A system and method for monitoring process water treated with a biocide is provided. The system includes a biocide feeding unit and a dissolved oxygen sensor. The dissolved oxygen sensor works in two modes, a biocide feeding mode and a background mode, and alerts an operator when the dissolved oxygen value indicates a fault in the system. A drop in dissolved oxygen during feeding can indicate faulty production of biocide or biocide degradation, both of which can lead to unwanted disinfection by-products.