The literature review described in this report is part of a larger
research project to assess STU performance with respect to
treatment of important wastewater constituents. The overall goal of
the project is to provide a toolkit and tool-use protocol that is
easy to implement and available to a wide range of users to assess
STU performance. This literature review is not a preview of tools
that we will develop and propose, but rather an analysis of the
information and data and the literature, to help guide our tool
development. All tools developed will be based on rigorous
experimental data and quantitative models verified with field data
from operating systems. In some cases, more sophisticated tools
(e.g., complex mathematical models) may be warranted depending on
the relative complexity of the problem and the relative risk
associated with a poor design. This literature review focused on
STU performance, key conditions or factors potentially affecting
STU performance, and the current best practices for using models
and other available tools to predict expected STU performance. The
information gained during this literature review will guide the
future direction of the project. Constituents of interest include
nitrogen (N), phosphorus (P), microbial pollutants, and emerging
organic wastewater contaminants (OWCs). Based on this literature
review, it is clear that due to the variability of data collected
at field sites, simple binary relationships (e.g., C/Co versus
depth for various soil types) for statistical predictions of the
attenuation of N, P, microorganisms or OWCs cannot be justified.
Specific to N, hydraulic loading rate appears to be more important
than soil texture or soil depth within the first 30-60 cm, although
both soil depth and texture remain important variables. Most of the
reported results related to the interaction of P with soil appear
to be from laboratory batch tests. Similarly, field-scale
evaluations of pathogen removal are limited. Finally, most of the
existing OWC work has focused on the occurrence and concentrations
of selected compounds in streams, lakes, and groundwater impacted
by wastewater treatment plant effluents. Currently very few models
have been developed for movement and treatment processes of N or P
in OWTS. However, adapting the CW2D model for STUs that will
predict the effect of different soil types (texture, structure, and
drainage class) appears promising. CW2D is a module of the well
known HYDRUS model designed to simulate nitrogen treatment in a
sand filter. This model incorporates most of the features one might
consider, including a comprehensive treatment of microbial growth,
the impact of oxygen mass transfer on nitrogen transformation, and
variable rates of denitrification due to changes in dissolved
oxygen concentrations, dissolved organic matter, and microbial
growth. The review of existing models demonstrates that simulation
of microbial characteristics in OWTS is still largely uncharted
territory.
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