Novel techniques to determine dilution ratios of raw wastewater and wastewater treatment plant effluent in the 5-day biochemical oxygen demand test.

The current 5-day biochemical oxygen demand (BOD) test methods lack detailed information on sample dilution, which typically leads to an incorrect dilution ratio of samples and failed BOD testing. This study proposed an improved dilution technique that effectively determines the dilution ratios for raw wastewater and wastewater treatment plants (WWTPs) effluent. The numbers of raw wastewater and WWTPs effluent samples used in the BOD tests were 201 and 61, respectively. The experimental results demonstrated that both the raw wastewater and WWTPs effluent varied in BOD values with the sample dilution ratio, and these changes were more noticeable when the dissolved oxygen consumption ratio (DOCR) was less than 20% or when the dilution ratio was large. Assuming that the BOD value over the DOCR range of 40%-70% was true, the optimal DOCR range was 40%-90% for the raw wastewater and 40%-70% and 80%-90% for the WWTPs effluent, where the relative error of BOD values in these DOCR ranges was less than 10% depending on the dilution ratio. The correlation between the sample dilution ratio and the BOD value over the optimal DOCR range was considerably higher than that over the entire DOCR range. This was combined with the correlation equation between BOD and chemical oxygen demand to propose an equation that could determine more accurate sample dilution ratios for raw wastewater and WWTPs effluent compared to the conventional sample dilution methods for the BOD test.