Misconnected industrial wastewater entry into rainwater pipe network results in the direct sewage discharge into rivers. Additionally, sudden industrial sewage discharge into sewers without treatment would disturb the normal operation of wastewater treatment plants. To trace the unexpected industrial discharge sources, with food manufacturing as the typical industrial trades, the chemical markers to indicate such kind of contamination were screened. For the selected agricultural and sideline processing or food manufacturing enterprises in China, it was found that potassium, sodium and chloride were ideal markers from the perspective of conservative and significant behavior. Specially, potassium and chloride have a good positive relationship with electric conductivity, providing feasibility for real-time monitoring of industrial wastewater discharge. With these markers, benchmark concentrations to indicate industrial flow of food manufacturing were presented. Accordingly, the method to locate industrial flow source was also proposed based on monitoring at manholes or junctions of urban drainage network.