This study aims to determine and evaluate the correlation between BOD5 and COD by calculating the R2 and r, mean (x), and standard deviation (SD) and to calculate the BI of each month as well as to evaluate the performance of the plant in terms of BOD5 and COD removal. Microsoft Excel spreadsheets were used for all data statistical analysis. The 95% confidence intervals we

For four months, including the summer (NOV- 2021 to DEC- 2021) and winter (JAN -2022 to FEB - 2022) seasons, composite samples were taken from the effluent tank of the Al-Hasa wastewater treatment plant. Before collecting the wastewater sample, the water bottle was washed with KMnO4. The estimation of several physicochemical characteristics required the immediate transfer of the wastewater samples to the laboratory.

The data of this study has been analyzed from the Al- Hasa common industrial wastewater treatment plant for one year NOV-2021 to OCT- 2022, following the standard methods (APHA) 14.

Standard techniques and statistical measures like mean, SD, R², and r were used to determine the COD and BOD5 relation in the samples. Microsoft Excel spreadsheets and origin lab were used for all statistical data analysis. The 95% confidence intervals were used.

The coefficient of determination is a word used in regression analysis and analysis of variance (often abbreviated R²)15. In traditional regression analysis the usage of R², also known as the multiple correlation coefficient, is well-established and its usefulness as a gauge of the effectiveness of predicting the dependent variable from the independent variables is derived from its definition as the proportion of variance "explained" by the regression model 16.

R² is a coefficient of determination that assesses the goodness of fit by calculating the proportion of explained variance from the regression equation vs the variance indicated by simply taking the mean value y that is mentioned by x "17.

Correlation coefficients describe the strength and direction of an association between variables18. The relationship between the two variables is referred to as the correlation and is measured with a value that ranges from -1 to +1. Zero denotes a lack of connection, +1 denotes a full or ideal correlation, and -1 denotes a weak correlation. The correlation’s strength rises from 0 to +1 and 0 to -119

In WWTP, equalizations tanks are generally built, because their primary purpose is to serve as a barrier, to gather the incoming raw wastewater, which is delivered at wildly varying rates, and transport it to the other parts of the effluent treatment plant at a constant flow rate (Figure 2. represents the working of equalization tank)20.

MBR (Figure 3 is a biological process combined with membrane filtration in this instance, where the breakdown of the biomass takes place inside the bioreactor tank and the membrane module completes the separation of the treated waste water from microorganism 22.

The correlations relation are indicated as below in following figures

Table 3, illustrates the R2, r, and regression equation. The value of r from NOV-2021 to OCT-2022 indicates a weak correlation between them, resulting in difficulties in predicting the BOD5 results depending on COD. However, in contradiction the AUG-2022 result shows a strong correlation of r = 0.8881, As BOD5 and COD have a good association, it is simple to anticipate the BOD5 results based on COD in this month. On the other hand, Figure 4,Figure 5,Figure 6,Figure 7,Figure 8,Figure 9, Figure 10,Figure 12,Figure 13,Figure 14, Figure 15 display the correlation between BOD5 and COD of industrial wastewater using the equation of y = MX + c. The confidence interval for the slope in Figure 6,Figure 7 indicates that 95% confidence in the BOD5 value resides between 14.4 mg/l to 18.3 mg/l and 1.0 mg/l to 13.2 mg/l of BOD5 for months NOV and DEC-2021 respectively, the R2 values reveal that more than 0.69% and 0.72% of the total variance is above the mean of 18.03 mg/l and 8.72 mg/l BOD for the months NOV and DEC-2021 respectively. Whereas for the month of JAN to FEB-2022 at a 95% confidence interval, BOD5 lies between 111.5 to 112 mg/l and 111 to 120 mg/l respectively, and the R2 values reveal that more than 4.85% and 4.04% of the overall variance exceeds the BOD mean 161.5 mg/l and 116.71 mg/l of JAN and FEB-2022 respectively (Figure 8,Figure 9). On moving for- ward in the months of MAR ,APR and MAY-2022, values of BOD5 at the same confidence interval lies between 16.24 mg/l to 20.33 mg/l ,14.74 mg/l to 18.52 mg/l and 19.29 mg/l and 22.26 mg/l respectively, and the R2 values shows that more than 1.48% ,9.8% an d6.16 %of the total variance is greater than 18.29 mg/l and 16.65 mg/l respectively. In a similar way months from JUN- to AUG-2022 shows a similar trend ,as per the R2 values more than 0.08%, and 14.15% of the total variance are above the BOD5 mean of 29.36 mg/l, 24.26 mg/l, and 27.22 mg/l, respectively. Finally in months of SEP and OCT-2022 the R2 value depicts that 6.84% and 1.11% of the total variance is greater than the 32.83 mg/l, 28.53mg/l respectively. table 2

These correlations are in accordance with the findings of previous studies, Thambavani ,et al. 2008 25 evaluates samples from OCT- 2010 to MAR-2021 to determine the relationship between COD and BOD5 of sugar mill effluents, they concluded that both parameters in sugar mill effluents had r of -0.94, indicating a very significant negative correlation between them. Although the R2 reveals a value of 0.88,showing a strong connection between them.

Similar to this, Ahmad ,et al. 26 their study determines a strong link by calculating the R2 between BOD5 and COD of influent samples from the Al Diwaniya WWTP from JAN- 2016 to MAR -2016. The R2 from JAN- 2016 to MAR- 2016 was 0.92, 0.97, 0.90, and 0.77, indicating a stronger correlation between BOD5 and COD. Dahamsheh ,et al. 12 also finds the relationship between the inlet BOD5 and inlet COD of the industrial WWTP at Al-Hussein bin Talal University, the average correlation between the BOD5 inlet and COD inlet, which was determined after collecting the 200 water samples over three years, was R2 = 0.0068, indicating a very weak correlation between them. A similar study was also carried out by Venkatesh, et al. 27 to establish a correlation between BOD5 and COD by collecting domestic sewage; they observed r of 0.98, showing a strong correlation. Whereas a coefficient of determination study was carried out by Haleem, et al.28 , by taking the samples from the Tigris River in Iran the value of R2= 0.91 shows a strong correlation between the BOD5 and COD. Daniel, et al.13 con- ducted a similar investigation by collecting the industrial effluent from a wastewater treatment plant in Rome and found that BOD5 and COD had a good association, with an R2 of 0.98 and r = 0.99 showing a strong correlation between them.

In terms of effluent quality, the performance and effectiveness of the Al -Hasa WWTP were assessed. Based on measurements of BOD5 and COD taken during plant operations over one year (NOV- 2021 to OCT- 2022) the evaluation was made.

The results in Table shows that, MAR-2022 had the good BOD5 removal efficiency of 91.52%, compared to all other months, however the result of JAN and FEB -22 had the lowest removal efficiency of 23.58% and 23.62% respectively. This low removal efficiency of plant may be due to the excessive organic loading coming to the influents. High or fluctuating salinity were also observed in the effluents of this plant ,which poses a challenge to biological treatment processes, and it is the major cause of plant failure29.

This analysis was supported by Ali,et al.30 in Haridwar, India, at an MBBR-equipped common wastewater treatment facility, where 77% of BOD was removed, demonstrating the plant's moderate removal effectiveness. Sundara, et al.31 also conducted a study at the Nesapaka STP by using activated sludge, this study endured 6 months (JUN -2009 to NOV -2009), and each month they observed a removal efficiency of more than 90% for BOD5, demonstrating the plant's good performance. In addition, Iwano, et al.32 conducted research to evaluate the efficacy of treating industrial wastewater, this study was conducted at the mechanical-biological wastewater treatment plant in Bystre near Giycko, which receives a mixture of household and dairy wastewater for one year, they found that the plant has an excellent removal efficiency of 97.86% to 99.75% for BOD.

Results shows that JAN- 2022, had low COD removal efficiencies of 7.03% followed by FEB and JUN-22. While MAY-2022, as compared to other months, had the best COD elimination efficiency of 91.22%. Ahmad, et al. 34 conducted a similar study in Tehran using wastewater from a petroleum refinery by using the activated sludge, they observed a 96% COD removal efficiency, showing the plant's good performance. Additionally, a three-year (2006-2008) study by Abma, et al. 35 was carried out at the Olburgen wastewater treatment facility in the Netherlands to evaluate the efficiency of COD removal, for the following three years, they found removal efficiencies of 46%, 67%, and 56%, respectively, demonstrating the plant's moderate removal efficiency. Ashutosh, et al.36 did an analysis like this one based on removal efficiency at Kaithal, India, from JAN- 2014 to APR- 2014, they found that the plant was operating at a good removal efficiency of 90.84% of COD.

Figure 16. represents the monthly variation of BI of the effluent from the Al-Hasa WWTP. From the month of NOV- 2021 to OCT-22 ,BI lies under the 0.5, this shows that the effluent still does not meet disposal criteria even after the current treatment, necessitating additional treatment options .This finding shows that the treated effluent quality is poor and the current treatment plant is completely unable to raise the effluent requirements to a disposable condition.