International Research Journal of Bioengineering and Biomedical Science Vol. 1(1) pp. 1-6, July, 2013
Available online http://www.interesjournals.org/IRJBBS
Copyright ©2013 International Research Journals
Full Length Research Paper
Evaluation of Screening Level Technique (SLT)
for pollutant removal in an engineered ecological
system: a case study in Kenya
*
Kennedy Omondi Otieno and Christian Thine Omuto
Department of Environmental and Biosystems Engineering, University of Nairobi
*Corresponding Author Email: otienook@yahoo.com
Abstract
This study was based on testing Screening Level Technique by way of evaluating biokinetic
performance of an operating tropical Vegetated Submerged Bed system with specific focus to a
pertinent 5 day Biochemical Oxygen Demand pollution indicator. The overall objective was to establish
effectiveness of the Screening Level Technique as a design concept for a reliable Vegetated Submerged
0
Bed system. The methodology applied was the estimation of the bulk removal rate at 20 C. According to
the concept of Screening Level Technique, the amount of pollutant removal in a Vegetated Submerged
Bed system is quantitatively expressed in terms of Removal Efficacy which basically depends on the
0
system’s hydraulic residence time and pollutant bulk removal rate. BOD5 bulk removal rate at 20 C was
-1
-1
established as 0.48 day and 0.53 day from both Screening Level Technique and Kickuth model
respectively. The validation of the Screening Level Technique was based on the test of statistical
significance at 95% confidence level as quantified by probability value of the null hypothesis that the
system BOD5 bulk removal rate as estimated from the two concepts are equal. Statistically, the
Screening Level Technique was found effective hence prospectively useful as a design concept for
tropical Vegetated Submerged Bed systems.
Keywords: Biokinetic, bioremediation, removal efficacy, wastewater, pollutant, vegetated submerged bed.
INTRODUCTION
Knowledge of pollutant removal in an Engineered
Ecological System (EES) is essential for evaluating
quality functions of existing systems or for pollution
abatement planning (Otieno, 2013). Analytical
Non-standard abbreviations
2
As; Surface area of the Vegetated Submerged Bed (m ), Df;
0
Degrees of Freedom, kBOD5; BOD5 Bulk Removal Rate at 20 C
-1
3 -1
(day ), Qd; Daily Average Hydraulic Loading Rate (m d ), RE;
System BOD5 Removal Efficacy (%), HRT; VSB wetland
Hydraulic Residence Time (t) (days), EES; Engineered
Ecological System(s), SLT; Screening Level Technique, VSB;
Vegetated Submerged Bed, PE; Population Equivalent,
USEPA; United States Environment Protection Agency, CSIR;
Council for Scientific and Industrial Research, GBH; Gravel Bed
Hydroponics, APHA; America Public Health Association, H0;
Null Hypothesis, Ha; Alternative Hypothesis, T; Temperature
0
( C).
quantitative techniques like Screening Level Technique
(SLT) are needed to provide this knowledge and assess
this function. Specifically, these techniques are needed to
estimate and appreciate the bioremediation performance
and level of public health and environmental protection
provided by an engineered ecological reactor especially
with respect to prediction of level of treatment and
compliance with legislative requirements. It is important
to point out that Kenya has wastewater disposal
problems typical of the developing world. In Nairobi, there
is evidence of severe contamination of Nairobi River both
from domestic and industrial waste. Therefore,
wastewater treatment is becoming increasingly important
due to an amplified pace of industrialization, rapid
urbanization, increased water supply service levels and
fast population growth rates (Nyakang’o and van
Bruggen, 1999). The appreciation of this importance has
brought about the need to search for appropriate
2 Int. Res. J. Bioeng. Biomed. Sci.
technology solutions suitable to local condition hence the
concepts of Screening Level Technique (SLT) and
Engineered Ecological Systems (EES) like Vegetated
Submerged Bed (VSB) systems.
Ideally, a predictive and design technique like SLT
should maximize the amount of information provided
while minimizing the amount of effort to obtain the
information (Otieno, 2013). In other words, rather
scientifically simplified and competent modeling
approaches that yield useful information are highly
desirable. A screening level technique refers to a
simplified, quantitative, predictive model that minimizes
time and effort for implementation (Dortch and Gerald,
1995). Simplification is achieved by making sound
scientific assumptions that reduce complexity of the
predictive mathematical formulations and input data
requirements. The intent of this study was therefore
based on testing effectiveness of this scientifically
simplified SLT by way of applying it in evaluation of
biokinetic performance of an operating Carnivore
Restaurant VSB system with specific focus to 5 day
Biochemical Oxygen Demand (BOD5). BOD5 was chosen
because it is a pertinent indicator of organic pollution of
water in biological treatment processes and generally,
VSB systems are sized based on it.
An Engineered Ecological System (EES) is an expertly
designed and built green system where flora and fauna
interact with each other and with the physical
environment (Otieno, 2013). The predominant energy
source in an EES is gravity, sunlight and diverse ecology.
In terms of diverse ecology, EES are designed as a
complex assemblage of wastewater, substrate,
vegetation and an array of microorganisms (most
importantly bacteria) specifically to replicate the clean up
function of natural ecological systems. Vegetation forms
the baseline of the ecology because besides taking up
pollutants as nutrients, it offers habitat and shelter for
many life forms including birds which introduce a
surprising range of small organisms into an EES via feet,
feathers or gut. Vegetation also helps to oxygenate water
in EES systems like VSB bioreactors which in turn aids
metabolism by bacteria. In the context of pollutant
removal, the interactions within an Engineered Ecological
System and the physical environment enable
contaminants transformations into nutrients leading to
decrease of toxicity of waste. Engineered Ecological
Systems have been used for pollution abatement in areas
such as solid waste and wastewater. The focus of this
study was on the wastewater treatment Vegetated
Submerged Bed (VSB) system. VSB systems (also
known as Subsurface Flow Wetlands) contain a bed of
media (such as crushed rock, small stones, gravel, sand
or soil) vegetated with aquatic plants such as cattails and
bulrushes (Vymazal, 2005). Wastewater stays beneath
the surface of the media, flows in contact with the roots
and rhizomes of the plants, and is not visible or available
to wildlife (USEPA, 2000). VSB wetland systems are
effective as wastewater treatment processes for a
number of reasons. First, bacterial growth attached to the
wetland gravel media and submerged roots is essential
for soluble and colloidal Biochemical Oxygen Demand
(BOD) reduction and other biologically driven processes.
Second, the quiescent water conditions found in the
subsurface media are conducive to removal of solids
constituents from wastewater (Regmi et al., 2003).
It should not be forgotten that an important part of the
modeling process is the field evaluation of an acquired
model. Once a bioremediation approach has proven
effective in a laboratory or pilot scale treatability study, it
must be monitored and evaluated at a field site. The
objective of field phase is to demonstrate that the
particular technology performs as expected in the field.
This is so because for most bioremediation technologies,
certain key parameters concerning applicability cannot be
thoroughly evaluated until the approach is scaled and
field tested on real or similar systems (USEPA, 1995).
Both engineering and statistics play an important role in
the verification process (USEPA, 2000). This study
therefore hoped to flow along this critical path of
reasoning putting emphasis on a chance for verification
failure of SLT rather than a chance to pass with the
intention of being insightful and useful to knowledge
growth of the SLT concept as was recommended by its
original developer. In so doing, the study was equally
cognizant of the notion that in field verification of models,
research engineers and scientists should be concerned
with reliability and usefulness rather than validity (Bell
and Senge, 1980; Mass and Senge, 1978). The question
should be whether the model serves the purpose for
which it was intended and whether it is helpful. Thus the
developers and users purpose must be kept in mind in
evaluating models usefulness or reliability (Otieno, 2013).
Criticisms of models also should reflect this perspective.
With this background, the following specific objectives
were found plausible for this study. (1) To establish the
biokinetic performance of an existing tropical VSB
wetland system treating restaurant wastewater from a
settling tank; and (2) To test the effectiveness and
reliability of the SLT.
MATERIALS AND METHODS
Description of the System
The study site is located in Nairobi, Kenya at longitude
0
0
36 48’E and latitude 1 20’S, at an altitude of 1785m in
the tropics, with an average annual rainfall of 200 800mm and average minimum and maximum
temperatures annual of 15.5°C and 26.5°C respectively.
The wetland system was designed for a temperate
climate based on 1200 Population Equivalent (PE) by
CSIR (Council for Scientific and Industrial Research) in
South Africa. It is a multi-stage system consisting of oil
Otieno and Omuto 3
filters, pretreatment settling tanks and a Vegetated
Submerged Bed (VSB) also called the subsurface
2
horizontal flow constructed wetland (area=1800m ).The
VSB system was originally followed by three free surface
flow wetland cells in series. The free surface flow wetland
cells were later done away with to create space for other
development.
The Carnivore VSB similarly referred to as Gravel Bed
Hydroponics (GBH) by the Carnivore staff, is simply a
sunken lined walled rectangular pond filled with 1m of
gravel covered with 10cm of soil to support growth of
bulrush (Typha spp) and other microphytes. Alternating
baffle walls are built across it at regular intervals. This
theoretically ensures that water takes a serpentine flow
course through the system thereby maximizing the flow
distance of an element of a pollutant that consequently
should increase the system’s pollutant(s) removal
efficacy (Otieno, 2013). The gravel does not simply
provide physical filtration of the pollutant and base for
vegetation but a home for bacteria that are carefully
dosed into the system at the inlet point. The planted
aquatic vegetation apart from other ecological roles,
principally by design, is expected to transfer oxygen from
above-surface leaves to sub-surface roots. The effluent
flowing through the VSB is then treated by the action of
the bacteria attached to the plant roots and gravel. The
aerobic bacteria are expected to be attached to the roots
while the anaerobic bacteria are expected to be attached
to the gravel/rocks (Otieno, 2013). Discharge from the
VSB enters a level control chamber that determines the
top water level within the VSB and from here it is
channeled by gravity to a holding pond.
Acceptance precision was defined as relative percent
differences of up to 20 percent. This benchmark of 20%
was informed by the reason that, at about 30% and
above difference between high and low values of
replicate samples; the variability is considered large and
may normally indicate the presence of toxic substances
or analytical problems (APHA, 1992). The study was
therefore merely being careful by considering this bench
mark.
Description of the Screening Level Technique (SLT)
As described by Dortch and Gerald (1995), a screeninglevel model is a quantitative and predictive analytical
model. The model idea is to estimate Removal Efficacy
(RE) for a specific pollutant given a limited amount of
basic information about an EES like the VSB system. The
model was built from the bottom up based on first
principles. The primary scientific assumption is existence
of steady state condition in EES. Also typically, two
conditions are assumed for spatial gradients in
concentration. That is, no existence of gradients and
existence of gradients along the main flow axis. The
relationship for RE, with spatial assumption of existence
of longitudinal gradients or plug flow is derived from the
one-dimensional mass transport equation (neglecting
dispersion) and is mathematically expressed as:
–K t
RE = (1 – e ) x 100.............................. [1]
Where;
RE = system pollutant removal efficacy;
0
k = pollutant volumetric reaction rate constant at 20 C
–1
(day ); and
t = hydraulic residence time (days);
Data collection and Laboratory Analysis
Water samples for laboratory analyses were collected
monthly over three months from March to May 2011.
Each set of sample for a particular date was a composite
of hourly grab samples taken at the VSB inlet and outlet
points at an interval of 4 hours from 0800 hrs to 1600hrs.
Because samples for BOD analysis may change greatly
during handling and storage and since laboratory test
could not be started within six hours, the samples were
incubated at 4°C for not more than 48 hours of its original
sampling time before beginning the BOD test. Secondary
data by Nyakang’o and van Bruggen for the same VSB
system taken from November 1996 to January 1997
when the system had achieved permanence status was
also used. In both cases, laboratory analysis for the
samples was based on the procedure outlined in
Standard Method 5210B for the analysis of samples for
biochemical oxygen demand (APHA, 1992). Validation of
results was achieved by replicate sample analysis
involving analyzing the same sample more than once and
comparing the results. The closer the results were, the
more accurate and precise they were considered.
Validation of SLT and Estimation of BOD5 Bulk
Removal Rate
Validation of SLT was based on testing it against the
universally accepted Kickuth model for sizing VSB
0
systems. The study used BOD5 bulk removal rate at 20 C
(kBOD5) as a descriptive parameter for validation.
According to Kickuth model, the basis for the design of a
functional Vegetated Submerged Bed system is the
surface area of the bed which basically is a function of
3
the daily average flow rate of sewage (m /d); VSB
drainable porosity and water depth; daily average BOD5
substrate of the feed (mg/l) and the required average
BOD5 substrate of the effluent (mg/l) with the constant
being the BOD5 bulk removal rate (USEPA, 1995; UN0
HABBITAT, 2008). The BOD5 bulk removal rate at 20 C
(kBOD5) according to the SLT was estimated using the
following first order plug flow equation developed from
equation (1) by making the pollutant volumetric reaction
rate parameter the subject:
kBOD5 = - ln (1- RE) / t and t = n h As / Qd................................... [2]
4 Int. Res. J. Bioeng. Biomed. Sci.
Where:
0
kBOD5 = BOD5 volumetric reaction rate constant at 20 C
–1
(d );
RE = system BOD5 removal efficacy (as a fraction);
t = hydraulic detention time (days);
n = average drainable bed porosity;
h = effective wetland water depth (m);
2
As = surface area of the bed (m ); and
3
Qd = daily average hydraulic loading rate (m /d).
An average system drainable porosity of 36% was
considered (Reed et al., 1995; Otieno, 2013). The t-test
statistics was used to test statistical significance (5%
significant level), as quantified by probability (p-value), of
the null hypothesis that the means of the bulk removal
rate parameter as estimated from the two equations are
equal (Kothari, 2004).
RESULTS
The Carnivore VSB wetland system received from
November 1996 to January 1997 and March 2011 to May
3
2011 an average of 97m wastewater/day. In table 1, an
overview is given of the inflow and outflow wastewater
characteristics of the Carnivore VSB. As can be seen,
BOD5 showed a rapid decrease from an inflow
concentration average of 610mg/l to an outflow
concentration average of 34mg/l. The results indicate a
positive treatment result by Carnivore Restaurant VSB
system. Based on the data, the average bulk loss value
from both the Kickuth model and screening level
-1
technique were computed and established as 0.53d and
-1
0.48d respectively. The data for the test of statistical
significance (5% significant level), as quantified by
probability (p-value), of the null hypothesis that the
means of the BOD5 bulk removal rate parameter as
estimated from the two equations are equal is presented
in table 2.
DISCUSSIONS
The influent wastewater into the Carnivore VSB system
with mean BOD5 concentrations of 610 mg/l falls under
the class of strong sewage (Mara, 1986; Metcalf et al.,
2003). This could be attributed to low specific water
consumption of about 80 litres per Population Equivalent
(PE) daily (150 litres per PE daily is normally expected)
assuming a total design PE served of 1200 for the
3
estimated wastewater production rate of 97m /day.
Figure 1 illustrates the expected biokinetic performance
of the Carnivore VSB system. The illustrated
performance is based on a high bioremediation
performance of 96% achieved despite system surface
overflow caused by clogging of the void spaces of the
gravel substrate in the about 35% entry zone.
Generally and while noting that this study is expected
to continue, the preliminary finding is that the BOD5
reduction performance for the Carnivore VSB system can
be said to be good for the reason that what is agreeably
expected is about 80% performance (Adrian and Frank,
2004). However, the Carnivore system performs nonconventionally in the overall biokinetic expectation since
-1
the evaluated BOD5 bulk loss value of 0.48 day could
-1
only compare favourably to that of 0.50 day for
conventional free surface flow wetlands as opposed to
-1
the characteristic value of 1.104d for conventional VSB
systems. Thus usage of gravel as a substrate in the
Carnivore VSB system did not add any technical value.
The nonstandard performance of the Carnivore VSB
system could not be associated with the probable
ineffectiveness on the part of the SLT but rather other
factors outside the scope of the study such as reactor
incomplete kinetics due to short circuiting of pollutants
travel path brought by system blockages and hydraulic
design deficiencies. For example and as can be seen
from figure 1, the system achieved an agreeably
expected 80% BOD5 reduction performance mark in less
than four days which possibly point to the fact that the
system could have been oversized in terms of longer
hydraulic residence time (HRT). The longer HRT perhaps
had a negative influence on the biokinetic performance of
the Carnivore VSB reactor. This is because VSB
biokinetic performance has a sensitive relationship with
the residence time an element of a pollutant takes in a
VSB reactor. After HRT of more than two days, the
reduction of BOD5 is not strongly dependent on HRT
since removal improves only slightly thereafter up to HRT
of 7.5 days (USEPA, 1995). Therefore, overdesign far
beyond the critical two day HRT increases biochemical
oxygen demand given that larger biodegradable particles
that have been quickly removed by physical mechanism
will be degraded over time and be converted into
particles in the soluble and small colloidal size range. As
such they become an internal source of BOD as they
degrade and reenter the water. Some material is also
incorporated into microbial biomass. True BOD removal
only occurs when the material causing the BOD is
completely converted by anaerobic biological processes
to gaseous end products and this is expected to take
place within the first two days for a conventional VSB
system.
The other significant reason Screening Level
Technique as a design and predictive model could not be
associated with the Carnivore nonstandard performance
is that it was subjected to risk of verification of failure by
field testing its effectiveness and prospective usefulness
against the universally accepted Kickuth model.
Statistically, it could not be invalidated by the finding of
the preliminary study. This implies that the SLT is
prospectively useful as a design concept for functional
wastewater VSB bioremediation systems in Kenya. Thus,
it could in future be applied in confidence in the screening
level assessment of biokinetic performance of existing
Otieno and Omuto 5
Table 1. Summary of performance data for Carnivore VSB
system
3
Date
Nov-96
Dec-96
Jan-97
Mar-11
Apr-11
May-11
Average
Daily Flow (m /day)
Inflow
Outflow
94
74
123
98
70
29
85
60
106
78
103
70
97
68
BOD5 (mg/l)
Inflow Outflow
550
40
600
20
750
50
620
25
560
38
580
32
610
34
Table 2. Hypothesis Test Data
H0: µ = µ0 =
Ha : µ ≠ µ0 =
(α)
Sample size (n)
Degree of freedom (Df)
Sample mean (µ)
Hypothesized mean (µ0)
Sample standard deviation (σs)
Calculated test statistic (t)
Critical t-value
Conclusion
-1
0.53 d
-1
0.53 d
0.05
6
5
-1
0.48 d
-1
0.53 d
-1
0.1021 d
-1.200
-2.571
Do not reject H0
Figure 1. Expected Biokinetic Performance for the Carnivore VSB Reactor
6 Int. Res. J. Bioeng. Biomed. Sci.
engineered ecological systems. It should be taken in
consideration that fundamental scientific knowledge of
the processes of pollutants removal in VSB
bioremediation systems is highly limited at present in
Kenya. The current design process for VSB systems is
also still largely based upon observational data rather
than scientific theories. The observational approach to
the design of VSB systems has led to the implementation
of many uneconomical non-performing systems. This has
in the past contributed to the problem of continued
pollution of fresh water sources and compromise of the
aquatic environment. This study is therefore expected to
make a positive contribution.
ACKNOWLEDGEMENTS
Acknowledgment is extended to the University of Nairobi
School of engineering staff and the Carnivore Restaurant
staff for the support during the period of this study. We
also extend acknowledgment to the various authors
whose publications have been cited. Specifically, we are
indebted to Nyakang’o and van Bruggen whose previous
work at the same case study formed an integral part of
the data used in this study.
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