Issues related to Fenton treatment of electroplating wastewater

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Our company handles the grinding wastewater from an electroplating plant; this wastewater is generated during the surface treatment of chromium-boron steel materials. Based on the chemicals added in the production process, the main pollutants in this wastewater are stone dust, iron, rust inhibitors (alkalis, trisodium phosphate), and grinding fluid (which mainly contains EDTA). The raw water is a relatively clear green liquid; the COD level is around 10,000 mg/l, the pH is 2–4, the total nickel content is 0.32 mg/l, and the total chromium content is 2–4 mg/l. I believe that heavy metals will definitely be complexed by EDTA. I want to use Fenton oxidation to break this complexation first; on one hand, this can release the heavy metals, which can then be removed through coagulation and sedimentation, and on the other hand, it can decompose part of the EDTA, thereby reducing the COD concentration. Based on the COD ratio: hydrogen peroxide:feverfew sulfate = 1:1 (by mass), with a ratio of hydrogen peroxide:ferrous sulfate = 8:1, the chemicals were mixed under pH conditions of 2–4. This resulted in the formation of many white flocculent particles; it took more than 3 hours for these particles to settle (the settlement rate was around 50%, and the sediment did not resemble ordinary sludge as it was quite light). The supernatant remained green, though its color was slightly lighter. I then added NaOH to raise the pH to around 10, at which point the color changed to orange-yellow (the color of rust). Adding lime milk, Na2S, PAC, and PAM did not yield good coagulation results; the resulting sediment particles were very fine, and it took an entire night for them to settle completely (the settlement rate was around 30–40%). The supernatant remained orange-yellow, again with a slightly lighter color. The COD removal rate was approximately 50%; there was almost no reduction in total nickel or total chromium levels, while total phosphorus levels actually increased (from 0.42 mg/l in the raw water to 15 mg/l in the treated water). I would like to ask the experts: what exactly are those white flocculent particles that form during the Fenton process? Why can’t it be effectively decolorized? Why is the coagulation effect so poor? Why does total phosphorus increase? Are there any ways to improve the COD removal rate?