Fentons Reagent tips: H₂O₂ + FeSO₄

I've been having trouble with reducing COD (Chemical Oxygen Demand) below approx. 500 ppm by using Fentons Reagent. 


For 100 gm of the sample (initial COD 3000 ppm) I use 5 gm of 30% $\ce{H2O2}$ & 0.1 gm of $\ce{FeSO4}$. pH is adjusted to between 3 to 5. Temp. is approx. Room Temp. & I've tried reaction times up to 6 hrs. 


Do these sound reasonable choices?


Any advice? 




I'm not an expert in Fenton chemistry but [it appears that difficulty in getting the Fenton reagent to reduce COD to below 500 ppm is not unusual.](http://www.h2o2.com/industrial/fentons-reagent.aspx?pid=143&name=General-Chemistry-of-Fenton-s-Reagent) 


1. Does your particular wastewater have chelants such as phosphates, EDTA, formaldehyde, and/or citric/oxalic acids?
2. Perhaps at long reaction times all the iron you add is being precipitated as iron (hydr)oxides. Could you lower the pH further?



> 
> Reaction rates with Fenton’s Reagent are generally limited by the rate
>  of $\ce{.OH}$ generation (i.e., concentration of iron catalyst) and less so
>  by the specific wastewater being treated. Typical $\ce{Fe:H2O2}$ ratios are
>  1:5-10 wt/wt, though iron levels < 25-50 mg/L can require excessive
>  reaction times (10-24 hours). This is particularly true where the
>  oxidation products (organic acids) sequester the iron and remove it
>  from the catalytic cycle. Fenton’s Reagent is most effective as a
>  pretreatment tool, where CODs are >500 mg/L. This is due to the loss
>  in selectivity as pollutant levels decrease...In addition to free radical scavengers,
>  the process is inhibited by (iron) chelants such as phosphates, EDTA,
>  formaldehyde, and citric/oxalic acids. Because of the sensitivity of
>  Fenton’s Reagent to different wastewaters, it is recommended that the
>  reaction always be characterized through laboratory treatability tests
>  before proceeding to plant scale.
> 
> 
> 


My tip would be to increase the concentration of both $\ce{H2O2}$ and $\ce{FeSO4}$, and, if possible, to make sure the pH is 3 (or maybe even lower). If that doesn't work, do you think iron chelators could be present in your particular wastewater sample(s)?


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