Refractory organic compounds refer to organic compounds that can not be degraded by unadapted activated sludge, but can be degraded to some extent after a certain period of acclimation. Some toxic macromolecular organics in wastewater, such as organic chlorides, organophosphorus pesticides, organic heavy metal compounds, polycyclic and other long-chain organic compounds represented by aromatics, are difficult to be degraded by microorganisms. There are also some organic compounds that can not be degraded by microorganisms, which can be called inert organic compounds.

Therefore, the wastewater containing this kind of organic matter should be treated separately by cultivating special microorganisms or by anaerobic and other special processes to convert part of its CODcr into BOD5, improve its biodegradability, and then mixed with other wastewater for secondary biological treatment.

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