# Predicting Optimum Dilution Factors for BOD Sampling and Desired Dissolved Oxygen for Controlling Organic Contamination in Various Wastewaters

Authors: Hisham A. Maddah
Journal: International Journal of Chemical Engineering
 (2022)

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## Abstract

High biochemical oxygen demand (BOD) concentrations in water minimize oxygen availability, damage ecosystem biodiversity, impair water quality, and spoil freshwater. The increased level of BOD is an indication of severe organic pollution of freshwater. Thus, this study aims to establish empirical correlations between the 5-day biochemical oxygen demand (BOD5) and organic decomposition time to control organic pollution in various wastewater effluents. Ultimate biochemical oxygen demand (UBOD) and minimum and average BODt data sets along with their reaction rates were collected from earlier sampling analyses in the plants used for industrial, domestic (sanitary), and storm (surface) wastewater treatment. Average BOD5/COD ratios were then utilized to calculate existing 5-day dissolved oxygen (DO5) concentration for the estimation of experimental dilution factors (dfs) as a good start in sampling analysis to reach an optimum DO5 concentration. Moreover, the relationships between average BOD5 vs. COD, and BOD5 vs. DO5, were obtained based on the literature with 60–70% oxygen consumption rates required for organic decomposition. Results showed that such BOD5 relationships with time (power equations) or with COD (linear correlations) are helpful for wastewater engineers to generate valuable and accurate results for quality control, without the need to conduct laboratory experiments. The proposed regression equations would facilitate effluent quality assessment, allowing selection of optimal processes to control microbiological contamination or organic constituents in wastewaters.


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## Body

## 1. Introduction

Biochemical oxygen demand (BOD) is the oxygen demand of the organism to break down organic matter over a given period. Typically, organic pollution in freshwater bodies can be detected by the experimentally measured BOD levels from sampling analysis, which is directly correlated to microbiological contamination in sampled water [1].The current projections of demographic growth and the increased demand for dairy products and consumption of meat prompted concerns regarding surface water quality due to the worsening of the global “sanitation crisis.” Such climate change is expected because of the introduced sanitary issues in surface water bodies (especially rivers) that require reduction of discharge flows and/or dilution capability. BOD rates have been modeled on a global scale, which concluded that chemical wastes and organic discharges have a major impact on the world’s economy if wastewater treatment progress is kept at current levels [2–4].In Europe, a previous study projected an increase in organic pollution, especially in the southern countries, where most rivers have lost their dilution power as a result of the lack of dilution capacity of wastewater discharge in water-scarce areas. The study indicated that the eastern part of Europe and the Black Sea were to be more impacted by decreased river dilution capability and possible water quality deterioration [5]. The BOD parameter was chosen by the United Kingdom (UK) in 1908 as an indicator of the organic pollution of rivers enforced by the UK “Royal Commission on River Pollution.” The traditional five-day period to estimate the BOD5 parameter was selected for this test because this is supposedly the longest time that river water takes to travel from its source to its delta (outlet end meeting a bay or an ocean) in the UK. Following that, the methods introduced in 1936 by the committee “American Public Health Association Standard” suggested using the BOD parameter as a reference indicator to evaluate the biodegradation of chemicals and hazardous substances. The use of the BOD5 parameter arises due to three major applications: (1) it is an indicator for the conformity of the wastewater discharge and the waste treatment procedures with the current up-to-date regulations, (2) it is required to obtain the ratio of BOD5 to chemical oxygen demand (COD) that would indicate the biodegradable fraction of effluent (discharge) from wastewater treatment plants, and (3) the ratio COD/BOD5 is an indicator of the size of a wastewater treatment plant required for a specific location [6–9].BOD5 is the amount of oxygen that is consumed by bacteria and other microorganisms to be aerobically degraded in a water medium over a 5-day span at a standard temperature of 20°C. Therefore, the BOD5 is an indirect metric method for quantifying existing organic or chemical wastes that are biodegradable in presence of oxygen in water, expressed in mg O2/L [10–13]. BOD5 indicates the amount of O2 dissolved in mg/L that is required in a particular time for biodegradation of the components of organic water waste. This value is an important parameter for the assessment of the degree of organic contamination that wastewater poses to the environment. The superficial flow of rainwater causes water to become contaminated, and this rainwater organic contamination can be analyzed by several parameters including BOD5. The importance of this parameter value lies in the evolution of the degree of the contamination of environmental wastewater. Since the wastewater contents or constituents in the receiving water are decomposed by the bacteria present in it, the oxygen is either completely or partially withdrawn from the water to carry such organic pollutants degradation [6–8].Two well-known methods are widely used for BOD measurement: (1) the dilution method, which is the most common and recent BOD measurement method based on the American Public Health Association (APHA) standards that have been certified by the US Environmental Protection Agency (USEPA), and (2) the manometric system, which has been commonly used in many sewage plants and other facilities around the world for over 75 years [14]. However, the USEPA denied the approval of the latter method for wastewater analysis, although in certain cases the USEPA has approved the manometric method due to lack of data consistency and progress in associated laboratory techniques [15]. In short, measuring BOD requires taking a minimum of two measurements, one measurement for the current (immediate) or initial dissolved oxygen (DO0), and the second measurement is after incubation of water samples in the lab for 5 days to be then tested for the remaining amount of final 5-day dissolved oxygen (DO5). Such experiments would allow us to quantify the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period [10–13].BOD is affected by the same factors that affect DO. High BOD-containing wastewaters will impact a minimum of 2.5 billion people by the year 2050. High BOD concentrations (e.g., 5 mg O2/L) in water minimize oxygen availability, degrade aquatic habitats as well as ecosystem biodiversity, impair water quality, and spoil freshwater [5]. Sources of BOD include topsoil, leaves and woody debris, animal manure, and effluents from pulp and paper mills [10–13]. The high BOD loadings to freshwater and/or watershed systems are mainly coming from anthropogenic sources, as mentioned, comprising domestic and livestock (animal) waste, industrial emissions, agricultural pollutants, and combined or mixed sewer overflows. During their path in the stream network, BOD concentrations become lower with continuous microbial degradation, leading to river self-purification, and self-revitalization, and dilution of BOD-containing wastewater before reaching the seas [5]. The higher the BOD value, the faster the oxygen in the stream is depleted. This suggests that higher levels of marine life have less oxygen available to them to be consumed aerobically. High BOD has the same effects as low dissolved oxygen where marine lives become suffocated and eventually die. Wastewater treatment plants, feedlots, food-processing plants, failing septic systems, and urban stormwater runoff can cause a spike in recorded BOD due to organic wastes in water [10–13].In the present study, we aim to establish empirical correlations between BOD5 and organic decomposition time to control organic pollution. Ultimate biochemical oxygen demand (UBOD) and minimum and average BODt data sets along with their reaction rates were collected from the plants used for industrial, domestic (sanitary), and storm (surface) wastewater treatment. The characteristics of various wastewater effluents (which are actually influents to the different wastewater treatment plants) have been thoroughly studied. The ultimate BOD and reaction rates were collected to be used as input to calculate average BOD5/COD ratios and DO5 concentration for the estimation of experimental dilution factors (dfs) at various temperatures and times, that is a very important factor to know how much oxygen should be introduced, based on existing BOD, for successful and complete organic decomposition, allowing us to find the relationships between average BOD5 vs. COD, and BOD5 vs. DO5. The proposed regression equations are believed to allow wastewater engineers to facilitate effluent quality assessment via selecting optimal processes that would control microbiological contamination, without the need for laboratory analysis.


## 2. Methodology and Study Framework

As discussed, the literature data were collected based on earlier BOD sampling analyses of various wastewater types including industrial, domestic (sanitary), and storm (surface) wastewater in different wastewater treatment plants. The previously collected laboratory data regarding BODt at their UBOD and minimum and average determined BODt values were then utilized to calculate the corresponding minimum and average constant reaction rates (k1) that would quantify organic decomposition rates in the studied BOD-containing sampled water. Then, the same identified k1 was applied in the BODt fundamental formula in equation (1), knowing UBOD, to obtain BODt at various times for t = 0–60 days with an increment rate of 5 days. The ultimate goal was to establish relationships between the BODt and organic decomposition time (t) to control organic pollution in various wastewater effluents. Reaction rates were assumed to be constant numbers since we are treating the same wastewater type at the same plant (i.e., similar wastewater characteristics).(1)BODt=UBOD1−e−k1t.We have also considered the impact of treating wastewater at lower temperatures than that already observed water temperatures in the various studied wastewater samples from selected wastewater plants. The same steps and/or train of calculations were followed to obtain [BODt vs. time] correlations when wastewater is treated at lower feed temperature, from using the common k1T temperature-dependent equation shown in equation (2) along with the predetermined θ constants and their temperature limits as reported in the literature (θ = 1.056 for T = 20–30°C and θ = 1.135 for T = 4–20°C) [16].(2)k1T=k120θT−20.The second established and adopted train of calculations starts with obtaining the average BOD5/COD ratios that were later used in estimating the average laboratory dilution factors (dfs). It is worth mentioning that we carried out our analysis with a selected ΔDO from x = 60–70% in equation (3), determined from DO0, where ΔDO corresponds to organic decomposition associated with the oxygen consumption in water (final dissolved oxygen minus initial dissolved oxygen), knowing that DO0 at 20°C = 9.1 mg/L for BOD5 (for domestic and industrial wastewaters) and that DO0 at 26.3°C = 8.1 mg/L for BOD5 (for storm wastewaters). The chosen organic decomposition rate of x = 60–70% is considered a reliable range that has been previously applied in the simulation of oxygen saturation concentrations in wastewater in a sludge treatment plant in Jakarta at various temperatures. Also, from the list of oxygen concentrations at a certain temperature, it was found there is a standard DO0 at a maximum of 9.1 mg/L and a minimum of 7.5 mg/L at 20°C and 30°C, respectively [13, 16, 17].(3)ΔDO=DO5−DO0⇒DO5=xDO0.We have calculated the estimated df from the known BOD5, COD, and DO0 and the determined DO5 at x = 65% (average) based on equation (3), which would be input in equation (4) for df.(4)df=BOD5/COD|avg×CODavgDO0−DO5mg/L.For each of the studied wastewater types from the various wastewater treatment plants, we have plotted the relationship between BOD5 and COD, BOD5 and df, as well as BOD5 against DO5 at x = 65%. Such identified trendlines are believed to allow wastewater engineers and plant technicians or operators to adjust operating conditions accordingly for desired DO corresponding to feed BOD5/COD ratios, which would ensure organic decomposition and smooth treatment operation. Figure 1 shows the study framework and the utilized step-by-step methods.Figure 1
Study framework starting with data sets collection for determining BOD5-related relationships in the studied wastewater types from various wastewater treatment plants.


### 2.1. Data Collection and Data Set Curation

Data sets of industrial wastewater were curated based on 3 factories located in Ghana (Kumasi Metropolis, the capital city of Ashanti) namely Kumasi Abattoir, Coca-Cola, and GGL factories. From sampling methods and adopted data collection for industrial wastewater type, the DO5 content of liquid was determined by the Azide modification of Winkler’s method before and after incubation for 5 days at 20°C. Such calculated differences would give the BOD5 values of the sample after allowance had been made for the dilution. For optimum biochemical oxidation, the pH values of the samples were kept in the range of 6.5–8 to have consistent analysis [18]. Three various industrial plants were selected to gather their industrial wastewater BOD5 and COD so that BOD5 can be recalculated at different k1 and different temperatures using the common k1T temperature-dependent equation shown in equation (2) along with the predetermined θ constants and their temperature limits as reported in the literature (θ = 1.056 for T = 20–30°C and θ = 1.135 for T = 4–20°C) [16]. Again, we calculated BOD5 for 60 days, and in the last, we have taken the average BOD5/COD for each plant to get dilution factors.Regarding stormwater data collection, data sets have been collected from stormwater sources found in different countries. We collected the stormwater quality from the previously carried out sampling of stormwater runoff in the street, highway, surfaces, and parking, collected from different catchments in the selected three different countries (Bialystok-Poland, Abeokuta-Nigeria, Luxembourg), and previously taken for the analysis of pollutants including the parameter of interest here, BOD5. Similar calculation steps were followed here to recalculate the BOD5 in several storm wastewater plants to identify the relationship between BOD5 vs. COD in ordinary wastewater runoff or rainwater while correlating BOD5 to the initial dissolved oxygen and required dilution factors [6–8].However, the collection of data and data set curation from various domestic wastewater treatment plants (three plants) was carried out based on data collected from Al-Diwaniyah Wastewater Treatment Plant, the wastewater treatment plants of Jordan, and the North Sewage Treatment Plant in Dhahran, Eastern Province, Saudi Arabia. Typically, domestic wastewater treatment plants collect their samples to be analyzed and find the level of water contamination, and then water quality is compared with other plants’ influents/effluents in the industry for the achievement of less polluted wastewatervia various treatment methods. For instance, wastewater stabilization ponds, activated sludge units, and trickling filters are some of the common strategies used for municipal wastewater treatment. Locally, 27 wastewater treatment plants distributed in the populated centers of Jordan are using the above-mentioned techniques for wastewater treatment. Such contamination can be investigated or attributed to the BOD5 level in collected samples for pilot-scaling to improve the treatment performance from changing organic loading rates or modifying designed hydraulic, or operating conditions, considering pH, turbidity, TSS, COD, and BOD, for better effluent quality that would potentially reduce contamination to the environment and risk of ecosystem pollution [10–13, 19, 20].


## 2.1. Data Collection and Data Set Curation

Data sets of industrial wastewater were curated based on 3 factories located in Ghana (Kumasi Metropolis, the capital city of Ashanti) namely Kumasi Abattoir, Coca-Cola, and GGL factories. From sampling methods and adopted data collection for industrial wastewater type, the DO5 content of liquid was determined by the Azide modification of Winkler’s method before and after incubation for 5 days at 20°C. Such calculated differences would give the BOD5 values of the sample after allowance had been made for the dilution. For optimum biochemical oxidation, the pH values of the samples were kept in the range of 6.5–8 to have consistent analysis [18]. Three various industrial plants were selected to gather their industrial wastewater BOD5 and COD so that BOD5 can be recalculated at different k1 and different temperatures using the common k1T temperature-dependent equation shown in equation (2) along with the predetermined θ constants and their temperature limits as reported in the literature (θ = 1.056 for T = 20–30°C and θ = 1.135 for T = 4–20°C) [16]. Again, we calculated BOD5 for 60 days, and in the last, we have taken the average BOD5/COD for each plant to get dilution factors.Regarding stormwater data collection, data sets have been collected from stormwater sources found in different countries. We collected the stormwater quality from the previously carried out sampling of stormwater runoff in the street, highway, surfaces, and parking, collected from different catchments in the selected three different countries (Bialystok-Poland, Abeokuta-Nigeria, Luxembourg), and previously taken for the analysis of pollutants including the parameter of interest here, BOD5. Similar calculation steps were followed here to recalculate the BOD5 in several storm wastewater plants to identify the relationship between BOD5 vs. COD in ordinary wastewater runoff or rainwater while correlating BOD5 to the initial dissolved oxygen and required dilution factors [6–8].However, the collection of data and data set curation from various domestic wastewater treatment plants (three plants) was carried out based on data collected from Al-Diwaniyah Wastewater Treatment Plant, the wastewater treatment plants of Jordan, and the North Sewage Treatment Plant in Dhahran, Eastern Province, Saudi Arabia. Typically, domestic wastewater treatment plants collect their samples to be analyzed and find the level of water contamination, and then water quality is compared with other plants’ influents/effluents in the industry for the achievement of less polluted wastewatervia various treatment methods. For instance, wastewater stabilization ponds, activated sludge units, and trickling filters are some of the common strategies used for municipal wastewater treatment. Locally, 27 wastewater treatment plants distributed in the populated centers of Jordan are using the above-mentioned techniques for wastewater treatment. Such contamination can be investigated or attributed to the BOD5 level in collected samples for pilot-scaling to improve the treatment performance from changing organic loading rates or modifying designed hydraulic, or operating conditions, considering pH, turbidity, TSS, COD, and BOD, for better effluent quality that would potentially reduce contamination to the environment and risk of ecosystem pollution [10–13, 19, 20].


## 3. Results and Discussion

Herein, the study of how BODt changes over time concerning the three different wastewater types has been thoroughly discussed. Regarding the BODt for stormwater influents (from three different plants), the increasing BODt trendline with time was justified from raw data in the three selected plants, indicating the strong reliability of the existence of power relationships between BODt and time as illustrated in Figure 2. Both average and minimum BODt values (k1 @ T = 20°C) were plotted from the various storm wastewater plants, showing a mean range of an average BODt ≈ 40–78 mg/L and a mean range of a minimum BODt ≈ 16–55 mg/L, as plotted in Figures 2(a), 2(c), and 2(e). Similarly, the same analysis was carried out with k1 @ T = 25°C to find the impact of temperature on the BODt mean ranges, which were found to increase the initial value (at t = 0) by +6 (with a mean range of an average BODt ≈ 46–78 mg/L and a minimum BODt ≈ 22–60 mg/L from k1 @ T = 25°C), as plotted in Figures 2(b), 2(d), and 2(f). This analysis justified the strong correlation and dependence of BODt on the water temperature, with an average of +1.2 mg/L per every 1°C as a heat added to the water for the initial BODt values. It is worth mentioning that the power relationships were expected in all plants since they refer to the organic decomposition rates occurring in storm wastewater that would follow an exponential or a power pattern according to the previously discussed or derived equation (1) for BODt.Figure 2
The determined fitted power relationships between BODt and time for 60 days of minimum and average BODt values collected from analysis of influents in various selected storm wastewater treatment plants: (a, b) Poland storm wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; (c, d) Luxembourg storm wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; and (e, f) Abeokuta-Nigeria storm wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively.
(a)(b)(c)(d)(e)(f)Moreover, the BODt for industrial wastewater influents were observed with an obvious increase of BODt trendline with time. However, much higher BODt concentrations were there because industrial wastes contain very high amounts of organic compounds generated from various chemical and industrial processes. The power relationships between BODt and time were only accurate at low or minimum BODt of previously analyzed industrial wastewaters as illustrated in Figure 3. Such inaccuracies have been attributed to the rich organic constituents found with average BODt samples, making it very difficult for the oxygen-containing wastewater to degrade organics over a short period of 60 days (i.e., it requires either much longer time or very high added oxygen concentrations to see the power relationship between maximum BODt and time, instead of the linear trends). Plotted data sets showed a mean average BODt >90,000 mg/L (with an average of 84,000 mg/L) and a mean minimum BODt <80,000 mg/L, both from k1 @ T = 20°C, as plotted in Figures 3(a), 3(c), and 3(e). Similarly, the same analysis was carried out with k1 @ T = 25°C to find the impact of temperature on the mean BODt, which was found to increase the initial BODt (at t = 0) by +400 mg/L or 10% more than that of k1 @ T = 20°C (with a mean average BODt >99,000 mg/L from k1 @ T = 25°C and a mean minimum BODt <88,000 mg/L from k1 @ T = 25°C), as plotted in Figures 3(b), 3(d), and 3(f). Thus, industrial wastewater BODt increases with temperature by almost +2 mg/L to +3 mg/L per every 1°C, justifying the impact of temperature on BODt as suggested. Again, the power relationships refer to the continuous organic decomposition rates as from equation (1).Figure 3
The determined fitted power relationships between BODt and time for 60 days of minimum and average BODt values collected from analysis of influents in various selected industrial wastewater treatment plants: (a, b) GGL industrial wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; (c, d) Coca-Cola industrial wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; and (e, f) Kumasi Abattoir industrial wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively.
(a)(b)(c)(d)(e)(f)The BODt for domestic wastewater influents were observed with a clear increase of BODt trendline with time, where both average and minimum BODt were found to intersect after a long period (60 days). Such behavior like this could be associated with the not low and not high BODt values for domestic wastewaters, resulting in semisimilar trendlines, which correspond to almost similar organic degradation rates for average and minimum BODt. Here, the power relationships between BODt and time were justified at low and high selected BODt from previously analyzed domestic wastewaters as illustrated in Figure 4. Plotted data sets showed a mean range of an average BODt ≈130–260 mg/L and a minimum BODt ≈50–170 mg/L from k1 @ T = 20°C, as plotted in Figures 4(a), 4(c), and 4(e). Similarly, the same analysis was carried out with k1 @ T = 25°C to find the impact of temperature on the BODt mean ranges, which was found to increase the initial BODt (at t = 0) by +20 mg/L or 25% more than that of k1 @ T = 20°C (with a mean range of an average BODt ≈ 150–250 mg/L and a minimum BODt ≈ 70–160 mg/L from k1 @ T = 25°C), as plotted in Figures 4(b), 4(d), and 4(f). Accordingly, it has been found that domestic wastewater BODt increases with temperature by almost +4 mg/L per every 1°C while having shorter required oxidation times at higher temperatures to achieve a near-complete organic oxidation rate from utilizing the introduced O2 in wastewater. Again, as discussed earlier, the power relationships refer to the continuous organic decomposition rates as illustrated in equation (1).Figure 4
The determined fitted power relationships between BODt and time for 60 days of minimum and average BODt values collected from analysis of influents in various selected domestic wastewater treatment plants: (a, b) North Sewage domestic wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; (c, d) Al-Diwaniyah domestic wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively; and (e, f) Jordan domestic wastewater treatment plant with k1 @ T = 20°C and k1 @ T = 25°C, respectively.
(a)(b)(c)(d)(e)(f)From correlating the average BOD5/COD ratios to the calculated existing DO5 concentration at 60–70% oxygen consumption rates, we established the optimum df to be taken into consideration before experimentation for BODt analysis of various wastewaters. The step V center connection lines (using ORIGIN software), illustrated as solid red lines in Figure 5, show the optimum from df points with an exact average of df ≈ 9.2 for storm wastewater (STM), df ≈ 12 × 103 for industrial wastewater (IND), and df ≈ 18.5–28.5 for domestic wastewater (DOM). The importance of df numbers arises from its usefulness as a starting point for wastewater operators and engineers to select the ideal df that would ensure the success of the experimental (sampling) analysis of BODt of various wastewaters for quantification of organic decomposition rates attributed to the introduced oxygen amounts. Despite that the DO5 parameter values were assumed to be constant at different observed BOD5 (due to the consideration of 60–70% oxygen consumption rates according to the literature [13, 16, 17]), this average oxygen consumption rate would give the best prediction towards estimating df values for each of the studied wastewater types. In other words, the relationships between average BOD5 vs. df, and BOD5 vs. DO5 were found to be both following linear trends with an increase in df when BOD5 becomes higher. According to the previously mentioned “standard methods for water & wastewater examination,” dilution techniques should be done by creating five samples with several dilutions with at least two bottles giving acceptable minimum DO depletion (>2 mg/L uptake after a 5-day incubation period) and residual limits (>1 mg/L) [18]. The plotted linear regressions are useful to facilitate the experimentation and analysis of wastewater samples for optimum selection of df that corresponds to the BOD5 and DO5 levels of wastewater influents. Meanwhile, finding the relationship between average BOD5 and COD is going to give much accurate and rapid analysis because such BOD5/COD ratios are known to serve as a guide in selecting proper dilutions for influents/effluents for the wastewater type of interest.Figure 5
The average 5-day dissolved oxygen (DO5) and the calculated experimental dilution factor (df), used in the BODt experimental analysis, correlated to the observed average BOD5via linear relationships for various wastewater influents: (a) storm wastewater (STM) where average BOD5 values were taken from Poland, Luxembourg, and Nigeria storm wastewater treatment plants; (b) industrial wastewater (IND) where average BOD5 values were taken from GGL, Coca-Cola, and Kumasi Abattoir industrial wastewater treatment plants; (c) domestic wastewater (DOM) where average BOD5 values were taken from North Sewage, Al-Diwaniyah, and Jordan domestic wastewater treatment plants. Note that the use of step V center connection lines (using ORIGIN software) in demonstrating the df points shows an exact average of df≈9.2 for STM, df≈12 × 103 for IND, and df≈18.5–28.5 for DOM that should be used as optimal starting trials in the experimental analysis of BODt of various wastewaters.
(a)(b)(c)The average BOD5/COD ratios for various studied wastewater types (effluents) have been estimated as illustrated in Figure 6(a). Storm wastewater reserved the minimum BOD5/COD ratio among the three wastewater types, which indicates existing of lower amounts of organic compounds (as compared to chemicals) in storm wastewater effluents. Storm, industrial, and domestic wastewaters had an average range of BOD5/COD ratios of 0.1∼0.35, 0.36∼0.5, and 0.4∼0.48, respectively. Obtaining the average BOD5/COD ranges are very useful since they can be used as indicator tools that would help experimentalists to accomplish accurate sampling analysis. Furthermore, using the corresponding df values would allow reaching an optimum design for the treatment of a certain type of influent (or even a mixed influent) in specific wastewater treatment plants. Precisely, the reported exact df averages in Figure 6(b) were justified from our analysis to be the initial trials used for BODt experimental analysis. This would allow supplying the required oxygen amounts to optimally control and disinfect microbiological contamination in wastewaters. Industrial wastewater had the highest average BOD5 and COD values in the order of 103, while storm wastewater had much less BOD5 and COD than that of domestic wastewater, illustrated in Figure 6(b), explaining the reasons for having the lowest BOD5/COD ratio for storm wastewater as shown in Figure 6(a). This has been also confirmed from the determined average BOD5/COD ratios from the various studied wastewater treatment plants presented in Figure 6(c), which were calculated from the previously conducted BODt analysis of wastewater influents used in generating such valuable and accurate standardized guidance and operation charts available in Figures 5 and 6 for quality control engineers who might not need to conduct the laboratory experiments prior taking design decisions.Figure 6
Dilution factor (df) correlated to average BOD5/COD ratios and the estimated average values for the different studied wastewater effluents as influents to various wastewater treatment plants: (a) the average BOD5/COD ratio ranges found from critical analysis and carried out BOD-related calculations for the three wastewater types, with the obtained df possible ranges to be used as trials in the experimental steps (IND, ×103); (b) a comparison between different wastewater characteristics in terms of their calculated average BOD5 corresponding to the wastewater average COD, note that reported average df values here are suggested to be the initial trials used for BODt experimental analysis; and (c) determined average BOD5/COD ratios from the various studied wastewater treatment plants.
(a)(b)(c)The correlation of both df and DO5 (mg/L) to the organic decomposition rates of the organic pollutants that exist in various wastewater types has been studied and plotted in Figure 7. As a rule of thumb, it is proven that there will be always a proportional relationship between existing oxygen (translated as DO5 content) and consumption or organic degradation whereas that the higher the DO5 there in the water, the more the organic decomposition occurs, as mathematically expressed previously in equation (3). One may observe that the df values here had the same optimum df average values for each wastewater type as predicted from Figure 5, with df≈9.2 for storm wastewater (STM), df ≈ 12 × 103 for industrial wastewater (IND), and df ≈ 18.5–28.5 for domestic wastewater (DOM). Moreover, df found to have an approximate range of 4.5–13.5, 1–22 × 103, and 12–35 for STM, IND, and DOM wastewater types, respectively, which can be used as a guidance or a reliable reference when diluting a specific wastewater sample for BODt sampling analysis. It is clear that the storm wastewater had the lowest df range and minimum df optimum average because of its lower BOD5 as well as BOD5/COD ratio, as shown in Figure 6, thus indicating the low quantity of existing organic constituents in storm wastewater influents to be treated at storm wastewater treatment plants. However, the obtained df mean values can serve as a good start for wastewater plant engineers to reach an optimum cost-effective DO5 concentration that would ensure maximum organic decomposition without the addition of excess or unnecessary extra oxygen amounts (Figure 7).Figure 7
The estimated organic decomposition rate (x) correlated to the calculated 5-day dissolved oxygen (DO5) and experimental dilution factor (df) for various wastewater influents: (a) storm wastewater (STM) data analysis from water characteristics in Poland, Luxembourg, and Nigeria storm wastewater treatment plants; (b) industrial wastewater (IND) data analysis from water characteristics in GGL, Coca-Cola, and Kumasi Abattoir industrial wastewater treatment plants; and (c) domestic wastewater (DOM) data analysis from water characteristics in North Sewage, Al-Diwaniyah, and Jordan domestic wastewater treatment plants.
(a)(b)(c)Table1 shows the summarized calculated results obtained from the collected experimental data sets based on the nine selected plants, as shown in Figure 6(c), for the three studied different wastewater types. All the data analysis has been conducted at the average wastewater temperature (T = 20°C) that is assumed to be similar to the average surrounding temperature. Domestic wastewater has low BOD5/COD values since BOD5 (organic contents) was found to be always less than half of the existing nonorganic contaminations (COD or chemicals contents) in wastewater. Typical values for the ratio of BOD5/COD for untreated municipal wastewater is 0.3 to 0.8 and If the BOD5/COD >0.5, the waste is considered easily treatable by biological means.Table 1
Estimated average and minimum BODt and water temperature rise impact with the corresponding BOD5/COD, experimental dilution factor (df), and average df range for the various studied wastewater types.
Wastewater type (Abbrev.)BODt∗ (mg/L)Min. BODt (mg/L)BODt∗ (mg/L) vs. temperature (per every 1°C)BOD5/COD∗df∗df range∗Storm (STM)40–7816–55+1.20.1∼ 0.359.24.5–13.5Industrial (IND)>90,000<80,000+2 to +30.36∼ 0.512 × 1031–22Domestic (DOM)130–26050–170+40.4∼ 0.4818.5–28.512–35∗ Average values from k1 @ T = 20°C, which correspond to the selected wastewater type experimental results.Knowing that UBOD is a parameter for water quality assessment that quantifies the oxygen required for the total biochemical degradation of organic matter by aquatic microorganisms, the depletion of DO5 is a primary water quality concern since the oxygen level is associated with disinfection (pathogen destruction). The DO5 depletion define the microbial use or consumption (i.e., demand) of oxygen during the aerobic oxidation of electron donors such as readily degradable organic carbon (e.g., sugars as glucose) and ammonia in waters as shown in the simplified reactions equations (5) and (6) [21], ignoring the substrate being incorporated into biomass. The general form of the previous equations for decomposition of organic matter can be written as in equation (7), where the rate of biodegradation can be modeled as a first-order reaction (i.e., as a function of the remaining oxygen demand). A portion of the organic matter (the growth substrate) is also converted into cell material, or biomass (the catalyst), so its concentration is changing as well [22].(5)C6H12O6+6O2⟶6CO2+6H2O(6)NH3+2O2⟶NO3−+H2O+H+(7)organic matter+nutrients+O2⟶CO2+H2O+biomass


## 4. Conclusion

We demonstrated the ability to establish empirical correlations between the 5-day biochemical oxygen demand (BOD5) and organic decomposition in wastewater effluents. Collected data sets from earlier sampling analyses included ultimate biochemical oxygen demand (UBOD) and minimum and average BODt along with their reaction rates. With a reaction rate k1 @ T = 20°C, wastewaters would have an average BODt of 40–78 mg/L, >90,000 mg/L, and 130–260 mg/L for the storm, industrial, and domestic wastewater, respectively. There is a strong correlation and dependence of BODt on the water temperature whereas that every 1°C added to the water yield in an increase of +1.2 mg/L, +2 mg/L to +3 mg/L, and +4 mg/L in the initial BODt values for the storm, industrial, and domestic wastewater, respectively. Thus, there will be shorter oxidation times required at higher temperatures to achieve a near-complete organic oxidation rate. The microorganisms’ oxygen uptake rate has to be less than the available oxygen for reduction of BOD. Based on the 60–70% oxygen consumption rates, the author estimated an optimum average of df ≈ 9.2 for storm wastewater (STM), df ≈ 12 × 103 for industrial wastewater (IND), and df ≈ 18.5–28.5 for domestic wastewater (DOM). These df values can serve as a good start in sampling analysis to reach an optimum cost-effective DO5 concentration without the addition of excess oxygen amounts. Such proposed regression equations and standardized guidance charts would bridge the gap between scientific observations and industrial best practices for optimal design and operation to control organic contamination in various wastewater treatment plants.



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