Correlating Biochemical and Chemical Oxygen Demand of Effluents
– A Case Study of Selected Industries in Kumasi, Ghana
123F. K. Attiogbe, Mary Glover-Amengor and K.T. Nyadziehe
1Department of Chemical Engineering, Kumasi Polytechnic, Kumasi, Ghana 2CSIR-Food Research Institute, P.O.Box M20, Accra, Ghana 3Department of Chemical Engineering, Kwame Nkrumah University of Science and
Technology, Kumasi, Ghana
This work aims at establishing an empirical correlation between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) of effluents from selected 5
industries in the Kumasi Metropolis to facilitate speedy effluent quality assessment or optimal process control. Hourly effluent samples were collected for an eight-hour period three times per week for analysis of the principal parameters BOD and COD 5
using the “Standard Methods for the Examination of Water and Wastewater”. Measured BOD and COD of the effluents were above the required discharged 5
guideline of 50 mg/l and 250 mg/l respectively specified by the Environmental Protection Agency, Ghana. These levels of BOD and COD of the wastewaters could 5
pose potential pollution to the waterbodies in which they are discharged since untreated. Results also indicate that the degree of common variation between the two variables COD and BOD is highly positive for the effluents from Ghana Brewery 5
Limited (GBL), Guinness Ghana Limited GGL) and The Coca-Cola Bottling Company of Ghana (Coca-Cola), thus the variables are said to be correlated. However, the degree of common variation between the variables was low for the effluent from the Kumasi Abattoir. The fitted equations were (GBL, y = 0.6558x – 270, r = 0.93;
Coca-Cola, y = 0.3536x + 113.6, r = 0.81; GGL, y = 0.225x + 1343.7, r = 0.83; Kumasi Abattoir, y = 0.1331x + 54806, r = 0.22) with x and y representing COD and BOD respectively. The BOD/COD ratio for the wastewater from the selected 55
industries can also be approximated to the gradient of their respective fitted equations i.e. GBL, 0.66; Coca-Cola, 0.35; GGL, 0.23. The fitted equations for GBL, Coca-Cola and GGL can be used to facilitate rapid effluent assessment or optimal process control by these industries once the chemical oxygen demand is measured.
Keywords: Empirical Correlation, Correlation Coefficient, Biochemical Oxygen
Demand, Chemical Oxygen Demand, Effluent, Selected Industries
Kumasi the capital city of Ashanti Region of Ghana is located in the south-central part of the country, about 250 km (by road) northwest of Accra and with an approximate 2size of 254km. Lake Bosumtwi, the largest natural lake in Ghana, is located approximately 32 km south of Kumasi. With a population of 1,517,000 (2005, source: United Nations Population Division), Kumasi is the second-largest city in the country
(http://en.wikipedia.org/wiki/kumasi). Figure 1 shows the position of Ashanti Region and Kumasi the capital on the map of Ghana.
Figure 1: Map of Ghana
The metropolis has many industries, ranging from small to large-scale enterprises. These industries are scattered in almost all the suburbs of the metropolis, with the majority concentrated at Ahensan, Kaase, Asokwa and Chirapatre areas. They are engaged in the manufacture of a variety of products including alcoholic beverages, soft drinks, nails, foam mattresses, lumber and plywood. There is also a slaughterhouse for animals (Sarfo-Afriyie, 1999).
Despite the economic gains derived from these industries routine visits made by the Environmental Protection Agency indicates that most of the industries use the natural resources (air, water and land) as sink for their wastes. Some of the food processing industries, including breweries discharge untreated wastewater into the public drainage system or nearby streams. This disposal practice could constitute potential or severe pollution problems to the waterbodies since the effluents contain organic compounds that require oxygen for degradation (Sarfo-Afriyie, 1999).
If water of high organic matter content or biochemical oxygen demand (BOD) value flows into a river, the bacteria in the river will oxidize the organic matter consuming oxygen from the water faster than it dissolves back in from the air. If this happens, fish will die from lack of oxygen, a consequence known as fish kill. A stream must have a minimum of about 2 mg/l of dissolved oxygen to maintain higher life forms. In addition to this life-sustaining aspect, oxygen is important because the end products of chemical and biochemical reactions in anaerobic systems often produce aesthetically displeasing colours, tastes and odours in water (Hach et al.,1997) .
Water is our most precious resource. The cleanliness of our lakes, rivers and oceans is one of the pressing goals for environmental protection. The balance of nature depends therefore on the comprehensiveness of our approach to solve the problem of wastewater disposal.
The most widely used parameter of organic pollution measurement applied to both
) wastewater and surface water is the 5-day biochemical oxygen demand (BOD5
(Metcalf and Eddy, 1995). The biochemical oxygen demand (BOD) is the amount of oxygen, expressed in mg/l or part per million (ppm) that bacteria take from water when they oxidised organic matter. The test is performed at a defined temperature (normally 20?C) and for a standard period, which is usually 5 days (hence BOD) but 5
can be longer for specific purposes (www.cefic.com). During the five-day period of
the BOD test, the bacteria oxidized mainly the soluble organic matter present in the water. Very little oxidation of solid (insoluble) matter occurs in that short time (Hach et al., 1997). Two methods are widely used for BOD measurement. One method, the dilution method, is a standard method of the American Public Health Association (APHA) and is approved by the U.S. Environmental Protection Agency (USEPA). The other method, the manometric method, has been used for over 75 years in many sewage plants and other installations throughout the world. The USEPA denied approval of this method when it selected methods for wastewater analysis, although in certain cases the USEPA has approved the manometric method (Hach et al., 1997).
The monitoring agencies such as Environmental Protection Agency of Ghana (EPA) require that industries submit their BOD results periodically. However, due to the 5
long incubation periods and low reproducibility, most industries find it very difficult to measure and report the BOD results of their effluents. If they measured it at all, 5
the results provide historical data only and do not facilitate rapid water quality assessment or optimal process control. However, chemical oxygen demand (COD) measurement is simple and can reliably be carried out in the laboratory. The COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong oxidant. The dichromate reflux method has been preferred over procedures using other oxidants because of superior oxidizing ability with a wide variety of samples, and ease of manipulation (Boyles, 1997).
Comparison of BOD with COD assesses whether the compound is readily
, an indication is that a COD:BOD ratio of greater than 100 biodegradable. For BOD5
means that the compound is relatively non-biodegradable and a ratio of less than 10 that it is relatively degradable. However, low BOD may merely mean that the test 5
microbes need longer than the test period to begin breaking the compound down and therefore ultimate BOD or other biodegradation testing is generally much more reliable (www.cefic.com).
Analysis of the effluents COD and BOD need to be carried out and the values 5
obtained used to establish an empirical relation for easy conversion of COD to BOD. 5
The results obtained would provide the tool for effective monitoring and evaluation of the effluent by the selected industries and the monitoring agencies. In addition, this work will facilitate rapid effluent assessment or process control by these industries once the chemical oxygen demand is measured or vice versa.
2. Materials and Methods
2.1 Location and Duration of Work
The study was carried out at the Ghana Brewery Limited, the Coca-Cola Bottling Company of Ghana, Guinness Ghana Limited and the Kumasi Abattoir Company Limited because of the large volume of effluents they generate and discharged into the Sisa River. These companies are located in Ahensan and Kaase areas of the metropolis. Figure 2 shows a map of the site (Kumasi Metropolis).
A period of two weeks was allocated to each of these industries. Sampling of wastewater was done on April 17 – 26, 2002 for GBL, May 2 -15, 2002 for Coca- Cola, May 17 – 29, 2002 for GGL and May 31– 27June 2002 for the Kumasi Abattoir.
The entire period of the study was four months, spanning from April to July 2002.
Figure 2: Map of Kumasi (Town and Country Planning Department, Kumasi)
2.2 Sample Collection
Effluent samples were collected in well-labelled plastic bottles on hourly basis. The collection times were usually from 0800-1500GMT. When not analyzed immediately, oC but analyses were all samples were preserved by keeping in the refrigerator at 4
carried out within twenty-four hour with the exception of samples from the abattoir, which were worked on promptly.
For Guinness Ghana Limited, hourly grab samples of soil were collected at the discharge point. In the case of the Kumasi Abattoir, grab samples of soil were taken from an open manhole on an hourly basis.
Hourly effluent samples were collected for an eight hour period three times per week and brought to the Environmental Laboratory of the Kwame Nkrumah University of Science and Technology (KNUST) for analysis of the principal parameters, COD BOD using “Standard Methods for the Examination of Water and Wastewater (1992). 5
The pH of the samples was also determined using a field pH measuring kit WTW pH 323-B/Set-2.
2.3 Five-Day Biochemical Oxygen Demand (The Dilution Method)
The dissolved oxygen content of liquid was determined by the Azide modification of 0the Winkler’s method before and after incubation for five days at 20C. The
difference gave the BOD of the sample after allowance had been made for the 5
dilution, if any, of the sample. For optimum biochemical oxidation, the pHs of the samples for analysis were 6.5 to 8 (APHA, 1992).
2.4 Chemical Oxygen Demand (COD) Using the Open Reflux Method
The sample, to be measured, was oxidized under reflux with a known amount of potassium dichromate in strong sulphuric acid with silver sulphate as a catalyst. Organic matter reduced part of the dichromate and the remainder was determined by titration with iron (II) ammonium sulphate or iron (II) sulphate using ferroin as indicator. Interferences from chloride were suppressed by the addition of mercuric sulphate to the reaction mixture. The chemical oxygen demand (COD) was expressed as milligrams of oxygen absorbed from standard dichromate per litre of sample (APHA, 1992).
variation of the effluent from GBL ranged from 323 mg/l to 33,426 The daily BOD5
mg/l. Most of the higher values for BOD were obtained during the cleaning of the 5
process vessels. The corresponding COD ranged from 430 mg/l to 44,800 mg/l. The same applied to effluent from GGL where higher BOD values were registered when 5
spent yeast was discharged together with the other processing wastewaters. The BOD 5
ranges from 1,000 mg/l to 40,000 mg/l and the corresponding COD from 1,356 mg/l to 112,000 mg/l. Values ranging from 10,000 mg/l to 230,000 mg/l for BOD and 5
8000 mg/l to 472,000 mg/l for COD were obtained for wastewater from the Kumasi Abattoir. The effluent from the Coca-Cola Bottling Company however, recorded the lowest daily BOD and COD values ranging 75 mg/l to 1,744 mg/l and 142 mg/l to 5
3,560 mg/l respectively. The average daily hourly BOD, COD and COD/BOD ratio 55
values for the selected industries are shown in tables 1-4.
Table 1: Average Daily Hourly BOD and COD Values for GBL 5
1 2 3 4 5 6
BOD mg/l 2,918 3719 5,259 4,281 2,624 4,941 5
COD mg/L 3,988 5519 7,478 7,338 6,405 6,667
COD/BOD 1.37 1.48 1.42 1.71 2.44 1.35 5
Table 2: Average Daily Hourly BOD and COD Values for Coca-Cola 5
1 2 3 4 5 6
BOD mg/l 336 678 317 202 603 345 5
COD mg/l 627 1,457 535 421 1,172 878
COD/BOD 1.87 2.18 1.69 2.02 1.94 2.54 5
and COD Values for GGL Table 3: Average Daily Hourly BOD5
1 2 3 4 5 6
BOD mg/l 2,844 3,946 4,938 4,425 3,775 9,213 5
COD mg/l 10,061 19,131 11,973 12,122 8,263 32,132 COD/BOD 3.54 4.85 2.42 2.74 2.19 3.49 5
Table 4: Average Daily Hourly BOD and COD Values for Kumasi Abattoir 5
1 2 3 4 5 6 7 8
BOD mg/l 87,756 71,875 37,800 48,921 128,813 52,875 70,750 33,875 5
COD mg/l 30,375 56,625 136,540 34,613 247,225 79,073 39,250 46,375
COD/BOD 0.35 0.79 3.61 0.71 1.92 1.50 0.55 1.37 5
The linear regression results obtained during the effluent analysis from the selected
industries in the Kumasi Metropolis are depicted in Figure 3 - 6.
A linear response was obtained between the data sets. The fitted equations on the
entire data (COD and BOD) set on a particular industry are generated with their 5
corresponding correlation coefficients (r).
Figure 3: Linear Regression of BOD and COD of Wastewater from GBL 5
y = 0.6558x - 270
2r = 0.856
and COD of Wastewater from the Coca-Cola Figure 4: Linear Regression of BOD5
1.8E+03y = 0.3536x + 113.6
21.6E+03r = 0.66311.4E+03
Figure 5: Linear Regression of BOD and COD of Wastewater from GGL 5
y = 0.225x + 1343.7
2r = 0.6856
and COD of Wastewater from Kumasi Figure 6: Linear Regression of BOD5
Abattoir Company Limited
y = 0.1331x + 54806
2r = 0.0495
The correlation coefficients of the fitted equations were generally positive for the results from the Ghana Brewery Limited samples analysis, being between 0.14 and 1.00.
For Guinness Ghana Limited, the correlation coefficients were between 0.21 and 0.95. However, the correlation coefficient of the entire data on GGL gave r = 0.83. In the case of Coca-Cola, the correlation coefficients of the fitted equations were positive being between 0.40 and 1.00.
The fitted equations correlation coefficients for the results from the daily analysis of the abattoir effluent were generally near zero being between 0.22 and 0.85. The exceptional correlation coefficient of 0.85 was obtained for a particular day analysis.
The following can be deduced:
The COD and the BOD results obtained for GBL, Coca-Cola and the GGL 5
showed that the degree of common variation between the two variables was highly
positive; thus the COD and the BOD are said to be highly correlated. 5
The fitted equations were y = 0.6558x - 270 (GBL), y = 0.3536x + 113.6 (Coca-
Cola) and y = 0.225x + 1343.7 (GGL).
The correlation coefficients of the equations for the GBL, Coca-Cola and GGL
were highly positive being 0.93, 0.81 and 0.83 respectively.
The BOD/COD ratio for GBL, Coca-Cola and GGL can be approximated to 5
the gradient of their respective fitted equations i.e. 0.66, 0.35 and 0.23.
However, the result obtained for the Kumasi Abattoir Company Limited showed
the degree of common variation between the COD and the BOD to be very low. 5
The fitted equation was y = 0.1331x + 54806 and the correlation coefficient being
0.22. For correlation coefficient r near zero, it means that there is no linear
correlation between the variables. In other words the correlation coefficient
measures the goodness of fit of the equation actually assumed to the data (Ayyub
and McCuen, 1997). Thus there is almost no linear correlation between the COD
for this effluent and thus a linear equation cannot be assumed. and the BOD5
The BOD and the COD values obtained on analysis of the effluents of the selected 5
industries have been found to be higher than expected from Environmental Protection Agency, Ghana Standards guideline for discharge ( BOD, 50 mg/l and the COD, 250 5
mg/l). These levels of BOD and COD could constitute potential pollution problems 5
to our waterbodies since they contain organic compounds that will require a large quantity of oxygen for degradation.
COD of some of the samples were lower than the BOD for the Abattoir wastewater 5
and this could be attributed to the presence of ammonia which is noted to significantly increase measured BOD as a result of microbial oxidation of the ammonia (ultimately to NO) (www.cefic.com). 3
The COD: BODratios for all the selected industries have been found to be less than 5
10. This implies that the compounds in the effluents of these industries were relatively degradable thus a possible depletion of the dissolved oxygen in the receiving river and a potential effect on aquatic life.
The COD correlates positively with the BOD of the wastewaters from GBL, GGL 5
and Coca-Cola. The correlation equations can be used to estimate the BOD for 5
reporting and for process control.
The negative correlation of the chemical oxygen demand (COD) to the five-day biochemical oxygen demand (BOD) at the instances that these occur could be 5
attributed to the composition or characteristics of the waste (effluent) not being constant (www.hach.com) and this is supported by the wide variation in correlation value from the linear regression of the Kumasi Abattoir wastewater (Fig. 6).
The degree of common variation between the COD and the BOD of the wastewaters 5
from the GBL, Coca-Cola and GGL was highly positive, thus the two parameters are correlated with good correlation coefficients of 0.93, 0.81 and 0.83 respectively.
The correlation coefficient of the fitted equation for prediction for Ghana Brewery Limited, Guinness Ghana Limited and The Coca-Cola Bottling Company of Ghana, may therefore be used to facilitate rapid effluent quality assessment or optimal process control by these industries once the chemical oxygen demand (COD) is measured or vice versa.