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Can metals affect the COD of wastewater?

The effluent wastewater from our FCC unit suddenly reached over 1000 ppm of COD.

It contains Fe(0.5ppm), Al(1.7ppm), Ca(3.1ppm), K(16.8 ppm), Na (23800 ppm), Ni (0.2ppm), P (6.6ppm), V(6.6ppm), Zn(0.4ppm), Cl(105.3ppm).

pH is about 6.8.

Can anyone explain whether these metals affect COD? and why?

Feb-2022

Answers

Prakash Pimparkar, Environmental Consulting Services, prakash.pimparkar@yahoo.in

Metal Ions do not contribute to COD. Also 1000 mg/l of COD is not high for FCC wastewater (sour condensate). COD in this wastewater is contributed by Sulphides and Ammoniacal Nitrogen (NH4 Ion).You have also not mentioned if it is VGO FCC or Residue FCC. Assuming it is Gas Oil It would have phenols and some aromatics contributing to COD. The quality of wastewater ( COD) would depend upon Feed quality to the FCC unit and the severity of cracking. So if you have changed the Feed corresponding quality change is expected.

As said earlier 1000 COD is reasonable also this waste is mixed with other waste streams before treatment ( variations in quality at different times of different units is also expected and the treatment plant can deal with it . Most treatment plants have surge or equalisation tanks further reducing the impact. Treatment plants also have built in additional load (BOD/COD) capacities as design margins.

Overall this is not an alarming situation and quite normal If it is expected that quality is going to be affected all time and the resulting in upset if treated wastewater quality then and only then you need to think of WWTP capacity enhancements which can be done many times without changing hardware.

May-2022
Nagarathinam S Murthy, McKinsey & Company, Chennai, nssvdvr@gmail.com

COD implies Chemical Oxygen demand which means any thing that can get oxidized from nascent state will fall under COD. However, the metals what one might see in FCC OH system will be one of the following viz. FCC E Cat micro fines, Corrosion products and CW leak in OH cooler / condenser. As regards, sudden spurt in COD, it need not be from any of the metals (which will be present in either oxide or sulphide form) in OH but it can be from dissolved organics like phenols, low mol wt acids, amines, etc. Look for changes in FCC feed quality too. Also, assess the sour water for TOC (Total Organic Carbon), which will give a reasonable correlation with COD. Additionally, it will be worth checking that there is no HC mix up while doing the COD test in lab.

Feb-2022