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The Malaysian Journal of Analytical Sciences, Vol 14 No 1 (2010): 24 - 31
24
EFFECTS OF INDUCED SALINITY ON BOD5 REACTION KINETICS OF
RIVER WATER SAMPLES
(Kesan Peningkatan Kemasinan Terhadap Kinetik Tindak Balas Keperluan Oksigen Biokimia
5 Hari Menggunakan Sampel Air Sungai)
Zaki Zainudin1*, Maketab Mohamed2, Mohd. Rosslim Ramli3
1Water Resources Technical Division, Institution of Engineers Malaysia, 46720 Petaling Jaya, Selangor Darul Ehsan
2Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
3Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
*Corresponding author: zakizainudin@gmail.com
Abstract
Biochemical Oxygen Demand (BOD) is a typical parameter used in assessing organic pollution strength in surface waters and is
normally tested over a 5-day period at an incubation temperature of 20°C (BOD5). The accuracy of this constituent, in assessing
organic contamination under brackish conditions has always been known to be somewhat limited as elevated concentrations of
chloride (Cl-) disrupts microbial activity from osmotic cellular degradation, causing the bottle decay rate, k1, to be effected. The
aim of this study was to quantify the effects of induced salinity on k1, with varying levels of sodium chloride (NaCl)
concentration (5 – 25 ppt), towards six mildly polluted to polluted tropical river water samples. The observed variations ranged
between 0.10 – 0.25/day of k1 for the stipulated samples using the Thomas graphical method for determination of the k
1 rate
constant. Sg. Rawang depicted the highest quantum of difference in k1, with decrement from 0.754/day (0 ppt) to 0.513/day (25
ppt), whereas Sg. Klang showed the lowest quantum, from 0.306/day (0 ppt) to 0.265/day (25 ppt).
Keywords : BOD5 saline, brackish, estuarine, bottle decay rate
Abstrak
Keperluan Oksigen Biokimia (BOD) adalah metodologi biasa yang digunakan untuk menilai kekuatan pencemaran bahan
organik dalam air dan biasanya diuji dalam jangka masa 5 hari pada suhu inkubasi 20°C (BOD5). Ketepatan BOD5, untuk
menilai kontaminasi organik dalam air masin sememangnya diketahui agak terhad akibat daripada kandungan klorida (Cl-) tinggi
yang mengganggu aktiviti mikrob, di mana berlakunya pelupusan sel dari proses osmosis, yang seterusnya menyebabkan
gangguan terhadap kadar pereputan dalam botol (k1). Tujuan kajian ini adalah untuk menghisab kesan kemasinan terhadap k1,
dengan meningkatkan kepekatan Natrium Klorida (NaCl) secara berperingkat, antara 5 – 25 bpj, terhadap enam sampel air
sungai yang diklasifikasikan sebagai sedikit tercemar hingga tercemar. Didapati variasi k1 umumnya berada antara 0.10 –
0.25/sehari menggunakan metodologi pengukuran grafikal Thomas. Sg. Rawang menunjukkan perbezaan ketara dalam nilai k1,
dengan kejatuhan daripada 0.754/sehari (0 bpj) ke 0.513/sehari (25 bpj), manakala Sg. Klang pula menunjukkan perbezaan
paling minima dari 0.306/sehari (0 bpj) ke 0.265/sehari (25 bpj).
Katakunci : BOD5 air masin, pencemaran organik di kawasan kuala sungai, kadar pereputan dalam botol
Introduction
Biochemical Oxygen Demand (BOD) is a fundamental parameter used in the assessment of organic contaminants
present in water and wastewater. The parameter was first used in the early 1900s as an indicator of organic
contamination from sewage sources in the United Kingdom (UK). An incubation time of 5 days at 20°C for testing,
brought about the acronym BOD5, with the primary justification that the maximum retention time of organic
pollutants from sewerage sources of rivers in the UK was in accordance to these conditions [1]. The test itself in-
turn, is primarily governed by three things; (1) the amount of biodegradable organic matter present, (2) mix culture
of microbial population that propagates the degradation and (3) acceptable dissolved oxygen levels for microbial
aerobic respiration. The amount of biodegradable organic matter present (left hand side of the Eq. 1.1), is the
primary constituent of concern measured in the test, as excess amounts of organic matter may contribute towards in-
stream oxygen depletion, commonly referred to as the DO sag [2] ;
Zaki Zainudin et al: EFFECTS OF INDUCED SALINITY ON BOD5 REACTION KINETICS OF
RIVER WATER SAMPLES
25
NewCellscNHOHc
2
3
2
a
nCOc)O
4
3
2
b
4
a
(nNOHC 3222cban ++
⎟
⎠
⎞
⎜
⎝
⎛−+→−−++
(1)
A universal qualifier used in BOD testing is that, only the carbonaceous fraction (or cBOD), is measured as this
portion truly reflects the biodegradable organics present. The resulting ammonia, NH3-N, which is a product of the
degradation, exhibits its own oxygen demand after a few days, during the transformation of NH3-N to NO2-N and
NO3-N (nitrification). This oxygen demand is referred to as nitrogenous BOD or nBOD. In order to inhibit the
effects of nBOD, nitrification inhibitors such as TCMP (2-chloro-6-(trichloro-methyl) pyridine) is utilized [3].
Throughout the degradation process, there must be sufficient levels of dissolved oxygen (DO) in the BOD test
bottle, preferably above 2 mg/l. Depletion of DO below this value at any time during the test, will incur anoxic
conditions, causing stress to the microbial population, hence affecting the BOD readings. If the value falls below 2
mg/l on the fifth day, the sample will simply be rejected and not considered to be part of the result. This is why
many analytical references on BOD testing often recommend preparation of serial dilutions of the same sample,
where incubation is done simultaneously [1].
The final variable for consideration is the quantity and type of microorganisms present. The microorganisms, which
drive the degradation process can either be introduced through seeding or assumed to be already present in ambient
water sample. As an added precautionary measure, seeding is often recommended by analytical references [3].
Though being the case, the quantification of the microbial population in the BOD tests remains arbitrary in many
practices. This does not mean that this variable is unimportant; after all it is the microorganisms that incur DO
depletion in the test bottle. Any disturbance, whether it is natural or otherwise, to microbial growth, will disrupt the
first-order reaction kinetics and hence affect the BOD results [4].
Problem Statement
The presence of reagents such as chlorine (Cl2), widely used as a disinfectant, in water and wastewater treatment
plants in many developing countries, is a good example of the disturbances discussed above. Chlorine is effective in
removing coliform organisms such as Escherichia coli (E. coli) and Enterococcus spp., by incurring osmotic
cellular protoplasmic decomposition [5]. The effects of these types of disinfectants on microbes are widely
recognized, though little is known on the implications towards the BOD test itself, when samples due for testing
contain elevated levels of the constituent. A chlorine check is typically recommended prior to commencement of
BOD5 analysis [3].
Another perspective is, to look at this in terms of application of the BOD test for assessment of ambient water
quality, particularly at the estuarine zone where salinity levels, as a result of chloride (NaCl) is predominant. It has
been long accepted that BOD, as a parameter of assessment for organic contamination under such conditions is not
preferable, where Total Organic Carbon (TOC) analysis is more preferred [6]. To what extent the chloride content
affects the BOD test under brackish conditions, remains ambiguous. TOC analysis though providing a viable, more
representative alternative is not necessarily a cost-effective solution, due to limited facilities and equipment [7].
This is even more so true when a comprehensive monitoring network is already in place. It is on this basis that the
extent of chloride influences on the BOD5 test, or more specifically the reaction kinetics involved needs to be
further scrutinized.
Methodology
Prior to conducting the analysis, suitable locations for grab sample collection were identified. Since the BOD5 test is
a bio-assay procedure, where the sensitivity of the analysis is directly related to the DO margin between the first and
fifth day, it was therefore necessary, to choose locations where organic contamination was known to be significant;
in order to encapsulate the maximum degradation, and hence view clear and distinct variations between runs. This
was done qualitatively, by correlation to specific land uses. Rivers and streams in the state of Selangor, Peninsular
Malaysia that receive significant amount of organic contributions, such as from sullage or greywater, sewage and
industrial sources were the best candidates to collect the grab samples. Based on historical monitoring data, these
stations were also known to exhibit significant BOD. Five sampling stations were identified; Sg. Rawang (Rawang
The Malaysian Journal of Analytical Sciences, Vol 14 No 1 (2010): 24 - 31
26
river), Sg. Serendah (Serendah river), Sg. Klang (Klang river) and Sg. Damansara (Damansara river, 2 stations,
upstream and downstream). The geographical coordinates of these stations are shown in Table 1 below:
Table 1: Location of Sampling Stations
River Basin Description Latitude
(N)
Longitude
(E)
Station
ID
Sg. Rawang Sg.
Selangor
Predominantly receives sewerage pollution
input from Rawang town, a tributary of Sg.
Serendah.
3° 19’00’’ 101° 4’00’’ S1
Sg. Serendah Sg.
Selangor
Identified as most polluting tributary
within Sg. Selangor particularly for
organic contaminants such as BOD, COD
and NH3-N
3° 21’00’’ 101° 33’00’’ S2
Sg. Klang Sg.
Klang
Receives input from various types of
pollution sources in Selangor state, border
transcends to Kuala Lumpur.
3° 2’50” 101° 30’43” S3
Sg. Damansara
(Upstream)
Sg.
Klang
A tributary of Sg. Klang, station is prior to
receiving industrial effluent from Shah
Alam industrial zone, located near TTDI
Jaya.
3° 4’25’’ 101° 33’16” S4
Sg. Damansara
(Downstream)
Sg.
Klang
Station located after industrial zone input,
but prior to Sg. Klang confluence.
Receives treated leachate discharge from
Waste Transfer Station.
3° 3’17” 101° 32’56” S5
All samples collected were incubated at 4°C for about 2 hours, during transit from site to the laboratory. The actual
BOD5 analysis was conducted in accordance with the American Public Health Association (APHA) Standard
Methods for the Examination of Water and Wastewater, Method 5210B.
Prior to incubation and analysis, sodium chloride (NaCl) solutions were prepared and mixed with the dilution water.
This was done with by using gravimetric method, factoring in the solubility limit of the constituent in a 300 ml
BOD5 test bottle under varying salinity levels from 5 parts per thousand (ppt) to 25 ppt for each of the sample
collected, at different dilutions. Table 2 below illustrates the amount of NaCl addition required to the achieve the
desired salinity ;
Table 2: Amount of Sodium Chloride (NaCl) added to 300ml BOD5 Test Bottle
Desired Salinity (ppt) NaCl addition (g)
5 1.5
10 3.0
15 4.5
20 6.0
25 7.5
Zaki Zainudin et al: EFFECTS OF INDUCED SALINITY ON BOD5 REACTION KINETICS OF
RIVER WATER SAMPLES
27
Lide [10], showed that the solubility of NaCl, at an incubation temperature of 20°C, based on the above desired
salinity levels should be close to 100%. DO levels in each of the BOD bottles were monitored daily, to view any
variation in decay rate, k1 and daily BOD. There are many proposed methodologies pertaining to k1 determination,
the one employed in this study is the Thomas’ graphical method [8]. This method relies on the following BOD rate
equation:
[
]
3
0t kt)6/1(1)kt(LBOD −
+= (2)
Rearranging this equation, and taking the cube root of both sides yields ;
)t(
)L(6
)k(
)kL(
1
BOD
t
3/1
0
3/2
3/1
0
3/1
t
+=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛ (3)
A plot of (t/BODt)1/3 over time is linear. The intercept and slope are defined as:
3/1
0)kL(A −
= (4)
3/1
0
3/2
)L(6
)k(
B= (5)
Finally solving for L0
1/3, in Eq. 3.11 by substitution of Eq. 3.12 yields:
⎟
⎠
⎞
⎜
⎝
⎛
=A
B
6k
(6)
To summarize, in calculating the bottle decay rate, k or k1, (t/BODt)1/3 versus time is plotted on an arithmetic graph
and a best-fit straight line is drawn, after which the intercept (A) and slope (B) from the plot is determined and
finally k, is calculated based on Eq. 6.
Results and Discussion
Referring to Figure 1 below, unsurprisingly, there is a noticeable difference for the BOD samples tested with
varying degrees of salinity. Generally, the bottle decay rate, k1, decreases as salinity increases, which in turn is an
indicator that the chloride is disrupting microbial activity. This hypothesis has been previously established, what is
interesting though, is the extent of the effect on the samples tested. After the fifth day, the margin, for Sg.
Damansara (downstream) and Sg. Klang samples, exhibited the maximum observable deficiency in BOD (ΔBOD),
between lowest and highest salinity at 8 mg/l each (Sg. Damansara (downstream) ; 0 ppt BOD5 = 15 mg/l, 25 ppt
BOD5 = 7 mg/l; Sg. Klang; 0 ppt BOD5 = 16 mg/l, 25 ppt BOD5 = 8 mg/l), a reduction of more than 50%. The
lowest variation was in the Sg. Serendah sample at about 1.5 mg/l (25%) which can be considered negligible as the
standard error for the BOD test is about 2 mg/l [3].
It should be noted that not all of samples followed the same inverted BOD-salinity relationship. There were also
some discrepancies, where higher salinity levels, actually incurred higher BOD (particularly for Sg. Rawang). This
phenomenon relates back to the fact that the BOD test is a bioassay procedure, and heterogeneous distribution of
organics and microbial populous, may be the root cause. Further observations are required to determine the root
cause of the anomaly.
The Malaysian Journal of Analytical Sciences, Vol 14 No 1 (2010): 24 - 31
28
Figure 1: BOD5 Analysis Results for Varying Induced Salinity Levels
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
12345
BOD(mg/l)
Days
Sg.Klang
0ppt 5ppt 10ppt 15ppt 20ppt 25ppt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
12345
BOD(mg/l)
Days
Sg.Damansara(Downstream)
0ppt 5ppt 10ppt 15ppt 20ppt 25ppt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
12345
BOD(mg/l)
Days
Sg.Damansara (Upstream)
0ppt 5ppt 10ppt 15ppt 20ppt 25ppt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
12345
BOD(mg/l)
Days
Sg.Rawang
0ppt 5ppt 10ppt 15ppt 20ppt 25ppt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
12345
BOD(mg/l)
Days
Sg.Serendah
0ppt 5ppt 10 ppt 15ppt 20ppt 25ppt
Zaki Zainudin et al: EFFECTS OF INDUCED SALINITY ON BOD5 REACTION KINETICS OF
RIVER WATER SAMPLES
29
The decay rate analysis, k1, in accordance with the Thomas graphical method was then conducted, the results of
which are summarized in Table 3 and illustrated in Figure 2:
Table 3: BOD Decay Rate, k1, Analysis Summary
BOD Decay Rate, k1 (1/day)
Salinity (ppt) Sg. Rawang Sg. Serendah Sg. Klang Sg. Damansara
(Upstream)
Sg. Damansara
(Downstream)
0 0.754 0.798 0.306 0.466 0.420
5 0.719 0.663 0.299 0.444 0.390
10 0.691 0.533 0.265 0.416 0.390
15 0.670 0.662 0.257 0.401 0.383
20 0.577 0.626 0.260 0.440 0.251
25 0.513 0.626 0.265 0.243 0.234
Δk1 (k25
ppm
– k0
ppm
) 0.241 0.172 0.041 0.223 0.186
%Δk1/k1
(
s=0
)
31.96% 21.55% 13.39% 47.85% 44.29%
Figure 2: BOD Decay Rate, k1, Graphical Analysis
The Malaysian Journal of Analytical Sciences, Vol 14 No 1 (2010): 24 - 31
30
Again, it is apparent there is a decrement in the bottle decay rate, k1, with regards to increasing salinity. The highest
quantum was observed in Sg. Rawang at 0.241/day, followed by Sg. Damansara (upstream) at 0.223/day, whereas
the lowest quantum was observed at Sg. Klang at 0.041/day. At first glance, this may seem anomalous, because, as
mentioned previously, Sg. Klang and Sg. Damansara (downstream) exhibited the highest reduction in terms of
overall BOD in the analytical proceedings. What needs to be understood here is that although there seems to be a
significant reduction in k1, (denoted as Δk1), the influence on the overall in-stream BOD magnitude, still remains
relative to the overall/original decay rate, vis-à-vis, the ratio Δk1/k1(s=0) is a more indicative contributor of the
influence of chloride towards overall BOD reduction. Sg. Rawang for example, though exhibiting a Δk1 of
0.241/day, only has a relative reduction or Δk1/k1(s=0) of 32% whereas Sg. Damansara (downstream) on the other
hand exhibited a Δk1/k1(s=0) of about 44%, an even more significant reduction than the former.
The rate of decrement itself (Δk1), varies from one sample to the next, which again may be attributed to the mix of
microbial populations already present in the sample, as well as the composition and biodegradability of the organic
constituents present, which more likely than not, is site specific and relative to input sources. However, it is clear,
for there to be any significant reduction in oxygen demand exerted by microbial organisms when stabilizing
biodegradable organic matter by salinity/chloride, the margin of relative reduction to the original decay rate
(Δk1/k1(s=0)) must be significant, whereas the magnitude of reduction (Δk1) alone is insufficient.
The decay rate in the bottle, k1 is often misinterpreted as kd, which is the in-stream decay rate. kd can differ to k1 by
as much as ten times [8], due to the unrestricted supply of oxygen transfer, occurring at the air-water interface,
attributed to re-aeration as well as photosynthesis. Therefore it would also be safe to assume that the decay rate ratio
affecting the bottle decay also applies under these conditions as well. The only unaccounted factor relating to BOD
kinetics under estuarine conditions is therefore tidal dilution of organic contaminants, which of course has a
substantial effect [9]. Albeit being the case, this case study has clearly shown that BOD is not a suitable parameter
for assessment of saline waters; the bio-kinetics is simply skewed, as elaborated above.
Conclusion
From the preliminary study conducted above, there is a significant drop in BOD as a result of increasing salinity in
all the river water samples collected. This was directly attributed to the influence of chloride in relation to microbial
cellular decomposition. Although the magnitude of Δk1 varies from one sample to the next, the end results subject
BOD to further scrutiny as a suitable water quality parameter for monitoring of estuarine zones. In consequence of
this observation, other water quality applications which cannot avoid using BOD as an indicator for organic matter,
such as in water quality modeling, need to account for the effects of salinity towards microbial activity [6]. A
reasonable approximation pertaining to the reduction in decay rate, in particular for tropical rivers can be done using
the above results.
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RIVER WATER SAMPLES
31
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