business
Penalty for Private Use $300
EPA/600/S2-86/060
aawir
60604
------- |