The Many Intricacies of Biochemical Oxygen Demand

Abstract

M any engineers who are involved in effluent compliance, wastewater treatment and water quality assessment have heard of the term BOD or Biochemical Oxygen Demand. However, unless the engineer is from technically specific branches of engineering, many delicate intricacies involved with BOD may elude them. In fact, the BOD 5 test (five days incubation at 20°C) which most engineers are familiar with is only the tip of the iceberg of a wider spectrum in relation to BOD. Thus, it is important for engineers to attain an adequate level of understanding of the components, kinetics and overall implication of the BOD test results not only for application in their daily work, but also in the interest of the environment. Back to Basics The history of the BOD 5 test dates back to 1908, when the Royal Commission on Sewage Disposal (UK) chose the parameter as an indicator for organic pollution in the Thames River, which in turn, has a nominal temperature of 20°C and retention time of five days at the tidal zone. By definition, the BOD test should be reflective of the oxygen uptake of microorganisms during decomposition of readily biodegradable organic matter under aerobic conditions. The reaction path is shown in the following Equation [1]. C n H a O b N c + (n + − − c)O 2 →

Full text

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efuent compliance, wastewater
treatment and water quality assessment
have heard of the term BOD or Biochemical
Oxygen Demand. However, unless the
engineer is from technically specic branches
of engineering, many delicate intricacies
involved with BOD may elude them.
In fact, the BOD? test (ve days
incubation at 20°C) which most engineers
are familiar with is only the tip of the
iceberg of a wider spectrum in relation to
BOD. Thus, it is important for engineers to
attain an adequate level of understanding
of the components, kinetics and overall
implication of the BOD test results not only
for application in their daily work, but also
in the interest of the environment.
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The history of the BOD? test dates back
to 1908, when the Royal Commission on
Sewage Disposal (UK) chose the parameter
as an indicator for organic pollution in the
Thames River, which in turn, has a nominal
temperature of 20°C and retention time of
ve days at the tidal zone.
By denition, the BOD test should
be reective of the oxygen uptake of
microorganisms during decomposition
of readily biodegradable organic matter
under aerobic conditions. The reaction path
is shown in the following Equation [1].
C6H5O/*c + (n + − − c)O1"→"
nCO
1
"@""""""""− c H
1
O + cNH
A
""
+ New Cells (1)
This common denition is rather
ambiguous as it makes no mention of
complete decomposition occurring within
ve days at 20°C, thus inhibiting the
oxygen uptake. However, it was later
discovered that at the said temperature
and time frame, most dissolved organic
matter was stabilised, typically between
70%-80% in most sample tests.
However, there remained the question
of the slowly biodegradable organic fraction,
which takes longer to decompose, typically
consisting of non-dissolved organics usually
found in domestic sewage as well as more
complex organic molecules from industry
[2]. The hypothesis that most of the organic
fraction is oxidised within ve days thus
becomes invalid for such cases.
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We shall now examine the BOD? test itself.
The oxygen uptake of microorganisms for
the BOD? test is measured as the depletion
in dissolved oxygen (DO) concentration
between the rst and the fth day, ∆DO,
in a 300ml BOD test bottle that can be
mathematically expressed as[1];
BOD?"B"∆DO = ,
P = (2)
Where DO! is the initial dissolved
oxygen concentration (with and without
dilution), and DO1 is the residual DO
measured after ve days, with P as the
dilution factor and V as the volume of
sample. DO is typically measured via a pre-
calibrated membrane probe or through the
Winkler Titration method. The reaction
kinetics that goes on during the ve-day
period is illustrated in Figure 1;
The above gure assumes that BOD?"
represents about 80% of the total BOD, or
more commonly referred to as Ultimate
BOD (uBOD). As discussed previously,
this only holds true if certain conditions
are met, where the uBOD value can be
experimentally determined via a prolonged
incubation time. Standards Methods for the
Examination of Water and Wastewater (21st
Edition) from the American Public Health
Association (APHA) recommends an
incubation time of 60 days [3]. Inherently,
this is impractical for operational and
regulatory purposes. However, since the
reaction follows a rst order rate kinetics
pathway, the uBOD is correlated to BOD?"
through the following formulae [4];
Figure 1: Graphical illustration of DO-BOD kinetics
The Many Intricacies of Biochemical
Oxygen Demand
...........................................................................................................................................................................................................................
By: Engr. Zaki bin Zainudin, Grad. IEM
a
4
a
2
3
2
b
2
3
4
DO1 − DO2
P
V
300

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BOD?"B"uBOD(1 − e-5k) (3)
Where k is the rst order rate constant
determined from experiment for various
types of water samples (available from
literary sources). This k value governs the
rate of reaction in the correlation, which
in turn, is independent of the amount of
BOD? representing uBOD. In other words,
regardless of where the nal BOD? value
is located in Figure 1, through utilisation
of the k value, the uBOD and, thus, the
actual organic pollution strength can be
estimated.
Why is this important to consider? The
answer lies in the travel time of the receiving
main stream or tributaries of rivers to its
downstream segment. If the travel time
of the organic pollutant is more than ve
days and consists mainly of the slowly
biodegradable fraction, an underestimation
of the organic pollution strength may
occur, particularly in Malaysia where the
primary pollution load contribution are
from sewage sources. Moreover, tropical
temperatures actually heighten microbial
activity and may incur higher BOD in a
shorter timeframe.
A reconciliation of this paradox is done
through the regulation of COD or Chemical
Oxygen Demand. Since the COD test utilises
a synthetic oxidising agent to replicate
the BOD oxidation process, the slowly
biodegradable fraction is also instantaneously
oxidised. This measure of control is prudent
towards water quality preservation.
Chapra et al. (2005)[4], however,
suggests for BOD to be differentiated to
two specic categories; fast-BOD and slow-
BOD, where the former would represent
the readily biodegradable fraction, and the
latter, the slowly biodegradable fraction.
This method of distinction was further
reafrmed when it was incorporated into
the United States’ Environmental Protection
Agency’s widely used water quality model,
QUAL2K.
Fast-BOD is determined via removal
of suspended organics through ltration.
Slow-BOD is then determined as a product
of the unltered sample versus the ltered
sample [4]. It is recommended, however,
that some nitrogenous inhibitation be
done to capture only the carbonaceous
fraction (cBOD).
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The preceding discussions only covered
BOD? and uBOD without consideration
for oxygen demand exhibited by nitrifying
microorganisms. Typically, for the BOD?"
test, this is not a cause for concern as this
type of oxygen demand only occurs after
a prolonged duration, after most of the
carbonaceous organic matter has been
stabilised.
However, APHA still recommends the
use of an inhibiting agent such as TCMP
(2-chloro-6-(trichloro methyl) pyridine)
[3]. This is because, for low level BOD
water that contains minute amount of
carbonaceous matter or high amount of
ammonia nitrogen, nitrication may occur
at an earlier stage inside the test bottle. In
addition, to be representative of owing
river conditions where nitrication does
not usually occur, the use of an inhibiting
agent would prove advantageous.
Nitrication is the transformation
process of ammonia nitrogen to nitrite
and nitrate by microorganisms from the
nitrosomonas" 56>" nitrobacter genus [1].
Since ammonia nitrogen is a by-product
of organic decomposition, the probability
of nitrication occurring and affecting the
BOD results increase as more time pass
(i.e. incubation time).
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A water sample due for BOD analysis must
be kept at 4°C onsite and analysed within
24 hours. This can be achieved through
the utilisation of a cooler box and some
ice cubes or cooling gel packs. Microbial
activity shall be kept to a minimum with
this procedure, thus reducing the amount
of organics stabilised during transit.
Bubbles and air pockets should also be
eradicated from the sample bottle to
ensure no oxygen transfer occurs [5].
A common misconception is that BOD
is a suitable parameter of assessment
for all types of water. This is not true,
especially for water with above normal
saline concentration such as brackish
water and seawater. The high chloride
content disrupts microbial activity
through protoplasmic degradation
(osmosis). This is one of the reasons
why the Interim Marine Water Quality
Standards (IMWQS) for Malaysia does
not prescribe BOD as a parameter of
provision. Total Organic Carbon (TOC)
is a more suitable parameter for such
conditions in substitute of BOD in
determining organic pollution.
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For the test results to hold water (pun
intended), several conditions have to
be met, one of which is that the residual
DO1 (on the fth day) cannot be less
than 2mg/l, otherwise the sample would
Figure 2: Carbonaceous and nitrogenous BOD[1]

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simply be rejected. This is because
such low oxygen levels would induce
stress to microorganisms stabilising the
organic matter and likely cause anaerobic
respiration. However, APHA has since
reviewed this number and the latest
edition of Standard Methods state that
DO1 must be more than 1mg/l.
In addition, APHA also states that for
the BOD? results to hold any meaning;
a minimum of 2mg/l oxygen depletion
must be met. In other words, the detection
limit for BOD should be set at 2mg/l
[3]. One reason behind this guideline
is to ensure that the indicated results
are actually from microbial respiration
instead of external inuences.
Finally, the analyser needs to
determine whether seeding is necessary.
Under typical circumstances, the microbial
population present within a water sample
is usually enough to incur oxygen demand.
However, there are instances when
induced seeding of these microorganisms
is required. Seeding ensures homogeneity
of the microorganisms stabilising the
organic matter, thus ensuring more
accurate results.
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Details pertaining to BOD have been
thoroughly explained. It is hoped that
the reader would now have a better
understanding and appreciation of the
many intricacies involved pertaining to
BOD and be able to utilise the knowledge
gained in their daily engineering practices
for the interest of the environment."n
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[1] C. N. Sawyer, P. L. McCarty
and G. F. Parkin, ‘Chemistry for
Environmental Engineering and
Science: Fifth Edition’, In : Chapter
23 : Biochemical Oxygen Demand,
McGraw-Hill Professional., USA,
2003, pp. 604 - 621.
[2] S. C. Chapra, ‘Surface Water-
Quality Modeling’, Prentice Hall,
USA, 1997.
[3] American Public Health
Association (APHA), American
Water Works Association (AWWA)
and Water Environment Federation
(WEF), ‘Standard Methods For
The Examination of Water and
Wastewater : 21st Edition’, APHA,
AWWA and WEF, 2005.
[4] S. C. Chapra, G. Pelletier and
H. Tao, QUAL2K: A Modeling
Framework for Simulating River
and Stream Water Quality, Version
2.04: Documentation and Users
Manual, Civil and Environmental
Engineering Dept., Tufts University,
Medford, MA., 2005.
[5] Recommended Holding Time
and Preservatives (Catalog), ALS
Technichem (M) Sdn. Bhd., Bukit
Jelutong, Shah Alam, Malaysia.