Most guidelines for urban wastewater treatment plants (WWTPs) are established on a time basis determination to determine performance and compliance with discharge standards and limits (i.e., maximum daily-average). Nevertheless, there is a lack of a systematic analysis of the relationship between instant sampling and daily average discharge concentration values. The present study used the chemical oxygen demand (COD) automated monitoring data that were collected from 1738 WWTPs in China to discuss the relationship between instant sampling and daily average values. A ratio model (K
value) was developed to study the relationship between the reliability of the instant sampling value and the daily average limit for the COD measurements. The K
value revealed that the ratio of COD instant and daily average measured concentrations for WWTPs in China collectively ranged from 1.00 to 1.45. The results of this study suggest setting the K
value of COD to 1.3 for WWTPs in China to estimate the corresponding instant sampling limit of COD, as well as for most WWTPs in China, in order to ensure the stability of compliance to the daily average limits. The instant sampling value of COD in 24 h should be controlled no more than five times out of the instant sampling limit, which is 1.3 times of the daily average limit.
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