Reduction of microorganisms in treated effluents
One of the goals of wastewater treatment is to reduce the presence of
microorganisms, including those that cause infectious diseases.
Following treatment, wastewater is discharged to lakes, rivers, and
ground waters, or it is used for beneficial purposes such as
irrigation. In the environment, treated effluents become part of the
larger water system and are a resource for many uses, including habitat
for aquatic life, water supply for various urban and agricultural
purposes, and a source for recreation. The possible presence of
microorganisms in treated wastewater is a concern for ecosystem and
public health. Ensuring safe water resources is especially pertinent
today, given the increasing demands on existing water sources. To
appropriately manage water supplies, microbial risk assessment is used
by public officials to assess the potential risks posed to human health
based on the anticipated presence of microorganisms and probable
exposure scenarios.
Following biological treatment of wastewater, a combination
of filtration and disinfection is often used to produce highly treated
water containing very low concentrations of viable microorganisms. One
of the challenges of reducing the presence of microorganisms in
wastewater effluents is that microorganisms can exist in a
free-floating state or they can be present inside of small particles.
The physical state of microorganisms drastically impacts the
feasibility of removal. In particular, the presence of microorganisms
within effluent particles protects the organisms from disinfectants.
However, filtration can be used to reduce the total number of effluent
particles, as well as the size of effluent particles.
Development of tools to optimize filtration and
disinfection systems
The goal of our research is to develop guidelines for designing and
operating filtration and disinfection systems such that the presence of
microorganisms is sufficiently reduced. The degree of removal necessary
is determined using a risk assessment framework that ensures protection
of public health. To better understand the relationships between
particles, microorganisms, and the effects of filtration and
disinfection, we collect biologically treated but undisinfected
wastewater from treatment plants and disinfect the samples in the
laboratory. A computer model is then used to better understand the
diffusion of disinfectants into particles and the disinfection of
microorganisms occluded within particles. Samples used in our studies
are collected from various treatment plants that represent a range of
treatment technologies. The type of biological treatment used impacts
the association of microorganisms in particles as well as the sizes and
concentrations of effluent particles.
Laboratory tests: particles and disinfection
characteristics
The disinfectants
that we use in the laboratory tests are chlorine, chloramines, and
ultraviolet light. During each test, the concentrations of
disinfectants and microorganisms are quantified at regular time
intervals. For each wastewater sample collected, a series of water
quality analyses is performed and information regarding the particle
characteristics is obtained. A dye is introduced into a subsample of
the wastewater prior to being filtered. The filter containing the
stained particles is placed under a microscope and the particles are
counted using computer-driven imaging software. Information on the
particle sizes and shapes is obtained using the software.
Computer modeling: diffusion of disinfectants in
particles
After completing the laboratory analyses, the data are used in a
computer
model to predict the extent of penetration of disinfectants into the
particles. Using information from the model, we develop criteria to
optimally integrate the filtration and disinfection systems to
collectively reduce the concentration of microorganisms in wastewater
effluents. Output from the computer model is used in combination with
published information on filtration performance to determine the most
suitable type of filtration technology for specific treatment plants.
Information obtained in this research will allow managers of treatment
systems to use information on particles (total counts and size
distribution) as a surrogate for effluent microbial presence.
Health-based criteria for optimizing filtration and
disinfection systems
The final stage of our project is to demonstrate use of the
optimization criteria using biologically treated wastewater collected
from a full scale facility in a bench-scale filtration and disinfection
reactor system. The ultimate goal of this research is to provide
operational guidelines consisting of risk-based criteria that describe
acceptable particle sizes for a range of disinfectant concentrations
and contact times. To implement optimization protocols at full-scale
treatment facilities, we will also develop methodologies for
calibrating electronic particle counters and provide documentation for
performing inactivation studies in conjunction with particle size
measurements to calibrate and apply the numerical model. The tools
developed in this project are intended to assist utility managers in
making process decisions that are feasible and meet risk management
objectives.
Background information:
Basagaoglu, H., McCoy, B. J., Ginn, T. R., Loge, F. J., and Dietrich,
J. P. (2002) "A
diffusion limited sorption kinetics model with polydispersed particles
of distinct sizes and shapes." Advances in Water Resources, 25(7),
755-772.
Dietrich, J. P., Basagaoglu, H., Loge, F. J., and Ginn, T.
R. (2003)
"Preliminary
assessment of transport processes influencing the penetration of
chlorine into wastewater particles and the subsequent inactivation of
particle-associated organisms." Water Research,
37(1), 139-149.
Emerick, R. W., Loge, F. J., Thompson, D., and Darby, J. L.
(1999) "Factors
influencing ultraviolet disinfection performance part II: Association
of coliform bacteria with wastewater particles." Water Environment
Research, 71(6), 1178-1187.
Loge, F. J., Emerick, R. W., Thompson, D. E., Nelson, D. C.,
and Darby, J. L. (1999) "Factors
influencing ultraviolet disinfection performance part I: Light
penetration to wastewater particles." Water Environment Research,
71(3), 377-381.
Loge, F. J., Emerick, R. W., Ginn, T. R., and Darby, J. L.
(2002) "Association
of
coliform, bacteria with wastewater particles: impact of operational
parameters of the activated sludge process." Water Research, 36(1),
41-48.
