Contact oxidation

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This is wastewater from pharmaceutical intermediates; the pretreatment process involves Fenton micro-electrolysis. There is a lot of foam in the contact oxidation tank, along with sediment on the surface. Initially, I thought it might be due to sludge aging (the sludge added by the client at the beginning of March) or an excessive amount of aeration. After reducing the aeration volume, the foam decreased. When performing sedimentation tests, fine sludge was found in the supernatant. The pH level in the contact oxidation tank drops rapidly, usually from 7 to 6 within one night. Out of concern that the operators on site might adjust the pH too much, I usually adjust it myself； The color of the water exiting the system is gray-green and it is clear and transparent. The COD level of the water after ABR treatment is 1300, while it ranges between 400 and 500 after contact oxidation treatment; the required standard is below 500, so the requirements can now be met ； It’s normal for the water volume to increase suddenly by 3 times due to improper operation the day before yesterday and yesterday; no water supply was scheduled for that night ； My questions are: 1. What causes foam in contact oxidation and how can it be resolved? ； 2. Color of normal sludge and effluent in contact oxidation ； I hope the experts here can help! Thank you so much!