Hello Mr. Barkhordary,

I think you should check organic loading rate (OLR) to system. If BOD or COD loading is too high the color of mixed liquor may turn to dark even you supply more than 2 mg/L DO. Mixing conditions were also important in these kind of system. You should also supply full mixing conditions for homogen oxygen concentration in this system. We can consider firstly mg COD value of one liter of wastewater (mgCOD/L or mgBOD/L). In addition to these, please make an experiment that showing the color of mix of varying concentrations of  latex-wastewater solution in outside of aeration tank.

We can contact again for your future works

The reasons could be the following,

1. Sulphate reducing bacteria could produce sulphates

2. Generally the colour is dependent on the pH but as you have stated it is 7, the organic loads as stated by Mr. Olcayto Keskinkan,  is to be analysed

3. As you have mentioned the colour is dark not the exact color the following conditions can also  be deduced from you observation like dull water indicates toxic ammonia levels, if dark greenish color is there then algal culture formations.

4. Check if there are any Stinky black sludge at the bottom of the aeration tank. If they are then the aeration is inefficient enabling the formation of anaerobic layers of fluid below like in aeration lagoons. 

Hope if these could solve your issue

Hello Mr. Barkhordary,

from your question it is not clear, what kind of aerobic wastewater treatment are you using. In case that there exists sludge recycling, the black color could be caused by high mineralization of returned sludge which is of black color.

Regards

Vit  

Dear Mr. Barkhordary,

besides the possible explanations given by the other researchers, brownish-black staining might be attributed to formation of iron and/or manganese oxides.

Due to aeration Fe2+ starts oxidizing even under slightly acid pH-conditions (pH 6.6) forming brownish iron oxides. Mn2+ oxidation in solution sets in at a pH higher than ~ 8.6, MnOx precipitation is black and might be responsible for staining your wastewater as well.

I do not not about initial concentration of Mn(II) and Fe(II) as well as pH-redox-conditions of your wastewater. Please check, this might be a possible explanation as well.

Kind regards

I think check a high loading of organic matter in wastewater,

If BOD or COD loading is too high the color of mixed liquor may turn to dark even you supply more than 2 mg/L DO. Mixing conditions were also important in these kind of system. However, besides the possible answers given by the other researchers above,  brownish-black staining might be attributed to formation of iron and/or manganese oxides.