In the design of distributed wastewater treatment systems with multiple contaminants, it is very important to minimize unnecessary stream mixing to reduce total treatment flow rate as much as possible. A new numerical indicator, total mixing influence potential (TMIP), to reflect the influence of the stream mixing caused by performing a process on the total treatment flow rate of a distributed wastewater system is introduced. In design procedure, the TMIP value is calculated based on pinch principle. The process with the smallest TMIP value will be performed first. The results of a few literature examples show that designs with very low (even minimum) total treatment flow rates can be obtained with the method proposed. In addition, the method proposed is simple and of clear engineering insight. The calculation effort does not increase significantly when the number of streams, contaminants, and/or treatment units increases. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 3223-3231, 2015