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FATE OF VIRUSES IN THE INFILTRATIVE SURFACE ZONE OF SYSTEMS THAT RELY ON SOIL TREATMENT FOR WASTEWATER RENOVATION
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Paper number 701P0104, . (doi: 10.13031/2013.15798)Authors: Sheila Van Cuyk, Robert Siegrist
Keywords:
Wastewater systems for onsite and small-scale applications are commonly designed for
application of primary treated wastewater into natural soil where it infiltrates and percolates
through the vadose zone before it recharges the underlying ground water. These systems are
widely used due to their high purification performance with respect to organics, solids and
nutrients, with relatively low cost and limited operation and maintenance requirements (Siegrist
et al. 2001). However, with such systems increasingly being used as permanent solutions for
wastewater treatment and at increasing numbers and densities, there is a growing awareness and
concern over system performance with respect to bacteria and viruses. Since human pathogens
are known to exist in sewage effluents, their removal during soil treatment of wastewater is
essential in preventing contamination of ground water, which may be used for drinking water
purposes. There have been incidences of disease outbreaks by contaminated drinking water due
to the source waters being contaminated (Craun 1985, US EPA 2000). In fact, some
investigators have claimed that septic systems are the most frequently reported cause of ground
water contamination associated with disease in the U.S. (Powelson and Gerba 1994).
Virus transport distance and transport times estimated by models are very sensitive to the choice
of attachment and inactivation rate coefficients (Yates 1995, Navigato 1999). These parameters
and the processes that control attachment and inactivation are not readily available or well
understood, particularly for soil systems used to treat domestic wastewater effluent onsite. This
paper describes laboratory experiments that attempt to understand what controls the natural
disinfection of virus in the subsurface, primarily in soils receiving septic tank effluent.
Figure 1 presents a generalized schematic of a soil-based treatment system that functions as an
insitu porous media biofilter (PMB). These systems include a broad spectrum of types and
designs. Soil may be used to treat very high quality effluent (e.g. sand filter effluent) that may
have low levels of carbon, nutrients and pathogens, or it may be amended with primary effluent
(e.g., septic tank effluent) that contains high levels of carbon, nutrient and pathogens. The
underlying soil and ground water may or may not be altered by the applied effluent. Important
zones for treatment in these systems include (1) the biozone or biomat, which may be formed at
the infiltrative surface, (2) the vadose zone, the depth of which may vary from 1 foot or less to
many hundreds of feet, and (3) the saturated ground water zone. The ground water transport
distance to receptors from such a system can vary anywhere from a hundred meters to many
kilometers.
The columns studies presented in this paper were designed and conducted to simulate the
unsaturated infiltrative surface of these systems where effluent is applied and where a biomat
may form. In order to address the issue of virus fate and transport, information on the
attachment and inactivation/die-off behavior has been gathered in the laboratory using material
and temperatures representative of field conditions following established methods (Harvey 1997,
Van Cuyk et al. 2001, Navigato 1999, Loveland et al. 1996). Two bacteriophages, MS-2 and
PRD-1, were used as surrogates for human pathogenic enteric viruses. The removal of viruses in
non-disinfected wastewater effluent released into the subsurface may depend almost completely upon the permanent attachment of viruses to subsurface solids and/or their inactivation due to are utilized to predict the transport of virus must include the loss of virus from soil solution or
ground water due to the attachment based on the physical and chemical properties of the soil and
the ground water. The work presented here involved a novel unsaturated column assembly used
to simulate the infiltrative surface alone. A vacuum manifold was used to simulate underlying
unsaturated soil, allowing for the collection of percolate samples immediately below the
infiltrative surface. This paper presents a summary of the research completed, while additional
details may be found in Van Cuyk (2003).