What is refractory COD (water recycling)?
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The contaminated streams will become more concentrated as good-quality water is recovered. If we take a typical industrial effluent with a refractory chemical oxygen demand (COD) of around 50 mg/l, this value increases to an average of 250 mg/l in an RO system that recovers 80% of the feed water. At such a concentration, the effluent no longer meets any of the specified discharge standards. What does \"refractory\" mean in this context?
I believe it can be understood as “difficult to dissolve”
Refractory: difficult to change, difficult to degrade. My understanding is that the COD of this industrial wastewater, which cannot be treated through degradation methods, is 50 mg/L; after reverse osmosis, the concentration rises to 250 mg/L, with an 80% water recovery rate.